Epitaxial graphene on SiC(0001): functional electrical microscopy studies and effect of atmosphere

Modeling Water Interactions with Graphene and Graphite via Force Fields Consistent with Experimental Contact Angles

Accurate simulation models for water interactions with graphene and graphite are important for nanofluidic applications, but existing force fields produce widely varying contact angles. Our extensive review of the experimental literature reveals extreme variation among reported values of graphene-water contact angles and a clustering of graphite-water contact angles into groups of freshly exfoliated (60° ± 13°) and not-freshly exfoliated graphite surfaces. The carbon-oxygen dispersion energy for a classical force field is optimized with respect to this 60° graphite-water contact angle in the infinite-force-cutoff limit, which in turn yields a contact angle for unsupported graphene of 80°, in agreement with the mean of the experimental results. Interaction force fields for finite cutoffs are also derived. A method for calculating contact angles from pressure tensors of planar equilibrium simulations that is ideally suited to graphite and graphene surfaces is introduced. Our methodology is widely applicable to any liquid-surface combination.

Transition of interfacial capacitors in electrowetting on a graphite surface by ion intercalation

The low voltage electrowetting response of a LiCl aqueous solution on a freshly cleaved surface of highly oriented pyrolytic graphite (HOPG) is presented. For applied voltages below 1 V, the energy stored in the electrical double layer (EDL) is insufficient to drive the spreading of the drop due to the pinning of the three phase contact line at the step edges. Electrochemical impedance spectroscopy shows a dramatic increase in capacitance above 1 V, which provides a sufficient electrowetting force for depinning the contact line, resulting in a subsequent decrease of the contact angle. The transition of the interfacial capacitance from the EDL to the many-fold high capacitance of the pseudocapacitor drives the electrowetting transition on the HOPG surface. The observed changes in the capacitances above 1 V are correlated with the cyclic voltammetry and atomic force microscopy results, which show that the Cl- ions intercalate into the graphite galleries upon acquiring sufficient energy to overcome the van der Waals attraction between the graphene layers through the side of the step edge of the basal planes. To the best of our knowledge, this is the first study on the voltage dependent intercalation mediated transition of interfacial capacitance driving the spreading of an aqueous electrolyte drop on the HOPG surface, which provides a fundamental understanding of the mechanism and opens up potential applications in microfluidics and charge storage technologies.

Layer number identification of CVD-grown multilayer graphene using Si peak analysis

Since the successful exfoliation of graphene, various methodologies have been developed to identify the number of layers of exfoliated graphene. The optical contrast, Raman G-peak intensity, and 2D-peak line-shape are currently widely used as the first level of inspection for graphene samples. Although the combination analysis of G- and 2D-peaks is powerful for exfoliated graphene samples, its use is limited in chemical vapor deposition (CVD)-grown graphene because CVD-grown graphene consists of various domains with randomly rotated crystallographic axes between layers, which makes the G- and 2D-peaks analysis difficult for use in number identification. We report herein that the Raman Si-peak intensity can be a universal measure for the number identification of multilayered graphene. We synthesized a few-layered graphene via the CVD method and performed Raman spectroscopy. Moreover, we measured the Si-peak intensities from various individual graphene domains and correlated them with the corresponding layer numbers. We then compared the normalized Si-peak intensity of the CVD-grown multilayer graphene with the exfoliated multilayer graphene as a reference and successfully identified the layer number of the CVD-grown graphene. We believe that this Si-peak analysis can be further applied to various 2-dimensional (2D) materials prepared by both exfoliation and chemical growth.

Tuning epitaxial graphene sensitivity to water by hydrogen intercalation

The effects of humidity on the electronic properties of quasi-free standing one layer graphene (QFS 1LG) are investigated via simultaneous magneto-transport in the van der Pauw geometry and local work function measurements in a controlled environment. QFS 1LG on 4H-SiC(0001) is obtained by hydrogen intercalation of the interfacial layer. In this system, the carrier concentration experiences a two-fold increase in sensitivity to changes in relative humidity as compared to the as-grown epitaxial graphene. This enhanced sensitivity to water is attributed to the lowering of the hydrophobicity of QFS 1LG, which results from spontaneous polarization of 4H-SiC(0001) strongly influencing the graphene. Moreover, the superior carrier mobility of the QFS 1LG system is retained even at the highest humidity. The work function maps constructed from Kelvin probe force microscopy also revealed higher sensitivity to water for 1LG compared to 2LG in both QFS 1LG and as-grown systems. These results point to a new field of applications for QFS 1LG, i.e., as humidity sensors, and the corresponding need for metrology in calibration of graphene-based sensors and devices.

The work function of few-layer graphene

A theoretical and experimental study of the work function of few-layer graphene is reported. The influence of the number of layers on the work function is investigated in the presence of a substrate, a molecular dipole layer, and combinations of the two. The work function of few-layer graphene is almost independent of the number of layers with only a difference between monolayer and multilayer graphene of about 60 meV. In the presence of a charge-donating substrate the charge distribution is found to decay exponentially away from the substrate and this is directly reflected in the work function of few-layer graphene. A dipole layer changes the work function only when placed in between the substrate and few-layer graphene through a change of the charge transfer between the two.

Investigation of the Differential Capacitance of Highly Ordered Pyrolytic Graphite as a Model Material of Graphene

A study of the differences among the capacitances of freshly exfoliated highly ordered pyrolytic graphite (HOPG, sample denoted FEG), HOPG aged in air (denoted AAG), and HOPG aged in an inert atmosphere (hereafter IAG) is presented in this work. The FEG is found to be more hydrophilic than AAG and IAG because the aqueous electrolyte contact angle (CA) increases from 61.7° to 72.5° and 81.8° after aging in Ar and air, respectively. Electrochemical impedance spectroscopy shows the FEG has an intrinsic capacitance (6.0 μF cm^-2 at the potential of minimum capacitance) higher than those of AAG (4.3 μF cm^-2) and IAG (4.7 μF cm^-2). The observed changes in the electrochemical response are correlated with spectroscopic characterization (Raman spectroscopy and X-ray photoelectron spectroscopy), which show that the surface of HOPG was doped or contaminated after exposure to air. Taken together, these changes upon atmospheric exposure are attributed to oxygen molecule, moisture, and airborne organic contaminations: high-vacuum annealing was applied for the removal of the adsorbed contaminants. It was found that annealing the aged sample at 500 °C leads to partial removal of the contaminants, as gauged by the recovery of the measured capacitance. To the best of our knowledge, this is first study of the effect of the airborne contaminants on the capacitance of carbon-based materials.

Carrier type inversion in quasi-free standing graphene: studies of local electronic and structural properties

We investigate the local surface potential and Raman characteristics of as-grown and ex-situ hydrogen intercalated quasi-free standing graphene on 4H-SiC(0001) grown by chemical vapor deposition. Upon intercalation, transport measurements reveal a change in the carrier type from n- to p-type, accompanied by a more than three-fold increase in carrier mobility, up to μh ≈ 4540 cm(2) V(-1) s(-1). On a local scale, Kelvin probe force microscopy provides a complete and detailed map of the surface potential distribution of graphene domains of different thicknesses. Rearrangement of graphene layers upon intercalation to (n + 1)LG, where n is the number of graphene layers (LG) before intercalation, is demonstrated. This is accompanied by a significant increase in the work function of the graphene after the H2-intercalation, which confirms the change of majority carriers from electrons to holes. Raman spectroscopy and mapping corroborate surface potential studies.

Graphene on mica - intercalated water trapped for life

In this work we study the effect of thermal processing of exfoliated graphene on mica with respect to changes in graphene morphology and surface potential. Mild annealing to temperatures of about 200°C leads to the removal of small amounts of intercalated water at graphene edges. By heating to 600°C the areas without intercalated water are substantially increased enabling a quantification of the charge transfer properties of the water layer by locally resolved Kelvin probe force microscopy data. A complete removal on a global scale cannot be achieved because mica begins to decompose at temperatures above 600°C. By correlating Kelvin probe force microscopy and Raman spectroscopy maps we find a transition from p-type to n-type doping of graphene during thermal processing which is driven by the dehydration of the mica substrate and an accumulation of defects in the graphene sheet.

Standardization of surface potential measurements of graphene domains

We compare the three most commonly used scanning probe techniques to obtain a reliable value of the work function in graphene domains of different thickness. The surface potential (SP) of graphene is directly measured in Hall bar geometry via a combination of electrical functional microscopy and spectroscopy techniques, which enables calibrated work function measurements of graphene domains in ambient conditions with values Φ1LG ~4.55 ± 0.02 eV and Φ2LG ~ 4.44 ± 0.02 eV for single- and bi-layer, respectively. We demonstrate that frequency-modulated Kelvin probe force microscopy (FM-KPFM) provides more accurate measurement of the SP than amplitude-modulated (AM)-KPFM. The discrepancy between experimental results obtained by different techniques is discussed. In addition, we use FM-KPFM for contactless measurements of the specific components of the device resistance. We show a strong non-Ohmic behavior of the electrode-graphene contact resistance and extract the graphene channel resistivity.