Control of doping by impurity Cchemical potentials: predictions for p-type ZnO

Defect physics investigations in bulk NaBiO3 photocatalysts via Heyd-Scuseria-Ernzerhof hybrid density functional theory calculations

This study employs Heyd-Scuseria-Ernzerhof hybrid density functional theory calculations to thoroughly investigate the n-type and p-type conductivity mechanisms of NaBiO3 photocatalysts. The results reveal that the intrinsic interstitial defect Na1+i is dominant under most growth conditions because of its lower formation energy. It is an excellent donor because of its shallower charge transition level. This makes it easily reach and even exceed the significant concentration of 1021 cm-3 with Na chemical potential regulation. Thus, in most circumstances, the intrinsic n-type conductivity of NaBiO3 found in experiments should primarily originate from the contribution of the interstitial defect Na1+i. The anti-site defect Bi2+Na also contributes to the unintentional n-type conductivity behavior. Especially under Na-poor and Bi-rich growth conditions, Bi2+Na becomes the dominant defect and is most responsible for the intrinsic n-type conductivity. The two major intrinsic defects, including Na1+i and Bi2+Na defects, can act as the photocatalytic reaction active sites or as a hole capture center (Bi2+Na) rather than as the recombination centers of the photo-generated electrons and holes in NaBiO3. On the other hand, based on thermodynamic simulation, the study examines the impacts of n-type and p-type doping at a fixed donor D+ or acceptor A- concentration on the conductive properties of NaBiO3 under different chemical potential conditions. It is indicated that p-type doping can convert the intrinsic n-type NaBiO3 into a p-type semiconductor only under non-thermal equilibrium growth conditions (quenching method). In contrast, n-type doping can easily enhance its n-type carrier concentration. Our results can guide optimizing the growth conditions to achieve high donor doping and high photocatalytic performance in NaBiO3 or NaBiO3-based materials.

Solving the Asymmetric Doping for Wide-Gap Semiconductors by Host Functionalization: Quantum Engineering Strategy

Asymmetric doping of wide-gap semiconductors has long been a major challenge, hindering their wider applications. Despite numerous attempts to address this issue through engineering doping levels, the results were still inconclusive. In this work, we propose a quantum engineering strategy based on the state-of-the-art spin-polarized HSE06 hybrid functional method. The local band offset between the host and quantum structures can considerably compensate for the large carrier activation energy (Ea). We chose the system of the AlN host embedded by GaN quantum dots as an example to validate the feasibility of this strategy. The Ea of Si (n-type) and Be (p-type) dopants can be reduced from 222 and 404 meV to negative values and 2 meV, respectively. Therefore, electron and hole density can be increased to more than 1019 and 1020 cm-3, respectively. We also tested potential dopants (C and Ge for the n-type, Mg and Ca for the p-type), and the technique is equally effective. This mechanism can also be used to understand the experimental observations of the superlattice doping strategy. Overall, our study demonstrates that the quantum engineering strategy provides a potential solution to overcome the asymmetric doping problem for universal wide-gap semiconductors and supports a feasible pathway for more efficient devices in the future.

Intrinsic point defects and the n- and p-type dopability in α- and β-Bi2O3 photocatalysts

In this work, all kinds of intrinsic point defects, unintentional N and H impurities and possible complex defects between impurities and native defects in α- and β-Bi₂O₃ with different growth conditions are systematically investigated using hybrid density functional calculations. And then, the n- or p-type doping mechanisms in α- and β-Bi₂O₃ are explored and discussed. It is found that α-Bi₂O₃ presents the n-type conductivity under O-poor conditions. The unintentional H interstitials as the shallow donors should be majorly responsible for the n-type conductivity character. While under O-rich conditions, α-Bi₂O₃ displays the p-type conductivity, and the unintentional complex defects V_Bi1 + 2H as the shallow acceptors should be the primary origins of the p-type conductivity. The hydrogenation of the Bi vacancy in α-Bi₂O₃ not only significantly lowers the formation energy of the Bi vacancy but also markedly decreases its acceptor transition level. This well explains the experimental observation that α-Bi₂O₃ changes from n-type to p-type conductivity with increasing O partial pressure. Compared to α-Bi₂O₃, β-Bi₂O₃ always presents the n-type conductivity behaviour regardless of the growth conditions. The native O1 vacancies (V_O1) and unintentional H interstitials in β-Bi₂O₃ are shallow and excellent donors. They are responsible for the n-type conductivity and further perfectly explain the observed unintentional n-type conductivity character in β-Bi₂O₃ experiments. Understanding the defect physics in α- and β-Bi₂O₃ could inspire more significant studies on developing Bi₂O₃-based photocatalysts.

Facile solvothermal synthesis of nitrogen-doped SnO2 nanorods towards enhanced photocatalysis

Heteroatom doping has proved to be one of the most effective approaches to further improve the photocatalytic activities of semiconducting oxides originating from the modulation of their electronic structures. Herein, nitrogen-doped SnO₂ nanorods were synthesized via facile solvothermal processes using polyvinylpyrrolidone (PVP) as a dispersing agent and ammonium water as the N source, respectively. Compared with pure SnO₂ sample, the as-synthesized nitrogen-doped SnO₂ nanorods demonstrated enhanced photocatalytic performances, evaluated by the degradation of rhodamine B (RhB), revealing the effectiveness of nitrogen doping towards photocatalysis. In particular, the optimal photocatalyst (using 0.6 g PVP and 1 mL ammonia water) could achieve up to 86.23% pollutant removal efficiency under ultraviolet (UV) light irradiation within 150 min, showing 17.78% higher efficiency than pure SnO₂. Detailed structural and spectroscopic characterization reveals the origin of activity enhancement of nitrogen-doping SnO₂ in contrast with pure SnO₂. Specifically, the bandgap and the morphologies of nitrogen-doped SnO₂ have changed with more chemisorbed sites, which is supposed to result in the enhancement of photocatalytic efficiency. Moreover, the possible formation mechanism of nitrogen-doped SnO₂ nanorods was discussed, in which PVP played a crucial role as the structure orientator.

Nitrogen-Doped Zinc Oxide for Photo-Driven Molecular Hydrogen Production

Due to its thermal stability, conductivity, high exciton binding energy and high electron mobility, zinc oxide is one of the most studied semiconductors in the field of photocatalysis. However, the wide bandgap requires the use of UV photons to harness its potential. A convenient way to appease such a limitation is the doping of the lattice with foreign atoms which, in turn, introduce localized states (defects) within the bandgap. Such localized states make the material optically active in the visible range and reduce the energy required to initiate photo-driven charge separation events. In this work, we employed a green synthetic procedure to achieve a high level of doping and have demonstrated how the thermal treatment during synthesis is crucial to select specific the microscopic (molecular) nature of the defect and, ultimately, the type of chemistry (reduction versus oxidation) that the material is able to perform. We found that low-temperature treatments produce material with higher efficiency in the water photosplitting reaction. This constitutes a further step in the establishment of N-doped ZnO as a photocatalyst for artificial photosynthesis.

Nitrogen-doped semiconducting oxides. Implications on photochemical, photocatalytic and electronic properties derived from EPR spectroscopy

Engineering defects in semiconducting metal oxides is a challenge that remains at the forefront of materials chemistry research. Nitrogen has emerged as one of the most attractive elements able to tune the photochemical and photocatalytic properties of semiconducting oxides, boosting visible-light harvesting and charge separation events, key elements in promoting solar driven chemical reactions. Doping with nitrogen is also a strategy suggested to obtain p-type conduction properties in oxides showing n-type features in their pristine state and to impart collective magnetic properties to the same systems. Here, we review the evolution in the understanding of the role of nitrogen doping in modifying the photochemical and electronic properties of the most common semiconducting oxides used in mentioned applications including: TiO2, ZnO, SnO2 and zirconium titanates. With an emphasis on polycrystalline materials, we highlight the unique role of Electron Paramagnetic Resonance (EPR) spectroscopy in the direct detection of open-shell N-based defects and in the definition of their structural and electronic properties. Synthetic strategies for the insertion of nitrogen defects in the various matrices are also discussed, along with the influence of the corresponding low-lying energy states on the general electronic properties of the doped solids.

N-Promoted Ru1/TiO2 single-atom catalysts for photocatalytic water splitting for hydrogen production: a density functional theory study

In our Ru₁–N₁/TiO₂ single-atom catalyst system, isolated Ru₁ atoms act as active sites for the reduction of protons, and the TiO₂ support offers the photogenerated carriers, allowing for a hydrogen evolution activity comparable to that of Pd.

Design Principles of p-Type Transparent Conductive Materials

Transparent conductive materials (TCMs) has always been playing a significant role in electronic and photovoltaic area, due to its prominent optical and electronic properties. To render those transparent materials highly conductive, efficient n- and p- type doping is critically needed to obtain high concentration of free electron and hole carriers. Despite extensive research over the past five decades, high-quality p-type doping of wide-band-gap transparent materials remains a challenge. Here, we summarize four proposed design principles to enhance the p-type conductivity of these wide band gap materials, including (i) reducing the formation energy of the acceptors to enhance the dopant concentration; (ii) lowering the ionization energy and, hence, increasing the ionization of the acceptors to increase the concentration of the free holes; (iii) increasing the VBM of the host material to approaching the pinned Fermi level; and (iv) suppressing the compensating donors to shifting the pinning Fermi level toward the VBM. For each mechanism, we discuss in detail its underlying physics and provided some examples to illustrate the design principles. From this review, one could learn the doping principles and have a strategic mind when designing other p-type materials.

Passivation Mechanism of Nitrogen in ZnO under Different Oxygen Ambience

Nitrogen-doped ZnO thin films were grown on a-plane Al2O3 by plasma-assisted molecular beam epitaxy. Hall-effect measurements indicated that the nitrogen-doped ZnO films showed p-type behavior first, then n-type, with the growth conditions changing from oxygen-radical-rich to oxygen-radical-deficient ambience, accompanied with the increase of the N/O ratio in the plasmas. The increasing green emission in the low temperature photoluminescence spectra, related to single ionized oxygen vacancy in ZnO, was ascribed to the decrease of active oxygen atoms in the precursor plasmas. CN complex, a donor defect with low formation energy, was demonstrated to be easily introduced into ZnO under O-radical-deficient ambience, which compensated the nitrogen-related acceptor, along with the oxygen vacancy.

Effective chemical potential for non-equilibrium systems and its application to molecular beam epitaxy of Bi2Se3

First-principles studies often rely on the assumption of equilibrium, which can be a poor approximation, e.g., for growth. Here, an effective chemical potential ([italic small mu, Greek, macron]) method for non-equilibrium systems is developed. A salient feature of the theory is that it maintains the equilibrium limits as the correct limit. In application to molecular beam epitaxy, rate equations are solved for the concentrations of small clusters, which serve as feedstock for growth. We find that [italic small mu, Greek, macron] is determined by the most probable, rather than by the lowest-energy, cluster. In the case of Bi₂Se₃, [italic small mu, Greek, macron] is found to be highly supersaturated, leading to a high nucleus concentration in agreement with experiment.

Chemical doping of the SnSe monolayer: a first-principle calculation

First-principles calculations were used to investigate the effect of doping on the electronic, magnetic and optical properties of the SnSe monolayer.

Organometallic Precursor-Derived SnO2/Sn-Reduced Graphene Oxide Sandwiched Nanocomposite Anode with Superior Lithium Storage Capacity

Benefiting from the reversible conversion reaction upon delithiation, nanosized SnO₂, with its theoretical capacity of 1494 mA h g^-1, has gained special attention as a promising anode material. Here, we report a self-assembled SnO₂/Sn-reduced graphene oxide (rGO) sandwich nanocomposite developed by organometallic precursor coating and in situ transformation. Ultrafine SnO₂ nanoparticles with an average diameter of 5 nm are sandwiched within the rGO/carbonaceous network, which not only greatly alleviates the volume changes upon lithiation and aggregation of SnO₂ nanoparticles but also facilitates the charge transfer and reaction kinetics of SnO₂ upon lithiation/delithiation. As a result, the SnO₂/Sn-rGO nanocomposite exhibited a superior lithium storage capacity with a reversible capacity of 1307 mA h g^-1 at a current density of 80 mA g^-1 in the potential window of 0.01-2.5 V versus Li⁺/Li and showed a reversible capacity of 767 mA h g^-1 over 200 cycles at a current density of 400 mA g^-1. When cycling at a higher current density of 1600 mA g^-1, the SnO₂/Sn-rGO nanocomposite showed a highly stable capacity of 449 mA g^-1 without obvious decay after 400 cycles.

Enhanced energy transfer in heterogeneous nanocrystals for near infrared upconversion photocurrent generation

The key to produce inorganic heterogeneous nanostructures, and to integrate multiple functionalities, is to enhance or at least retain the functionalities of different components of materials. However, this ideal scenario is often deteriorated at the interface of the heterogeneous nanostructures due to lattice mismatches, resulting in downgraded performance in most hybrid nanomaterials. Here, we report that there is a narrow window in controlling temperature in a Lewis acid-base reaction process to facilitate epitaxial alignment during the synthesis of hybrid nanomaterials. We demonstrate a perfectly fused NaYF₄:Yb,Tm@ZnO heterogeneous nanostructure, in which the semiconductor ZnO shell can be epitaxially grown onto lanthanide-doped upconversion nanoparticles. By achieving a matched crystal lattice, the interface defects and crystalline grain boundaries are minimized to enable more efficient energy transfer from the upconversion nanoparticles to the semiconductor, resulting in both enhanced upconversion luminescence intensity and superior photoelectrochemical properties. This strategy provides an outstanding approach to endow lanthanide-doped upconversion nanoparticles with versatile properties.

Excitation Dependent Phosphorous Property and New Model of the Structured Green Luminescence in ZnO

The copper induced green luminescence (GL) with two sets of fine structures in ZnO crystal has been found for several decades (i.e., R. Dingle, Phys. Rev. Lett. 23, 579 (1969)), but the physical origin of the doublet still remains as an open question up to now. In this paper, we provide new insight into the mechanism of the structured GL band in terms of new experimental findings and theoretical calculations. It is found, for the first time, that the GL signal exhibits persistent afterglow for tens of minutes after the switch-off of below-band-gap excitation light but it cannot occur under above-band-gap excitation. Such a phosphorous property may be interpreted as de-trapping and feeding of electrons from a shallow trapping level via the conduction band to the Cu-related luminescence centers where the Cu³⁺ ion is proposed to work as the final state of the GL emission. From first-principles calculation, such a Cu³⁺ ion in wurtzite ZnO prefers a high spin 3d⁸ state with two non-degenerated half-filled orbitals due to the Jahn-Teller effect, probably leading to the double structures in photoluminescence spectrum. Therefore, this model gives a comprehensively new understanding on the mechanism of the structured GL band in ZnO.

Spin-dependent electron transport in C and Ge doped BN monolayers

The aliovalent doping in h-BN monolayers leads to unique features in the electron transport characteristics including significant enhancement of current at the dopant site, diode-like asymmetric current–voltage response, and spin-dependent current.

Photocatalysis of C, N-doped ZnO derived from ZIF-8 for dye degradation and water oxidation

The C, N-doped ZnO derived from ZIF-8 via two-step pyrolysis showed excellent performances in photocatalytic dye degradation and oxygen evolution.

Morphological evolution of 2D Rh nanoplates to 3D Rh concave nanotents, hierarchically stacked nanoframes, and hierarchical dendrites

Impurity doping has yielded a number of useful optical and catalytic alloy nanoparticles, by providing synthetic routes to unprecedented nanostructures. However, Zn is difficult to use as a dopant in alloy nanoparticles due to the difficulty in reduction, and therefore little has been reported on Zn-doped alloy nanoparticles and their potential applications. Herein we report an unusual role of the dopant Zn as a crystal growth modifying agent to cause the formation of novel concave Rh nanostructures, namely nanotents. We could further prepare unprecedented hierarchically stacked Rh nanoframes and dendritic nanostructures derived from them by understanding the role of various surface-stabilizing moieties. We also report the usage of new Rh nanostructures in selective hydrogenation of phthalimides.

Nature of charge transport and p-electron ferromagnetism in nitrogen-doped ZrO2: an ab initio perspective

Zirconium dioxide provides an exceptional prototype material for studying the redistribution of the polaron holes and its magnetic coupling with their nearby anions owning to the difference oxygen binding behavior in the monoclinic phase. Here, we perform a comprehensive study of the p-electron magnetism in the nitrogen doped 2 × 2 × 2 monoclinic ZrO₂ based on spin-polarized density functional theory. Nitrogen substitutions make the system display half-metallic properties, and the origin of room temperature ferromagnetism ascribes to the p-p coupling interaction between N 2p and the host 2p states. The charge density difference and Mülliken population analyses provide evidences of charge redistributions. Our results reveal that the polaron transfer may alter the magnetic properties and it is greatly facilitated ferromagnetic coupling if the polaron holes are localized around a single anion dopant.

Ab initio study on the stability of N-doped ZnO under high pressure

We perform first-principles density functional theory calculations to examine the stability of nitrogen-doped wurtzite ZnO under pressure.

Oxygen vacancy diffusion in bare ZnO nanowires

Oxygen vacancies (VO) are known to be common native defects in zinc oxide (ZnO) and to play important roles in many applications. Based on density functional theory, we present a study for the migration of oxygen vacancies in ultra-thin ZnO nanowires (NWs). We find that under equilibrium growth conditions VO has a higher formation energy (Ef) inside the wire than that at shallow sites and surface sites, with different geometric relaxations and structural reconstructions. The migration of VO has lower barriers in the NW than in the bulk and is found to be energetically favorable in the direction from the bulk to the surface. These results imply a higher concentration of VO at surface sites and also a relative ease of diffusion in the NW structure. Our results support the previous experimental observations and are important for the development of ZnO-based devices in photocatalysis and optoelectronics.

P-type nitrogen-doped ZnO nanostructures with controlled shape and doping level by facile microwave synthesis

We report herein the development of a facile microwave irradiation (MWI) method for the synthesis of high-quality N-doped ZnO nanostructures with controlled morphology and doping level. We present two different approaches for the MWI-assisted synthesis of N-doped ZnO nanostructures. In the first approach, N-doping of Zn-poor ZnO prepared using zinc peroxide (ZnO2) as a precursor is carried out under MWI in the presence of urea as a nitrogen source and oleylamine (OAm) as a capping agent for the shape control of the resulting N-doped ZnO nanostructures. Our approach utilizes the MWI process for the decomposition of ZnO2, where the rapid transfer of energy directly to ZnO2 can cause an instantaneous internal temperature rise and, thus, the activation energy for the ZnO2 decomposition is essentially decreased as compared to the decomposition under conductive heating. In the second synthesis method, a one-step synthesis of N-doped ZnO nanostructures is achieved by the rapid decomposition of zinc acetate in a mixture of urea and OAm under MWI. We demonstrate, for the first time, that MWI decomposition of zinc acetate in a mixture of OAm and urea results in the formation of N-doped nanostructures with controlled shape and N-doping level. We report a direct correlation between the intensity of the Raman scattering bands in N-doped ZnO and the concentration of urea used in the synthesis. Electrochemical measurements demonstrate the successful synthesis of stable p-type N-doped ZnO nanostructures using the one-step MWI synthesis and, therefore, allow us to investigate, for the first time, the relationship between the doping level and morphology of the ZnO nanostructures. The results provide strong evidence for the control of the electrical behavior and the nanostructured shapes of ZnO nanoparticles using the facile MWI synthesis method developed in this work.

High Mg effective incorporation in Al-rich AlxGa1 - xN by periodic repetition of ultimate V/III ratio conditions

According to first-principles calculations, the solubility of Mg as a substitute for Ga or Al in AlxGa1 - xN bulk is limited by large, positive formation enthalpies. In contrast to the bulk case, the formation enthalpies become negative on AlxGa1 - xN surface. In addition, the N-rich growth atmosphere can also be favorable to Mg incorporation on the surface by changing the chemical potentials. On the basis of these special features, we proposed a modified surface engineering technique that applies periodical interruptions under an ultimate V/III ratio condition (extremely N-rich), to enhance Mg effective incorporation. By optimizing the interruption conditions (2 nm interruption interval with 2 s interruption time), the enhancement ratio can be up to about 5 in the Al0.99Ga0.01N epilayer.

The delocalized nature of holes in (Ga, N) cluster-doped ZnO

A spin-polarized density-functional theory study is presented here, revealing that a single hole state created by (Ga, N) cluster doping in ZnO contains the contributions from all of the N atoms in the cluster. This is in contrast to the situation where N atoms alone are doped into ZnO, and have a highly localized hole state centered around the dopant N atoms. Hence, this study shows that an enhanced delocalized hole state can be obtained if an appropriate electronic environment is provided.

p-Type conductivity in N-doped ZnO: the role of the N(Zn)-V(O) complex

Although nitrogen-doped zinc oxide has been fabricated as a light-emitting diode, the origin of its p-type conductivity remains mysterious. Here, by analyzing the surface reaction pathway of N in ZnO with first-principles density functional theory calculations, we demonstrate that the origin of p-type conductivity of N-doped ZnO can originate from the defect complexes of N(Zn)-V(O) and N(O)-V(Zn). Favored by the Zn-polar growth, the shallow acceptor of N(O)-V(Zn) actually evolves from the double-donor state of N(Zn)-V(O). While N(Zn)-V(O) is metastable, the p-doping mechanism of N(Zn)-V(O)→N(O)-V(Zn) in ZnO will be free from the spontaneous compensation from the intrinsic donors. The results may offer clearer strategies for doping ZnO p-type more efficiently with N.

Phase stabilization in nitrogen-implanted nanocrystalline cubic zirconia

The phase stability of nanocrystallites with metastable crystal structures under ambient conditions is usually attributed to their small grain size. It remains a challenging problem to maintain such phase integrity of these nanomaterials when their crystallite sizes become larger. Here we report an experimental-modelling approach to study the roles of nitrogen dopants in the formation and stabilization of cubic ZrO(2) nanocrystalline films. Mixed nitrogen and argon ion beam assisted deposition (IBAD) was applied to produce nitrogen-implanted cubic ZrO(2) nanocrystallites with grain sizes of 8-13 nm. Upon thermal annealing, the atomic structure of these ZrO(2) films was observed to evolve from a cubic phase, to a tetragonal phase and then a monoclinic phase. Our X-ray absorption near edge structure study on the annealed samples together with first-principle modelling revealed the significance of the interstitial nitrogen in the phase stabilization of nitrogen implanted cubic ZrO(2) crystallites via the soft mode hardening mechanism.

Assorted analytical and spectroscopic techniques for the optimization of the defect-related properties in size-controlled ZnO nanowires

In this article, the important role of the intrinsic defects in size-controlled ZnO nanowires (NWs) which play a critical role in the properties of the NWs, was studied with a combined innovative experimental analysis. The NWs prepared by both the aqueous solution method and chemical vapour deposition process were of increasing length and decreasing size-to-volume (S/V) ratio. The combined approach involved different analytical and spectroscopic techniques and from the correlation between the different measurements, the concentration of the oxygen vacancies jointly with the zinc interstitials defects and the zinc vacancy defects was observed to be positively or negatively correlated, respectively, with the magnitude of the photoluminescence intensity and radiative lifetimes. Furthermore, the experimental results also suggest that the oxygen vacancy defects are not only spatially located on the surface of the NW but an increasing fraction of the total oxygen vacancy defects connected with the green emission is also located in an annulus region beneath the surface as the ZnO NWs elongate. On the other hand, as the donor concentration plays a critical function in the properties of the ZnO NWs, an analytical model was derived for the calculation of the donor concentration of the NWs directly from its reverse-biased current-voltage characteristics obtained from the conductive atomic force microscopy measurements.

Compensation mechanism in N-doped ZnO nanowires

N-doped ZnO nanowires are synthesized at a relatively low growth temperature of 500 degrees C by directly heating zinc powder using NH(3) as the dopant. The incorporation of N into the ZnO nanowires is experimentally confirmed by x-ray photoelectron spectroscopy, Raman spectra and photoluminescence measurements. By combining post annealing experiments after growth with first-principles calculations, the detailed migration mechanism of N and compensation mechanism in N-doped ZnO nanowires are systematically studied. The larger aspect ratio of nanowires favors the formation of oxygen vacancy and out-diffusion of substitutional N (N(O)), making N(O) in ZnO nanowires always compensated by hydrogen interstitials (H(I)). Our results can help to explain the challenge in getting p-type ZnO and shed new light on the possible realization of p-type doping of ZnO in the future.

Nitrogen-doped ZnO nanowire arrays for photoelectrochemical water splitting

We report the rational synthesis of nitrogen-doped zinc oxide (ZnO:N) nanowire arrays, and their implementation as photoanodes in photoelectrochemical (PEC) cells for hydrogen generation from water splitting. Dense and vertically aligned ZnO nanowires were first prepared from a hydrothermal method, followed by annealing in ammonia to incorporate N as a dopant. Nanowires with a controlled N concentration (atomic ratio of N to Zn) up to approximately 4% were prepared by varying the annealing time. X-ray photoelectron spectroscopy studies confirm N substitution at O sites in ZnO nanowires up to approximately 4%. Incident-photon-to-current-efficiency measurements carried out on PEC cell with ZnO:N nanowire arrays as photoanodes demonstrate a significant increase of photoresponse in the visible region compared to undoped ZnO nanowires prepared at similar conditions. Mott-Schottky measurements on a representative 3.7% ZnO:N sample give a flat-band potential of -0.58 V, a carrier density of approximately 4.6 x 10(18) cm(-3), and a space-charge layer of approximately 22 nm. Upon illumination at a power density of 100 mW/cm(2) (AM 1.5), water splitting is observed in both ZnO and ZnO:N nanowires. In comparison to ZnO nanowires without N-doping, ZnO:N nanowires show an order of magnitude increase in photocurrent density with photo-to-hydrogen conversion efficiency of 0.15% at an applied potential of +0.5 V (versus Ag/AgCl). These results suggest substantial potential of metal oxide nanowire arrays with controlled doping in PEC water splitting applications.

Observation of unintentionally incorporated nitrogen-related complexes in ZnO and GaN nanowires

We report the observation of unintentionally incorporated nitrogen-related complexes in ZnO and GaN nanowires grown by the catalytic vapor-phase transport method. In particular, our experimental findings from Raman scattering spectroscopy and mass-selected time-of-flight particle emission measurements suggest the presence of interstitial nitrogen molecules that are formed during the nanowire growth. These results may be relevant for many nanowire systems, emphasizing the necessity of more studies on unintentional impurity incorporations in these nanomaterials.

The fabrication of a ZnO nanowire/La0.65Sr0.35MnO3 heterojunction and characterization of its rectifying behavior

We have fabricated a ZnO nanowire (NW)/La(0.65)Sr(0.35)MnO(3) (LSMO) p-n heterojunction by growing the NWs by an easy aqueous chemical route on a pressed powdered pellet of LSMO. The NWs have hexagonal wurtzite structure with good optical emission properties. The current-voltage (I-V) curves of a single NW junction measured by a scanning tunneling microscope (STM) probe show excellent rectifying behavior with rectification ratio approximately 40, which is comparable to the characteristics of the junction made by large area NW array junctions. Different voltage-dependent current transport mechanisms have been found, which are explained through the use of a band diagram.

Possible approach to overcome the doping asymmetry in wideband gap semiconductors

The asymmetry doping problem has severely hindered the potential applications of many wideband gap (WBG) materials. Here, we propose a possible approach to overcome this long-standing doping asymmetry problem for WBG semiconductors. Our approach is based on the reduction of the ionization energies of dopants through introduction and effective doping of mutually passivated impurity bands, which can be realized by doping the host with passive donor-acceptor complexes or isovalent impurities. Our density-functional theory calculations demonstrate that this approach provides excellent explanations for the n-type doping of diamond and p-type doping of ZnO, which could not be understood by previous theories. In principle, this approach can be applied to any WBG semiconductors and therefore will open a broad vista for the application of WBG materials.

Resonant coupling of bound excitons with LO phonons in ZnO: Excitonic polaron states and Fano interference

We report on a photoluminescence observation of robust excitonic polarons due to resonant coupling of exciton and longitudinal optical (LO) phonon as well as Fano-type interference in high quality ZnO crystal. At low enough temperatures, resonant coupling of excitons and LO phonons leads to not only traditional Stokes lines (SLs) but also up to second-order anti-Stokes lines (ASLs) besides the zero-phonon line (ZPL). The SLs and ASLs are found to be not mirror symmetric with respect to the ZPL, strongly suggesting that they are from different coupling states of exciton and phonons. Besides these spectral features showing the quasiparticle properties of exciton-phonon coupling system, the first-order SL is found to exhibit characteristic Fano lineshape, caused by quantum interference between the LO components of excitonic polarons and the continuous phonon bath. These findings lead to a new insight into fundamental effects of exciton-phonon interactions.