Wide Band Gap Metal Oxide Research Articles

(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.

Through-Process Analytical Modeling of Photoconductance Spectrum for Porous ZnO Nanocrystalline Film

In the past decades, many models have been developed to describe the photoconductance and the related photoresponse phenomena in wide-band gap metal oxides, but the related mechanisms and kinetics are still confused. Here, based on the persistent photoconductance observed in the porous ZnO nanocrystalline film, a donor photoionization model (DPM) is proposed to decipher the photoelectric kinetics both upon the sub-band gap and the above-band gap illuminations. In the DPM, the reaction rate equations and the related analytical functions are employed to numerically model the through processes of photoconductance spectra in a time-domain by comprehensively considering the photoelectric processes in ZnO such as the band-to-band transition, the donor photoionization, the surface oxygen adsorption/desorption, and the photogenerated electron recapture by donors. In the DPM simulation processes, some essential parameters, such as the electron yield from donor photoionization, the electron capture rates by donors, and the oxygen adsorption reaction rates on ZnO surface, are extracted quantitatively from the photoconductance spectra. The DPM analytical process herein reveals the photoresponse kinetics in detail and proposes a new insight into the basic theory of metal oxide photoresponse that might be helpful for further understanding the photoexcited carrier kinetics and the related photochemical processes.

Kelvin probe force microscopy of nanocrystalline TiO2 photoelectrodes

Dye-sensitized solar cells (DSCs) provide a promising third-generation photovoltaic concept based on the spectral sensitization of a wide-bandgap metal oxide. Although the nanocrystalline TiO2 photoelectrode of a DSC consists of sintered nanoparticles, there are few studies on the nanoscale properties. We focus on the microscopic work function and surface photovoltage (SPV) determination of TiO2 photoelectrodes using Kelvin probe force microscopy in combination with a tunable illumination system. A comparison of the surface potentials for TiO2 photoelectrodes sensitized with two different dyes, i.e., the standard dye N719 and a copper(I) bis(imine) complex, reveals an inverse orientation of the surface dipole. A higher surface potential was determined for an N719 photoelectrode. The surface potential increase due to the surface dipole correlates with a higher DSC performance. Concluding from this, microscopic surface potential variations, attributed to the complex nanostructure of the photoelectrode, influence the DSC performance. For both bare and sensitized TiO2 photoelectrodes, the measurements reveal microscopic inhomogeneities of more than 100 mV in the work function and show recombination time differences at different locations. The bandgap of 3.2 eV, determined by SPV spectroscopy, remained constant throughout the TiO2 layer. The effect of the built-in potential on the DSC performance at the TiO2/SnO2:F interface, investigated on a nanometer scale by KPFM measurements under visible light illumination, has not been resolved so far.

Photoassisted defluorination of fluorinated substrates and pharmaceuticals by a wide bandgap metal oxide in aqueous media

Persistent fluorinated substances, such as the fluorine-bearing pharmaceutical drugs Fluoxetine (FLX; Prozac) and Fluvoxamine maleate (FOM) together with several other substrates (fluorobenzoic acid and fluoroaliphatic model compounds), were photochemically defluorinated and degraded under UVC illumination in relatively good yields in the presence of a wide band gap metal oxide (β-Ga2O3) in heterogeneous aqueous media. The formation of fluoride ions increased with increasing illumination time under an inert nitrogen atmosphere, the transformation of the aromatic moiety was slower under these conditions, but nonetheless it did occur. The optimal amount of β-Ga2O3 loading for defluorination was 50 mg in aqueous media (0.10 mM, 100 mL); the optimal pH to defluorinate FLX was pH 6. Platinization (1 wt%) of the gallium oxide particles enhanced defluorination under an inert nitrogen atmosphere, but was decreased under an oxygen atmosphere; however, in the latter case the degradation of the substrates was facilitated as witnessed by loss of the aromatic moiety. The Ames test on the intermediate products from the photodegradation of FLX and 4-(trifluoromethyl)benzoic acid after long illumination times revealed that none were mutagenic.

Plasmonic Gold Nanocrystals Coupled with Photonic Crystal Seamlessly on TiO2 Nanotube Photoelectrodes for Efficient Visible Light Photoelectrochemical Water Splitting

A visible light responsive plasmonic photocatalytic composite material is designed by rationally selecting Au nanocrystals and assembling them with the TiO(2)-based photonic crystal substrate. The selection of the Au nanocrystals is so that their surface plasmonic resonance (SPR) wavelength matches the photonic band gap of the photonic crystal and thus that the SPR of the Au receives remarkable assistance from the photonic crystal substrate. The design of the composite material is expected to significantly increase the Au SPR intensity and consequently boost the hot electron injection from the Au nanocrystals into the conduction band of TiO(2), leading to a considerably enhanced water splitting performance of the material under visible light. A proof-of-concept example is provided by assembling 20 nm Au nanocrystals, with a SPR peak at 556 nm, onto the photonic crystal which is seamlessly connected on TiO(2) nanotube array. Under visible light illumination (>420 nm), the designed material produced a photocurrent density of ~150 μA cm(-2), which is the highest value ever reported in any plasmonic Au/TiO(2) system under visible light irradiation due to the photonic crystal-assisted SPR. This work contributes to the rational design of the visible light responsive plasmonic photocatalytic composite material based on wide band gap metal oxides for photoelectrochemical applications.

Band engineered ternary solid solution CdSxSe1−x-sensitized mesoscopic TiO2 solar cells

The optical band gap of the light absorber and the alignment of its bands with the underlying wide band gap metal oxide are critical for efficient light harvesting and charge separation in semiconductor-sensitized solar cells (SSCs). In practice, these two requirements are however not always fulfilled concomitantly in SSCs. Favourable band alignment in CdSe-sensitized TiO2 requires utilization of quantum sized CdSe, which causes great losses in the harvesting of long wavelength photons due to quantum confinement effects. In the present study, ternary cadmium sulfoselenide (CdSxSe1-x), which has a tunable band gap between those of CdSe and CdS without reducing the dimension, was proposed as a sensitizer for TiO2. CdSxSe1-x was successfully synthesized by alternately depositing CdS and CdSe layers under ambient conditions. SSCs utilizing CdSxSe1-x-sensitized TiO2 yielded a power conversion efficiency of 4.05% under simulated AM1.5 100 mW cm(-2) illumination, rivalling the well-studied cascaded CdS/CdSe electrodes when an aqueous polysulfide solution was used as the electrolyte and Cu2S as the counter electrode. The findings of the present study provide an alternative and viable approach for optimizing the energetics of semiconductor sensitizers for efficient charge separation, while also maintaining good light harvesting.

A ZnO/N-doped carbon nanotube nanocomposite charge transport layer for high performance optoelectronics

Metal oxide charge transport layers are widely used to promote the interfacial charge transport of organic optoelectronics. Nevertheless, frequently used wide-bandgap metal oxides with low electrical conductivities reveal inherent limitations in the charge transport enhancement. We present the remarkable electro-conductivity enhancement of solution processable ZnO charge transport layers upon dispersing a tiny amount (less than 0.1 wt%) of chemically doped CNTs and the corresponding device performance improvement of light-emitting diodes (OLEDs). Using various undoped or doped CNTs, whose work function was systematically tuned by substitutional doping of electron deficient B or electron rich N,N-doped CNT (N-CNT), the composite showed a lowered work function matching well with the conduction band of ZnO. Consequently, the ZnO/N-CNT nanocomposite transport layer with 0.08 wt% N-CNT showed a five-fold enhancement of electron mobility, while maintaining the intrinsic bandgap energy levels, optical transparency and solution processability of pure ZnO. The inverted OLEDs employing ZnO/N-CNT nanocomposite electron transport layers could facilitate well-balanced electron–hole injection and, thus, more than two-fold enhancement of maximum luminance (from 21 000 cd m−2 at 14.6 V to 46 100 cd m−2 at 14.0 V) and efficiency (from 6.9 cd A−1 at 13.4 V to 14.3 cd A−1 at 13.6 V). This highly effective charge mobility enhancement enabled by work function tunable, chemically doped CNTs would be beneficial for various organic and inorganic charge transport materials with different energy levels.

Investigation of the fill factor of dye-sensitized solar cell based on ZnO nanowire arrays

The fill factor of dye-sensitized solar cells based on the ZnO nanowire array is very low, which is usually ascribed to a rapid charge recombination. In this article, the influence on the fill factor of ZnO nanowire array cell is investigated and discussed by comparing dark current and decay rate of open circuit potential of the ZnO nanowire array cell with those of the ZnO nanoparticle cell, TiO 2 nanoparticle cell and TiO 2-coated ZnO nanowire array cell. The results demonstrate that the low fill factor of the ZnO nanowire array cell is largely caused by a rapid decrease of electron injection efficiency rather than a rapid charge recombination, which is decided by the absorption nature of Ru-complexed dye molecules on ZnO surface and repellency of radial electric field. The fill factor of the ZnO nanowire array cell can be improved by coating ZnO nanowires with a wide band gap semiconductor material or metal oxide insulator.

Stabilization Principles for Polar Surfaces of ZnO

ZnO is a wide band gap metal oxide with a very interesting combination of semiconducting, transparent optical and catalytic properties. Recently, an amplified interest in ZnO has appeared due to the impressive progress made in nanofabrication of tailored ZnO nanostructures and functional surfaces. However, the fundamental principles governing the structure of even the clean low-index ZnO surfaces have not been adequately explained. From an interplay of high-resolution scanning probe microscopy (SPM), X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy experiments, and density functional theory (DFT) calculations, we identify here a group of hitherto unresolved surface structures which stabilize the clean polar O-terminated ZnO(0001) surface. The found honeycomb structures are truly remarkable since their existence deviates from expectations using a conventional electrostatic model which applies to the opposite Zn-terminated (0001) surface. As a common principle, the differences for the clean polar ZnO surfaces are explained by a higher bonding flexibility of the exposed 3-fold coordinated surface Zn atoms as compared to O atoms.

Persistent conductivity in post-growth doped ZnO films following pulsed UV laser irradiation

Solution and rf sputter deposited doped ZnO films were subjected to cumulative 4-ns pulses of 355 nm light from a pulsed Nd:YAG laser at fluences between 5 and 150 mJ/cm 2. Film densification, change in refractive index, and an increase in conductivity were observed following room temperature irradiation in air, a carbon monoxide reducing environment, or under vacuum. At fluences between 20 and 80 mJ/cm 2, the films did not damage catastrophically under irradiation and high visible transparency persisted. The increase in conductivity is attributed to creation of oxygen vacancies and subsequent promotion of free carriers into the conduction band. Effects were most pronounced for films treated in vacuum. All treated films became insulating again upon equilibration in air at room temperature after several days. Films were characterized by means of UV–VIS–NIR transmission spectroscopy, Raman spectroscopy, and Hall measurements. Analysis of interference fringes in measured transmission spectra allowed evaluation of optical properties. Raman measurements showed an increase of LO mode intensity with respect to TO mode intensity as the films became more conducting in accord with previous work. Results of this study are not only important for continued development of transparent conducting oxides, but also provide compelling evidence for the role of free carriers as initiators of laser damage in wide bandgap metal oxide films.

Hybrid organic-inorganic light emitting diodes: effect of the metal oxide

Hybrid organic-inorganic light emitting diodes (HyLEDs), employing metal oxides as the electron injecting contacts, are interesting as an alternative to OLEDs. Until recently, the metal oxide of choice was either titanium dioxide or zinc oxide. In this work two wide bandgap metal oxides, HfO2 and MgO, are employed as electron injecting layer in HyLEDs. It is demonstrated that both the current density and the luminance values obtained are directly related to the barriers for electron injection (from the ITO to the metal oxide) and for hole transfer to the same metal oxide, outlining a new design rule for the optimization of HyLEDs. Record device efficacies (3.3 cd/A, >10000 cd/m2) using the commonly used emitting polymer (F8BT) are obtained.

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.

Thermal ionisation of bond-centred muonium in diamond?

Abstract A positive muon implanted into an insulator or semiconductor often binds an electron to form muonium ( μ + e - ≡ Mu ) , a paramagnetic “atom” electronically similar to hydrogen. Evidence was sought for thermal ionisation of bond-centred muonium ( Mu BC ) in natural type Ia single crystal diamond using the longitudinal magnetic field muon spin relaxation (LF- μ SR ) and a similar study was made of the diamagnetic state and tetrahedral muonium ( Mu T ) using the transverse field muon spin rotation (TF- μ SR ) techniques. Optical characterisation methods revealed the presence of A, B, P1, and P2 centres in the samples. The Mu BC state in diamond is easily observed and there is a good correlation between theoretical and experimental hyperfine parameters [J.W. Schneider, et al., Phys. Rev. Lett. 71(4) (1993) 557 [1] ]. Curiously, despite its predicted stability, the bond-centred hydrogen state has not yet been observed in diamond. Following the discovery of hydrogen dopant states in certain wide band gap metal oxides, and the possibility of hydrogen related molecular dopants in diamond, the study of hydrogen in diamond is important. Although it is evident from its hyperfine parameters that Mu BC is not a shallow donor, the question still arises as to where the Mu BC state in diamond might lie in the band gap. Accordingly, measurements of high temperature stability of Mu BC have been performed in a search for its possible ionisation. The longitudinal-field signature of Mu BC begins to disappear above 1000 K. The possible onset of diffusion between bond-centre sites has still to be discounted but, interpreted as ionisation, the data place limits of 0.60 and 1.8 eV on the muonium donor depth in diamond.

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.

The gap level of bond-centred muonium in diamond

Muonium, with a positive muon as the nucleus is considered a light isotope of hydrogen displaying a close chemical analogy to this atom. It offers a unique opportunity to study the behaviour of hydrogen in diamond at very low concentrations. The mass difference, however, implies that dynamical effects will be distinct. The bond centred muonium (Mu BC ) state in diamond is easily observed and there is a very good correlation between theoretical and experimental hyperfine parameters ((4)). Curiously, despite its predicted stability, the bond centred hydrogen state has not yet been observed in diamond. Following the discovery of hydrogen dopant states in certain wide band gap metal oxides, and the possibility of hydrogen related molecular dopants in diamond, the study of hydrogen in diamond is important. Although it is evident from its hyperfine parameters that (Mu BC ) is not a shallow donor, the question still arises as to where the (Mu BC ) state in diamond might lie in the band gap. Accordingly, measurements of the high temperature stability of (Mu BC ) have been performed in a search for its possible ionization. The results are consistent with such an ionization, as the disappearance of Mu BC polarisation (setting in near 1000 K) is correlated with the slight increase in the population of the diamagnetic μ+ species.

Photophysical Processes Related to Photoadsorption and Photocatalysis on Wide Band Gap Solids: A Review

AbstractFor Abstract see ChemInform Abstract in Full Text.

Effect of Surface Photoreactions on the Photocoloration of a Wide Band Gap Metal Oxide: Probing Whether Surface Reactions Are Photocatalytic

A nonphotocatalytic reaction occurring on the surface of an irradiated wide band gap metal oxide, such as ZrO2, can affect the process of photoinduced formation of Zr3+, F- and V-type color centers. The effect of such reactions is seen as the influence of photostimulated adsorption on the photocoloration of the metal oxide specimen. In particular, photoadsorption of electron donor molecules leads to an increase of electron color centers, whereas photoadsorption of electron acceptor molecules leads to an increase of hole color centers. Monitoring the photocoloration of a metal oxide during a surface photochemical reaction probes whether the reaction is photocatalytic: accordingly, the influence of simple photoreactions on the photocoloration of ZrO2, reactions that involved the photoreduction of molecular oxygen, the photooxidation of molecular hydrogen, the photooxidation of hydrogen by adsorbed oxygen, and the photoinduced transformation of ammonia and carbon dioxide. Kinetics of the photoprocesses are reported, as well as the photoinduced chesorluminscence (PhICL effect) of ammonia. Thermoprogrammed desorption and mass spectral monitoring of the photoreaction involving NH3 identified hydrazine as an intermediate and molecular nitrogen as the final product. The photoreactions involving NH3 and CO2 are nonphotocatalytic processes, in contrast to the photooxidation of hydrogen which is photocatalytic. Carbon dioxide and carbonate radical anions are formed by interaction of CO2 with Zr3+ centers and hole states (OS-*), respectively. Mechanistic implications are discussed.

Electrochromic devices

This paper reviews a number of recently discussed electrochromic devices, mainly for modulating optical transmittance. Data are given on all-solid-state and polymer-laminated constructions, with and without self-powering by integrated solar cells. Special attention is devoted to a novel flexible polyester-based device incorporating electrochromic tungsten oxide and nickel oxide. In particular the exposition covers some critical manufacturing aspects, including gas treatments for pre-charging of the individual electrochromic films in order to make them ready for facile device assembly as well as enhancements of the bleached-state transmittance through additions of wide band gap metal oxides to the nickel oxide.

Modeling and Experimental Examination of the Solonitsyn Memory Effect on the Surface of Wide Band Gap Metal Oxides

When the surface of a solid semiconductor or dielectric metal oxide (or other) specimen is preirradiated, the solid often retains its photochemical activity after termination of irradiation through formation of long-lived surface-active adsorption centers. This effect has two origins, viz., the so-called Kugel'sberg memory effect and the Solonitsyn memory effect. The former denotes preirradiation in the presence of the adsorbate molecules, whereas the latter refers to preirradiation in vacuo followed by subsequent introduction of adsorbate molecules into the reactor. This article reports results of detailed studies on the Solonitsyn memory effect in gas/solid heterogeneous systems with respect to photostimulated adsorption (i.e. reductive or oxidative adsorption) of molecular oxygen, molecular hydrogen, and methane on the surface of a dielectric metal oxide such as zirconia. The memory effect has been quantified for several metal oxides and alkali halides by means of an experimentally determined postadsor...

Photophysical processes related to photoadsorption and photocatalysis on wide band gap solids: A review

During the last two decades, various pathways describing photoexcitation of small molecules’ surface reactions at the wide band gap metal oxides and halides (Eg > 3 eV) have been recognized. Photogeneration of excitons and free charge carriers may occur in bands of: i) fundamental absorption; ii) extrinsic and intrinsic defect absorption, including those related to surface states; and iii) in UV‐induced color centers. Considerable red shifts relative to the fundamental absorption threshold of wide band gap solids have been observed for the spectral limits of surface photoreactions induced in extrinsic absorption bands. This allows thinking about the wide band gap solids as a potential competitors for the relatively narrow band gap photocatalysts. This review discusses the concept of surface photoadsorption (photocatalytic) center while differentiating active and inactive states of the center. Electronically excited defect, surface self‐trapped or bound exciton, and the surface defect with trapped photo carrier are considered as the active states of photoadsorption (photocatalytic) centers of different types. The decay pathway of active state determines the lifetime of a photocatalytic center, and in this connection the Langmuir‐Hinshelwood kinetic approach is discussed.