Subband States Research Articles

Lead-Free Cs2AgBiCl6 Double Perovskite: Experimental and Theoretical Insights into the Self-Trapping for Optoelectronic Applications.

Lead-free double perovskites (DPs) will emerge as viable and environmentally safe substitutes for Pb-halide perovskites, demonstrating stability and nontoxicity if their optoelectronic property is greatly improved. Doping has been experimentally validated as a powerful tool for enhancing optoelectronic properties and concurrently reducing the defect state density in DP materials. Fundamental understanding of the optical properties of DPs, particularly the self-trapped exciton (STEs) dynamics, plays a critical role in a range of optoelectronic applications. Our study investigates how Fe doping influences the structural and optical properties of Cs2AgBiCl6 DPs by understanding their STEs dynamics, which is currently lacking in the literature. A combined experimental-computational approach is employed to investigate the optoelectronic properties of pure and doped Cs2AgBiCl6 (Fe-Cs2AgBiCl6) perovskites. Successful incorporation of Fe3+ ions is confirmed by X-ray diffraction and Raman spectroscopy. Moreover, the Fe-Cs2AgBiCl6 DPs exhibit strong absorption from below 400 nm up to 700 nm, indicating sub-band gap state transitions originating from surface defects. Photoluminescence (PL) analysis demonstrates a significant enhancement in the PL intensity, attributed to an increased radiative recombination rate and higher STE density. The radiative kinetics and average lifetime are investigated by the time-resolved PL (TRPL) method; in addition, temperature-dependent PL measurements provide valuable insights into activation energy and exciton-phonon coupling strength. Our findings will not only deepen our understanding of charge carrier dynamics associated with STEs but also pave the way for the design of some promising perovskite materials for use in optoelectronics and photocatalysis.

Unraveling Defect-Mediated Enhancement of Transient Photoconductivity and Slower Carrier's Mobility Decay in Cu-Doped Cs2AgBiBr6 Nanocrystals Using Ultrafast Pump-Probe Spectroscopy.

Lead-free double perovskite nanocrystals (A2B'(III)B″(I)X6 NCs) address the instability and toxicity concerns of lead-based counterparts, but their device performance is limited by subpar absorption and unexplored carrier dynamics. Impurity ion doping offers a route to tune electrical conductivity and charge carrier transport. Herein, we synthesized Cu-doped Cs2AgBiBr6 (CABB) nanocrystals using a hot-injection approach and investigated the charge carrier's dynamics through ultrafast pump-probe spectroscopy. Copper introduction into the CABB lattice enhanced absorption in the near-infrared region and introduced sub-band gap defect states in CABB NCs. The transient absorption study revealed a faster bleach decay with increased copper doping, as a result of charge transfer from the conduction band to copper defect states. Also, an optical pump terahertz probe study displays higher photoconductivity and mobility in Cu-doped CABB NCs. Slower mobility decay in Cu-doped systems was attributed to the charge carrier's entrapment at the defect state. These findings suggest that copper-doped CABB is a superior contender for optoelectronic applications over conventional CABB.

Charge transport through the multiple end zigzag edge states of armchair graphene nanoribbons and heterojunctions.

This comprehensive study investigates charge transport through the multiple end zigzag edge states of finite-size armchair graphene nanoribbons/boron nitride nanoribbons (n-AGNR/w-BNNR) junctions under a longitudinal electric field, where n and w denote the widths of the AGNRs and the BNNRs, respectively. In 13-atom wide AGNR segments, the edge states exhibit a blue Stark shift in response to the electric field, with only the long decay length zigzag edge states showing significant interaction with the red Stark shift subband states. Charge tunneling through such edge states assisted by the subband states is elucidated in the spectra of the transmission coefficient. In the 13-AGNR/6-BNNR heterojunction, notable influences on the energy levels of the end zigzag edge states of 13-AGNRs induced by BNNR segments are observed. We demonstrate the modulation of these energy levels in resonant tunneling situations, as depicted by bias-dependent transmission coefficient spectra. Intriguing nonthermal broadening of tunneling current shows a significant peak-to-valley ratio. Our findings highlight the promising potential of n-AGNR/w-BNNR heterojunctions with long decay length edge states in the realm of GNR-based single electron transistors at room temperature.

Modulation of electron transport and quantum lifetimes in symmetric and asymmetric AlGaAs/InGaAs double quantum well structures

The low-temperature electron transport (τ t ) and quantum (τ q ) lifetimes are modulated as a function of applied electric field (F) in Al0.3Ga0.7As/In0.15Ga0.85As-based symmetric and asymmetric double quantum well structures by considering ionized impurity, alloy disorder and interface roughness scatterings. The asymmetry in the structure potential is achieved through differences in the well widths, i.e. W w1 ≠ W w2. We show that in a symmetric structure (W w1 = W w2 = 120 Å) resonance of subband states (RSS) occurs at F = F rss = 0, whereas in asymmetric structures [(W w1, W w2) = (90 Å, 150 Å) and (W w1, W w2) = (150 Å, 90 Å)], the RSS is shifted and occurs at |F rss| = 16.4 kV cm−1. The symmetric and antisymmetric nature of the subband wave functions at their corresponding F rss makes τ t a minimum and this increases with change in F during multisubband occupancy. However, the absence of the factor 1 – cosθ in the quantum lifetime makes τ q one order less than the magnitude of τ t , but τ q shows a maximum at the corresponding F rss and decreases with change in F. The above changes in τ t and τ q with F make their ratio τ t /τ q maximum at the transition of subband states.

Fractal microstructure of semiconducting Sb2S3 by surfactant-assisted spin coating and its advantage in optoelectronic properties

Antimony sulfide, Sb2S3 is a mixed band gap semiconductor having a great potential in photovoltaic applications. Its thin films and nanostructures are also a proven anode material in batteries and supercapacitors. This paper presents the synthesis of a unique fern-shaped fractal microstructure of Sb2S3 grown over bare glass substrate using a spin-coating assisted by Triton-X-100 surfactant. The FE-SEM images confirm the fractal formation spanning over hundreds of micrometers. The film exhibits a direct band gap of 1.62 eV. The photoluminescence spectra reveal a strong emission peak at 800 nm (1.55 eV) indicating radiative transitions from sub-band gap states. The surfactant, Triton-X-100, is found to be primarily responsible for forming the ‘Fern’ shaped fractals which have a fractal dimension of 1.84. Without the surfactant, a porous microstructure of Sb2S3 film is observed. The fractality is also associated with lower strain in the film leading to a reduced point defects which would otherwise act as potential recombination centers.

Origin of surface and subband states at the InAs(111)A surface

The atomic structure of surfaces and interfaces plays a vital role in the electronic quality and properties of quantum devices. The interplay between the surface and confined bulk subband states in terms of their susceptibility has been investigated in relation to crystal defects on an InAs(111)A-$(2\ifmmode\times\else\texttimes\fi{}2)$ reconstructed surface, using low-temperature scanning tunneling microscopy and spectroscopy. We measure the two-dimensional quantized subband states arising from the confined potential imposed by downward bending of the conduction band edge. Furthermore, we show evidence of the existence of surface Bloch states within the confined bulk band gap projected on the surface spectrum which have originated from the surface reconstruction. As expected, larger confined bulk band gaps at the surface and conduction band offset are measured to be 0.58 and 0.31 eV, respectively. We further show the scattering of these quantum states at different surface defects and demonstrate that surface states are more susceptible to the defect potential when compared with the corresponding subband states. This apparent contrast follows from the length scale at which these defect potentials actively interact on or near the surface. Our observed experimental results are supported by empirical tight-binding simulations for the subband states and first-principles density functional theory simulations for the surface states present on the surface.

Electric field induced non-monotonic electron transport in pseudomorphic Al0.3Ga0.7As/In0.15Ga0.85As coupled quantum well structure

In this article, the non-monotonic electron mobility (μ) is modulated by applying an external electric field (Fext) in pseudomorphic Al0.3Ga0.7As/In0.15Ga0.85As asymmetric coupled quantum well (ACQW) structure. The structural asymmetry is achieved through dissimilar doping in the side barriers. Application of a specific Fext (here, Fext = 6.66 kV/cm) makes the proposed structure potentially symmetric causing resonance of subband states (RSS) between the wells. Near resonance, the drastic change in the distribution of subband wave functions leads to cusp like variation of the scattering rate matrix elements resulting non-monotonic nature of μ. We show that the variation of μ is mostly due to the ionized impurity scattering mediated by intersubband effects. However, there is a sudden drop in mobility due to interface roughness scattering i.e., μir at RSS and subsequently, it reduces the overall magnitude of μ. We show that μ can be enhanced with a sharp drop at the resonance point by increasing the well width, central barrier width, spacer width, or doping region width of the proposed structure. The results obtained can be utilized to improve the performance of negative resistance based low dimensional device structures.

THz Response of Charge Carriers in Nanoparticles: Microscopic Master Equations Reveal an Unexplored Equilibration Current and Nonlinear Mobility Regimes

Herein, the THz mobility of charge carriers in low‐dimensional semiconductors based on a density matrix approach involving master equations for population and polarization dynamics is modeled. Pulsed THz fields induce intraband transitions between quantized subband states, creating polarization and subsequent charge transport that governs the electron mobility. It is shown that an equilibration current emerges—a purely quantum mechanical contribution understood via the Ehrenfest theorem in 1D—reshaping the low‐frequency mobility. Apart from thermal population, the results further demonstrate that the frequency‐dependent mobility becomes THz field strength and spectrum, as well as pulse width and chirp dependent, already at moderate THz probe fields of 1 kV cm−1, e.g., for 1D CdSe or GaAs nanostructures. The parametric nature of the underlying master differential equations for polarization and population, driving the intraband conductivity, results in a nonlinear, third‐order mobility and susceptibility, causing a nontrivial field dependence as well as power broadening even at moderate field strength. The obtained results are in good agreement with experiments. The observed high nonlinearities strongly impact the design and interpretation of THz charge carrier mobility experiments and further allow applications like coherent control, frequency mixing, or synthesis through a field‐controlled nonlinearity or high harmonics generation, especially interesting for future 6G telecommunication.

Impurity photo-ionization cross section and stark shift of ground and two low-lying excited electron-states in a core/shell ellipsoidal quantum dot

A new investigation on photo-ionization cross section (PICS) of a donor impurity confined in a GaAs/AlGaAs core/shell elliptic quantum dots (CSEQDs) has been performed under electric field (EF) effect. By means of the finite element method (FEM), we have computed the binding energy (BE), oscillator strength Pif and PICS within the framework of the effective mass theory (EMT). Our numerical results reveal that the stark shift is greatly dependent on the strength of the EF, the degree of ellipticity β, and the states of electronic transitions. It can take positive or negative values depending on the situation. In addition, the photo-ionization describing the transition from the donor impurity ground-state (1simp) to one of the conduction sub-band states (1s0, 2s0, and 2p0) is affected by the shape of the quantum dots (QDs) and the EF. We demonstrated that varying internal and external parameters may control the photo-ionization cross-section. These characteristics can be exploited to fabricate optoelectronic devices like QD infrared photodetectors.

Study of nonmonotonic electron mobility due to influence of asymmetric structure parameters in pseudomorphic heterojunction field effect transistors

Nonmonotonic electron mobility is obtained in InxGa1-xAs/GaAs double quantum well pseudomorphic heterostructure field effect transistor by changing the structure parameters. We show that a rapid drop in mobility occurs at the point of resonance of sub-band states due to asymmetric variation of doping concentrations. The sub-band wave function distributions change significantly near the resonance and influence the sub-band mobilities through the scattering potentials, there by causing the dip in μ. The depth of nonlinearity in μ enhances by increasing the central barrier width and the difference between the well widths. On the other hand, variation of μ as a function of asymmetric change of well widths leads to a hump like raise in μ under unequal doping concentrations. Our results of nonlinear mobility can be utilized in low temperature transistor applications.

Resonant Tunneling between Quantized Subbands in van der Waals Double Quantum Well Structure Based on Few-Layer WSe2.

We demonstrate van der Waals double quantum well (vDQW) devices based on few-layer WSe2 quantum wells and a few-layer h-BN tunnel barrier. Due to the strong out-of-plane confinement, an exfoliated WSe2 exhibits quantized subband states at the Γ point in its valence band. Here, we report resonant tunneling and negative differential resistance in vDQW at room temperature owing to momentum- and energy-conserved tunneling between the quantized subbands in each well. Compared to single quantum well (QW) devices with only one QW layer possessing quantized subbands, superior current peak-to-valley ratios were obtained for the DQWs. Our findings suggest a new direction for utilizing few-layer-thick transition metal dichalcogenides in subband QW devices, bridging the gap between two-dimensional materials and state-of-the-art semiconductor QW electronics.

Study of sub-band states formation in the optical band gap of CuGaS2 thin films by electronic excitations

The impacts of electronic excitations with three different ion beams, namely Au, Ag, and O ions at 100–120 MeV and fluences of 5 × 1012 ions/cm2, on the structural, optical, morphological, and electrical properties of spray-deposited CuGaS2 (CGS) thin films have been investigated. Powder X-ray diffraction and micro-Raman spectroscopy measurements have revealed a significant modification in crystallinity and an increase in crystallite sizes after irradiation. Atomic force microscopy (AFM) has revealed variations in surface roughness root-mean-square values and grain size following ion irradiation. UV/Vis-NIR spectroscopy has shown that ion-irradiated films exhibit a redshift compared to pristine CGS, attributable to the creation of new sub-band states between the conduction and valence bands. The resistivity of a CGS thin film changed from 5.33 × 10−2 Ωcm to 1.30 × 10−2 Ωcm, 9.6 × 10−2 Ωcm, and 9.63 × 10−2 Ωcm, respectively, after Au-, Ag-, and O-ion-irradiations. The highest photocurrent of 1.196 mA was observed for the Au-ion-irradiated CGS film. By comparing all of the results, it is concluded that an Au-ion-irradiated CGS thin film may act as a suitable photo-absorber for photovoltaic applications.

Superfluorescence of Sub-Band States in C-Plane In0.1Ga0.9N/GaN Multiple-QWs.

Superfluorescence is a collective emission from quantum coherent emitters due to quantum fluctuations. This is characterized by the existence of the delay time ( for the emitters coupling and phase-synchronizing to each other spontaneously. Here we report the observation of superfluorescence in c-plane In0.1Ga0.9N/GaN multiple-quantum wells by time-integrated and time-resolved photoluminescence spectroscopy under higher excitation fluences of the 267 nm laser and at room temperature, showing a characteristic from 79 ps to 62 ps and the ultrafast radiative decay (7.5 ps) after a burst of photons. Time-resolved traces present a small quantum oscillation from coupled In0.1Ga0.9N/GaN multiple-quantum wells. The superfluorescence is attributed to the radiative recombination of coherent emitters distributing on strongly localized subband states, Ee1→Ehh1 or Ee1→Elh1 in 3nm width multiple-quantum wells. Our work paves the way for deepening the understanding of the emission mechanism in the In0.1Ga0.9N/GaN quantum well at a higher injected carrier density.

The Deep Eutectic Solvent Precipitation Synthesis of Metastable Zn4V2O9.

A precipitation method involving a deep eutectic solvent (DES)─a mixture of hydrogen bond donor and acceptor─is used to synthesize a ternary metal oxide. Without toxic reagents, precipitates consisting of Zn3(OH)2V2O7·nH2O and Zn5(OH)6(CO3)2 are obtained by simply introducing deionized H2O to the DES solution containing dissolved ZnO and V2O5. Manipulation of the synthetic conditions demonstrates high tunability in the size/morphology of the two-dimensional nanosheets precipitated during the dynamic equilibrium process. According to differential scanning calorimetry and high-temperature powder X-ray diffraction, Zn3V2O8 and ZnO obtained by the annealing of the precipitate are intermediates in the reaction pathway toward metastable Zn4V2O9. Intimate mixing of the metal precursors achieved by the precipitation method allows access to the metastable zinc-rich vanadate with unusually rapid heat treatment. The UV-vis and surface photovoltage spectra reveal the presence of sub-band gap states, stemming from the reduced vanadium (V4+) center. Photoelectrochemical measurements confirm weak photoanodic currents for water and methanol oxidation. For the first time, this work shows the synthesis of a metastable oxide with the DES-precipitation route and provides insight into the structure-property relationship of the zinc-rich vanadate.

Asymmetric doping induced nonlinear electron mobility in double V-shaped quantum well based FET structure under external electric field

The effect of asymmetric doping profile on the electron mobility μ is studied in a Field Effect Transistor (FET) structure based on Al x Ga 1−x As double V-shaped quantum well (D-VQW) in presence of an external electric field F. We introduce asymmetry in the structure by considering different doping concentrations in the side barriers along the substrate and surface. The resultant asymmetric potential is varied as a function of F. Accordingly the energy levels E l and wave functions ξ l within the coupled structure alter. By varying F suitably the resonance of subband states can be realized between the wells. Around the point of resonance, ξ l rearranges significantly. The change in ξ l influences the screened scattering potentials through intersubband effects leading to nonlinear μ. We consider ionized impurity (Imp-) scattering and alloy disorder (Al-) scattering for the D-VQW structure and show that under double subband occupancy, μ has an oscillatory behavior, basically due to Imp-scattering. The oscillatory μ enhances by increasing the well width and central barrier width and also by decreasing the height of the V-shaped well. Our results of nonlinear electron mobility can be employed to tune the transistor performance with gate electric field by adopting an optimized set of structure parameters.

The influence of surface roughness on the presence of polymorphs and defect states in P3HT layers

Despite extensive studies on the archetypal poly(3-hexylthiophene-2,5-diyl) (P3HT) over the past decade, a concurrent investigation of the surface roughness influence on polymorphism, density of states (DOS), photoluminescence (PL), and hole mobility is missing. We report on the effect of substrate roughness on the optical and electrical properties of P3HT thin films deposited on smooth c-Si (roughness < 1 nm) and rough ITO (roughness ∼ 6 nm) substrates. Grazing-incidence wide-angle X-ray scattering and the energy-resolved electrochemical impedance spectroscopy were used to correlate the two polymorphs with the defect DOS in P3HT thin film grown on ITO substrate. The Jablonski diagram reconstructed from PL spectra elucidates the PL bleaching associated with sub-bandgap defect states above the highest occupied molecular orbital found in the case of the rough ITO substrate. A decrease in the PL intensity of the 0–1 transition caused by this extra band of defect states is presumably due to a non-radiative transition from the excited to the ground state or reduction of transition probability. The observed sub-band gap states have no effect on hole mobility. However, the continuum of gap states, which increases with decreasing layer thickness on both substrates, lowers the hole mobility by one order.

Majorana bound states in topological insulators without a vortex

We consider a three-dimensional topological insulator (TI) wire with a nonuniform chemical potential induced by gating across the cross section. This inhomogeneity in chemical potential lifts the degeneracy between two one-dimensional surface state subbands. A magnetic field applied along the wire, due to orbital effects, breaks time-reversal symmetry and lifts the Kramers degeneracy at zero momentum. If placed in proximity to an $s$-wave superconductor, the system can be brought into a topological phase at relatively weak magnetic fields. Majorana bound states (MBSs), localized at the ends of the TI wire, emerge and are present for an exceptionally large region of parameter space in realistic systems. Unlike in previous proposals, these MBSs occur without the requirement of a vortex in the superconducting pairing potential, which represents a significant simplification for experiments. Our results open a pathway to the realization of MBSs in present-day TI wire devices.

Controlling Photoluminescence and Photocatalysis Activities in Lead‐Free Cs2PtxSn1−xCl6 Perovskites via Ion Substitution

Lead-free halide perovskites have triggered interest in the field of optoelectronics and photocatalysis because of their low toxicity, and tunable optical and charge-carrier properties. From an application point of view, it is desirable to develop stable multifunctional lead-free halide perovskites. We have developed a series of Cs2 Ptx Sn1-x Cl6 perovskites (0≤x≤1) with high stability, which show switchable photoluminescence and photocatalytic functions by varying the amount of Pt4+ substitution. A Cs2 Ptx Sn1-x Cl6 solid solution with a dominant proportion of Pt4+ shows broadband photoluminescence with a lifetime on the microsecond timescale. A Cs2 Ptx Sn1-x Cl6 solid solution with a small amount of Pt4+ substitution exhibits photocatalytic hydrogen evolution activity. An optical spectroscopy study reveals that the switch between photoluminescence and photocatalysis functions is controlled by sub-band gap states. Our finding provides a new way to develop lead-free multifunctional halide perovskites with high stability.

Hybridization three subbands at Dirac point in special designed strained HgTe thin films with structural inversion asymmetry

We study specially designed strained thin HgTe layers with structural inversion asymmetry (SIA) which allow us to distinguish the topological surface states (TSS) typical for a two dimensional (2D) quantum well system in a subband state. To obtain such a dispersion relation on the basis of the eight-band kp model, the theoretical investigation calculations of thin (below 25 nm wide) HgTe strained films with SIA are investigated. The numerical band-gap engineering and dispersion relation allow us to obtain a new class of materials that are characterized by a Dirac-like dispersion and hybridization of the three different charges describing two TSS and one quantum well subband at Γ-point (zero gap). This opens up many possibilities from the applications point of view. An external electric field removed this degeneration and opened a band gap between Γ6, (lh—light hole) and (hh—heavy hole) subbands characteristic for TSS and the subband characteristic for the 2D quantum well state, respectively. The width of the band gap as a function of the external electric field is also considered. Due to consideration being given to the possible applications, analysis of the dispersion relation and Landau levels (LL) energy shape with SIA is also investigated. The possibility of tuning a band gap is promising from the point of view of, for example, THz detectors and emitters. What is very important, the proposed structures allow the avoidance of the coexistence of TSS with bulk states, as very often occurs in so-called 3D strained HgTe-like materials. Analysis of the wave function as a function of the width of the investigated structure as well as the external electric field is also presented. Due to the strong correlation between both states (2D and TSS), and their very well known properties, we expect that such HgTe films can be used as optical active layers in the THz region.

Majorana zero modes in a cylindrical semiconductor quantum wire

We study Majorana zero modes properties in cylindrical cross-section semiconductor quantum wires based on the $k \cdot p$ theory and a discretized lattice model. Within this model, the influence of disorder potentials in the wire and amplitude and phase fluctuations of the superconducting order-parameter are discussed. We find that for typical wire geometries, pairing potentials, and spin-orbit coupling strengths, coupling between quasi-one-dimensional sub-bands is weak, low-energy quasiparticles near the Fermi energy are nearly completely spin-polarized, and the number of electrons in the active sub-bands of topological states is small.