Gholamreza Yazdi

Surface functionalization and biomedical applications based on SiC

The search for materials and systems, capable of operating long term under physiological conditions, has been a strategy for many research groups during the past years. Silicon carbide (SiC) is a material, which can meet the demands due to its high biocompatibility, high inertness to biological tissues and to aggressive environment, and the possibility to make all types of electronic devices.This paper reviews progress in biomedical and biosensor related research on SiC. For example, less biofouling and platelet aggregation when exposed to blood is taken advantage of in a variety of medical implantable materials while the robust semiconducting properties can be explored in surface functionalized bioelectronic devices.

Organosilane-functionalized wide band gap semiconductor surfaces

Surface functionalization of wide band gap semiconductors, SiC, ZnO, and GaN, with organosilane is reported. Formation of self-assembled monolayers of mercaptopropyltrimethoxysilane is confirmed by x-ray photoelectron spectroscopy and atomic force microscopy. The molecules are adsorbed on the surfaces through the silane groups with the free thiol groups molecularly oriented away from the surface. Moreover, chemisorption via the thiolate is observed for the ZnO surface. Immobilization of a model biomolecule to the functionalized surface is demonstrated. An amino acid derivative, i.e., phosphotyrosine derived thiol, is linked on the functionalized ZnO and GaN surfaces via formation of disulfide bridges.Surface functionalization of wide band gap semiconductors, SiC, ZnO, and GaN, with organosilane is reported. Formation of self-assembled monolayers of mercaptopropyltrimethoxysilane is confirmed by x-ray photoelectron spectroscopy and atomic force microscopy. The molecules are adsorbed on the surfaces through the silane groups with the free thiol groups molecularly oriented away from the surface. Moreover, chemisorption via the thiolate is observed for the ZnO surface. Immobilization of a model biomolecule to the functionalized surface is demonstrated. An amino acid derivative, i.e., phosphotyrosine derived thiol, is linked on the functionalized ZnO and GaN surfaces via formation of disulfide bridges.

Aligned AlN nanowires by self-organized vapor–solid growth

Highly oriented AlN single crystal nanowires with aspect ratio up to 600, diameter in the range of 40-500 nm, and 100 microm lengths, have been synthesized via a vapor-solid growth mechanism. The results were obtained at 1750 degrees C and 850 mbar nitrogen pressure on vicinal SiC substrates pretreated by SiC sublimation epitaxy in order to attain distinguishable terraces. It was found that the nanowires change in thickness after they have reached a critical length, and this fact contributes to an understanding of the growth mechanism of AlN nanowires. The nanowires are hexagonally shaped and perfectly aligned along the [0001] direction with a small tilt given by the substrate vicinality. Under nitrogen excess a preferential growth along the c-axis of the wurtzite structure takes place while below some critical value of nitrogen pressure the growth mode switches to lateral. The AlN nanowires are shown to have a dislocation free wurtzite crystal structure. Some possible applications are discussed.

Heteroepitaxial ZnO nano hexagons on p-type SiC

ZnO single crystal nanohexagons have been grown heteroepitaxially on p-type Si-face 4H-SiC substrates with 8 degrees miscut from to [0 0 0 1] by catalyst-free atmospheric pressure metalorganic chemical vapor deposition and characterized by x-ray diffraction, scanning and transmission electron microscopy as well as energy disperse x-ray and cathodoluminescence analyses. The as-grown ZnO nanohexagons have a pillar shape terminated by a and c plane facets, and are aligned along the growth direction with the epitaxial relation [0 0 0 1](ZnO) parallel to[0 0 0 1](4H-SiC) and [1 0 (1) over bar 0](ZnO) parallel to[1 0 (1) over bar 0](4H-SiC). The ZnO nanohexagons demonstrate intense UV emission (lambda(NBE)=376 nm) and negligible defect-related luminescence.

Aligned AlN nanowires and microrods by self-patterning

Self-patterned AlN microrods and nanowires were grown on 4H-SiC substrate by a physical vapor transport method. AlN hexagonal pyramids were found to be nucleation sites for the evolution of the obs ...

Growth and morphology of AlN crystals

This study focused on growth dependencies, morphological forms and initial nucleation of aluminium nitride (AlN) crystals. Epitaxial layers of AlN have been grown on 4H-SiC substrates by sublimation recondensation in a radio frequency (RF) heated graphite furnace. Both AlN nuclei size and growth rate increased as temperature was increased and decreased as the pressure was increased. The results of these effects are different kinds of surface morphology. We have observed three modes of AlN single crystals: plate-like, columnar and needle-like. Optical microscopy and scanning electron microscopy (SEM) along with atomic force microscopy (AFM) were used to characterize the crystal surface morphology. Cathodoluminescence (CL) and x-ray diffraction (XRD) were applied to determine crystal quality and crystallographic orientation of the grown crystals.

Freestanding AlN single crystals enabled by self-organization of 2H-SiC pyramids on 4H-SiC substrates

A sublimation-recondensation process is presented for high quality AlN (0001) crystals at a high growth rate by employing 4H-SiC substrates with a predeposited epilayer. It is based on the coalesce ...

Tuning the Emission Energy of Chemically Doped Graphene Quantum Dots

Tuning the emission energy of graphene quantum dots (GQDs) and understanding the reason of tunability is essential for the GOD function in optoelectronic devices. Besides material-based challenges, the way to realize chemical doping and band gap tuning also pose a serious challenge. In this study, we tuned the emission energy of GQDs by substitutional doping using chlorine, nitrogen, boron, sodium, and potassium dopants in solution form. Photoluminescence data obtained from (Cl- and N-doped) GQDs and (B-, Na-, and K-doped) GQDs, respectively exhibited red- and blue-shift with respect to the photoluminescence of the undoped GQDs. X-ray photoemission spectroscopy (XPS) revealed that oxygen functional groups were attached to GQDs. We qualitatively correlate red-shift of the photoluminescence with the oxygen functional groups using literature references which demonstrates that more oxygen containing groups leads to the formation of more defect states and is the reason of observed red-shift of luminescence in GQDs. Further on, time resolved photoluminescence measurements of Cl- and N-GQDs demonstrated that Cl substitution in GQDs has effective role in radiative transition whereas in N-GQDs leads to photoluminescence (PL) quenching with non-radiative transition to ground state. Presumably oxidation or reduction processes cause a change of effective size and the bandgap.

Surface engineering of functional materials for biosensors

Single crystal SiC with suitable surface termination and ZnO films on top have been grown. Biomolecular immobilization on the grown surfaces has been performed. APTES on Si terminated SiC surfaces and reaction with a compound containing a pre-activated carboxylic group was used. The rate of success for surface functionalization was monitored by XPS. A successful two-step functionalization including binding of MPA through an amide bond formation, between the amine group of APTES and the carboxylic group in MPA on ZnO epilayers, was shown as a proof of concept for biofunctionalization

Surface Functionalization of SiC for Biosensor Applications

SiC is a biocompatible material and a candidate as a transducer for biosensors. Here we have investigated the possibility to functionalize SiC with biomolecules. We have also processed very simple devices and performed electrical characterization. Double polished SiC samples with a C-face substrate and Si-face low doped epilayer have been functionalized on both sides. The SiC was first treated by HF in order to remove the native oxide, partly successful on the Si-face side but probably not on the C-face side. MPTMS, 3-mercaptopropyl trimethoxysilane, was chosen as the biomolecule since it has both a silanol group to be used as an anchoring group to the substrate and a thiol group available for further linking possibilities. The functionalization was evaluated by XPS, contact angle experiments, AFM and electrical measurements. The MPTMS molecules attached with the thiol (or sulphur containing) group pointing out from the surface on both faces of the SiC. Interesting differences between the two faces are however revealed by the analysis.