Fourier-transform infrared and photoluminescence spectroscopies of self-assembled monolayers of long-chain thiols on (001) GaAs J. Appl. Phys. 99, 054701 (2006); 10.1063/1.2178659Passivation of InP surfaces of electronic devices by organothiolated self-assembled monolayers Self-assembled monolayers ͑SAMs͒ of octadecanethiol ͑ODT͒, CH 3 ͑CH 2 ͒ 17 SH, were deposited on GaAs ͑100͒ substrates from liquid and vapor phases. Liquid-phase-deposited SAMs were prepared by immersing the substrate in a dilute solution of ODT and ethanol, while vapor-phase-deposited monolayers were prepared by exposing the GaAs surface to a stream of ODT vapor in an ultrahigh vacuum environment. The structural and optical properties of the resulting SAMs were examined with contact angle ͑CA͒ analysis, photoluminescence ͑PL͒ spectroscopy, high-resolution x-ray photoelectron spectroscopy ͑HRXPS͒, and spectroscopic ellipsometry. Although well-ordered films were formed from both deposition techniques, PL, CA analysis, and ellipsometry measurements revealed that the overall quality, structure, and long-term durability of the resulting SAMs depended on the preparation method. Specifically, time-dependent PL and CA analysis indicated an enhanced stability for vapor-deposited films stored under ambient conditions. Through HRXPS measurements, the attachment of the thiolate molecules to the GaAs substrates was shown to proceed through the formation of chemical bonds at both Ga and As surface sites, with the percentage of each bonding configuration dictated by the surface termination produced via the cleaning process used prior to the SAM deposition. Collectively, the results suggested that more robust monolayers exhibiting greater surface coverage, and therefore increased passivation and stability characteristics, are assembled from vapor phase.

The present work describes the development of a hybrid GaAs-aptamers biosensor for the label-free detection of adenosine 5′-triphosphate (ATP). The implemented sensing strategy relies on the sensitivity of the GaAs photoluminescence (PL) emission to the local environment at its surface. Specifically, GaAs substrates were chemically modified with thiol-derivatized oligonucleotide aptamers following conventional condensed-phase deposition techniques and exposed to the target ATP molecules. The resulting modification in the PL intensity is attributed to a specific biorecognition interaction between the aptamer receptors and the ATP target and, more importantly, the accompanying ligand-induced structural change in the aptamer conformation. Since the negatively charged aptamer probes are covalently anchored to the substrate surface, the sensing mechanism can be understood in terms of a change in the surface charge distribution and thereby, the width of the nonemissive GaAs surface depletion layer. Biosensors fabricated with aptamer probes of various lengths indicate a strand length-dependent nature of the luminescence response to the biorecognition events, with longer aptamers yielding a greater PL enhancement. Results provided by several control experiments demonstrate the sensitivity, specificity, and selectivity of the proposed biosensor in accurately identifying ATP. Modeling the performance data by means of Poisson–Boltzmann statistics in combination with the GaAs depletion layer model shows a good correlation between the structural conformation of the aptamers and the PL yield of the underlying substrate. Collectively, the results described within indicate the promise of the prospective luminescence-based GaAs-aptamer biosensor for use in real-time sensing assays requiring a straightforward and efficient means of label-free analytical detection.

Octadecanethiol (ODT), CH3(CH2)17SH, self-assembled monolayers (SAMs) were deposited on GaAs (100) substrates from solution and from vapor phase. The monolayers assembled from solution were prepared by incubating the substrate in a dilute millimolar ODT/ethanol solution, while vapor-deposited monolayers were prepared from vapor phase transport of ODT in an ultrahigh vacuum environment. The structural and optical characteristics of the resulting SAMs were examined with contact angle (CA) analysis, photoluminescence (PL) spectroscopy, atomic force microscopy (AFM), and spectroscopic ellipsometry. Static CA measurements of the SAMs indicated the successful formation of highly organized monolayers by both preparation routes. In contrast, PL spectroscopy, AFM, and ellipsometry measurements showed differences in the optical and structural properties of SAMs prepared from liquid and vapor phases. The data obtained indicate that more densely packed monolayers exhibiting greater surface coverage and therefore, increased passivation characteristics are assembled from vapor phase.

The templated growth of GaAs nanowires based on a catalytic seed particle is described. The optimum growth conditions for biosensor applications of the nanowires are presented including the catalyst formation process and nanowire growth temperature. We have also observed growth conditions that result in nanowires oriented parallel to the surface, which may result in a new paradigm for biosensor fabrication.