Victor Yu

Experimental Review of Graphene

This review examines the properties of graphene from an experimental perspective. The intent is to review the most important experimental results at a level of detail appropriate for new graduate students who are interested in a general overview of the fascinating properties of graphene. While some introductory theoretical concepts are provided, including a discussion of the electronic band structure and phonon dispersion, the main emphasis is on describing relevant experiments and important results as well as some of the novel applications of graphene. In particular, this review covers graphene synthesis and characterization, field-effect behavior, electronic transport properties, magnetotransport, integer and fractional quantum Hall effects, mechanical properties, transistors, optoelectronics, graphene-based sensors, and biosensors. This approach attempts to highlight both the means by which the current understanding of graphene has come about and some tools for future contributions.

Raman spectroscopy of the internal strain of a graphene layer grown on copper tuned by chemical vapor deposition

Strain can be used as an alternate way to tune the electronic properties of graphene. Here we demonstrate that it is possible to tune the uniform strain of graphene simply by changing the chemical vapor deposition growth temperature of graphene on copper. Due to the cooling of the graphene on copper system, we can induce a uniform compressive strain on graphene. The strain is analyzed by Raman spectroscopy, where a shift in the 2D peak is observed and compared to our ab initio calculations of the graphene on copper system as a function of strain.

Large contrast enhancement of graphene monolayers by angle detection

Exfoliated graphene monolayers are identified by optical inspection. In order to improve the monolayer detection, we investigate the angle dependence of the optical contrast of graphene on a 90 nm SiO2/Si substrate. We observe a significant enhancement of the visibility of graphene by changing the polarization and the angle of optical incidence. This method can be used to detect graphene on arbitrary substrates such as GaAs/AlAs based materials, which have a much cleaner surface.

Probing the experimental phonon dispersion of graphene using12C and13C isotopes

Using very uniform large scale chemical vapor deposition grown graphene transferred onto silicon, we were able to identify 15 distinct Raman lines associated with graphene monolayers. This was possible thanks to a combination of different carbon isotopes and different Raman laser energies and extensive averaging without increasing the laser power. This allowed us to obtain a detailed experimental phonon dispersion relation for many points in the Brillouin zone. We further identified a D+D’ peak corresponding to a double phonon process involving both an interand intra-valley phonon.

Reduced crystallinity and enhanced charge transport by melt annealing of an organic semiconductor on single layer graphene

We report on the effect of the annealing temperature on the crystallization and the electrical properties of the semiconducting polymer poly(3-hexylthiophene) (P3HT) on single layer graphene. Electrical characterization showed that heating the P3HT film above the melting point (Tm) resulted in a higher vertical charge carrier mobility. Grazing incidence X-ray diffraction (GIXD) revealed that the film was actually less crystalline overall, but that it consisted of a much higher number of face-on crystallites. We moreover show that annealing above Tm removes the existing seeds still present in the film at lower temperatures and enhances face-on formation. These results provide a better understanding of the influence of the annealing temperature on polythiophene crystallization on graphene, and it shows that the annealing at higher temperature induces a more favorable crystalline orientation which enhances charge transport, despite the reduction in the overall crystallinity. These results should help in the design of more efficient graphene based organic electronic devices by controlling the crystalline morphology of the semiconducting film.

Weak localization in graphene: theory, simulations, and experiments.

We provide a comprehensive picture of magnetotransport in graphene monolayers in the limit of nonquantizing magnetic fields. We discuss the effects of two-carrier transport, weak localization, weak antilocalization, and strong localization for graphene devices of various mobilities, through theory, experiments, and numerical simulations. In particular, we observe a minimum in the weak localization and strong localization length reminiscent of the minimum in the conductivity, which allows us to make the connection between weak and strong localization. This provides a unified framework for both localizations, which explains the observed experimental features. We compare these results to numerical simulation and find a remarkable agreement between theory, experiment, and numerics. Various graphene devices were used in this study, including graphene on different substrates, such as glass and silicon, as well as low and high mobility devices.

Weak Localisation in Clean and Highly Disordered Graphene

We look at the magnetic field induced weak localisation peak of graphene samples with different mobilities. At very low temperatures, low mobility samples exhibit a very broad peak as a function of the magnetic field, in contrast to higher mobility samples, where the weak localisation peak is very sharp. We analyze the experimental data in the context of the localisation length, which allows us to extract, both the localisation length and the phase coherence length of the samples, regardless of their mobilities. This analysis is made possible by the observation that the localisation length undergoes a generic weak localisation dependence with striking universal properties.

Weak localization in graphene: Experiments and the localization length

We compare the density dependence of the weak localization peak of graphene samples with the one of the computed localization length. The samples show a uniform density dependence of the relative magneto-resistance, which is similar to the computed relative localization length dependence. This analysis is performed for high mobility samples, such as large single crystal flakes as well as low mobility devices and polycrystalline large scale graphene. The magnetic field dependence of the relative localization length exhibits a striking universal behavior.

Graphlocons: large dendritic graphene crystals and their electronic properties

Submillimeter dendritic-shaped crystals were synthesized by low pressure vapor chemical vapor deposition inside a copper enclosure. With their sixfold symmetry and fractal-like shape, the resulting crystals resemble snowflakes. The electronic properties of the devices were investigated down to sub-Kelvin temperatures, showing mobilities over 5000 cm2/Vs and the quantum Hall effect at 8T. The magnetoresistance also displayed a sharp peak at zero field which we attribute to weak localization.