Optical probing of the electronic interaction between graphene and hexagonal boron nitride

Building a cm2scale CVD graphene-based gas sensor: modelling the kinetic with a three-site adsorption/desorption Langmuir model

In this article, we aim to develop and study a highly sensitive and selective cm2scale graphene-based gas sensor. We present the technology used to fabricate sensors which integrate monolayer chemical vapour deposition graphene: photolithography and transfer of layers. Characterization techniques (optical microscopy, AFM, micro-Raman spectroscopy, transport electrical measurements) ensure a diagnosis of graphene ribbons and allow good reproducibility of technological processes. We present the results of gas characterizations after a 200 ppm NO2exposure. We propose a novel approach for the modelling of the sensor response with a three-site adsorption/desorption Langmuir model. This innovative way of modelling the sensor response should provide a better understanding of the sensor's kinetic and help to overcome the long response time observed with graphene gas sensors.

Optical properties of two-dimensional tin nanosheets epitaxially grown on graphene

Heterostacks formed by combining two-dimensional materials show novel properties which are of great interest for new applications in electronics, photonics and even twistronics, the new emerging field born after the outstanding discoveries on twisted graphene. Here, we report the direct growth of tin nanosheets at the two-dimensional limit via molecular beam epitaxy on chemical vapor deposited graphene on Al2O3(0001). The mutual interaction between the tin nanosheets and graphene is evidenced by structural and chemical investigations. On the one hand, Raman spectroscopy indicates that graphene undergoes compressive strain after the tin growth, while no charge transfer is observed. On the other hand, chemical analysis shows that tin nanosheets interaction with sapphire is mediated by graphene avoiding the tin oxidation occurring in the direct growth on this substrate. Remarkably, optical measurements show that the absorption of tin nanosheets exhibits a graphene-like behavior with a strong absorption in the ultraviolet photon energy range, therein resulting in a different optical response compared to tin nanosheets on bare sapphire. The optical properties of ultra-thin tin films therefore represent an open and flexible playground for the absorption of light in a broad range of the electromagnetic spectrum and technologically relevant applications for photon harvesting and sensors.

Localised strain and doping of 2D materials

There is a growing interest in 2D materials-based devices as the replacement for established materials, such as silicon and metal oxides in microelectronics and sensing, respectively. However, the atomically thin nature of 2D materials makes them susceptible to slight variations caused by their immediate environment, inducing doping and strain, which can vary between, and even microscopically within, devices. One of the misapprehensions for using 2D materials is the consideration of unanimous intrinsic properties over different support surfaces. The interfacial interaction, intrinsic structural disorder and external strain modulate the properties of 2D materials and govern the device performance. The understanding, measurement and control of these factors are thus one of the significant challenges for the adoption of 2D materials in industrial electronics, sensing, and polymer composites. This topical review provides a comprehensive overview of the effect of strain-induced lattice deformation and its relationship with physical and electronic properties. Using the example of graphene and MoS₂ (as the prototypical 2D semiconductor), we rationalise the importance of scanning probe techniques and Raman spectroscopy to elucidate strain and doping in 2D materials. These effects can be directly and accurately characterised through Raman shifts in a non-destructive manner. A generalised model has been presented that deconvolutes the intertwined relationship between strain and doping in graphene and MoS₂ that could apply to other members of the 2D materials family. The emerging field of straintronics is presented, where the controlled application of strain over 2D materials induces tuneable physical and electronic properties. These perspectives highlight practical considerations for strain engineering and related microelectromechanical applications.

Optical Imaging of Redox and Molecular Diffusion in 2D van der Waals Space

Understanding charge transfer (CT) between two chemical entities and the subsequent change in their charge densities is essential not only for molecular species but also for various low-dimensional materials. Because of their extremely high fraction of surface atoms, two-dimensional (2-D) materials are most susceptible to charge exchange and exhibit drastically different physicochemical properties depending on their charge density. In this regard, spontaneous and uncontrollable ionization of graphene in the ambient air has caused much confusion and technical difficulty in achieving experimental reproducibility since its first report in 2004. Moreover, the same ambient hole doping was soon observed in 2-D semiconductors, which implied that a common mechanism should be operative and apply to other low-dimensional materials universally. Notably, a similar CT reaction has long been known for carbon nanotubes but is still controversial in its mechanism.In this Account, we review our breakthroughs in unraveling the chemical origin and mechanistic requirements of the hidden CT reactions using 2-D crystals. As a first step, we have developed in situ optical methods to quantify charge density using Raman and photoluminescence (PL) spectroscopy and imaging. To overcome the multimodal sensitivity of Raman frequencies, we established a novel analytical method based on theory and experiments with excellent resolution for the charge density (∼1 × 10¹² cm^-2) and lattice strain (∼0.02%) of graphene. For 2-D transition-metal dichalcogenides, PL spectroscopy and imaging provided a high precision and sensitivity that enabled rapid kinetic measurements in a spatially resolved manner.Using gas- and temperature-controlled in situ measurements, we revealed that the electrical holes are injected by the oxygen reduction reaction (ORR) O₂ + 4H⁺ + 4e^- ⇄ 2H₂O, which was independently verified by the pH dependence in HCl solutions. In addition to oxygen and water vapor, the overall CT reaction requires hydrophilic dielectric substrates, which assist the hydration of the sample-substrate interface. We also found that the CT reaction is substantially enhanced when samples are thermally annealed. The amplification is due to the interfacial hydrophilicity increased by the thermal hydroxylation of substrates, which indicates that the CT reaction is localized at the interface and boosted by interfacial water.The interface-localized CT allowed us to study and control molecular diffusion through the 2-D van der Waals space between samples and substrates. Wide-field PL imaging showed how fast oxygen molecules diffuse through the interfacial space, subsequently inducing the CT reaction. By increasing the 2-D gap spacing, the diffusion kinetics could be accelerated. The rate of CT could also be enhanced by introducing defects on the basal plane of 2-D crystals, which demonstrates the decisive role of defects as CT centers.Because of their unique geometry, low-dimensional materials are highly susceptible to external perturbation including charge exchange. Because the vulnerability can be exploited to modify material properties, the complete mechanism of the fundamental charge exchange summarized in this Account will be essential to exploring material and device properties of other low-dimensional materials.

Design and fabrication of a large-range graphene/hexagonal boron nitride heterostructure based pressure sensor with poly(methyl methacrylate) substrate

Aiming at overpressure measurement, this paper presents a large-range graphene/hexagonal boron nitride (h-BN) heterostructure-based pressure sensor with a poly(methyl methacrylate) (PMMA) substrate. Graphene and h-BN are chosen as sensitive materials because they both have large Young's modulus, high intrinsic strength, high natural frequency, and atomic thickness at the same time. These characteristics provide favorable conditions for the application of the sensor in the high pressure and high frequency dynamic environment. Moreover, the photoresist-assisted transfer technology is proposed for transferring graphene from the growth substrate to the PMMA substrate and the lift-off method with exposure and development is developed to achieve metal patterning on the PMMA substrate. The sensor characterization results suggest that the graphene and h-BN films have good transfer qualities and the heterojunction possesses excellent electrical performance. The static pressure loading experiments confirm that the sensor has a pressure range of up to 85 MPa and its piezoresistive coefficient is 0.7 GPa^-1, which indicates that the designed sensor is suitable for overpressure fields. This study provides a novel method for determining overpressure and lays a foundation for the fabrication of graphene-based electronic devices with an organic substrate.

Turn of the decade: versatility of 2D hexagonal boron nitride

The era of two-dimensional (2D) materials, in its current form, truly began at the time that graphene was first isolated just over 15 years ago. Shortly thereafter, the use of 2D hexagonal boron nitride (h-BN) had expanded in popularity, with use of the thin isolator permeating a significant number of fields in condensed matter and beyond. Due to the impractical nature of cataloguing every use or research pursuit, this review will cover ground in the following three subtopics relevant to this versatile material: growth, electrical measurements, and applications in optics and photonics. Through understanding how the material has been utilized, one may anticipate some of the exciting directions made possible by the research conducted up through the turn of this decade.

Oxygen intercalation in PVD graphene grown on copper substrates: A decoupling approach

We investigate the intercalation process of oxygen in-between a PVD-grown graphene layer and different copper substrates as a methodology for reducing the substrate-layer interaction. This growth method leads to an extended defect-free graphene layer that strongly couples with the substrate. We have found, by means of X-ray photoelectron spectroscopy, that after oxygen exposure at different temperatures, ranging from 280 °C to 550 °C, oxygen intercalates at the interface of graphene grown on Cu foil at an optimal temperature of 500 °C. The low energy electron diffraction technique confirms the adsorption of an atomic oxygen adlayer on top of the Cu surface and below graphene after oxygen exposure at elevated temperature, but no oxidation of the substrate is induced. The emergence of the 2D Raman peak, quenched by the large interaction with the substrate, reveals that the intercalation process induces a structural undoing. As suggested by atomic force microscopy, the oxygen intercalation does not change significantly the surface morphology. Moreover, theoretical simulations provide further insights into the electronic and structural undoing process. This protocol opens the door to an efficient methodology to weaken the graphene-substrate interaction for a more efficient transfer to arbitrary surfaces.

Electron Tunneling through Boron Nitride Confirms Marcus-Hush Theory Predictions for Ultramicroelectrodes

Marcus-Hush theory of electron transfer is one of the pillars of modern electrochemistry with a large body of supporting experimental evidence presented to date. However, some predictions, such as the electrochemical behavior at disk ultramicroelectrodes, remain unverified. Herein, we present a study of electron tunneling across a hexagonal boron nitride acting as a barrier between a graphite electrode and redox mediators in a liquid solution. This was achieved by the fabrication of disk ultramicroelectrodes with a typical diameter of 5 μm. Analysis of voltammetric measurements, using two common outer-sphere redox mediators, yielded several electrochemical parameters, including the electron transfer rate constant, limiting current, and transfer coefficient. They depart significantly from the Butler-Volmer kinetics and instead show behavior previously predicted by the Marcus-Hush theory of electron transfer. In addition, our system provides a noteworthy experimental platform, which could be applied to address a number of scientific problems such as identification of reaction mechanisms, surface modification, or long-range electron transfer.

Spectroscopic and Electrical Characterizations of Low-Damage Phosphorous-Doped Graphene via Ion Implantation

Development of n-/p-type semiconducting graphenes is a critical route to implement in graphene-based nanoelectronics and optronics. Compared to the p-type graphene, the n-type graphene is more difficult to be prepared. Recently, phosphorous doping was reported to achieve air-stable and high mobility of n-typed graphene. The phosphorous-doped graphene (P-Gra) by ion implantation is considered as an ideal method for tailoring graphene due to its IC compatible process; however, for a conventional ion implanter, the acceleration energy is in the order of kiloelectron volts (keV), thus severely destroys the sp² bonding of graphene owing to its high energy of accelerated ions. The introduced defects, therefore, degrade the electrical performance of graphene. Here, for the first time, we report a low-damage n-typed chemical vapor deposition (CVD) graphene by an industrial-compatible ion implanter with an energy of 20 keV where the designed protection layer (thin Au film) covered on as-grown CVD graphene is employed to efficiently reduce defect formation. The additional post-annealing is found to heal the crystal defects of graphene. Moreover, this method allows transferring ultraclean and residue-free P-Gra onto versatile target substrates directly. The doping configuration, crystallinity, and electrical properties on P-Gra were comprehensively studied. The results indicate that the low-damaged P-Gra with a controllable doping concentration of up to 4.22 at % was achieved, which is the highest concentration ever recorded. The doped graphenes with tunable work functions (4.85-4.15 eV) and stable n-type doping while keeping high-carrier mobility are realized. This work contributes to the proof-of-concept for tailoring graphene or 2D materials through doping with an exceptional low defect density by the low energy ion implantation, suggesting a great potential for unconventional doping technologies for next-generation 2D-based nanoelectronics.

A smart drug-delivery nanosystem based on carboxylated graphene quantum dots for tumor-targeted chemotherapy

Aim: Constructing a new drug-delivery system using carboxylated graphene quantum dots (cGQDs) for tumor chemotherapy in vivo. Materials & methods: A drug-delivery system was synthesized through a crosslink reaction of cGQDs, NH2-poly(ethylene glycol)-NH2 and folic acid. Results: A drug delivery system of folic acid-poly(ethylene glycol)-cGQDs was successfully constructed with ideal entrapment efficiency (97.5%) and drug-loading capacity (40.1%). Cell image indicated that the nanosystem entered into human cervical cancer cells mainly through macropinocytosis-dependent pathway. In vivo experiments showed the outstanding antitumor ability and low systemic toxicity of this nanodrug-delivery system. Conclusion: The newly developed drug-delivery system provides an important alternative for tumor therapy without causing systemic adverse effects.

Strain-Engineering of Twist-Angle in Graphene/hBN Superlattice Devices

The observation of novel physical phenomena such as Hofstadter's butterfly, topological currents, and unconventional superconductivity in graphene has been enabled by the replacement of SiO₂ with hexagonal boron nitride (hBN) as a substrate and by the ability to form superlattices in graphene/hBN heterostructures. These devices are commonly made by etching the graphene into a Hall-bar shape with metal contacts. The deposition of metal electrodes, the design, and specific configuration of contacts can have profound effects on the electronic properties of the devices possibly even affecting the alignment of graphene/hBN superlattices. In this work, we probe the strain configuration of graphene on hBN in contact with two types of metal contacts, two-dimensional (2D) top-contacts and one-dimensional edge-contacts. We show that top-contacts induce strain in the graphene layer along two opposing leads, leading to a complex strain pattern across the device channel. Edge-contacts, on the contrary, do not show such strain pattern. A finite-elements modeling simulation is used to confirm that the observed strain pattern is generated by the mechanical action of the metal contacts clamped to the graphene. Thermal annealing is shown to reduce the overall doping while increasing the overall strain, indicating an increased interaction between graphene and hBN. Surprisingly, we find that the two contact configurations lead to different twist-angles in graphene/hBN superlattices, which converge to the same value after thermal annealing. This observation confirms the self-locking mechanism of graphene/hBN superlattices also in the presence of strain gradients. Our experiments may have profound implications in the development of future electronic devices based on heterostructures and provide a new mechanism to induce complex strain patterns in 2D materials.

Large-Area High-Quality AB-Stacked Bilayer Graphene on h-BN/Pt Foil by Chemical Vapor Deposition

Large-area, high-quality bilayer graphene (BLG) has attracted great interest because of its immense potential for many viable applications. However, its growth is still greatly limited owing to its small size and low carrier mobility. In this article, we report the successful growth of large-area, high-quality AB-stacked BLG on hexagonal boron nitride (h-BN)/Pt foil by chemical vapor deposition (CVD). Optical microscopy and scanning electron microscopy observations reveal the formation of uniform and continuous BLG films with sizes of up to 500 μm, which are 4-5 times larger than those reported elsewhere for CVD-grown BLG films. A large carrier mobility of up to 9000 cm² V^-1 s^-1 is observed for the BLG films grown on h-BN/Pt foils under ambient conditions. We also propose a plausible growth mechanism of BLG growth on h-BN/Pt foils. Our findings will contribute for the better understanding of the fundamental BLG physics and the development of BLG-based devices.

Sensitive Raman Probe of Electronic Interactions between Monolayer Graphene and Substrate under Electrochemical Potential Control

In situ electrochemical Raman spectroscopic measurements of defect-free monolayer graphene on various substrates were performed under electrochemical potential control. The G and 2D Raman band wavenumbers (ω_G, ω_2D) of graphene were found to depend upon the electrochemical potential, i.e., the charge density of graphene. The values of ω_G and ω_2D also varied depending on the choice of substrates. On metal substrates where graphene was synthesized by chemical vapor deposition, a strong blue shift of ω_2D was induced, which could not account for the strain and charge doping. We attributed the blue shift of ω_2D to a change in the electronic properties of graphene induced by distinct electronic interactions with the metal substrates. To explain the unique characteristics in the Raman spectrum of graphene on various substrates, a novel mechanism is proposed considering reduction of the Fermi velocity in graphene owing to dielectric screening from the metal substrates.

Growth of Graphene/h-BN Heterostructures on Recyclable Pt Foils by One-Batch Chemical Vapor Deposition

High-quality large-area graphene/h-BN vertical heterostructures are promising building blocks for many viable applications such as energy harvesting/conversion, electronics and optoelectronics. Here, we successfully grew high-quality large-area graphene/h-BN vertical heterostructures on Pt foils by one-batch low-pressure chemical vapor deposition (LPCVD). We obtained the high quality of about 200-µm-wide graphene/h-BN film having uniform layer thickness. Moreover, the obtained graphene/h-BN heterostructures exhibited field effect mobility of up to 7,200 cm²V⁻¹s⁻¹ at room temperature. These results suggest that such graphene/h-BN heterostructures on recyclable Pt foils grown by LPCVD are promising for high-performance graphene-based electronics.

Effect of molybdenum disulfide nanoribbon on quantum transport of graphene

Based on the density functional theory method in combination with the nonequilibrium green's function formalism, the quantum transport properties in graphene-[Formula: see text] vertical heterojunction were investigated in this work. The leads are boron doped graphene and seamlessly connect to the graphene nanoribbon in central scattering region. Although there is a weak graphene-[Formula: see text] interaction, molybdenum disulfide can smooth the electrostatic potential and enlarge the transport properties of the whole device. However, another competitive factor is that of the edge states of the [Formula: see text] nanoribbon. When the transport is along the zigzag direction of graphene, the armchair [Formula: see text] nanoribbon simply enlarges the transmission coefficient. Nevertheless, in the armchair transport system, there is an asymmetric electrostatic potential induced by the different atomic potentials of S and Mo atoms at both edges in the zigzag [Formula: see text] nanoribbon, whose potential can lead to obvious scattering from graphene to [Formula: see text] and suppress the transmission probability. Therefore, it also suppresses the influence of zigzag [Formula: see text] nanoribbon on the transmission coefficient. Our first principles simulations provide useful predictions for the application of graphene based emerging electronics, which may stimulate further experimental exploration.

2D Raman band splitting in graphene: Charge screening and lifting of the K-point Kohn anomaly

Pristine graphene encapsulated in hexagonal boron nitride has transport properties rivalling suspended graphene, while being protected from contamination and mechanical damage. For high quality devices, it is important to avoid and monitor accidental doping and charge fluctuations. The 2D Raman double peak in intrinsic graphene can be used to optically determine charge density, with decreasing peak split corresponding to increasing charge density. We find strong correlations between the 2D ₁ and 2D ₂ split vs 2D line widths, intensities, and peak positions. Charge density fluctuations can be measured with orders of magnitude higher precision than previously accomplished using the G-band shift with charge. The two 2D intrinsic peaks can be associated with the "inner" and "outer" Raman scattering processes, with the counterintuitive assignment of the phonon closer to the K point in the KM direction (outer process) as the higher energy peak. Even low charge screening lifts the phonon Kohn anomaly near the K point for graphene encapsulated in hBN, and shifts the dominant intensity from the lower to the higher energy peak.

Continuous Films of Self-Assembled Graphene Quantum Dots for n-Type Doping of Graphene by UV-Triggered Charge Transfer

The demands to examine components serving as one of the active layers in heterostructures of 2D materials have been recently increasing. Nanomaterials synthesized from a solution process and their self-assembly can provide a promising route to build a new type of mixed dimensional heterostructures, and several methodologies have been reported previously to construct 2D assemblies from colloidal nanostructures in solution. Graphene quantum dots (GQDs), receiving much interest due to the tunable optical band gap and the capability of chemical functionalization, are considered as emerging nanomaterials for various optoelectronic and biological applications. This study fabricates a closely packed GQDs film (GQDF) from colloidal solutions using a solvent-assisted Langmuir Blodgett method, and investigates the optical and electrical characteristics of the heterostacked graphene/GQD film (G/GQDF) structures. It is observed that the GQDF plays a role not only as a buffer layer that isolates Chemical Vapor Deposited graphene (CVD graphene) from undesired p-doping but also as a photoactive layer that triggers n-doping of the heterostacked CVD graphene film. The n-doping density of the G/GQDF device is proportional to UV irradiation time, but its carrier mobility remains constant regardless of doping densities, which are unique characteristics that have not been observed in other doping methods.

The integration of graphene into microelectronic devices

Since 2004 the field of graphene research has attracted increasing interest worldwide. Especially the integration of graphene into microelectronic devices has the potential for numerous applications. Therefore, we summarize the current knowledge on this aspect. Surveys show that considerable progress was made in the field of graphene synthesis. However, the central issue consists of the availability of techniques suitable for production for the deposition of graphene on dielectric substrates. Besides, the encapsulation of graphene for further processing while maintaining its properties poses a challenge. Regarding the graphene/metal contact intensive research was done and recently substantial advancements were made towards contact resistances applicable for electronic devices. Generally speaking the crucial issues for graphene integration are identified today and the corresponding research tasks can be clearly defined.

Large-Scale Graphene on Hexagonal-BN Hall Elements: Prediction of Sensor Performance without Magnetic Field

A graphene Hall element (GHE) is an optimal system for a magnetic sensor because of its perfect two-dimensional (2-D) structure, high carrier mobility, and widely tunable carrier concentration. Even though several proof-of-concept devices have been proposed, manufacturing them by mechanical exfoliation of 2-D material or electron-beam lithography is of limited feasibility. Here, we demonstrate a high quality GHE array having a graphene on hexagonal-BN (h-BN) heterostructure, fabricated by photolithography and large-area 2-D materials grown by chemical vapor deposition techniques. A superior performance of GHE was achieved with the help of a bottom h-BN layer, and showed a maximum current-normalized sensitivity of 1986 V/AT, a minimum magnetic resolution of 0.5 mG/Hz(0.5) at f = 300 Hz, and an effective dynamic range larger than 74 dB. Furthermore, on the basis of a thorough understanding of the shift of charge neutrality point depending on various parameters, an analytical model that predicts the magnetic sensor operation of a GHE from its transconductance data without magnetic field is proposed, simplifying the evaluation of each GHE design. These results demonstrate the feasibility of this highly performing graphene device using large-scale manufacturing-friendly fabrication methods.

Large-Scale Synthesis of a Uniform Film of Bilayer MoS2 on Graphene for 2D Heterostructure Phototransistors

The large-scale synthesis of atomically thin, layered MoS2/graphene heterostructures is of great interest in optoelectronic devices because of their unique properties. Herein, we present a scalable synthesis method to prepare centimeter-scale, continuous, and uniform films of bilayer MoS2 using low-pressure chemical vapor deposition. This growth process was utilized to assemble a heterostructure by growing large-scale uniform films of bilayer MoS2 on graphene (G-MoS2/graphene). Atomic force microscopy, Raman spectra, and transmission electron microscopy characterization demonstrated that the large-scale bilayer MoS2 film on graphene exhibited good thickness uniformity and a polycrystalline nature. A centimeter-scale phototransistor prepared using the G-MoS2/graphene heterostructure exhibited a high responsivity of 32 mA/W with good cycling stability; this value is 1 order of magnitude higher than that of transferred MoS2 on graphene (2.5 mA/W). This feature results from efficient charge transfer at the interface enabled by intimate contact between the grown bilayer MoS2 (G-MoS2) and graphene. The ability to integrate multilayer materials into atomically thin heterostructures paves the way for fabricating multifunctional devices by controlling their layer structure.

Electronic and Mechanical Properties of Graphene-Germanium Interfaces Grown by Chemical Vapor Deposition

Epitaxially oriented wafer-scale graphene grown directly on semiconducting Ge substrates is of high interest for both fundamental science and electronic device applications. To date, however, this material system remains relatively unexplored structurally and electronically, particularly at the atomic scale. To further understand the nature of the interface between graphene and Ge, we utilize ultrahigh vacuum scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) along with Raman and X-ray photoelectron spectroscopy to probe interfacial atomic structure and chemistry. STS reveals significant differences in electronic interactions between graphene and Ge(110)/Ge(111), which is consistent with a model of stronger interaction on Ge(110) leading to epitaxial growth. Raman spectra indicate that the graphene is considerably strained after growth, with more point-to-point variation on Ge(111). Furthermore, this native strain influences the atomic structure of the interface by inducing metastable and previously unobserved Ge surface reconstructions following annealing. These nonequilibrium reconstructions cover >90% of the surface and, in turn, modify both the electronic and mechanical properties of the graphene overlayer. Finally, graphene on Ge(001) represents the extreme strain case, where graphene drives the reorganization of the Ge surface into [107] facets. From this work, it is clear that the interaction between graphene and the underlying Ge is not only dependent on the substrate crystallographic orientation, but is also tunable and strongly related to the atomic reconfiguration of the graphene-Ge interface.

Solution-Processed Dielectrics Based on Thickness-Sorted Two-Dimensional Hexagonal Boron Nitride Nanosheets

Gate dielectrics directly affect the mobility, hysteresis, power consumption, and other critical device metrics in high-performance nanoelectronics. With atomically flat and dangling bond-free surfaces, hexagonal boron nitride (h-BN) has emerged as an ideal dielectric for graphene and related two-dimensional semiconductors. While high-quality, atomically thin h-BN has been realized via micromechanical cleavage and chemical vapor deposition, existing liquid exfoliation methods lack sufficient control over h-BN thickness and large-area film quality, thus limiting its use in solution-processed electronics. Here, we employ isopycnic density gradient ultracentrifugation for the preparation of monodisperse, thickness-sorted h-BN inks, which are subsequently layer-by-layer assembled into ultrathin dielectrics with low leakage currents of 3 × 10(-9) A/cm(2) at 2 MV/cm and high capacitances of 245 nF/cm(2). The resulting solution-processed h-BN dielectric films enable the fabrication of graphene field-effect transistors with negligible hysteresis and high mobilities up to 7100 cm(2) V(-1) s(-1) at room temperature. These h-BN inks can also be used as coatings on conventional dielectrics to minimize the effects of underlying traps, resulting in improvements in overall device performance. Overall, this approach for producing and assembling h-BN dielectric inks holds significant promise for translating the superlative performance of two-dimensional heterostructure devices to large-area, solution-processed nanoelectronics.

Tuning and Persistent Switching of Graphene Plasmons on a Ferroelectric Substrate

We characterized plasmon propagation in graphene on thin films of the high-κ dielectric PbZr0.3Ti0.7O3 (PZT). Significant modulation (up to ±75%) of the plasmon wavelength was achieved with application of ultrasmall voltages (< ±1 V) across PZT. Analysis of the observed plasmonic fringes at the graphene edge indicates that carriers in graphene on PZT behave as noninteracting Dirac Fermions approximated by a semiclassical Drude response, which may be attributed to strong dielectric screening at the graphene/PZT interface. Additionally, significant plasmon scattering occurs at the grain boundaries of PZT from topographic and/or polarization induced graphene conductivity variation in the interior of graphene, reducing the overall plasmon propagation length. Lastly, through application of 2 V across PZT, we demonstrate the capability to persistently modify the plasmonic response of graphene through transient voltage application.

Direction-controlled chemical doping for reversible G-phonon mixing in ABC trilayer graphene

Not only the apparent atomic arrangement but the charge distribution also defines the crystalline symmetry that dictates the electronic and vibrational structures. In this work, we report reversible and direction-controlled chemical doping that modifies the inversion symmetry of AB-bilayer and ABC-trilayer graphene. For the "top-down" and "bottom-up" hole injection into graphene sheets, we employed molecular adsorption of electronegative I2 and annealing-induced interfacial hole doping, respectively. The chemical breakdown of the inversion symmetry led to the mixing of the G phonons, Raman active Eg and Raman-inactive Eu modes, which was manifested as the two split G peaks, G(-) and G(+). The broken inversion symmetry could be recovered by removing the hole dopants by simple rinsing or interfacial molecular replacement. Alternatively, the symmetry could be regained by double-side charge injection, which eliminated G(-) and formed an additional peak, G(o), originating from the barely doped interior layer. Chemical modification of crystalline symmetry as demonstrated in the current study can be applied to other low dimensional crystals in tuning their various material properties.

Graphene on hexagonal boron nitride

The field of graphene research has developed rapidly since its first isolation by mechanical exfoliation in 2004. Due to the relativistic Dirac nature of its charge carriers, graphene is both a promising material for next-generation electronic devices and a convenient low-energy testbed for intrinsically high-energy physical phenomena. Both of these research branches require the facile fabrication of clean graphene devices so as not to obscure its intrinsic physical properties. Hexagonal boron nitride has emerged as a promising substrate for graphene devices as it is insulating, atomically flat and provides a clean charge environment for the graphene. Additionally, the interaction between graphene and boron nitride provides a path for the study of new physical phenomena not present in bare graphene devices. This review focuses on recent advancements in the study of graphene on hexagonal boron nitride devices from the perspective of scanning tunneling microscopy with highlights of some important results from electrical transport measurements.

Gate dependent Raman spectroscopy of graphene on hexagonal boron nitride

Raman spectroscopy, a fast and nondestructive imaging method, can be used to monitor the doping level in graphene devices. We fabricated chemical vapor deposition (CVD) grown graphene on atomically flat hexagonal boron nitride (hBN) flakes and SiO2 substrates. We compared their Raman response as a function of charge carrier density using an ion gel as a top gate. The G peak position, the 2D peak position, the 2D peak width and the ratio of the 2D peak area to the G peak area show a dependence on carrier density that differs for hBN compared to SiO2. Histograms of two-dimensional mapping are used to compare the fluctuations in the Raman peak properties between the two substrates. The hBN substrate has been found to produce fewer fluctuations at the same charge density owing to its atomically flat surface and reduced charged impurities.

Nanogap based graphene coated AFM tips with high spatial resolution, conductivity and durability

After one decade of analyzing the intrinsic properties of graphene, interest into the development of graphene-based devices and micro electromechanical systems is increasing. Here, we fabricate graphene-coated atomic force microscope tips by growing the graphene on copper foil and transferring it onto the apex of a commercially available AFM tip. The resulting tip exhibits surprising enhanced resolution in nanoscale electrical measurements. By means of topographic AFM maps and statistical analyses we determine that this superior performance may be related to the presence of a nanogap between the graphene and the tip apex, which reduces the tip radius and tip-sample contact area. In addition, the graphene-coated tips show a low tip-sample interaction, high conductivity and long life times. The novel fabrication-friendly tip could improve the quality and reliability of AFM experiments, while reducing the cost of AFM-based research.

Large current modulation and spin-dependent tunneling of vertical graphene/MoS2 heterostructures

Vertical graphene heterostructures have been introduced as an alternative architecture for electronic devices by using quantum tunneling. Here, we present that the current on/off ratio of vertical graphene field-effect transistors is enhanced by using an armchair graphene nanoribbon as an electrode. Moreover, we report spin-dependent tunneling current of the graphene/MoS2 heterostructures. When an atomically thin MoS2 layer sandwiched between graphene electrodes becomes magnetic, Dirac fermions with different spins feel different heights of the tunnel barrier, leading to spin-dependent tunneling. Our finding will develop the present graphene heterostructures for electronic devices by improving the device performance and by adding the possibility of spintronics based on graphene.