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The Pennsylvania State University

The Graduate School

Department of Chemistry

THE SYNTHESIS AND CHARACTERIZATION

OF FUNCTIONALIZED NANOTHREADS

A Thesis in

Chemistry

Daniel Koeplinger

Submitted in Partial Fulfillment of the Requirements for the Degree of

Master of Science

May 2019

The thesis of Daniel Koeplinger was reviewed and approved* by the following:

John V. Badding Professor of Chemistry, Physics, and Materials Science and Engineering Thesis Advisor

John B. Asbury Associate Professor of Chemistry

Ben J. Lear Associate Professor of Chemistry

Philip C. Bevilacqua Distinguished Professor of Chemistry, Biochemistry and Molecular Biology Head of the Department of Chemistry

*Signatures are on file in the Graduate School.

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ABSTRACT

Carbon nanothreads are a relatively new type of carbon material which has an sp3 structure similar to diamond, but extends in only one dimension. The predicted properties of nanothreads are extraordinary, rivaling other recent carbon materials such as carbon nanotubes and graphene in both strength and stiffness. Nanothreads are synthesized experimentally by slow compression of benzene to about 20 GPa, followed by slow decompression, resulting in a crystalline product in which the polymer is arranged into a hexagonally close-packed structure.

The material can be identified via its characteristic six-fold diffraction pattern, as well as its

Raman and IR spectra.

In addition to nanothreads produced from benzene, several new types of nanothreads have also been synthesized from other precursors, such as pyridine and aniline. In Chapter 2.1 of this work, experiments involving the compression-induced polymerization of three additional aromatic molecules are described. Toluene produced the most promising results, yielding a transparent amorphous solid which has similar IR and Raman spectra to those of previous nanothreads. Nanothread syntheses were also attempted using benzoic acid and hexabromobenzene as precursors, but no significant reaction was seen for either material.

Nanothread structure is complex, and there are many structures of similar energy which are thermodynamically possible. In addition to the fully-saturated sp3 products from a Degree-6 polymerization, products which result from incomplete polymerization, including unsaturated

Degree-4 and Degree-2 segments, are observed. Each of these structures has varying amounts of sp2 character. A series of experiments are described in Chapter 2.2 in which the ratio of sp2 to sp3 bonds in the nanothread product was measured via advanced solid-state NMR. These ratios were used to approximate the relative quantities of Degree-2, -4, and -6 structures for both benzene iv and pyridine nanothreads. NMR data was also used to constrain which isomers of each Degree might be found in experimentally-synthesized nanothreads.

When nanothreads are formed under pressure, they produce crystals of close-packed fibers. These fibers are bound together primarily by Van-der-Waals forces, and application of mechanical force could theoretically exfoliate individual threads from the bulk. In Chapter 3, exfoliation experiments are described in which three separate methods attempted to separate individual threads from the bulk: applying shearing forces by twisting samples between two planes, peeling fibers away from the bulk using tape adhesive, and crushing the nanothread crystals with the tip of a diamond. Samples from each of these methods were examined by atomic force microscopy. Identification of individual fibers separated from the bulk nanothread crystal proved problematic because of the small radii of nanothreads, particularly when compared to the resolution of techniques used to characterize nanothreads in the bulk. As a result of these limitations, the only way to prove that a fiber was a nanothread would be to find a thread still partially attached to the bulk. No partially-attached fibers were observed by AFM using the three preparation methods listed above, but the possibility of identifying such a feature in the future was not excluded.

TABLE OF CONTENTS

List of Figures ………………………………………………………………....………………... vi

List of Tables …………………………………………………………………....…....……..….. ix

List of Abbreviations ……………………………………………………………………...…...... x

Acknowledgments …………………………………………………………...... ……….…….... xii

CHAPTER 1 Introduction ……………………………………………………………………...... 1

1.1 Computational Predictions of Structure and Properties

1.2 Synthesis of Benzene Nanothreads

1.3 Synthesis of Other Nanothreads

1.4 Potential Applications for Nanothreads

CHAPTER 2 Use of Benzene Derivatives and Isotopes in Synthesis ……..………………...… 17

2.1 Synthesis of Nanothreads from Benzene Derivatives

2.2 Solid-State NMR Studies on 13C Benzene and 15N Pyridine Nanothreads

CHAPTER 3 Mechanical Exfoliation of Benzene Nanothreads ………………………...... 41

CHAPTER 4 Conclusions and Future Outlook ………………………………………………... 49

References ………………………………………………………………………...….………… 50

Appendix: Additional Copyright Information …………………………………………………. 54

LIST OF FIGURES

Figure 1: (a) Computationally predicted stress-strain curves for three nanothread isomers, which predict the relative strength and stiffness of each. (b-d) Diagrams of each of isomer with each bond colored according to its stress near the ultimate tensile strength of its structure. Adapted with permission from [5] Zhan, et al. 2016…………………………………………….…….….. 5

Figure 2: Example structures of each possible nanothread Degree, including Degree-0, which is simply an organized stack of benzene rings which are not bonded to one another. Adapted with permission from [8] Chen, et al. 2017……………………………………………...………....…. 7

Figure 3: A TEM micrograph of one of the original nanothread products showing ordered striations about 6.4 Å apart. Reprinted with permission from [9] Fitzgibbons, et al. 2014.…..… 9

Figure 4: (a) Models of polytwistane arranged into hexagonal packing structures, demonstrating the 6.5 Å distance between threads and the 5.6 Å interplanar spacing. (b) A six-fold x-ray diffraction pattern resulting from diffraction in the c-axis of the crystal. (c) A two fold x-ray diffraction pattern resulting from diffraction in the b-axis. Reprinted with permission from [13] Li, et al. 2017………………………………………………………………………………….... 10

Figure 5: Sketch of a diamond anvil cell, as used in a typical nanothread synthesis. A sample is enclosed inside of a gasket between two diamonds. These diamonds are then pressed together, concentrating force into the tips (culets) of the diamonds. This generates extreme pressures within the sample, which is used as the driving force behind nanothread polymerization…….. 11

Figure 6: Raman spectra for a sample of toluene excited at a wavelength of 532 nm under increasing amounts of pressure within a DAC. The starting pressure was 4.56 GPa, moving upward to 22.97 GPa, where the photoluminescent background overtook the samples. All subsequent measurements during the remainder of the compression and decompression showed only an elevated background with no significant peaks…………………………………...…… 19

Figure 7: The mid-IR absorption spectrum of (Blue) the toluene compression product and of (Red) a sample of benzene nanothreads………………………………………………………… 21

Figure 8: Image of the toluene compression product (darker center circle) in its stainless steel DAC gasket (bright outer ring). Image captured on an Olympus BX62 Microscope in reflection mode…………………………………………………………………………………………….. 23

Figure 9: Raman spectra for a sample of HBB excited at a wavelength of 532 nm under increasing amounts of pressure within a DAC. The starting pressure was 3.73 GPa, moving upward to 25.11 GPa. The sample was then decompressed and the recovered sample was measured at ambient pressure…………………………………………………………………... 29

Figure 10: The Raman spectra from 0-1800 cm-1 of (Blue) the HBB Compression Sample and (Red) a sample of unmodified HBB, each excited at a wavelength of 514 nm. The wavelengths of peaks common to both of the samples are labeled. (Green) The Raman spectrum of a sample of benzene nanothreads excited at 633 nm is included for comparison………………………... 30 vii

Figure 11: Reaction scheme for the proposed monomer chain transfer reaction in HBB para polymer, a Degree-2 nanothread. In Step (a), a bromine radical is eliminated in the position para one of the end radicals, leaving a fully aromatic chain-end residue with five remaining bromides which will no longer grow unless it comes in contact with the growing end of another chain. In Step (b) the bromine radical reacts with an HBB monomer to produce either (1) a Br2 molecule plus a new growing chain with an identical chain-end residue from the one produced in Step (a), or (2) a new growing chain with a chain end residue containing seven bromides. A similar monomer chain transfer reaction would be expected for a diradical-mediated 4+2 reaction…... 35

Figure 12: The 13C NMR spectra of 13C-enriched benzene nanothreads. (a) The proton-decoupled multiCP spectrum of all carbons in the sample, which allows calculation of the relative prevalence of sp2- and sp3-hybridized carbon in the sample. (b) The dipolar DEPT spectrum, giving signal only from CH features. This is compared to the dipolar DEPT spectrum of amorphous polystyrene (PS). (c) The three-spin coherence spectrum, giving signal only from CH2 features. (d) The dipolar dephased multiCP spectrum, giving signal only from non- protonated or mobile carbon. Reprinted with permission from [42] Duan, et al. 2018.………... 37

Figure 13: The percentage of carbon atoms involved in various structural features in the 13C benzene nanothread sample, as calculated through a combination of pulse-sequence and 2D NMR techniques. Reprinted with permission from [42] Duan, et al. 2018…….………………. 38

Figure 14: One possible structure for a benzene nanothread, consistent with the length of the Degree-6 segments and relative proportions of each structure calculated through various NMR techniques. The sp3-CH units are represented in blue, sp2-CH in yellow, and non-protonated sp2- C in magenta. Reprinted with permission from [42] Duan, et al. 2018…….…………………... 39

Figure 15: (a) The solid-state 15N NMR spectrum from 15N-enriched pyridine nanothreads. A selective spectrum for NH is shown as a thin purple line beneath the spectrum. (b) The solid- state 13C for the same sample. A selective spectrum for non-protonated/mobile C is shown as a thin green line beneath the spectrum. In both (a) and (b), the signals of unreacted pyridine are indicated by the dashed blue lines, demonstrating large changes in the product compared to the reactant. Reprinted with permission from [21] Li, et al. 2018………………….………….…… 40

Figure 16: Depictions of each exfoliation method attempted with benzene nanothreads. Top: A nanothread crystal is placed onto a germanium wafer. Another surface is placed against the first and twisted, applying shear forces to pull the crystal apart. Middle: A nanothread crystal is stuck to a piece of scotch tape. The tape is folded onto itself, then peeled back. Adhesive from each area interacts with the crystal, pulling threads away from the bulk crystal. Bottom: The tip of a diamond is used to crush the nanothread crystal, pushing threads away from the bulk……...… 42

Figure 17: AFM height maps of two germanium surfaces. Left: The surface topography of an unused germanium wafer showing surface height variation of less than 1 nm. Right: The topography of the same wafer following the shearing force exfoliation procedure showing severe surface damage………………………………………………………………………………….. 44

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Figure 18: AFM height maps including an additional germanium surface. Left: The surface topography of the same unused germanium wafer. Right: The topography of a germanium wafer following the scotch tape exfoliation method and an acetone rinse. While surface damage is significantly reduced using this method, significant amounts of adhesive appear to be left behind by the rinse step………………………………………………………………………………… 46

LIST OF TABLES

Table 1: The structures of the most stable achiral nanothreads. The structures include (a) (3,0) tube and (b) Polymer I. Computationally-predicted energies of formation, Young’s Moduli, and effective radius are given for each structure. Potential energy was calculated relative to graphane and is given in eV per six-membered ring taking part in the structure. Additional low-energy structures can be found in Table 2. Adapted with permission from [4] Xu, et al. 2015……..…... 3

Table 2: The structures of the most stable chiral nanothreads, sorted by stiffness. The structures include (c) polytwistane. Computationally-predicted energies of formation, Young’s moduli, and effective radius are given for each structure. Potential energy was calculated relative to graphane and is given in eV per six-membered ring taking part in the structure. Young’s Moduli were calculated with both free polymer ends and fixed ends. Adapted with permission from [4] Xu, et al. 2015…………………………………………………………………………………………… 4

Table 3: The tensile strengths and elasticities of several materials compared to the predicted properties of benzene nanothreads……………………………………………………………… 16

LIST OF ABBREVIATIONS

AFM (Atomic Force Microscopy)

APS (Advanced Photon Source, Argonne National Lab)

CHESS (Cornell High Energy Synchrotron Source, Cornell University)

DAC (Diamond Anvil Cell)

DEPT (Distortionless Enhancement by Polarization Transfer)

DOE (United States Department of Energy)

EDG (Electron Donating Group)

EDS (Energy Dispersive x-ray Spectroscopy)

EFree (Energy Frontier Research in Extreme Environments Center, DOE Office of Science)

EWG (Electron Withdrawing Group)

HBB (Hexabromobenzene)

HPCAT (High Pressure Collaborative Access Team)

MAS (Magic Angle Spinning)

MRI (Materials Research Institute, Penn State University)

MultiCP (Cross Polarization by Multiple Contact Periods)

NMR (Nuclear Magnetic Resonance spectroscopy)

NSF (National Science Foundation)

PAN (Polyacrylonitrile)

PE Cell (Paris-Edinburgh Cell)

PS (Polystyrene)

PVC (Polyvinyl chloride)

SAXS (Small-Angle X-ray Scattering) xi

SNS (Spallation Neutron Source, Oak Ridge National Lab)

STM (Scanning Tunneling Microscopy)

TEM (Transmission Electron Microscopy)

TERS (Tip-Enhanced Raman Spectroscopy)

WAXS (Wide-Angle X-ray Scattering)

XRD (X-Ray Diffraction) xii

ACKNOWLEDGMENTS

I would first like to thank my advisor, Dr. John Badding, for providing me with the opportunities that I have had during the course of my graduate career at Penn State. I would also like to thank my committee members for their support. Additionally, I would like to thank our collaborators at each of the off-campus labs that I traveled to during my experiments. Without their guidance, these experiments would not have been possible. In particular, I would like to thank Jamie Molaison at SNS, Maria Baldini at APS, and Zhongwu Wang and Xin Huang at

CHESS. I would also like to thank our collaborators at Brandeis University, Pu Duan and Dr.

Klaus Schmidt-Rohr, for contributing their expertise with NMR, which is presented in Chapter

2.2 of this work. I would also like to thank the members of the Badding group. In particular, the other members of the nanothread project during my time, Xiang Li, Steve Juhl, and Sikai Wu.

Working with you to gather data, interpret results and discuss experimental plans made this work possible.

Most of all, I would like to thank my friends and family who supported me throughout my graduate experience. It has not been easy, but you all stuck with me and encouraged me throughout. I would especially like to thank my mother for her emotional support and my father for our many sessions of practical advice. I would also like to thank Caitlyn, who has turned out to be the world’s best lab partner, consistently making herself available at all hours of the day and night for any manner of celebration or commiseration. And finally Lynne, who has stood by my side through it all: I’ll have that piece of paper soon.

A portion of this research is based upon work supported by the Energy Frontier Research in Extreme Environments (EFree) Center, an Energy Frontier Research Center funded by the

U.S. Department of Energy (DOE) Office of Science under award number DE-SC0001057. This xiii material is also based upon work supported in part by the National Science Foundation (NSF)

Center for Nanothread Chemistry under Grant No. CHE-1832471. Any opinions, findings, and conclusions or recommendations expressed here are those of the author and do not necessarily reflect the views of the DOE or the NSF.

A portion of this research used resources at the Spallation Neutron Source (SNS), a DOE

Office of Science User Facility operated by the Oak Ridge National Laboratory. Another portion of this research used resources of the Advanced Photon Source (APS), a DOE Office of Science

User Facility operated by Argonne National Laboratory under Contract No. DE-AC02-

06CH11357. This work is also partially based upon research conducted at the Cornell High

Energy Synchrotron Source (CHESS) which is supported by the NSF under award DMR-

1332208.

The author declares no competing financial interests. Additional copyright information for figures and tables reproduced or adapted from other publications can be found in the appendix at the end of this work.

CHAPTER 1

Introduction

Carbon nanothreads are a relatively new type of carbon material which has an sp3 structure similar to diamond, but extending in only one dimension. The predicted properties of nanothreads are extraordinary, rivaling other recent carbon materials such as carbon nanotubes and graphene in both strength and stiffness. However, nanothreads include a broad range of similar-energy structures, and it has been found that many different structures can be formed within a single fiber. Experimental nanothread synthesis has been developed over the past few years. The procedure involves the slow compression of benzene to about 20 GPa, followed by slow decompression, resulting in the polymerization of benzene into a crystalline product. These crystals are made up of hexagonally close-packed fibers, resulting in a characteristic six-fold diffraction pattern. The material can also be identified by its Raman, IR, or NMR spectra under certain conditions. Nanothread synthesis has expanded to other aromatic precursors as well. Most notable among these is pyridine.

1.1 Computational Predictions of Structure and Properties

Nanothreads were predicted computationally long before they were synthesized. Several variants were proposed by different researchers before the concept of a nanothread was differentiated from that of a carbon nanotube. (3,0) sp3 tube was the first predicted nanothread.

Proposed in 2001 as a structural variant of carbon nanotubes, the purpose was originally to find the extreme minimum diameter of carbon nanotubes. Using sp3 bonding hybridization rather than sp2 to allows for smaller bond angles, and therefore fewer carbon atoms in the tube’s 2 circumference. The structure was named (3,0) sp3 tube, following carbon nanotube procedural naming conventions 1. A second type of nanothread was independently proposed in 2011 by

Wen, et al. Proposed to form through the polymerization of pressurized benzene, Polymer I was computationally predicted to be thermodynamically accessible from benzene because it has a lower potential energy per carbon atom 2. Finally, polytwistane was proposed in 2014 by Barua, et al. as an extended form of the diamondoid twistane. It was proposed that twistane could be extended in one dimension by connecting individual molecules to one another using hydrocarbon bridges. Ultimately, this was predicted to form a new type of chiral, sp3 nanotube 3.

Upon further examination, these three molecules were found to be variations on the same

3 idea: they are each fully-saturated, one-dimensional sp polymers with molecular formulas CnHn, whose unit cells could be viewed as a series of six-membered carbon rings linked to one another via six total bonds. Compounds meeting these conditions were designated as Degree-6 nanothreads. It was found that this definition for nanothreads encompassed a large number of possible structural isomers. Degree-6 nanothreads, can be divided into three major classes, based on the distribution of the bonds between a six-membered ring and its neighbors. In Class I, a ring is bonded once with its first neighbor, and five times with its next. Class I typically results in a high-energy structure and as a result, these structures are considered thermodynamically unfavorable and unlikely to be seen experimentally. Class II compounds consist of two connections in one direction, and four in the other. Class III nanothreads have three bonds in each direction 4.

Xu, et al enumerated fifty possible Degree-6 structures. Of these, polytwistane was found to be the lowest energy isomer, but many other structures were similar in energy. A total of fifteen structures, including tube (3,0), Polymer I, and polytwistane, were found to be within 80 3 meV per carbon atom of the most stable structure, indicating that any or all of these low-energy structures could be formed experimentally 4. These structures can be found in Tables 1 and 2, along with the predicted potential energies, stiffnesses, and effective radii of each structure.

Table 2: The structures of the most stable chiral nanothreads, sorted by stiffness. The structures include (c) Polytwistane. Computationally-predicted energies of formation, Young’s moduli, and effective radius are given for each structure. Potential energy was calculated relative to graphane and is given in eV per six-membered ring taking part in the structure. Young’s Moduli calculated with free polymer ends and fixed ends. Adapted with permission from [4] Xu, et al. 2015.

The fifteen lowest-energy structures can be broken down into three major subcategories:

Achiral, Stiff Chiral, and Soft Chiral. The Stiff and Soft Chiral species have helical structures, while Achiral structures are not helical. Helical structures can be divided into Stiff and Soft

Chiral molecules based on their effective radii and Young’s moduli. Soft Chiral structures have larger radii, and their constituent atoms are further from the central axis. The larger radii result in polymers that can stretch more easily as force is applied, giving rise to the term “Soft” Chiral.

Correspondingly, the structures with higher predicted Young’s moduli (Stiff Chiral) tend have smaller radii. The Stiff Chiral molecule with the highest modulus is polytwistane. Most of the achiral structures have stiffnesses to polytwistane 4.

The tensile strengths of many of the Degree-6 nanothreads have also been predicted, and have been found to vary significantly with structure. In simulations done in Zhan, et al., 2016, polytwistane was calculated to have an excellent tensile strength of 141 GPa, while Polymer I and an unspecified soft chiral polymer were predicted to have only moderate tensile strengths of 6

86 and 79 GPa, respectively. The result illustrates the massive impact that structural conformation has on a nanothread’s mechanical properties. Polytwistane’s structure can effectively distribute load evenly across the three bonds that connect each unit cell together.

Most achiral compounds, such as Polymer I, are Class II nanothreads and therefore concentrate stress into just two bonds between unit cells. Tube (3,0) would likely have a larger tensile strength than other achiral nanothreads because is a Class III nanothread. Class III is not necessarily enough to increase tensile strength, however; soft chiral structures generally have 3 bonds between each unit cell, but their tensile strengths are limited by their helical structure.

When stress is applied to a soft chiral nanothread, the fiber stretches out, pushing the atoms toward the center of the helix. As a result, the tensile force concentrates in the interior of the helix, while the exterior undergoes compression as the molecule is forced into a linear conformation. Torsion is also generated on the helix as a result of this straightening action, further disrupting the structure. Overall, in all three categories, ineffective load distribution is the strength-limiting factor for nanothread structures 5. Other studies have found similar tensile strengths and stiffnesses 6, 7.

As will be discussed in Section 1.2, nanothreads are currently synthesized through the linear polymerization of cyclic aromatics under very high pressure. The net result of the reaction is that the π-bonding electrons in the aromatic molecule form new sigma bonds to nearby rings.

However, there is no single-step process which could lead to the formation of six new bonds per ring to form Degree-6 nanothreads. Instead, the reaction is predicted to follow a multi-step polymerization mechanism in which each ring reacts to form one bond to each adjacent ring

(Degree-2), then a second (Degree-4), and then a third (Degree-6) 8. Examples of each of these structures can be found in the following figure. 7

The mechanism to go from Degree-0 to Degree-2 generates a diradical intermediate which reacts in a cascade to produce the para polymer. This would be followed by two “zipper” reactions to saturate the remaining double bonds and give a Degree-6 structure. If the first step in a nanothread polymerization consists of a 4+2 cycloaddition, the process can proceed straight from Degree-0 benzene to Degree-4 through a single step, then use a single zipper step to

Degree-6. The initial 4+2 cycloaddition has been predicted to utilize a concerted Diels-Alder mechanism, though significant rearrangement of the crystal structure is required to orient the aromatic rings properly. As is the case with any multi-step reaction, each step is not 100% efficient. Some threads (or repeat units along threads) will be trapped in Degree-2 or -4 structures which act as local energy minima, where the remaining double-bonds are positioned such that they cannot undergo further intramolecular reaction. In practice, this means that nanothreads could include a variety of unsaturated defects along their length, in addition to a mix of the Degree-6 structures discussed previously 8. These defects would limit the strength of the 8 overall polymer, as a Degree-4 unit cell would logically have similar or lower strength compared to a Class-II Degree 6 structure. The stiffness and tensile strength of Degree-2 threads would fall even lower 6. These unsaturated structures could also form at the end of a polymerizing chain, inhibiting further growth. Depending on the prevalence of this phenomenon, this could be a limiting factor for chain length 8.

1.2 Synthesis of Benzene Nanothreads

The first successful synthesis of nanothreads was reported in 2014, and was carried out by the polymerization of benzene under high pressure 9. In this publication, the polymers produced from the compression of benzene were referred to as “carbon nanothreads”. They have also been referred to as “diamond nanothreads” to emphasize their sp3 bonding hybridization and to evoke their theorized diamond-like structure. Experimentation has since revealed that these molecules are very structurally complex, making the implication of an idealized diamond structure less accurate. More importantly, nanothreads have since been synthesized from other molecules, yielding similar structures. Many of these structures could be described as “diamond” or “carbon” nanothreads with a similar accuracy to the polymer which originally bore those names. A large portion of this work is concerned with the synthesis of these alternative types of nanothreads, and so, to avoid confusion, each nanothread variant in this work will be named using a simple procedural naming system of “(precursor) nanothreads.” Any alternative names from the original publications will be stated, but not used in this work. Following this convention, nanothreads synthesized by the compression of benzene will be referred to as

“benzene nanothreads.”

The synthesis of benzene nanothreads was achieved by Fitzgibbons, et al. through the slow compression of benzene to 20 GPa at room temperature before slowly decompressing to 9 ambient pressure 9. Reactions had been induced through the compression of benzene in the past; however, previous experiments had all produced a waxy product of hydrogenated amorphous carbon 10-12. The Fitzgibbons synthesis instead produced a white, insoluble product. When examined by TEM, the material was found to be made up of individual thin fibers with aligned orientations. Parallel striations ran along its surface and individual fibrous strands were even found protruding from the edges of the material. The order of the structure was evident from the diffraction patterns of the material, the most characteristic of which was a set of six-fold diffraction peaks, indicating a hexagonally close-packed structure 9. The crystalline order of the product nanothreads has been found to be aligned with the pressure axis, rather than the precursor crystal axes, making the reaction non-topochemical 13.

Figure 3: A TEM micrograph of one of the original nanothread products showing ordered striations about 6.4 Å apart. Reprinted with permission from [9] Fitzgibbons, et al. 2014. 10

Li, et al. 2017.

Over the course of the Fitzgibbons TEM experiments, the structure of the material appeared to degrade under the electron beam until it became amorphous 9. This is a common phenomenon among carbon materials, and especially polymers. As the electron dose increases in a given area of the sample, the sp2 character of the material increases and the sp3 decreases. This indicates that the electrons are breaking bonds within the nanothreads and causing them to convert back to degree-4 or degree-2 forms. Such degradation can make working with the material under the TEM problematic; however, low electron dose techniques have been 11 developed by Juhl, et al. which allow for longer observation windows with less damage to the structure of the sample 14-16.

Infrared (IR) and Raman spectral absorption bands attributed to abundant sp3 C-H and sp2

C=C-H functionality are observed in nanothreads. These spectra are distinct from the IR and

Raman spectra of benzene, but are fairly similar to spectra observed for hydrogenated amorphous carbon, the alternative product of benzene compression reactions. This makes the identification of the product of a compression reaction difficult using purely IR or Raman techniques 12, 13.

Additionally, because a large number of structural isomers are thermodynamically possible for both Degree-4 and Degree-6 nanothreads 8, it is likely that a number of different structures form during synthesis. These are likely intermixed within individual threads. Benzene nanothreads have shown negligible solubility in all solvents evaluated to date 9. This has prevented the collection of many types of measurements common to other polymers, including averages and dispersities for number of repeat units.

Figure 5: Sketch of a diamond anvil cell, as used in a typical nanothread synthesis. A sample is enclosed inside of a gasket between two diamonds. These diamonds are then pressed together, 12 concentrating force into the tips (culets) of the diamonds. This generates extreme pressures within the sample, which is used as the driving force behind nanothread polymerization.

At the time of writing, nanothreads have been synthesized through two methods. The first method is the slow compression of a sample in a Diamond Anvil Cell (DAC). In this method, a hole with a diameter on the order of 100 microns is drilled into a metal gasket, the sample of interest is placed in this opening, and the gasket is compressed between two opposing diamonds.

The flat tips (culets) of the diamonds range from about 200 to 500 um in diameter. The diameter of the culet determines how high of a pressure it can induce on the sample, with thinner tips concentrating force more effectively, resulting in higher pressures in the sample chamber.

Modern DACs have been reported to be capable of generating up to 1000 GPa of pressure, though more common DACs have a safe limit of closer to 100 GPa 17. The advantages of using a

DAC for nanothread synthesis include a capability for in-situ photon-based measurement with a variety of instrumentation. This is possible because two sides of the cell are made up of diamond, which has weak absorbance at most wavelengths. In-situ pressure measurements also take advantage of this feature. The fluorescence peak of a ruby, when excited at a wavelength of approximately 500 nm, changes depending on the pressure it is experiencing, and can be used to find the pressure within the sample chamber. The primary disadvantage of a DAC is the miniscule sample size. The sample chamber is only 100 um across, and as a result, the yield is on the order of micrograms.

The second method used to synthesize nanothreads uses the Paris-Edinburgh cell (PE

Cell). The sample chamber in this method consists of two metal cups which surrounds the sample, forming a roughly spherical chamber around the sample. The gasket is reinforced on the seams by a series of bands to prevent pressure from releasing through these areas. The sample is 13 compressed between two diamond-tipped pistons, the lower of which is pressed upward by a hydraulic oil pump. The PE cells used in our experiments were just barely capable of generating the pressures required for nanothread synthesis. Their maximum pressures are typically about

17-20 GPa. As a result, PE Cells are useful for benzene nanothread synthesis, but it may not be suitable for other starting materials which require higher pressures to react. The primary advantage of the PE Cell is its larger yield. A successful run can produce as much as 2 mg of nanothreads. However, in-situ reaction monitoring is much more difficult in a PE Cell than in a

DAC. Because the metal gasket surrounds the sample on all sides, photon-based measurement is not possible until the gasket has been reopened at the end of the experiment. Instead, the only way to measure the in-situ properties during a PE Cell reaction is through neutron diffraction, which requires the use of specialized facilities and expensive neutron-transparent TiZr alloy gaskets, rather than the stainless steel of unmonitored reactions. Neutron-opaque stainless steel gaskets also prevent the measurement of pressure within a sample during a compression cycle.

1.3 Synthesis of Other Nanothreads

In addition to nanothreads made from carbon and hydrogen, it is possible to incorporate other elements and functionalities into the structure. These molecules could be synthesized using the same methods as traditional nanothreads, but yield compounds with slightly different chemistries. One computational study calculated the energy of the polymerization using several different types of functionalized benzene molecules and found that changing functionalities or adding heteroatoms to the ring would not greatly affect the energy input required, and would only impact the strength of the final material in a minor way 18. Functionalized nanothreads could have important applications in the future. The ability to select side groups allows for 14 greater compatibility in composite materials 18, and may allow for the tuning of the material’s electronic band gap for use in semiconductive applications, in parallel with methods being developed for band gap tuning in nanodiamond 19.

The first published synthesis of a nanothread from non-benzene starting materials was achieved with aniline by compression to 33 GPa at 550 K. This was reported to result in a nanothread in which the hydrogen of one carbon per unit cell is replaced with an –NH2 functionality 20. Pyridine nanothreads have also been reported from the compression of pyridine to 23 GPa at room temperature. The synthesis route for this molecule was very similar to benzene nanothreads, involving similar compression and decompression profiles. Published under the name “carbon nitride nanothreads,” these polymers retained many similar characteristics to the original benzene nanothreads. Some new properties also emerged, including a new photoluminescent spectrum. This spectrum may be indicative of a reduction in the electronic bandgap of the material 21, which has been predicted to be smaller in pyridine than in benzene 18, and could be a step toward conductive or semiconductive nanothreads. The photoluminescence could also be caused by defects in the structure caused by the nitrogen 21.

The substitutions in these new nanothreads add even more structural complexity, as there are a large number of structural permutations based on which position is replaced in each six- membered ring 22.

Most aromatic molecules can be divided into several broad categories based on how they are anticipated to affect the polymerization, as compared to benzene. One category included any molecule, such as pyridine, in which one or more of the carbons in a benzene ring are replaced by a heteroatom. This change was expected to modify the symmetry and aromaticity of the molecule. A second category includes compounds in which one or more hydrogens are replaced 15 by a functional group, such as toluene or aniline. These molecules were anticipated to change the symmetry, stereochemistry, and allow for potential tuning of the electronic properties of the ring.

A third category includes hydrocarbon rings with either a greater or lesser number of carbon atoms compared to benzene, such as cyclooctatetraene. The chemistry of this category would be highly dependent on number of carbon atoms in the ring. A fourth category includes multi-ring aromatic molecules, including fused-ring systems, such as naphthalene, rings connected by linkers, such as biphenyl. Although these materials are predicted to yield nanothreads with even greater strength than benzene nanothreads 23, they would also be expected react at higher pressure, as more aromatic stabilization and steric resistance would need to be overcome to start the reaction. Nanothreads polymerized from multi-ring precursors would also have a greater risk of forming unsaturated products. For example, while naphthalene could theoretically polymerize to form a Degree-10 nanothread, if the precursors are not perfectly aligned, it may be more likely to form side-products, such as a Degree-4 polymer with the remaining aromatic rings protruding from alternating sides of the chain. This could also result in crosslinking between threads, since these unsaturated features may interact with those of nearby chains. In addition to the molecules that fit in a single one of the preceding four categories, many aromatics also combine features from multiple categories, resulting in a large variety of chemistries available for potential polymerization.

1.4 Potential Applications for Nanothreads

Regardless of the exact composition or structure, nanothreads have been predicted to have exceptional tensile strengths. Though some variants of nanothreads have relatively low elastic moduli, others are some of the stiffest materials ever predicted. Nanothreads 5-7 rival other 16 modern carbon materials, such as carbon nanotubes 24 and graphene 25, which have gained widespread public recognition for these qualities.

Table 3: The tensile strengths and elasticities of several materials compared to the predicted properties of benzene nanothreads.

One obvious future application for nanothreads is in structural components. Due to the predicted mechanical properties, nanothreads will likely make ideal candidates for construction plastics and composites 28, 29. Additionally, the properties of a nanothread may be easily tuned by changing functionalities or heteroatoms within the ring, yielding a wide range of chemistries to utilize. Studies have predicted that these functionalized nanothreads could have increased compatibility with other materials without sacrificing significant amounts of mechanical performance 18. In addition to the chemical properties, a nanothread’s physical properties may also be tunable. For example, it has been suggested that the electronic bandgap of nanodiamond could be tunable by functionalization or introduction of heteroatoms into the structure 19.

Because the structures of the two materials are similar, it could be possible to replicate this effect in this system, producing conductive or semiconductive nanothreads. 17

CHAPTER 2

Use of Benzene Derivatives and Isotopes in Synthesis

2.1 Synthesis of Nanothreads from Benzene Derivatives

A significant portion of this work focused on high-pressure Diamond Anvil Cell (DAC) syntheses of nanothreads from different forms of functionalized benzene. Three aromatics were chosen from the second category listed in Chapter 1.3, in which one or more hydrogens are replaced by a functional group. The specific compounds chosen for these experiments were toluene, benzoic acid, and hexabromobenzene. In theory, each of these molecules may be able to react to yield nanothreads in which one or more exterior hydrogens per unit cell are replaced by the corresponding functional group from the precursor.

One important aspect to consider when picking a functional group is its electronic properties. Electron withdrawing groups (EWGs) reduce electron density within the ring system, and therefore reduce its aromatic stabilization. It was conjectured that reducing the aromaticity would reduce the pressure of polymerization by increasing the localized character of the carbon- carbon double bonds. Another consideration is the effect of the electronic properties on the mechanism of the reaction. Nanothread polymerization has been suggested to proceed through a series of successive 2+2 or 4+2 cycloaddition reactions 8. One potential mechanism for the reaction is a concerted Diels-Alder type reaction. If two benzene molecules were reacting with one another, 4 of the 6 positions of any given ring will act as a dienophile, and the reaction would be made more favorable by having electron donating groups (EDGs) in those positions.

Any given position could potentially act as either a dienophile or a diene, but on average it was predicted that the reaction could be slightly improved by adding an electron withdrawing group 18 to any given position. Overall, EWGs would be predicted to decrease aromaticity, but make a

Diels-Alder reaction slightly less favorable, while EDGs would do the opposite. Neither type is therefore favored, so weak to moderate EDGs and EWGs were chosen for this experiment.

Alternatively, the products of this polymerization may be explained by a non-concerted mechanism proceeding through diradical intermediates. The reactivity of the putative diradical intermediates may favor different monomer functionalities than those which are most favorable in a concerted Diels-Alder reaction. Further discussion of these two polymerization mechanisms will appear in the Future Synthetic Studies subsection of this chapter.

The experiments in this chapter were conducted using the High Pressure Collaborative

Access Team (HPCAT) facilities at the Advanced Photon Source (APS) in Argonne National

Lab and at the Cornell High Energy Synchrotron Source (CHESS) B1 SAXS/WAXS Beamline at Cornell University.

Toluene (C6H5CH3)

Toluene was chosen as a precursor due to the simple functionality that it provides.

Replacing one hydrogen with a methyl group has the potential to produce interesting variations on the benzene nanothread. Because toluene has the same symmetry group as pyridine, it will have the same permutations in possible structure, though the lowest energy structures may differ due to steric and electronic differences between toluene and pyridine.

Reagent-grade toluene (≥99.5% purity, CAS Number 108-88-3) was loaded at 4.56 GPa in a DAC and slowly compressed to about 27 GPa over the course of approximately 12 hours. It was held at this pressure for about 2 hours before being slowly decompressed back to ambient pressure for another 12 hours. The product was recovered as a transparent, colorless solid. This solid was stable in air at room temperature and pressure, and maintained its appearance and 19 spectroscopic properties for at least ten months before being utilized in destructive testing methods. In-situ Raman spectroscopy was done during the compression, the results of which can be found in the following figure.

Toluene was loaded into the DAC at 4.56 GPa. The Raman spectrum at this pressure holds good agreement with the peaks expected in the literature 30, with the exception of the large, broad peak centered near 160 cm-1. Peaks in this wavenumber range typically result from lattice vibrations, which are controlled by the crystal structure of the chemical. This broad peak could indicate a lack of crystallinity, which would be reasonable, as toluene forms a glass phase when quickly compressed as a liquid to pressures higher than 2 GPa. This sample was loaded at over 20 twice that pressure, so the sample may have formed a glass phase as a result. As toluene was compressed, each of the peaks in its Raman spectrum widened slightly; however, no significant changes were seen in the spectrum between the loading pressure through 16.96 GPa. The chemistry of the sample appeared to begin changing around 20 GPa. Although no changes were seen in the locations of the peaks, the level of the photoluminescent background began to drastically increase at this pressure. In the following measurement at 23 GPa, the photoluminescent background had completely overpowered all peaks. This photoluminescent background dominated the spectra for the rest of the experiment, and remained the same as the sample was decompressed and recovered, which likely indicates that an irreversible chemical reaction took place. Due to equipment restrictions, the excitation wavelength used for these measurements was 532 nm. Wavelengths in this region have also been known to lead to a large photoluminescence in other aromatic compression products. Subsequent Raman spectroscopy on the recovered product also showed a similar photoluminescent background because it was performed using a 514 nm excitation wavelength. 633 nm is known to be a better excitation wavelength for nanothreads of this type, and its use in future experiments could lead to lower photoluminescence.

Figure 7: The mid-IR absorption spectrum of (Blue) the toluene compression product and of

(Red) a sample of benzene nanothreads.

The IR absorbance spectrum of the product was measured using a Hyperion 3000 FT-IR microscope in transmittance mode. The IR spectrum was dominated by absorbances associated with sp3 C-H bands, including the large peak at 2924 cm-1, which corresponds to a strong alkane

C-H stretch, and the peaks seen at 1453 and 1377 cm-1, which may correspond to the weaker alkane –C-H bending vibrations. The IR spectrum shows that product likely has some sp2 character. The peak at 727 cm-1 could be an alkene =C-H bending peak, while the absorbance in the region around 1732 cm-1 could be an alkene C=C stretch. The peak shoulder at 3019 cm-1 could be caused by an alkene =C-H stretch, though the size of the peak at 2924 cm-1 makes this hard to confirm. The shouldering may also be caused by small amounts of unreacted toluene, which has an aromatic C-H stretching peak near that value. Toluene would also absorb around

1600 and 1475 cm-1, which are caused by its aromatic C=C stretch 31, and small peaks are seen at similar wavenumbers ] 32. Overall, the IR absorbance spectrum for the toluene compression 22 product appears almost identical to the spectrum of a benzene nanothread, suggesting that the toluene product may have a similar structure. However, under uncontrolled conditions, compression of benzene yields hydrogenated amorphous carbon which has similar IR absorbance spectrum to both of those seen above 12.

To determine which product was formed, the toluene compression sample was also examined by TEM for the distinctive six-fold diffraction pattern which has been characteristic for all nanothreads synthesized in the past. In this experiment, none of the examined particles showed significant diffraction spots, indicating that the product was amorphous. Based on the

Raman, IR, and electron diffraction evidence, it is likely that the product of the toluene compression was a form of hydrogenated amorphous carbon; however, it is also possible that toluene nanothreads formed, but were damaged in the electron beam before their diffraction pattern could be found. The methyl functionalities of a toluene nanothread could potentially cause greater interactions with the electron beam. Polypropylene has previously been observed to be much more sensitive to electron damage than polyethylene 33, and the relationship between toluene nanothreads and benzene nanothreads may mimic this trend. This could be tested by repeating this compression experiment and searching for diffractions by XRD rather than by

TEM to avoid beam damage. 23

Figure 8: Image of the toluene compression product (darker center circle) in its stainless steel

DAC gasket (bright outer ring). Image captured on an Olympus BX62 Microscope in reflection mode.

In addition to instrumental analysis, the toluene product was surveyed visually. While the amorphous product of an uncontrolled benzene reaction typically appears waxy, the toluene product appeared relatively colorless and glass-like, as seen in the above figure. The toluene product has similar IR signals to both benzene nanothreads and hydrogenated amorphous carbon.

Because the measured electron diffraction patterns indicated that the product was amorphous, the most likely product for this reaction is hydrogenated amorphous carbon, with the caveat that toluene nanothreads may have been formed and then amorphized in an electron beam. In addition to these possibilities, tt may also be possible that the toluene product initially polymerized to form a nanothread product whose higher order crystalline structure collapsed 24 spontaneously after synthesis. Such degradation could be caused by the methyl functionalities on the sides of a toluene nanothread, which could make ordered interaction between threads less energetically favorable. However, methyl groups are likely not large enough to cause decrystallization of this type, and such a mechanism would likely leave behind crystalline regions in the product which would have been detected in the TEM electron diffraction experiment.

Formation of a hydrogenated amorphous carbon product is encouraging, and is an indication that nanothread formation may be possible at similar maximum pressures to those used in this experiment. Adjustments to the compression and decompression pressure profiles could result in polymerization of toluene to nanothread products in follow-up experiments. As stated previously, toluene forms a disordered glass phase if compressed directly from the liquid phase to a pressure of 2 GPa or higher. In this experiment, the toluene was loaded into the DAC and immediately pressurized to 4.56 GPa. As a result, the starting material of this experiment was almost certainly a disordered solid, while all previous nanothread polymerizations have used a crystalline starting material. It is possible that the lack of order in the starting material prevented an ordered nanothread polymerization. Packing toluene at a lower pressure would allow a crystalline starting material to be used, which may allow polymerization. Because toluene also has both a crystalline solid phase and an easily-accessible glass phase, it could be useful in future experiments to expand on these results. Successfully producing ordered nanothreads from a disordered starting material would conclusively prove that the order in nanothreads is related only to the unilateral pressure in the reaction vessel. Alternatively, if nanothreads could only be produced from crystalline toluene and not from the glass phase, this would provide strong evidence that higher-order structure in nanothreads requires at least some 25 degree of crystallinity in the precursor. This could be taken another step further by preparing toluene samples where some regions are crystalline and some are disordered, and seeing whether this forms nanothreads, hydrogenated amorphous carbon, or a combination of the two.

A second experiment involving the compression of toluene was attempted at the CHESS

B1 X-ray Diffraction beamline, but the experiment was not completed because toluene proved to be a difficult precursor to measure using diffraction methodology. In XRD methods, samples must be either single crystals or powders in order to align the diffraction planes of the crystals and produce diffraction spots or rings. In order to create a single crystal sample within a DAC at room temperature from liquid starting materials such as toluene, a seed crystal is typically required. For this experiment, seed crystals were not available due to material and equipment constraints. Therefore a powder sample would be required. Standard procedure to create a powder from a liquid sample in a DAC involves first freezing a sample by applying pressure.

The sample is then partially melted and refrozen repeatedly by quickly raising and lowering the pressure. This process melts the edges of large crystalline sections and reforms them as new crystals grains, making the sample more and more multicrystalline to a point where the sample approximates a powder. This procedure was difficult to execute on toluene using the DAC that was available. Toluene freezes into a crystal around 1 GPa, but lowering the DAC much lower than 1 GPa risks breaking the seal of the gasket and allowing air into the sample. Repeated imprecise actions in this pressure region, such as quickly cycling the pressure up and down, also increase the risk of seal rupture. Additionally, if liquid toluene is quickly compressed to 2 GPa or higher, a disordered glass phase forms, as the molecules are compressed too quickly to rearrange themselves into ordered crystals. When pressure is lowered, these disordered regions slowly crystallize. To form a toluene powder, it is therefore necessary to cycle the pressure from just 26 under 1 GPa to just under 2 GPa. If the pressure is lowered too far, the cell releases and most be reloaded, and if it is raised too high or the pressure is changed too slowly, the pressure cycling will be ineffective at increasing the crystallinity. The DAC’s pressure control screws are very sensitive in this pressure range, and this obstacle ultimately could not be overcome during the allotted beam time. This incident provides useful insight into a potential problem for toluene compression experiments, and the lessons learned from it could be applied to future precursors with easily-accessible glass phases as well.

Benzoic Acid (C6H5COOH)

Benzoic acid was chosen as a precursor because of potentially useful solubility of a nanothread made from it. As mentioned in Chapter 1.2, nanothreads have previously been found to be largely insoluble. This has prevented the application of common polymer analysis techniques, such as size exclusion chromatography and solution-based light scattering. These methods would provide valuable insight into the structure of nanothreads in general, giving estimates for the average chain length and variability in chain length. A nanothread with carboxylic acid functionalities on the exterior would likely be soluble in water and other polar solvents. Carboxylation of the surface of nanodiamond has previously been demonstrated to increase its solubility 34. Given the similarities between the structures of nanodiamond and nanothreads, it is likely carboxylation of the exterior of a nanothread would also increase its solubility.

Benzoic acid (≥99.5% purity, CAS Number 65-85-0) was loaded in a DAC as a solid powder and slowly compressed by hand to 19 GPa and monitored by XRD using the CHESS B1

X-Ray beamline. The only diamonds available for the DAC at the time were cracked, and as a result higher pressures were not available without risking further damage to the diamond. The 27 sample was held at pressure for fourteen hours before being returned to ambient pressure. The sample was recovered as a solid, with no visually apparent changes occurring in the sample between the start and end of the run.

The primary change in the diffraction pattern during the run was expansion of the angle of each diffraction ring, indicating a contraction of the spacing between molecules. Upon decompression, the rings returned to their original positions. No new diffraction rings were seen during the experiment, and the diffraction pattern of the recovered sample matched the pattern of the original. This indicates that no major phase change occurred during the compression. It also suggests that no chemical reaction occurred in the analyte during the compression. If a reaction did take place, its product would have to have the same crystal structure as the original sample.

No Raman spectrum for the product was observed due to excitation limitations with the instrument used. When excited at 514 nm, the compression product of benzoic acid exhibited an intense photoluminescent background which masked the Raman spectrum. The use of a higher wavelength excitation source may reduce the photoluminescence, allowing Raman spectral measurement. The thickness of the sample precluded direct IR transmittance measurements.

Available evidence suggests that benzoic acid nanothreads were not synthesized by direct polymerization under the conditions in this experiment. To determine whether higher induction pressures may be necessary to induce polymerization, further studies at higher pressure will be needed, utilizing in-situ Raman measurement. If the carboxylic acid functionality is found to prevent nanothread formation, even at high pressure, a variety of other simple benzene derivatives could be used in its place, including phenols or benzenesulfonates. Alternatively, esters or masked polar groups could be used in a polymerization before transformation to carboxyl groups after the reaction. Beyond functionalization, other strategies could also be used 28 to increase nanothread solubility. In general, polymer solubility decreases with chain length, so intentionally generating short chain-length nanothreads could be a viable method to produce a soluble polymer. This could be done by mixing two aromatics with vastly different polymerization pressures, and then compressing the solution in a DAC, generating a crystal with each precursor randomly interspersed with the other. When the polymerization pressure of the more reactive precursor is reached, only that precursor should react to form oligomers, while the second reagent would cause breaks in the threads. Chain length in such a reaction could be controlled by the concentration of the less-reactive reagent.

Hexabromobenzene (C6Br6)

Hexabromobenzene (HBB) was selected as a potential precursor because a nanothread derived from it would feature exterior C-Br bonds which could be used for post-polymerization functionalization. These functionalizations would be possible through use of simple SN1-type reaction mechanisms or through radical mechanisms. This reactivity would allow for the synthesis of any number types of nanothread, including a multitude which may not be feasible through polymerization of functionalized precursors. In addition to HBB, other hexahalobenzenes could be used in this role, including C6Cl6 and C6I6. HBB was chosen because it may offer a wider range of post-polymerization reactivities. For the polymerization reaction itself, HBB was anticipated to have similar reactivity to precursors used previously, including benzene and hexafluorobenzene.

Solid hexabromobenzene (97% purity, CAS Number 87-82-1) was loaded in a DAC at

3.73 GPa and slowly compressed to about 25 GPa over the course of 12 hours. It was held at maximum pressure for about an hour, then slowly decompressed to ambient pressure over 14 hours. In-situ Raman measurement of the material was done periodically during the compression 29 and decompression cycle. For these measurements, the sample was excited at 532 nm.

Representative spectra from the compression and decompression can be found in the following figure.

At 3.73 GPa, the start of the run, the Raman spectrum of the HBB sample shows peaks which match with those of literature values for the chemical 35. The large peak in each spectrum at about 1300 cm-1 was generated by the diamond anvils of the DAC, which act as windows into the sample chamber which the Raman lasers can pass through. As the pressure is increased, each peak experienced a slightly higher Raman shift. In particular, the peaks with the highest Raman shifts move far enough that they leave the range of the detector and disappear. The photoluminescent background also slightly increases with pressure, but much less severely than 30 the increase seen in toluene. This intensity of this photoluminescence decreased slightly by the time the same was recovered, but did not disappear entirely. Overall, there did not appear to be any major change in peak positions from the start to the end of the reaction. After recovery, the

Raman spectrum of the HBB sample was taken again using a 514 nm excitement wavelength and compared with the spectra of a sample of unmodified HBB and a sample of benzene nanothreads.

Figure 10: The Raman spectra from 0-1800 cm-1 of (Blue) the HBB Compression Sample and

(Red) a sample of unmodified HBB, each excited at a wavelength of 514 nm. The wavelengths of peaks common to both of the samples are labeled. (Green) The Raman spectrum of a sample of benzene nanothreads excited at 633 nm is included for comparison.

The Raman spectrum of the recovered HBB compression sample was found by exciting the sample with a 514 nm laser. The samples were measured up to a Raman shift of 3200 cm-1; however, no significant peaks were seen past 1750 cm-1. The peaks of the compressed matched 31 up perfectly with those of an unmodified HBB sample, indicating that no significant change in structure took place. The peaks at 147, 207, 230, and 1485 cm-1 also match up well with literature values for HBB 35. The photoluminescence of the sample remained higher intensity in the compressed sample than in the unmodified HBB, indicating that some minor reaction may have occurred which did not significantly impact the structure of the HBB. The compressed sample also exhibits a broad absorbance peak at 1548 cm-1 which does not match up to the unmodified sample, but also does not match up with peaks from the benzene nanothread sample.

The thickness of the sample precluded direct IR transmittance measurements.

Overall, it is clear that the Raman spectrum of the compressed HBB sample matches up very well with the unmodified HBB, and does not share any features with the spectrum for the benzene nanothread sample. Therefore, it can be concluded that polymerization did not take place during this experiment, though it is possible that a minor side reaction may have occurred.

It is possible that the large size of bromine atoms may increase the activation energy for the polymerization of HBB. Follow-up studies will need to be done to determine whether higher activation pressure is required to induce polymerization. There is also the possibility that the bromide groups of this molecule could cause chain transfer reactions which may interfere with nanothread formation. This possibility will be explored in the Future Synthesis Studies in this chapter. Overall, these side reactions could be investigated by NMR, or by looking for evidence of diatomic bromine, radical bromine or bromide in compression products. Incomplete polymerization due to chain termination might be expected if nanothread polymerization proceeds through the non-concerted diradical mechanism mentioned in the introduction to this chapter. Halogenated monomers such as HBB may be more susceptible to free radical chain termination, leading to lower average molecular weight than other types of nanothreads. 32

Future Synthesis Studies

While the direct production of nanothreads from functionalized monomers was unsuccessful in this work, functionalized nanothreads remain a desirable synthetic target.

Optimization of pressure rates could potentially yield nanothreads from each of the precursors that were studied. In these studies, toluene was most likely amorphized, but may have formed a novel electron-beam sensitive type of nanothread. Soluble nanothreads, such as those that may be produced from benzoic acid, have the potential to open up new characterization possibilities using established polymer science methodologies. Finally, nanothreads which can react to change their functionalities after polymerization could allow the synthesis of a much broader range of polymers through a variety of simple methods.

Significant improvements, such as reducing reaction pressures and increasing yields are a few steps required to make nanothread synthesis practical. These could conceivably be accomplished in a number of ways, including detailed studies on the exact reaction conditions required to trigger the polymerization reaction, investigation into the effect on heat and light on the reaction, or the development of catalysts and initiators.

In particular, initiators may be an interesting topic of future study. As discussed at the start of this this chapter, each of the precursors in this section were selected under the assumption that the reaction takes place through a concerted cycloaddition mechanism. If this is the case, each monomer (i.e. benzene in the case of benzene nanothreads) would act as both a diene and a dienophile in a Diels-Alder reaction. Therefore, small quantities of more-reactive dienes or dienophiles could be added to the reactants and may be able to act as initiators for the polymerization. The reaction of these initiators with the monomer would provide a lower 33 activation pressure than the monomers have with one another. Use of initiators could allow for better control over reaction mechanism and the structure of the final product, as well as give opportunities to functionalize the ends of the nanothreads.

Diels-Alder reactions typically occur between two molecules of different potential energies. The dimerization, oligomerization, and polymerization of simple linear or cross- conjugated alkenes has been previously suggested to proceed through a step-wise non-concerted diradical mechanism 36-39. In dilute organic solutions, dimerization of simple linear or cross- conjugated alkenes to 2+2, 2+4, or 4+4 dimers is observed. Unconstrained, highly reactive monomers typically favor formation of the more thermodynamically stable 4+4 dimer products, forming an 8 membered ring; however, in sterically strained systems, the kinetically-favored 4+2 dimeric products are often the predominant products 36-39. In organic solutions, higher monomer concentration favors the formation of oligomers and higher order polymers over the formation of dimers due to the proximity of diradical intermediates to one another 36, 37. A solid-state reaction utilizing this mechanism might mimic these high concentration conditions due to the close proximity of nearby monomers, making polymers the expected product.

By analogy to unsaturated hydrocarbon polymerization discussed above, under high- pressure conditions, reaction of crystalline benzene would likely form a similar sterically- or motionally-hindered system, potentially resulting in a 4+2 dimer being the dominant product of a diradical reaction. The diradical mechanism may be operative in the 4+2 product given that a separate diradical product, the para polymer, has also been predicted 8. Thus either a concerted

Diels-Alder cycloaddition 8 or step-wise diradical mechanism 36-39 could potentially be the mechanism of the pressure-induced polymerization of benzene. The mechanism of this reaction might be confirmed experimentally in a number of ways, including addition of radical-trapping 34 or radical-detecting agents during synthesis, although detection of diradical intermediates in such reactions can be difficult experimentally 36. If the reaction is found to proceed through a diradical mechanism, a variety of well-developed reaction initiation systems (i.e., light, ionizing radiation, radical initiators) or catalysts could potentially be utilized to develop the synthesis of nanothreads on an industrially-useful scale.

Regardless of which mechanism produces the 4+2 product, the para polymerization is known to occur through diradical means, and care should therefore be taken to choose precursor functionalities which stabilize the diradical intermediates during the reaction. This stabilization would reduce the overall reaction’s activation energy, potentially decreasing the amount of pressure required to start the reaction. It would also increase the lifetime of the intermediate, which could increase the rate of the polymerization. On the other hand, functionalities such as chlorine and bromine may be undesirable due to predicted side reactions. To draw parallels to other polymerizations, Teflon is commonly made through free radical polymerization; however, polyvinyl chloride (PVC) is typically synthesized through other means 40. This is because PVC has a very high rate of monomer chain transfer in free radical reactions. Overall, chain transfer reactions do not inhibit the polymerization, but they do reduce the average molecular weight of the products. Chain transfer reactions terminate the growth of the original growing chain, but also give rise to a new growing chain in its place 40. The rate of chain transfer is dependent on the stability of the radical involved in the reaction. In the case of PVC, the species in question is a chlorine radical, but in the case of Teflon, it would be a fluorine radical, which is much more stable 41. Therefore monomer chain transfer in Teflon is typically a minor concern, but in PVC it is a major consideration which limits chain length. Chain transfer rates for polyvinyl bromine would be even larger than for PVC. Also note that in PVC, in order to do a chain transfer 35 reaction, a relatively uncommon head-to-head polymerization step typically has to occur for a chlorine atom to be in the right position to do the chain transfer. This slows the rate of chain transfer compared to a hypothetical monomer, such as 1,2-dichloroethene which would always have a chloride in position to do chain transfer 40.

Step (b) the bromine radical reacts with an HBB monomer to produce either (1) a Br2 molecule plus a new growing chain with an identical chain-end residue from the one produced in Step (a), or (2) a new growing chain with a chain end residue containing seven bromides. A similar monomer chain transfer reaction would be expected for a diradical-mediated 4+2 reaction. 36

Because nanothread formation goes at least partially through a diradical mechanism, monomer chain transfer reactions would likely be observed when using chlorinated or brominated aromatic precursors. In HBB, for example, there will always be a bromine atom in position to be eliminated by the radical, regardless of whether the para polymer or 4+2 route is active. Therefore use of chlorine or bromine functionalities may result in large monomer chain transfer rates during nanothread formation, leading to a drop in average molecular weight. As seen in Figure 11, these chain transfers would result in characteristic chain ends that may be measurable by mass spectrometry or similar methods. Presence of Br2 or Cl2 in the product may also be an indication of these side reactions. Fluorobenzenes, analogous to Teflon, would be predicted to have molecular weight profiles closer to that of unfunctionalized benzene because the instability of the fluorine radical would greatly disfavor the chain transfer reaction.

2.2 Solid-State NMR Studies on 13C Benzene and 15N Pyridine Nanothreads

As discussed in Chapter 1.1, nanothread structure is complex, and there are many similar- energy structures which could be thermodynamically possible during synthesis. In addition to fully-saturated sp3 products from a Degree-6 polymerization, there are also products which result from incomplete polymerization, including unsaturated Degree-4 and Degree-2 segments, which have varying amounts of sp2 character. In this section, experiments are described in which NMR spectroscopy was used to estimate the relative quantities of sp3 and sp2 carbon in a nanothread sample, find the percentages of carbon which were part of various types of structures, and measure the length of pure Degree-6 nanothread segments.

13 15 C6-enriched benzene (99% enriched) and N-enriched pyridine (98%+ enriched) from

Cambridge Isotope Laboratories were used to prepare nanothreads to be examined by solid-state 37

NMR. Each of these isotopic precursors was compressed in a PE cell in off-line experiments at the Spallation Neutron Source (SNS) at Oak Ridge National Lab. The samples were each compressed to about 23 GPa over the course of approximately 8 hours, held at pressure for an hour, and decompressed back to ambient pressure in about 6-8 hours before the nanothread samples were collected and sent to the Schmidt-Rohr lab at Brandeis University for analysis.

CH2 features. (d) The dipolar dephased multiCP spectrum, giving signal only from non- protonated or mobile carbon. Reprinted with permission from [42] Duan, et al. 2018.

Advanced solid state MAS NMR with specialized pulse sequence routines were used to analyze to analyze the 13C benzene nanothread and 15N pyridine nanothread products that were produced by our laboratory. MultiCP (cross polarization by multiple contact periods), together with magic-angle spinning (MAS), was used to acquire proton-decoupled 13C NMR spectra. As is shown in Figure 13, two carbon resonances were observed. One was near 40 ppm and was characteristic of sp3 carbon, and a smaller peak near 130 ppm was characteristic of sp2 carbon.

The integrated signals were in an approximately 3:1 ratio, suggesting that approximately 72% of the carbon was sp3 hybridized while 28% was sp2 hybridized. A 1H 13C dipolar Distortionless

Enhancement by Polarization Transfer (DEPT) double resonance pulse experiment was used to

42 demonstrate that most carbons were bound to a single hydrogen (C3-CH or C=C-H) .

Figure 13: The percentage of carbon atoms involved in various structural features in the 13C benzene nanothread sample, as calculated through a combination of pulse-sequence and 2D

NMR techniques. Reprinted with permission from [42] Duan, et al. 2018. 39

A variety of specialized pulse sequence routines and 2D NMR techniques were utilized to estimate the percentage of total carbon atoms present in various structural types in the nanothread samples. These percentages are reported in Figure 14. It was found that approximately 33% of carbon atoms were involved in Degree-4 and Degree-6 structures each.

The remaining carbon was involved in Degree-2 structures, unreacted benzene, or a variety of minor side-products. The minimum length of pure Degree-6 nanothread was estimated to be at least 2.5 nm, or 12 benzene rings, based on spin exchange rate, as detailed in Duan, et al. 2018.

Based on the length of the Degree-6 segments and the relative proportions of each structure,

Duan, et al. also predicted a possible structure for benzene nanothreads, which can be seen in the following figure 42.

Figure 14: One possible structure for a benzene nanothread, consistent with the length of the

Degree-6 segments and relative proportions of each structure calculated through various NMR techniques. The sp3-CH units are represented in blue, sp2-CH in yellow, and non-protonated sp2-

C in magenta. Reprinted with permission from [42] Duan, et al. 2018.

Similar solid-state NMR characterization of 15N-enriched pyridine nanothreads prepared in our laboratory was reported by Li, et al. The 15N and 13C NMR spectra from this sample is shown in Figure 16. Based on these experiments, it was found that approximately 40% of the atoms in the sample were involved in Degree-6 structures 21. 40

CHAPTER 3

Mechanical Exfoliation of Benzene Nanothreads

Mechanical exfoliations of benzene nanothreads were investigated in this project. In principle, mechanical exfoliation of nanothreads should be possible in a manner roughly analogous to the exfoliation of other types of single crystal polymers 43, or, more famously, graphene 44. Nanothreads, as synthesized, consist of crystals of aligned threads which should only be held together by Van-der-Waals forces, and possibly some minor cross-linking. Several exfoliation methods were attempted in this work. These included applying of shearing force to nanothread samples by rubbing them between two surfaces, peeling nanothread particles apart using tape, and crushing nanothread crystals using the tip of a diamond. Each method provided different challenges to overcome, but ultimately, evidence of successful exfoliation was not found using any of the attempted methods due to the difficulties effectively manipulating the samples, and because of an inability to differentiate the resultant nanothread particles from other carbon debris, such as dust. Each of the exfoliation methods is depicted in Figure 13. 42

Figure 16: Depictions of each exfoliation method attempted with benzene nanothreads.

Top: A nanothread crystal is placed onto a germanium wafer. Another surface is placed against the first and twisted, applying shear forces to pull the crystal apart. Middle: A nanothread crystal is stuck to a piece of scotch tape. The tape is folded onto itself, then peeled back. Adhesive from each area interacts with the crystal, pulling threads away from the bulk crystal. Bottom: The tip of a diamond is used to crush the nanothread crystal, pushing threads away from the bulk. 43

The first method used in this experiment involved placing a nanothread crystal between two parallel surfaces, then sliding the surfaces along one another to generate shear forces on the crystal. The surfaces were then separated from one another and examined using a Bruker Icon

Atomic Force Microscope (AFM) in tapping mode. A Bruker ScanAsyst-Air probe was used, with a silicon tip on a nitride cantilever. This same AFM/Probe combination was used for all experiments in this chapter. The first version of this exfoliation procedure utilized undoped, N- type germanium wafers in the (100) orientation for each surface. Germanium was chosen because it has a high atomic weight, and should therefore exhibit strong Van-der-Waals interactions with the nanothread crystal. Commercially-available germanium wafers also have very little surface height variation, which could allow for imaging of nanothread fibers on the surface. A surface with nearly atomic smoothness is required because nanothreads have a radius of approximately 0.6 nm, and could easily be invisible on even moderately rough surfaces.

Unfortunately, using this method, each germanium wafer suffered severe surface damage during the exfoliation process. Surface damage included both pitting and scratching, roughing up the surface and making it impossible to find any nanothreads that may have separated from the bulk.

The most common damage features were scratches about 20 nm wide. The surface was also imaged by EDS, where it was found that there was an approximately equal number of germanium and carbon particles of similar size intermixed on the surface of the wafer. Therefore, identification of even moderately sized particles by AFM would be impossible without preventing surface damage. 44

It was unclear what the cause of the damage was. While germanium can, in principle, scratch another piece of germanium, generating a scratch would require some sort of raised area on one of the germanium surfaces to concentrate stress onto the opposing wafer. In this case, the raised areas should be fairly noticeable by AFM because they would have to be at least 20 nm in both height and width to leave scratches of similar dimensions. The germanium wafers were examined prior to experimentation, and, as advertised by the manufacturer, they appeared to be nearly atomically smooth, with no especially notable bumps. It was hypothesized that damage might instead be occurring because the surface of the wafer had oxidized in the air and was instead germanium oxide, or because some sort of dirt or dust particle had settled on the surface.

To account for these possibilities, a cleaning procedure was developed where, in the Penn State

Materials Research Institute (MRI) nanofabrication lab, all debris was cleaned from the surface 45 of the germanium wafer with methanol. Following this, the wafer placed into water for several minutes to remove the oxide coating. Germanium oxide is soluble in water and is readily removed as a result. The removal of the oxide layer was confirmed via ellipsometer. The water was then rinsed off of the wafer with a second methanol rinse and the same exfoliation procedure was repeated. Similar surface damage was seen in these trials as in the untreated wafers.

Another possible solution was the use two surfaces made from different materials to reduce damage to the germanium surface. Germanium was kept for one surface, while the other was replaced with Teflon. In addition to its low hardness, Teflon was chosen because it has very small Van-der-Waals interactions, and separated nanothreads would therefore interact preferentially with the germanium surface. In practice, the Teflon surface was damaged by the germanium wafer, producing a significant amount of debris which remained on the germanium and interfered with any attempts to locate nanothreads on the surface.

Another method investigated to exfoliate nanothreads was to peel nanothreads from the bulk using scotch tape. This method has famously been used to wide success in graphene, and also in the exfoliation of single crystal polymers. The method that was used was adapted from a similar procedure which was used by Dou, et al. to separate a number of single crystal polymers.

Nanothread particles were placed onto the sticky side of a piece of scotch tape. The tape was then folded over so that a sticky section further down the same piece of tape was stuck to the top of the particles. This was then removed, bringing with it a percentage of the original particle.

This could be stuck with another piece of tape, continuing on until the desired level of exfoliation has been achieved. Then a germanium wafer was placed face down into the tape and peeled back off to deposit the nanothreads on the surface. The adhesive was then washed away using acetone or another organic solvent. This wash step appeared to be less effective than in the 46 original work, as a large amount of debris was left behind when the wafer was examined by

AFM. It is possible that the substitution of germanium wafers for the original silicon wafers may have resulted in interaction between the germanium and the acetone during the rinse step; however, no sources could be found to support the notion that germanium is incompatible with this solvent. It is also possible that germanium has increased interaction with the scotch tape adhesive, resulting in surface damage and/or incomplete removal of the adhesive during the wash step.

One of the major difficulties found when exfoliating nanothreads was the difficulty proving that any given particle is made up of nanothreads. In other characterization methods, one of the primary methods of identifying a nanothread is by finding its crystal’s characteristic six- 47 fold diffraction pattern, as this is not a pattern seen in dust. However, this experiment involves removing this higher order crystallinity, so this cannot be used for identification of exfoliated particles. The next two most useful means of characterization are Raman and IR spectroscopy.

Nanothreads typically have unique signals in these wavelengths when compared to debris; however, these methods are both light-based and therefore cannot be used if particle size is below several hundred nanometers.

Using current methods, the only way to conclusively identify a fiber as a nanothread would be to find it attached to the side of a larger particle that could be characterized via one or more of the previous methods. To try to produce these conditions, one final exfoliation method was developed. This method involved crushing nanothread crystals with the tip of a diamond to try to force the strands apart from one another. Several “diamond crushing” experiments were performed, but no loose strands were found by AFM during these experiments.

Future exfoliation experiments could be improved by improving characterization methodology. One potentially useful technique improves on the limited resolution of Raman spectroscopy. Tip-Enhanced Raman Spectroscopy (TERS) takes Raman measurements at the tip of a scanning tunneling microscope (STM) probe, which provides Raman data on the sample surrounding the probe during an STM run. The resolution of the method is improved to about 1.7 nm 45. Unfortunately, this resolution is still 3-10 times larger than the predicted radius of a single nanothread 4, 9, 13, but it could be a useful method for the characterization of larger bundles of threads. Used in combination with higher-resolution AFM, this method would provide the means to identify small bundles of nanothreads, which could be more likely to have loose threads hanging from the sides. Use of an STM on its own may also help to differentiate nanothreads from other carbon material on the basis of electronic bandgap. However, current benzene 48 nanothreads are insulating materials with similar bandgaps to those of carbon debris. In the future, if bandgap doping is achieved in nanothreads, STM may become a useful way to characterize exfoliated nanothreads of these types.

Despite problems encountered with exfoliating and characterizing non-crystalline nanothreads, the discovery of new methods to separate fibers or bundles of fibers from the bulk and identify them may enable the use of established mechanical tests that could be done via

AFM. These include three-point deformation testing and nanoindentation. Both of these methods have been successfully used in the past with nanoscale fibers, but would have to be adapted to work with nanothreads, which are about an order of magnitude smaller than the smallest fibers typically used in these methods 46-48. Tensile testing by pulling on a nanothread with an AFM tip may also be possible in the future. However, AFM tensile testing is typically limited by the length of the sample fibers, which are required to be on the order of hundreds of microns 46. These types of

AFM tests could help to better understanding of the morphology of nanothreads, and to experimentally confirm the mechanical properties that have been suggested by computational predictions.

CHAPTER 4

Conclusions and Future Outlook

Nanothreads are a growing area of interest, and there are many possible experiments that could expand on current knowledge. In addition the lines of research that directly build on the research presented in this work, there are many opportunities to improve the polymerization reaction itself. One possible frontier includes advancements in reaction conditions to reduce the pressures required for the reaction, improve the yield, and reduce the variation in structures so that specific products can be formed. Reducing pressures and increasing yields will be necessary to make nanothreads feasible for use in any of their potential applications. A few reasonable starting places to achieve these goals might be the investigation into the mechanism of the reaction, development of initiator/catalyst systems, and studies on the impact of pressure, temperature, and light conditions on the activation pressure and product structure.

Characterizing nanothreads once they have been synthesized remains a challenge. The single most useful metric is the presence of a characteristic six-fold diffraction pattern brought about by the higher-order hexagonal packing structure of the threads. Raman and IR spectroscopy can be useful for characterization as well, but it is difficult to confirm the existence of nanothreads using these methods alone, as nanothreads show signals very similar to dust or carbon debris for each of these methods. Because of these uncertainties, several results in this work could not be confirmed, including the possible synthesis of disordered toluene nanothreads and the exfoliation of individual nanothreads from the bulk. The use or combination of other characterization methods, such as Tip-Enhanced Raman Spectroscopy (TERS) or Scanning

Tunneling Microscopy (STM) may help to strengthen characterization capabilities in the future. 50

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APPENDIX

Additional Copyright Information

Figure 1 of this work was adapted from Carbon, 107, Zhan, H.; Zhang, G.; Bell, J.M.; Gu, Y., The morphology and temperature dependent tensile properties of diamond nanothreads, 304-309, Copyright 2016, with permission from Elsevier.

Figure 2 of this work was adapted with permission from Chen, B.; Hoffmann, R.; Ashcroft, N.W.; Badding, J.; Xu, E.; Crespi, V. Linearly polymerized benzene arrays as intermediates, tracing pathways to carbon nanothreads. J. Am. Chem. Soc. 2015, 137, 14373-14386. Copyright 2015 American Chemical Society.

Figure 3 of this work was reprinted by permission from Springer Nature: Nature Materials, 14, 1, Benzene-derived carbon nanothreads, Fitzgibbons, T.C.; Guthrie, M.; Xu, E.S.; Crespi, V.H.; Davidowski, S.K.; Cody, G.D.; Alem, N.; Badding, J.V., 43-47, Copyright 2014.

Figure 4 of this work was reprinted with permission from Li, X.; Baldini, M.; Wang, T.; Chen, B.; Xu, E.; Vermilyea, B.; Crespi, V.H.; Hoffmann, R.; Molaison, J.J.; Tulk, C.A.; Guthrie, M.; Sinogeikin, S.; Badding, J.V. Mechanochemical synthesis of carbon nanothread single crystals. J. Am. Chem. Soc. 2017, 139, 16343-16349. Copyright 2017 American Chemical Society.

Figures 12, 13, and 14 of this work were reprinted with permission from Duan, P.; Li, X.; Wang, T.; Chen, B.; Juhl, S.J.; Koeplinger, D.; Crespi, V.H.; Badding, J.V.; Schmidt-Rohr, K. The chemical structure of carbon nanothreads analyzed by advanced solid-state NMR. J. Am. Chem. Soc. 2018, 140, 7658-7666. Copyright 2018 American Chemical Society.

Figure 15 of this work was reprinted with permission from Li, X.; Wang, T.; Duan, P.; Baldini, M.; Huang, H.T.; Chen, B.; Juhl, S.J.; Koeplinger, D.; Crespi, V.H.; Schmidt-Rohr, K.; Hoffmann, R.; Alem, N.; Guthrie, M.; Zhang, X.; Badding, J.V. Carbon nitride nanothread crystals derived from pyridine. J. Am. Chem. Soc. 2018, 140, 4969-4972. Copyright 2018 American Chemical Society.

Tables 1 and 2 of this work were adapted with permission from Xu, E.S.; Lammert, P.E.; Crespi, V.H. Systematic enumeration of sp3 nanothreads. Nano Lett. 2015, 15, 5124-5130. Copyright 2015 American Chemical Society.