Eunho Kim

Methane as an effective hydrogen source for single-layer graphene synthesis on Cu foil by plasma enhanced chemical vapor deposition

A single-layer graphene is synthesized on Cu foil in the absence of H(2) flow by plasma enhanced chemical vapor deposition (PECVD). In lieu of an explicit H(2) flow, hydrogen species are produced during the methane decomposition process into their active species (CH(x<4)), assisted with the plasma. Notably, the early stage of growth depends strongly on the plasma power. The resulting grain size (the nucleation density) has a maximum (minimum) at 50 W and saturates when the plasma power is higher than 120 W because hydrogen partial pressures are effectively tuned by a simple control of the plasma power. Raman spectroscopy and transport measurements show that decomposed methane alone can provide a sufficient amount of hydrogen species for high-quality graphene synthesis by PECVD.

Intrinsic characteristics of transmission line of graphenes at microwave frequencies

In this paper, we have quantitatively evaluated the effective surface conductivity of chemical vapor deposition-grown graphene through a full-wave electromagnetic method and also investigated the intrinsic characteristics of the transmission line (TL) of the graphene at frequency ranging from 0.5 to 40 GHz. According to the simulated data based on the measured S-parameters, the effective conductivity of single- and multi-layer graphene (MLG) was about 4.3 × 106 S/m and 1.2 × 106 S/m, respectively. Furthermore, we confirm that multi-layer graphene is more suitable for use in transmission lines compared to single-layer graphene in the observed frequency region.

Effects of hydrogen in the cooling step of chemical vapor deposition of graphene

We investigated the effects of hydrogen gas flow rate in the cooling step of CVD on graphene film. We discovered that hydrogen gas in the cooling step influences the quality of graphene. The D peak (defects of graphene) intensity of the Raman spectrum tends to increase with an increasing hydrogen flow rate. Whether methane gas as a hydrocarbon source is maintained during the cooling step or not, the hydrogen flow rate during the cooling step strongly affects the quality of CVD-grown graphene.

Graphene film growth on sputtered thin Cu–Ni alloy film by inductively coupled plasma chemical vapor deposition

Graphene film growth on a Cu–Ni alloy thin film with various alloy compositions is reported in this paper. A magnetron co-sputtered thin film was applied to precisely vary the alloy composition, and graphene film was grown at conventional growth conditions by inductively coupled plasma chemical vapor deposition. A highly uniform single-layer graphene, mostly bilayer graphene with ∼70% coverage, and mostly few-layer graphene were obtained on the Cu–Ni alloys with 8.6, 21, and 34.8 atom% Ni, respectively. The measured resistance decreased from ∼1429 Ω sq−1, to ∼756 Ω sq−1 and to ∼240 Ω sq−1, and the transmittance decreased from 97%, to 95% and to 89%, respectively, as the Ni composition increased. The film thickness of the graphene layers could be controlled by tuning the alloy composition via the co-sputtering process. But the analysis also indicates that, at the same time, the degree of disordered stacking between layers also tends to increase with increasing Ni content in the Cu–Ni film.

Effects of alloying 30 at. % Ni using a Cu catalyst on the growth of bilayer graphene

A small percentage graphene bilayer where the first layer was fully covered with a graphene sheet was grown on alloy foils via an inductively coupled plasma-chemical vapor deposition chamber. Compared with Cu foils, the alloy foils led to faster growth of the graphene film, while maintaining the same quality, homogeneity, and thickness uniformity over the whole area synthesized as the growth characteristics of a monolayer graphene grown on Cu. Moreover, the combined catalyst had a graphene film simultaneously grown with a metallic compound of magnesium silicate in some regions and of 200 nm Cu2.4S in other regions. Nevertheless, graphene was grown continuously and highly homogenously over the entire large area synthesized without boundaries between regions. Thus the resulting graphene growth is affected primarily by the Cu catalyst and partly by the Ni and that the quality of the graphene is dependent on the Cu catalyst.

High frequency transmission properties of graphene monolayer with different coplanar waveguide electrode configurations

We present the transmission properties of graphene monolayer for three different coplanar waveguide electrode configurations at microwave frequencies. It is confirmed that the transmission level can be dependent on the ground shape due to electromagnetic coupling effect between graphene and ground pad in the CPW electrode. Among the three CPW electrodes with and without graphene, the CPW electrode having a deep ground pad shows the most superior transmission characteristics. From the resultant, we know that it is very important to consider the graphene as well as RF electrode design for the desired transmission level.