Facile graphene transfer directly to target substrates with a reusable metal catalyst.

High-throughput, roll-to-roll growth and transferring of high-quality, large-area chemical vapor deposited (CVD) graphene directly onto a target substrate with a reusable metal catalyst is an enabling technology for flexible optoelectronics. We explore the direct transfer via hot lamination of CVD graphene onto a flexible substrate, followed by electrochemical delamination (bubble transfer) of the graphene. The transfer method investigated here does not require any intermediate transfer layer and allows the copper to be reused, which will reduce the production cost and avoid the generation of chemical waste. Such integration is one necessary step forward toward the economical and industrial scale production of graphene. Our method bares promise in various applications. As an example, we fabricated flexible solution-gated graphene field-effect-transistors, which exhibited transconductance as high as 200 µS.

Phonon self-energy corrections to nonzero wave-vector phonon modes in single-layer graphene.

Phonon self-energy corrections have mostly been studied theoretically and experimentally for phonon modes with zone-center (q=0) wave vectors. Here, gate-modulated Raman scattering is used to study phonons of a single layer of graphene originating from a double-resonant Raman process with q≠0. The observed phonon renormalization effects are different from what is observed for the zone-center q=0 case. To explain our experimental findings, we explored the phonon self-energy for the phonons with nonzero wave vectors (q≠0) in single-layer graphene in which the frequencies and decay widths are expected to behave oppositely to the behavior observed in the corresponding zone-center q=0 processes. Within this framework, we resolve the identification of the phonon modes contributing to the G(⋆) Raman feature at 2450 cm(-1) to include the iTO+LA combination modes with q≠0 and also the 2iTO overtone modes with q=0, showing both to be associated with wave vectors near the high symmetry point K in the Brillouin zone.

Using the G' Raman cross-section to understand the phonon dynamics in bilayer graphene systems.

The G' (or 2D) Raman band of AB stacked bilayer graphene comes from a double resonance Raman (DRR) process and is composed of four peaks (P(11), P(12), P(21), and P(22)). In this work, the integrated areas (IA) of these four peaks are analyzed as a function of the laser power for different laser lines. We show that the dependence of the IA of each peak on temperature is different for each distinct laser excitation energy. This special dependence is explained in terms of the electron-phonon coupling and the relaxation of the photon-excited electron. In this DRR process, the electron is scattered by an iTO phonon from a K to an inequivalent K' point of the Brillouin zone. Here, we show that this electron relaxes while in the conduction band before being scattered by an iTO phonon due to the short relaxation time of the excited electron, and the carrier relaxation occurs predominantly by emitting a low-energy acoustic phonon. The different combinations of relaxation processes determine the relative intensities of the four peaks that give rise to the G' band. Some peaks show an increase of their IA at the expense of others, thereby making the IA of the peaks both different from each other and dependent on laser excitation energy and on power level. Also, we report that the IA of the G' mode excited at 532 nm, shows a resonance regime involving ZO' phonons (related to the interlayer breathing mode in bilayer graphene systems) in which a saturation of what we call the P(12) process occurs. This effect gives important information about the electron and phonon dynamics and needs to be taken into account for certain applications of bilayer graphene in the field of nanotechnology.