First-principles Study of Adsorption Energetics of Alkanethiols on GaAs(001)
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First-Principles Study of Adsorption Energetics of Alkanethiols on GaAs(001) Oleksandr Voznyy and Jan J. Dubowski Department of Electrical and Computer Engineering, UniversitÈ de Sherbrooke, 2500, boul. de l'UniversitÈ, Sherbrooke, J1K 2R1, Canada ABSTRACT Adsorption of alkanethiols on GaAs (001) surface under low coverage conditions was studied using density functional calculations in a periodic supercell approach. The study of physisorbed precursor and transition to chemisorption revealed that hydrogen atoms stay on the surface upon S-H bond cleavage and significantly affect desorption products and energies, in agreement with available experimental data. Binding of thiols to GaAs is found to be comparable or stronger than that of thiols to noble metals surfaces. Calculated thiolate-surface binding energies are found to be higher for Ga-rich than for As-rich surfaces, and are strongly dependent on surface reconstruction, adsorption site and coverage. This dependence is explained by the violation or fulfillment of electron counting rule, rehybridization of surface atom orbitals and strain relaxation upon addition of extra electrons brought by adsorption. INTRODUCTION In addition to widely used gold nanoparticles and thiolated gold substrates, semiconductors could offer potential benefits in using semiconductor-specific electronic properties for biosensing. For example, photoluminescence (PL) is expected to change upon adsorption of bioactive material on the surface and, thus, it can be used to monitor the event of adsorption. Development of new biosensors for in-vivo applications based on quantum dots (QDs) requires materials that would provide a PL signal in the wavelength region of the biological transparency window, i.e., around 1 µm. This opens the potential for the use of InAs QDs for biosensing. Consequently, passivation and biofunctionalization of GaAs surface becomes highly important as this is the material of choice for capping of InAs QDs. Self-assembled monolayers (SAMs) of thiols on GaAs can prevent degradation of the GaAs surface and enhance the performance of the device by protecting it from harmful exposure to oxygen. Thiols can also increase the PL signal from the substrate by removing the surface states which is important for a PL-based biosensor. At the same time, thiols with different endgroups can provide a link for the attachment of specific biomolecules. Thus, investigation of SAMs of alkanethiols on GaAs surface is interesting from both a fundamental perspective and for potential applications, including passivation of GaAs, creation of transition layers for Schottky diodes and nanolithography [1, 2]. Due to specific properties of semiconductors and the high sensitivity of thiol-surface bonding to the substrate properties, very little information from relatively well studied thiols on noble metals systems [3, 4] can be applied to thiols on GaAs. However, theoretical modeling of the semiconductor-thiol interface can be used as a supplementary characterization tool, which can help to
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