Surface Review and Letters

We report the generation and detection of bipolar organic electroluminescence of porphyrin molecules from a nanoscale junction in an ultrahigh vacuum scanning tunneling microscope (STM). Clear molecular fluorescence from porphyrin molecules near metal substrates has been realized through highly localized electrical excitation of molecules in proximity to a sharp tip apex. The molecular origin of the luminescence, arising from the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) radiative transitions of neutral molecules, is clearly established by the observed well-defined vibrationally resolved fluorescence spectra that match perfectly with conventional photoluminescence data from molecular thin films. The molecules fluoresce at low onset voltages for both bias polarities, presenting an example of bipolar organic electroluminescence at the nanoscale. Such bipolar operation suggests a double-barrier model for electron transport, with hot electron injection into unoccupied states of molecules in both polarities. The optical behavior of molecules in the tunnel junction is also found sensitive to the electronic properties of molecules and energy level alignment at the interface. These results offer new information to the optoelectronic behavior of molecules in a nanoscopic environment and may open up new routes to the development of single-molecule science and molecular scale electronics.

We present and discuss the results of calculations of the CaF₂ bulk and surface electronic structure. Our results show, that the best agreement with experiment for the optical band gap (12.1 eV) can be obtained using a hybrid Hartree–Fock and density functional theory exchange functional, in which Hartree–Fock exchange is mixed with density functional theory exchange functionals, using Becke's three-parameter method, combined with the nonlocal correlation functionals by Perdew and Wang (10.96 eV). We also present calculations of CaF₂(111), (110), and (100) surfaces. Our calculated surface energies confirm that the CaF₂(111) surface is the most stable one, in agreement with the experiment.

X-rays are ideal to study the structure of crystals due to their weak interaction with matter and in most cases allow a quantitative analysis using kinematical theory. To study the incorporation of atoms during crystal growth and to analyze the kinetics on the crystal surface high primary beam intensities available at synchrotrons are required. Our studies of the molecular beam epitaxy growth of III–V semiconductors reveal that, despite their similarity in crystal structure, the surface kinetics of GaAs(001), InAs(001) and GaSb(001) differ strongly. GaAs shows an unexpectedly large coarsening exponent outside the predicted range of Ostwald ripening models during recovery. GaSb exhibits dramatically different surface morphology variations during growth and recovery. Overgrowth of GaAs by epitaxial MnAs demonstrates the ability of X-ray diffraction to follow an interface as it is buried during heteroepitaxy, which is not possible by reflection high-energy electron diffraction.

Thin films of single c-domain/single crystal (PbMg_1/3Nb_2/3O₃)_1-x(PbTiO₃)_x, x = 0–0.4 (PMNT) were heteroepitaxially grown on (001)SrTiO₃ and (001)MgO substrates by magnetron sputtering using a quenching process. The lattice parameters of the quenched PMNT thin films were almost the same to the bulk values independently to the lattice parameters of substrates. The quenched PMNT thin films showed stress free structural properties, although the crystal structure of thin films is modified from bulk PMNT. The electromechanical properties are the same to the bulk single c-domain single crystals.

The influence of substrate rotary speed, species of reaction gases and flows on nanoindentation, nanoscratch fracture, and residual stress were explored on reactive sputtered nanoscale CrN/ZrN multilayered coatings. Hardness and critical fracture load as high as 32 GPa and 85 mN with desirable compressive stress were achieved for this model. A proper percentage of NH₃ in N₂ reaction gas was also proved to be of benefit to synthesize high-hard and fracture-resistant CrN/ZrN coatings. The low-angle XRD patterns provided the layer modulation period of the coatings. A marked polycrystallite of two-cubic NaCl phase CrN + ZrN as well as probably hexagonal Cr₂N with small modulation period corresponded to the enhanced mechanical properties.

We study the electronic states and current–voltage (I–V) characteristics of junction systems with atomic-scale nanocontacts as a function of the distance between electrodes by using the first-principles recursion-transfer-matrix (RTM) method combined with nonequilibrium Green function (NEGF) method. We observe a strong nonlinear behavior in the I–V characteristics and correspondingly a gap structure emerges in the differential conductance. We find that such a nonlinear behavior appears when the transport properties change from tunneling to ballistic regimes.

Adsorption of oxygen on α-Mo₂C(0001) is investigated with Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and angle-resolved photoemission spectroscopy (ARPES) utilizing synchrotron radiation. It is found that C KLL Auger peak intensity does not change during O₂ exposure, indicating that the depletion of C atoms does not proceed. It is deduced from ARPES and LEED results that adsorbed oxygen atoms from a well-ordered (1 × 1) lattice on the α-Mo₂C(0001) surface. The ARPES study shows that oxygen adsorption induces a peculiar state around Fermi level (E_F). Off-normal-emission measurements prove that the state is a half-filled metallic state.

We studied adsorption and reaction of nitric oxide on Si(111)7 × 7, 5 × 2-Au, , , and 6 × 6-Au surfaces with low-energy electron diffraction, Auger electron spectroscopy, and scanning tunneling microscopy (STM). We found that NO gas reacts most strongly with the 7 × 7 surface, strongly with the 5 × 2-Au surface, and little with the other Si(111)-Au surfaces. STM results indicated that the NO exposure removes adatoms and erodes the row structure on the 5 × 2-Au surface at room temperature.

The electronic structures of L-cysteine adsorbed on Au(111)/Si(111) and surfaces were studied by using X-ray photoelectron spectroscopy (XPS). L-cysteine multilayers on Au(111)/Si(111) and surfaces were prepared under UHV condition. L-cysteine monolayers were obtained by annealing such multilayer sample up to 130°C. In the case of the Au(111)/Si(111) surface, we observed no chemical shift and no change in the peak intensity ratios among C1s, N1s, O1s and S 2p before and after annealing. On the other hand, the component attributed to COOH group in C1s spectra decreased after annealing. In addition, O1s XPS spectra of monolayer on the surface was composed of two distinct peaks, which indicates that there are two kinds of O atoms in different conformations. These observations indicate that the COOH group adsorbs directly to a surface atoms and dissociate by the interaction with the substrate Si(111) trimer atoms.

We have investigated the effect of Bi on the heteroepitaxial growth of Co on Cu by reflection high-energy electron diffraction (RHEED) measurements. It was found that Bi enhanced the layer-by-layer growth of Co on the Cu(111) surfaces at 100°C. The dependence of the growth on Bi layer thickness suggested that there existed a suitable amount of Bi surfactant layer that enhanced smoother layered growth. On the contrary, for the case of Co growth on Cu(100), Bi depressed the layer-by-layer growth of Co on Cu(100). The surface segregation effect of Bi was also studied by Auger electron spectroscopy (AES).

Three-dimensional atomic arrangements of ultra thin Fe silicide films were directly revealed. By using circularly polarized light with opposite helicities, forward focusing peaks with their positions shifted in photoelectron angular distribution (PEAD) patterns can be obtained. We successfully observed the PEAD patterns of ultra thin Fe silicide films from different core levels of Fe and Si atoms by display-type spherical mirror analyzer (DIANA). The element selective stereo photographs indicate similar hexagonal atomic arrangement with three-fold symmetry for Si and Fe atoms, which is consistent with a model based on a CsCl-type structure with B-type stacking. This is the first observation of stereoscopic atomic arrangements for ultra thin compound films, which implies that the stereo atomscope is very powerful structure analysis tool also for complex structures on surface.

Chlorine is one of the most used species to produce n-type ZnSe epilayers. In this paper, we present Secondary Ion Mass Spectrometry (SIMS) profiles of a series of Cl-doped ZnSe samples, which were grown by Molecular Beam Epitaxy (MBE) technique on GaAs substrates. These profiles have been used to examine the limitation of SIMS analysis of narrow Cl-delta layers. In order to convert SIMS raw data to quantified data, the depth profile from a Cl-implanted standard sample has been used to estimate the "useful ion yield" of chlorine and thus the instrumental sensitivity for chlorine in a ZnSe matrix. The "useful ion yield" and detection limit of chlorine in the ZnSe host matrix were calculated to be 4.7 × 10^-7 and 5 × 10¹⁷ atoms/cm³, respectively.

Transparent and conducting oxide films find many applications because of their excellent properties such as high optical transparency, low surface resistance, high infrared reflectance, etc. Realization of these properties depend upon the choice of the deposition technique and the control of deposition parameters. In this paper, we report the preparation of highly transparent and conducting films of indium oxide (In₂O₃) and indium tin oxide (ITO) by activated reactive evaporation on glass substrates. These films were deposited by evaporating pure indium and 90% In + 10% Sn alloy using an electron gun in the presence of oxygen ions at ambient temperature. Films of different thickness have been prepared and their optical, electrical and structural properties are studied. In₂O₃ films showed higher transparency (90%) compared to ITO films (85%) but the electrical resistivity was observed to be little higher (2.5 × 10^-3 Ω cm) compared to ITO films (6 × 10^-4 Ωcm). Hall measurements on aged ITO films gave the charge density of 3 × 10²⁰ per cm³ and mobility 35.6 cm²/V-s. The refractive index and extinction coefficient were found to be around 2.0 and 0.005 for ITO films and 2.10 and 0.001 for In₂O₃ films at 550 nm respectively. ITO and In₂O₃ films were amorphous in nature for lesser thickness, but for thicker films, the partial crystallinity was observed.

The growth mode and electronic structure of the vapor-deposited Ag overlayer on the surface is investigated by angle-resolved photoemission spectroscopy utilizing synchrotron radiation. The coverage dependent measurements of the two-dimensional band structure by the Ag 4d states reveal that the bands with a dispersing feature are developed from the low coverage region, suggesting that the Ag adatoms aggregate to form clusters with a laterally ordered structure. The Ag clusters grow in size with increasing the Ag coverage whereas the ordered atomic structure of the Ag cluster is preserved. From the analysis of the Ag 5sp band, the Ag clusters have a nonmetallic electronic structure at low coverages, and the metallic nature is developed as the size of the clusters becomes enlarged.

Nanoscale frictional phenomena at solid–solid surfaces with lubricants are studied numerically using a lattice model which consists of two rigid substrates and a monolayer of lubricant molecules. The maximum static frictional force, which works on the driven upper solid, is always finite and obeys a certain scaling relation. The lubricant layer, however, shows a kind of phase transition from a pinned state to free sliding state when the strength of the interaction potential with the substrates decreases. We discuss the peculiar pinning mechanism of the upper substrate in the presence of a lubricant monolayer.

Germanium was deposited onto highly oriented pyrolytic graphite (HOPG) with and without antimony in ultra-high vacuum. The surface morphology was analyzed using in situ scanning tunneling microscopy (STM) at room temperature (RT). The film grows exclusively in 3D island mode and was affected significantly by substrate defects. At initial stage, nucleation of cluster occurred at step edges and defect sites. Later, we found various types of Ge nanostructures on HOPG in different deposition conditions and stages, including cluster chains, cluster islands, nanowires, and double layer ramified islands at RT. Compact Ge islands were observed when depositing at a substrate temperature of 450 K or after an annealing at 600 K following RT deposition. In addition, the pre-deposited Sb on graphite enhances the sticking probability and suppresses the surface diffusion of Ge atoms, resulting in a significant increase in Ge cluster island density on HOPG terraces.

Amphiphilic comb-shaped 2-hydroxyethyl methacrylate-graft-poly(styrene)-block-poly(poly (ethylene glycol) monomethacrylate) (P(HEMA)–g–PS–b–P(PEGMA)) copolymer brushes, covalently tethered on Si(100) substrate, have been successfully prepared by (i) UV-induced hydrosilylation of 4-vinylbenzyl chloride (VBC) with the hydrogen-terminated Si(100) (Si–VBC surface), (ii) surface-initiated atom transfer radical polymerization (ATRP) of 2-hydroxyethyl methacrylate (HEMA) to give the Si–VBC–g–P(HEMA) surface, (iii) coupling of 2-bromoisobutyryl bromide with the HEMA units via esterification to provide the comb-shaped macroinitiators (Si–VBC–g–P(HEMA)–R₃Br) for the subsequent ATRP, (iv) surface-initiated ATRP of styrene (St) to produce the Si–VBC–g–P(HEMA)–g–PS surface, and (v) the persisting of an active PS chain end can be used as the initiator for the subsequent ATRP of poly(ethylene glycol) monomethacrylate (PEGMA) to give the amphiphilic Si–VBC–g–P(HEMA)–g–PS–b–P(PEGMA) brush surface. The chemical composition and functionality of the silicon surface were tailored by the well-defined comb-shaped PS and P (PEGMA) brushes.

Poly(vinylidene fluoride) (PVDF, –(CH₂–CF₂)_n–) shows the effective H⁺ desorption induced by the irradiation of photon corresponding to the transition from carbon (C) 1s to σ(C–H)*. In order to clarify the effect of the C–H bond scission by the irradiation, near-edge X-ray absorption fine structure (NEXAFS) spectra and the kinetic energy (E_k) distribution of desorbed ion were observed. By the irradiation of photon near C 1s region, a new peak appears in the C 1s NEXAFS spectra at photon energy of 285 eV, which is about 3 eV lower than that of the lowest peak in the NEXAFS spectrum of the pristine PVDF film. The appearance of the lowest NEXAFS peak of irradiated PVDF film is assigned to the transition to π*. It indicates that the irradiation of photons near C 1s region introduces carbon–carbon double bonds into the backbone chain of PVDF. At early stage of X-ray exposure the yield of desorbed ion with low E_k (~ 2 eV) decreases rapidly. The ion with low E_k is assigned to H⁺ desorbed from the sp³-hybrid state, which is characteristics of the pristine PVDF. It indicates that formation of double bonds in PVDF backbone chain makes the number of sp³-hybrid state decrease. This variation occurs by irradiation of photons corresponding to the transition from C 1s to σ(C–H)* more rapidly than that to the transition to σ(C–F)*.

The interface between biaxially oriented poly (ethylene terephthalate) (PET) films and poly (styrene-co-maleic anhydride) (PSMA) was reinforced by nitrogen plasma treatment of PET film and subsequent annealing treatment of the PET/PSMA bi-material. The fracture toughness, Gc, of the interface was quantitatively measured using an asymmetric double cantilever beam test (ADCB). X-ray photoelectron spectrometry (XPS) was used to measure the change in the surface composition of PET films upon plasma treatment and correlate the fracture toughness of the interface. The fracture energy of PET/PSMA interface is significantly enhanced by annealing the plasma treated PET with PSMA at a temperature greater than the glass transition temperature of PSMA (~ 120°C). At an annealing temperature of 150°C, Gc increases with increasing plasma treatment time and reaches a plateau value of ~ 100–120 J/m², a two order of magnitude increase in Gc compared with that of samples annealed at 130°C. The enhancement of the adhesion is resulted from the in-situ formation of copolymers due to reaction between amine functional groups from the plasma treatment and anhydride groups from PSMA. For plasma treatment time < 10 s, scanning electron microscope (SEM) measurement show that the fracture surface is relatively smooth indicating an interfacial failure between PET/PSMA. With increasing plasma treatment time and therefore increasing the amount of nitrogen functional groups on PET surface, large plastic deformation takes place at the PET/PSMA interface. For treatment time ≥ 100–150 s, the PET/PSMA interface becomes stronger than PET bulk material and consequently crack deviates from the interface and the failure occurs within the PET film. The interlayer fracture energy of a biaxially oriented PET film can thus be quantitatively measured with a Gc value of roughly 120 J/m².

To study some of the interfacial properties of PtSi/Si diodes, Schottky structures were fabricated on (100) crystalline silicon substrates by conventional thermal evaporation of Pt on Si followed by annealing at different temperatures (from 400°C to 700°C) to form PtSi. The PtSi/n-Si diodes, all yielded Schottky barrier (SB) heights that are remarkably temperature dependent. The temperature range (20–290 K) over which the I–V characteristics were measured in the present study is broader with a much lower limit (20 K), than what is usually reported in literature. These variations in the barrier height are adequately interpreted by introducing spatial inhomogeneity into the barrier potential with a Gaussian distribution having a mean barrier of 0.76 eV and a standard deviation of 30 meV. Multi-frequency capacitance–voltage measurements suggest that the barrier is primarily controlled by the properties of the silicide–silicon interface. The forward C–V characteristics, in particular, show small peaks at low frequencies that can be ascribed to interface states rather than to a series resistance effect.

In order to study the mass transfer phenomena for water vapor absorbed by triethylene glycol (TEG) solution and to elucidate effect of interfacial disturbance on absorption performance, the interfacial phenomena for water drop instilling on the surface of TEG solution and mass transfer process were observed and operated in this study. Besides liquid and gas flow rates, the interfacial disturbance can also affect the mass transfer performance in the absorption system with continuous liquid phase, and the advanced mass transfer phenomena must be discussed from the interface of matter. Therefore, the surface tensions of desiccant solutions were measured to analyze the disturbed phenomena of water drop instilling on the surface of TEG solution. Since the Marangoni-Index (MI) means the maximum surface tension difference on the surface of liquid layer, the values of MI were calculated and compared with the mass transfer performance of packed-bed absorber. Generally speaking, the interfacial disturbance, resulted from the surface tension difference, would promote the contacting area of gas and liquid phases. The observation of water drop instilling on surface of TEG solution found that the interfacial disturbance existed between water drop and TEG solution, and the interfacial instability was found in the peripheral region of the water drop. On the other hand, the values of MI were increased by the increased TEG concentrations, and it can be deduced that the surface tension effect should be significant in the higher concentration of TEG solution and the interfacial disturbance would be heavier above 95 wt.% TEG solutions. Finally, the absorption performance for 95 and 96 wt.% TEG solutions were much larger than that of 90 and 88.6 wt.% TEG solutions in the operation of packed-bed absorber.

Growth of Ge, Al and Sb on highly oriented pyrolytic graphite (HOPG) was systematically investigated using in situ scanning tunneling microscopy (STM). At room temperature (RT), three dimensional (3D) clusters of all three elements nucleate and grow at the step edges and defect sites of HOPG. The clusters of Al and Ge form chains, while Sb islands are mostly isolated. With further deposition at RT, Al clusters grow and coarsen into faceted islands with craters on the top (111) facets, whereas ramified single- and double-layer cluster islands are observed for Ge. When deposited or annealed at T ≥ 175°C, Ge forms crystallites but with randomly oriented facets. As spherical Sb islands grow beyond certain size, (111) facets appear on the top. Additionally, crystalline 2D films and 1D nanorods are observed for Sb deposited at RT. At T ≈ 100°C and higher flux, only the 2D and 1D Sb islands are formed. These different growth behaviors reflect the unique nature in which the atoms (molecules), clusters and crystallites of each element interact with HOPG surface and with each other.

Gold is widely used in nanotechnology, for example as a substrate in forming self-assembled monolayers or as nanoparticles for their unique optical and chemical properties. In this paper we give an overview of the properties of gold relevant to its potential application in molecular-scale devices and present some of our recent computational predictions. Density functional calculations of molecular adsorption onto gold surfaces were used to investigate the effect of surface symmetry and identify new linking schemes for self-assembled monolayers. Adsorption energies of methylthiolate (SCH₃) onto the (111), (100) and (110) surfaces of gold are predicted to be 39.3, 48.4 and 51.1 kcal/mol respectively and demonstrate that selective functionalization of the surfaces is possible. Phosphine molecules with at least two hydrogen atoms substituted for methyl groups are predicted to form Au–P surface bonds with energies of about 13–20 kcal/mol.

We present the first-principles calculation of transport properties of a semiconducting carbon nanotube connected to aluminum metallic electrodes. The transmission spectrum of the nanotube shows a decrease in the band-gap energy level but has a finite value due to the hybridization of the metallic states of the electrodes.

Crosslinked poly(4-vinylbenzyl chloride) (PVBC) microspheres of about 550 μm in diameter were first synthesized by suspension copolymerization of 4-vinylbenzyl chloride (VBC) in the presence of a crosslinking agent, ethylene glycol dimethacrylate (EGDMA). Subsequent surface-initiated atom transfer radical polymerization (ATRP) of 4-vinylpyridine (4VP), using the VBC units of PVBC on the microsphere surface as the macroinitiators, produced well-defined (nearly monodisperse) and covalently tethered poly(4-vinylpyridine) (P4VP) brushes. Quaternization of the tertiary amine groups of the P4VP brushes with hexyl bromide gave rise to a high concentration of quaternary ammonium salt (QAS) on the microsphere surfaces. The chemical composition of the microsphere surfaces at various stages of surface modification was characterized by X-ray photoelectron spectroscopy (XPS). The bactericidal effect of the QAS-functionalized microspheres on Escherichia coli (E. coli) was demonstrated. No significant loss in the surface bactericidal activity of the microsphere was observed during repeated applications.

Polypyrrole (PPy) films have been successfully electrodeposited on stainless steel substrates in aqueous solution. In this work, three systems of electrolytes were studied: oxalic acid, dodecylbenzenesulfonic acid (DBSA) and a mixture of oxalic acid and DBSA. A combination of XPS and TOF–SIMS revealed the formation of an iron oxalate layer at the interface between the oxalic acid-doped PPy (PPy(Ox)) and stainless steel and a thin layer of DBSA was observed at the interface between DBSA-doped PPy (PPy(DBSA)) and stainless steel. Similar to the PPy(Ox) system, an iron oxalate was also present at the co-doped PPy/stainless steel interface. Cyclic voltammetry indicated that an iron oxalate layer initially formed at the surface of the stainless steel when the co-doping system was used. The adhesion strength and corrosion performance of the PPy coating on stainless steel were evaluated by lap shear tests and an anodic potentiodynamic polarization technique, respectively. The co-doped PPy-coated stainless steel exhibited the best adhesion and a significant shift of corrosion potential to the positive direction. This finding opens the possibility for the co-doped PPy coating to be deployed as a strongly adherent corrosion inhibitor by using a simple one-step electropolymerization process.

Several different species of ions, Au, Fe, Ag, Ti and Si, were implanted into austenite stainless steel sheets at an energy of 3 MeV respectively. The martensite transformation induced with the ion implantation was investigated with transmission electron microscopy equipped with an energy dispersive X-ray spectrometer. The residual stresses induced with ion implantation were evaluated by the curvature technique. The effects of irradiation doses and ion species on the residual stress near surface induced by ion implantation were investigated. It is found that compressive residual stresses were induced by all the ions, and Fe and Au ions, among these ions, produced a higher level of residual stress at the same implantation. It shows that ion implantation can be employed to control and modify the internal stress near surface by changing the irradiation dose and selecting ion specie of the ion implantation.