Showing papers on "Overlayer published in 2001"

Epitaxy and Molecular Organization on Solid Substrates

Abstract: The recent emergence of molecular films as candidates for functional electronic materials has prompted numerous investigations of the underlying mechanisms responsible for their structure and formation. This review describes the role of epitaxy in molecular organization on crystalline substrates. A much-needed grammar of epitaxy is presented that classifies the various modes of epitaxy according to transformation matrices that relate the overlayer lattice to the substrate lattice. The different modes of epitaxy can be organized hierarchically to reflect the balance of overlayer–substrate and molecule–molecule energies. In the case of molecular overlayers, the mismatch of overlayer and substrate symmetries commonly leads to coincident epitaxy in which some of the overlayer lattice points do not reside on substrate lattice points. Analyses of numerous reported epitaxial molecular films reveal that coincidence is quite common even though, based on overlayer–substrate interface energies alone, not as energetically favorable as commensurism. The prevalence of coincidence can be attributed to overlayer elastic constants, associated with molecule–molecule interactions within the overlayer, that are larger than the elastic constants of the overlayer–substrate interface. This condition facilitates prediction of the epitaxial configuration and overlayer structure through simple and comparatively efficient geometric modeling that does not require the input of potential energies, while revealing the role of phase coherence between the overlayer and substrate lattices.

Switchable mirrors based on nickel–magnesium films

Abstract: A new type of electrochromic mirror electrode based on reversible uptake of hydrogen in nickel magnesium alloy films is reported. Thin,magnesium-rich Ni-Mg films prepared on glass substrates by cosputtering from Ni and Mg targets are mirror-like in appearance and have low visible transmittance. Upon exposure to hydrogen gas or on reduction in alkaline electrolyte, the films take up hydrogen and become transparent. When hydrogen is removed, the mirror properties are recovered. The transition is believed to result from reversible formation of Mg2NiH4 and MgH2. A thin overlayer of palladium was found to enhance the kinetics of hydrogen insertion and extraction,and to protect the metal surface against oxidation.

Flat conduction-band alignment at the CdS/CuInSe2 thin-film solar-cell heterojunction

Abstract: By combining ultraviolet and x-ray photoelectron spectroscopy with inverse photoemission spectroscopy, we find that the conduction-band alignment at the CdS/CuInSe2 thin-film solar-cell heterojunction is flat (0.0±0.2 eV). Furthermore, we observe a valence-band offset of 0.8±0.2 eV. The electronic level alignment is dominated by (1) an unusually large surface band gap of the CuInSe2 thin film (1.4 eV), (2) by a reduced surface band gap of the CdS overlayer (2.2 eV) due to intermixing effects, and (3) by a general influence of the intermixing on the chemical state near the interface.

Surface-enhanced Raman scattering on uniform platinum-group overlayers: preparation by redox replacement of underpotential-deposited metals on gold.

Abstract: The preparation of Pt-group metal films on roughened gold electrodes by utilizing spontaneous redox replacement of an underpotential-deposited (upd) copper or lead monolayer with a Pt-group metal cation solute is described. The resulting films display intense surface-enhanced Raman scattering (SERS) for adsorbates bound to the overlayer and free from substrate interferences. This strategy provides a useful alternative, at least for platinum, to the constant-current electrodeposition method commonly utilized to prepare SERS-active Pt-group metal films (Zou, S.; Weaver, M. J. Anal. Chem. 1988, 70. 2387). Similarly to related earlier studies, the film uniformity (specifically, the absence or otherwise of residual Au “pinhole” sites) was tested by employing carbon monoxide, and also ethylene, as “probe” chemisorbates, since they yield vibrational frequencies on Au that are blue-shifted from the corresponding bands for adsorbate bound to Pt-group metal sites. While a single redox replacement of upd Cu with Pt(...

Method of creating a Pd-covered single-atom sharp W pyramidal tip: Mechanism and energetics of its formation

Abstract: Single-atom sharp pyramidal W tips, wrapped in a Pd overlayer, having atom-perfect wedges can be routinely and repeatedly created using a surface-science technique. This single-atom tip is thermally stable up to 1000 K because of the exceptionally large surface-energy anisotropy of the Pd covered W(111) surface. We conclude from atom-by-atom analysis that the W tip is covered with only one physical monolayer of Pd. We also investigate the mechanism and energetics of the atomic processes involved in its formation.

A Chiral Metal Surface.

Abstract: Pronounced surface restructuring leads to the formation of chiral kink sites when a chiral molecular overlayer (2,5,8,11,14,17-hexa-(tert-butyl)decacyclene) is adsorbed onto an extended, flat metal surface (Cu{110}). The process is proposed to happen as a result of the adsorbed molecule inducing chirality in the achiral surface.

Ab initio molecular dynamics simulation of the Ag(111)-water interface

Abstract: The results of a first-principles study of the structure of the Ag(111)–water interface are presented. The calculations were carried out using an ab initio Car–Parrinello molecular dynamics simulation within a pseudopotential formalism and the generalized gradient approximation to the exchange-correlation potential. Periodic five layer slabs covered with 48 water molecules were employed to simulate the structure of the interface. An analysis of the structural properties of the interface shows that the preferable places of residence of H2O molecules in the contact overlayer are the metal top sites. The electronic structure of the interface has also been explored. It was found that there is a strong coupling of the water overlayer with the metal crystal electronic states. However, the surface-state charge density is only slightly disturbed by the presence of water. The empty surface states are seen to not be quenched by the presence of water, which is in qualitative agreement with existing experiments. The electrons donated to the metal by the water fill the metal crystal electronic states, which is responsible for the dominant coupling of the metal with the water system.

Ab initio molecular dynamics simulation of the Cu(110)–water interface

Abstract: The results of a first principles simulation of the Cu(110)–water interface are presented. The calculations were carried out using ab initio Car–Parrinello molecular dynamics simulation within a pseudopotential formalism and the BLYP generalized gradient approximation to the exchange-correlation potential. An analysis of the structural properties of the interface shows that absorption of H2O molecules occurs on the metal top sites which is in agreement with experimental results. The electronic structure of the interface has also been explored. It was found that there is a strong coupling of the water overlayer with the metal crystal electronic states. However, the surface-states charge density is undisturbed by the presence of water. The empty surface states are seen to not be quenched by the presence of water, which is also in qualitative agreement with existing experiments.

Bonding mechanism and atomic geometry of an ordered hydroxyl overlayer on Pt(111).

Abstract: Exposing water to a (2 × 2)−O precovered Pt(111) surface at 100 K and subsequently annealing at 155 K led to the formation of a well-ordered (√3 × √3)R30° overlayer. The structure of this overlayer...

Probing interface electronic structure with overlayer quantum-well resonances: Al/Si(111).

Abstract: The dispersion of quantum-well resonances in ultrathin epitaxial Al films on Si(111) reveals energy- and wave vector-dependent reflection properties at the Al/Si interface. The substrate electronic structure strongly influences the phase shift of the electron waves upon reflection at the interface. Thus the details of the substrate electronic structure need to be taken into account for a complete analysis of metallic quantum-well resonances. Furthermore, the assumption of loss of parallel wave vector information upon reflection or transmission through a lattice-mismatched interface is challenged. The changes induced in the electronic structure of the overlayer can be used to probe the ground-state substrate band edges.

Patterning effect of a sol-gel TiO2 overlayer on the photocatalytic activity of a TiO2/SnO2 bilayer-type photocatalyst

Abstract: TiO2 films with a thickness of 75 ± 5 nm (anatase) were formed on SnO2-film (580 ± 80 nm) coated soda-lime glass substrates (SnO2/SL-glass) by a sol-gel method. Although the photocatalytic activity for CH3CHO oxidation (λex > 300 nm) significantly exceeded that of a standard TiO2/quartz sample, it decayed with illumination time (t) at t > 0.75 h. Stripes of anatase TiO2 films of 40 nm in thickness and 1 mm in width were prepared on the SnO2/SL-glass substrate in a 1-mm pitch by photolysis of an organically modified sol-gel film. The TiO2 patterning further increased the photocatalytic activity by a factor of 4.1 as compared to the non-patterned sample, and it was also maintained at 0 < t < 2 h. The flat band potentials of the TiO2 and SnO2 films are determined to be −0.34 and +0.07 V (vs. SHE), respectively, at pH = 7 by the Mott-Schottky plots. On the basis of the results, the outstanding patterning effects could be rationalized in terms of the vectorial charge separation at the interface between TiO2 and SnO2.

Interface engineering of a ZrO2/SiO2/Si layered structure by in situ reoxidation and its oxygen-pressure-dependent thermal stability

Abstract: Reactions at ZrO2/SiO2/Si interfaces during fabrication and postannealing have been studied in detail. The layered structures were fabricated by deposition of a thin Zr layer on a chemical oxide, followed by oxidation in an UHV chamber without air exposure (i.e., in situ reoxidation). On-line x-ray photoelectron spectroscopy was used to show that in situ reoxidation can be used for precisely designing interfacial structures. It was found that the thermal stability of ZrO2/SiO2/Si interfaces crucially depends on oxygen ambient; that is, while the interfaces are stable up to 900 °C under UHV conditions, annealing in 1×10−4 Torr oxygen results in formation of interfacial Zr silicate. Moreover, the ZrO2 overlayer was found to accelerate the interfacial oxidation reaction.

Enhanced effect of vacuum-deposited SiO2 overlayer on photo-induced hydrophilicity of TiO2 film

Abstract: Photo-induced hydrophilicity of SiO2/TiO2 multilayer film prepared by using the vacuum deposition method was investigated by means of water contact angle measurement. Using black light irradiation of the films centering at a wavelength of 365 nm, an extreme photo-induced hydrophilicity was achieved when the TiO2 film was covered by SiO2 overlayer ranging from 10 to 20 nm in thickness. These multilayer films exhibited much more extreme hydrophilicity than the TiO2 film without SiO2 overlayer. The surface analyses revealed that the enhanced photo-induced hydrophilic surface of the multilayer films exhibited an improved photo-catalytic activity towards decomposition of organic substances on their surfaces. It was found that significant growth of the SiOH group occurred in the uppermost surface of the SiO2 overlayer of the multilayer films through the depth profile measurement of TOF-SIMS. This result suggests that the photo-generated reactive species such as hole created in the TiO2 film may transmit the SiO2 layer to reach the surface. The enhanced photo-induced hydrophilicity of the films can be explained by a synergetic effect of the improved photo-catalytic activity of the multilayer film and the stable hydrophilicity of SiO2 itself.

Interfacial chemistry and the performance of bromine-etched CdZnTe radiation detector devices

Abstract: The interfacial chemistry and composition of Pt electrodes sputter deposited on bromine-etched CdZnTe surfaces was studied by X-ray photoelectron spectroscopy. The interfacial composition of a functioning and a nonfunctioning CdZnTe detector shows significant differences. The degree of cation out-diffusion into the Pt overlayer and the in-diffusion of Pt into the CdZnTe correlate with the degree of oxidation found at the metal-semiconductor interface. Most of the oxide present at the interface was found to be TeO/sub 2/. The results suggest that the interdiffusion of the atoms and associated charges contribute to stoichiometric variations at the metal-semiconductor interface and influence the electrical performance of the devices.

Reliable Ti/Al and Ti/Al/Ni/Au ohmic contacts to n-type GaN formed by vacuum annealing

Abstract: In this work, we investigated Ti/Al and Ti/Al/Ni/Au ohmic contact metallizations to n-GaN which were formed by a vacuum annealing method. Our effort was focused on minimizing the oxidation of the Al, which limits the performance of the Al-based contacts. We have also investigated the effect of the Ni/Au overlayers on the performance and morphological characteristics of the contacts. Vacuum annealing was found to require temperatures similar to those used in halogen lamp rapid thermal annealing for forming ohmic contacts on n-type GaN, but with minimal oxidation of the Al surface. It was found that the Ni/Au overlayer on the Ti/Al reduces the specific contact resistivity values slightly and at the same time causes an upward shift, by ∼200 °C, in the contact formation temperature. For the Ti/Al and Ti/Al/Ni/Au on n-GaN doped at 1.1×1018 cm−3, a minimum specific contact resistivity of 1.2×10−5 Ω cm2 was achieved after 950 and 1100 °C anneals, respectively. Auger electron spectroscopic studies revealed that obtaining optimum ohmic contact resistance requires the penetration of Al through the Ti layer to reach the GaN surface in addition to N outdiffusing from the GaN surface and into the metal layers. Atomic force microscopy investigations indicated that the introduction of the Ni/Au overlayer on the Ti/Al had the effect of increasing the surface roughness after annealing.

Temperature stability of c-axis oriented LiNbO3/SiO2/Si thin film layered structures

Abstract: Theoretical calculations have been performed for the temperature stability of the c-axis oriented LiNbO3 thin film layered structures on passivated silicon (SiO2/Si) substrate with and without a non-piezoelectric SiO2 overlayer. The phase velocity, electromechanical coupling coefficient and temperature coefficient of delay (TCD) have been calculated. The thicknesses of various layers have been determined for optimum SAW performance with zero TCD. The presence of a non-piezoelectric SiO2 overlayer on LiNbO3 film is found to significantly enhance the coupling coefficient. The optimized results reveal that a high coupling coefficient of K2 = 3.45% and a zero TCD can be obtained in the SiO2/LiNbO3/SiO2/Si structure with a 0.235λ thick LiNbO3 layer sandwiched between 0.1λ thick SiO2 layers.

Quantitative determination of the metastability of flat Ag overlayers on GaAs(110).

Abstract: Atomically flat ultrathin Ag films on GaAs(110) can be formed through a kinetic pathway. However, such films are metastable and will transform to 3D islands upon high temperature annealing. Using scanning tunneling microscopy, we have measured quantitatively the layer-resolved metastability of flat Ag overlayers as they evolve toward their stable state, and deduced the corresponding kinetic barrier the system has to overcome in reaching the stable state. These results indicate that the metastability of the Ag overlayer is defined by the quantum nature of the conduction electrons confined within the overlayer.

Single crystalline silicon dioxide films on Mo(1 1 2)

Abstract: A preparation is reported which, for the first time, results in a thin, crystalline SiO 2 film on a Mo(1 1 2) single crystal. The procedure consists of repeated cycles of Si deposition and subsequent oxidation, followed by a final annealing procedure. LEED pictures of high contrast show a crystalline SiO 2 overlayer with a commensurate relationship to the Mo(1 1 2) substrate. Surface imperfections have been studied by SPA-LEED and a structure model, consistent with the appearance of antiphase domain boundaries as preferential disorder, is proposed. AES and XPS have been used to control film stoichiometry. A spatial dependence of the Si 4+ core level shift with distance from the interface plane is observed and well explained by image charge interaction across the interface. Furthermore, the theoretically predicted insensitivity of the Si 4+ core level shift with respect to the degree of crystallinity is experimentally verified for the first time. The wetting of the substrate by the film has been investigated by XPS and TDS. The results prove that the film covers the substrate completely.

Scanning tunneling spectroscopy of Na on Cu(111)

Abstract: Monolayer films of Na evaporated onto Cu(111) under UHV conditions are investigated with scanning tunneling microscopy and spectroscopy (STS) at low temperature. The STS data clearly reveal a two-dimensional overlayer state of Na on Cu(111). From STS and spectroscopic imaging, the binding energies and effective masses of this state are determined for various coverages. The state is demonstrated to be affected by spatial inhomogeneities such as Cu steps or local coverage variations. Drastic bias-dependent variations of apparent step heights and tunneling barrier heights are reported and interpreted in terms of the large contributions of the overlayer state to the tunneling current.

Functionalization of silicon step arrays II: Molecular orientation of alkanes and DNA

Abstract: Alkanes and DNA are adsorbed via thiol groups on ultrathin gold films that have been templated on stepped Si(111)7×7 surfaces. The orientation of the adsorbed molecules is determined by polarization-dependent near edge x-ray absorption fine structure spectroscopy from the C 1s and N 1s core levels. An anisotropy in the polar distribution is found which is consistent with models in which the molecules have a preferred tilt angle with respect to normal but a random azimuthal distribution. In order to obtain maximum coverage and an oriented overlayer it is necessary to deposit more than 3 nm of Au (12 monolayers).