Highly Oriented Pyrolytic Graphite Surface Research Articles

Tip convolution on HOPG surfaces measured in AM-AFM and interpreted using a combined experimental and simulation approach

Amplitude modulated atomic force microscopy (AM-AFM) was used to examine the influence of the size of the AFM tip apex on the measured surface topography of single highly oriented pyrolytic graphite (HOPG) atomic steps. Experimental measurements were complemented by molecular dynamics simulations of AM-AFM and the results from both were evaluated by comparison of the measured or simulated width of the topography at the step to that predicted using simple rigid-body geometry. The results showed that the step width, which is a reflection of the resolution of the measurement, increased with tip size, as expected, but also that the difference between the measured/simulated step width and the geometric calculation was tip size dependent. The simulations suggested that this may be due to the deformation of the bodies and the effect of that deformation on the interaction force and oscillation amplitude. Overall, this study showed that the resolution of AM-AFM measurements of atomic steps can be correlated to tip size and that this relationship is affected by the deformation of the system.

Defects of clean graphene and sputtered graphite surfaces characterized by time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy

Clean surface of graphene was obtained at 500 °C and characterized by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). In the XPS C1s spectrum of graphene, besides an asymmetric sp2 carbon peak and a π-π∗ shake-up peak appeared, an additional sp3 carbon peak representing sp3 defects was also present. In the ToF-SIMS positive ion spectrum of graphene, a series of CxH2+ ions originated from the defects of graphene was found. To determine the origin of the CxH2+ ions, defects were created on the surface of nearly defect-free highly oriented pyrolytic graphite (HOPG) by bombarding it with a Cs+ ion beam at various sputtering doses. A detailed examination of the positive ion spectra of ion-bombarded HOPG surfaces reveals the presence of the CxH2+ ions, confirming that these CxH2+ ions, which came from the defects created on the sputtered HOPG surfaces, are similar to the defects present on graphene surface. A sp3 carbon peak at 285.3 eV, representing sp3 defects appeared in the XPS spectra of sputtered HOPG samples, confirms that the surface of the sputtered HOPG is similar to that of graphene. Fragmentation mechanisms of selected ions in the ToF-SIMS spectra of graphene and HOPG were proposed.

Thermal melting and ablation dynamics on a femtosecond laser-heated highly-oriented pyrolytic graphite surface

Time-resolved optical reflection microscopy studies demonstrate spatiotemporal dynamics of melting and ablation of graphite surface molten by single IR femtosecond laser pulses, which are revealed by monitoring picosecond oscillations of the probe reflectivity modulated by transient acoustic reverberations in the surface melt. Temporal periods and amplitudes of the reverberations are affected through transient variations of melt thickness and acoustic impedance by melting, thermal expansion, spallation and fragmentation processes, thus enabling quantitative evaluation of their contributions and basic parameters.

Self-assembly of a sulfur-bridged annulene: Substrate effect and donor-acceptor complex

We have studied the self-assembly structure of meso-diphenyltetrathia [22]-annulene [2,1,2,1] (DPTTA) on highly oriented pyrolytic graphite (HOPG), Au(111), and single-layer graphene (SLG) modified Au(111) substrates. High resolution scanning tunneling microscopy (STM) reveals that DPTTA molecules pack into one dimensionally ordered row structure on graphene and HOPG surfaces, while assemble into two dimensional close-packed structure on Au(111) surface. We ascribe this difference to the effect of the substrate. The addition of C60 molecules on these DPTTA modified substrates further reveals that the structural difference in DPTTA adlayer can affect its ability to form donor-acceptor (D-A) hierarchical structure with C60 molecules. The results provide an example of substrate effect in self-assemblies of functional molecules, which is significant for the design of molecular based devices.

Extended monolayer of cyano-ended oligo(para-phenylenes) at the air/HOPG interface investigated by high-resolution AFM

The formation of functional networks on surfaces is one of the main challenges in the field of nanotechnologies. In this paper, we shall propose a very simple process which can be used to achieve the formation of extended monolayer of functional oligo(para-phenylenes) molecules at the air/graphite interface. By developing a convergent strategy, we successfully achieved the synthesis of oligo(para-phenylenes) molecules with a tuneable length. The photophysical properties of these new oligomers were characterized by UV–vis absorption and fluorescence spectroscopy. Deposition of these molecules by a simple spin-coating process on a highly oriented pyrolytic graphite (HOPG) surface leads to the formation of extended monolayered 2D networks. These networks were characterized by atomic force microscopy experiments under ambient conditions with submolecular resolution thus providing the adsorption model of these molecules on an HOPG surface.

In-Situ AFM Observation of Highly Oriented Pyrolytic Graphite in PC-Based Electrolyte Solution Containing Magnesium Ion

Introduction Graphite is used as a negative active material in lithium-ion batteries. The surface film, called solid electrolyte interphase (SEI) formed in ethylene carbonate (EC)-based electrolyte solution, plays an important role in reversible lithium-ion intercalation and de-intercalation reaction. When propylene carbonate (PC) is used as organic solvent, electrolyte decomposition and exfoliation of graphite layers occurred. We reported that lithium-ion intercalation and de-intercalation in PC-based electrolytes was succeeded using high concentrated lithium salt electrolytes or containing multivalent cations[1-3]. However, the SEI formation mechanism in PC-based electrolyte solution containing multivalent cation was not clarified. In this study, we investigate the surface morphology changes of highly oriented pyrolytic graphite (HOPG) electrode in PC-based electrolytes containing Mg2+ by in-situ Atomic Force Microscopy (AFM) to clarify the SEI formation mechanism. Experimental A three-electrode cell was used for electrochemical measurements. A HOPG was used as a working electrode and lithium foils were used as a counter electrode and a reference electrode. Electrolytes were prepared from lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2, LiTFSA), magnesium bis(trifluoromethanesulfonyl)imide (Mg(N(SO2CF3)2)2, Mg(TFSA)2), and PC. The molar ratio of two cation was fixed to be Mg2+/Li = 1.2. AFM observation was conducted during the CV measurements. All potentials were referred to Li/Li+. Results Figure 1 shows the cyclic voltammograms and in-situ AFM image at 1st cycle of HOPG. Large reduction peak at around 0.7 V appeared in the first cycle but disappeared in the second cycle. This result indicated that irreversible electrolyte decomposition, which was related to SEI formation, occurred at the initial cycle. In the AFM image obtained between 0.508 V and 1.204 V, precipitates appeared on the HOPG surface at 0.66 V. In EC-based electrolyte solution, hill-like structures and blister structures by the co-intercalation and decompostion of solvated-lithium ion were observed around 0.9 V. However, these structures were not observed in this system. Therefore, it is proposed that the addition of Mg2+ suppressed the co-intercalation of solvated lithium ion and SEI was formed by the surface reaction. Reference [1] S.-K. Jeong et al., J. Power Sources, 175 (2008) 540. [2] S. Takeuchi et al., Electrochim. Acta, 56 (2011) 10450. [3] S. Takeuchi et al., submitted. Acknowledgment This work was partially supported by CREST, JST and JSPS KAKENHI Grant Number 16H04216. Figure 1

Origin of Asymmetry of Paired Nanogap Voltammograms Based on Scanning Electrochemical Microscopy: Contamination Not Adsorption.

Formation of a nanometer-wide gap between tip and substrate electrodes by scanning electrochemical microscopy (SECM) enables voltammetric measurement of ultrafast electron-transfer kinetics. Herein, we demonstrate the advantage of SECM-based nanogap voltammetry to assess the cleanness of the substrate surface in solution by confirming that airborne contamination of highly oriented pyrolytic graphite (HOPG) causes the nonideal asymmetry of paired nanogap voltammograms of (ferrocenylmethyl)trimethylammonium (Fc(+)). We hypothesize that the amperometric response of a 1 μm-diameter Pt tip is less enhanced in the feedback mode, where more hydrophilic Fc(2+) is generated from Fc(+) at the tip and reduced voltammetrically at the HOPG surface covered with airborne hydrophobic contaminants. The tip current is more enhanced in the substrate generation/tip collection mode, where less charged Fc(+) is oxidized at the contaminated HOPG surface. In fact, symmetric pairs of nanogap voltammograms are obtained with the cleaner HOPG surface that is exfoliated in humidified air and covered with a nanometer-thick water adlayer to suppress airborne contamination. This result disproves a misconception that the asymmetry of paired nanogap voltammograms is due to electron exchange mediated by Fc(2+) adsorbed on the glass sheath of the tip. Moreover, weak Fc(+) adsorption on the HOPG surface causes only the small hysteresis of each voltammogram upon forward and reverse sweeps of the HOPG potential. Significantly, no Fc(2+) adsorption on the HOPG surface ensures that the simple outer-sphere pathway mediates ultrafast electron transfer of the Fc(2+/+) couple with standard rate constants of ≥12 cm/s as estimated from symmetric pairs of reversible nanogap voltammograms.

Metal dependent motif transition in a self-assembled monolayer of bipyridine derivatives via coordination: An STM study.

Low-dimensional molecular motifs with diversity developed via the on-surface chemistry are attracting growing interest for their potential in advanced nanofabrication. In this work, scanning tunneling microscopy was employed to investigate the in situ and ex situ metal coordinations between 4,4'-ditetradecyl-2,2'-bipyridine (bpy) and Zn(ii) or Cu(ii) ions at a highly oriented pyrolytic graphite (HOPG)/1-phenyloctane interface under ambient conditions. The results demonstrate that the bpy adopts a flat-lying orientation with its substituted alkyl chains in a tail-to-tail arrangement in a bpy monolayer. For the in situ coordination, the bpy/Zn(ii) and bpy/Cu(ii) complexes are aligned in edge-on fashions, wherein the bpy stands vertically on the HOPG surface and interdigitates at the alkyl chains. In the two-dimensional arrays of ex situ coordinated complexes, metal dependent motifs have been observed with Zn(ii) and Cu(ii), wherein the bipyridine moieties are parallel to the graphite surface. These results suggest that the desired on-surface coordination architectures may be achieved by the intentional selection of the metal centers.

Self-Assembly of Concentric Hexagons and Hierarchical Self-Assembly of Supramolecular Metal–Organic Nanoribbons at the Solid/Liquid Interface

In an effort to exert more precise control over structural features of supramolecules, a series of giant concentric hexagons were assembled as discrete structures using tetratopic terpyridine (tpy) ligands. In preparation of tetratopic ligand, pyrylium and pyridinium salts chemistry significantly facilitated synthesis. The key compounds were obtained by condensation reactions of pyrylium salts with corresponding primary amine derivatives in good yields. These discrete metallo-supramolecular concentric hexagons were fully characterized by NMR, ESI-MS, TWIM-MS, and TEM, establishing their hexagon-in-hexagon architectures. The combination of different tetratopic ligands also assembled hybrid concentric hexagons with increasing diversity and complexity. Furthermore, these concentric hexagon supramolecules with precisely controlled shapes and sizes were utilized as building blocks to hierarchically self-assemble supramolecular metal-organic nanoribbons (SMON) at solid-liquid interfaces. Ambient STM imaging showed the formation of long 1D SMON rather than 2D assembly on the basal plane of highly oriented pyrolytic graphite (HOPG) surface after simple dropcasting of the solution of preassembled concentric hexagons onto a freshly cleaved surface of HOPG. This wet chemical method based on self-assembly may offer simple, economical, and scalable routes to deliver complex materials.

Protolytic decomposition of n-octane on graphite at near room temperature.

Graphite basal surface is inert, and decomposition of n-alkanes on the graphite surface has not been discovered. We here report the evidence of decomposition of n-octanes on highly oriented pyrolytic graphite (HOPG) surface, heat-treated up to 1200 °C under high vacuum (10−7 Pa), at near room temperatures. Using a temperature programmed desorption apparatus equipped with a quadrupole mass spectrometer showed the production of hydrogen molecules, methane, and ethane, suggesting that the protonation of n-octane takes place on graphite surface at near room temperature. It is known that acidic functional groups are terminated at edges on the air-cleaved HOPG surface and they increase their acidity via reactions with water. However, it is most unlikely that they protonate n-alkanes at near room temperature such as superacids. We anticipate that superacidic protons, which can protonate n-octanes, are produced on the graphite surface through a novel reaction mechanism.

Apparent corrugation variations on moiré patterns on highly oriented pyrolytic graphite

Many different moiré patterns were reported on highly oriented pyrolytic graphite (HOPG) surfaces in scanning tunneling microscopy (STM) studies. Such observations were attributed to the dislocation of the top most graphene layers of the crystal with respect to the lower layers. So far, the corrugation height difference of a moiré pattern observed by STM was reported to be independent of the tunnel junction bias regardless of the periodicity of the pattern. With the method we developed to generate moiré patterns on HOPG surfaces quite easily and unharmfully to the crystals, we investigated a large number of different moiré patterns. In this work we report on the apparent corrugation height variations of moiré patterns, as a function of tunnel junction bias in STM images. We show that this change varies from moiré pattern to moiré pattern, and does not necessarily follow a continuous increase or decrease. We show strong experimental evidence that while several moiré patterns may have structural origins; most of them have almost purely electronic character.

Formation and composition of adsorbates on hydrophobic carbon surfaces from aqueous laccase-maltodextrin mixture suspension

A robust procedure for the surface bio-functionalization of carbon surfaces was developed. It consists on the modification of carbon materials in contact with an aqueous suspension of the enzyme laccase from Trametes versicolor and the lyophilization agent maltodextrin, with the pH value adjusted close to the isoelectric point of the enzyme. We report in-situ investigations applying Quartz Crystal Microbalance with Dissipation (QCM-D) for carbon-coated sensor surfaces and, moreover, ex-situ measurements with static contact angle measurements, X-ray Photoelectron Spectroscopy (XPS) and Scanning Force Microscopy (SFM) for smooth Highly Oriented Pyrolytic Graphite (HOPG) substrates, for contact times between the enzyme formulation and the carbon material surface ranging from 20s to 24h. QCM-D studies reveals the formation of rigid layer of biomaterial, a few nanometers thin, which shows a strongly improved wettability of the substrate surface upon contact angle measurements. Following spectroscopic characterization, these layers are composed of mixtures of laccase and maltodextrin. The formation of these adsorbates is attributed to attractive interactions between laccase, the maltodextrin-based lyophilization agent and the hydrophobic carbon surfaces; a short-term contact between the aqueous laccase mixture suspension and HOPG surfaces is shown to merely result in de-wetting patterns influencing the results of contact angle measurements. The new enzyme-based surface modification of carbon-based materials is suggested to be applicable for the improvement of not only the wettability of low energy substrate surfaces with fluid formulations like coatings or adhesives, but also their adhesion in contact with hardened polymers.

Contact Angle Hysteresis on Graphene Surfaces and Hysteresis-free Behavior on Oil-infused Graphite Surfaces

Contact angle hysteresis (CAH) on graphitic surfaces, including chemical vapor deposition (CVD) graphene, reduced electrophoretic deposition (EPD) graphene, highly oriented pyrolytic graphite (HOPG), and polished graphite sheet, has been investigated. The hysteresis loops of water drops on the first three samples are similar but the receding contact angle is particularly small for the polished graphite sheet.The significant CAH observed on CVD graphene and HOPG associated with atom-scale roughness has to be attributed mainly to adhesion hysteresis (surface relaxation), instead of roughness or defects.The difference of the wetting behavior among those four graphitic samples has been further demonstrated by hexadecane drops. On the surface of HOPG or CVD graphene,the contact line expands continuously with time, indicating total wetting for which the contact angle does not exist and contact line pinning disappears. In contrast, on the surface of reduced EPD graphene, spontaneous spreading is halted by spikes on it and partial wetting with small contact angle (θ≈4°) is obtained. On the surface of polished graphite sheet, the superlipophilicity and porous structure are demonstrated by imbibition and capillary rise of hexadecane. Consequently, an oil-infused graphite surface can be fabricated and the ultralow CAH of water (∆θ≈2°) is achieved.

Amorphous nanostructuralization in HOPG by 1014 W cm-2 laser

This reports provide an amorphous nanostructuralization technique on the surface modification in Highly Oriented Pyrolytic Graphite (HOPG) by using a femtosecond laser. We showed, for the first time, that the surface of HOPG is changed to the amorphous nanostructuralization graphite by using a femtosecond laser-driven compression technique. Our results also suggest that the HOPG surface is changed until the deeper area from the surface by the laser-driven shock wave. A single shot of a femtosecond laser beam (1.27 ∼ 1.33×1014 Wcm∼2 in intensity, with 2 mm-diameter, and 110 fs in pulse width) is irradiated under the vacuum ambience onto a 2 mm-thick of HOPG. The calculated impact pressures on a sample was 8.3 ∼ 8.7 GPa. Crystal structure in the HOPG were analyzed using a Raman spectroscopy and an X-ray diffraction, those analyzing depth from the surface were 50 nm and 350 μm, respectively.

Incorporation of low energy activated nitrogen onto HOPG surface: Chemical states and thermal stability studies by in-situ XPS and Raman spectroscopy

In this paper we report the chemical states analysis of activated nitrogen incorporated highly oriented pyrolytic graphite (HOPG) surface under well-controlled conditions. Nitrogen incorporation is carried out by two different processes: an indirect RF nitrogen plasma and low energy (1keV) N2+ implantation. Bonding configuration, concentration and thermal stability of the incorporated nitrogen species by aforesaid processes are systematically compared by in-situ X-ray photoelectron spectroscopy (XPS). Relatively large concentration of nitrogen is incorporated onto RF nitride HOPG surface (16.2 at.%), compared to N2+ implanted HOPG surface (7.7 at.%). The evolution of N 1s components (N1, N2, N3) with annealing temperature is comprehensively discussed, which indicates that the formation and reorganization of local chemical bonding states are determined by the process of nitridation and not by the prior chemical conditioning (i.e., amorphization or hydrogenation) of the HOPG surface. A combined XPS and Raman spectroscopy studies revealed that N2+ implantation process resulted in a high level of defects to the HOPG surface, which cannot be annealed-out by heat treatment up to 1000°C. On the other hand, the RF nitrogen plasma process did not produce a high level of surface defects, while incorporating nearly the same amount of stable nitrogen species.

Study of the electrode tool wear and the probe tip sharpening phenomena during the nanoscale STM electric discharge lithography of the bulk HOPG surface

Scanning tunneling microscope (STM)-based electric discharge lithography or machining is capable of fabricating nanostructures or nano-devices out of the carbon-based materials with low cost and high accuracy. As a tip-based nanofabrication method, the evolution of the STM probe tip, which functionalized as the electrode tool, is of great importance to the machining capability and performance of the STM electric discharge lithography. In this work, both the electrode tool wear (ETW) and the probe tip sharpening phenomena were investigated during the SPM electric discharge lithography of the bulk highly oriented pyrolytic graphite (HOPG) sample surface. The influence of the electrode tool wear evolution on the machining results was experimentally studied. The geometric parameters of the STM probe tip were measured from SEM imaging. Moreover, the probe tip sharpening phenomenon was specifically observed by applying the negative polarity machining. The evidences of the underlying mechanisms for the tip sharpening effect were revealed experimentally in this study, which have never been reported in the previous work. It is further proved that this effect could be utilized to the in-situ sharpening of the STM probe tip. Finally, the effective methods to improve the machining consistency and the tool lifetime were proposed.

Well-controlled preparation of evenly distributed nanoporous HOPG surface via diazonium salt assisted electrochemical etching process

Evenly distributed nanoporous highly oriented pyrolytic graphite (HOPG) surfaces with controllable pore size were successfully prepared via diazonium salt assisted electrochemical etching method. The porous HOPG was investigated by atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) Raman spectroscopy and X-ray diffraction. The size of these pores can be tuned by manipulating the electrochemical etching time. These porous HOPG substrates also demonstrated the enhanced electrocatalytical behaviour and were employed as benign arena for the immobilization of Ru(bpy)32+ for electrochemiluminescence (ECL) sensing applications.

Electrochemistry of Fe3+/2+ at highly oriented pyrolytic graphite (HOPG) electrodes: kinetics, identification of major electroactive sites and time effects on the response.

The electrochemistry of the Fe3+/2+ redox couple has been studied on highly oriented pyrolytic graphite (HOPG) samples that differ in step edge density by 2 orders of magnitude, to elucidate the effect of surface structure on the electron transfer (ET) kinetics. Macroscopic cyclic voltammetry measurements in a droplet-cell arrangement, highlight that the Fe3+/2+ process is characterised by slow ET kinetics on HOPG and that step edge coverage has little effect on the electrochemistry of Fe3+/2+. A standard heterogeneous ET rate constant of ∼5 × 10-5 cm s-1 for freshly cleaved HOPG was derived from simulation of the experimental results, which fell into the range of the values reported for metal electrodes, e.g. platinum and gold, despite the remarkable difference in density of electronic states (DOS) between HOPG and metal electrodes. This provides further evidence that outer-sphere redox processes on metal and sp2 carbon electrodes appear to be adiabatic. Complementary surface electroactivity mapping of HOPG, using scanning electrochemical cell microscopy, reveal the basal plane to be the predominant site for the Fe3+/2+ redox process. It is found that time after cleavage of the HOPG surface has an impact on the surface wettability (and surface contamination), as determined by contact angle measurements, and that this leads to a slow deterioration of the kinetics. These studies further confirm the importance of understanding and evaluating surface structure and history effects in HOPG electrochemistry, and how high resolution measurements, coupled with macroscopic studies provide a holistic view of electrochemical processes.

XPS study of gold oxidation with nitrogen dioxide in model Au/C samples

The interaction of NO2 with model samples obtained by the gold sputter deposition onto the surface of highly oriented pyrolytic graphite (HOPG) has been studied by X-ray photoelectron spectroscopy (XPS). It has been shown that 3D metal particles characterized by an Au4f7/2 binding energy (BE) of 84.0 eV form on the initial smooth graphite surface. During the sputter deposition onto the surface of HOPG preliminarily activated by means of ion etching, gold atoms chemically bound to the carbon atoms form in the area of surface defects. Such atoms are characterized by a positive shift of BE(Au4f7/2). It has been established that the 3D particles are resistant to the action of NO2 under the pressure XXXXXXXX mbar at room temperature. On the contrary, atomic gold oxidizes under these conditions into Au(III) complexes bound to the graphite surface. It is assumed that gold atoms on the support surface play the role of active sites of gold catalysts in oxidation reactions.

Effect of adsorption of butyl xanthate on galena, PbS, and HOPG surfaces as studied by atomic force microscopy and spectroscopy and XPS

Atomic force microscopy (AFM) and force spectroscopy techniques have emerged as powerful tools for in situ studies of interactions between solid surfaces, reactants and air bubbles which are of fundamental importance for understanding the mechanisms and increasing the efficiency of froth flotation. Such studies, however, have very rarely involved real mineral sulfides and flotation collectors to date. Here, the tapping mode AFM, force spectroscopy with conventional silicon cantilevers, and ex situ XPS were used to characterize surfaces of single crystals of natural PbS and, for comparison, highly oriented pyrolytic graphite (HOPG) in pure water before and after their pre-treatment in 0.1mM and 10mM potassium n-butyl xanthate (KBX) solutions. Submicrometer domains with the height about 3–8nm were observed on the PbS surfaces with chemisorbed xanthate. The force–distance curves measured upon retraction of the tip from PbS and HOPG surfaces pre-treated in KBX solutions revealed the attraction forces, often with two sudden pull-off steps, acting at the separation distances longer than 100nm. We attributed the soft domains to gas nanobubbles; the long-range attraction was explained in terms of the capillary forces induced by a vapor cavity arising between the tip and the hydrophobic substrates upon their disconnection, and coalescence of the cavity with the pre-existing nanobubbles. The role of these phenomena in flotation is briefly discussed.