Al-doped ZnO (AZO) thin films have been prepared by mist chemical vapor deposition and magnetron sputtering. The band gap shift as a function of carrier concentration in n-type zinc oxide (ZnO) was systematically studied considering the available theoretical models. The shift in energy gap, evaluated from optical absorption spectra, did not depend on sample preparations; it was mainly related to the carrier concentrations and so intrinsic to AZO. The optical gap increased with the electron concentration approximately as ne2∕3 for ne≤4.2×1019 cm−3, which could be fully interpreted by a modified Burstein–Moss (BM) shift with the nonparabolicity of the conduction band. A sudden decrease in energy gap occurred at 5.4−8.4×1019 cm−3, consistent with the Mott criterion for a semiconductor-metal transition. Above the critical values, the band gap increased again at a different rate, which was presumably due to the competing BM band-filling and band gap renormalization effects, the former inducing a band gap widening and the latter an offsetting narrowing. The band gap narrowing (ΔEBGN) derived from the band gap renormalization effect did not show a good ne1∕3 dependence predicated by a weakly interacting electron-gas model, but it was in excellent agreement with a perturbation theory considering different many-body effects. Based on this theory a simple expression, ΔEBGN=Ane1∕3+Bne1∕4+Cne1∕2, was deduced for n-type ZnO, as well as p-type ZnO, with detailed values of A, B, and C coefficients. An empirical relation once proposed for heavily doped Si could also be used to describe well this gap narrowing in AZO.

͑Zn,Al͒O thin films have been prepared by a dc reactive magnetron sputtering system with the Al contents in a wide range of 0 -50 at. %. The structural, optical, and electrical properties of ͑Zn,Al͒O films were detailedly and systematically studied. The amount of Al in the film was nearly the same as, but often lower than, that in the sputtering target. The growth rate of films monotonically decreased as the Al content increased. In a low Al content region ͑Ͻ10 at. % ͒, Al-doped ZnO ͑AZO͒ thin films could be obtained at 400°C in an Ar-O 2 ambient with good properties. The optimal results of n-type AZO films were obtained at an Al content of 4 at. %, with low resistivity ϳ10 −4 ⍀ cm, high transmittance ϳ90% in the visible region, and acceptable crystal quality with a high c-axis orientation. The band gap could be widened to 3.52 eV at 4 at. % Al due to the Burstein-Moss shift ͓E. Burstein, Phys. Rev. 93, 632 ͑1954͔͒ modulated by many-body effects. An appropriate Al-doping concentration served effectively to release the residual, compressive stress in film, which may be the reason for the improvement in film stability and the increment in grain size as well. In a medium Al content region ͑10-30 at. % ͒, however, the film quality was degraded, which was presumably due to the formation of clusters or precipitates in the grains and boundaries. Besides the ͑002͒ plane, other diffraction peaks such as ͑100͒ and ͑101͒ planes of ZnO were observed, but the ͑Zn,Al͒O films still exhibited a single-phase wurtzite ZnO structure. An intragrain cluster scattering mechanism was proposed to interpret the reduction of carrier mobility in films with the Al contents in the 7 -20 at. % region. The solubility limit of Al in ZnO film was identified to be in the 20-30 at. % range, much higher than the thermodynamic solubility limit of 2 -3 at. % in ZnO. In a high Al content region ͑ജ30 at. % ͒, there were distinct observations for ͑Zn,Al͒O films. As the Al content was 30 at. %, the film appeared in a two-phase nature with ZnO hexagonal and Al 2 O 3 rhombohedral structures. At the 50 at. % Al content, the matrix of the ͑Zn,Al͒O film was Al 2 O 3 , and no evident trace of wurtzite ZnO was observed. The electrical and optical properties for both cases were also very different from those at the Al contents below 30 at. %.

We report a breakthrough in fabricating ZnO homojunction light-emitting diode by metal organic chemical vapor deposition. Using NO plasma, we are able to grow p-type ZnO thin films on n-type bulk ZnO substrates. The as-grown films on glass substrates show hole concentration of 10 16 -10 17 cm −3 and mobility of 1 -10 cm 2 V −1 s −1 . Room-temperature photoluminescence spectra reveal nitrogen-related emissions. A typical ZnO homojunction shows rectifying behavior with a turn-on voltage of about 2.3 V. Electroluminescence at room temperature has been demonstrated with band-to-band emission at I = 40 mA and defect-related emissions in the blue-yellow spectrum range.

Li-doped, p-type ZnO thin films have been realized via dc reactive magnetron sputtering. An optimized result with a resistivity of 16.4Ωcm, Hall mobility of 2.65cm2∕Vs, and hole concentration of 1.44×1017cm−3 was achieved, and electrically stable over a month. Hall-effect measurements supported by secondary ion mass spectroscopy indicated that the substrate temperature played a key role in optimizing the p-type conduction of Li-doped ZnO thin films. Furthermore, ZnO-based p-n homojunction was fabricated by deposition of a Li-doped p-type ZnO layer on an Al-doped n-type ZnO layer.

A Li–N dual-acceptor doping method has been developed to prepare p-type ZnO thin films by pulsed laser deposition. The lowest room-temperature resistivity is found to be ∼0.93Ωcm, much lower than that of Li or N monodoped ZnO films. The p-type conductivity of ZnO:(Li,N) films is very reproducible and stable, with acceptable crystal quality. The acceptor activation energy in ZnO:(Li,N) is about 95meV. ZnO-based homostructural p-n junctions were fabricated by depositing an n-type ZnO:Al layer on a p-type ZnO:(Li,N) layer, confirmed by secondary ion mass spectroscopy. The current-voltage characteristics exhibit their inherent rectifying behaviors.

Novel CoO nanostructure arrays on nickel foam with needle-like and rhombic morphologies have been prepared by using urea and hexamethylenetetramine as hydrolysis agents through fluoride-assisted hydrothermal method, respectively. The possible formation mechanism and effect factors of the novel arrays were systematically investigated by X-ray diffraction, scanning and transmission electron microscopies, N 2 sorption, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The precursor Co(CO 3 ) 0.5 (OH)•0.11H 2 O or Co(OH)F plays a crucial role in the formation of needle-like or rhombic arrays. As-prepared precursor arrays can be further converted to corresponding CoO arrays by annealing at 450 °C for 2 h without signification alteration of one-dimensional (1D) morphology. When tested as anodes for lithium ion batteries (LIBs) without the addition of other ancillary materials (carbon black and binder), the synthesized CoO arrays with needle-like and rhombic morphologies deliver ultrahigh initial discharge capacities of 1973.3 and 1447.9 mAh g −1 , respectively. In addition, they also maintain high reversible capacities of 710 and 719 mAh g −1 at 0.2 C after 50 cycles, respectively.

ZnFe 2 O 4 is a promising visible-light-driven photocatalyst with high energy level CBM for hydrogen generation. However, it could only be synthesized in the bulk form and its photocatalytic activities are limited by the rapid recombination of photo-generated carriers. Here, we report the preparation of a ZnFe 2 O 4 nanostructure in ultrathin hollow sphere morphology with a 10 nm thick shell, which could shorten the diffusion distance of photo-generated carriers to minimize their interior recombination.Furthermore, ZnFe 2 O 4 /ZnO nanoheterostructures, containing 15 nm size heterojunctions, could be synthesized facilely. We have found that this unique structure endows ZnFe 2 O 4 /ZnO with a large separation of photo-generated carriers. The hydrogen generation rate of ZnFe 2 O 4 /ZnO nanoheterostructures without co-catalysts is up to 2.15 mmol h À1 g À1 under visible light irradiation (l > 420 nm), which is 45 times higher than the best yields ever reported for ZnFe 2 O 4 -based photocatalysts. These research results provide a general and effective route to synthesize other nanoheterostructures for a variety of applications.

For colloidal semiconductor nanocrystals (NCs), shape control and doping as two widely applied strategies are crucial for enhancing and manipulating their functional properties. Here we report a facile and green synthetic approach for high-quality colloidal Mn doped ZnO NCs with simultaneous control over composition, shape and optical properties. Specifically, the shape of doped ZnO NCs can be finely modulated from three dimensional (3D) tetrapods to 0D spherical nanoparticles in a single reaction scheme. The growth mechanism of doped ZnO NCs with interesting shape transition is explored. Furthermore, we demonstrate the tunable optical absorption features of Mn doped ZnO NCs by varying the Mn doping levels, and the enhanced photocatalytic performance of Mn doped ZnO NCs under visible light, which can be further optimized by delicately controlling their shapes and Mn doping concentrations. Our results provide an improved understanding of the growth mechanism of doped NCs during the growth process and can be potentially extended to ZnO NCs doped with other metal ions for various applications.