Synthesis of Noble Metal Nanoparticles By Atomic Layer Deposition Utilizing Self-Assembled Monolayers Templates

Self-assembled monolayers (SAMs) are widely used as surface modification structures for thin films synthesis and designed nanostructures fabrication¹. Alkyltrichlorosilanes is a group of SAMs widely applied in the field of area-selective atomic layer deposition (AS-ALD). The closely packed octadecyltrichlorosilane (ODTS) SAMs could act as monolayer resist to prohibit ALD², while the pinholes on unsaturated ODTS SAMs could act as nucleation sites to initiate ALD growth. Thus the distribution and structure of pinholes, including its size and density, are important factors for utilizing SAMs template to fabricate nanoparticles via ALD. Many studies characterize SAMs and pinholes by atomic force microscopy (AFM), scanning electron microscopy (SEM), scanning tunneling microscope (STM), etc³. Most of these methods could provide the microscopic information on the pinhole structures, yet the information from these methods is quite localized and hard to obtain statistical average and full-scale information about the pinhole size and distribution density.        

Here we report utilizing electrochemical impedance spectroscopy (EIS) to study the size distribution and loading of initial nanoparticles grown in those pinhole sites.  EIS is a powerful and surface sensitive characterization technique to obtain the average and global characters of the detected surfaces^4,5. In this study, the properties of unsaturated ODTS SAMs are characterized with EIS methods. The relationship between SAMs growth time and the pinholes properties has been established. With the SAMs immersing time increasing, the sizes and density distribution are both reduced. For SAMs sample immersed over long time, at least 24 hours, negligible pinholes are observed. Furthermore, Pd ALD process has been carried on those SAMs modified substrates. The distribution of Pd nanoparticles corresponds well with the pinholes. The results show that the nanoparticles grown on SAMs modified substrates depend on the pinholes properties. With SAMs modified substrate, ALD grown noble metal nanoparticles with uniform size and controlled loading density can be achieved.  

 References:

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5. Xiaoquan Lu, Huiqing Yuan, Guofang Zuo, Jiandong Yang, Thin Solid Films 2008, 516, 6476–6482