Highly Oriented Pyrolitic Graphite Substrate Research Articles

Two polymorphic forms of 10,12-pentacosadiyn-1-ol molecular layers on graphite transferred from an aqueous surface

Molecular layers of the diacetylene (DA) 10,12-pentacosadiyn-1-ol (PCDYol) were transferred from an aqueous surface onto a highly oriented pyrolitic graphite (HOPG) substrate. Two polymorphic forms, having herringbone and parallel molecular arrangements, were observed by scanning tunneling microscopy (STM) in air. In contrast to DA molecules having the COOH end group, where only the parallel arrangement is realized, the herringbone and parallel polymorphic forms arise from the flexibility of hydrogen bonds between the OH end groups of PCDYol, as observed for alkanols. The area ratio for these arrangements on the HOPG surface depended on the density of the molecules on the water surface and the sample preparation temperature. By one-dimensional (1D) contrast modulation (moiré pattern), the point-on-line coincidence between the lattice points of the PCDYol layers and the HOPG surface was determined for the herringbone and parallel arrangements. Because it promotes stability of the alkane, the epitaxy is primarily controlled by the coincidence of the molecular zigzag skeleton in the parallel arrangement with the benzenoid rings of graphite. In contrast, the herringbone arrangement was lacking in this coincidence with the graphite structure. Therefore, as is evident from the fluctuation of the orientation of molecular arrays (lamella axes), the herringbone arrangement is relatively unstable on the HOPG substrate. Consequently, we conclude that the herringbone arrangement is quasi-stable while the parallel arrangement is stable.

Nanostructuration of self-assembled poly(styrene-b-isoprene-b-styrene) block copolymer thin films in a highly oriented pyrolytic graphite substrate

Highly oriented pyrolitic graphite (HOPG) is a useful substrate to visualize epitaxial formation due to its crystallographic structure. The morphology of a poly(styrene-b-isoprene-b-styrene) block copolymer thin film on a HOPG substrate was investigated by atomic force microscopy. Block copolymer domains generated a morphology with triangular regularity. This arrangement was induced by the HOPG substrate structure due to van der Waals attraction between the HOPG π-conjugated system and aromatic ring of polystyrene domains. However, increasing the film thickness, the substrate effect on the surface morphology decreased. As a consequence, film surfaces showed the coexistence of different structures such as highly aligned cylinders and perforated lamellae. When film thickness exceeded a threshold value, the substrate did not have effect in the surface arrangements and the surface showed a similar morphology to that existing in bulk.

Morphology and Wettability of [Bmim] [PF6] Ionic Liquid on HOPG Substrate

We investigate the morphology and wettability of [Bmim][PF6] ionic liquid (IL) on a highly oriented pyrolitic graphite (HOPG) substrate using atomic force microscopy. Thin films, nanometer-sized droplets, and “drop-on-layer" structures of the IL are found on the substrate. Films with a thickness of up to 2 nm (about 4 IL layers) show the solid-like behavior. In contrast, a dewetting phenomenon is observed for thicker IL films, indicating that the IL films retain liquid properties. The contact angle of a buck IL droplet on the HOPG is measured to be about 35°. The wettability of the bulk IL droplet on the HOPG is found to be quite different from that of IL films. These results indicate that the IL molecules can be organized into various micro-morphologies when they are confined to a solid substrate and show characteristic behavior at nanometer scales.

Manipulation of cadmium selenide nanorods with an atomic force microscope

We have used an atomic force microscope (AFM) to manipulate and study ligand-cappedcadmium selenide nanorods deposited on highly oriented pyrolitic graphite (HOPG). TheAFM tip was used to manipulate (i.e., translate and rotate) the nanorods by applying aforce perpendicular to the nanorod axis. The manipulation result was shown todepend on the point of impact of the AFM tip with the nanorod and whether thenanorod had been manipulated previously. Forces applied parallel to the nanorodaxis, however, did not give rise to manipulation. These results are interpretedby considering the atomic-scale interactions of the HOPG substrate with theorganic ligands surrounding the nanorods. The vertical deflection of the cantileverwas recorded during manipulation and was combined with a model in order toestimate the value of the horizontal force between the tip and nanorod duringmanipulation. This horizontal force is estimated to be on the order of a few tens of nN.

Characterization of gold nanoclusters deposited on HOPG by atmospheric plasma treatment

AbstractThe interest of gold nanoparticles in the field of nanocatalysis and nanosensors is growing. For example, carbon nanotubes covered with gold nanoclusters could present interesting properties in catalysis or/and in devices based on catalytic reaction such as chemical gas sensor. Unfortunately, the characterization of nanoparticles deposited onto a substrate is generally not a trivial exercise. Indeed, in many cases, the amount and size of material deposited is in the range of the sensitivity limits and the spatial resolutions of the techniques used. In that respect, our system, i.e. gold deposited onto a carbon support, is a favourable case, due to the high difference between the atomic numbers of gold and carbon.In this work, gold nanoparticles were deposited onto a HOPG (Highly Oriented Pyrolitic Graphite) substrate using an atmospheric plasma. In this preliminary study, HOPG sample has been used as a model surface that could present the same chemical properties as multiwall carbon nanotubes (MWCNTs).The surface composition was analysed using X‐ray photoelectron spectroscopy (XPS). Narrow region electron spectra were used to extract the chemical‐state information from the C 1s, O 1s and Au 4f peaks. The Au 4f spectral line shape was also analysed using the QUASES–Tougaard software package in order to obtain the in‐depth concentration profile of the resulting nanomaterial. This result evidences a good agreement between experiment and modelling. Indeed, field emission scanning electron microscope (FE‐SEM) and atomic force microscopy (AFM) images highlight a homogeneous distribution of 10 nm‐size gold clusters with a surface coverage of 12% on the HOPG surface, both in agreement with the results obtained with the Tougaard and coworkers method. Copyright © 2008 John Wiley & Sons, Ltd.

STM observation of the quantum interference effect in finite-sized graphite

Superperiodic patterns were observed by STM on two kinds of finite-sized graphene sheets. One is nanographene sheets inclined from a highly oriented pyrolitic graphite (HOPG) substrate and the other is a several-layer-thick graphene sheets with dislocation-network structures against a HOPG substrate. As for the former, the in-plane periodicity increased gradually in the direction of inclination, and it is easily changed by attachment of a nanographite flake on the nanographene sheets. The oscillation pattern can be explained by the interference of electron waves confined in the inclined nanographene sheets. As for the latter, patterns and their corrugation amplitudes depended on the bias voltage and on the terrace height from the HOPG substrate. The interference effect by the perturbed and unperturbed waves in the overlayer is responsible for the patterns whose local density of states varies in space.

Surface structure of lithiated graphite by X-ray photoelectron diffraction

The surface composition and structure of a stage-one lithium intercalation compound (Li-GIC) was studied by X-ray photoelectron diffraction (XPD) from Li 1s and C 1s core levels. The Li-GIC was prepared in situ by vapor phase intercalation of lithium into highly oriented pyrolitic graphite (HOPG) in an ultra-high vacuum system. Stage-one Li-GIC, LiC 6, having a characteristic golden color, was obtained using Li evaporation onto a HOPG substrate at 400 K, while a metallic lithium overlayer was observed at depositions below room temperature. XPD patterns of Li 1s and C 1s intensities as a function of the polar emission angle were obtained at a fixed photon energy of 1253.6 eV. The experimental XPD patterns on the HOPG and the stage-one Li-GIC were both in quite good agreement with calculated XPD patterns based on Rehr–Albers separable representation for scattering of the emitted photoelectrons. The structural model for the surface of the Li-GIC producing the best fit to the experimental data has the bulk LiC 6 lattice with surface termination in the graphene plane.

AuSb2 alloy formation on HOPG by successive deposition of Sb clusters and Au atoms

The behaviour of gold atoms that alloy into amorphous antimony (aSb) islands is studied by transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDS). In a first step, antimony thin films are produced by low-energy cluster beam deposition (LECBD). The antimony clusters (of diameter ≈5 nm, with 2300 atoms) are generated in a thermal source by the gas aggregation technique and deposited onto highly-oriented pyrolitic graphite (HOPG). For cluster submonolayer coverage (<0.2 M.L.), the diffusion and interaction of clusters on substrate lead to the formation of amorphous ramified Sb islands, formed by the aggregation of spherical particles about 10 nm in diameter. In a second step, gold is vapour-deposited onto the aSb–HOPG substrate. The influence of gold atom deposition on the morphology of the islands is studied. Moreover, varying the gold thickness, we characterize the AuSb2 alloy formation.

Spontaneous alloying on HOPG by successive deposition of Sb clusters and Au atoms

Alloying of gold atoms into nanometer-sized antimony clusters deposited on highly oriented pyrolitic graphite (HOPG) has been studied by transmission electron microscopy (TEM) for morphological and structural characterisations and by energy dispersive X-ray spectroscopy (EDX) for analysis of composition. First, large antimony clusters (≈5.4 nm) generated by the gas aggregation technique in a thermal source were deposited on HOPG. These antimony clusters diffused on graphite and formed, by collision and irreversible sticking, ramified islands the density of which can be predicted, prior to deposition, with suitable cluster flux and substrate temperature. Then, the gold atoms were evaporated at various mean thicknesses on the same Sb N –HOPG substrate. For this post-deposition of gold, the morphology of the deposit was quite different, depending on the evaporation method (Knudsen cell heated by electron bombardment or spiral tungsten filament heated by the Joule effect): ramified islands of Sb–Au alloys plus gold supported particles when using the first method and only ramified Sb–Au islands for the second one. In this paper, we give an explanation of these two different morphologies and we study the alloy formation versus the gold content.

Using a new kind of organic complex system of electrical bistability for ultrahigh density data storage

We present a new kind of organic complex system of electrical bistability, m-nitrobenzal malononitrile and diamine benzene (mNBMN-DAB), which can be employed as ultrahigh density data storage devices by scanning tunneling microscopy (STM). The mNBMN-DAB thin film was prepared on highly oriented pyrolitic graphite (HOPG) substrates using physical vacuum deposition method. A critical voltage pulse between the STM tip and the surface of the HOPG substrate can make marks on the thin films. The size of the marks is about 1.3 nm and the corresponding data storage density is larger than 1013 bits/cm2. A new mechanism of the charge transfer in the system for the data storage is suggested. These results show a great potential of this type of organic composite systems in the application of the ultrahigh density data storage.

Chemical and morphological modifications of silicon wafers treated by ultrasonic impacts of powders: consequences on diamond nucleation

The mechanical enhancement of diamond nucleation on silicon wafers by the use of ultrasonic impacts of different abrasive powders in various liquid media is investigated via surface-enhanced Raman spectroscopy (SERS), atomic-force microscopy (AFM) and scanning electron microscopy (SEM). The growth of diamond nuclei is carried out in a bell-jar microwave-assisted plasma set-up. The nucleation densities obtained are compared to those resulting from a highly oriented pyrolitic graphite (HOPG) substrate, a virgin silicon wafer and a silicon wafer previously in-situ etched by a hydrogen plasma. The results are analyzed in terms of free carbon formation, surface morphology and particle implantation. The role of the morphological aspect is seen to be negligible compared with the chemical role. When non-diamond powders are used, the nucleation density is seen to depend on the amount of amorphous carbon phases formed in the physical sites (defects, stresses), at the silicon surface. When diamond powder is used, nucleation density up to 1.5 × 1010 cm−2 is obtained. It is attributed to an implantation of the diamond particles occurring during the ultrasonic impacts. On the HOPG substrate, a low and non-uniform crystal density, related to the distribution of scratches on the surface, is obtained. This highlights the role played by carbon phases, located at highly defective regions, in promoting diamond nucleation.