Highly Oriented Pyrolytic Graphite Surface Research Articles

Phosphole modified pentathienoacene: Synthesis, electronic properties and self-assembly

A series of new ladder π-conjugated materials, phosphole modified pentathienoacene (PO-PTA), are synthesized and characterized. Single-crystal X-ray results demonstrate that methyl-disubstituted PO-PTA forms a face-to-face dimer structure driven by π-π interactions. The investigations of optical properties showed that the oxidized phosphole moiety in this ladder system can effectively narrow the band gap. PO-PTA is a promising building block in π-conjugated polymers and oligomers for optoelectronic applications. The derivative of PO-PTA, obtained by introducing four long alkyl chains, can self-assemble into one-dimensional (1D) fibers based on intermolecular π-π interactions, dipole-dipole interactions and van der Waals interactions. Interestingly, the uniform and well-ordered monolayers were also obtained for PO-PTA derivative on a HOPG (highly oriented pyrolytic graphite) surface.

Ultrafast Direct Ablative Patterning of HOPG by Single Laser Pulses to Produce Graphene Ribbons

AbstractUltrafast, single step and direct patterning of highly oriented pyrolytic graphite (HOPG) is achieved through pulsed laser interference ablation using a near field transmitting phase mask. Periodic arrays of lines are patterned on the HOPG surface over large areas by spatially modulating the laser intensity through the mask. Thus patterned surface serve as a source for multi and few layer graphene ribbons for transferring onto desired substrates using polydimethylsiloxane as transferring agent. The transferred regions are contained with few layer graphene (5–6 layers) ribbons as well as thick graphitic ribbons (30–40 nm), with widths ∼1 μm and lengths of several micrometers. Raman, TEM and electrical measurements have confirmed that the transferred ribbons are highly crystalline in nature. Using combinations of shadow and transmitting phase masks, other patterns such as checker boards and diamond‐shaped pits are produced.

Influence of Mg2+, Ni2+, and Cu2+ on DNA assembly on HOPG surfaces: atomic force microscopy study

Adsorption of circular DNA onto bare highly oriented pyrolytic graphite (HOPG) surfaces by the addition of Mg²⁺, Ni²⁺, and Cu²⁺ has been investigated by atomic force microscopy (AFM). AFM results revealed that the topography and height of DNA on HOPG surface by the addition of different metal ions are quite different. After the addition of Mg²⁺ for incubation, DNA molecules tend to form many loops on HOPG surfaces, which are derived from the crossover of intramolecular and intermolecular chains. After the addition of Ni²⁺, DNA molecules can form network on HOPG surfaces, and the density of DNA network was significantly increased with increasing DNA concentration. Consequently, dense DNA network can be obtained by using relatively low concentration of DNA and Ni²⁺. As for the addition of Cu²⁺, angular DNA loops composed of flat chains were observed. The observed flat DNA chains with an average height of 0.52 nm can be ascribed to Cu²⁺ insert into the site between bases and phosphate group of DNA inducing denaturation of DNA molecules. This study is very helpful for understanding the interactions of metal ions and DNA molecules, and for constructing various DNA structures on the carbonaceous surfaces.

Size-Dependent Intercalation of Ions into Highly Oriented Pyrolytic Graphite in Ionic Liquids: An Electrochemical Atomic Force Microscopy Study

Morphological changes of the highly oriented pyrolytic graphite (HOPG) surface during electrochemical intercalation–deintercalation process in pure ionic liquids (ILs) have been studied using electrochemical atomic force microscopy (EC-AFM). The results show that electrochemical intercalation of ions into HOPG is strongly dependent on anionic size of the ILs used. Up to 0.8 V versus the Ag quasi reference, drastic reversible morphological changes of HOPG surface during anion intercalation–deintercalation were observed in [BMIM][HSO4], whereas these changes were absent in [BMIM][N(Tf)2]. However, at and above 1.2 V, the HOPG surface degrades in both the ILs to form carbon nanoparticles.

Preparation of monolayers of [MnIII 6CrIII]3+ single-molecule magnets on HOPG, mica and silicon surfaces and characterization by means of non-contact AFM

We report on the characterization of various salts of [MnIII6CrIII]3+ complexes prepared on substrates such as highly oriented pyrolytic graphite (HOPG), mica, SiO2, and Si3N4. [MnIII6CrIII]3+ is a single-molecule magnet, i.e., a superparamagnetic molecule, with a blocking temperature around 2 K. The three positive charges of [MnIII6CrIII]3+ were electrically neutralized by use of various anions such as tetraphenylborate (BPh4-), lactate (C3H5O3-), or perchlorate (ClO4-). The molecule was prepared on the substrates out of solution using the droplet technique. The main subject of investigation was how the anions and substrates influence the emerging surface topology during and after the preparation. Regarding HOPG and SiO2, flat island-like and hemispheric-shaped structures were created. We observed a strong correlation between the electronic properties of the substrate and the analyzed structures, especially in the case of mica where we observed a gradient in the analyzed structures across the surface.

Growth mechanism and interface magnetic properties of Co nanostructures on graphite

We investigated structural, electronic, and magnetic properties of Co adsorbed on highly oriented pyrolytic graphite (HOPG). Distribution and atomic sites of 3 $d$ transition-metal Co nanoislands and adatoms on HOPG were experimentally investigated by scanning tunneling microscopy with atomic resolution. In the very low thickness regime (\ensuremath{\le}0.6 \AA{}), a strong nucleation mechanism and a preferred Co nanoisland diameter of \ensuremath{\sim}3.4 nm have been observed. Co adatoms were found to preferentially occupy \ensuremath{\beta} sites of the HOPG surface graphene layer and the atoms aggregated by further occupation of either \ensuremath{\alpha} or overbond sites. This is in contrast to predictions based on density functional theory, which indicates that the hollow sites are the most energetically stable sites for Co adsorption. The presence of surface hydrocarbon contamination on graphite might be one possible cause of the observed active nucleation and stabilized nanoisland diameter of Co. The formation of Co carbide was evidenced by x-ray absorption spectroscopy. More importantly, the Co magnetic spin moment at the interface of Fe-capped ferromagnetic Co nanostructures and graphite, as determined by x-ray magnetic circular dichroism and sum-rule analysis, was found to be only 63% of the bulk value, implying a magnetically defective spin contact for carbon spintronics applications.

Surface-Confined Conformers and Coassembly-Induced Conformer Resolution

Stereoisomerism is a fundamental chemistry issue and has been intensively investigated because of its importance in organic chemistry, biology, and pharmacology. Molecules with freely rotatable single bonds have many interconvertable conformers. Herein, we report the surface-adsorption-induced conformer resolution by employing azobenzene-3,3-dicarboxylic acid (ADA-33) as a model compound. Two linear assembly phases composed of trans conformers on a highly oriented pyrolytic graphite (HOPG) surface are observed by scanning tunneling microscopy. With the codeposition with 1-octanoic acid (OA), only one trans conformer of ADA-33 can be recognized by OA to form a two-component assembly with alternately arranged ADA-33 and OA stripes, which can be attributed to the epitaxial assembly of ADA-33 and OA on the HOPG surface, and weak hydrogen bonding exists between conformer I and OA molecules. The results are of significance with respect to the discrimination and resolution of conformers on a solid surface and provide molecular insights into the coadsorption assembly on the surface.

Palladium nano-clusters grown on prestructured HOPG substrates

The growth of Palladium nano-clusters prepared by atomic beam deposition on prestructured highly oriented pyrolytic graphite (HOPG) surfaces has been investigated by means of scanning tunnelling microscopy (STM). Preformed nanosized pits created on the HOPG surfaces are used as localized pinning sites for Pd cluster nucleation and growth at room temperature. We succeeded in obtaining Pd clusters of nanometric size and with rather sharp size distributions. A systematic morphological study conducted by STM reveals a linear dependence between the height and the diameter of the Pd nanostructures. Finally, Pd nano-clusters stabilized on prestructured HOPG surfaces were found to be active catalysts in the Heck cross-coupling reaction.

Self-Assemblies of meso-Tetraphenylporphine Ligand on Surfaces of Highly Oriented Pyrolytic Graphite and Single-Walled Carbon Nanotubes: Insights from Scanning Tunneling Microscopy and Molecular Modeling

The self-assembly of porphyrins into highly organized functional arrays supported on appropriate solid substrates is an area of research with multiple potential applications in the "bottom-up" approach to manufacturing. In order to analyze the self-assembly of meso-tetraphenylporphine (H2TPP) on the surfaces of highly oriented pyrolytic graphite (HOPG) and single-walled carbon nanotubes (SWNTs), we performed molecular mechanics modeling (by MM+ force field) and scanning tunneling microscopy (STM) imaging. Molecular modeling predicted an energetic preference of the H2TPP molecules to adsorb in monolayers on the surfaces of graphite and SWNT sidewall, rather than their stacking or separation. On graphite, the most favorable arrays were predicted to be ribbons composed of interacting parallel chains of H2TPP molecules. On the SWNT sidewall, the energetic preference pointed toward the formation of parallel and interacting long-period helixes, resulting in an almost full coverage of the SWNT surface. These preferable arrays on both carbon materials assure the interaction of every porphyrin unit with as many neighbors as possible, thus lowering the potential energy of the adsorption complexes. STM imaging results are in good agreement with molecular modeling predictions. The formation of self-assembled ribbons was a frequently observed phenomenon on the HOPG surface, while on the SWNT surface a full coverage of the exposed portion of the sidewalls was observed, suggesting the formation of interacting long-period helixes. A preferential adsorption of H2TPP molecules near graphite topographic defects was also observed.

SEM and ESR measurements on HOPG surfaces irradiated with Ar11+ and Ar1+

Highly oriented pyrolytic graphite (HOPG) samples were irradiated with a highly charged ion (HCI) Ar11+ at a fluence of 1014 cm−2. After irradiation, scanning electron microscopy (SEM) was used to observe the irradiated trace. During SEM measurement, an electron acceleration voltage of 0.5, 1 and 5 kV was applied to measure the samples, respectively. The irradiated contrast can be observed in the Si surface at 0.5 kV, which could not be found at 1 kV or higher. The irradiated area becomes brighter than that of the unirradiated area. When Ar1+ was used to bombard the solid surface, the fluence should become of the order of 1015 cm−2. In this case, the irradiated contrast could be observed. It means HCI is able to enhance more effectively the surface modification. Furthermore, electron spin resonance was used to measure the surface of highly oriented pyrolytic graphite (HOPG). A clear resonant peak appears in the irradiated sample, which cannot be found in the pristine sample. The phenomenon could be related to defects induced by the HCI impact.

Potential and Kinetic Electron Emissions from HOPG Surface Irradiated by Highly Charged Xenon and Neon Ions

Highly charged 129Xeq+ (q = 10−30) and 40Neq+ (q = 4−8) ion-induced secondary electron emissions on the surface of highly oriented pyrolytic graphite (HOPG) are reported. The total secondary electron yield is measured as a function of the potential energy of incident ions. The experimental data are used to separate contributions of kinetic and potential electron yields. Our results show that about 4.5% and 13.2% of ion's potential energies are consumed in potential electron emission due to different Xeq+ -HOPG and Neq+ -HOPG combinations. A simple formula is introduced to estimate the fraction of ion's potential energy for potential electron emission.

Enhanced Nucleation of Pt Particles on Boron-Treated Highly Oriented Pyrolytic Graphite via Chemical Vapor Deposition

The nucleation of Pt particles from CH3CpPt(CH3)3 on boron-treated highly oriented pyrolytic graphite (HOPG) was investigated. HOPG was enhanced with B from diborane using hot wire chemical vapor deposition to generate BDx fragments that dissociatively adsorbed on the HOPG surface and subsequently reacted with the sp2 C lattice. Unreacted BDx fragments either reacted with the HOPG or desorbed during annealing. Pt was deposited using chemical vapor deposition. Platinized B-treated HOPG samples exhibited enhanced Pt dispersion and uniform nucleation as compared with cleaved and annealed HOPG samples. Pt particle density is shown to be tunable using varying B exposures. This study demonstrates that the optimization of Pt dispersion and adhesion to HOPG substrates is achievable using a scalable chemical enhancement method.

Quasi in situ XPS study of electrochemical oxidation and reduction of highly oriented pyrolytic graphite in [1-ethyl-3-methylimidazolium][BF 4] electrolytes

Highly oriented pyrolytic graphite (HOPG) electrodes were electrochemically oxidised and reduced in 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF 4] electrolytes and studied by X-ray photoelectron spectroscopy (XPS). Sample preparation and transfer have been performed under inert nitrogen atmosphere in a preparation chamber directly attached to an ultra high vacuum system. After electrochemical treatment, both, the electrolyte and the electrode surface were investigated. While on the oxidised HOPG surface the core levels of the detected elements shift towards lower energies, on the reduced samples a shift towards higher binding energies is observed. These shifts refer to a Fermi level shift proving that graphite intercalation compounds were formed. Intercalation occurs together with co-intercalation of the ionic liquid. XPS analysis of the ionic liquids before and after electrochemical treatment reveals changes in electrolyte composition. The influence of impurities on electrochemical behaviour and XPS data is discussed.

In situ STM study of Pt-nanodot arrays on HOPG prepared by electron-beam lithography

Patterned model electrodes, consisting of platinum nanodots on highly oriented pyrolytic graphite (HOPG) surface, were prepared by electron beam lithography and subsequent platinum deposition via sputtering. The diameter of obtained Pt dots was 30 nm with a distance (pitch) of 200 nm. The electrodes were investigated by scanning tunneling microscopy (STM) in air as well as in situ in 0.5 M sulphuric acid electrolyte. The Pt dot pattern remained largely unchanged during STM measurements in air, displaying good anchoring of Pt dots to the smooth HOPG surface. Stepwise increase of the electrode potential in the electrolyte led to platinum dissolution; this process was recorded by in situ STM. In course of Pt dissolution, a hole pattern corresponding to the formerly existing dot pattern appeared on the HOPG surface, as imaged by in situ STM.

Formation of Gallium-induced nanostructures on single crystal HOPG surface

The room temperature growth of gallium atoms on the highly oriented pyrolytic graphite (HOPG) surface has been performed. The gallium atoms were deposited by thermal evaporation method in an ultra high vacuum system at a base pressure 5 × 10−10 torr. The X-ray photo electron spectroscopy (XPS) studies had been performed to confirm the presence of gallium atoms on HOPG surface. Scanning tunneling spectroscopy (STM) technique was employed to study the surface morphology of the clean HOPG surface and gallium covered HOPG surfaces which recognize the formation of gallium induced nanostructures. The deconvoluted XPS core level spectra of C (1s) and Ga (3d) demonstrate the possible interaction between substrate and the adsorbate atoms. The STM analysis revealed that the gallium deposition on HOPG led to significant change in the surface morphology. It was observed that the Ga atoms adsorbed as layer structure on HOPG surface for low coverage while quasi one-dimensional chain like nanostructure (1 ± 0.2 nm) has been formed for higher Ga coverage. The nanostructured surfaces induced by Ga deposition are found to be stable and could be used as a template for the growth of metallic nanostructures.

The structure and properties of the Si nanostructures on an HOPG surface

The morphology, electronic structure, and optical properties of self-assembled silicon nanostructures grown on the surface of Highly Oriented Pyrolytic Graphite (HOPG) by molecular beam epitaxy were studied by ultra high vacuum (UHV) scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS) in situ, and by Raman spectroscopy ex situ. At coverages of less than 1 monolayer (ML), the formation of monolayered silicon nanoislands with an atomic structure similar to that of graphene was observed.

Intermolecular band dispersion in quasi-one-dimensional adenine assemblies

Highly-ordered, hydrated adenine multilayer films grown on the surface of highly-oriented pyrolytic graphite, HOPG(0001), display extended electronic states, affording anisotropic band-like charge transport along the π-π stacking direction.

Modification of HOPG Surface on Irradiation by Highly Charged Ar11+ and Xe26+ Ions Investigated by SEM, ESR, SQUID, and Raman Measurements

Highly oriented pyrolytic graphite (HOPG) samples were irradiated by highly charged ions Ar11+ and Xe26+. Scanning electron microscopy (SEM), electron spin resonance (ESR), a superconducting quantum interference device (SQUID), and Raman spectroscopy were used to investigate the effect of this irradiation. SEM observations revealed that the contrast between irradiated and unirradiated regions became discernible at a fluence of about 1014cm -2. On the other hand, for samples irradiated by Ar+ ions, a fluence of the order of 1015cm -2 was necessary to obtain a similar contrast to that obtained by Ar11+ ion irradiation. This demonstrates that highly charged ions effectively enhance modification of solid surfaces. Furthermore, the ESR and SQUID measurements revealed the formation of defects and magnetization of the irradiated surface. Raman measurements were also performed to investigate the structural transformation at different fluences. © 2011 The Surface Science Society of Japan.

Metallic Nanostructure Formation Using Self-Assembled Chemically Anchored Gold Nanoparticles

In the present work, the formation of silver nano- and mesopourous structures on highly oriented pyrolytic graphite (HOPG) modified with self-assembled gold nanoparticles (Au NPs) is demonstrated. To illustrate the patterning effect with nanometer resolution of Au NPs, a comparison between silver electrodeposition on bare HOPG (0001) substrate and Au NP-modified HOPG (0001) surface is performed. On bare HOPG substrate the electrodeposition occurs on terraces, steps, and surface defects according to the well known Volmer–Weber island growth mechanism. Whereas on the Au NP-modified HOPG substrate, silver deposition results in a very different morphology: nanostructures (mesoporous films for longer deposition time) are formed on the entire electrode surface. Field emission gun scanning electron microscopy, energy dispersive spectrometry and X-ray photoelectron spectroscopy are used to reveal the selective deposition of silver on Au NPs during the first stage of the electrodeposition process. At the later stage due to coalescence, mesoporous silver film is formed. The possible mechanism for silver ions penetration and reduction on single Au NP with -dodecanethiol shell is discussed.

Electrochemical preparation and characterization of gold-polyaniline core-shell nanocomposites on highly oriented pyrolytic graphite

A simple electrochemical method for the in situ preparation of homogeneously dispersed gold-polyaniline core/shell nanocomposite particles with controlled size on the highly oriented pyrolytic graphite (HOPG) was demonstrated. The HOPG surface was modified preferentially by covalent bonding of a two-dimensional 4-aminophenyl monolayer employing diazonium chemistry. AuCl 4 − ions were attached to the Ar-NH 2 termination and reduced electrochemically. This results in the formation of Au nuclei that could be further grown into gold nanoparticles. The formation of polyaniline as the shell wrap of Au nanoparticle was established by localized electro-polymerization. These core-shell nanocomposites prepared were characterized by AFM and cyclic voltammetry. The results show that the gold-polyaniline core-shell composites on HOPG have a mean particle size of 100 nm in diameter and the polyaniline shell thickness is about 15 nm.