We have used molecular dynamics to calculate the thermal conductivity of symmetric and asymmetric... more We have used molecular dynamics to calculate the thermal conductivity of symmetric and asymmetric graphene nanoribbons (GNRs) of several nanometers in size (up to ~4 nm wide and ~10 nm long). For symmetric nanoribbons, the calculated thermal conductivity (e.g. ~2000 W/m-K @400K for a 1.5 nm {\times} 5.7 nm zigzag GNR) is on the similar order of magnitude of the experimentally measured value for graphene. We have investigated the effects of edge chirality and found that nanoribbons with zigzag edges have appreciably larger thermal conductivity than nanoribbons with armchair edges. For asymmetric nanoribbons, we have found significant thermal rectification. Among various triangularly-shaped GNRs we investigated, the GNR with armchair bottom edge and a vertex angle of 30{\deg} gives the maximal thermal rectification. We also studied the effect of defects and found that vacancies and edge roughness in the nanoribbons can significantly decrease the thermal conductivity. However, substantial thermal rectification is observed even in the presence of edge roughness.
Physical Review B Condensed Matter and Materials Physics, Sep 1, 2008
We calculate the vibrational spectra for typical nanostructures including thin films, nanowires, ... more We calculate the vibrational spectra for typical nanostructures including thin films, nanowires, and quantum dots of yttria, an important laser host material. Lattice dynamics calculations show that the vibrational spectra of nanocrystals are distinct from that of the bulk phase in the enhanced tails at low and high frequencies. Some of the low-frequency and high-frequency modes only involve surface atoms. Molecular-dynamics simulations are used to further decompose the contributions of surface and internal atoms, and the results confirm the evident broadening of the surface spectra. The broadening in vibrational spectra is attributed to the broadened atomic spring constants, further to the lowered symmetry of nanostructures.
Volume 10: Heat and Mass Transport Processes, Parts A and B, 2011
... The band gap of semiconduct-ing SWCNTs has been found to vary linearly with CNT diameter, wit... more ... The band gap of semiconduct-ing SWCNTs has been found to vary linearly with CNT diameter, with the band gap of ... PREDICTING THE PROPERTIES OF NANOSTRUCTUREDMETAMATERIALS: VERTICALLY ALIGNED SINGLE-WALLED CARBON NANOTUBE ARRAYS ...
We demonstrate that laser peening coupled with sintering of CdTe nanowire films substantially enh... more We demonstrate that laser peening coupled with sintering of CdTe nanowire films substantially enhances film quality and charge transfer while largely maintaining basic particle morphology. During the laser peening phase, a shockwave is used to compress the film. Laser sintering comprises the second step, where a nanosecond pulse laser beam welds the nanowires. Microstructure, morphology, material content, and electrical conductivities of the films are characterized before and after treatment. The morphology results show that laser peening can decrease porosity and bring nanowires into contact, and pulsed laser heating fuses those contacts. Multiphysics simulations coupling electromagnetic and heat transfer modules demonstrate that during pulsed laser heating, local EM field enhancement is generated specifically around the contact areas between two semiconductor nanowires, indicating localized heating. The characterization results indicate that solely laser peening or sintering can o...
We study the effects of stress and strain on thermal transport and mechanical properties of armch... more We study the effects of stress and strain on thermal transport and mechanical properties of armchair and zigzag graphene nanoribbons (AGNR and ZGNRs) using non-equilibrium molecular dynamics simulations. Both nanoribbons buckle at low temperatures (100K) and the buckling height decreases as the temperature increases. Moreover they experience an intrinsic tensile stress which decreases as the temperature increases. The thermal conductivity of AGNR decreases significantly with applied symmetric tensile strains below $ 0.1 $. On the other hand, thermal conductivity of ZGNR shows anomalous behavior we attribute to its small width. Asymmetric stress along the transport direction is achieved by exerting a lateral force $F_y$ on the side atoms of one half of the AGNR. We observe significant thermal rectification up to $65%$ and interestingly, it is tunable with the $F_y$. The observed heat flux is larger from the less stressed region to the more stressed region than {\it vice versa}. This ...
Volume 4: Energy Systems Analysis, Thermodynamics and Sustainability; Combustion Science and Engineering; Nanoengineering for Energy, Parts A and B, 2011
Volume 1: Heat Transfer in Energy Systems; Theory and Fundamental Research; Aerospace Heat Transfer; Gas Turbine Heat Transfer; Transport Phenomena in Materials Processing and Manufacturing; Heat and, 2012
Volume 1: Heat Transfer in Energy Systems; Theory and Fundamental Research; Aerospace Heat Transfer; Gas Turbine Heat Transfer; Transport Phenomena in Materials Processing and Manufacturing; Heat and, 2012
ABSTRACT High-crystallinity 0D, 1D and 2D Bi2Te3 nanocrystals have been synthesized using the pyr... more ABSTRACT High-crystallinity 0D, 1D and 2D Bi2Te3 nanocrystals have been synthesized using the pyrolysis of organometallic compound method. The growth process of Bi2Te3 nanocrystals was revealed by transmission electron microscopy (TEM) images. Samples synthesized at the temperature of 35°C show a dominant morphology of 0D nanoparticle or 1D nanorod, while samples synthesized at the temperature above 75°C show a dominant morphology of 2D nanoplate. Phonon vibrational behavior was investigated by Raman spectroscopy. 2D nanoplates show similar Raman features to few-quintuple thick Bi2Te3 layers, while 0D and 1D nanostructures show a blueshifted A1g2 mode and a much stronger A1u mode. This is the report about Raman spectra obtained on small Bi2Te3 nanoparticle and nanorod whose size is within the strong quantum confinement region.
Volume 10: Heat and Mass Transport Processes, Parts A and B, 2011
ABSTRACT The Lorentz oscillator model is well-known for its effectiveness to describe the far inf... more ABSTRACT The Lorentz oscillator model is well-known for its effectiveness to describe the far infrared optical properties of ionic materials. The parameters including oscillator strength and damping factor in the model are usually obtained by fitting to experimental results. In this paper, a new method, based on static and dynamic first-principle simulations, is developed to parameterize the Lorentz oscillator model with the initial atomic structure as the only input parameter. This method is then applied to predict the far infrared reflectance of GaAs, which shows excellent agreement with experimental measurements.
We have used molecular dynamics to calculate the thermal conductivity of symmetric and asymmetric... more We have used molecular dynamics to calculate the thermal conductivity of symmetric and asymmetric graphene nanoribbons (GNRs) of several nanometers in size (up to ~4 nm wide and ~10 nm long). For symmetric nanoribbons, the calculated thermal conductivity (e.g. ~2000 W/m-K @400K for a 1.5 nm {\times} 5.7 nm zigzag GNR) is on the similar order of magnitude of the experimentally measured value for graphene. We have investigated the effects of edge chirality and found that nanoribbons with zigzag edges have appreciably larger thermal conductivity than nanoribbons with armchair edges. For asymmetric nanoribbons, we have found significant thermal rectification. Among various triangularly-shaped GNRs we investigated, the GNR with armchair bottom edge and a vertex angle of 30{\deg} gives the maximal thermal rectification. We also studied the effect of defects and found that vacancies and edge roughness in the nanoribbons can significantly decrease the thermal conductivity. However, substantial thermal rectification is observed even in the presence of edge roughness.
Physical Review B Condensed Matter and Materials Physics, Sep 1, 2008
We calculate the vibrational spectra for typical nanostructures including thin films, nanowires, ... more We calculate the vibrational spectra for typical nanostructures including thin films, nanowires, and quantum dots of yttria, an important laser host material. Lattice dynamics calculations show that the vibrational spectra of nanocrystals are distinct from that of the bulk phase in the enhanced tails at low and high frequencies. Some of the low-frequency and high-frequency modes only involve surface atoms. Molecular-dynamics simulations are used to further decompose the contributions of surface and internal atoms, and the results confirm the evident broadening of the surface spectra. The broadening in vibrational spectra is attributed to the broadened atomic spring constants, further to the lowered symmetry of nanostructures.
Volume 10: Heat and Mass Transport Processes, Parts A and B, 2011
... The band gap of semiconduct-ing SWCNTs has been found to vary linearly with CNT diameter, wit... more ... The band gap of semiconduct-ing SWCNTs has been found to vary linearly with CNT diameter, with the band gap of ... PREDICTING THE PROPERTIES OF NANOSTRUCTUREDMETAMATERIALS: VERTICALLY ALIGNED SINGLE-WALLED CARBON NANOTUBE ARRAYS ...
We demonstrate that laser peening coupled with sintering of CdTe nanowire films substantially enh... more We demonstrate that laser peening coupled with sintering of CdTe nanowire films substantially enhances film quality and charge transfer while largely maintaining basic particle morphology. During the laser peening phase, a shockwave is used to compress the film. Laser sintering comprises the second step, where a nanosecond pulse laser beam welds the nanowires. Microstructure, morphology, material content, and electrical conductivities of the films are characterized before and after treatment. The morphology results show that laser peening can decrease porosity and bring nanowires into contact, and pulsed laser heating fuses those contacts. Multiphysics simulations coupling electromagnetic and heat transfer modules demonstrate that during pulsed laser heating, local EM field enhancement is generated specifically around the contact areas between two semiconductor nanowires, indicating localized heating. The characterization results indicate that solely laser peening or sintering can o...
We study the effects of stress and strain on thermal transport and mechanical properties of armch... more We study the effects of stress and strain on thermal transport and mechanical properties of armchair and zigzag graphene nanoribbons (AGNR and ZGNRs) using non-equilibrium molecular dynamics simulations. Both nanoribbons buckle at low temperatures (100K) and the buckling height decreases as the temperature increases. Moreover they experience an intrinsic tensile stress which decreases as the temperature increases. The thermal conductivity of AGNR decreases significantly with applied symmetric tensile strains below $ 0.1 $. On the other hand, thermal conductivity of ZGNR shows anomalous behavior we attribute to its small width. Asymmetric stress along the transport direction is achieved by exerting a lateral force $F_y$ on the side atoms of one half of the AGNR. We observe significant thermal rectification up to $65%$ and interestingly, it is tunable with the $F_y$. The observed heat flux is larger from the less stressed region to the more stressed region than {\it vice versa}. This ...
Volume 4: Energy Systems Analysis, Thermodynamics and Sustainability; Combustion Science and Engineering; Nanoengineering for Energy, Parts A and B, 2011
Volume 1: Heat Transfer in Energy Systems; Theory and Fundamental Research; Aerospace Heat Transfer; Gas Turbine Heat Transfer; Transport Phenomena in Materials Processing and Manufacturing; Heat and, 2012
Volume 1: Heat Transfer in Energy Systems; Theory and Fundamental Research; Aerospace Heat Transfer; Gas Turbine Heat Transfer; Transport Phenomena in Materials Processing and Manufacturing; Heat and, 2012
ABSTRACT High-crystallinity 0D, 1D and 2D Bi2Te3 nanocrystals have been synthesized using the pyr... more ABSTRACT High-crystallinity 0D, 1D and 2D Bi2Te3 nanocrystals have been synthesized using the pyrolysis of organometallic compound method. The growth process of Bi2Te3 nanocrystals was revealed by transmission electron microscopy (TEM) images. Samples synthesized at the temperature of 35°C show a dominant morphology of 0D nanoparticle or 1D nanorod, while samples synthesized at the temperature above 75°C show a dominant morphology of 2D nanoplate. Phonon vibrational behavior was investigated by Raman spectroscopy. 2D nanoplates show similar Raman features to few-quintuple thick Bi2Te3 layers, while 0D and 1D nanostructures show a blueshifted A1g2 mode and a much stronger A1u mode. This is the report about Raman spectra obtained on small Bi2Te3 nanoparticle and nanorod whose size is within the strong quantum confinement region.
Volume 10: Heat and Mass Transport Processes, Parts A and B, 2011
ABSTRACT The Lorentz oscillator model is well-known for its effectiveness to describe the far inf... more ABSTRACT The Lorentz oscillator model is well-known for its effectiveness to describe the far infrared optical properties of ionic materials. The parameters including oscillator strength and damping factor in the model are usually obtained by fitting to experimental results. In this paper, a new method, based on static and dynamic first-principle simulations, is developed to parameterize the Lorentz oscillator model with the initial atomic structure as the only input parameter. This method is then applied to predict the far infrared reflectance of GaAs, which shows excellent agreement with experimental measurements.
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