Highly Oriented Pyrolytic Graphite Basal Plane Research Articles

Bifunctional Oxygen Electrodes with Highly Step-Enriched Surface of Fe–Nx Containing Carbonaceous Thin Film

Carbonaceous air cathodes in zinc-air batteries have attracted attention due to their high electron conductivity and low cost; however, the increment in the catalytic activity for the oxygen reduction and evolution in addition to the durability are demanded. Based on a fundamental study about the substantial effect of a three-dimensional active site at a step on a highly-oriented pyrolytic graphite basal plane on the electrode reactions, we attempted to extend the idea to a graphitic carbon fiber surface as a more applied form for the air electrode. Coating of a carbonaceous thin film derived from iron phthalocyanine on finely-etched graphitic carbon fibers produced a highly step-enriched surface demonstrated by the field-emission scanning electron microscope image and Raman spectra with a developed defect peak. The iron K-edge X-ray absorption fine structure showed a step-oriented iron atom coordinated by two nitrogen atoms and a basal plane oriented iron atom coordinated by 4 nitrogen atoms. The step-enrichment enhanced the oxygen reduction and evolution; in particular, the low overpotential phenomenon observed for the fundamental graphite system was reproduced in the carbon fiber paper, both in a half cell and a full cell of the zinc-air battery, which also showed a favorable cycling performance.

Cobalt pyridinoporphyrazine film as a platinum group metal-free mediator in hydrogen electrochemistry

The cobalt pyridinoporphyrazine film deposited on basal plane highly oriented pyrolytic graphite (HOPG) and Au(111) electrodes exhibited electrocatalytic activity to hydrogen oxidation reaction compared to the bare electrode surfaces. The deposition performed by spontaneous adsorption of the cobalt pyridinoporphyrazine complex from its aqueous solution led on basal plane HOPG to the formation of a uniform homogeneous film with simultaneous reduction of cobalt centre, while at Au(111) electrode preferential aggregation of pyridinoporphyrazine complex took place without any change of reduction state of cobalt. Accordingly, both substrates affected the pathway of electrocatalytic hydrogen oxidation. Potentiometric detection of hydrogen based on equilibrium potential of pyridinoporphyrazine Co(II)/Co(I) electrode upon alternation presence of hydrogen and argon in solution was examined.

Atomically-Resolved Oxidative Erosion and Ablation of Basal Plane HOPG Graphite Using Supersonic Beams of O2 with Scanning Tunneling Microscopy Visualization

The detailed mechanism and kinetics for the oxidative erosion and ablation of highly oriented pyrolytic graphite (HOPG) with molecular oxygen has been examined by monitoring the spatiotemporal evolution of the reacting interface. This has been accomplished using a new, unique gas-surface scattering instrument that combines a supersonic molecular beam with a scanning tunneling microscope (STM) in ultrahigh vacuum. Using this new instrument, we are able to tightly control the energy, angle, and flux of impinging oxygen along with the surface temperature and examine the reacted surface spanning atomic, nano, and mesocopic length-scales. We observe that different oxidation conditions produce morphologically distinct etching features: anisotropic channels, circular pits, and hexagonal pits faceted along crystallographic directions. These outcomes depend upon independent effects of oxygen energy, incident angle, and surface temperature. Reaction probability increased with beam energy and demonstrated non-Arrhen...

(Invited) AC Impedance Studies on Electrode/Electrolyte Interface in Lithium-Ion Batteries

Lithium-ion batteries have been extensively used for portable devices and electric vehicles. For the latter use of lithium-ion batteries, rapid charge is required in addition to long cycle lives, high safety, etc. We have focused on the rate capabilities of lithium-ion batteries by considering the lithium-ion transfer at electrode/electrolyte interface and also ion transport inside composite electrodes. Lithium-ion transfer at electrode/electrolyte interface was studied by using various model electrodes. Highly oriented pyrolytic graphite (HOPG) was used as a model electrode of graphite negative electrode. Since the basal plane of HOPG has limited reaction sites, we can observe large lithium-ion (charge) transfer resistances by Ac impedance spectroscopy. Then, it is quite easy to evaluate the activation energies for lithium-ion transfer at HOPG electrode. The values are ranging from 50-60 kJ/mol. For the positive electrodes, we used thin films prepared by pulsed laser deposition (PLD). We fabricated LiCoO2 and LiMn2O4 thin film electrodes. By using the thin films, the lithium-ion transfer resistances become also clear. Again, the activation energies were obtained and the values are almost the same with those of graphite. Based on the activation energies for lithium-ion transfer at negative and positive electrodes, it becomes quite clear that the rate capabilities of lithium-ion batteries at the lower temperature region are quite dependent on the lithium-ion transfer resistances. We then focused on the ion transfer resistances inside the composite negative and positive electrodes. We used the four-probe cell to evaluate the ion transport resistances inside composite electrodes by Ac impedance spectroscopy. Consequently, the ion transport inside composite electrodes was found to be very slow for the high density electrodes. By the above results, I will talk about the rate determining steps of lithium-ion batteries at the conference. Acknowledgment This work was partially supported by CREST, JST.

Nanoscale Electrochemistry of sp2 Carbon Materials: From Graphite and Graphene to Carbon Nanotubes

Carbon materials have a long history of use as electrodes in electrochemistry, from (bio)electroanalysis to applications in energy technologies, such as batteries and fuel cells. With the advent of new forms of nanocarbon, particularly, carbon nanotubes and graphene, carbon electrode materials have taken on even greater significance for electrochemical studies, both in their own right and as components and supports in an array of functional composites. With the increasing prominence of carbon nanomaterials in electrochemistry comes a need to critically evaluate the experimental framework from which a microscopic understanding of electrochemical processes is best developed. This Account advocates the use of emerging electrochemical imaging techniques and confined electrochemical cell formats that have considerable potential to reveal major new perspectives on the intrinsic electrochemical activity of carbon materials, with unprecedented detail and spatial resolution. These techniques allow particular features on a surface to be targeted and models of structure-activity to be developed and tested on a wide range of length scales and time scales. When high resolution electrochemical imaging data are combined with information from other microscopy and spectroscopy techniques applied to the same area of an electrode surface, in a correlative-electrochemical microscopy approach, highly resolved and unambiguous pictures of electrode activity are revealed that provide new views of the electrochemical properties of carbon materials. With a focus on major sp(2) carbon materials, graphite, graphene, and single walled carbon nanotubes (SWNTs), this Account summarizes recent advances that have changed understanding of interfacial electrochemistry at carbon electrodes including: (i) Unequivocal evidence for the high activity of the basal surface of highly oriented pyrolytic graphite (HOPG), which is at least as active as noble metal electrodes (e.g., platinum) for outer-sphere redox processes. (ii) Demonstration of the high activity of basal plane HOPG toward other reactions, with no requirement for catalysis by step edges or defects, as exemplified by studies of proton-coupled electron transfer, redox transformations of adsorbed molecules, surface functionalization via diazonium electrochemistry, and metal electrodeposition. (iii) Rationalization of the complex interplay of different factors that determine electrochemistry at graphene, including the source (mechanical exfoliation from graphite vs chemical vapor deposition), number of graphene layers, edges, electronic structure, redox couple, and electrode history effects. (iv) New methodologies that allow nanoscale electrochemistry of 1D materials (SWNTs) to be related to their electronic characteristics (metallic vs semiconductor SWNTs), size, and quality, with high resolution imaging revealing the high activity of SWNT sidewalls and the importance of defects for some electrocatalytic reactions (e.g., the oxygen reduction reaction). The experimental approaches highlighted for carbon electrodes are generally applicable to other electrode materials and set a new framework and course for the study of electrochemical and interfacial processes.

Protein sensors based on reversible π–π stacking on basal plane HOPG electrodes

In this study, the modification of basal planes of highly oriented pyrolytic graphite (HOPG) electrodes with pyrene-functionalised biotin (PFB), via π–π stacking, and ethylene glycol antifouling molecules, via covalent bonding, for detection of streptavidin is presented. Biotin was first conjugated to the pyrene moieties by an esterification reaction in order to enable the self-assembly of biotin onto the surface of HOPG via non-covalent π–π stacking. The as-prepared biotinylated electrode was used as the sensing probe to analyze the concentration of streptavidin via the diminution in pyrene electrochemistry resulted from the desorption of pyrene from the surface that is mediated by the biotin–streptavidin recognition. X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), square wave voltammetry (SWV) measurements, and electrochemical impedance spectroscopy (EIS) were used to characterize the amount of surface bound pyrene modified biotin and the concentration of streptavidin. This simple strategy can be applied to the detection of other biomolecules via bio-recognition and the reversible π–π stacking process.

Voltammetric and in situ frequency modulation atomic force microscopic investigation of phenalenyl derivatives adsorbed on graphite surfaces

A combined electrochemical and in situ frequency modulation atomic force microscopy (FM-AFM) study of phenalenyl derivatives, tetra tert-butyl derivatives of s-indacenodiphenalene (TTB-IDPL), adsorbed on a highly oriented pyrolytic graphite (HOPG) electrode was investigated. The cyclic voltammetry (CV) showed stable and adsorption-type redox peaks from TTB-IDPL at the double-layer region of the HOPG basal plane, but the peak became much more intense by adding the anodic potential. In situ FM-AFM images of the molecular film revealed the casted molecules formed aggregated islands on the HOPG surface at cathodic potentials. When anodic potentials were applied, the height of aggregates increased and finally became mobile on the surface. By stepping back to the cathodic potentials, the aggregates dispersed as isolated molecules that were invisible by AFM measurements. This morphology change resulted in the increase of the CV peak originated from TTB-IDPL. Reversible dispersion–aggregation processes depending on the electrode potential were imaged by FM-AFM. The size of aggregates at each potential indicates that the origin of the dispersion of aggregates could be the change of the interfacial free energy, which is dependent on the electrode potential. These AFM results aid in the correlation of the structure of the adsorbate layers with their electrochemical response.

Comparison and Reappraisal of Carbon Electrodes for the Voltammetric Detection of Dopamine

The electro-oxidation of dopamine (DA) is investigated on the unmodified surfaces of five different classes of carbon electrodes: glassy carbon (GC), oxygen-terminated polycrystalline boron-doped diamond (pBDD), edge plane pyrolytic graphite (EPPG), basal plane pyrolytic graphite (BPPG), and the basal surface of highly oriented pyrolytic graphite (HOPG), encompassing five distinct grades with step edge density and coverage varying by more than 2 orders of magnitude. Surfaces were prepared carefully and characterized by a range of techniques, including atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), and Raman spectroscopy. Although pBDD was found to be the least susceptible to surface fouling (even at relatively high DA concentrations), the reaction showed sluggish kinetics on this electrode. In contrast, DA electro-oxidation at pristine basal plane HOPG at concentrations ≤100 μM in 0.15 M PBS, pH 7.2, showed fast kinetics and only minor susceptibility toward surface fouling from DA byproducts, although the extent of HOPG surface contamination by oxidation products increased substantially at higher concentrations (with the response similar on all grades, irrespective of step edge coverage). EPPG also showed a fast response, with little indication of passivation with repeated voltammetric cycling but a relatively high background signal due to the high capacitance of this graphite surface termination. Of all five carbon electrode types, freshly cleaved basal plane HOPG showed the clearest signal (distinct from the background) at low concentrations of DA (<10 μM) as a consequence of the low capacitance. Studies of the electrochemical oxidation of DA in the presence of the common interferents ascorbic acid (AA) and serotonin (5-HT), of relevance to neurochemical analysis, showed that the signals for DA were still clearly and easily resolved at basal plane HOPG surfaces. In the presence of AA, repetitive voltammetry caused products of AA electro-oxidation to adsorb onto the HOPG surface, forming a permselective film that allowed the electrochemical oxidation of DA to proceed unimpeded, while greatly inhibiting the electrochemical response of AA itself. The studies presented provide conclusive evidence that the pristine surface of basal plane HOPG is highly active for the detection of DA, irrespective of the step edge density and method of cleavage, and adds to a growing body of evidence that the basal plane of HOPG is a much more active electrode for many classes of electrode reactions than previously believed.

Morphology and modulus evolution of graphite anode in lithium ion battery: An in situ AFM investigation

We investigated the interfacial electrochemical processes on graphite anode of lithium ion battery by using highly oriented pyrolytic graphite (HOPG) as a model system. In situ electrochemical atomic force microscopy experiments were performed in 1 M lithium bis(trifluoromethanesulfonyl)imide/ethylene carbonate/diethyl carbonate to reveal the formation process of solid electrolyte interphase (SEI) on HOPG basal plane during potential variation. At 1.45 V, the initial deposition of SEI began at the defects of HOPG surface. After that, direct solvent decomposition took place at about 1.3 V, and the whole surface was covered with SEI. The thickness of SEI was 10.4 ± 0.2 nm after one cycle, and increased to 13.8 ± 0.2 nm in the second cycle, which is due to the insufficient electron blocking ability of the surface film. The Young’s modulus of SEI was measured by a peak force quantitative nanomechanical mapping (QNM). The Young’s modulus of SEI is inhomogeneous. The statistic value is 45 ± 22 MPa, which is in agreement with the organic property of SEI on basal plane of HOPG.

Electron transfer of phenalenyl derivative molecules adsorbed at the graphite electrode/aqueous solution interface

The phenalenyl derivative molecules, diphenyl derivatives of 1,2-bis-(phenalen-1-ylidene) ethene (Ph2-BPLE) and tetra t-butyl derivatives of s-indacenodiphenalene (TTB-IDPL), casted on a highly oriented pyrolytic graphite (HOPG) electrode, were characterized by using cyclic voltammetry (CV) in aqueous electrolyte solution. Stable and adsorption type redox peaks at the double-layer region of a HOPG basal plane electrode revealed that the redox peaks originated from the molecules. It was found that the peak area of the CV curve, the amount of redox-active adsorbates, apparently increased after anodic potential scans up to +1.4V vs. RHE without any additional redox-active molecules in the liquid phase. Quantitative analyses indicated that it was due to potential-induced dispersion of the phenalenyl derivative molecules on the electrode, and effective electron transfer occurred only from the molecule directly attached to the electrode surface.

Bovine serum albumin film as a template for controlled nanopancake and nanobubble formation: In situ atomic force microscopy and nanolithography study

Air nanobubbles and nanopancakes were investigated in situ by both tapping mode atomic force microscopy (TM AFM) and atomic force nanolithography techniques employing bovine serum albumin (BSA) film supported by highly oriented pyrolytic graphite (HOPG). The BSA denaturation induced by the water-to-ethanol exchange served for conservation of nanobubble and nanopancake sites appearing as imprints in BSA film left by gaseous cavities formerly present on the interface in the aqueous environment. Once the BSA film was gently removed by the nanoshaving technique applied in ethanol, a clean basal plane HOPG area with well-defined dimensions was regenerated. The subsequent reverse ethanol-to-water exchange led to the re-formation of nanopancakes specifically at the nanoshaved area. Our approach paves the way for the study of gaseous nanostructures with defined dimensions, formed at solid–liquid interface under controlled conditions.

Insight into STM image contrast of n-tetradecane and n-hexadecane molecules on highly oriented pyrolytic graphite

Two-dimensional ordered patterns of n-tetradecane (n-C14H30) and n-hexadecane (n-C16H34) molecules at liquid/graphite interface have been directly imaged using scanning tunneling microscope (STM) under ambient conditions. STM images reveal that the two different kinds of molecules self-organize into ordered lamellar structures in which alkane chains of the molecules extend along one of three equivalent lattice axes of highly oriented pyrolytic graphite (HOPG) basal plane. For n-C14H30 molecules, the molecular axes are observed to tilt by 60° with respect to inter-lamellar trough lines and the carbon backbones of the alkane chains are perpendicular to the HOPG basal plane in an all-trans conformation. However, for n-C16H34 molecules, the molecular axes are perpendicular to lamellar borders (90°) and the planes of the all-trans carbon skeletons are parallel to the graphite basal plane. The results clearly indicate that outmost hydrogen atoms of the alkane chains dominate atom-scaled features of the STM images. That is, in the case of long-chain alkane molecules, topographic effects dominantly determine STM image contrast of the methylene regions of the alkane chains that are adsorbed on HOPG.

Nanoparticles Change the Ordering Pattern of n-Carboxylic Acids into Nanorods on HOPG

This paper describes the formation of organic nanorods induced by monolayer-protected inorganic nanoparticles. Alkanes and alkane derivatives, such as n-carboxylic acids, self-assemble on highly oriented pyrolytic graphite (HOPG) into a persistent molecular packing structure that is dictated by the epitaxial interaction between the carbon chain plane and the HOPG basal plane. Carboxylic acids form 2-D crystalline layers consisting of nanostripe domains whose periodicity is one or two times the molecular chain length. However, when the molecular ordering occurs in the vicinity of a nanoparticle, this persistent HOPG-dominated nanostripe pattern is disrupted, and nanorods attached to the nanoparticles become the dominant structure. In order to understand the underlying mechanism of the nanoparticle-mediated nanorod formation, the effects of film-forming conditions, carboxylic acid chain length, nanoparticle size, and chemical composition of the nanoparticle are examined. It is determined that carboxylic acid nanorods can be induced by nanoparticles of different core materials including CdSe, CdS, and Au, as long as the protecting monolayer allows sufficient dispersion and colloidal stability of the nanoparticles in solution. A carboxylic chain length range amenable to the nanorod formation is identified, as is the relationship between the nanoparticle size and the number of nanorods per nanoparticle. This study contributes to the understanding of seed-mediated crystallization and molecular ordering. Moreover, it defines the parameters governing solution-based formation of hybrid nanostructures and nanopatterns incorporating dual functionality as defined by the inorganic nanoparticle and organic nanorod, respectively.

Role of Edge Orientation in Kinetics of Electrochemical Intercalation of Lithium-Ion at Graphite

A relation between the amount of edge orientations and the kinetics of lithium-ion intercalation at the basal plane of highly oriented pyrolytic graphite (HOPG) was studied. The fraction of edge orientations (f(edge)) exposed on the basal plane of HOPG was evaluated from the kinetics of heterogeneous electron transfer of Ru(NH(3))(6)(2+/3+). Obtained f(edge) values ranged from 0.025 to 0.067, which were in good agreement with those previously reported. The charge-transfer resistance (R(ct)) for lithium-ion intercalation at the same HOPG samples was evaluated by ac impedance spectroscopy in an electrolyte consisting of 1 mol dm(-3) LiClO(4) dissolved in a mixture of ethylene carbonate/dimethyl carbonate (1:1 by vol). A clear correlation was observed between the f(edge) and R(ct) values at the basal plane of 15 HOPG samples, and the edge-area specific resistance was evaluated to be ca. 200 Ω cm(2) at the electrode potential of 0.2 V versus Li/Li(+). These results indicate that the amount of edge orientations is one of the determining factors in the kinetics of lithium-ion intercalation at graphite.

Platinum nanostructured HOPG – Preparation, characterization and reactivity

A present challenge in electrocatalysis is to identify highly active catalysts. Therefore it is of fundamental importance to achieve a better understanding of the activity of catalyst materials in dependence of morphology and supporting material. Here, we used platinum nanoparticles supported on highly oriented pyrolytic graphite (HOPG) to investigate catalytic properties for hydrogen related reactions. The activity of Pt nanoparticles supported on carbon for application in fuel cells and platinum on glassy carbon has been investigated extensively for hydrogen related reactions in the past. However, there are only few experimental investigations on the activity of Pt nanoparticles supported on a HOPG basal plane, a well-defined support regarding geometric and electronic structure. Platinum was deposited electrochemically from solution containing K 2PtCl 6 by means of potentiostatic single pulse method to produce nanostructured HOPG surfaces with different particle sizes (in the range between <10 nm and 50 nm) and particle densities. The samples were characterized electrochemically and via SPM techniques with respect to morphology, size and distribution of the platinum nanoparticles. The results on the catalytic activity of Pt/HOPG regarding HER show no pronounced dependence on the particle size or coverage of the substrate. This differs from results obtained for the nanostructured systems Pt/Au(1 1 1) and Pd/Au(1 1 1) reported in literature, which show a strong enhancement in HER activity for decreasing Pt or Pd coverage. We therefore conclude that the support can strongly influence the activity of platinum nanoparticles.

Slow Diffusion Reveals the Intrinsic Electrochemical Activity of Basal Plane Highly Oriented Pyrolytic Graphite Electrodes

This paper reports a method for distinguishing the electroactivity of different types of sites on heterogeneous electrode surfaces, exemplified through studies of basal plane highly oriented pyrolytic graphite (HOPG) electrodes. By depositing a thin film of Nafion with incorporated redox species (i.e., tris(2-2′-bipyridyl)ruthenium(II), Ru(bpy)32+, and hexaaminoruthenium(III), [Ru(NH3)6]3+) onto HOPG, diffusion is greatly slowed down. On the time scale of cyclic voltammetry, one can then distinguish between different scenarios of electrode activity because sites on the electrode, with different activity, become diffusionally decoupled. In particular, we show that one can discriminate readily between limiting cases in which the basal plane of HOPG is considered to be either (i) completely active or (ii) inert (with only step edges active). Experimental measurements coupled to modeling show unequivocally that the basal plane of HOPG is electrochemically active. The methodology described and the results obtained have important implications for understanding the intrinsic activity of the basal plane and step edges of graphite electrodes and related carbon-based electrode materials.

Scanning Micropipet Contact Method for High-Resolution Imaging of Electrode Surface Redox Activity

A scanning micropipet contact method (SMCM) is described which promises wide-ranging application in imaging and quantifying electrode activity at high spatial resolution. In SMCM, a moveable micropipet probe (diameter 300 nm to 1 microm) containing an electroactive species in electrolyte solution is brought to a sample electrode surface so that the liquid meniscus makes contact. The micropipet contains a reference-counter electrode, and the sample is connected as the working electrode to make a two-electrode voltammetric measurement. SMCM thus makes possible highly localized electrochemical experiments, and furthermore, heterogeneous electrode surfaces may be investigated without the substrate being completely immersed in solution. This opens up the possibility of making measurements on a wide range of electrode materials without having to encapsulate the electrode. Furthermore, the electrode/solution contact can be made rapidly and briefly, which is useful for situations where the electrode would be unstable for longer periods (e.g., due to corrosion or surface adsorption). For heterogeneously active surfaces the technique is particularly powerful as it allows defined areas to be targeted and individual sites to be probed. To exemplify the approach, the electroactivity of basal plane highly oriented pyrolytic graphite (HOPG) and two types of aluminum alloy were investigated. SMCM measurements indicate that basal plane HOPG shows much greater activity than present consensus. Measurements of chemically heterogeneous aluminum alloy surfaces with SMCM allow variations in redox activity to be mapped with high spatial resolution.

Mechanism for Electrochemical Oxidation of Highly Oriented Pyrolytic Graphite in Sulfuric Acid Solution

In order to clarify the oxidation mechanism of carbon materials in polymer electrolyte fuel cells (PEFCs), electrochemical oxidation of highly oriented pyrolytic graphite (HOPG) was investigated in sulfuric acid solution at less than vs normal hydrogen electrode. Surface oxidation was accelerated at , whereas oxidation kinetics seemed to be slow at . In situ electrochemical atomic force microscopy clearly showed that hillock structure was developed with a formation of C–O bonding on the basal plane of HOPG in the early stage of the oxidation process. Hence, the hydrolysis reaction to produce alcohol and other functional groups was considered as the initiation reaction for electrochemical oxidation of HOPG. Accumulation of the surface strain by the formation of hillocks may lead to development of the oxidized layer. The oxidation kinetics of further process accompanied with an evolution of CO or could be slower than that of the hydrolysis reaction, which might be ascribed to the low electron conductivity of the oxidized HOPG surface. The mechanism for electrochemical oxidation of HOPG contributes an elucidation of carbon oxidation reactions in PEFCs.

Chemical sputtering of room temperature ATJ graphite and HOPG by slow atomic and molecular D ions

We present experimental results for methane production from ATJ graphite and highly oriented pyrolytic graphite (HOPG) produced by atomic and molecular D ions in the energy range 5–150eV/D. For the ATJ graphite target, a systematic trend of the methane yields for the different molecular species compared at the same impact energy/D is observed. While all three species lead to methane yields that coincide within the experimental uncertainty at the high energy end of the investigated range, at lower energies the yields diverge progressively; the incident triatomic molecular ion leads to the largest yields per atom, and the atomic ion to the smallest. The difference at the lowest investigated energy (10eV/D) is about a factor of two. The measured yields were found to be in good agreement with recent in house molecular dynamics simulations. Results are presented for two different HOPG basal plane orientations relative to the surface plane, one parallel, the other perpendicular. Differences in the methane yields for the two orientations are observed at the higher investigated energies that suggest reduced diffusion from the sample bulk when the basal planes are oriented parallel to the surface plane. A similar reduction is not observed in case of acetylene production, suggesting that the acetylene is produced mainly at the target surface.

Growth and Reactions of SiOx/Si Nanostructures on Surface-Templated Molecule Corrals

Surface-templated nanostructures on the highly oriented pyrolytic graphite (HOPG) basal plane were created by controlled Cs+- or Ga+)ion bombardment, followed by subsequent oxidation at high temperature, forming molecule corrals. The corrals were then used for template growth of SiOx/Si nanostructures. We demonstrate here that, for SiOx/Si nanostructures formed in controlled molecule corrals, the amount of silicon deposited on the surface is directly correlated with the corral density, making it possible to generate patterned SiOx/Si nanostructures on HOPG. Since the size, depth, position, and surface density of the nanostructures can be controlled on the HOPG, it is possible to produce surfaces with patterned or gradient functionalities for applications in fields such as biosensors, microelectronics, and biomaterials (e.g., neuron pathfinding). If desired, the SiOx structures can be reduced in size by etching in dilute HF, and further oxidation of the nanostructures is slow enough to provide plenty of time to functionalize them using ambient and solution reactions and to perform surface analysis. Organosilane monolayers on surface-templated SiOx/Si nanostructures were examined by X-ray photoelectron spectroscopy, time-of-flight secondary ion mas spectrometry, and atomic force microscopy. Silanes with long alkyl chains such as n-octadecyltrichlorosilane (C18) were found to both react on SiOx/Si nanostructures and to condense on the HOPG basal plane. Shorter-chain silanes, such as 11-bromoundicyltrimethoxysilane (C11) and 3-mercaptopropyltrimethoxysilane (C3) were found to react preferentially with SiOx/Si nanostructures, not HOPG. The SiOx/Si nanostructures were also found to be stable toward multiple chemical reactions. Selective modification of SiOx/Si nanostructures on the HOPG basal plane is thus achievable.