Bandgap Metal Oxide Semiconductors Research Articles

Atomic Layer Deposition of Ultra-Thin Crystalline Electron Channels for Heterointerface Polarization at Two-Dimensional Metal-Semiconductor Heterojunctions

Atomic layer deposition (ALD) has emerged as a promising technology for the development of the next generation of low-power semiconductor electronics. The wafer-scaled growth of two-dimensional (2D) crystalline nanostructures is a fundamental step toward the development of advanced nanofabrication technologies. Ga2O3 is an ultra-wide bandgap metal oxide semiconductor for application in electronic devices. The polymorphous Ga2O3 with its unique electronic characteristics and doping capabilities is a functional option for heterointerface engineering at metal-semiconductor 2D heterojunctions for application in nanofabrication technology. Plasma-enhanced atomic layer deposition (PE-ALD) enabled the deposition of ultra-thin nanostructures at low-growth temperatures. The present study used the PE-ALD process for the deposition of atomically thin crystalline ß-Ga2O3 films for heterointerface engineering at 2D metal-semiconductor heterojunctions. Via the control of plasma gas composition and ALD temperature, the wafer-scaled deposition of ~5.0 nm thick crystalline ß-Ga2O3 at Au/Ga2O3-TiO2 heterointerfaces was achieved. Material characterization techniques showed the effects of plasma composition and ALD temperature on the properties and structure of Ga2O3 films. The following study on the electronic characteristics of Au/Ga2O3-TiO2 2D heterojunctions confirmed the tunability of this metal/semiconductor polarized junction, which works as functional electron channel layer developed based on tunable p-n junctions at 2D metal/semiconductor interfaces.

Three-terminal nonvolatile memory photodetectors based on rationally engineered heterostructured tin zinc oxide nanowires

One-dimensional wide band-gap metal oxide semiconductors demonstrate enormous potential in high-performance ultraviolet photodetectors. Here, via tuned zinc oxide (ZnO) concentration ratio and heterojunction structure as well as decent VTH value, we electrospun tin zinc oxide (SnZnOX) nanowires for photoelectronic conversion function. The constructed optimized photodetectors based on Sn8Zn2OX nanowires can exhibit a remarkable detectivity of 1.77 × 1016 Jones, excellent responsivity of 2.23 × 103 A/W and relative high sensitivity over 104. In addition to the excellent photodetection performance, Sn8Zn2OX nanowire devices exhibit nonvolatile memory effect, which can maintain the light-induced low resistance state at 5 V without light. Moreover, a voltage of 30 V was applied to Sn8Zn2OX nanowires devices to realize light-writing state switching to bias-erasing state, and the device could come back to their initial high resistance state. These results indicate SnZnOX nanowires devices possess a tremendous potential in the circuit design of the artificial visual system.

Synthesis and photocatalytic activity of polymer stabilized cadmium selenide quantum dots-titanium dioxide nanocomposites

In this work, we report the synthesis of the nanocomposites of cadmium selenide (CdSe) quantum dots (QDs) and titanium dioxide nanoparticles (TiO2 NPs), and their use in the photocatalytic degradation of an organic dye. Titanium dioxide nanoparticles are a multifaceted large band gap metal oxide semiconductor that has been subjected to innumerable research studies in the past decades. TiO2 NPs exhibit characteristic properties depending upon their morphology, crystal structure, size and the method of synthesis that makes them suitable and efficient for different applications. The synthesized CdSe QDs were then used to sensitize TiO2 NPS in order to form CdSe QDs-TiO2 nanocomposites. The morphology of the QDs were investigated by high resolution-transmission electron microscopy. Fluorescence spectroscopy studies were performed to determine the absorption and emission peaks of the CdSe QDs. The band gap increased of pristine TiO2 NPs and CdSe decorated TiO2 NPs were evalued using UV–visible diffuse reflectance spectroscopy. The XRD pattern of the synthesized nanocomposites depicted the formation of anatase phase of TiO2. These nanocomposites were then used for the photocatalytic degradation of methylene blue (MB) dye.

Enhanced piezoelectric-effect-assisted photoelectrochemical performance in ZnO modified with dual cocatalysts

Promising strategies are of great significance for designing photoelectrodes with high charge separation efficiency and low charge recombination rate in photoelectrochemical (PEC) water splitting. In this paper, we firstly propose a method of combining piezoelectric effect with photoelectrochemical cell by utilizing of direct band gap metal-oxide semiconductor ZnO as the photoanode. After introducing piezoelectric effect resulted from ultrasonic vibrations, the current density value reaches 0.45 mA·cm−2 at 1.23 VRHE by the production of strain-induced charge and the enhancement of the electric field, 1.7 times higher than that of ZnO without ultrasonic vibrations (0.27 mA·cm−2 at 1.23 VRHE). The deposition of spatially separated dual cocatalysts, of which the bottom Pt act as electron collection and transport layer while the outmost Co-Pi serve as hole-transfer layer, further facilitates the separation of charge (strain-induced and photogenerated) and accelerates the reaction kinetics. The utilization of dual cocatalysts efficiently improves the performance of photoanodes and leads to a current density of 0.80 mA·cm−2 at 1.23 VRHE, 1.8 and 3.0 times higher than the values of ZnO with and without ultrasonic vibrations. The strategy provides a promising method and idea for the manufacture of practical and high-performance electrodes.

Plasmonic Au Nanoparticles/KCa2Nb3O10 nanosheets 0D/2D heterojunctions with enhanced photocatalytic activity towards the degradation of tetracycline hydrochloride

Exploration of an effective strategy to enhance the visible-light-driven photocatalytic activity of wide band gap metal oxide semiconductors is of great importance. In this work, plasmonic Au nanoparticles/KCa2Nb3O10 nanosheets 0D/2D heterojunctions were constructed by a facile precipitation–calcination process. In these heterojunctions, Au nanoparticles not only act a photosensitizer to harvest the visible light but also assist in the charge separation, therefore enhancing the visible light photocatalytic activity. The optimized Au/KCa2Nb3O10 0D/2D plasmonic heterojunction showed a remarkable photocatalytic activity towards the degradation of tetracycline hydrochloride. The photocatalytic activity of the optimized 2%-Au/KCNO heterojunction is 3.9 times that of bare KCa2Nb3O10. This work provides an alternative approach to improve the light utilization efficiency and photocatalytic activity of KCa2Nb3O10 semiconductor for photocatalytic degradation of organic pollutant.

Fundamental Aspects of Photoinduced Charge Flow at a Quantum-Dot-Sensitized Single-Crystal TiO2 Semiconductor Interface

The fundamental aspects of charge transfer from photoexcited CdSe quantum dots to a single crystal of TiO2, a wide band gap metal oxide semiconductor, were investigated and compared with that of a dye-sensitized system in relation to the operation of quantum-dot-sensitized solar cells (QDSCs) and dye-sensitized solar cells (DSSCs). Due to the stark differences in both physical and electronic properties of quantum dots versus molecular dyes, it was hypothesized that the fundamental behavior of the two systems could differ greatly. The large size and surface area of the quantum dots relative to molecular dyes present the possibility for the positively charged hole to move a greater distance away from the QD/oxide interface during the electron injection process. This increased distance influences the Coulombic interaction between the trapped hole and injected electron, leading to differences and increased complexity of the recombination pathways when compared to the dye system.

Improved efficiency of PbS quantum dot sensitized NiO photocathodes with naphthalene diimide electron acceptor bound to the surface of the nanocrystals

Hybrid materials combining a wide bandgap metal oxide semiconductor, metal chalcogenide nanocrystals and molecular systems represent very attractive materials for fabricating devices with new function or improved photoelectrochemical performance. This study deals with sensitization of NiO, which is a p-type semiconductor, by quantum dots (QDs) of PbS with an average diameter of 3nm. The PbS QDs were attached to the monocrystalline film of NiO by mercaptopropionic acid linker and were subsequently capped with methyl-pyridine naphthalene diimide (NDI) units to prepare quantum dot sensitized solar cells (p-QDSSCs) on NiO electrodes. Time-resolved photoluminescence measurements of the PbS emission were used to determine the rate constants for charge transfer from the PbS exciton to the NiO, cobalt based redox mediator and NDI. Notably, it was shown that NDI quenches the PbS exciton by electron transfer with a quite fast rate constant (6.9 × 107s−1). The PbS QDs sensitized NiO films were finally used to fabricate solar cells with tris(4,4′-ditert-butyl-2,2′-bipyridine) cobalt(III/II) as redox mediator. It was observed that the presence of NDI on PbS improved the photovoltaic performance by 50% relative to that of cells without NDI, leading to a device with the following characteristics: Jsc = 5.75mA/cm2, Voc = 226mV, ff = 34% and PCE = 0.44%. This study demonstrates that photogalvanic processes can be a productive pathway to better performing sensitized p-type semiconductor for p-QDSSC. In other words, photoinduced electron transfer from the QDs towards the electrolyte rather than initial photoinduced charge injection into the p-type semiconductor can be a favorable operative mechanism in QD sensitized NiO films and might be exploited further for the construction of better performing solar cells or photocatalytic devices.

Rutile TiO2 as an Anode Material for Water-Splitting Dye-Sensitized Photoelectrochemical Cells

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) use a wide bandgap metal oxide semiconductor functionalized with a light-absorbing dye and water-oxidation catalyst to harvest light and drive water oxidation, respectively. We demonstrate here that the rutile polymorph of TiO2 (r-TiO2) is a promising anode material for WS-DSPECs. Recombination between the injected electron and oxidized sensitizer with r-TiO2 is an order of magnitude slower than with anatase TiO2 (a-TiO2), with injection yields approaching 100%. Studies with a reductive quencher demonstrate that r-TiO2 is significantly more efficient than a-TiO2, while exhibiting greater dye stability. Furthermore, comparison of direct band gap excitation photocurrent generation for bare and sensitized r-TiO2 suggests that the sensitizer functions as a light harvester and redox mediator.

(Plenary) Wide Band Gap Semiconductor Transistors for FPD

Wide band gap metal oxide semiconductor TFTs will play a major role in the development of the high resolution and high performance flat panel display. Metal oxide semiconductor TFTs with high mobility and good stability were developed at ETRI through extensive investigation of the materials, device structure, and interface engineering.

Photoreactivity of ZnO nanoparticles in visible light: Effect of surface states on electron transfer reaction

Wide band gap metal oxide semiconductors such as zinc oxide (ZnO) show visible band photolysis that has been employed, among others, to degrade harmful organic contaminants into harmless mineral acids. Metal oxides show enhanced photocatalytic activity with the increase in electronic defects in the crystallites. By introducing defects into the crystal lattice of ZnO nanoparticles, we observe a redshift in the optical absorption shifting from the ultraviolet region to the visible region (400–700 nm), which is due to the creation of intermediate defect states that inhibit the electron hole recombination process. The defects were introduced by fast nucleation and growth of the nanoparticles by rapid heating using microwave irradiation and subsequent quenching during the precipitation reaction. To elucidate the nature of the photodegradation process, picosecond resolved time correlated single photon count (TCSPC) spectroscopy was carried out to record the electronic transitions resulting from the de-excitation of the electrons to their stable states. Photodegradation and TCSPC studies showed that defect engineered ZnO nanoparticles obtained through fast crystallization during growth lead to a faster initial degradation rate of methylene blue as compared to the conventionally synthesized nanoparticles.

Ultrafast Electron Transfer between Conjugated Polymer and Antimony-Doped Tin Oxide (ATO) Nanoparticles

Nanocomposites of conjugated polymers and wide band gap metal oxide semiconductors have received intense interests in recent years because of their potential application in solar cells. Photoinduced charge separation and recombination in conjugated poly[2-methoxy-5-(2-ethyl-hexyloxy)-(phenylene vinylene)] (MEH-PPV) sensitized Sb:SnO2 (ATO) nanocrystalline thin films were investigated by ultrafast transient IR spectroscopy. For ATO films with 0%, 1%, 3%, and 5% Sb dopants, the rates of electron injection from MEH-PPV to ATO were similar (ranging from 0.77 to 0.46 ps), while the rates of the charge recombination increased at higher doping levels. The effects of the doping level on the injection and the recombination rates were discussed.

An aqueous solution approach to advanced metal oxide arrays on substrates

Materials chemistry has emerged as one of the most consistent fabrication tools for the rational delivery of high purity functional nanomaterials, engineered from molecular to microscopic scale at low cost and large scale. An overview of the major achievements and latest advances of a recently developed growth concept and low temperature aqueous synthesis method, for the fabrication of purpose-built large bandgap metal oxide semiconductor materials and oriented nano-arrays is presented. Important insight of direct relevance for semiconductor technology, optoelectronics, photovoltaics and photocatalysis for solar hydrogen generation, are revealed by in-depth investigations of the electronic structure of metal oxide nanostructures with new morphology and architecture, carried out at synchrotron radiation facilities.

Subnanosecond Relaxation Dynamics of 2,2'-Azinobis(3-ethylbenzothiazoline-6-sulfonate) and Chlorpromazine. Assessment of Photosensitization of a Wide Band Gap Metal Oxide Semiconductor TiO2 [Erratum to document cited in CA122:251875

Subnanosecond Relaxation Dynamics of 2,2'-Azinobis(3-ethylbenzothiazoline-6-sulfonate) and Chlorpromazine. Assessment of Photosensitization of a Wide Band Gap Metal Oxide Semiconductor TiO2 [Erratum to document cited in CA122:251875