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2019 Annual Report

Cover Image

A Graphene Superconductor That Plays More Than One Tune

Professor Feng Wang’s group and collaborators have developed a graphene device that’s thinner than a human hair but has a depth of special traits. It easily switches from a superconducting material that conducts electricity without losing any energy, to an insulator that resists the flow of electric current, and back again to a superconductor – all with a simple flip of a switch. The material is composed of three 2D layers of graphene (gray) on 2D layers of boron nitride (red and blue) to form a repeating pattern called a moiré superlattice. Superconductivity is indicated by the light-green circles, which represent the positive charge (the hole) sitting on each unit cell of the moiré superlattice.

Article Citation: Chen G, Sharpe AL, Gallagher P, Rosen IT, Fox EJ, Jiang L, Lyu B, Li H, Watanabe K, Taniguchi T, Jung J, Shi Z, Goldhaber-Gordon D, Zhang Y, Wang F. Signatures of tunable superconductivity in a trilayer graphene moiré superlattice. Nature. 2019 Jul 17. DOI: 10.1038/s41586-019-1393-y.

Table of Contents

3 Mission

4 Director’s Perspective

6 Programs

7 Kavil ENSI Faculty

11 Student Engagement

12 Spotlight on Fellows

26 Thesis Prize

27 Faculty Awards

28 Winton Program for Physics and Sustainability / Kavli ENSI Exchange Program

32 Institute for Basic Science / Kavil ENSI Exchange Program

33 Research Highlights

72 Publications

Mission

Nature is brimming with examples of efficiency, from the way plants capture energy in sunlight to the molecular machines that make muscles contract. The Kavli Energy NanoScience Institute (ENSI) at UC Berkeley, partnered with the Lawrence Berkeley National Laboratory, brings together some of the world’s top researchers from across the fields of materials science, physics, engineering, and biology. Their investigations into nature’s ways of managing energy at the nanoscale will lead to real change in our capacity to generate, store, and use energy. Together, these researchers aim to improve the performance of existing energy technologies and develop entirely new ways of harnessing energy for the world’s growing population.

Director’s Perspective

Peidong Yang, Director, Kavli Energy

That Ubiquitous Nano-Energy

Nano-Energy is ubiquitously present in our living environment. There are many good examples of nanoscale energy transformation, interconversion, and control, from the way plants capture energy in sunlight to the molecular machines that make muscles contract.

Energy Nanocircuitry is about directional transport of energy carriers (photon, electron, phonon, ions/molecules) and their coupling and conversion. At the nanometer scale, we are exploring new energy conversion units, guiding units, as well as directional transport and amplification units (e.g. thermal, photonic, chemical, and nanofluidic diodes and transistors). The insights gained will be used to establish the fundamental principles and ultimate limits of nanoscale energy conversion processes, and to investigate new architectures for nanoscale energy conversion systems.

Currently, the Kavli ENSI’s research is focused around six themes:

Artificial Energy Conversion and Circuits: Create energy-producing circuits that employ tailored nanoscale components.

Chemical Transformation and Catalysis: Develop networks of complex 4 artificial catalysts that produce fuel and other intricate molecules.

Nanoscale Control of Energy Flow: Use the quantum mechanics of photosynthesis to develop efficient ways to harvest light energy.

Nanoscale Motors: Devised manmade tiny motors that can mimic that efficiency of molecular motors found in nature.

Thermal Energy and Circuitry: Build new circuits from specially designed nanostructures to transmit, store, control, and convert thermal energy.

Energy Systems Design: Craft entire synthetic nanoscale systems with multiple energy-related functions.

Energy research, by its nature, is highly interdisciplinary. We are happy that this year we have four additional outstanding scientists joining our ENSI faculties. They are Dr. Alessandra Lanzara; Dr. Ali Javey; Dr. Ting Xu; and Dr. Kristin Persson. Their participation will continue to enrich our ENSI research portfolio and bring unique and deep insights towards our fundamental understanding of the nanoscale energy transformation.

Kavli ENSI has received matching fund gifts from the Heising-Simons Foundation, establishing a Heising-Simons Junior Fellowship Program, and a donation from the Philomathia Foundation, establishing the Philomathia Graduate Student Fellowship Program. We are going to continue our strong community building efforts. Starting academic year 2019-2020, we will inaugurate the Kavli ENSI Lectureship. Each year, we will bring in two distinguished speakers in the general area of nanoscience and energy science. This will be a great platform for the ENSI students, postdocs and faculties to interact with other leading experts in the field, and to openly exchange ideas and opinions. We will also initiate the annual ENSI retreat where the ENSI students, postdocs and faculties can share their thoughts on the challenges and opportunities in their research.

Energy Nanoscience also presents a great opportunity for global engagement and collaboration. A globalized interdisciplinary approach will significantly push the scientific boundary and open up new exciting research frontiers. In this regard, we will continue to expand our scientific collaboration with many other institutes as the international science community has now renewed its engagement on problems of energy conversion and utilization.

-Peidong Yang

Strong Partnerships for the Future

Kavli ENSI has implemented numerous programs to increase interdisciplinary collaborations including:

KAVLI ENSI HEISING-SIMONS JUNIOR FELLOWSHIP Established in 2016, the Heising-Simons Junior Fellowships are two-year postdoctoral fellowship positions designed to attract the most talented and promising young researchers in nanoscale science.

KAVLI ENSI PHILOMATHIA GRADUATE STUDENT FELLOWSHIP Established in 2016, the Philomathia Graduate Fellowships are one-year graduate fellowship positions designed to give promising young researchers the opportunity to work across the disciplines of nanoscience to discover new ways in probing and/or controlling energy processes on the nanoscale.

UC Berkeley – Kavli ENSI Graduate Student Fellowship in NanoScience In 2018, the Kavli ENSI established a new four-year fellowship award in nanoscience. Candidates for the Kavli ENSI Fellowship in Nanoscience will participate in a campus-wide, cross-disciplinary competition for multi-year fellowships administered by the Graduate Division. This competition is recognized as one of the pillars of Berkeley’s comprehensive excellence, as uniformly high standards are applied to the nominees from all programs — including the range of fields that intersect with nanoscience research, including chemistry, engineering, physics, and environmental science.

Student Thesis Prize Awards The Kavli ENSI Thesis Prize is awarded annually to a graduate student for his/her outstanding PhD thesis based on their quality of work, publication status, strength of up to two supporting letters, CV of the nominee, and relevance to the thesis of the Kavli mission.

Winton Program for the Physics of Sustainability, U.K. - Kavli ENSI Exchange Program An Exchange program between the Winton Program for the Physics of Sustainability (Winton Program) at the University of Cambridge and the Kavli Energy NanoScience Institute (Kavli ENSI) was established in 2016 to promote and further academic links between the two institutions. In that time, this partnership has produced a shared commitment to exchange ideas and solutions in order to advance science through joint workshops, sharing resources, and academic exchanges of postdoctoral researchers and faculty members annually. The exchange program would run a competitive processes, explore joint hires and resources, and would continue in subsequent years.

Institute for Basic Science, South Korea - Kavli ENSI Exchange Program In 2017, Kavli ENSI and IBS signed a memorandum of understanding to establish an informal partnership between the two organizations. The scope of the program is to develop activities that advance scholarship through cooperative relationships.

Kavli ENSI Faculty

Pa ul Alivisatos Carlos Bustamante Michael Crommie Founding Director, Kavli ENSI Professor of Chemistry Co-Director, Professor of Physics Professor of Chemistry & Materials Science and Engineering

Felix Fischer Graham Fleming Naomi Ginsberg Assistant Professor of Chemistry Professor of Chemistry Assistant Professor of Chemistry and Physics

Tsu-Jae King Liu David Limmer Jeffrey Neaton Professor of Electrical Assistant Professor of Professor of Physics Engineering Chemistry

Kavli ENSI Faculty

Irfan Siddiqi Gabor Samorjai Feng Wang

Professor of Physics Professor of Chemistry Professor of Physics

Birgitta Whaley Eli Yablonovitch Omar Yaghi Professor of Chemistry Professor of Electrical Co-Director, Kavli ENSI Engineering Professor of Chemistry

Piedong Yang Alex Zettl Xiang Zhang Director, Kavli ENSI Professor of Physics Professor of Mechanical Professor of Chemistry Engineering

New Faculty Joining in 2019

KRISTIN PERSSON

Associate Professor, Materials Science and Engineering

Persson obtained her Ph.D. in Theoretical Physics at the Royal Institute of Technology in Stockholm, Sweden in 2001. She is a Professor in Materials Science and Engineering at UC Berkeley with a joint appointment as Faculty Staff Scientist at LBNL. Persson is the Director and co-founder of the Materials Project (www.materialsproject.org); one of the most visible of the Materials Genome Initiative (MGI) programs, attracting over a hundred thousand users worldwide. She is known for her advancement of data-driven materials design and materials informatics. Applications include materials in extreme pH environments, novel multivalent electrode and electrolyte materials, novel photocatalysts for water splitting and CO2 reduction, and new polar materials. Persson serves as an Associate Editor for Chemistry of Materials, on the NSF Advisory Committee for Cyberinfrastructure, on the MRS Program Development Subcommittee and is the MGI ambassador for The Metal, Minerals, and Materials Society (TMS). She is one of five thrust leads in the Joint Center for Energy Storage Research (JCESR) and heads the NSF-funded Local Spectroscopy Data Infrastructure. She has received the 2018 DOE Secretary of Energy’s Achievement Award, the 2017 TMS Faculty Early Career Award, the LBNL Director’s award for Exceptional Scientific Achievement (2013) and she is a 2018 Kavli Fellow. She holds several patents in the clean energy space and has co-authored more than 150 peer-reviewed publications.

ALI JAVEY

Professor, Electrical Engineering and Computer Sciences

Ali Javey received a Ph.D. degree in chemistry from Stanford University in 2005, and was a Junior Fellow of the Harvard Society of Fellows from 2005 to 2006. He then joined the faculty of the University of California, Berkeley where he is currently a professor of Electrical Engineering and Computer Sciences. He is also a senior faculty scientist at the Lawrence Berkeley National Laboratory where he serves as the program leader of Electronic Materials (E-Mat). He is a co-director of Berkeley Sensor and Actuator Center (BSAC). He is an associate editor of ACS Nano. Javey's research interests encompass the fields of chemistry, materials science, and electrical engineering. His work focuses on the integration of nanoscale electronic materials for various technological applications, including low power electronics, flexible circuits and sensors, and energy generation and harvesting. He is the recipient of MRS Outstanding Young Investigator Award (2015), Nano Letters Young Investigator Lectureship (2014); UC Berkeley Electrical Engineering Outstanding Teaching Award (2012); APEC Science Prize for Innovation, Research and Education (2011); Netexplorateur of the Year Award (2011); IEEE Nanotechnology Early Career Award (2010); Alfred P. Sloan Fellow (2010); Mohr Davidow Ventures Innovators Award (2010); National Academy of Sciences Award for Initiatives in Research (2009); Technology Review TR35 (2009); NSF Early CAREER Award (2008); U.S. Frontiers of Engineering by National Academy of Engineering (2008); and Peter Verhofstadt Fellowship from the Semiconductor Research Corporation (2003).

TING XU

Professor, Chemistry and Materials Science & Engineering

Ting Xu received her PhD degree from the Department of Polymer Science and Engineering at University of Massachusetts, Amherst, and was a joint postdoctoral fellow at University of Pennsylvania and National Institute of Standards and Technology.

Ting has been actively involved with many societies, including MRS, the American Physics Society (APS), American Chemical Society (ACS) and Biophysics Society (BPS). She organized and chaired symposia at both APS and ACS national meetings and is one of organizers of BPS US-Brazil thematic meeting. She also serves on the Scientific Advisory Committee at Center of Integrated Nanoscience Center (CINT). She has also contributed to several roadmap reports for various federal agencies including NSF, DOE, and DOD. She has authored over 100 peer-reviewed papers and books and has been recognized with several awards.

Ting’s research interests rest at the interface between materials chemistry, biology and systems engineering with a focus on soft materials. Her research aims to (1) understand assembly processes in multi-component systems, and (2) apply the fundamental knowledge gained to control the assembly kinetics and pathways to generate hierarchically structured hybrid materials. Her research group develops functional materials for life science, environment and energy applications.

ALESSANDRA LANZARA

Professor, Physics

Dr. Alessandra Lanzara is the Charles Kittel Professor of Physics at the University of California Berkeley and Senior Faculty Scientists at the Lawrence Berkeley National Laboratory. She is also the Chair of the American Physical Society Far West Section and the Director of the International School of Non-Equilibrium Phenomena at the Ettore Majorana Institute in Italy. She serves in several scientific councils such as the National Research Council of Italy.

She is a founder of a quantum detection company, QUAD. Lanzara has made groundbreaking contributions to the fundamental physics of quantum materials. Her accomplishments include discovery of spin momentum locking and electron-phonon interaction in high temperature superconductors, studies of symmetry breaking in graphene resulting in ability to drive bandgap and Fermi velocity engineering; discovery of spin orbit entanglement and spin photocurrents with high efficient optical control in topological insulators. She has also developed novel methods for the synthesis of large size graphene wafers and developed novel state of the art tools for detection and direct imaging of spin in materials. She serves in the Editorial Board of Europhysics Letters, Nature- Scientific Reports and Solid State Communication.

Dr. Lanzara has been recognized for her accomplishments, with several awards such as the Fibonacci Prize, the Marie Goepert Award from the American Physical Society, Fellow of the American Physical Society, Shirley Award and the McMillan Prize. Dr. Lanzara received a Bachelor's degree in Physics in 1995 from the University of Rome La Sapienza and a Ph.D. in Physics and Materials Science from the same university in 1999. She was a post-doctoral fellow at Stanford University between 1999 and 2002. She began her career with UC Berkeley and LBNL in 200

Student Engagement with the Global Scientific Community

For the 2018-19 academic year, the Kavli ENSI graduate students and postdocs further cemented the role of the institute in establishing collaborations and scientific exchanges at UC Berkeley. This year we focused on three areas to enhance the role of the Kavli ENSI in our community: 1.) fostering communication internally between students, postdocs, and Kavli ENSI Resources within our institution, 2.) engaging with researchers at other universities to bring outside ideas to UC Berkeley, and 3.) promoting the work of our students and postdocs externally through conference attendance and exchange programs.

Internally, we promoted efforts to speed the exchange of ideas and resources within the Kavli ENSI at Berkeley. Over the past year, our graduate student and postdoctoral scholar group has continued with 100% attendance for monthly meetings, which include both research talks and institute planning. In these meetings, students and postdocs take the opportunity to share their research and connect with other groups in the institute as well as steer the direction of the Kavli ENSI. The students and postdocs this year also hosted an open house in which research groups presented posters of projects that could benefit from collaboration, and campus resources such as the Chemistry Molecular Computation and Graphics Facility advertised their capabilities. In addition, we continued events including a “Flash Talk” series to promote the work of students and postdocs within UC Berkeley. Finally, the Kavli ENSI continued to push for truly interdisciplinary and collaborative work through its graduate and postdoctoral fellowships.

This year we also focused on bringing new ideas to the Kavli ENSI at UC Berkeley. The institute hosted three seminars in which external speakers engaged with our community. In August 2018, Kavli ENSI hosted Prof. A.K. Sood from the Indian Institute of Science, Bangalore to discuss Ultrafast Photoresponses of 2D Materials. Also in 2018, Dan Congreve, the Rowland Fellow at Harvard University, spoke about Next Generation Nanomaterials for Light and Energy. In November 2018, Prof. Tobias Hertel of Julius-Maximilians University, Würzburg presented regarding doping in carbon nanotubes.

Our third focus was to encourage students and postdocs to promote research at the Kavli ENSI beyond Berkeley. This year we began a new program to provide travel grants to students and postdocs to attend conferences or give invited talks.

Stephen (Matt) Gilbert 2018-2019

Stephen (Matt) Gilbert was awarded the Philomathia Graduate Student Fellowship, from Fall 2018- Spring 2019.

Advisor: Prof. Alex Zettl

As the 2018-2019 Kavli ENSI Philomathia Graduate Fellow, I have had the opportunity to have a particularly productive year thanks to the ability to direct my own research and disseminate my findings to the community. This year I have co-authored 5 publications (2 under review) including two first-author papers (with two more in preparation). I presented my research around the world including invitations to speak at Sandia National Laboratories, Cambridge University, Tel Aviv University, and Intel, in addition to contributed presentations at the Graphene 2019 conference and the Gordon Research Conference on Two-Dimensional Materials.

My work has been recognized through several awards including the Lars Commins Memorial Award for excellence in experimental physics, the Winton-Kavli Graduate Exchange Scholarship, and a Graphene 2019 student award.

My key discoveries this year mainly centered around finding new methods for controlling two- dimensional materials with electron and ion irradiation. Using a newly discovered orientation for a two-dimensional insulator, hexagonal boron nitride, I found that we can push the limits of nano-lithography towards atomic precision. Collaborating extensively within the Kavli Institute, we found that patterning this material can unleash new and unexpected electronic and structural properties. I also invented a new graphene actuator that allows for the remote control of rotors using an electron beam.

In addition to growing my own research, I have had the opportunity to give back to the university community through mentoring. In the past year, I have mentored 5 undergraduate students in the laboratory, tailoring projects to suit their interests and pushing them towards success in their academic careers. Next year, two will be attending graduate school at Harvard and UC Irvine, two will begin research careers at Lawrence Berkeley National Laboratory, and one will remain with the Zettl Group here at UC Berkeley.

Justin Ondry 2019-2020

Justin Ondry was awarded the Philomathia Graduate Student Fellowship, from Fall 2019- Spring 2020.

Advisor: Prof. Paul Alivisatos

Controlling Quantum Dot Coupling in Coherent Arrays of Core-Shell Nanocrystals

Colloidal semiconductor nanocrystals have many impressive size-tunable properties when considered as individual isolated materials. Building on this, many interesting emergent properties may arise if the electronic states in the individual quantum dots are strongly and controllably coupled. This endeavor is difficult because the native organic ligands on colloidal nanocrystals present a large (>1eV) tunneling barrier preventing strong optical or electronic coupling between quantum dots. Groups have successfully lowered these barriers by removing the organic ligands and letting the inorganic cores crystallographically fuse.(1) For these materials, tight binding calculations predict many interesting emergent properties such as a Dirac-like mini-band structure with a band gap that result from the Figure 1: (A) Scheme for assembling and attaching well-faceted core shell CdSe/CdS nanocrystals into superlattices with ordered nanocrystal positions and coupling of the quantum atomic ordering of the underlying crystal lattice. Nanocrystals are assembled on a dots.(2) While this DMF subphase with their native oleate ligands, then the ligands are exchanged to strategy is powerful, it is tert-butylthiol while floating on the DMF subphase. Next the nanocrystals are difficult to controllably vary transferred to a substrate and gently annealed to remove the ligands and immobilize the coupling between the particles, locking in the order. Finally, SILAR is used to infill the voids between particles, necessary to the particles with the desired material leading to attached nanocrystals. (B) Scheme tune their properties. showing how the initial nanocrystal shell thickness can be tuned to modulate the To circumvent some of coupling between the CdSe cores. these limitations, we have developed a different strategy to prepare coupled quantum dot systems. In this strategy we use well-faceted core

shell nanocrystals which because of their shape can readily assemble into superlattices with nanocrystal and atomic ordering on A DMF subphase (Figure 1A). Once the superlattices are formed, the ligands are replaced with tert- butylthiol, an easily thermally degradable ligand, and the superlattice is transferred to a substrate and annealed to lock the superlattice in place. Next the voids between the particles are infilled with either CdS or ZnS via a Figure 2: (A) The structure of native ligand capped nanocrystals, (B) TEM SILAR treatment, which image of and assembled superlattice of oleate capped CdSe nanocrystals, (C) is similar to atomic layer Small angle electron diffraction pattern collected over an 11µm2 area showing a single crystal hexagonal pattern. (D) Wide angle electron diffraction pattern over deposition. Next an the same area and (E) HRTEM image of assembled nanocrystals with and inset annealing step can be Fourier transform of the image. (F)-(J) Same characterization for the t- used to improve the butylthiol exchanged particles, (K)-(O) for particles joined with 6 monolayers of connectivity between CdS SILAR, and (P)-(T) 6ML of ZnS SIALR. the particles and possibly heal defects which may have formed at the interfaces. Since this strategy is modular, the initial particles used can be interchanged and for example, the inter-core spacing can be tuned to modulate the coupling between the quantum dots (Figure 1B).

Figure 2 shows structural characterization for the different steps of the developed nanocrystal assembly process. For the native particles, separated by the ligands, TEM imaging shows well- ordered particles over short small fields of view (Figure 2B). Small angle electron diffraction collected over an 11µm2 area (Figure 2C), shows a single crystal hexagonal pattern confirming long range nanocrystal ordering. Wide angle electron diffraction (Figure 2D) collected over the same area also shows single crystal-like diffraction from the wurtzite atomic lattice indicating the atomic lattices are also well-ordered over long range. HRTEM (Figure 2E) and the inset FFT show the atomic lattices of the particles are well aligned initially. In Figure 3F-J, we show the same characterization for the t-butylthiol exchanged particles, in Figure 3K-O particles joined with 6 monolayers of CdS SILAR, and Figure 3P-T for 6ML of ZnS SIALR.

In Figure 3, we show radial integration of the diffraction patterns which further informs us on the structure of the materials. In the case of small angle diffraction (Figure 3D) collected in transmission geometry (Figure 3A), we see that after the t-butylthiol exchange, the peak shifts to higher Q consistent with a decrease in spacing due to the shorter ligands.

Figure 3: (A) Sample geometry for small angle and wide angle electron For Wide angle diffraction characterization leading to diffraction from the (XXX0) planes of diffraction (Figure 3E) we wurtzite. (B) Schematic showing the radial integration of the diffraction pattern; see only peaks which I(Q) and the azimuthal integration of an annulus; I(Φ). (C) Sample geometry correspond to strong for reflection wide angle X-ray diffraction leading to diffraction from (000X) perpendicular orientation planes in wurtzite. (D) radially integrated small angle electron diffraction of the [0001] relative to patterns. (E) Radially integrated wide angle electron diffraction. (F) the substrate. Finally, Azmuthally integrated winde angle electon diffraction for the (101 0)̅ diffraction azimuthally integrating ring. (G) Wide angle X-ray diffraction pattern of the native assembled particles. (Figure 3B) the (1100) peak (Figure 3F) shows increased intensity at 60° intervals consistent with near single crystal ordering of the individual nanocrystals. Wide angle x-ray diffraction (Figure 3F) collected in reflection geometry (Figure 3C) shows a strong (0002) peak, consistent with perpendicular alignment of the [0001] relative to the substrate.

Taken together our data shows that we have successfully prepared highly ordered atomically attached nanocrystals with defined structure across multiple length scales. Going forward, the control we have developed will allow us to carefully and controllably study coupled quantum dot arrays and hopefully identify materials with emergent electronic properties.

Trevor Roberts 2019-2020

Trevor Roberts was awarded the Philomathia Graduate Student Fellowship, from Fall 2019- Spring 2020.

Advisor: Prof. Naomi Ginsberg

Correlating Nanoscale Quantum Dot Superlattice Morphology with Energy Transport Using Ultrafast STED Imaging Quantum dot superlattices (QDSLs), mesoscopic 2D structures formed by the self-assembly of QDs, are a promising class of nanomaterials for solar light harvesting.1 However, the electronic coupling between QDs in lattices is typically weak and the influence structural defects have on the energy migration properties of such materials is still unclear. I propose a collaboration between the Alivisatos and Ginsberg groups, wherein we will use an adaptation of stimulated emission depletion (STED) microscopy to weakly-coupled and new, strongly- coupled QDSLs to gain unprecedented insight into the role and extent nanoscopic defects play in altering energy transport in nanocrystal assemblies.

During QD self-assembly, structural anomalies like point defects (~10 nm), larger regions of disorder (~50 nm), and grain boundaries form.2 How these impact the resulting energetic landscape and excitation energy transport in QDSLs, and specifically, if these defects impact weakly- and strongly-coupled systems differently is of fundamental importance for light- harvesting design. Correlating QDSL morphology with its energetic landscape and transport functionality necessitates optical resolution on both spatial scales relevant to defects and temporal scales relevant to transport in their vicinity. Only by leveraging spectroscopic (Ginsberg group) and synthetic (Alivisatos group) expertise will we uncover how different superlattices and lattice defects influence the electronic energy landscape/transport.

We will employ a highly modular, ultrafast STED microscopy method to optically resolve QDSLs on unprecedented time and length scales. STED uses a pump light pulse to create a diffraction- limited excitation region and a red-detuned donut-shaped depletion pulse to drive stimulated emission around the periphery of the excitation to yield a sub-diffraction excitation volume. Because re-excitation by the depletion pulse can occur if absorption and emission maxima are too similar, we will engineer core-shell QDs with large Stokes shifts, with a feedback relay between synthesis and spectroscopy to optimize said parameter. While ligand-decorated QDSLs can be formed through self-assembly, the electronic coupling between adjacent dots is weak due to the significant tunneling barriers imposed by the organic ligands. In an effort to enhance this coupling, Justin Ondry of the Alivisatos group is developing a means to crystallographically fuse QDs with a hexagonal packing arrangement in 2D superlattices. STED imaging will allow us to resolve structural features of each type of lattice on the order of 10’s of nm, and we will modify conventional STED microscopy by 1) spectrally resolving our images, determining if nanoscale defects shift the local emission profile of a given material, and 2), using a time resolved ultrafast adaptation of STED, (TRU)STED, to measure the time and length scales of excitation diffusion, both close to and far away from defects. Here, a second, time- delayed depletion pulse quenches excitations that diffuse out of the initial excitation volume, enabling energy migration length scales to be obtained by recording the remaining fluorescence vs. delay time. This combination of novel synthesis and spectroscopy will reveal key differences on the extent defects impede energy transport in weakly and strongly coupled systems.

I predict superlattices with fused QDs will exhibit faster energy migration timescales and a higher tolerance of defects as a consequence of a more delocalized electronic wavefunction. It is possible, however, that because of the greater degree of delocalization, the defects present in the lattice may be “felt” from greater distances and may actually function as more severe energetic sinks. Comparing the timescales of energy flow in regions of uniformity as well as

near various defects will suggest whether we should be more or less concerned about the presence of defects in more strongly coupled superlattices. STED microscopy has only recently super-resolved commercial QDs3 in spite of their relatively small Stokes shifts. Resolution is compromised by direct excitation by the depletion pulse via 1 and/or 2 photon absorption. Fortunately, a greater Stokes shift generated by Alivisatos/Ondry coupled with our ability to remove 2-photon absorption in our detection scheme will improve the achievable resolution of QDSLs.

Super-resolving both weakly- and strongly-coupled QDSLs as well as probing the time and length scales of energy migration will serve to elucidate the nature by which and degree to which defects in such materials hamper energy transport. These findings will inform design principles for numerous 2D light harvesting architectures, a class of materials of interest to many Kavli researchers. Kavli ENSI support will afford me the financial flexibility to augment our current instrumentation to address this inquiry as well as provide a collaborative network to discuss and share my findings. Finally, embarking on this project will mark a significant new step in my research, as it will be the first application of STED to QD superlattices and the first time our group studies an all inorganic system with STED.

(1) Science. 317, 355-358 (2007), (2) Chem. Mater. 30, 4831-4837 (2018), (3) Nat. Comm. 6, 1-6 (2015)

Archana Raja 2017-2019

Archana Raja was awarded the Heising-Simons Postdoctoral Fellowship, from Fall 2017 to Fall 2019.

Advisor: Prof. Paul Alivisatos

Exploring the Fundamental Optoelectronics of 2D Materials

This past year (2018-2019), I have worked on a number of projects involving optoelectronic processes of sub-nanometer thick two-dimensional (2D) materials, including uncovering a fundamentally new type of material disorder at the limit of atomically thin crystals, termed “dielectric disorder”. This work was recently accepted at the journal Nature Nanotechnology. I published one lead author paper on how optical excitations couple to lattice degrees of freedom (“phonons”) as you add an extra layer of material to the 2D monolayer (published in Nano Letters [1]), and was a co-author on a paper discerning excited state coupling to phonons (published in Nanoscale [2]). Additionally, the work I covered in my previous year’s progress report on correlated optical and angle-resolved photoemission spectroscopy of 2D materials has been submitted to the journal Physical Review Letters.

Dielectric disorder in 2D materials The understanding and eventual control of material disorder is a key enabler of technologies, including nanoscale devices. In my recently accepted paper entitled “Dielectric disorder in 2D

materials”, where I am the lead author, we present a completely new form of disorder that is not based on imperfections in the material but on fluctuations in the local dielectric environment. The prototypical 2D semiconductor, a monolayer of WS2, was investigated using optical spectroscopy, where correlations between lower energy and higher energy excitations give a distinct signature for dielectric disorder. We also showed the significant consequences of this type of disorder on optical properties and energy transport.

Enhanced exciton-phonon coupling from monolayer to bilayer 2D semiconductors

Transition metal dichalcogenides (TMDC) undergo a unique transition from being an indirect-gap semiconductor with low light emission to a direct- gap semiconductor with orders of magnitude higher relative emission efficiencies from the few-layer to the monolayer limit. Understanding the fate of excitations across the band-gap is crucial for developing a complete, fundamental understanding of the optoelectronic properties. In this study [1], I was able to show how the sensitivity to the local environment at the atomically thin limit introduces new, unexpected relaxation channels for excited electron-hole pairs, or “excitons”, in monolayer and Fig 1. Schematic illustration of the electron- bilayer WS2. I participated in another theory- electron and electron-hole interaction affected experiment collaboration where we were able to by environmental screening fluctuations due to extend this understanding to a variety of higher the substrate roughness, impurities, and excited exciton states [2].

Correlated optical and photoemission spectroscopy to uncover new properties of 2D materials

The influence of their immediate surrounding upon 2D materials therefore offers new ways to design multicomponent systems. In the recently submitted paper, where I am a co-lead author, we investigate the effects of dielectric screening on the electronic dispersion and the bandgap in the atomically thin 2D semiconductor WS2 using correlative optical spectroscopy and angle- resolved photoemission spectroscopy together with first-principles calculations. We find the main effect of an increase in the environmental dielectric screening to be a rigid, symmetric reduction of the bandgap, with little change to the electronic dispersion of the occupied and unoccupied states. We explain our observations through the spatial dependence of the changes in the Coulomb potential induced by the dielectric environment. The results suggest dielectric engineering as a truly non-invasive way of tailoring the band structure of 2D semiconductors and give guidance to understanding electronic bands of 2D materials embedded in vertical heterojunctions. Top-down patterning of 2D materials

I have also started a project where I utilize a unique tool on the UC Berkeley campus that can pattern materials with sub-nm lateral using energetic helium ions. This project extends my previous studies on synthesizing laterally confined 2D nanocrystals that will exhibit Fig 2. Gas-assisted focused ion beam induced quantum-confined properties compared to the extended etching (a) Topography of XeF2 assisted helium ion milled trench on bulk MoS2 (b) cross-section height profile of the milling regions different Helium ion milling time 18

2D monolayer. In this top-down approach, a thin TMDC flake will be patterned with a XeF2 gas assisted helium focused ion beam induced etching (FIBIE) to create well-defined quantum dots down to the 10nm scale. Fig. 2 (a,b) shows preliminary results of gas-assisted FIBIE on thin MoS2, which indicates the fabrication leaves little residue on the edge of the defined features. The dosage of the FIB can be adjusted to control the depth of the FIBIE etch, as shown by the line cut in Fig. 2 (b). The quality of the patterned quantum dots will be examined by other optical and electron microscopy methods. Metal contacts will be deposited onto a graphene substrate, allowing electrical contact to be made to quantum dots that can be transferred on to them using typical polymer-based transfer methods for future investigations.

[1] Archana Raja, Malte Selig, Gunnar Berghäuser, Jaeeun Yu, Heather M. Hill, Albert F. Rigosi, Louis E. Brus et al. "Enhancement of Exciton–Phonon Scattering from Monolayer to Bilayer WS2." Nano letters 18, no. 10 (2018): 6135-6143.

[2] Samuel Brem, Jonas Zipfel, Malte Selig, Archana Raja, Lutz Waldecker, Jonas D. Ziegler, Takashi Taniguchi, Kenji Watanabe, Alexey Chernikov, and Ermin Malic. "Intrinsic lifetime of higher excitonic states in tungsten diselenide monolayers." Nanoscale (2019).

Peng-Cheng Chen 2019-2021

Peng-Cheng Chen was awarded the Heising-Simons Postdoctoral Fellowship, from Spring 2019 to Spring 2021.

Advisor: Prof. Peidong Yang

Developing Hybrid Nanocatalysts for Sustainable Nitrogen Fixation

Modern human civilization relies heavily on nitrogen fixation to produce ammonia, an indispensable feedstock for fertilizer synthesis and crop production, as well as a carbon-free fuel molecule. Industrially, nitrogen fixation is primarily performed through the catalytic reduction of nitrogen with hydrogen molecules, which is known as the Haber-Bosch Process and accounts for nearly 50% of the nitrogen atoms in humans. Nonetheless, this approach is constrained by its harsh reaction conditions (consuming 1% of global energy), fossil fuel consumption as the hydrogen source, and high carbon dioxide emission.

In view of these limitations, alternative approaches such as electrocatalytic and photoelectrocatalytic nitrogen fixation, that operate at mild conditions, utilize water as the hydrogen source, and are powered by electricity from renewable wind/solar energy, have been considered as sustainable ways to Figure 1. A hybrid nanocatalyst system that integrate achieve nitrogen fixation. The bottleneck of these semiconductor nanowires and multimetallic nanoparticle co-catalysts.

methods is the lack of effective catalytic materials. Traditional catalysts are usually limited by the scaling relation. The catalysts exhibit either strong or weak binding for all reactants and products, while an ideal catalyst should have strong activation of reactants but relatively weak binding for products. In order to realize sustainable N2 fixation, it is imperative to develop catalytic materials with superior activity and selectivity. The Yang group at UC Berkeley has significant experience and expertise in developing catalytic systems that integrate semiconductor nanowires and nanoparticle co-catalysts for electrocatalysis and photoelectrocatalysis (Figure 1). The inherent properties of nanowires, including large surface area and enhanced light scattering/absorption, make nanowire arrays promising candidates for (photo-)electrodes. Upon combination with select nanoparticle co-catalysts, such catalytic systems have shown superior catalytic performance for water splitting and carbon dioxide fixation. Based on the Yang group’s strength in catalysis, Pengcheng’s research at UC Berkeley will focus on integrating multimetallic nanoparticle catalysts and semiconductor nanowires for enhanced nitrogen fixation. Since conventional catalysts are limited by the scaling relation, one potential strategy to overcome this limitation is to incorporate multiple catalytic components into a single catalyst. In Pengcheng’s PhD research, he has utilized nanoreactor-mediated synthesis to systematically investigate multimetallic nanoparticles. Not only does his research pioneer the study of complex phase-separation behavior of multiple metals at the nanoscale, it also paves new routes to synthesize libraries of Figure 2. Polymer nanoreactor-mediated combinatorial synthesis unprecedented multimetallic nanostructures of multimetallic nanoparticles. (Figure 2). In view of the promise of multimetallic nanoparticles, the integration of nanowire arrays and multimetallic nanoparticles could become a powerful platform for nitrogen fixation, where nanoparticles are synthesized on nanowires in a bottom-up manner, and the effect of catalyst structure and composition on the catalytic activity can be comprehensively studied. In addition, the proposed catalytic system will offer the opportunity to improve many other reactions such as water splitting, carbon dioxide fixation, and alcohol oxidation, which will increase the sustainability and efficiency of chemical processes that will benefit the academic and industrial communities.

Shaowei Li 2018-2020

Shaowei Li was awarded the Heising-Simons Postdoctoral Fellowship, from Fall 2018 to Fall 2020.

Advisor: Prof. Mike Crommie

Scanning Probe Optical Spectroscopy and Microscopy

A. Research Scope

The main goal of the proposed project by Shaowei Li is to shatter the diffraction limit in the optical measurements and probe the inhomogeneous properties in spatial resolvable optical phenomena. The desire for observing finer details using optical microscopy, particularly in bio- science, has pushed technology developments toward the joint spatial-temporal resolution. The combination of optical excitation with scanning probe techniques provides a new window for viewing the unique properties of individual nano-scale objects. The optically excited atoms or molecules can be locally probed by a scanning probe microscope (SPM). Specific experiments include single molecule infrared absorption spectro-microscopy and single molecule stimulated Raman spectro-microscopy.

B. Project Rationale and Justification

Optical spectroscopy, such as infrared and Raman spectroscopy, detects the vibrations and rotations of atoms and molecules which serve as a fingerprint for chemical identification and as a basis for understanding their behavior. Due to the diffraction-limited spatial resolution, these methods cannot resolve the inhomogeneous properties of individual atoms or molecules. Inelastic tunneling electron spectroscopy (IETS) with scanning tunneling microscope (STM) can resolve the molecular vibration at the sub-molecular scale, but lacks the sensitivity of many molecular modes compared to the optical methods. An ideal solution is to excite the molecule vibration with photons while probing it with an SPM, which combines the versatility of optical excitation and the spatial sensitivity of an SPM. Previously, Shaowei had observed the single C- H bond scission induced by photo-assisted electron tunneling, and further detected the dynamics of single molecule vibration excited by a broadband pulse laser. With support from Kavli ENSI, Shaowei will keep expanding the capability of a laser-SPM by extending the laser excitation into the infrared range as well as probing the excitation using various SPMs.

C. Experimental Arrangement

The first experiment Shaowei plans to pursue is to demonstrate single molecule infrared spectroscopy and microscopy. Infrared irradiation can easily vibrate the molecules since its spectra range covers the typical molecular vibration modes from a few meV to a few hundred meV. A schematic diagram of the apparatus used for this experiment is shown in Fig. 1A. The apparatus contains a tunable infrared light source and an SPM. Previous infrared-STM studies failed to achieve molecular level resolution, mainly due to the difficulty in separating the weak laser induced current signal from large tunneling background. This hurdle can be overcome by monitoring the laser induced actions of a molecule. Excitation of molecular vibrations can often trigger molecular actions including diffusions, rotations, or structural transitions. These actions usually lead to large changes in molecular geometry which can easily be monitored by SPMs. The energy diagram of a molecule undergoing a two-state switching mediated by a vibrationally excited state is shown in Fig 1B. By tuning the wavelength of the incoming light, different vibrational modes can be selectively excited which will change the two-state transition rate as well as the dwell time of each state of the molecule. Therefore, the vibrational energy of individual molecules can be locally detected by measuring the action rate as a function of wavelength. A similar experimental set-up can also be applied to perform single molecule

stimulated Raman spectroscopy and microscopy by substituting the infrared light source with a two-color laser beam separated by an energy equal to a specific vibrational mode as indicated in Fig. 1B.

The experiment can be further extended to molecular systems without large vibration mediated atomic displacements, with the aid of lock-in techniques. Lock-in technique has been widely applied in optical measurements using a sinusoidal power modulation applied by a chopper. Unfortunately, the thermal expansion of SPM tip in response to the power modulation can cause an unstable junction. Instead, Shaowei will use polarization modulation (periodically change the laser polarization angle of as indicated in Fig. 1A) while maintaining the power constant. Since the SPM junction selectively enhances the electric field alignment with the tip axis, this technique will allow us to lock-in to the laser induced effect without changing the laser power.

Fig. 1 (A) Schematic diagram of the experimental set-up. The laser illumination can excite the vibration of individual molecules in the STM junction. The vibrationally excited molecule is probed by the localized tunneling electrons.

(B) Schematic diagram of the stimulated Raman process.

Luis Pazos-Outon 2017-2019

Luis Pazos-Outon was awarded the Heising-Simons Postdoctoral Fellowship, from Fall 2017 to Fall 2019.

Advisor: Prof. Eli Yablonovitch

Lead Halide perovskites:

On my first year I wrote a theoretical manuscript in which I studied the performance limitations of methylammonium lead halide perovskites. Lead halide materials have seen a recent surge of interest from the photovoltaics community following the observation of surprisingly high photovoltaic performance, with optoelectronic properties similar to GaAs. This begs the question: What is the limit for the efficiency of these materials?

It has been known that under 1-sun illumination the efficiency limit of crystalline silicon is 29%, despite the radiative (ideal) limit for its bandgap being 33%. This discrepancy is due to strong Auger decay. We demonstrated that this phenomenon in lead halide perovskites is greatly∼ ∼

reduced, largely due to its strong absorption, which allows to make very thin devices, suppressing Auger decay. Therefore, we showed that lead halide perovskites can achieve nearly ideal performance.

This work was published as an invited paper at the Journal of Physical Chemistry Letters.

Following this work, I was contacted by researchers at the University of Washington to help them develop a theoretical model to understand their experimentally measured luminescence in perovskite films. Reducing non-radiative recombination in semiconducting materials is a prerequisite for achieving the highest performance in light-emitting and photovoltaic applications.

We demonstrated that with a simple surface treatment we could synthesize lead healide materials with an internal luminescene efficiency of 91.9 ± 2.7% under 1 Sun illumination. These results suggest hybrid perovskite solar cells are inherently capable of further increases in power conversion efficiency if surface passivation can be combined with optimized charge carrier selective interfaces. This is a new benchmark for this class of materials, and sets lead halides in a similar performance level as GaAs devices, the top performing materials known to date. This work was published in Nature Photonics. More recently I finished two projects that I had started at the University of Cambridge:

1. We studied the diffusion length of electrons in perovskite materials, showing diffusion lengths in excess of 12 µm. For such demonstration we used a device architecture that I had developed at Cambridge during my PhD.

2. One source of instability in perovskite solar cells is interfacial defects, particularly those that exist between the perovskite and the hole transport layer. We demonstrate that using thermally evaporated dopant-free tetracene we can reach an efficiency of up to 21.6% and an extended power output of over 550 hours of continuous illumination at AM1.5G, retaining more than 90% of the initial performance and thus validating our approach. Our findings represent a breakthrough in the construction of stable perovskite solar cells with minimized nonradiative losses.

These studies were published in the journals Joule and in Science Advances respectively.

Thermophotovoltaics:

Thermophotovoltaic power conversion utilizes thermal radiation from a local heat source to generate electricity in a photovoltaic cell. It was shown in recent years that the addition of a highly reflective rear mirror to a solar cell maximizes the extraction of luminescence. This, in turn, boosts the voltage, enabling the creation of record-breaking devices. Now we report that the rear mirror can be used to create thermophotovoltaic systems with unprecedented high efficiency. This mirror reflects low-energy infrared photons back into the heat source, recovering their energy. This radically improves thermophotovoltaic efficiency. Therefore, the rear mirror serves a dual function; boosting the voltage and reusing infrared thermal photons. This allows the possibility of a practical >50% efficient thermophotovoltaic system. Based on this reflective rear mirror concept, we demonstrated a thermophotovoltaic efficiency of 29.1 ± 0.4% at an emitter temperature of 1,207 °C.

This work has been accepted for publication at the journal Proceedings of the National Academy of Sciences (PNAS).

Recently, we started a collaboration with a Berkeley-based startup (located at LBL), and we are building a system to use these engines for large scale energy storage. Energy storage is one of the greatest challenges for the effective implementation of renewable energies, and therefore we believe this could have great impact on the viability of alternative energy sources.

Cong Su 2019-2021

Cong Su was awarded the Heising-Simons Postdoctoral Fellowship, from Spring 2019 to Spring 2020.

Advisor: Prof. Alex Zettl

Atomic Engineering on 2D Materials Using High-Energy Electrons

Atomic engineering explores methods of creating new material structures atom-by-atom. Particularly, it includes (1) doping single atom onto a targeted location, and (2) manipulating atom movement to a desired spot. These techniques, once established, will provide a powerful platform of material engineering to the whole scientific community. Up to now, people have used ion bombardment as doping method, but the technique creates more unwanted damages rather than functional dopants. For atom manipulation, scanning tunneling microscope (STM) is the only practical technique, but it has several limitations to be scaled up: (1) strenuous helium- cooling; (2) slow manipulation speed; (3) only applicable to surface-adsorbed atoms. All of these problems, in principle, can be resolved by utilizing scanning transmission electron microscope (STEM).

Figure: A two-year research proposal of Atomic Engineering. Phase 1 is completed. Phase 2 and 3 are in progress. “E-beam” means electron beam.

Cong Su is going to conduct his research in four difference phases:

• Phase 1 (Dynamics Space): Construct the fundamental theories of single atom control. A theory based on experimental evidences called Dynamics Space was proposed during my Ph.D research for such purpose. The theory uses atom species, host materials, and electron voltages as inputs and yields a decision tree to maximize the controlability.

• Phase 2-1 (Atom-precise doping): A novel stacking configuration is proposed for precise doping as shown in the figure above. The beam focuses on the lower layer to create defects while the upper layer can provide dopant source by electron sputtering. Phase 2-2 (single- atom manipulation): Demonstrate the controllability of single atoms and expand the library of controllable atoms. Susi etc. have shown that silicon dopant atoms in graphene can be manipulated by electron irradiation in STEM, and we later extended the list to phosphorus. More atom species (e.g. Al, N) and host materials (e.g. hBN, TMDs) will be investigated. [Phase 2 can be done in Molecular Foundry using TEAM 0.5 at 80 keV and FEI ThemIS at 60 and 80 keV.]

• Phase 3 (Fast-speed manipulation): The speed for moving atom has to be accelerated for large scale manipulation. One of the prediction of Dynamics Space is that thermal vibration of lattice could increase the transition rate of structural dynamics. This implies that by increasing temperature up to 1100 °C, we could increase the speed of atom movement by several orders of magnitude. [This in-situ experiment can be done by using Protochip holder available in FEI ThemIS microscope in Molecular Foundry.]

• Phase 4 (Applications): (1) Single photon emitters can be created in hexagonal boron nitride by intentionally doping certain atoms (e.g. carbon, oxygen) into lattice. [The optical spectroscopy of single photon emitters could be done in collaboration with Prof. Feng Wang and Prof. Michael Crommie in Berkeley.] (2) New structures of atoms can be engineered to create novel single atom catalysis. [EELS in STEM (monochromated TEAM 0.5) and STS measurement (Crommie Group) can be used for determining the physical properties of these defects. All of the sample preparations and STEM operations will be supervised by Prof. Alex Zettl.]

Thesis Prize

This year’s student thesis prize was awarded to Dohyung Kim

Dr. Dohyung Kim

Advisor: Prof. Peidong Yang, Department of Chemistry, and Materials Science and Engineering

Electrochemical or photochemical conversion of carbon dioxide to value-added products has the potential to fundamentally change our traditional ways of harvesting energy and manufacturing chemical products.

Kim's thesis focuses on the use of nanoparticles as catalysts for CO2 conversion and their structural factors affecting catalytic properties. The shift in the electrocatalytic behavior of gold-copper nanoparticles made by the change of material composition and atomic orderedness has been studied to illustrate the catalytic importance of structural precision down to the atomic level. The possible ways of interfacing nanoparticle electrocatalysts with light-absorbing platforms were explored for CO2 valorization using renewable energy sources. Nanoparticles can also be integrated with other materials such as the metal-organic frameworks or molecular complexes for the creation of a CO2 catalytic system. More importantly, the dynamic restructuring of nanoparticles was utilized to induce favorable electrocatalytic properties for CO2 to conversion of multicarbon products. Overall, the projects pursued in the thesis not only illustrate the structural complexity of nanoscale catalytic systems but also underline the need to develop a comprehensive understanding of all the factors for the development of CO2 catalysis.

Faculty Awards

Paul Alivisatos 2019, Honorary Doctorate, the Hebrew University of Jerusalem. The Hebrew University awards honorary degrees to persons who have distinguished themselves by academic or creative achievement, who have rendered outstanding service to the University, or whose activities have been of notable benefit to humanity, the State of Israel, or the Jewish people.

2019, F. A. Cotton Medal, for Excellence in Chemical Research is awarded annually and recognizes accomplishments in research.

2019, The Glenn T. Seaborg Medal. The Glenn T. Seaborg Medal was established in 1987 by the UCLA Department of Chemistry and Biochemistry to honor individuals for their significant contributions to chemistry and biochemistry.

Tsu-Jae King Liu 2018, Named fellow of the National Academy of Inventors, an organization that champions the societal benefits of university research.

Eli Yablonovitch 2018, Edison Medal of the IEEE, for “leadership, innovation, and entrepreneurial achievements in photonics, semi-conductor lasers, antennas, and solar-cells”.

2018, Named Fellow of the National Academy of Inventors, an organization that champions the societal benefits of university research.

2019, The Franklin Institute 2019 Benjamin Franklin Medal in Electrical Engineering, for “widely used scientific improvements to radio- and light-based technologies in wireless communications and solar energy applications.”

2019, Frederic Ives Medal/Jarus W. Quinn Prize, the highest Award of the Optical Society, for “diverse and deep contributions to optical science including photonic crystals, strained semiconductor lasers, and new record-breaking solar cell physics.”

Omar Yaghi 2019, The Royal Swedish Academy of Sciences, Gregori Aminoff Prize in Crystallography, “for fundamental contributions to the development of reticular chemistry”

2019, Member, The National Academy of Sciences “in recognition of distinguished and continuing achievements in original research.”

Peidong Yang 2019, Received the CO2 Conversion Challenge Grant from NASA

Winton Program for Physics and Sustainability, UK – Kavli ENSI Exchange Program

2018 - 2019 Participants:

Antonios Alvertis Graduate Student Supervisor: Dr. Akshay Rao, Winton Fellow and Optoelectronics Group, Department of Physics, University of Cambridge Host: Professor Jeffrey Neaton, Department of Physics, UC Berkeley

Exchange Proposal The project “Phonon effects in solid state singlet fission” will investigate the effect that vibrations in solid-state structures of organics semiconductors have on single exciton fission, which is a process that can be utilized in solar cells to increase device efficiency. The research will aim to provide an accurate description of the electronic structure of materials suitable for singlet fission and then calculate the effect of vibrations on their properties. The final phase of the work will be to model the real-time dynamics of singlet fission in a solid with the insight used to inform how to improve the performance of solar cells.

Chloe Gao Graduate Student Supervisor: Professor David Limmer, Chemistry Faculty and Kavli ENSI, UC Berkeley Host: Professor Dr. Alpha Lee, Winton Fellow, and Theory of Condensed Matter Group, Department of Physics, University of Cambridge

Exchange Proposal In her research in the Limmer group, she has developed a method to compute transport coefficients using large deviation functions (LDFs). This method relies heavily on the accurate evaluation of LDF and can be costly and inefficient for complex systems. As an alternative approach to tackling these complex problems, she will work with the Lee group in Cambridge to develop enhanced sampling algorithms employing machine learning to estimate LDFs. Once a machine learning aided sampling algorithm is devised, it can be used to study complex chemical system such as transport in ionic liquids based supercapacitors.

Matt Gilbert Graduate Student Supervisor: Professor Alex Zettl, Department of Physics and Kavli ENSI Host: Professor Ulrich Keyser, Biological and Soft Systems Group, Department of Physics, University of Cambridge

Exchange Proposal In this project, he will bring his expertise on creating structural devices using 2D materials and work with the Keyser group and Dr. Hannah Stern to work on two projects to couple h-BN defects and pores to the outside world. The first will be to perform DNA translocation experiments on nanopores made with atomic precision. The second will be to perform correlated microscopy studies of quantum emission from h-BN defects that will be first imaged with HR-TEM at Berkeley and then bring them to Cambridge to correlate the atomic structure with the local fluorescence.

Stephanie Mack Graduate Student Supervisor: Professor Jeffrey Neaton, Department of Physics and Kavli ENSI Host: Dr. Bartomeu Monserrat, Winton Fellow, and Theory of Condensed Matter Group, Department of Physics, University of Cambridge

Exchange Proposal In this project, the first-principles based techniques that Stephanie has utilized to date will be expanded through working with experts in Cambridge in structure prediction and advanced electron-phonon coupling techniques. These techniques will be applied to exploring how electronic and topological features can be manipulated with pressure in materials. The pressure is known to have enormous effects on material properties, but its use for tuning topology remains an open field that could yield surprising results for controlling these unique properties for future device functionality.

Alex Casalis de Pury Graduate Student Supervisor: Professor Jeremy Baumberg, Department of Physics, University of Cambridge Host: Professor Paul Alivisatos, Department of Chemistry and Kavli ENSI, UC Berkeley

Exchange Proposal Plasmonic nanogaps can be formed by having Au nanoparticles on an ultrathin material (0.4 – 10nm) placed onto an Au ‘mirror’. The material acts as a spacer between the nanoparticle and Au ‘mirror’. In this way the field is confined to below the diffraction limit inside the spacer material, leading to large field enhancements (up to ~104). Irradiation of this nanoparticle-on-mirror (NPoM) system enables novel interactions with the field, and scattering gives us information on how the material interacts with this field. This proposal aims to combine work in the Alivisatos group on the modification of Au nanoparticles in graphene liquid cells with plasmonic techniques from the Baumberg group.

Hsin-Zon Tsai Postdoctoral Researcher Supervisor: Professor Michael Crommie, Department of Physics and Kavli ENSI, UC Berkeley Host: Professor Stephan Hofmann, Department of Engineering, University of Cambridge

Exchange Proposal This project will combine electron microscopy and local probe microscopy to characterize the synthesis process and post-growth atomic structure. Hofmann group at the University of Cambridge have unique expertise on growth dynamics and electron microscopy. Hsin-Zon will work closely with the Cambridge team to investigate novel synthesis and patterning of 2D material heterostructures, transition metal dichalcogenides (TMDs) and covalent organic frameworks (COFs) using gas injection equipped scanning electron microscope and image seeding and subsequent propagation of the growth.

Zhixin Alice Ye Graduate Student Supervisor: Professor Tsu-Jae King Liu, Electrical Engineering and Computer Sciences Department and Kavli ENSI, UC Berkeley Host: Dr. Giuliana Di Martino, Winton Fellow and Nanophotonics Group, Department of Physics, University of Cambridge

Exchange Proposal In this project, Alice will explore the properties of the valence change mechanism (VCM) memristive switches that have high endurance and energy-efficiency. The systems require optimization and a better understanding of filament development and dissolution. To study these process non-destructive optical techniques, developed in the Di Martino group will be utilized to reveal real-time information about the formation of oxygen vacancies in a model materials stack of TiN/HfOx/Au nanoparticle.

Camille Stavrakas Graduate Student Supervisor: Dr. Sam Stranks, Optoelectronics Group, Cambridge Host: Dr. Edward Barnard, Molecular Foundry, LBNL

Exchange Proposal The project aims to “Explore non-radiative loss mechanisms and recombination pathways in metal-halide perovskite through 3D photoluminescence tomography.” The Strank’s group recently found that light and atmospheric treatments on polycrystalline perovskite thin films resulted in large enhancements in the luminescence of the dark grains, with macroscopic optoelectronic properties approaching those of the best crystalline semiconductors reported to date. The visit of Camille for three months would enable samples produced in Cambridge to be studied with subsurface two-photon microscopy, a technique pioneered by Edward Barnard to study photovoltaic materials. The results may lead to a detailed understanding of the charge dynamics that are crucial for further improvements in device performance of perovskite-based solar cells and LEDs.

Mustafa Caglar Graduate Student Supervisor: Professor Ulrich Keyser, Biological and Soft Systems Group, Cambridge Host: Professor Alex Zettl, Department of Physics and Kavli ENSI

Exchange Proposal The project is closely linked to the Ph.D. research of Mustafa, to understand and control ionic selectivity across semi-permeable membranes that are crucial to key applications such as batteries and reverse osmosis power generation. Within the Keyser group, he is studying the behavior of graphene and hexagonal boron nitride (hBN) as 2D porous membranes, utilizing intrinsic defects within chemical vapor deposition (CVD). The Zettl group also has a wealth of expertise in creating, imaging and manipulating nanopores using a different technique involving TEM drilling and dielectric breakdown pore creation. This project will provide an opportunity to explore the benefits and drawbacks of the respective methods and apply these to studies of ionic flux across these membranes to study the reverse osmosis process.

Dr. Aditya Sadhanala Postdoctoral Research Associate Supervisor: Professor Richard Friend, Optoelectronics Group, Cambridge Host: Professor Peidong Yang, Department of Chemistry and Kavli ENSI

Exchange Proposal The advent of perovskite semiconductors has brought a paradigm shift in semiconductor science and technology opening up a unique set of opportunities to realize novel high performing optoelectronic applications. The magic of these polycrystalline thin-film perovskites is their demonstration of intrinsic semiconductor behavior, perfect clean characteristics and low disorder similar to those obtained in extremely purified single crystals of inorganic semiconductors. Aditya’s research at Berkeley is to synthesize and explore lead-free perovskite semiconductors based nanostructures like nano-crystals, nano-wires, and nano-platelets. There are various synthesis routes available for making this possible and Prof. Yang’s group has strong expertise on this front. Aditya’s extensive expertise in working with perovskite-based semiconductors will be deployed to studying their vastly varying photo-physical properties for applications including solar cells, LEDs and FETs.

Dr. Hannah Stern Postdoctoral Researcher Supervisor: Dr. Akshay Rao Optoelectronics Group, Cambridge Host: Professor Naomi Ginsberg, Department of Chemistry and Kavli ENSI

Exchange Proposal While in Berkeley Hannah worked primarily within the group of Professor Naomi Ginsberg, using a combination of diffraction-limited optical spectroscopic techniques to probe quantum emission in monolayers of hexagonal boron nitride (h-BN). h-BN is a 2D wide band-gap semiconductor that has recently been shown to display bright room-temperature single photon emission in the visible, sparking immense interest in the material as a solid-state quantum emitter for use in quantum communications. As part of this work, in collaboration with The Zettl research group in Berkeley, Hannah is investigating the structural properties and morphological dependence of h-BN single photon emission.

Professor Stephen Elliott Faculty Department of Chemistry,Cambridge Host: Professor Jeffrey Neaton, Physics and Kavli ENSI and Professor Xiang Zhang, Mechanical Engineering and Kavli ENSI

Exchange Proposal The aim of the exchange is to extend Stephen’s research interests in thermal functionality of materials to thermoelectric generation (TEG) for energy-harvesting applications. He plans to use a DFT-based computational approach to design and discover new (families of) TEG materials with ‘engineered’ optimized thermoelectric and cognate properties in the first instance. To establish this new research the exchange will enable learning at first-hand about various relevant techniques employed by researchers at Kavli ENSI, and to forge collaborations with colleagues there which will further strengthen links between the Winton Programme for the Physics of Sustainability and Kavli ENSI.

Institute for Basic Science, South Korea – Kavli ENSI Exchange Program

Myounghwan Oh Postdoctoral Researcher Supervisor: Prof. Taeghwan Hyeon, Institute of Basic Science (IBS), Seoul National University Host: Prof. Paul Alivisatos, Chemistry, UC Berkeley

Exchange Proposal The goal of this project is to investigate the three-dimensional (3D) self- organization of the grains and the emergent phenomena from their geometry such as magnetic/electric ordering and enhancement of catalytic activity at the grain boundaries (GBs). Grains can be considered as the functional and structural building block of polycrystalline materials. From this perspective, the multigrain nanocrystals are a small cluster of the building blocks. Our approach to design and synthesize multigrain nanocrystals with well-organized grain structure provided a fundamental model system for understanding the GB properties and the collective behavior of the grains. We were able to observe the structure of GB at the atomic level and obtain a correlation between the GB defects and bulk electrocatalytic property using the multigrain nanocrystal system. This is hard to achieve with bulk polycrystalline materials or nanocrystal random aggregates. Our next step is to study the collective elastic behavior of the grains which can lead to an unusual atomic structure of crystals. Our research group expects a new way of lattice engineering of nanocrystal to improve the electromagnetic and catalytic property in this study.

Min Gee Cho Graduate Student Supervisor: Prof. Taeghwan Hyeon, Institute of Basic Science (IBS), Seoul National University Host: Prof. Paul Alivisatos, Chemistry, UC Berkeley

Exchange Proposal The project aims to investigate the relationship between strain and morphology in heterostructured oxide nanocrystals and their physical/chemical properties for various applications. The Hyeon’s group recently developed over 20 different kinds of heterostructured oxide nanocrystals with combinations of spinel oxides and lanthanide oxides and correlated the grain boundary defects in core-shell structure and their electrocatalytic activity. And very recently, they also produced multigrain monometallic nanocrystal. This can be of great importance in that multigrain core nanocrystal is largely strained and can further provide structural diversification in core-shell nanocrystals with improved catalytic properties. During the visit to Berkeley, the three-dimensional atomic structure determination of unusually strained oxide nanocrystals produced in SNU will be studied. The study may lead to propose a principle that the strained multigrain nanocrystal can determine the final morphology of the nanocrystal.

Alivisatos Group

The Alivisatos group is engaged in the study of fundamental structural, chemical and optoelectronic properties of nanocrystals or quantum dots (QDs), along with their applications in light-emitting devices and harvesting solar energy. The group is also engaged in pushing limits of measurement techniques for structural and optical properties at this nanoscale limit. Below is a summary of their research activities conducted over the past year, which can be broadly categorized into three topics:

(1) Measuring and controlling radiative and non-radiative pathways in QDs

Highly efficient semiconductor QDs have been an area of focus due to their potential for exploring the fundamental thermodynamic limits of semiconductor efficiencies and also for device applications. Improvements to synthetic methods are not easy to measure due to the limitations of standard techniques as efficiencies near 100%, like that for core/shell CdSe/CdS QDs. A new characterization method based on sensitive measurement of heat released through non-radiative processes enables measurement of emission efficiency with ±0.2% error. The feedback loop between careful post-synthetic treatments and its effect on the QD efficiency was facilitated by this method.

The group is also pursuing schemes for efficient light-harvesting applications of QDs, where the transfer of both the electron and hole out of the QD either to electrodes or to drive photocatalytic reactions follows the excitation. The efficiency of this charge transfer step is the bottleneck for improving the overall solar-to-chemical conversion efficiency. Studies of charge transfer rates in two classes of CdSe/CdS nanorods are being pursued to tease out mechanistic details.

InP QDs are used in consumer products due to its high stability and low toxicity. However, as synthesized, they have notoriously low radiative efficiencies due to surface defects. Recently, new chemistries have been discovered to improve InP QD emission efficiency by over 10x. The mechanism by which this process operates is still unknown and the group is currently studying this treatment via a number of methods, including isothermal titration calorimetry, nuclear magnetic resonance, and X-ray photoelectron spectroscopy.

(2) Understanding structure, defects, and rational growth of quantum dots

The group uses transmission electron microscopy (TEM) to look at the positions of atoms in nanocrystals and identify atomic level imperfections. This is used to observe the different pathways through which these atomic imperfections can heal and become a perfect material. Direct observation of the defect healing pathways helps design better materials processing strategies such that defects don’t form in the first place or are easily removed from the nanoscale crystals.

Furthermore, observing nanocrystals in their native liquid environment is important for understanding the mechanisms of shape control. Encapsulating QDs in solution between sheets of graphene enables the solution to be imaged in the high vacuum of a TEM. Over the past year, the group has been engaged in studying the mechanism of non-equilibrium shape transformations of gold nanocrystals.

Many novel properties of nanocrystalline materials result from the self-organization of the grains, such as enhanced catalytic activity at grain boundary (GB). The group has extensively investigated GB formation using advanced TEM imaging. This, along with the feedback loop with synthesis, has enabled exquisite control of the number, size, and orientation of the individual grains.

(3) Applications in light-emitting fabrics

Smart fabrics that combine traditional clothing with functional electronic and optoelectronic devices are a topic of a growing field of research with a broad range of applications in areas including healthcare, sports, and the internet of things. Two common approaches for producing smart fabrics are 1) to adhere fabricated devices on top of a pre-woven fabric, and 2) to develop fiber-shaped devices that can be then integrated into large area textile. The latter approach requires fabricating fiber-shaped devices that are not only flexible but also are weavable into textiles. The group has been developing wearable and all-solution processable electrically pumped light-emitting fibers (Fig. 1A). In addition, the group has used nanowires of highly emissive perovskite nanocrystals embedded in polymers and aligned through shear forces in 3D printing to construct images, pixels, and stretchable fabrics with a particular polarization of light (Fig. 1B).

(A) Schematic of a coaxially-coated, electrically-pumped light-emitting fiber along with a picture of a light- emitting fiber emitting in green. A library of nanomaterials can be used to tune the emission wavelength. (B) Image created using 3-D printed, aligned perovskite nanowires viewed in different polarizatios.

Photos of researchers

Jason Calvin, Graduate Fellow Chang Yan, Postdoctoral Fellow

Justin Ondry, Graduate Fellow

Bustamante Group

Alexander, L. M., Goldman, D. H., Wee, L. M. & Bustamante, C. Non-equilibrium dynamics of a nascent polypeptide during translation suppress its misfolding. Nat. Commun. 10, 2709 (2019).

Protein folding can begin co-translationally. Due to the difference in timescale between folding and synthesis, co-translational folding is thought to occur at equilibrium for fast-folding domains. In this scenario, the folding kinetics of stalled ribosome-bound nascent chains should match the folding of nascent chains in real time. To test if this assumption is true, we compare the folding of a ribosome-bound, multi-domain calcium-binding protein stalled at different points in translation with the nascent chain as is it being synthesized in real-time, via optical tweezers. On stalled ribosomes, a misfolded state forms rapidly (1.5 s). However, during translation, this state is only attained after a long delay (63 s), indicating that, unexpectedly, the growing polypeptide is not equilibrated with its ensemble of accessible conformations. Slow equilibration on the ribosome can delay premature folding until adequate sequence is available and/or allow time for chaperone binding, thus promoting productive folding.

Desai, V. P., Frank, F., Lee, A., Righini, M., Lancaster, L., Noller, H. F., Tinoco Jr., I., Bustamante, C. Co-temporal Force and Fluorescence Measurements Reveal a Ribosomal Gear-shift Mechanism of Translation Regulation by mRNA Secondary Structures. Mol. Cell (Accepted, 2019).

The movement of ribosomes on an mRNA is often interrupted by secondary structures that present mechanical barriers and play a central role in translation regulation. Here, we investigate how ribosomes couple the activity of translocation factor EF-G and internal conformational changes to unwind mRNA secondary structures using high-resolution optical tweezers with single-molecule fluorescence capability. We find that hairpin opening occurs during EF-G catalyzed translocation and is driven by the forward rotation of the small subunit head. Moreover, we modulate the magnitude of the hairpin barrier by force and surprisingly find that ribosomes respond to strong barriers by shifting their operation to an alternative 7-fold slower kinetic pathway prior to translocation. Such a slower gear results from an allosteric switch in the ribosome that may allow it to exploit thermal fluctuations to overcome mechanical barriers. Finally, we observe that ribosomes occasionally open the hairpin in two successive sub-codon steps, revealing a previously unobserved translocation intermediate.

Chen, Z., Gabizon, R., Brown, A. I., Lee, A., Song, A., Diaz-Celis, C., Kaplan, C. D., Koslover, E. F., Yao, T., Bustamante, C. (2019) BioRxiv, 641506; Submitted to eLife. High- resolution and high-accuracy topographic and transcriptional maps of the nucleosome barrier

Nucleosomes represent mechanical and energetic barriers that RNA Polymerase II (Pol II) must overcome during transcription. A high-resolution description of the barrier topography, its modulation by epigenetic modifications, and their effects on Pol II nucleosome crossing dynamics, is still missing. Here, we obtain topographic and transcriptional (Pol II residence time) maps of canonical, H2A.Z, and monoubiquitinated H2B (uH2B) nucleosomes at near base-pair resolution and accuracy. Pol II crossing dynamics are complex, displaying pauses at specific loci, backtracking, and nucleosome hopping between wrapped states. While H2A.Z widens the barrier, uH2B heightens it, and both modifications greatly lengthen Pol II crossing time. Using 36

the dwell times of Pol II at each nucleosomal position we extract the energetics of the barrier. The orthogonal barrier modifications of H2A.Z and uH2B, and their effects on Pol II dynamics rationalize their observed enrichment in +1 nucleosomes and suggest a mechanism for selective control of gene expression.

Fig. 6 from Chen, Z. et al. Transcriptional Maps of the Nucleosome Reveal that H2A.Z Enhances the Width and uH2B the Height of the Barrier (A) Median residence time histograms of Pol II transcription through bare NPS DNA (black), xWT (orange), hWT (red), H2A.Z (blue) and uH2B (green) nucleosomes. Bar width is 1 bp and major peak positions are labeled (in bp) above the corresponding peaks. NPS entry, dyad, NPS exit are marked with blue dashed lines. The polar plots on the right are the corresponding transcriptional maps of the nucleosome, formed by projecting the residence time histogram onto the surface of nucleosomal DNA. The top axis (red) indicates corresponding positions of the first half of nucleosome expressed as superhelical locations (SHL). n = 35, 23, 26, 21, 31, respectively for NPS DNA, xWT, hWT, H2A.Z and uH2B nucleosomes.

Postdoctoral researcher Juan Pablo Castillo working with high-resolution optical tweezers with single-molecule fluorescence capabilities in the Bustamante optics lab.

Graduate student Robert Sosa and undergraduate research assistant Weiyi Tang preparing benign buffers in the Bustamante wet lab for a protein purification.

Crommie Group

Scanned Probe Microscopy of Novel 1D and 2D Materials

During the past year, the Crommie research group has focused on exploring the structural and electronic properties of low dimensional materials including two-dimensional (2D) Transition Metal Dichalcogenides (TMDs), gate-tunable Graphene/Hexagonal Boron Nitride devices, and single molecules by means of Scanning Tunneling Microscopy/Spectroscopy (STM/STS).

2D TMDs host a variety of exotic quantum phenomena such as the Mott insulating phase. Bulk 1T-TaS2 and the surface of bulk 1T-TaSe2 have long been known to exhibit unusual insulating phases in the star-of-David charge density wave (CDW) state. The nature of these insulating phases, however, is complicated by the interlayer stacking orders. We have performed combined STM/STS, angle-resolved photoemission spectroscopy (ARPES), and theoretical studies of the electronic structure of single-layer 1T-TaSe2 grown via molecular beam epitaxy. We have discovered a new insulating phase and confirmed its Mott nature through comparison of experimental and theoretical local density of states (LDOS) maps at different energies. Exotic orbital textures discovered in the lowest empty states suggest pronounced correlation effects that are unique to 2D, and which disappear as soon as interlayer coupling is added back in. We have shown that the Mott phase is intrinsic to this 2D TMD and provides an ideal platform to study doped Mott insulators via gate-tuning. Our measurements show evidence of 2D quantum spin liquid behavior in this material, a completely new phase of matter.

(A) STM image of topological GNR superlattice grown on Au(111). (B) (B) STM image of COF366-OME and dI/dV maps of the COF LUMO and HOMO orbitals.

We have also shown that by fusing 2D materials with different compositions in a lateral heterojunction, novel 1D interfaces can be created that may host a variety of interesting phenomena. In the past year we have focused on studying lateral heterostructures of semiconducting WS2/WSe2 monolayers by means of STM/STS measurement. Topographic 38

images show that the interface is coherent, dislocation free, and atomically precise. To accommodate lattice mismatch on the order of 4%, both materials are highly strained which manifests itself in the electronic structure. Their band gaps are found to significantly deviate from their unstrained counterparts while a sharp lateral transition of the order of only 2nm is observed at the interface. This type of strain-tunable, clean and sharp interfaces can potentially host novel electronic states and has great potential in future device applications.

We have found graphene on BN to be a useful new platform for studying electromigration of adsorbed molecules due to its weak van der Waals interaction, which greatly decreases adsorption and diffusion energy, enabling the observation of superlubricity at the atomic scale. In the past year, we have fabricated gate-tunable two-terminal graphene/BN devices to study the motion of adsorbed molecules and atoms on the device under electromigration forces. We have also explored molecules that exhibit conformational change with the application of an electric field.

We have also explored self-assembled 1D graphene nanoribbons (GNRs) and shown that it is possible to manipulate their electronic ground state by tuning the precursor molecule building block. This has allowed us to fabricate the first metallic GNRs, and to show that it is possible to manipulate their bandwidth over orders of magnitude through small structural changes.

Graduate students Ryan Lee (left) and Wei Ruan (right) are loading the two-dimensional (2D) Transition Metal Dichalcogenides (TMDs) samples.

Graduate student Franklin Liou is examining the adsorbates on Graphene surface.

Graduate students Danel Rizzo (back) and Peter Waller (front) are growing graphene nano ribbons (GNRs).

Fischer Group

The Fischer group focuses on the rational design and synthesis of atomically precise carbon nanomaterials and their incorporation into functional electronic devices. Using low-temperature (~4 K) Scanning Probe Microscopy (SPM) techniques, the group is able to study exotic physical phenomena emerging from the effects of confinement. These nanomaterials are constructed using a “bottom-up” approach in which a single building block, or monomer, is used to piece together the larger structure. The precision in this method enables the group to probe the downstream effects resulting from changes in atomic structure. Over the past year research concentrated upon three areas: Graphene Nanoribbons, Molecular Machines, and Nanomagnets.

Graphene Nanoribbons

Graphene, a single atom thick sheet of carbon atoms, shows incredible promise for electronics, and when narrow strips of graphene, also known as graphene nanoribbons, are used they offer a potential alternative to silicon semiconductors. These graphene nanoribbons (GNRs) are synthesized on a gold surface in ultra-high vacuum allowing for high-quality, reproducible GNRs. The Fischer group targets functionalized GNRs in order to correlate changes in atomic structure with changes in electronic structure. The group recently synthesized a monomer that forms a GNR with segments of different width. 40

The distance between the segments is dictated by the initial monomer, and the final GNR can be either a metal or semiconductor depending on that distance. Additionally, the Fischer group has been working to incorporate dopant atoms (i.e. N atoms) into the core of the GNR. The addition of dopant atoms disrupts the all carbon structure the GNR inherits from graphene. The group has developed a method to form C-N bonds on the gold surface. Finally, the ribbons are studied using SPM, which gives the group atomic and electronic structure information. This gives the Fischer group unprecedented Figure 1. b) synthesis of molecular precursor c) Schematic control over the electronics and the ability to representation of the stepwise thermally induced on-surface map out the effects of various structural growth of a 7/9-AGNR superlattice from molecular precursor changes. 1. Poly-1 is formed upon annealing at 200 °C and full cyclization occurs at 3

Molecular Machines

Among the family of primitive machines, mechanical levers consisting of a rigid beam pivoted at a fulcrum represent one of the simplest mechanisms that employs mechanical advantage to translate and multiply a force input. The underlying principle, first documented by Archimedes, relates the ratio of forces (FA, FB) applied at points A and B to the ratio of the distances (a, b) between the fulcrum and the point where a force is applied. Variations of this principle are at the heart of even the most complex geared mechanisms and machines. Despite synthetic efforts toward the design of molecular machines, the extent to which these fundamental mechanical laws translate to the nanoscale remains poorly understood. The Fischer group has begun designing and synthesizing molecular lever machines that can be actuated either through an external mechanical input or through electromechanical forces. We are also taking advantage of organic synthetic tools to alter the ratio of the mechanical lever length (a and b) and determine the resulting force multiplication. This research will enable the functional integration of molecular lever action machines capable of translocating molecules or nanoparticles across a surface.

Nanomagnets

The Fischer group has designed monomers to create 1-dimensional chains and 2-dimensional networks of isolated spin-centers that can act like nanomagnets. These pyramid-shaped all- carbon monomers (phenalene, triangularine, etc.) are typically very reactive, but creating these chains and networks in a controlled environment allows the group to study these molecules more thoroughly. These arrays of nanomagnets could be used in next-generation data storage devices (magnetic RAM, MRAM) and storing data in the electron spin gives us an additional degree of freedom for data storage / energy efficient electronics.

Fleming Group

Nonlinear Spectroscopy of Photosynthetic Complexes and Low-dimensional Materials

The Fleming group continued its efforts to understand various aspects of photosynthetic light- harvesting and low-dimensional materials using spectroscopic techniques, including snapshot transient absorption (TA) spectroscopy, 2DES (Two Dimensional Electronic Spectroscopy) and 2DEV (Two Dimensional Electronic Vibrational) spectroscopy, supported by theoretical calculations for a better perspective.

Photosynthetic organisms regularly experience fluctuations in light intensities, which require the activation of photoprotective energy dissipation mechanisms in periods of excess light. This dissipation, referred to as non-photochemical quenching (NPQ), is known to impact biomass and biofuel yields. However, the molecular details remain controversial due to the challenge of detecting molecular signals relevant to the quenching in fully intact photosynthetic samples.

In order to gain insight into the molecular mechanisms behind NPQ, the Fleming group developed TA spectroscopy, which monitors the excited-state population and dynamics throughout the quenching response on the seconds-to-minutes time scale (Park et al. Photosynth. Res. 2019). Following the successful detection of the carotenoid (Car) radical cation signal (Park et al. JPCL 2017), the snapshot TA technique was extended to investigate the dynamics of the Car S1 state in intact plant thylakoid membranes throughout the process of high-light exposure (Park et al. JACS 2018).

Additionally, the Fleming group has reported the first in vivo investigation of photoprotective quenching mechanisms in a photosynthetic algae (Park et al. PNAS 2019). Specifically, evidence was found for the involvement of both the Car radical cation and the Car S1 state in the quenching response of the microalgae Nannochloropsis oceanica. By using knockout mutants, two proteins that are essential for photoprotection in N. oceanica were identified: the enzyme VDE which produces the carotenoid zeaxanthin under high light and the pH-sensing LHCX1 protein. Beyond the experimental efforts to investigate the photophysical quenching mechanism, the Fleming group also approached this question from the perspective of multiscale modeling, which revealed that quenching can be accounted for by the decrease in the excitation diffusion length (Bennett et al. PNAS 2018). The group has recently reviewed combined modeling and

spectroscopic efforts (Bennett et al. Open Biology 2019) which, together, aim to develop a more complete understanding of the process of photosynthetic regulation.

The Fleming group used the relatively new 2DEV spectroscopy to investigate the excited state dynamics of the lowest singlet excited state of the triphenylmethane dyes, crystal violet and malachite green, by monitoring the C=C aromatic stretch (Wu et al. PCCP 2018). Electronic structure calculations were performed to assign the observed 2DEV transitions to specific geometries. The studies suggest that the evolution in the excited state involves intramolecular charge transfer and the nuclear wavepacket diffuses on the electronic excited state before relaxing through a conical intersection to an intermediate twisted ground state, which then proceeds to the equilibrium ground state through twisting of the aniline rings.

The group conducted 2DEV spectroscopy of the LHCII complex and is working on understanding the Center Line Slope (CLS) information obtained from the experiments (Wu et al. Faraday Discuss. 2019). The CLS describes the correlation between initial electronic coherence created by the first visible pulse and the final vibrational coherence created by the IR probe pulse, and can be positive, negative, or zero. CLSs with a rich variety of dynamics were observed for the LHCII complex.

The Fleming group performed independent theoretical investigations that evaluate the 2DEV spectra of coupled molecular complexes (Bhattacharyya et al. JPCL 2019). These studies were done to understand the role played by a vibrational mode that is resonant with the electronic exciton energy gap in the ultra-efficiency of light-harvesting. A back-and-forth transport of excitation between the IR mode and the electronic excitons could explain the long-lived oscillations previously reported for 2DES experiments on photosynthetic complexes. It is seen that a resonant IR mode results in the creation of vibronic excitons that have a strong presence in the 2DEV spectra. The vibronic excitons form stronger coherences amongst them, compared to electronic-only excitons, with rapid decoherence. Population relaxation among vibronic excitons is also found to be significantly larger than population relaxation between the electronic-only excitons.

The Fleming group also performed 2DES experiments on monolayer MoS2 in an attempt to elucidate the dynamics of exciton formation in monolayer transition metal dichalcogenides (Guo et al. Nat. Phys. 2018). The studies hint at the presence of strong intravalley exchange interaction, resulting in mixing of excitons.

Ginsberg Group

The Ginsberg group develops spectroscopic imaging tools that access new combinations of spatial and temporal resolution appropriate for elucidating dynamic processes in self-assembled materials, in particular within disordered semiconductors that may find use in next generation solar cells, light emitting diodes, or other optoelectronic applications. One example is our low- dose cathodoluminescence (CL) microscope, which was used in collaboration with Kavli ENSI members Prof. Limmer and Prof. Yang to directly visualize a structural phase transition in metal halide perovskite nanowires. The kinetic pathways that underly these transitions are difficult to probe because they occur over multiple disparate length and time scales. Using scanning

electron CL imaging with in situ heating on perovskite nanowires created by the Yang group and combining results with calculations from the Limmer group, we determined that the elusive mechanism for the Y- to B-phase transformation is facilitated via the presence of a disordered interface, simultaneously providing the first detailed experimental-numerical characterization of a non-martensitic phase transition driven by halide ion diffusion. A manuscript is currently in preparation. Understanding heterojunction properties in perovskite nanowires is key to uncovering design principles relevant for precise control of carrier transport in functional optoelectronic devices. We applied CL imaging to characterize the distribution and propagation of a localized phase transition in single-crystal perovskite nanowires. The intersection of a “yellow” and heat-induced “black” phase forms a p-n junction without chemical doping, highlighting the remarkable functional tunability of these materials. [Kong 2018] Perovskite nanowires are interesting also because novel electronic behaviors emerge when the material dimensions are small enough to induce quantum confinement effects. In collaboration with the Yang group, we directly imaged rapid one-dimensional diffusion of excitations along aligned bundles of quantum confined perovskite nanowires using stroboSCAT (see below), which revealed that energy prefers to propagate along the long axis of the nanowires while coupling between neighboring nanowires is weak. We furthermore characterized how the anisotropic confinement affects the electronic structure and dynamics using broadband polarization-resolved transient absorption microscopy. With this technique that measures ultrafast spectra of excitations on the microscale we observe a polarization-dependent splitting of the band edge exciton line, and from the polarized fluorescence of nanowires in solution we determine that the exciton transition dipole moments are anisotropic in strength. This anisotropy is not seen to become more significant in thinner nanowires, suggesting that the nanowires exhibit weak confinement in relation to the exciton radius. A manuscript is currently in preparation. Directly visualizing diffusion of energy carriers in complex solid environments is crucial to many energy applications, and our group has developed a new ultrafast microscopy technique called stroboSCAT to study such intricate processes on relevant ultrasmall and ultrafast scales. The apparatus leverages the sensitivity of interferometric scattering microscopy (iSCAT) to capture an ultrafast snapshot of energy migration in the material in addition to a correlative baseline image of the sample morphology. It is non-damaging and easy to use, revolutionizing the study of diffusion in disordered solids. We applied this technique to a wide range of semiconducting materials – single crystal and polycrystalline, inorganic and organic – to characterize how energy navigates through disordered landscapes. One important case study in perovskite thin films revealed that grain boundaries appear to promote out-of-plane carrier transport. Energy follows the path of least resistance deeper into the material to cross grain boundaries, suggesting a possible mechanism for enhanced photovoltaic power conversion efficiency if energy can be channeled toward extraction layers. A manuscript is currently in review at Nature Materials.

0 ns 1 ns 2 ns 5 ns

1µm

Ultrafast stroboSCAT images of perovskite thin films with ~1 μm size grains reveal that carrier diffusion is more heterogeneous due to a strong dependence on inter-grain connectivity and depth-dependent grain boundary resistivity. Bright contrast in the image, which encodes depth information, correlates with the position of grain boundaries that appear to direct energy carriers deeper into the film.

Pictures of students doing cutting-edge research

From left to right: Jenna Tan, Rebecca Wai and Hannah Weaver from the Ginsberg group visit the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory. This beamline uses short pulses of visible and x- ray light to look at the evolution of very fast processes in materials. Many optics and specialized vacuum equipment are necessary to precisely control light and maintain a vacuum environment.

From left to right: Lena Blackmon, Shwetadwip Chowdhury, Hannah Weaver and Milan Delor celebrate the culmination of a successful collaboration which combined two imaging techniques in the same microscope. The Ginsberg group specializes in building instruments that image dynamic processes in nanoscale objects. Being able to characterize a sample in many ways in the same microscope will help us to develop a more complete picture of how energy moves through disordered environments so that we can design better materials for the semiconductors and solar cells of tomorrow.

King - Liu Group

Ultra-Low-Energy Operation of Relays for Quantum Computing

Quantum computers require electronic switches operating under cryogenic conditions to manipulate and digitally read individual quantum bits (qubits). Ideally, this electronic control circuitry should be monolithically integrated with the qubits and hence should dissipate as little energy as possible in order to maintain the computer at milli-Kelvin temperatures. In contrast to transistors, micrometer-scale electro-mechanical switches (i.e., relays) have zero off-state leakage, which provides for zero passive power dissipation; relays also can operate with ultra- low voltages (milli-Volts) to provide for ultra-low active power dissipation. In this project, milli- Volt operation of relay-based digital circuits at cryogenic temperatures is investigated, toward the goal of practically enabling a quantum computer.

Microscope image of relay test chip with wire-bonded pads for testing a 4-terminal relay at ultra-low temperatures.

Experimental results show that relays operate stably with relatively low contact resistance (less than 1 kilo-ohms) and low hysteresis voltage (less than 20 milli-Volts) at temperatures below 90K – the boiling point of oxygen (O2) – down to 1.8K (the lower temperature limit for micro- probe stations at UC Berkeley). Accordingly, milli-Volt operation of relay integrated circuits has been successfully achieved at cryogenic temperatures. Since quantum computers require operation at milli-Kelvin temperatures, relays will be wire-bonded to chip-carrier packages for further testing at such extremely low temperatures, in the coming reporting period. A superconducting metallic material may be necessary to use as the contacting electrode material

in order to achieve proper functionality at such extremely low temperatures; for this reason approaches to integrating niobium into the relay fabrication process will be investigated as well.

Postdoctoral researcher Sergio Almeida (left) and graduate student Xiaoer Hu (right) preparing to test relays at cryogenic temperatures in a vacuum wafer probe station cooled with liquid nitrogen.

Graduate students Zhixin “Alice” Ye (left) and Tsegereda Esatu (right) programming a Keithley 4200A Semiconductor Parameter Analyzer Characterization System for electrical characterization of relay performance across a wide range of operating temperatures.

Limmer Group

Over the last year, the Limmer group has made strides in the understanding of diverse nanoscale nonequilibrium processes of relevance to energy sciences.

In collaboration with Kavli members Naomi Ginsberg and Peidong Yang, the Limmer group has investigated the interactions between photo-physical and structural dynamics in lead halide perovskite nanocrystals. Specifically, the nanoscale photoinduced phase separation in single- crystal mixed Br-/I- methylammonium (MA+) lead halide perovskite (MAPb(BrxI1- x)3) nanoplates was studied, and it was established that phase separation can be tuned by lowering the electron-phonon coupling strength by partially exchanging MA+ for Cs+. These results, supported by multiscale modeling, demonstrated that photoinduced phase separation is an intrinsic property of mixed halide perovskites, the extent and dynamics [1]. In other collaborative work with the Yang and Ginsberg groups, the direct visualization of halide anion inter-diffusion in CsPbCl3–CsPbBr3 single crystalline perovskite nanowire heterojunctions was accomplished in conjunction with molecular modeling to establish the mechanism of the high ionic conductivity in these materials [2]. This work elucidated the intrinsic solid-state ion diffusion mechanisms in this class of semisoft materials, and offered guidelines for engineering materials with long-term stability in functional devices.

In a study with the Fischer and Yablonovitch groups, the Limmer group helped to clarify the role of non-Lorenzian line shapes in tunnel field effect transistors [3]. Electronic switches generally demand an on/off ratio of at least a million. Steep exponentially falling spectral tails would be needed for rapid off-state switching. This requires a new electronic feature, not previously recognized: narrowband, heavy-effective mass, quantum wire electrical contacts, to the tunneling quantum states. In order to study these effects numerically, the Limmer group, in collaboration with the Rabani group at UC Berkeley, has developed molecular simulation tools to simulate nanoscale junctions in cases where electron correlation and electron phonon effects are important [4-5].

Finally, the Limmer group has made a number of recent theoretical discoveries related to how nanoscale systems driven far from equilibrium respond to external perturbations. In one early example, a system of interacting active Brownian particles, a minimal model for bacterial organization, was shown to exhibit Fickian mass transport in that the diffusion constant determines the response of a small density gradient, despite the statistics of displacements being non-Gaussian [6]. The non-Gaussian distribution was shown to give rise to nonlinear responses at large concentration gradients. In other work, the Limmer group introduced a way to compute arbitrarily high order transport coefficients of nanoscale systems, using the framework of large deviation theory [7]. Leveraging time reversibility in the microscopic dynamics, it was found that nonlinear response was computable from equilibrium multi-time correlation functions among both time reversal symmetric and asymmetric observables. This connection established a thermodynamic-like relation for nonequilibrium response and provided a practical route to its evaluation. The utility of this formalism was demonstrated in a model of thermal rectification.

Images of self-trapped polarons in CsPbI3 (left) and MAPbI3 (right) demonstrating the decrease in size accompanying the increased electron-phonon coupling in the latter.

Group Members and Project descriptions:

Post docs Mirza Galib: Using machine learning based molecular dynamics simulations, Mirza is investigating the mechanisms of reactive uptake of atmospherically relevant molecules into sea spray aerosols.

Sam Niblett: Sam is developing tools to study chemical reactions in heterogeneous environments, including the aqueous chemistry of carbon dioxide.

Graduate Students

Chloe Gao: Chloe is studying the transport of energy, mass and charge on nanoscales using molecular simulation and large deviation theory.

Trevor GrandPre: Trevor works on developing theoretical tools to study the collective behavior of biological systems driven far from equilibrium.

Avishek Das: Avishek studies the self-assembly of materials driven far from equilibrium by developing variational statements.

Yoonjae Park: Yoon is developing a path integral framework to study polaronic effects of optical excitations in contemporary semiconductors like the lead halides.

Ben Kuznetz Speck: Ben studies how biological systems tune the rate of active processes by burning chemical fuel.

Neaton Group

The Neaton group is a theoretical and computational group working at the intersection of physics, chemistry, and materials science. We primarily draw on contemporary “first-principles” approaches based on density functional theory (DFT) which have the ability to predict measurable properties of materials with good accuracy without adjustable empirical parameters. We interact closely with experimental groups to guide and be inspired by studies of new materials and phenomena in nanoscience, renewable energy, and quantum materials, reflecting a breadth consistent with the flexibility of first-principles methods. Below is an overview of the previous year’s progress:

Metal-Organic Frameworks: Carbon capture Metal organic frameworks are a class of highly tunable porous crystalline materials which readily adsorb CO2 and are promising candidates for gas capture and separation. This year, in collaboration with Foundry scientists David Prendergast and Steve Whitelam, the Neaton group demonstrated how cooperative adsorption of CO2 arises in certain MOFs even in the absence of structural phase transitions [19]. In a separate study, state-of-the-comp techniques to simulate NMR measurement in MOFs after CO2 chemio-absorption were performed and shown to be in good agreement with experimental measurements made by the Long and Remier groups [10].

Method development for predicting excited state properties and transport at nanoscale interfaces The Neaton group has continued to develop state-of-the-art computational method based on non-equilibrium green function formalism for predicting charge transport at the nanoscale. This past year these methods were generalized to treat covalently bound metal-molecule junctions, while still accounting for many-body electron-electron interactions at a level consistent with the GW method [7]. In a separate work, in collaboration with Leeor Kronik at the Weizmann Institute, the Neaton group demonstrated that time-dependent DFT in conjunction with range- separated hybrid functionals are able to predict absorption spectra in simple solid state systems at a fraction of the cost of more sophisticated many-body techniques [1].

Two-dimensional transition metal-dichalcogenides: Defect characterization Two-dimensional transition metals exhibit a unique symmetry which allows researchers to selectively excite electrons with different crystal momenta by varying the polarization of incoming light. This selectivity has immense implications for device design, specifically next- generation transistors. This past year, in collaboration with Steven Louie’s group, the Neaton group studied the robustness of these selection rules in the presence of point defects explicitly including many-body electron-electron interactions [13].

Perovskites: Photophysics for solar energy conversion This past year, the Neaton group, in collaboration with Hemamala Karunadasa at Stanford, put forward a set of design principles for synthesizing halide double perovskites with smaller band gaps more suitable for optical absorption in the visible range. With these principles, the Neaton group predicted Cs2AgTlBr6 to have a direct bandgap of 0.95 eV, which to the best of our knowledge is the lowest band gap ever reported for a halide perovskite [17]. In a separate work in collaboration with Karen Rabe, the Neaton group predicted that in SrTiO4 biaxial strain can induce a structural distortion which induces a spontaneous internal electric field. This

phenomenon, known as ferroelectricity, has applications for efficient capacitor design and more generally memory storage devices [5].

Topological States in Quantum Materials The classification of material by a topological index rather than a symmetry group represents a recent radical shift in the community’s understanding of phase transitions. This past year the Neaton group investigated the connection between non-trivial topology and multiferroic phases in a class of Hexagonal Manganites [2]. In separate work, the Neaton group demonstrated that by applying pressure to systems as simple as bulk elemental Lithium, one could drive the system from a normal to topologically non-trial phase [4].

Siddiqi Group

The Quantum Nanoelectronics Laboratory (QNL) conducts research on control and measurement of quantum systems using microwave resonators and microwave quantum bits known as transmons. Since these systems do not have strong coupling to external noise, they can stay entangled for extended periods of time. With a sufficient number of qubits, one could perform computations that would be infeasible on a classical computer. QNL is also using quantum chemistry algorithms on these systems to simulate the energy structure of small molecules. These algorithms map different configurations of molecules, such as interatomic distances and angles, onto different states of the transmon system. By looking at the statistics of the transmons for different configurations, one can determine the structure and energy levels of the molecule. As the number of qubits and their interactions increase, more and more complex molecules can be simulated.

In order to improve the capabilities of the quantum systems, QNL has been working to increase the number of usable qubits in a single system. A critical step in fabricating such systems is controlling the resonant frequency of the transmons. QNL’s current standard is a set of silicon chips with niobium microwave structures lithographically defined on the surface. In order to complete the transmon qubit fabrication, small (100 nm on a side) aluminum-aluminum oxide- aluminum tunnel barriers must be deposited. The frequency of the transmon is highly dependent on the size of the junction, as was as the growth of the intermediate oxide layer. In order to have many qubits come out at a set of desired frequencies, QNL has worked on improving fabrication techniques to increase uniformity of tunnel barriers across a six-inch silicon wafer. This improvement in frequency tolerance will allow faster and more efficient computational operations between the transmons.

In addition to improving fabrication, QNL is also improving the controllability of a small number of qubits. Leveraging the computational power of quantum systems requires the use of operations that will entangle qubits. QNL is using a system with transmon qubits whose frequencies are tunable in-situ to study the behavior of a particular two-qubit entangling interaction known as the cross resonance gate with qubits at different operating frequencies. This study will further inform target qubit tolerances during initial fabrication.

Qubit_chip.jpg: Quantum information processor with 8 transmon style quantum bits.

QNL is studying a different entangling operation known as the Mølmer–Sørensen interaction. Although well known in the ion-trapping field, QNL is working to demonstrate this interaction in transmon systems for the first time. This particular interaction distinguishes itself from many other transmon operations in that it can intrinsically generate entanglement among multiple qubits simultaneously.

QNL is also increasing the computational capacity of current systems. Although transmons are usually operated as qubits (using two quantum levels), the physical system actually has more than two levels that can be used for quantum information processing. One project is using three levels of their transmons as a qutrit-based system. They are studying the dynamics of quantum scrambling, a particular operation in which quantum information is rapidly spread throughout a system, but is always recoverable. Using qutrits rather than qubits, they can perform their experiment with a fewer number of physical transmons.

By both improving control of current transmon systems and developing the fabrication techniques for creating larger systems, QNL is hoping to increase the experimental space for quantum computation. With these capabilities, more complex physical processes reliant on quantum behavior can be modeled and simulated using the well-controlled system of transmons.

Depositing aluminum to form Josephson junctions, an integral part in fabricating transmon style qubits

Room temperature electrical characterization of a quantum device.

Cryogenic testing of quantum limited microwave detectors

Somorjai Group

Integration of Heterogeneous, Homogenous, and Enzyme Catalysis on the Nanoscale

The main focus of Somorjai Lab is developing new catalytic systems and studying the reaction mechanism via characterizing the catalysts under reaction conditions.

This includes integration of three fields of catalysis (i.e. heterogeneous, homogeneous, and enzymatic), done in collaboration with Matthew Francis and Dean Toste. It is beneficial to heterogenize homogeneous catalysts and enzymes in order to create a catalyst with the selectivity of homogeneous catalysts and enzymes and the reusability of heterogeneous catalysts. For example, heterogenized homogeneous catalysts are achieved by 1Schematics showing the evolution of the complexity of catalytic systems leading to synthesizing sub-1nm metal increased catalytic selectivity for multipath and multiproduct catalytic transformations. nanoparticles encapsulated into dendrimer supports. Heterogenized enzymes are produced though DNA directed immobilization 55

where one short DNA strand is attached to a glass surface and its complementary strand is attached to the enzyme.

Dimerization of isobutene and other olefins are investigated by the alkylation reactions using acidified metal-organic frameworks (MOFs) as catalysts in collaboration with Omar Yaghi. Sulfated MOF-808 can work as strong Bronsted acid for catalyzing oligomerization reaction of light olefins at relatively low temperatures. The deactivation and regeneration mechanisms were also investigated. In another project, pristine MOF-808 was used as catalyst for dehydration of alcohols (e.g. methanol, ethanol). The size effect of nanoparticles on catalytic performance were studied extensively by the Somorjai group over the past two decades, from sub-nanometer to a hundred nanometers. To complete the study of size effect, noble metal single atom catalysts supported by cerium oxide were synthesized and their catalytic performance were studied by methanol dehydrogenation reaction.

We also have collaborative projects focused on alternative energy sources. In collaboration with Sandia National Laboratory we are studying hydrogen storage on surfaces for fuel cell applications. In collaboration with the Honda Research Institute we are working on understanding the mechanism in lithium-ion batteries of how lithium diffuses between the anode and cathode through the organic electrolyte solution.

Luning Chen, a visiting student in the Somorjai Group, works on testing catalytic performance of CeO2 supported Pt single atom catalysts via continuous flow reactor by gas chromatography.

Wang Group

Feng Wang’s group specializes in studies of low dimensional materials using a variety of transport and optical techniques, including energy-, polarization-, and time-resolved measurements on micrometer and nanometer length scales. The group made many scientific contributions this year, and three particularly exciting directions are highlighted below.

In the past year, the Wang group has made groundbreaking contributions to the study of strongly correlated phenomena in stacks of two dimensional (2D) materials, with an eye towards high temperature superconductivity. Understanding the mechanism of high temperature superconductivity is a central problem in condensed matter physics. It is often speculated that high temperature superconductivity arises from a doped Mott insulator, where interactions between electrons make the system unexpectedly insulating. Obtaining an exact solution that describes this system (the Hubbard model) is extremely challenging due to the strong electron- electron correlation. Therefore, it is highly desirable to experimentally study a model system in which the unconventional superconductivity can be continuously tuned by varying several parameters. The Wang group, for the first time, realized a tunable Mott insulator in a stack of trilayer graphene and hexagonal boron nitride (TLG/hBN), which forms a moiré superlattice (Fig. 1, Ref 8). Around the 1/4 filling Mott state, which corresponds to one hole per moiré unit cell, the Wang group observed two superconductivity domes. Upon tuning the width of the electronic bands by vertical electric field, the Wang group observed clear phase transitions of metal – Mott insulator – superconductor (1). These results provide TLG/hBN as an ideal platform to study strongly correlated physics, such as the Hubbard model and high temperature superconductivity, and bridges two-dimensional materials and strongly correlated physics.

The Wang group also investigated stacks of semiconducting 2D materials that form a moiré superlattice. The moiré superlattice formed between two-dimensional (2D) materials provides a powerful tool to engineer novel quantum phenomena by introducing new length and energy scales associated with the moiré periodicity. As above, the most striking phenomena emerge in the “strong-coupling” regime, where the periodic moiré potential dominates over the relevant kinetic energy and qualitatively changes the electron behaviors in both real and momentum space. The Wang group reported the first observation of moiré excitons in the “strong coupling” regime (5). They showed that the kinetic energy of excitons, i.e. bosons composed of tightly- bound electron-hole pairs, is completely dominated by the moiré potential in nearly aligned WSe2/WS2 superlattices (Fig. 2) This causes the original exciton state to split into multiple moiré excitons, leading to multiple emergent peaks in the absorption spectra with distinctively different gate dependence. Furthermore, they found that the moiré exciton states are tightly confined around well-separated points in the moiré pattern, forming a periodic boson lattice in the real space. Combing the formation of a well-defined exciton superlattice, the great tunability of the van der Waals heterostructures, and the strong light-matter interaction of 2D semiconductors, the WSe2/WS2 superlattices provide a promising platform to realize novel quantum optoelectronics and excitonic states, such as topological exciton bands and a strongly correlated exciton Hubbard model.

Studying and understanding how electrons interact in many-body systems at a fundamental level is at the core of condensed matter physics. Interacting electrons in three- and two- dimensional metals are well described by almost independent quasi-particles within the Fermi liquid theory. However, when electrons are confined in one-dimensional systems such as metallic single-walled carbon nanotubes (SWNTs), electrons cannot move without pushing all other electrons around. Therefore, the properties of SWNTs cannot be described by a single- particle model and instead require Luttinger liquid theory. Luttinger liquid exhibits many fascinating physical properties: a power-law decay of tunneling probability and spin-charge separation, where the spin and charge degrees of freedom propagate with different velocities. Both the power index of the tunneling probability and the velocity ratio between charge and spin modes are uniquely defined by a single Luttinger interaction parameter. The Wang group carried out parameter-free test of the Luttinger liquid theory by correlating two completely distinct physical properties: electron tunneling and plasmon propagation (12). This is achieved by a direct combination of electrical transport and optical nanoscopy on the same metallic 57

SWNT cross junctions, which provides the first parameter-free experimental determination of Luttinger parameters.

Fig. 1. (a) Schematic of TLG/hBN moiré superlattice. Only atoms of the top hBN layer and the bottom graphene layer are shown for image clarity. In the Mott insulating state at 1/4 filling, each electron (or hole) occupies one superlattice site and they are separated by the dominating Coulomb repulsion. (b) Color plot of resistance as a function of top and bottom gates. The highlighted straight lines correspond to the charge-neutrality point (CNP), 1/4 filling, 1/2 fillings, fully-filled point (FFP) and 3/2 fillings resistance peaks. (c) Superconductivity phase diagram of TLG/hBN as a function of doping and temperature. a b

Fig. 2. (a) A schematic of a moiré superlattice between WSe2 and WS2 monolayers, with moiré wavevectors a1 and a2. (b) The absorption spectra for aligned (top) and misaligned (bottom) WSe2/WS2 heterostructures, showing dramatic splitting of the lowest energy peak due to the moiré superlattice.

Graduate student Danqing Wang and postdoc Wenyu Zhao open the near-field optical cryostat used to investigate the optical response of materials at small length scales.

Graduate students Emma Regan and Danqing Wang pose next to their femto-second laser that is used to study the optical response of two-dimensional materials on ultrafast time scales.

Whaley Group

Two research projects of Prof. B. Whaley’s group fall under the energy nanoscience umbrella. Both projects involve theoretical and computational modeling for specific experimental collaborations.

Single photon absorption by photosynthetic light-harvesting complexes This project is a study of fully quantum dynamics of a photosynthetic system, both as it absorbs an incident photon and the subsequent transport of the excitation across the complex. Our collaborators in the group of Prof. G. Fleming are setting up experiments to create nonclassical light pulses that contain one and only one photon (a Researchers at Whaley group discussion. Left to right: Robert Cook multimode Fock state). The time delay (postdoc), Liwen Ko (graduate student, Chemistry), Zengzhao Li between the pulse’s arrival time and the (postdoc). Inset: Lu Yang (undergraduate student, Chemistry). detection of any subsequent fluorescence contains information about the (in)efficiencies in the transport of a single excitation across the light-harvesting complex.

In a proof of concept study with a minimal toy model, we have derived a tractable solution for the joint system- light wave function (R. Cook and K. B. Whaley). From this joint wave function, shot-by-shot trajectories can be numerically generated. These simulations show that in trajectories where the outgoing fluorescent photon is significantly delayed, a dramatic increase in the excitation probability occurs, and near perfect transfer occurs between the sites (see upper panel in Fig. 1).

Additionally, research has continued to study what effect a vibrational environment has on the energy Fig. 1. Sample trajectories of dimer evolution. The probability of transport. This is done in the context of the exciton to occupy site 1 (bottom) and site 2 (top), given a a model where the electronic degrees of simulated measurement record. Ensemble averages are also shown. freedom couple to a non-Markovian phonon bath. Past work has utilized the Hierarchical Equations of Motion (HEOM) to study the probability of electronic excitation under different states of light, both with and without the phonon coupling. We have combined this with a multi-mode Fock state master equation, which models a local system interacting with pulses of light that contain a definite number of photons. Averaging over input Fock states with a thermal weighting of photon numbers, provides an approximate model of incoherent sunlight. This constitutes a generalization of our previous work combining the HEOM with a multi-mode coherent state containing an average number of one photon.i Our current research in progress investigates what role the pulse’s phase coherence plays in the dynamics and how a (thermal averaged) pulsed model compares to other states of light, such as squeezed vacuum states. This work is being carried out by postdoc Rob Cook and undergraduate student Lu Yang.

Vibrationally assisted energy transfer (VAET) In VAET a structured vibrational reservoir (such as molecular vibrations) improves energy transfer efficiency in molecules and molecular complexes. This phenomenon is believed to be critical in the efficiency of photosynthetic light harvesting. Direct computer simulations are particularly difficult to implement as the vibrational environment is mesoscopic in scale and exhibits strongly non-Markovian correlations. Fig. 2. The three-site system coupled to two vibrational modes in the absence of noise. The dashed arrows In collaboration with the experimental group of indicate cross couplings induced in an effective H. Häffner, this project uses emerging quantum Hamiltonian in the single-excitation subspace. 60

computing hardware, as implemented by trapped atomic ions, to develop a new approach to understanding VAET. Previous experimentsii have implemented a minimal model, where a vibrational model assisted in the transport of energy from a donor to an acceptor ion.

Current theoretical studies have explored the interplay between quantum VAET and a classical version of noise-assisted energy transfer (ENAQT). In general, classical noise has been found to weaken vibrationally assisted energy transfer and destroys its quantum signature.

Current efforts are directed at moving beyond the two-ion system. A model that combines three sites and two vibrational modes has been derived, which is a logical extension of the two-site experimental demonstration (Fig. 2). The larger system exhibits interesting new phenomena such as interference between vibrational quanta at weak electron-phonon coupling. In addition, numerical simulations reveal several features that have no analog in the two-site system. This work is being carried out by postdoc Zengzhao Li and graduate students Liwen Ko and Zhibo Yang.

1 H. C. H. Chan, O. E. Gamel, G. R. Fleming, and K. B. Whaley, “Single photon absorption by single photosynthetic light-harvesting complexes”, J. Phys. B: Atomic, Molecular and Optical Physics, Special Issue on Problems of light energy conversion and light harvesting, 51, 054002 (2018). 1 D. J. Gorman, B. Hemmerling, E. Megidish, S. A. Moeller, P. Schindler, M. Sarovar, and H. Haeffner, “Engineering Vibrationally Assisted Energy Transfer in a Trapped-Ion Quantum Simulator”, Phys. Rev. X, 8, 011038 (2018).

Yablonovitch Group

Tunnel Transistors

Quantum dot tunnel transistors are one type of high-quality, low-defect density tunnel transistors. In such devices, the lineshape of the broadening of the energy levels of the quantum dots, which interact with the contacts, is usually assumed to be Lorentzian. Lorentzian lineshapes lead to unacceptably slow, algebraic (that is, slower than the exponential switching seen in MOSFETs) subthreshold switching in tunnel transistors (Fig. 1.). In this work it was found that a narrow band, heavy effective mass metallic wire has to be introduced as an intermediary between the Figure 1: Due to interaction with the contacts, the quantum dots and the main metallic contact to energy levels in the quantum dots are broadened, ensure non-Lorentzian switching in tunnel and this leads to leakage in the off state. transistors (Fig. 2.). This work provides a deeper understanding of the quantum mechanics dictating the fundamental switching characteristics of tunnel transistors.

Figure 2: The quantum dots need to be coupled to the main metallic wires through a narrow-band, heavy effective mass wire.

*Sri Krishna Vadlamani ; Sapan Agarwal ; David T. Limmer ; Steven G. Louie ; Felix R. Fischer ; Eli Yablonovitch, “Tunnel-FET Switching Is Governed by Non-Lorentzian Spectral Line Shape”, Proceedings of the IEEE (2019).

Analog computing Patrick Xiao, a fresh alumnus of the group, Sri Krishna, and Prof. Yablonovitch are currently designing analog electrical oscillator circuits, via simulations, to find good approximate solutions for hard discrete optimization problems. The focus is on the NP-hard Ising problem that involves finding the global optimal configuration of a set of magnetic spins interacting via pairwise exchange interactions. The promise of analog hardware is that it can be several orders of magnitude faster than algorithms run on digital computers, as has already been demonstrated in previous work by research groups in the US and Japan. The results obtained by our group so far have been encouraging, and efforts are being made to improve the system.

Optical Antennas In the fast few decades, optical interconnects have replaced electrical wires to facilitate fast and efficient telecommunications on a long-distance scale. However, due to increasing needs for efficient data communication to extend the information age, optical interconnects are now being implemented within data centers and potentially on a wafer-level scale. These integrated optical interconnects require fast and efficient nanoscale light sources that are electrically injected and capable of being efficiently coupled to a photonic waveguide.

While lasers would conventionally be used for this purpose, scaling them down to the nanoscale poses issues due to high loss. LEDs are capable of scaling down to the nanoscale and can operate efficiently without a threshold, but they are limited in speed by their spontaneous emission rate to about 1GHz. However, by coupling the LED to an optical antenna, we can enhance the spontaneous emission rate, which would allow for >100GHz direct modulation. While such a device has been demonstrated to work as a proof of concept, coupling the antenna-LED to a photonic waveguide is non-trivial. However, by leveraging computational inverse design techniques, we demonstrated that the antenna-LED is capable of being very efficiently coupled to an optical waveguide -- ultimately achieving a waveguide coupling efficiency of 94% and an antenna efficiency of 64%, while maintaining a large spontaneous emission enhancement to enable high speed operation. The figure below depicts the side view of the coupling scheme.

*Nicolas M. Andrade, Sean Hooten, Seth A. Fortuna, Kevin Han, Eli Yablonovitch, and Ming C. Wu, "Inverse design optimization for efficient coupling of an electrically injected optical antenna-LED to a single-mode waveguide," Opt. Express 27, 19802-19814 (2019).

Thermophotovoltaics

We have established a new efficiency record to convert heat radiation to electricity, 29%, using thermophotovoltaics, where the thermal radiation excites photovoltaic cells. The thermophotovoltaic efficiency extrapolates to over 50% using known scientific concepts, and existing material properties.

This scientific breakthrough was inspired by the previous scientific recognition that high solar cell performance requires the creation of a dense infrared luminescent photon gas within the solar cell. That inspiration has held all solar cell efficiency records since 2011. It demands a highly reflective rear mirror to trap the internal luminescence.

Now we recognize that such a mirror can serve double-duty. It also solves the greatest problem in thermophotovoltaics, which is how to exploit the thermal photons that are too small to produce electricity. The mirror reflects those small photons to reheat the thermal source providing a second chance to make electricity, leading to unprecedented efficiency.

*Zunaid Omair, Gregg Scranton, Luis M. Pazos-Outόn, T. Patrick Xiao, Myles A. Steiner, Vidya Ganapati, Per F. Peterson, John Holzrichter, Harry Atwater, Eli Yablonovitch. “Ultra-Efficient Thermophotovoltaic Power Conversion by Band-Edge Spectral Filtering”, Proceedings of the National Academy of Sciences (2019).

Yaghi Group

(1) Single-crystal x-ray diffraction structures of covalent organic frameworks1 The grand challenge for covalent organic frameworks (COFs) roots in obtaining high-quality single crystals of COFs. The Yaghi group first developed a general method that uses aniline as the modulator to grow large single crystals amenable to single crystal X-ray diffraction characterization of up to 0.83-angstrom resolution, which has never been reported. This significant advance leads COF chemistry to unambiguous solution and precise anisotropic refinement in defining the structures.

Figure 1. Large single crystals of imine-based COFs synthesized under aniline modulation.

(2) 3D Covalent Organic Frameworks of Interlocking 1D Square Ribbons2 In this work, a new mode of mechanical interlocking in extended structure has been achieved by using copper complexes as metal templates to hold 1D organic ribbons in a well-defined arrangement, where corner-sharing squares are mutually interlocked, to afford a 3D woven structure with platinum sulfide (pts) topology. Demetalated COF-500 in a purely organic woven form can be obtained upon removal of the copper(I) ions without unzipping the total structure.

Figure 2. (a) Synthesis scheme to yield COF-500-Cu. (b) Purely organic extended woven COF-500 is formed upon removal of copper (I) ions.

(3) Bioinspired Metal−Organic Framework Catalysts for Selective Methane Oxidation to Methanol (Collaboration with Gabor Somorjai Group)3 The Yaghi group reported the utilization of MOF-808 to post-synthetically incorporate highly active copper−oxygen complexes, bis(μoxo) dicopper species inspired by particulate methane monooxygenase (pMMO). The MOF-808 catalysts with the stabilized active sites show high selectivity for methane oxidation to methanol under isothermal conditions at 150 °C.

Figure 3. MOF-808 catalyst inspired by pMMO for methane oxidation to methanol. Atom labeling scheme: C, black; O, red; N, green; Cu, orange; Zr, blue polyhedra. (H atoms are omitted for clarity.)

(4) Linking Molybdenum-Sulfur Clusters for Electrocatalytic Hydrogen Evolution4 To control the spatial arrangement of the catalyst on the electrode surface for higher performance in hydrogen generation, metal−organic sulfides (MOS) were synthesized by connecting Mo−S clusters with organic linkers through strong and directional bonds. Extended structures including cluster dimers, 1D chains, and cages were obtained. Significantly, the cluster chain displays a 40-fold enhancement in turnover frequency compared with the unlinked cluster, and requires an overpotential of only 89 mV to achieve a current density of 10 mA cm-2. (a) (b)

Figure 4. (a) Mo3S7 clusters connected by organic linkers into dimers and a chain. (b) Mo3S4 clusters connected by organic linkers into a dimer and a cage. (Color code for ball-and-stick models: Mo, blue; Br, orange; S, yellow; R group, purple; Zn, pink; C, gray; N, green; O, red. Color code for topology illustrations: SBUs, blue triangles; linkers, red lines; Zn centers, pink squares; cavity, yellow sphere.)

(5) Cytoprotective metal-organic frameworks for anaerobic bacteria (Collaboration with Peidong Yang Group)5 Yaghi group reported a strategy to uniformly wrap Morella thermoacetica bacteria with an ultrathin monolayer of metal-organic framework to provide cytoprotection against oxidative stress. This nanometer-thick monolayer of MOF not only significantly enhanced the lifetime of bacteria in the presence of oxygen, but also enables the bacteria to continuously produce acetic acid from CO2.

Figure 5. Design and synthesis of the M. thermoacetica–MOF wrapping system. In the space-filling model, atoms of cell wall and ROS are represented in cyan and green spheres, respectively. (Hydrogen atoms on zirconium clusters are omitted for clarity. Color code: blue, Zr; red, O; gray, C; white, H; yellow, P.)

Figure 6. Professor Yaghi and group members are discussing the single crystal growth of MOFs.

Figure 7. Professor Yaghi and Christian Diercks are discussing the topology of the structure model.

Yang Group

The Yang group's research is divided into three main branches, each with a substantial focus on developing new nanotechnology for energy applications and exploring fundamental nanoscience that assists in the future development of such technology. These branches are nanocrystal catalysis, nano-bio interfaces, and perovskite nanomaterials.

Nanocrystal electrocatalysis

Electrocatalysis aims to funnel electricity into driving chemical reactions with high selectivity and energy efficiency. With this electricity becoming increasingly renewable-derived, electrocatalysis is a key part of establishing a fully renewable energy economy where chemicals can be created using renewable inputs. In the Yang group, research efforts focus on the rational design of nanomaterials with controlled structure for the electrocatalysis of (1) oxygen reduction reaction (ORR), (2) carbon dioxide reduction reaction (CO2RR), and (3) nitrogen reduction reaction

Gold nanoclusters can harvest light and protect bacteria against reactive oxygen species to maintain high bacterium viability. By combining gold nanoclusters with CO2 fixing bacteria, this new generation of photosynthetic biohybrid system can efficiently absorb sunlight and transfer photogenerated electrons to cellular metabolism, realizing CO2 fixation continuously over several days.

(NRR). Studying the effect of catalyst processing has demonstrated a delicate trade-off between ORR activity and Pt-Ni nanoframe electrocatalysts stability. On this basis, Pt-Co nanoframes have been introduced to further enhance ORR performance. With regards to CO2RR, the group has developed a Cu nanoparticle ensemble catalyst which can effectively convert CO2 to C2+ products. Probing the evolutions of Cu nanoparticles during this process using in situ x-ray techniques yields insights into the active sites for CO2RR to multi-carbon products. In addition, sulfur-doped supports have been employed to stabilize Au nanoclusters, which exhibit remarkable activity and stability for NRR. The fundamental relationship between structure and performance of electrocatalysts explored in our studies advances the pressing initiative of reducing global CO2 emission and fossil fuel consumption.

Nano-bio interfaces

In the Yang Group, researchers have devised strategies to pair the high light-harvesting efficiency from solid-state semiconductors with the selectivity and replication of whole-cell biological catalysts. In a recently published work, highly biocompatible gold nanoclusters (AuNCs) were introduced into CO2 fixing bacteria. These AuNCs act as intracellular photosensitizers and provide the bacteria with photogenerated reducing equivalents. The photosynthetic biohybrid system fixes CO2 into acetate with 2.86% quantum yield. Recently, the 67

group has also demonstrated the ability to pair surface modified AuNCs with N2 bacteria. The close association of AuNCs with the cells enables light-driven N2 to NH3 conversion and other N-containing organics. In addition to the bacteria-AuNCs hybrid systems, the group has also reported on the synthesis of a 2-dimensional MOF structure that closely wraps anaerobic bacteria. The MOF nanosheets serve to reduce concentration of reactive oxygen species and thus allow facultative anaerobes bacteria the ability to function in oxygen-rich environments.

Perovskite nanomaterials

Lead-halide perovskites are an emerging class of soft ionic crystals with excellent optoelectronic properties for next-generation photovoltaic and light-emitting applications. The Yang group has developed advanced synthetic methodology of CsPbX3 nanostructure with desired size, composition, and properties, using colloidal, solution-phase, vapor-phase, and solid-state approaches. Brightly emitting colloidal nanowires (NWs) with well-controlled morphology and anisotropic photoluminescence can be 3D printed in a programmed print path, enabling the

Researchers in the Yang group use Raman spectroscopy to elucidate detailed structural information about new environmentally-benign perovskite analogue materials.

Pictured, left to right: Dr. Michael Ross (UCB), Ms. Maria Folgueras (UCB), and Ms. Ye Zhang (UCB)

creation of nanocomposites for applications including information storage and encryption, mechano-optical sensing, and optical displays. CsSnI3 NWs synthesized from solution-phase were discovered to undergo a p-type/n-type phase transition upon localized heating, enabling the formation of current-rectifying p-n heterojunctions on a single nanowire without compositional change. The robust halide perovskites synthesis methods also allow for investigation of fundamental properties of perovskite nanostructures. Vapor-phase anion exchange reaction was achieved and resolved in individual 2D perovskite nanoplates to elucidate the fundamental atomic kinetics and dynamics of perovskite structures. Since surface condition plays an important role in the optical performance of semiconductor materials, an effective strategy to passivate surface defect sites can improve materials performance. The researchers in the Yang group successfully enhanced the light-emission efficiencies in CsPbX3 NWs by nearly three orders of magnitude through oxygen passivation. In addition to synthesizing CsPbX3 perovskites, the Yang group has also devoted efforts in developing new environmentally benign luminescent materials for future solid-state lighting, sensor, and display applications. Most recently, a new Cu(I)-based all-inorganic rare-earth halide material has been synthesized by a solid-state reaction method. This discovery establishes a new strategy for constructing promising emissive halide perovskite analogues.

Setting up an electrochemical reactor for the conversion of CO2 and water into value-added chemicals using electricity. Inside the reactor is a unique nanomaterial-based catalyst designed by the Yang group with enhanced efficiency for making multicarbon products. Pictured, Mr. Yifan Li (UCB).

Zettl Group

The Zettl Group is an experimental condensed matter physics research group at the University of California, Berkeley, and the associated Lawrence Berkeley National Laboratory. The main focus is on the synthesis, characterization, and possible application of novel low dimensional materials that have relevance to energy harvesting, energy flow, and energy transduction.

Reconfigurable all-liquid electronics

Conventional electronics consist of metallic wires connecting solid state devices on a semiconducting chip. Imagine purely liquid 3-D electronics, where the charge flow is along electrically conducting liquid channels immersed in another (say insulating) liquid. The walls of the channel are defined by jammed nanoparticles. The Zettl group has developed a method by which liquid electronics can be 3-D printed. Such "all liquid" electronics are surprisingly robust and easily reconfigurable, not to mention easily recycled. Fig. 1 shows examples of a liquid coil (inductor), and the word "Cal", printed in 3-D.

Magic wand localized doping of graphene

Graphene, a one-atom-thick sheet of strongly bonded carbon atoms, could form the basis of high efficiency electronic and mechanical devices. Using an externally applied electric field or impurity atoms, it is Fig. 1 (a) All-liquid 3D printing of Ti3C2Tx possible to "dope" graphene to have an excess of MXene stabilized interfacial assemblies in negative electrons (n-type) or an excess of positive silicone oil. holes (p-type). The Zettl group has discovered a

method by which doping can be achieved using a fine electron beam. With this "magic wand", arbitrary n and p devices can be co-written into a single specimen of graphene. No reactive chemical species are required. Fig. 2 illustrates the process.

Fig. 2. Using an electron beam to locally dope graphene. The letter "B" is an n-type region in a p-type background.

Pushing linear-chain materials to the single-chain limit

There is much current interest in the two-dimensional limit of layered, quasi 2-D materials (e.g. graphite, MoS2). Interesting new physics and chemistries can emerge when the few- to single-layer limits are realized. There exists a complementary class of quasi1-D chain-like materials, of which transition-metal- trichalcogenides (TMTs) are the best known examples. These materials have been extensively studied in bulk, i.e. in samples containing many, many aligned and bonded chains. What happens if individual chains of such materials are isolated? Will they still have the properties of Fig. 3. Transmission electron microscopy images for single chain, double the bulk, parent material? The chain, and triple chain NbSe3 isolated within nanotubes. For the single chain Zettl group has explored this case, an atomic model illustrates the torsional wave. question. Individual chains of the TMT NbSe3 have been isolated within carbon or boron nitride nanotubes and studied theoretically and structurally via high resolution transmission electron microscopy. Interestingly, single chains do not have the same atomic or electronic structure as for bulk samples. For example, a new dynamic charge- induced torsional wave is observed. Fig. 3 shows such a torsional wave. 70

Students at work in the Zettl lab.

Zhang Group

To explore next-generation energy efficient nano-mechanical devices, the Zhang group developed a way to trap objects at nanometer scale without any external energy input. This novel trapping platform will enable a variety of applications including contact-free nanomachines, ultrasensitive force sensors, and reliable nanoscale manipulation.

In this work, two gold plates are used to demonstrate the trapping platform, where the larger substrate gold plate is coated with a low-refractive-index material, Teflon, and the smaller gold plate serves as the object being trapped. Depending on the thickness of the Teflon, the smaller gold plate can be trapped at tens of nanometer distance away from the substrate for an impressively long time. The trapping is achieved by harnessing a phenomenon of theoretical quantum physics known as the Casimir effect. Until recently, researchers thought of the Casimir effect as an attractive force. But for the first time, the Zhang group proposed theoretically, and demonstrated experimentally, a repulsive Casimir force that eventually leads to a stable Casimir trap, without the input of extra energy.

Mechanical systems, such as gears and wheels, are core components of machines that we use daily. But these machines often require expensive maintenance or replacement because friction caused by physical contact — gears grinding, or wheels rubbing against another material — can wear these systems down, causing them to malfunction or work inefficiently. The Zhang group’s discovery paves the way for exploring new forms of non-friction mechanics which could have a variety of applications, such as contact-free nanomachines and ultrasensitive force sensors.

In another project, the Zhang group demonstrated that a chain of disordered resonators could organize themselves to form a device with designed functionalities via proper photothermal energy management. In conventional practice, actively tuning a device with multiple elements requires a sophisticated control feedback loop that is not energy efficient. However, the Zhang group demonstrated that those different elements can “communicate” with each other and then collectively form desired functionality. This novel self-organization system is completely passive; therefore, it does not require any additional control electronics and it is extraordinarily energy efficient.

The device before self-organization is a random chain of silicon resonators where each can transmit and reflect light. The device is then lit by a laser, inducing a thermos-optical feedback among all the resonators. As a result, the local temperature of each resonator increases differently and collectively changes the system’s overall property. In the paper, the Zhang group shows that they can cause this system to form a depletion band in the transmission spectrum at a desired wavelength, functioning as a bandpass filter.

Nano-fabrication defects and wasted heat are both ubiquitous in benchwork at this scale. With this demonstration, the Zhang group’s work opens a route for exploring functional nanophotonics by using wasted heat energy to correct fabrication defects. This means future integrated photonics chips could be fabricated more energy efficiently, improving device yield and enabling improved resiliency to environmental perturbations.

Figure 1: (A) By coating a thin layer of Teflon on a gold substrate, a stable Casimir equilibrium is formed so that a gold nanoplate can be trapped at an equilibrium position in ethanol. (B) Casimir interaction energy between the gold nanoplate and the Teflon-coated gold surface. The Casimir force given by the derivative of the Casimir energy with respect to the distance is repulsive at short distances and attractive at long distances. (C) Electron microscope image of five thermally insulated resonators (each dissipating heat, orange arrows), driven by a 1,064 nm laser (red arrow) and probed with a counter-propagating supercontinuum laser (yellow arrow). (D) Transmission spectrum before (black curve) and after (red curves) self-organization showing the emergence of a depletion band (shaded in red) fixed by the 1,064 nm drive (dashed line).

* “Stable Casimir equilibria and quantum trapping” Science Jun 2019 “Dissipative self-organization in optical space” Nature Photonics Oct 2018

Publications 2018-2019

Alivisatos Group

1. Ondry JC, Philbin JP, Lostica M, Rabani E, 10. Eliminating Surface Halide Vacancies with Softer Alivisatos AP. Resilient Pathways to Atomic Lewis Bases. J Am Chem Soc. 2018 Dec 2. DOI: Attachment of Quantum Dot Dimers and Artificial 10.1021/jacs.8b11035. Solids from Faceted CdSe Quantum Dot Building 11. Needell DR, Ilic O, Bukowsky CR, Nett Z, Xu L, He J, Blocks. ACS Nano. 2019 Jun 27. DOI: Bauser H, Lee BG, Geisz JF, Nuzzo RN, Alivisatos 10.1021/acsnano.9b03052. AP and Atwater HA. Design Criteria for Micro-Optical 2. Gao J, Kidon L, Rabani E, Alivisatos AP. Tandem Luminescent Solar Concentrators IEEE Ultrahigh Hot Carrier Transient Photocurrent in Journal of Photovoltaics 2018. DOI: Nanocrystal Arrays by Auger Recombination. 10.1109/JPHOTOV.2018.2861751 Nano Lett. 2019 Jun 22. DOI: 12. Kim TG, Zherebetskyy D, Bekenstein Y, Oh MH, 10.1021/acs.nanolett.9b02374. Wang LW, Jang E, Alivisatos AP. Trap Passivation 3. Fischer S, Mehlenbacher RD, Lay A, Siefe C, in Indium-Based Quantum Dots through Surface Alivisatos AP, Dionne JA. Small Alkaline-Earth- Fluorination: Mechanism and Applications. ACS based Core/Shell Nanoparticles for Efficient Nano. 2018 Oct 18. DOI: 10.1021/acsnano.8b06692. Upconversion. Nano Lett. 2019 Jun 13. Hauwiller MR, Zhang X, Liang WI, Chiu CH, Zhang 12;19(6):3878-3885. DOI: Q, Zheng W, Ophus C, Chan EM, Czarnik C, Pan M, 10.1021/acs.nanolett.9b01057. Ross FM, Wu WW, Chu YH, Asta M, Voorhees PW, 4. Zhou N, Bekenstein Y, Eisler CN, Zhang D, Alivisatos AP, Zheng H. Dynamics of Nanoscale Schwartzberg AM, Yang P, Alivisatos AP, Lewis Dendrite Formation in Solution Growth Revealed JA. Perovskite nanowire-block copolymer Through in Situ Liquid Cell Electron Microscopy. composites with digitally programmable Nano Lett. 2018 Oct 10;18(10):6427-6433. DOI: polarization anisotropy. Sci Adv. 2019 May 10.1021/acs.nanolett.8b02819. 31;5(5):eaav8141. DOI: 10.1126/sciadv.aav8141 14. Osowiecki W and Alivisatos AP. Introductory 5. Prigozhin MB, Maurer PC, Courtis AM, Liu N, Wisser Perspective. Electrochemical Engineering: MD, Siefe C, Tian B, Chan E, Song G, Fischer S, Electrochemical Engineering: From Discovery to Aloni S, Ogletree DF, Barnard ES, Joubert LM, Rao Product Nov. 2018. ISBN 978-3-527-34206-8 J, Alivisatos AP, Macfarlane RM, Cohen BE, Cui Y, 15. Hauwiller MR, Frechette LB, Jones MR, Ondry JC, Dionne JA, Chu S. Bright sub-20-nm Rotskoff GM, Geissler P, Alivisatos AP. Unraveling cathodoluminescent nanoprobes for electron Kinetically-Driven Mechanisms of Gold Nanocrystal microscopy. Nat Nanotechnol. 2019 Mar 4. DOI: Shape Transformations Using Graphene Liquid Cell 10.1038/s41565-019-0395-0. Electron Microscopy. Nano Lett. 2018 Aug 20. DOI: 6. Jurow MJ, Morgenstern T, Eisler C, Kang J, Penzo 10.1021/acs.nanolett.8b02337. E, Do M, Engelmayer M, Osowiecki WT, Bekenstein 16. Gu XW, Hanson LA, Eisler CN, Koc MA, Alivisatos Y, Tassone C, Wang LW, Alivisatos AP, Brütting W, AP. Pseudoelasticity at Large Strains in Au Liu Y. Manipulating the Transition Dipole Moment of Nanocrystals. Phys Rev Lett. 2018 Aug CsPbBr(3) Perovskite Nanocrystals for Superior 3;121(5):056102. DOI: Optical Properties. Nano Lett. 2019 Mar 15. DOI: 10.1103/PhysRevLett.121.056102. 10.1021/acs.nanolett.9b00122 17. Porter IJ, Cushing SK, Carneiro LM, Lee A, Ondry 7. Hanifi DA, Bronstein ND, Koscher BA, Nett Z, JC, Dahl JC, Chang HT, Alivisatos AP, Leone SR. Swabeck JK, Takano K, Schwartzberg AM, Maserati Photoexcited Small Polaron Formation in Goethite L, Vandewal K, van de Burgt Y, Salleo A, Alivisatos (α-FeOOH) Nanorods Probed by Transient Extreme AP. Redefining near-unity luminescence in quantum Ultraviolet Spectroscopy. J Phys Chem Lett. 2018 Jul dots with photothermal threshold quantum yield. 19;9(14):4120-4124. DOI: .1021/acs.jpclett.8b01525. Science. 2019 Mar 15;363(6432):1199-1202. DOI: 18. Lay A, Siefe C, Fischer S, Mehlenbacher RD, Ke F, 10.1126/science.aat3803. Mao WL, Alivisatos AP, Goodman MB, Dionne JA. 8. Hauwiller MR, Ondry JC, Chan CM, Khandekar P, Yu Bright, Mechanosensitive Upconversion with Cubic- J, Alivisatos AP. Gold Nanocrystal Etching as a Phase Heteroepitaxial Core-Shell Nanoparticles. Means of Probing the Dynamic Chemical Nano Lett. 2018 Jul 11;18(7):4454-4459. DOI: Environment in Graphene Liquid Cell Electron 10.1021/acs.nanolett.8b01535. Microscopy. J Am Chem Soc. 2019 Mar 19. Swabeck JK, Fischer S, Bronstein ND, Alivisatos 13;141(10):4428-4437. DOI: 10.1021/jacs.9b00082. AP. Broadband Sensitization of Lanthanide Emission 9. Nenon DP, Pressler K, Kang J, Koscher BA, with Indium Phosphide Quantum Dots for Visible to Olshansky JH, Osowiecki WT, Koc MA, Wang LW, Near-Infrared Downshifting. J Am Chem Soc. 2018 Alivisatos AP. Design Principles for Trap-Free Jul 25;140(29):9120-9126. DOI: CsPbX(3) Nanocrystals: Enumerating and 10.1021/jacs.8b02612.

20. Osowiecki WT, Ye X, Satish P, Bustillo KC, Clark EL, Crommie, F. R. Fischer, Topological Band Alivisatos AP. Tailoring Morphology of Cu-Ag Engineering of Graphene Nanoribbons, Nature Nanocrescents and Core-Shell Nanocrystals Guided 560 204-208, 2018. DOI: 10.1038_s41586-018- by a Thermodynamic Model. J Am Chem Soc. 2018 0376-8.ris Jul 11;140(27):8569-8577. DOI: 3. M. Ugeda, A. Pulkin, S. Tang, H. Ryu, Q. Wu, Y. 10.1021/jacs.8b04558. Zhang, D. Wong, Z. Pedramrazi, A. Martín-Recio, 21. Gibson NA, Koscher BA, Alivisatos AP, and Leone Y. Chen, F. Wang, Z-X. Shen, S-K. Mo, O. V. SR. Excitation Intensity Dependence of Yazyev, M. F. Crommie,Observation of Photoluminescence Blinking in CsPbBr3 Perovskite topologically protected states at crystalline phase Nanocrystals J. Phys. Chem. C 2018. DOI: boundaries in single-layer WSe2,Nature 10.1021/acs.jpcc.8b03206 Communications 9 3401, 2018. DOI: 10.1038/s41467-018-05672-w Bustamante Group 4. D. Wong, Y. Wang, W. Jin, H-Z. Tsai, A. Bostwick, E. Rotenberg, R. K. Kawakami, A. Zettl, A. A. 1. Gabizon R, Lee A, Vahedian-Movahed H, Ebright Mostofi, J. Lischner, M. F. Crommie, Microscopy R, and Bustamante C. Pause sequences facilitate of hydrogen and hydrogen-vacancy defect entry into long-lived paused states by reducing structures on graphene devices, Physical Review, RNA polymerase transcription rates. Nature B 98 155436, 2018.DOI: Communications 9 2018. DOI: 10.1038/s41467- 10.1103/PhysRevB.98.155436 018-05344-9 5. J. Lu, H-Z. Tsai, A. Tatan, S. Wickenburg, A. 2. Bustamante C, and Tafoya S. Biochemistry and Omrani, D. Wong, A. Riss, E. Piatti, K. Watanabe, biophysics in singulo: when less is more”. T. Taniguchi, A. Zettl, V. Pereira, M. Crommie, Proceedings of the Workshop Cell Biology and Frustrated supercritical collapse in tunable charge Genetics 137. 2018. Print. arrays on graphene, Nature Communications, 10 3. Schöneberg J, Pavlin M, Yan S, Righini M, Lee I, 477, 2019. DOI:10.1002/adma.201805941 Carlson L, Bahrami A, Goldman D, Ren X, 6. Daniel J. Rizzo, Meng Wu, Hsin-Zon Tsai, Tomas Hummer G, Bustamante C, and Hurley J. ATP- Marangoni, Rebecca A. Durr, Arash A. Omrani, dependent force generation and membrane Franklin Liou, Christopher Bronner, Trinity Joshi, scission by ESCRT-III and Vps4. Science 362. Giang D. Nguyen, Griffin F. Rodgers, Won-Woo 2018. DOI: 10.1101/262170 Choi, Jakob H. Jørgensen, Felix R. Fischer, 4. Zhang H, Wang Y, Zhang H, Liu X, Lee A, Huang Steven G. Louie, and Michael F. Crommie, Q, Wang F, Chao J, Liu H, Li J, Shi J, Zuo X, Length-Dependent Evolution of Type II Wang L, Wang L, Cao Z, Bustamante C, Tian ZQ, Heterojunctions in BottomUp-Synthesized and Fan C. Programming chain-growth co- Graphene Nanoribbons, Nano Letters 19 3221 polymerization of DNA hairpin tiles for in-vitro (2019) DOI:10.1021/acs.nanolett.9b00758 hierarchical supramolecular organization. Nature Communications 10. 2019.DOI: Fischer Group 10.1103/PhysRevLett.108.112003 5. Tafoya S, Large S, Liu S, Bustamante C, and 1. D. J. Rizzo, G. Veber, T. Cao, C. Bronner, T. Sivak D.Using a system’s equilibrium behavior to Chen, F. Zhao, H. Rodriguez, S. G. Louie, M. F. reduce its energy dissipation in nonequilibrium Crommie, F. R. Fischer, Topological Band processes. PNAS. 2019 DOI: Engineering of Graphene Nanoribbons, Nature 10.1073/pnas.1817778116. 560 204-208, 2018. DOI: 10.1038_s41586-018- 6. Alexander L, Goldman D, Wee L, and 0376-8.ris Bustamante C. Non-equilibrium dynamics of a 2. Zhu, J., German, R., Senkovsky, B. V., Haberer, nascent polypeptide during translation suppress its D., Fischer, F. R., Grüneis, A., van Loosdrecht, P. misfolding. Nature Communications 10. 2019. H. M. Exciton and Phonon Dynamics in Highly DOI: 10.1038_s41467-019-10647-6.ris Aligned 7-Atom Wide Armchair Graphene Nanoribbons as Seen by Time-Resolved Crommie Group Spontaneous Raman Scattering. Nanoscale 2018, 10, 17975-17982. DOI: 10.1039/C8NR05950K 1. Jairo Velasco, Jr., Juwon Lee, Dillon Wong, 3. Pfeiffer, M. Senkovsky, B. V., Haberer, D., Salman Kahn, Hsin-Zon Tsai, Joseph Costello, Fischer, F. R., Yang, F.; Meerholtz, K., Ando, Y., Torben Umeda, Takashi Taniguchi, Kenji Grüneis, A., Lindfors, K., Observation of Room Watanabe, Alex Zettl, Feng Wang, and Michael F. Temperature Photoluminescence Blinking in Crommie, Visualization and Control of Single- Armchair-Edge Graphene Nanoribbons. Nano Electron Charging in Bilayer Graphene Quantum Lett. 2018, 18, 7038-7044. DOI: Dots, Nano Letters 18 5104-5110 (2018) DOI: 10.1021/acs.nanolett.8b03006 10.1021/acs.nanolett.8b01972 4. Alavi, S., Senkovski, B., Pfeiffer, M., Haberer, D., 2. D. J. Rizzo, G. Veber, T. Cao, C. Bronner, T. Fischer, F., Grüneis, A., Lindfors, K., Probing the Chen, F. Zhao, H. Rodriguez, S. G. Louie, M. F. Origin of Photoluminescence Brightening in 75

Graphene Nanoribbons. 2D Mater. 2019, 6, 4. M. Lai, A. Obliger, D. Lu, C.S. Kley, C.G. Bischak, 035009. DOI:10.1088/2053-1583/ab1116 Q. Kong, T. Lei, L. Dou, N.S. Ginsberg, D.T. 5. Wan, L., Cho, E.-S., Marangoni, T., Shea, P., Limmer, P. Yang, Intrinsic anion diffusivity in lead Kang, S.Y., Rogers, C., Zaia, E., Cloke, R., Wood, halide perovskites is facilitated by a soft lattice, B., Fischer, F., Urban, J., Prendergast, D., Edge- Proc. Natl. Acad. Sci., 115, 11929-11934 2018. Functionalized Graphene Nanoribbon DOI:10.1073/pnas.1812718115 Encapsulation to Enhance Stability and Control 5. H. Stern, R. Wang, Y. Fan, R. Mizuta, J.C. Kinetics of Hydrogen Storage Materials. Chem. Stewart, L.-M. Needham, T.D. Roberts, R. Wai, Mater. 2019, 31, 2960-2970. DOI: N.S. Ginsberg, D. Klenerman, S. Hofmann, S.F. 10.1021/acs.chemmater.9b00494 Lee, Spectrally Resolved Photodynamics of 6. Zhao, L., Kaiser, R. I., Xu, B.; Ablikim, U., Lu, W., Individual Emitters in Large-Area Monolayers of Ahmed, M. Evseev, M. M., Bashkirov, E. K.; Hexagonal Boron Nitride, ACS Nano, 13, 4538- Azyazov, V. N.; Zagidullin, M. V.; Morozov, A. N.; 4547 2019. DOI: 10.1021/acsnano.9b00274 Howlander, A. H.; Wnuk, S. F.; Mebel, A. M., Joshi, D., Veber, G., Fischer, F. R., Gas Phase Liu Group Synthesis of [4]-helicene. Nat. Commun. 2019, 10, 1510. DOI: 10.1038/s41467-019-09224-8 1. C. Qian and T.-J. K. Liu, “Nanoelectromechanical 7. Vadlamani, S. K., Agawal, S., Limmer, D. T.; Switches,” Chapter in Device Physics and Louie, S. G., Fischer , F. R., Yablonovitch, E., Technologies for New Computing Architectures Tunnel-FET Switching is Governed by Non- and Systems, S. Deleonibus, Editor (Pan Stanford Lorentzian Spectral Line Shape. Proc. IEEE 2019, Publishing), 2018. Print. early access. DOI: 10.1109/JPROC.2019.2904011 8. Rizzo, D., Wu, M., Tsai, H.-Z., Marangoni, T., Limmer Group Durr, R., Omrani, A., Liou, F., Bronner, C., Joshi, T., Nguyen, Giang, Rodgers, G., Choi, W., 1. C. G. Bischak, A. B. Wong, E. Lin, D. T. Limmer, Jørgensen, J., Fischer, F. R., Louie, S. G., P. Yang, and N. S. Ginsberg Crommie, M., Length-dependent Evolution of Type Tunable polaron distortions control the extent of II Heterojunctions in Bottom-Up-Synthesized halide demixing in lead halide perovskites,J. Phys. Graphene Nanoribbons. Nano Lett. 2019, 19, Chem. Lett, 2018. DOI: 3221-2228. DOI: 10.1021/acs.nanolett.9b00758 10.1021/acs.jpclett.8b01512 9. Von Kugelgen, S., Piskun, I., Griffin, J. H., 2. T. Grand-Pre and D. T. Limmer PRE, Current fluctuations of active Brownian Eckdahl, C. T., Jarenwattananon, N. N., Fischer, Particles Rapid Communications 2018. F. R. J., Templated Synthesis of End- DOI:10.1073/pnas.1812718115 Functionalized Graphene Nanoribbons Through 1. S. K. Vadlamani, S. Agarwal, D. T. Limmer, S. G. Living Ring-Opening Alkyne Metathesis Louie, F. R. Fischer, and E. Yablonovitch, Tunnel- Polymerization. Am. Chem. Soc. 2019, just FET Switching is Governed by Non-Lorentzian accepted. Spectral Line-Shape, iEEE, 2018. DOI:10.1109/JPROC.2019.2904011 Fleming Group 2. A.J. Schile, and D.T. Limmer. Studying rare nonadiabatic dynamics with transition path 1. J. M. Morris and G. R. Fleming, Quantitative sampling quantum jump trajectories, J. Chem. modeling of energy dissipation in Arabidopsis Phys., 2018. DOI: 10.1063/1.5058281 thaliana Env. and Exp. Botany, 2018. DOI: 3. N. Yunger Halpern and D.T. Limmer, 10.1016/j.envexpbot.2018.03.021 Fundamental limitations on photoisomerization from thermodynamic resource theories, arXiv, Ginsberg Group 2018. 4. C.Y. Gao, and D.T. Limmer. Nonlinear transport 1. C.G. Bischak, A.B. Wong, E. Lin, D.T. Limmer, P. coefficients from large deviation functions, J. Yang, N. S. Ginsberg, Tunable polaron distortions Chem. Phys., 2018. DOI: 10.1063/1.5110507 control the extent of halide demixing in lead halide 5. A. Levy, L Kidon, D.T. Limmer and E. Rabani J. perovskites, J. Phys. Chem. Lett., 9, 3998-4005 Absence of Coulomb Blockade in the Anderson 2018. DOI: 10.1021/acs.jpclett.8b01512 Impurity Model at the Symmetric Point, Phys. 2. M. D. Delor, H. L. Weaver, Q. Yu, N. S. Ginsberg, Chem. C, 2019. DOI: 10.1021/acs.jpcc.9b04132 Universal imaging of material functionality through 6. A.J. Schile, and D.T. Limmer Communication: nanoscale tracking of energy flow,submitted 2018. Rate constants in spatially Inhomogenious 3. Q. Kong, W. Lee, M. Lai, C.G. Bischak, G. Gao, systems, J. Chem. Phys. 2019. A.B. Wong, T. Lei, Y. Yu, L.-W. Wang, N.S. DOI:10.1063/1.5092837 Ginsberg, P. Yang, Phase-transition–induced p-n 7. A. Levy, W Dou, E. Rabani, and D.T. Limmer A junction in single halide perovskite nanowire, complete quasiclassical map for the dynamics of Proc. Natl. Acad. Sci., 115, 8889-8894 2018. interacting fermions, J. Chem. Phys., 2019. DOI:10.1073/pnas.1806515115 76

8. A.J. Schile, and D.T. Limmer, Simulating conical Karunadasa, Small-Band-Gap Halide Double intersection dynamics in the condensed phase Perovskites,Angew. Chem. Int. Ed. 57, 12765 with hybrid quantum master equations J. Chem. 2018. DOI: 10.1002/anie.201807421 Phys., 2019. 9. J. K. Cooper, S. E. Reyes-Lillo, L. H. Hess, C.-M. 9. U. Ray and D.T. Limmer, Heat current fluctuations Jiang, J. B. Neaton, and I. D. Sharp, Physical and anomalous transport in low dimensional Origins of the Transient Absorption Spectra and carbon lattices arXiv. 2019. Dynamics in Thin-Film Semiconductors: The Case of BiVO4, J. Phys. Chem. C 122, 20642 2018. Neaton Group DOI: 10.1021/acs.jpcc.8b06645 10. J. Kundu, J. F. Stilck, J.-H. Lee, J. B. Neaton, D. 1. A. C. Forse, P. J. Milner, J.-H. Lee, H. N. Prendergast, and S. Whitelam, Cooperative Gas Redfearn, J. Oktawiec, R. L. Siegelman, J. D. Adsorption without a Phase Transition in Metal- Martell, B. Dinakar, L. B. Porter-Zasada, M. I. Organic Frameworks, Phys. Rev. Lett, 121, Gonzalez, , J. R. Long, and J. A. J. B. Neaton 015701 2018. DOI: Reimer, Elucidating CO2 Chemisorption in 10.1103/PhysRevLett.121.015701 Diamine-Appended Metal-Organic Frameworks,J. 11. D. Wing, J. B. Haber, R. Noff, B. Barker, D. A. Am. Chem. Soc. 140, 18016. 2018. DOI: Egger, A. Ramasubramaniam, S. G. Louie, J. B. 10.1021/jacs.8b10203 Neaton, and L. Kronik, Comparing time- 2. J. Witte, , and M. Head-Gordon, Push J. B. Neaton dependent density functional theory with many- it to the limit: comparing periodic and local body perturbation theory for semiconductors: approaches to density functional theory for Screened range-separated hybrids and the GW intermolecular interactions, Mol. Phys. 1. 2018. plus Bethe-Salpeter approach, Phys. Rev. Mater., DOI: 10.1080/00268976.2018.1542164 3, 064603 2019. DOI:

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