Tunable properties of few-layer graphene-N-methylpyrrolidone hybrid structures

Sensitive Transfer-Free Wafer-Scale Graphene Microphones

During the past decades micro-electromechanical microphones have largely taken over the market for portable devices, being produced in volumes of billions yearly. Because performance of current devices is near the physical limits, further miniaturization and improvement of microphones for mobile devices poses a major challenge that requires breakthrough device concepts, geometries, and materials. Graphene is an attractive material for enabling these breakthroughs due to its flexibility, strength, nanometer thinness, and high electrical conductivity. Here, we demonstrate that transfer-free 7 nm thick multilayer graphene (MLGr) membranes with diameters ranging from 85-155 to 300 μm can be used to detect sound and show a mechanical compliance up to 92 nm Pa^-1, thus outperforming commercially available MEMS microphones of 950 μm with compliances around 3 nm Pa^-1. The feasibility of realizing larger membranes with diameters of 300 μm and even higher compliances is shown, although these have lower yields. We present a process for locally growing graphene on a silicon wafer and realizing suspended membranes of patterned graphene across through-silicon holes by bulk micromachining and sacrificial layer etching, such that no transfer is required. This transfer-free method results in a 100% yield for membranes with diameters up to 155 μm on 132 fabricated drums. The device-to-device variations in the mechanical compliance in the audible range (20-20000 Hz) are significantly smaller than those in transferred membranes. With this work, we demonstrate a transfer-free method for realizing wafer-scale multilayer graphene membranes that is compatible with high-volume manufacturing. Thus, limitations of transfer-based methods for graphene microphone fabrication such as polymer contamination, crack formation, wrinkling, folding, delamination, and low-tension reproducibility are largely circumvented, setting a significant step on the route toward high-volume production of graphene microphones.

New graphene derivative with N-methylpyrrolidone: suspension, structural, optical and electrical properties

In the present study, a new graphene derivative created by the interaction of 2-5 nm thick multi-graphene suspension flakes with N-methylpyrrolidone (NMP) was examined. It was found that the conductivity of the films prepared from the suspensions obtained using the treatment of initial graphene suspensions in NMP decreased after additional annealing in the temperature range from 60 to 200 °C by six to seven orders of magnitude. The annealing temperature required for this decrease of conductivity shows a decrease with diminishing suspension flake sizes. The obtained high-resistivity films demonstrate low leakage currents (10^-7-10^-8 A cm^-2), an ultra-low charge in the functionalized films (-(1-4) × 10¹⁰ cm^-2), a relatively high dielectric constant of 7.0-9.0, and a breakdown electric-field strength of (2-3) × 10⁵ V cm^-1. The possibility to restore the conductivity to a resistance value of 43 kΩ sq^-1 in the upper layer of the films prepared from the functionalized suspension and the possibility to obtain an unexpectedly high value of electron mobility μ≈ 55-65 cm² V^-1 s^-1 in the recovered few top layers (in comparison with the electron mobility of 0.1-1 cm² V^-1 s^-1 in the films prepared from the initial suspension) was shown. The combination of the useful properties of the films prepared from the functionalized graphene suspension together with the possibility to form heterostructures using the simple chemical recovery of the top layer and the expected flexibility of such heterostructures, makes these films promising for a wide range of applications, including the fabrication of dielectric films for van der Waals heterostructures and structures intended for flexible and printed electronics.