Conduction Band Energy Levels Research Articles

Spectroelectrochemistry of Colloidal CdSe Quantum Dots

Solution-phase spectroelectrochemistry was used to study electron injection into colloidal CdSe quantum dots (QDs) with sizes ranging from 3.4 to 11.1 nm in tetrahydrofuran (THF). The absorbance and photoluminescence of the QDs were monitored in response to both charging and discharging cycles, and the optical changes were reversible on a timescale of minutes. Bleaching of the QD 1S3/2h1Se exciton state was used to determine the conduction band energy levels. We found that the negative trion state was stable in THF for hours under an applied cathodic potential. Both the degree of bleaching and the recovery of the exciton state depended on the applied potential. Based on the current and charge measurements, we found that between 10 and 150 electrons were injected into the QDs, depending on the electrode potential and QD size. Most of the electron injection occurred below the band edge and led to quenching of the QD photoluminescence. The potential at which injection into QDs occurred depended on the nature of the QD ligands.

Zn‐Cu‐In‐S‐Se Quinary “Green” Alloyed Quantum‐Dot‐Sensitized Solar Cells with a Certified Efficiency of 14.4 %

AbstractThe photoelectronic properties of quantum dots (QDs) have a critical impact on the performance of quantum‐dot‐sensitized solar cells (QDSCs). Currently, I‐III‐VI group QDs have become the mainstream light‐harvesting materials in high‐performance QDSCs. However, it is still a great challenge to achieve satisfactory efficiency for light‐harvesting, charge extraction, and charge collection simultaneously in QDSCs. We design and prepare Zn0.4Cu0.7In1.0SxSe2−x (ZCISSe) quinary alloyed QDs by cation/anion co‐alloying strategy. The critical photoelectronic properties of target QDs, including band gap, conduction band energy level, and density of defect trap states, can be conveniently tailored. Experimental results demonstrate that the ZCISSe quinary alloyed QDs can achieve an ideal balance among light‐harvesting, photogenerated electron extraction, and charge‐collection efficiencies in QDSCs compared to its single anion or cation quaternary alloyed QD counterparts. Consequently, the quinary alloyed QDs boost the certified efficiency of QDSCs to 14.4 %, which is a new efficiency record for liquid‐junction QD solar cells.

Zn-Cu-In-S-Se Quinary "Green" Alloyed Quantum-Dot-Sensitized Solar Cells with a Certified Efficiency of 14.4 .

The photoelectronic properties of quantum dots (QDs) have a critical impact on the performance of quantum-dot-sensitized solar cells (QDSCs). Currently, I-III-VI group QDs have become the mainstream light-harvesting materials in high-performance QDSCs. However, it is still a great challenge to achieve satisfactory efficiency for light-harvesting, charge extraction, and charge collection simultaneously in QDSCs. We design and prepare Zn0.4 Cu0.7 In1.0 Sx Se2-x (ZCISSe) quinary alloyed QDs by cation/anion co-alloying strategy. The critical photoelectronic properties of target QDs, including band gap, conduction band energy level, and density of defect trap states, can be conveniently tailored. Experimental results demonstrate that the ZCISSe quinary alloyed QDs can achieve an ideal balance among light-harvesting, photogenerated electron extraction, and charge-collection efficiencies in QDSCs compared to its single anion or cation quaternary alloyed QD counterparts. Consequently, the quinary alloyed QDs boost the certified efficiency of QDSCs to 14.4 %, which is a new efficiency record for liquid-junction QD solar cells.

Photocatalytic hydrogen evolution of nanoporous CoFe2O4 and NiFe2O4 for water splitting

The spinels CoFe2O4 and NiFe2O4 of nanoporous photocatalysts were prepared by dealloying and calcination. The photocatalytic performance for the hydrogen generation rate via water splitting was measured. The results revealed that CoFe2O4 exhibits a sheet-like nanoporous structure and that abundant mesopores are distributed in the nanosheets. NiFe2O4 shows a typical pore-ligament structure. The measurements show that hydrogen generation is exhibited by both oxides because the bandgap of CoFe2O4 and NiFe2O4 is higher than the water oxidization potential. The hydrogen generation rate is approximately 0.088 mmol h−1g−1 for CoFe2O4 and 0.026 mmol h−1g−1 for NiFe2O4 when the TEOA (10 vol%) sacrificial agent is adopted. This performance is significantly higher than that of methanol as the scavenger because TEOA increases the pH value of the solution, changes the negative shift in the conduction band energy level and improves the electron transport efficiency. The higher performance of CoFe2O4 is attributed to its larger specific surface area, ample unimpeded penetration diffusion paths and higher electron transfer rate.

Photo-recycling the Sacrificial Electron Donor: Towards Sustainable Hydrogen Evolution in a Biphasic System.

H2 may be evolved biphasically using a polarised liquid|liquid interface, acting as a "proton pump", in combination with organic soluble metallocenes as electron donors. Sustainable H2 production requires methodologies to recycle the oxidised donor. Herein, the photo-recycling of decamethylferrocenium cations (DcMFc+ ) using aqueous core-shell semiconductor CdSe@CdS nanoparticles is presented. Negative polarisation of the liquid|liquid interface is required to extract DcMFc+ to the aqueous phase. This facilitates the efficient capture of electrons by DcMFc+ on the surface of the photo-excited CdSe@CdS nanoparticles, with hydrophobic DcMFc subsequently partitioning back to the organic phase and resetting the system. TiO2 (P25) and CdSe semiconductor nanoparticles failed to recycle DcMFc+ due to their lower conduction band energy levels. During photo-recycling, CdS (on CdSe) may be self-oxidised and photo-corrode, instead of water acting as the hole scavenger.

Dipole-templated homogeneous grain growth of CsPbIBr2 films for efficient self-powered, all-inorganic photodetectors

Self-powered, all-inorganic photodetectors (PDs) have attracted ever-increasing research attention recently, because of their upgraded stability, promising performance, and free of external power supply. However, most of the inorganic perovskite films used for such PDs are prepared by solution methods, for which the crystal sizes vary significantly among different areas. This issue is more serious among different samples and eventually stands a substantial hindrance to PDs’ applications. In this paper, we introduced an amount of polyethyleneimine (PEI) species into the CsPbIBr2 precursor, which enabled a thin film with the homogeneous grain growth. Moreover, with the PEI modification, the conduction band energy level of resulting CsPbIBr2 film was also reduced, which leads to a more favored interfacial energy alignment between it and the TiO2 electron transporting layer (ETL). Consequently, the champion PD yielded the dark current of 3.12 × 10−10 A, the peak responsivity (R) of 0.20 A/W, the maximum specific detectivity (D*) of 6.01 × 1012 Jones, and the response time of 2.07 μs, which are far beyond those of the one prepared without PEI additive.

G-C3N4@PMo12 composite material double adjustment improves the performance of perovskite-based photovoltaic devices

At present, reducing the defect density at grain boundaries is still the main method to improve the performance of perovskite photodetectors. The increase of grain size can also improve the performance of perovskite photodetectors. In this work, we added POM @ g-C3N4 as a dopant into the perovskite precursor solution for the first time. The doping of g-C3N4 can control the crystallization rate of perovskite, thereby passivating the charge recombination center at the grain boundary to reduce the defect density. However, the conduction band energy level of g-C3N4 is higher than that of perovskite, which is not conducive to electron transmission. Polyoxometalate (POM) H3PMo12O40 (PMo12) has very good ultraviolet–visible light absorption. Adding it to perovskite can improve the ultraviolet–visible light absorption of the perovskite layer. Furthermore, the energy level of the polyoxometalate PMo12 matches the perovskite layer, which promotes the electron transport. Adding the composite material to the perovskite precursor solution not only increases the grain size of the perovskite crystals from 100 nm to 300 nm, but also increases the light absorption of the perovskite layer. Consequently, the photocurrent increased from 4.5 μA to 11.5 μA under the double regulation of the composite material, which is 2.6 times of the photocurrent of the reference group. This work provides a method of double adjustment of composite materials to increase the photocurrent of photodetectors.

Construction of p-n type Ag3PO4/CdWO4 heterojunction photocatalyst for visible-light-induced dye degradation

Regarding the high stability of Ag3PO4 and CdWO4, their valence band and conduction band energy levels match each other, a novel p-n type Ag3PO4/CdWO4 heterojunction photocatalyst was synthesized by conventional hydrothermal strategy, and analyzed by PXRD, SEM TEM and XPS, respectively. The corresponding results suggested that the nanorods of CdWO4 were successfully intergrown on the surface of Ag3PO4. The as-synthesized Ag3PO4/CdWO4 exhibited enhanced photocatalytic performance and reusability than that of two independent precursors via visible-light-induced photodegradation of Rhodamine B (RhB). The improved photocatalytic performance may due to expanded visible-light response range and enhanced the efficiency of carrier pairs separation induced by the adjustment of band-gap in p-n type Ag3PO4/CdWO4 heterojunction. Moreover, a putative photocatalytic mechanism of such heterojunction in degradation with dye was proposed.

Effects of triaxial strain on carrier activity, carrier lifetime, and absorption spectrum of (S, Se, Te) and 2 N Co-doped ZnO

The acceptor (N)-isovalent (S, Se, Te) elements that separately co-dope ZnO is a novel doping technology. However, the effects of triaxial strain on the photocatalytic performance and mechanism of doped systems are often neglected in experimental studies. Moreover, the influence of interstitial H on the doped ZnO system is not considered. This paper adopted first-principles and selected the appropriate strain range and step length. Assuming the equivalent effect of several strain points of tensile strain or compressive strain, we used simulation calculation to fit the complex strain problem in the actual process (such as doping, the strain problem caused by the difference of the lattice constant and thermal expansion coefficient of the substrate and the doping system), and then studied physical properties of the doped system. The planar wave ultrasoft pseudopotential + U method based on the first-principles generalized gradient approximation under the framework of density functional theory was used to calculate (S, Se, Te) and 2 N co-doped ZnO. The interstitial H-doped Zn36SO33N2 was studied as a representative ZnO system. Results showed that the binding energy of all doping systems under unstrained conditions was negative with a relatively high stability. The strain increased under tensile strain and compressive strain. As the formation energy of the doping systems increased, system stability decreased. Although the band gap of all compressive strain systems widened, impurities formed in the band gap, and valence band electrons absorbed photons of low energy to transition to the energy level of the impurities and then to the energy level of conduction band. The photoexcited electron-graded transitions induced the absorption spectrum to red-shift. The band gap of the system widened, whereas the band gaps of other tensile strain systems narrowed, except for when the tensile strain was 1%. However, regardless of whether the band gap of the tensile strain system became narrow or wide, the photoexcited electrons were still able to transition in stages. Hence, the absorption spectrum of the tensile strain system still red-shifted. The stability, activity, carrier separation, and lifetime of Zn36SHiO33N2 were relatively better than those of Zn36SO33N2 at 5% tensile strain. These properties make Zn36SHiO33N2 an ideal photocatalyst. This study provides a theoretical reference for the experimental design and preparation of novel ZnO photocatalysts.

Ligand-Dependent Tuning of Interband and Intersubband Transitions of Colloidal CdSe Nanoplatelets

Although surface ligands of colloidal nanocrystals are known to adjust the absolute energy levels of valence and conduction bands of semiconductor nanocrystals, they typically have only minor influ...

Reactively sputtered Zn(O,S) buffer layers for controlling band alignment of Cu(In,Ga)Se2 thin-film solar cell interface

Abstract Buffer layers in Cu(In,Ga)Se2 (CIGS) solar cells are highly preferable to be vacuum-compatible and use non-toxic materials not only for the film but also for the manufacturing process. Moreover, high controllability for the energy levels of buffer layers is greatly desirable for achieving higher solar cell efficiency because the buffer layer is formed directly onto the absorber layer which generates carriers. In this study, the S/(O + S) composition ratio of the Zn(O,S) thin films was finely controlled by varying O2/(Ar + O2) gas flow ratio using a reactive sputtering method including an Ar/O2 mixture gas and a single ZnS sputter target. The structural, electrical, and photovoltaic performance properties of the Zn(O,S) thin films and their CIGS solar cells were investigated at varied S/(O + S) composition ratios. As the S/(O + S) composition ratio of the Zn(O,S) films increased, bandgap bowing occurred with the increase of the conduction band and valence band energy levels. The photovoltaic performance was greatly influenced by the difference of the conduction band energies between the Zn(O,S) buffer and the CIGS absorber. When the conduction band energy of the Zn(O,S) was too high or too low compared to that of the CIGS, it increased the carrier recombinations or series resistance, respectively, which induced losses of the open-circuit voltage and the fill factor. The controlling and optimizing the energy levels at the Zn(O,S)/CIGS interface were crucial for improved solar cell performances.

Dye-sensitized photocatalytic hydrogen evolution by using copper-based ternary refractory metal chalcogenides

The exploration of photocatalytic transformation of solar energy into H2 through water splitting is an important direction towards sustainable and non-polluting energy in order to cover energy necessity partially. Ternary transition metal chalcogenides have been attracted attention among the other chalcogenides due to their potential applications in the photocatalytic and electrocatalytic hydrogen evolution. Herein, Cu2WS4 nanocubes and Cu2WSe4 nanosheets have been synthesized through a facile hot-injection method to benefit from the advantages such as minimizing the required pressure and reaction time by this technique. The photocatalytic hydrogen evolution activities of Cu2WS4 and Cu2WSe4 have been investigated under the visible light irradiation by using eosin-Y (EY) dye and triethanolamine (TEOA) as a photosensitizer and an electron donor, respectively. Cu2WS4 nanocubes have exhibited higher photocatalytic activity and stability than Cu2WSe4 nanosheets. The photocatalytic HER rates of Cu2WS4 and Cu2WSe4 have been determined as 1260 μmol g−1 h−1 and 861 μmol g−1 h−1, respectively. Photocatalytic HER activities were figured out in the order of Cu2WS4 > Cu2WSe4 which could be attributed to differences between proton reduction potential and the conduction band energy levels.

Fabrication of Novel Z-Scheme Bi₂WO6/NaBiO₃ Nanocomposites with Enhanced Photocatalytic Activity in the Degradation of 2,3-Dichlorophenol.

The Z-scheme Bi₂WO6/NaBiO₃ nanocomposites were first fabricated by a facile hydrothermal method, and were then characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, energy dispersive spectrometer, Fourier-transform-infrared spectroscopy, X-ray photoelectron spectroscopy and N₂ adsorption-desorption. The as-prepared Bi₂WO6/NaBiO₃ nanocomposites exhibit outstanding photocatalytic activity and recyclability. A 98.4% photodegradation of 2,3-dichlorophenol (50 mg·L-1) was attained in the presence of Bi₂WO6/NaBiO₃ (1:10) under the visible-light irradiation in 30 min. In particular, the photocatalytic mechanism has been discussed in detail, based on four aspects: (1) oxidative species, (2) photoelectrochemical performance, (3) conduction band and valence band energy levels and (4) possible transition states and reactions. In conclusion, O-₂ is the main active oxidative species in the Bi₂WO6/NaBiO₃ nanocomposite. The material has higher photocurrent and visible light adsorption but lower electron-hole pairs recombination, which contributes to distinguished photocatalytic efficiency. The Z-scheme photocatalytic path was proposed and the possible degradation process and routes have been summarized.

Synthesize of gadolinium-doped ZnO nano particles for energy applications by enhance its optoelectronic properties

In recent research, the zinc oxide (ZnO) semiconductor nano particles have received a lot of attention due to its unique physical, chemical, magnetic and optoelectronic properties. The present study focuses on the photo physical and optoelectronic properties of ZnO nanostructures. To attain better optical and optoelectronic properties for light emitting diodes, solar cells and photo detectors, the rare earth transition metal ions, specifically gadolinium (Gd) were doped into ZnO nano material. Gadolinium-doped ZnO nano materials have been synthesized by a simple chemical deposition method. The structural, morphological, optical properties of synthesized samples have been investigated by various techniques, including x-ray diffraction (XRD), UV-absorption spectroscopy, and photo luminescence spectroscopy. X-ray diffraction analysis confirmed the formation of pure single phase with a particle size in the range of 30–42 nm. Optical absorption studies demonstrates that the absorption edge of the Gd:ZnO appeared to shift towards the longer wavelength due to the charge transfer between the ZnO valence or conduction band and rare earth ion 4f energy level. The band gap of the doped ZnO samples was found to be decreases and it varies from 3.2 eV to 3.1 eV due to the quantum confinement of the charge carriers in ZnO. These results provide a new approach to design ZnO nano particles with enhanced photochemical properties, light emitting electronics and solar cells.

Synthesis of crystalline zinc hydroxystannate and its thermally driven amorphization and recrystallization into zinc orthostannate and their phase-dependent cytotoxicity evaluation

Engineered metal-oxide nanoparticles have greatly been used in various bio-applications, but their physicochemical properties change dramatically within a bio-system. Therefore, the development of new metal-oxide nanoparticles and proper understanding of their toxicity profile in bio-environment are very important for advancement of cancer-related diagnostic and therapeutic approaches. Nanostructured wide-bandgap semiconductors ZnSn(OH)6 and Zn2SnO4 are fabricated by one-pot hydrothermal synthesis and tested for the first time in anti-cancer treatments. The crystalline zinc-hydroxystannate is converted by high-temperature annealing into amorphous and then crystalline zinc-orthostannate, as confirmed by Rietveld refinement analyses. A dose-dependent cytotoxicity is observed when human cervical carcinoma cell lines are exposed to these nanomaterials, mainly due to the elevation of intracellular reactive-oxygen-species levels in the treated cells, which leads to oxidative stress and cell damage. The crystalline phases of the nanomaterials reveal better cell-killing efficacy due to the overlap of the conduction-band energy levels with the cellular redox potential, leading to favorable electron transfer from the biological redox couples to the conduction-band of the semiconductor nanoparticles, thus producing more reactive-oxygen-species for cell damage. Importantly, this higher reactive-oxygen-species production by the crystalline samples significantly reduces cancer cell migration and proliferation and decreases metastasis, which remains an unmet challenge in cancer therapy.

Epitaxial Cubic Silicon Carbide Photocathodes for Visible-Light-Driven Water Splitting.

Cubic silicon carbide (3C‐SiC) material feature a suitable bandgap and high resistance to photocorrosion. Thus, it has been emerged as a promising semiconductor for hydrogen evolution. Here, the relationship between the photoelectrochemical properties and the microstructures of different SiC materials is demonstrated. For visible‐light‐derived water splitting to hydrogen production, nanocrystalline, microcrystalline and epitaxial (001) 3C‐SiC films are applied as the photocathodes. The epitaxial 3C‐SiC film presents the highest photoelectrochemical activity for hydrogen evolution, because of its perfect (001) orientation, high phase purity, low resistance, and negative conduction band energy level. This finding offers a strategy to design SiC‐based photocathodes with superior photoelectrochemical performances.

Cd-free Cu(InGa)Se2 solar cells with eco-friendly a-Si buffer layers

CdS, In2S3 and ZnO-based materials are typically adopted as buffer layers in high efficiency CIGS solar cells. However, the drawbacks involved toxicity, complicated synthesis process and environmental damage of aforementioned materials would someday limit the development themselves. Here, for the first time, eco-friendly and low-cost a-Si thin films prepared by e-beam evaporation were used as buffer layers in CIGS solar cells. Thickness of a-Si films was changed to optimize the performance of CIGS solar cells through investigating their optical, structural and electrical properties. It revealed that the conduction band alignment of CIGS and a-Si is a favorable spike and a 60 nm a-Si film is optimal for CIGS/a-Si interface properties. Following the SCAPS simulation, bandgaps and conduction band energy level of CIGS films were facilely modulated through altering the Ga/(Ga + In) ratios. The conversion efficiency of solar cell with modified CIGS film was relatively improved by over 30% compared to the pristine one. The present work provides a new and eco-friendly strategy for fabrication of CIGS solar cells and expands the application of a-Si thin films as buffer layers for solar cells.

The band bending effect of LiI/NaI treated TiO2 photoanodes on the performance of dye-sensitized solar cells.

A new method has been developed for surface treatment of TiO2 photoanodes with LiI/NaI to enhance the photocurrent and, subsequently, the performance efficiency of the fabricated dye-sensitized solar cells (DSSCs). Three different concentrations (0.1, 0.25, and 0.5 mmol%) of LiI and NaI solutions were used to investigate the effect of this surface treatment on the device performance of DSSCs. A positive shift in the energy level of TiO2 has been experienced by surface treated devices, which is predominantly supported by the decrease in VOC. Furthermore, the introduction of LiI/NaI onto the TiO2 surface resulted in a reduction in the crystallite size, indicating an increase in the surface area which helps in more dye adsorption leading to higher JSC values of the devices. Besides, modification of the conduction band energy level, it also allows a fast electron injection process by shifting the density of states. Thus, this approach offers a simple but efficient route to enhance the photocurrent and efficiency of DSSCs.

A novel strategy to construct a visible-light-driven Z-scheme (ZnAl-LDH with active phase/g-C3N4) heterojunction catalyst via polydopamine bridge (a similar "bridge" structure)

A novel strategy to construct a visible-light-driven Z-scheme heterojunction catalyst was employed by crosslinking ZnAl-layered double hydroxide (ZnAl-LDH) nanosheets with the active phase on carbon nitride (g-C3N4) substrates via a polydopamine bridge (a similar "bridge" structure). In this paper, multiple optical and electrochemical detection methods indicated that the 0.5P-LDH_500CN photocatalyst demonstrated excellent visible-light absorption properties, photo-generated electron-hole separation ability and photocatalytic activity for p-nitrophenol under visible-light (> 420 nm), etc. A Z-scheme charge transfer mechanism via PDA bridge was proposed to achieve heterojunction charge separation. This mechanism involved the recombination of photo-induced electrons directly on the ZnAl-LDH_500 valence band through the PDA channel and the holes were captured at the conduction band energy level of the g-C3N4. The detection of active species, including O2-, h+ and OH, further proofed the Z-scheme charge transfer mechanism, which could be speculated that all active species affected the photocatalytic reaction with the order of h+ >OH >O2-. Meanwhile, this work also exposed that the formation of active phase in ZnAl-LDH could synergize with PDA to promote the application of visible-light-active photocatalysts based on g-C3N4 materials in high-efficiency energy.

Electrical tuning of helical edge states in topological multilayers

Mainstream among topological insulators, GaSb/InAs quantum wells present a broken gap alignment for the energy bands which supports the quantum spin Hall insulator phase and forms an important building block in the search of exotic states of matter. Such structures allow the band-gap inversion with electrons and holes confined in adjacent layers, providing a fertile ground to tune the corresponding topological properties. Using a full 3D eight-band method we investigate the inverted band structure of GaSb/InAs/GaSb and InAs/GaSb/InAs multilayers and the behavior of the helical edge states, under the influence of an electric field applied along the growth direction. By tuning the electric field modulus, we induce the change of the energy levels of both conduction and valence bands, resulting in a quantum spin Hall insulator phase where the helical edge states are predominantly confined in the GaSb layer. In particular, we found that InAs/GaSb/InAs has a large hybridization gap of about and, therefore, are promising to observe massless Dirac fermions with a large Fermi velocity. Our comprehensive characterization of GaSb/InAs multilayers creates a basis platform upon which further optimization of III–V heterostructures can be contrasted.