Vacancy Research Articles

Nitrogen vacancies in GaN templates and their critical role on the luminescence efficiency of blue quantum wells.

In InGaN-based LEDs, an InGaN layer underlying active region has been widely used to improve the luminescence efficiency of the quantum wells (QWs). It has been reported recently that the role of InGaN underlayer (UL) is to block the diffusion of point defects or surface defects in n-GaN into QWs. The type and the source of the point defects need further investigations. In this paper, using temperature-dependent photoluminescence (PL) measurements, we observe emission peak related to nitrogen vacancies (VN) in n-GaN. In combination with secondary ion mass spectroscopy (SIMS) measurement and theoretical calculation, it is found that VN concentration in n-GaN is as high as about 3 × 1018 cm-3 in n-GaN grown with low growth V/III ratio and can be suppressed to about 1.5 × 1016 cm-3 by increasing growth V/III ratio. Luminescence efficiency of QWs grown on n-GaN under high V/III ratio is greatly improved. These results indicate high density of nitrogen vacancies are formed in n-GaN layer grown under low V/III ratio and diffuse into quantum wells during epitaxial growth and reduce the luminescence efficiency of the QWs.

Dynamic reactions of defects in ion-implanted 4H-SiC upon high temperature annealing

Single-photon emitters based on intrinsic defects in silicon carbide (SiC) are promising as solid-state qubits for the quantum information storage, whereas defect engineering in a controllable manner still remains challenging. Herein, the thermally-driven defect dynamic reaction in the ion implanted 4H-SiC has been exploited through the optical emission spectra of defects. For the heavy-ion (Si or Ar) implanted samples with abundant Frenkel pairs, the silicon vacancies (VSi) are energetically converted into the carbon antisite-vacancy pair (CSi-VC) upon annealing till 1300 °C for 30 min, accompanied with the gradual lattice recovery and local strain relaxation. The further temperature elevation dissociates the metastable CSi-VC into carbon antisite (CSi) and carbon vacancy (VC), as supported by the consequent quenching of the (CSi-VC)-related emission at 700 nm. Thus, the whole defect reaction is probed as the vacancy interconversion from VSi to VC with the byproduct of stacking faults. In contrast, the intermediate CSi-VC complexes are not energetically favorable during the annealing of the H-implanted sample, which results from the negligible generation of Frenkel pairs, as supported by the x-ray diffraction patterns and Raman scattering analysis. These findings provide guidance for defect engineering in SiC toward the creation of reliable single photon emitters.

Fabrication of direct Z-scheme Cu2O@V-CN (octa) heterojunction with exposed (1 1 1) lattice planes and nitrogen-rich vacancies for rapid sterilization

The Z-scheme heterojunction has demonstrated significant potential for promoting photogenerated carrier separation. However, the rational design of all-solid Z-scheme heterojunctions catalysts and the controversies about carrier transfer path of direct Z-scheme heterojunctions catalysts face various challenges. Herein, a novel heterojunction, Cu2O@V-CN (octa), was fabricated using V-CN (carbon nitride with nitrogen-rich vacancies) in-situ electrostatic self-wrapping Cu2O octahedra. Density functional theory (DFT) calculations revealed that the separation of carriers across the Cu2O@V-CN (octa) heterointerface was directly mapped to the Z-scheme mechanism compared to Cu2O/V-CN (sphere). This is because the Cu2O octahedra expose more highly active (111) lattice planes with more terminal Cu atoms and V-CN with abundant nitrogen vacancies to form delocalized electronic structures like electronic reservoirs. This facilitates the wrapping of Cu2O octahedra by V-CN and protects their stability via tighter interfacial contact, thus enhancing the tunneling of carriers for rapid photocatalytic sterilization. These findings provide novel approaches for designing high-efficiency Cu2O-based photocatalytic antifoulants for practical applications.

Proton-irradiation-induced degradation in GaN-based UV LEDs: Role of unintentionally doped carbon

Degradation and defect evolution in GaN-based UV LEDs under 3 MeV proton irradiation were throughly investigated in this work. Combined with the yellow luminescence band at ∼2.2 eV in photoluminescence spectra with the energy level of Ev + 0.16 eV extracted by deep-level transient spectroscopy measurement, an intrinsic CN-related defect with an abnormally decreased concentration after irradiation was identified. Based on energy levels, the spatial configuration of defects, and their correlations, several possible origins and evolution processes of the defects are systematically discussed. A defect evolution model under proton irradiation was established: C atoms departed from the CN-related defect aligned along the dislocation in the active region stimulated by the displacement damage effect, leaving behind a nitrogen vacancy (VN) aligned along the dislocation, accompanied by the generation of a non-interacting carbon interstitial (Ci). Both the increased VN and Ci were found to contribute jointly to the decrease in optical power and the increase in leakage current. This indicates that carbon—a common unintentional dopant in GaN-based optical devices—can be an important factor in the degradation of GaN-based UV LEDs under proton irradiation, and should be further noted in radiation resistance applications.

Self-Assembled p–n Homojunction in SnS2 Nanosheets for Enhanced Visible Light-Driven Photocatalysis

Defect engineering and hetero-interfacial bending are standard approaches for enhancing carrier separation and increasing visible light absorption in a semiconductor photocatalyst. Here, we demonstrate the evolution of sulfur vacancies (VS) and the self-assembly of a p–n homojunction through prolonged aqueous washing of SnS2 nanosheets. The experimental studies have validated increased hydrophilicity, layer thinning, VS generation, and extended photocarrier lifetime in washed SnS2 nanosheets. Density functional theory has revealed the changes in the local electronic structure of SnS2 in the presence of structural defects. An electrochemical impedance spectroscopy study has shown that VS actuates the self-assembly of a pseudo-p-type SnS layer over n-type SnS2 to generate the p–n homojunction. The p-type carrier densities are increased twice as the number of washings is increased from 3 to 10 times. The photocatalytic efficacy of the SnS2 homojunction is evaluated in the study of the degradation of several water contaminants under white light and monochromatic excitations. The proposed mechanism shows that the built-in electric field at the p–n junction promotes the separation of the photogenerated electrons and holes. Meanwhile, VS traps the electrons and transfers them to the catalytically reactive centers to participate in photocatalysis. Our findings demonstrate that a simple approach that involves prolonged aqueous washing of as-produced SnS2 promotes the formation of VS and the self-assembly of a p–n homojunction in SnS2 nanosheets with improved visible light photocatalytic performances.

Origin of Enhanced Nonradiative Carrier Recombination Induced by Oxygen in Hybrid Sn Perovskite

Oxygen ingression has been shown to substantially decrease the carrier lifetime of Sn-based perovskites, behind which the mechanism remains yet unknown. Our first-principles calculations reveal that in prototypical MASnI3 (MA = CH3NH3), oxygen by itself is not a recombination center. Instead, it tends to form substitutional OI through combining with native I vacancies (VI) and remarkably increases the original recombination rate of VI by 2-3 orders of magnitude. This rationalizes the experimentally observed sharp decline of carrier lifetime in perovskites exposed to air. The significantly enhanced carrier recombination is due to a smaller electron capture barrier of OI, resulting from lattice strengthening and the suppressed structural relaxation upon electron capture. These insights offer a route to further improve device performance via anion engineering in broad Sn-based perovskite optoelectronics operating in ambient air. Moreover, our results highlight the important role of lattice relaxation for nonradiative carrier capture in materials in general.

Theoretical investigations of Ti4C3 and Ti4C3T2 (T = F, O and OH) monolayers as anode materials for Li-ion batteries

Two-dimensional transition metals carbides have gained widespread researching attention as promising alternative anodes for rechargeable batteries due to their high reversible capacity. Herein, we study the electrochemical properties of Ti4C3 monolayer and its functionalized derivatives (Ti4C3T2) in Li-ion batteries by first-principles methods. The Ti4C3 and Ti4C3O2 monolayers show large capacities, able to adsorb two layers of Li on both sides of the monolayer, generating theoretical capacities of 470 and 412 mA·h/g, respectively. The diffusion barriers on bare Ti4C3 monolayer are 19.5 meV, which is lower than most of the 2D materials that have been reported. Finally, we also investigated the impact of intrinsic defects on adsorption and diffusion of Li on Ti4C3 and Ti4C3T2 monolayers. Calculations show that the introduction of carbon vacancies (VC) hinders Li adsorption on Ti4C3 and Ti4C3O2 monolayers but promotes Li adsorption on Ti4C3F2 monolayer. Titanium vacancies (VTi) facilitate the adsorption of Li on the Ti4C3F2 and Ti4C3O2 monolayers. The presence of vacancies also changes the diffusion behavior of Li. This work has a theoretical significance for the design of lithium-ion battery anode materials.

Strategy for reducing the carriers transfer antagonistic effect between heterojunction and plasmonic effect and weakening photocorrosion of Cu2O for excellent photocatalytic bacteriostasis.

Heterogeneous catalysis composed of plasmonic metal and semiconductor has been utilized to tune local surface electron density in MOA (Molecular oxygen activation). However, there is a severe antagonistic effect between Schottky junction carriers and SPR (Surface Plasmon Resonance) induced hot carriers transfer routers when metal and semiconductor are both excited to dramatically reduce carriers separation efficiency. Hence, a highly effective photocatalytic antifoulant obtained by V-CN (carbon nitride with nitrogen vacancies) in-situ loading Cu2O and Ag nanoparticles (Cu2O/Ag/V-CN) was introduced to promote MOA to assist the metal ions sterilization. The DFT calculations (Density Functional Theory) and FEM calculations (Finite Element Method) intuitively proved the photocatalytic antifoulant belonged to a ternary Z-scheme heterojunction and could visibly weaken the antagonistic effect of hot carriers and Schottky carriers transport routes. The delocalized electron structure caused by V-CN and the effective electron mediator of Ag were the key to the formation of Z-scheme interfacial heterojunctions. These conclusions were also supported by experimental data, like more ∙O2- production capacity, efficient carriers separation, and higher carriers lifetime (27% higher than Cu2O and Cu2O/V-CN) as well as the weakened Cu2O photocorrosion tendency (Cu2O turning into CuO). Additionally, except for increasing nearly-three times adsorption energy of O2 for rapid activation, Cu2O/Ag/V-CN with abundant nitrogen vacancies can more significantly slow metal ions release (less about 97% to pure Cu2O and at least 22% higher than reported systems), which can observably save the amount of catalyst and heavy metals content. Therefore, Cu2O/Ag/V-CN has great potential for practical antifouling applications.

Cross‐Sectional Carrier Lifetime Profiling and Deep Level Monitoring in Silicon Carbide Films Exhibiting Variable Carbon Vacancy Concentrations

The carrier lifetime control over 150 μm thick 4H‐SiC epitaxial layers via thermal generation and annihilation of carbon vacancy (VC) related Z1/2 lifetime killer sites is reported. The defect developments upon typical SiC processing steps, such as high‐ and moderate‐temperature anneals in the presence of a carbon cap, are monitored by combining electrical characterization techniques capable of VC depth‐profiling, capacitance–voltage (CV) and deep‐level transient spectroscopy (DLTS), with a novel all‐optical approach of cross‐sectional carrier lifetime profiling across 4H‐SiC epilayer/substrate based on imaging time‐resolved photoluminescence (TRPL) spectroscopy in orthogonal pump‐probe geometry, which readily exposes in‐depth efficacy of defect reduction and surface recombination effects. The lifetime control is realized by initial high‐temperature treatment (1800 °C) to increase VC concentration to ≈1013 cm−3 level followed by a moderate‐temperature (1500 °C) post‐annealing of variable duration under C‐rich thermodynamic equilibrium conditions. The post‐annealing carried out for 5 h in effect eliminates VC throughout the entire ultra‐thick epilayer. The reduction of VC‐related Z1/2 sites is proven by a significant lifetime increase from 0.8 to 2.5 μs. The upper limit of lifetimes in terms of carrier surface leakage and the presence of other nonradiative recombination centers besides Z1/2, possibly related to residual impurities such as boron are discussed.

Unraveling the Defect-Dominated Broadband Emission Mechanisms in (001)-Preferred Two-Dimensional Layered Antimony-Halide Perovskite Film

By adding molar-controlled SbCl3 in a Cs3Sb2Cl9 precursor, we employed a low-temperature solution-processed approach to prepare high-quality (001)-preferred Cs3Sb2Cl9 thin film, which demonstrates a stable defect-dominated broadband emission at room temperature. Density functional theory calculations reveal that the defect emission originates from the donor-acceptor pair (DAP) recombination between chlorine vacancy (VCl) and cesium vacancy (VCs). Furthermore, VCl + VCs DAP is more stable on the (001) surface. The improved film quality and the more stable VCl + VCs DAP increase the activation energy related to defect states, resulting in an enhancement of the defect emission for the high-quality (001)-preferred film. This work provides deep insight into the key role of the (001) surface in defect emission and a feasible strategy to enhance the defect emission in 2D halide perovskites A3B2X9 (A = CH3NH3, Cs, Rb; B = Bi, Sb; X = Cl, Br, I) by control of the thin film preferred orientation.

Interspersing CdS nanodots into iodine vacancy-rich BiOI sphere for photocatalytic lignin valorization

Flower-like BiOI was decorated by CdS nanodots and followed by the introduction of iodine vacancies (VI) for photocatalytic sodium lignosulfonate (SLS) valorization under visible light. The iodine vacancies could adjust the band configuration, strengthen the light absorption and act as electron traps, while the intimate contact between BiOI and CdS nanodots provides a high-speed channel for charge transfer. As a consequence, the photocatalytic performance of SLS conversion into value-added vanillin was greatly improved over CdS/BiOI-VI compared with those of CdS, BiOI and CdS/BiOI. The highest yield of vanillin is 10.95 mg/gSLS over CdS/BiOI-VI, about 5, 8.7, 1.3 times those of CdS, BiOI, CdS/BiOI, respectively, and exceeding most related photocatalysts reported elsewhere. More significantly, as to the lignin from Masson pine and alkali lignin, the corresponding vanillin yield can reach 7.04 and 6.54 mg/glignin, respectively, under the same condition, which suggests the great potential and universality for photocatalytic lignin valorization over such CdS/BiOI-VI heterostructure.

Understanding effect of distortions and vacancies in wurtzite AlScN ferroelectric memory materials: Vacancy-induced multiple defect state types and relaxation dependence in transition energy levels

Energy-efficient compact alternatives to fully digital computing strategies could be achieved by implementations of artificial neural networks (ANNs) that borrow analog techniques. In-memory computing based on crossbar device architectures with memristive materials systems that execute, in an analog way, multiply-and-accumulate operations prevalent in ANN is a notable example. Ferroelectric (FE) materials are promising candidates for achieving ANN thanks to their excellent down-scalability, improved electrical control, and high energy efficiency. However, it remains challenging to develop a crossbar device architecture using FE materials. The difficulty stems from decreasing the leakage current of FE hardware and, simultaneously, reducing the film thickness for achieving compact systems. Here, we have performed density-functional-theory calculations to investigate the electronic, energy-based, and structural signatures of wurtzite FE material Al0.75Sc0.25N with a nitrogen vacancy (VN) in different charge states. We find that VN can introduce two defect states, viz., the singlet state above the valence band maximum (VBM) and a triplet state below the conduction band minimum in wurtzite AlScN models. The calculations reveal that the group of transition levels E3+/2+/E2+/1+ with small formation energies occur at ∼0.78/1.03 eV above the VBM in the wurtzite AlScN with a relaxed configuration, which may shift by a large degree to lower energy levels if atoms surrounding the defect are not fully relaxed. Theoretical studies elucidate the vacancy-enhanced increase in the leakage current utilizing large AlScN supercells. These findings render atomistic insights that can provide a path forward for the design of next-generation portable low-power electronic systems.

The structural, mechanical and electrical properties of 2D SiC with C-related point defects and substitution of C by foreign atoms

As a wide and direct bandgap semiconductor material, two-dimensional (2D) graphene-like SiC has attracted extensive research attention recently. Native defects and foreign impurities often have remarkable effects on the properties of semiconductors. In this paper, the structural, mechanical and electrical properties of 2D SiC with carbon antisite (CSi), vacancy (VC), interstitial (Ci) and substitution of C by N (NC) and B (BC) have been studied systematically using first-principles calculations. It is found that defects do not cause structural reconstruction for 2D SiC. The formation energies of CSi, VC, NC and BC are not sensitive to the uniaxial strain. CSi and Ci are likely to be formed spontaneously due to the negative formation energy values. In addition, the phonon spectra and elastic constant calculations show that the properties of 2D SiC with VC and Ci are unstable. Furthermore, from the calculations of the band structures and density of states, we observed that the bandgap of 2D SiC is slightly reduced by CSi, VC, Ci, and NC, however is increased by BC. Prominent additional charge states are generated in the bandgap of 2D SiC except for NC. Moreover, the overall carrier mobility of 2D SiC could be significantly reduced by the defects.

Defect emission in Cs3Cu2I5 and CsCu2I3 halide films

Cs3Cu2I5 and CsCu2I3 have shown potential applications as optoelectronic materials, but the mechanism of their emission remains controversial. It is widely believed that the Stokes shifts of Cs3Cu2I5 (1.45 eV) and CsCu2I3 (1.63 eV) are caused by self-trapped excitons, but the apparent changes in emission intensity observed experimentally are difficult to explain by this theory. In this paper, the defect emission mechanism is investigated: Iodine vacancies (VI) are widely present in iodide materials, but the effect of such point defects on Cs3Cu2I5 and CsCu2I3 has been little studied. Theoretical calculations based on the first-principles suggest that the VI defect (3.05 eV) may be directly related to the Stokes shift of Cs3Cu2I5 (similar conclusions were obtained for CsCu2I3). In the experiments related to VI, the emission quenching in the iodination treatment is associated with the decrease of defect concentration, while the emission recovery in the annealing treatment stems from the increase of defect concentration caused by iodine decomposition. The iodination and annealing treatments for CuI shows that VI defects play an important role in the emission of iodide. In addition, experiments to control the defect concentration by RTP treatment also showed that VI defects have an effect on the emission intensity of Cs3Cu2I5 and CsCu2I3. Based on the experimental results and theoretical calculations, the emission mechanism of Cs3Cu2I5 and CsCu2I3 may be the semiconductor defect emission theory. We believe that our research on the defect emission theory will provide new insights into the application of Cs3Cu2I5 and CsCu2I3.

Filling Selenium into Sulfur Vacancies in Ultrathin Tungsten Sulfide Nanosheets for Superior Potassium Storage.

The development of WS2 as an anode for potassium-ion batteries (PIBs) is severely confined by the low K+ storage capacity and poor intrinsic electrical conductivity. Our previous study demonstrated that the creation of sulfur vacancies (VS) in WS2 can enhance its K+ storage capability. However, it is a big challenge to keep the stability of VS while reserving the excellent activity. Herein, we design Se-filled WS2 nanosheets with VS (VS-WS2-Se NS) for PIBs. The Se heteroatom filling into the VS can not only stabilize and activate them, rendering more active sites to adsorb K+, but also further enhance the electrical conductivity. Consequently, the VS-WS2-Se NS anode presents significantly promoted storage capacity and reaction kinetics, superior to the pristine WS2 and WS2 with only VS. Remarkably, the VS-WS2-Se NS anode exhibits the highest specific capacity of 363.9 mA h g-1 at 0.05 A g-1. Simultaneously, a high reversible capacity of 144.2 mA h g-1 after 100 cycles at 2.0 A g-1 is shown. Ex situ analyses demonstrated that the potassium storage mechanism involves the intercalation and conversion reaction between WS2 and K+. Moreover, DFT calculations revealed that the Se filling into VS can further enhance the electrical conductivity and reduce the K-insertion energy barriers of WS2 and thus account for the outstanding electrochemical performance. This study demonstrates that engineering the vacancies by the heteroatom filling strategy offers a novel and feasible route for designing high-performance electrode materials in various energy-storage systems.

Blue-light-excited red emission in a CaO:Eu phosphor

Introducing multiple luminescent centers into the matrix crystal is an effective measure to endow this class of materials with new optical properties. Here, the coexistence of Eu2+ and Eu3+ was achieved in CaO matrix crystal from CaCO3:Eu3+ by a double crucible carbon reduction method. Under NUV-light excitation, the PL spectrum of CaO:Eu consists both a broad band of Eu2+ and several sharp lines of Eu3+. More significantly, CaO:Eu exhibits broadband emission at λmax = 661 nm upon excitation at 466 nm. Calcium vacancies (VCa) in CaCO3:Eu3+ increase the reduction rate, thereby enhancing the emission intensity of Eu2+. The high Ra (91.2) and LE (49.36 lm/w), and low CCT (3435 K) of as-fabricated WLEDs suggest that the prepared CaO:Eu are promising materials for WLEDs applications. The findings provide a new avenue for exploring novel Eu2+-activated simple oxide-based red phosphors.

Effect of hydrogen on the unintentional doping of 4H silicon carbide

High-purity semi-insulating (HPSI) 4H silicon carbide (4H-SiC) single crystals are critical semiconductor materials for fabricating GaN-based high-frequency devices. One of the major challenges for the growth of HPSI 4H-SiC single crystals is the unintentional doping of nitrogen (N) and boron (B). The addition of hydrogen has been supposed to mitigate unintentional doping. However, the underlying mechanism has not been well understood. In this work, the role of hydrogen in the growth of HPSI 4H-SiC single crystals is investigated by first-principles formation-energy calculations. We find that the addition of hydrogen significantly mitigates N doping while hardly affecting B doping. Once hydrogen is added, hydrogen may adsorb at the growing surface of 4H-SiC, leading to surface passivation. Since N can react with hydrogen to form stable NH3 (g), the chemical potential of N is reduced, so that the formation energy of N in 4H-SiC increases. Hence, the critical partial pressure of nitrogen required for the growth of HPSI 4H-SiC single crystals increases by two orders of magnitude. Moreover, we reveal that the adjustment of relative B and N doping concentrations has a substantial impact on the Fermi energy of HPSI 4H-SiC. When the doping concentration of N is higher than that of B, N interacts with carbon vacancies (VC) to pin the Fermi energy at Z1/2. When the doping concentration of B is higher than that of N, the Fermi energy is pinned at EH6/7. This explains that the resistivity of unintentionally doped HPSI 4H-SiC may vary.

Large-scale synthesis of visible light responsive ZnS by one-step molten salt method

It still remains a big challenge for the large-scale synthesis of visible light responsive ZnS. In this work, a simple one-step molten salt method is developed for large-scale synthesis of visible light responsive ZnS, in which the as-obtained sample at T oC is named as ST (T = 200, 300, 400, 500, 600 °C). The degradation reaction of Rhodamine B (RhB) is used to evaluate photocatalytic activity under visible light (λ ＞ 400 nm). Among the samples, S200 shows the highest visible light activity. Under visible light irradiation (λ ＞ 400 nm), 70% of RhB can be degraded by S200 after 90 min. Typically, the visible light activity of S200 is 28.2 times higher than that of S600. The higher activity of S200 is mainly attributed to more sulfur vacancies (VS), wider light absorption range, higher charge separation efficiency and larger BET area. Specifically, the BET area of S200 (44.4 m2 g−1) is 4.1 times higher than that of S600 (8.7 m2 g−1), thus S200 can provide more active sites. Meanwhile, the photocurrent for S200 is 6 times higher than that of S600, demonstrating that S200 has a higher charge separation efficiency mainly caused by more surface VS. Density functional theory (DFT) calculation further demonstrates that VS is favorable thermodynamically for the adsorptions of O2 and H2O, which benefits the generation of •OH and •O2– active species, leading to an improved activity. After three cycles of RhB degradation, S200 shows outstanding stability with 95% visible-light activity remained. The molten salt method is simple and productive, which can be used for large-scale synthesis of photocatalysts in practice.

Sulfur Vacancies‐Triggered High SERS Activity of Molybdenum Disulfide for Ultrasensitive Detection of Trace Diclofenac

AbstractDue to the efficient photoinduced charge transfer properties of nonmetals, nonmetallic surface‐enhanced Raman scattering (SERS) substrates have been advanced rapidly these years. And among of these, molybdenum disulfide (MoS2) is perceived as one of the most representative nonmetallic SERS substrates due to its abundant active sites. However, its progress in SERS field is severely hindered because of the lower SERS activity. Herein, one type of vacancies‐triggered highly sensitive SERS substrate based on nanostructured MoS2 with sulfur vacancies (VS) is proposed. The VS‐MoS2 substrates are intended to promote the SERS performance through the VS density regulation. Intrinsically, moderate VS can change the bandgap structure of MoS2, thus promoting the separation of internal carriers and enhancing the interaction with target species. Externally, the presence of VS can bring abundant adsorption sites on MoS2 surface, which greatly improves its ability to enrich target molecules. Both internal and external properties make VS‐MoS2 a high SERS activity substrate. Moreover, with the antibiotic contaminant diclofenac as a model probe molecule, the VS‐MoS2 substrate can accurately perceive its nanomolar concentration level with an enhancement factor up to 5.85 × 103. This work demonstrates exciting potential for analysis in real water environments based on nonmetallic SERS applications.

Improved photoconductive gain and high responsivity in LT-GaAs on UHV annealing without arsenic overpressure

The study reports peculiar light absorption in LT-GaAs due to the prominence of arsenic vacancies (VAs), interstitials (Asi), and neutral arsenic anti-sites (AsGa0) after UHV annealing under no Arsenic vapour pressure. We compared unannealed and annealed samples’ transient absorption behaviour and vibrational modes through ultrafast transient absorption spectroscopy (UFTS) (over 2.75 eV–0.775 eV spectral range) and RAMAN spectroscopy, respectively. We found that the UHV heating of LT-GaAs has augmented the tensile strain in the film by incorporating split Asi. However, there is overall defect mitigation, as evident from the reduced TO phonon peak (RAMAN) and ∼59 meV reduction in Fermi level (UFTS). Even with reduced defects, the UFTS data for the annealed sample shows a higher switching speed and responsivity (1.75 ps faster) in the NIR range and a higher photoconductive gain in the visible region.