Exfoliated Bilayer Graphene Research Articles

Exploring the Role of Molecular Intercalants in Electrochemical Production of Graphene-Based 2D Materials

Since the discovery of graphene, two-dimensional (2D) materials have gained extensive attention due to the unique material properties from their reduced dimensionality. Numerous techniques have been thoroughly investigated for the fabrication of graphene-based 2D materials. Among them, electrochemical synthesis is one of the most promising approaches for the large-scale production of 2D materials with tunable properties [1-3]. Especially, intercalation of various ions/molecules electrochemically driven in between the graphene sheets plays the key role in the synthesis of single or few-layer 2D materials. However, the influence of molecular intercalants is still poorly understood.In this work, we studied different types of intercalants (i.e., ClO4 -, PF6 -, BF4-, HSO4 -) and explored the inserting process of molecular anions in-between the weekly-bonded layered materials driven by electrochemistry. Combining the cyclic voltammetry, XRD, Raman and density functional theory computation modeling, we can monitor the intercalation behavior of different anions and evaluate the contribution of intercalants in the high-yield production of graphene by electrochemical exfoliation. By optimizing the use of certain molecular intercalants, high exfoliation yield (>80 %) of graphene nanosheets with moderate oxidation damage and good electrochemical stability was achieved assisted by electrochemical exfoliation. Our study provides new insights into the electrochemical production of graphene-based 2D materials and suggests that the introduction of molecular intercalants is a promising tool for the controlled synthesis of layered 2D materials.Reference:(1) Xia, Z. Y.; Pezzini, S.; Treossi, E.; Giambastiani, G.; Corticelli, F.; Morandi, V.; Zanelli, A.; Bellani, V.; Palermo, V. The Exfoliation of Graphene in Liquids by Electrochemical, Chemical, and Sonication-Assisted Techniques: A Nanoscale Study. Advanced Functional Materials 2013, 23 (37), 4684-4693. DOI: 10.1002/adfm.201203686.(2) Xia, Z. Y.; Maccaferri, G.; Zanardi, C.; Christian, M.; Ortolani, L.; Morandi, V.; Bellani, V.; Kovtun, A.; Dell'Elce, S.; Candini, A.; et al. Dispersion Stability and Surface Morphology Study of Electrochemically Exfoliated Bilayer Graphene Oxide. Journal of Physical Chemistry C 2019, 123 (24), 15122-15130. DOI: 10.1021/acs.jpcc.9b03395.(3) Cao, J. Y.; He, P.; Mohammed, M. A.; Zhao, X.; Young, R. J.; Derby, B.; Kinloch, I. A.; Dryfe, R. A. W. Two-Step Electrochemical Intercalation and Oxidation of Graphite for the Mass Production of Graphene Oxide. Journal of the American Chemical Society 2017, 139 (48), 17446-17456. DOI: 10.1021/jacs.7b08515.

Dispersion Stability and Surface Morphology Study of Electrochemically Exfoliated Bilayer Graphene Oxide

During the last decade, electrochemical exfoliation of graphite has aroused great interest from both academia and industry for mass production of graphene sheets. Electrochemically exfoliated graphene oxide (EGO) features a much better tunability than chemically EGO, or even than graphene obtained with ultrasonic exfoliation. Chemical and electrical properties of EGO can be modified extensively, thanks to its step-controllable oxidation process, varying the electrolytes and/or the applied potential. It is thus possible, using tunable electrochemical oxidation, to produce low-defect EGO sheets, featuring both good electric conductivity and good dispersibility in common solvents (e.g., acetonitrile or isopropanol). This greatly facilitates its application in several fields, for example, in flexible electronics. In this work, we correlate the dispersion behavior of EGO with its chemical properties using the relative Hansen solubility parameter, zeta potential values, X-ray photoemission spectroscopy, and Raman analysis. A surface morphology study by atomic force microscopy and transmission electron microscopy analyses also reveals that EGO sheets are multiple structures of (partially oxidized) graphene bilayers. Conductive EGO films could be easily prepared by vacuum filtration on different substrates, obtaining electrical conductivity values of up to ∼104 S/m with no need for further reduction procedures.

Phonon stiffen and soften at zigzag- and armchair-dominated edges of exfoliated bilayer graphene ribbon presented by Raman spectra

The Raman spectra at the edge of the exfoliated bilayer graphene ribbon (GR) were investigated in detail. Results show that both G and 2D phonons stiffen (wave number increases) at zigzag-dominated edge, while they soften at armchair-dominated edge compared with those at the middle position in the GR. Furthermore, the full widths at half maximum intensity of both G and 2D Raman peaks narrow at the zigzag-dominated edge, while they broaden at the armchair-dominated edge. The stiffness and softness are attributed to the C–C bonds at the edge. For zigzag-dominated edge, the stiffness may originate in the increase of the force constant induced by the shrinking of C–C bond. For armchair-dominated edge, the softness may be due to the decrease of the force constant induced by the unsaturated hanging bonds at edge, which is different from Kohn anomaly and charge doping. The analysis is in agreement well with others calculation results about C–C bonds and the edge energy. These results may be useful to understand physical properties at the bilayer graphene edge and for applications in the device by taking advantage of the edge states in bilayer graphene.

Manipulation of electrical properties in CVD-grown twisted bilayer graphene induced by dissociative hydrogen adsorption

We report hydrogen adsorption on twisted bilayer graphene (tBLG). Raman spectroscopy and the electrical transport properties (electrical resistance and thermoelectric power) confirm the electron doping by hydrogen adsorption, in agreement with the previous report involving exfoliated bilayer graphene (BLG). Common electron doping behaviors were observed at various twist angles (0°, 5°, 12.5°, and 30°), and the adsorptions follow the first-order Langmuir-type adsorption model. Specifically, we analyzed the off-state currents, with band-gap openings of around 13 meV in tBLG with twist angle of 0°, as in Bernal-stacked BLG.

Strain Induced Intrinsic Band Gap in Chemically Exfoliated Bilayer Graphene

Strain Induced Intrinsic Band Gap in Chemically Exfoliated Bilayer Graphene

Anomalous spectral features of a neutral bilayer graphene.

Graphene and its bilayer are two-dimensional systems predicted to show exciting many-body effects near the neutrality point. The ideal tool to investigate spectrum reconstruction effects is angle-resolved photoemission spectroscopy (ARPES) as it probes directly the band structure with information about both energy and momentum. Here we reveal, by studying undoped exfoliated bilayer graphene with ARPES, two essential aspects of its many-body physics: the electron-phonon scattering rate has an anisotropic k-dependence and the type of electronic liquid is non-Fermi liquid. The latter behavior is evident from an observed electron-electron scattering rate that scales linearly with energy from 100 meV to 600 meV and that is associated with the proximity of bilayer graphene to a two-dimensional quantum critical point of competing orders.

Reexamination of basal plane thermal conductivity of suspended graphene samples measured by electro-thermal micro-bridge methods

Thermal transport in suspended graphene samples has been measured in prior works and this work with the use of a suspended electro-thermal micro-bridge method. These measurement results are analyzed here to evaluate and eliminate the errors caused by the extrinsic thermal contact resistance. It is noted that the room-temperature thermal resistance measured in a recent work increases linearly with the suspended length of the single-layer graphene samples synthesized by chemical vapor deposition (CVD), and that such a feature does not reveal the failure of Fourier’s law despite the increase in the reported apparent thermal conductivity with length. The re-analyzed apparent thermal conductivity of a single-layer CVD graphene sample reaches about 1680 ± 180 W m−1 K−1 at room temperature, which is close to the highest value reported for highly oriented pyrolytic graphite. In comparison, the apparent thermal conductivity values measured for two suspended exfoliated bi-layer graphene samples are about 880 ± 60 and 730 ± 60 Wm−1K−1 at room temperature, and approach that of the natural graphite source above room temperature. However, the low-temperature thermal conductivities of these suspended graphene samples are still considerably lower than the graphite values, with the peak thermal conductivities shifted to much higher temperatures. Analysis of the thermal conductivity data reveals that the low temperature behavior is dominated by phonon scattering by polymer residue instead of by the lateral boundary.

Bilayer graphene quantum dot defined by topgates

We investigate the application of nanoscale topgates on exfoliated bilayer graphene to define quantum dot devices. At temperatures below 500 mK, the conductance underneath the grounded gates is suppressed, which we attribute to nearest neighbour hopping and strain-induced piezoelectric fields. The gate-layout can thus be used to define resistive regions by tuning into the corresponding temperature range. We use this method to define a quantum dot structure in bilayer graphene showing Coulomb blockade oscillations consistent with the gate layout.

Observation of Quantum Hall Effect and weak localization in p-type bilayer epitaxial graphene on SiC(0001)

We investigated the magnetotransport properties of a bilayer graphene Hall bar sample epitaxially grown on Si-terminated silicon carbide. Integer quantum Hall effect and weak localization effect were observed. From the weak localization effect, the carrier scattering and phase coherence were extracted. The extracted phase coherence length is 0.5μm at 2K, which is comparable with that of the exfoliated graphene. The temperature dependence of the phase coherence rate shows that electron–electron interaction is the main inelastic-scattering factor and direct Coulomb interaction also exists. Compared with exfoliated bilayer graphene, no sign of weak antilocalization was observed at such high carrier concentration of 1013cm−2 due to the strong intervally scattering. Both the strong intervally scattering and Coulomb interaction of phase coherence are due to the large amount of atomically sharp defects in epitaxial graphene on Si-terminated SiC.

Unraveling Quantum Hall Breakdown in Bilayer Graphene with Scanning Gate Microscopy

Investigating the structure of quantized plateaus in the Hall conductance of graphene is a powerful way of probing its crystalline and electronic structure and will also help to establish whether graphene can be used as a robust standard of resistance for quantum metrology. We use low-temperature scanning gate microscopy to image the interplateau breakdown of the quantum Hall effect in an exfoliated bilayer graphene flake. Scanning gate images captured during breakdown exhibit intricate patterns where the conductance is strongly affected by the presence of the scanning probe tip. The maximum density and intensity of the tip-induced conductance perturbations occur at half-integer filling factors, midway between consecutive quantum Hall plateau, while the intensity of individual sites shows a strong dependence on tip-voltage. Our results are well-described by a model based on quantum percolation which relates the points of high responsivity to tip-induced scattering in a network of saddle points separating localized states.

High-Yield Chemical Vapor Deposition Growth of High-Quality Large-Area AB-Stacked Bilayer Graphene

Bernal-stacked (AB-stacked) bilayer graphene is of significant interest for functional electronic and photonic devices due to the feasibility to continuously tune its band gap with a vertical electric field. Mechanical exfoliation can be used to produce AB-stacked bilayer graphene flakes but typically with the sizes limited to a few micrometers. Chemical vapor deposition (CVD) has been recently explored for the synthesis of bilayer graphene but usually with limited coverage and a mixture of AB- and randomly stacked structures. Herein we report a rational approach to produce large-area high-quality AB-stacked bilayer graphene. We show that the self-limiting effect of graphene growth on Cu foil can be broken by using a high H(2)/CH(4) ratio in a low-pressure CVD process to enable the continued growth of bilayer graphene. A high-temperature and low-pressure nucleation step is found to be critical for the formation of bilayer graphene nuclei with high AB stacking ratio. A rational design of a two-step CVD process is developed for the growth of bilayer graphene with high AB stacking ratio (up to 90%) and high coverage (up to 99%). The electrical transport studies demonstrate that devices made of the as-grown bilayer graphene exhibit typical characteristics of AB-stacked bilayer graphene with the highest carrier mobility exceeding 4000 cm(2)/V · s at room temperature, comparable to that of the exfoliated bilayer graphene.

Interlayer coupling in rotationally faulted multilayer graphenes

This paper reviews progress in the theoretical modelling of the electronic structure of rotationally faulted multilayer graphenes. In these systems the crystallographic axes of neighbouring layers are misaligned so that the layer stacking does not occur in the Bernal structure observed in three-dimensional graphite and frequently found in exfoliated bilayer graphene. Notably, rotationally faulted graphenes are commonly found in other forms of multilayer graphene including epitaxial graphenes thermally grown on SiC , graphenes grown by chemical vapour deposition, folded mechanically exfoliated graphenes, and graphene flakes deposited on graphite. Rotational faults are experimentally associated with a strong reduction of the energy scale for coherent single particle interlayer motion. The microscopic basis for this reduction and its consequences have attracted significant theoretical attention from several groups that are highlighted in this review.

Infrared Spectroscopy of Wafer-Scale Graphene

We report spectroscopy results from the mid- to far-infrared on wafer-scale graphene, grown either epitaxially on silicon carbide or by chemical vapor deposition. The free carrier absorption (Drude peak) is simultaneously obtained with the universal optical conductivity (due to interband transitions) and the wavelength at which Pauli blocking occurs due to band filling. From these, the graphene layer number, doping level, sheet resistivity, carrier mobility, and scattering rate can be inferred. The mid-IR absorption of epitaxial two-layer graphene shows a less pronounced peak at 0.37 ± 0.02 eV compared to that in exfoliated bilayer graphene. In heavily chemically doped single-layer graphene, a record high transmission reduction due to free carriers approaching 40% at 250 μm (40 cm(-1)) is measured in this atomically thin material, supporting the great potential of graphene in far-infrared and terahertz optoelectronics.

Raman study on the interlayer interactions and the band structure of bilayer graphene synthesized by alcohol chemical vapor deposition

We investigated the electronic band structure and interlayer interactions in graphene synthesized by alcohol-chemical vapor deposition (a-CVD) using microprobe Raman spectroscopy and tight-binding band-structure calculations. The number of graphene layers was determined from the spectrally integrated intensity ratios of the G phonon to 2D phonon peaks. We found that the value of the parameter determining interlayer interactions in a-CVD bilayer graphene was less than half that of exfoliated bilayer graphene. The weak interlayer interaction in a-CVD bilayer graphene was attributed to non-AB stacking order.

Mapping the Dirac point in gated bilayer graphene

We have performed low temperature scanning tunneling spectroscopy measurements on exfoliated bilayer graphene on SiO2. By varying the back gate voltage we observed a linear shift of the Dirac point and an opening of a bandgap due to the perpendicular electric field. In addition to observing a shift in the Dirac point, we also measured its spatial dependence using spatially resolved scanning tunneling spectroscopy. The spatial variation of the Dirac point was not correlated with topographic features and therefore we attribute its shift to random charged impurities.