Showing papers by "Mildred S. Dresselhaus published in 2012"
TL;DR: The results suggest that the growth here occurs via surface-mediated growth, which is similar to graphene growth on Cu under low pressure, which are particularly attractive for use as atomic membranes or dielectric layers/substrates for graphene devices.
Abstract: Hexagonal boron nitride (h-BN) is very attractive for many applications, particularly, as protective coating, dielectric layer/substrate, transparent membrane, or deep ultraviolet emitter. In this work, we carried out a detailed investigation of h-BN synthesis on Cu substrate using chemical vapor deposition (CVD) with two heating zones under low pressure (LP). Previous atmospheric pressure (AP) CVD syntheses were only able to obtain few layer h-BN without a good control on the number of layers. In contrast, under LPCVD growth, monolayer h-BN was synthesized and time-dependent growth was investigated. It was also observed that the morphology of the Cu surface affects the location and density of the h-BN nucleation. Ammonia borane is used as a BN precursor, which is easily accessible and more stable under ambient conditions than borazine. The h-BN films are characterized by atomic force microscopy, transmission electron microscopy, and electron energy loss spectroscopy analyses. Our results suggest that the...
1,069 citations
TL;DR: In this article, Minnich et al. reviewed the progress made in thermoelectrics over the past two years on charge and heat carrier transport, strategies to improve the thermiolectric figure of merit, with new discussions on device physics and applications.
Abstract: This review is an update of a previous review (A. J. Minnich, et al., Energy Environ. Sci., 2009, 2, 466) published two years ago by some of the co-authors, focusing on progress made in thermoelectrics over the past two years on charge and heat carrier transport, strategies to improve the thermoelectric figure of merit, with new discussions on device physics and applications, and assessing challenges on these topics. Understanding of phonon transport in bulk materials has advanced significantly as the first-principles calculations are applied to thermoelectric materials, and experimental tools are being developed. Some new strategies have been developed to improve electron transport in thermoelectric materials. Fundamental questions on phonon and electron transport across interfaces and in thermoelectric materials remain. With thermoelectric materials reaching high ZT values well above one, the field is ready to take a step forward and go beyond the materials' figure of merit. Developing device contacts and module fabrication techniques, developing a platform for efficiency measurements, and identifying applications are becoming increasingly important for the future of thermoelectrics.
1,049 citations
01 Jan 2012
TL;DR: In this article, Minnich et al. reviewed the progress made in thermoelectrics over the past two years on charge and heat carrier transport, strategies to improve the thermiolectric figure of merit, with new discussions on device physics and applications.
Abstract: This review is an update of a previous review (A. J. Minnich, et al., Energy Environ. Sci., 2009, 2, 466) published two years ago by some of the co-authors, focusing on progress made in thermoelectrics over the past two years on charge and heat carrier transport, strategies to improve the thermoelectric figure of merit, with new discussions on device physics and applications, and assessing challenges on these topics. Understanding of phonon transport in bulk materials has advanced significantly as the first-principles calculations are applied to thermoelectric materials, and experimental tools are being developed. Some new strategies have been developed to improve electron transport in thermoelectric materials. Fundamental questions on phonon and electron transport across interfaces and in thermoelectric materials remain. With thermoelectric materials reaching high ZT values well above one, the field is ready to take a step forward and go beyond the materials' figure of merit. Developing device contacts and module fabrication techniques, developing a platform for efficiency measurements, and identifying applications are becoming increasingly important for the future of thermoelectrics.
826 citations
TL;DR: It is shown here that GB strength can either increase or decrease with the tilt, and the behaviour can be explained well by continuum mechanics.
Abstract: Graphene is often referred to as the strongest material in existence. That may be so for a perfect crystal, but most graphene sheets are polycrystalline, and the grain boundaries affect their mechanical properties. A new study reveals that both the density and detailed arrangement of the defects that form the grain boundaries play a significant part in determining the strength of a polycrystalline graphene sheet.
550 citations
TL;DR: It will be demonstrated that defects play a key role in graphene physicochemical properties and could even be critical to generate biocompatible materials.
Abstract: Defects are usually seen as imperfections in materials that could significantly degrade their performance. However, at the nanoscale, defects could be extremely useful since they could be exploited to generate novel, innovative and useful materials and devices. Graphene and graphene nanoribbons are no exception. This review therefore tries to categorize defects, emphasize their importance, introduce the common routes to study and identify them and to propose new ways to construct novel devices based on 'defective' graphene-like materials. In particular, we will discuss defects in graphene-like systems including (a) structural (sp(2)-like) defects, (b) topological (sp(2)-like) defects, (c) doping or functionalization (sp(2)- and sp(3)-like) defects and (d) vacancies/edge type defects (non-sp(2)-like). It will be demonstrated that defects play a key role in graphene physicochemical properties and could even be critical to generate biocompatible materials. There are numerous challenges in this emerging field, and we intend to provide a stimulating account which could trigger new science and technological developments based on defective graphene-like materials that could be introduced into other atomic layered materials, such as BN, MoS(2) and WS(2), not discussed in this review.
548 citations
TL;DR: Signals from a G-SERS substrate were demonstrated to have interesting advantages over normal SERS, in terms of cleaner vibrational information free from various metal-molecule interactions and being more stable against photo-induced damage, but with a comparable enhancement factor.
Abstract: Surface enhanced Raman spectroscopy (SERS) is an attractive analytical technique, which enables single-molecule sensitive detection and provides its special chemical fingerprints. During the past decades, researchers have made great efforts towards an ideal SERS substrate, mainly including pioneering works on the preparation of uniform metal nanostructure arrays by various nanoassembly and nanotailoring methods, which give better uniformity and reproducibility. Recently, nanoparticles coated with an inert shell were used to make the enhanced Raman signals cleaner. By depositing SERS-active metal nanoislands on an atomically flat graphene layer, here we designed a new kind of SERS substrate referred to as a graphene-mediated SERS (G-SERS) substrate. In the graphene/metal combined structure, the electromagnetic “hot” spots (which is the origin of a huge SERS enhancement) created by the gapped metal nanoislands through the localized surface plasmon resonance effect are supposed to pass through the monolayer graphene, resulting in an atomically flat hot surface for Raman enhancement. Signals from a G-SERS substrate were also demonstrated to have interesting advantages over normal SERS, in terms of cleaner vibrational information free from various metal-molecule interactions and being more stable against photo-induced damage, but with a comparable enhancement factor. Furthermore, we demonstrate the use of a freestanding, transparent and flexible “G-SERS tape” (consisting of a polymer-layer-supported monolayer graphene with sandwiched metal nanoislands) to enable direct, real time and reliable detection of trace amounts of analytes in various systems, which imparts high efficiency and universality of analyses with G-SERS substrates.
513 citations
TL;DR: The experimental observation of coherent heat conduction through the use of finite-thickness superlattices with varying numbers of periods is reported, which is consistent with a coherent phononHeat conduction process.
Abstract: The control of heat conduction through the manipulation of phonons as coherent waves in solids is of fundamental interest and could also be exploited in applications, but coherent heat conduction has not been experimentally confirmed. We report the experimental observation of coherent heat conduction through the use of finite-thickness superlattices with varying numbers of periods. The measured thermal conductivity increased linearly with increasing total superlattice thickness over a temperature range from 30 to 150 kelvin, which is consistent with a coherent phonon heat conduction process. First-principles and Green’s function–based simulations further support this coherent transport model. Accessing the coherent heat conduction regime opens a new venue for phonon engineering for an array of applications.
508 citations
TL;DR: The synthesis of large-area h-BN film is reported using atmospheric pressure chemical vapor deposition on a copper foil, followed by Cu etching and transfer to a target substrate, and the mobility of the CVD graphene device remains the same before and after device integration.
Abstract: Hexagonal boron nitride (h-BN) is a promising material as a dielectric layer or substrate for two-dimensional electronic devices. In this work, we report the synthesis of large-area h-BN film using atmospheric pressure chemical vapor deposition on a copper foil, followed by Cu etching and transfer to a target substrate. The growth rate of h-BN film at a constant temperature is strongly affected by the concentration of borazine as a precursor and the ambient gas condition such as the ratio of hydrogen and nitrogen. h-BN films with different thicknesses can be achieved by controlling the growth time or tuning the growth conditions. Transmission electron microscope characterization reveals that these h-BN films are polycrystalline, and the c-axis of the crystallites points to different directions. The stoichiometry ratio of boron and nitrogen is close to 1:1, obtained by electron energy loss spectroscopy. The dielectric constant of h-BN film obtained by parallel capacitance measurements (25 μm2 large areas) ...
500 citations
TL;DR: An alternative materials design is reported, using alloy Si(70) Ge(30) instead of Si as the nanoparticles and Si(95)Ge(5) as the matrix, to increase the power factor but not the thermal conductivity, leading to a ZT of 1.3 ± 0.1 at 900 °C.
Abstract: Modulation-doping was theoretically proposed and experimentally proved to be effective in increasing the power factor of nanocomposites (Si80Ge20)70(Si100B5)30 by increasing the carrier mobility but not the figure-of-merit (ZT) due to the increased thermal conductivity. Here we report an alternative materials design, using alloy Si70Ge30 instead of Si as the nanoparticles and Si95Ge5 as the matrix, to increase the power factor but not the thermal conductivity, leading to a ZT of 1.3 ± 0.1 at 900 °C.
465 citations
TL;DR: The enhanced thermoelectric properties of selenium (Se) doped bismuth telluride (Bi(2)Te(3-x)Se(x)) nanoplatelet (NP) composites synthesized by the polyol method are reported on.
Abstract: We report on the enhanced thermoelectric properties of selenium (Se) doped bismuth telluride (Bi2Te3–xSex) nanoplatelet (NP) composites synthesized by the polyol method. Variation of the Se composition within NPs is demonstrated by X-ray diffraction and Raman spectroscopy. While the calculated lattice parameters closely follow the Vegard’s law, a discontinuity in the shifting of the high frequency (Eg2 and A1g2) phonon modes illustrates a two mode behavior for Bi2Te3–xSex NPs. The electrical resistivity (ρ) of spark plasma sintered pellet composites shows metallic conduction for pure Bi2Te3 NP composites and semiconducting behavior for intermediate Se compositions. The thermal conductivity (κ) for all NP composites is much smaller than the bulk values and is dominated by microstructural grain boundary scattering. With temperature dependent electrical and thermal transport measurements, we show that both the thermoelectric power S (−259 μV/K) and the figure of merit ZT (0.54) are enhanced by nearly a facto...
333 citations
TL;DR: In this paper, the authors used isotopic defects to probe materials' properties by separating their electronic properties from behaviors associated with their phonon dynamics, which can also be beneficial to materials in providing dopants to control both carrier concentration and whether the carriers are n-type or p-type 1-16.
TL;DR: It is reported that boron atoms can be efficiently substituted for carbon in graphene and Boron-doped graphene appears to be a useful tool for engineering the physical and chemical properties of graphene.
Abstract: The introduction of foreign atoms, such as nitrogen, into the hexagonal network of an sp2-hybridized carbon atom monolayer has been demonstrated and constitutes an effective tool for tailoring the intrinsic properties of graphene. Here, we report that boron atoms can be efficiently substituted for carbon in graphene. Single-layer graphene substitutionally doped with boron was prepared by the mechanical exfoliation of boron-doped graphite. X-ray photoelectron spectroscopy demonstrated that the amount of substitutional boron in graphite was ∼0.22 atom %. Raman spectroscopy demonstrated that the boron atoms were spaced 4.76 nm apart in single-layer graphene. The 7-fold higher intensity of the D-band when compared to the G-band was explained by the elastically scattered photoexcited electrons by boron atoms before emitting a phonon. The frequency of the G-band in single-layer substitutionally boron-doped graphene was unchanged, which could be explained by the p-type boron doping (stiffening) counteracting the...
TL;DR: This method opens up a facile route for aerogel production with a wide variety of materials and tremendous opportunities for bio-scaffold, energy storage, thermoelectric, catalysis, and hydrogen storage applications.
Abstract: Aerogels have numerous applications due to their high surface area and low densities. However, creating aerogels from a large variety of materials has remained an outstanding challenge. Here, we report a new methodology to enable aerogel production with a wide range of materials. The method is based on the assembly of anisotropic nano-objects (one-dimensional (1D) nanotubes, nanowires, or two-dimensional (2D) nanosheets) into a cross-linking network from their colloidal suspensions at the transition from the semi-dilute to the isotropic concentrated regime. The resultant aerogels have highly porous and ultrafine three-dimensional (3D) networks consisting of 1D (Ag, Si, MnO2, single-walled carbon nanotubes (SWNTs)) and 2D materials (MoS2, graphene, h-BN) with high surface areas, low densities, and high electrical conductivities. This method opens up a facile route for aerogel production with a wide variety of materials and tremendous opportunities for bio-scaffold, energy storage, thermoelectric, catalysis, and hydrogen storage applications.
TL;DR: This study highlights an optimized temperature range for the pelletization of the nanoplatelet composites for TE applications and provides a subtle understanding of the grain growth mechanism and the filtering of low energy electrons and phonons with thermoelectric interfaces.
Abstract: Control of competing parameters such as thermoelectric (TE) power and electrical and thermal conductivities is essential for the high performance of thermoelectric materials. Bulk-nanocomposite materials have shown a promising improvement in the TE performance due to poor thermal conductivity and charge carrier filtering by interfaces and grain boundaries. Consequently, it has become pressingly important to understand the formation mechanisms, stability of interfaces and grain boundaries along with subsequent effects on the physical properties. We report here the effects of the thermodynamic environment during spark plasma sintering (SPS) on the TE performance of bulk-nanocomposites of chemically synthesized Bi2Te2.7Se0.3 nanoplatelets. Four pellets of nanoplatelets powder synthesized in the same batch have been made by SPS at different temperatures of 230, 250, 280, and 350 °C. The X-ray diffraction, transmission electron microscopy, thermoelectric, and thermal transport measurements illustrate that the ...
TL;DR: Using scanning tunneling microscopy and spectroscopy techniques, a pristine structure was ruled out in favor of a reconstructed edge structure composed of 5-7 member rings, showing a better match with experimental results, and thereby suggesting the possibility of a defective morphology at the edge of CVD grown nanoribbons.
Abstract: We used scanning tunneling microscopy and spectroscopy (STM/S) techniques to analyze the relationships between the edge shapes and the electronic structures in as-grown chemical vapor deposition (CVD) graphene nanoribbons (GNRs). A rich variety of single-layered graphene nanoribbons exhibiting a width of several to 100 nm and up to 1 μm long were studied. High-resolution STM images highlight highly crystalline nanoribbon structures with well-defined and clean edges. Theoretical calculations indicate clear spin-split edge states induced by electron–electron Coulomb repulsion. The edge defects can significantly modify these edge states, and different edge structures for both sides of a single ribbon produce asymmetric electronic edge states, which reflect the more realistic features of CVD grown GNRs. Three structural models are proposed and analyzed to explain the observations. By comparing the models with an atomic resolution image at the edge, a pristine (2,1) structure was ruled out in favor of a recons...
TL;DR: In this paper, the authors calculated the resonance Raman intensity for twisted bilayer graphene as a function of laser excitation energy based on the extended tight binding method, which can be used for the experimental identification of the twisting angle.
Abstract: The $G$-band Raman intensity is calculated for twisted bilayer graphene as a function of laser excitation energy based on the extended tight binding method. Here we explicitly consider the electron-photon and electron-phonon matrix elements of twisted bilayer graphene to calculate the resonance Raman intensity. The $G$-band Raman intensity is sensitive to the laser excitation energy and the twisting angle between the layers as a result of folding the electronic energy band structure. The Van Hove energy singularity, which is an electron transition energy between the conduction and valence bands, depends on $n\ensuremath{-}m$ of the twisting vector $(n,m)$. The relative intensity of the $G$ band as a function of twisting vectors is presented, which should be useful for the experimental identification of the twisting angle.
TL;DR: Graphene, which is intrinsically stretchable up to 10%, will enable high-performance composites to be fabricated at relatively low cost and it is envisage that such composites may replace carbon nanotubes for various applications in the near future.
Abstract: We report a mechanically strong, electrically and thermally conductive, and optically transparent shape-memory polyurethane composite which was fabricated by introducing a small amount (01 wt%) of high-quality graphene as a filler Geometrically large (≈46 μm(2)), but highly crystallized few-layer graphenes, verified by Raman spectroscopy and transmission electron microscopy, were prepared by the sonication of expandable graphite in an organic solvent Oxygen- containing functional groups at the edge plane of graphene were crucial for an effective stress transfer from the graphene to polyurethane Homogeneously dispersed few-layered graphene enabled polyurethane to have a high shape recovery force of 18 MPa cm(-3) Graphene, which is intrinsically stretchable up to 10%, will enable high-performance composites to be fabricated at relatively low cost and we thus envisage that such composites may replace carbon nanotubes for various applications in the near future
TL;DR: A review of 50 years of research in carbon materials physics and carbon-based nanostructures can be found in this article, where the review topics include an early history of studies of graphene and graphite, graphite intercalation compounds, forerunners of nano-carbons, fullerenes, carbon nanotubes and, finally, graphene and graphene nanoribbons.
Abstract: A review is presented based on our 50 year involvement in studying carbon materials physics and carbon-based nanostructures. The review topics include an early history of studies of graphene and graphite, graphite intercalation compounds, forerunners of nano-carbons, fullerenes, carbon nanotubes and, finally, graphene and graphene nanoribbons.
TL;DR: In this paper, the authors present electrical and thermal specific heat measurements that show superconductivity in double-wall carbon nanotube (DWCNT) bundles and suggest an intrinsic superconducting transition below 6.8ÅK for one particular sample.
Abstract: We present electrical and thermal specific heat measurements that show superconductivity in double-wall carbon nanotube (DWCNT) bundles. Clear evidence, comprising a resistance drop as a function of temperature, magnetoresistance and differential resistance signature of the supercurrent, suggest an intrinsic superconducting transition below 6.8 K for one particular sample. Additional electrical data not only confirm the existence of superconductivity, but also indicate the Tc distribution that can arise from the diversity in the diameter and chirality of the DWCNTs. A broad superconducting anomaly is observed in the specific heat of a bulk DWCNT sample, which yields a Tc distribution that correlates well with the range of the distribution obtained from the electrical data. As quasi one dimensionality of the DWCNTs dictates the existence of electronic density of state peaks, confirmation of superconductivity in this material system opens the exciting possibility of tuning the Tc through the application of a gate voltage.
TL;DR: In this article, a general approach to address individual graphene sheets in few-layer graphene by Raman spectroscopy was developed, based on isotope labeling of individual layers during their synthesis and subsequent transfer to form multilayered graphene.
Abstract: We developed a new general approach to address individual graphene sheets in few-layer graphene by Raman spectroscopy. Our method is based on isotope labeling of individual layers during their synthesis and subsequent transfer to form multilayered graphene. The power of the procedure is demonstrated in the analysis of the interactions of individual layers with the substrate and with the environment. In addition, we measured Raman spectra of individual graphene layers in 3-LG during electrochemical doping. We show that they do not exhibit the same level of doping as one another and the doping level is dependent on layer position with respect to the substrate.
TL;DR: The results suggest that the enhancement of the Raman signal of the G mode in a twisted graphene bilayer (2-LG) on a hexagonal single-crystalline boron nitride substrate comes from new states in the electronic structure, which are a consequence of a local specific rotation of the grains in graphene layers.
Abstract: We report an unusual enhancement of the Raman signal of the G mode in a twisted graphene bilayer (2-LG) on a hexagonal single-crystalline boron nitride substrate. We used an isotopically engineered 2-LG, where the top layer was composed of 13C atoms and the bottom layer of 12C atoms. Consequently, it was possible by Raman spectroscopy to distinguish between the enhancement coming from the top and bottom layers. The enhancement of the G mode was, however, found to be similar for the top and bottom layers, and this enhancement effect was observed only at certain locations on the substrate. The experiment with two different laser excitation energies showed that the location of the enhanced spots is dependent on the laser excitation energy. Therefore our results suggest that the enhancement comes from new states in the electronic structure, which are a consequence of a local specific rotation of the grains in graphene layers.
Journal Article•
TL;DR: In this paper, the authors demonstrate the use of film-induced frustrated etching (FIFE) as a facile, scalable method to reveal and quantify structural defects in continuous thin sheets.
Abstract: The metrology of two-dimensional (2D) materials such as graphene, boron nitride or molybdenum disulfide grown by chemical vapor deposition (CVD) is critical for the optimization of their synthesis. We demonstrate the use of film-induced frustrated etching (FIFE) as a facile, scalable method to reveal and quantify structural defects in continuous thin sheets. The sensitivity of the analysis technique to intentionally induced lattice defects in graphene compares favorably to the sensitivity of Raman spectroscopy. A strong correlation between the measured defectiveness and the maximum carrier mobility in graphene emphasizes the importance of the technique for growth optimization. Due to its ease and widespread availability, we anticipate that FIFE will find wide application in the characterization of CVD-synthesized 2D materials.
TL;DR: In this paper, the authors investigate phonon mean free path distribution inside fully unfilled (Co4Sb12) and fully filled (LaFe4S b12) bulk skutterudites by fitting their thermal conductivity to analytical models which employ different phonon dispersions.
Abstract: Experimental thermal conductivity of bulk materials are often modeled using Debye approximation together with functional forms of relaxation time with fitting parameters. While such models can fit the temperature dependence of thermal conductivity of bulk materials, the Debye approximation leads to large error in the actual phonon mean free path, and consequently, the predictions of the thermal conductivity of the nanostructured materials using the same relaxation time are not correct even after considering additional size effect on the mean free path. We investigate phonon mean free path distribution inside fully unfilled (Co4Sb12) and fully filled (LaFe4Sb12) bulk skutterudites by fitting their thermal conductivity to analytical models which employ different phonon dispersions. We show that theoretical thermal conductivity predictions of the nanostructured samples are in agreement with the experimental data obtained for samples of different grain sizes only when the full phonon dispersion is considered.
TL;DR: In this paper, an iterative two-dimensional-two-band model was developed to get a consistent inverse-effective-mass-tensor and band gap, which can be used in a general 2D system that has a nonparabolic dispersion relation as in the Bi1−Sbx thin film system.
Abstract: The electronic band structures of Bi1–xSbx 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 Bi1–xSbx 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 nonparabolic dispersion relation as in the Bi1–xSbx thin film system.
01 Nov 2012
TL;DR: Araujo et al. as mentioned in this paper proposed a method to solve the problem of high dimensional image recognition in the context of physics and applied it in the field of Artificial Intelligence.
Abstract: D. L. Mafra,1,2 J. Kong,2 K. Sato,3 R. Saito,3 M. S. Dresselhaus,2,4 and P. T. Araujo2,* 1Departamento de Fisica, Universidade Federal de Minas Gerais, 30123-970 Belo Horizonte, Brazil 2Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA 3Department of Physics, Tohoku University, Sendai 980-8578, Japan 4Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA (Received 18 October 2012; published 30 November 2012; corrected 4 December 2012)
TL;DR: In this paper, gate-modulated and laser-dependent Raman spectroscopy have been widely used to study $q=0$ zone center phonon modes, their self-energy, and their coupling to electrons in graphene systems.
Abstract: Gate-modulated and laser-dependent Raman spectroscopy have been widely used to study $q=0$ zone center phonon modes, their self-energy, and their coupling to electrons in graphene systems. In this work we use gate-modulated Raman of $q\ensuremath{
e}0$ phonons as a technique to understand the nature of five second-order Raman combination modes observed in the frequency range of 1700--2300 cm${}^{\ensuremath{-}1}$ of single-layer graphene (SLG). Anomalous phonon self-energy renormalization phenomena are observed in all five combination modes within this intermediate frequency region, which can clearly be distinguished from one another. By combining the anomalous phonon renormalization effect with the double resonance Raman theory, which includes both phonon dispersion relations and angular dependence of the electron-phonon scattering matrix elements, and by comparing it to the experimentally obtained phonon dispersion, measured by using different laser excitation energies, we can assign each Raman peak to the proper phonon combination mode. This approach should also shed light on the understanding of more complex structures such as few-layer graphene (FLG) and its stacking orders as well as other two-dimensional (2D)-like materials.
Journal Article•
TL;DR: In this paper, the authors analyzed the integrated areas (IA) of the four peaks of the double resonance Raman (DRR) process and showed that the dependence of the IA of each peak on temperature is different for each distinct laser excitation energy.
Abstract: The G' (or 2D) Raman band of AB stacked bilayer graphene comes from a double resonance Raman (DRR) process and is composed of four peaks (P(11), P(12), P(21), and P(22)). In this work, the integrated areas (IA) of these four peaks are analyzed as a function of the laser power for different laser lines. We show that the dependence of the IA of each peak on temperature is different for each distinct laser excitation energy. This special dependence is explained in terms of the electron-phonon coupling and the relaxation of the photon-excited electron. In this DRR process, the electron is scattered by an iTO phonon from a K to an inequivalent K' point of the Brillouin zone. Here, we show that this electron relaxes while in the conduction band before being scattered by an iTO phonon due to the short relaxation time of the excited electron, and the carrier relaxation occurs predominantly by emitting a low-energy acoustic phonon. The different combinations of relaxation processes determine the relative intensities of the four peaks that give rise to the G' band. Some peaks show an increase of their IA at the expense of others, thereby making the IA of the peaks both different from each other and dependent on laser excitation energy and on power level. Also, we report that the IA of the G' mode excited at 532 nm, shows a resonance regime involving ZO' phonons (related to the interlayer breathing mode in bilayer graphene systems) in which a saturation of what we call the P(12) process occurs. This effect gives important information about the electron and phonon dynamics and needs to be taken into account for certain applications of bilayer graphene in the field of nanotechnology.
TL;DR: In this article, the authors theoretically predict that a large variety of Dirac-cone materials can be constructed in Bi1−xSbx thin films and show how to construct single-, bi-and tri-Dirac cone materials with various amounts of wave vector anisotropy.
Abstract: We theoretically predict that a large variety of Dirac-cone materials can be constructed in Bi1−xSbx 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 device design, quantum electrodynamics and other fields.
TL;DR: In this paper, the electronic interactions between the [Cu(opba)]2− anions (where opba is orthophenylenebis (oxamato)) and single-wall carbon nanotubes (SWCNTs) were investigated by resonance Raman spectroscopy.
Abstract: The electronic interactions between the [Cu(opba)]2− anions (where opba is orthophenylenebis (oxamato)) and single-wall carbon nanotubes (SWCNTs) were investigated by resonance Raman spectroscopy. The opba can form molecular magnets, and the interactions of opba with SWCNTs can produce materials with very different magnetic/electronic properties. It is observed that the electronic interaction shows a dependence on the SWCNT diameter independent of whether they are metallic or semiconducting, although the interaction is stronger for metallic tubes. The interaction also is dependent on the amount of complex that is probably adsorbed on the carbon surface of the SWCNTs. Some charge transfer can be also occurring between the metallic complex and the SWCNTs. Copyright © 2012 John Wiley & Sons, Ltd.