Epitaxial Overlayers Research Articles

Enhanced Photoelectrochemical Performance of Coaxial-Nanocoupled Strontium-Rich SrTiO3/TiO2 {001} Nanotube Arrays

The composition of a photocatalyst is the fundamental determinant of its photoability. Recently, significant attention has been focused on the ternary oxide epitaxial overlayers. Here, a highly active {001} facet dominant coaxial-nanocoupled SrTiO3/TiO2 nanotube array thin film was fabricated by a modified two-step anodization combined with a hydrothermal process. Through photoelectrochemical (PEC) measurements, such a coaxial-nanocoupled SrTiO3/TiO2 {001} surface reconstruction can definitely improve its PEC performance. The enhanced PEC performance and strongest photocurrent response of SrTiO3/TiO2 nanotubes (STNs) with a 2 h hydrothermal treatment were ascribed to the fast electron transfer in coaxial nanoarchitecture.

(Invited) In Situ Stress and Nanogravimetric Measurements during Underpotential Deposition

The underpotential deposition (UPD) of metals onto foreign metal substrates involves monolayer or submonolayer formation at potentials that are positive of the reversible electrodeposition of the bulk metal. UPD is particularly important to the understanding of electrodeposition where processes in the UPD region can be expected to influence the growth and subsequent properties of bulk thin films. In electrocatalysis, sub-monolayers of metals such as Bi, Pb, and Tl on noble metal surfaces have shown enhanced catalytic activity for a variety of electroreduction processes. Modified catalytic activity has also been observed for Pd and Pt overlayers consisting of a few monolayers. For this reason UPD processes are being examined by a variety of in situ techniques, including wafer curvature, in order to better understand the relationship between the structure of the UPD adlayers and the electrocatalytic activity. Changes in surface stress can be measured by monitoring the curvature of a wafer or cantilever while in solution and under potential control. Surface stresses on the order of 0.01 N/m can be resolved, sufficient to study the adsorption of molecular monolayers onto the electrode surface. Equation (1) quantifies the change in surface stress associated with epitaxial overlayer growth1, ΔF = Δfo + Δfpzc + Δh + Σcohtf (1) where Δfo is the intrinsic surface stress difference between the overlayer and the substrate, Δfpzc is an electrocapillary term that reflects the change in capacitance and potential of zero charge (pzc) associated with changing the surface from the substrate metal to the overlayer metal, Δh is the interface stress, and ΔΣcohtf is the product of the coherency stress and the film thickness. For sub-monolayers, Eqn. (1) may also contain an additional term that captures the contribution of the compressive strain due to island size effects2. Although the misfit term in Eqn. (1) is often assumed to dictate the overall change in surface stress, this is not always the case. The upd of Al and Cu onto Au are clear examples where the stress change is compressive even though the lattice misfit is positive. In some cases, pseudomorphic film growth of several monolayers allows the coherency term to be distinguished from the interfacial terms, which are independent of film thickness. Measurements of this type have been made for the electrodeposition of Pd onto (111)-textured Au3, and have resulted in estimates of Δh for the Pd-Au interface to be about -1.2 N/m. This is similar in both sign and magnitude to values reported for Ag-Cu4. This talk will examine the surface stress changes associated with underpotential deposition, paying particular attention to the interfacial contributions, in addition to that associated with lattice misfit.

Energetics and Atomic Structures of Cu2Te Overlayers on CdTe(111)

Although Cu2Te is widely used as a back contact material in CdTe solar cells, the exact atomic structure of the Cu2Te/CdTe interface remains unclear. Using first-principles calculations within density functional theory, we search for possible structures of Cu2Te overlayers on CdTe(111) surfaces based on our recent theoretical determination of a stable layered structure of bulk Cu2Te. We discover that the unstrained bulklike configuration is energetically more favorable than various epitaxial configurations because of the significant energy costs associated with the in-plane strain, even though the first epitaxial overlayer is already more stable than bulk Cu2Te. Our thermodynamic analysis further confirms that the unstrained bulklike interface structure is still stable at typical experimental growth temperatures (∼700 K). It is also found that van der Waals forces play a considerable role in the stacking of the Cu2Te overlayers. These findings are discussed in connection with existing and new experimental...

Synergistic oxygen evolving activity of a TiO2-rich reconstructed SrTiO3(001) surface.

In addition to composition, the structure of a catalyst is another fundamental determinant of its catalytic reactivity. Recently, anomalous Ti oxide-rich surface phases of ternary oxides have been stabilized as nonstoichiometric epitaxial overlayers. These structures give rise to different modes of oxygen binding, which may lead to different oxidative chemistry. Through density functional theory investigations and electrochemical measurements, we predict and subsequently show that such a TiO2 double-layer surface reconstruction enhances the oxygen evolving activity of the perovskite-type oxide SrTiO3. Our theoretical work suggests that the improved activity of the restructured TiO2(001) surface toward oxygen formation stems from (i) having two Ti sites with distinct oxidation activity and (ii) being able to form a strong O-O moiety (which reduces overbonding at Ti sites), which is a direct consequence of (iii) having a labile lattice O that is able to directly participate in the reaction. Here, we demonstrate the improvement of the catalytic performance of a well-known and well-studied oxide catalyst through more modern methods of materials processing, predicted through first-principles theoretical modeling.

Helium Atom Scattering from Graphene Grown on Rh(111)

Graphene growth on transition-metal surfaces leads to an ordered superlattice that appears as a series of hills and valleys in scanning tunneling microscopy. On Rh(111), the graphene forms an incommensurate epitaxial overlayer; the crystallographic axes of the graphene are closely aligned with the Rh(111) surface lattice vectors, but the Rh and graphene lattice constants are significantly different. Thus, the C atoms sit over different positions of the Rh(111) lattice and their height above the Rh(111) surface varies, but the different binding sites also affect the local electronic structure. Scanning tunneling microscopy is affected by both of these factors. He scattering is more a measure of the geometrical corrugation, the variation of the height of the graphene sheet relative to the Rh(111) surface. We compare the results of He diffraction measurements with those of previous scanning tunneling microscopy measurements to assess the relative geometric and electronic corrugations.

Silicene-derived phases on Ag(111) substrate versus coverage:Ab initiostudies

Silicene is systematically investigated as an epitaxial overlayer on an Ag(111) substrate based on the ab initio density functional theory. The geometry and stability of five silicene-silver adsorbate systems with four coincidence lattices, $\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7}$ on $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}$, $3\ifmmode\times\else\texttimes\fi{}3$ on $4\ifmmode\times\else\texttimes\fi{}4$, $2\ifmmode\times\else\texttimes\fi{}2$ on $\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7}$, and $\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7}$ on $2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3}$, are related to the Si coverage, biaxial strain, and preparation conditions. Their phase diagram is calculated for varying chemical potential of the Si reservoir. The scanning tunneling microscopy images calculated for the optimized atomic geometries agree with those observed experimentally. The destruction of the original honeycomb symmetry and the strong adsorbate-substrate interaction significantly influence the electronic structure. Four peeled-off silicene sheets show conical linear bands, with small gaps. However, the band edges of the $3\ifmmode\times\else\texttimes\fi{}3$ on $4\ifmmode\times\else\texttimes\fi{}4$ geometry cannot be explained in terms of gap opening between Dirac cones for symmetry reasons. We confirm the conclusion that the linear bands observed by ARPES are due to folded Ag bands.

Effect of structural modulation and thickness of a graphene overlayer on the binding energy of the Rashba-type surface state of Ir(111)

The Ir(111) surface is known to host a surface state with a giant spin–orbit splitting due to the Rashba effect. This surface state is stable even in air when Ir is protected with an epitaxial graphene overlayer. In the present paper, we reveal an effect allowing one to tune the binding energy of this spin-split surface state up and down and demonstrate the practical application of this effect by two different approaches. The first approach is related to a decoration of the moiré pattern of single-layer graphene on Ir(111) by self-assembled nanoclusters of different compositions. The clusters locally pin graphene to the Ir substrate and enhance the amplitude of its structural corrugation, which, in turn, leads to an increase in the surface state binding energy. The second approach is related to the synthesis of few-layer graphene on Ir(111) by segregation of carbon. Additional graphene layers induce a shift of the Ir surface state towards lower binding energies and bring it almost to the Fermi level. Based on density functional calculations performed for the graphene/Ir(111) system, we show that in both cases the effect causing the binding energy shifts is intimately related to the distance between graphene and the Ir surface, which is subject to change due to deposition of clusters or by increasing the amount of graphene overlayers. In contrast, the observed spin–orbit splitting of the Ir(111) surface state remains remarkably robust and constant in all cases. Our theoretical analysis reveals that such stability can be explained by the localization properties of the Ir surface state that is a deep surface resonance.

Rapid liftoff of epitaxial thin films

Abstract

Simultaneous N-intercalation and N-doping of epitaxial graphene on 6H-SiC(0001) through thermal reactions with ammonia

Surface functionalization of epitaxial graphene overlayers on 6H-SiC(0001) has been attempted through thermal reactions in NH3. X-ray photoelectron spectroscopy and micro-region low energy electron diffraction results show that a significant amount of N is present at the NH3-treated graphene surface, which results in strong band bending at the SiC surface as well as decoupling of the graphene overlayers from the substrate. The majority of the surface N species can be removed by annealing in vacuum up to 850 A degrees C, weakening the surface band bending and resuming the strong coupling of graphene with the SiC surface. The desorbed N atoms can be attributed to the intercalated species between graphene and SiC. Low temperature scanning tunneling spectroscopy and density functional theory simulations confirm the presence of N dopants in the graphene lattice, which are in the form of graphitic substitution and can be stable above 850 A degrees C. This is the first report of simultaneous N intercalation and N doping of epitaxial graphene overlayers on SiC, and it may be employed to alter the surface physical and chemical properties of epitaxial graphene overlayers.

Position dependant critical thickness in finite epitaxial systems

During the growth of an epitaxial overlayer, interfacial misfit dislocations become energetically favourable on exceeding the critical thickness. In the case of finite systems (i.e. either the substrate or the epitaxial overlayer is finite), different positions along the interface are not equivalent and the critical thickness will be position dependent. Minimum of these thickness values can be visualized as the global critical thickness. The current work aims at simulating the stress state of a growing epitaxial overlayer on a finite substrate using finite element method and further use the numerical model to calculate the position dependent critical thickness along the interface. Eigenstrains will be imposed in selected regions in the domain towards this end. The variation of shear stress along the interface will be computed from the model to understand the issues related to the mechanism of formation of misfit dislocations. This includes the important question: “why interfacial dislocations cannot ‘punch-in’ directly from the free lateral surface?” The simulation methodology and associated concepts can readily be extended to other finite epitaxial systems like stripes and islands.

Finite Substrate Effects on Critical Thickness in Epitaxial Systems

During the growth of an epitaxial overlayer on a thick substrate (GeSi on Si), an interfacial misfit dislocation becomes energetically favourable on exceeding the critical thickness. In substrates of finite thickness, the value of critical thickness is altered with respect to thick substrates. Thin substrates can bend and partially relax the coherency stresses, thus contributing to the altered value of the critical thickness. The current work aims at simulating the stress state of a growing finite epitaxial overlayer on a substrate of finite thickness, using finite element method. The numerical model is used to calculate the critical thickness for substrates with finite thickness. Eigenstrains will be imposed in selected regions in the domain towards this end. Size of the substrate for which it is not energetically favourable to accommodate a misfit dislocation is determined from the simulations (i.e. the system remains coherent for substrates below this thickness). Important effects arising in the transition regime of substrate thicknesses are also investigated.

Anomalous magneto-optical behavior of uniaxial Co/CoO bilayer films

We study the magneto-optical properties of thin epitaxially grown Co/CoO bilayer films with in-plane uniaxial anisotropy by means of Magneto-Optical Kerr Effect (MOKE) measurements. The magneto-optical response obtained from these samples exhibits a surprising and atypical behavior, in that the polarizer setting, at which signal inversion occurs, is a function of the sample orientation. This behavior even occurs for identical orientations of the magnetization with respect to the plane-of-incidence. Using Generalized Magneto-Optical Ellipsometry, we show that the origin of this behavior is the presence of optical anisotropy, which we attribute to the strained epitaxial CoO overlayer in our samples.

First-principles studies of AlSb, GaSb, and InSb on Si(111) and Si(100)

We study the energetics and the atomic structures of MSb, where M is Al, Ga, or In, monolayers or thicker layers on Si(111)1×1 substrates and Si(100)2×1 substrates from first-principles. The calculated surface energies of MSb on Si(111) differ less than those of MAs on Si(111), for each M. Si(100)2×1:Sb is the most stable for each M among Si(100)2×1, Si(100)2×1:Sb, Si(100)2×1:(MSb), and Si(100)2×1:M. The relative surface energies of thicker epitaxial overlayer films on Si(100) are larger.

Energetics of island formation of AlAs, GaAs, and InAs on Si(100)

We study the energetics of island formation of AlAs, GaAs, and InAs on Si(100)2×1 substrates from first-principles. Si(100)2×1:As is stable under As-rich conditions in all cases. Si(100)2×1:(AlAs) and Si(100)2×1:(GaAs) are stable under Al-rich and Ga-rich conditions, respectively. However, the surface energy of Si(100)2×1:InAs is higher than that of Si(100)2×1:As under In-rich conditions. The energies of thicker epitaxial overlayer films of AlAs, GaAs, and InAs are predicted to be higher than the corresponding energies of these monolayer films.

Hydrogen electrocatalysis on overlayers of rhodium over gold and palladium substrates—more active than platinum?

We have investigated the stability and catalytic activity of epitaxial overlayers of rhodium on Au(111) and Pd(111). Both surfaces show a strong affinity for hydrogen. We have calculated the energy of adsorption both for a strongly and a more weakly adsorbed species; the latter is the intermediate in the hydrogen evolution reaction. Both the energy of activation for hydrogen adsorption (Volmer reaction) and hydrogen recombination (Tafel reaction) are very low, suggesting that these overlayers are excellent catalysts.

XPS investigation of ion beam induced conversion of GaAs(0 0 1) surface into GaN overlayer

For the advance of GaN based optoelectronic devices, one of the major barriers has been the high defect density in GaN thin films, due to lattice parameter and thermal expansion incompatibility with conventional substrates. Of late, efforts are focused in fine tuning epitaxial growth and in search for a low temperature method of forming low defect GaN with zincblende structure, by a method compatible to the molecular beam epitaxy process. In principle, to grow zincblende GaN the substrate should have four-fold symmetry and thus zincblende GaN has been prepared on several substrates including Si, 3C–SiC, GaP, MgO, and on GaAs(0 0 1). The iso-structure and a common shared element make the epitaxial growth of GaN on GaAs(0 0 1) feasible and useful. In this study ion-induced conversion of GaAs(0 0 1) surface into GaN at room temperature is optimized. At the outset a Ga-rich surface is formed by Ar + ion bombardment. Nitrogen ion bombardment of the Ga-rich GaAs surface is performed by using 2–4 keV energy and fluence ranging from 3 × 10 13 ions/cm 2 to 1 × 10 18 ions/cm 2. Formation of surface GaN is manifested as chemical shift. In situ core level and true secondary electron emission spectra by X-ray photoelectron spectroscopy are monitored to observe the chemical and electronic property changes. Using XPS line shape analysis by deconvolution into chemical state, we report that 3 keV N 2 + ions and 7.2 × 10 17 ions/cm 2 are the optimal energy and fluence, respectively, for the nitridation of GaAs(0 0 1) surface at room temperature. The measurement of electron emission of the interface shows the dependence of work function to the chemical composition of the interface. Depth profile study by using Ar + ion sputtering, shows that a stoichiometric GaN of 1 nm thickness forms on the surface. This, room temperature and molecular beam epitaxy compatible, method of forming GaN temperature can serve as an excellent template for growing low defect GaN epitaxial overlayers.

Quantum oscillations in surface properties

Many properties of metallic thin films have been shown to oscillate with film thickness due to quantum size effects, i.e. the confinement of electrons inside epitaxial metal overlayers which causes quantization of the electronic states. This is a very general phenomenon and it affects both bulk properties of the films, such as resistivity or superconducting transition temperatures, and surface properties, such as chemical reactivity, diffusivity, thermal stability, i.e surface roughening transitions. In this paper we describe some of these thickness-dependent properties which affect the stability of nanostructures and allow us to tailor their properties. We shall concentrate in the paradigmatic example of thin films of Pb grown on metallic and semiconducting substrates and how one can achieve the growth of highly perfect, atomically flat, epitaxial films on different substrates, due to the kinetic constrains imposed by the presence of QSE, a topic increasingly important in the production of nanoscale quantum devices.

Graphene on metal surfaces

The article reviews work on graphene monolayers adsorbed on metal surfaces. Graphene layers on metals have been prepared by surface segregation of carbon and by decomposition of hydrocarbons. The films are often not rotationally aligned to the metal surface. However, for a number of hexagonally close-packed surfaces perfectly ordered epitaxial overlayers have been obtained, with domains larger than the terraces of the metal substrate. In most cases the well-ordered overlayers display moiré structures with large periodicities, resulting from the lattice mismatch between graphene and the underlying metal. These structures are connected with a buckling of the graphene layer indicating local variations of the binding to the metal. For the metal–graphene spacings values between approximately 2.1 and 3.8 Å have been found, depending on the metal. Reasons for these strong variations are not yet clear, but there are indications that the systems fall into two classes that differ qualitatively with respect to the metal/graphene interaction. These variations are also reflected by the electronic structure. There are metal–graphene systems in which the π band is significantly downshifted in energy compared to the free-standing graphene, and a band gap of order eV has opened at the K ¯ point of the Brillouin zone. In other systems, the electronic structure of free-standing graphene is almost intact. The perfectness of the epitaxial moiré phases offers promising applications, e.g., as templates for nanostructures.

Growth of ultra-thin cerium oxide layers on Cu(1 1 1)

The reactive vacuum deposition of CeO 2 on Cu(1 1 1) surface in oxygen atmosphere provides high quality epitaxial ceria overlayers. We report the growth characteristics of Ce oxide, the structures, and the temperature stability of the oxide phases as investigated by low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy. We find that Ce oxide on the Cu(1 1 1) grows initially in the form of islands giving sharp hexagonal LEED pattern of the CeO 2(1 1 1) structure corresponding to the (1.5 × 1.5) structure. The CeO 2–Cu(1 1 1) films exhibited mixed valence states and temperature dependent CeO 2–Ce 2O 3 transition above 900 K due to the vacuum annealing. The transition progressed more rapidly at the surface, probably by formation of oxygen vacancies.

Zn Adsorption on Pd(111): ZnO and PdZn Alloy Formation

The adsorption and thermal desorption of Zn and ZnO on Pd(111) was studied in the temperature range between 300 and 1300 K with TDS, LEED, and CO adsorption measurements. At temperatures below 400 K, multilayer growth of Zn metal on the Pd(111) surface takes place. At a coverage of 0.75 ML of Zn, a p(2 x 2)-3Zn LEED structure is observed. Increasing the coverage to 3 ML results in a (1 x 1) LEED pattern arising from an ordered Zn multilayer on Pd(111). Thermal desorption of the Zn multilayer state leads to two distinct Zn desorption peaks: a low-temperature desorption peak (400-650 K) arising from upper Zn layers and a second peak (800-1300 K) originating from the residual 1 ML Zn overlayer, which is more strongly bound to the Pd(111) surface and blocks CO adsorption completely. Above 650 K, this Zn adlayer diffuses into the subsurface region and the surface is depleted in Zn, as can be deduced from an increased amount of CO adsorption sites. Deposition of >3 ML of Zn at 750 K leads to the formation of a well-ordered Pd-Zn alloy exhibiting a (6 x 4 square root 3/3)rect. LEED structure. CO adsorption measurements on this surface alloy indicate a high Pd surface concentration and a strong reduction of the CO adsorption energy. Deposition of Zn at T > 373 K in 10(-6) mbar of O2 leads to the formation of an epitaxial (6 x 6) ZnO overlayer on Pd(111). Dissociative desorption of ZnO from this overlayer occurs quantitatively both with respect to Zn and O2 above 750 K, providing a reliable calibration for both ZnO, Zn, and oxygen coverage.