Showing papers by "Mildred S. Dresselhaus published in 2011"
TL;DR: In this article, the power of Raman spectroscopy as a probe and a characterization tool for sp2 carbon materials is discussed, with particular emphasis given to the field of photophysics.
Abstract: This paper reviews progress that has been made in the use of Raman spectroscopy to study graphene and carbon nanotubes These are two nanostructured forms of sp2 carbon materials that are of major current interest These nanostructured materials have attracted particular attention because of their simplicity, small physical size and the exciting new science they have introduced This review focuses on each of these materials systems individually and comparatively as prototype examples of nanostructured materials In particular, this paper discusses the power of Raman spectroscopy as a probe and a characterization tool for sp2 carbon materials, with particular emphasis given to the field of photophysics Some coverage is also given to the close relatives of these sp2 carbon materials, namely graphite, a three-dimensional (3D) material based on the AB stacking of individual graphene layers, and carbon nanoribbons, which are one-dimensional (1D) planar structures, where the width of the ribbon is on the nano
780 citations
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21 Feb 2011TL;DR: In this article, a detailed analysis of Raman Spectroscopy in Graphene Releated Systems is presented, including the G-band and time-independent perturbations.
Abstract: Part I Materials Science and Raman Spectroscopy Background The sp^2 Nanocarbons: Prototypes for Nanoscience and Nanotechnology Electrons in sp^2 Nanocarbons Vibrations in sp^2 Nanocarbons Raman Spectroscopy: From Graphite to sp^2 Nanocarbons Quantum Description of Raman Scattering Symmetry Aspects and Selection Rules: Group Theory Part II Detailed Analysis of Raman Spectroscopy in Graphene Releated Systems The G-band and time-Independent Perturbations The G-band and the Time-Dependent Perturbations Resonance Raman Scattering: Experimental Observations of the Radial Breathing Mode Theory of Excitons in Carbon Nanotubes Tight-Binding Method for Calculating Raman Spectra Dispersive G'-band and Higher-Order Processes:the Double Resonance Process Disorder Effects in the Raman Spectra of sp^2 Carbons Summary of Raman on sp^2 Nanocarbons
471 citations
TL;DR: The concept of modulation doping in three-dimensional nanostructured bulk materials to increase the thermoelectric figure of merit is introduced via experiment using composites made of doped silicon nanograins and intrinsic silicon germanium grains.
Abstract: We introduce the concept of modulation doping in three-dimensional nanostructured bulk materials to increase the thermoelectric figure of merit. Modulation-doped samples are made of two types of nanograins (a two-phase composite), where dopants are incorporated only into one type. By band engineering, charge carriers could be separated from their parent grains and moved into undoped grains, which would result in enhanced mobility of the carriers in comparison to uniform doping due to a reduction of ionized impurity scattering. The electrical conductivity of the two-phase composite can exceed that of the individual components, leading to a higher power factor. We here demonstrate the concept via experiment using composites made of doped silicon nanograins and intrinsic silicon germanium grains.
459 citations
TL;DR: This work introduces the first experimental technique which can measure MFP distributions over a wide range of length scales and materials and obtains good agreement with first-principles calculations.
Abstract: Size effects in heat conduction, which occur when phonon mean free paths (MFPs) are comparable to characteristic lengths, are being extensively explored in many nanoscale systems for energy applications. Knowledge of MFPs is essential to understanding size effects, yet MFPs are largely unknown for most materials. Here, we introduce the first experimental technique which can measure MFP distributions over a wide range of length scales and materials. Using this technique, we measure the MFP distribution of silicon for the first time and obtain good agreement with first-principles calculations.
454 citations
TL;DR: A significant reduction in the phonon lifetime of the in-plane vibrational modes is probably due to an enhanced electron–phonon coupling in the few quintuple layer regime.
Abstract: We report on Raman spectroscopy of few quintuple layer topological insulator bismuth selenide (Bi2Se3) nanoplatelets (NPs), synthesized by a polyol method. The as-grown NPs exhibit excellent crystalline quality, hexagonal or truncated trigonal morphology, and uniformly flat surfaces down to a few quintuple layers. Both Stokes and anti-Stokes Raman spectroscopy for the first time resolve all four optical phonon modes from individual NPs down to 4 nm, where the out-of-plane vibrational A(1g)(1) mode shows a few wavenumbers red shift as the thickness decreases below ~15 nm. This thickness-dependent red shift is tentatively explained by a phonon softening due to the decreasing of the effective restoring force arising from a decrease of the van der Waals forces between adjacent layers. Quantitatively, we found that the 2D phonon confinement model proposed by Faucet and Campbell cannot explain the red shift values and the line shape of the A(1g)(1) mode, which can be described better by a Breit–Wigner–Fano resonance line shape. Considerable broadening (~17 cm(–1) for six quintuple layers) especially for the in-plane vibrational mode E(g)(2) is identified, suggesting that the layer-to-layer stacking affects the intralayer bonding. Therefore, a significant reduction in the phonon lifetime of the in-plane vibrational modes is probably due to an enhanced electron–phonon coupling in the few quintuple layer regime.
417 citations
TL;DR: Considering the short history of graphene edge research, the progress has been impressive, and many further advances in this field are anticipated.
Abstract: The current status of graphene edge fabrication and characterization is reviewed in detail. We first compare different fabrication methods, including the chemical vapor deposition method, various ways of unzipping carbon nanotubes, and lithographic methods. We then summarize the different edge/ribbon structures that have been produced experimentally or predicted theoretically. We discuss different characterization tools, such as transmission electron microscopy and Raman spectroscopy, that are currently used for evaluating the edge quality as well as the atomic structures. Finally, a detailed discussion of defective and folded edges is also presented. Considering the short history of graphene edge research, the progress has been impressive, and many further advances in this field are anticipated.
396 citations
01 Aug 2011
TL;DR: In this paper, the authors proposed a new approach for energy efficiency in renewable energy systems, which is supported by the U.S. Dept. of Energy (DE-SC0001299/DE-FG02-09ER46577).
Abstract: United States. Dept. of Energy. Office of Basic Energy Sciences (Grant No. DE-SC0001299/DE-FG02-09ER46577)
332 citations
TL;DR: A systematic experimental and theoretical analysis of the Raman modes in bothBernal (ABA stacking order) and rhombohedral trilayer graphene (3LG) shows that the G band, G' (2D) band, and the intermediate-frequency combination modes of 3LGs are sensitive to the stacking order of 3LG.
Abstract: Bernal (ABA stacking order) and rhombohedral (ABC) trilayer graphene (3LG) are characterized by Raman spectroscopy. From a systematic experimental and theoretical analysis of the Raman modes in both of these 3LGs, we show that the G band, G' (2D) band, and the intermediate-frequency combination modes of 3LGs are sensitive to the stacking order of 3LG. The phonon wavevector q, that gives the double resonance Raman spectra is larger in ABC than ABA, which is the reason why we get the different Raman frequencies and their spectral widths for ABA and ABC 3LG. The weak electron-phonon interaction in ABC-stacked 3LG and the localized strain at the boundary between ABC- and ABA-stacked domains are clearly reflected by the softening of the G mode and the G' mode, respectively.
197 citations
TL;DR: It is found that the appearance of the M band strictly depends on the AB stacking, which could be used as a fingerprint for AB-stacked bilayer graphene, and provides another effective way to probe phonon dispersion, electron-phonon coupling, and to exploit the electronic band structure of graphene layers.
Abstract: Though graphene has been intensively studied by Raman spectroscopy, in this letter, we report a study of the second-order overtone and combination Raman modes in a mostly unexplored frequency range of 1690-2150 cm(-1) in nonsuspended commensurate (AB-stacked), incommensurate (folded) and suspended graphene layers. On the basis of the double resonance theory, four dominant modes in this range have been assigned to (i) the second order out-of-plane transverse mode (2oTO or M band), (ii) the combinational modes of in-plane transverse acoustic mode and longitudinal optical mode (iTA+LO), (iii) in-plane transverse optical mode and longitudinal acoustic mode (iTO+LA), and (iv) longitudinal optical mode and longitudinal acoustic mode (LO+LA). Differing from AB-stacked bilayer graphene or few layer graphene, single layer graphene shows the disappearance of the M band. Systematic analysis reveals that interlayer interaction is essential for the presence (or absence) of the M band, whereas the substrate has no effect on the presence (or absence) of the M band. Dispersive behaviors of these "new" Raman modes in graphene have been probed by laser excitation energy-dependent Raman spectroscopy. It is found that the appearance of the M band strictly depends on the AB stacking, which could be used as a fingerprint for AB-stacked bilayer graphene. This work expands upon the unique and powerful abilities of Raman spectroscopy to study graphene and provides another effective way to probe phonon dispersion, electron-phonon coupling, and to exploit the electronic band structure of graphene layers.
154 citations
TL;DR: It is shown that the bottom layer of the bilayer graphene is significantly doped from the substrate, while the top layer does not exhibit a signature of the doping from the environment.
Abstract: Bilayer graphene was prepared by the subsequent deposition of a 13C single-layer graphene and a 12C single-layer graphene on top of a SiO2/Si substrate. The bilayer graphene thus prepared was studied using Raman spectroscopy and in situ Raman spectroelectrochemistry. The Raman frequencies of the 13C graphene bands are significantly shifted with respect to those of 12C graphene, which allows us to investigate the single layer components of bilayer graphene individually. It is shown that the bottom layer of the bilayer graphene is significantly doped from the substrate, while the top layer does not exhibit a signature of the doping from the environment. The electrochemical doping has the same effect on the charge carrier concentration at the top and the bottom layer despite the top layer being the only layer in contact with the electrolyte. This is here demonstrated by essentially the same frequency shifts of the G and G′ bands as a function of the electrode potential for both the top and bottom layers. Nev...
103 citations
TL;DR: Observations suggest that the Raman enhancement in GERS occurs through a charge-transfer (CT) enhancement mechanism and the CT process can be modulated by the graphene EFE.
Abstract: The modulation of charger-transfer (CT) enhancement in graphene-enhanced Raman scattering (GERS) by an electric field under different atmospheres is reported. The GERS spectra of cobalt phthalocyanine (CoPc) molecules were collected by in situ Raman measurements under ambient air, vacuum, NH3 atmosphere, and O2 atmosphere, in which the Fermi level of graphene was modulated by an electrical field effect (EFE). The Raman scattering intensities of adsorbed molecules can be tuned to be stronger or weaker as the graphene Fermi level down-shifts or up-shifts under electrical field modulation. However, the Raman intensity modulation in GERS is seriously influenced by the hysteresis effect in graphene EFE, which makes the modulation ability small and shows strong gate voltage sweep rate dependence in ambient air. Fortunately, the hysteresis effect in graphene EFE can be decreased by performing the measurement under vacuum conditions, and thus the Raman modulation ability in GERS can be increased. Furthermore, compared with the vacuum condition, the Raman modulation ability shows an increase under an NH3 atmosphere, while it shows a decrease under an O2 atmosphere, which is due to the different Fermi level modulation region in different atmospheres. More interestingly, this Raman intensity modulation in GERS shows a hysteresis-like behavior that is the same as the graphene Fermi level modulation under the EFE in a different atmosphere. All these observations suggest that the Raman enhancement in GERS occurs through a charge-transfer (CT) enhancement mechanism and the CT process can be modulated by the graphene EFE. This technique will benefit the study of the basic properties of both graphene and chemical enhancement mechanism in surface-enhanced Raman spectroscopy (SERS).
University of Trento1, Sharif University of Technology2, Karlsruhe Institute of Technology3, University of Science and Technology of China4, Budapest University of Technology and Economics5, Dresden University of Technology6, University of Vienna7, London Centre for Nanotechnology8, University College London9, University of Camerino10, Massachusetts Institute of Technology11
TL;DR: In this paper, angle-resolved photoemission spectroscopy was used to show that a new energy band appears in the electronic structure of electron-doped hydrogenated monolayer graphene (H-graphene).
Abstract: We show with angle-resolved photoemission spectroscopy that a new energy band appears in the electronic structure of electron-doped hydrogenated monolayer graphene (H-graphene). Its occupation can be controlled with the hydrogen amount and allows for tuning of graphene's doping level. Our calculations of the electronic structure of H-graphene suggest that this state is largely composed of hydrogen 1s orbitals and remains extended for low H coverages despite the random chemisorption of H. Further evidence for the existence of a hydrogen state is provided by x-ray absorption studies of undoped H-graphene which are clearly showing the emergence of an additional state in the vicinity of the pi* resonance.
TL;DR: In this article, the electronic contribution to the Raman spectra of individual metallic single-walled carbon nanotubes (MSWNTs) has been investigated, where electron-hole pairs are inelastically scattered across the graphenelike linear electronic subbands of these one-dimensional quasimetals.
Abstract: We present experimental measurements of the electronic contribution to the Raman spectra of individual metallic single-walled carbon nanotubes (MSWNTs). Photoexcited carriers are inelastically scattered by a continuum of low-energy electron-hole pairs created across the graphenelike linear electronic subbands of the MSWNTs. The optical resonances in MSWNTs give rise to well-defined electronic Raman peaks. This resonant electronic Raman scattering is a unique feature of the electronic structure of these one-dimensional quasimetals.
TL;DR: The results suggest that the nanostructures of Kondo insulators can be designed for high performance thermoelectric cooling devices at low temperatures.
Abstract: We predict a large thermoelectric figure-of-merit in Kondo insulator nanowires at low temperatures. The high ZT values are due to the Kondo effect for electrons and boundary scattering on phonons. We simulated the electron properties of the bulk Kondo insulators within the framework of dynamical mean field theory and found that electrons have short mean free path. In nanowire structures, electron transport is hardly affected by the boundary scattering due to their small intrinsic mean free paths while phonons are strongly scattered due to classical size effect. The results suggest that the nanostructures of Kondo insulators can be designed for high performance thermoelectric cooling devices at low temperatures.
01 Oct 2011
TL;DR: Experimental measurements of the electronic contribution to the Raman spectra of individual metallic single-walled carbon nanotubes (MSWNTs) show resonant electronic Raman scattering is a unique feature of the Electronic structure of these one-dimensional quasimetals.
Abstract: We present experimental measurements of the electronic contribution to the Raman spectra of individual metallic single-walled carbon nanotubes (MSWNTs). Photoexcited carriers are inelastically scattered by a continuum of low-energy electron-hole pairs created across the graphenelike linear electronic subbands of the MSWNTs. The optical resonances in MSWNTs give rise to well-defined electronic Raman peaks. This resonant electronic Raman scattering is a unique feature of the electronic structure of these one-dimensional quasimetals.
TL;DR: The chirality-dependent transport mechanisms in peapod- and chemical vapor deposition-grown double-walled carbon nanotubes (DWNTs) films are discussed by identifying the chiralities of the inner and the outer tubes using fast Fourier transform image processing, as well as optical studies.
Abstract: A fundamental understanding of the electrical properties of carbon nanotubes is vital when fabricating high-performance polymeric composites as well as transparent conductive films. Herein, the chirality-dependent transport mechanisms in peapod- and chemical vapor deposition-grown double-walled carbon nanotubes (DWNTs) films are discussed by identifying the chiralities of the inner and the outer tubes using fast Fourier transform image processing, as well as optical studies (e.g., Raman, UV, and photoluminescence spectroscopies). The observed conduction mechanisms are strongly dependent on the total fraction of the metallic inner and outer tubes within the DWNT samples. Furthermore, the contribution of the inner tubes to the electronic transport properties of DWNT films is confirmed by photochemically deactivating the outer tubes in both types of DWNT samples.
TL;DR: An iterative-two-dimensional- two-band model is developed to get a consistent inverse-effective-mass-tensor and band gap that can be used in a general two-dimensional system that has a nonparabolic dispersion relation as in the Bi(1-x)Sb(x) thin film system.
Abstract: The electronic band structures of Bi${}_{1-x}$Sb${}_{x}$ thin films can be varied as a function of temperature, pressure, stoichiometry, film thickness and growth orientation. We here show how different anisotropic single-Dirac-cones can be constructed in a Bi${}_{1-x}$Sb${}_{x}$ thin film for different applications or research purposes. For predicting anisotropic single-Dirac-cones, we have developed an iterative-two-dimensional-two-band model to get a consistent inverse-effective-mass-tensor and band-gap, which can be used in a general two-dimensional system that has a non-parabolic dispersion relation as in a Bi${}_{1-x}$Sb${}_{x}$ thin film system.
TL;DR: In this paper, a method of dispersing strongly bundled double-walled carbon nanotubes via a homogeneous coating of mussel protein in an aqueous solution is presented.
Abstract: A method of dispersing strongly bundled double-walled carbon nanotubes (DWNTs) via a homogeneous coating of mussel protein in an aqueous solution is presented. Optical activity, mechanical strength, as well as electrical conductivity coming from the nanotubes and the versatile biological activity from the mussel protein make mussel-coated DWNTs promising as a multifunctional scaffold and for anti-fouling materials.
TL;DR: The thermal treatment of DW NTs encapsulating fullerenes is expected to result in high-quality TWNTs, similar to the growth of DWNTs from the coalescence of fullererenes, but both the quality and morphology of TW NTs strongly depend on their growth conditions, such as the precursor that is used and the fi lling ratio of the fulle Renes.
Abstract: Carbon nanotubes exhibit unique physicochemical properties and electronic structure according to their chirality and the number of layers. [ 1–3 ] Triple-walled carbon nanotubes (TWNTs) consisting of three concentric graphene nanotubes provide many opportunities for systematic studies of carbon nanotube systems by comparing their behaviors with that of single-, double-, and multiwalled carbon nanotubes (SWNTs, DWNTs, and MWNTs, respectively). Their versatile chirality confi gurations, due to their three respective components, each of which can be either semiconducting or metallic, will allow TWNTs to have intrinsically unique physics that is not observed in SWNTs or DWNTs. Very recently, several attempts at synthesizing TWNTs have been reported by chemical vapor deposition (CVD), [ 4–7 ] thermal treatment of DWNTs encapsulating fullerenes, [ 8 ] and thermal decomposition of DWNTs encapsulating ferrocene [ 9 ] . Among these methods, the thermal treatment of DWNTs encapsulating fullerenes is expected to result in high-quality TWNTs, similar to the growth of DWNTs from the coalescence of fullerenes. [ 9–13 ] However, both the quality and morphology of TWNTs strongly depend on their growth conditions, such as the precursor that is used and the fi lling ratio of the fullerenes, the annealing temperature, and
TL;DR: In this article, the effects of halogen dopants (bromine and iodine) at different concentrations on the higher frequency modes (the so-called G and G′ bands) of the Raman spectra of double-wall carbon nanotube (DWCNT) “buckypaper” are investigated.
TL;DR: A personal review of 50 years of my research experiences in working on carbon science and carbon-based nanostructures is presented in this paper, based on a talk given in Warsaw, Poland, on this topic, presented at a meeting of the European Materials Research Society.
Abstract: A personal review is presented on 50 years of my research experiences in working on research on carbon science and carbon-based nanostructures. The text is based on a talk given in Warsaw, Poland, on this topic, presented at a meeting of the European Materials Research Society in September 2010.
TL;DR: In this article, the influence of electron (hole) doping on the electronic structure of metallic single-walled carbon nanotubes (m-SWCNTs) is discussed by first principles calculations.
TL;DR: This work shows how to construct single-, bi- and tri-Dirac-cone materials with various amounts of wave vector anisotropy, which can be of special interest to electronic device design, quantum electrodynamics and other fields.
Abstract: We theoretically predict that a large variety of Dirac-cone materials can be constructed in Bi${}_{1-x}$Sb${}_{x}$ thin films, and we here show how to construct single-, bi- and tri- Dirac-cone materials with various amounts of wave vector anisotropy. These different types of Dirac cones can be of special interest to electronic devices design, quantum electrodynamics and other fields.
TL;DR: Dynamic in‐column energy filtering in an AC environment provides an integral new approach to perform dynamic in situ studies with aberration correction and opens up striking new opportunities for atomic scale studies of nanomaterials.
Abstract: Structural and compositional studies of nanomaterials of technological importance have been carried out using advanced electron microscopy methods, including aberration-corrected transmission electron microscopy (AC-TEM), AC-high angle annular dark field scanning TEM (AC-HAADF-STEM), AC-energy filtered TEM, electron-stimulated energy dispersive spectroscopy in the AC-(S)TEM and high-resolution TEM (HRTEM) with scanning tunneling microscopy (STM) holder The AC-EM data reveal improvements in resolution and minimization in image delocalization A JEOL 2200FS double-AC field emission gun TEM/STEM operating at 200 kV in the Nanocentre at the University of York has been used to image single metal atoms on crystalline supports in catalysts, grain boundaries in nanotwinned metals, and nanostructures of tetrapods Joule heating studies using HRTEM integrated with an STM holder reveal in situ crystallization and edge reconstruction in graphene Real-time in situ AC-HAADF-STEM studies at elevated temperatures are described Dynamic in-column energy filtering in an AC environment provides an integral new approach to perform dynamic in situ studies with aberration correction The new results presented here open up striking new opportunities for atomic scale studies of nanomaterials and indicate future development directions Microsc Res Tech, 2011 © 2010 Wiley-Liss, Inc
31 Jan 2011
TL;DR: The nitrogenated sites within N-doped MWCNTs provided additional sites for interactions that are important to disperse nanotubes in DNA solutions, supported by the larger amount of adsorbed DNA on N- doped M WCNTs, as well as by the higher binding energy established between nucleobases and the N-Doped CNTs.
Abstract: The dispersibility in a DNA solution of bundled multiwalled carbon nanotubes (MWCNTs), having different chemical functional groups on the CNT sidewall, was investigated by optical spectroscopy. We observed that the dispersibility of nitrogen (N)-doped MWCNTs was significantly higher than that of pure MWCNTs and MWCNTs synthesized in the presence of ethanol. This result is supported by the larger amount of adsorbed DNA on N-doped MWCNTs, as well as by the higher binding energy established between nucleobases and the N-doped CNTs. Pure MWCNTs are dispersed in DNA solution via van der Waals and hydrophobic interactions; in contrast, the nitrogenated sites within N-doped MWCNTs provided additional sites for interactions that are important to disperse nanotubes in DNA solutions.
TL;DR: In this article, it was shown that the monotonic component of the σ(T) dependence can be satisfactorily approximated by theoretical calculations based on the classical Fuchs - Sondheimer theory.
Abstract: The room-temperature dependences of the electrical conductivity σ, Seebeck coefficient S, Hall coefficient RH, and the thermoelectric power factor P on the thickness (d=10–300 nm) of the thin films grown on mica substrates by thermal evaporation in vacuum of Bi-Sb solid solutions crystals with 4.5 at.% Sb were obtained. It was established that an increase in d up to ~ 200 nm leads to a change in kinetic coefficients and that in the thickness dependences of the thermoelectric properties, quantum oscillations were observed. It was shown that the monotonic component of the σ(T) dependence can be satisfactorily approximated by theoretical calculations based on the classical Fuchs - Sondheimer theory. The theoretically estimated period of oscillations is in a good agreement with the experimentally observed period.