xO Thin Films Research Articles

Comparison of electrochromic properties of Ni1−xO in lithium and lithium-free aprotic electrolytes: From Ni1−xO pigment coatings to flexible electrochromic devices

Electrochromic nickel oxide (Ni1−xO) pigment powder which was made from nickel acetate reacting with H2O2/urea solution followed by heat treatment of xerogels at 400°C (24h) was milled with zirconia beads in pure water or water with added nickel oxyhydroxy precipitate acting simultaneously as the dispersant and the coating binder. The corresponding pigment particle suspensions were spin coated on FTO glass and flexible ITO-PET foils, respectively, and cured at 150°C. The optical properties of the deposited pigment coatings on FTO glass substrates were determined with an UV VIS spectrometer, providing total direct transmittance (TT), total diffuse transmittance (DT) and haze (in %). Haze increased with the coating thickness but did not exceed 4%. SEM micrographs of the deposited pigment coatings revealed that they consisted of agglomerated Ni1−xO pigment with voids.Potential cycling of Ni1−xO pigment coatings and Ni1−xO thin films was performed in 1M LiClO4/PC and 0.1M TBA+ triflate/PC electrolytes, revealing that the pigment coatings exhibited electrochemical and electrochromic activity also in lithium free (aprotic) electrolyte, while variations of the oxidation and reduction reaction peaks at various scan rates confirmed the presence of surface electrochemical reactions. The near-grazing incidence angle reflection–absorption spectroscopic technique (IR RA) was employed for identifying surface and inner-grain Ni–O stretching modes, the former being responsible for observed electrochemical properties, which was also demonstrated from the ex situ IR RA measurements performed for coatings charged/discharged in 1M LiClO4/PC and 0.1M TBA+ triflate/PC electrolytes. The results disclosed in this study suggest that the electrochromic effect was not in direct correlation with the de-insertion/insertion of small ions into the NiO (bunsenite) grains but rather the pigment coatings behave similarly to nickel oxide electrochemical capacitors. The electrochromic response of a flexible EC devices made either of PEDOT (self-standing foil) or WO3 and Ni1−xO pigment coatings glued with PMMA based electrolytes without and with lithium salt were recorded for the demonstration of the possible practical application of wet deposited Ni1−xO pigment coatings made as “electrochromic” paints.

Local structure relaxation in nanocrystalline Ni1 − xO thin films

Non-stoichiometric nickel oxide (Ni1−xO) thin films were prepared by DC magnetron sputtering technique in mixed Ar/O2 atmosphere and studied by synchrotron radiation Ni K-edge x-ray absorption spectroscopy, x-ray diffraction and scanning electron microscopy. The use of advanced modelling technique, combining classical molecular dynamics with ab initio multiple-scattering extended x-ray absorption fine structure calculations, allowed us to describe the structure relaxation and dynamics in nanocrystallites and to estimate their size and the concentration of nickel vacancies.

On the growth of Zn1–xMnxO thin films by plasma‐assisted MBE

AbstractThe growth of ZnxMn1–xO thin films by plasma‐assisted MBE has been investigated. Mn incorporation results in drastic changes in the surface morphology, the distribution of the atoms at the surface and the epilayer growth rates, which have not been reported so far and have not been observed in any other ZnO‐based alloy. These marked changes are readily observable in diluted alloys with Mn content below 1%. It is shown that Mn acts as a catalyst for oxygen incorporation into ZnO. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescent properties of Cd x Zn1−x O thin films prepared by sol-gel spin-coating method

CdxZn1−xO thin films were prepared on Si (100) substrates by the sol-gel spin coating method. Temperaturedependent photoluminescence (PL) measurements were carried out to investigate the luminescent properties of the CdxZn1−xO thin films. The PL peaks of the CdxZn1−xO thin films decrease as the Cd concentration increases and the near-band edge emission (NBE) PL peaks of the CdxZn1−xO thin films are shifted toward the red region. In the temperature-dependent PL measurement, three components at 2.855, 3.038, and 3.148 eV in the PL emission peak of the Cd0.2Zn0.8O thin films were observed at 12 K. With increasing temperature, the emission peak at 3.148 eV at 12 K becomes red-shifted and the monotonic PL peak at 12 K divides into three clear peaks as the temperature increases. The activation energy for the 3.148 eV peak is 69.54 meV corresponding to the energy for the frozen-out donors.

Role of epitaxial strain on the magnetic structure of Fe-doped CoFe2O4

The magnetic structure of Fe-doped CoFe2O4 (Co1−xFe2+xO4) grown on MgO (001) and SrTiO3 (001) substrates is studied with superconducting quantum interference device magnetometry and soft x-ray magnetic spectroscopies. X-ray and electron diffraction show that the choice of substrate has large effects on the strain, crystal structure and surface morphology of Co1−xFe2+xO4 thin films. Samples grown on MgO have small, coherent strains and surfaces that are nearly atomically flat, whereas films grown on SrTiO3 have large tensile strains and surfaces terminated with islands, which indicate the presence of a large density of misfit dislocations. These differences in structural properties correlate with the large differences seen in the magnetic structure; samples grown on SrTiO3 have larger magnetic moments and increased anisotropies compared to those grown on MgO. Most strikingly, the large magnetic spin and orbital moments found in the films grown on SrTiO3 suggest a suppression of anti-phase boundary formation, which we attribute to the large compressive lattice mismatch and the formation of misfit dislocations during the film growth in order to relieve the epitaxial strain. This results in the films grown on SrTiO3 having magnetic properties that are more similar to bulk Co1−xFe2+xO4 than those grown on MgO, demonstrating that epitaxial strain can result in large changes in the magnetic structure of Co1−xFe2+xO4.

Effect of annealing temperature and CuO on microstructure and optical band gap of CuxZn1−xO thin films

CuxZn1−xO thin films were prepared by the sol–gel process. Microstructure, surface topography and optical properties were studied by X-ray diffractometry, atom force microscopy, UV–Vis spectrophotometer and fluorescence spectrometer. The monoclinic CuO phase has been observed in CuxZn1−xO (x=0.05, 0.10, and 0.15) thin films annealed at 800°C. Preferential c-axis orientation of the CuxZn1−xO thin films annealed at 800°C increases with the increase of x value. The annealing temperature and CuO component have great effect on the absorption coefficient of CuxZn1−xO thin films in the visible region. The decrease of optical band gap may be attributed to the reduction of the fraction of the amorphous phase and stoichiometry deficiency of ZnO due to CuO doping. The deep level emission in ZnO thin film originates from oxygen vacancy and oxygen interstitial defects. The violet emission peaked at about 400nm has been assigned to the electron transition from zinc interstitial level and the valence band.

Pyroelectric electron emission from nanometer-thick films of PbZrxTi1−xO3

We report pyroelectric emission from PbZrxTi1−xO3 (PZT) thin films on nanometer-sharp tips. The epitaxial PZT films are 30 nm thick and grown directly on single-crystal silicon tips. Pyroelectric emission occurs for heating rates of ≥50 °C/min in a 20 V/μm external field. The emission current is a maximum of 240 nA when the heating rate is 100 °C/min and the electric field strength is ≥6.7 V/μm. The emitted charge is ∼7% of that expected for a perfect thin film emitter of epitaxial PbZrxTi1−xO3. We calculate that pyroelectric emission can occur without an applied field if the heating rate exceeds 4.0 × 107 °C/min.

Semiconducting p-type MgNiO:Li epitaxial films fabricated by cosputtering method

Li-doped ternary MgxNi1−xO thin films were deposited on (0001) Al2O3 substrates by a radio frequency (RF) magnetron cosputtering method with MgO and NiO:Li targets. The Mg mole fraction and Li content were relatively controlled by changing RF power for the MgO target over a range of 0–300 W, while the NiO:Li target was kept at 150 W. As a result, all films were epitaxially grown on (0001) Al2O3 substrates with the relationship of [1¯1¯0]NiO||[111¯0]Al2O3, [1¯12¯]NiO||[21¯1¯0]Al2O3 (in-plane), and [1¯11]NiO||[0001¯]Al2O3 (out-of-plane), and showed p-type semiconducting properties. Furthermore, from x-ray diffraction patterns, the authors found that MgO was effectively mixed with NiO:Li without structural deformation due to low lattice mismatch (0.8%) between NiO and MgO. However, the excess Li contents degraded the crystallinity of the MgNiO films. The band-gap of films was continuously shifted from 3.66 eV (339 nm) to 4.15 eV (299 nm) by the RF power of the MgO target. A visible transmittance of more than 80% was exhibited at RF powers higher than 200 W. Ultimately, the electrical resistivity of p-type MgNiO films was improved from 7.5 to 673.5 Ωcm, indicating that the Li-doped MgNiO films are good candidates for transparent p-type semiconductors.

Nanostructures and thin films of transparent conductive oxides studied by perturbed angular correlations

AbstractThe versatility of perturbed angular correlations (PAC) in the study of nanostructures and thin films is demonstrated, namely for the specific cases of ZnO/CdxZn1−xO thin films and Ga2O3 powder pellets and nanowires, examples of transparent conductive oxides. PAC measurements as a function of annealing temperature were performed after implantation of 111mCd/111Cd (T1/2 = 48 min) and later compared to density functional theory simulations. For ZnO, the substitution of Cd probes at Zn sites was observed, as well as the formation of a probe‐defect complex. The ternary CdxZn1−xO (x = 0.16) showed good macroscopic crystal quality but revealed some clustering of local defects around the probe Cd atoms, which could not be annealed. In the Ga2O3 samples, the substitution of the Cd probes in the octahedral Ga‐site was observed, demonstrating the potential of ion‐implantation for the doping of nanowires.

Nanostructured GdxZn1−xO thin films by nebulizer spray pyrolysis technique: Role of doping concentration on the structural and optical properties

Nanostructured GdxZn1−xO thin films with different Gd concentration from 0% to 10% deposited at 400°C using the NSP technique. The films were characterized by structural, surface and optical properties, respectively. X-ray diffraction analysis shows that the Gd doped ZnO films have lattice parameters a=3.2497Å and c=5.2018Å with hexagonal structure and preferential orientation along (002) plane. The estimated values compare well with the standard values. When film thickness increases from 222 to 240nm a high visible region transmittance (>70%) is observed. The optical band gap energy, optical constants (n and k), complex dielectric constants (εr and εi) and optical conductivities (σr and σi) were calculated from optical transmittance data. The optical band gap energy is 3.2eV for pure ZnO film and 3.6eV for Gd0.1Zn0.9O film. The PL studies confirm the presence of a strong UV emission peak at 399nm. Besides, the UV emission of ZnO films decreases with the increase of Gd doping concentration correspondingly the ultra-violet emission is replaced by blue and green emissions.

Influence of magnesium doping on the structural and optical properties of tin (II) oxide thin films deposited by electron beam evaporation

Single-phase polycrystalline magnesium-doped tin oxide (MgxSn1−xO; x=0, 0.04, and 0.08) thin films were deposited by electron beam evaporation on the glass substrate. X-ray diffraction analysis showed that the peaks intensity of the polycrystalline α-SnO thin films increased along with the increasing Mg content. The crystallite size calculated from X-ray diffraction data decreased by increasing the Mg doping concentration, which was also confirmed by atomic force microscopy. The stoichiometry and thickness of the thin films were determined by Rutherford backscattering spectroscopy. An increase in both the optical transmission (57–95%) and band gap (2.5–2.82eV) of the MgxSn1−xO thin films were observed which were investigated by UV–vis spectroscopy. Photoluminescence of MgxSn1−xO thin films revealed that there were two extra peaks at 482nm and 550nm due to the crystal defects introduced by the Mg doping and these peaks become weaker and shifted to longer wavelength by increasing the Mg doping concentration.

Electrodeposition of CoxFe3–xO4 Epitaxial Films and Superlattices

Spinel ferrites are of interest because of their potential applications in spintronics (spin-based electronics) and solid-state memories. Cobalt ferrite (CoFe2O4) is an inverse spinel ferrite. Currently, the utility of CoFe2O4 is mainly based on its high coercivity and magnetocrystalline anisotropy. The magnetic and electrical properties of CoFe2O4 depend on its Co:Fe ratio. In this paper, a one-step electrodeposition of CoxFe3-xO4 (0 < x < 1) thin films from an alkaline Fe3+- and Co2+-triethanolamine solution is presented. The Co:Fe ratio in the CoxFe3–xO4 thin films can be tuned by controlling the deposition potential. That is, CoxFe3–xO4 thin films with tunable chemical and physical properties can be produced from a single solution. Superlattices in the CoxFe3–xO4 system were also electrodeposited from the same solution by simply pulsing between two potentials. Compared to CoxFe3–xO4 individual films, superlattices exhibit resistance switching and a more pronounced negative differential resistance (NDR) feature at lower current during perpendicular transport measurements, which could be used in resistive random access memories.

Characterization of Manganese Ions in Mn-substituted Ba0.67Sr0.33MnxTi1−xO3 Thin Films

The correlation of chemical valence, occupied site and the solid solubility of Mn in Ba0.67Sr0.33MnxTi1−xO3 thin films are investigated by X-ray diffraction and X-ray photoelectron spectroscopy technologies. The chemical shifts of Ba3d, Sr3d, Ti2p and O1s dependence of Mn content in the films were systemically collected and studied. The results reveal that the Mn replace Ti preferentially in the Ba0.67Sr0.33MnxTi1−xO3 thin films and can occupy A-site as well as B-site substituting Ba and Ti, respectively, while excess Mn ion is introduced in the thin films. The solid solubility of B-site substitution in Ba0.67Sr0.33MnxTi1−xO3 thin films is about 0.04.

Tunable Optical and Structural Properties of MgxZn1−xO Films Prepared by In Situ Atomic Layer Doping Technique

Magnesium zinc oxide (MgxZn1−xO) thin films with tunable optical and structural properties have been prepared by in-situ atomic layer doping technique. The linear dependence of the Mg content on the Mg atomic layer doping percentage indicates this in-situ atomic layer doping is a precisely controlled technique for the Mg incorporation into ZnO. X-ray characterization reveals that solubility limit of MgO in the MgxZn1−xO films is approximately x = 0.1. Photoluminescence (PL) measurement shows the near-band-edge emission of MgxZn1−xO films shifts from 378 nm to 346 nm with an increase of the Mg content. The temperature dependent PL spectroscopy shows that the activation energy of thermal quenching process increases from 17 meV in pure ZnO to 48 meV in Mg0.1Zn0.9O films, indicating that thermal quenching is suppressed by the Mg incorporation. The optically-pumped stimulated emission at λ ∼ 356 nm was observed from the Mg0.1Zn0.9O film, manifesting the high optical quality of the MgxZn1−xO films grown by the in-situ atomic layer doping technique.

Highly‐Tunable Nickel Cobalt Oxide as a Low‐Temperature P‐Type Contact in Organic Photovoltaic Devices

AbstractWe report on the investigation of nickel cobalt oxide (NixCo3−xO4) thin films grown by pulsed laser deposition as hole‐transport interlayers (HTL) in organic photovoltaic (OPV) devices. Films of 7 nm thickness were grown under various oxygen deposition pressures (pO2) in the range of 2–200 mTorr. We explore both bulk and surface properties of these thin films. The workfunction (ϕ) for each of the films was statistically similar (∼4.7 eV), regardless of pO2. There was not a strong dependence of the power conversion efficiency (η) on the conductivities of the HTLs varying between 0.009 ‐ 10 S/cm. The observed differences in OPV efficiencies (ranging from 1.16 to 2.46%) were correlated to the near surface chemical composition of the NixCo3−xO4 HTL, as observed by differences in the relative surface hydroxyl concentration. The critical role of the near‐surface composition of the HTL at the HTL/organic interface was further explored by modifying the hydroxyl concentration using an oxygen plasma treatment. This treatment mitigated the impact of surface hydroxyl coverage, demonstrating either identical or increased values for ϕ and η, regardless of initial pO2 in the creation of the NixCo3−xO4 HTL. To further explore this we also employed a phosphonic acid surface modification agent on the HTL, increasing ϕ to 5.2 eV producing the best η value of 3.4%, equivalent to the PEDOT:PSS control devices. These results indicate that nickel cobalt oxide is a promising p‐type oxide for carrier‐selective interlayers in organic solar cells; however, for this to be fully realized the specific surface chemistry at the oxide/polymer interface must be controlled to increase ϕ and optimize device performance.

Electrical and Optical Properties of p-Type Li,Cu-Codoped NiO Thin Films

Li,Cu-codoped NiO thin films were deposited on fused silica substrates using a chemical solution deposition method. These thin films were phase-pure and (200) orientated with cubic structure. The transmittance and conductivity for the LixCu0.1Ni0.9−xO thin films increased as the Li content (x) increased. Transmittance of about 80% in the visible region and an optical bandgap of 3.27 eV were obtained for the Li0.1Cu0.1Ni0.8O thin film. Furthermore, the resistivity was reduced to 0.046 Ω cm for the Li0.1Cu0.1Ni0.8O thin film. The high transmittance and low resistivity resulted from larger grain size, increased as the Li content increased. These results suggest that Li,Cu-codoped NiO thin films can be utilized as p-type transparent conducting oxides.

Precise calibration of Mg concentration in MgxZn1−xO thin films grown on ZnO substrates

The growth techniques for MgxZn1−xO thin films have advanced at a rapid pace in recent years, enabling the application of this material to a wide range of optical and electrical applications. In designing structures and optimizing device performances, it is crucial that the Mg content of the alloy be controllable and precisely determined. In this study, we have established laboratory-based methods to determine the Mg content of MgxZn1−xO thin films grown on ZnO substrates, ranging from the solubility limit of x ∼ 0.4 to the dilute limit of x &amp;lt; 0.01. For the absolute determination of Mg content, Rutherford backscattering spectroscopy is used for the high Mg region above x = 0.14, while secondary ion mass spectroscopy is employed to quantify low Mg content. As a lab-based method to determine the Mg content, c-axis length is measured by x-ray diffraction and is well associated with Mg content. The interpolation enables the determination of Mg content to x = 0.023, where the peak from the ZnO substrate overlaps the MgxZn1−xO peak in standard laboratory equipment, and thus limits quantitative determination. At dilute Mg contents below x = 0.023, the localized exciton peak energy of the MgxZn1−xO films as measured by photoluminescence is found to show a linear Mg content dependence, which is well resolved from the free exciton peak of ZnO substrate down to x = 0.0043. Our results demonstrate that x-ray diffraction and photoluminescence in combination are appropriate methods to determine Mg content in a wide Mg range from x = 0.004 to 0.40 in a laboratory environment.

On the radiation hardness of (Mg,Zn)O thin films grown by pulsed-laser deposition

We report on electrical properties and the generation of the E4 defect in pulsed-laser deposited MgxZn1−xO thin films irradiated with 2.25 MeV protons. Whereas the electrical properties of the Schottky diodes as well as the net doping density of the samples did not change due to irradiation, the concentration of the E4 defect increased proportional to the applied dose as revealed by deep level transient spectroscopy. The generation rate η, is for binary ZnO thin films about 40 cm−1, a factor of 3 higher than in melt-grown single crystals, and increases to about 100 cm−1 for the Mg-alloyed thin films.

Controllable band-gap engineering of the ternary MgxNi1 − xO thin films deposited by radio frequency magnetron sputtering for deep ultra-violet optical devices

The ternary MgxNi1−xO thin films were deposited on glass substrates by radio frequency (RF) magnetron co-sputtering with NiO and MgO targets. The Mg mole fraction was controlled by RF power of MgO target from 0 to 200W at 600°C. As a result, the absorption edge was proportionally shifted from 330nm (3.75eV) to 314nm (3.95eV), when RF power of MgO target increased. The Mg composition was easily mixed up to 4.9% in the MgxNi1−xO thin films when the RF power for MgO was 200W, which was confirmed by secondary ion mass spectrometry data showing similar changes in the Mg mole fraction. In addition, the transmittance of all films maintained at an average value of 80% in the visible region and no significant structural changes were observed, regardless of the Mg contents, because of the identical rock-salt structure and low lattice mismatch (0.8%) between NiO and MgO.

Growth and properties of HfxZn1−xO thin films on flexible PET substrate using pulsed laser deposition

Hf-doped ZnO thin films with different Hf contents of target (HfxZn1−xO 0 ≤ x ≤ 10 at.%) were deposited on flexible PET substrates under different oxygen pressure using pulsed laser deposition. Microstructures, optical and electrical properties of the films were studied. The results show that the as-deposited HfxZn1−xO (0 ≤ x ≤ 5 at.%) films crystallized in ZnO hexagonal wurtzite structure with a highly preferred c-axis orientation. The films exhibited a transmission of higher than 80 % in the visible region. With increasing Hf content, the lattice constants increased; the morphology of the films deteriorated. The sheet resistance reached a minimum value of 16 Ω/□ when the doping level was about 0.5 at.%.