Inkyung Song

Homogeneous and stable p-type doping of graphene by MeV electron beam-stimulated hybridization with ZnO thin films

In this work, we demonstrate a unique and facile methodology for the homogenous and stable p-type doping of graphene by hybridization with ZnO thin films fabricated by MeV electron beam irradiation (MEBI) under ambient conditions. The formation of the ZnO/graphene hybrid nanostructure was attributed to MEBI-stimulated dissociation of zinc acetate dihydrate and a subsequent oxidation process. A ZnO thin film with an ultra-flat surface and uniform thickness was formed on graphene. We found that homogeneous and stable p-type doping was achieved by charge transfer from the graphene to the ZnO film.

Direct momentum-resolved observation of one-dimensional confinement of externally doped electrons within a single subnanometer-scale wire.

Cutting-edge research in the band engineering of nanowires at the ultimate fine scale is related to the minimum scale of nanowire-based devices. The fundamental issue at the subnanometer scale is whether angle-resolved photoemission spectroscopy (ARPES) can be used to directly measure the momentum-resolved electronic structure of a single wire because of the difficulty associated with assembling single wire into an ordered array for such measurements. Here, we demonstrated that the one-dimensional (1D) confinement of electrons, which are transferred from external dopants, within a single subnanometer-scale wire (subnanowire) could be directly measured using ARPES. Convincing evidence of 1D electron confinement was obtained using two different gold subnanowires with characteristic single metallic bands that were alternately and spontaneously ordered on a stepped silicon template, Si(553). Noble metal atoms were adsorbed at room temperature onto the gold subnanowires while the overall structure of the wires was maintained. Only one type of gold subnanowire could be controlled using external noble metal dopants without transforming the metallic band of the other type of gold subnanowires. This result was confirmed by scanning tunnelling microscopy experiments and first-principles calculations. The selective control clearly showed that externally doped electrons could be confined within a single gold subnanowire. This experimental evidence was used to further investigate the effects of the disorder induced by external dopants on a single subnanowire using ARPES.

Indium-induced triple-period atomic wires on a vicinal Si(111) surface: In/Si(557)

An indium-induced one-dimensional (1D) surface reconstruction on a Si(557) surface was studied by the combined approach of scanning tunneling microscopy (STM) and first principles calculations. Low-energy electron diffraction revealed a (1◊3) phase with a triple-period along the step edge direction, which was also confirmed by STM. The STM images showed that the 1D structure consists of two atomic chains. One is located on the terrace and consists of triple-period bright protrusions. The other shows a weak ◊3 modulation at the step edge. Five atomic structure models based on the In adatom of a In/Si(111)- p 3◊ p 3 surface were considered to figure out the underlying structure of the STM images of the In/Si(557)-1◊3 surface. Interestingly, a heterogeneous In-Si adatom chain model reproduced most of the features of STM images and was the most stable energetically at a wide range of In chemical potential.

Influence of graphene-substrate interactions on configurations of organic molecules on graphene: Pentacene/epitaxial graphene/SiC

Pentacene has been used widely in organic devices, and the interface structure between pentacene and a substrate is known to significantly influence device performances. Here we demonstrate that molecular ordering of pentacene on graphene depends on the interaction between graphene and its underlying SiC substrate. The adsorption of pentacene molecules on zero-layer and single-layer graphene, which were grown on a Sifaced 6H-SiC(0001) wafer, was studied using scanning tunneling microscopy (STM). Pentacene molecules form a quasi-amorphous layer on zero-layer graphene which interacts strongly with the underlying SiC substrate. In contrast, they form a uniformly ordered layer on the single-layer graphene having a weak graphene-SiC interaction. Furthermore, we could change the configuration of pentacene molecules on the singlelayer graphene by using STM tips. The results suggest that the molecular ordering of pentacene on graphene and the pentacene/graphene interface structure can be controlled by a graphene-substrate interaction.

P-Type Doping of Graphene Films by Hybridization with Nickel Nanoparticles

Here, we demonstrate the decoration of Ni nanoparticles (NPs) on graphene films by simple annealing for p-type doping of graphene. Scanning electron microscopy and atomic force microscopy revealed that high-density, uniformly sized Ni NPs were formed on the graphene films. The density of the Ni NPs increased gradually, whereas the size of the Ni NPs decreased with increasing NiCl26H2O solution concentration. The formation of Ni NPs on graphene films was explained by heat-driven dechlorination and subsequent nano-particlization, as investigated by X-ray photoelectron spectroscopy. The doping effect of Ni NPs onto graphene films was verified by Raman spectroscopy and electrical transport measurements. This method may provide a facile and universal way to obtain metal NPs on graphene if the metal forms a compound with Cl.

Spontaneous assembly of ordered atomic wires with a long interwire distance on a stepped atomic template

Indium atomic wires with a long interwire distance of 5.73 nm were ordered spontaneously at room temperature on a stepped atomic template using a Si(557) surface. The long interwire distance is very interesting because, in general, interwire interactions are needed to order atomic wires in such a way that ordered atomic wires have a short interwire distance of just a few A. The Si(557) surface is composed of four steps, i.e., one (111) step and three (112) steps, with a very similar local structure to each other. However, mobile indium atoms at room temperature were adsorbed specifically onto the second Si(112) step while maintaining the overall structure of the stepped atomic template, as observed by scanning tunneling microscopy, which results in the ordered atomic wires with the long interwire distance. This was supported by first-principles calculations.