JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS Vol. 9, No. 3, March 2007, p. 733 - 736 Density functional theory investigation of p-aminothiophenol molecules adsorbed on gold nanoparticles D. MANIU * , V. CHIS, M. BAIA, F. TODERAS, S. ASTILEAN Physics Faculty, Babes-Bolyai University, M. Kogalniceanu 1, 400084, Cluj-Napoca, Romania The current investigation is focused on quantum-chemical calculation and modelling of the SERS spectra for p- aminothiophenol molecules on gold nanoparticles. The DFT calculations are undertaken for p-aminothiophenol with S-H groups replaced by an S-Au bond. This model allows for more appropriate vibrational modes, electronic levels and transition moments description of the adsorbed p-aminothiophenol. Strong S-Au covalent bonding is indicated by the large concentration of electron density between S and bonded Au atoms and by distinctly directional Au-S-C bond whose bond angle is 105º. (Received November 15, 2006; accepted December 21, 2006) Keywords: Self-assembled, P-aminothiophenol, Density functional theory 1. Introduction The assembly of the nanosized metal nanoparticles with functionalized molecules has received considerable attention and shown extensive applications in various fields as molecular and biomolecular recognition, nanolithography and sensing. It is well known that thiols are irreversibly adsorbed on gold surfaces and form compact molecular monolayers. The understanding of such self-assembled monolayer (SAM) thin films is necessary to enable successful and reliable devices such as label-free biosensors to be constructed. A variety of experimental methods have been used to probe the quality and chemical nature of SAM on metal surface. Among them, surface-enhanced Raman scattering (SERS) plays an important role due to its richness of molecular structure information and high sensitivity. Nowadays, nanoparticles and nanoarrays design has become more sophisticated and new platforms for SERS are being made available [1-2]. We have recently reported self-assembled small polystyrene spheres coated with gold shells as ideal substrate for SERS studies [2]. These metal nanoshells are of high interest since the SERS effect is mediated by the nanoshell surface-plasmon resonances and these are tunable throughout the visible and near-infrared regions of the spectrum via appropriate choice of inner and outer radii. Furthermore, SERS spectra of the p- aminothiophenol (p-ATP) molecule adsorbed on the immobilized gold nanoparticles have been successfully recorded by using the 532, 633, 830 and 1064 nm laser lines [3]. Recent ab-initio density functional theory (DFT) calculations of thiolate molecules on the Au surface show that S head groups prefer to bond at bridge with a strong chemical bonding [4]. The DFT calculations for the binding between thiolate molecules and gold clusters also find that sulphur forms strong chemical bonds with only one or two gold atoms [4]. In this work we address similar issues by comparing the experimental measurements with quantum- chemical calculation and modelling of SERS spectra. We find good agreement between our simulated spectrum and the experimental SERS spectrum. High concentration of electron density between S and bonded Au atom confirm strong S-Au covalent bonding. 2. Experimental Aminopropyltriethoxysilane (APS), p-Aminothiophenol (p-ATP), and HAuCl 4 ·3H 2 O were purchased from commercial sources (Aldrich) and used as received. Ultra pure water was used throughout the experiments. All other reagents employed for substrate and solutions preparation were of analytical grade. The gold colloidal suspension was prepared by the following procedure [5]: 500 ml of 10 -3 M HAuCl 4 was brought to a boil with vigorous stirring on a magnetic stirring hot plate. Ten millilitres of 38.8 mM Na citrate was added to the solution all at once with vigorous stirring. The yellow solution turned clear, dark blue and then a deep red-burgundy colour within a few minutes. Stirring and boiling has been continued for 10-15 minutes after the burgundy colour was observed. The solution has been removed from heat and kept stirring until is getting cold and then the volume was adjusted to 500 ml with water. Colloidal solutions were stored in clean brown glass bottles until used. The assembling protocol of the gold nanoparticles was reported in a previously work [3]. The Raman spectrum was recorded using a Bruker Equinox 55 spectrometer with integrated FRA 106 module