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Microscopy and spectroscopy studies of (Sim̳Gen̳)p̳ superlattices and alkylthiol monolayers

Authors Zhang, Chun
Issue Date 1999
Summary The main ingredients of this thesis are divided into two parts. In the first part, we have investigated the second-order nonlinear optical susceptibility of short-period (SimGen)p strained layer superlattices (SLS). The measured X[(squared)]311 for (Si5Ge5)3 sample is 1.0 x 10[to the power of negative seven] esu at 1.17 eV (1064 nm) and 7.2 x 10[to the power of negative seven] esu at 1.55 eV (800 nm). In addition to the Si-Ge bond contribution from Si/Ge interfaces, we attribute an inhomogeneous strain due to lattice mismatch in the superlattice layers to be the main source of the SH responses we observed. Theoretical calculation of strain induced X[squared (2)] gives the value comparable to the one experimentally determined. This strain model can also explain the larger SH responses from superlattices samples containing stacking fault defects. We finally show that a multistructure of alternative strained superlattices and Si buffer layers can coherently generate SH, hence make such material potentially useful in practice. In the second part, we have studied the frictional property for alkyl-thiol monolayers CH3(CH2)nSH (n = 6, 9, 17) self-assembled on Au(111) with different molecular configurations under annealing. From scanning tunneling microscopy (STM) studies, it is known that these molecules form ordered ([square root] 3 x [square root] 3)R30° structure after self-assembly at room temperature. With annealing at sufficient high temperatures for an extended time period, the ordering of the films would first degrade and then form another order stripe-phase as partial desorption of the molecules takes place. Such thermally induced irreversible structural changes are further confirmed by using X-ray photoelectron spectroscopy (XPS) and Fourier transform inferred spectroscopy (FTIR). Our friction results on these films measured by frictional force microscope (FFM) with a Si3N4 tip show that friction increases as the monolayer ordering deteriorates and then reaches to a saturation level of [spacing tide]4 times larger for the stripe-phase than that for the original ([square root] 3 x [square root] 3)R30° phase. The increase of the friction as the monolayer ordering degrades can be understood as due to the increased accessibility to excite Gauche kinks in the hydrocarbon chain by the AFM tip, which opens up an energy dissipation channel.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 1999
Language English
Format Thesis
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