Session J12: Focus Session: Graphene: Growth, Mechanical Exfoliation, and Properties - Strain and Structure

Growth-induced electronic properties of epitaxial graphene

The growth of epitaxial graphene on silicon carbide is challenging to understand and control, yet rife with scientific and technological opportunities. This is due in part to different growth-induced structures such as the ``moire'' alignment of graphene layers in multilayer epitaxial graphene on SiC($000\bar{1)}$ and the formation of sidewall ribbons at natural and lithographically-defined SiC(0001) step-bunches (nanofacets). We apply scanning tunneling microscopy (STM) and spectroscopy (STS) to probe the local energy bands of such growth-induced structures. STS at cryogenic temperatures and large magnetic fields creates a comb of discrete Landau level energies that we use to quantitatively characterize the local electronic properties.   

Colossal Corrugations in Freestanding Graphene Observed with STM

The discovery of graphene, a unique two-dimensional electron system with extraordinary physical properties, has ignited tremendous research activity in both science and technology. Graphene interactions with a substrate, such as SiO$_{2}$/Si, are known to significantly degrade the electrical performance of graphene devices. Alternatively, suspending a graphene device eliminates the substrate interaction, thereby yielding a 10-fold increase in mobility. However, a detailed investigation on the microscopic scale explaining the origin of these improvements has yet to be completed. In this talk, we present for the first time atomic-resolution STM images of a freestanding graphene membrane. Samples were prepared by direct CVD growth and by large graphene sheet transfer, both onto a 2000-mesh copper grid. Atomic-scale corrugation amplitudes were observed in perfect registry with, yet 50 times larger than the expected electronic corrugations. Density functional theory revealed that charge localization occurs directly beneath the STM tip due to bond angles rotating away from sp$^{2}$ hybridization as graphene flexes in response to the electrostatic attraction. A detailed model of the 3-way interaction which accounts for the observed behavior will be discussed.   

Melon: A carbon-nitride analog to graphene

Although graphene remains the premier 2-D material, many others have been shown to exist. A close analog to graphene would be a two-dimensional sheet composed of carbon and nitrogen, known as melon. Bulk melon, also known as graphitic carbon-nitride, has been successfully synthesized and shown to be an organic semiconductor with a band-gap around 2.7 eV. We report on the successful synthesis of single layer and few layer melon. The physical and electrical characteristics of this close cousin to graphene will be presented along with the synthesis method.   

AB-stacked multilayer graphene synthesized via chemical vapor deposition: a characterization by hot carrier transport

We report the synthesis of AB-stacked multilayer graphene via ambient pressure chemical vapor deposition on Cu foil. Four-terminal devices were fabricated from such graphene and characterized by hot carrier transport at temperatures down to 240 mK and in magnetic fields up to 14 T. The differential conductance (dI/dV) shows a characteristic dip at longitudinal voltage bias V=0 at low temperatures, indicating the presence of hot electron effect due to a weak electron-phonon coupling. Under magnetic fields, the magnitude of the dI/dV dip diminishes through the enhanced intra-Landau level cyclotron phonon scattering. Our results provide new perspectives in obtaining and understanding AB-stacked multilayer graphene, important for future graphene-based applications.   

Signatures of indentation strain in graphene conductance

We investigate effects on the conductance of a graphene sheet of electron scattering from a localized indentation. Strain in graphene creates effective magnetic fields, and the scattering from a radial strain profile is distinct from other short range scatterers. Through tight-binding calculations, we examine the expected conductance in several experimental geometries, including within graphene nano-ribbons and suspended graphene.   

Transport in Suspended graphene under strain

Suspended graphene devices with graphene flakes were fabricated using acid etching approach, and strained via application of gate voltage. The stretching procedure was observed and verified by in situ SEM imaging. We observe a change in the devices' minimum conductance and mobility values. Latest experimental results will be compared with theoretical models.   

Suppression of Weak-Localization Effect in Strained CVD --grown Graphene

We investigate the magnetic field and temperature-dependent transport properties of CVD-grown graphene subjected to different strains. The graphene is transferred to kapton substrates to which a blending force can be applied. In zero magnetic field, the prefactor to the logarithmic-in-temperature conductivity correction decreases by an approximate factor of 3 for strains as high as 0.6~{\%}. There is also a concomitant decrease in diffusivity by a factor of 7. At 5~K we observe negative magnetoresistance for fields up to 0.2 Tesla followed by positive magnetoresistance at higher fields. We attribute the low field negative magnetoresistance to weak-localization and find that it is well described by theory. The strains resulting from the applied blending force inhibit the intervalley scattering rate more than an order of magnitude, thereby leading to a suppression of weak-localization.   

Probing Mechanical Properties of Chemical Vapor Deposition Graphene Membranes Using Indentation Methods

Recent experimental studies have shown that two-dimensional pristine graphene is the strongest material ever measured. We used Atomic Force Microscopy (AFM) and Agilent G200 nanoindenter to measure the mechanical properties of graphene films obtained by Chemical Vapor Deposition (CVD). CVD graphene with different grain size and number of layers were produced in controlled synthetic conditions and transferred onto silicon dioxide substrate with holes of various diameters. Nano-indentation measurement revealed that stiffness and fracture strength of CVD graphene membranes are similar to those of pristine graphene membranes under the condition that suspended graphene membrane is within a single grain boundary without defects. Furthermore, elastic modulus and fracture strength of multi-layer graphene membranes increase with respect to the number of layers.   

Electron transport measurement of graphene under one-dimensional local strain

Introducing a nonuniform strain is a promising technique for controlling electron transport in graphene. Theories have predicted the formation of band gaps with properly designed strain; however, reports on experimental transport properties of strained graphene are quite limited. In this presentation, we report the measurement of electron transport in graphene under one-dimensional local strain. The local strain was introduced by inserting a one-dimensional dielectric nanorod between a graphene film and its substrate, using a technique reported in [1]. We found that the conductivity across the strained region decreases around the Dirac point in comparison with the unstrained graphene attached to the substrate, although the mobility far from the Dirac point is almost unchanged. The results cannot be explained by the change of the capacitance between the graphene film and the gate electrode, indicating that the strain affects the electron transport. The experimental results on strained and unstrained graphene devices from the same graphene film as well as the numerical results will be discussed. \\[4pt] [1] H. Tomori {\it et al.}, Appl. Phys. Express {\bf 4}, 075102 (2011).   

Introducing designed local strain to graphene using dielectric nanostructures

Strain engineering is a promising method for controlling electron transport in graphene. In this presentation, we report a simple and easy method for inducing designed local strain in graphene films [1]. Nanostructures made of a dielectric material (electron beam resist) are placed between graphene and the substrate, and graphene sections between nanostructures are attached to the substrate. The strength and spatial pattern of the strain can be controlled by the size and the separation of nanostructures. Application of strain is confirmed by the Raman spectroscopy as well as from scanning electron microscope (SEM) images. The Raman 2D peak shows spatially nonuniform downshift, which corresponds to the positions of the resist nanostructures. From SEM images, the maximum stretch of the graphene film reaches about 20\%. This technique can be applied to formation of band gaps in graphene. \\[4pt] [1] H. Tomori {\it et al.}, Appl. Phys. Express {\bf 4}, 075102 (2011).   

Mechanical-Electric-Magnetic Coupling Effects in Black and White Graphenes

Nanoscale multifield couplings can turn very common materials such as carbon and boron nitride into promising functional materials for many device applications. We recently found that the magnetism in graphene nanoribbons on silicon substrates can be tuned linearly by applied bias voltage (\textbf{\textit{Phys.Rev.Lett}}, \textbf{103}, 187204, 2009), and this novel magnetoelectric effect is robust to material and geometry variations. Adsorbed graphene nanoribbons can also create tunable magnetism on silicon surface (\textbf{\textit{Phys.Rev.B}} \textbf{82}, 235423, 2010). Strain tunable magnetism has also been found in defect graphene (\textbf{\textit{ACS Nano}} \textbf{4}, 2124, 2010; \textbf{\textit{Phys. Rev. B}} \textbf{82}, 085425, 2010). Contrast to the zero-gap graphene, Hexagon-BN layers (white graphene) and rolled-up nanotubes are generally insulating, we show that the wide gap in them can be tuned into semiconducting range, even closed in BN nanoribbons by electric fields and narrowed by reduced tube diameter or local curvature radius (\textbf{\textit{Nano Lett.}} \textbf{10}, 5049, 2010; \textbf{\textit{Phys. Rev. B}} \textbf{82}, 035412, 2010). What is more, our recent experiments have demonstrated that flow-induced-voltage in graphene can be 20 folds higher than in graphite (\textbf{\textit{Appl.Phys.Lett}}. 99, 073103 (2011)). Such extraordinary mechanical-electric-magnetic coupling effects in graphene and BN systems open up new vistas in functional devices compatible with silicon-based technology for efficient energy conversion and novel functional systems.