We generalize and systematize basic experimental data on optical and luminescence properties of ZnO single crystals, thin films, powders, ceramics, and nanocrystals. We consider and study mechanisms by which two main emission bands occur, a short-wavelength band near the fundamental absorption edge and a broad long-wavelength band, the maximum of which usually lies in the green spectral range. We determine a relationship between the two luminescence bands and study in detail the possibility of controlling the characteristics of ZnO by varying the maximum position of the short-wavelength band. We show that the optical and luminescence characteristics of ZnO largely depend on the choice of the corresponding impurity and the parameters of the synthesis and subsequent treatment of the sample. Prospects for using zinc oxide as a scintillator material are discussed. Additionally, we consider experimental results that are of principal interest for practice.
The set of experimental data on the X-ray-excited luminescence and X-ray induced conductivity of ZnSe are compared to the data on the photoluminescence and photoconductivity. It is experimentally established that the current-voltage characteristics and the kinetics of phosphorescence and current relaxation depend on the type of excitation. It is found that the external electric field influences the intensity and shape of bands in the luminescence spectra. It is shown that the character of excitation defines the kinetics of recombination, charge carrier trapping, and conductivity in wide-gap se…
Achieving efficient and stable ultraviolet emission is a challenging goal in optoelectronic devices. Herein, we investigate the UV luminescence of zinc germanate Zn2GeO4 microwires by means of photoluminescence measurements as a function of temperature and excitation conditions. The emitted UV light is composed of two bands (a broad one and a narrow one) associated with the native defects structure. In addition, with the aid of density functional theory (DFT) calculations, the energy positions of the electronic levels related to native defects in Zn2GeO4 have been calculated. In particular, ou…
The problem of detecting the ability of some metal oxides as photocatalysts is receiving considerable attention in the world. The studied zinc oxide is considered as one of the most utilized semiconductors, due to its direct band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature. The characteristic formulation of Copper doped zinc oxide is implemented to obtain an improved adsorption kinetics and photocatalytic activity of zinc oxide photocatalyst capable of evolving estimated photoluminescence (PL) spectra of zinc oxide nanoparticles to explore the effect of Copp…
GaAs nanowires (NWs) grown by molecular-beam epitaxy may contain segments of both the zincblende (ZB) and wurtzite (WZ) phases. Depending on the growth conditions, we find that optical emission of such NWs occurs either predominantly above or below the band gap energy of ZB GaAs [E(g,ZB)]. This result is consistent with the assumption that the band gap energy of wurtzite GaAs [E(g,WZ)] is larger than E(g,ZB) and that GaAs NWs with alternating ZB and WZ segments along the wire axis establish a type II band alignment, where electrons captured within the ZB segments recombine with holes of the ne…
Zinc oxide and Aluminum Ferrite were prepared Chemical route. The samples were characterized by XRD and VSM. Simulation of M-H plots of Co/CoO thin films were performed. Effect of parameters was observed on saturation magnetization.
The various decay mechanisms of excitons in cuprous oxide (Cu2O) are highly sensitive to defects which can relax selection rules. Here we report cryogenic hyperspectral imaging of exciton luminescence from cuprous oxide crystals grown via the floating zone method showing the samples have few defects. Some locations, however, show strain splitting of the 1s orthoexciton triplet polariton luminescence. Strain is reduced by annealing. In addition, annealing causes annihilation of oxygen and copper vacancies, which leads to a negative correlation between luminescence of unlike vacancies.
The Zn K-edge X-ray absorption spectroscopy has been used to investigate in situ the decomposition of zinc peroxide (ZnO$_2$) to zinc oxide (ZnO). Principal component and linear combination analyses of the EXAFS spectra have been employed to identify the phase composition of the oxide upon heating to 900$^\circ$C. Only the ZnO$_2$ phase has been found up to 180$^\circ$C, whereas only the nanocrystalline ZnO phase has occurred above 250$^\circ$C. Detailed structural information on the temperature dependence of the local environment of zinc atoms has been obtained using the reverse Monte Carlo s…
During a bioInspired crystallization process, incorporation of amino acids into the crystal structure of ZnO induces lattice strain that leads to linear bandgap shifts.
Emerging technologies are demanding innovative properties of glasses. In this work, Cerium Oxide is used as a dopant in Zinc-Tellurite samples and its effects on the properties of the glass are investigated. Thermal analysis and x-ray diffraction confirmed the amorphous nature of all samples. The bivalent nature of Cerium is investigated spectroscopically and a strong redshift induced by the dopant is attributed to charge transfers O$^{2-}{\rightarrow}$Ce$^{4+}$, while the 4f-5d transition of Ce$^{3+}$ could not be identified in absorption or luminescence measurements. Yellow/red (570/650 nm) …
The micro-properties (structure and composition), and macro-properties (electrical and optical properties) of zinc oxide (ZnO) thin films deposited on glass substrates using a filtere vacuum arc deposition (FVAD) system were investigated as a function of oxygen pressure (0.37 - 0.5 Pa) and arc current (100 - 300 A). Correlations between the various properties were investigated.
We use real-time spectroscopic polarimetric observations of growth and a chemical model derived therefrom, to develop a method of growing dense, two-dimensional zinc oxide epitaxially on sapphire by metalorganic chemical vapor deposition. Particulate zinc oxide formed in the gas phase is used to advantage as the deposition source. Our real-time data provide unequivocal evidence that: a seed layer is required; unwanted fractions of ZnO are deposited; but these fractions can be removed by cycling between brief periods of net deposition and etching. The transition between deposition and etching o…
The spectral characteristics of ZnO:Ga and ZnO:Ga,N ceramics prepared by uniaxial hot pressing have been investigated. At room temperature, the edge (exciton) band at 3.12 eV dominates in the luminescence spectra of ZnO:Ga, while a wide luminescence band at 2.37 eV, which is likely to be due to zinc vacancies, is observed in the spectra of ZnO:Ga,N. Upon heating, the edge band maximum shifts to lower energies and the bandwidth increases. The extrapolated position of the edge-band maximum at zero temperature, Em(0) = 3.367 +/- 0.005 eV, is in agreement with the data for thin zinc oxide films. T…
We report a method to promote photoluminescence emission in graphene materials by enhancing carrier scattering instead of directly modifying band structure in multilayer reduced graphene oxide (rGO) nanospheres. We intentionally curl graphene layers to form nanospheres by reducing graphene oxide with spherical polymer templates to manipulate the carrier scattering. These nanospheres produce hot-carrier luminescence with more than ten-fold improvement of emission efficiency as compared to planar nanosheets. With increasing excitation power, hot-carrier luminescence from nanospheres exhibits abn…
The enthalpies of dissolution of metastable rock salt and thermodynamically stable wurtzite polymorphs of zinc oxide in aqueous H2SO4 have been measured in direct calorimetric experiments at 303 K and 0.1 MPa and the obtained results enabled determination of the standard enthalpy of the rock salt-to-wurtzite phase transition in ZnO, ΔtrH = -11.7+/-0.3 kJ/mol.
Single photon sources are required for a wide range of applications in quantum information science, quantum cryptography and quantum communications. However, so far majority of room temperature emitters are only excited optically, which limits their proper integration into scalable devices. In this work, we overcome this limitation and present room temperature electrically triggered light emission from localized defects in zinc oxide (ZnO) nanoparticles and thin films. The devices emit at the red spectral range and show excellent rectifying behavior. The emission is stable over an extensive pe…
Basal-plane stacking faults are an important class of optically active structural defects in wurtzite semiconductors. The local deviation from the 2H stacking of the wurtzite matrix to a 3C zinc-blende stacking induces a bound state in the gap of the host crystal, resulting in the localization of excitons. Due to the two-dimensional nature of these planar defects, stacking faults act as quantum wells, giving rise to radiative transitions of excitons with characteristic energies. Luminescence spectroscopy is thus capable of detecting even a single stacking fault in an otherwise perfect wurtzite…
Uniaxial hot pressing has been used to obtain ceramics based on zinc oxide, and their optical, x-ray-structure, luminescence, and scintillation characteristics have been studied. It is shown that, by changing the concentration of the dopant (Ga) and the codopant (N), it is possible to change the intensities of the edge band (397.5 nm) and the intraband luminescence (510 nm) of the ZnO luminescence, as well as their ratio. Undoped ZnO ceramic has good transparency in the visible region and fairly high luminous yield: 9050 photons per MeV. Ceramic ZnO:Ga possesses intense edge luminescence with …
An original modification of the standard Pulse Laser Deposition (PLD) method for preparing both undoped and indium doped zinc oxide (ZnO:In) thin films at low substrate temperature is proposed. This preparation method does not demand any further post-deposition annealing treatment of the grown films. The developed method allows to grow thin films at low substrate temperature that prevents them from the considerable loss of their intrinsic electrical and optical properties. The influence of deposition parameters on the electrical and optical parameters of the undoped and the indium doped ZnO th…
In this work we present a detailed Raman scattering investigation of zinc oxide and aluminum-doped zinc oxide (AZO) films characterized by a variety of nanoscale structure and morphology and synthesized by pulsed laser deposition (PLD) under different oxygen pressure conditions. The comparison of Raman data for pure ZnO and AZO films with similar morphology at the nano/mesoscale allows to investigate the relation between Raman features (peak or band positions, width, relative intensity) and material properties such as local structural order, stoichiometry and doping. Moreover Raman measurement…
Rare-earth-doped optical materials are important for light sources in optoelectronics, as well as for efficient optical amplification elements and other elements of photonics. On the basis of the previously developed method of anhydrous, low-temperature synthesis of Er/Yb oxides from their chlorides we fabricated proper nanoparticles with defined parameters and used them for the development of optically transparent, luminescent polymer nanocomposite with low optical scattering, suitable for direct, light-induced formation of photonic elements. Introduction of preformed gold nanoparticles in su…
Flexible and transparent electronics presents a new era of electronic technologies. Ubiquitous applications involve wearable electronics, biosensors, flexible transparent displays, radio-frequency identifications (RFIDs), etc.Zinc oxide (ZnO) and related materials are the most commonly used inorganic semiconductors in flexible and transparent devices, owing to their high electrical performance, together with low processing temperature and good optical transparency.In this paper, we review recent advances in flexible and transparent thin-film transistors (TFTs) based on ZnO and related material…
The dielectric permittivity of a material encapsulates the essential physics of light-matter interaction into the material's local response to optical excitation. Dynamic, photo-induced modulation of the permittivity can enable an unprecedented level of control over the phase, amplitude, and polarization of light. Therefore, the detailed dynamic characterization of technology-relevant materials with substantially tunable optical properties and fast response times is a crucial step in the realization of tunable optical devices. This work reports on the extraordinarily large permittivity changes…
We present experimental results on the photoluminescence (PL) of gas-suspended zinc oxide nanoclusters prepared during ablation of sintered ZnO targets by a pulsed ArF laser in the presence of oxygen ambient gas. The PL spectra in the UV spectral region correspond to the exciton recombination in the nanoclusters which are crystallized and cooled down to the temperature of the ambient gas in the ablation chamber. The time evolution of the spectra as well as their dependence on the ambient gas pressure are discussed.
We report persistently increased conduction in ZnO nanowires irradiated by ion beam with various ion energies and species. This effect is shown to be related to the already known persistent photo conduction in ZnO and dubbed persistent ion beam induced conduction. Both effects show similar excitation efficiency, decay rates, and chemical sensitivity. Persistent ion beam induced conduction will potentially allow countable (i.e., single dopant) implantation in ZnO nanostructures and other materials showing persistent photo conduction.
Ceramics ZnO:Zn of 20mm diameter and 1.6mm thickness with an optical transparency up to 0.33 in the visible region have been prepared by hot pressing technique. Scintillating and luminescent characteristics such as emission spectra, decay time, yield, and TSL glow curve have been measured under X-ray excitation. Two emission bands peaking at 500 and 380 nm were detected, the light output was about 80% of that for standard BGO scintillator, main decay constant was 10.4 +/- 0.1 ns. The obtained data allow us to consider the ZnO:Zn ceramics as a perspective scintillator. Finally, the investigatio…
Hybrid field effect transistors based on the organic polymer poly(3-hexylthiophene) (P3HT) and inorganic zinc oxide were investigated. In this report we present one of the first studies on hybrid transistors employing one polymeric transport layer. The sol-gel processed ZnO was modified via Al doping between 0.8 and 10 at.%, which allows a systematic variation of the zinc oxide properties, i.e. electron mobility and morphology. With increasing doping level we observe on the one hand a decrease of the electron mobilities by two orders of magnitude,on the other hand doping enforces a morphologic…
Nanostructured Zinc oxide (ZnO) was synthesized via a ball milling for 10 hours using high energy planetary ball mill. Phase purity and homogeneity of all the samples have been investigated by X-ray diffraction (XRD) and Field Emission Scanning Electron Microscopy (FE-SEM). All the diffraction peaks can be indexed to the hexagonal phase ZnO with hexagonal symmetry (space group P63mc). Average crystallite size was observed to be 20 nm. There was a remarkable suppression in thermal conductivity (κ) compared to the bulk values by a factor of ~50 at room temperature. This suggests to the possibili…
In this work we use density functional theory (DFT) to investigate the influence of semi-local exchange and correlation effects on the electronic and optical properties of zinc oxide. We find that the inclusion of such effects using the Tran-Blaha modified Becke-Johnson potential yields an excellent description of the electronic structure of this material giving energy band gap which is systematically larger than the one obtained with standard local functionals such as the generalized gradient approximation. The discrepancy between the experimental and theoretical band gaps is then significant…
The elasticity and piezoelectricity of zinc oxide (ZnO) crystals and single layers are investigated from the first-principles calculations. It is found that a ZnO thin film less than three Zn-O layers prefers a planar graphite-like structure to the wurtzite structure. ZnO single layers are much more flexible than graphite single layers in the elasticity and stronger than boron nitride single layers in the piezoelectricity. Single-walled ZnO nanotubes (SWZONTs) can exist in principle because of their negative binding energy. The piezoelectricity of SWZONTs depends on their chirality. For most Z…
Present paper reports the effect of ZnO additives on humidity and temperature sensing properties of cuprous oxide. The cuprous oxide powder was mixed with 10% and 25% ZnO by weight and these samples were pelletized by using hydraulic pressing machine. The sensing materials were also investigated by Scanning Electron Microscope (SEM) and X-ray Diffraction (XRD). SEM images show morphology and porosity of material. The average particles size of cuprous oxide was found to be 1.2 micron. The sheet like structures of ZnO is evident in micrographs. From XRD all peaks are well identified and crystall…
Growth driven phase transitions in Zinc Oxide nanoparticles through machine-learning assisted simulations
CEMES, CNRS and Université de Toulouse（图卢兹大学）；CNRS, Sorbonne Université, Institut des NanoSciences de Paris, UMR 7588（索邦大学）；Univ. Lille, CNRS, INRA, ENSCL, UMR 8207, UMET, Unité Matériaux et Transformations（里尔大学）
The local magnetic moment of Ti:ZnO is calculated from first principles by using the corrected-band-gap scheme (CBGS). The results shows that the system is magnetic with the magnetization of 0.699 $μ_B$ per dopant. The origin of the local magnetic moment is considered to be the impurity band partially occupied by the donor electrons in the conduction band. Further, the impacts of applying Hubbard U to Ti-d orbital on the magnetic moment have been investigated.
The electrical and structural characteristics of 50 nm zinc oxide (ZnO) metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors subjected to proton irradiation at different temperatures are reported and compared. We irradiated the devices with 200 keV protons to a fluence of 1016 cm-2. Examination of the X-ray diffraction (XRD) rocking curves indicates a strongly preferred (100) orientation for the grains of the as-deposited film, with decreases in crystal quality for all irradiated samples. In addition, peak shifts in XRD and Raman spectra of the control sample relative to well-known …
Based on first-principles calculation, it has been predicted that the magnetic anisotropy energy (MAE) in Co-doped ZnO (Co:ZnO) depends on electron-filling. Results show that the charge neutral Co:ZnO presents a "easy plane" magnetic state. While modifying the total number of electrons, the easy axis rotates from in-plane to out-of-plane. The alternation of the MAE is considered to be the change of the ground state of Co ion, resulting from the relocating of electrons on Co d-orbitals with electron-filling.
Objective of this work was to synthesize Copper doped Zinc Oxide (CZO) films and optimization of process parameters by varying molarity of zinc acetate dehydrate from 0.5 M to 1.0 M, concentration of copper acetate monohydrate from 1% to 5 % and annealing temperature from 200 C to 300 C to measure the sensitivity of CZO films for CO (Carbon Monoxide) gas. The concentration of CO gas was maintained at 5 ppm and operating temperature of 250 oC was used for sensing. Analysis for sensitivity showed highest grading for parametric combination of 0.75 molarity, 3% copper concentration and 300 C annea…
Density functional theory (DFT) calculations are carried out to study the structure and electronic structure of amorphous zinc oxide (a-ZnO). The models were prepared by the "melt-quench" method. The models are chemically ordered with some coordination defects. The effect of trivalent dopants in the structure and electronic properties of a-ZnO is investigated. Models of a-X_0.375 Z_0.625 O (X=Al, Ga and In) were also prepared by the "melt- quench" method. The trivalent dopants reduce the four-fold Zn and O, thereby introducing some coordination defects in the network. The dopants prefer to bon…
Size-property relations in plasma-modified ZnO thin films have been investigated as a function of plasma treatment duration. The correlations between crystallite sizes and the morphological characteristics of films have been extracted on the basis of the frontier computational analysis of the scanning probe microscope (SPM) data matrices. The nanocluster structure of oxide films have been studied in detail with high accuracy. The strong plasma-induced changes in crystallite sizes have been interpreted as a size-structure phase transition. Direct measurements of X-ray diffraction and optical tr…
Observing non-classical nucleation pathways remains challenging in simulations of complex materials with technological interests. This is because it requires very accurate force fields that can capture the whole complexity of their underlying interatomic interactions and an advanced structural analysis. Here, we first report the construction of a machine-learning force field for zinc oxide interactions using the Physical LassoLars Interaction Potentials approach which allows us to be predictive even for untrained structures. Then, we carried out freezing simulations from a liquid and observed …
The paper presents results for zinc oxide films grown at low temperature regime by Atomic Layer Deposition (ALD). We discuss electrical properties of such films and show that low temperature deposition results in oxygen-rich ZnO layers in which free carrier concentration is very low. For optimized ALD process it can reach the level of 10^15 cm-3, while mobility of electrons is between 20 and 50 cm2/Vs. Electrical parameters of ZnO films deposited by ALD at low temperature regime are appropriate for constructing of the ZnO-based p-n and Schottky junctions. We demonstrate that such junctions are…
We review pitfalls in recent efforts to make a conventional semiconductor, namely ZnO, ferromagnetic by means of doping with transition metal ions. Since the solubility of those elements is rather low, formation of secondary phases and the creation of defects upon low temperature processing can lead to unwanted magnetic effects. Among others, ion implantation is a method of doping, which is highly suited for the investigation of those effects. By focussing mainly on Fe, Co or Ni implanted ZnO single crystals we show that there are manifold sources for ferromagnetism in this material which can …
The substitution of zinc for iron in YBaFe4O7 has allowed the oxide series YBaFe4-xZnxO7, with 0.40 < x < 1.50, belonging to the "114" structural family to be synthesized. These oxides crystallize in the hexagonal symmetry (P63mc), as opposed to the cubic symmetry (F-43m) of YBaFe4O7. Importantly, the d.c. magnetization shows that the zinc substitution induces ferrimagnetism, in contrast to the spin glass behaviour of YBaFe4O7. Moreover, a.c. susceptibility measurements demonstrate that concomitantly these oxides exhibit a spin glass or a cluster glass behaviour, which increases at the expense…
Defect pyrochlore oxides have attracted a great interest as promising luminescent materials due to their flexible composition and high electron/hole mobility. In this work, we investigate the structural and electronic properties of lanthanide-doped (Ln) defect pyrochlore oxides \ce{KNbWO6}:0.125Ln$^{3+}$ by using first-principles calculations. We perform structural optimizations of various defect pyrochlore models and calculate their electronic structures, revealing that hydration has a significant influence on both local symmetry around Eu$^{3+}$ ion and band structures with an alteration of …
The fixed node diffusion Monte Carlo (DMC) method has attracted interest in recent years as a way to calculate properties of solid materials with high accuracy. However, the framework for the calculation of properties such as total energies, atomization energies, and excited state energies is not yet fully established. Several outstanding questions remain as to the effect of pseudopotentials, the magnitude of the fixed node error, and the size of supercell finite size effects. Here, we consider in detail the semiconductors ZnSe and ZnO and carry out systematic studies to assess the magnitude o…
Positively charged zinc oxide clusters ZnnOm (up to n = 16, m <= n) of various stoichiometry were synthesized in the gas phase by excimer ArF laser ablation of a ZnO target and investigated using time-of-flight mass spectrometry. Depending on ablation conditions, either metal rich or stoichiometric clusters dominate in the mass spectrum. When the irradiated target surface is fairly fresh, the most abundant clusters are metal rich with Zn(n+1)On and Zn(n+3)On being the major series. The stoichiometric clusters are observed with an etched ablated surface. The magic numbers at n = 9, 11 and 15 in…
We demonstrate heavy and hyper doping of ZnO by a combination of gallium (Ga) ion implantation using a focused ion beam (FIB) system and post-implantation laser annealing. Ion implantation allows for the incorporation of impurities with nearly arbitrary concentrations, and the laser-annealing process enables dopant activation close to or beyond the solid-solubility limit of Ga in ZnO. We achieved heavily doped ZnO:Ga with free-carrier concentrations of ~10^21 cm^(-3), resulting in a plasma wavelength of 1.02 um, which is substantially shorter than the telecommunication wavelength of 1.55 um. T…
Room-temperature ferromagnetism is observed in (110) oriented ZnO films containing 5 at % of Sc, Ti, V, Fe, Co or Ni, but not Cr, Mn or Cu ions. There are large moments, 1.9 and 0.5 muB/atom for Co- and Ti-substituted oxides, respectively. Sc-substituted ZnO shows also a moment of 0.3 muB/Sc. Magnetization is very anisotropic, with variations of up to a factor three depending on the orientation of the applied field relative to the R-cut sapphire substrates. Results are interpreted in terms of a spin-split donor impurity band model, which can account for ferromagnetism in insulating or conducti…
RF sputtered ZnO and Al:ZnO films are attractive transparent conductive oxides for fabrication of opto-electronic devices. In this paper we present efforts to control carrier concentration and mobility of ZnO/Al:ZnO thin films by controlling deposition parameters (RF power, pressure and substrate temperature. Al:ZnO thin film with resistivity as low as $ρ$ = $3.8\times 10^{-4}$ $Ω$.cm at deposition temperature of 250°C has been achieved. Zinc oxide thin film with low resistivity of $ρ$ = $3.7\times 10^{-2}$ $Ω$.cm and high electron mobility of $30$ $\mathrm{cm^{-2}V^{-1}s^{-1}}$ at deposition …
The coordination polyhedra around the cations are the building blocks of ionic solids. In context of amorphous InGaZn oxide (a-IGZO), even though the coordination polyhedra are irregularly arranged, it will be beneficial to identify them. In this work, we address the questions, (a) is it possible to classify all the polyhedra that occur in a-IGZO into only a few distinct groups? and (b) are these the same polyhedral motifs as those observed in the crystalline indium gallium zinc oxide (c-IGZO) or other related crystalline oxides of indium, gallium and zinc? Therefore, in this first principles …
The mechanism responsible for the emission of clusters from heavy ion irradiated solids is proposed to be thermal spikes. Collision cascade-based theories describe atomic sputtering but cannot explain the consistently observed experimental evidence for significant cluster emission. Statistical thermodynamic arguments for thermal spikes are employed here for qualitative and quantitative estimation of the thermal spike-induced cluster emission from silicon, germanium and zinc oxide. The evolving cascades and spikes in elemental and molecular semiconducting solids are shown to have fractal charac…
The wide bandgap semiconductor ZnO is interesting for spintronic applications because of its small spin-orbit coupling implying a large spin coherence length. Utilizing vertical spin valve devices with ferromagnetic electrodes (TiN/Co/ZnO/Ni/Au), we study the spin-polarized transport across ZnO in all-electrical experiments. The measured magnetoresistance agrees well with the prediction of a two spin channel model with spin-dependent interface resistance. Fitting the data yields spin diffusion lengths of 10.8nm (2K), 10.7nm (10K), and 6.2nm (200K) in ZnO, corresponding to spin lifetimes of 2.6…
Herein, we report an easy, straight forward, and versatile approach to build 0D/2D hybrid nanoparticle/graphene architectures by means of non-covalent chemistry and a modified Layer-by-Layer assembly. Three water soluble perylene diimides were employed to efficiently exfoliate pristine graphite into positively charged few- and multilayer graphene flakes. Further combination of these cationic building blocks with anionic zinc oxide nanoparticles led to the formation of tailor-made hybrid films via electrostatic and van der Waals interactions. These supramolecular hybrid nano-structures were tho…
An approach is presented to modify the WF of solution-processed sol-gel derived ZnOover an exceptionally wide range of more than 2.3 eV. This approach relies on the formation of dense and homogeneous self-assembled monolayers based on phosphonic acids with different dipole moments. This allows us to apply ZnO as charge selective bottom electrodes in either regular or inverted solar cell structures, using P3HT:PCBM as the active layer. These devices compete with or even exceed the performance of the reference cell on ITO/PEDOT:PSS. Our finding challenges the current view that bottom electrodes …
Recently, Shih et al. [Phys. Rev. Lett. 105, 146401 (2010)] published a theoretical band gap for wurtzite ZnO, calculated with the non-selfconsistent GW approximation, that agreed surprisingly well with experiment while deviating strongly from previous studies. They showed that a very large number of empty bands is necessary to converge the gap. We reexamine the GW calculation with the full-potential linearized augmented-plane-wave method and find that even with 3000 bands the band gap is not completely converged. A hyperbolical fit is used to extrapolate to infinite bands. Furthermore, we eli…
The exploration of new catalysts for the vapor-liquid-solid (VLS) synthesis of one-dimensional (1-D) materials promises to yield new morphologies and functionality. Here, we show, for the model ZnO system, that this possible using a semiconductor (Ge) catalyst. In particular, two unusual morphologies are described: twisted nanowires and twisted nanotubes, in addition to the usual straight nanowires. The twisted nanotubes show large hollow cores and surprisingly high twisting rates (up to 9o/μm), which cannot be easily explained through the Eshelby twist model. A combination of ex situ and in s…
