Compositional Superlattices Research Articles

Photostimulated Nernst effect in two-dimensional compositional semiconductor superlattices under the influence of confined phonons

The photostimulated Nernst effect in compositional semiconductor superlattices under the influence of a confined acoustic phonon is studied by using the quantum kinetic equation. The case of the confined electron-confined acoustic phonon scattering process is examined in detail. The obtained analytical results reveal that the Nernst coefficient (NC) depends on the external field such as magnetic field B, the frequency Ω, and the amplitude E 0 of the electromagnetic wave in a complicated way but also like a function of the temperature of the system and the period of the superlattice. It also depends on the quantum number m-describing the confined acoustic phonon. The theoretical results are depicted and discussed for GaAs/AlxGa 1 − xAs compositional semiconductors superlattices. The results indicate that the Shubnikov-De hass oscillations have appeared. The confined phonons make the magnitude of the Nernst coefficient higher and more obvious than the case of a bulk phonon. In addition, the magneto-photon resonance condition is also proved. Besides, the Nernst coefficient decreases significantly as the temperature increases. The oscillations Nernst coefficient in a magnetic field are consistent with the previous experimental.

Theoretical Study of the Nernst Effect in Compositional Superlattices in the Presence of Strong Electromagnetic Wave

The Nernst effect has been theoretically studied in compositional superlattice in the presence of a strong electromagnetic wave. Using the quantum kinetic equation for electrons with two cases electrons-acoustic phonons scattering and electrons-optical phonons scattering, we obtained the analytic expression of the Nernst coefficient and kinetic tensors as a function of the magnetic field, temperature, frequency, and amplitude of the electromagnetic wave and parameters of the compositional superlattice. The dependence of the Nernst coefficient on the magnetic field and temperature is achieved by numerical calculations for AlGa/AlGaAs material. The result indicates that in the case of electron-acoustic phonon scattering, the Shubnikov-de Hass oscillation appears. Whereas, in the case of electrons-optical phonons scattering, the peak of magneto-photon-phonon resonance appears. In both cases, when temperature increases, the Nernst coefficient decreases rapidly, and for electrons-optical phonons scattering, the resonance peak has a movement.

MXene Nanosheet/Organics Superlattice for Flexible Thermoelectrics

Two-dimensional (2D) materials with outstanding electronic transport properties are rigid against bending because of strong in-plane covalent bonding and intrinsically flexible because of the lack of out-of-plane constraint and thus are considered to be promising for flexible thermoelectrics (TEs). As a typical 2D material, MXene, however, exhibited a restricted TE performance because the termination groups and guest molecules in MXene nanosheets introduced by acid etching and reassembly deteriorate intra/interflake conduction. This work realized increases in both the carrier concentration and intra/interflake mobility by the construction of a MXene nanosheet/organic superlattice (SL) and composition engineering, attributed to electron injection, intercoupling strengthening, and defect reduction at the nanosheet edges. An electrical conductivity increased by 5 times, to 2.7 × 105 S m–1, led to power factors of up to ∼33 μW m–1 K–2, which is above the state-of-the-art for similar materials, almost by a factor of 10. A TE module comprising four SL film legs could yield 58.6 nW power at a temperature gradient of 50 K. Additionally, both the annealed film and the corresponding module exhibited excellent reproducibility and stability. Our results provide a strategy to tailor the TE performance of 2D-material films through SL construction and composition engineering.

Erratum: Theoretical study of photo-stimulated thermo-magnetoelectric effects in two-dimensional compositional superlattices using quantum kinetic equation

Erratum: Theoretical study of photo-stimulated thermo-magnetoelectric effects in two-dimensional compositional superlattices using quantum kinetic equation

Theoretical study of photo-stimulated thermo-magnetoelectric effects in two-dimensional compositional superlattices using quantum kinetic equation

Theoretical study of photo-stimulated thermo-magnetoelectric effects in two-dimensional compositional superlattices using quantum kinetic equation

Size effect on interfacial pseudocapacitive contributions to lithium-ion storage in microscale carbon/TiO2 nanosheet composite

The research on pseudocapacitive characteristics is mostly focused on the nano- or submicron-scale metal-oxide electrode materials. However, the studies on related properties are still rare in the microscale materials. Electrode materials with a microscale particle size can exhibit higher tap density and lower specific surface areas, which are more conducive to commercial applications. Therefore, here we present a study about the effects of particle size on interfacial pseudocapacitive contributions to lithium-ion storage and the internal dynamics associated with the interfacial ion diffusions in two-dimensional microscale carbon/TiO2 nanosheet superlattice composites. The results reveal the initial particle size of the layered material exhibits significant effects on the kinetics of the pseudocapacitive redox reactions that lead to different rate behaviors. The higher reversible capacity will be achieved with the smaller particle size for the microscale-layered carbon/TiO2 nanosheets. A specific capacity of 512.5 mAh/g and a high pseudocapacitive reaction at a current of 0.1 A/g are achieved for carbon/TiO2 with a particle size of 4.3 μm. The excellent electrochemical properties of the microscale-layered carbon/TiO2 are attributed to the increase in the interfacial pseudocapacitive contribution, and the decrease in the ion diffusion path resulted from the decrease in the particle sizes.

Enhanced thermoelectric performance and tunable polarity in 2D Cu2S-phenol superlattices composites for solar energy conversion

Solar energy conversion is of great interest in developing green and sustainable energy. Herein, we report the solar-to-electricity capability of a new two dimensional (2D) Cu2S-phenol superlattice (CP-SL) and carbon nanotubes (CNTs) composites for the first time. CP-SL is demonstrated to have a high Seebeck coefficient and a low thermal conductivity comparable to that of metal-organic frameworks. CP-SL based solar thermoelectric generator (STEG) exhibits stable output voltages for long time usages superior to previously reported STEGs. The device is made of p- and n-type modules that are composed of CP-SL/CNT and CP-SL/polyethyleneimine (PEI) doped CNTs (PEI-CNT) composites, respectively. The polarity of the composites is dominated by the CNTs which have higher carrier concentration. The carrier transport mechanism in the composites matches well with a parallel model, indicating the CP-SL and CNT interfaces play a minor role in carrier transport. The maximum ZT value of CP-SL/CNT is achieved by an in-situ growth method, which is about 35 times higher than that of the original CP-SL. These results indicate that 2D CP-SL is a new material with tunable thermoelectric properties and polarities, which may lay a foundation to realize p- and n-type properties in one material for single-material organic electronic devices development.

Self-formed compositional superlattices triggered by cation orderings in m-plane Al1\u2212xInxN on GaN

Immiscible semiconductors are of premier importance since the source of lighting has been replaced by white light-emitting-diodes (LEDs) composed of thermodynamically immiscible InxGa1−xN blue LEDs and yellow phosphors. For realizing versatile deep-ultraviolet to near-infrared light-emitters, Al1−xInxN alloys are one of the desirable candidates. Here we exemplify the appearance and self-formation sequence of compositional superlattices in compressively strained m-plane Al1−xInxN films. On each terrace of atomically-flat m-plane GaN, In- and Al-species diffuse toward a monolayer (ML) step edge, and the first and second uppermost < stackrel{-}{1}stackrel{-}{1}20> cation-rows are preferentially occupied by Al and In atoms, respectively, because the configuration of one In-N and two Al-N bonds is more stable than that of one Al-N and two In-N bonds. Subsequent coverage by next < stackrel{-}{1}stackrel{-}{1}20> Al-row buries the < stackrel{-}{1}stackrel{-}{1}20> In-row, producing nearly Al0.5In0.5N cation-stripe ordering along [0001]-axis on GaN. At the second Al0.72In0.28N layer, this ordinality suddenly lessens but In-rich and In-poor < stackrel{-}{1}stackrel{-}{1}20>-rows are alternately formed, which grow into respective {0001}-planes. Simultaneously, approximately 5-nm-period Al0.70In0.30N/Al0.74In0.26N ordering is formed to mitigate the lattice mismatch along [0001], which grow into approximately 5-nm-period Al0.70In0.30N/Al0.74In0.26N {10stackrel{-}{1}2} superlattices as step-flow growth progresses. Spatially resolved cathodoluminescence spectra identify the emissions from particular structures.

A synergetic promotion of sodium-ion storage in titania nanosheets by superlattice assembly with reduced graphene oxide and Fe-doping strategy

Titania has been taken as one promising anode material for sodium ion batteries because of its long cycling stability and suitable working voltage. One of the main bottlenecks for its use is the low capacity. To overcome this challenge, in this work, restacked two dimensional (2D) titania nanosheets, which has an open edge and enlarged interlayer spacing for the ion diffusion, have been doped with Fe elements and hetero-assembled with the conductive reduced graphene oxide (rGO). The conductive rGO added increased the charge transfer at the interface of anode and electrolyte, decreased the diffusion barrier of Na+ inside the composites and increased the stability of titania nanosheet during the charge and discharge process, increasing the capacity and long cycle stability of the anode. The high concentration doping of Fe in the titania nanosheets further decreased the Na+ diffusion barrier, increased the electrical conductivity of the composites and greatly increased the storage sites of Na+. The as obtained superlattice composites of Ti0.67Fe0.3O2/rGO showed a sodium ion capacity of 378.8 mA h g−1 at 0.1 A g−1 after 100 cycles, which is the highest values among all reported Ti and Fe oxide materials. It still delivered a stable capacity of 287.4 mA h g−1 at high current charge density (1 A g−1) after a long cycling process (3000 cycles).

A superlattice-based resonant cavity-enhanced photodetector operating in the long-wavelength infrared

The design, fabrication, and characterization of a resonant cavity-enhanced photodetector (RCE PD) operating in the long-wavelength infrared regime are demonstrated. The incorporation of the low bandgap InAs/InAs0.70Sb0.30 type-II strained-layer superlattice into the absorber layer of the detector cavity, along with the high-reflectivity (Rm &amp;gt; 0.9) AlAs0.08Sb0.92/GaSb distributed Bragg reflector pairs, results in resonant enhancement at 7.7–7.8 μm, which is a spectral region relevant in applications in sensing of chemical warfare agents and in medical biomarker diagnostics. These resonant wavelength peaks also display a high quality factor in the range of 76–86 and a small temperature coefficient of 0.52 nm K−1. An nBn architecture, where an Al0.71Ga0.29As0.08Sb0.92 layer acts as a barrier for majority electrons while minimizing the valence band offset with the absorber, is also incorporated into the cavity in order to improve the electrical properties of the detector. Spectral response measurements yield a peak external quantum efficiency of 14.6% and a peak responsivity of 0.91 A W−1 at 77 K and −0.8 V; meanwhile, a dark current density of 2.0 × 10−4 A cm−2 at 77 K results in a specific detectivity of 3.7 × 1010 cm Hz1/2 W−1, coming close to the theoretical background-limited D* of an ideal broadband photovoltaic detector with the superlattice composition as that of the RCE PD.

Study of the Quantum Magneto-Thermoelectric Effect in the Two-Dimensional Compositional Superlattice GaAs/AlGaAs under the influence of an Electromagnetic Wave by Using the Quantum Kinetic Equation

The quantum magneto-thermoelectric effect in a two-dimensional compositional superlattice under the influence of an electromagnetic wave (EMW) in two cases is investigated. Two cases of the electron scattering mechanism are considered: the electron-acoustic phonon scattering and electron-optical phonon scattering. Analytical expressions for the quantum Ettingshausen coefficient (EC), the thermopower tensor, the thermoelectric tensor and the kinetic tensor are obtained by using a quantum kinetic equation. These expressions are numerically solved for the two-dimensional compositional superlattice GaAs/AlGaAs and the results are discussed. The results show that in the case of electron-acoustic phonon scattering, the Shubnikov-de Haas oscillations appear when we examine the dependences of the quantum EC, the thermopower tensor and the thermoelectric tensor on the magnetic field. In the case of electron-optical phonon scattering, resonance peaks that satisfy the condition of the inter-subband magneto-phonon resonance appear. In the two cases, the superlattice period (a parameter specific to the material) strongly affects the quantum magneto-thermoelectric effect. When the superlattice period is small, quantum EC oscillations (in the case of electron-acoustic phonon interaction) and quantum EC resonance peaks (in the case of electron-optical phonon interaction) appear. However, when the superlattice period is large, these oscillations and resonance peaks are not observed. Especially, the influence of electromagnetic waves on the quantum magneto-thermoelectric effect is also clarified. The quantum theory of the magneto-thermoelectric effect has been studied from low temperature to high temperature. This overcomes the limitations of the Boltzmann kinetic equation which was studied at high temperature. The results are new and can serve as a basis for further development of the theory of quantum magneto-thermoelectric effects in low-dimensional semiconductor systems.

Fabrication and Characterization of an InAs(Sb)/InxGa1−xAsySb1−y Type‐II Superlattice

Infrared optoelectronic devices based on type‐II superlattice structures of III–V semiconductor materials have progressed significantly during the past decades. Exploring and further expanding the material space is of great significance for the development of infrared applications. Herein, a superlattice structure based on InAs(Sb)/InxGa1−xAsySb1−y grown using a fractional monolayer alloy process to control the superlattice composition and structure is presented. High‐order satellite peaks with a narrow full width at half maximum (31 arcs) in the X‐ray diffraction patterns indicate the good crystal quality of the superlattices. Transmission electron microscopy (TEM) with energy‐dispersive spectroscopy (EDS) analysis validate the fractional monolayer alloy growth method. Photoluminescence measurements and k·p model calculations reveal the superior optical properties of the superlattice (sevenfold higher intensity than that of InAs/GaSb). In addition, the flexible control over the superlattice structure afforded by the method allows miniband engineering to cover the mid‐wavelength and long‐wavelength infrared regions. This work highlights the great potential of InAs(Sb)/InxGa1−xAsySb1−y superlattices in infrared optoelectronic devices.

Magneto – thermoelectric Effects in Compositional Superlattice in the Presence of Electromagnetic Wave

Ettingshausen coefficient (EC) in the compositional semiconductor superlattice under the influence of electromagnetic wave (EMW) is surveyed by using the quantum kinetic equation for electrons. The analytical expressions of the Ettingshausen coefficient are numerically calculated for the GaAs/AlGaAs compositional semiconductor superlattice. It have been showed that the appearance of EMW has changed the EC’s value and the EC decreases nonlinearly when the temperature increases. Studying the dependence of EC on the magnetic field, we discovered that the resonance peaks have appeared and the superlattice period strongly affects the Ettingshausen coefficient.&#x0D;

Improved spectral and temporal response of MSM photodetectors fabricated on MOCVD grown spontaneous AlGaAs superlattice

A co-planar metal-semiconductor-metal nonsymmetrical back to back Schottky diode photodetector using natural superlattice AlGaAs grown by metalorganic vapor phase epitaxy on GaAs (100) has been reported. The detection efficiency and photoresponse of the superlattice based device are found significantly superior compared to the one based on high temperature annealed homogeneous AlGaAs. Under a forward bias of 1 V, the peak values of responsivity, detectivity and sensitivity were 10.133 mA/W, 7.6 × 1011 cmHz1/2W−1, 81.06 cm2/W and 1.14 mA/W, 7.05 × 1010 cmHz1/2W−1, 2.82 cm2/W for the device with as-grown natural superlattice and homogeneous composition of AlGaAs, respectively. Besides, the device with natural superlattice structure showed much faster response to the pulsed light with rise and decay time of 560 μs and 1 ms as compared to 2 and 7 ms, respectively for the device with disordered bulk AlGaAs. The superior spectral and temporal characteristics of the device are explained by a model based on a third diode representing the net effect due to the superlattice modulations along with two Schottky diodes at the metal-semiconductor junctions. The third barrier, which is basically due to the periodic modulation in aluminium composition, plays an important role in enhancement of the photocurrent owing to the activation of the superlattice channels under light while keeping the dark current small. The fast sweeping of the photogenerated carries by the intrinsic electric field at the heterointerfaces in the active semiconducting layer makes the characteristic times of the device with the superlattice structures much smaller than one with homogeneous AlGaAs. Degradation in photoresponse and speed is attributed to the interdiffusion as an effect of thermal annealing.

High operating temperature InAs/GaSb type-II superlattice detectors on GaAs substrate for the long wavelength infrared

We report on the development of InAs/GaSb type-II superlattice infrared photodetectors for operation under temperatures reachable with thermoelectric cooling. We investigate optically immersed, laterally operated photoconductors with a cutoff wavelength around 10 μm at an operating temperature of 200 K. The identification of a suitable superlattice composition, the growth of a linearly graded metamorphic buffer layer and the transfer of the device concept from GaSb to GaAs are motivated and described. We show that immersion lens technology even for non-doping optimized devices enables a peak spectral detectivity above 6 × 109 cm Hz0.5 W−1 at 195 K, approaching the performance of commercially available HgCdTe-based photoconductors.

X-ray reflectivity and photoelectron spectroscopy of superlattices with silicon nanocrystals

The structural properties and features of the chemical composition of SiO x N y /SiO2, SiO x N y /Si3N4, and SiN x /Si3N4 multilayer thin films with ultrathin (1–1.5 nm) barrier SiO2 or Si3N4 layers are studied. The films have been prepared by plasma chemical vapor deposition and have been annealed at a temperature of 1150°С for the formation of silicon nanocrystals in the SiO x N y or SiN x silicon-rich layers with a nominal thickness of 5 nm. The period of superlattices in the studied samples has been estimated by X-ray reflectivity. The phase composition of superlattices has been studied by X-ray electron spectroscopy using the decomposition of photoelectron spectra of the Si 2p, N 1s, and O 1s levels into components corresponding to different charge states of atoms.

Influence of confined optical phonons and laser radiation on the hall effect in a compositional superlattice

The effect of confined optical phonons and laser radiation are taken into account to study the Hall effect in a compositional semiconductor superlattice (CSSL). The new theoretical expressions of the Hall conductivity, the Hall coefficient and the magnetoresistance are obtained as functions of the external fields (the magnetic field B, the laser frequency Ω and amplitude E), the CSSL parameters (the well length dI, the barrier length dII and height U) and the quantum number m characterizing the phonon confinement effect. The magneto-confined phonon resonance condition in a CSSL is also carried out to explain the numerical evaluation results for the GaAs/AlxGa1−xAs CSSL. When optical phonons are confined, the Hall conductivity has more resonance peaks in comparison with that for bulk phonons. In addition, these sub-peaks are more separate from the main peaks as the CSSL layers thickness decreases. The tighter CSSL period is, the more forcefully the phonon confinement makes. When the CSSL period d is smaller than 30nm, the contribution of confined optical phonons is significant and could not be neglected. The magnetophonon resonance for bulk semiconductor could be achieved in the limit the CSSL period goes to infinity. Furthermore, the Hall conductivity increases and the Hall coefficient, the magnetoresistance decrease due to the laser radiation and the effect of confined optical phonons. Especially, the near-linear dependence of the magnetoresistance on temperature is in good agreement with the experimental result.

Nonlinear current density in a compositional semiconductor superlattice under crossed electric and magnetic fields

Nonlinear current density in a compositional semiconductor superlattice under crossed electric and magnetic fields

Investigation of the magnetoresistivity in compositional superlattices under the influence of an intense electromagnetic wave

The magnetoresistivity (MR) is theoretically calculated in a compositional semiconductor superlattice (CSSL), subjected to a crossed DC electric field and magnetic field, in the presence of an intense electromagnetic wave (EMW). The magnetic field is oriented along the growth direction of the CSSL and the electron–acoustic phonon interaction is taken into account at low temperature. Numerical results for the GaN/AlGaN CSSL show the Shubnikov–de Haas (SdH) oscillations in the MR whose period does not depend on the temperature and amplitude decreases with increasing temperature. The temperature dependence of the relative amplitude of these oscillations is in good agreement with other theories and experiments in some two-dimensional (2D) electron systems. The influence of the EMW as well as superlattice structure on the MR is discussed and compared with available theoretical and experimental results.

Development of efficient electrocatalysts via molecular hybridization of NiMn layered double hydroxide nanosheets and graphene.

Ni(2+)Mn(3+) layered double hydroxide (LDH) nanoplatelets have been hydrothermally synthesized in a homogeneous precipitation of mixed Ni(2+)/Mn(2+) salts at a molar ratio of 2 : 1 via the hydrolysis of hexamethylenetetramine (HMT) and in situ oxidation with H2O2. After anion-exchange, NiMn LDH was exfoliated into unilamellar nanosheets. Subsequent flocculation of NiMn LDH nanosheets with (reduced) graphene oxide (GO/rGO) into superlattice composites was achieved and further tested as electrocatalysts for oxygen evolution reaction (OER). The face-to-face heteroassembly of NiMn LDH nanosheets with conductive rGO at an alternating sequence resulted in a small overpotential of 0.26 V and a Tafel slope of 46 mV per decade, which is much superior to as-exfoliated nanosheets. The analyses of electrochemical activity surface area (ECSA) and impedance spectra clearly indicated that the superlattice structure was ideal in facilitating the migration/transfer of the charge and reactants, revealing the electrochemical energetics and mechanism behind the synergistic effect arising from molecular hybridization. The proof of concept toward total water splitting using the newly developed hybrid electrocatalyst was demonstrated by an electrolysis cell powered by a single AA battery.