Thin Graphite Flakes Research Articles

Mesoscopic scale rearrangements of graphite nanoflake open edges under mild annealing treatments

The paper presents an effect of mild annealing treatments on structural ordering and open edge morphology of graphite nanoflakes (GNF). Thin graphite flakes were obtained from the bulk highly oriented pyrolytic graphite by a prolonged grinding in a pure argon atmosphere and were annealed at T = 670 K for 24 h either in vacuum or in the air. Modification of GNF open edges was studied with high-resolution transmission electron microscopy (HRTEM) and 2D-Fourier analysis of the open edge area HRTEM-images. The morphology of the GNF edges evolve differently under annealing, revealing itself in the azimuthal width of the two-dimensional fast-Fourier transform (2D-FFT) maxima. After annealing in the air, the width decreased, while the vacuum treatment lead to its growth. We relate the former to the open edge flattening via removal of the edge defects by transformation to the gaseous carbon oxides, the latter – to the opposite process of defect accumulation in vacuum with the transformation of the near-edge structure to the less ordered one. The correlation is found between the intensity of the (101) powder X-ray diffraction maximum and the azimuthal width of the FFT-peaks indicating the direct relation of these quantities.

Documenting capacity and cyclic stability enhancements in synthetic graphite potassium-ion battery anode material modified by low-energy liquid phase ball milling

Graphite is the most promising anode material for potassium-ion batteries due to the attractive theoretical capacity of 278 mAh g−1 and its low potential operation with respect to a potassium reference electrode. However, it is reported that its practically achievable capacities and cyclic stabilities are often relatively poor, which may impact the practical application of graphite in this new type of batteries. The recent results indicate that mechanical milling can be a possible modification method for graphitic materials to improve their electrochemical behaviors. In this work, we document the effects of liquid-phase ball milling conducted under low-energy conditions on the capacity levels and cyclic stability of the commercially available synthetic graphite. The obtained thin graphite flakes with high surface area lead to a much improved electrochemical performance of the electrodes with high reversible capacity of 227 mAh g−1 after 500 cycles at 100 mA g−1 (capacity decay as slow as 0.0043% per cycle and a capacity retention of ~98%) and de-potassiation capacity of 226 mAh g−1 at 4 A g−1 current. This study demonstrates the possibility of capacity and cyclic stability enhancement in graphitic materials by mechanical milling under appropriately selected milling conditions.

Influence of rhombohedral stacking order in the electrical resistance of bulk and mesoscopic graphite

The electrical, in-plane resistance as a function of temperature $R(T)$ of bulk and mesoscopic thin graphite flakes obtained from the same batch was investigated. Samples thicker than $\sim 30$ nm show metalliclike contribution in a temperature range that increases with the sample thickness, whereas a semiconductinglike behavior was observed for thinner samples. The temperature dependence of the in-plane resistance of all measured samples and several others from literature can be very well explained between 2 K and 1100 K assuming three contributions in parallel: a metalliclike conducting path at the interfaces between crystalline regions, composed of two semiconducting phases, i.e. Bernal and rhombohedral stacking. From the fits of $R(T)$ we obtain a semiconducting energy gap of $110 \pm 20$meV for the rhombohedral and $38\pm 8 $meV for the Bernal phase. The presence of these crystalline phases was confirmed by x-ray diffraction measurements. We review similar experimental data from literature of the last 33 years and two more theoretical models used to fit $R(T)$.

Influence of ion irradiation on temperature dependent electrical transport behavior of thin graphite flakes

AbstractIn this article, the influence of ion irradiation on temperature dependent electrical transport characteristics of thin graphite flakes was investigated. Thin graphite flakes were exfoliated by mechanical exfoliation method. Scanning electron microscopy was used to study surface morphology of the graphite flakes. The resistance versus temperature studies revealed that the graphite flake not subjected to Ga+ion-irradiation showed a perfect metallic behavior, while the graphite flake after ion-irradiation showed a semiconducting behavior. The current-voltage (I-V) characteristics of bare and ion-irradiated graphite flakes were investigated. The bare graphite flake showed an ohmic-type I-V characteristics representing metallic behavior, while the ion-irradiated graphite flake showed a non-linear type diode-like characteristics. The temperature-dependent conductance measurements of ion-irradiated graphite flake were also performed and discussed in detail. The effect of Ga+ions on the electronic transport behavior of thin graphite flakes has been discussed based on the investigation results.

Mechanical buckling induced periodic kinking/stripe microstructures in mechanically peeled graphite flakes from HOPG

Mechanical exfoliation is a widely used method to isolate high quality graphene layers from bulk graphite. In our recent experiments, some ordered microstructures, consisting of a periodic alternation of kinks and stripes, were observed in thin graphite flakes that were mechanically peeled from highly oriented pyrolytic graphite. In this paper, a theoretical model is presented to attribute the formation of such ordered structures to the alternation of two mechanical processes during the exfoliation: (1) peeling of a graphite flake and (2) mechanical buckling of the flake being subjected to bending. In this model, the width of the stripes L is determined by thickness h of the flakes, surface energy $$\gamma $$ , and critical buckling strain $$\varepsilon _{\mathrm{cr}}$$ . Using some appropriate values of $$\gamma $$ and $$\varepsilon _{\mathrm{cr}}$$ that are within the ranges determined by other independent experiments and simulations, the predicted relations between the stripe width and the flake thickness agree reasonably well with our experimental measurements. Conversely, measuring the L–h relations of the periodic microstructures in thin graphite flakes could help determine the critical mechanical buckling strain $$\varepsilon _{\mathrm{cr}}$$ and the interface energy $$\gamma $$ .

Experimental evidence of very long intrinsic phonon mean free path along the c-axis of graphite

We report on experimental studies of the average phonon mean free path in the c-axis direction of graphite. Through systematically measuring the cross-plane thermal conductivity of thin graphite flakes with thickness ranging from 24 nm to 714 nm via a differential three omega method, we demonstrate that the average phonon mean free path in the c-axis direction of graphite is around 200 nm at room temperature, much larger than the commonly believed value of just a few nanometers. This study provides direct experimental evidence for the recently projected very long phonon mean free path along the c-axis of graphite.

Flexible Sheet Heater with Carbon Nanotube Networked Thin Graphite Flakes

Carbon nanomaterials have attracted much attention for heater applications due to their outstanding thermal characteristics [1-3]. Also, sheet heaters made from carbon nanomaterials have drawn on going interest of automobile industry as a promising candidate for heater applications due to the flexibility, low power consumption, and rapid response. Despite of potential possibilitis of carbon nanomaterials for heater applications, it is needed to improve the electrical conductivity and mechanical durability for practical use such as seat heater.In this study, we fabricated sheet heater using thin graphite flakes to improve electrical conductivity and multi walled carbon nanotubes (MWCNT) to ehnance the mechanical stability of sheet heaters. The thin graphite flakes and amide functionalized MWCNT were blended with polyester resin and printed on a PET film by screen printing method. Fabricated sheet heaters show uniform heat distribution compared to conventional wire heater. And then, we fabricated large area heater same as seat heater for automobile application to evaluate the feasibility of the sheet heater. Heating characteristics of the sheet heater was analyzed in correlation with microstructure and electrical characteristics. This research was financially supported by the Ministry of Trade, Industry and Energy(MOTIE) and Korea Institute for Advancement of Technology(KIAT) through the Promoting Regional specialized IndustryReferences[1] Daewoong Jung, Donghyun Kim, Kyung H. Lee, Lawrence J. Overzet, Gil S. Lee, Sensors and Actuators: A 199, 176 (2013).[2] Hoon-Sik Jang, Sang Koo Jeon, Seung Hoon Nahm, CARBON 49, 111 (2011).[3] Hyeongwook Im, Eui Yun Jang, Ajeong Choi, Wal Jun Kim, Tae June Kang, Yung Woo Park, and Yong Hyup Kim, Appl. Mater. Interfaces 4, 2338 (2012).

Lateral homoepitaxial growth of graphene

In this paper, we present electron diffraction and high-resolution transmission electron microscopy (HRTEM) evidence that graphene can be laterally homoepitaxially grown from homo-seeds. We demonstrate that exfoliated thin graphite flakes and CVD-grown graphene grains can both serve as seed crystals. Raman spectra indicate that the epitaxial graphene is 1–2 layers thick, regardless of seed thickness. A two-step growth procedure is developed to epitaxially synthesize large graphene grains. Lateral homoepitaxial growth makes it a reality to duplicate the structure and increase the size of small high-quality graphene crystals.

Possible room temperature superconductivity in conductors obtained by bringing alkanes into contact with a graphite surface

Electrical resistances of conductors obtained by bringing alkanes into contact with a graphite surface have been investigated at room temperatures. Ring current in a ring-shaped container into which n-octane-soaked thin graphite flakes were compressed did not decay for 50 days at room temperature. After two HOPG plates were immersed into n-heptane and n-octane at room temperature, changes in resistances of the two samples were measured by four terminal technique. The measurement showed that the resistances of these samples decrease to less than the smallest resistance that can be measured with a high resolution digital voltmeter (0.1μV). The observation of persistent currents in the ring-shaped container suggests that the HOPG plates immersed in n-heptane and n-octane really entered zero-resistance state at room temperature. These results suggest that room temperature superconductor may be obtained by bringing alkanes into contact with a graphite surface.

One-pot synthesis of carbon coated-SnO2/graphene-sheet nanocomposite with highly reversible lithium storage capability

We report a one-pot hydrothermal approach to synthesize carbon coated-SnO2/graphene-sheet (SnO2-C/GNS) nanocomposite. Strong oxidation–reduction reactions to produce GNS by the traditional Hummers' method are avoided. The experiments show that the glucose and tin tetrachloride can intercalate into the thin graphite flake to exfoliate graphite and form SnO2-C/GNS nanocomposite simultaneously during the hydrothermal process. The approach is quite simple and green. Meanwhile, the prepared SnO2-C/GNS nanocomposite as an anode material of lithium-ion batteries exhibits higher lithium storage capacity and better cycling performance compared to SnO2 nanoparticle and SnO2-C microsphere. It still delivers the reversible capacity of 703 mA h g−1 after 80 cycles at a current density of 100 mA g−1 and maintains 443 mA h g−1 after 100 cycles at a current density of 1000 mA g−1. The improvement in the performance of SnO2-C/GNS nanocomposite can be attributed to the fully confinement of SnO2 nanoparticles between the GNS and the carbon layer, which can effectively prevent the detachment and agglomeration of SnO2 and preserve the integrity of the nanostructure during charge/discharge cycling.

Investigation of Temperature Dependent Current–Voltage Characteristics of Thin Graphite Flakes Under Low- and High-Current Biasing

We have investigated the temperature dependent current–voltage (I–V ) characteristics of thin graphite flakes (size in micron-range) under lowand high-current biasing at various temperatures from 25 K to 300 K. We observed a clear metallic behavior which is an identical behavior of graphite as reported previously. From the I–V characteristics, we observed an Ohmic-like linear characteristics at low biasing; however, the same behavior is turned into curve-like nonlinear characteristics when the sample is biased with higher current. We also noticed a clear kink-type structure from the I–V curves when the temperature goes down. Our experimental points are fitted on a straight line and no kink was observed at 300 K. We suspect the appearance of kink effect is mainly due to interaction between conduction electrons and various modes of phonons or partially arises from thermal activation.

X-ray absorption spectroscopy by full-field X-ray microscopy of a thin graphite flake: Imaging and electronic structure via the carbon K-edge

We demonstrate that near-edge X-ray-absorption fine-structure spectra combined with full-field transmission X-ray microscopy can be used to study the electronic structure of graphite flakes consisting of a few graphene layers. The flake was produced by exfoliation using sodium cholate and then isolated by means of density-gradient ultracentrifugation. An image sequence around the carbon K-edge, analyzed by using reference spectra for the in-plane and out-of-plane regions of the sample, is used to map and spectrally characterize the flat and folded regions of the flake. Additional spectral features in both π and σ regions are observed, which may be related to the presence of topological defects. Doping by metal impurities that were present in the original exfoliated graphite is indicated by the presence of a pre-edge signal at 284.2 eV.

Spin transport in a thin graphite flake

We have studied the spin transport on a 30-nm thick and several micrometer long oriented graphite flake using a spin-valve configuration with four ferromagnetic Co electrodes of different widths and several \(\upmu\hbox{m}\) separation. A 5-nm thin Pt layer has been introduced in between the ferromagnetic Co injector/detector and the graphite surface. In spite of the conductivity mismatch problem, efficient electrical spin injection and detection in graphite has been achieved. The magnetoresistance in the local and half-local electrodes shows clear maxima with symmetry around zero field. The spin transport can be detected up to 150 K.

Nanoscale Fabrication of a Three-Dimensional Stack of Graphene Layers Using a Focused Ion Beam

A top-down nanoscale stack fabrication approach involving the high-precision carving of a stack of graphene layers from a thin graphite flake via focused-ion-beam (FIB) three-dimensional (3D) etching is presented. Using this technology, etching was performed in the c-axis with the height of a few tens of nanometres. By reducing the stack height to 50 nm, a 3D stack of graphene layers coupled in a stacked manner was fabricated. The transport characteristics of the stack in the c-axis were studied and compared with the transport characteristics of the stack with a larger in-plane area and stack height value. The observed conductivity of the nanostack with a 50 nm height was one order smaller than the conductivity of the stack with a larger in-plane area and stack height value. Nonlinear current-voltage (I-V) characteristics were observed in all the studied temperatures, but the stack with a larger in-plane area and stack height value showed nonlinear characteristics only at lower temperatures. The observation of these temperature-dependent I-V characteristics was discussed in detail, and the tunneling characteristics were investigated using the Fowler-Nordheim tunneling model, which showed the best fit to the experiment data in this study.

Fabrication and Characteristics of Nanoscale Stacked-Tunneling-Junctions on Graphite Flake Using Focused Ion Beam

We report the successful fabrication and characteristics of a nanoscale stack of tunneling junctions formed along the c-axis of thin graphite flakes. The stack was made by a three-dimensional focused-ion-beam etching method. First, a submicron bridge was patterned in a required junction width by normal-direction etching. By tilting the sample stage to 90°, two grooves on the bridge were etched from the lateral direction in order to create the required stack size. The stacked junctions showed a perfect c-axis transport behavior and nonlinear current–voltage (I–V) characteristics at all studied temperatures. We discussed the nonlinear I–V characteristics in detail using the Schottky barrier model and Fowler–Nordheim (FN) tunneling theory. The stack transport characteristics showed good agreement with FN tunneling behavior.

Enhancement of the ferromagnetic order of graphite after sulphuric acid treatment

We have studied the changes in the ferromagnetic behavior of graphite powder and graphite flakes after treatment with diluted sulphuric acid. We show that this kind of acid treatment enhances substantially the ferromagnetic magnetization of virgin graphite micrometer size powder as well as in graphite flakes. The anisotropic magnetoresistance amplitude at 300 K measured in a micrometer size thin graphite flake after acid treatment reaches values comparable to polycrystalline cobalt.

Fabrication of nanoscale three-dimensional graphite stacked-junctions by focused-ion-beam and observation of anomalous transport characteristics

We report on the fabrication and transport characteristics of nanoscale stacked-junctions of thin graphite flake. The stacked-junctions were fabricated using a three-dimensional focused-ion-beam milling. By varying the effective in-plane area down to submicron scale, the stacked-junctions with in-plane area A (from 2 down to 0.25 μm 2) and stack height–length (from 300 to 100 nm) along c-axis were fabricated. The nano-stack shows perfect c-axis transport characteristics in which we observed a semiconducting behavior for T > 65 K and metallic behavior for T < 65 K. The obtained results were well fitted with the c-axis electrical conduction mechanism. The stack with in-plane area A of 0.25 μm 2 showed nonlinear concave-like I– V characteristics even at 300 K; however the stack with A ⩾ 1 μm 2 were shown an ohmic-like I– V characteristic at 300 K for both low and high-current biasing. It turned into nonlinear characteristics when the temperature goes down. The observation of this anomalous transport characteristics were discussed in detail with stack capacitance calculations. The nonlinear characteristics observed at 300 K for the stack with A of 0.25 μm 2 were shown best fit with Fowler–Nordheim tunneling model.

Fabrication and Characteristics of Submicron Stacked-Junctions on Thin Graphite Flakes

We report on the fabrication and transport characteristics of submicron-size stacks along c-axis of thin graphite flakes. The stacks were fabricated using a three-dimensional focused-ion-beam (FIB) etching technique. The stack with in-plane area A of 0.5 microm2 showed nonlinear concave-like I-V characteristics even at 300 K; however the stack with A of > 0.5 microm2 were shown an ohmic-like I-V characteristic at 300 K for both low and high-current biasing. It turned into nonlinear characteristics when the temperature goes down. The in-plane area dependence of stack capacitance were discussed and the observed capacitance of stack with A of 0.5 microm2 is smaller than the capacitance of stack with A of 1 microm2 which causes the nonlinear I-V characteristics in stack with A of 0.5 microm2 even at 300 K.

The influence of Ga + irradiation on the transport properties of mesoscopic conducting thin films

We studied the influence of 30 keVGa + -ions—commonly used in focused-ion-beam (FIB) devices—onthe transport properties of thin crystalline graphite flakes, andLa0.7Ca0.3MnO3 and Co thin films. The changes in electrical resistance were measured in situ during irradiationand also the temperature and magnetic field dependence before and after irradiation. Ourresults show that the transport properties of these materials strongly change atGa + fluences much below those used for patterning and ion-beam-induced deposition(IBID), seriously limiting the use of FIB when the intrinsic properties of thematerials of interest are of importance. We present a method that can be used toprotect the sample as well as to produce selectively irradiation-induced changes.

High-resolution electron microscopy of metal-intercalated “Graphimets”

Samples of some commercially available catalysts (known as “Graphimets”), in the form of graphite which had been intercalated with chlorides of Fe, Co, Cu, Ni, and Pt and subsequently reduced at low temperatures, have been examined by high-resolution electron microscopy and other techniques. The observations revealed that the intercalated element was present primarily as small islands of either metal or metal oxide on the surfaces of thin graphite flakes which appeared to have separated as a consequence of the manufacturing procedure. The direct resolution of lattice fringes in electron micrographs of the small (10–100 Å) particles, together with subsequent optical diffraction, provided a means of microdiffraction analysis which enabled identification of the phases present. The very high reaction rate measured for graphite gasification in the case of the Fe Graphimet, which will be described in detail elsewhere, was in keeping with the small particle size and the correspondingly large surface area. Finally, the potential of electron microscopy at the lattice resolution level for resolving a number of outstanding questions concerning the role of small metal particles in catalysis is briefly discussed.