Characterization of immobilized DNA on sulfur-passivated InAs surfaces

Abstract

The immobilization of DNA on passivated n-type InAs (100) surfaces has been studied using X-ray and ultraviolet photoelectron spectroscopy. The benefits of sulfur passivation using ammonium sulfide solution ((NH₄)₂S) for DNA immobilization were examined. The XPS/UPS data carried out on non-functionalized and functionalized surfaces demonstrate that the DNA probes reacted with the sulfur-passivated InAs surface. The XPS data in combination with fluorescently-tagged DNA indicate that the sulfur passivation process leads to a higher and more uniform attachment of DNA over the surface compared to non-sulfur-passivated InAs surfaces. The XPS data obtained immediately after sulfur passivation clearly observes In-S bonding, with little or no As-S. In addition, the XPS spectra of As 3d core-levels immediately after sulfur passivation shows that there is a negligible amount of As-O_x, but the peak become considerable after exposure to the aqueous DNA probe solution. The increase in As-O_x is likely due to the presence of non-sulfur bonded As atoms present on the surface. The presence of sulfur on the surface does lead to the high areal density of attached ssDNA. This system forms the basis of a DNA sensing system. While chemically passivating the surface against oxidation and facilitating probe attachment, the changes in Fermi level position were also monitored by UPS. UPS spectra show that the Fermi level of a clean InAs surface is located ~0.6 eV above the valence band maximum. The changes in electronic states induced by sulfur passivation and the pinning of E_F are discussed.