Multilayer Epitaxial Graphene Research Articles

Modifications of Epitaxial Graphene on SiC for the Electrochemical Detection and Identification of Heavy Metal Salts in Seawater.

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.

The structural modification and magnetism of many-layer epitaxial graphene implanted with low-energy light ions

Modifying the properties of graphene has gained wide interest for a plethora of potential applications, including spintronics. One approach has demonstrated that proton irradiation can induce ferromagnetism in graphene as well as in graphite. However, little is known about how the protons interact with graphene, the mechanism that creates the ferromagnetism, or whether the protons remain in the graphene. Here we report an investigation, broadly relevant to graphitic carbon, using low-energy (360–2000 eV) ions of hydrogen, deuterium, and helium implanted into multilayer epitaxial graphene. Complementary x-ray and neutron reflectivity demonstrate that essentially all of the implanted hydrogen remains chemisorbed in graphene. In situ x-ray diffraction reveals significantly different rates of interlayer expansion of the multilayer graphene. Analysis of these data demonstrates that the interlayer expansion arises entirely from the interstitials created by the ions and not from hydrogen that remains in the graphene. The results also establish a quantitative measure of the layer expansion due to carbon interstitials. Magnetometry and x-ray diffraction studies show that the magnetic moment relates to the amount of interstitial carbon rather than the amount of hydrogen, demonstrating that the induced room-temperature ferromagnetism arises directly from the disrupted bonding of the carbon lattice.

Electron transport property of epitaixial bilayer graphene on SiC substrate

Graphene can find great potential applications in the future electronic devices. In bilayer graphene, the relative rotation angle between graphene layers can modulate the interlayer interaction and hence induces rich physical phenomena. We systematically study the temperature dependent magnetoresistance (MR) properties in the epitaxial bilayer graphene (BLG) grown on the SiC substrate. High quality BLG is synthesized by molecular beam epitaxy in ultra-high vacuum. We observe the negative MR under a small magnetic field applied perpendicularly at temperature < 80 K, which is attributed to a weak localization effect. The weak localization effect in our epitaxial BLG is stronger than previously reported ones in epitaxial monolayer and multilayer graphene system, which is possibly because of the enhanced interlayer electron transition and thus the enhanced valley scattering in the BLG. As the magnetic field increases, the MR exhibits a classical Lorentz MR behavior. Moreover, we observe a linear magnetoresistance behavior in a large field, which shows no saturation for the magnetic field of up to 9 T. In order to further investigate the negative and linear magnetoresistance, we conduct angle-dependent magnetoresistance measurements, which indicates the two-dimensional magnetotransport phenomenon. We also find that the negative MR phenomenon occurs under a parallel magnetic field, which may correspond to the moiré pattern induced local lattice fluctuation as demonstrated by scanning tunneling microscopy measurement on an atomic scale. Our work paves the way for investigating the intrinsic properties of epitaxial BLG under various conditions.

Twistronics in Graphene, from Transfer Assembly to Epitaxy

The twistronics, which is arising from the moiré superlattice of the small angle between twisted bilayers of 2D materials like graphene, has attracted much attention in the field of 2D materials and condensed matter physics. The novel physical properties in such systems, like unconventional superconductivity, come from the dispersionless flat band that appears when the twist reaches some magic angles. By tuning the filling of the fourfold degeneracy flat bands, the desired effects are induced due to the strong correlation of the degenerated Bloch electrons. In this article, we review the twistronics in twisted bi- and multi-layer graphene (TBG and TMG), which is formed both by transfer assembly of exfoliated monolayer graphene and epitaxial growth of multilayer graphene on SiC substrates. Starting from a brief history, we then introduce the theory of flat band in TBG. In the following, we focus on the major achievements in this field: (a) van Hove singularities and charge order; (b) superconductivity and Mott insulator in TBG and (c) transport properties in TBG. In the end, we give the perspective of the rising materials system of twistronics, epitaxial multilayer graphene on the SiC.

Valley and Zeeman Splittings in Multilayer Epitaxial Graphene Revealed by Circular Polarization Resolved Magneto-infrared Spectroscopy.

Circular-polarization-resolved magneto-infrared studies of multilayer epitaxial graphene (MEG) are performed using tunable quantum cascade lasers in high magnetic fields up to 17.5 T. Landau level (LL) transitions in the monolayer and bilayer graphene inclusions of MEG are resolved, and considerable electron-hole asymmetry is observed in the extracted electronic band structure. For monolayer graphene, a four-fold splitting of the n = 0 to n = 1 LL transition is evidenced and attributed to the lifting of the valley and spin degeneracy of the zeroth LL and the broken electron-hole symmetry. The magnetic field dependence of the splitting further reveals its possible mechanisms. The best fit to experimental data yields effective g-factors, gVS* = 6.7 and gZS* = 4.8, for the valley and Zeeman splittings, respectively.

Nanotribological Properties Of Epitaxial Graphene Grown On C-Terminated Face Of Silicon Carbide Semiconductor

The frictional properties of mono-layer and multilayer epitaxial graphene grown on the C terminated face of SiC has been investigated by using atomic force microscopy measurements. Epitaxially grown graphene samples were characterized by Raman spectroscopy measurements. Atomic force microscopy has been employed in ambient conditions for friction measurements using pre-calibrated cantilevers. Both Raman spectroscopy and atomic force microscopy analysis showed that the number of defects, which increases consistent with increasing number of graphene layers, plays an important role on the tribological properties of epitaxial graphene.

Damping of Landau levels in neutral graphene at low magnetic fields: A phonon Raman scattering study

Landau level broadening mechanisms in electrically neutral and quasineutral graphene were investigated through micro-magneto-Raman experiments in three different samples, namely, a natural single-layer graphene flake and a back-gated single-layer device, both deposited over Si/SiO2 substrates, and a multilayer epitaxial graphene employed as a reference sample. Interband Landau level transition widths were estimated through a quantitative analysis of the magnetophonon resonances associated with optically active Landau level transitions crossing the energy of the E_2g Raman-active phonon. Contrary to multilayer graphene, the single-layer graphene samples show a strong damping of the low-field resonances, consistent with an additional broadening contribution of the Landau level energies arising from a random strain field. This extra contribution is properly quantified in terms of a pseudomagnetic field distribution Delta_B = 1.0-1.7 T in our single-layer samples.

Evidence of Fermi level pinning at the Dirac point in epitaxial multilayer graphene

We investigate the temperature-dependent conductivity of epitaxial multilayer graphene using THz time-domain spectroscopy and find evidence that the Fermi level in quasineutral graphene layers is pinned at the Dirac point by midgap states. We demonstrate that the scattering mechanisms result from the interplay between midgap states that dominate in the vicinity of the Dirac point and short-range potentials that govern at higher energies (>8 meV). Our results highlight the potential of multilayer epitaxial graphene for probing low-energy Dirac particles and also for THz optics.

Role of Transient Reflection in Graphene Nonlinear Infrared Optics

Understanding the optical response of graphene at terahertz frequencies is of critical importance for designing graphene-based devices that operate in this frequency range. Here we present a terahertz pump–probe measurement that simultaneously measures both the transmitted and reflected probe radiation from multilayer epitaxial graphene, allowing for an unambiguous determination of the pump-induced absorption change in the graphene layers. The photon energy in the experiment (30 meV) is on the order of the doping level in the graphene, which enables the exploration of the transition from interband to intraband processes, depending on the amount of pump-induced heating. Our findings establish the presence of a large, photoinduced reflection that contributes to the change in sign of the relative transmitted terahertz radiation, which can be purely positive or predominantly negative depending on the pump fluence, while the change in absorption is found to be negative at all fluences. We develop a hot carrier...

Effects of substrate on the domains and electrical properties of epitaxial graphene formed on on-axis C-face 4H-SiC

The effect of substrate surface preparation on the quality of epitaxial graphene grown on on-axis C-face 4H-SiC has been investigated. Structural epitaxial graphene has been achieved using a graphite enclosure. The homogeneity and domain size of epitaxial graphene on different pretreated substrates were studied using Raman spectroscopy, field emission scanning electron microscopy and hall measurements. The etching of C-face SiC substrate using C3H8/H2 is shown to have a pronounced effect on the domain size and hall mobility of multilayer epitaxial graphene. From Raman measurements, it is found that the pretreatment of substrate has a great influence on the domain size of epitaxial graphene. From hall results, it is found that the effect of substrate pretreatment on carrier mobility follows the same tendency reflected in the results of Raman spectra. It is concluded that the pretreatment of substrate surface, mainly the etching for generating high density, regular and periodic step structure with flatter terrace, would allow the growth of graphene with significantly large area and high mobility, even on large on-axis C-face 4H-SiC wafer.

Probing electron–electron interactions in multilayer epitaxial graphene grown on SiC using temperature-dependent Hall slope

Abstract We have studied electron–electron (e–e) interactions in multilayer graphene grown on SiC(0001). We find that the observed logarithmic temperature (ln T) dependence of the Hall slope is a good physical quantity for probing e–e interactions since it is not affected by electron–phonon scattering at high temperatures. By subtracting the weak localization correction term, we are able to study e–e interactions independently. It is found that the interaction correction terms determined by two methods, which both show ln T dependences, agree better with each other in the high-temperature regime. Our approach is applicable to other two-dimensional materials which do not have buckled structures.

Scanning Tunneling Spectroscopy of Proximity Superconductivity in Epitaxial Multilayer Graphene.

We report on spatial measurements of the superconducting proximity effect in epitaxial graphene induced by a graphene-superconductor interface. Superconducting aluminum films were grown on epitaxial multilayer graphene on SiC. The aluminum films were discontinuous with networks of trenches in the film morphology reaching down to exposed graphene terraces. Scanning tunneling spectra measured on the graphene terraces show a clear decay of the superconducting energy gap with increasing separation from the graphene-aluminum edges. The spectra were well described by Bardeen-Cooper-Schrieffer (BCS) theory. The decay length for the superconducting energy gap in graphene was determined to be greater than 400 nm. Deviations in the exponentially decaying energy gap were also observed on a much smaller length scale of tens of nanometers.

Charge Trapping in Monolayer and Multilayer Epitaxial Graphene

We have studied the carrier densitiesnof multilayer and monolayer epitaxial graphene devices over a wide range of temperaturesT. It is found that, in the high temperature regime (typicallyT≥ 200 K),ln⁡(n)shows a linear dependence of 1/T, showing activated behavior. Such results yield activation energiesΔEfor charge trapping in epitaxial graphene ranging from 196 meV to 34 meV. We find thatΔEdecreases with increasing mobility. Vacuum annealing experiments suggest that both adsorbates on EG and the SiC/graphene interface play a role in charge trapping in EG devices.

Weak localization and microwave-irradiated transport in multilayer epitaxial graphene grown on SiC

We have studied weak localization (WL) and microwave-irradiated transport in multilayer graphene grown on SiC(0001). Different scattering channels are identified by analyzing the WL data. Moreover, we have shown that at a fixed ambient temperature, irradiating graphene with a microwave appears to be equivalent to changing the ambient temperature without microwave. We find that both the zero-field resistance of graphene and the WL correction term can be used as reliable thermometers which agree well with each other.

Near-edge X-ray absorption spectroscopy signature of image potential states in multilayer epitaxial graphene

Single layer behavior in multilayer epitaxial graphene has been a matter of intense investigation. This is due to the layer decoupling that occurs during growth of graphene on some types of substrates, such as carbon-terminated silicon carbide. We show here that near-edge X-ray absorption spectroscopy can be used to observe the signature of this decoupling. To this end, samples of multilayer graphene from silicon carbide sublimation were grown with different degrees of decoupling. Raman spectroscopy was used to infer the degree of structural decoupling. X-ray grazing-incidence diffraction and scanning tunneling microscopy showed that growth initiates with the presence of bilayer graphene commensurate structures, while layer decoupling is associated to the formation of incommensurate structures observed for longer sublimation time. Near-edge X-ray absorption spectroscopy was used to probe the electronic states above the Fermi energy. Besides the σ* and π* empty states, image potential states are observed and show a clear change of intensity as a function of incident angle. These image potential states evolve from a graphite- to graphene-like behavior as a function of growth time and can be used to infer the degree of structural coupling among layers.

Electronic cooling via interlayer Coulomb coupling in multilayer epitaxial graphene.

In van der Waals bonded or rotationally disordered multilayer stacks of two-dimensional (2D) materials, the electronic states remain tightly confined within individual 2D layers. As a result, electron–phonon interactions occur primarily within layers and interlayer electrical conductivities are low. In addition, strong covalent in-plane intralayer bonding combined with weak van der Waals interlayer bonding results in weak phonon-mediated thermal coupling between the layers. We demonstrate here, however, that Coulomb interactions between electrons in different layers of multilayer epitaxial graphene provide an important mechanism for interlayer thermal transport, even though all electronic states are strongly confined within individual 2D layers. This effect is manifested in the relaxation dynamics of hot carriers in ultrafast time-resolved terahertz spectroscopy. We develop a theory of interlayer Coulomb coupling containing no free parameters that accounts for the experimentally observed trends in hot-carrier dynamics as temperature and the number of layers is varied.

SU(4) symmetry breaking revealed by magneto-optical spectroscopy in epitaxial graphene

Refined infrared magnetotransmission experiments have been performed in magnetic fields B up to 35 T on a series of multilayer epitaxial graphene samples. Following the main optical transition involving the n=0 Landau level (LL), we observe a new absorption transition increasing in intensity with magnetic fields B>26 T. Our analysis shows that this is a signature of the breaking of the SU(4) symmetry of the n=0 LL. Using a quantitative model, we show that the only symmetry-breaking scheme consistent with our experiments is a charge density wave (CDW).

Intraband carrier dynamics in Landau-quantized multilayer epitaxial graphene

We investigate the low-energy carrier dynamics in Landau quantized multilayer epitaxial graphene on SiC, using 14 meV photons. The THz absorption is dominated by Landau-level transitions within the conduction bands of several graphene layers with different doping. Varying the magnetic field allows us to tune the THz-induced response from induced transmission around B = 0 to induced absorption at intermediate fields (1.5 T–3.3 T) and back to induced transmission at higher fields (3.3 T–7 T). The main features of this complex response are explained by a strong dependence of the absorption on the electron temperature. Furthermore a prolonged relaxation at high fields, which is attributed to reduced scattering via optical phonons, is observed.

Synthesis and Characterization of (3-Aminopropyl)Triethoxysilane-Modified Epitaxial Graphene

Electrochemical immunosensor devices comprise of an antibody immobilised onto a semiconducting or conducting substrate. The use of epitaxial graphene in immunosensors allows for the detection of an antigen specifically bound to the immobilised antibody by monitoring the current modulation of lithographically fabricated graphene channel devices. Multilayer epitaxial graphene (MEG) was produced on semi-insulating 4H-SiC(0001) substrates by annealing at 1700°C at 1x 10-5 mbar using a graphite cap. Thickness and morphology of the graphene was studied using Raman spectroscopy, XPS, AFM, and SEM. Selective areas of graphene were targeted for modification by adding a protective window of PMMA. In order to immobilise the antibody to the graphene substrate, an amine-terminated surface is required. (3-aminopropyl) triethoxysilane (APTES), is used to achieve amine termination, which is itself bound to a hydroxyliated graphene surface. Hydroxylation was achieved using Fenton chemistry and changes in surface hydrophobicity are confirmed using contact angle measurements. Attachment of APTES to the hydroxyl terminated graphene channel was confirmed using cyclic voltammetry (CV), XPS, and Raman spectroscopy. This functionalization method can be used to attach any antibody to the graphene substrate that can bind to an amine group. This platform is therefore easily adaptable for the fabrication of a range of immunosensor devices for the detection of different biomarkers.

Generic epitaxial graphene biosensors for ultrasensitive detection of cancer risk biomarker

A generic electrochemical method of ‘bioreceptor’ antibody attachment to phenyl amine functionalized graphitic surfaces is demonstrated. Micro-channels of chemically modified multi-layer epitaxial graphene (MLEG) have been used to provide a repeatable and reliable response to nano-molar (nM) concentrations of the cancer risk (oxidative stress) biomarker 8-hydroxydeoxyguanosine (8-OHdG). X-ray photoelectron spectroscopy, Raman spectroscopy are used to characterize the functionalized MLEG. Confocal fluorescence microscopy using fluorescent-labelled antibodies indicates that the anti-8-OHdG antibody selectively binds to the phenyl amine-functionalized MLEG’s channel. Current–voltage measurements on functionalized channels showed repeatable current responses from antibody–biomarker binding events. This technique is scalable, reliable, and capable of providing a rapid, quantitative, label-free assessment of biomarkers at nano-molar (<20 nM) concentrations in analyte solutions. The sensitivity of the sensor device was investigated using varying concentrations of 8-OHdG, with changes in the sensor’s channel resistance observed upon exposure to 8-OHdG. Detection of 8-OHdG concentrations as low as 0.1 ng ml−1 (0.35 nM) has been demonstrated. This is five times more sensitive than reported enzyme linked immunosorbent assay tests (0.5 ng ml−1).