Nanotechnology February 2012 2011 November February

Briefings tm Nanotechnology february 2012 2011 November february www.ScientificAmerican.com

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Microengineering with Swirls 2 CSI: Nanoparticles 2 A Trap for Ions and Atoms 4 Cooling to the Quantum Ground State 5 Hydrodynamics of Writing 5 Diamonds for Better Qubits 6 tm BriefingS | Nanotechnology 2

carry orbital angular moment NANOTECHNOLOGY and magnetic moment, which Briefings leads to unique interactions with Swirls move matter. Jo Verbeeck of the Uni- tiny objects versity of Antwerp and col- Scientific American Briefings: Nanotechnology leagues from Austria, the Neth- consists of summaries of recent peer-reviewed k p h oto toc articles from the scientific literature. It draws Individual cells can be ma- erlands and Canada have now these summaries from the journals of Nature nipulated by tiny vortices gener- demonstrated an electron vortex

Publishing Group, including Nature, Nature ated in fluids, rather than by the beam with a diameter of less k i / S Ka z miers

Chemistry, Nature Materials, Nature am

Nanotechnolgy and the Nature Reviews journals. potentially harmful lasers or than 1.2 Å. Ad Mariette DiChristina electric fields typically used. The Electron vortex beams were liva, semen and blood on various Senior Vice President and Editor-in-Chief, concept is the brainchild of Li first created by passing a plane objects at the crime scene. At Scientific American Zhang and his colleagues at the wave beam through a graphite present, methods and tests used Philip Yam Managing Editor, Online, Scientific American Swiss Federal Institute of Tech- film that spontaneously formed to analyze body fluids are de- nology in Zurich, who used the a spiral structure, and acted as a structive and have a low speci- John Rennie Contributing Editor, Scientific American vortices to control the movement phase plate. This was difficult to ficity. Now, Nunzianda Frascione Michael Mrak of microscopic objects. reproduce and gave limited con- and colleagues at King’s College Design Director, Scientific American trol over the resulting beam. Ver- London have shown that mag- Philip Campbell beeck and co-workers had im- netic nanoparticles conjugated Editor-in-Chief, Nature proved on this approach by cre- with specific antibodies can de- Steven Inchcoombe ating a vortex beam with a tect and identify blood and sa- Managing Director, Nature Publishing Group, and President, Scientific American holographic mask inside a trans- liva in situ on different types of mission electron microscope. substrates. Mike Florek k p h oto toc Executive Vice President, Scientific American / i S However, the effective beam di- The researchers functional- ans Bruce Brandfon ameter was several micrometers. ized magnetic nanoparticles

Vice President and Publisher, Scientific American Ev M ar k Verbeek and colleagues have with fluorescently labeled anti- Wendy Elman The team placed a tank of now reduced this beam diameter bodies that recognize specific Vice President, Digital Solutions, Scientific American water in a rotating magnetic to atomic dimensions by placing components of red blood cells, Michael Voss Vice President and Associate Publisher, Marketing field, which triggered nickel a holographic mask in the con- white blood cells or saliva. They and Business Development, Scientific American nanowires in the tank to rotate denser plane of a state-of-the-art applied the nanoparticles to Christian Dorbandt in turn, generating microvor- microscope with double aberra- human blood or saliva that had Managing Director, Consumer Marketing, tices. The vortices trapped poly- tion correction. At 1.2 Å, the been smeared onto a glass slide. Scientific American styrene microbeads in the water. beam size is comparable to the After 30 minutes the unbound Matt Hansen Senior Production Editor, Nature Publishing Group By controlling the movement of size of the 2p orbital in a ni- nanoparticles were removed by a Kerrissa Lynch the nanowires, the authors could trogen atom (see image; left and magnet and the bound conju- Web Production Editor, Scientific American tightly control the movement of right panels show the beam and gates were visualized under a How to contact us the beads. the 2p orbital respectively, drawn fluorescent microscope. The an- For subscription correspondence, including They also successfully manip- approximately to scale). The tiny tibodies showed good specificity change of e-mail addresses: U.S. and Canada: 800-333-1199 ulated Escherichia coli bacteria vortex beam may allow atomic- and had little cross reactivity Outside the North America: +1-515-248-7684 using a pair of microspheres in resolution mapping of magnetic with other body fluids. Further- Email: [email protected] Postal address: Scientific American Briefings, Box place of the nanowires. states. more, bloodstains that were 3187, Harlan, IA 51537 —Daniel Cressey, Nature —Michael Segal, treated with the nanoparticles Annual subscription (12 issues): $19.95 (USD) For editorial comments: Nature Nanotechnology could still be used for DNA pro- ■■ Nano Lett. 10.1021/nl2032487 (2011) Email: [email protected] filing, suggesting that this www.ScientificAmerican.com ■■ Appl. Phys. Lett. doi: 10.1063/1.3662012 method could potentially save (2011) ELECTRON BEAMS money as DNA profiling would Scientific American is a trademark of only be carried out on identified Scientific American, Inc., used with permission. An atom-sized MAGNETIC NANOPARTICLES sections of the samples. The method also worked on samples vortex At the crime on substrates such as ceramic, tm Electron beams are typically paper and dark fabrics, thereby plane waves. This means that the scene increasing the likelihood of un- beam phase is identical for all DNA profiling is widely used covering important evidence at points in a plane perpendicular by forensic investigators to iden- the crime scene. to the beam direction. The phase tify an offender from just a single —Ai Lin Chun, of an electron vortex beam, on cell. However, equally valuable is Nature Nanotechnology the other hand, describes a the ability to detect and identify spiral. As a result, vortex beams traces of body fluids such as sa- ■■ Analyst doi: 10.1039/c1an15200a (2011) SUBSCRIBE >> tm BriefingS | Nanotechnology 3

NANOELECTROMECHANICAL SYSTEMS SENSORS LIQUID CRYSTALS Keeping the Photon counting Light-driven noise down color change

Any device that amplifies a The development of choles- signal inevitably adds noise, and teric liquid crystals that offer quantum mechanics prevents this dynamic color tuning of their k p h oto toc added noise being reduced below reflection across the visible On the Cover a certain value. It is possible to ap- spectrum would be of great in- Extremely small vortices of proach this quantum limit by terest for many applications in / i S J onsson Henri k swirling fluid offer a new way using superconducting devices to imaging, display technology Optical fiber. for microengineers and amplify electrical signals, but and electronics. Unfortunately, biomedical researchers to these devices are complex. Now Distributed temperature this task has so far required manipulate tiny objects such Francesco Massel and co-workers sensing can be useful for moni- the use of three distinct liquid as individual cells. See page 2. at Aalto University and the VTT toring structures such as build- crystal films, each serving the Technical Research Centre of Fin- ings and pipelines. Michael red, green or blue regions sepa- Credit: PLAINVIEW/ land have shown that nanome- Tanner and colleagues from Her- rately and being switched on or iStockphoto chanical resonators can amplify iot-Watt University in the UK off electronically. In contrast, microwave signals, and that it may and the National Institute of using light as a means of be possible to reach the quantum Standards and Technology tuning reflectivity is attractive limit with this approach. (NIST) in Colorado, USA, have because it offers remote, spa- now built a distributed fiber tial and temporal control. So Raman sensor that can simulta- far, however, reports of such neously measure absolute tem- light-driven color changes are perature at over 100 1.2-cm- dynamic and continuous; that k p h oto toc

/ i S spaced positions along a single- is, the material remains a par- mode optical fiber. Although ticular color only for as long as orres distributed fiber sensors have the controlling light is incident T arlos been around for a while, the re- on it. Quan Li and co-workers C uis L searchers claim that previous re- at Kent State University in the The Finnish team starts by search focused on using multi- USA have now synthesized two using lithography and focused ion- mode fibers and avalanche pho- light-driven chiral molecular beam etching to define a mechan- todiodes, which limited switches with very high helical ical resonator and a microwave detection wavelengths to around twisting powers that quickly cavity in a 150-nm-thick layer of 800–900 nm. In contrast, this and reversibly modulate their aluminum on a silica surface. latest work uses pulses of 1,550 structural reflection from blue When a pump signal is fed into nm light and measures the to red, via green, when irradi- this system, energy is transferred single-photon level Raman back- ated with light of ultraviolet or from the cavity to the resonator if scattered signal using supercon- visible wavelengths. Such the pump frequency is higher than ducting nanowire single-photon wavelength-selective reflection the resonance frequency of the detectors. The researchers used a provides a simple and easy way cavity, and vice versa. And if a time-of-flight approach to deter- of simultaneously achieving weak probe signal is sent into the mine the temperature profile red, green and blue reflection system when energy is being along the fiber. They recorded colors in a single light-driven transferred to the resonator, this temperature measurements at self-organized thin film. The probe can also be amplified. various positions along the fiber researchers say that this work Massel and co-workers show that with an uncertainty of less than will encourage the develop-

Scientific AmericanBriefings,Nanotechnology, Volume 1, Number 2, approximately 20 noise quanta are 3 K over a period of 1 minute. ment of light-driven chiral mo- February 2012, published monthly by Scientific American, a division of Nature America, Inc., 75 Varick Street, 9th Floor, New York, NY 10013-1917. added to the signal, and predict They hope that a 1-km-range dis- lecular switches or motors for Subscription rates: 1 year (12 issues) $19.95 (USD). that it should be possible to reach tributed sensor will be possible a range of practical Please send subscription correspondence, including change of e-mail and postal addresses to: the quantum limit of adding just in the near future. applications. Scientific American Briefings, Box 3187, Harlan, IA 51537. E-mail address for subscription inquires: half a quantum of noise. —David Pile, —James Baxter, [email protected]. E-mail address for general inquires: —Peter Rodgers, Nature Photonics Nature Photonics [email protected]. Subscription inquires: Nature Nanotechnology U.S. and Canada: 800-333-1199; other: +1-515-248-7684. ■■ Appl. Phys. Lett. doi: ■■ Adv. Mater. doi: 10.1002/ Copyright © 2012 Scientific American, a division of Nature America, Inc. ■■ Nature doi: 10.1038/nature10628 (2011) 10.1063/1.3656702 (2011) adma.201103362 (2011) All rights reserved. SUBSCRIBE >> tm BriefingS | Nanotechnology 4

a conventional microscope. With The researchers also suggest the ELECTRON MICROSCOPY the instrument, the platinum– possibility to engineer the cell cobalt nanoparticles — which such that the Fano interference Mapping are promising as a fuel-cell cata- is robust against environmental ensembles lyst but are known to degrade decoherence. The challenge now over time — were mapped at var- is for the research community to ious stages of ageing in a proton- realize such a cell in practice. exchange-membrane fuel cell. By —Oliver Graydon, mapping ensembles of nanopar- Nature Photonics ticles, the precise structure and composition of the catalyst could ■■ Phys. Rev. A doi: 10.1103/ be linked to its bulk electro- PhysRevA.83.053818 (2011) chemical performance with sta- tistical confidence. TRAPPING —Owain Vaughan, Nature Nanotechnology A marriage of

■■ Nano Lett. doi: 10.1021/nl203975u (2011) atoms and ions Scientists from the Raman SOLAR CELLS Research Institute in India and the Johannes Gutenberg Univer- Fano-enhanced sity in Germany have unveiled a device that can simultaneously k p h oto toc / i S performance trap both ions and cold atoms. be- The apparatus combines a linear tep h ens Quantum coherence tween competing energy-level radiofrequency Paul trap for ions S acom J pathways is known to help opti- with a six-laser-beam magneto- Electron microscope. mize photosynthesis and enable optical trap for atoms. Spatial Electron microscopy is rou- phenomena such as lasing overlap between the traps allows tinely used to characterize the without inversion. Researchers the investigation of ion–atom in- structure of metal nanoparticles, from Texas A&M University and teractions at low temperatures. and with the help of electron en- Princeton University in the USA The researchers cooled the atoms ergy-loss spectroscopy, chemical have now suggested that by laser Doppler cooling using maps with atomic resolution can quantum coherence could also light from an amplified home- also be obtained. A chemical map be used to enhance the perfor- made external-cavity diode laser. of a single particle can, however, mance of solar cells. One idea is They tested their design with 85Rb take hours to record. Therefore, to split degenerate excited en- atoms and 85Rb+ ions, which were acquiring a statistically signifi- ergy levels by exploiting tun- made by using two-photon ion- cant sample of a system that con- neling between two adjacent ization to strip electrons tains nanoparticles with a variety quantum dots. The two split en- from 85Rb atoms. The researchers of different compositions, such as ergy levels (known as an energy say that the scheme also lends it- a heterogeneous catalyst, is im- level doublet) both couple to the self to any species that can be practical. David Muller, Zhongyi conduction state. Fano interfer- laser-cooled in a vapor cell, and Liu and colleagues at Cornell Uni- ence between the two pathways that experiments with multiple versity, General Motors and minimizes unwanted radiative species may even be possible. Florida International University transitions and helps to maxi- —Oliver Graydon, have now shown that the im- mize the generated photocur- Nature Photonics proved electron optics of an aber- rent. An alternative approach is ration-corrected electron micro- to create an energy level doublet ■■ Appl. Phys. B doi: 10.1007/s00340-011- scope can allow hundreds of plat- from two lower-energy levels 4726-6 (2011) inum–cobalt nanoparticles to be and use Fano interference to en- chemically mapped. hance the absorption of solar en- NANOTECHNOLOGY The US team used a scanning ergy. Theoretical analysis of this transmission electron micro- scheme suggests that the photo- Size matters scope that can correct up to the current and peak power of a pho- Although multiwalled fifth-order of aberrations and al- tovoltaic cell employing fully co- carbon nanotubes show promise lows data to be collected around herent Fano interference could for delivering cancer therapy, a thousand times faster than on be improved by up to 50 percent. there have been concerns raised SUBSCRIBE >> tm BriefingS | Nanotechnology 5

Teams from Caltech in the USA array, which served as their and the University of Vienna in model of paper. Austria have now used photonics The basic hydrodynamics of to reach the quantum regime in the writing process is governed an experiment that operates at by the capillary force that makes ommons

C an environmental temperature the ink flow and by the viscous of 20 K — around 1,000 times force exerted by the substrate. higher than previous experi- Considering the balance of the ments. Jasper Chan and col- two forces, Kim et al. have suc-

/W i k ime d ia W ieser E ric leagues fabricated a silicon ceeded in reproducing both the Model of a triple-walled carbon nanotube. nanobeam cavity and placed it dynamics of the blot that is cre- into a helium cryostat at a tem- ated before the pen starts over the potential of these nano- perature of 20 K. They then fed moving, and the frontal shape tubes to cause cancer owing to light from a tunable laser into and final width of the line pro- their similarity in structure to the nanobeam via a tapered fiber duced by their simple pen. asbestos. New research by nanoprobe. By tuning the laser —Andreas Trabesinger, Nagai et al. indicates that thin wavelength to a slightly longer Nature Physics (approximately 50 nm in diam- frequency than the resonance of eter) multiwalled carbon nano- the nanobeam optical cavity, the ■■ Phys. Rev. Lett. doi: 10.1103/ tubes with a crystalline-like light can be used to perform op- PhysRevLett.107.264501 structure can pierce the mem- tically induced damping of the brane of mesothelial cells and mechanical motion, thereby SILICON NANOCRYSTALS can lead to inflammation and the cooling the oscillator to its generation of mesotheliomas in quantum ground state. The re- More is better vivo. Thick-walled (approxi- searchers say that experiments The poor optical properties of mately 150 nm) or tangled (2–20 for preparing and measuring the silicon have restricted its use in nm) multiwalled carbon nano- non-classical quantum states of photonics applications. How- tubes caused less damage and the mechanical system are now ever, computer simulations by were less carcinogenic. Thus, within reach. Holger Vach of the Ecole Poly- controlling the diameter of mul- —Oliver Graydon, technique now suggest a sur- tiwalled carbon nanotubes might Nature Photonics prising way to improve the op- reduce potential carcinogenic tical properties of hydrogenated risk in humans. ■■ Nature doi: 10.1038/nature10461 (2011) silicon nanocrystals — add more —Nicola McCarthy, silicon! Nature Reviews Cancer QUANTUM PHYSICS Vach first computed the properties of nanocrystals that con- ■■ Proc. Natl Acad. Sci. doi: 10.1073/ Let it flow tain three parallel hexagons of pnas.1110013108 (2011) It’s simple, really: the basic silicon atoms, with hydrogen principle of writing with ink is atoms bonded to the silicon OPTOMECHANICS the delivery of a liquid from a atoms in the top and bottom reservoir to an absorbent sur- hexagons. These Si18H12 nano- Ultimate cooling face, and then the spreading of crystals absorb light in the ultra- The field of optomechanics, in that liquid as the source moves violet region of the spectrum but which light is used to influence across the porous substrate. To not at any other wavelengths. the behavior of miniature me- study the hydrodynamics in Vach then computed what would chanical devices, has taken a sig- more detail, however, Jungchul happen if an extra silicon atom nificant step forward with the re- Kim and colleagues have used a was inserted into the middle of cent news that researchers have “minimal pen” made of a capil- these nanostructures. He found successfully laser-cooled a nano- lary tube, brought close to a that the resulting Si19H12 nano- mechanical oscillator to its highly hydrophilic micropillar crystals also absorbed light at quantum ground state. Although certain visible and infrared physicists are keen to explore the wavelengths. Furthermore, he quantum behavior of a mechan- found that the Si19H12 nano- ical oscillator, the effects are crystals should be more stable k p h oto toc

hidden from view at normal tem- / i S than any other known silicon peratures and can only be ac- nanocrystals. umming cessed at very low temperatures, C Vach attributes the increased

which are difficult to reach. A nt h ia stability and improved optical SUBSCRIBE >> tm BriefingS | Nanotechnology 6

properties of the Si19H12 nano- size. The researchers considered alignment, and measured hole crystals to the increased electron a device in which this quantum- mobilities about 500 times delocalization caused by the ad- dot array is sandwiched between larger than in unconfined dition of the extra silicon atom. two electrical leads and calcu- blends. As a result, they found This delocalization means that lated that the device’s conduc- that, relative to an untemplated the silicon hexagons resemble tance per quantum dot in the device of the same thickness, benzene rings in many ways. array is significantly enhanced their device absorbed only 38 Moreover, the stability of the over the single-quantum-dot percent of the light, but pro- nanocrystals, together with their case. They attribute this to the duced 80 percent of the photo- compatibility with the existing fact that some of the quantum- current. Normalized to volume, infrastructure for making silicon dot electron states become delo- the photocurrent density was chips, could lead to a variety of calized because of coherent cou- double that of the untemplated applications. pling to the leads. device. —Peter Rodgers, —David Gevaux, Further performance im- Nature Nanotechnology Nature Physics provements may result from increasing the electron mobility of ■■ Nano Lett. doi: 10.1021/nl203275n (2011) ■■ Phys. Rev. Lett. doi: 10.1103/ PCBM, and increasing the areal PhysRevLett.107.196802 (2011) fraction accounted for by the MATERIALS PHYSICS cylinders. PHOTOVOLTAICS —Michael Segal, Dots work Nature Nanotechnology Performance on together ■■ Appl. Phys. Lett. doi: 10.1063/1.3651509 (2011) When a beam of light hits a order thin metal film, some of it is ab- An intimate and random mix- sorbed and the rest is reflected. ture of two organic semiconduc- PHOTONICS However, pierce the same film tors is used in many of the best- with a regular array of holes performing organic solar cells. Diamond and, even if the hole size is These bulk heterojunction de- smaller than the wavelength of vices benefit from a large active cavities the light, more light is trans- surface area and are easy to mitted than would be expected. make. However, the disordered Researchers have now shown nature of the mixture makes it that an effect similar to this so- difficult to extract charges, called extraordinary optical which hinders power-conver-

transmission might also be seen sion efficiencies. Now Charles k p h oto toc with electrons. Black and colleagues at d ee / i S oy In the analogy put forward Brookhaven National Labora- R by L. S. Petrosyan and col- tory and State University of New leagues, the optical apertures York, Stony Brook, have im- The leading proposal for a are replaced by quantum dots — proved charge extraction from a solid-state quantum computer nanostructures that can support bulk heterojunction device by that works at room temperature discrete and localized electron reducing disorder. relies on the electronic, spin and states by virtue of their small The researchers mixed the optical characteristics of discrete hole-conducting polymer P3HT point defects in diamond. Dia- and the electron-conducting mond is a good host for spin- polymer PCBM into a templated based quantum computing be- array of vertically aligned cylin- cause it has very few nuclear spins drical pores, each with a diam- of its own, and it is able to host eter of approximately 75 nm. defects — most notably nitrogen– Previous work had demon- vacancy centers — that barely in- strated that P3HT was a better teract with the surrounding lat- hole-conductor when confined tice. By creating an array of these to nanoscale volumes because defects, it might be possible to en-

v elis individual polymer chains were code an ensemble of coupled / pa better aligned. Black and col- quantum bits in their spins. leagues confirmed that their One way of enhancing the k p h oto toc i S confined P3HT had improved coupling between qubits of such SUBSCRIBE >> tm BriefingS | Nanotechnology 7

an array is to embed them in an elastic ink shell made from an optical cavity — most obviously WAVEGUIDES aqueous triblock copolymer so- by forming a cavity out of dia- lution, which acts as a sacrificial mond itself. What’s needed is a Liquid flexibility support for the core fluid. They narrow-resonance, high-quality Integrated optical systems then print the core fluid and vis- cavity that avoids losses to require waveguides that are able coelastic fugitive ink shell simul- phonon modes and other non- to channel light along defined taneously while curing the wave- radiative channels, but this has pathways with minimal loss and guide using an ultraviolet LED. proved difficult to achieve in negligible crosstalk. Traditional The intensity of the ultraviolet diamond. techniques for fabricating light can be varied in the range Janine Riedrich-Möller and channel waveguides, such as soft of 1–19 mW cm−2, with a higher colleagues have gone some way lithography, direct lithographic curing intensity leading to lower to overcoming the difficulties by patterning or photoresist-tem- optical loss. Waveguides printed using focused ion-beam milling plated etching, are limited to in- through this technique exhibit to create one- and two-dimen- plane configurations or require low optical loss throughout the sional photonic-crystal cavities repeated developing or etching visible spectrum. The re- in suspended single-crystal dia- steps to produce multiple layers searchers say that this approach mond membranes. They can of waveguides. Jennifer Lewis offers a flexible way of pro- then tune the characteristics of and co-workers in the USA have ducing waveguide networks for the cavities to enhance the zero- now developed a technique for integration with high-band- phonon photoluminescent emis- fabricating optical waveguides width next-generation optical sion of silicon–vacancy defects in arbitrary planar and non- systems and optical sensor present in the diamond. planar configurations through a arrays. —Ed Gerstner, technique called photocurable —James Baxter, Nature Physics liquid core–fugitive shell Nature Photonics printing. The researchers first ■■ Nature Nanotech. doi: 10.1038/ encapsulate a hybrid organic–in- ■■ Adv. Mater. doi: 10.1002/ nnano.2011.190 (2011) organic fluid within a visco- adma.201102411 (2011)

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