Thermostability and Related Issues of Self-Aligned Metal Nanoparticles: Research by Temperature Programmed Real-Time AES, LEED, and Desorption
Date Issued
2007
Date
2007
Author(s)
Liu, Kuan-Te
DOI
zh-TW
Abstract
A previous series of studies of our lab[1~3] on vapor depositing several kinds of metals on Al2O3/NiAl(100) had created metal nanopar-ticles(NPs) with following characteristics: well-ordered alignment, self-limiting size distribution with average size of ~2.7nm, and high thermostability. And these features can be attributed to peculiar one-dimensional long stripes with ~4 nm interdistance on the surface of the ultrathin Al2O3 template. Above conclusions are results of STM re-search. This thesis includes researches on thermostability and related issues of the same system studied by methods mentioned in thesis title. It contains two major parts:
The first part is about finding the method which can greatly en-hance thermostability of NP assembly. One of the conclusions of STM experiments of a previous study[3] is that Co NPs have high thermosta-bility, e.g. 1.5ML Co NP assembly sustains the density of ~260/104 nm2 even after 800 – 1090 K annealing. But in this independent experiment, We employed Temperature Programmed real-time Auger (TPA) and Temperature Programmed thermal Desorption (TPD) and found that pure Co NP assembly would not desorb in any form but start diffusing into the NiAl bulk at ~600K and it’s auger signal disappears almost completely at about 1000K. This disagrees with the previous conclu-sion[3]. After reviewing data and records of previous experiment, we found rational explanation and proved it: thermostability of Co NPs can be enhanced nearly 400 K by exposing them to oxygen. The final result can be attributed to two mechanisms: I. Oxygen can passivate the boundary of each Co NP. II. Co NPs work as catalyst to catalyze a redox reaction between part of CoOx and NiAl substrate, which increases the thickness of the Al2O3 film between them. Combining these two mechanisms can prevent Co NPs from sintering with each other and diffusing into substrate. This method may be applied on several differ-ent kinds of metal NPs. The unexpected oxygen surfactant in previous STM experiment[3] possibly come from longer experimental time (overnight), STM sample holder outgasing, and smaller Co coverage. We also show the effect of CO as surfactant and TPA results of Fe NPs here.
The second part is about changing the interdistance of peculiar one-dimensional long strips on the surface of the single-crystalline Al2O3/NiAl(100) by altering lattice mismatch between alumina over-layer and substrate. The idea is to vary the ratio of Ni to Al in surface layer of NiAl(100) and alloying it with Co or Fe then we expect some new stable alloy phase with lattice constant differing from original would form on surface layer of NiAl(100). Although we got some achievements here, they need to be analyzed further by instruments with higher surface analyzing abilities like SPA-LEED and STM.
There is an appendix in the final of this thesis. It contains some supplementary data from experiments of alumina layer on NiAl(100). The research in alumina layer includes exposing to oxygen at different temperatures and desorbing behaviors under different heating rates.
Results in this thesis may open a new door to create more thermal stable magnetic metal NPs of special size, density, and alignment in UHV. First part just like what chemists usually do to isolate particles from each other in solution. And 2nd part also has lots of possibilities waiting for further studying.
Subjects
自發有序排列
奈米顆粒
熱穩定性
程式控溫即時AES
晶格錯位
self-aligned
nanoparticle
thermostability
temperature programmed real-time AES
TPA
lattice mismatch
Type
thesis
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