Somsakul Watcharinyanon

Changes in structural and electronic properties of graphene grown on 6H-SiC(0001) induced by Na deposition

The effects of Na deposited on monolayer graphene on SiC(001) were investigated by synchrotron-based photoelectron spectroscopy and angle resolved photoelectron spectroscopy. The experimental resul ...

A low-energy electron microscopy and x-ray photo-emission electron microscopy study of Li intercalated into graphene on SiC(0001)

The effects induced by the deposition of Li on 1 and 0 ML graphene grown on SiC(0001) and after subsequent heating were investigated using low-energy electron microscopy (LEEM) and x-ray photo-emis ...

Control of Epitaxial Graphene Thickness on 4H-SiC(0001) and Buffer Layer Removal through Hydrogen Intercalation

We report graphene thickness, uniformity and surface morphology dependence on the growth temperature and local variations in the off-cut of Si-face 4H-SiC on-axis substrates. The transformation of the buffer layer through hydrogen intercalation and the subsequent influence on the charge carrier mobility are also studied. A hot-wall CVD reactor was used for in-situ etching, graphene growth in vacuum and the hydrogen intercalation process. The number of graphene layers is found to be dependent on the growth temperature while the surface morphology also depends on the local off-cut in the substrate and results in a non-homogeneous surface. Additionally, the influence of dislocations on surface morphology and graphene thickness uniformity is also presented.

Nanoscale 3-D (E, k x , k y ) band structure imaging on graphene and intercalated graphene

An x-ray photoemission electron microscope (X-PEEM) equipped with a hemispherical energy analyzer is capable of fast acquisition of momentum-resolved photoelectron angular distribution patterns in a complete cone. We have applied this technique to observe the 3-D (E, kx, ky) electronic band structure of zero-, one-, and two-monolayer (ML) graphene grown ex situ on 6H-SiC(0001) substrates where a carbon buffer layer (zero ML) forms underneath the graphene layer(s). We demonstrate that the interfacial buffer layer can be converted into quasi-free-standing graphene upon intercalation of Li atoms at the interface and that such a graphene is structurally and electronically decoupled from the SiC substrate. High energy and momentum resolution of the X-PEEM, along with short data acquisition times from submicrometer areas on the surface demonstrates the uniqueness and the versatility of the technique and broadens its impact and applicability within surface science and nanotechnology.

Soft X-ray Exposure Promotes Na Intercalation in Graphene Grown on Si-Face SiC

An investigation of how electron/photon beam exposures affect the intercalation rate of Na deposited on graphene prepared on Si-face SiC is presented. Focused radiation from a storage ring is used for soft X-ray exposures while the electron beam in a low energy electron microscope is utilized for electron exposures. The microscopy and core level spectroscopy data presented clearly show that the effect of soft X-ray exposure is significantly greater than of electron exposure, i.e., it produces a greater increase in the intercalation rate of Na. Heat transfer from the photoelectrons generated during soft X-ray exposure and by the electrons penetrating the sample during electron beam exposure is suggested to increase the local surface temperature and thus the intercalation rate. The estimated electron flux density is 50 times greater for soft X-ray exposure compared to electron exposure, which explains the larger increase in the intercalation rate from soft X-ray exposure. Effects occurring with time only at room temperature are found to be fairly slow, but detectable. The graphene quality, i.e., domain/grain size and homogeneity, was also observed to be an important factor since exposure-induced effects occurred more rapidly on a graphene sample prepared in situ compared to on a furnace grown sample.

Ytterbium oxide formation at the graphene–SiC interface studied by photoemission

Synchrotron-based core level and angle resolved photoemission spectroscopy was used to study the formation of ytterbium (Yb) oxide at the graphene–SiC substrate interface. Oxide formation at the interface was accomplished in two steps, first intercalation of Yb into the interface region and then oxygen exposure while heating the sample at 260 °C to oxidize the Yb. After these processes, core level results revealed the formation of Yb oxide at the interface. The Yb 4f spectrum showed upon oxidation a clear valence change from Yb2+ to Yb3+. After oxidation the spectrum was dominated by emission from oxide related Yb3+ states and only a small contribution from silicide Yb2+ states remained. In addition, the very similar changes observed in the oxide related components identified in the Si 2p and Yb 4f spectra after oxidation and after subsequent heating suggested formation of a Si-Yb-O silicate at the interface. The electronic band structure of graphene around the K¯-point was upon Yb intercalation found to ...

The registry of graphene layers grown on SiC(000-1)

Graphene samples were grown on the C-face of SiC, at high temperature in a furnace and an Ar ambient, and were investigated using LEEM, XPEEM, LEED, XPS and ARPES. Formation of fairly large grains (crystallographic domains) of graphene exhibiting sharp 1x1 patterns in m-LEED was revealed and that different grains showed different azimuthal orientations. Selective area constant initial energy photoelectron angular distribution patterns recorded showed the same results, ordered grains and no rotational disorder between adjacent layers. A grain size of up to a few mm was obtained on some samples.