Highly Oriented Pyrolytic Graphite Surface Research Articles

RHEED Study of the Epitaxial Growth of Silicon and Germanium on Highly Oriented Pyrolytic Graphite

Two-dimensional silicon (silicene) and germanium (germanene) have attracted special attention from researchers in recent years. At the same time, highly oriented pyrolytic graphite (HOPG) and graphene are some of the promising substrates for growing silicene and germanene. However, to date, the processes occurring during the epitaxial growth of silicon and germanium on the surface of such substrates have been poorly studied. In this work, the epitaxial growth of silicon and germanium is studied directly during the process of the molecular beam epitaxy deposition of material onto the HOPG surface by reflection high-energy electron diffraction (RHEED). In addition, the obtained samples are studied by Raman spectroscopy and scanning electron microscopy. A wide range of deposition temperatures from 100 to 800 °C is considered and temperature intervals are determined for various growth modes of silicon and germanium on HOPG. Conditions for amorphous and polycrystalline growth are distinguished. Diffraction spots corresponding to the lattice constants of silicene and germanene are identified that may indicate the presence of areas of graphene-like 2D phases during epitaxial deposition of silicon and germanium onto the surface of highly oriented pyrolytic graphite.

Toward a Comprehensive Understanding of the Anomalously Small Contact Angle of Surface Nanobubbles.

Experimental studies have demonstrated that the gas phase contact angle (CA) of a surface nanobubble (SNB) is much smaller than that of a macroscopic gas bubble. This reduced CA plays a crucial role in prolonging the lifetime of SNBs by lowering the bubble pressure and preventing gas molecules from dissolving in the surrounding liquids. Despite extensive efforts to explain the anomalously small CA, a consensus about the underlying reasons is yet to be reached. In this study, we conducted experimental investigations to explore the influence of gas molecules adsorbed at the solid-liquid interface on the CA of SNBs created through the solvent exchange (SE) method and temperature difference (TD). Interestingly, no significant change is observed in the CA of SNBs on highly oriented pyrolytic graphite (HOPG) surfaces. Even for nanobubbles on micro/nano pancakes, the CA only exhibited a slight reduction compared to SNBs on bare HOPG surfaces. These findings suggest that gas adsorption at the immersed solid surface may not be the primary factor contributing to the small CA of the SNBs. Furthermore, the CA of SNBs formed on polystyrene (PS) and octadecyltrichlorosilane (OTS) substrates was also investigated, and a considerable increase in CA was observed. In addition, the effects of other factors including impurity, electric double layer (EDL) line tension, and pinning force upon the CA of SNBs were discussed, and a comprehensive model about multiple factors affecting the CA of SNBs was proposed, which is helpful for understanding the abnormally small CA and the stability of SNBs.

Atomic force microscopy investigation of DNA denaturation on a highly oriented pyrolytic graphite surface

Understanding of DNA interaction with carbonaceous surfaces (including graphite, graphene and carbon nanotubes) is important for the development of DNA-based biosensors and other biotechnological devices. Though many issues related to DNA adsorption on graphitic surfaces have been studied, some important aspects of DNA interaction with graphite remain unclear. In this work, we use atomic force microscopy (AFM) equipped with super-sharp cantilevers to analyze the morphology and conformation of relatively long DNA molecule adsorbed on a highly oriented pyrolytic graphite (HOPG) surface. We have revealed the effect of DNA embedding into an organic monolayer of N,N′-(decane-1,10-diyl)-bis(tetraglycinamide) (GM), which may “freeze” DNA conformation on a HOPG surface during drying. The dependence of the mean squared point-to-point distance on the contour length suggests that DNA adsorbs on a bare HOPG by a “kinetic trapping” mechanism. For the first time, we have estimated the unfolded fraction of DNA upon contact with a HOPG surface (24 ± 5 %). The obtained results represent a novel experimental model for investigation of the conformation and morphology of DNA adsorbed on graphitic surfaces and provide with a new insight into DNA interaction with graphite.

Graphite surface modification by quasi-periodically structured diazonium monolayer

In this study, a novel functionalization protocol of highly oriented pyrolytic graphite (HOPG) was proposed, in which a two dimensional (2D) porous network formed by alkoxy-substituted dehydrobenzo[12]annulene (DBA-OC12) was served as a sacrificial template for electrografting of 3,5-terbutyl diazonium (3,5-TBD). The 2D DBA-OC12 porous network was synthesized by drop-casting of the corresponding precursor followed by a drying process. The structural and electrochemical properties of pristine as well as functionalized HOPG surfaces were investigated by means of cyclic voltammetry (CV) and scanning tunneling microscopy (STM).

The regulation of surface nanobubble generation via solvent exchange on different substrates

Surface nanobubbles (SNBs) play an essential role in fluid transport, interfacial reactions, electrochemistry, material fabrication, surface cleaning, etc. Controllable preparation of SNBs is very crucial for their applications, such as controlling material preparation and increasing the efficiency of surface cleaning. Ethanol-water exchange, as a typical example of solvent exchange, is one of the most widely used methods to produce SNBs. By controlling the conditions, it is possible to manipulate the size and number density of nanobubbles. In this work, we systematically investigated the formation of SNBs on hydrophobic and hydrophilic surfaces by ethanol-water exchange at different rates using atomic force microscopy. Results indicate that the exchange rate has a significant effect on the size and number density of SNBs. On both hydrophobic surface of highly oriented pyrolytic graphite (HOPG) and hydrophilic surface of mica, formed nanobubbles exhibit smaller sizes and higher number density with increased exchange rates. But at the same solvent exchange rate, SNBs generated on HOPG surface were in greater number and width than on mica, while their heights change much less. To understand the effect of exchange rate on nanobubble production, we proposed a mechanism of non-uniform nucleation and diffusion-driven growth. A simulation of the gas concentration near SNBs was performed, and the results show good agreement between numerical calculations and experimental data. We believe that our results will contribute to the understanding of nanobubble generation and stabilization processes. It also provides a new perspective on the factors that affect the generation of SNBs, which allows the production and application of nanobubbles to be controlled in a reliable manner.

Fabricating 2D Host −Guest Kagomé Packing for C2-Symmetric Aromatic Carboxylic Acids with Different Spatial Configuration: Different Ways, Same Destination

The self-assembly of two kinds of C2-symmetric aromatic carboxylic acids named 4,4′,4″,4‴-(1,4-phenylenebis(azanetriyl))tetrabenzoic acid (H4PTA) and 5′,5″-bis(4-carboxyphenyl)-[1,1′:3′,1″:3″,1‴-quaterphenyl]-4,4‴-dicarboxylic acid (H4QDA) and the coadsorption with coronene (COR) molecules at different solution concentrations were investigated at the heptanoic acid (HA)/highly oriented pyrolytic graphite (HOPG) interface by scanning tunneling microscopy (STM). H4PTA molecules with nonplanar conformation self-assembled into a highly ordered rhombus structure at variational concentrations and subsequently could be regulated into a Kagomé network by the coadsorption of COR molecules. H4QDA molecules with planar conformation self-assembled into two various nanostructures (rhombus structure and Kagomé network) coexisting on the HOPG surface at different concentrations. The Kagomé architecture of H4QDA could act as a rigid host template to trap the COR molecules. Meanwhile, COR exhibited preferential adsorption in the porous template: COR only entered the hexagonal cavities at low concentrations and filled in all hexagonal and triangular cavities at high concentrations. Density functional theory (DFT) calculations and molecular dynamics (MD) simulations showed that the host–guest co-assembled structures were more thermodynamically and kinetically stable. The formation of different self-assembly and co-assembly processes of two molecules could be attributed to the dissimilar molecular conformation. Our work is of significance to further explore the formation mechanism of two-dimensional (2D) porous arrangements and provides an ideal way to regulate the adsorption of porous templates.

Study on the fabrication of diazonium molecular monolayer material on graphite substrate viaelectrochemical grafting approach

Surface denaturation of carbon surfaces with molecular monolayer is appointed as one of the most effective bottom-up techniques to enlarge the applicability of the carbon-based electrodes in the electrochemical sensor field. Diazonium molecules have been often employed to denature the surface of materials such as graphite and graphene through the formation of C-C chemical bonds between them. However, the layered formation of these molecules is not well controlled yet due to their intrinsic high reactivity. This report demonstrated a practical approach for covalent functionalisation of the highly oriented pyrolytic graphite (HOPG) surface, a model system of multilayered graphene, by using 3,5-bis-tert-butylbenzenediazonium molecules. Due to the substituents at the 3,5 positions the aryl radicals prefer bonding directly to the HOPG surface than attacking the grafted aryls towards the monolayered formation. The efficiency of this approach was determined by the combined cyclic voltammetry, atomic force microscopy and scanning tunnelling microscopy.

Growth and Characterization of Ultrathin Vanadium Oxide Films on HOPG.

Integration of graphene into various electronic devices requires an ultrathin oxide layer on top of graphene. However, direct thin film growth of oxide on graphene is not evident because of the low surface energy of graphene promoting three-dimensional island growth. In this study, we demonstrate the growth of ultrathin vanadium oxide films on a highly oriented pyrolytic graphite (HOPG) surface, which mimics the graphene surface, using (oxygen-assisted) molecular beam epitaxy, followed by a post-annealing. The structural properties, surface morphology, and chemical composition of the films have been systematically investigated by in situ reflection high-energy electron diffraction during the growth and by ex situ techniques, such as atomic force microscopy, scanning tunneling microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS). Crystalline monolayer vanadium oxide can be achieved on HOPG by systematically tuning the deposition time of V atoms and by subsequent annealing at 450 °C in controlled atmospheres. Increasing the partial pressure of O2 during the deposition seems to decrease the mobility of V atoms on the graphitic surface of HOPG and promote the formation of a two-dimensional (2D) vanadium oxide. The obtained oxide layers are found to be polycrystalline with an average grain size of 15 nm and to have a mixed-valence state with mainly V5+ and V4+. Moreover, XPS valence band measurements indicate that the vanadium oxide is insulating. These results demonstrate that a 2D insulating vanadium oxide can be grown directly on HOPG and suggest vanadium oxide as a promising candidate for graphene/oxide heterostructures.

Quantitative Characterization of Oxygen-Containing Groups on the Surface of Carbon Materials: XPS and NEXAFS Study

The results of the comparative quantitative study of oxygen-containing groups adsorbed on the surface of carbonized sponge scaffold (CSS), highly oriented pyrolytic graphite (HOPG), fullerite C60 and multi-walled carbon nanotubes (MWCNTs) introduced into a high vacuum from the atmosphere without any pre-treatment of the surface are discussed. The studied materials are first tested by XRD and Raman spectroscopy, and then quantitatively characterized by XPS and NEXAFS. The research results showed the presence of carbon oxides and water-dissociation products on the surfaces of materials. It was shown that main source of oxygen content (~2%) on the surface of HOPG, MWCNTs, and C60 powder is water condensed from the atmosphere in the form of an adsorbed water molecule and hydroxyl group. On the CSS surface, oxygen atoms are present in the forms of carbon oxides (4–5%) and adsorbed water molecules and hydroxyl groups (5–6%). The high content of adsorbed water on the CSS surface is due to the strong roughness and high porosity of the surface.

Probing tip-induced attractive deformation of graphite surfaces through wave function dissipation in field emission resonance

We studied wave function dissipation (WFD) in field emission resonance (FER) by performing scanning tunneling microscopy on the highly oriented pyrolytic graphite (HOPG) and Ag(111) surfaces under two conditions: (1) the same current and FER number; (2) the same tip structure but different currents. Under the first condition, we observed that the decay rate corresponding to the WFD exhibited a larger variation on the HOPG surface than it did on the Ag(111) surface. Under the second condition, the decay rate was nearly independent of the FER electric field for the Ag(111) surface; by contrast, it was linearly proportional to the FER electric field for the HOPG surface. These remarkable differences can be attributed to the factors that the tip-induced attractive deformation caused by the electrostatic force was considerably more prominent on the HOPG surface than on the Ag(111) surface and that the deformed HOPG top layer had a unique electronic structure similar to that of single-layer graphene.

Preparation and characterization of non-aromatic ether self-assemblies on a HOPG surface

On-surface self-assemblies of aromatic organic molecules have been widely investigated, but the characterization of analogous self-assemblies consisting of fully sp3-hybridized molecules remains challenging. The possible on-surface orientations of alkyl molecules not exclusively comprised of long alkyl chains are difficult to distinguish because of their inherently low symmetry and non-planar nature. Here, we present a detailed study of diamondoid ethers, structurally rigid and fully saturated molecules, which form uniform 2D monolayers on a highly oriented pyrolytic graphite (HOPG) surface. Using scanning tunneling microscopy, various computational tools, and x-ray structural analysis, we identified the most favorable on-surface orientations of these rigid ethers and accounted for the forces driving the self-organization process. The influence of the oxygen atom and London dispersion interactions were found to be responsible for the formation of the observed highly ordered 2D ether assemblies. Our findings provide insight into the on-surface properties and behavior of non-aromatic organic compounds and broaden our understanding of the phenomena characteristic of monolayers consisting of non-planar molecules.

Hole-injection barrier across the intermolecular interaction mediated interfacial DNTT layer

The electronic structures and morphology of dinaphthothienothiophene (DNTT) thin films on highly oriented pyrolytic graphite (HOPG) and polycrystalline Au (pc-Au) substrates have been investigated using photoelectron spectroscopy and atomic force microscopy techniques to better correlate the charge-injection barrier with the interfacial structure. DNTT molecules follow Stranski–Krastanov-type growth mode, i.e., highly-dewetted islands/fibers above a relatively-wetted interfacial layer, on both the substrates, but distinctively different electronic structures. On the HOPG surface, the as-grown and thermal-annealed films show a broad and a sharp highest occupied molecular orbital (HOMO) bands, respectively. On the pc-Au surface, a significantly split HOMO band and a reduced hole-injection barrier are observed in a monolayer thick film, for the first time, which are absent in other thick films. Such splitting indicates a presence of a strong intermolecular interaction in the interfacial layer, presumably due to the frontier π-orbital overlapping arising from partially-covered densely-packed herringbone-like arranged molecules. The dewetted-fibers seem to form with up-right oriented molecules having negligible interaction, which desorb easily by thermal annealing to retain the highly-stable interfacial layer. The reduction of the hole-injection barrier at the DNTT/pc-Au interface, due to the formation of such interfacial layer having strong intermolecular interaction, has an immense importance in increasing the charge-injection efficiency from a practical electrode.

Tuning the morphological and electrical properties of graphite surface by self-assembled viologen nanostructures

In this contribution, a combination of cyclic voltammetry (CV), atomic force microscopy (AFM), electrochemical scanning tunneling microscopy (EC-STM) and Kelvin probe force microscopy (KPFM) is used to characterize the electrochemical, structural and electronic properties of the dibenzyl viologen (DBV) molecules based adlayers electrochemically deposited on highly oriented pyrolytic graphite (HOPG) surface. The adlayer formations on HOPG are dependent on its redox states in-situ modulated by the electrode potential and the used concentration. Relying on the intrinsic solubility of individual reduced molecules one enables permanent modulation the modification degree of HOPG surface. In addition, at the potentials where HOPG surface is merely modified by DBV0 molecules, the electrical properties of HOPG surface can also be modulated by various DBV2+ concentrations as the variable.

Ionic liquids on uncharged and charged surfaces: In situ microstructures and nanofriction

In situ changes in the nanofriction and microstructures of ionic liquids (ILs) on uncharged and charged surfaces have been investigated using colloid probe atomic force microscopy (AFM) and molecular dynamic (MD) simulations. Two representative ILs, [BMIM][BF4] (BB) and [BMIM][PF6] (BP), containing a common cation, were selected for this study. The torsional resonance frequency was captured simultaneously when the nanoscale friction force was measured at a specified normal load; and it was regarded as a measure of the contact stiffness, reflecting in situ changes in the IL microstructures. A higher nanoscale friction force was observed on uncharged mica and highly oriented pyrolytic graphite (HOPG) surfaces when the normal load increased; additionally, a higher torsional resonance frequency was detected, revealing a higher contact stiffness and a more ordered IL layer. The nanofriction of ILs increased at charged HOPG surfaces as the bias voltage varied from 0 to 8 V or from 0 to —8 V. The simultaneously recorded torsional resonance frequency in the ILs increased with the positive or negative bias voltage, implying a stiffer IL layer and possibly more ordered ILs under these conditions. MD simulation reveals that the [BMIM]+ imidazolium ring lies parallel to the uncharged surfaces preferentially, resulting in a compact and ordered IL layer. This parallel “sleeping” structure is more pronounced with the surface charging of either sign, indicating more ordered ILs, thereby substantiating the AFM-detected stiffer IL layering on the charged surfaces. Our in situ observations of the changes in nanofriction and microstructures near the uncharged and charged surfaces may facilitate the development of IL-based applications, such as lubrication and electrochemical energy storage devices, including supercapacitors and batteries.

Toward Single-Atomic-Layer Lithography on Highly Oriented Pyrolytic Graphite Surfaces Using AFM-Based Electrochemical Etching

Atomic force microscopy (AFM)-based electrochemical etching of a highly oriented pyrolytic graphite (HOPG) surface is studied toward the single-atomic-layer lithography of intricate patterns. Electrochemical etching is performed in the water meniscus formed between the AFM tip apex and HOPG surface due to a capillary effect under controlled high relative humidity (~ 75%) at otherwise ambient conditions. The conditions to etch nano-holes, nano-lines, and other intricate patterns are investigated. The electrochemical reactions of HOPG etching should not generate debris due to the conversion of graphite to gaseous CO and CO2 based on etching reactions. However, debris is observed on the etched HOPG surface, and incomplete gasification of carbon occurs during the etching process, resulting in the generation of solid intermediates. Moreover, the applied potential is of critical importance for precise etching, and the precision is also significantly influenced by the AFM tip wear. This study shows that the AFM-based electrochemical etching has the potential to remove the material in a single-atomic-layer precision. This result is likely because the etching process is based on anodic dissolution, resulting in the material removal atom by atom.

The surface defects of HOPG induced by low-energy Ar+ ion irradiation

Low-energy heavy ion irradiation is a promising surface modification technique due to its ability of precise controlling and tuning the density and location of defects in materials. In this work, 1-keV Ar+ ions with irradiation doses from 9.6 × 1012 ions/cm2 to 3.5 × 1015 ions/cm2 has been used to bombard the pristine highly oriented pyrolytic graphite (HOPG) at elevated temperature. The irradiated HOPG surfaces have been analyzed by multi-methods. Huge and sub-nm hillock-like surface defects are mainly concentrated on the irradiated HOPG surface, and are well consistent with the results from the Monte Carlo simulation. In particular, the surface defects exhibit a high thermal stability during the annealing procedure. The number of the surface defects increases as the irradiation dose increases and reaches the saturation point at a high dose, which is explained by the formation of vacancy clusters in collision cascades. The enhancement of light absorption in the ultraviolet-visible range for the irradiated surface is observed and mainly ascribed to the synergistic effect of the huge and sub-nm hillocks-like surface defect and defect energy levels in the forbidden bandgaps.

In-situ high-speed atomic force microscopy observation of dynamic nanobubbles during water electrolysis

The nanobubbles (NB) formed on a solid surface has been reported, although the stability in a solution is still discussed. The atomic force microscopy (AFM) has shown the unique shape of the flatted NB, however the dynamic behavior has not investigated yet. Recently, the high scanning speed AFM (HS-AFM) has been developed and applied to the several interfaces. Here, we present in-situ HS-AFM observation during water electrolysis. The hydrogen and oxygen NB evolution on highly oriented pyrolytic graphite (HOPG) electrode are studied. Our video data is the first time to demonstrate the NB nucleation and the growth. The processes are different between both gases. The hydrogen NB grows with active coalescence, while the oxygen one is smaller and irregularly moves on HOPG surface. Using this technique, we will be able to study the NB stability influence by some factors, such as the surface potential and electric capillarity.

Nucleation, growth, and stability of WSe2 thin films deposited on HOPG examined using in situ, real-time synchrotron x-ray radiation

We have examined the nucleation and growth of WSe2 thin films in ultrahigh vacuum on highly oriented pyrolytic graphite (HOPG) using in situ real-time x-ray fluorescence (XRF), and ex situ x-ray diffraction, x-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy. We employed W(CO)6 as the W source delivered via a supersonic molecular beam, Sen delivered via an effusion cell, and we examined substrate temperatures from 400 to 540 °C. Crystalline, near stoichiometric thin films were formed at temperatures Ts ≤ 470 °C, whereas those formed at 540 °C were very W-rich. The thin films were not continuous but consisted of filamentlike features with spikelike edges. A focus of our work was to examine the initial stages of growth and the effects of extinguishing one of the species (W or Se) both before and during growth. First, in all cases examined, there was a delay in the onset of a measurable rate of growth on the clean HOPG surface following the introduction of both species, W(CO)6 and Sen. In cases where the incident flux of W(CO)6 was gated, once WSe2 growth had commenced, extinguishing the flux of W(CO)6 quenched growth immediately and did not result in the deposition of additional Se. Once the incident flux of W(CO)6 was re-started, growth began essentially immediately. The pattern with Sen gating was strikingly different. In this case, once WSe2 growth had commenced, extinguishing the flux of Sen resulted in a continuing uptake of W essentially unabated, while the amount of Se in the thin film decreased, which resulted in an oscillation in the Se-to-W content in the thin film. As the thin films were stable in UHV in the absence of both species, the incident W(CO)6 is responsible for the etching of Se, which we postulate is due to a ligand exchange reaction forming volatile SeCO.

Influence of supporting electrolyte on electrochemical formation of copper nanoparticles and their electrocatalytic properties

Comparative analysis of copper nanoparticles (CuNPs) obtained by electrodeposition on highly oriented pyrolytic graphite (HOPG) substrates from different supporting electrolytes containing sulphate anions, was performed. Voltammetric results indicated that Cu electrodeposition follows a diffusion-controlled nucleation and crystal growth model for three solutions studied (Na2SO4, H2SO4 and Na2SO4+H2SO4). Na2SO4 solution was found to be most effective because the copper reduction occurs at most positive potential value, reaching the highest current density. Analysis of potentiostatic current transients revealed that the process can be described predominantly by a model involving 3D-progressive nucleation mechanism, which was corroborated by scanning electron microscopy (SEM) analysis. SEM images showed high density of hemispherical shaped Cu particles of different sizes (mostly between 80-150 nm), randomly distributed on the HOPG surface for Na2SO4 electrolyte solution. In the presence of H2SO4, the size dispersion decreased, resulting in particles with greater diameters (up to 339 nm). The use of electrolyte solution with Na2SO4+H2SO4 revealed lower particle density with a considerable crystal size dispersion, where very small crystallites are prevailing. Cyclic voltammetry was used to evaluate qualitatively the catalytic activity of CuNPs deposited from three electrolyte solutions towards the nitrate reduction reaction. An enhanced catalytic effect was obtained when copper particles were prepared from either Na2SO4 or H2SO4 supporting electrolytes.

Double Lamellar Morphologies and Odd–Even Effects in Two- and Three-Dimensional N,N′-bis(n-alkyl)-naphthalenediimide Materials

The fabrication of highly ordered nanostructured surfaces is desirable in supramolecular chemistry and envisaged to bolster advances in heterogeneous catalysis and microelectronic applications. Here, we report on a novel set of alkylated double N,N′-bis(n-alkyl)-naphthalenediimides (NDIs) for the functionalization of highly oriented pyrolytic graphite (HOPG) with precise double lamellar morphologies. A detailed analysis of the two-dimensional (2D) self-assembled monolayers by scanning tunneling microscopy (STM) reveals that the structural repeating unit of the double lamellae is tuned precisely by the length of the alkyl chain that is connecting the NDI units. However, the expected odd–even effect is disturbed within the monolayers of a series of homologues. In contrast, a clear odd–even effect is observed for the melting temperatures of the respective bulk materials. Small-angle X-ray scattering reveals that these bulk materials exhibit nanophase-separated lamellar phases with domain spacings that are slightly larger than the repeating units of the double lamellar structures formed on the HOPG surface. The discrepancy is assigned to a partial desorption of the alkyl spacer from the HOPG surface, which becomes more pronounced when increasing its length. Our findings suggest that this lengthening increases the conformational freedom of the molecules on the surface while retaining a double lamellar morphology.