- 标题
- 摘要
- 关键词
- 实验方案
- 产品
-
Radially resolved electronic structure and charge carrier transport in silicon nanowires
摘要： The electronic structure of silicon nanowires is studied using density functional theory. A radially resolved density of states is discussed for different nanowire diameters and crystal orientations. This approach allows the investigation of spatially varying electronic properties in the radial direction and extends previous studies, which are usually driven by a one-dimensional band structure analysis. We demonstrate strong differences in the electronic structure between the surface and the center of the nanowire, indicating that the carrier transport will mainly take place in the center. For increasing diameters, the density of states in the center approaches the bulk density of states. We find that bulk properties, such as the indirect nature of the band gap, become significant at a nanowire diameter of approximately 5 nm and beyond. Finally, the spatial characteristic of the current is visualized in terms of transmission pathways on the atomic scale. Electron transport is found to be more localized in the nanowire center than the hole transport. It also depends on the crystal orientation of the wire. For the growing demand of silicon nanowires, for example in the field of sensors or field-effect transistors, multiple conclusions can be drawn from the present work, which we discuss towards the end of the publication.
关键词： charge carrier transport,radially resolved density of states,silicon nanowires,density functional theory,electronic structure
更新于2025-09-09 09:28:46
-
Two-level system damping in a quasi-one-dimensional optomechanical resonator
摘要： Nanomechanical resonators have demonstrated great potential for use as versatile tools in a number of emerging quantum technologies. For such applications, the performance of these systems is restricted by the decoherence of their fragile quantum states, necessitating a thorough understanding of their dissipative coupling to the surrounding environment. In bulk amorphous solids, these dissipation channels are dominated at low temperatures by parasitic coupling to intrinsic two-level system (TLS) defects; however, there remains a disconnect between theory and experiment on how this damping manifests in dimensionally reduced nanomechanical resonators. Here, we present an optomechanically mediated thermal ringdown technique, which we use to perform simultaneous measurements of the dissipation in four mechanical modes of a cryogenically cooled silicon nanoresonator, with resonant frequencies ranging from 3–19 MHz. Analyzing the device’s mechanical damping rate at fridge temperatures between 10 mK and 10 K, we demonstrate quantitative agreement with the standard tunneling model for TLS ensembles con?ned to one dimension. From these ?ts, we extract the defect density of states (P0 ～ 1?4 × 1044 J?1 m?3) and deformation potentials (γ ～ 1–2 eV), showing that each mechanical mode couples on average to less than a single thermally active defect at 10 mK.
关键词： Quantum technologies,Nanomechanical resonators,Optomechanical,Mechanical damping rate,Deformation potentials,Tunneling model,Defect density of states,Two-level system (TLS) defects,Silicon nanoresonator
更新于2025-09-09 09:28:46
-
Applied Nanophotonics || Spontaneous emission of photons and lifetime engineering
摘要： Excited atoms, molecules, and solids emit spontaneously photons, with the rate of emission being dependent both on their intrinsic properties and the properties of ambient space. The latter is capable or not capable of carrying on certain electromagnetic modes. This property of space is described by means of the density of electromagnetic modes or, in quantum language, photon density of states. Photon density of states can be engineered in a desirable way using spatially arranged components with different dielectric permittivity on the wavelength scale. This chapter provides a brief introduction to spontaneous photon emission control in nanostructures based on confinement of electromagnetic waves to get photon density of states’ enhancement or inhibition.
关键词： nanostructures,electromagnetic waves,spontaneous emission,photon density of states,lifetime engineering
更新于2025-09-09 09:28:46
-
Phase diagram of a general biaxial nematic model based on density of states computation
摘要： The phase diagram of a general biquadratic Hamiltonian model of biaxial nematic liquid crystals was investigated both analytically through mean-field approximation and with computer simulations. However, their largely concurrent predictions are not borne out by experimentation, and the issue is still debated. We revisited this problem with Monte Carlo simulations based on the computation of density of states of the system through entropic sampling procedure, traversing through the relevant model parameter space (S) along representative trajectories. Our recent work indicated that the competing roles of different contributions in the Hamiltonian over significant regions of S could be the underlying entropic reason defying the earlier predictions. We find that our data differ from the reported results qualitatively, specifically as the trajectories approach the so-called partly repulsive regions of S. The complex free-energy profiles that we obtain in such cases, as a function of system order parameters, indicate entropic barriers to the development of the biaxial order to the expected degree. Significant increase in the influence of the intermolecular interactions between the uniaxial and biaxial tensors, at the expense of contributions from pure biaxial couplings, is indicated to be the inhibiting factor for the onset of the biaxial phase.
关键词： Biaxial nematics,free-energy curves,entropic ensemble,phase diagrams,Wang–Landau algorithm,density of states computation,frontier sampling
更新于2025-09-09 09:28:46
-
Modulation of the electronic properties of two-dimensional MoTe2/WSe2 heterostructure by electrical field
摘要： The structure and electronic properties of two-dimensional MoTe2/WSe2 van der Waals heterostructure under external electric field have been investigated by first-principles calculation. The interlayer distance between MoTe2 and WSe2 is 3.613? and the binding energy per unit is ?0.183eV, which indicated that MoTe2 is bound to WSe2 via van der Waals interaction. The interlayer distance can be modified by external electric field and the band gap of MoTe2/WSe2 heterostructure continuously decreases with increasing external electric field, eventually a transition from semiconductor to metal is observed, particularly, the band alignment of the MoTe2/WSe2 heterostructure can be effectively tuned from intrinsic type-II to type-I. Applying external electric field along +z direction and ?z direction has different effects on the band gap due to the intrinsic spontaneous polarization in MoTe2/WSe2 heterostructure. Our study indicates that the external electric field can significantly tune the band offsets and modify the band alignment between MoTe2 and WSe2. The present study would be helpful for application of such transition-metal dichalcogenides heterostructures in nano- and optoelectronics.
关键词： band gap,MoTe2/WSe2 heterostructure,external electric field,Density of States
更新于2025-09-09 09:28:46
-
Periodic polymers with increasing repetition unit: Energy structure and carrier transfer
摘要： We study the energy structure and the transfer of an extra electron or hole along periodic polymers made of N monomers, with a repetition unit made of P monomers, using a tight-binding wire model, where a site is a monomer (e.g., in DNA, a base pair), for P even, and deal with two categories of such polymers: made of the same monomer (GC …, GGCC …, etc.) and made of different monomers (GA …, GGAA …, etc.). We calculate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) eigenspectra, density of states, and HOMO-LUMO gap and find some limiting properties these categories possess, as P increases. We further examine the properties of the mean over time probability to find the carrier at each monomer. We introduce the weighted mean frequency of each monomer and the total weighted mean frequency of the whole polymer, as a measure of the overall transfer frequency content. We study the pure mean transfer rates. These rates can be increased by many orders of magnitude with appropriate sequence choice. Generally, homopolymers display the most efficient charge transfer. Finally, we compare the pure mean transfer rates with experimental transfer rates obtained by time-resolved spectroscopy.
关键词： LUMO,periodic polymers,weighted mean frequency,pure mean transfer rates,energy structure,carrier transfer,HOMO,density of states,HOMO-LUMO gap,tight-binding wire model
更新于2025-09-04 15:30:14
-
First Principle Calculations of Optical Properties of CdS:Al system (A DFT+U study)
摘要： In current research, we perform a computational study to calculate optical properties of Al doped CdS system for 1 × 1 × 2, 2 × 2 × 1, 2 × 2 × 2 supercell configurations and for DFT+U (1 × 2 × 2). Cd atoms are substituted with Al atoms and the result for optical properties shows significant changes in the case of 2 × 2 × 1 supercell configuration. This study has been executed with PBE-GGA and GGA+U approaches, used in Wien2K code employed within the framework of DFT. In comparison to 1 × 1 × 2 supercell configuration, optical absorption for 2 × 2 × 1 supercell configuration and DFT+U shows blueshift while for 2 × 2 × 2 redshift is observed. Study explore that optical properties are enhanced and absorption increases in visible region. Hence, Al doping into host CdS lattice suggest that opto-electrical properties are enhanced which advocate candidacy of this material for nanotechnology and optoelectronic devices.
关键词： DFT calculations,Density of states,Al-doping,Cadmium sulfide,Optical properties
更新于2025-09-04 15:30:14
-
Band-fluctuations model for the fundamental absorption of crystalline and amorphous semiconductors: a dimensionless joint density of states analysis
摘要： We develop a band-fluctuations model which describes the absorption coefficient in the fundamental absorption region for direct and indirect electronic transitions in disordered semiconductor materials. The model accurately describes both the Urbach tail and absorption edge regions observed in such materials near the mobility edge in a single equation with only three fitting parameters. An asymptotic analysis leads to the universally observed exponential tail below the bandgap energy and to the absorption edge model at zero Kelvin above it, for either direct or indirect electronic transitions. The latter feature allows the discrimination between the absorption edge and absorption tails, thus yielding more accurate bandgap values when fitting optical absorption data. We examine the general character of the model using a dimensionless Joint Density of States formalism with a quantitative analysis of a large amount of optical absorption data. Both heavily doped p-type GaAs and nano-crystalline Ga1?xMnxN, as examples for direct bandgap materials, as well as amorphous Si:Hx, SiC:Hx and SiNx, are modeled successfully with this approach. We contrast our model with previously reported empirical models, showing in our case a suitable absorption coefficient shape capable of describing various distinct materials while also maintaining the universality of the exponential absorption tail and absorption edge.
关键词： band-fluctuations model,fundamental absorption,disordered semiconductor materials,absorption edge,Joint Density of States,Urbach tail
更新于2025-09-04 15:30:14
-
Pressure-Induced Phase Transitions in TiO <sub/>2</sub> Rutile Nanorods
摘要： TiO2 nanorods (diameter of 6-8 nm) with the rutile-type structure have been investigated by Raman spectroscopy under high pressure. Additional bands in the Raman spectrum at ambient pressure are interpreted as resulting from defects that break the Raman selection rules as the simulated vibrational density of states (vDOS) is in good agreement with the experimental spectrum. Under pressure, the rutile structure transforms to the baddeleyite one at P~16 GPa. After pressure cycle, the α-PbO2 phase is recovered. The sequence of phase is discussed in terms of thermodynamics and kinetics based on crystallographic relationships. Contrary to other 1D TiO2 nanomaterials that adopt a metastable structure, the nanorods studied in this work show little impact of the morphology on the phase transitions sequence compared to the bulk TiO2.
关键词： phase transitions,vibrational density of states,TiO2 nanorods,high pressure,Raman spectroscopy
更新于2025-09-04 15:30:14
-
Quantum and spectral properties of the Labyrinth model
摘要： We consider the Labyrinth model, which is a two-dimensional quasicrystal model. We show that the spectrum of this model, which is known to be a product of two Cantor sets, is an interval for small values of the coupling constant. We also consider the density of states measure of the Labyrinth model and show that it is absolutely continuous with respect to Lebesgue measure for almost all values of coupling constants in the small coupling regime.
关键词： coupling constant,density of states measure,quasicrystal,Labyrinth model,spectrum
更新于2025-09-04 15:30:14