Gate-tunable Graphene Research Articles

Charge carrier density dependent Raman spectra of graphene encapsulated in hexagonal boron nitride

We present low-temperature Raman measurements on gate tunable graphene encapsulated in hexagonal boron nitride, which allows to study in detail the Raman G and 2D mode frequencies and line widths as function of the charge carrier density. We observe a clear softening of the Raman G mode (of up to 2.5 cm$^{-1}$) at low carrier density due to the phonon anomaly and a residual G~mode line width of $\approx$ 3.5 cm$^{-1}$ at high doping. From analyzing the G mode dependence on doping and laser power we extract an electron-phonon-coupling constant of $\approx$ 4.4 $\times$ 10$^{-3}$ (for the G mode phonon). The ultra-flat nature of encapsulated graphene results in a minimum Raman 2D peak line width of 14.5 cm$^{-1}$ and allows to observe the intrinsic electron-electron scattering induced broadening of the 2D peak of up to 18 cm$^{-1}$ for an electron density of 5$\times$10$^{12}$ cm$^{-2}$ (laser excitation energy of 2.33 eV). Our findings not only provide insights into electron-phonon coupling and the role of electron-electron scattering for the broadening of the 2D peak, but also crucially shows the limitations when it comes to the use of Raman spectroscopy (i.e. the use of the frequencies and the line widths of the G and 2D modes) to benchmark graphene in terms of charge carrier density, strain and strain inhomogenities. This is particularly relevant when utilizing spatially-resolved 2D Raman line width maps to assess substrate-induced nanometer-scale strain variations.

Ultrafast intrinsic optical-to-electrical conversion dynamics in a graphene photodetector

Optical-to-electrical (O-E) conversion in graphene is a central phenomenon for realizing anticipated ultrafast and low-power-consumption information technologies. However, revealing its mechanism and intrinsic time scale require uncharted terahertz (THz) electronics and device architectures. Here, we succeeded in resolving O-E conversion processes in high-quality graphene by on-chip electrical readout of ultrafast photothermoelectric current. By suppressing the RC time constant using a resistive zinc oxide top gate, we constructed a gate-tunable graphene photodetector with a bandwidth of up to 220 GHz. By measuring nonlocal photocurrent dynamics, we found that the photocurrent extraction from the electrode is instantaneous without a measurable carrier transit time across several-micrometer-long graphene, following the Shockley-Ramo theorem. The time for photocurrent generation is exceptionally tunable from immediate to > 4 ps, and its origin is identified as Fermi-level-dependent intraband carrier-carrier scattering. Our results bridge the gap between ultrafast optical science and device engineering, accelerating ultrafast graphene optoelectronic applications.

Band conductivity oscillations in a gate-tunable graphene superlattice

Electrons exposed to a two-dimensional (2D) periodic potential and a uniform, perpendicular magnetic field exhibit a fractal, self-similar energy spectrum known as the Hofstadter butterfly. Recently, related high-temperature quantum oscillations (Brown-Zak oscillations) were discovered in graphene moiré systems, whose origin lies in the repetitive occurrence of extended minibands/magnetic Bloch states at rational fractions of magnetic flux per unit cell giving rise to an increase in band conductivity. In this work, we report on the experimental observation of band conductivity oscillations in an electrostatically defined and gate-tunable graphene superlattice, which are governed both by the internal structure of the Hofstadter butterfly (Brown-Zak oscillations) and by a commensurability relation between the cyclotron radius of electrons and the superlattice period (Weiss oscillations). We obtain a complete, unified description of band conductivity oscillations in two-dimensional superlattices, yielding a detailed match between theory and experiment.

Polaritons in low-dimensional materials and their coupling characteristics

Polaritons, i.e. new collective modes formed by the strong coupling between light and electrons, phonons, excitons, or magnons in matter, have recently received extensive attention. Polaritons in low-dimensional materials exhibit strong spatial confinement, high quality factor, and gate-tunability. Typical examples include gate-tunable graphene surface plasmon polaritons, high-quality hyperbolic phonon polaritons in hexagonal boron nitride, topological phonon polaritons in <i>α</i>-MoO<sub>3,</sub> and one-dimensional Luttinger-liquid plasmon polaritons in carbon nanotubes. These unique properties make polaritons an excellent candidate for future nano-photonics devices. Further, these polaritons can significantly interact with each other, resulting in a variety of polariton-polariton coupling phenomena, greatly expanding their applications. In this review paper, we first introduce scanning near-field optical microscopy, i.e. the technique used to probe polaritons in low-dimensional materials, then give a brief introduction to the basic properties of polaritons. Next, we discuss in detail the coupling behavior between various polaritons. Finally, potential applications of polaritons coupling are proposed.

Design of gate-tunable graphene electro-optical reflectors based on an optical slot-antenna coupled cavity

The unique properties of graphene offer an exciting opportunity towards tunable photonic surfaces for flexible devices. In this paper, we design a gate-tunable, free-space graphene electro-optical reflector based on cavity resonator structures. We firstly calculate the graphene refractive index n and k as a function of Fermi level and external gating voltage. Then, we designed the structure of the single-layer graphene reflective resonator by carefully selecting suitable materials and device parameters to maximize the reflectance differences before and after electro-optical tuning. We also developed a theoretical model to discuss this system based on the optical transition matrix method. Moreover, we used field enhancement to further increase the reflectance differences by incorporating Sn nanodots based optical slot-antenna coupled cavities. The maximum broadband, incident angle insensitive reflectance differences could reach 28% with an extinction ratio of 1.62 dB at a low insertion loss of 0.45 dB, and the spectral range is tunable by changing the optical cavity length. We also used an indium tin oxide layer as part of the optical cavity and the electrode simultaneously to reduce the voltage applied. To our best knowledge, this work is the first one on tunable two-dimensional (2D) material reflectors for free-space applications, apart from using liquid crystals or magnetic metasurfaces. This new design of tunable 2D electro-optical reflectors also reduces the complexity of fabrication steps, having promising applications in tunable flexible photonic surfaces and devices for variable optical attenuators and light detection and ranging systems.

Enhanced tunable plasmonic resonance in crumpled graphene resonators loaded with gate tunable metamaterials.

Graphene devices have been widely explored for photonic applications, as they serve as promising candidates for controlling light interactions resulting in extreme confinement and tunability of graphene plasmons. The ubiquitous presence of surface crumples in graphene, very less is known on how the crumples in graphene can affect surface plasmon resonance and its absorption properties. In this article, a novel approach based on the crumpled graphene is investigated to realize broadband tunability of plasmonic resonance through the mechanical reconfiguration of crumpled graphene resonators. The mechanical reconfiguration of graphene crumples combined with dual electrostatic gating (i.e. raising the Fermi level from 0.2-0.4 eV) of graphene serves as a tuning knob enabling broad spectral tunability of plasmonic resonance in the wavelength range of 14-24 µm. The crumpled region in the resonators exhibits an effective trapping potential where it extremely confines the surface plasmonic field on the surfaces of crumples providing localized surface plasmon resonance at the apices of these crumples. Finally, to achieve near-unity absorption >99% at the resonance wavelengths (17 µm and 22 µm) crumpled graphene resonators are loaded with four ring shaped metamaterials which result in the enhanced near-field intensity of ≈1.4×106. This study delivers insight into the tunability of crumpled graphene and their coupling mechanism by providing a new platform for the flexible and gate tunable graphene sensors at the infrared region.

Electrical transport properties of gate tunable graphene lateral tunnel diodes

A detailed study of the electrical transport properties of gate tunable graphene lateral tunnel diodes is presented. The graphene-Al2O3-graphene lateral tunnel diodes are fabricated on Si/SiO2 substrates, and the fabricated devices show rectifying characteristics at the low voltage below 1 V. The rectifying behavior can be controlled by applying back gate voltages. As a result, the devices show high asymmetry and strong nonlinearity current–voltage (I–V) characteristics, which are desirable properties for applications such as optical rectennas and infrared detectors. The electrical transport mechanism of the graphene lateral diodes is analyzed by extracting parameters from the measured I–V characteristics. We find that trap-assisted tunneling from the defect levels in the Al2O3 layer is the most likely mechanism of the forward current of the fabricated graphene lateral diodes.

Complete Complex Amplitude Modulation with Electronically Tunable Graphene Plasmonic Metamolecules

Dynamic high-resolution wavefront modulation of light is a long-standing quest in photonics. Metasurfaces have shown potential for realizing light manipulation with subwavelength resolution through nanoscale optical elements, or metaatoms, to overcome the limitations of conventional spatial light modulators. State-of-the-art active metasurfaces operate via phase modulation of the metaatoms, and their inability to also independently control the scattered amplitude leads to an inferior reconstruction of the desired wavefronts. This fundamental problem posed severe performance limitations particularly for applications relying on subwavelength spatiotemporal complex field modulation, which includes dynamic holography, high-resolution imaging, optical tweezing, and optical information processing. Here, we present the "metamolecule" strategy, which incorporates two independent subwavelength scatterers composed of noble metal antennas coupled to gate-tunable graphene plasmonic nanoresonators. The two-parametric control of the metamolecule secures the complete control of both amplitude and phase of light, enabling 2π phase shift as well as large amplitude modulation including perfect absorption. We further develop a generalized graphical model to examine the underlying requirements for complete complex amplitude modulation, offering intuitive design guidelines to maximize the tunability in metasurfaces. To illustrate the reconfigurable capability of our designs, we demonstrate dynamic beam steering and holographic wavefront reconstruction in periodically arranged metamolecules.

High-performance monolayer MoS2 field-effect transistor with large-scale nitrogen-doped graphene electrodes for Ohmic contact

A finite Schottky barrier and large contact resistance between monolayer MoS2 and electrodes are the major bottlenecks in developing high-performance field-effect transistors (FETs) that hinder the study of intrinsic quantum behaviors such as valley-spin transport at low temperature. A gate-tunable graphene electrode platform has been developed to improve the performance of MoS2 FETs. However, intrinsic misalignment between the work function of pristine graphene and the conduction band of MoS2 results in a large threshold voltage for the FETs, because of which Ohmic contact behaviors are observed only at very high gate voltages and carrier concentrations (∼1013 cm−2). Here, we present high-performance monolayer MoS2 FETs with Ohmic contact at a modest gate voltage by using a chemical-vapor-deposited (CVD) nitrogen-doped graphene with a high intrinsic electron carrier density. The CVD nitrogen-doped graphene and monolayer MoS2 hybrid FETs platform exhibited a large negative shifted threshold voltage of −54.2 V and barrier-free Ohmic contact under zero gate voltage. Transparent contact by nitrogen-doped graphene led to a 214% enhancement in the on-current and a fourfold improvement in the field-effect carrier mobility of monolayer MoS2 FETs compared with those of a pristine graphene electrode platform. The transport measurements, as well as Raman and X-ray photoelectron spectroscopy analyses before and after thermal annealing, reveal that the atomic C-N bonding in the CVD nitrogen-doped graphene is responsible for the dominant effects of electron doping. Large-scale nitrogen-doped graphene electrodes provide a promising device platform for the development of high-performance devices and the study of unique quantum behaviors.

Coexistence of two graphene-induced modulation effects on surface plasmons in hybrid graphene plasmonic nanostructures.

Integrating gate-tunable graphene with plasmonic nanostructures or metamaterials offers a great potential in achieving dynamic control of plasmonic response. While remarkable progress has been made in realizing efficient graphene-induced modulations of plasmon resonances, a full picture of graphene-plasmon interactions and the consequent deep understanding on graphene-enabled tuning mechanism remain largely unexplored. Here, we theoretically identify, for the first time, two distinct modulation effects that can coexist in graphene-based plasmonic nanostructure: graphene can influence the plasmon resonances by either acting as equivalent nanocircuit elements or effectively altering their excitation environment, leading to totally different tuning behaviors. A general dependency of tuning features on the graphene-induced impedance, irrespective of structure geometries, is established when graphene serves as nanocircuit elements. We demonstrate that these two modulation effects can be dynamically controlled by appropriately integrating graphene with plasmonic nanostructures, which provide an active window for efficient modulation of surface plasmons. Our findings may pave the way towards realizing dynamic control of plasmonic response, which holds great potential applications in graphene-based active nanoplasmonic devices.

Manipulating line waves in flat graphene for agile terahertz applications

AbstractReducing open waveguides enabled by surface waves, such as surface plasmon polaritons, to a one-dimensional line is attractive due to the potentially enhanced control over light confinement and transport. This was recently shown to be possible by simply interfacing two co-planar surfaces with complementary surface impedances, which support transverse-magnetic and transverse-electric modes, respectively. Attractively, the resultant “line wave” at the interface line features singular field enhancement and robust direction-dependent polarizations. Current implementations, however, are limited to microwave frequencies and have fixed functionality due to the lack of dynamic control. In this article, we examine the potential of using gate-tunable graphene sheets for supporting line waves in the terahertz regime and propose an adequate graphene-metasurface configuration for operation at room temperature and low voltage conditions. In addition, we show the occurrence of quasi-line wave under certain conditions of non-complementary boundaries and qualify the degradation in line wave confinement due to dissipation losses. Furthermore, we show the possibility to alter the orientation of the line wave’s spin angular momentum on demand unlike conventional surface waves. Our results on active manipulation of electromagnetic line waves in graphene could be useful for various applications including reconfigurable integrated circuits, modulation, sensing and signal processes.

Low-energy high-speed plasmonic enhanced modulator using graphene.

Graphene, as a type of flexible and electrically adjustable two-dimensional material, has exceptional optical and electrical properties that make it possible to be used in modulators. However, the poor interaction between optical fields and a single atom graphene layer prevents the easy implementation of graphene modulators. Currently available devices often require a larger overlap area of graphene to obtain the desired phase or amplitude modulation, which results in a rather large footprint and high capacitance and consequently increases the energy consumption and reduces the modulation speed. In this paper, a localized plasmonic-enhanced waveguide modulator with high-speed tunability using graphene is proposed for telecommunication applications. Strong modulation of the transmission takes place due to the enhanced interaction between the ultrathin plasmon patches and the graphene, when the plasmons are tuned on- and off-resonance by the gate-tunable graphene. A 400 GHz modulation rate using low gated-voltages with an active device area of 0.2 μm2 and a low consumption of only 0.5 fJ/bit is achieved, which paves the way for ultrafast low-energy optical waveguide modulation and switching.

Current-Phase Relation of Ballistic Graphene Josephson Junctions.

The current-phase relation (CPR) of a Josephson junction (JJ) determines how the supercurrent evolves with the superconducting phase difference across the junction. Knowledge of the CPR is essential in order to understand the response of a JJ to various external parameters. Despite the rising interest in ultraclean encapsulated graphene JJs, the CPR of such junctions remains unknown. Here, we use a fully gate-tunable graphene superconducting quantum intereference device (SQUID) to determine the CPR of ballistic graphene JJs. Each of the two JJs in the SQUID is made with graphene encapsulated in hexagonal boron nitride. By independently controlling the critical current of the JJs, we can operate the SQUID either in a symmetric or asymmetric configuration. The highly asymmetric SQUID allows us to phase-bias one of the JJs and thereby directly obtain its CPR. The CPR is found to be skewed, deviating significantly from a sinusoidal form. The skewness can be tuned with the gate voltage and oscillates in antiphase with Fabry-Pérot resistance oscillations of the ballistic graphene cavity. We compare our experiments with tight-binding calculations that include realistic graphene–superconductor interfaces and find a good qualitative agreement.

Performance optimization in gate-tunable Schottky junction solar cells with a light transparent and electric-field permeable graphene mesh on n-Si

Gate-tunable graphene mesh-on-Si Schottky junction solar cells are fabricated and the effect of the external gate voltage on the photovoltaic characteristics is investigated.

Millimeter-scale gate-tunable graphene nanoribbon devices as a platform for mid-infrared and bio sensing applications

The experimental observation of graphene plasmon resonance has generated tremendous interest due to numerous potential applications such as photosensors, detectors, biosensors, and switches from THz to mid-infrared regime. However, practical applications require much larger dimensions (mm to cm scale) than that demonstrated in the proof-of-concept devices. Moreover, such devices also require a detailed understanding of ribbon-to-ribbon interaction, which has not been investigated so far. Here we demonstrate gate tunable plasmon resonance in the mid-infrared spectral region on millimeter-scale graphene nanoribbon (GNR) array devices fabricated using graphene monolayer. Gate dependent Fourier-transform infrared (FTIR) transmission measurements on GNR of various widths were investigated experimentally. The shift in plasmon resonance peaks of wave number 100cm−1 at applied external gate voltage −100V was observed. This shift is attributed to strong gate modulation. Our investigation of ribbon-to-ribbon interaction by tuning the aspect ratio reveals strong modulation of surface plasmon resonance peaks in GNR. This suggests that plasmon resonances are coupled as evidenced by blue-shifted plasmon resonance. These studies demonstrate that large-area GNR devices can serve as an ideal platform for ultrasensitive sensing and detector applications.

Gate-tunable graphene quantum dot and Dirac oscillator

We obtain the solution of the Dirac equation in (2+1) dimensions in the presence of a constant magnetic field normal to the plane together with a two-dimensional Dirac-oscillator potential coupling. We study the energy spectrum of graphene quantum dot (QD) defined by electrostatic gates. We give discussions of our results based on different physical settings, whether the cyclotron frequency is similar or larger/smaller compared to the oscillator frequency. This defines an effective magnetic field that produces the effective quantized Landau levels. We study analytically such field in gate-tunable graphene QD and show that our structure allows us to control the valley degeneracy. Finally, we compare our results with already published work and also discuss the possible applications of such QD.

High-speed gate-tunable terahertz coherent perfect absorption using a split-ring graphene.

We exploit split-ring graphene to realize coherent perfect absorption (CPA) in the terahertz (THz) regime. By controlling the relative phase of two counter-propagating coherent beams, the coherent absorption at resonant frequency of 2.91 THz can be tuned continuously from 99.7% to less than 2.1×10(-4)%, which gives a modulation contrast of 56.7 dB. Moreover, the coherent absorption also can be tuned by varying the gate-controlled Fermi energy based on the electro-absorption effect of graphene, giving a modulation contrast of 19 dB. Center frequency tunable CPA is also achieved using gate-tunable split-ring graphene. We discuss the tolerance of the geometric parameters (radii of the inner and outer circles, gap aperture) and study the device performance dependence on the relative intensities of two input beams and the angle between two input beams. Additionally, the response time of the device is analyzed to be ∼36 ps, which indicates possible high-speed gate-tunable THz CPA operations.

Fabrication of Gate-tunable Graphene Devices for Scanning Tunneling Microscopy Studies with Coulomb Impurities.

Owing to its relativistic low-energy charge carriers, the interaction between graphene and various impurities leads to a wealth of new physics and degrees of freedom to control electronic devices. In particular, the behavior of graphene's charge carriers in response to potentials from charged Coulomb impurities is predicted to differ significantly from that of most materials. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) can provide detailed information on both the spatial and energy dependence of graphene's electronic structure in the presence of a charged impurity. The design of a hybrid impurity-graphene device, fabricated using controlled deposition of impurities onto a back-gated graphene surface, has enabled several novel methods for controllably tuning graphene's electronic properties. Electrostatic gating enables control of the charge carrier density in graphene and the ability to reversibly tune the charge and/or molecular states of an impurity. This paper outlines the process of fabricating a gate-tunable graphene device decorated with individual Coulomb impurities for combined STM/STS studies. These studies provide valuable insights into the underlying physics, as well as signposts for designing hybrid graphene devices.

Fabrication of Gate-tunable Graphene Devices for Scanning Tunneling Microscopy Studies with Coulomb Impurities

Fabrication of Gate-tunable Graphene Devices for Scanning Tunneling Microscopy Studies with Coulomb Impurities

Multi-terminal transport measurements of MoS2 using a van der Waals heterostructure device platform.

Atomically thin two-dimensional semiconductors such as MoS2 hold great promise for electrical, optical and mechanical devices and display novel physical phenomena. However, the electron mobility of mono- and few-layer MoS2 has so far been substantially below theoretically predicted limits, which has hampered efforts to observe its intrinsic quantum transport behaviours. Potential sources of disorder and scattering include defects such as sulphur vacancies in the MoS2 itself as well as extrinsic sources such as charged impurities and remote optical phonons from oxide dielectrics. To reduce extrinsic scattering, we have developed here a van der Waals heterostructure device platform where MoS2 layers are fully encapsulated within hexagonal boron nitride and electrically contacted in a multi-terminal geometry using gate-tunable graphene electrodes. Magneto-transport measurements show dramatic improvements in performance, including a record-high Hall mobility reaching 34,000 cm(2) V(-1) s(-1) for six-layer MoS2 at low temperature, confirming that low-temperature performance in previous studies was limited by extrinsic interfacial impurities rather than bulk defects in the MoS2. We also observed Shubnikov-de Haas oscillations in high-mobility monolayer and few-layer MoS2. Modelling of potential scattering sources and quantum lifetime analysis indicate that a combination of short-range and long-range interfacial scattering limits the low-temperature mobility of MoS2.