Chia-Hao Chen

Multilength-Scale Chemical Patterning of Self-Assembled Monolayers by Spatially Controlled Plasma Exposure: Nanometer to Centimeter Range

We present a generic and efficient chemical patterning method based on local plasma-induced conversion of surface functional groups on self-assembled monolayers (SAMs). Here, spatially controlled plasma exposure is realized by elastomeric poly(dimethylsiloxane) (PDMS) contact masks or channel stamps with feature sizes ranging from nanometer, micrometer, to centimeter. This chemical conversion method has been comprehensively characterized by a set of techniques, including contact angle measurements, X-ray photoelectron spectroscopy (XPS), scanning photoelectron microscopy (SPEM), scanning electron microscopy (SEM), and scanning Kelvin probe microscopy (SKPM). In particular, XPS and SPEM can be used to distinguish regions of different surface functionalities and elucidate the mechanism of plasma-induced chemical conversion. In the case of an octadecyltrichlorosilane (OTS) monolayer, we show that exposure to low-power air plasma causes hydroxylation and oxidation of the methyl terminal group on an OTS-covered Si surface and generates polar functional groups such as hydroxyl, aldehylde, and carboxyl groups, which can allow subsequent grafting of dissimilar SAMs and adsorption of colloid nanoparticles onto the patterned areas with an achievable resolution down to the 50 nm range.

Enhanced light–matter interaction of graphene–gold nanoparticle hybrid films for high-performance SERS detection

By simply coating graphene films on Au nanoparticles, the optical properties of the hybrid films are investigated. It is found that the coverage of a monolayer graphene film leads to a decreased transmittance of up to 15.8% in the visible range, much higher than the 2.3% transmittance loss for intrinsic graphene. At the same time, the plasmonic resonance of the hybrid films experiences a red-shift in resonance frequency and a broadening in the transmission dip. By means of finite element simulations, these observations are attributed to strong light–matter interaction at the interface between graphene and Au nanoparticles, as indicated by the increased absorption cross section and higher electric field intensity. The electron transfer between graphene and Au nanoparticles is confirmed by high resolution X-ray photoelectron spectroscopy studies. Furthermore, the enhanced electromagnetic hot spots at the interface between graphene and Au nanoparticles make such graphene–Au nanoparticle hybrid films cost-effective and high-performance surface-enhanced Raman scattering substrates for detecting organic molecules such as rhodamine-6G, for which an enhancement factor of ∼107 is achieved.

Is electron accumulation universal at InN polar surfaces

Recent experiments indicate the universality of electron accumulation and downward surface band bending at as-grown InN surfaces with polar or nonpolar orientations. Here, we demonstrate the possibility to prepare flatband InN ( 000 1 ¯ ) surfaces. We have also measured the surface stoichiometry of InN surfaces by using core-level photoelectron spectroscopy. The flatband InN ( 000 1 ¯ ) surface is stoichiometric and free of In adlayer. It implies that the removal of In adlayer at the InN ( 000 1 ¯ ) surface leads to the absence of downward surface band bending. On the other hand, the stoichiometric InN (0001) surface still exhibits surface band bending due to the noncentrosymmetry in the wurtzite structure.

Preparation and characterization of an ordered 1-dodecanethiol monolayer on bare Si(111) surface.

We have grown 1-dodecandthiol (DDT) monolayer on a bare Si(111) surface through ultraviolet-assisted photochemical reaction. The resulting monolayer was investigated by means of water contact angle measurement, synchrotron radiation-based high-resolution X-ray photoelectron spectroscopy, and polarization-dependent near-edge X-ray absorption fine structure spectroscopy. These combined probes for characterization reveal a hydrophobic ambient surface; the DDT was directly attached to Si through a Si-S bond, and the molecules formed an ordered monolayer with an average tilt angle of 57° of the alkyl chains relative to the substrate surface.

Graphene as tunable transparent electrode material on GaN: Layer-number-dependent optical and electrical properties

Graphene has been regarded as a prospective transparent electrode for a GaN-based light-emitting diode (LED), but fundamental knowledge about the intrinsic properties of the graphene/GaN contact is lacking. We have studied the optical and electronic properties of graphene exfoliated on an n-type GaN surface. The graphene visibility was simulated based on Fresnels law and confirmed with an optical microscope and micro-Raman spectra. The interfacial electronic property was studied with a scanning photoelectron microscope. We found that the Schottky barrier height of the graphene/n-GaN is decreased with decreasing graphene number of layers, yielding an improved GaN-based LED performance.

Experimental Determination of Electron Affinities for InN and GaN Polar Surfaces

We have measured the electron affinities of clean, stoichiometric InN and GaN polar surfaces via ultraviolet photoelectron spectroscopy. The electron affinities of InN were measured to be 4.7 and 4.6 eV for In- and N-polar surfaces, respectively. In contrast, the electron affinities of GaN vary greatly with the film polarity, i.e., 3.8 and 3.3 eV for Ga- and N-polar surfaces, respectively. We propose that the difference between polar surfaces originates from the spontaneous polarization effect. Furthermore, its closely related to the film carrier concentration. With the measured electron affinities, we are able to confirm the known polar heterojunction band alignments.

Valence band offset and interface stoichiometry at epitaxial Si3N4/Si(111) heterojunctions formed by plasma nitridation

Ultrathin {beta}-Si{sub 3}N{sub 4}(0001) epitaxial films formed by N{sub 2}-plasma nitridation of Si(111) substrates have been studied by photoelectron spectroscopy using synchrotron radiation. The valence band offset at the {beta}-Si{sub 3}N{sub 4}/Si interface was determined by valence-band photoelectron spectra to be 1.8 eV. Furthermore, the Si 2p core-level emissions were analyzed for nitride (Si{sup 4+}) and subnitride (Si{sup 3+} and Si{sup +}) components to characterize the interface stoichiometry. In contrast to the interfaces formed by ammonia thermal nitridation and N{sub 2}-plasma nitridation at room temperature, the interface formed by N{sub 2}-plasma nitridation at high substrate temperature is very close to subnitride free with an abrupt composition transition.

Mapping polarization induced surface band bending on the Rashba semiconductor BiTeI

Surfaces of semiconductors with strong spin-orbit coupling are of great interest for use in spintronic devices exploiting the Rashba effect. BiTeI features large Rashba-type spin splitting in both valence and conduction bands. Either can be shifted towards the Fermi level by surface band bending induced by the two possible polar terminations, making Rashba spin-split electron or hole bands electronically accessible. Here we demonstrate the first real-space microscopic identification of each termination with a multi-technique experimental approach. Using spatially resolved tunnelling spectroscopy across the lateral boundary between the two terminations, a previously speculated on p-n junction-like discontinuity in electronic structure at the lateral boundary is confirmed experimentally. These findings realize an important step towards the exploitation of the unique behaviour of the Rashba semiconductor BiTeI for new device concepts in spintronics.

Natural band alignments of InN/GaN/AlN nanorod heterojunctions

Valence band alignments of wurtzite III-nitride semiconductorheterojunctions are investigated using cross-sectional scanning photoelectron microscopy and spectroscopy on the nonpolar side-facet of a vertically −c-axis-aligned heterostructurenanorod array. The nonpolar measurement geometry and near fully relaxed lattice structure allow for the determination of “natural” band alignments without the influence of spontaneous and piezoelectricpolarization fields. The valence band offsets of InN/GaN, GaN/AlN, and InN/AlN are measured to be 0.8 ± 0.1, 0.6 ± 0.1, and 1.4 ± 0.1 eV, respectively. These results are in good agreement with previous data for heteroepitaxial films and obey the expected transitivity rule.

Direct imaging of GaN p-n junction by cross-sectional scanning photoelectron microscopy and spectroscopy

uted by them to the existence of high-density steps and defects at the cleavage surface. In this work, we provide direct evidence for the unpinned nature of cleaved a-plane GaN surfaces. Furthermore, we report on the application of XSPEM/S technique for direct imaging of GaN p-n junction. The sample structure for this study is N-polar p-GaN 1.5 m /n-GaN 1.5 m /AlN 25 nm /Si 3 N 4 /Si111, which was grown by plasma-assisted molecular beam epitaxy PA-MBE on a Si wafer. Details of the growth technique can be found elsewhere. 11 The n- and p-type doping was performed by using high-purity Si and Mg solid cells during the PA-MBE growth process. The Mg concentration as determined by secondary ion mass spectroscopy is 7