Midgap Level Research Articles

NdxZn1-xO nanomaterial: Relating the effects of hydrazine-based synthesis to the improved structure, morphology and optical properties

The current paper reports a convenient method to obtain NdxZn1-xO (x = 0.05 and 0.1 % wt.) nanomaterial useful for optoelectronic applications. The material prepared from hydrazine-Nd/Zn-oxalate exhibited enhanced photoluminescence (PL) properties compared to their counterpart material prepared from Nd/Zn-oxalate precursor. The significant improvements in the microstructure and blue-light PL emissions of NdxZn1-xO were owed to the hydrazine-modified precursor. The XRD of materials confirmed the hexagonal wurtzite ZnO crystal structure. The microstructural analysis by SEM/TEM indicated spherically shaped nanosized crystal growth with narrow particle size distribution (10–70 nm). The HR-TEM revealed superior crystallinity with well-aligned lattice planes. EDAX, XPS and Raman confirmed the elemental composition (Zn2+, O2- and Nd3+) and their chemical bonding in the NdxZn1-xO. The band gap of materials were reduced due to the introduction of newly created mid-gap energy levels by Nd3+ dopant. The RT-PL spectra exhibited dual emission, the first UV band attributed to excitonic recombination while the second visible blue-green band assigned to deep-level emissions caused by intrinsic defects (O and Zn vacancies) and shallow Nd3+ and Zni levels. Improved PL intensities could be linked to the combined effect of Nd3+ doping and extra defects formed by the hydrazinate precursor. The blue-PL band (∼ λ = 468 nm) intensified 1.5 times with a narrow bandwidth of ̴ 15 nm. Hence, the current studies elucidated the optimistic use of hydrazine precursor for the production of Nd-doped ZnO nanomaterials with improved particle morphology, intrinsic defects and optical light emission properties.

Compositional engineering of interfacial charge transfer in van der Waals heterostructures of graphene and transition metal dichalcogenides.

Owing to their unique optical and electronic properties, vertical van der Waals heterostructures (vdWHs) have attracted considerable attention in optoelectronic applications, such as photodetection, light harvesting, and light-emitting diodes. To fully harness these properties, it is crucial to understand the interfacial charge transfer (CT) and recombination dynamics across vdWHs. However, the effects of interfacial energetics and defect states on interfacial CT and recombination processes in graphene-transition metal dichalcogenide (Gr-TMD) vdWHs remain debated. Here, we investigate the interfacial CT dynamics in Gr-TMD vdWHs with different chemical compositions (W, Mo, S, and Se) and tunable interfacial energetics. We demonstrate, using ultrafast terahertz spectroscopy, that while the photo-induced electron transfer direction is universal with graphene donating electrons to TMDs, its efficiency is chalcogen-dependent: the CT efficiency of S atom-based vdWHs is 3-5 times higher than that of Se-based vdWHs thanks to the lower Schottky barrier present in S-based vdWHs. In contrast, the electron back transfer process from TMD to Gr, which defines the charge separation time, is transition metal-dependent and dominated by the mid-gap defect level of TMDs: W transition metal-based vdWHs possess extremely long charge separation, well beyond 1ns, which is significantly longer than Mo-based vdWHs with only 10s of ps charge separation. This difference can be traced to the much deeper mid-gap defect reported in W-based TMDs compared to Mo-based ones, resulting in modified energetics for the back electron transfer from the trapped states to graphene. Our results shed light on the role of interfacial energetics and defects by tailoring chemical compositions of TMDs on the interfacial CT and recombination dynamics in Gr-TMD vdWHs, which is pivotal for optimizing optoelectronic devices, particularly in the field of photodetection.

A Multiscale Simulation on Aluminum Ion Implantation-Induced Defects in 4H-SiC MOSFETs

Aluminum (Al) ion implantation is one of the most important technologies in SiC device manufacturing processes due to its ability to produce the p-type doping effect, which is essential to building p–n junctions and blocking high voltages. However, besides the doping effect, defects are also probably induced by the implantation. Here, the impacts of Al ion implantation-induced defects on 4H-SiC MOSFET channel transport behaviors are studied using a multiscale simulation flow, including the molecular dynamics (MD) simulation, density functional theory (DFT) calculation, and tight-binding (TB) model-based quantum transport simulation. The simulation results show that an Al ion can not only replace a Si lattice site to realize the p-doping effect, but it can also replace the C lattice site to induce mid-gap trap levels or become an interstitial to induce the n-doping effect. Moreover, the implantation tends to bring additional point defects to the 4H-SiC body region near the Al ions, which will lead to more complicated coupling effects between them, such as degrading the p-type doping effect by trapping free hole carriers and inducing new trap states at the 4H-SiC bandgap. The quantum transport simulations indicate that these coupling effects will impede local electron transports, compensating for the doping effect and increasing the leakage current of the 4H-SiC MOSFET. In this study, the complicated coupling effects between the implanted Al ions and the implantation-induced point defects are revealed, which provides new references for experiments to increase the accepter activation rate and restrain the defect effect in SiC devices.

Dual-Photosensitizer Synergy Empowers Ambient Light Photoactivation of Indium Oxide for High-Performance NO2 Sensing.

Light-activated chemiresistors offer a powerful approach to achieving lower-temperature gas sensing with unprecedented sensitivities. However, an incomplete understanding of how photoexcited charge carriers enhance sensitivity obstructs the rational design of high-performance sensors, impeding the practical utilization under commonly accessible light sources instead of ultravioletor higher-energy sources. Here, a rational approach is presented to modulate the electronic properties of the parent metal oxide phase, exemplified by this model system of Bi-doped In2O3 nanofibers decorated with Au nanoparticles (NPs) that exhibit superior NO2 sensing performance. Bi doping introduces mid-gap energy levels into In2O3, promoting photoactivation even under visible blue light. Additionally, green-absorbing plasmonic Au NPs facilitate electron transfer across the heterojunction, extending the photoactive region toward the green light. It is revealed that the direct involvement of photogenerated charge carriers in gas adsorption and desorption processes is pivotal for enhancing gas sensing performance. Owing to the synergistic interplay between the Bi dopants and the Au NPs, the Au-BixIn2-xO3 (x=0.04) sensing layers attain impressive response values (Rg/Ra=104 at 0.6ppm NO2) under green light illumination and demonstrate practical viability through evaluation under simulated mixed-light conditions, all of which significantly outperforms previously reported visible light-activated NO2 sensors.

COF/In2S3 S-Scheme Photocatalyst with Enhanced Light Absorption and H2O2-Production Activity and fs-TA Investigation.

Photocatalytic hydrogen peroxide (H2O2) synthesis from water and O2 is an economical, eco-friendly, and sustainable route for H2O2 production. However, single-component photocatalysts are subjected to limited light-harvesting range, fast carrier recombination, and weak redox power. To promote photogenerated carrier separation and enhance redox abilities, an organic/inorganic S-scheme photocatalyst is fabricated by in situ growing In2S3 nanosheets on a covalent organic framwork (COF) substrate for efficient H2O2 production in pure water. Interestingly, compared to unitary COF and In2S3, the COF/In2S3 S-scheme photocatalysts exhibit significantly larger light-harvesting range and stronger visible-light absorption. Partial density of state calculation, X-ray photoelectron spectroscopy, and femtosecond transient absorption spectroscopy reveal that the coordination between In2S3 and COF induces the formation of mid-gap hybrid energy levels, leading to smaller energy gaps and broadened absorption. Combining electron spin resonance spectroscopy, radical-trapping experiments, and isotope labeling experiments, three pathways for H2O2 formation are identified. Benefited from expanded light-absorption range, enhanced carrier separation, strong redox power, and multichannel H2O2 formation, the optimal composite shows an impressive H2O2-production rate of 5713.2µmol g-1 h-1 in pure water. This work exemplifies an effective strategy to ameliorate COF-based photocatalysts by building S-scheme heterojunctions and provides molecular-level insights into their impact on energy level modulation.

Plasma-assisted annealing of Pt-doped rutile TiO2 nanoparticles for enhanced decomposition and bacterial inactivation under general lighting

Enhanced photocatalytic activity of rutile-based TiO2 materials under general lighting is practically desired. O2 plasma-assisted annealing (PAA) effects on Pt-doped rutile TiO2 nanoparticles were clarified along with its visible-light-driven photocatalytic activity enhancement. The PAA-treated samples were mainly analyzed using optical spectroscopy and x-ray photoelectron spectroscopy (XPS). The photocatalytic activity was assessed by decomposing methylene blue dye and inactivating Bacillus subtilis under general lighting. The PAA treatment changed the O 1s, Ti 2p, and Pt 4f spectra of XPS from those of the pristine sample. This change indicated that the PAA treatment introduced more oxygen deficiency or oxygen vacancies and more oxygen groups adsorbed on the surface. The introduced oxygen vacancies and adsorbed oxygen groups would change the band structure, which primarily narrowed the bandgap energy or broadened the valence band edge, increased the number of electron-trapping sites from the shallow to midgap levels, and enhanced the upward band-bending at the surface. The PAA-induced change in the band structure enhanced the decomposition and bacterial inactivation because it facilitated the separation and concentration of photoexcited carriers. The findings provide a new perspective on enhancing the photocatalytic activities of rutile-based TiO2 nanoparticles under general lighting.

Controlled p-Type Doping of Pyrite FeS2.

Pyrite FeS2 has extraordinary potential as a low-cost, nontoxic, sustainable photovoltaic but has underperformed dramatically in prior solar cells. The latter devices focus on heterojunction designs, which are now understood to suffer from problems associated with FeS2 surfaces. Simpler homojunction cells thus become appealing but have not been fabricated due to the historical inability to understand and control doping in pyrite. While recent advances have put S-vacancy and Co-based n-doping of FeS2 on a firm footing, unequivocal evidence for bulk p-doping remains elusive. Here, we demonstrate the first unambiguous and controlled p-type transport in FeS2 single crystals doped with phosphorus (P) during chemical vapor transport growth. P doping is found to be possible up to at least ∼100 ppm, inducing ∼1018 holes/cm3 at 300 K, while leaving the crystal structure and quality unchanged. As the P doping is increased in crystals natively n-doped with S vacancies, the majority carrier type inverts from n to p near ∼25 and ∼55 ppm P, as detected by Seebeck and Hall effects, respectively. Detailed temperature- and P-doping-dependent transport measurements establish that the P acceptor level is 175 ± 10 meV above the valence band maximum, explain details of the carrier inversion, elucidate the relative mobility of electrons and holes, reveal mid-gap defect levels, and unambiguously establish that the inversion to p-type occurs in the bulk and is not an artifact of hopping conduction. Such controlled bulk p-doping opens the door to pyrite p-n homojunctions, unveiling new opportunities for solar cells based on this extraordinary semiconductor.

Study on characteristics of neutron-induced leakage current increase for SiC power devices

In this paper, the displacement damage degradation characteristics of silicon carbide (SiC) Schottky barrier diode (SBD) and metal oxide semiconductor field effect transistor (MOSFET) are studied under 14-MeV neutron irradiation. The experimental results show that the neutron irradiation with a total fluence of 1.18×10<sup>11</sup> cm<sup>–2</sup> will not cause notable degradation of the forward <i>I-V</i> characteristics of the diode, but will lead to a significant increase in the reverse leakage current. A defect with energy level position of <inline-formula><tex-math id="Z-20230916130504">\begin{document}$ E_{\rm{C}} - 1.034 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230976_Z-20230916130504.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230976_Z-20230916130504.png"/></alternatives></inline-formula> eV is observed after irradiation by deep level transient spectroscopy (DLTS) testing, which is corresponding to the neutron-induced defect cluster in SiC. This deep level defect may cause the Fermi level of n-type doping drift region to move toward the mid-gap level. It ultimately results in the decrease of the Schottky barrier and the increase of the reverse leakage current. In addition, neutron-induced gate leakage increase is also observed for SiC MOSFET. The gate current corresponding to <i>V</i><sub>gs</sub> = 15 V after irradiation increases nearly 3.3 times that before irradiation. The donor-type defects introduced by neutron irradiation in the oxide layer result in the difference in the conductivity mechanism of gate oxygen between before and after irradiation. The defects have an auxiliary effect on carrier crossing the gate oxide barrier, which leads to the increase of gate leakage current. The defects introduced by neutron irradiation are neutral after capturing electrons. The trapped electrons can cross a lower potential well and enter the conduction band to participate in conduction when the gate is positively biased, thus causing the gate current to increase with the electric field increasing. After electrons captured by donor-type defects enter the conduction band, positively charged defects are left from the gate oxide, leading to the negative shift of the transfer characteristics of SiC MOSFET. The results of DLTS testing indicate that the neutron irradiation can not only cause the intrinsic defect state of SiC material to change near the channel of MOSFET, but also give rise to new silicon vacancy defects. However, these defects are not the main cause of device performance degradation due to their low density relative to the intrinsic defect’s.

Molecular-level insights on NIR-driven photocatalytic H2 generation with ultrathin porous S-doped g-C3N4 nanosheets

Unraveling how mid-gap state energy level of graphitic carbon nitride (g-C3N4) promote near-infrared (NIR) driven photochemical energy conversion at the molecular level remains a grand challenge. Here, we report a series of S double-site-doped ultrathin g-C3N4 nanosheets (SUCN) with adjustable intermediate band gap benefits from light response over NIR region. The SUCN produced after optimizing S double-site doping can effectively generate hydrogen (H2) under NIR-light irradiation. The highest H2 generation rate achieved was respectively 9.35 and 17.46 μmol g−1 h−1 under λ = 765 and λ > 800 nm, which is firstly expended photocatalytic activity of S-doped g-C3N4 to NIR region beyond λ > 765 nm. We proposed a molecular-level method, i.e., the localized oxidation state of proton acceptor triethanolamine (TEOA) in the mid-gap state to ensure the NIR-driven H2 generating behavior.

The effect of two intermediate band energy levels in ZnTe solar cell

The intermediate band (IB) effects on the ZnTe(O) solar cells with oxygen defects have been widely investigated. A favorable influence on the conversion efficiency was predicted numerically, provided that oxygen doping concentrations are very high to allow a tunneling effect between IB energy levels for more light absorption. By incorporating more than one intermediate band energy level in a direct band gap materials such as ZnTe material, the effects of the intermediate bands are predicted by numerical simulation to be more pronounced on the performance of the solar cells, we realized a simulation study on an intermediate band solar cell (IBSC) based on ZnTe(O) with oxygen impurities. Indeed, oxygen incorporates multiple acceptor and donor mid-gap levels in ZnTe, we investigated in this study on the influence of two intermediate band (IB) energy levels: the first at 0.53 eV (acceptor energy level) below the conduction band edge and the second at 0.33 eV (donor energy level) above the maximum of the valence band.

High Thermoelectric Performance in Chalcopyrite Cu1–xAgxGaTe2–ZnTe: Nontrivial Band Structure and Dynamic Doping Effect

The understanding of thermoelectric properties of ternary I-III-VI2 type (I = Cu, Ag; III = Ga, In; and VI = Te) chalcopyrites is less well developed. Although their thermal transport properties are relatively well studied, the relationship between the electronic band structure and charge transport properties of chalcopyrites has been rarely discussed. In this study, we reveal the unusual electronic band structure and the dynamic doping effect that could underpin the promising thermoelectric properties of Cu1-xAgxGaTe2 compounds. Density functional theory (DFT) calculations and electronic transport measurements suggest that the Cu1-xAgxGaTe2 compounds possess an unusual non-parabolic band structure, which is important for obtaining a high Seebeck coefficient. Moreover, a mid-gap impurity level was also observed in Cu1-xAgxGaTe2, which leads to a strong temperature-dependent carrier concentration and is able to regulate the carrier density at the optimized value for a wide temperature region and thus is beneficial to obtaining the high power factor and high average ZT of Cu1-xAgxGaTe2 compounds. We also demonstrate a great improvement in the thermoelectric performance of Cu1-xAgxGaTe2 by introducing Cu vacancies and ZnTe alloying. The Cu vacancies are effective in increasing the hole density and the electrical conductivity, while ZnTe alloying reduces the thermal conductivity. As a result, a maximum ZT of 1.43 at 850 K and a record-high average ZT of 0.81 for the Cu0.68Ag0.3GaTe2-0.5%ZnTe compound are achieved.

Tuning the surface states of TiO2 using Cu5 atomic clusters

If the ability of Cu5 atomic quantum clusters (AQCs) to create states within the bulk gap of TiO2 persists in the presence of oxygen, then the absorption spectrum of the AQC-TiO2 complex could be matched to the peak of the solar spectrum, thereby enhancing its photocatalytic activity under ambient conditions. Here we demonstrate that this is indeed the case, by examining the electronic structure of Cu5 AQCs adsorbed on a rutile (110) TiO2 surface in the presence of oxygen. We find that adsorbed oxygen changes the most stable AQC isomer from a 2-dimensional trapezoidal structure to a 3-dimensional bipyramidal structure. Furthermore, the bipyramidal AQC creates two ferromagnetically coupled polarons in the TiO2. In contrast, the trapezoidal AQC only creates a single polaron. In the presence of oxygen, these polaronic states associated with the bare cluster are joined by further mid-gap energy levels, formed from hybrid states located on copper and oxygen atoms. Dissociated oxygen-dimer adsorption on Cu5 generally leads to significant deformation of the whole cluster, which accounts for the dramatic drop of the total energy compared to molecular oxygen dimer adsorption. The ability of Cu5 AQCs to create states within the bulk gap of TiO2 under ambient conditions demonstrates that the absorption spectrum of the AQC-TiO2 complex is more closely matched to the peak of the solar spectrum, than either the AQC or TiO2 alone.

Synergistic effects on d-band center via coordination sites of M-N3P1 (M = Co and Ni) in dual single atoms that enhances photocatalytic dechlorination from tetrachlorobispheonl A

Transition metal single atoms (TM-SAs) coordinated with highly electronegative N atoms often suffer from low activity and poor stability, which limiting their application in catalysis. To solve it, a PH3-assisted annealing strategy is designed to synthesize atomically dispersed TM-SAs (CCoNiP), which is stemmed from a pyrolysis approach of pre-designed CoNi layered double hydroxide (LHD) as a soft-template, and further coordinated with P atoms for adjusting the coordination environment. Characterization results show that the atomically dispersed Co and Ni atoms anchor on the carbon nitride substrate with Co/Ni-N3P1 coordination sites. Combined with density functional theory calculations, it is confirmed that multiple coordination sites of Co/Ni-N and Co/Ni-P can modulate d-band center position which increases the catalytic activity of TM-SAs. The formed multiple midgap levels can extend optical absorption ranges. Meanwhile, P-introduction can change the coordination environment, suppress the conversion trend of SAs to high valence state and improve electron separation. All the above characteristics can improve effective degradation from Tetrachlorobisphenol A (TCBPA) under visible light irradiation, achieving 100% removal and 44.1% dechlorination rate.

Insight into the defective sites of TiO2/sepiolite composite on formaldehyde removal and H2 evolution

TiO2/sepiolite composites (T–S–R) with different contents of Ti3+ and oxygen vacancy were successfully prepared through 0D/1D structural assembly and thermal treatment under various atmospheres (R = Air, O2, N2, Ar and H2). The content of Ti3+ and oxygen vacancy was confirmed through electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS), which exhibited significant effect on the photocatalytic formaldehyde (HCHO) removal and H2 evolution. The T–S–Ar composite exhibited much higher formaldehyde degradation efficiency (91.8% within 90 min) and H2 production performance (1974.8 μmol/h/g) than other samples under simulated solar light. According to the material characterizations and DFT calculations, the improved photocatalytic activity could be ascribed to the synergistic effect between natural sepiolite and TiO2. Besides, the generation of mid-gap levels induced by the appropriate content of Ti3+ and oxygen vacancy led to the larger specific surface area, smaller nano-sized TiO2 and higher photogenerated carriers’ separation efficiency. Furthermore, it was found that higher content of oxygen vacancies would act as the recombination centers for electron–hole pairs, thereby greatly reducing the photocatalytic performance. Notably, our study provides a promising candidate for effectively addressing the air pollution (e.g. HCHO removal) and energy issues (e.g. H2 production) based on the defect engineering and mineral support technology.

Defect tolerance in chalcogenide perovskite photovoltaic material BaZrS3

Chalcogenide perovskites (CPs) exhibiting lower band gaps than oxide perovskites and higher stability than halide perovskites are promising materials for photovoltaic and optoelectronic applications. For such applications, the absence of deep defect levels serving as recombination centers (dubbed defect tolerance) is a highly desirable property. Here, using density functional theory (DFT) calculations, we study the intrinsic defects in BaZrS3, a representative CP material. We compare Hubbard-U and hybrid functional methods, both of which have been widely used in addressing the band gap problem of semi-local functionals in DFT. We find that tuning the U value to obtain experimental bulk band gap and then using the obtained U value for defect calculations may result in over-localization of defect states. In the hybrid functional calculation, the band gap of BaZrS3 can be accurately obtained. We observe the formation of small S-atom clusters in both methods, which tend to self-passivate the defects from forming mid-gap levels. Even though in the hybrid functional calculations several relatively deep defects are observed, all of them exhibit too high formation energy to play a significant role if the materials are prepared under thermal equilibrium. BaZrS3 is thus expected to exhibit sufficient defect tolerance promising for photovoltaic and optoelectronic applications.

Charge trapping at Fe due to midgap levels in Ga2O3

Fe acts as an electron trap in gallium oxide (Ga2O3), thereby producing a semi-insulating material that can be used in device fabrication. However, such trapping can lead to negative effects when Fe is unintentionally incorporated into bulk crystals or thin films. In this work, photoinduced electron paramagnetic resonance (photo-EPR) is used to investigate carrier capture at Fe in β-Ga2O3. Two crystals doped with 8 × 1017 cm−3 and 5 × 1018 cm−3 Fe and one Mg-doped crystal containing 7 × 1016 cm−3 unintentional Fe are studied by illuminating with LEDs of photon energies 0.7–4.7 eV. Steady state photo-EPR results show that electrons excited from Ir, an unintentional impurity in bulk crystals, are trapped at Fe during illumination with photon energy greater than 2 eV. Significantly, however, trapping at Fe also occurs in the crystals where Ir does not participate. In such cases, we suggest that excitation of intrinsic defects such as oxygen or gallium vacancies are responsible for trapping of carriers at Fe. The results imply that the investigation of intrinsic defects and their interaction with Fe is necessary to realize stable and reliable Ga2O3:Fe devices.

Efficient photocatalytic H2 evolution over NiS-PCN Z-scheme composites via dual charge transfer pathways

In this study, both narrow band-gap semiconductor NiS and S and O defects modification were introduced into the design of PCN photocatalyst to enhance its photocatalytic performance. The O and S defects, on one hand, provided the higher specific area and more charge separation sites, and constituted the dual fast charge transfer pathways with NiS through the N–C–S–Ni bonds and N–C–O–Ni–S bonds, which greatly increased the carrier transfer kinetics at the interface and restricted the carrier recombination. On the other hand, the midgap level derived from S and O defects also enhanced the absorption of visible light and accommodated more excited electrons. Furthermore, Z-scheme mode could form as Ni2+ species were reduced to metallic Ni0 to acts as charge transfer mediator under irradiation. As such, a maximum hydrogen evolution rate of 1239.3 μmol/g/h was achieved over 5%NiS/SO-Melon composite, nearly 31.4 times the rate of pristine PCN.

Unscrambling for Subgap Density-of-States in Multilayered MoS2Field Effect Transistors under DC Bias Stress via Optical Charge-Pumping Capacitance-Voltage Spectroscopy

Herein, we quantitatively analyze the evolution of the subgap density of states (DOSs) for multilayered molybdenum disulfide (m-MoS2) field effect transistors (FETs) with bilayered SiNx/SiOx gate dielectrics under positive bias stress (PBS) and negative bias stress (NBS) by using optical charge-pumping capacitance-voltage spectroscopy. To decouple external gas ambient effects on device instability, hydrophobic fluoropolymers (cyclized transparent optical polymer; CYTOP) are employed for m-MoS2 FETs followed by the evaluation of subgap-DOSs in the devices for the respective PBS (or NBS). Through extraction of subgap-DOSs and their deconvolution with an analytical model of acceptor (or donor)-like states, it is shown that the device instability is closely correlated with state transitions of DOSs, corresponding to shallow (or midgap) levels for monosulfur vacancy (VS) (or disulfur vacancy ( $\text{V}_{\mathrm {S2}}$ )). Moreover, after PBS, the initial states of VS(0) (or $\text{H}_{\mathrm {S2}}$ (0)) transit toward VS (−1) (or $\text{H}_{\mathrm {S2}}(-1$ )) via electron trapping, whereas the transition toward VS (0) and $\text{H}_{\mathrm {S2}}$ (+1) during NBS is assessed from the initial $\text{V}_{\mathrm {S}}(-1$ ) and $\text{H}_{\mathrm {S2}}$ (0) states. Furthermore, technology computer-aided design (TCAD) simulation based on the extracted DOSs properly replicates the measured I-V characteristics of m-MoS2 FETs with (and without) CYTOP encapsulation. In this study, subgap-DOS characterization via optical charge pumping and validation of the quantitative evolution of subgap-DOSs suggest that this platform can be potentially beneficial for an in-depth understanding of the origins of device instability for transition metal dichalcogenide (MX2)-based FETs, where M is a transition metal and X is a chalcogenide.

Double Charge Polarity Switching in Sb‐Doped SnSe with Switchable Substitution Sites

AbstractTin mono‐selenide (SnSe) is one of the most promising thermoelectric materials; however, it experiences difficulty in controlling the carrier polarity, which is inevitable for realizing p‐n homojunction devices. Herein, double switching of charge polarity in (Sn1−xSbx)Se by varying x is reported; pure SnSe shows p‐type conduction, whereas the polarity of (Sn1−xSbx)Se switches to n‐type conduction for 0.005 < x < 0.05, and then re‐switches to p‐type conduction for x > 0.05. The major Sb substitution site switches from the Se (SbSe) to Sn site (SbSn) with increasing x. SbSn (Sb3+ at Sn2+) works as a donor, but SbSe (Sb3− at Se2−) does not produce a hole because of the Sb–Sb dimer formation. The mechanism of double polarity switching is explained by native p‐type conduction in pure SnSe due to Sn‐vacancy formation, whereas (Sn1−xSbx)Se exhibits n‐type behavior due to conduction through the SbSe impurity band formed above the valence band maximum, and finally re‐switches to weak p‐type, where the Fermi level approaches the midgap level between the SbSe band and conduction band minimum. Clarification of the Sb doping mechanism will provide a crucial guide for developing more sophisticated doping routes for SnSe and high‐performance energy‐related devices.

Tuning the electronic and magnetic properties of pentagraphene through the C1 vacancy

Pentagraphene (PG), a two-dimensional allotrope of carbon with only five-membered rings, was recently predicted to exhibit a wide band gap, an unusual negative Poisson’s ratio, and robust dynamical and thermal stability. However, its potential properties arising from defect engineering of five-membered rings have not been explored yet. In this work, we explore the C1 vacancy in PG using first-principles density functional calculations. The formation energy of this vacancy is surprisingly within the formation energy window of monovacancy (MV) in graphene. This defect introduces eight midgap levels indicating the possibility of studying the charge states. The charged defect is amphoteric with deep donor and deep acceptor levels. Introducing the vacancy in the sheet renders it inherently ferromagnetic (FM) with a magnetic moment of 4 µB. Upon further investigating spin polarization in PG, three possible unique magnetic configurations are found to exist: FM, ferrimagnetic (FiM), and anti-ferromagnetic (AFM). The energy difference between the FM and the FiM (AFM) states is ∼−62.4 meV atom−1 (∼−81.9 meV atom−1). Despite FiM and AFM being lower in energy, interestingly, this intrinsic FM ordering of spins is preserved even when the defect concentration is increased. This observation would probably hint at the existence of a kinetic barrier between FM and AFM states which merits further investigations.