Wide Band Gap Oxide Research Articles

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.

Wire-supported CdSe nanowire array photoelectrochemical solar cells

Previous fiber-shaped solar cells are based on polymeric materials or dye-sensitized wide band-gap oxides. Here, we show that efficient fiber solar cells can be made from semiconducting nanostructures (e.g. CdSe) with smaller band-gap as the light absorption material. We directly grow a vertical array of CdSe nanowires uniformly around a core metal wire and make the device by covering the top of nanowires with a carbon nanotube (CNT) film as the porous transparent electrode. The CdSe-CNT fiber solar cells show power conversion efficiencies of 1-2% under AM 1.5 illumination after the nanowires are infiltrated with redox electrolyte. We do not use a secondary metal wire (e.g. Pt) as in conventional fiber-shaped devices, instead, the end part of the CNT film is condensed into a conductive yarn to serve as the secondary electrode. In addition, our CdSe nanowire-based photoelectrochemical fiber solar cells maintain good flexibility and stable performance upon rotation and bending to large angles.

Directing photophysics and structure in semiconductor nanowires with erbium and germanium

Recent efforts from our labs have focused on the fundamental properties of erbium incorporated into Ge nanowires (Ge NWs) and a diverse number of radial core/shell platforms containing these two elements. In this review we focus on two beneficial outcomes that can be exploited from such structures to even broader families of nanostructures: (a) the useful chemical and photophysical utility of providing wide bandgap oxide shells onto germanium nanowire cores containing rare earth ions such as erbium; (b) the unique combination of germanium and tin in an erbium doped oxide nanowire to engage in a spontaneous oxidation–reduction reaction that enhances the erbium near infrared photoluminescence (PL). A broad range of spectroscopic (PL, PL excitation) and structural (high resolution transmission electron microscopy, scanning electron microscopy, Raman spectroscopy, and energy dispersive X-ray analysis) tools are employed for this evaluation.

Origin of room temperature d0 ferromagnetism and characteristic photoluminescence in pristine SnO2 nanowires: A correlation

Arrays of SnO2 nanowires are fabricated by employing a wet chemical template assisted sol–gel route using ordered nanoporous anodic aluminium oxide as the host. The origin of room temperature d0 ferromagnetism in pristine polycrystalline SnO2 nanowires is investigated by correlating photoluminescence and electron paramagnetic resonance (EPR) studies. It has been found that the naturally grown structural defects of oxygen vacancies namely singly ionised oxygen vacancy (VO) clusters induce the characteristic photoluminescence and contribute in ferromagnetism of pristine SnO2 nanowires at room temperature. The presence of the VO structural defects in the pure SnO2 nanowires is also assured by the EPR spectroscopy. Present study will help understand the puzzle about the unexpected magnetic phenomenon in these undoped wide band gap oxide semiconductors.

Tuning magnetoresistance and exchange coupling in ZnO by doping transition metals

A systematic study focused on the magneto-transport properties of transition metal (Cu or Co) doped ZnO thin films is performed to elucidate the role of doping on tuning the band structure and exchange coupling in wide band gap oxides. Detailed theoretical fittings suggest that the negative magnetoresistance (MR) originates from the spin-dependent scattering due to the high-order sp-d exchange interaction, while the positive MR can be well described by a model invoking two spin split subbands. Our results suggest that with different dopants both the electronic band structure and the exchange coupling in ZnO can be rationally tailored.

Polaronic transport and magnetism in Ag-doped ZnO

Magnetic polarons have been proposed to play important roles in doped wide band gap oxides that exhibit weak room temperature ferromagnetism. Here, we report the experimental evidence supporting polaronic magnetism and transport in Ag-doped ZnO. Temperature dependent resistivity data suggest the conduction mechanism of Mott and Efros variable range hopping. The observed weak ferromagnetism and its temperature dependence are explained using the percolation-based model of partially ordered bound magnetic polarons.

Sources of Conductivity and Doping Limits in CdO from Hybrid Density Functional Theory

CdO has been studied for decades as a prototypical wide band gap transparent conducting oxide with excellent n-type ability. Despite this, uncertainty remains over the source of conductivity in CdO and over the lack of p-type CdO, despite its valence band maximum (VBM) being high with respect to other wide band gap oxides. In this article, we use screened hybrid DFT to study intrinsic defects and hydrogen impurities in CdO and identify for the first time the source of charge carriers in this system. We explain why the oxygen vacancy in CdO acts as a shallow donor and does not display negative-U behavior similar to all other wide band gap n-type oxides. We also demonstrate that p-type CdO is not achievable, as n-type defects dominate under all growth conditions. Lastly, we estimate theoretical doping limits and explain why CdO can be made transparent by a large Moss-Burstein shift caused by suitable n-type doping.

Defect-induced magnetism in undoped wide band gap oxides: Zinc vacancies in ZnO as an example

To shed light on the mechanism responsible for the weak ferromagnetism in undoped wide band gap oxides, we carry out a comparative study on ZnO thin films prepared using both sol-gel and molecular beam epitaxy (MBE) methods. Compared with the MBE samples, the sol-gel derived samples show much stronger room temperature ferromagnetism with a magnetic signal persisting up to ∼740 K, and this ferromagnetic order coexists with a high density of defects in the form of zinc vacancies. The donor-acceptor pairs associated with the zinc vacancies also cause a characteristic orange-red photoluminescence in the sol-gel films. Furthermore, the strong correlation between the ferromagnetism and the zinc vacancies is confirmed by our first-principles density functional theory calculations, and electronic band alteration as a result of defect engineering is proposed to play the critical role in stabilizing the long-range ferromagnetism.

Bright room-temperature green luminescence from YSZ:Tb3+

The room temperature spectroscopic properties of terbium doped zirconia fibres grown by Laser Floating Zone method are reported. The fibres were found to be stabilised in the tetragonal crystalline phase as measured by X-ray diffraction and Raman spectroscopy techniques. The doping with terbium ions was performed during the growth process and their incorporation on the wide band gap oxide host assumes an important role on the stabilisation of the high temperature tetragonal crystalline phase. Under ultraviolet optical pumping a bright green luminescence was observed by naked eye at room temperature, corresponding to the 4f→4f transitions between the crystal-field split energy levels of the 5D4 and 7F5 manifolds of the 4f8 electronic configuration of the Tb3+ ions. These results suggest that YSZ:Tb3+ could be considered as a promising candidate for applications in green electroluminescent devices.

The Role of Oxide Thin Layer in Inverted Structure Polymer Solar Cells

The role of wide band gap oxide thin layer in inverted structure polymer solar cells was investigated by employing oxide films of TiO2 and Nb2O5approximately 10 nm in thickness deposited onto FTO substrates. The experimental results demonstrated that the thin oxide layer serving to separate the electron collecting electrode and the photoactive film of a blend of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) was necessary to promote the formation of continuous uniform PCBM film to block holes in P3HT from being recombined with electrons in collecting electrode. A use of TiO2 buffer layer leads to power conversion efficiency as high as 2.8%. As for Nb2O5, in spite the fact that its conduction band is higher than the LUMO level of PCBM polymer acting as electron transport material, a power conversion of 2.7%, which was only slightly different from the 2.8% achieved for the cell employing TiO2. These experimental results suggest a tunneling mechanism for the electrons to transport from the PCBM to collecting electrode over the oxide film, instead of a diffusion through the oxide film arising from either energy or concentration difference of the photogenerated electrons.

A comparative study of semiconductor-based plasmonic metamaterials

Recent metamaterial (MM) research faces several problems when using metal-based plasmonic components as building blocks for MMs. The use of conventional metals for MMs is limited by several factors: metals such as gold and silver have high losses in the visible and near-infrared (NIR) ranges and very large negative real permittivity values, and in addition, their optical properties cannot be tuned. These issues that put severe constraints on the device applications of MMs could be overcome if semiconductors are used as plasmonic materials instead of metals. Heavily doped, wide bandgap oxide semiconductors could exhibit both a small negative real permittivity and relatively small losses in the NIR. Heavily doped oxides of zinc and indium were already reported to be good, low loss alternatives to metals in the NIR range. Here, we consider these transparent conducting oxides (TCOs) as alternative plasmonic materials for many specific applications ranging from surface-plasmon-polariton waveguides to MMs with hyperbolic dispersion and epsilon-near-zero (ENZ) materials. We show that TCOs outperform conventional metals for ENZ and other MM-applications in the NIR.

Electronic transport properties ofSrTiO3and its alloys:Sr1−xLaxTiO3andSrTi1−xMxO3(M=Nb,Ta)

Wide band-gap oxide strontium titanate $({\text{SrTiO}}_{3})$ is highly recognized as potential thermoelectric material for its large Seebeck coefficient. Recently, several experimental studies emerged on how to improve the thermoelectric transport properties of ${\text{SrTiO}}_{3}$ by doping with La and Nb. To aid this effort, a better understanding of the effect of doping on electronic structure and transport properties is essential. In this paper, the electronic transport properties of ${\text{SrTiO}}_{3}$, ${\text{Sr}}_{1\ensuremath{-}x}{\text{La}}_{x}{\text{TiO}}_{3}$, and ${\text{SrTi}}_{1\ensuremath{-}x}{M}_{x}{\text{O}}_{3}$ $(M=\text{Nb},\text{Ta})$ are determined by detailed first-principles electronic band structure and Boltzmann transport calculations. Furthermore, the effect of uniaxial and simple shear deformations on the transport properties of pure strontium titanate is also investigated. For the treatment of the substitutional alloys, on-site Coulomb interaction added versions of electronic exchange and correlation functionals are preferred to obtain an accurate electronic band structure. Calculated Seebeck coefficient and its variation as a function of temperature for the alloy systems are compared with available experimental results; an excellent agreement is observed. Additionally, the electronic relaxation time and electronic contribution to thermal conductivity are predicted by fitting the calculated electrical conductivity to the experimental electrical conductivity. A significant increase in electronic thermal conductivity is observed for high carrier concentrations. Overall, the necessity of excessive carrier concentrations that can only be achieved by highly doped structures is verified.

High-sensitivity photovoltage based on the interfacial photoelectric effect in the SrTiO3−δ/Si heterojunction

A high sensitivity photovoltaic effect has been observed in a heterojunction composed of n-type wide bandgap oxide SrTiO3−δ and p-type Si fabricated by laser molecular beam epitaxy. The responsivity of open-circuit photovoltage can reach 104 V/W without any amplification under zero bias for the wavelength range from visible to near infrared light in nW-μW order. We attribute the high performance of the photovoltage responsivity to the interfacial photoelectric effects in the SrTiO3−δ/Si heterojunction. From the experimental results, some ideas can be generalized to improve photovoltaic efficiency and develop high sensitivity photodetectors with wide bandgap oxide materials and Si.

RIXS approach to local environment around impurity atoms in diluted magnetic semiconductors and dielectrics

The use of resonant inelastic X-ray scattering (RIXS) to understand local environment around magnetic atoms in various diluted magnetic semiconductors (DMS) and dielectrics (DMD) has been overviewed. The overall spectroscopic results of transition-metal-doped GaAs DMS and wide band-gap oxide DMD systems show that the magnetic transition-metal dopants can occupy not only cation sites but also interstitial ones and the interactions between substituted and interstitial magnetic ions play a very important role on the nature of magnetic properties of these materials. This suggests that a careful verification of presence/absence of structural defects is required, especially for when the size of defect configuration is very small (less than few nm).

Effect of substrate temperature on the photoelectrochemical responses of Ga and N co-doped ZnO films

Ga–N co-doped ZnO thin films with reduced bandgaps were deposited on F-doped tin-oxide-coated glass by radio-frequency magnetron sputtering at different substrate temperatures in mixed N2 and O2 gas ambient. We found that Ga–N co-doped ZnO films exhibited enhanced crystallinity when compared to undoped ZnO films grown under the same conditions. Furthermore, Ga–N co-doping ensured enhanced N-incorporation ZnO thin films as the substrate temperature is increased. As a result, Ga–N co-doped ZnO thin films exhibited much improved photoelectrochemical (PEC) response, compared to ZnO thin films. Our results therefore suggest that the passive co-doping approach could be a means to improve PEC response for bandgap-reduced wide-bandgap oxides through impurity incorporation.

Color tunability of the upconversion emission in Er–Yb doped the wide band gap nanophosphors ZrO2 and Y2O3

Recently an increasing interest in the investigation of nanocrystalline wide band gap oxides for both fundamental research and potential applications in upconversion phosphors, fluorescent labels for sensitive detection of biomolecules, white light generation, and two-photon confocal-microscope imaging. Er 3+ /Yb 3+ doped Y 2 O 3 and ZrO 2 present favorable physical properties, such as a wide band gap between 5.6 and 7.0 eV, and ease of synthesis in the nanometer regime. In particular, the intensity of the visible rare earth luminescence depends on dopant concentration, crystalline phase and size of the nanocrystals. As a result, it is possible to control the emission color between red and green by adjusting the Er and Yb dopant concentrations. In this work, based on a stationary solution of the rate equations that govern the luminescence emission, we derive a simple model that allows the prediction of the overall color emission depending on the Yb and Er concentration. In addition, our model allows the facile quantification of the upconversion mechanisms responsible of the visible color emission in these nanophosphors under 970 nm excitation.

Tuning the properties of complex transparent conducting oxides: Role of crystal symmetry, chemical composition, and carrier generation

The electronic properties of single- and multi-cation transparent conducting oxides (TCOs) are investigated using first-principles density functional approach. A detailed comparison of the electronic band structure of stoichiometric and oxygen deficient In$_2$O$_3$, $\alpha$- and $\beta$-Ga$_2$O$_3$, rock salt and wurtzite ZnO, and layered InGaZnO$_4$ reveals the role of the following factors which govern the transport and optical properties of these TCO materials: (i) the crystal symmetry of the oxides, including both the oxygen coordination and the long-range structural anisotropy; (ii) the electronic configuration of the cation(s), specifically, the type of orbital(s) -- $s$, $p$ or $d$ -- which form the conduction band; and (iii) the strength of the hybridization between the cation's states and the p-states of the neighboring oxygen atoms. The results not only explain the experimentally observed trends in the electrical conductivity in the single-cation TCO, but also demonstrate that multicomponent oxides may offer a way to overcome the electron localization bottleneck which limits the charge transport in wide-bandgap main-group metal oxides. Further, the advantages of aliovalent substitutional doping -- an alternative route to generate carriers in a TCO host -- are outlined based on the electronic band structure calculations of Sn, Ga, Ti and Zr-doped InGaZnO$_4$. We show that the transition metal dopants offer a possibility to improve conductivity without compromising the optical transmittance.

Location and catalytic role of iron species in TiO2:Fe photocatalysts: An EPR study

Abstract A usual approach to improve the photocatalytic activity of wide band gap oxide semiconductors is through doping with transition metals. Dopants are primarily selected according to their capacity to introduce energy levels in the oxide band gap, promoting light absorbance. In the present work, we carefully characterize iron-doped titania powders obtained by a sol–gel route. Iron cations are introduced in the initial solution, before gelification, what promotes their lattice localization. X-ray diffraction and electron spin resonance spectroscopy are used to determine the temperature evolution of the crystalline structure and the dopant local environment. These results allow us to correlate the structural, electronic and chemical properties of the iron-doped titania powders with their activity towards the photocatalytic degradation of methylene blue (MB). The obtained results showed that Fe 3+ cations far from improve the photocatalytic properties of bare TiO 2 , decreased the activity towards methylene blue degradation. We associated such decrease with the role played by the iron cations depending on their position inside the lattice of TiO 2 . And second, iron cations located near or at the surface tend to form iron-based structures, such as iron titanate or pseudobrookite which are highly inactive as photocatalyst.

Synthesis and characterization of band gap-reduced ZnO:N and ZnO:(Al,N) films for photoelectrochemical water splitting

ZnO thin films with significantly reduced band gaps were synthesized by doping N and codoping Al and N at 100 °C. All the films were synthesized by radiofrequency magnetron sputtering on F-doped tin-oxide-coated glass. We found that codoped ZnO:(Al,N) thin films exhibited significantly enhanced crystallinity compared with ZnO doped solely with N, ZnO:N, at the same growth conditions. Furthermore, annealed ZnO:(Al,N) thin films exhibited enhanced N incorporation over ZnO:N films. As a result, ZnO:(Al,N) films exhibited better photocurrents than ZnO:N films grown with pure N doping, suggesting that charge-compensated donor–acceptor codoping could be a potential method for band gap reduction of wide-band gap oxide materials to improve their photoelectrochemical performance.

Electron and molecular processes on the surface of wide-bandgap oxides induced by photoexcitation of point defects

Abstract The step-by-step experimental investigations of mechanisms of interrelated processes in electronic and atomic subsystems on UV-VIS illuminated surface of wide-bandgap oxides are presented. A variety of complementary experimental methods such as mass-spectrometry, optical and ESR spectroscopies and UV-photoelectron spectroscopy have been adapted to carry out in situ investigations in three phases: gas-adsorbate-surface. The spectral selective and site sensitive excitation of intrinsic anion and cation vacancies in the sub-bandgap range of photon energy is discussed. The kinetic parameters of three channels of surface relaxation: radiative, non-radiative and chemi-relaxation are measured. The structure of photoadsorbed species is determined. The potentialities to regulate the number and composition of surface defects and their use for “self-sensitization” of real photocatalysts are demonstrated.