Investigation of structure and chemical states of self-assembled Au nanoscale particles by angle-resolved X-ray photoelectron spectroscopy
Introduction
In recent years, self-assembled nanostructures have received considerable interest for their favorable physical, chemical, electronic and optical properties [1], [2], [3], [4], [5]. The self-assembly technique of metal or semiconductor nanoparticles onto insulating or conducting substrates through the interactions between colloidal particles and functional groups within the self-assembled monolayer (SAM) has been fully developed and various uniform and stable self-assembled nanoparticles have been obtained [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. The self-assembled nanoscale architectures are routinely characterized by transmission electron microscope (TEM), scanning electron microscope (SEM), surface-enhanced Raman scattering (SERS) spectra, absorbance spectra, ellipsometry measurement, advancing contact angle measurement and other methods [1], [2], [13], [14], [15]. It was asserted from the above analysis techniques that gold nanoparticles were covalently assembled onto the thiol-terminated surface. However, there has been no direct experimental evidence for the formation of AuS bonds between gold nanoparticles and thiol-groups on the surface of SAMs. A systematic study of the structure and chemical states of the self-assembled gold nanoparticles is essential to understand fully the mechanism of the self-assembly process and obtain the optimum performance of such structures in numerous applications [16], especially in the field of molecular electronics and single-electron materials [1].
X-ray photoelectron spectroscopy (XPS) has been widely used to obtain chemical information of SAMs [17], [18], [19], [20]. However, the peak intensity of the investigated atoms under nanoparticles is usually too weak to be observed on XPS spectra. The typical advantage of angle-resolved XPS (ARXPS) is that the peak intensity can be enhanced by lowering the electron take-off angles. So, the chemical states of the signal atoms under the nanoparticles can be determined and direct experimental evidence about the AuS bonds between the gold particles and the surface thiol-groups can be obtained. In addition, the analysis of angle-dependent data can provide an estimate of the thickness of the SAM and indicate whether the SAM is continuous.
In our work, Au nanoparticles with diameter of 2.6 nm were assembled onto a silicon substrate using 3-mercaptopropyl-trimethyl-silane (MPTMS) as a coupling agent. The thickness of the silane SAM was determined according to the method developed by Fadely [21] and the sulfur species of the self-assembled gold nanoparticles were characterized by ARXPS at an electron take-off angle of 5°. S 2p spectra were curve-fitted to separate the different types of sulfur species. The analysis of peak positions was also compared with two similar samples and with other groups [22], [23], [24].
Section snippets
Preparation of gold nanoparticles [25]
0.3 ml of 1 M thiocyanate (NaSCN) was added to 50 ml aqueous solution containing 0.5 ml of 1% (g/g) HAuCl4 and 0.75 ml of 0.2 M K2CO3 with stirring. The mixture developed a yellowish color after 15–30 min and was left for 15 h at 22°C in the dark to come to completion. Thus, colloidal Au particles with a diameter of about 2.6 nm were formed.
Preparation of substrate Si(111)
The Si (111) substrate (1 cm by 1 cm) was degreased using toluene, acetone, and deionized water sequentially with ultrasonic agitation. Then, it was immersed in
The film thickness measurement
It is well known that, when a sample is irradiated by soft X-rays, the intensity of photoelectrons emitted from the sample surface is dependent on the flux of the X-rays, the density of the investigated atoms in samples, the take-off angles and some other factors. The photoelectron intensity is only a function of the take-off angle when the other factors are fixed. Analysis of the angle-dependent data can provide an estimate of the thickness of the overlayer and indicate whether the overlayer
Conclusions
This paper has for the first time reported direct experimental evidence for the existence of AuS bonds between the gold particles (d=2.6 nm) and the thiol-groups of the silane SAM by ARXPS. At an electron take-off angle of 5°, the S 2p spectrum of the self-assembled gold nanoparticles gives two typical peaks, which are assigned to be the unbound thiol and bound thiolate with the binding energies of 164.6 eV and 162.6 eV respectively. In addition, the angle-dependent data, based on the ratio of
Acknowledgements
We thank Dr. Zhaiwei Liu for the collection and analysis of the XPS data and Zhijun Zhang for the supply of sample I. This work was financially supported by the National Natural Science Foundation of China (No.29733080) and National Climb Projects.
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