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45761-AC5
The Role of Surface Modification on Nanoparticle Formation by Atomic Layer Deposition

Stacey F. Bent, Stanford University

Nanoparticles exhibit many interesting new chemical, physical, electronic, and mechanical properties and are being pursued for a number of applications.  For many of these applications, such as catalysis, the assembly or distribution of the nanoparticles may be as important as the particle size and composition.  We are carrying out fundamental studies into a novel method for growing nanoparticles on surfaces in which the size and average spacing of the nanoparticles can be controlled.  The process utilizes chemical modification of the substrate surface combined with atomic layer deposition (ALD).  ALD is a vapor phase deposition process consisting of an alternating series of self-limiting chemical reactions between gas phase precursors and the substrate.   ALD can be used to grow a large range of catalytically important materials. 

In order to achieve controlled nanoparticle growth by ALD, we have focused our efforts on using self assembled monolayers (SAM) formed from octadecyltrichlorosilane (ODTS) to modify silicon oxide-terminated silicon substrates.  We have shown in earlier work that a very well-packed SAM can resist atomic layer deposition completely, whereas less well-packed SAMs allow some ALD to occur.  Over the past year we have investigated the effect of intentionally forming defective SAMs to allow for nucleation of nanoparticles by ALD.  Specifically, the self assembled monolayers were imparted with increasing defect concentrations by intentionally shortening the SAM formation time.  These defects, in turn, served as potential nucleation sites for ALD.  

In these studies, a series of ODTS SAMs were deposited on the silicon substrates over formation times ranging from 2 hours to 12 hours.  The SAMs were characterized by a combination of ellipsometry and water contact angle measurements.  Following SAM formation, the substrates were placed into a custom built ALD reactor to perform Pt deposition.  Pt ALD was achieved using (methylcylopentadienyl)-trimethylplatinum (IV) and oxygen precursors, delivered in separate pulses, with the substrate at approximately 300^oC.  Depositions were carried out for 20-200 binary ALD cycles. After Pt ALD, the substrates were characterized by x-ray photoelectron spectroscopy, scanning electron microscopy, and scanning Auger electron spectroscopy.

The results show that Pt nanoparticles can be successfully deposited on the SAM-modified planar substrates.  The XPS data show that the Pt concentration at the surface after ALD decreases with increasing formation time of the surface-modifying SAM.  Moreoever, the SEM image results suggest a trend in which shorter SAM formation times (more defects) lead to a higher density of Pt nanoparticles, while longer SAM formation time (fewer defects) leads to a lower density of nanoparticles.

The effect of ALD cycle number was also investigated.  For a fixed SAM formation time, the XPS data show an increase in Pt atomic percent at the surface with increasing cycle number.  SEM data show that while the Pt still grows as nanoparticles, the average size of those nanoparticles increases with cycle number.  Interestingly, preliminary analysis indicates that the increase in the amount of Pt at the surface with increasing cycle number deviates from the functional form expected based upon a simple nanoparticle volume analysis.  This deviation suggests that other factors are contributing to the nucleation and growth processes. 

During the remaining period of the grant, we will continue to examine the effect of the defect concentration in SAMs and the number of ALD cycles on the density and size of Pt nanoparticles.  We will also investigate other ALD systems in addition to Pt, such as TiO₂.  Finally, we intend to model the nanoparticle formation process at SAM-modified surfaces to better understand nucleation phenomena in these systems.

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