Abstract
The interaction of SH-, or S2- with Au surface has been phenomenon of considerable interest in last decade. The Au-S adsorption bond was studied as convenient linker for many complex molecules such as alkane-thiols, proteins, 3D molecular structures for molecular electronics, or for biological sensors applications [[1]]. The adsorption of SH-, or S2- to the Au is enabled by formation of strong S-Au bond (few eV energy) [[2]], which is stable in a wide potential range from Au surface oxidation (1.2 V vs. SHE) to reductive dissociation of Au-S bond (~-0.6 V vs. SHE). The change in the energetics of the surface states due to HS-, S2- adsorption does affect the surface resistance of ultrathin Au films. This effect is mainly due to S-adsorbate induced changes to quality of electron scattering from the surface states [[3]]. Therefore, an adsorption process of SH-, or S2-on Au can be indirectly observed by measuring the change in the overall resistance of the Au thin films.
In this talk we will present data studying chemo-resistive properties of Au ultrathin films exposed to solutions containing Cl- and HS- ions in 0 – 100 ppm range, Figure 1. The surface modification of these films was done by deposition of few monolayers of Au, Pd and AuPd via SLRR of Pb UPD [[4]]. The change in resistivity of Au films as a function of the solution composition was measured by AC method in order to minimize the effect of parasitic current path through the solution. As deposited Au films showed very small surface chemo-resistance while the films modified by few monolayers of Au, Pd and AuPd deposit have shown much better performance. To analyze our data we have developed comprehensive analytical model which links the adsorption process of different anions from the solution to the change in the diffused scattering coefficient for electrons from the surface [[5]]. Experimental results and analysis show that modifications of the Au surface, with several monolayers of Au and Pd, increases probability for electron scattering from the surface and improves the chemo-resistance of ultrathin Au films. The significance of our findings is discussed for chemical sensors design shining a new light on the importance of surface preparation for chemo-resistive sensor performance, Figure 1.
The authors acknowledge the support from Baker Hughes Corporation and NSF Chemistry division under the contract # 0955922.
[1] Nanoelectronics, Nanowires, Molecular Electronics, and Nanodevices, K. Iniewski, Mc Graw Hill, New York (2013).
[2] Y. Jin, C. Yu, R.J. Denman, W. Zhang, Recent Advances in Dynamic Covalent Chemistry, Chemical Society Reviews 42, 6634 (2013)
[3] Ultrathin Electrochemical Chemo- and Biosensors, eds. O.S. Wolfbeis, and V. M. Mirsky, Springer (2013), p. 120
[4] S. R. Brankovic , J. X. Wang and R.R. Adzic, Surface Science , 474, L173, (2001).
[5] E. H. Sondheimer, Advances in Physics, 1, 1 (1952).
Figure 1