Single Crystal Graphite Research Articles

Wear-free sliding electrical contacts with ultralow electrical resistivity

Sliding electrical contacts are commonly applied in electrical connectors, such as conductive slip rings, pantographs, switches, and commutators. However, they suffer from several unavoidable problems caused by friction and wear, including high energy consumption, intermittent failures, limited life, and even failure disasters. In this study, we realized an ultralow-friction and long-distance wear-free state, defined as structural superlubricity (SSL), between sliding electrical interfaces under ambient conditions. A conductive SSL can be implemented in experiments with single-crystal graphite flakes on flattened metals, such as Au and Ni films. Furthermore, we found that depositing a 2 to 3-nm-thick diamond-like carbon (DLC) film on a nickel alloy can lead to an even lower resistivity than that of metals alone. In addition, we revealed the mechanism by which DLC films can improve the conductivity between graphite and metals through density functional-theory simulations. In addition, we prepared a prototype of the SSL slip ring and proved that it possessed ultralow friction, was wear-free, and had no intermittent failures. Consequently, our results demonstrate a unique type of electrical-contact interface for applications requiring conduction while sliding. Thus, we opened the door for SSL electromechanical coupling.

Macroscopic perspective on phase transition behavior of natural single-crystal graphite under different pressure environments

Comprehensive understanding of the direct transformation pathway from graphite to diamond under high temperature and high pressure has long been one of the fundamental goals in materials science. Despite considerable experimental and theoretical progress, current experimental studies have mainly focused on the local microstructural characterizations of recovered samples, which has certain limitations for high-temperature and high-pressure products, which often exhibit diversity. Here, we report on the pressure-induced phase transition behavior of natural single-crystal graphite under three distinct pressure-transmitting media from a macroscopic perspective using in situ two-dimensional Raman spectroscopy, scanning electron microscopy, and atomic force microscopy. The surface evolution process of graphite before and after the phase transition is captured, revealing that pressure-induced surface textures can impede the continuity of the phase transition process across the entire single crystal. Our results provide a fresh perspective for studying the phase transition behavior of graphite and greatly deepen our understanding of this behavior, which will be helpful in guiding further high-temperature and high-pressure syntheses of carbon allotropes.

Intrinsic interlayer shear strength of graphite

Graphite holds significant values in the energy and electronics industries due to its unique properties. As a quintessential example of highly anisotropic materials, the shear strength measures one of its most fundamental mechanical properties. However, the lack of ideal materials and testing methods has led to a wide dispersion in the reported values. To address this issue, we utilized epitaxially grown single-crystal graphite and developed a high-throughput sample preparation method, along with a novel loading technique in this work. The intrinsic shear strength of AB-stacked graphite was determined to be τs = 62 MPa, by excluding the size effect in measurements. The results are further compared to highly oriented pyrolytic graphite specimens processed down to nanoscale thickness, highlighting the adverse impact of twisted single-crystalline interfaces between the graphitic layers. Additionally, we observed a distinctive failure mechanism with continuous and uniform cascade plastic slips across the thickness of graphite samples, which corresponds to an interlayer shear strength approaching τs. The intrinsic shear strength characterized in our work sets an upper limit for the interlayer shear resistance of graphite. The experimental procedure for measuring shear strength can be applied to other van der Waals materials.

Acoustic shock wave-induced sp2-to-sp3-type phase transition: a case study of a graphite single crystal

A technique for the exposure of low-pressure acoustic shock waves was applied for the first time in the exploration of the phase transition from graphite to disordered graphite, which was well characterized by Raman, XPS and HR-TEM measurements.

Quantized thermoelectric Hall plateau in the quantum limit of graphite as a nodal-line semimetal

We performed thermoelectric Hall conductivity ${\ensuremath{\alpha}}_{xy}$ measurements on single-crystal graphite in the quantum limit up to 13 T. Both electrical and thermoelectric transport measurements were performed on the same crystal to extract pure ${\ensuremath{\alpha}}_{xy}$, avoiding any sample quality dependence. The ${\ensuremath{\alpha}}_{xy}$ converges to a plateau in the quantum limit with a linear dependence on temperature. This behavior is analogous to the quantized thermoelectric Hall effect (QTHE) observed in three-dimensional Dirac/Weyl nodal-point semimetals, and experimentally confirms a theoretical proposal on the QTHE in semimetals with nodal lines as in graphite.

Enhance high harmonic generation (HHG) efficiency via compact multi-plate continuum post-compression for time-resolved angle-resolved photoemission spectroscopy.

Time-resolved angle-resolved photoemission spectroscopy (Tr-APRES) gives direct insight into electron dynamics by providing temporal-, energy-, and momentum-resolved information in one experiment. A major obstacle to using high harmonic generation (HHG) probe pulses for photoemission spectroscopy is the low conversion efficiency, that is, the low flux of probe photons. We use a Yb-KGW based duo-laser source with an oscillator to pump two separate amplifiers and generate two synchronized pulsed laser sources with average energies of 7.5 and 6W. By using the multiplate continuum method and chirped mirrors, the resulting flux of HHG photons at 33-70eV can be increased 50-fold (up to 1011 photons/s) by using post-compressed 30fs pulses compared with the photon flux generated by the fundamental 190fs pulses. Moreover, pulses from the 6W amplifier are used to pump an optical parametric amplifier that can vary the wavelengths for photoexcitation. The system performance is demonstrated by applying Tr-ARPES to single-crystal graphite. The front tilt broadening is significantly suppressed by the off-plane mounted conical grating, leading to a 184fs temporal resolution that is mainly limited by the pump pulse. The energy resolution is 176meV.

Interference of excitons and surface plasmons in the optical absorption spectra of monolayer and bilayer graphene

By using spectroscopy ellipsometry, we investigated the optical absorption spectra of monolayer and bilayer graphene in the deep-ultraviolet region up to 6.42 eV as a function of temperature. The optical absorption spectra of monolayer and bilayer graphene can be fitted by two Breit-Wigner-Fano (BWF) spectral functions, in which each BWF is originated by the interference of discrete excitonic spectra and continuous surface plasmon spectra. Each BWF peak shows narrower linewidth and larger peak intensity with increasing temperature. The unusual temperature dependence of the BWF peaks is understood by a shorter lifetime of the surface plasmon at a higher temperature, which gives a smaller BWF asymmetric parameter. The optical absorption by the surface plasmon is usually observed only at the surface of the three-dimensional crystal or two-dimensional material. The contribution of the surface plasmon becomes relatively larger in the case of monolayer or bilayer graphene than that of single-crystal graphite because of the two-dimensional nature of electrons. The observed optical absorption spectra are confirmed by calculating the complex dielectric function using the first-principles calculations.

Continuous epitaxy of single-crystal graphite films by isothermal carbon diffusion through nickel.

Multilayer van der Waals (vdW) film materials have attracted extensive interest from the perspective of both fundamental research1-3 and technology4-7. However, the synthesis of large, thick, single-crystal vdW materials remains a great challenge because the lack of out-of-plane chemical bonds weakens the epitaxial relationship between neighbouring layers8-31. Here we report the continuous epitaxial growth of single-crystal graphite films with thickness up to 100,000 layers on high-index, single-crystal nickel (Ni) foils. Our epitaxial graphite films demonstrate high single crystallinity, including an ultra-flat surface, centimetre-size single-crystal domains and a perfect AB-stacking structure. The exfoliated graphene shows excellent physical properties, such as a high thermal conductivity of ~2,880 W m-1 K-1, intrinsic Young's modulus of ~1.0 TPa and low doping density of ~2.2 × 1010 cm-2. The growth of each single-crystal graphene layer is realized by step edge-guided epitaxy on a high-index Ni surface, and continuous growth is enabled by the isothermal dissolution-diffusion-precipitation of carbon atoms driven by a chemical potential gradient between the two Ni surfaces. The isothermal growth enables the layers to grow at optimal conditions, without stacking disorders or stress gradients in the final graphite. Our findings provide a facile and scalable avenue for the synthesis of high-quality, thick vdW films for various applications.

Catalytic transformation of coconut husk into single-crystal graphite and its application for the removal of antibiotics from wastewater

Ciprofloxacin is a top-priority pollutant due to its hazardous effects and frequent detection in the environment. The objective of this study was to examine the adsorption capacity of graphite made from coconut husk (CHG) for the removal of the antibiotic, ciprofloxacin. Analytical techniques were used to characterize the material and the effects of operating conditions on the adsorption behaviour were investigated. In fixed-bed studies, the adsorbent's regeneration capacity was assessed. Maximum removal of ciprofloxacin was obtained at a pH of 10. Kinetic studies revealed that the adsorption was fast and experimental data best fit the pseudo second-order kinetic model (R2 = 0.99). Adsorption was found to follow the predictions of the Sips isotherm model with a maximum adsorption capacity of 124 mg g−1. The fixed bed system reached an adsorption capacity of 79 mg g−1 at saturation time and a mass transfer zone of 2 cm (flow rate of 5 mL min−1, a bed height of 5 cm, and a diameter of 1.9 cm). The Thomas model displayed the best fit to the experimental data (R2>0.9). Furthermore, regeneration studies revealed that after three cycles, CHG can retain up to 70% efficiency. The adsorption mechanism was investigated using desorption kinetic models, which supported the predictions that adsorption occurred via multilayer adsorption on heterogeneous active sites. The results obtained show that this adsorbent has the potential for the efficient removal of ciprofloxacin from aqueous solution on an industrial scale.

(Digital Presentation) Understanding the Impact of High-Nickel Cathode Microstructure on Battery Safety and Cycling Performance

To meet the increasing energy demands of portable devices and electric vehicles, high-nickel lithium-ion cathode materials with the general formula Li(NixMnyCoz)O2 (NMC) have been extensively researched. Currently NMC811 is used commercially for high-energy applications. The energy density of NMC also comes with concerns over cycle life and safety1,2. To improve the cycle life of NMC-based cells, single-crystal materials have recently gained attention to tackle the particle cracking issues found in polycrystalline cathodes3. However, for successful introduction to the lithium-ion battery market, inherent safety over a material’s lifetime also needs to be proven. Failure and degradation mechanisms both need to be fully understood to improve the stability of future cathode materials. Abusive testing, such as overheating, overcharge and nail penetration, has been used in conjunction with in-situ and ex-situ X-ray computed tomography (CT) 3D imaging to perform post-mortem studies and understand the relationship between thermal failure and cathode microstructure4,5. However, the interplay between safety characteristics, microstructural properties and material degradation remains unclear.This work first aims to compare the safety performance of polycrystalline and single-crystal NMC811 in 200 mAh pouch cells. Accelerating rate calorimetry (ARC) with a heat-wait-search (HWS) technique is used to heat cells and determine the onset of self-heating, onset of thermal runaway and the peak thermal runaway temperature. Laboratory-based pre- and post-mortem in-situ and ex-situ X-ray CT is also used for non-destructive imaging at multiple length scales to determine how failure propagates through the cells and the impacts on the electrodes and microstructure. Pouch cells containing polycrystalline and single-crystal NMC811 cathode and graphite anode are electrochemically cycled to induce material degradation. EIS measurements and diagnostic cycles are performed to identify prevalent degradation modes in both types of cathode materials. Finally, the same ARC and X-ray CT characterisations are performed on the aged cells to determine how degradation and changes to the material structure affect the safety performance in high-nickel cathode materials.The results of this work will improve the current understanding of capacity fade in high-nickel cathodes and the safety behaviour over the lifetime of a battery cell. This information can then be used to inform future materials development and strategies for mitigating thermal runaway in batteries. References L. Ma, M. Nie, J. Xia, and J. R. Dahn, J. Power Sources, 327, 145–150 (2016).H. J. Noh, S. Youn, C. S. Yoon, and Y. K. Sun, J. Power Sources, 233, 121–130 (2013).J. Langdon and A. Manthiram, Energy Storage Mater., 37, 143–160 (2021).D. Patel, J. B. Robinson, S. Ball, and D. J. L. Brett, (2020).D. P. Finegan et al., Phys. Chem. Chem. Phys., 18, 30912–30919 (2016).

Exploring unusual temperature-dependent optical properties of graphite single crystal by spectroscopic ellipsometry

This study investigated the optical properties of graphite single crystal as a function of temperature between 4.5 and 500 K and within the spectral range of 0.73–6.42 eV through spectroscopic ellipsometry. At room temperature, the complex dielectric function of graphite exhibited broad spectra in the ultraviolet energy region. The Breit–Wigner–Fano (BWF) line shape analysis of the optical absorption spectrum demonstrated that two BWF line shapes were fitted with central energies at 4.85 ± 0.01 eV and 6.21 ± 0.03 eV. These two features were attributed to interference between an excitonic transition at the saddle point (M) in the band structures and collective excitations of the surface plasmons. Analysis of the temperature-dependent asymmetric BWF line shape indicated that the peak position shifted to lower energies under increasing temperatures, with the linewidth becoming narrow and the intensity increasing; this can be explained by the asymmetric factor 1/qBWF, which decreased under increasing temperatures. The temperature dependence of optical absorption for graphite is opposite to that of the interband absorptions observed in silicon. First-principles calculations verified the presence of two resonant conditions in the ultraviolet energy region. The lifetime of surface plasmons was expected to decrease with increased temperatures, leading to a decrease of the 1/qBWF. These results offer insight into quasiparticle band structures and collective excitations of graphite and provide valuable information for the technological development of graphite-based ultraviolet optoelectronic and photonic devices at different temperatures.

Cutting of Graphite at Atomic and Close-to-Atomic Scale Using Flexible Enhanced Molecular Dynamics

Atomic and close-to-atomic scale manufacturing is the key technology for the production of next-generation devices with atomic precision. As an important approach of mechanical processing, cutting has evolved as a potential candidate to generate an atomically smooth surface; thus, exploring its ultimate capability is significant. In this paper, single-crystal graphite, whose lattice structure and chemical bond property are of representation for demonstration, is selected to study the mechanism of atomic layer removal using molecular dynamics. A localized workpiece, which is dynamically updated on the basis of the tool position, is used to improve the computation efficiency. The principle and bullet points of this modeling method are first introduced, followed by a series of simulations under various undeformed chip thicknesses and tool edge radii. In addition, different potentials for the tool–workpiece interaction are tested, and the effect on the material response is presented. Based on the analysis of deformation, the number of carbon layers removed, and cutting forces, the chip formation mechanism and further understanding of the controllability of cutting at atomic and close-to-atomic scale can be achieved.

Chloroaluminate Anion Intercalation in Graphene and Graphite: From Two-Dimensional Devices to Aluminum-Ion Batteries.

A rechargeable aluminum-ion battery based on chloroaluminate electrolytes has received intense attention due to the high abundance and chemical stability of aluminum. However, the fundamental intercalation processes and dynamics in these battery systems remain unresolved. Here, the energetics and dynamics of chloroaluminate ion intercalation in atomically thin single crystal graphite are investigated by fabricating mesoscopic devices for charge transport and operando optical microscopy. These mesoscopic measurements are compared to the high-performance rechargeable Al-based battery consisting of a few-layer graphene-multiwall carbon nanotube composite cathode. These composites exhibit a 60% capacity enhancement over pyrolytic graphite, while an ∼3-fold improvement in overall ion diffusivity is also obtained exhibiting ∼1% of those in atomically thin single crystals. Our results thus establish the distinction between intrinsic and ensemble electrochemical behavior in Al-based batteries and show that engineering ion transport in these devices can yet lead to vast improvements in battery performance.

Robustness of structural superlubricity beyond rigid models

Structural superlubricity is a theoretical concept stating that the friction force is absent between two rigid, incommensurate crystalline surfaces. However, elasticity of the contact pairs could modify the lattice registry at interfaces by nucleating local slips, favoring commeasure. The validity of structural superlubricity is thus concerned for large-scale systems where the energy cost of elastic distortion to break the lattice registry is low. In this work, we study the size dependence of superlubricity between single-crystal graphite flakes. Molecular dynamics simulations show that with nucleation and propagation of out-of-plane dislocations and strained solitons at Bernal interfaces, the friction force is reduced by one order of magnitude. Elastic distortion is much weaker for non-Bernal or incommensurate ones, remaining notable only at the ends of contact. Lattice self-organization at small twist angles perturbs the state of structural superlubricity through a reconstructed potential energy surface. Theoretical models are developed to illustrate and predict the interfacial elastoplastic behaviors at length scales beyond those in the simulations. These results validate the rigid assumption for graphitic superlubricity systems at microscale, and reveal the intrinsic channels of mechanical energy dissipation. The understandings lay the ground for the design of structural superlubricity applications.

Graphite microcrystals growth in naturally graphitized coal from Hunan, China

The transformation of coal to graphite is a process through which carbon atoms are arranged from amorphous into a crystalline form. However, there is still no study focusing on how the graphite single crystal grow up in graphitized coals during natural graphitization. Herein, a series of naturally graphitized coals were thoroughly studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) to uncover the morphological and atomic structural evolution from anthracite to coaly graphite. The anthracite and meta-anthracite samples showed an amorphous carbon structure, whereas the carbon arrangement becomes more and more orderly from semi-graphite to coaly graphite as confirmed by XRD. The crushed particles of anthracite and meta-anthracite show granular and tabular texture with uneven surface under SEM. Small layered graphite flake appears from semi-graphite stage with co-occurrence of dense coal particles, HRTEM data shows the layered graphite flakes are the single graphite crystals. Almost all the dense coal particles change into layered microcrystalline graphite in the most graphitized coals (coaly graphite), and the flaky graphite microcrystals always show up as aggregates. The results will benefit the future applications of the naturally graphitized coals.

Electrical conductivity of a single parallel contact between carbon nanotubes.

Interfacial resistance plays a critical role in the transport properties of nanomaterial-based assemblies. However, understanding of them remains limited due to the difficulty of experimental approaches. Here we report in situ measurements of the electrical resistance of a single parallel contact between carbon nanotubes. By varying the contact length systematically, we derive the electrical conductivities of carbon nanotubes and interfaces. The interface between nanotubes exhibits conductivity intermediate between those of pyrolytic and single-crystal graphite. The threshold contact length between interface- and bulk-dominant electrical transport is quantitatively estimated.

Lattice preferred orientation of graphite determined by the anisotropy of out-of-phase magnetic susceptibility

Single crystals of graphite are diamagnetic with a strong crystalline anisotropy. Their magnetic susceptibility measured in alternating fields exhibits a weak, negative in-phase component and a relatively strong out-of-phase component due to electric eddy currents. Consequently, if graphite crystals are oriented preferentially by their crystal lattices (LPO) in graphite ore, one would expect a strong anisotropy of magnetic susceptibility (AMS) of the bulk rock. However, the standard AMS, which is the anisotropy of the in-phase component of susceptibility (ipAMS), reflects not only the LPO of graphite, but also the preferred orientation of the paramagnetic and ferromagnetic admixtures. On the other hand, the anisotropy of out-of-phase susceptibility (opAMS) in rocks such as these should directly indicate the graphite LPO due to its strong electrical conductivity, and can therefore be considered free from the contributions of non-conductive paramagnetic and ferromagnetic minerals. These competing factors were tested on natural metamorphic graphite ores near the South Bohemian town of Český Krumlov in the Moldanubian Zone. The ipMS of the ore is very low, 10−6 to 10−4 [SI], being positive in some specimens and negative in others, which indicates mixtures with variable proportions of graphite and paramagnetic-ferromagnetic minerals. On the other hand, opMS is exclusively positive, on the order of 10−5 to 10−4, and is strongly indicative of graphite. The degree of opAMS is distinctly high at 200–300%, and the opAMS lineation is closely perpendicular to the metamorphic foliation in ores and wall rocks. All this indicates a conspicuous LPO of graphite in the ore that was probably created during Variscan regional metamorphism and associated ductile deformation.

Catalytically transformed low energy intensive 2D-layered and single crystal-graphitic renewable carbon cathode conductors

This is the first study of the catalytic graphitization of Black Spruce (Picea mariana) which has successfully discovered the formation of single crystal graphitic carbon structures with a very high conductivity over 850 S/m implemented in the cathode of a coin cell battery. Renewable carbon with this conductivity is suitable for use in bio-electronics, organic thin film transistors, fuel cells, organic batteries, supercapacitors and sensing device applications. The P. mariana was doped with iron nitrate nanoparticle precursor, and sequentially thermo-catalyzed in presence of helium at temperatures between 300 and 800 °C. Transmission electron micrographs reveal formation of graphitic structures with an interplanar distance of ∼0.33 nm resembling single crystal graphite structure. Raman spectroscopy and X-ray diffraction studies confirm the presence of nano-layered carbon, and the high conductivity was observed in Fe-free residual graphite. Thus, using iron nitrate as a catalyst promotes the formation of single crystal graphitic structures at a significantly reduced thermal energy than traditional pyrolysis treatment and opens a new frontier for sustainable bio-electronics and energy materials manufacturing.

Effects of maturity and temperature on soot density and specific heat

This paper presents an analysis of measurements of density and specific heat, coupled with X-ray crystallography data, for 47 solid polycyclic aromatic hydrocarbons (PAHs) spanning a range in carbon-to-hydrogen ratio (C/H) of 1.0–2.4. The results provide estimates of the dependence of the density and specific heat on C/H and temperature, parameterized for use in models of soot formation and chemical evolution. They are also applicable to models of the optical properties of soot particles at different levels of maturity at temperatures ranging from 0 to 4300 K. Comparisons of these parameterizations, extrapolated to values of C/H representative of highly carbonized particles, demonstrate excellent agreement with measured densities and specific heats of mature soot particles and polycrystalline and single-crystal graphite. This paper also presents a parameterization for the dependence of C/H on the dispersion exponent to facilitate the estimation of density and specific heat from optical measurements.

Elastic instability in graphite single crystal under dynamic triaxial compression: Effect of strain-rate on the resulting microstructure

We focus on the behavior of graphite under triaxial loading at a constant strain-rate using large scale molecular dynamics simulations. Buckling patterns (chevrons) in graphite nucleate from an elastic instability strongly related to the material anisotropy and subsequently grow until the first diamond nuclei appear. We show that the phase transition completely inhibits the growth of chevrons in buckled graphite, the diamond grain size being determined by the size of chevrons at the onset of nucleation. Cubic-diamond clusters nucleate within chevrons of buckled graphite and grow until the parent phase is entirely transformed. This phenomenon leads to nano-structured diamond polycrystals, with orientations of interfaces given by those of the buckled material right before the nucleation process. The buckled graphite microstructure is shown to strongly influence the final microstructure/size of nano-diamonds.