Micromechanical Cleavage Research Articles

Unique graphene‐carbon black hybrid nanofiller by a micromechanical cleavage technique as a reinforcing agent in elastomers: Fundamental and experimental studies

AbstractIn this work, a synthetic technique was developed which effected delamination of graphite into graphene with simultaneous disaggregation of carbon black clusters and formation of secondary attachment between the two. Styrene butadiene rubber nanocomposite with this hybrid filler exhibited substantial reinforcement in the mechanical properties, wear resistance and rubber‐filler interactions. The hybrid nanofiller was superior to conventional carbon black, imparting 57% increase in tensile strength of the nanocomposite. The composite with the hybrid filler also exhibited improved modulus and traction. A detailed analysis of mechanism of reinforcement by the hybrid filler is proposed by the computation of root mean square end‐to‐end network chain distance and the lateral tube dimension employing the entangled‐bound‐rubber‐tube (EBT) model. This neoteric hybrid nanofiller overcame the so‐called dispersibility issues and could be exploited as a part of replacement of carbon black, a synthetic material from fossil fuel by sustainable natural filler.

Exfoliation mechanisms of 2D materials and their applications

Due to the strong in-plane but weak out-of-plane bonding, it is relatively easy to separate nanosheets of two-dimensional (2D) materials from their respective bulk crystals. This exfoliation of 2D materials can yield large 2D nanosheets, hundreds of micrometers wide, that can be as thin as one or a few atomic layers thick. However, the underlying physical mechanisms unique to each exfoliation technique can produce a wide distribution of defects, yields, functionalization, lateral sizes, and thicknesses, which can be appropriate for specific end applications. The five most commonly used exfoliation techniques include micromechanical cleavage, ultrasonication, shear exfoliation, ball milling, and electrochemical exfoliation. In this review, we present an overview of the field of 2D material exfoliation and the underlying physical mechanisms with emphasis on progress over the last decade. The beneficial characteristics and shortcomings of each exfoliation process are discussed in the context of their functional properties to guide the selection of the best technique for a given application. Furthermore, an analysis of standard applications of exfoliated 2D nanosheets is presented including their use in energy storage, electronics, lubrication, composite, and structural applications. By providing detailed insight into the underlying exfoliation mechanisms along with the advantages and disadvantages of each technique, this review intends to guide the reader toward the appropriate batch-scale exfoliation techniques for a wide variety of industrial applications.

Prospects of Application and Global Significance of Graphene

The review article is an excursus into the world publications describing the properties of graphene, methods of synthesis of it and variety of its application fields. The paper describes in detail the structure of graphene as well as the methods for its fabrication: micromechanical cleavage, chemical stratification, epitaxial growth, and chemical gas-phase deposition, including their advantages and disadvantages. In addition, the review contains information on the electronic, mechanical, optical, and chemical properties of graphene, which lend its uniqueness. Due to its unique properties, graphene and its modified quasi-two-dimensional structures are the objects of increased scientific interest in various fields of science, such as energy, electronics, optoelectronics, medicine, bioengineering, aerospace, aviation, ecology, materials engineering, etc. In order to expand the journal readership among the physicists, chemists, and materials scientists, who are not deep specialists in graphene science, the style of the present review is somewhere close to popular science one.

Bull's eye grating integrated with optical nanoantennas for plasmonic enhancement of graphene long-wave infrared photodetectors

Two-dimensional (2D) materials have exhibited potential for infrared detection at room temperature, yet their low light absorption impedes their widespread application. In addition, micromechanical cleavage, which is the main method by which high-quality 2D layers are achieved, typically leads to small-area flakes, hampering their application as photodetectors. In this work, we designed a hybrid plasmonic structure, comprising a metallic bull's eye grating and optical nanoantennas, to collect and concentrate light into a piece of single-layer graphene with sub-wavelength lateral extent. This boosts the interaction between the graphene and light, thereby improving its photodetection performance in the technologically important long-wave infrared (LWIR) region. Finite-difference time-domain electromagnetic simulations were performed to this end. The plasmonic structure we present is predicted to enhance the absorption of light by the graphene by ∼558 times, which in turn is predicted to enhance the detectivity of the LWIR photodetector by ∼32 times.

Tunable Schottky barrier width and enormously enhanced photoresponsivity in Sb doped SnS2 monolayer

Doping, which is the intentional introduction of impurities into a material, can improve the metal-semiconductor interface by reducing Schottky barrier width. Here, we present high-quality two-dimensional SnS2 nanosheets with well-controlled Sb doping concentration via direct vapor growth approach and following micromechanical cleavage process. X-ray photoelectron spectroscopy (XPS) measurement demonstrates that Sb contents of the doped samples are approximately 0.22%, 0.34% and 1.21%, respectively, and doping induces the upward shift of the Fermi level with respect to the pristine SnS2. Transmission electron microscopy (TEM) characterization exhibits that Sb-doped SnS2 nanosheets have a high-quality hexagonal symmetry structure and Sb element is uniformly distributed in the nanosheets. The phototransistors based on the Sb-doped SnS2 monolayers show n-type behavior with high mobility which is one order of magnitude higher than that of pristine SnS2 phototransistors. The photoresponsivity and external quantum efficiency (EQE) of Sb-SnS2 monolayers phototransistors are approximately three orders of magnitude higher than that of pristine SnS2 phototransistor. The results suggest that the method of reducing Shottky barrier width to achieve high mobility and photoresponsivity is effective, and Sb-doped SnS2 monolayer has significant potential in future nanoelectronic and optoelectronic applications.

2D Transition Metal Dichalcogenide Thin Films Obtained by Chemical Gas Phase Deposition Techniques

AbstractUltrathin 2D transition metal dichalcogenide (TMD) thin films have attracted much attention due to their very good electrical, optical, and electrochemical properties. Chemical vapor deposition (CVD) and atomic layer deposition (ALD), which is in some regards an enhanced version of CVD, are techniques that can provide exceptionally conformal large‐area coatings, even for complex surface geometries. Besides, these techniques include the transport of one or more precursor chemicals in the gas phase onto a substrate. Subsequently, a chemical reaction occurs, resulting in the deposition of a film of a solid material on the substrate. One of the advantageous aspects of chemical deposition methods, such as CVD and ALD, is the growth of thin films onto a variety of substrates as well as 3D structures. Because of their chemical approach, these techniques are well suited to synthesizing 2D materials (2DMs) with a low defect concentration. Furthermore, the scalability would allow industrial application, as opposed to, e.g., micromechanical cleavage. Here, the recent progress in 2D TMD thin films is reviewed and the current applications of these materials fabricated by CVD and ALD are surveyed.

Crystallization behavior of polypropylene/graphene nanoplatelets composites

Interest in graphite fillers has grown since the separation of graphene from graphite by micromechanical cleavage. The object of the paper is to understand the influence of graphene nanoplatelets (GNPs) with different sizes on the crystallization behavior of a polyolefin matrix such as polypropylene (PP), after elaboration by melt mixing and compression molding. Composites with volume fractions of graphene nanoplatelets ranging from 0.3 to 2 vol% were prepared. The particle dispersion states in the composites were characterized at different scales using Scanning and Transmission Electron Microscopies (SEM and TEM). Polypropylene crystallization and orientation were investigated using optical microscopy, Differential Scanning Calorimetry (DSC) and X-ray diffraction. This paper discusses the strong acceleration of crystallization kinetics due to the presence of GNPs. The micrometric flake-shaped GNPs act as nucleating agent and induce an epitaxial growth of alpha (α) crystalline phase of PP. The nucleating effect is related to the surface of the particles available for heterogeneous nucleation. Radial spherulitic growths are observed from the smallest micrometric particles. The coarsest GNPs, easily oriented by flow, favor PP transcrystallinity, in such a way that (010) plane of PP is parallel to (001) plane of graphene nanoplatelets.

Two-dimensional plumbum-doped tin diselenide monolayer transistor with high on/off ratio

Doping can effectively regulate the electrical and optical properties of two-dimensional semiconductors. Here, we present high-quality Pb-doped SnSe2 monolayer exfoliated using a micromechanical cleavage method. X-ray photoelectron spectroscopy measurement demonstrates that Pb content of the doped sample is ∼3.6% and doping induces the downward shift of the Fermi level with respect to the pure SnSe2. Transmission electron microscopy characterization exhibits that Pb0.036Sn0.964Se2 nanosheets have a high-quality hexagonal symmetry structure and Pb element is uniformly distributed in the nanosheets. The current of the SnSe2 field effect transistors (FETs) was found to be very difficult to turn off due to the high electron density. The FETs based on the Pb0.036Sn0.964Se2 monolayer show n-type behavior with a high on/off ratio of 106 which is higher than any values of SnSe2 FETs reported at the moment. The estimated carrier concentration of Pb0.036Sn0.964Se2 is approximately six times lower than that of SnSe2. The results suggest that the method of reducing carrier concentration by doping to achieve high on/off ratio is effective, and Pb-doped SnSe2 monolayer has significant potential in future nanoelectronic and optoelectronic applications.

Graphene: from synthesis to engineering to biosensor applications

Graphene is a fascinating material of recent origin whose first isolation was being made possible through micromechanical cleavage of a graphite crystal. Owing to its fascinating properties, graphene has garnered significant attention in the research community for multiple applications. A number of methods have been employed for the synthesis of single-layer and multi-layer graphene. The extraordinary properties of graphene such as its Hall effect at room temperature, high surface area, tunable bandgap, high charge mobility and excellent electrical, conducting and thermal properties allow for the development of sensors of various types and also opened the doors for its use in nanoelectronics, supercapacitors and batteries. Biological aspects of graphene have also been investigated with particular emphasis on its toxicity and drug delivery. In this review, many of the salient aspects of graphene, such as from synthesis to its applications, primarily focusing on sensor applications which are of current interest, are covered.

New Layered Triazine Framework/Exfoliated 2D Polymer with Superior Sodium-Storage Properties.

The efficient synthesis of 2D polymers (2DPs) with tailorable structures and properties is highly desired but remains a considerable challenge. Here, the first solution synthesis of millimeter-size crystalline covalent triazine frameworks (CTFs) with a clear lamellar structure, which can be exfoliated into micrometer-size few-layer 2DP sheets via both micromechanical cleavage and liquid sonication, is reported. The obtained CTFs or 2DPs show a unique staggered AB stacking with a dominant pore size of ≈0.6 nm, which is different from the common eclipsed AA stacking in various covalent organic frameworks. The preference for AB stacking is due to the specific interaction of triflic acid with CTFs as revealed computationally. When explored as new polymeric anodes for sodium-ion batteries, both crystalline bulk CTF and exfoliated 2DP exhibit very high capacities (225 and 262 mA h g-1 at 0.1 A g-1 , respectively), impressive rate capabilities (67 and 119 mA h g-1 at 5.0 A g-1 , respectively), and excellent cycling stability (95% capacity retention after 1200 cycles) due to their robust conjugated porous structure, outperforming most organic/polymeric sodium-ion battery anodes ever reported.

A two-dimensional Fe-doped SnS2 magnetic semiconductor

Magnetic two-dimensional materials have attracted considerable attention for their significant potential application in spintronics. In this study, we present a high-quality Fe-doped SnS2 monolayer exfoliated using a micromechanical cleavage method. Fe atoms were doped at the Sn atom sites, and the Fe contents are ∼2.1%, 1.5%, and 1.1%. The field-effect transistors based on the Fe0.021Sn0.979S2 monolayer show n-type behavior and exhibit high optoelectronic performance. Magnetic measurements show that pure SnS2 is diamagnetic, whereas Fe0.021Sn0.979S2 exhibits ferromagnetic behavior with a perpendicular anisotropy at 2 K and a Curie temperature of ~31 K. Density functional theory calculations show that long-range ferromagnetic ordering in the Fe-doped SnS2 monolayer is energetically stable, and the estimated Curie temperature agrees well with the results of our experiment. The results suggest that Fe-doped SnS2 has significant potential in future nanoelectronic, magnetic, and optoelectronic applications.

Rapid and nondestructive layer number identification of two-dimensional layered transition metal dichalcogenides

MoS2, MoSe2 and WSe2 thin flakes were fabricated by the standard micromechanical cleavage procedures. The thickness and the optical contrast of the atomic thin dichalcogenide flakes on SiO2/Si substrates were measured by atomic force microscopy (AFM) and spectroscopic ellipsometer. A rapid and nondestructive method by using reflection spectra was proposed to identify the layer number of 2D layered transition metal dichalcogenides on SiO2 (275 nm)/Si substrates. The contrast spectra of 2D nanosheets with different layer numbers are in agreement with theoretical calculations based on Fresnel’s law, indicating that this method provides an unambiguous and nondestructive contrast spectra fingerprint for identifying single- and few-layered transition metal dichalcogenides. The results will greatly help in fundamental research and application.

Solution Processed Boron Nitride Nanosheets: Synthesis, Assemblies and Emerging Applications

In the last decade, few‐layer boron nitride nanosheets and monolayer BN nanosheet (BNNS) have gained much attention due to their unique physical and chemical properties. To date, BNNS can be produced by micromechanical cleavage of BN crystal, unzipping BN nanotubes, chemical vapor deposition (CVD), and solution processed exfoliation from bulk BN powder. Due to the low cost and abundance of bulk BN powder and its simple processing for potentially scalable production, great efforts have been devoted toward solution processed exfoliation. In this feature article, recent significant advances in solution processed synthesis of BNNS are summarized. In particular, the solvent choice for one‐step BN exfoliation is highlighted. Multi‐dimensional assemblies consisting of BNNS are discussed, such as BN fiber, BN paper, and BN aerogel. The emerging applications of BNNS in different fields are then focused on, especially in barrier materials, dielectrics, catalysts, and thermal management.

Electrical characterization of two-dimensional materials and their heterostructures

Two-dimensional (2D) materials have gained enormous attention in recent years owing to their huge potential in future electronics and optics. On the one hand, conventional 2D materials like graphene, MoS2, h-BN are being intensively studied, on the other hand, search for novel 2D materials is at a rapid pace. In this study, we have investigated electrical properties of 2D nanosheets of ultrathin Indium Selenide (InSe), a member of the III-VI chalcogenides’ family. The InSe layers were prepared via micromechanical cleavage of its bulk crystal and were integrated into a field-effect transistor (FET) device as the transport channel. On characterizing the InSe-based FETs, InSe showed n-type conductance with the mobility of 2.1×10−4 cm2V−1s−1.

Electronic structure and mechanical properties of layered compound YB2C2: A promising precursor for making two dimensional (2D) B2C2 nets

Layered compounds play pivotal roles as precursors for producing 2D materials through mechanical exfoliation (micro-mechanical cleavage) or chemical approaches. Therefore, searching for layered compounds with sharp anisotropic chemical bonding and properties becomes emergent. In this work, the stability, electronic structure, elastic properties, and lattice dynamics of YB2C2 were investigated. Strong anisotropy in elastic properties is revealed, i.e., high Young's modulus in a-b plane but low Young's modulus in c direction. The maximum to minimum Young's modulus ratio is 2.41 and 2.45 for YB2C2 with P42/mmc and P4/mbm symmetry, respectively. The most likely systems for shear sliding or micro-delaminating are (001)[100] and (001)[010]. The anisotropic elastic properties are underpinned by the anisotropic chemical bonding, i.e., strong bonding within the B2C2 nets and weak bonding between Y atom layers and B2C2 nets. YB2C2 is electrically conductive and the contributions to the electrical conductivity are from delocalized Y 4deg as well as B 2pz and C 2pz electrons. The layered crystal structure, sharp anisotropic mechanical properties, and metallic conductivity endorse YB2C2 promising as a precursor for new 2D B2C2 nets.

Optical contrast spectra studies for determining thickness of stage-1 graphene-FeCl3 intercalation compounds

Because of novel features in their structural, electronic, magnetic and optical properties, especially potential applications in nanoelectronics, the few-layer graphene intercalation compounds (FLGICs) have been intensively studied recently. In this work, the dielectric constant of the doped graphene of stage-1 FeCl3-GIC is obtained by fitting the optical contrast spectra. And fully intercalated stage-1 FeCl3-FLGICs were prepared by micromechanical cleavage method from graphite intercalation compounds (GICs) for the first time. Finally, we demonstrated that the thickness of stage-1 FeCl3-GICs by micromechanical cleavage can be determined by optical contrast spectra. This method also can be used to other FLGICs, such as SbCl5-FLGICs and AuCl5-FLGICs, etc.

Graphene‐Based Nanocomposites for Energy Storage

Since the first report of using micromechanical cleavage method to produce graphene sheets in 2004, graphene/graphene‐based nanocomposites have attracted wide attention both for fundamental aspects as well as applications in advanced energy storage and conversion systems. In comparison to other materials, graphene‐based nanostructured materials have unique 2D structure, high electronic mobility, exceptional electronic and thermal conductivities, excellent optical transmittance, good mechanical strength, and ultrahigh surface area. Therefore, they are considered as attractive materials for hydrogen (H2) storage and high‐performance electrochemical energy storage devices, such as supercapacitors, rechargeable lithium (Li)‐ion batteries, Li–sulfur batteries, Li–air batteries, sodium (Na)‐ion batteries, Na–air batteries, zinc (Zn)–air batteries, and vanadium redox flow batteries (VRFB), etc., as they can improve the efficiency, capacity, gravimetric energy/power densities, and cycle life of these energy storage devices. In this article, recent progress reported on the synthesis and fabrication of graphene nanocomposite materials for applications in these aforementioned various energy storage systems is reviewed. Importantly, the prospects and future challenges in both scalable manufacturing and more energy storage‐related applications are discussed.

Exfoliated 2D black phosphorus nanosheets: Field emission studies

A few layer black phosphorus (BP) nanosheets are obtained by micromechanical cleavage from a bulk BP crystal. In a typical exfoliation procedure, a layer of BP is peeled off from bulk crystal with the help of Scotch tape and transferred onto SiO2/Si and pristine Si substrates. The morphological and structural analyses of the samples were carried out using an optical microscopy, a transmission electron microscopy, and a Raman spectroscopy. Field emission investigations on a few layered BP nanosheets on Si substrate were carried out at the base pressure of 1 × 10−8 mbar. The turn-on value, corresponding to emission current density of ∼1 μA/cm2, is found to be ∼5.1 V/μm for BP nanosheets/Si emitter and high field enhancement factor (β) ∼1164, attributed to atomically thin/sharp edges of the BP nanosheets. The emission current shows good stability at a preset value of ∼5 μA over a period of more than 8 h. The present results demonstrate the potential of the mechanically exfoliated BP nanosheets/Si field emitter for the development of practical electron sources.

(Invited) Liquid-Phase Exfoliated Two-Dimensional Nanosheets: From Large Scale Production to Energy Storage Applcations

Graphene has generated huge interest in recent years due to its unique physics properties [1]. We have shown that high-quality monolayer graphene can be produced at significant yields by non-chemical, solution-phase exfoliation of graphite in certain organic solvents. Until a few years ago the standard procedure used to make graphene was micromechanical cleavage [2], which is a very low yield production method. In order to fully exploit graphene’s outstanding properties, a mass production method was necessary and the development of a method to exfoliate cheap, commercial graphite in organic solvents down to large area single graphene flakes with high yield was one major achievement [3]. Recently this work has been extended to a wide range of two-dimensional inorganic nanomaterials [3,4]. These are potentially important because they occur in >100 different types with a wide range of electronic properties, varying from metallic to semiconducting. The liquid-phase exfoliation method has now been up-scaled to produce grams of a variety of exfoliated materials per day [5]. This talk will first discuss the galaxy of existent layered materials, with emphasis on synthesis and liquid-phase exfoliation to finish off with some key applications recently developed in our laboratories. A. K. Geim, Science 324, 1530 (Jun, 2009). S. Stankovich et al., Nature 442, 282 (Jul, 2006).Y. Hernandez et al., Nature Nanotechnology, 3, 9 (2008) 563 J. N. Coleman et al., Science, 331, pp. 568-571 (2011) V. Nicolosi et al., Science, 340, 6139, 1420 (2013) K. R. Paton et al., Nature Materials, 13, 6, 624-30 (2014)

Measurements of the in-plane coefficient of thermal expansion of freestanding single-crystal natural graphite

The in-plane coefficient of thermal expansion (CTE) of freestanding single-crystal natural graphite was directly determined using the thermal bulge method. The Scotch tape method was used to obtain 40-nm-thick graphite flakes by the micromechanical cleavage of high-quality graphite flakes. The naturally bulged shape of the samples enabled us to easily measure the CTE without any need for transfer or additional processes. The measured CTE was negative and gradually decreased in magnitude from 1.8×10−6/°C to 0.7×10−6/°C when the samples were heated from 25°C to 225°C.