Mobility Of Graphene Research Articles

Anomalous redshift of graphene absorption induced by plasmon-cavity competition.

Anomalous redshift of the absorption peak of graphene in the cavity system is numerically and experimentally demonstrated. It is observed that the absorption peak exhibits a redshift as the Fermi level of graphene increases, which is contrary to the ordinary trend of graphene plasmons. The influencing factors, including the electron mobility of graphene, the cavity length, and the ribbon width, are comprehensively analyzed. Such anomalous redshift can be explained by the competition between the graphene plasmon mode and the optical cavity mode. The study herein could be beneficial for the design of graphene-based plasmonic devices.

Hydrogen-Assisted Fast Growth of Large Graphene Grains by Recrystallization of Nanograins.

Chemical vapor deposition has been highlighted as a promising tool for facile graphene growth in a large area. However, grain boundaries impose detrimental effects on the mechanical strength or electrical mobility of graphene. Here, we demonstrate that high-pressure hydrogen treatment in the preannealing step plays a key role in fast and large grain growth and leads to the successful synthesis of large grain graphene in 10 s. Large single grains with a maximum size of ∼160 μm grow by recrystallization of nanograins, but ∼1% areal coverage of nanograins remains with 28–30° misorientation angles. Our findings will provide insights into mass production of high-quality graphene.

Graphene Mobility Dependence on the Resistivity of Si Wafer

Graphene field effect transistors (GFETs) are typically benchmarked based on their field effect mobility. Constant mobility model proposed by Kim1 et.al is extensively used to derive this mobility parameter by directly fitting the transfer curves of graphene devices. Various methodologies were suggested to supplant and improve this existing mobility model. Among them Zhang2 et.al suggested the need for using measured gate oxide capacitance in order to extract mobility for top-gated GFETs. One of the commonly overlooked features, in the case of back gated GEFTs is the dopant concentration or the resistivity of Si substrate used. Herein, we explore the dependence of graphene carrier mobility on the induced carrier concentration and depletion capacitance of underlying Si substrate. Si with resistivities 0.01-0.02 Ω-cm and 0.001-0.005 Ω-cm were used to grow 10 nm thermal SiO2. GFETs were fabricated using CVD graphene on top of this thermal oxide substrate and the corresponding transfer curves are co-related with the capacitance density measured from corresponding MOSCAP structure. Detailed carrier mobility extraction and results are presented.(1) Kim, S.; Nah, J.; Jo, I.; Shahrjerdi, D.; Colombo, L.; Yao, Z.; Tutuc, E.; Banerjee, S. K. Realization of a High Mobility Dual-Gated Graphene Field-Effect Transistor with Al2O3 Dielectric. Appl. Phys. Lett. 2009, 94 (6). https://doi.org/10.1063/1.3077021.(2) Zhang, Z.; Xu, H.; Zhong, H.; Peng, L. M. Direct Extraction of Carrier Mobility in Graphene Field-Effect Transistor Using Current-Voltage and Capacitance-Voltage Measurements. Appl. Phys. Lett. 2012, 101 (21). https://doi.org/10.1063/1.4768690.

Modulation Doping via a Two-Dimensional Atomic Crystalline Acceptor.

Two-dimensional nanoelectronics, plasmonics, and emergent phases require clean and local charge control, calling for layered, crystalline acceptors or donors. Our Raman, photovoltage, and electrical conductance measurements combined with ab initio calculations establish the large work function and narrow bands of α-RuCl3 enable modulation doping of exfoliated single and bilayer graphene, chemical vapor deposition grown graphene and WSe2, and molecular beam epitaxy grown EuS. We further demonstrate proof of principle photovoltage devices, control via twist angle, and charge transfer through hexagonal boron nitride. Short-ranged lateral doping (≤65 nm) and high homogeneity are achieved in proximate materials with a single layer of α-RuCl3. This leads to the best-reported monolayer graphene mobilities (4900 cm2/(V s)) at these high hole densities (3 × 1013 cm-2) and yields larger charge transfer to bilayer graphene (6 × 1013 cm-2).

Reduced thermal conductivity of supported and encased monolayer and bilayer MoS2

Electrical and thermal properties of atomically thin two-dimensional (2D) materials are affected by their environment, e.g. through remote phonon scattering or dielectric screening. However, while it is known that mobility and thermal conductivity (TC) of graphene are reduced on a substrate, these effects are much less explored in 2D semiconductors such as MoS2. Here, we use molecular dynamics to understand TC changes in monolayer (1L) and bilayer (2L) MoS2 by comparing suspended, supported, and encased structures. The TC of monolayer MoS2 is reduced from ∼117 W m−1 K−1 when suspended, to ∼31 W m−1 K−1 when supported by SiO2, at 300 K. Encasing 1L MoS2 in SiO2 further reduces its TC down to ∼22 W m−1 K−1. In contrast, the TC of 2L MoS2 is not as drastically reduced, being >50% higher than 1L both when supported and encased. These effects are due to phonon scattering with remote vibrational modes of the substrate, which are partly screened in 2L MoS2. We also examine the TC of 1L MoS2 across a wide range of temperatures (300 K to 700 K) and defect densities (up to 5 × 1013 cm−2), finding that the substrate reduces the dependence of TC on these factors. Taken together, these are important findings for all applications which will use 2D semiconductors supported or encased by insulators, instead of freely suspended.

Plasmonically enhanced mid-IR light source based on tunable spectrally and directionally selective thermal emission from nanopatterned graphene

We present a proof of concept for a spectrally selective thermal mid-IR source based on nanopatterned graphene (NPG) with a typical mobility of CVD-grown graphene (up to 3000 hbox {cm}^2,hbox {V}^{-1},hbox {s}^{-1}), ensuring scalability to large areas. For that, we solve the electrostatic problem of a conducting hyperboloid with an elliptical wormhole in the presence of an in-plane electric field. The localized surface plasmons (LSPs) on the NPG sheet, partially hybridized with graphene phonons and surface phonons of the neighboring materials, allow for the control and tuning of the thermal emission spectrum in the wavelength regime from lambda =3 to 12 upmum by adjusting the size of and distance between the circular holes in a hexagonal or square lattice structure. Most importantly, the LSPs along with an optical cavity increase the emittance of graphene from about 2.3% for pristine graphene to 80% for NPG, thereby outperforming state-of-the-art pristine graphene light sources operating in the near-infrared by at least a factor of 100. According to our COMSOL calculations, a maximum emission power per area of 11times 10^3 W/hbox {m}^2 at T=2000 K for a bias voltage of V=23 V is achieved by controlling the temperature of the hot electrons through the Joule heating. By generalizing Planck’s theory to any grey body and deriving the completely general nonlocal fluctuation-dissipation theorem with nonlocal response of surface plasmons in the random phase approximation, we show that the coherence length of the graphene plasmons and the thermally emitted photons can be as large as 13 upmum and 150 upmum, respectively, providing the opportunity to create phased arrays made of nanoantennas represented by the holes in NPG. The spatial phase variation of the coherence allows for beamsteering of the thermal emission in the range between 12^circ and 80^circ by tuning the Fermi energy between E_F=1.0 eV and E_F=0.25 eV through the gate voltage. Our analysis of the nonlocal hydrodynamic response leads to the conjecture that the diffusion length and viscosity in graphene are frequency-dependent. Using finite-difference time domain calculations, coupled mode theory, and RPA, we develop the model of a mid-IR light source based on NPG, which will pave the way to graphene-based optical mid-IR communication, mid-IR color displays, mid-IR spectroscopy, and virus detection.

Novel graphene electrochemical transistor with ZrO2/rGO nanocomposites functionalized gate electrode for ultrasensitive recognition of methyl parathion

A novel graphene electrochemical transistor (GECT) based on zirconia/reduced graphene oxide (ZrO2/rGO) nanocomposites functionalized gate electrode and graphene as channel was successfully designed and fabricated for highly sensitive detection of methyl parathion (MP). ZrO2/rGO nanocomposites were co-electrodeposited on a glassy carbon electrode considering the special affinity of ZrO2 toward phosphate groups and high specific surface area of rGO. The sensing mechanism is attributed to the change of effective gate voltage applied to GECT induced by the electrochemical reactions of MP at the functionalized gate electrode, thus tuning the channel carrier concentration and the channel current. Due to the excellent electrocatalytic activity of ZrO2/rGO on the gate and the high carrier mobility of graphene in the channel, the developed sensor exhibited excellent sensing performances with ultra-low detection limit (10 pg/mL) and ultra-wide linear range from 10 pg/mL to 10 μg/mL, which especially stand out of the reported devices. The GECT sensor also showed good selectivity toward usual interferents and strong practicality through testing MP in Chinese cabbage samples. The developed GECT devices provided not only the novel platform for pesticide detection with high sensitivity but the new method of enhancing the sensing performance through specifically functionalized gate electrode.

Dual-frequency on–off modulation and slow light analysis based on dual plasmon-induced transparency in terahertz patterned graphene metamaterial

A dual-frequency on–off modulator with considerable modulation depth (MD) and relatively low insertion loss (IL) is performed with patterned monolayer graphene metamaterial. Destructive interference in this structure gives rise to the dual plasmon-induced transparency (DPIT) phenomenon. The coupled mode theory, confirmed by simulated values, is comprehensively introduced to expound the physical mechanism of the DPIT effect. In addition, the influences of the Fermi level on the DPIT transmission spectrum and the carrier mobility of graphene on the on–off modulation are researched. It is found that the dual-frequency on–off modulator exhibits remarkable modulation performance on both switches and is easier to fabricate in operation than other multi-layer graphene-based modulators. In the ‘on1/off1’ state, the MD and IL are 93%, 0.32 dB, respectively. In the ‘on2/off2’ state, the MD and IL are 85%, 0.25 dB, separately. Moreover, the property of slow light reflected by the group index is analyzed. It exhibits that the group index of the proposed structure with multi-channel can reach 358. Thus, the proposed structure stretches the versatile applications in multi-function modulators and multi-channel slow light devices at the terahertz band.

Interplay of filling fraction and coherence in symmetry broken graphene p-n junction

Graphene p–n junction (PNJ) with co-propagating spin-valley polarized quantum Hall (QH) edges is a promising platform for studying electron interferometry. Though several conductance measurements have been attempted for such PNJs, the edge dynamics of the spin-valley symmetry broken edge states remain unexplored. In this work, we present the measurements of conductance together with shot noise, an ideal tool to unravel the dynamics, at low temperature, in a dual graphite gated hexagonal boron nitride encapsulated high mobility graphene device. The conductance data show that the symmetry broken QH edges at the PNJ follow spin selective equilibration. The shot noise results as a function of both p and n side filling factors reveal the unique dependence of the scattering mechanism. Remarkably, the scattering is found to be fully tunable from incoherent to coherent regime with the increasing number of QH edges at the PNJ, shedding crucial insights of edge dynamics.

Resolved Infrared Spectroscopy of Aqueous Molecules Employing Tunable Graphene Plasmons in an Otto Prism.

Real-time and in situ detection of aqueous solution is essential for bioanalysis and chemical reactions. However, it is extremely challenging for infrared microscopic measurement because of the large background of water absorption. Here, we proposed a wideband-tunable graphene plasmonic infrared biosensor to detect biomolecules in an aqueous environment, employing attenuated total reflection in an Otto prism configuration and tightly confined plasmons in graphene nanoribbons. Benefiting from the graphene plasmonic electric field enhancement, such a biosensor is able to identify the molecular chemical fingerprints without the interference of water absorption. As a proof of concept, the recombinant protein AG and goat anti-mouse immunoglobulin G (IgG) are used as the sensing analytes, of which the vibrational modes (1669 and 1532 cm-1) are very close to the OH-bending mode of water (1640 cm-1). Simulation results show that the fingerprints of protein molecules in the water environment can be selectively enhanced. Therefore, the water absorption is successfully suppressed so that two protein modes can be resolved by sweeping graphene Fermi energy in a wide waveband. By further optimizing the incident angle and graphene mobility to improve the mode energy of graphene plasmons, maximum enhancement factors of 112 and 130 can be achieved for amide I and II bands. Our work provides an effective approach for the highly sensitive and selective in situ identification of aqueous-phase molecular fingerprints in fields of healthcare, food safety, and biochemical sensing.

Reference-free THz-TDS conductivity analysis of thin conducting films.

We present a reference-free method to determine electrical parameters of thin conducting films by steady state transmission-mode terahertz time-domain spectroscopy (THz-TDS). We demonstrate that the frequency-dependent AC conductivity of graphene can be acquired by comparing the directly transmitted THz pulse with a transient internal reflection within the substrate which avoids the need for a standard reference scan. The DC sheet conductivity, scattering time, carrier density, mobility, and Fermi velocity of graphene are retrieved subsequently by fitting the AC conductivity with the Drude model. This reference-free method was investigated with two complementary THz setups: one commercial fibre-coupled THz spectrometer with fast scanning rate (0.2-1.5 THz) and one air-plasma based ultra-broadband THz spectrometer for greatly extended frequency range (2-10 THz). Certain propagation correction terms for more accurate retrieval of electrical parameters are discussed.

Fractional quantum Hall effect in CVD-grown graphene

We show the emergence of fractional quantum Hall states in graphene grown by chemical vapor deposition (CVD) for magnetic fields from below 3 T to 35 T where the CVD-graphene was dry-transferred. Effective composite-fermion filling factors up to ν * = 4 are visible and higher order composite-fermion states (with four flux quanta attached) start to emerge at the highest fields. Our results show that the quantum mobility of CVD-grown graphene is comparable to that of exfoliated graphene and, more specifically, that the p/3 fractional quantum Hall states have energy gaps of up to 30 K, well comparable to those observed in other silicon-gated devices based on exfoliated graphene.

Camphor-Based CVD Bilayer Graphene/Si Heterostructures for Self-Powered and Broadband Photodetection.

This work demonstrates a self-powered and broadband photodetector using a heterojunction formed by camphor-based chemical vaper deposition (CVD) bilayer graphene on p-Si substrates. Here, graphene/p-Si heterostructures and graphene layers serve as ultra-shallow junctions for UV absorption and zero bandgap junction materials (<Si bandgap (1.1 eV)) for long-wave near-infrared (LWNIR) absorption, respectively. According to the Raman spectra and large-area (16 × 16 μm2) Raman mapping, a low-defect, >95% coverage bilayer and high-uniformity graphene were successfully obtained by camphor-based CVD processes. Furthermore, the carrier mobility of the camphor-based CVD bilayer graphene at room temperature is 1.8 × 103 cm2/V·s. Due to the incorporation of camphor-based CVD graphene, the graphene/p-Si Schottky junctions show a good rectification property (rectification ratio of ~110 at ± 2 V) and good performance as a self-powered (under zero bias) photodetector from UV to LWNIR. The photocurrent to dark current ratio (PDCR) value is up to 230 at 0 V under white light illumination, and the detectivity (D*) is 8 × 1012 cmHz1/2/W at 560 nm. Furthermore, the photodetector (PD) response/decay time (i.e., rise/fall time) is ~118/120 μs. These results support the camphor-based CVD bilayer graphene/Si Schottky PDs for use in self-powered and ultra-broadband light detection in the future.

Edge-dependent ballistic transport through copper-decorated carbon-nanotube–graphene covalent junction with low Schottky barrier

The ultrahigh carrier mobility and matchable work function of graphene have positioned this material as a leading candidate for the ideal contact material for carbon nanotubes (CNTs). Highly efficient carrier transport through CNT–graphene junctions is facilitated by covalently bonded contacts. This paper, therefore, proposes covalently bonded CNT–graphene junctions and investigates their characteristics theoretically. In these junctions, partially unzipped CNTs are longitudinally or radially bonded with graphene. By exploiting nonequilibrium Green's functions with density-functional theory, we examine ballistic electron transport (∼1.38 × 105 cm2/V s) and edge-dependent transport. Moreover, the contact properties of the junctions with adsorbed Cu atoms are investigated. Electron transfer from Cu to the junction turns the p-type Schottky contact into an n-type contact and decreases the Schottky barrier height from 0.2 to 0.08 eV. Furthermore, the junction resistance decreases by one to three orders of magnitude. The proposed design of Cu-decorated CNT–graphene junctions and first-principles calculations suggest an approach for low-power, high-performance CNT-based electronics.

Highly sensitive methyl parathion sensor based on Au-ZrO2 nanocomposites modified graphene electrochemical transistor

The excessive use of pesticides does great harm to environment and human health, a high performance sensor is urgently needed for the management of pesticides contamination. Herein, a novel graphene electrochemical transistor (GECT) for the detection of methyl parathion (MP) was successfully fabricated for the first time. A single layer of graphene was covered between the source and drain electrodes by the wet transfer method to act as channel for the transistor. Au-ZrO2 nanocomposites were electrodeposited at graphene channel of the device, considering the special affinity of ZrO2 nanoparticles toward phosphate group and electrocatalytic ability of Au nanoparticles. The sensing mechanism was attributed to the change of carrier concentration in channel induced by the electrochemical reaction of MP at the graphene channel, which built up the mathematic relationship between the change of channel current and the concentration of MP. Different modified materials and electrodeposition conditions were compared and optimized. Due to the high carrier mobility of graphene and excellent catalytic performance of Au-ZrO2 nanocomposites in the channel, the developed sensor showed a limit of detection as low as 0.1 ng/mL and ultra-wide linear range from 0.4 ng/mL to 7 μg/mL, which are comparable or superior to those using the large-scale instruments. Meanwhile, the device exhibited good selectivity toward usual interferents and high accuracy for detection of MP in Chinese cabbage samples, demonstrating the promising practicality of the devices. It is expected that GECT can act as a desirable transducer platform for highly sensitive and selective detection of pesticides.

Multiple-graphene layer based p++–n–n−–n++ device on Si/3C-SiC (100) substrate: a highly sensitive visible photo-sensor

The current paper reports the prospects of a multiple-graphene-layer (MGL) based vertically doped p++–n–n––n++ avalanche photo-sensor in visible wavelength region (300 nm–800 nm). High carrier mobility, intrinsic optical and mechanical properties in graphene at room temperature, have made this material promising for various novel applications. The authors have used an indigenously developed and experimentally verified quantum-corrected field maximum classical drift-diffusion (QCFMCDD) mathematical model for studying the optical characteristics of MGL based p++–n–n––n++ avalanche photo-diode sensor. The validity of the simulator is established through a comparative study between the experimental and analytical observations under similar operating (structural/electrical/thermal) conditions. Additionally, the superiority of the QCFMCDD model over the conventional classical drift-diffusion model is established through this comprehensive study. The analysis reveals that the opto-electrical properties of the device-under-test improve significantly as a result of the incorporation of MGL in the low doped active region. The authors have compared the results with those of a Si/4H-SiC super-lattice avalanche photo-sensor at 500 nm wavelength of incident radiation. It is observed that graphene outperforms its Si/4H-SiC counterpart in terms of photo-responsivity (0.820 A W−1 vs. 0.650 A W−1) and quantum efficiency (84% vs. 62%) for optical irradiation with visible laser source. The opto-electrical performance of the 3 × 3 array type graphene photo-sensor is further analyzed and compared with those of its single array counterpart. The photo-responsivity as well as quantum efficiency increases by at least 30% in the case of the 3 × 3 photo-sensor array. In comparison to available photo detectors, reported so far in published literature, the designed photo-sensor shows overall performance betterment in the visible wavelength region. To the best of authors’ knowledge, this is the first report on MGL based 3 × 3 array type ultra fast highly sensitive visible avalanche diode photo-sensor.

Triple plasmon-induced transparency and outstanding slow-light in quasi-continuous monolayer graphene structure

We propose a simple quasi-continuous monolayer graphene structure and achieve a dynamically tunable triple plasmon-induced transparency (PIT) effect in the proposed structure. Additional analyses indicate that the proposed structure contains a self-constructed bright-dark-dark mode system. A uniform theoretical model is introduced to investigate the spectral response characteristics and slow light-effects in the proposed system, and the theoretical and the simulated results exhibit high consistency. In addition, the influences of the Fermi level and the carrier mobility of graphene on transmission spectra are discussed. It is found that each PIT window exhibits an independent dynamical adjustability owing to the quasi-continuity of the proposed structure. Finally, the slow-light effects are investigated based on the calculation of the group refractive index and phase shift. It is found that the structure displays excellent slow-light effects near the PIT windows with high-group indices, and the maximum group index of each PIT window exceeds 1000 when the carrier mobility of graphene increases to 3.5 m2 V−1 s−1. The proposed structure has potential to be used in multichannel filters, optical switches, modulators, and slow light devices. Additionally, the established theoretical model lays a theoretical basis for research on multimode coupling effects.

Relationship between mobility and strain in CVD graphene on h-BN

This study examines the relationship between the electrical properties and Raman spectra of field effect transistors (FETs) produced using chemical vapor deposited (CVD) graphene transferred onto hexagonal boron nitride (h-BN) structures. Carrier mobility values were calculated based on the electrical properties of the fabricated FETs, where the highest carrier mobility was 39 989 cm2/Vs. Carrier mobility increased with a decrease in the full width at half maximum (FWHM) of the 2D-band peak of CVD graphene. A linear relationship with a slope of 2.18 between the G-band and 2D-band peak positions was detected, indicating that a uniaxial strain existed in the CVD graphene FETs. Based on the peak shifts in the 2D-band, it was determined that both compressive and tensile strains were responsible for limiting carrier mobility. Ultimately, the analysis of peak positions and FWHMs of 2D-bands enabled us to evaluate the uniformity of electrical properties of CVD graphene without fabricating specialized measurement devices.

Atomic-Scale Carving of Nanopores into a van der Waals Heterostructure with Slow Highly Charged Ions.

The growing family of 2D materials led not long ago to combining different 2D layers and building artificial systems in the form of van der Waals heterostructures. Tailoring of heterostructure properties postgrowth would greatly benefit from a modification technique with a monolayer precision. However, appropriate techniques for material modification with this precision are still missing. To achieve such control, slow highly charged ions appear ideal as they carry high amounts of potential energy, which is released rapidly upon ion neutralization at the position of the ion. The resulting potential energy deposition is thus limited to just a few atomic layers (in contrast to the kinetic energy deposition). Here, we irradiated a freestanding van der Waals MoS2/graphene heterostructure with 1.3 keV/amu xenon ions in high charge states of 38, which led to nanometer-sized pores that appear only in the MoS2 facing the ion beam, but not in graphene beneath the hole. Reversing the stacking order leaves both layers undamaged, which we attribute to the high conductivity and carrier mobility in graphene acting as a shield for the MoS2 underneath. Our main focus is here on monolayer MoS2, but we also analyzed areas with few-layer structures and observed that the perforation is limited to the two topmost MoS2 layers, whereas deeper layers remain intact. Our results demonstrate that in addition to already being a valuable tool for materials processing, the usability of ion irradiation can be extended to mono- (or bi)layer manipulation of van der Waals heterostructures when the localized potential energy deposition of highly charged ions is also added to the toolbox.

Graphene ballistic rectifiers: Theory and geometry dependence

The graphene ballistic rectifier uses the ultra-high carrier mobility of graphene to rectify ballistic carriers in an asymmetric nanostructure, and has been previously shown to operate beyond 650 GHz. Unlike conventional diodes and transistors, it does not require a bandgap. Building on the previously developed extended Büttiker-Landauer formula for semiconductors, we derive an analytical theory suitable for coexistence of electrons and holes in semi-metal graphene. Four ballistic rectifier designs are fabricated and compared. The developed theory fits their characteristics well, with derived parameters showing good agreement with device geometries. We also predict achievable responsivities of at least 50,800 V/W and noise-equivalent powers of 0.51 pW/Hz1/2 using these designs, far better than has so far been achieved. Importantly, this theory predicts increased responsivities with a large difference in carrier mobilities. Using the theory presented here, other graphene based ballistic nanodevices may be designed and optimized.