Highly Oriented Pyrolytic Graphite Research Articles

Design and performance of the high-resolution stress and texture neutron diffractometer HETU

The high-resolution stress and texture neutron diffractometer HETU at the China Mianyang Research Reactor is dedicated to measuring and analysing the residual stress and texture of engineering materials. The instrument can provide a high instrument resolution and a high neutron flux at the sample position. This article presents the HETU neutron optics design based on Monte Carlo simulation and the first assessment of its experimental performance. The technical parameters of the monochromators, slit system, radial collimators and detector were determined. A double-focusing silicon monochromator with (311) and (400) planes and a highly oriented pyrolytic graphite (HOPG) monochromator with (002) and (004) planes were selected to provide either high instrument resolution or high neutron flux modes. The highest instrument resolution of Δd/d = 1.65 × 10−3 is obtained when using Si(400), while the highest neutron flux at the sample position is 3.0 × 107 n s−1 cm−2 at the wavelength of 2.37 Å reflected by HOPG(004). The HETU diffractometer started operation in early 2022.

Microfluidic-Assisted Alignment of Graphene Nanosheets for Microfiber Formation

Recently, our lab developed graphene oxide microfibers for ultrasensitive neurochemical detection with fast-scan cyclic voltammetry (FSCV). Graphene-fiber microelectrodes provide a significant advance over traditional carbon-fiber microelectrodes due to their resistance to detrimental fouling, fast electron transfer kinetics, and frequency independent behavior. Despite our recent advancement, there still exists a gap in our understanding of how specific changes in the carbon chemical structure influences adsorption and interaction of various neurochemicals at the surface. Understanding the extent to which the carbon chemical structure influences electrochemical detection is critical for designing tailor-made electrodes for ultrasensitive analyte detection. From a molecular level point-of-view, the graphene sheet consists of two regions with different structure: i) the basal plane, consisting of conjugated sp2 hybridized carbon atoms, and characterized by an atomic flatness and low defect density; and ii) the edge plane, that forms sp3 sites with a high level of defects and a variety of functional groups and dangling bonds. It is generally believed that the electrochemical properties of the basal and edge plane are significantly different, however, there has been debate concerning the electrochemical activity at the basal and edge plane. The conventional consensus is that electron transfer kinetics is faster on the edge plane in comparison with the basal plane sites, while the opposite barrier of this debate provides evidence for fast electron transfer at pristine basal plane. Moreover, how these factors influence neurochemical detection, particularly at rapid scan rates, is still lacking due to the necessity of using microelectrodes with FSCV. To date, most studies analyzing the influence of basal vs. edge plane surfaces on electrochemical detection have used macroelectrodes like highly ordered pyrolytic graphite (HOPG) which are not amenable to FSCV detection due to the large capacitive currents obtained at macroelectrodes at scan rates consisting of hundreds of volts per second. Therefore, to address this existing knowledge gap in understanding how edge vs. basal planes influence subsecond neurochemical detection, the ability to develop microelectrodes with controlled surfaces is crucial. Here, I will discuss our labs latest effort in developing graphene oxide microfibers with controllable nanosheet alignment to study the influence of basal vs. edge plane availability on neurochemical detection at rapid scan rates. To assist in nanosheet alignment, we use microfluidic channels to assist in fluid-driven control of nanosheet orientation. Using microfluidics to help develop highly controllable electrode materials provides a critical advance in our ability to develop novel carbon surfaces for detection.

Electronic and Vibrational Properties of Episulfide-Graphene Materials

Our group has investigated the stability of sulfur adatoms on graphene obtained by a direct reaction of highly oriented pyrolytic graphite (HOPG) with elemental sulfur. With the help of density functional theory (DFT) calculations, we have developed different models of the structure and evaluated the electronic and vibrational properties of synthesized graphene-sulfur (GS) materials. Our structural models are based on sulfur atoms covalently attached to graphene. The DFT electronic structure calculations support the formation of a strong and stable sulfur bonding to graphene and give a chemical composition in good agreement with the experimental proportion of sulfur to carbon. In addition, ab-initio molecular dynamics simulations allow us to confirm the stability on S-G bonds. The vibrational modes calculated with DFT support the vibrational signatures of the GS materials observed using Raman spectroscopy.

Work function measurement by ultraviolet photoelectron spectroscopy: Versailles project on advanced materials and standards interlaboratory study

We present the results of an interlaboratory study on work function (WF) measurements by ultraviolet photoelectron spectroscopy (UPS) conducted under the auspices of the Versailles Project on Advanced Materials and Standards. Two samples, gold (Au) film deposited on a flat Si(100) and highly oriented pyrolytic graphite (HOPG), were distributed to six different laboratories. Prior to UPS measurements, the samples underwent common sample pre-cleaning procedures: wet treatment and Ar+-sputtering in a vacuum for Au, and mechanical peeling in air for HOPG. Instrumental settings are adjusted for energy-scale calibration and sample bias optimization. The average reference WF value (WFref) and its expanded uncertainty (Uref) were calculated from each dataset that participants provided. The results of the Au sample demonstrated 5.40 ± 0.13 eV after Ar+ sputtering in vacuum, while the HOPG sample showed 4.62 ± 0.16 eV after mechanical peeling-off. Even though the participants used slightly different sample bias voltages and sample cleaning methods with various instruments, the results demonstrate remarkable consistency. Their consistency among the laboratories for both samples with high and low WF values provides a basis for establishing a new international standard for UPS in the surface chemical analysis community.

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.

Gas concentration in rarefied flows: Experiments and modeling

The relationship between gas-surface scattering dynamics and flow through conical systems in the rarefied regime has been explored with experimental and computational tools. Molecular beam-surface scattering experiments were used to characterize the inelastic scattering dynamics of Ar and N2 on a highly oriented pyrolytic graphite (HOPG) surface. These data were then used to develop a gas-surface scattering approach for kinetic fluid simulations, which combines the diffuse scattering and Cercignani-Lampis-Lord (CLL) models. The translational energy and angular flux distributions from the beam-surface scattering experiments were used to obtain the normal energy accommodation (αn) parameter as well as the effective surface mass (M) in the CLL model. Constant values of αn=0.9, M = 40 amu for Ar and αn=0.75, M = 28 amu for N2 were found to capture the experimental angular flux scattering distributions over a wide range of incident energies and angles. The gas-surface model was subsequently used in direct simulation Monte Carlo (DSMC) simulations of the flux of directed gas flow through a conical concentrator and compared with measurements of gas concentration with a prototype concentrator in separate molecular beam experiments. Comparisons of predicted concentration pressure ratios (pressure in an accommodation chamber with vs. without concentrator) with measured values for three free-stream velocities of Ar and N2 showed maximum deviations of 4.8% and 9.8% for Ar and N2, respectively. The simulations show that gas-surface collisions resulting in the loss of the normal component of the incident energy while preserving the tangential momentum of the impinging atom or molecule, resulting in super-specular scattering, lead to higher concentration of gases than purely specular, energy conserving gas-surface interactions. For conical concentrators, with inlet-to-outlet area ratios of 133 and 600, specular reflections result in concentration factors (flux through an orifice with vs. without concentrator) of only ∼8, while super-specular scattering results in concentration factors of 64 and 127, respectively.

Guest-induced structural phase transformations of the multicomponent network at liquid/solid interfaces

The fabrication of bi-component and three-component supramolecular networks based on hydrogen bonds was investigated by scanning tunnelling microscope (STM). Induced by successively introduced coronene (COR) and 1,3,5-benzenetricarboxylic acid (TMA), the intermolecular interactions between [1,1′-biphenyl]-3,4′,5-tricarboxylicacid (BPTC) were changed, leading to the structural transformation of the lamellar self-assembled nanostructure of BPTC on highly oriented pyrolytic graphite (HOPG). Eventually, the flower-like nanostructure of BPTC/COR and kagomé nanostructure of BPTC/TMA/COR were obtained on HOPG substrates respectively. This investigation of the multicomponent host–guest network could be useful for the construction of novel heterogenous molecular structures.

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.

Surface production of negative deuterium ions from plasma-exposed boron doped diamond and graphite: work function measurements using photoemission yield spectroscopy

Negative-ion sources are of considerable interest for applications such as materials processing and neutral beam injection for magnetic confinement fusion. The efficient production of negative ions in these sources often relies on surface production. Work function measurements are critical to enable a detailed understanding of the mechanisms that underpin this. In this study we used a combination of photoemission yield spectroscopy and the Fowler method to determine the work functions of boron doped diamond (BDD) and highly oriented pyrolytic graphite (HOPG) directly after exposure to a low-pressure inductively coupled deuterium plasma (150 W, 2 Pa). A magnetised retarding field energy analyser is used to measure the negative ion current from the samples. During plasma exposure, samples are biased at −130 V or −60 V and their temperature is varied between 50 ∘C and 750 ∘C. The results show that the increasing work function of the plasma exposed HOPG occurs over the same sample temperature range as the decreasing negative-ion current. In contrast, the work function of BDD does not show a clear relationship with negative-ion current, suggesting that different mechanisms influence the negative-ion production of metal-like HOPG and dielectric-like BDD. The necessity for an additional fitting parameter for the Fowler fits to BDD suggests that its electronic properties are changing under plasma exposure, unlike HOPG. For both materials, the maximum photocurrent measured from the samples displays a strong similarity with negative-ion current, suggesting they are driven by a common mechanism. The in-situ measurement of the work function using non-invasive techniques is of interest for the development of negative ion sources.

Preparation of Electrochemical Sensor for Paracetamol Detection Based on a New Piperazine Substituted Triazole-Coumarin Hybrid Doped PEDOT Modified Flexible HOPG Electrode

This study includes the electrochemical determination of paracetamol (PC), using novel piperazine substituted triazole-coumarin hybrids doped PEDOT coated highly oriented pyrolytic graphite (HOPG) electrode. For the electrochemical determination of paracetamol, using an electrode prepared by a one step electropolymerization of the new triazole-tethered piperazine coumarin hybrid doped PEDOT conductive polymer on the HOPG substrate was mentioned. The electrodes were characterized by FT-IR, SEM and XRD analysis. The possible interference effects of Ascorbic acid, Uric acid, Dopamine, Glucose and Caffeine were investigated by chronoamperometry in the presence of paracetamol. The sensor (PEDOT-MC1/HOPG-10 Scan) has a high sensitivity to paracetamol, with a limit of detection (LOD) of 0.312 μm and a RSD % of 1.22 (S/N = 3). In addition, real sample analyzes were performed on pharmaceutical samples for PC determination using the prepared sensor (PEDOT-MC1/HOPG-10 Scan). The novelty of this paper is the use of new piperazine substituted triazole-coumarin hybrid doped PEDOT material for the first time in the literature, as well as a new electrode substrate material for the determination of paracetamol and the use of HOPG (Highly oriented pyrolytic graphite), which exhibits higher electroanalytical performance than other carbon-based materials.

Electrochemical grafting assisted modification of graphite surface by 3,4,5-trimethoxybenzenediazonium based monolayer

A conventional route to create such nanoscale electronic devices using the autonomous ordering and assembly of organic molecules on atomically well-defined surfaces has been proposed. However, these thin films are unstable in realistic environments due to weak interaction between the organic molecules and crystal surfaces. Therefore, enhancing the interfacial interaction between them is assigned as the key approach to extend the scope of application of these promising monolayer thin films. In this work, we demonstrate the formation of an organic monolayer, namely 3,4,5-trimethoxy diazonium (3,4,5-TMD), on a Highly Ordered Pyrolytic Graphite (HOPG) electrode by using the power of electrochemical method. The structural properties at the nano scale as well as the bonding nature between 3,4,5-TMD adlayer and HOPG electrode at the interface were investigated by a combination of cyclic voltammetry (CV), atomic force spectroscopy (AFM) and Raman spectroscopy. As a result, the 3,4,5-TMD molecules covalently bond to carbon atoms located at the HOPG interface for forming a monolayer with its thickness of 0.92 ± 0.02 nm. This finding opens a partway to apply the electrochemical grafting for covalent functionalization of 2D materials on HOPG and other metallic surfaces.

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.

Two-dimensional assemblies of saccharide-derived molecules on highly oriented pyrolytic graphite revealed by scanning tunneling microscopy

Saccharides play important roles as structural components of plants and for molecular recognition in living organisms, and they are increasingly being used as environmentally-friendly bio-based materials. In this study, two-dimensional (2D) assemblies of 1,5-anhydro-D-glucitol (AG) derivatives were examined by scanning tunneling microscopy at the highly oriented pyrolytic graphite (HOPG)/solvent interface. The effects of the solvent, alkyl chain linkage, and alkyl chain length on the 2D structures were investigated. Grid-like and columnar structures were periodically formed depending on the odd–even nature of the carbon atoms in the alkyl chain unit, highlighting the importance of in-plane intermolecular interactions between adsorbed molecules in determining the 2D molecular arrangement. The calculated adsorption energies for the alkyl chain and pyranose ring units with the graphene model indicated that the former was significantly greater than the latter. Despite the strong interactions between adsorbates and substrate, relatively weak in-plane interactions between adsorbates result in the 2D molecular arrangements of the AG derivatives.

Active Probe Atomic Force Microscopy with Quattro-Parallel Cantilever Arrays for High-Throughput Large-Scale Sample Inspection.

An Atomic Force Microscope(AFM) isa powerful and versatile tool for nanoscale surface studies to capture 3D topography images of samples. However, due to their limited imaging throughput, AFMs have not been widely adopted for large-scale inspection purposes. Researchers have developed high-speed AFM systems to record dynamic process videos in chemical and biological reactions at tens of frames per second, at the cost of a small imaging area of up to several square micrometers. In contrast, inspecting large-scale nanofabricated structures, such as semiconductor wafers, requires nanoscale spatial resolution imaging of a static sample over hundreds of square centimeters with high productivity. Conventional AFMs use a single passive cantilever probe with an optical beam deflection system, which can only collect one pixel at a time during AFM imaging, resulting in low imaging throughput. This work utilizes an array of active cantilevers with embedded piezoresistive sensors and thermomechanical actuators, which allows simultaneous multi-cantilever operation in parallel operation for increased imaging throughput. When combined with large-range nano-positioners and proper control algorithms, each cantilever can be individually controlled to capture multiple AFM images. With data-driven post-processing algorithms, the images can be stitched together, and defect detection can be performed by comparing them to the desired geometry. This paper introduces principles of the custom AFM using the active cantilever arrays, followed by a discussion on practical experiment considerations for inspection applications. Selected example images of silicon calibration grating, highly-oriented pyrolytic graphite, and extreme ultraviolet lithography masks are captured using an array of four active cantilevers ("Quattro") with a 125 µm tip separation distance. With more engineering integration, this high-throughput, large-scale imaging tool can provide 3D metrological data for extreme ultraviolet (EUV) masks, chemical mechanical planarization (CMP) inspection, failure analysis, displays, thin-film step measurements, roughness measurement dies, and laser-engraved dry gas seal grooves.

The relationship between surface microstructure and super-lubrication performance based on 2D LDHs

The mechanism of frictional energy dissipation suggests that layered double hydroxides (LDHs) could be ideal materials for achieving solid state superlubricity due to their strong adsorption effect and weak internal interactions. In this work, three metal ion-coordinated LDHs nanosheets were designed to investigate the specific effect of surface microstructure on frictional properties. The experimental results showed that the lubrication performance is determined by the surface structure, and the friction coefficient doubles (2 × 10−3 to 3.9 × 10−3) and the adhesion force increases by 62.3% (6.9–11.2 nN) when the divalent metal ions change from tetrahedral to octahedral coordination. Through crystal field stability energy analysis and density functional theory (DFT) calculations, the superlubricity of LDHs materials is attributed to two aspects: firstly, the existence of strong charge density transfer between LDHs nanosheets and the probe, known as the anchoring effect; secondly, the formation of an ultra-low sliding energy barrier interface between LDHs nanosheets and highly oriented pyrolytic graphite (HOPG). Therefore, the relationship between tribological properties and surface structure can guide future research on superlubricity materials.

Using a Neural Network to Study the Effect of the Means of Synthesizing Exfoliated Graphite on Its Macropore Structure

Graphite intercalated compounds (GICs) with different stage numbers are prepared chemically from highly oriented pyrolytic graphite (HOPG), natural flaked graphite (FG) and nitric acid. Exfoliated graphite samples (EG-T) are synthesized from GICs via water treatment followed by thermal shock. The aim of this work is to investigate the dependence of the inner EG-T pore structure on the extent of oxidation and type of graphite by processing scanning electron microscopy (SEM) micrographs of EG-T cross sections. A procedure is developed on the basis of a deep convolutional neural network that speeds up image processing with no appreciable loss of accuracy. A strong correlation is found between EG-T pore structure parameters, the depth of oxidation, and the type of graphite.

Electrostimulation and Nanomanipulation of Two-Dimensional MoO3-x Layers Grown on Graphite

Molybdenum trioxide shows many attractive properties, such as a wide electronic band gap and a high relative permittivity. Monolayers of this material are particularly important, as they offer new avenues in optoelectronic devices, e.g., to alter the properties of graphene electrodes. Nanoscale electrical characterization is essential for potential applications of monolayer molybdenum trioxide. We present a conductive atomic force microscopy study of an epitaxially grown 2D molybdenum oxide layer on a graphene-like substrate, such as highly oriented pyrolytic graphite (HOPG). Monolayers were also investigated using X-ray photoelectron spectroscopy, atomic force microscopy (semi-contact and contact mode), Kelvin probe force microscopy, and lateral force microscopy. We demonstrate mobility of the unpinned island under slight mechanical stress as well as shaping and detachment of the material with applied electrical stimulation. Non-stoichiometric MoO3-x monolayers show heterogeneous behavior in terms of electrical conductivity, which can be related to the crystalline domains and defects in the structure. Different regions show various I–V characteristics, which are correlated with their susceptibility to electrodegradation. In this work, we cover the existing gap regarding nanomanipulation and electrical nanocharacterization of the MoO3 monolayer.

Investigation of the Pore Structure of Exfoliated Graphite Based on Highly Oriented Pyrolytic Graphite Nitrate

Graphite intercalated compounds (GICs) with different stage numbers were prepared from highly oriented pyrolytic graphite (HOPG) and nitric acid using a chemical method. Exfoliated graphite (EG-T) was synthesized from GICs by water treatment followed by thermal shock. The effects of the graphite oxidation depth on the EG-T thermal expansion coefficient, volatile content, and total porosity were examined. However, the main purpose of this work was investigation of the dependence of the inner EG-T pore structure on the level of oxidation. Thus, we studied the micro- and mesopore structure and specific surface area by nitrogen porosimetry and the modern 2D-NLDFT method to calculate the pore size distribution and pore volume. As well, we performed a mercury porosimetry experiment to determine the macropore characteristics. We examined the pore space using a number of scanning electron micrographs of EG-T particle cross-sections using an image processing technique. In this way we showed the strong correlation between the EG-T pore structure parameters and oxidation depth of graphite.

Using a Neural Network to Study the Effect of the Means of Synthesizing Exfoliated Graphite on Its Macropore Structure

Graphite intercalated compounds (GICs) with different stage numbers are prepared chemically from highly oriented pyrolytic graphite (HOPG), natural flaked graphite (FG) and nitric acid. Exfoliated graphite samples (EG-T) are synthesized from GICs via water treatment followed by thermal shock. The aim of this work is to investigate the dependence of the inner EG-T pore structure on the extent of oxidation and type of graphite by processing scanning electron microscopy (SEM) micrographs of EG-T cross sections. A procedure is developed on the basis of a deep convolutional neural network that speeds up image processing with no appreciable loss of accuracy. A strong correlation is found between EG-T pore structure parameters, the depth of oxidation, and the type of graphite.

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.