Midgap Energy Research Articles

Strong Dependence of Point Defect Properties in Metal Halide Perovskites on Description of van der Waals Interaction.

Weaker than ionic and covalent bonding, van der Waals (vdW) interactions can have a significant impact on structure and function of molecules and materials, including stabilities of conformers and phases, chemical reaction pathways, electro-optical response, electron-vibrational dynamics, etc. Metal halide perovskites (MHPs) are widely investigated for their excellent optoelectronic properties, stemming largely from high defect tolerance. Although MHPs are primarily ionic compounds, we demonstrate that vdW interactions contribute ∼5% to the total energy, and that static, dynamics, electronic and optical properties of point defects in MHPs depend significantly on the vdW interaction model used. Focusing on widely studied CsPbBr3 with the common Br vacancy and interstitial defects, we compare the PBE, PBE+D3, PBE+TS, PBE+TS/HI and PBE+MBD-NL models and show that vdW interactions strongly alter the global and local geometric structure, and change the fundamental bandgap, midgap state energies and electron-vibrational coupling. The vdW interaction sensitivity stems from involvement of heavy and highly polarizable chemical elements and the soft MHP structure.

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.

Midgap states and energy alignment at interconnect are crucial for perovskite tandem solar cells

Midgap states and energy alignment at interconnect are crucial for perovskite tandem solar cells

Selenium doping to improve internal electric field for excellent photocatalytic efficiency of g-C3N4 nanosheets

Graphitic carbon nitride is a promising photocatalyst to address environmental issues and energy applications. Herein, we develop an efficient defect-containing functionalized system of 2D selenium doped g-C3N4 porous nanosheets via one pot with two-step temperature for the first time. During heat treatment, Se-infused g-C3N4 nanosheets loosen stacking and form thinner nanosheets, which could effectively shorten the electronic path of charge migration. The introduction of nitrogen vacancies creates a midgap energy state below the CB, which improves the light-harvesting efficiency. Moreover, the dual effect of N vacancy and Se doping provides a driving force for the dissociation of excitons and achieves an effective spatial charge separation. High specific surface area (104.1 m2/g), suitable bandgap (2.65 eV), and good photocurrent response (0.45 µA cm−2) help to enhance the photocatalytic efficiency of 1-SeCN remarkably. Henceforth, high photocatalytic H2 production rate of 5441 µmol g−1 h−1 and CO evolution rate of 31.5 µmol g−1 h−1 verifies the effective Se doping enhances the photocatalytic efficiency of g-C3N4. Furthermore, DFT calculations are consistent with experimental results and this facile engineering strategy provides a scalable way to develop a novel, efficient g-C3N4-based photocatalyst for H2 production and CO2 reduction.

On the Voltage Dependent Series Resistance, Interface Traps, and Conduction Mechanisms in the Al/(Ti-doped DLC)/p-Si/Au Schottky Barrier Diodes (SBDs)

In this study, Al-(Ti:DLC)-pSi/Au Schottky barrier diode (SBD) was manufactured instead of conventional metal / semiconductor (MS) with and without an interlayer and then several fundamental electrical-characteristics such as ideality factor (n), barrier height B series and shunt resistances (Rs, Rsh), concentration of acceptor atoms (NA), and width of depletion-layer (Wd) were derived from the forward-reverse bias current/voltage (I-V), capacitance and conductance as a function of voltage (C/G-V) data using various calculation-methods. Semi logarithmic IF-VF plot shows a linear behavior at lower-voltages and then departed from linearity as a result of the influence of series resistance/Rs and organic-interlayer. Three linear regions can be seen on the double-logarithmic IF-VF plot. with different slopes (1.28, 3.14, and 1.79) in regions with low, middle, and high forward bias, which are indicated that Ohmic-mechanism, trap-charge-limited-current (TCLC) mechanism, and space-charge-limited-current (SCLC) mechanism, respectively. Energy dependent surface states (Nss) vs (Ess-Ev) profile was also obtained from the Card-Rhoderick method by considering voltage-dependence of n and B and they were grown from the mid-gap energy up to the semiconductor's valance band (Ev). To see the impact of Rs for 1 MHz, the measured C/G-V graphs were amendment. All results are indicated that almost all electrical parameters and conduction mechanism are quite depending on Rs, Nss, and calculation method due the voltage dependent of them.

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.

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.

Understanding the Heterointerfaces in Perovskite Solar Cells via Hole Selective Layer Surface Functionalization.

Interfaces in perovskite solar cells (PSCs) play a pivotal role in determining device performance by influencing charge transport and recombination. Understanding the physical processes at these interfaces is essential for achieving high-power conversion efficiency in PSCs. Particularly, the interfaces involving oxide-based transport layers are susceptible to defects like dangling bonds, excess oxygen, or oxygen deficiency. To address this issue, the surface of NiOx is passivated using octadecylphosphonic acid (ODPA), resulting in improved charge transport across the perovskite hole transport layer (HTL) interface. This surface treatment has led to the development of hysteresis-free devices with an impressive ≈13% increase in power conversion efficiency. Computational studies have explored the halide perovskite architecture of ODPA-treated HTL/Perovskite, aiming to unlock superior photovoltaic performance. The ODPA surface functionalization has demonstrated enhanced device performance, characterized by superior charge exchange capacity. Moreover, higher band-to-band recombination in photoluminescence and electroluminescence indicates presence of lower mid-gap energy states, thereby increasing the effective photogenerated carrier density. These findings are expected to promote the utilization of various phosphonic acid-based self-assembly monolayers for surface passivation of oxide-based transport layers in perovskite solar cells. Ultimately, this research contributes to the realization of efficient halide PSCs by harnessing the favorable architecture of NiOx interfaces.

Oxygen-vacancy defect in 4H-SiC as a near-infrared emitter: An ab initio study

Optically active spin defects in semiconductors can serve as spin-to-photon interfaces, key components in quantum technologies. Silicon carbide (SiC) is a promising host of spin defects thanks to its wide bandgap and well-established crystal growth and device technologies. In this study, we investigated the oxygen-vacancy complexes as potential spin defects in SiC by means of ab initio calculations. We found that the OCVSi defect has a substantially low formation energy compared with its counterpart, OSiVC, regardless of the Fermi level position. The OCVSi defect is stable in its neutral charge state with a high-spin ground state (S = 1) within a wide energy range near the midgap energy. The zero-phonon line (ZPL) of the OCVSi0 defect lies in the near-infrared regime, 1.11–1.24 eV (1004–1117 nm). The radiative lifetime for the ZPL transition of the defect in kk configuration is fairly short (12.5 ns). Furthermore, the estimated Debye–Waller factor for the optical transition is 13.4%, indicating a large weight of ZPL in the photoluminescence spectrum. All together, we conclude that the OCVSi0 defect possesses desirable spin and optical properties and thus is potentially attractive as a quantum bit.

Enhanced photocatalytic activity in RhB dye degradation by Mn and B co-doped mixed phase TiO2 photocatalyst under visible light irradiation

Here, we report synthesis of undoped, manganese (Mn) doped, boron (B) doped and Mn-B co-doped TiO2 nanoparticles (NPs) by a cost-effective sol-gel method. The structural properties, phase, micro-morphology and particle size of the synthesized nanoparticles have been characterized by X-ray diffraction, Raman spectroscopy, Transmission Electron Microscopy (TEM) and Dynamic Light Scattering (DLS) techniques. These studies revealed that undoped and mono (Mn/B) doped TiO2 NPs exhibit only anatase phase, whereas (Mn and B) co-doped TiO2 NPs exhibit anatase and rutile mixed phase structure. Furthermore, co-doping reduces the particle size, agglomeration in aqueous medium and the sample possesses crystal defects. UV–Visible diffusive reflectance spectroscopy (DRS) revealed that the co-doped sample demonstrates extended optical response in the visible wavelength range of the solar spectrum and is red shifted when compared to undoped and mono doped TiO2 NPs. The obtained optical band gap (2.11 eV) and the Urbach energy (1.12 eV) for the Mn and B co-doped TiO2 NPs suggest the formation of mid-gap energy states, which might make this NPs a suitable candidate for photocatalysis in the visible light spectrum. We investigated the photocatalytic degradation of rhodamine B (RhB) dye by the synthesized NPs under visible light irradiation. Our studies revealed that co-doped TiO2 photocatalyst can degrade 99% of RhB dye within 90 min of visible light irradiation and can retain 95% degradation efficiency up to the third consecutive cycles. Finally, we provide a plausible explanation for such enhanced visible light assisted photocatalytic performance.

Effect of Ni and Mn dopant on thermoelectric power generation performance of ZnO nanostructures synthesized via hydrothermal method

In this article, we have presented a low-cost hydrothermal approach to enhance the thermoelectric performance of ZnO nanostructures via modulation doping. For this purpose, we have prepared a series of pure and X:ZnO (X = Ni & Mn) samples. The Seebeck value of the Mn-doped samples possesses the maximum Seebeck coefficient of −36 μV/°C compared to the pure and Ni-doped samples (−22 μV/°C & −27 μV/°C) at room temperature. The highest value of the Seebeck coefficient for the Mn-doped samples is related to the formation of mid-gap energy band states due to the substitution of Mn2+ with Zn2+. These mid-band states induce an imbalance in the DOS, by producing a spin polarization effect that leads to a high Seebeck value. In terms of electrical conductivity, the Ni-doped ZnO sample exhibits the highest electrical conductivity of about 122 S/cm, due to the incorporation of Ni metal ions inside the ZnO matrix (confirmed by XRD) and leads to a high carrier concentration. However, the highest Seebeck value for the Mn-doped sample results in the maximum thermoelectric power factor ∼1.12 × 10−5 Wm−1C−2 at room temperature.

Revealing the impact of the host-salt non-stoichiometry on the performance of perovskite solar cells

In this study, we are able to fine tune I-rich (AX-rich, such as CH3NH3I) or I-poor (Pb-rich) growth conditions, which allowing us to deposit highly crystalline “phase-pure” 1.53eV bandgap α-FA0.95Cs0.05PbI3composition.

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.

Computational studies of Ag5 atomic quantum clusters deposited on anatase and rutile TiO2 surfaces

Aiming at boosting the photocatalytic activities of rutile and anatase TiO2 surfaces, an in-depth investigation of stoichiometric and reduced rutile TiO2 (110) and anatase TiO2 (101) decorated with bipyramidal and trapezoidal Ag5 atomic quantum clusters (AQCs) is carried out. In this study, density functional theory (DFT) plus a Hubbard correction (U) is implemented to explore the geometric and electronic properties of such systems. It is found that the silver AQCs donate electrons to both stoichiometric and reduced TiO2 surfaces resulting in the formation of a single polaron at either a fivefold coordinated (Ti5c) atom or a sixfold coordinated (Ti6c) atom, indicating improved surface activity. Depositing Ag5 AQCs on both TiO2 surfaces can produce mid-gap states within the band gap of the bulk, thereby improving the optical response of the composite in the visible and infrared. As expected, the number of mid-gap energy states increases further by introducing a single oxygen vacancy into the studied surfaces, which means that Ag5 AQCs and oxygen vacancies can reinforce each other, leading to higher efficient photocatalytic activity. We also find that upon adsorption of Ag5 AQCs on an anatase TiO2 (101) surface, the energy required to form an oxygen vacancy is lower than that of rutile TiO2 (110). Moreover, the adsorption of both bipyramidal and trapezoidal Ag5 AQCs on both TiO2 surfaces generally leads to significant distortion of the clusters, which accounts for the significant reduction in the total energy as compared to the pristine TiO2. This detailed investigation provides insight into new mechanisms for enhancing photocatalytic efficiency of both rutile TiO2 (110) and anatase TiO2 (101) surfaces.

P-Terminated InP (001) Surfaces: Surface Band Bending and Reactivity to Water.

Stable InP (001) surfaces are characterized by fully occupied and empty surface states close to the bulk valence and conduction band edges, respectively. The present photoemission data show, however, a surface Fermi level pinning only slightly below the midgap energy which gives rise to an appreciable surface band bending. By means of density functional theory calculations, it is shown that this apparent discrepancy is due to surface defects that form at finite temperature. In particular, the desorption of hydrogen from metalorganic vapor phase epitaxy grown P-rich InP (001) surfaces exposes partially filled P dangling bonds that give rise to band gap states. These defects are investigated with respect to surface reactivity in contact with molecular water by low-temperature water adsorption experiments using photoemission spectroscopy and are compared to our computational results. Interestingly, these hydrogen-related gap states are robust with respect to water adsorption, provided that water does not dissociate. Because significant water dissociation is expected to occur at steps rather than terraces, surface band bending of a flat InP (001) surface is not affected by water exposure.

Activation energy dependence on doping concentration in PVA-MgCl2 composites

Solid polymer electrolytes are fast replacing the liquid ones because of their numerous advantages such as high energy density, long cycling life, fast charging, low leakage currents and easy fabrication in a variety of shapes. Although most of the reports are based on Li-ion batteries, focus is slowly shifting to develop electrolytes with Group II elements such as Magnesium and Calcium because of their easy availability and moderate ionic activity.In this work, we have complexed polymer polyvinyl alcohol with Magnesium Chloride (MgCl2·6H2O) at different weight percentages. DC conductivity studies were conducted using a two probe setup. It is seen that the activation energy of the system depends on ionic conductivity which in turn depend on the percolating interfacial effect, i.e. anions adsorb on the surface of fillers (and then break up the ion pair and lead to increased interfacial ionic conductivity.This increased conductivity is partly due to the doping and partly due to midgap energy levels developing due to formation of polarons and bipolarons in the composite gel polymer electrolyte .

New disordered anyon phase of doped graphene zigzag nanoribbon

We investigate interacting disordered zigzag nanoribbons at low doping, using the Hubbard model to treat electron interactions within the density matrix renormalization group and Hartree-Fock method. Extra electrons that are inserted into an interacting disordered zigzag nanoribbon divide into anyons. Furthermore, the fractional charges form a new disordered anyon phase with a highly distorted edge spin density wave, containing numerous localized magnetic moments residing on the zigzag edges, thereby displaying spin-charge separation and a strong non-local correlation between the opposite zigzag edges. We make the following new predictions, which can be experimentally tested: (1) In the low doping case and weak disorder regime, the soft gap in the tunneling density of states is replaced by a sharp peak at the midgap energy with two accompanying peaks. The e^-/2 fractional charges that reside on the boundary of the zigzag edges are responsible for these peaks. (2) We find that the midgap peak disappears as the doping concentration increases. The presence of e^-/2 fractional charges will be strongly supported by the detection of these peaks. Doped zigzag ribbons may also exhibit unusual transport, magnetic, and inter-edge tunneling properties.

Nitrogen doping of indium oxide for enhanced photocatalytic reduction of CO2 to methanol

Herein, the nitrogen-doped indium oxide (N-In2O3) photocatalyst was confirmed to be highly active and stable for photocatalytic reduction of CO2 to methanol in an aqueous solution at ambient conditions. The efficiency of N-In2O3 in producing methanol can be flexibly improved by tuning the nitrogen doping content. The highest formation rate of methanol reaches to 394 μmol gcat-1 h-1, with a methanol selectivity of 63%, at a nitrogen doping content of 3.74%. Nitrogen doping generates mid-gap energy states and reduces the bandgap of In2O3, thus boosting photon absorption and electron-hole separation. Nitrogen doping creates more oxygen vacancies on In2O3, thus forming more active sites for CO2 adsorption and conversion. Nitrogen doping also enhances the activity of the surface frustrated Lewis pairs (SFLPs), which further promotes CO2 adsorption and activation. The multiple-role of nitrogen doping results in the highly active and stable photocatalytic reduction of CO2 to methanol.

Experimental and theoretical studies on atomic structures of the interface states at SiO2/4H-SiC(0001) interface

We investigated the atomic structures of the interface states (gap states) at the SiO2/4H-SiC(0001) interface using hard x-ray photoelectron spectroscopy (HAXPES), operando hard x-ray photoelectron spectroscopy, extended x-ray absorption fine structure, and first principles molecular dynamics (FPMD) calculations. For the interface states, two types were observed: continuous interface states in the whole gap and interface states with discrete energy levels near the conduction band minimum (CBM). From HAXPES, we found that carbon clusters and carbon vacancies were formed at the SiO2/4H-SiC(0001) interface. FPMD calculations on the SiO2/4H-SiC(0001) interface showed that the interface states in the whole gap were attributed to the various atomic geometries of the CßSi3 species and the carbon clusters with various sizes and surrounding atoms. For the interface states with a discrete energy level near the CBM, we could not find their atomic structure using our current calculations. We calculated the carbon vacancies prepared on the side of an SiC substrate at the SiO2/4H-SiC(0001) interface, indicating the formation of a discrete energy level in the midgap. It is likely that carbon vacancies formed at the step of the interface may modulate the midgap energy level to energy below the CBM. Therefore, we propose that the interface states with discrete energy levels near the CBM could be attributed to the carbon vacancies formed on the steps at the SiO2/4H-SiC(0001) interface.

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.