Pyrolytic Graphite Substrate Research Articles

Domain wall currents and ferroelectric domain structures of PbTiO3 thin films on highly oriented pyrolytic graphite (HOPG) substrates

Domain wall currents and ferroelectric domain structures of PbTiO3 thin films on highly oriented pyrolytic graphite (HOPG) substrates

Growth and ferroelectric properties of SrBi2Ta2O9 thin films on highly oriented pyrolytic graphite substrates with atomic structure like graphene surface

Growth and ferroelectric properties of SrBi2Ta2O9 thin films on highly oriented pyrolytic graphite substrates with atomic structure like graphene surface

Nghiên cứu tính chất điện hóa, cấu trúc và khả năng xúc tác khử điện hóa oxygen của vật liệu Fe-Porphyrin trên bề mặt HOPG

This paper describes an efficient approach to fabricate Fe-Porphyrin thin film deposited on highly oriented pyrolytic graphite substrate (HOPG/FePP) via the dip-coating method serving as a novel electrocatalyst for oxygen reduction reaction (ORR) in acidic medium. The electrochemical, morphological behaviors and surface structure at the molecular level of the HOPG/FePP as well as its catalytic activities were characterized upon employing a state-of-the-art toolbox including cyclic voltammetry (CV), atomic force microscopy (AFM), electrochemical scanning tunneling microscopy (EC-STM) and linear sweep voltammetry (LSV). Consequently, the HOPG/ FePP thin film exhibited a significantly enhanced catalytic activity for ORR under applied experimental conditions.

Molecular beam epitaxy synthesis of In2Se3 films

The effects of substrate choice, substrate temperature, Se/In flux ratio, and cooling rate after deposition on the phase composition, surface morphology, and stoichiometry of indium selenide films synthesized via molecular beam epitaxy are presented. In2Se3 films were synthesized on sapphire, Si(111) and highly oriented, pyrolytic graphite (HOPG) substrates. The phase composition, stoichiometry, and surface morphology of the films were characterized via Raman spectroscopy, x-ray photoelectron spectroscopy, and atomic force microscopy, respectively. Higher substrate temperature combined with higher Se/In ratio promoted formation of β-In2Se3 over γ and/or κ-In2Se3 on all substrates. Higher Se/In ratio also independently promoted β-In2Se3 over γ and/or κ-In2Se3 on all substrates at 673 K. The lateral dimensions of In2Se3 flakes increased as the substrate temperature increased on all substrates, and the largest lateral dimensions were observed for β-In2Se3 flakes on HOPG at 973 K. No evidence of α-In2Se3 was observed in the Raman spectra of any of the films at any of the synthesis conditions in this study. β-In2Se3 films on HOPG were cooled at 1200, 120, and 12 K/h and no evidence of a β to α-In2Se3 phase transition was observed. Some evidence of β to α-In2Se3 phase transition was observed in temperature-dependent XRD of In2Se3 powders, suggesting that another parameter besides cooling rate is locking the In2Se3 films into the β-phase.

Synthesis of Cd-Cu nanoparticles supported on HOPG for use as electrocatalytic material for nitrate reduction

Bimetallic nanoparticles application has attracted great attention in the field of electrocatalysis because they can enhance the catalytic properties toward certain reactions of technological interest, such as nitrate reduction. For this purpose, Cd-Cu bimetallic nanoparticles were prepared on highly oriented pyrolytic graphite substrates by sequential electrodeposition of both metals. The process was studied by conventional electrochemical techniques, and the generated deposits were characterized by scanning electron microscopy − energy dispersive X-ray spectroscopy analysis. Initially, Cu nanoparticles were prepared by a simple potentiostatic pulse applied to the carbonaceous substrate. Scanning electron microscopy images showed high density of mainly hemispherical-shaped Cu crystals distributed preferably on step edges and surface defects of the highly oriented pyrolytic graphite electrode. Subsequently, Cd particles were prepared in the same way by immersing the Cu nanoparticles / highly oriented pyrolytic graphite modified substrate in an electrolyte solution containing Cd2+ ions, exploiting the phenomenon of underpotential deposition. It was evidenced that the existing crystals grew in size, most of them showing the typical shapes of those for Cd nanoparticles. The application of the Cd-Cu nanoparticles / highly oriented pyrolytic graphite electrode as a feasible electrocatalyst material for nitrate reduction was investigated by cyclic voltammetry for different nitrate concentrations, also considering the possibility of using this procedure as a method for the anion detection.

Electronic properties of individual CsPbI2Br nanocrystals investigated by LT-STM

Solution-processed metal halide perovskite nanocrystals show promise for various potential optoelectronic applications and the exploration of the fundamental physics underlying them. However, the electronic properties of individual nanocrystals have not been thoroughly studied. Here, we applied low-temperature scanning tunneling microscopy to investigate the properties of metal halide perovskite CsPbI2Br nanocrystals with a diameter in the range of 10–20 nm. Sub-monolayer dispersions of the nanocrystal on highly oriented pyrolytic graphite and gold thin film substrates were achieved. Using scanning tunneling microscopy, we resolved topographies of individual nanocrystals on the gold film, and their electronic properties were probed by scanning tunneling spectroscopy. In our experiment, no obvious dependence of the extracted energy gap on the nanocrystal size and shape was found, which is consistent with the reported small exciton Bohr radius in metal halide perovskite materials. Additionally, we observed that the energy gaps of some nanocrystals are smaller than that of the bulk, suggesting the influence of factors such as deep-level defects/traps, ion migration, etc. on the electronic structure.

Evidence of thickness-dependent surface-induced ferroelectricity in few-layer germanium sulfide obtained via scanning tunneling spectroscopy.

The discovery of ferroelectricity in two-dimensional van der Waals materials has sparked enormous interest from the scientific community, due to its possible applications in next-generation nanoelectronic devices, such as random-access memory devices, digital signal processors, and solar cells, among others. In the present study, we used vapor phase deposition to synthesize ultrathin germanium sulfide nano-flakes on a highly oriented pyrolytic graphite substrate. Nanostructures of variable thicknesses were characterized using scanning tunneling microscopy and spectroscopy. Tunneling currents under forward and backward biases were measured as a function of nano-flake thickness. Remarkably, we clearly observed a hysteresis pattern, which we attributed to surface ferroelectric behavior, consistent with the screening conditions of polarization charges. The effect increases as the number of layers is reduced. This experimental result may be directly applicable to miniaturized memory devices, given the two-dimensional nature of this effect.

A Computational Study of Cluster Dynamics in Structural Lubricity: Role of Cluster Rotation

We present a computational study of sliding between gold clusters and a highly oriented pyrolytic graphite substrate, a material system that exhibits ultra-low friction due to structural lubricity. By means of molecular dynamics, it is found that clusters may undergo spontaneous rotations during manipulation as a result of elastic instability, leading to attenuated friction due to enhanced interfacial incommensurability. In the case of a free cluster, shear stresses exhibit a non-monotonic dependency on the strength of the tip-cluster interaction, whereby rigid clusters experience nearly constant shear stresses. Finally, it is shown that the suppression of the translational degrees of freedom of a cluster’s outermost-layer can partially annihilate out-of-plane phonon vibrations, which leads to a reduction of energy dissipation that is in compliance with Stokesian damping. It is projected that the physical insight attained by the study presented here will result in enhanced control and interpretation of manipulation experiments at structurally lubric contacts.

Electronic Structure and Surface Chemistry of Hexagonal Boron Nitride on HOPG and Nickel Substrates.

The effect of point defects and interactions with the substrate are shown by density functional theory calculations to be of significant importance for the structure and functional properties of hexagonal boron nitride (h-BN) films on highly ordered pyrolytic graphite (HOPG) and Ni(111) substrates. The structure, surface chemistry, and electronic properties are calculated for h-BN systems with selected intrinsic, oxygen, and carbon defects and with graphene hybrid structures. The electronic structure of a pristine monolayer of h-BN is dependent on the type of substrate, as h-BN is decoupled electronically from the HOPG surface and acts as bulk-like h-BN, whereas on a Ni(111) substrate, metallic-like behavior is predicted. These different film/substrate systems therefore show different reactivities and defect chemistries. The formation energies for substitutional defects are significantly lower than for intrinsic defects regardless of the substrate, and vacancies formed during film deposition are expected to be filled by either ambient oxygen or carbon from impurities. Significantly lower formation energies for intrinsic and oxygen and carbon substitutional defects were predicted for h-BN on Ni(111). In-plane h-BCN hybrid structures were predicted to be terminated by N-C bonding. Substitutional carbon on the boron site imposes n-type semiconductivity in h-BN, and the n-type character increases significantly for h-BN on HOPG. The h-BN film surface becomes electronically decoupled from the substrate when exceeding monolayer thickness, showing that the surface electronic properties and point defect chemistry for multilayer h-BN films should be comparable to those of a freestanding h-BN layer.

Comprehensive characterization of different metallic thin films on highly oriented pyrolytic graphite substrate

Thin metallic films supported on highly oriented pyrolytic graphite (HOPG) with low surface roughness are needed to improve the performance of different industrial applications and devices. As a result, an experimental investigation of electron beam physical vapour deposition of M (= Ti, Au, and Ag) metals on HOPG substrate to form HOPG-M thin films with a nominal thickness of 10 nm is reported. The comprehensive characterizations of the films were performed using different characterization techniques. The findings show that the films formed had different morphologies and topographies where Ti film was continuous while Au and Ag films were made up of islands with some discontinuities. The surface chemistry shows that the Ti film existed in both metallic and oxidized states while the Au and Ag films existed in metallic states. Raman analysis shows that there were some variations in the molecular structures after deposition while FTIR analysis shows that different functional groups existed on the thin films. The surface wettability behaviour of the Ag film based on water contact angles was hydrophobic while the other samples were hydrophilic. This work provides an opportunity for further examination of the HOPG as a substrate for the preparation of different M thin films.

Cu-phthalocyanine long-range ordered bulk growth due to the weak interaction with highly oriented pyrolytic graphite substrate

We investigate the relationship between the ordered growth of copper-phthalocyanine (CuPc) on highly oriented pyrolytic graphite (HOPG) and the electronic interaction at the interface. Via spot profiles analysis of electron diffraction patterns we determine an ordered growth for all measured CuPc films ranging from sub-monolayer to 3.3 nm. In particular we found that for CuPc thick films the ordered domains have a dimension of 50 nm. Electronic structure of CuPc/HOPG interface was studied by means of photoelectron and electron energy loss spectroscopies (EELS). From photoelectron spectroscopy we observe that molecules at the interface are weakly physisorbed. This weak interaction contributes to the formation of ordered 2-D islands. Since the geometry of the surface unit cell does not change with film thickness, the second CuPc layer grows stacked flat on the first one, forming ordered 3-D islands. It follows that the ordered bulk phase derives from the weak interaction between Pcs and HOPG. The Q band of CuPc/HOPG, measured via EELS, is different from the ones of the most common Pc phases. Therefore, electron diffraction and EELS measurements indicate the formation of a novel Pc long-range ordered structure observed only at this interface.

Atomistic simulations of graphene origami: Dynamics and kinetics

Origami offers two-dimensional (2D) materials with great potential for applications in flexible electronics, sensors, and smart devices. However, the dynamic process, which is crucial to construct origami, is too fast to be characterized by using state-of-the-art experimental techniques. Here, to understand the dynamics and kinetics at the atomic level, we explore the edge effects, structural and energy evolution during the origami process of an elliptical graphene nano-island (GNI) on a highly ordered pyrolytic graphite (HOPG) substrate by employing steered molecular dynamics simulations. The results reveal that a sharper armchair edge is much easier to be lifted up and realize origami than a blunt zigzag edge. The potential energy of the GNI increases at the lifting-up stage, reaches the maximum at the beginning of the bending stage, decreases with the formation of van der Waals overlap, and finally reaches an energy minimum at a half-folded configuration. The unfolding barriers of elliptical GNIs with different lengths of major axis show that the major axis should be larger than 242 Å to achieve a stable single-folded structure at room temperature. These findings pave the way for pursuing other 2D material origami and preparing origami-based nanodevices.

Controllable growth of two-dimensional NbSe2 flakes with irregular geometries under ion etching

Nanostructured NbSe2 has gained enormous interest in recent decades owning to its exotic properties and potential applications. Although two-dimensional (2D) NbSe2 structures have been successfully synthesized on various substrates, it remains a great challenge to prepare NbSe2 nanostructures with high edge-to-surface ratio, which is highly desirable for potential applications in catalysis. Here, we demonstrate that 2D NbSe2 with irregular geometries can be controllably grown on highly oriented pyrolytic graphite substrates via tuning the subtle balance between growth and ion etching. Atomic force microscopy, transmission electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy were employed to characterize the morphology and composition of the as-grown NbSe2 flakes. Our work highlights the important role of ion etching and paves the way for the growth of 2D materials with various geometries.

Effect of a Single Platinum Atom within a Small Metal Oxide Cluster: Reaction of DMMP with Size-Selected Pt1Zr2O7 Supported on HOPG.

Chemical warfare agents (CWAs) are a persistent threat facing civilians and military personnel across the modern geopolitical landscape. The development of the next generation of protective and sensing materials stands to benefit from an improved fundamental understanding of the interaction of CWA molecules with the active components of such candidate materials. The use of model systems in well-controlled environments offers a route to glean such information and has been applied here to investigate the fundamental interaction of a nerve agent simulant molecule, dimethyl methylphosphonate (DMMP), with a small cluster model of a single atom catalyst (SAC) active site. The cluster models, Pt1Zr2O7, were prepared by depositing mass-selected cluster anions synthesized in the gas phase onto a 100 K highly oriented pyrolytic graphite (HOPG) substrate surface prepared in ultra-high vacuum (UHV) at sub-monolayer coverage. Upon deposition, the cluster anions lost their charge to the electrically conductive surface to yield free-standing neutral clusters. The HOPG-supported clusters were characterized by X-ray photoelectron spectroscopy (XPS) to determine the oxidation states and chemical environment of the metal atoms present within the clusters. The reactivity of the clusters with DMMP was investigated via temperature-programmed desorption/reaction (TPD/R) and XPS experiments in which the clusters were exposed to DMMP and incrementally heated to higher temperatures. In contrast to two other HOPG-supported clusters, (ZrO2)3 and Pt1Ti2O7, recently investigated in our laboratory, Pt1Zr2O7 decomposed DMMP to primarily evolve a methane species, which was completely absent for the other clusters.

Interlayer hybridization in a van der Waals quantum spin-Hall insulator/superconductor heterostructure

In this work, we present an angle-resolved photoemission spectroscopy study of a 1T′-WTe2 monolayer epitaxially grown on NbSe2 substrates, a prototypical quantum spin Hall insulator (QSHI)/superconductor heterojunction. Angle-resolved photoemission spectroscopy data indicate the formation of electronic states in the bulk bandgap of WTe2, which are absent in the nearly free-standing WTe2 grown on the highly oriented pyrolytic graphite substrate, where an energy gap of ∼100 meV is reported. The results are explained in terms of hybridization effects promoted by the QSHI–superconductor interaction at WTe2/NbSe2 interfaces, in line with recent scanning probe microscopy investigation and theoretical band structure calculations. Our findings highlight the important role of interlayer interaction on the electronic properties and ultimately on the engineering of topological properties of the QSHI/superconducting heterostructure.

Edge states of α-bismuthene nanostructures

We present a systematic investigation of the edge states (ESs) of two-dimensional α-bismuthene (α-Bi) structures self-assembled on highly oriented pyrolytic graphite substrates, using scanning tunnelling microscopy and scanning tunnelling spectroscopy. The measurements are carried out for 3ML, 5ML and 7ML thick Bi structures. Our spectroscopy studies reveal clear features at the edges of the 5ML and 7ML thick structures, and the positions of the ESs coincide with the topographical step edges. In contrast, in 3ML structures the ESs appear to be absent and instead new states are sometimes observed, far from the topographical edge. These states are associated with a moiré pattern and result from strain-induced modulation of the topology. Our observations demonstrate the impact on the ESs of coupling to adjacent structures.

Self‐Assembly and Oriented Growth of Conductive Ni‐CAT‐1 Metal‐Organic Framework at Solid–Liquid Interfaces

AbstractTwo‐dimensional (2D) conductive metal‐organic frameworks (MOF) represent a unique class of electrode materials with high capacity and power density. Understanding molecular mechanisms and pathways for heterogeneous nucleation of 2D π‐conjugated MOFs is highly desirable for controlling the structure and properties of conductive MOFs on solid substrates. Herein, a systematic study of nucleation and growth of 2D π‐conjugated Ni‐catecholate (Ni‐CAT‐1) MOFs on highly oriented pyrolytic graphite (HOPG) and copper substrates is reported. It is discovered that the nucleation density and growth kinetics of the MOF film can be controlled by varying substrate interactions with the organic linker. Specifically, π–π interactions between the linker and the HOPG dictate lower nucleation density, whereas π–metal interactions between the linker and the copper substrate dictate faster nucleation and higher nucleation densities. These studies reveal the key mechanism for Ni‐CAT‐1 nucleation on different surfaces and provide insights into interfacial control over the growth of other 2D π‐conjugated MOF films on solid substrates to inform synthesis of functional materials.

Comparative study of electrodeposited copper nanoparticles on different substrates for their use in the reduction of nitrate ions

A comparative analysis of the electrodeposition of copper nanoparticles (CuNPs) was studied on highly oriented pyrolytic graphite (HOPG) and vitreous carbon (VC) substrates from solutions containing different supporting electrolytes (Na2SO4 and H2SO4).Voltammetric results indicated the presence of a single cathodic process, corresponding to the reduction reaction of Cu2+ ions to Cuo, for the solutions studied, using the HOPG substrate. The solution containing Na2SO4 as supporting electrolyte was found to be more effective because the copper reduction process occurs at more positive potential values, reaching the highest current density. For the VC substrate, two cathodic processes were evidenced, with the formation of Cu+ ions as intermediate species. As in the HOPG substrate, the use of the solution containing Na2SO4 showed the better behavior for copper electrodeposition, being the most favored process on VC. This behavior could be explained by considering the existence of structural defects in the VC, which would facilitate Cu nucleation.The nucleation and growth kinetics of CuNPs on HOPG electrodes corresponded predominantly to a model including progressive nucleation on active sites, and diffusion controlled growth, presenting a good correlation with the scanning electron microscopy images. For VC substrates, the kinetics analysis did not yield conclusive results. Different morphological features of Cu deposits were observed on HOPG and VC, presenting the former a larger covered area.The HOPG/CuNPs modified electrodes evidenced an enhancement in the catalytic activity towards the nitrate reduction reaction.

The self-assembly and structural regulation of a hydrogen-bonded dimeric building block formed by two N[sbnd]H···O hydrogen bonds on HOPG

The self-assembled behavior of an unsymmetric molecule (BCDTDA) with one imidazole group as center and benzoic acid group as functional group is studied, and the regulatory behaviors of coronene (COR) and three bipyridine derivatives (named BP, PEBP-C4 and PEBP-C8) on BCDTDA self-assembly structures are also investigated. Based on highly oriented pyrolytic graphite (HOPG) substrate, scanning tunneling microscopy (STM) is used to observe the variation of assembled behaviors at the solid-liquid interface. Because of the concentration effect, BCDTDA molecules can assemble into grids and Kagomés structures in the form of NH···O hydrogen bonded dimers. BCDTDA molecules still maintain dimeric structures in the regulation of COR and BP molecules to BCDTDA self-assembly. However, PEBP-C4 and PEBP-C8 destroy the structure of the dimers, and form a variety of co-assembled structures with BCDTDA. Different guest molecules coordinate the host molecules differently, which makes the experiment more meaningful. Combined with density functional theory (DFT) calculation, the discovery of molecular interactions provides a promising strategy for the construction of functional nanostructures and devices.

Probing the correlation between morphology and optical anisotropy in ZnTPP films grown at different temperatures

Films of tetraphenylporphyrins (TPP) have been recently investigated for their ability in protecting graphite electrode surfaces from degradation. The effectiveness of the protection depends on both the molecule–substrate and molecule–molecule interactions, which determine the type of film growth and its morphology. For instance, meso-tetraphenyl porphyrin-Zn(II) (ZnTPP) arranges differently depending on the substrate, conditions used for deposition and post-growth treatments. Since many parameters influence film morphology and strategies to reach an efficient protective coverage, here we investigate the role of the substrate temperature in determining morphological variations in thick ZnTPP films. ZnTPP molecules were sublimated by an organic molecular beam epitaxy (OMBE) system on a highly oriented pyrolytic graphite (HOPG) substrate, controlled in temperature by a variable temperature cryostat. The film morphology was characterized by atomic force microscopy (AFM), whereas the type of growth by reflectance anisotropy spectroscopy (RAS), which is highly sensitive to the orientation and type of arrangement of molecules on the substrate; changes in the film valence electronic structure were monitored in situ by UV photoemission spectroscopy (UPS). The comparison between AFM and RAS investigations clearly correlates a ZnTPP film morphology to the characteristic optical signal, both influenced by temperature conditions and diffusivity of molecules on the substrate.