Low-temperature low-power PECVD synthesis of vertically aligned graphene.

The need for 2D vertical graphene nanosheets (VGNs) is driven by its great potential in diverse energy, electronics, and sensor applications, wherein many cases a low-temperature synthesis is preferred due to requirements of the manufacturing process. Unfortunately, most of today's known methods, including plasma, require either relatively high temperatures or high plasma powers. Herein, we report on a controllable synthesis of VGNs at a pushed down low-temperature boundary for synthesis, the low temperatures (450 °C) and low plasma powers (30 W) using capacitively coupled plasma (CCP) driven by radio-frequency power at 13.56 MHz. The strategies implemented also include unrevealing the role of Nickel (Ni) catalyst thin film on the substrates (Si/Al). It was found that the Ni catalyst on Si/Al initiates the nucleation/growth of VGNs at 450 °C in comparison to the substrates without Ni catalyst. With increasing temperature, the graphene nanosheets become bigger in size, well-structured and well separated. The role of Ni catalysts is hence to boost the growth rate, density, and quality of the growing VGNs. Furthermore, this CCP method can be used to synthesize VGNs at the lowest temperatures possible so far on a variety of substrates and provide new opportunities in the practical application of VGNs.

Laser-based setup for simultaneous measurement of the Seebeck coefficient and electrical conductivity for bulk and thin film thermoelectrics.

In this paper, an original homemade system is presented in detail for the electrical and thermoelectrical characterizations of several types of materials from bulk to thin films. This setup was built using a modulated CO2 laser beam to probe the thermoelectric properties at different depths below the surface. It allows a simultaneous measurement of the electrical conductivity (σ) and the Seebeck coefficient (S), from room temperature up to 250 °C. A commercial sample of Bi2Te3 was first used to validate the Seebeck coefficient measurement. Single crystalline silicon (sc-Si) was used for the uncertainty quantification during the simultaneous measurement of the Seebeck coefficient and the electrical conductivity. At the micrometer scale, thermoelectric characterization of the mesoporous Si (50 µm thickness) was achieved and results gave very promising values (S ≈ 700 µV K-1) for micro-thermo-generator fabrication. In the case of thin film materials, metals (copper and constantan) and oxide thin films (titanium oxide) were also characterized in the in-plane configuration in order to determine the metrology limits of our thermoelectric setup. In this case, a typical sensitivity of about 2µV K-1 was achieved.