Highly Oriented Pyrolytic Graphite Substrate Research Articles

Epitaxial growth of p-sexiphenyl film on highly oriented pyrolytic graphite surface studied by scanning tunneling microscopy

The epitaxial growth of p-sexiphenyl (C 36H 26, 6P) on highly oriented pyrolytic graphite (HOPG) surface has been investigated by scanning tunneling microscopy (STM). 6P molecules prefer epitaxial growth with the long axis along the [110] direction (armchair direction) of the HOPG substrate, with the unit cell parameters b 1 = 0.67 ± 0.06 nm, b 2 = 5.97 ± 0.06 nm and angle of 88 ± 3° between them. The relation of the 6P overlayer lattice vectors with the HOPG substrate has also been deduced, i.e. the 5 × 1 supercell is in a point-on-point commensurate relation with respect to the HOPG substrate surface.

Preparation of thin films of graphite-like B/C/N materials on Ni (111) and HOPG substrates

Thin films of graphite-like layered materials containing boron, carbon and nitrogen (B/C/N materials) were prepared on substrates of Ni (111) single crystal and highly oriented pyrolytic graphite (HOPG) by low pressure CVD method using acrylonitrile and boron trichloride as starting materials. Electron spectroscopy for chemical analysis (ESCA) and reflective high energy electron diffraction (RHEED) suggest that highly oriented and crystalline thin film of B/C/N material deposited on Ni (111) substrate having catalytic ability, while small amount of B/C/N film deposited on a part of HOPG substrate. Transmission electron microscopy (TEM) indicates the B/C/N thin film had a layered structure whose ab-axis was parallel to the Ni (111) surface. These results of ESCA, RHEED and TEM strongly suggest that the B/C/N film deposited on Ni (111) substrate in a manner of hetero-epitaxial growth.

Detection of Novel Gaseous States at the Highly Oriented Pyrolytic Graphite−Water Interface

We report a novel form of the gaseous state at the interface of water and highly oriented pyrolytic graphite (HOPG) that is induced by local supersaturation of gas. Such local supersaturation of gas next to the HOPG substrate can be achieved by (1) displacing an organic liquid with a gentle flow of water, (2) displacing cold water (approximately 0 degrees C) with a gentle flow of warm water (approximately 40 degrees C), or (3) preheating the HOPG substrate to approximately 80 degrees C before exposing it to water at room temperature. In addition to the spherical-cap-shaped nanobubbles reported by many researchers, flat (quasi-two-dimensional, pancake-like) gas layers and nanobubble-flat gas layer composites (spherical-cap-shaped nanobubbles sitting on top of the quasi-two-dimensional gas layers) were detected. These entities disappeared after the system was subjected to a moderate level of degassing (approximately 0.1 atm for 1.5 h), and they did not form when the liquids involved in the aforementioned displacing procedure (to induce local supersaturation of gas) had been predegassed (to approximately 0.1 atm). The stability and some physical properties of these newly found gaseous states are examined.

Study of the growth of NiO on highly oriented pyrolytic graphite by X-ray absorption spectroscopy

In this work, we present a X-ray absorption spectroscopy (XAS) study of the growth of NiO on highly oriented pyrolytic graphite (HOPG). NiO has been grown by reactive evaporation of metallic Ni in an oxygen atmosphere (2 × 10 −5 Torr) at room temperature. We paid special attention to the study of the early stages of growth. Both, Ni 2p and O 1s core-level XAS spectra were measured. For large NiO coverages, the spectra resemble that of a NiO single crystal, thus indicating the formation of a stoichiometric NiO thin film on the HOPG substrate. The Ni 2p XAS spectra remain similar during the whole growth process, indicating that Ni atoms are present in the high spin Ni 2+ form, as supported by multiplet calculations. In contrast, for low coverages the line-shape of the O 1s XAS spectra differ strongly from that of bulk NiO. Cluster calculations of the spectra in octahedral and pyramidal symmetries support the formation of nanometric planar NiO islands at the graphite steps as previously observed by atomic force microscopy (AFM) in this system.

Electrodeposition of Pd–Ag alloy nanowires on highly oriented pyrolytic graphite

This paper reports findings of an investigation of Pd–Ag alloy nanowires on the step edges of highly oriented pyrolytic graphite (HOPG) by electrochemical deposition at room temperature. Scanning electron microscopy (SEM) images reveal that these alloy nanowires (109–430 nm) are uniform in diameter, and have lengths up to 100–500 μm. The electrodeposition process involves the initial formation of nanowires induced at the step edges of the oxidized HOPG substrate at a very negative potential and subsequent growth at a constant low current density to coalesce the discontinuous nanowires. Alloy nanowires with a 20–25% silver content can be obtained when the ratio of Pd and Ag in the solution is carefully controlled. The SEM images demonstrate that the alloy nanowire arrays are continuous, parallel, ordered, well-aligned and have a narrow distribution of diameters. The Pd–Ag alloy nanowire arrays are promising materials for fabricating hydrogen nanosensors.

Passivant dependence of nanoparticle-substrate interactions of alkanethiolate-passivated Au nanoparticles on graphite substrates

We have carried out the photoemission study of monodisperse Au nanoparticles passivated with the various alkanethiolate on the highly oriented pyrolytic graphite (HOPG) substrate. The photoemission spectra in the vicinity of the Fermi level of alkanethiolate-passivated Au nanoparticles on the HOPG substrates are not the metallic Fermi edge. The steep slopes of Fermi level onsets move away from the Fermi level and depend on the surface-passivant molecules. We have analyzed the obtained spectra with the dynamic final state effect model that takes into account the Coulomb interaction between the photoelectron and photohole with a finite lifetime during the photoemission process. The calculated results based on the dynamic final state effect model reproduce the experimental ones fairly well. Furthermore, it is found that the photohole lifetime depends on the molecular length of surface-passivant. We attribute the passivant dependence of photoemission spectra to the surface-passivant dependent nanoparticle-substrate interaction.

Single molecule spectrum of rhodamine 6G on highly oriented pyrolytic graphite

We have measured the scanning tunneling microscope (STM) light emission spectrum of a single molecule of rhodamine 6G (R6G) adsorbed on highly oriented pyrolytic graphite (HOPG). Since the HOPG substrate radiates no STM light, we have succeeded in observing the spectrum radiated by R6G alone. The spectrum agrees well with the photoluminescence spectrum of R6G on HOPG with the exception of two structures that may arise from a triplet state whose transition is forbidden in photoluminescence. Based on this agreement, we have determined the STM light emission mechanism of adsorbed R6G.

Surface-Passivant Dependence of Dynamic Charging Effect in Alkanethiolate-Passivated Au Nanoparticles on Graphite Substrates Studied by Photoelectron Spectroscopy

AbstractWe have performed a photoemission study of various alkanethiolate- (AT-) passivated Au nanoparticles with diameter of 4 nm on the highly oriented pyrolytic graphite (HOPG) substrates. It is found that the photoemission spectra in the vicinity of Fermi level of all the present AT-passivated Au nanoparticles on the HOPG substrates do not exhibit the usual metallic Fermi edge, with the steep slope being away from the Fermi level. Moreover, it is found that these spectral features depend on the surface-passivant molecules. We attribute the unusual spectral features in the vicinity of Fermi level to the dynamic charging effect in photoemission final-state, indicative of the interaction between the nanoparticle and substrate through the surface-passivants on a femtosecond time scale.

Charge Injection Barriers at a Ribonucleic Acid/Inorganic Material Contact Determined by Photoemission Spectroscopy

Ribonucleic acid (RNA) homopolymer thin films on highly oriented pyrolytic graphite (HOPG) were prepared in ultrahigh vacuum (UHV) directly from aqueous solution by electrospray (ES) injection. The polyadenosine (poly rA) films were prepared in several steps of increasing thickness without breaking the vacuum. Before deposition and between deposition steps, the samples were characterized with photoemission spectroscopy (PES). Both X-ray and ultraviolet photoemission spectroscopy (XPS and UPS) were employed. XPS enabled the detailed measurement of core level peaks, giving insight into the chemical interaction at the interface and the layer morphology. The corresponding UP-spectra sequence allowed us to directly follow the transition from HOPG valence bands to the poly rA highest occupied molecular orbital (HOMO) structure. This enabled the determination of the poly rA ionization energy and work function as well as the charge injection barriers between the Fermi level of the HOPG substrate and the poly rA HOMO. The injection barrier between the lowest unoccupied molecular orbital (LUMO) and the HOPG Fermi level was determined using the HOMO-LUMO gap value determined by optical absorption. The results indicate that significant injection barriers exist between HOPG and the poly rA overlayer, limiting conductivity across this interface.

Femtosecond dynamic final-state effect in photoemission of surface-passivated metallic nanoparticles on graphite substrates

We have carried out systematic photoemission studies of alkanethiolate- (AT-)passivated metallic (Ag and Au) nanoparticles supported on the highly oriented pyrolytic graphite (HOPG) substrates. The Fermi-level onsets in the photoemission spectra of AT-passivated Ag nanoparticles on the HOPG substrates are not the metallic Fermi-edge, and depend on the nanoparticle diameter and surface-passivant molecules. Furthermore, the core-level photoemission spectra also depend on the nanoparticle diameter. We attribute these unusual spectral features to the dynamic final-state effect in photoemission, indicative of the interaction between the nanoparticle and substrate through the surface-passivants on a femtosecond timescale.

Mesoscopic Structures of Nanocrystals: Collective Magnetic Properties Due to the Alignment of Nanocrystals

Maghemite nanocrystals, deposited by slow evaporation on HOPG (highly oriented pyrolytic graphite) substrate, form mesoscopic structures which strongly depend on the nanocrystal coating. When the c...

C58on HOPG: Soft-landing adsorption and thermal desorption

Electron-impact induced dissociation/ionization of C60 molecules was used to produce an intense beam of C58+ ions. This was directed towards a HOPG (highly oriented pyrolytic graphite) substrate under nominally perpendicular impact conditions in order to generate deposits by soft-landing (kinetic energy < 0.1 eV atom−1). Deposited C58 molecules could subsequently be thermally desorbed intactly. Thermal desorption mass spectra of the deposits exhibit only C58. Surface deposited C58 can react with background gases to generate hydride derivatives C58Hn which are also desorbable. The apparent desorption energy of C58 and C58Hn molecules from the HOPG surface varies with increasing adsorbate coverage from 2 ± 0.1 to 2.2 ± 0.1 eV as determined by a Redhead analysis. These values are 0.7 ± 0.1 eV larger than found for C60 desorbed from the same substrate.

Infrared reflection spectroscopy of thin films on highly oriented pyrolytic graphite.

The properties of highly oriented pyrolytic graphite (HOPG) as a substrate for external reflection infrared spectroscopy in the mid-infrared region were investigated. Clean HOPG substrates, physisorbed hydrocarbon multilayers, and chemisorbed monolayers of p-substituted aryl radicals on HOPG were used as samples, and the experimental spectra were compared and complemented with the results of spectral simulations. From reflectivity measurements of clean HOPG surfaces with polarized light as a function of the light incidence angle and the frequency, the anisotropic optical constants n (refractive index) and k (absorption index) were determined for in-plane and out-of-plane directions with respect to the graphite basal plane. These constants express the semimetallic properties of HOPG, indicated by an intermediate reflectivity between a typical metal and a dielectric substrate and by asymmetric, distorted peak shapes in adsorbate film spectra, which represent a transition state between symmetrical, positive absorptions on metals and inverted, negative peaks on dielectric substrates. Regarding spectral sensitivity and surface selection rules, HOPG behaves much like a metal and is therefore an equally suitable substrate for external reflection infrared (IR) measurements.

Dynamic final-state effect on the Au4fcore-level photoemission of dodecanethiolate-passivated Au nanoparticles on graphite substrates

We have carried out a photoemission study using synchrotron radiation of dodecanethiolate- (DT-) passivated Au nanoparticles supported on the highly oriented pyrolytic graphite (HOPG) substrates. From the detailed line-shape analyses, it is found that Au $4f$ core-level photoemission spectra of DT-passivated Au nanoparticles on the HOPG substrates consist of the two components, which originate from the inner Au atoms of Au nanoparticles and surface Au atoms bonded to surface passivants of DT molecules. Furthermore, it is found that Au $4f$ core-level spectra shift to higher binding energy with decreasing the nanoparticle diameter. We attribute these energy shifts to the dynamic final-state effect on the Au $4f$ core-level photoemission due to the positive charge with a finite lifetime left behind in the nanoparticles in the photoemission final state.

Mineralization behaviour of collagen type I immobilized on different substrates

Collagen type I as a robust fibre protein and main component of the extracellular matrix of most tissues is increasingly utilized for surface engineering of biomaterials using different immobilization methods. In the present work we studied the mineralization behaviour of fibrillar collagen type I in simulated body fluid as a measure for conformational changes caused by adsorptive immobilization or immobilization by partial incorporation into the anodic oxide layer on c.p.-titanium using microscopic and vibration spectroscopic methods. Adsorptive immobilization on highly oriented pyrolytic graphite (HOPG) and c.p.-titanium without collagen were used as references. In the initial phase (1–24 h) the kinetics of formation and the morphology of calcium phosphate phases (CPP) are strongly influenced both by the substrate and the immobilization method. Compared to HOPG both types of immobilization on titanium increasingly inhibit the formation of CPP. For longer times (30 d) these initial differences disappear-mineralization product on titanium, irrespective of the presence of collagen, is a mixture of amorphous calcium phosphate and octacalcium phosphate. Contrary to this the mineralization of HOPG substrates results in hydroxy apatite. This is discussed with respect to the conditions during the immobilization as well as the resulting interactions between substrate and immobilized collagen. It is shown that the mineralization process exhibits a high sensitivity with respect to conformational changes caused by these interactions. Possible cell biological relevance of these conformational changes is discussed.

Electrochemical Deposition of Gold Metal Particles into 3‐Dimensional Polypyrrole Film Deposited on a Highly Oriented Pyrolytic Graphite

AbstractElectrochemical techniques and lateral friction microscopy (LFM) are exploited to characterize the deposition of gold metal particles onto the 3‐dimensional (3‐D) polypyrrole (PPy) film deposited on 2‐dimensional (2‐D) highly oriented pyrolytic graphite (HOPG) substrate surface in an aqueous solution involving 0.01 M pyrrole and 0.1 M LiClO4⋅ 3H2O. Cyclic voltammetry is utilized to find the gold deposition potential onto the PPy film from 0.001 M KAu(CN)2/KOH solution. The gold deposition potential is found to be in the range of −1.2 V to −1.4 V. Chronoamperometry is used to find out the nucleation and growth mechanism of gold metal particles onto PPy film. When the PPy film is thin, the mechanism follows the 3‐D instantaneous and moved towards 3‐D progressive as the film thickness increases. Considering the high resistance of thick PPy film and insulating and compact nature of the film at more cathodic potentials, it is suggested that the gold nuclei are formed first on the HOPG substrate surface, move to the PPy film surface and then distributed inside the PPy matrix. Since the friction of gold and the PPy film is different, the LFM is found to be an effective tool to see the distribution of gold particles in the domain boundaries of the PPy film.

Structures of Self-Assembled Monolayer of Squaraine Molecules Adsorbed on Highly Oriented Pyrolytic Graphite and Au(111) Substrates

Molecular arrangements of squaraine (SQ) molecules formed on highly oriented pyrolytic graphite (HOPG) and Au(111) substrates in phenyloctane solution were studied by scanning tunneling microscopy (STM). On HOPG, SQ molecules without hydroxyl groups formed row arrangements, in contrast to the herringbone structures previously observed for the SQ molecules with hydroxyl groups, indicating that the chemically added hydroxyl groups play an important role in the formation of the herringbone arrangement. On the other hand, the observed molecular structures on the Au(111) substrate were completely different from those on the HOPG substrate, which was attributed to the specific interactions between the two anilino moieties in a SQ molecule and Au(111) lattices. The obtained results indicate the possibility of structural control of SQ molecules, namely, the photo-absorption property, by changing the kinds of functional groups and substrates.

Growth of epitaxial nanowires at the junctions of nanowalls.

Growth of epitaxial nanowires at the junctions of nanowalls.

Electrodeposition of Ti Nanowires on Highly Oriented Pyrolytic Graphite from an Ionic Liquid at Room Temperature

Ti nanowires have been deposited electrochemically, with in situ monitoring by scanning tunneling microscopy, at the step edge of highly oriented pyrolytic graphite (HOPG) at room temperature from 0.24 M TiCl4 in the ionic liquid 1-butyl-3-methyl imidazolium bis((trifluoromethyl)sulfonyl)amide. To our knowledge, this is the first time that a transition metal nanowire has been prepared in this way, which is enabled by the wide electrochemical windows of ionic liquids. The formation of the first nanowire is induced at the step edge of the HOPG substrate at a potential of −1.0 V versus the [Fc]+/[Fc] redox couple; subsequent straight and highly aligned wires grow in parallel to the first. This cooperative nucleation process of nanowires is unusual and has not been reported before. Up to six wires grow at constant potential over a period of about 20 min. The wires exhibit a narrow width distribution of 10 ± 2 nm and have a length of more than 100 nm.

Photoemission study of metallic nanoparticles passivated by dodecanethiolates: silver nanoparticles

Photoemission study of dodecanethiolate-passivated Ag nanoparticles supported on the highly oriented pyrolytic graphite (HOPG) has been carried out in order to investigate their nanoparticle–substrate interactions. It is found that the Fermi-level onset in the photoemission spectrum for the present Ag nanoparticle is not the Fermi edge, with the steep slope being away from the Fermi-level. The observed photoemission spectra are well characterized by a theoretical calculation that takes into account the influence of the photohole left behind in the Ag nanoparticle in the photoemission final-state and nanoparticle–substrate interaction on a femotosecond timescale. From these results, we discuss the nanoparticle–substrate interactions through the surface-passivants of the surface-passivated Ag nanoparticles on the HOPG substrates.