Spectroscopic properties of semiconductor crystals with direct forbidden energy gap

Critical Review of Cu-Based Hole Transport Materials for Perovskite Solar Cells: From Theoretical Insights to Experimental Validation

Despite the remarkable efficiency of perovskite solar cells (PSCs), long-term stability remains the primary barrier to their commercialization. The prospect of enhancing stability by substituting organic transport layers with suitable inorganic compounds, particularly Cu-based inorganic hole-transport materials (HTMs), holds promise due to their high valence band maximum (VBM) aligning with perovskite characteristics. This review assesses the advantages and disadvantages of these five types of Cu-based HTMs. Although Cu-based binary oxides and chalcogenides face narrow bandgap issues, the "chemical modulation of the valence band" (CMVB) strategy has successfully broadened the bandgap for Cu-based ternary oxides and chalcogenides. However, Cu-based ternary oxides encounter challenges with low mobility, and Cu-based ternary chalcogenides face mismatches in VBM alignment with perovskites. Cu-based binary halides, especially CuI, exhibit excellent properties such as wider bandgap, high mobility, and defect tolerance, but their stability remains a concern. These limitations of single anion compounds are insightfully discussed, offering solutions from the perspective of practical application. Future research can focus on Cu-based composite anion compounds, which merge the advantages of single anion compounds. Additionally, mixed-cation chalcogenides such as CuxM1-xS enable the customization of HTM properties by selecting and adjusting the proportions of cation M.

Highly Monochromatic Ultraviolet LED Based on the SnO2 Microwire Heterojunction Beyond Dipole-Forbidden Band-Gap Transition

SnO2 has been extensively applied in the fields of optoelectronic devices because of its large band gap, high exciton binding energy, and outstanding optical/electrical properties. However, its applications in ultraviolet light-emitting diodes (LEDs) are still hindered by the dipole-forbidden rule. Herein, the dipole-forbidden rule can be conquered by synthesizing Sb-incorporated SnO2 microwires (SnO2:Sb MWs), which are examined by ultraviolet photoluminescence emitting at 363.2 nm and a line width of 11.3 nm. Subsequently, a highly monochromatic ultraviolet light-emitting diode (LED) based on a SnO2:Sb MW heterojunction was constructed with a p-GaN film serving as the hole supplier. In the LED, the presence of a MgO intermediate layer can modulate carrier transport and recombination path, thus achieving band-edge optical transition in the SnO2:Sb MW. As the LED is modified using Ag nanowires, electrical properties, especially for the hole injection efficiency, were dramatically boosted, contributing significantly to the device high brightness. The LED emits at 365.9 nm and a line width of 12.4 nm. Therefore, we have realized a high-brightness and narrow-band ultraviolet LED with the shortest peak wavelength never seen in previously reported SnO2 LEDs. This work will promote the potential applications of low-dimensional SnO2 optoelectronic devices and provide an effective exemplification to overcome the dipole-forbidden rule in metal-oxide materials with "forbidden" energy gaps.

Unraveling the enhancement mechanisms of H2S sensing on a SnO2 surface: an ab initio perspective

Accurate and effective sensing of H2S is one of the most complex and challenging tasks. Recent studies have demonstrated that the development of an Sb-doped SnO2 nanoribbon sensor can enhance the detection limit of H2S. To clarify the enhancement mechanism, various factors that regulate the sensing processes, such as the Sb-doped sites, surface oxygen defects and possible pre-adsorbed oxygen species, are considered in this study. Theoretical calculations reveal a thorough and extensive understanding of the gas-sensing mechanisms. Once the H2S gas interacted with the Sb-doped SnO2(110) surface, the molecule released electrons back to the surface, together with a decreased resistance level and strengthened detection signal. In addition, the dissociative adsorption of the O2 molecule also plays a significant role in the sensing processes. It is expected that the present work can provide effective guidance for improving the sensitivity of metal oxide surfaces and shed new light on the development of next generation gas-sensing materials.

Reduction of carbon dioxide with mesoporous SnO2 nanoparticles as active photocatalysts under visible light in water

We demonstrated the photocatalytic reduction of CO₂ to HCOOH using mesoporous SnO₂ nanoparticles as active photocatalysts in water which acted as a sacrificial electron source as well as a solvent under atmospheric pressure.

Local symmetry breaking in SnO2 nanocrystals with cobalt doping and its effect on optical properties

X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) have been used to study the structural and morphological characteristics of cobalt doped tin(iv) oxide (Sn1-xCoxO2; 0 ≤ x ≤ 0.04) nanocrystals synthesized by a chemical co-precipitation technique. Electronic structure analysis using X-ray photoemission spectroscopy (XPS) shows the formation of tin interstitials (Sni) and reduction of oxygen vacancies (VO) in the host lattice on Co doping and that the doped Co exists in mixed valence states of +2 and +3. Using XRD, the preferential position of the Sni and doped Co in the unit cell of the nanocrystals have been estimated. Rietveld refinement of XRD data shows that samples are of single phase and variation of lattice constants follows Vegard's law. XRD and TEM measurements show that the crystallite size of the nanocrystals decrease with increase in Co doping concentration. SAED patterns confirm the monocrystalline nature of the samples. The study of the lattice dynamics using Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy shows the existence of many disorder activated forbidden optical phonon modes, along with the corresponding classical modes, signifying Co induced local symmetry breaking in the nanocrystals. UV-Vis spectroscopy shows that the optical band gap has red shifted with increase in doping concentration. The study of Urbach energy confirms the increase in disorder in the nanocrystals with Co doping. Local symmetry breaking induced UV emission along with violet, blue and green luminescence has been observed from the PL study. The spectral contribution of UV emission decreases and green luminescence increases with increase in doping. Using PL, in conjunction with Raman spectroscopy, the type of oxygen vacancy induced in the nanocrystals on Co doping has been confirmed and the position of the defect levels in the forbidden zone (w.r.t. the optical band gap) has been studied.

SnO2 Nanostructures: Effect of Processing Parameters on Their Structural and Functional Properties

Zero- and 1D (one-dimensional) tin (IV) oxide nanostructures have been synthesized by thermal evaporation method, and a comparison of their morphology, crystal structure, sorption properties, specific surface area, as well as electrical characteristics has been performed. Synthesized SnO₂ nanomaterials were studied by X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM), N₂ sorption/desorption technique, IR spectroscopy and, in addition, their current-voltage characteristics have also been measured. The single crystalline structures were obtained both in case of 0D (zero-dimensional) SnO₂ powders and in case of 0D nanofibers, as confirmed by electron diffraction of TEM. It was found that SnO₂ synthesis parameters significantly affect materials' properties by contributing to the difference in morphology, texture formation, changes in IR spectra of 1D structure as compared to 0D powders, increases in the specific surface area of nanofibers, and the alteration of current-voltage characteristics 0D and 1D SnO₂ nanostructures. It was established that gas sensors utilizing of 1D nanofibers significantly outperform those based on 0D powders by providing higher specific surface area and ohmic I-V characteristics.

Stoichiometry, Morphology, and Size-Controlled Electrochemical Fabrication of CuxO (x = 1, 2) at Underpotential

A new one-step electrochemical approach has been developed for the morphology, size, and stoichiometry-controlled synthesis of Cu₂O, CuO, and Cu₂O/CuO composite structures at room temperature without using surfactants, capping agents, or any other additives. The electrochemical deposition of a Cu monolayer using underpotential deposition (UPD) and the flow rate of oxygen gas bubbled through the deposition solution used for oxidation of the Cu layer are the key parameters for controlling the stoichiometry of the Cu_xO (x = 1, 2) structures. The morphologies, crystallinity, stoichiometries, optical properties, and photoelectrochemical properties of the as-electrodeposited Cu₂O and CuO materials were analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS), UV-vis absorption, and photoelectrochemical (PEC) techniques. The results indicated that the Cu₂O and CuO materials electrodeposited on both indium tin oxide coated (ITO) quartz and gold electrodes using this new electrochemical technique exhibit high-quality single crystalline structures and high photoactivity with rapid photoelectrical response to light irradiation.

DFT calculations of tin dioxide crystals containing heavily-doped fluorine

First-principles calculations based on the density functional theory (DFT) within the generalized gradient approximation have been used in the present research. Fluorine doping in the SnO 2 crystals has been carried out considering a number of different defect concentrations. Dopant influence upon structural, electronic and electrical properties of the tin dioxide has been discussed in detail. The system presents n-type electrical conductivity relating our work directly to a number of empirical studies in this area. An experimental fact that n-type conductivity tends to decrease at rather high fluorine impurity rates has been explained at the theoretical level.

Luminescence studies on SnO2 and SnO2:Eu nanocrystals grown by laser assisted flow deposition

Eu³⁺ optically-activated transparent conductive tin oxide nanocrystals were produced by the innovative laser assisted flow deposition technique.

Enhanced visible light photocatalytic activity of Cu2O via cationic–anionic passivated codoping

To improve the photocatalytic activity of Cu₂O for hydrogen production through water splitting, the band edges of Cu₂O should be modified to meet the electronic transition of angular momentum selection rules (Δl = ±1) and match with the hydrogen or oxygen production levels.

Bose-Einstein condensation of excitons in Cu2O: progress over 30 years

Experiments on Bose-Einstein condensation (BEC) of excitons in the semiconductor Cu2O started over 30 years ago, as one of the first serious attempts at exciton BEC. Early claims were based on spectroscopic signatures and transport data which have since been reinterpreted, in large part because the Auger recombination process for excitons was not well understood. Understanding of the Auger process has advanced, and recent experiments have made significant progress toward exciton BEC. We review the history of experiments on exciton BEC in Cu2O, the Auger recombination process, and the prospects for observing exciton BEC in this system in the near future.

Functionalization of selectively grown networked SnO2 nanowires with Pd nanodots by γ-ray radiolysis

γ-ray radiolysis is applied to synthesizing Pd nanodots on networked SnO(2) nanowires. The growth behavior of Pd nanodots is systematically investigated as a function of the precursor concentration, illumination intensity, and exposure time of the γ-rays. These factors greatly influence the growth behavior of the Pd nanodots. Selectively grown networked SnO(2) nanowires are uniformly functionalized with Pd nanodots by the radiolysis process. The NO(2) sensing characteristics of the Pd-functionalized SnO(2) nanowires are compared with those of bare SnO(2) nanowires. The results indicate that γ-ray radiolysis is an attractive means of functionalizing the surface of oxide nanowires with catalytic Pd nanodots. Moreover, the Pd-functionalization greatly enhances the sensitivity and response time in SnO(2) nanowire-based gas sensors.

Ultraviolet electroluminescence from randomly assembled n-SnO(2) nanowiresp-GaN:Mg heterojunction

Electroluminescence characteristics of a heterojunction light-emitting diode, which was fabricated by depositing a layer of randomly assembled n-SnO(2) nanowires on p-GaN:Mg/sapphire substrate via vapor transport method, were investigated at room temperature. Peak wavelength emission at around 388 nm was observed for the diode under forward bias. This is mainly related to the radiative recombination of weakly bounded excitons at the shallow-trapped states of SnO(2) nanowires, Under reverse bias, near bandedge emission from the p-GaN:Mg/sapphire leads to the observation of emission peak at around 370 nm.

Growth of SnO(2) nanocrystals controlled by erbium doping in silica

We investigate the effects of erbium doping on SnO(2) nanoclustering in Sn-doped silica. Vibrational spectroscopy data from Raman and infrared absorption measurements show nanostructuring effects on the SnO(2) nanophase. Ultraviolet absorption spectra evidence a gap shift ascribable to size-dependent quantum confinement, also suggesting a role of erbium doping in determining cluster sizes and the amount of localized states on the nanophase boundary. Transmission electron microscopy confirms and details the spectroscopic data. As a result of these measurements, we find that the nanocrystal size distribution becomes narrower, increasing the erbium concentration, while the density of localized states at the nanocrystal surface decreases. The distribution of erbium ions among the possible environments is then examined through simultaneous spectroscopy of luminescence excited by nanocrystal-to-erbium energy transfer and the absorption of nanocrystal luminescence by erbium ions. This analysis shows that erbium behaves as an extrinsic nucleation centre of the SnO(2) nanophase at low doping levels, whereas at high concentrations it modifies the matrix, hindering the growth of SnO(2) crystals and passivating the interface.

Wave-vector-dependent exciton exchange interaction

The exchange interaction for the yellow 1S orthoexciton in Cu2O is derived up to the order K2. The resulting exchange splittings are verified experimentally using high resolution spectroscopy. In agreement with theory the fine structure shows a characteristic dependence on the direction of the wave vector.