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Roles of Carrier Doping, Band Gap, and Electron Relaxation Time in the Boltzmann Transport Calculations of a Semiconductor’s Thermoelectric Properties
摘要： Although there is a growing demand for first-principles predictions of the thermoelectric properties of materials, the contribution of various errors in Boltzmann transport calculations is not negligible. We conducted a typical first-principles calculation and a Boltzmann transport analysis on a typical semiconductor (Si) at various temperatures T while varying the band gap ?g, electron relaxation time ?el, and phonon thermal conductivity ?ph to demonstrate how the calculated thermoelectric properties, which are functions of the carrier doping level, are affected by these parameters. Bipolar conduction drastically decreased zT via a degradation of the Seebeck coefficient S and an increase in the effective Lorenz factor Leff, indicating the importance of a wide enough ?g (several multiples of kBT or higher) for high zT. Thus, the underestimation of ?g, which frequently happens in first-principles calculations, could induce large errors in calculations for narrow-gap semiconductors. The calculation of the electron thermal conductivity without Peltier thermal conductivity was found to limit the zT of typical semiconductors to below 1. A small value of ?ph/?el, where ?ph/?el is the degree to which a material is a phonon-glass electron-crystal, was necessary to achieve a high zT. Fitting the calculations with experimental thermoelectric properties showed that ?el can vary by an order of magnitude from 10115 to 10114 s, depending on both T and the samples. This indicates that the use of a fixed relaxation time is inappropriate for thermoelectric materials.
关键词： Boltzmann transport calculation,first-principles calculation,thermoelectric properties
更新于2025-09-23 15:23:52
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Light-induced EPR study of spin-assisted charge transport in PFOT:PC61BM composite
摘要： Magnetic resonance, relaxation and dynamic parameters of polaron spins and methanfullerene radical anions, initiated by the infrared-visible-ultraviolet photons in bulk heterojunctions of the composite formed by narrow-band poly[(9,9-dioctylfluorenyl-2.7-diyl)-co-(bithiophene)] (PFOT, F8T2) copolymer and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) globules were investigated by the direct Light-Induced Electron Paramagnetic Resonance (LEPR) spectroscopy. It was shown that some of the polarons are captured by spin traps formed in the copolymer matrix due to its disordering. The number, spatial distribution and energy depth of such traps depend on the structure and morphology of composite. It is shown that the main parameters of both charge carriers are determined by the exchange interaction of the spin ensembles, as well as the energy of the photons. The formation of spin traps in the copolymer matrix and the exchange interaction between different spin packets cause the extreme sensitivity of the composite's magnetic resonance and electronic parameters to the number and energy of the initiating photons. The predominant photoinitiation of localized polarons in the copolymer matrix was demonstrated. This process is substantially accelerated when the composite is illuminated by photons with the energy lying near 1.8 and 2.7 eV. The recombination of both charge carriers can be described in terms of a bimolecular process of the second order. It was found that the contributions of polarons and methanofullerene radical anions to the effective paramagnetic susceptibility increase substantially near the photon energy of 2.6 and 2.1 eV due to the exchange interaction of these charge carriers. It is shown that the mobility of polarons varies monotonically throughout all the photon energy range, whereas the librational spin dynamics of the methanofullerene globules is substantially accelerated near the photon energy of 2.0 and 2.7 eV.
关键词： Polymer nanocomposites,Spin recombination,Transport properties,Spin interaction,Polaron,Light-induced EPR
更新于2025-09-23 15:23:52
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High infrared transmittance CdS single crystal grown by physical vapor transport
摘要： Φ55 × 15 mm2 CdS bulk single crystal with high infrared transmittance was grown by physical vapor transport. The single crystal has a consistent structure from top to bottom, which was confirmed by X-ray diffraction. The (002) full-width at half-maximum of the X-ray diffraction was measured to be 60.00 arcsec, indicating a good quality of the structure. Hall mobility, specific resistivity, and carrier concentration for the top and bottom of the crystal were observed as well. Transmittance for the CdS single crystal was measured to be higher than 70% from 2.5 to 4.5 μm, making the single crystal an important candidate for infrared window materials. Furthermore, the absorption mechanism of the CdS single crystal was analyzed.
关键词： physical vapor transport,X-ray diffraction,semiconducting materials,single crystal growth
更新于2025-09-23 15:23:52
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Transport properties of doped zigzag graphene nanoribbons
摘要： Numerous studies on materials have driven the development of modern nanoelectronic devices. And research also shown that the integrated circuits have entered the era of the nanoelectronic scales from the scale of microelectronics. But the limitations of copper as a traditional connection, such as the resistivity increases a lot, further causing a lot of heat in the interconnect, have been highlighted. Therefore, we need new materials as the substitution of copper. The metallic properties exhibited by the zigzag graphene nanoribbons (ZGNRs) can be controlled by the edge states, doping and different widths of the nanoribbons. In this paper, we applied simulation to dope copper atom chains on ZGNRs. We found an energetic phenomenon that after doping the nanoribbons conductivity have increased significantly than the original. In addition, the transmission channels are mainly concentrated near the doping position, and the width used for transmission is greatly reduced after doping. It is expected to be used as an inter-connect application in nano-integrated circuits in the future.
关键词： Density functional theory,Interconnect,Electronic transport property,Non-equilibrium Green's function,Zigzag graphene nanoribbons,Doping
更新于2025-09-23 15:23:52
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Charge transport in graphene-based mesoscopic realizations of Sachdev-Ye-Kitaev models
摘要： We consider a recent proposal for a physical realization of the Sachdev-Ye-Kitaev (SYK) model in the zeroth-Landau-level sector of an irregularly shaped graphene flake. We study in detail charge transport signatures of the unique non-Fermi-liquid state of such a quantum dot coupled to noninteracting leads. The properties of this setup depend essentially on the ratio p between the number of transverse modes in the lead M and the number of the fermion degrees of freedom N on the SYK dot. This ratio can be tuned via the magnetic field applied to the dot. Our proposed setup gives access to the nontrivial conformal-invariant regime associated with the SYK model as well as a more conventional Fermi-liquid regime via tuning the field. The dimensionless linear-response conductance acquires distinct p dependencies for the two phases, respectively, in the low-temperature limit, with a universal jump at the transition. We find that corrections scale linearly and quadratically in either temperature or frequency on the two sides of the transition. In the weak-tunneling regime, we find differential conductance proportional to the inverse square root of the applied voltage bias U for bias energy eU larger than temperature scale kB T. This dependence is replaced by a conventional Ohmic behavior with constant conductance proportional to 1/sqrt(T) for bias energy eU smaller than temperature scale kB T. We also describe the out-of-equilibrium current-bias characteristics and discuss various crossovers between the limiting behaviors mentioned above.
关键词： Sachdev-Ye-Kitaev model,tunneling conductance,graphene,quantum dot,charge transport,non-Fermi-liquid,conformal invariance
更新于2025-09-23 15:23:52
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Long-Range Activationless Photostimulated Charge Transport in Symmetric Molecular Junctions
摘要： Molecular electronic junctions consisting of nitroazobenzene oligomers covalently bonded to a conducting carbon surface using an established 'all-carbon' device design were illuminated with UV?vis light through a partially transparent top electrode. Monitoring junction conductance with a DC bias imposed permitted observation of photocurrents while varying the incident wavelength, intensity, molecular layer thickness, and temperature. The photocurrent spectrum tracked the in situ absorption spectrum of nitroazobenzene, increased linearly with light intensity, and depended exponentially on applied bias. The electronic characteristics of the photocurrent differed dramatically from those of the same device in the dark, with orders of magnitude higher conductance and very weak attenuation with molecular layer thickness (β = 0.14 nm?1 for thickness above 5 nm). The temperature dependence of the photocurrent was opposite that of the dark current, with a 35% decrease in conductance between 80 and 450 K, while the dark current increased by a factor of 4.5 over the same range. The photocurrent was similar to the dark current for thin molecular layers but greatly exceeded the dark current for low bias and thick molecular layers. We conclude that the light and dark mechanisms are additive, with photoexcited carriers transported without thermal activation for a thickness range of 5?10 nm. The inverse temperature dependence is likely due to scattering or recombination events, both of which increase with temperature and in turn decrease the photocurrent. Photostimulated resonant transport potentially widens the breadth of conceivable molecular electronic devices and may have immediate value for wavelength-specific photodetection.
关键词： charge transport,optoelectronics,photocurrent,molecular electronics,molecular orbital energy,tunneling barrier,HOMO?LUMO gap,photoinduced transport
更新于2025-09-23 15:22:29
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(Bz) <sub/><i>n</i> </sub> and (VBz) <sub/><i>n</i> </sub> covalent functionalized MoS <sub/>2</sub> monolayer: electronic and transport properties
摘要： Inspired by Benzene (Bz) derivatives dramatically enhancing MoS2 monolayer electronic properties (ACS Nano. 2015, 9, 6018–6030), we have investigated electronic and transport properties of (Bz)n/MoS2 and (VBz)n/MoS2, which are designed by grafting (Bz)n and (VBz)n arrays onto 2D monolayer MoS2 (ML-MoS2), respectively, using density functional theory (DFT) and non-equilibrium Green’s function (NEGF) methods. ML-MoS2 provides a perfect substrate for grafting (Bz)n and (VBz)n arrays upon its surface as a result of stable covalent binding energy with -3.841 eV and -1.953 eV for (Bz)n/MoS2 and (VBz)n/MoS2 respectively. From the electronic properties, we can find that grafting (Bz)n onto the ML-MoS2 surface turns ML-MoS2 from typical semiconductor to metallic properties because four wide bands coupled by (Bz)n and MoS2 in (Bz)n/MoS2 show better delocalization in heterointerface, resulting to these bands across the Fermi level (Ef). Furthermore, the introduction of metal V. Transport properties of ML-MoS2, (Bz)n/MoS2 or (VBz)n/MoS2 for two-probe devices are all studied in zigzag and armchair direction. The ferromagnetic (VBz)n/MoS2 shows a spin polarized transport characteristic, spin-down state gives a higher conductivity than spin-up state. By comparison the zigzag direction is the preferential pathway for electron transport. Finally this work suggests that the novel (VBz)n nanowire grafted on MoS2 should have potential application in low-dimensional magnetic nanoelectronic devices.
关键词： (Bz)n/MoS2,(VBz)n/MoS2,covalent functionalized,electronic property,transport property
更新于2025-09-23 15:22:29
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Energy relaxation of hot carriers near the charge neutrality point in HgTe-based 2D topological insulators
摘要： We present experimental results of non-linear transport in HgTe-based 2D topological insulators, where the conductance is dominated by Dirac-like helical edge states when the Fermi level is pinned to the bulk insulating gap. We find that hot carrier’s energy relaxation is faster close to the charge neutrality point (CNP) which can be attributed to localized nature and incompressibility of charge puddles resulting from inhomogeneous charge distribution near CNP. The tunnel-coupling of these puddles (quantum dots) to 1D edge channels can randomize phase memory leading to incoherent inelastic processes. Hot edge carriers, excited by the electric field, relax to equilibrium via thermalization in multiple puddles resulting in the emission of phonons in the puddles. At relatively low temperature (T≤ 10K), the energy relaxation time shows strong temperature dependence (τ_e ∝ T^{-3.5}), which is interpreted as small angle scattering, consistent with resistance saturation at low temperatures.
关键词： Non-linear transport,electron-phonon scattering,energy relaxation mechanisms,inelastic processes,Topological insulators
更新于2025-09-23 15:22:29
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The Temperature Dependences of the Electron-Piezoelectric Potential Phonon Interacting System of Quasi Two Dimensional System in GaN and ZnS
摘要： We investigated theoretically the temperature dependence of the quantum optical transition of quasi 2-Dimensional Landau splitting system, in GaN and ZnS. We apply the Quantum Transport theory (QTR) to the system in the con?nement of electrons by square well con?nement potential. We use the projected Liouville equation method with Equilibrium Average Projection Scheme (EAPS). Through the analysis of this work, we found the increasing properties of Quantum Transition Line Shapes (QTLSs) and the Quantum Transition Line Widths (QTLWs) of CdS and GaN with the temperature we also found that QTLW, (cid:2)(cid:3)T (cid:4) of ZnS < (cid:2)(cid:3)T (cid:4) of GaN in (cid:5) = 394 (cid:6)m.
关键词： Quantum Transport Theory,Quantum Transition Line Widths (QTLW),Quantum Transition Line Shapes (QTLS),GaN and ZnS,Cyclotron Resonance,Equilibrium Average Projection Scheme (EAPS),Electron Phonon Coupling System
更新于2025-09-23 15:22:29
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Electron transport through phenylene sandwiched between zigzag graphene nanoribbons
摘要： We study systematically the electron transport through a phenylene rotor with an axis of atomic carbon chain (CPC) connected to twofold symmetric electrodes of nonmagnetic zigzag graphene nanoribbons. The density functional theory combined with the nonequilibrium Green’s function method is employed for the simulation. The CPC rotor is conductive with parabolic I–V characteristic when its ring is coplanar with the electrodes. Its rotation modulates the symmetry of its electron states and their matching to the states in the electrodes. The I–V curve then becomes characterized by sharp peaks with strong negative differential resistance (NDR) in a large range of the rotation angle. The corresponding shift of transport modes in energy with the rotation opens a way to efficient and accurate manipulation of NDR.
关键词： Electron transport,Graphene,Zigzag nanoribbon,Phenylene
更新于2025-09-23 15:22:29