Results 1 - 10 of 11
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[en] The effect of lithium adsorption on the phase transition from 1H-MoS_2 to 1T-MoS_2 is studied by first-principles computations. The results indicate the possibilities of the phase transition for the lithium adsorption. Based on the results of charge density difference and charge-transfer of molybdenum disulfide with lithium adsorption, we elucidated that the mechanism of the changes of electronic property accompanying the phase transition is attributed to the electron transfer of different atoms. According to the result of transition state, it can be found that the phase-transition barrier is related to the coverage of lithium atoms on MoS_2 surface. It may be helpful to obtaining experimentally the stable 1T-MoS_2 structure. - Highlights: • The transformation of MoS_2 from semiconducting to metallic phases is elucidated on the essence of the electron transfer. • The relationship between Li coverage and phase transition barrier is obtained. • The decomposing of Lithiated 1T-MoS_2 at a high concentration of Li adsorption is explained. • The results can be helpful to obtaining experimentally the stable 1T-MoS_2 structure.
[en] Highlights: An ab-initio-thermodynamic-combined approach was applied to study the polytypism. Classical nucleation theory was parameterized by ab initio calculations. Chemical potential exhibits important influence on the polytypism of GaAs nanowires. We have studied the polytypism induced by the chemical potential through an ab-initio-thermodynamic-combined approach in Au-catalyzed GaAs nanowire growth. The nucleation in nanowire growth is numerically discussed by classical nucleation theory parameterized by ab initio calculations on the formation energies of the GaAs surfaces and steps. By using the approach we found that the polytypism induced by chemical potential could be explained by the energy competition of lateral walls of the nucleus very well. Furthermore, the obtained results confirmed that the driving force of wurtzite phase nucleation originates from the lower formation energy of its (1 0 -1 0) surface.
[en] First-principles calculations based on density-functional theory have been performed on the spacer-dependent magnetic properties of δ-doped GaAs/(Ga, Mn)As ferromagnetic heterostructures (DFH). It is found that all the structures show ferromagnetic (FM) alignment as the favoured configuration. Their electronic structures are half-metallic independent of the spacer thickness d. The interlayer exchange coupling (IEC) between two magnetic layers decreases sharply at first with increasing d, then reaches a stationary value at d = 4a0 (a0 is the lattice constant of GaAs), where the FM and antiferromagnetic states become energetically degenerate. The charge density and the strong spin-polarized holes are concentrated mostly in the vicinity of MnAs magnetic layers for all the structures. After the injection of free holes into the system, we confirm the free-hole mediated mechanism for the intralayer exchange coupling and IEC. Furthermore, the injection of holes enhances the intralayer exchange coupling much more than the IEC. From these results, it is definitely shown that the magnetic coupling of DFH can get across a nonmagnetic spacer. It is closely related to the spacer thickness d, and the redistribution of injection holes plays a key role when d is small.
[en] This paper presents a review of recent advances in quantum well and quantum cascade infrared photodetectors developed in Shanghai Institute of Technical Physics, Chinese Academy of Sciences (SITP/CAS). Firstly, the temperature- and bias-dependent photocurrent spectra of very long wavelength (VLW) GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) are studied using spectroscopic measurements and corresponding theoretical calculations in detail. We confirm that the first excited state, which belongs to the quasi-bound state, can be converted into a quasi-continuum state induced by bias and temperature. Aided by band structure calculations, we propose a model of the double excited states that determine the working mechanism in VLW QWIPs. Secondly, we present an overview of a VLW QWIP focal plane array (FPA) with 320×256 pixels based on the bound to quasi-bound (BTQB) design. The technology of the manufacturing FPA based on the QWIP structures has been demonstrated. At the operating temperature of 45 K, the detectivity of QWIP FPA is larger than with a cutoff wavelength larger than . Finally, to meet the needs of space applications, we proposed a new long wavelength quantum cascade detector with a broadband detection ranging from to . With a pair of identical coupled quantum wells separated by a thin barrier, acting as absorption regions, the relative linewidth () of response can be dramatically broadened to 30.7%. It is shown that the spectral shape and linewidth can be tuned by the thickness of the thin barrier, while it is insensitive to the working temperature. The device can work above liquid nitrogen temperature with a peak responsivity of 63 mA/W and Johnson noise limited detectivity of . (topical review)
[en] A novel structure to achieve long wavelength infrared broadband detection ranging from 7.6–10.4 µm is proposed via quantum cascade detector. With a pair of identical coupled quantum wells separated by a thin barrier, acting as absorption regions, the relative linewidth (ΔE/E) of response can be dramatically broadened to 30.7%. Moreover, this structure introduces about 30% enhancement in blackbody responsivity. It is shown that the spectral shape can be tuned by the bias voltage effectively, while it is insensitive to the working temperature. The device can work at liquid nitrogen temperature with Johnson noise limited blackbody detectivity of 5.4 × 108 Jones. (letter)
[en] We report on the effects of two different molecular beam epitaxy growth modes on the performance of In0.14Ga0.86As/GaAs quantum well infrared photodetectors (QWIPs). The performance of quantum well (QW) materials are characterized by photoluminescence (PL), x-ray diffraction, and high resolution transmission electron microscope, and a systematic photoelectric characterization is carried out for these InGaAs/GaAs QWIPs. The results indicate that the introduction of continuous low temperature growth can effectively reduce In atom interdiffusion while maintaining the higher PL intensity. QWIPs grown by the low temperature method show the bound-to-bound intraband transition mode as initially designed, whereas the high temperature during the growth makes the operating mode of the device changing to bound-to-quasi continuous mode, which affects the performance of the quantum well infrared photodetectors. Compared with InGaAs/GaAs QWIP fabricated by the temperature-changed growth method, the peak responsivity of the low-temperature grown sample is increased by a factor of 38 and reaches 5.67 A W−1 at 20 K, indicating high responsivity of InGaAs/GaAs QWIP. The reason is attributed to the pronounced increase of photoconductive gain in the device with the B–B working mode. Furthermore, the background limited performance temperature (T BLIP) of low-temperature grown QWIPs is improved by ~10 K. (paper)
[en] We report the dependence of the near-field optical modes in metal-insulator-metal quantum well infrared photodetector (MIM-QWIP) on the incident angles. Three optical modes are observed and attributed to the 2nd- and the 3rd-order surface plasmon polariton (SPP) modes and the localized surface polariton (LSP) mode. In addition to the observation of a responsivity enhancement of 14 times by the LSP mode, the varying pattern of the three modes against the incident angle are revealed, in which the LSP mode is fixed while the 2nd SPP mode splits into two branches and the 3rd SPP mode red-shifts. The detailed mechanisms are analyzed and numerically simulated. The results fit the experiments very well, demonstrating the wavevector coupling effect between the incident light and the metal gratings on the SPP modes. Our work will pave the way to fully understanding the influence of incident angles on a detector’s response for applying the MIM-QWIP to focal plane arrays.
[en] Quantum dots/graphene (QDs/Gr) composites have become the research hotspot recently due to their unique synergistic effect as optical absorption material for next-generation electronic and optoelectronic devices. In this work, Ge QDs/Gr composite is prepared by a simple and effective ion-beam sputtering deposition technique. The intact growth evolution process is detailly investigated by means of the effect of Ge deposition amount, which will induce the enhanced crystallinity in QDs and the reduced defects in graphene. Moreover, a feasible and inspiring strategy to effectively tune doping in graphene by artificial control through changing the deposition amount of Ge atoms on graphene is demonstrated. In addition, charge transfer and interaction strength at the interface of Ge QD and graphene is influenced via the oxygen defect in the QD surface, which is consistent with field-effect transistor test and first-principle calculations. The p-doping characteristics of graphene decorated by Ge QDs may have significant application prospects in energy band engineering of graphene-based building blocks for graphene-based composite development and near-infrared detector applications. (paper)
[en] We report the investigation of exciton dynamics in type-II self-assembled GaSb/GaAs quantum dots. The GaSb/GaAs quantum dots (QDs) were grown using a modified liquid phase epitaxy technique. Statistical size distributions of the uncapped QDs were investigated experimentally by field-emission scanning electron microscopy (SEM) and atomic force microscopy (AFM), and theoretically by an eight-band k · p calculation, which demonstrated a dissolution effect. Furthermore, the low-temperature luminescence spectra of type-II GaSb/GaAs QDs with a thick capping layer exhibit well-resolved emission bands and LO-phonon-assisted transitions in the GaSb wetting layer. However, the luminescence lines quench at temperatures above 250 K, which is attributed to the weak quantum confinement of electrons participating in indirect exciton recombination. It was demonstrated that the room temperature stability of the excitons in type-II GaSb/GaAs QDs could be achieved by growing thin a capping layer, which provides strong quantum confinement in the conduction band and enhances the electron–hole Coulomb interaction, stabilizing the excitons. (paper)
[en] In this work, the crystal structure of GaAs nanowires grown by molecular beam epitaxy has been tailored only by bismuth without changing the growth temperature and V/III flux ratio. The introduction of bismuth can lead to the formation of zinc-blende GaAs nanowires, while the removal of bismuth changes the structure into a 4H polytypism before it turns back to the wurtzite phase eventually. The theoretical calculation shows that it is the steadiest for bismuth to adsorb on the GaAs(111)B surface compared to the liquid gold catalyst surface and the interface between the gold catalyst droplet and the nanowire, and these adsorbed bismuth could decrease the diffusion length of adsorbed Ga and hence the supersaturation of Ga in the gold catalyst droplet.