Electron Subbands Research Articles

Bi2WO6 hollow microspheres with high specific surface area and oxygen vacancies for efficient photocatalysis N2 fixation

In this work, Bi2WO6 hollow microspheres containing oxygen vacancies (OVs-BWO) was synthesized by a solvothermal template-free method. Raman, EPR and XPS spectra provide sufficient evidence for the existence of OVs in the material. Its activity toward photocatalytic nitrogen fixation was evaluated. Experimental results showed that the OVs-BWO with a BET surface area of 80.5 m2·g−1 exhibited the ammonia yield of 106.4 μmol·gcat−1 under simulated sunlight for 2 h, which is ca. 18 times higher than that of equal-area nest-like Bi2WO6 without oxygen vacancies. The enhanced activity is mainly attributed to the metastable electrons within the sub-band (i.e., defect energy level) induced by oxygen vacancies to π anti-bonding orbitals of N2 with a manner of the non-radiative transfer, leading to the activation of nitrogen molecules. The activation and hydrogeneration processes of nitrogen at active site of OVs-BWO were studied by DFT calculation and the possible mechanisms were proposed. The sub-band extends the light absorption region of OVs-BWO to 700 nm. Transient photocurrent response showed that the sub-band electron excitation accelerates the transfer of charges and hinders the electron-hole recombination. It can be perfectly reused five times when air and nitrogen are used as nitrogen sources, thus the OVs-BWO hollow microspheres offer a promising alternative for efficient N2 fixation under ambient conditions.

Conductance in a Nanoribbon of Topologically Insulating MoS2 in the 1T’ Phase

The use of new materials with advanced properties has become mandatory to meet the needs for higher electronics performance at reduced power. Topological insulators (TIs) possess highly conductive topologically protected edge states which are insensitive to scattering and thus suitable for energy-efficient high-speed devices. Here, we evaluate the subband structure in a narrow nanoribbon of 1 $\text {T}^\prime$ molybdenum disulphide using an effective k.p Hamiltonian. Highly conductive topologically protected edge modes whose energies lie within the bulk band gap are investigated on equal footing with traditional electron and hole subbands. Due to the interaction between the edge modes at opposite sides, a small gap in their linear spectrum opens up in a narrow nanoribbon. This gap is shown to sharply increase with the perpendicular out-of-plane electric field, in contrast to the behavior in a wide nanoribbon with negligible edge modes’ interaction. This increase leads to a rapid decrease in the ballistic nanoribbon conductance and current with the electric field, which can be used for designing molybdenum disulphide nanoribbon-based current switches.

Modifying the band gap of an armchair graphene nanoribbon by edge bond relaxation

The edge bond relaxation is an interesting techniques used to modify the band gap of an armchair graphene nanoribbon (AGNR) which is not possible for the case of a carbon nanotube. In this work, the modification of band gap of an AGNR by edge bond relaxation has been investigated via tight-binding calculation. When edge bond relaxation effects taken into account the carbon-carbon (CC) bond length at the left and right edges is slightly reduced as compared to the CC bond length in the middle of the AGNR as a result the electronic subbands found within their corresponding band structures shifts either upward or downward that leads to the change in their band gap due to the change in positions of both the conduction band minimum and the valence band maximum. The results show that without edge bond relaxation the AGNR exhibits direct band gap semiconductor behavior with a very small band gap of a u-v = 3 m + 2 family close to metallic property. The results further show that with edge bond relaxation the all AGNRs maintain their superior direct band gap semiconducting properties with band gap of the u-v = 3 m + 1 and u-v = 3 m + 2 families increases while the band gap of a u-v = 3 m family decreases as compared to the without edge bond relaxation for similar chirality values (u, v). Finally, it has been observed that the modification in band gap of an AGNR by edge bond relaxation opens up their availability for designing improved version of a good field-effect transistor and due to their direct band gap they are suitable nanostructured material for light-emitting diodes, lasers, and solar cells.

Dual-Wavelength Y-Branch DBR Lasers With 100 mW of CW Power Near 2 μm

The interband GaSb-based diode lasers emitting simultaneously in two narrow bands separated by either ~1.6 or ~3.3 THz were designed, fabricated and characterized. The device active region contained one asymmetric tunnel-coupled double quantum well with separation between two lowest electron subbands controlled by thickness of the tunnel barrier. The Y-branch 6th order distributed Bragg reflector devices have been fabricated with either deep or shallow etched ridge waveguides. The increase of the deeply etched ridge waveguide width from 10 to $20~\mu \text{m}$ improved laser threshold and efficiency thanks to reduction of the relative role of the sidewall defect recombination. Further improvement of the device performance parameter was achieved by shallow etching. The shallow etched lasers with stable dual-wavelength emission spectrum generated 100 mW of continuous wave output power at 20 °C.

Domino Effect of Thickness Fluctuation on Subband Structure and Electron Transport within Semiconductor Cascade Structures.

Effectively restraining random fluctuation of layer thickness (RFT) during the thin-film epitaxy plays an essential part in improving the quality of low-dimensional materials for device application. While it is already challenging to obtain an ideal growth condition for thickness control, the tangle of RFT with interfacial problems makes it even more difficult to guarantee the properties of heterostructures and the performance of devices. In our research, the RFT of potential barriers and wells within a semiconductor multilayer is demonstrated to correlate with the interfacial grading effect (IFG) and to affect the band offset strongly. Then, the synergetic effect of RFT and IFG that serves as the first domino is shown to impact the subband structure and the electron transport successively. On the basis of an investigation of a quantum cascade structure, statistical results indicate a normal distribution of RFT with a standard deviation of about 1 Å and an extreme value of 3 Å (about one monolayer) for all the layers within 38 cascade periods. The "seemingly negligible" RFT could actually reduce the conduction band offset for tens to hundreds of meV and alter the subband gaps at a rate of 40 meV/monolayer at most. Furthermore, the dependence of different subband gaps on the barrier/well thickness differs from one another. In addition, the distribution of wave function could also be regulated dramatically by RFT to change the type of electron transition and thus the carrier lifetime. Further impacts of RFT and the RFT-modulated subband alignment on electron transport result in two different mechanisms (injection-dominant and extraction-dominant) of electron population inversion (PI), which is manifested by comparatively discussing the results of in situ electron holography and macro performances.

Gate-Voltage-Induced Switching of the Spin-Relaxation Rate in a Triple-Quantum-Well Structure

Switching of the Dyakonov-Perel spin relaxation owing to the Rashba spin-orbit interaction (SOI) is theoretically studied in a semiconductor heterostructure with three quantum wells. The action of the present spin-relaxation switching is based on the gate-voltage induced electron transfer from the central well with vanishing Rashba SOI to the left or right well with large Rashba SOI. The spin-relaxation rate is calculated by extending the Dyakonov-Perel theory of the spin relaxation in a single subband to that in more than one subband in order to take into account the contribution from electrons in excited subbands at higher temperatures. It is shown that the on:off ratio of the spin-relaxation rate at room temperature reaches ${10}^{6}$ by choosing widths and compositions of well and barrier layers so as to reduce electron population in excited subbands, which gives an undesirable spin relaxation. The present spin-relaxation switching is expected to improve the on:off ratio of the current in the spin-lifetime field effect transistor.

Nonlocal Quantum Effects in Plasmons of Graphene Superlattices.

By using a nonlocal, quantum mechanical response function we study graphene plasmons in a one-dimensional superlattice (SL) potential V_{0}cosG_{0}x. The SL introduces a quantum energy scale E_{G}∼ℏv_{F}G_{0} associated with electronic subband transitions. At energies lower than E_{G}, the plasmon dispersion is highly anisotropic; plasmons propagate perpendicularly to the SL axis, but become damped by electronic transitions along the SL direction. These results question the validity of semiclassical approximations for describing low energy plasmons in periodic structures. At higher energies, the dispersion becomes isotropic and Drude-like with effective Drude weights related to the average of the absolute value of the local chemical potential. Full quantum mechanical treatment of the kinetic energy thus introduces nonlocal effects that delocalize the plasmons in the SL, making the system behave as a metamaterial even near singular points where the charge density vanishes.

Boron Impurity Effect on the Structural, Elastic, and Electronic Properties of Titanium Carbide

Atomic, structural, and elastic properties of titanium carbide with the boron impurity have been studied in the framework of the density functional theory in the general gradient approximation, by using the software ABINIT. The calculations of the total energy of a TiC supercell with boron impurity atoms showed that the latter do not tend to form clusters in titanium carbide. The equilibrium distances between the adjacent planes of titanium atoms were found to increase in the presence of the boron impurity. The electronic spectra of TiC supercells with various numbers and positions of boron impurity atoms are analyzed. The presence of boron impurity atoms is found to result in the appearance of a subband of their electron states, which is located between the local electronic spectra of the 2s and 2p carbon states by about 0.24 Hartree below the Fermi level. The coordination positions of boron impurity atoms affect only the shape and half-width of their electronic subband. An insignificant increase in the electron density of states just below the Fermi level also takes place. The bulk modulus of a titanium carbide supercell with boron impurity atoms is calculated and analyzed.

Solution of Self-Consistent Kohn–Sham and Poisson Equations for Quasi Two-Dimensional Electron Gas in the Accumulation Layer of Semiconductor with Nonparabolic Conduction Band

We generalize the method proposed previously for a self-consistent solution of the system of the Kohn–Sham and Poisson equations to the case of high density of electrons localized near the surface of a semiconductor due to size quantization of the electron spectrum in a potential well in the band banding region. We obtain a completely self-consistent iterative solution for a quasi two-dimensional electron gas in the accumulation layer at the surface of a semiconductor with a degenerate electron gas in the bulk. Both parabolic and non-parabolic energy spectra of conduction electrons are considered. For a parabolic conduction band, we obtain the spatial distributions of the electron density and electrostatic potential, as well as the energies of size-quantized levels and their dependence on the depth of the near-surface potential well. A significant decrease in the density of three-dimensional electrons was revealed in the region where the quasi two-dimensional electron gas occupied the accumulation layer. We calculated the dependencies of the excess electron surface density and the capacitance of the structure on the surface potential. The results are presented in dimensionless form that can be used to semi-quantitatively evaluate the parameters of accumulation layers by the known level of doping and the value of band bending. In the case of the nonparabolic conduction band, due to the finite bandgap width, the spectrum of quasi two-dimensional electron gas is obtained in the two-band Franz–Kane approximation by solving the eigenvalue problem for the new single-band equation of the effective-mass method. The density of states and the effective mass of electrons in the 2D subbands is found to increase linearly with energy. We have derived the expression that analytically approximates the calculated energy spectrum. The position of levels, the dispersion of energy spectrum, and the dependence of effective mass on energy, which were found for the size-quantized sub-bands, are in agreement with the published results for characteristics of quasi two-dimensional electrons at n-InAs surface that were directly measured by angle resolved photoelectron spectroscopy and magnetotunneling spectroscopy. Conditions are considered when the nonparabolicity of the conduction band enables quasi two-dimensional electrons to absorb the normally incident radiation due to intersubband transitions.

Persistent spin textures and currents in wurtzite nanowire-based quantum structures

We explore the spin and charge properties of electrons in wurtzite semiconductor nanowires where radial and axial confinement leads to tubular or ring-shaped quantum structures. Accounting for spin-orbit interaction induced by the wurtzite lattice as well as a radial potential gradient, we analytically derive the corresponding low-dimensional Hamiltonians. It is demonstrated that the resulting tubular spin-orbit Hamiltonian allows to construct spin states that are persistent in time and robust against disorder. We find that these special scenarios are characterized by distinctive features in the optical conductivity spectrum, which enable an unambiguous experimental verification. In both types of quantum structures, we discuss the dependence of the occurring persistent charge and spin currents on an axial magnetic field and Fermi energy which show clear fingerprints of the electronic subband structure. Here, the spin-preserving symmetries become manifest in the vanishing of certain spin current tensor components. Our analytic description relates the distinctive features of the optical conductivity and persistent currents to bandstructure characteristics which allows to deduce spin-orbit coefficients and other band parameters from measurements.

Giant anisotropy of Gilbert damping in a Rashba honeycomb antiferromagnet

Giant Gilbert damping anisotropy is identified as a signature of strong Rashba spin-orbit coupling in a two-dimensional antiferromagnet on a honeycomb lattice. The phenomenon originates in spin-orbit induced splitting of conduction electron subbands that strongly suppresses certain spin-flip processes. As a result, the spin-orbit interaction is shown to support an undamped non-equilibrium dynamical mode that corresponds to an ultrafast in-plane N\'eel vector precession and a constant perpendicular-to-the-plane magnetization. The phenomenon is illustrated on the basis of a two dimensional $s$-$d$ like model. Spin-orbit torques and conductivity are also computed microscopically for this model. Unlike Gilbert damping these quantities are shown to reveal only a weak anisotropy that is limited to the semiconductor regime corresponding to the Fermi energy staying in a close vicinity of antiferromagnetic gap.

Terahertz luminescence and photoconductivity associated with the impurity electron transitions in GaAs/AlGaAs quantum wells

The photoconductivity and photoluminescence spectra of GaAs/AlGaAs quantum wells doped with shallow donors are studied at low lattice temperatures. The optical electron transitions between the first electron subband and donor ground states, as well as between the excited and ground donor states, are revealed in the terahertz photoluminescence and photoconductivity spectra. The temperature evolution of the impurity-related photocurrent in the terahertz spectral range is also studied.

Competition between BCS and FFLO States in Magnetic Superconductors in a Cryptoferromagnetic Phase

The possibility of appearance of inhomogeneous superconducting Fulde—Ferrell—Larkin—Ovchinnikov (FFLO) states in magnetic superconductors in a cryptoferromagnetic phase with helical magnetic ordering has been analyzed. The dependence of the critical temperature on the angle between the wave vectors of the spatial modulation of the FFLO state and helical magnetic structure has been calculated within the proposed model. It has been shown that their mutually perpendicular orientation corresponds to the most energetically favorable state. The numerical calculations have also shown the existence of a tricritical point on a line separating the Bardeen—Cooper—Schrieffer and FFLO phases on the phase diagram of states. Furthermore, FFLO states can appear in a magnetic superconductor even at fairly strong exchange fields because of the difference between the effective masses of conduction electrons in different spin subbands and the anisotropy of the Fermi surface.

Dual wavelength operation of the GaSb-based Y-branch distributed Bragg reflector lasers near 2.1 μm

The GaSb-based diode lasers operating simultaneously at two wavelengths near 2.1 μm have been designed and fabricated. The Y-branch devices used the 6th order distributed Bragg reflectors (DBR) to provide spectrally selective feedback. The laser active region contained two asymmetric quantum wells with allowed optical transitions between two lowest electron (e1 and e2) and top hole (hh1) subbands. The laser emission lines loosely matched the e1-hh1 and e2-hh1 optical transitions with separation corresponding to ∼3.1 THz. The 10 μm wide deeply etched straight ridge DBR lasers demonstrated stable single mode operation with more than 50 mW of output power.

Electronic transport in n-type modulation-doped AlGaAs/GaAsBi quantum well structures: influence of Bi and thermal annealing on electron effective mass and electron mobility

We investigate electronic transport properties of as-grown and annealed n-type modulation-doped Al0.15Ga0.85As/GaAs1−xBix (x = 0 and 0.04) quantum well (QW) structures using magnetotransport measurements in the temperature range 4.2 K and 60 K and at magnetic fields up to 18 T. Thermal annealing process was applied at two different temperatures, 700 °C and 350 °C during 60 s and 180 s, respectively. We find that electron effective mass and 2D electron density in as-grown Bi-containing sample are slightly lower than that in Bi-free one. Furthermore, quantum electron mobility and quantum scattering time are observed to be decreased in Bi-containing samples. The annealing process at 700 °C causes a slight increase in electron effective mass and 2D electron density. A negligible decrease in electron effective mass and an increase in 2D electron density are determined following annealing at 350 °C. The observed change in electron effective mass following thermal annealing process is attributed to changing 2D electron density in the samples. No improvement on quantum electron mobility and quantum scattering time are observed following thermal annealing at both process temperatures. We determine that one electron subband (e1) for as-grown and annealed (at 700 °C for 60 s) Bi-containing QWs and two electron subbands (e1 and e2) for the annealed (at 350 °C for 180 s) GaAsBi QW sample and the Bi-free QW sample contribute to electronic transport. Our results reveal that there is no significant direct effect of Bi on effective electron mass, but an indirect effect, in which Bi can provoke changes in 2D electron density and hence causes not to observe actual band-edge electron mass but a deviation from its band-edge value. Therefore, it can be concluded that dispersion curve of conduction band does not change as an effect of Bi incorporation in GaAs.

Electron-hole hybridization in bilayer graphene.

Band structure determines the motion of electrons in a solid, giving rise to exotic phenomena when properly engineered. Drawing an analogy between electrons and photons, artificially designed optical lattices indicate the possibility of a similar band modulation effect in graphene systems. Yet due to the fermionic nature of electrons, modulated electronic systems promise far richer categories of behaviors than those found in optical lattices. Here, we uncovered a strong modulation of electronic states in bilayer graphene subject to periodic potentials. We observed for the first time the hybridization of electron and hole sub-bands, resulting in local band gaps at both primary and secondary charge neutrality points. Such hybridization leads to the formation of flat bands, enabling the study of correlated effects in graphene systems. This work may provide a novel way to manipulate electronic states in layered systems, which is important to both fundamental research and application.

Effects of thickness, impurity, surface roughness, and temperature on transport and thermoelectric coefficients of ultra-thin silicon nanowires: An analytic study

Abstract A comprehensive study of electronic, thermal, and thermoelectric coefficients of ultra-thin Si nanowires based on the analytic procedure is presented, and the effects of temperature and doping density variation that were not addressed in the previous simulated work, are reported. The quantum-wire electronic subbands and chemical potential are obtained via effective mass and envelope function approximation. To determine the low-field electron mobility and lattice thermal conductivity, relaxation time approximation within linear response theory is employed. The electron mobility is found to reach a peak value at 8 nm wire thickness, and reduces asymptotically to its bulk value with the increasing wire cross-section. Thermoelectric properties are studied over a wide range of thicknesses (3–14 nm), doping densities ( 3 × 1 0 15 – 1 . 6 × 1 0 19 cm−3), and temperatures (200–700 K). The thermoelectric figure of merit ZT increases modestly with the temperature in the range investigated, while its increase with the doping density is more distinct. The ZT variation with quantum confinement or nanowire cross-section reaches a peak value of 0.53 at 4 nm thickness at room temperature for rms surface roughness height Δ = 0 . 5 nm and doping density 1 . 6 × 1 0 19 cm−3, which is consistent with the previous work. The results of this comprehensive analytic study are found to be consistent with the available experimental data and in excellent agreement with elaborate Monte Carlo simulations, where available, while the method also allows for better physical understanding of the effects of different factors involved.

Electronic States in Cylindrical Core-Multi-Shell Nanowire

The recent advances in nanowire (NW) growth technology have made possible the growth of more complex structures such as core-multi-shell (CMS) NWs. We propose the approach for calculation of electron subbands in cylindrical CMS NWs within the simple effective mass approximation. Numerical results are presented for ${\rm GaAs/Al_{0.3}Ga_{0.7}As}$ radial heterostructure with AlGaAs-core and 4 alternate GaAs and AlGaAs shells. The influence of an effective mass difference in heterolayers is discussed.

Background impurities and a delta-doped QW. Part I: Center doping

The influence of shallow background donor impurities on the energy characteristics of the SiGe/Si/SiGe quantum well structure with centered delta-doping is studied numerically. The description of the self-consistent method includes the calculation of the donors impurity binding energy in the delta-layer. The delta-layer impurity binding energy as well as the energy differences between the first quantized electron subbands in the well demonstrate a significant dependence on the characteristics of the background doping. Therefore, the background doping cannot be neglected when studying phenomena like intersubband optical transitions.

Light absorption and pseudospin density generation in graphene nanoribbons

Light absorption in graphene nanoribbons is studied analytically and numerically. Due to size quantization, the electronic states in the nanoribbon form a set of one-dimensional subbands modifying remarkably the absorption coefficient. The electronic subbands are calculated using the Berry-Mondragon boundary conditions at the nanoribbon edges, which are applicable for nanoribbons of the armchair type. Numerical results on pseudospin (sublattice) density generation by light absorption are presented and discussed.