Publications by authors named "Kirill I Bolotin"

28 Publications

  • Page 1 of 1

Spin/Valley Coupled Dynamics of Electrons and Holes at the MoS-MoSe Interface.

Nano Lett 2021 Sep 19;21(17):7123-7130. Epub 2021 Aug 19.

Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany.

The coupled spin and valley degrees of freedom in transition metal dichalcogenides (TMDs) are considered a promising platform for information processing. Here, we use a TMD heterostructure MoS-MoSe to study optical pumping of spin/valley polarized carriers across the interface and to elucidate the mechanisms governing their subsequent relaxation. By applying time-resolved Kerr and reflectivity spectroscopies, we find that the photoexcited carriers conserve their spin for both tunneling directions across the interface. Following this, we measure dramatically different spin/valley depolarization rates for electrons and holes, ∼30 and <1 ns, respectively, and show that this difference relates to the disparity in the spin-orbit splitting in conduction and valence bands of TMDs. Our work provides insights into the spin/valley dynamics of photoexcited carriers unaffected by complex excitonic processes and establishes TMD heterostructures as generators of spin currents in spin/valleytronic devices.
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http://dx.doi.org/10.1021/acs.nanolett.1c01538DOI Listing
September 2021

The patterning toolbox FIB-o-mat: Exploiting the full potential of focused helium ions for nanofabrication.

Beilstein J Nanotechnol 2021 6;12:304-318. Epub 2021 Apr 6.

Corelab Correlative Microscopy and Spectroscopy, Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany.

Focused beams of helium ions are a powerful tool for high-fidelity machining with spatial precision below 5 nm. Achieving such a high patterning precision over large areas and for different materials in a reproducible manner, however, is not trivial. Here, we introduce the Python toolbox FIB-o-mat for automated pattern creation and optimization, providing full flexibility to accomplish demanding patterning tasks. FIB-o-mat offers high-level pattern creation, enabling high-fidelity large-area patterning and systematic variations in geometry and raster settings. It also offers low-level beam path creation, providing full control over the beam movement and including sophisticated optimization tools. Three applications showcasing the potential of He ion beam nanofabrication for two-dimensional material systems and devices using FIB-o-mat are presented.
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http://dx.doi.org/10.3762/bjnano.12.25DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8042487PMC
April 2021

Tunable Graphene Phononic Crystal.

Nano Lett 2021 Mar 23;21(5):2174-2182. Epub 2021 Feb 23.

Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany.

In the field of phononics, periodic patterning controls vibrations and thereby the flow of heat and sound in matter. Bandgaps arising in such phononic crystals (PnCs) realize low-dissipation vibrational modes and enable applications toward mechanical qubits, efficient waveguides, and state-of-the-art sensing. Here, we combine phononics and two-dimensional materials and explore tuning of PnCs via applied mechanical pressure. To this end, we fabricate the thinnest possible PnC from monolayer graphene and simulate its vibrational properties. We find a bandgap in the megahertz regime within which we localize a defect mode with a small effective mass of 0.72 ag = 0.002 m. We exploit graphene's flexibility and simulate mechanical tuning of a finite size PnC. Under electrostatic pressure up to 30 kPa, we observe an upshift in frequency of the entire phononic system by ∼350%. At the same time, the defect mode stays within the bandgap and remains localized, suggesting a high-quality, dynamically tunable mechanical system.
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http://dx.doi.org/10.1021/acs.nanolett.0c04986DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7953378PMC
March 2021

Giant Tunable Mechanical Nonlinearity in Graphene-Silicon Nitride Hybrid Resonator.

Nano Lett 2020 Jun 1;20(6):4659-4666. Epub 2020 Jun 1.

Department of Physics, Indian Institute of Technology, Kanpur UP-208016, India.

High quality factor mechanical resonators have shown great promise in the development of classical and quantum technologies. Simultaneously, progress has been made in developing controlled mechanical nonlinearity. Here, we combine these two directions of progress in a single platform consisting of coupled silicon nitride (SiNx) and graphene mechanical resonators. We show that nonlinear response can be induced on a large area SiNx resonator mode and can be efficiently controlled by coupling it to a gate-tunable, freely suspended graphene mode. The induced nonlinear response of the hybrid modes, as measured on the SiNx resonator surface is giant, with one of the highest measured Duffing constants. We observe a novel phononic frequency comb which we use as an alternate validation of the measured values, along with numerical simulations which are in overall agreement with the measurements.
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http://dx.doi.org/10.1021/acs.nanolett.0c01586DOI Listing
June 2020

Intrinsic and Extrinsic Defect-Related Excitons in TMDCs.

Nano Lett 2020 Apr 24;20(4):2544-2550. Epub 2020 Mar 24.

Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany.

We investigate the excitonic peak associated with defects and disorder in low-temperature photoluminescence of monolayer transition metal dichalcogenides (TMDCs). To uncover the intrinsic origin of defect-related (D) excitons, we study their dependence on gate voltage, excitation power, and temperature in a prototypical TMDC monolayer MoS. Our results suggest that D excitons are neutral excitons bound to ionized donor levels, likely related to sulfur vacancies, with a density of 7 × 10 cm. To study the extrinsic contribution to D excitons, we controllably deposit oxygen molecules onto the surface of MoS kept at cryogenic temperature. We find that, in addition to trivial p-doping of 3 × 10 cm, oxygen affects the D excitons, likely by functionalizing the defect sites. Combined, our results uncover the origin of D excitons, suggest an approach to track the functionalization of TMDCs, to benchmark device quality, and pave the way toward exciton engineering in hybrid organic-inorganic TMDC devices.
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http://dx.doi.org/10.1021/acs.nanolett.9b05323DOI Listing
April 2020

Selective Functionalization of Graphene at Defect-Activated Sites by Arylazocarboxylic tert-Butyl Esters.

Angew Chem Int Ed Engl 2019 Mar 25;58(11):3599-3603. Epub 2019 Jan 25.

Institut für Chemie und Biochemie, Freie Universität Berlin, Takustraße 3, 14195, Berlin, Germany.

The development of versatile functionalization concepts for graphene is currently in the focus of research. Upon oxo-functionalization of graphite, the full surface of graphene becomes accessible for C-C bond formation to introduce out-of-plane functionality. Herein, we present the arylation of graphene with arylazocarboxylic tert-butyl esters, which generates aryl radicals after activation with an acid. Surprisingly, the degree of functionalization is related to the concentration of lattice vacancy defects in the graphene material. Consequently, graphene materials that are free from lattice defects are not reactive. The reaction can be applied to graphene dispersed in solvents and leads to bitopic functionalization as well as monotopic functionalization when the graphene is deposited on surfaces. As the arylazocarboxylic tert-butyl ester moiety can be attached to various molecules, the presented method paves the way to functional graphene derivatives, with the density of defects determining the degree of functionalization.
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http://dx.doi.org/10.1002/anie.201811192DOI Listing
March 2019

Motion Transduction with Thermo-mechanically Squeezed Graphene Resonator Modes.

Nano Lett 2018 11 29;18(11):6719-6724. Epub 2018 Oct 29.

Department of Physics , Indian Institute of Technology , Kanpur , Uttar Pradesh 208016 , India.

There is a recent surge of interest in amplification and detection of tiny motion in the growing field of opto- and electromechanics. Here, we demonstrate widely tunable, broad bandwidth, and high gain all-mechanical motion amplifiers based on graphene/silicon nitride (SiNx) hybrids. In these devices, a tiny motion of a large-area SiNx membrane is transduced to a much larger motion in a graphene drum resonator coupled to SiNx. Furthermore, the thermal noise of graphene is reduced (squeezed) through parametric tension modulation. The parameters of the amplifier are measured by photothermally actuating SiNx and interferometrically detecting graphene displacement. We obtain a displacement power gain of 38 dB and demonstrate 4.7 dB of squeezing, resulting in a detection sensitivity of 3.8 [Formula: see text], close to the thermal noise limit of SiNx.
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http://dx.doi.org/10.1021/acs.nanolett.8b02293DOI Listing
November 2018

Publisher Correction: Controlled dynamic screening of excitonic complexes in 2D semiconductors.

Sci Rep 2018 Apr 12;8(1):6093. Epub 2018 Apr 12.

Department of Physics and Astronomy, Vanderbilt University, Nashville, TN-37235, USA.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.
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http://dx.doi.org/10.1038/s41598-018-23600-2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5897401PMC
April 2018

Controlled dynamic screening of excitonic complexes in 2D semiconductors.

Sci Rep 2018 01 15;8(1):768. Epub 2018 Jan 15.

Department of Physics and Astronomy, Vanderbilt University, Nashville, TN-37235, USA.

We report a combined theoretical/experimental study of dynamic screening of excitons in media with frequency-dependent dielectric functions. We develop an analytical model showing that interparticle interactions in an exciton are screened in the range of frequencies from zero to the characteristic binding energy depending on the symmetries and transition energies of that exciton. The problem of the dynamic screening is then reduced to simply solving the Schrodinger equation with an effectively frequency-independent potential. Quantitative predictions of the model are experimentally verified using a test system: neutral, charged and defect-bound excitons in two-dimensional monolayer WS, screened by metallic, liquid, and semiconducting environments. The screening-induced shifts of the excitonic peaks in photoluminescence spectra are in good agreement with our model.
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http://dx.doi.org/10.1038/s41598-017-18803-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5768700PMC
January 2018

Hidden Area and Mechanical Nonlinearities in Freestanding Graphene.

Phys Rev Lett 2017 Jun 27;118(26):266101. Epub 2017 Jun 27.

Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA.

We investigated the effect of out-of-plane crumpling on the mechanical response of graphene membranes. In our experiments, stress was applied to graphene membranes using pressurized gas while the strain state was monitored through two complementary techniques: interferometric profilometry and Raman spectroscopy. By comparing the data obtained through these two techniques, we determined the geometric hidden area which quantifies the crumpling strength. While the devices with hidden area ∼0% obeyed linear mechanics with biaxial stiffness 428±10  N/m, specimens with hidden area in the range 0.5%-1.0% were found to obey an anomalous nonlinear Hooke's law with an exponent ∼0.1.
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http://dx.doi.org/10.1103/PhysRevLett.118.266101DOI Listing
June 2017

The effect of intrinsic crumpling on the mechanics of free-standing graphene.

Nat Commun 2015 Nov 6;6:8789. Epub 2015 Nov 6.

Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA.

Free-standing graphene is inherently crumpled in the out-of-plane direction due to dynamic flexural phonons and static wrinkling. We explore the consequences of this crumpling on the effective mechanical constants of graphene. We develop a sensitive experimental approach to probe stretching of graphene membranes under low applied stress at cryogenic to room temperatures. We find that the in-plane stiffness of graphene is 20-100 N m(-1) at room temperature, much smaller than 340 N m(-1) (the value expected for flat graphene). Moreover, while the in-plane stiffness only increases moderately when the devices are cooled down to 10 K, it approaches 300 N m(-1) when the aspect ratio of graphene membranes is increased. These results indicate that softening of graphene at temperatures <400 K is caused by static wrinkling, with only a small contribution due to flexural phonons. Together, these results explain the large variation in reported mechanical constants of graphene devices and pave the way towards controlling their mechanical properties.
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http://dx.doi.org/10.1038/ncomms9789DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4667622PMC
November 2015

Hot Electron-Based Near-Infrared Photodetection Using Bilayer MoS2.

Nano Lett 2015 Nov 6;15(11):7440-4. Epub 2015 Oct 6.

Department of Electrical Engineering and Computer Science and ∥Department of Mechanical Engineering, Vanderbilt University , Nashville, Tennessee 37212, United States.

Recently, there has been much interest in the extraction of hot electrons generated from surface plasmon decay, as this process can be used to achieve additional bandwidth for both photodetectors and photovoltaics. Hot electrons are typically injected into semiconductors over a Schottky barrier between the metal and semiconductor, enabling generation of photocurrent with below bandgap photon illumination. As a two-dimensional semiconductor single and few layer molybdenum disulfide (MoS2) has been demonstrated to exhibit internal photogain and therefore becomes an attractive hot electron acceptor. Here, we investigate hot electron-based photodetection in a device consisting of bilayer MoS2 integrated with a plasmonic antenna array. We demonstrate sub-bandgap photocurrent originating from the injection of hot electrons into MoS2 as well as photoamplification that yields a photogain of 10(5). The large photogain results in a photoresponsivity of 5.2 A/W at 1070 nm, which is far above similar silicon-based hot electron photodetectors in which no photoamplification is present. This technique is expected to have potential use in future ultracompact near-infrared photodetection and optical memory devices.
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http://dx.doi.org/10.1021/acs.nanolett.5b02866DOI Listing
November 2015

Electrical Control of near-Field Energy Transfer between Quantum Dots and Two-Dimensional Semiconductors.

Nano Lett 2015 Jul 3;15(7):4374-80. Epub 2015 Jun 3.

∇Vanderbilt Institute for Nanoscale Science and Engineering, Nashville, Tennessee 37235, United States.

We investigate near-field energy transfer between chemically synthesized quantum dots (QDs) and two-dimensional semiconductors. We fabricate devices in which electrostatically gated semiconducting monolayer molybdenum disulfide (MoS2) is placed atop a homogeneous self-assembled layer of core-shell CdSSe QDs. We demonstrate efficient nonradiative Förster resonant energy transfer (FRET) from QDs into MoS2 and prove that modest gate-induced variation in the excitonic absorption of MoS2 leads to large (∼500%) changes in the FRET rate. This in turn allows for up to ∼75% electrical modulation of QD photoluminescence intensity. The hybrid QD/MoS2 devices operate within a small voltage range, allow for continuous modification of the QD photoluminescence intensity, and can be used for selective tuning of QDs emitting in the visible-IR range.
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http://dx.doi.org/10.1021/acs.nanolett.5b00514DOI Listing
July 2015

Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers.

Nat Nanotechnol 2014 Jun 28;9(6):436-42. Epub 2014 Apr 28.

1] Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA [2] Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.

In the pursuit of ultrasmall electronic components, monolayer electronic devices have recently been fabricated using transition-metal dichalcogenides. Monolayers of these materials are semiconducting, but nanowires with stoichiometry MX (M = Mo or W, X = S or Se) have been predicted to be metallic. Such nanowires have been chemically synthesized. However, the controlled connection of individual nanowires to monolayers, an important step in creating a two-dimensional integrated circuit, has so far remained elusive. In this work, by steering a focused electron beam, we directly fabricate MX nanowires that are less than a nanometre in width and Y junctions that connect designated points within a transition-metal dichalcogenide monolayer. In situ electrical measurements demonstrate that these nanowires are metallic, so they may serve as interconnects in future flexible nanocircuits fabricated entirely from the same monolayer. Sequential atom-resolved Z-contrast images reveal that the nanowires rotate and flex continuously under momentum transfer from the electron beam, while maintaining their structural integrity. They therefore exhibit self-adaptive connections to the monolayer from which they are sculpted. We find that the nanowires remain conductive while undergoing severe mechanical deformations, thus showing promise for mechanically robust flexible electronics. Density functional theory calculations further confirm the metallicity of the nanowires and account for their beam-induced mechanical behaviour. These results show that direct patterning of one-dimensional conducting nanowires in two-dimensional semiconducting materials with nanometre precision is possible using electron-beam-based techniques.
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http://dx.doi.org/10.1038/nnano.2014.81DOI Listing
June 2014

Bandgap engineering of strained monolayer and bilayer MoS2.

Nano Lett 2013 Aug 9;13(8):3626-30. Epub 2013 Jul 9.

Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, United States.

We report the influence of uniaxial tensile mechanical strain in the range 0-2.2% on the phonon spectra and bandstructures of monolayer and bilayer molybdenum disulfide (MoS2) two-dimensional crystals. First, we employ Raman spectroscopy to observe phonon softening with increased strain, breaking the degeneracy in the E' Raman mode of MoS2, and extract a Grüneisen parameter of ~1.06. Second, using photoluminescence spectroscopy we measure a decrease in the optical band gap of MoS2 that is approximately linear with strain, ~45 meV/% strain for monolayer MoS2 and ~120 meV/% strain for bilayer MoS2. Third, we observe a pronounced strain-induced decrease in the photoluminescence intensity of monolayer MoS2 that is indicative of the direct-to-indirect transition of the character of the optical band gap of this material at applied strain of ~1%. These observations constitute a demonstration of strain engineering the band structure in the emergent class of two-dimensional crystals, transition-metal dichalcogenides.
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http://dx.doi.org/10.1021/nl4014748DOI Listing
August 2013

Three-dimensional graphene foams promote osteogenic differentiation of human mesenchymal stem cells.

Nanoscale 2013 May;5(10):4171-6

Department of Biomedical Engineering, Vanderbilt University, VU Station B #351631, 2301 Vanderbilt Place, Nashville, TN 37235, USA.

Graphene is a novel material whose application in biomedical sciences has only begun to be realized. In the present study, we have employed three-dimensional graphene foams as culture substrates for human mesenchymal stem cells and provide evidence that these materials can maintain stem cell viability and promote osteogenic differentiation.
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http://dx.doi.org/10.1039/c3nr00803gDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3672224PMC
May 2013

Photosystem I on graphene as a highly transparent, photoactive electrode.

Langmuir 2013 Apr 22;29(13):4177-80. Epub 2013 Mar 22.

Interdisciplinary Materials Science Graduate Program, Vanderbilt University, Nashville, Tennessee 37235, United States.

We report the fabrication of a hybrid light-harvesting electrode consisting of photosystem I (PSI) proteins extracted from spinach and adsorbed as a monolayer onto electrically contacted, large-area graphene. The transparency of graphene supports the choice of an opaque mediator at elevated concentrations. For example, we report a photocurrent of 550 nA/cm(2) from a monolayer of PSI on graphene in the presence of 20 mM methylene blue, which yields an opaque blue solution. The PSI-modified graphene electrode has a total thickness of less than 10 nm and demonstrates photoactivity that is an order of magnitude larger than that for unmodified graphene, establishing the feasibility of conjoining these nanomaterials as potential constructs in next-generation photovoltaic devices.
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http://dx.doi.org/10.1021/la305020cDOI Listing
April 2013

High-field electrical and thermal transport in suspended graphene.

Nano Lett 2013 Oct 20;13(10):4581-6. Epub 2013 Feb 20.

Micro and Nanotechnology Lab and ‡Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign , Illinois 61801, United States.

We study the intrinsic transport properties of suspended graphene devices at high fields (≥1 V/μm) and high temperatures (≥1000 K). Across 15 samples, we find peak (average) saturation velocity of 3.6 × 10(7) cm/s (1.7 × 10(7) cm/s) and peak (average) thermal conductivity of 530 W m(-1) K(-1) (310 W m(-1) K(-1)) at 1000 K. The saturation velocity is 2-4 times and the thermal conductivity 10-17 times greater than in silicon at such elevated temperatures. However, the thermal conductivity shows a steeper decrease at high temperature than in graphite, consistent with stronger effects of second-order three-phonon scattering. Our analysis of sample-to-sample variation suggests the behavior of "cleaner" devices most closely approaches the intrinsic high-field properties of graphene. This study reveals key features of charge and heat flow in graphene up to device breakdown at ~2230 K in vacuum, highlighting remaining unknowns under extreme operating conditions.
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http://dx.doi.org/10.1021/nl400197wDOI Listing
October 2013

Using Voronoi tessellations to assess nanoparticle-nanoparticle interactions and ordering in monolayer films formed through electrophoretic deposition.

J Phys Chem B 2013 Feb 5;117(6):1664-9. Epub 2012 Sep 5.

Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA.

Monolayers of iron oxide nanoparticles of two different sizes, 9.6 nm and 16.5 nm, were fabricated through electrophoretic deposition. The arrangements of nanoparticles within the films were analyzed using the technique of Voronoi tessellations. These analyses indicated that the films possessed equivalent degrees of ordering, and that the films were uniform over centimeter length scales. Precise measurements of the interparticle spacing were obtained, and the magnitudes of magnetic dipole interactions were calculated. The dipole-dipole interaction among the larger nanoparticles was 14 times larger than that of the smaller nanoparticles, indicating that magnetic coupling interactions could not have been the lone source of ordering in the system.
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http://dx.doi.org/10.1021/jp305958wDOI Listing
February 2013

Probing charge scattering mechanisms in suspended graphene by varying its dielectric environment.

Nat Commun 2012 Mar 13;3:734. Epub 2012 Mar 13.

Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA.

Graphene with high carrier mobility μ is required both for graphene-based electronic devices and for the investigation of the fundamental properties of Dirac fermions. An attractive approach to increase the mobility is to place graphene in an environment with high static dielectric constant κ that would screen the electric field due to the charged impurities present near graphene's surface. Here we investigate the effect of the dielectric environment of graphene and study electrical transport in multi-terminal graphene devices suspended in liquids with κ ranging from 1.9 to 33. For non-polar liquids (κ<5), we observe a rapid increase of μ(κ), with room-temperature mobility reaching ~60,000 cm(2) Vs(-1) for devices in anisole (κ = 4.3). We associate this trend with dielectric screening of charged impurities adsorbed on graphene. We observe much lower mobility μ~20,000 cm(2) Vs(-1) for devices in polar liquids (κ ≥ 18) and explain it by additional scattering caused by ions present in such liquids.
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http://dx.doi.org/10.1038/ncomms1740DOI Listing
March 2012

Graphene: corrosion-inhibiting coating.

ACS Nano 2012 Feb 10;6(2):1102-8. Epub 2012 Feb 10.

Interdisciplinary Graduate Program in Materials Science, Vanderbilt University, Nashville, Tennessee 37235, United States.

We report the use of atomically thin layers of graphene as a protective coating that inhibits corrosion of underlying metals. Here, we employ electrochemical methods to study the corrosion inhibition of copper and nickel by either growing graphene on these metals, or by mechanically transferring multilayer graphene onto them. Cyclic voltammetry measurements reveal that the graphene coating effectively suppresses metal oxidation and oxygen reduction. Electrochemical impedance spectroscopy measurements suggest that while graphene itself is not damaged, the metal under it is corroded at cracks in the graphene film. Finally, we use Tafel analysis to quantify the corrosion rates of samples with and without graphene coatings. These results indicate that copper films coated with graphene grown via chemical vapor deposition are corroded 7 times slower in an aerated Na(2)SO(4) solution as compared to the corrosion rate of bare copper. Tafel analysis reveals that nickel with a multilayer graphene film grown on it corrodes 20 times slower while nickel surfaces coated with four layers of mechanically transferred graphene corrode 4 times slower than bare nickel. These findings establish graphene as the thinnest known corrosion-protecting coating.
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http://dx.doi.org/10.1021/nn203507yDOI Listing
February 2012

Graphene bimetallic-like cantilevers: probing graphene/substrate interactions.

Nano Lett 2011 Nov 24;11(11):4748-52. Epub 2011 Oct 24.

Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, United States.

The remarkable mechanical properties of graphene, the thinnest, lightest, and strongest material in existence, are desirable in applications ranging from composite materials to sensors and actuators. Here, we demonstrate that these mechanical properties are strongly affected by the interaction with the substrate onto which graphene is deposited. By measuring the temperature-dependent deflection of graphene/substrate "bimetallic" cantilevers we determine strain, thermal expansion coefficient, and the adhesion force acting on graphene films attached to a substrate. Graphene deposited on silicon nitride (SiN(x)) is under much larger strain, ε(g) ∼ 1.5 × 10(-2), compared to graphene on gold (Au), ε(g) < 10(-3). The thermal expansion coefficient α(g) of graphene attached to SiN(x) is found to be negative, in the range from (- 5... - 1) × 10(-6)K(-1) and smaller in magnitude than α(g) of suspended graphene. We also estimate the interfacial shear strength of the graphene/SiN(x) interface to be ∼1 GPa at room temperature.
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http://dx.doi.org/10.1021/nl202562uDOI Listing
November 2011

Performance of monolayer graphene nanomechanical resonators with electrical readout.

Nat Nanotechnol 2009 Dec 20;4(12):861-7. Epub 2009 Sep 20.

Department of Mechanical Engineering, Columbia University, New York, NY 10027, USA.

The enormous stiffness and low density of graphene make it an ideal material for nanoelectromechanical applications. Here, we demonstrate the fabrication and electrical readout of monolayer graphene resonators, and test their response to changes in mass and temperature. The devices show resonances in the megahertz range, and the strong dependence of resonant frequency on applied gate voltage can be fitted to a membrane model to yield the mass density and built-in strain of the graphene. Following the removal and addition of mass, changes in both density and strain are observed, indicating that adsorbates impart tension to the graphene. On cooling, the frequency increases, and the shift rate can be used to measure the unusual negative thermal expansion coefficient of graphene. The quality factor increases with decreasing temperature, reaching approximately 1 x 10(4) at 5 K. By establishing many of the basic attributes of monolayer graphene resonators, the groundwork for applications of these devices, including high-sensitivity mass detectors, is put in place.
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http://dx.doi.org/10.1038/nnano.2009.267DOI Listing
December 2009

Observation of the fractional quantum Hall effect in graphene.

Nature 2009 Nov 1;462(7270):196-9. Epub 2009 Nov 1.

Department of Physics, Columbia University, New York, New York 10027, USA.

When electrons are confined in two dimensions and subject to strong magnetic fields, the Coulomb interactions between them can become very strong, leading to the formation of correlated states of matter, such as the fractional quantum Hall liquid. In this strong quantum regime, electrons and magnetic flux quanta bind to form complex composite quasiparticles with fractional electronic charge; these are manifest in transport measurements of the Hall conductivity as rational fractions of the elementary conductance quantum. The experimental discovery of an anomalous integer quantum Hall effect in graphene has enabled the study of a correlated two-dimensional electronic system, in which the interacting electrons behave like massless chiral fermions. However, owing to the prevailing disorder, graphene has so far exhibited only weak signatures of correlated electron phenomena, despite intense experimental and theoretical efforts. Here we report the observation of the fractional quantum Hall effect in ultraclean, suspended graphene. In addition, we show that at low carrier density graphene becomes an insulator with a magnetic-field-tunable energy gap. These newly discovered quantum states offer the opportunity to study correlated Dirac fermions in graphene in the presence of large magnetic fields.
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http://dx.doi.org/10.1038/nature08582DOI Listing
November 2009

Measurement of discrete energy-level spectra in individual chemically synthesized gold nanoparticles.

Nano Lett 2008 Dec;8(12):4506-12

Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, New York 14853, USA.

We form single-electron transistors from individual chemically synthesized gold nanoparticles, 5-15 nm in diameter, with monolayers of organic molecules serving as tunnel barriers. These devices allow us to measure the discrete electronic energy levels of individual gold nanoparticles that are, by virtue of chemical synthesis, well-defined in their composition, size and shape. We show that the nanoparticles are nonmagnetic and have spectra in good accord with random-matrix-theory predictions taking into account strong spin-orbit coupling.
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http://dx.doi.org/10.1021/nl802473nDOI Listing
December 2008

Imaging electromigration during the formation of break junctions.

Nano Lett 2007 Mar 17;7(3):652-6. Epub 2007 Feb 17.

Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, New York 14853, USA.

Using a scanning electron microscope, we make real-time movies of gold nanowires during the process of electromigration. We confirm the importance of using a small series resistance when employing electromigration to make controlled nanometer-scale gaps suitable for molecular-electronics studies. We are also able to estimate the effective temperature experienced by molecular adsorbates on the nanowire during the electromigration process.
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http://dx.doi.org/10.1021/nl062631iDOI Listing
March 2007

Anisotropic magnetoresistance and anisotropic tunneling magnetoresistance due to quantum interference in ferromagnetic metal break junctions.

Phys Rev Lett 2006 Sep 20;97(12):127202. Epub 2006 Sep 20.

Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY 14853, USA.

We measure the low-temperature resistance of permalloy break junctions as a function of contact size and the magnetic field angle in applied fields large enough to saturate the magnetization. For both nanometer-scale metallic contacts and tunneling devices we observe large changes in resistance with the angle, as large as 25% in the tunneling regime. The pattern of magnetoresistance is sensitive to changes in bias on a scale of a few mV. We interpret the effect as a consequence of conductance fluctuations due to quantum interference.
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http://dx.doi.org/10.1103/PhysRevLett.97.127202DOI Listing
September 2006

From ballistic transport to tunneling in electromigrated ferromagnetic breakjunctions.

Nano Lett 2006 Jan;6(1):123-7

Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, New York 14853, USA.

We fabricate ferromagnetic nanowires with constrictions whose cross section can be reduced gradually from 100 x 30 nm(2) to the atomic scale and eventually to the tunneling regime by means of electromigration. The contacts are mechanically and thermally stable. We measure low-temperature magnetoresistances (MR) < 3% for contacts < 400 Omega, reproducible MR variations that are nonmonotonic in the regime 400 Omega - 25 kOmega, and a maximum MR of 80% for atomic-scale widths. These results for devices > 400 Omega differ from previous room-temperature studies of electrodeposited devices. For samples in the tunneling regime, we observe large fluctuations in MR, between -10 and 85%.
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http://dx.doi.org/10.1021/nl0522936DOI Listing
January 2006
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