Single-crystal Graphene Research Articles

Fast single-crystallization of Cu foils Facilitated by graphene growth

The production of large-area single-crystal metal foils such as Cu and Ni has recently gained momentum through thermally annealing of commercially available polycrystalline foils. Surface energy plays a pivotal role for grain growth, but is usually insufficient, particularly for adjacent grains with similar surface orientations, causing prolonged annealing time and reproducibility issues. Our method accelerates Cu foil single-crystallization through graphene growth, where graphene coverage on Cu modulates surface energy, enhancing driving forces and accelerating grain growth. This is a critical breakthrough in overcoming the traditional grain growth limitations. We successfully prepare decimeter-sized single-crystal Cu (111) foils with high efficiency and reproducibility and use it for single-crystal graphene growth. Our method paves the way for the scalable production of large-area single-crystal Cu foils, with potential applications extending to other metals, contributing to the ongoing efforts to overcome the challenges associated with traditional single-crystal production methods.

Biosensing beyond Debye screening length using epitaxial graphene field-effect transistors on SiC substrate

We demonstrated the antigen detection beyond Debye screening length using antibody-modified epitaxial graphene field-effect transistors (FETs), which have been considered to be impossible because of the Debye screening length. The transfer characteristics of the graphene FETs with a single crystal epitaxial graphene film showed no concentration dependence of buffer solutions. The capacitance measurements in an epitaxial graphene FET also showed no concentration dependence, indicating that the solution-gate capacitance of the epitaxial graphene FET was independent on the Debye screening length. In addition, the minimum capacitance values were also almost independent for the concentration change. Since the Debye screening length depends on the ionic strength of the solution, which is proportional to the buffer solution concentration, the electrical characteristics of solution-gated epitaxial graphene FETs are almost independent on the Debye screening length owing to their small quantum capacitance. The antibody-modified epitaxial graphene FETs indeed detected the antigen, indicating that the epitaxial graphene FETs can detect the target beyond the Debye screening length without any complicated device fabrication processes and other desalted apparatuses.

Transfer-free p-type graphene field-effect transistors with high mobility and on/off ratio

Graphene is considered a promising material because of the novel functionalities associated with its outstanding charge transport properties. However, because graphene has no bandgap, its electrical conductivity cannot be controlled as in semiconductors, at present. Although many attempts have been made to achieve a bandgap opening in graphene, a meaningful bandgap opening for p-type field-effect-transistors (FETs) still remains a challenge. In this study, boron-doping in transfer-free monolayer graphene was successfully demonstrated for digital logic devices. Our approach is highly versatile, as it allows the fabrication of p-type single-crystal graphene FETs having a mobility of ∼290 cm2V−1s−1, an on/off ratio of 1.9 × 105, and a subthreshold swing of 70 mVdec−1. The scalability and versatility of this transfer-free approach for the fabrication of p-type graphene FETs pave the way for high-performance p-type graphene-based digital logic circuits.

Growth of centimeter-scale single-crystal graphene on polycrystalline copper foil for ultrahigh sensitive sweat sensors

The low-cost growth of wafer-scale single-crystal (SC) shows great potential in its practical application. Epitaxial growth of large-area SC graphene on pre-produced SC metal foil with a specific index surface has been reported, which requires a high cost and complexity of operational processes. Here, we demonstrate an easy and low-cost approach for the growth of wafer-scale SC graphene on polycrystalline copper foil, the growth kinetics and the lattice orientation for graphene growth controlled by induced surface steps. Despite graphene domains grown across the Cu grain boundary, the domains have a common crystal orientation at the macroscale induced by the surface steps. This work demonstrates the polycrystalline Cu foil can also grow graphene domains with unidirectional alignment and seamless stitching on the large area. The wearable 2D graphene array sensors have also been developed for the detection of interleukin-6 (IL-6) in sweat, which can locate and detect infection or disease in a non-invasive way. Our growth method and novel application put up a new way of production and utilization of SC two-dimensional (2D) materials.

Study on the effects of the oxidized copper foil on graphene nucleation and growth based on the restricted space

In this study, the oxidized copper foil and the original copper foil were used as substrates to fabricate graphene samples by low pressure chemical vapor deposition (LPCVD) in a restricted growth space. The results demonstrated that in the restricted growth chamber, the oxidized layer on the oxidized copper foil inhabited graphene nucleation in the early nucleation stage, but with the oxidized layer heavily decomposing, decomposed oxygen would accelerate graphene growth. Because the decomposition rate of the oxidized layer would increase with growth temperature increasing, the acceleration effect of oxygen atoms on graphene growth becomes more obvious at higher growth temperature. So millimeter-scale graphene single crystals could be easily fabricated at 1060 °C on the oxidized copper foils in the restricted growth chamber. With the help of density functional theory (DFT) calculations, the influence mechanism of oxygen atoms on graphene nucleation and growth on copper substrate was investigated. The results showed that the oxygen atoms could not only react with CH fragments to form CO fragments to consume the number of carbon monomers, but also accelerate the diffusion and clustering of CH fragments.

Twist angle dependence of graphene-stacked junction characteristics

Vertically stacked graphene diodes are fabricated using epitaxially grown graphene with twist angles ranging from 0° to 30°. Their switching behavior and negative differential conductance are observed at all the measured angles. The junction conductance in the initial state does not indicate clear angle dependence and is almost constant, i.e. 231 μS for all devices. The junction conductance in the high-bias region exhibits a steep peak at 12°. The on/off ratio of the stacked junction diode indicates a maximum value of 142 at 12°. Therefore, the fabricated stacked graphene device with a simple structure exhibits strong nonlinear electrical properties and negative differential conductance at all twist angles. The on/off ratio of the stacked junction diodes is controlled by the twist angle between two single-crystal graphene layers.

Graphene synthesis by electromagnetic induction heating: Domain size and morphology control

In this paper we discuss the effect of hydrogen and methane content during low-pressure chemical vapor deposition (LPCVD) of graphene on inductively heated copper foils. By increasing the H2/CH4 ratio by a factor of 5 from 25 to 125, different graphene morphologies ranging from dendritic fractals to compact hexagonal islands are obtained. In addition, increasing the hydrogen concentration allows the nucleation rate to be slowed down by a factor of ∼10 thereby high-quality regular hexagonal graphene single crystals of significant size of 0.1 mm are found. From these measurements, we estimate the activation energy for graphene nucleation in low-pressure CVD (2 eV) and propose a phenomenological law for graphene nucleation. As compared to conventional CVD methods, considerable advantages of inductive heating are outlined, and some fundamental aspects of this approach are discussed.

Towards growth of pure AB-stacked bilayer graphene single crystals

Towards growth of pure AB-stacked bilayer graphene single crystals

Single-crystal Cu(1 1 1) foil preparation by direct bonding technology

Considering the significant advantages of single-crystal Cu(111) in the synthesis of 2D materials and many other fields, its controllable preparation has attracted widespread attention. However, the formation of a single-crystal Cu foil has always been hindered by competition among grains with different orientations. Herein, single-crystal Cu(111) was prepared on the basis of the traditional direct bonding copper method. Cu foil and c-plane sapphire substrate were bonded through Cu2O at high temperature. After the reduction of Cu2O by H2, the surface crystal structure of sapphire will guide Cu to form single-crystal Cu(111). The single-crystallinity of Cu(111) and the specific orientation relationship between Cu(111) and sapphire were confirmed by LEED and XRD. Furthermore, the growth of high-quality single-crystal graphene was performed on single-crystal Cu(111). This work not only facilitates the mass production of 2D materials such as single-crystal graphene, but also provides greater application potential to the direct bonding copper technology and single-crystal Cu.

Wafer scale growth of single crystal two-dimensional van der Waals materials.

Two-dimensional (2D) van der Waals (vdW) materials, including graphene, hexagonal boron nitride (hBN), and metal dichalcogenides (MCs), form the basis of modern electronics and optoelectronics due to their unique electronic structure, chemical activity, and mechanical strength. Despite many proof-of-concept demonstrations so far, to fully realize their large-scale practical applications, especially in devices, wafer-scale single crystal atomically thin highly uniform films are indispensable. In this minireview, we present an overview on the strategies and highlight recent significant advances toward the synthesis of wafer-scale single crystal graphene, hBN, and MC 2D thin films. Currently, there are five distinct routes to synthesize wafer-scale single crystal 2D vdW thin films: (i) nucleation-controlled growth by suppressing the nucleation density, (ii) unidirectional alignment of multiple epitaxial nuclei and their seamless coalescence, (iii) self-collimation of randomly oriented grains on a molten metal, (iv) surface diffusion and epitaxial self-planarization and (v) seed-mediated 2D vertical epitaxy. Finally, the challenges that need to be addressed in future studies have also been described.

(Invited) Vertically Stacked Graphene Junction Diodes

Graphene is expected to be nanomaterial of the post-silicon era because of its numerous advantageous properties. The single crystal graphene thermally grown on SiC substrate is considered the most promising platform for future electric devices. Large area epitaxial graphene on SiC substrate has been successfully fabricated using high-temperature rapid thermal annealing [1]. In this study, we present vertically stacked graphene junction diodes fabricated using direct bonding technique [2–4]. Figure 1(a) shows the schematic of a stacked graphene junction diode. Two graphene layers on SiC are directly bonded to each other. Gold–coated contact pins are attached to the graphene surface to establish ohmic contact. The graphene samples and contact probes are fixed by acrylic molds. Strong nonlinear current–voltage (I-V) characteristics are observed for this simple device configuration, as shown in Fig. 1(b). In the low bias voltage (Vb) region, the resistance of the graphene diode is approximately 20 GΩ. The current abruptly increases when the bias voltage is increased to approximately 30 V. The two-terminal resistance at a high bias voltage (100 V) is approximately 21 kΩ. The vertically stacked graphene junction exhibits bidirectional thyristor-type characteristics with a high on-off ratio of approximately 106. The Fowler–Nordheim (F-N) tunneling phenomenon is observed in the sub-threshold region. The gap distance estimated from the F-N plot is approximately 6 nm, which can be attributed to the large off resistance. The Coulomb force generated by applying the bias voltage causes the two graphene layers to attract each other, resulting in an electrically conductive state. The junction voltage (Vj) of the device was measured using a four-terminal configuration. As shown in Fig. 1(c), negative resistance is observed in the low-resistance region. The effective internal resistance of the junction diode is reduced to approximately 450 Ω. The off resistance of the diodes is considerably modified by the initial distance between the graphene layers. However, the I-Vj characteristics in the negative resistance region are identical for each device. This result suggests that the cause of the negative resistance phenomena is closely related to the band structure of graphene. A vertically stacked graphene junction diode with a high on-off ratio was demonstrated. The main component of the diode comprised a pair of single-crystal graphene layers fabricated through direct bonding. The negative resistance of the graphene diode presents new possibilities for the application of functional devices, such as terahertz emitters.[1] T. Aritsuki, et al., Jpn. J. Appl. Phys. 55 (2016) 06GF03.[2] J. Du, eta al., Jpn. J. Appl. Phys. 58 (2019) SDDE01.[3] N. Murakami, et al., Jpn. J. Appl. Phys. 60 (2021) SCCD01.[4] M. Ohi, et al., Jpn. J. Appl. Phys. 62 (2023) to be published. Figure 1

In Situ Growth of High-Quality Single-Crystal Twisted Bilayer Graphene on Liquid Copper.

Twisted bilayer graphene (TBG) generates significant attention in the fundamental research of 2D materials due to its distinct twist-angle-dependent properties. Exploring the efficient production of TBG with a wide range of twist angles stands as one of the major frontiers in moiré materials. Here, the local space-confined chemical vapor deposition growth technique for high-quality single-crystal TBG with twist angles ranging from 0° to 30° on liquid copper substrates is reported. The clean surface, pristine interface, high crystallinity, and thermal stability of TBG are verified by using comprehensive characterization techniques including optical microscopy, electron microscopy, and secondary-ion mass spectrometry. The proportion of TBG in bilayer graphene reaches as high as 89%. In addition, the stacking structure and growth mechanism of TBG are investigated, revealing that the second graphene layer develops beneath the first one. A series of comparative experiments illustrates that the liquid copper surface, with its excellent fluidity, promotes the growth of TBG. Electrical measurements show the twist-angle-dependent electronic properties of as-grown TBG, achieving a room-temperature carrier mobility of 26640 cm2 V-1 s-1 . This work provides an approach for the in situ preparation of 2D twisted materials and facilitates the application of TBG in the fields of electronics.

Mechanical and fracture behaviour of pristine and defective single/bi-crystal graphene/Ti nanocomposites using molecular dynamics simulations

Metal matrix nanocomposites (MMNCs) are emerging as lightweight but strong propositions with high specific strength and elastic modulus for specific design applications. In the present paper, single-crystal (GrNSs) and bi-crystalline graphene nanosheets (5|7GrNSs) were reinforced into titanium (Ti) matrices to investigate the mechanical and fracture characteristics of Ti-based MMNCs i.e. TiGrNCs and 5|7TiGrNCs, using molecular dynamics (MD). Based on our findings it can be observed that the use of GrNSs helps improve the strength of Ti in a significant manner. The results further demonstrate that the presence of grain boundaries (GBs) did not exert a significant impact on the mechanical properties of nanocomposites (NCs). Moreover, the presence of vacancies and grain boundaries did not deteriorate the mechanical properties of NCs in a deleterious manner. These outcomes will help in the development of NCs that are both lightweight and structurally stronger for the automotive, marine, and aerospace industries.

Controlled Growth of Single-Crystal Graphene Wafers on Twin-Boundary-Free Cu(111) Substrates.

Single-crystal graphene (SCG) wafers are needed to enable mass-electronics and optoelectronics owing to their excellent properties and compatibility with silicon-based technology. Controlled synthesis of high-quality SCG wafers can be done exploiting single-crystal Cu(111) substrates as epitaxial growth substrates recently. However, current Cu(111) films prepared by magnetron sputtering on single-crystal sapphire wafers still suffer from in-plane twin boundaries, which degrade the SCG chemical vapor deposition. Here, it is shown how to eliminate twin boundaries on Cu and achieve 4 in. Cu(111) wafers with ≈95% crystallinity. The introduction of a temperature gradient on Cu films with designed texture during annealing drives abnormal grain growth across the whole Cu wafer. In-plane twin boundaries are eliminated via migration of out-of-plane grain boundaries. SCG wafers grown on the resulting single-crystal Cu(111) substrates exhibit improved crystallinity with >97% aligned graphene domains. As-synthesized SCG wafers exhibit an average carrier mobility up to 7284 cm2 V-1 s-1 at room temperature from 103 devices and a uniform sheet resistance with only 5% deviation in 4 in. region.

Competing mechanisms govern the thermal rectification behavior in semi-stochastic polycrystalline graphene with graded grain-size distribution

Thermal rectifiers are devices that have different thermal conductivities in opposing directions of heat flow. The realization of practical thermal rectifiers relies significantly on a sound understanding of the underlying mechanisms of asymmetric heat transport, and two-dimensional materials offer a promising opportunity in this regard owing to their simplistic structures together with a vast possibility of tunable imperfections. However, the in-plane thermal rectification mechanisms in 2D materials like graphene having directional gradients of grain sizes have remained elusive. In fact, understanding the heat transport mechanisms in polycrystalline graphene, which are more practical to synthesize than large-scale single-crystal graphene, could potentially allow a unique opportunity, in principle, to combine with other defects and designs for effective optimization of thermal rectification. In this work, we investigate the thermal rectification behavior in periodic atomistic models of polycrystalline graphene whose grain arrangements were generated semi-stochastically to have different gradient grain-density distributions along the in-plane heat flow direction. We employ the centroidal Voronoi tessellation technique to generate realistic grain boundary structures for graphene, and the non-equilibrium molecular dynamics simulations method is used to calculate the thermal conductivities and rectification values. Additionally, detailed phonon characteristics and propagating phonon spatial energy densities are analyzed based on the fluctuation-dissipation theory to elucidate the competitive interplay between two underlying mechanisms, namely, (1) propagating phonon coupling and (2) temperature-dependence of thermal conductivity that determine the degree of asymmetric heat flow in graded polycrystalline graphene.

Direct Electrochemical Functionalization of Graphene Grown on Cu Including the Reaction Rate Dependence on the Cu Facet Type.

We present an electrochemical method to functionalize single-crystal graphene grown on copper foils with a (111) surface orientation by chemical vapor deposition (CVD). Graphene on Cu(111) is functionalized with 4-iodoaniline by applying a constant negative potential, and the degree of functionalization depends on the applied potential and reaction time. Our approach stands out from previous methods due to its transfer-free method, which enables more precise and efficient functionalization of single-crystal graphene. We report the suggested effects of the Cu substrate facet by comparing the reactivity of graphene on Cu(111) and Cu(115). The electrochemical reaction rate changes dramatically at the potential threshold for each facet. Kelvin probe force microscopy was used to measure the work function, and the difference in onset potentials of the electrochemical reaction on these two different facets are explained in terms of the difference in work function values. Density functional theory and Monte Carlo calculations were used to calculate the work function of graphene and the thermodynamic stability of the aniline functionalized graphene on these two facets. This study provides a deeper understanding of the electrochemical behavior of graphene (including single-crystal graphene) on Cu(111) and Cu(115). It also serves as a basis for further study of a broad range of reagents and thus functional groups and of the role of metal substrate beneath graphene.

Optical-fiber-integrated high-speed organic phototransistor with broadband imaging capacity.

Fiber optic communication is becoming the central pillar of modern high-speed communication technology, which involves the abundant fiber components. Currently, most of photodetectors are fabricated on the silicon chip, so mass fiber-to-chip interfaces increase the complexity of advanced optoelectronic system, and also grow the risk of optical information loss. Here, we report an all-fiber organic phototransistor by employing rubrene single crystal and few-layer graphene to realize the "plug-to-play" operation. The device shows a broadband photoresponse from the ultraviolet to visible range, with fast response times of approximately 130/170 µs and reasonable specific detectivity of 6 × 109 Jones, which is close to the level of commercial on-chip device. Finally, several imaging applications are successfully demonstrated by deploying this all-fiber device. Our work provided an efficient strategy for fabricating all-fiber organic devices, and confirmed their significant potential in future optical fiber optoelectronics.

Van der Waals Interaction Mediated Redox on Graphene Electrode

Graphene, unlike (defective) graphene oxide, is generally not considered to be a highly efficient electrochemical electrode except for activity at the edges. This is because, graphene forms a highly stable hydration bilayer that screens the electric field emanating from the electrode to drive the electrochemical process. A remarkable electrochemical electrode property of graphene is discovered and explored. The property emerges from graphene’s single crystal, aromatic structure. The redox behavior of an aromatic compound, methylene blue (MB) and non-aromatic compounds, such as , potassium ferric cyanide are compared. The electrochemical behavior on monolayer of graphene is quantitatively measured by a novel opto-electrochemical instrument for both electrochemical and structural information. The instrument called, the Scanning Electrometer for electrical Double-layer (SEED) allows local measurement of redox on a 10 micron laser spot to exclusively measure redox signal away from the edges of graphene. Two distinct peaks for both reduction and oxidation during conventional cyclic voltametric (CV) waveform are observed by SEED (Figure 1). The two peaks were attributed to two distinct orientation of MB chemisorption on graphene and subsequent electron transfer. One of the peak depended on precise orientation of the MB on single crystal graphene surface indicating pi-pi interaction mediated chemisorption (Figure 2). The other peak is independent of graphene orientation. MB redox on Au showed a single peak (Figure 1) and was independent of orientation (Figure 2). The effect of the two modes of chemisorption on ionic strength was studied and compared with MB redox on pure gold performed on the same chip. The two peaks were not distinguishable by CV or differential pulse voltammetry (DPV) measured on the same sample under similar conditions. This indicated the sensitivity of SEED to obtain structural information and its ability to measure electrochemical activity on micro-spot. All the three measurements, SEED, DPV and CV were performed without taking the sample out of the solution. The redox of non-aromatic compounds showed no orientation effect. The distinct van der Waals interaction mediated chemisorption by pi-pi stacking on graphene electrode may have distinct advantage to design highly sensitive and specific sensors and biosensors in future, for example, in DNA and RNA sensing. Figure 1

Transparent single crystal graphene flexible strain sensor with high sensitivity for wearable human motion monitoring

Human-machine interfaces, smart glasses, touch screens and some electronic skins all require highly transparent and flexible strain sensing components. Flexible strain sensors usually use microstructures or porous active materials to improve sensitivity and response speed, resulting in low transparency of the device. Graphene has attracted extensive attention due to its special physical properties, especially excellent flexibility, and high transmittance. However, previous studies found that it is difficult to obtain high gauge factor in graphene. Thus, graphene cannot be used to monitor subtle deformations. Although previous research has shown that the strain sensors based on the combination of graphene and other materials have high sensitivity, but lack of transmittance. In this paper, we report a single-crystal graphene (SCG) strain sensor fabricated by encapsulating single-crystal hexagonal graphene in a flexible substrate. The strain sensor shows excellent performance with a high gauge factor (16.9) because it is not affected by wrinkles, grain boundaries and additional layers. Further, the strain sensor retains the advantage of high light transmittance of graphene. This sensitive SCG strain sensor has been demonstrated to detect a range of human motions, including finger gestures, tiny vibration of different sounds and heartbeats with high performance. This work further confirms the application of graphene in strain sensors, and provides new ideas for the research of graphene-based transparent strain sensors.

Hydrogen-modulation method for wafer-scale few-layer single-crystal graphene growth

Single-crystal few-layer graphene films have potential applications in microelectronic and optoelectronic devices. Considerable efforts have been taken to prepare large-area bilayer graphene through chemical vapor deposition, but the growth of uniform graphene films with increased layers remains challenging. Herein, a hydrogen-modulation method for growing wafer-scale few-layer single-crystal graphene through chemical vapor deposition was proposed. By controlling the hydrogen flow rate at different stages, the dissolution amount and segregation process of carbon atoms in a CuNi substrate can be regulated separately. The hydrogen-modulation method fully exploits the effect of the dissolution–segregation mechanism, and the number of graphene layers can be controlled by modulating the hydrogen flow rate. Through the proposed method, the coverages of bilayer and trilayer graphene can reach more than 99% and 95%, respectively. The stacking and uniformity of few-layer graphene were evaluated by Raman and microstructure characterization, and bilayer and trilayer graphene were confirmed as AB and ABA stacks, respectively. Transmission line test structure and field effect transistor were prepared to verify the electrical properties of bilayer graphene. Results show that the square resistance of bilayer graphene is significantly lower than that of single-layer graphene, and it can also achieve high on-state current on field effect transistor.