RESUMO
Electron emission from quasi-freestanding bilayer epitaxial graphene (QFEG) on a silicon carbide substrate is reported, demonstrating emission currents as high as 8.5µA, at â¼200 °C, under 0.3 Torr vacuum. Given the significantly low turn-on temperature of these QFEG devices, â¼150°C, the electron emission is explained by phonon-assisted electron emission, where the acoustic and optical phonons of QFEG causes carrier acceleration and emission. Devices of differing dimensions and shapes are fabricated via a simple and scalable fabrication procedure and tested. Variations in device morphology increase the density of dangling bonds, which can act as electron emission sites. Devices exhibit emission enhancement at increased temperatures, attributed to greater phonon densities. Devices exhibit emission under various test conditions, and a superior design and operating methodology are identified.
RESUMO
Chemiresistive graphene sensors are promising for chemical sensing applications due to their simple device structure, high sensitivity, potential for miniaturization, low-cost, and fast response. In this work, we investigate the effect of (1) ZnO nanoparticle functionalization and (2) engineered defects onto graphene sensing channel on device resistance and low frequency electrical noise. The engineered defects of interest include 2D patterns of squares, stars, and circles and 1D patterns of slots parallel and transverse to the applied electric potential. The goal of this work is to determine which devices are best suited for chemical sensing applications. We find that, relative to pristine graphene devices, nanoparticle functionalization leads to reduced contact resistance but increased sheet resistance. In addition, functionalization lowers 1/f current noise on all but the uniform mesa device and the two devices with graphene strips parallel to carrier transport. The strongest correlations between noise and engineering defects, where normalized noise amplitude as a function of frequency f is described by a model of AN/fγ, are that γ increases with graphene area and contact area but decreases with device total perimeter, including internal features. We did not find evidence of a correlation between the scalar amplitude, AN, and the device channel geometries. In general, for a given device area, the least noise was observed on the least-etched device. These results will lead to an understanding of what features are needed to obtain the optimal device resistance and how to reduce the 1/f noise which will lead to improved sensor performance.
RESUMO
The electrochemical detection of heavy metal ions is reported using an inexpensive portable in-house built potentiostat and epitaxial graphene. Monolayer, hydrogen-intercalated quasi-freestanding bilayer, and multilayer epitaxial graphene were each tested as working electrodes before and after modification with an oxygen plasma etch to introduce oxygen chemical groups to the surface. The graphene samples were characterized using X-ray photoelectron spectroscopy, atomic force microscopy, Raman spectroscopy, and van der Pauw Hall measurements. Dose-response curves in seawater were evaluated with added trace levels of four heavy metal salts (CdCl2, CuSO4, HgCl2, and PbCl2), along with detection algorithms based on machine learning and library development for each form of graphene and its oxygen plasma modification. Oxygen plasma-modified, hydrogen-intercalated quasi-freestanding bilayer epitaxial graphene was found to perform best for correctly identifying heavy metals in seawater.
Assuntos
Grafite , Metais Pesados , Grafite/química , Hidrogênio , Oxigênio , Sais , Água do MarRESUMO
Free-standing crystalline membranes are highly desirable owing to recent developments in heterogeneous integration of dissimilar materials. Van der Waals (vdW) epitaxy enables the release of crystalline membranes from their substrates. However, suppressed nucleation density due to low surface energy has been a challenge for crystallization; reactive materials synthesis environments can induce detrimental damage to vdW surfaces, often leading to failures in membrane release. This work demonstrates a novel platform based on graphitized SiC for fabricating high-quality free-standing membranes. After mechanically removing epitaxial graphene on a graphitized SiC wafer, the quasi-two-dimensional graphene buffer layer (GBL) surface remains intact for epitaxial growth. The reduced vdW gap between the epilayer and substrate enhances epitaxial interaction, promoting remote epitaxy. Significantly improved nucleation and convergent quality of GaN are achieved on the GBL, resulting in the best quality GaN ever grown on two-dimensional materials. The GBL surface exhibits excellent resistance to harsh growth environments, enabling substrate reuse by repeated growth and exfoliation.
Assuntos
Grafite , Cristalização , SemicondutoresRESUMO
The electrochemical response of multilayer epitaxial graphene electrodes on silicon carbide substrates was studied for use as an electrochemical sensor for seawater samples spiked with environmental contaminants using cyclic square wave voltammetry. Results indicate that these graphene working electrodes are more robust and have lower background current than either screen-printed carbon or edge-plane graphite in seawater. Identification algorithms developed using machine learning techniques are described for several heavy metals, herbicides, pesticides, and industrial compounds. Dose-response curves provide a basis for quantitative analysis.
RESUMO
The transparency of two-dimensional (2D) materials to intermolecular interactions of crystalline materials has been an unresolved topic. Here we report that remote atomic interaction through 2D materials is governed by the binding nature, that is, the polarity of atomic bonds, both in the underlying substrates and in 2D material interlayers. Although the potential field from covalent-bonded materials is screened by a monolayer of graphene, that from ionic-bonded materials is strong enough to penetrate through a few layers of graphene. Such field penetration is substantially attenuated by 2D hexagonal boron nitride, which itself has polarization in its atomic bonds. Based on the control of transparency, modulated by the nature of materials as well as interlayer thickness, various types of single-crystalline materials across the periodic table can be epitaxially grown on 2D material-coated substrates. The epitaxial films can subsequently be released as free-standing membranes, which provides unique opportunities for the heterointegration of arbitrary single-crystalline thin films in functional applications.
RESUMO
Using square wave voltammetry, we show an increase in the electrochemical detection of trinitrotoluene (TNT) with a working electrode constructed from plasma modified graphene on a SiC surface vs. unmodified graphene. The graphene surface was chemically modified using electron beam generated plasmas produced in oxygen or nitrogen containing backgrounds to introduce oxygen or nitrogen moieties. The use of this chemical modification route enabled enhancement of the electrochemical signal for TNT, with the oxygen treatment showing a more pronounced detection than the nitrogen treatment. For graphene modified with oxygen, the electrochemical response to TNT can be fit to a two-site Langmuir isotherm suggesting different sites on the graphene surface with different affinities for TNT. We estimate a limit of detection for TNT equal to 20 ppb based on the analytical standard S/N ratio of 3. In addition, this approach to sensor fabrication is inherently a high-throughput, high-volume process amenable to industrial applications. High quality epitaxial graphene is easily grown over large area SiC substrates, while plasma processing is a rapid approach to large area substrate processing. This combination facilitates low cost, mass production of sensors.
RESUMO
We report a large area terahertz detector utilizing a tunable plasmonic resonance in subwavelength graphene microribbons on SiC(0001) to increase the absorption efficiency. By tailoring the orientation of the graphene ribbons with respect to an array of subwavelength bimetallic electrodes, we achieve a condition in which the plasmonic mode can be efficiently excited by an incident wave polarized perpendicular to the electrode array, while the resulting photothermal voltage can be observed between the outermost electrodes.
RESUMO
We report here a new type of plasmon resonance that occurs when graphene is connected to a metal. These new plasmon modes offer the potential to incorporate a tunable plasmonic channel into a device with electrical contacts, a critical step toward practical graphene terahertz optoelectronics. Through theory and experiments, we demonstrate, for example, anomalously high resonant absorption or transmission when subwavelength graphene-filled apertures are introduced into an otherwise conductive layer. These tunable plasmon resonances are essential yet missing ingredients needed for terahertz filters, oscillators, detectors, and modulators.
RESUMO
Femtosecond-THz optical pump probe spectroscopy is employed to investigate the cooling dynamics of hot carriers in quasi-free standing bilayer epitaxial graphene with hydrogen interacalation. We observe longer decay time constants, in the range of 2.6 to 6.4 ps, compared to previous studies on monolayer graphene, which increase nonlinearly with excitation intensity. The increased relaxation times are due to the decoupling of the graphene layer from the SiC substrate after hydrogen intercalation which increases the distance between graphene and substrate. Furthermore, our measurements show that the supercollision mechanism is not related to the cooling process of the hot carriers, which is ultimately achieved by electron optical phonon scattering.
RESUMO
Strong circularly polarized excitation opens up the possibility to generate and control effective magnetic fields in solid state systems, e.g., via the optical inverse Faraday effect or the phonon inverse Faraday effect. While these effects rely on material properties that can be tailored only to a limited degree, plasmonic resonances can be fully controlled by choosing proper dimensions and carrier concentrations. Plasmon resonances provide new degrees of freedom that can be used to tune or enhance the light-induced magnetic field in engineered metamaterials. Here we employ graphene disks to demonstrate light-induced transient magnetic fields from a plasmonic circular current with extremely high efficiency. The effective magnetic field at the plasmon resonance frequency of the graphene disks (3.5 THz) is evidenced by a strong ( ~ 1°) ultrafast Faraday rotation ( ~ 20 ps). In accordance with reference measurements and simulations, we estimated the strength of the induced magnetic field to be on the order of 0.7 T under a moderate pump fluence of about 440 nJ cm-2.
RESUMO
The initial stages of epitaxial graphene growth were studied by characterization of graphene formed in localized areas on C-face 6H-SiC substrates. The graphene areas were determined to lie below the level of the surrounding substrate and showed different morphologies based on size. Employing electron channeling contrast imaging, the presence of threading screw dislocations was indicated near the centers of each of these areas. After the graphene was removed, these dislocations were revealed to lie within the SiC substrate. These observations suggest that screw dislocations act as preferred nucleation sites for graphene growth on C-face SiC.
RESUMO
We report the rapid detection of SARS-CoV-2 in infected patients (mid-turbinate swabs and exhaled breath aerosol samples) in concentrations as low as 60 copies/mL of the virus in seconds by electrical transduction of the SARS-CoV-2 S1 spike protein antigen via SARS-CoV-2 S1 spike protein antibodies immobilized on bilayer quasi-freestanding epitaxial graphene without gate or signal amplification. The sensor demonstrates the spike protein antigen detection in a concentration as low as 1 ag/mL. The heterostructure of the SARS-CoV-2 antibody/graphene-based sensor is developed through a simple and low-cost fabrication technique. Furthermore, sensors integrated into a portable testing unit distinguished B.1.1.7 variant positive samples from infected patients (mid-turbinate swabs and saliva samples, 4000-8000 copies/mL) with a response time of as fast as 0.6 s. The sensor is reusable, allowing for reimmobilization of the crosslinker and antibodies on the biosensor after desorption of biomarkers by NaCl solution or heat treatment above 40 °C.
Assuntos
Técnicas Biossensoriais , COVID-19 , Grafite , Humanos , SARS-CoV-2RESUMO
We report the first observation of linear magnetoresistance (LMR) in multilayer epitaxial graphene grown on SiC. We show that multilayer epitaxial graphene exhibits large LMR from 2.2 K up to room temperature and that it can be best explained by a purely quantum mechanical model. We attribute the observation of LMR to inhomogeneities in the epitaxially grown graphene film. The large magnitude of the LMR suggests potential for novel applications in areas such as high-density data storage and magnetic sensors and actuators.
RESUMO
We present the first microscopic transport study of epitaxial graphene on SiC using an ultrahigh vacuum four-probe scanning tunneling microscope. Anisotropic conductivity is observed that is caused by the interaction between the graphene and the underlying substrate. These results can be explained by a model where charge buildup at the step edges leads to local scattering of charge carriers. This highlights the importance of considering substrate effects in proposed devices that utilize nanoscale patterning of graphene on electrically isolated substrates.
Assuntos
Compostos Inorgânicos de Carbono/química , Cristalização/métodos , Grafite/química , Microscopia de Varredura por Sonda/métodos , Nanoestruturas/química , Nanoestruturas/ultraestrutura , Nanotecnologia/métodos , Compostos de Silício/química , Condutividade Elétrica , Substâncias Macromoleculares/química , Teste de Materiais , Conformação Molecular , Tamanho da Partícula , Propriedades de SuperfícieRESUMO
Point defects in SiC are an attractive platform for quantum information and sensing applications because they provide relatively long spin coherence times, optical spin initialization, and spin-dependent fluorescence readout in a fabrication-friendly semiconductor. The ability to precisely place these defects at the optimal location in a host material with nano-scale accuracy is desirable for integration of these quantum systems with traditional electronic and photonic structures. Here, we demonstrate the precise spatial patterning of arrays of silicon vacancy ([Formula: see text]) emitters in an epitaxial 4H-SiC (0001) layer through mask-less focused ion beam implantation of Li+. We characterize these arrays with high-resolution scanning confocal fluorescence microscopy on the Si-face, observing sharp emission lines primarily coming from the [Formula: see text] zero-phonon line (ZPL). The implantation dose is varied over 3 orders of magnitude, leading to [Formula: see text] densities from a few per implantation spot to thousands per spot, with a linear dependence between ZPL emission and implantation dose. Optically-detected magnetic resonance (ODMR) is also performed, confirming the presence of V2 [Formula: see text]. Our investigation reveals scalable and reproducible defect generation.
RESUMO
We report a direct correlation between carrier mobility and Raman topography of epitaxial graphene (EG) grown on silicon carbide (SiC). We show the Hall mobility of material on SiC(0001) is highly dependent on thickness and monolayer strain uniformity. Additionally, we achieve high mobility epitaxial graphene (18100 cm(2)/(V s) at room temperature) on SiC(0001) and show that carrier mobility depends strongly on the graphene layer stacking.
RESUMO
Control of atomic-scale interfaces between materials with distinct electronic structures is crucial for the design and fabrication of most electronic devices. In the case of two-dimensional materials, disparate electronic structures can be realized even within a single uniform sheet, merely by locally applying different vertical gate voltages. Here, we utilize the inherently nano-structured single layer and bilayer graphene on silicon carbide to investigate lateral electronic structure variations in an adjacent single layer of tungsten disulfide (WS2). The electronic band alignments are mapped in energy and momentum space using angle-resolved photoemission with a spatial resolution on the order of 500 nm (nanoARPES). We find that the WS2 band offsets track the work function of the underlying single layer and bilayer graphene, and we relate such changes to observed lateral patterns of exciton and trion luminescence from WS2.
RESUMO
Metal films deposited on graphene are known to influence its electronic properties, but little is known about graphene's interactions with very low work function rare earth metals. Here we report on the work functions of a wide range of metals deposited on n-type epitaxial graphene (EG) as measured by Kelvin Probe Force Microscopy (KPFM). We compare the behaviors of rare earth metals (Pr, Eu, Er, Yb, and Y) with commonly used noble metals (Cr, Cu, Rh, Ni, Au, and Pt). The rare earth films oxidize rapidly, and exhibit unique behaviors when on graphene. We find that the measured work function of the low work function group is consistently higher than predicted, unlike the noble metals, which is likely due to rapid oxidation during measurement. Some of the low work function metals interact with graphene; for example, Eu exhibits bonding anomalies along the metal-graphene perimeter. We observe no correlation between metal work function and photovoltage, implying the metal-graphene interface properties are a more determinant factor. Yb emerges as the best choice for future applications requiring a low-work function electrical contact on graphene. Yb films have the strongest photovoltage response and maintains a relatively low surface roughness, ~5 nm, despite sensitivity to oxidation.