Ultrathin Oxide Films Research Articles

Combined spectroscopic and ab initio investigation of monolayer-range Cr oxides on Fe(001): The effect of ordered vacancy superstructure

We investigated the electronic structure of an ultrathin Cr oxide film prepared by growing about 0.8 monolayers of Cr on the oxygen-terminated $\mathrm{Fe}(001)\text{\ensuremath{-}}p(1\ifmmode\times\else\texttimes\fi{}1)\mathrm{O}$ surface and characterized by the formation of an ordered array of Cr vacancies producing a $(\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5})R{27}^{\ensuremath{\circ}}$ superstructure. We combined experimental techniques such as angle- and spin-resolved photoemission spectroscopy, low-energy electron diffraction, and scanning tunneling spectroscopy with ab initio calculations, focusing on (i) the peculiar energy dispersion of $\mathrm{O}\phantom{\rule{0.16em}{0ex}}2p$ states and (ii) the orbital and spin character of $\mathrm{Cr}\phantom{\rule{0.16em}{0ex}}3d$ states. We show that the experimental $\mathrm{O}\phantom{\rule{0.16em}{0ex}}2p$ dispersion can be related to the presence of an ordered vacancy lattice. The comparison with the existing literature on the oxidation of bulk Cr(001), where a network of Cr vacancies with a short-range crystallographic order is present, reveals a similar effect on O states. The valence electronic structure of the Cr oxide layer is mostly composed by spin-minority Cr states, consistent with an antiferromagnetic coupling with the Fe substrate.

Thin and ultrathin films of palladium oxide for oxidizing gases detection

Abstract Thin and ultrathin films of palladium oxide are set forth as promising materials for oxidizing gas detection. Thin and ultra thin PdO films were prepared by thermal oxidation at dry oxygen of previously formed Pd layers on polished poly-Al2O3, SiO2/Si (100), optical quality quartz, and amorphous carbon/KCl substrates. By high-resolution transmission electron microscopy and high-resolution scanning electron microscopy it has been found that thickness was about 5–10 nm and 30–35 nm for ultrathin and thin PdO films, respectively. By high-energy electron diffraction and X-ray analysis it has been established that oxidation of initial Pd layers at T = 770–1070 K led to formation of homogenous polycrystalline PdO films. At ozone and nitrogen dioxide detection PdO films prepared by oxidation at T = 870 K have shown the good values of sensitivity, signal stability, operation speed, and reproducibility of sensor response. In comparison with other materials, palladium oxide thin and ultrathin films have some advantages at gas sensor fabrication. Firstly, at rather low operating temperatures PdO films have shown good values of functional parameters. Secondly, the synthesis procedure of binary PdO films is rather simple and is compatible with planar processes of microelectronic industry.

Toward plasma enhanced atomic layer deposition of oxides on graphene: Understanding plasma effects

Integration of dielectrics with graphene is essential for the fulfillment of graphene based electronic applications. While many dielectric deposition techniques exist, plasma enhanced atomic layer deposition (PEALD) is emerging as a technique to deposit ultrathin dielectric films with superior densities and interfaces. However, the degree to which PEALD on graphene can be achieved without plasma-induced graphene deterioration is not well understood. In this work, the authors investigate a range of plasma conditions across a single sample, characterizing both oxide growth and graphene deterioration using spectroscopic analysis and atomic force microscopy. Investigation of graphene and film quality produced under these conditions provides insight into plasma effects. Using their method, the authors achieve ultrathin (<1 nm) aluminum oxide films atop graphene.

Spectroscopic ellipsometry investigation of microcrystalline fractions in p-type hydrogenated microcrystalline silicon oxide (p-μc-SiOx:H) ultra-thin films

Abstract Spectroscopic ellipsometry (SE) was used to characterize the effect of the CO2/SiH4 ratios on the p-type hydrogenated microcrystalline silicon oxide (p-μc-SiOx:H) thin films. The p-μc-SiOx:H thin films were fabricated by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) method on the soda-lime glass substrates. The CO2/SiH4 ratios were varied from 0.0 to 0.5 with constant H2-dilution during the deposition. The p-μc-SiOx:H samples were analyzed for thickness, microstructure, and optical characteristics by SE. The ellipsometric measurements were performed in the range of 0.75–5.5 eV with the incident angle of 50, 60 and 70 degrees. The physical structures were proposed as either the double- and triple-layer models. The optical dispersion was proposed based on Tauc-Lorentz (TL) model in order to extract the information of interest including dielectric dispersion and crystalline fraction. The results from the SE analyses showed that the thin films prepared with pure SiH4 could only be fitted by the triple-layer model, whose effect refractive index was gradually decreased from 3.92 to 2.53 with the increased CO2/SiH4 ratio. With the introduction of the CO2, the obtained films were instead best represented by the double-layer model. The calculated crystalline fractions could be determined from the SE results, which accurately corresponded to those from the Raman spectroscopy and high-resolution transmission electron microscope (HRTEM). In addition, the dielectric dispersion of the thin films was related directly with CO2/SiH4 ratios, but inversely with crystalline fraction with the values of 53 down to 18% with the increased gas mixture ratio.

Ferroelectricity in ultrathin yttrium-doped hafnium oxide films prepared by chemical solution deposition based on metal chlorides and alcohol

Abstract We suggest a facile way to prepare precursor solutions with metal salts and alcohol and fabricate ultrathin ferroelectric yttrium-doped hafnium oxide films on Pt(111)/TiO 2 /SiO 2 /Si substrates by chemical solution deposition and post-annealing treatment. The samples were prepared with 5.2 mol% yttrium-doping and had a thickness ranging from 3 nm to 9 nm. We also varied the post-annealing temperature from 600 °C to 800 °C. The ultrathin films were characterized by transmission electron microscopy (TEM) and Raman spectroscopy. Their local ferroelectric properties were investigated by piezoresponse force microscopy (PFM) for domain imaging and polarization switching at nanoscale.

Nanoscale analysis of the oxidation state and surface termination of praseodymium oxide ultrathin films on ruthenium(0001)

The complex structure and morphology of ultrathin praseodymia films deposited on a ruthenium(0001) single crystal substrate by reactive molecular beam epitaxy is analyzed by intensity-voltage low-energy electron microscopy in combination with theoretical calculations within an ab initio scattering theory. A rich coexistence of various nanoscale crystalline surface structures is identified for the as-grown samples, notably comprising two distinct oxygen-terminated hexagonal Pr2O3(0001) surface phases as well as a cubic Pr2O3(111) and a fluorite PrO2(111) surface component. Furthermore, scattering theory reveals a striking similarity between the electron reflectivity spectra of praseodymia and ceria due to very efficient screening of the nuclear charge by the extra 4f electron in the former case.

Stabilization of ultrathin (hydroxy)oxide films on transition metal substrates for electrochemical energy conversion

Design of cost-effective electrocatalysts with enhanced stability and activity is of paramount importance for the next generation of energy conversion systems, including fuel cells and electrolysers. However, electrocatalytic materials generally improve one of these properties at the expense of the other. Here, using density functional theory calculations and electrochemical surface science measurements, we explore atomic-level features of ultrathin (hydroxy)oxide films on transition metal substrates and demonstrate that these films exhibit both excellent stability and activity for electrocatalytic applications. The films adopt structures with stabilities that significantly exceed bulk Pourbaix limits, including stoichiometries not found in bulk and properties that are tunable by controlling voltage, film composition, and substrate identity. Using nickel (hydroxy)oxide/Pt(111) as an example, we further show how the films enhance activity for hydrogen evolution through a bifunctional effect. The results suggest design principles for this class of electrocatalysts with simultaneously enhanced stability and activity for energy conversion. Development of electrocatalysts with high stability and activity is a critical challenge. Here, the authors combine simulations with in situ experiments to identify principles underlying simultaneously enhanced stability and activity of ultrathin (hydroxy)oxide films on transition metal substrates.

The variation of electrical transport properties with thickness for ultrathin indium oxide films

AbstractWe have systematically investigated the electrical transport properties of a series of InO films (with thickness t ranging from 4 to 45 nm) grown on yttrium stabilized ZrO single crystal substrates. Those nm films reveal metallic characteristics in electrical transport properties, and electron–electron interaction effects govern the low temperature behaviors of the resistivity. For the 6.3 and 3.7 nm thick films, the resistivities variation with temperature [] curves show insulator behaviors in the whole measured temperature range (2–300K). In addition, the two‐dimensional (2D) Mott type variable‐range‐hopping (VRH) dominates the temperature behavior of resistivity in the temperature range , and a crossover to the 2D Efros–Shklovskii (ES) VRH occurs below . Our results are quantitatively consistent with the theoretical predications of the 2D Mott‐VRH and 2D ES‐VRH theories in the corresponding temperature regions.

Structure and morphology of magnetron sputter deposited ultrathin ZnO films on confined polymeric template

Abstract The structure and morphology of ultra-thin zinc oxide (ZnO) films with different film thicknesses on confined polymer template were studied through X-ray reflectivity (XRR) and grazing incidence small angle X-ray scattering (GISAXS). Using magnetron sputter deposition technique ZnO thin films with different film thicknesses ( R g film thickness, where R g ∼ 20 nm ( R g is the unperturbed radius of gyration of polystyrene, defined by R g = 0.272 √M 0 , and M 0 is the molecular weight of polystyrene). The detailed internal structure, along the surface/interfaces and the growth direction of the system were explored in this study, which provides insight into the growth procedure of ZnO on confined polymer and reveals that a thin layer of ZnO, with very low surface and interface roughness, can be grown by DC magnetron sputtering technique, with approximately full coverage (with bulk like electron density) even in nm order of thickness, in 2–7 nm range on confined polymer template, without disturbing the structure of the underneath template. The resulting ZnO-polystyrene hybrid systems show strong ZnO near band edge (NBE) and deep-level (DLE) emissions in their room temperature photoluminescence spectra, where the contribution of DLE gets relatively stronger with decreasing ZnO film thickness, indicating a significant enhancement of surface defects because of the greater surface to volume ratio in thinner films.

Microstructure and electrical properties of palladium oxide thin films for oxidizing gases detection

Abstract Palladium oxide thin and ultrathin films have been presented as promising materials for detection of oxidizing gases. PdO films were prepared by thermal oxidation of previously formed Pd layers on different substrates: SiO 2 /Si (100), Si (100), optical quality quartz, and KCl (100) with a buffer layer of amorphous carbon. By X-ray analysis, reflection high-energy electron diffraction, and high resolution transmission electron microscopy investigations it has been established that homogenous polycrystalline PdO films (thickness was about 5–40 nm) were formed by oxidation at dry oxygen at temperatures T = 770–870 K. The experimental results of X-ray analysis and transmission electron microscopy have shown that the increase in oxidation temperature led to uniformity and enlargement of crystalline grains in homogeneous PdO films. It has been found that the energy band gap values increased monotonically from Δ E g = 2.15 eV to 2.3 eV for PdO films with p -type conductivity oxidized at Т an = 670 K and 1070 K, respectively. At ozone detection PdO films fabricated on polished poly-Al 2 O 3 have shown high values of sensitivity, signal stability, and reproducibility of sensor response. For fabrication of gas sensors the application of PdO thin and ultrathin films has some advantages in comparison with other materials because PdO films have shown good values of sensor functional parameters, and their synthesis procedure is rather simple and is compatible with planar processes of the microelectronic industry also.

A dry process for forming ultrathin silicon oxide film on gold nanoparticle

A simple dry process for preparing an ultrathin SiO2 shell on gold nanoparticle (Au NP) has been developed. The adsorption and reaction of 1,3,5,7-tetramethylcyclotetrasiloxane on Au NP-loaded ZnO (Au/ZnO) from gas phase at 353 K yields a multilayer of polymethylsiloxane (PMS) on the Au surface, while a monolayer is formed on the ZnO surface. The postheating in the air at 773 K transforms the PMS layer to a uniform SiO2 layer with thickness (lSiO2) of ∼2 nm on the surface of every Au NP (Au@SiO2/ZnO). UV-visible absorption spectra show that the SiO2 shell enhances the localized surface plasmon resonance of Au NP with its peak redshifted from 530 nm to 571 nm. The 3D finite-difference time-domain calculations for Au@SiO2(lSiO2 = 2 nm)/ZnO indicate that a strong local electric field is generated at the Au-SiO2-ZnO three-phase interface along the peripheral edge of Au NP with an enhancement factor of ∼107.

A ternary functional Ag@GO@Au sandwiched hybrid as an ultrasensitive and stable surface enhanced Raman scattering platform

The graphene-mediated surface enhanced Raman scattering (SERS) substrates by virtues of plasmonic metal nanostructures and graphene or its derivatives have attracted tremendous interests which are expected to make up the deficiency of traditional plasmonic metal substrates. Herein, we designed and fabricated a novel ternary [email protected]@Au sandwich hybrid wherein the ultrathin graphene oxide (GO) films were seamlessly wrapped around the hierarchical flower-like Ag particle core and meanwhile provided two-dimensional anchoring scaffold for the coating of Au nanoparticles (NPs). The surface coverage density of loading Au NPs could be readily controlled by tuning the dosage amount of Au particle solutions. These features endowed the sandwiched structures high enrichment capability for analytes such as aromatic molecules and astonishing SERS performance. The Raman signals were enormously enhanced with an ultrasensitive detection limit of rhodamine-6G (R6G) as low as 10−13 M based on the chemical enhancement from GO and multi-dimensional plasmonic coupling between the metal nanoparticles. In addition, the GO interlayer as an isolating shell could effectively prevent the metal–molecule direct interaction and suppress the oxidation of Ag after exposure at ambient condition which enabled the substrates excellent reproducibility with less than 6% signal variations and prolonged life-time. To evaluate the feasibility and the practical application for SERS detection in real-world samples based on GO sandwiched hybrid as SERS-active substrate, three different prohibited colorants with a series of concentrations were measured with a minimum detected concentration down to 10−9 M. Furthermore, the prepared GO sandwiched nanostructures can be used to identify different types of colorants existing in red wine, implying the great potential applications for single-particle SERS sensing of biotechnology and on-site monitoring in food security.

Band Bending in Mg-Colored and O2-Activated Ultrathin MgO(001) Films

Ultrathin MgO films grown on Ag(001) have been investigated using X-ray and ultraviolet photoemission spectroscopies for oxide films successively exposed to Mg and O2 flux. Studying work functions and layer-resolved Auger shifts allows us to keep track of band profiles from the oxide surface to the interface and reveal the charge-transfer mechanisms underlying the controlled creation of Mg-induced surface color centers and the catalytic enhancement of O2 activation. Our results demonstrate that one can intimately probe the catalytic properties of metal-supported ultrathin oxide films by studying the electronic band alignment at interfaces.

Efficient phase matching algorithm for measurements of ultrathin indium tin oxide film thickness in white light interferometry

A novel method is proposed to measure the thickness of the indium tin oxide (ITO) film, which is less than 20 nm, using valid Fourier’s phase information of white light correlogram and curve matching algorithm. Based on the Fourier transform amplitude information, the valid phase distribution function that contains the thin transparent electrode ITO film thickness information has been successfully extracted. A curve matching algorithm based on standard deviation is employed to accurately calculate the thickness of such thin ITO films. The experimental results show that the thickness values were consistent with that determined using the stylus instruments, indicating that this method can be applied to measure the ITO film thickness ranging from 5 to 100 nm.

Energy Level Alignment at the Fullerene/Titanium Oxide Ultrathin Film Interface

The performance of molecule based electronic devices can be improved by use of transition metal oxides (TMO) as charge injection buffer layers between electrodes and organic semiconductors. It is known that the appropriate energy level alignment at the molecule/TMO interface determines the efficiency of the respective TMO. Herein, scanning tunneling microscopy is employed to characterize the interface formed by fullerenes (C60) deposited on a titanium oxide (TiO) ultrathin film, which is created by oxidation of a Pt3Ti(111) surface. Individual C60 are identified with orbital resolution, and the interfacial and intermolecular interactions are characterized in detail. Furthermore, the energy level alignment at the C60/TiO ultrathin film interface is deduced based on scanning tunneling spectroscopy data. The results demonstrate that the C60/TiO interface corresponds to a type-I heterojunction and, thus, is useful to decouple C60 molecules from the metallic alloy surface.

Processing and charge state engineering of MoOx

The effects of wet chemical processing employed in device fabrication standards are studied on molybdenum oxide (MoOx) ultra-thin films. We have combined x-ray photoelectron spectroscopy (XPS), angle resolved XPS and x-ray reflectivity to gain insight into the changes in composition, structure and electronic states upon treatment of films with different initial stoichiometry prepared by reactive sputtering. Our results show significant reduction effects associated with the development of gap states in MoOx, as well as changes in the composition and structure of the films, systematically correlated with the initial oxidation state of Mo.

Localization-driven metal–insulator transition in epitaxial hole-doped Nd1−xSrxNiO3 ultrathin films

Advances in thin film growth technologies make it possible to obtain ultra-thin perovskite oxide films and open the window for controlling novel electronic phases for use in functional nanoscale electronics, such as switches and sensors. Here, we study the thickness-dependent transport characteristics of high-quality ultrathin Nd0.9Sr0.1NiO3 (Sr-NNO) films, which were grown on LaAlO3 (0 0 1) single-crystal substrates by using pulsed laser deposition method. Thick Sr-NNO films (25 unit cells) exhibit metallic behavior with the electrical resistivity following the T n (n < 2) law corresponding to a non-Fermi liquid system, while a temperature driven metal–insulator transition (MIT) is observed with films of less than 15 unit cells. The transition temperature increases with reducing film thickness, until the insulating characteristic is observed even at room temperature. The emergence of the insulator ground state can be attributed to weak localization driven MIT expected by considering Mott–Ioffe–Regel limit. Furthermore, the magneto-transport study of Sr-NNO ultrathin films also confirms that the observed MIT is due to the disorder-induced localization rather than the electron–electron interactions.

Parameter Space of Atomic Layer Deposition of Ultrathin Oxides on Graphene.

Atomic layer deposition (ALD) of ultrathin aluminum oxide (AlOx) films was systematically studied on supported chemical vapor deposition (CVD) graphene. We show that by extending the precursor residence time, using either a multiple-pulse sequence or a soaking period, ultrathin continuous AlOx films can be achieved directly on graphene using standard H2O and trimethylaluminum (TMA) precursors even at a high deposition temperature of 200 °C, without the use of surfactants or other additional graphene surface modifications. To obtain conformal nucleation, a precursor residence time of >2s is needed, which is not prohibitively long but sufficient to account for the slow adsorption kinetics of the graphene surface. In contrast, a shorter residence time results in heterogeneous nucleation that is preferential to defect/selective sites on the graphene. These findings demonstrate that careful control of the ALD parameter space is imperative in governing the nucleation behavior of AlOx on CVD graphene. We consider our results to have model system character for rational two-dimensional (2D)/non-2D material process integration, relevant also to the interfacing and device integration of the many other emerging 2D materials.

Electrical Properties of Ultrathin Indium Tin Oxide Films in Relation to Sputtering Voltage–Current

Electrical Properties of Ultrathin Indium Tin Oxide Films in Relation to Sputtering Voltage–Current

Work function mediated by deposition of ultrathin polar FeO on Pt(111)

Abstract Significant work function changes from bare Pt(111) surface to 1 monolayer and 2 monolayers of ultrathin iron oxide (FeO) films on it are investigated by means of scanning tunneling microscopy/spectroscopy (STM/STS). With FeO layer-by-layer growth, a continuous reduction of the work function along with the surface vacuum level (VL) shifting is observed. We found that the compression of the electron spill-out at the metal-oxide interface and the substantial reconstruction of 2 ML FeO film, respectively, make major contributions to the first and the second reductions of the work function. The rectifying effect in FeO films is also observed, which is attributed to the downward shift of band alignment imposed by the total change in surface dipole. Our work shows that the polar oxide films play an important role to adjust surface electronic structures for enhancing device functionality.