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Shape dependence of physical properties of polar semiconducting nanowires

by Xiong, Qihua.

Abstract (Summary)
Semiconducting nanowires (SNWs) show promises as building blocks for nanoelectronics, optoelectronics and biological or chemical sensors. Due to their quasione-dimensional nature, research into the fundamental physical properties of nanowires has focused on quantum size effects, e.g., transverse confinement for electrons or phonons. However, the effects of the shape of nanowires on the physical properties of SNWs have not yet been systematically studied. In this thesis, we collect results from a series of experiments whose outcome is predicted to be, or shown to be, dependent on the shape of the nanowire. The term “shape” is taken to refer not only to the cross sectional geometric form (e.g., circle, hexagon, square, etc.) but also the aspect ratio A of the nanowire, where A is the ratio of the nanowire length to its minimum lateral dimension. Five studies addressed in this work point to the conclusion that “shape matters” in the physical properties of polar SNWs. These five studies are summarized in four chapters: (1) Automatic twinning superlattice formation in GaP and InP nanowires (Chapter 4); (2) Activation of surface optical phonons in cylindrical and rectangular GaP and ZnS nanowires (Chapter 5); (3) Extra phonon modes in cylindrical GaP nanowires (Chapter 5); (4) Raman scattering in individual cylindrical GaP nanowires by antenna effect (Chapter 6); (5) Nanomechanical properties of rectangular cross section ZnS nanowires (Chapter 7). In four of these studies, a clear experimental case is given for the impact of “shape” on the observations. In the 5th, the theory for small bending oscillations of a doubly-clamped nanowire has an explicit shape dependence for the resonant frequency, as predicted by the Euler-Bernoulli equation. Through Raman scattering studies of ensembles, or individual polar SNWs, we have demonstrated that the shape (cross sectional shape and aspect-ratio) matters for the phonon properties. For example, we show that the modulation of the cross sectional area iii activates surface optical (SO) phonon Raman scattering. The modulation is identified with a nanowire growth instability. We show that the SO phonon dispersion is strongly dependent on the cross sectional shape. Furthermore, Raman data on individual polar SNWs reveal extra phonon modes, suggesting that the long-range dipolar interaction in SNWs is shape-dependent. Our interpretation of our Raman scattering results in GaP nanowires is that a high aspect ratio coupled with a sufficiently small nanowire diameter must lead to a splitting of the bulk LO and TO phonons into LOx+LOz and TOx+TOz where z is the direction parallel to the nanowire axis. This thesis also presents several first observations, such as the automatic growth of twinning superlattices (TSLs) in III-V SNWs. The periodicity of the superlattice appears to be affected by ?T=Tm - T, i.e., the degree of undercooling of the liquid phase in contact with the solid phase during VLS growth, where Tm is the melting temperature of the nanowire. We present results from two III-V systems (GaP, InP) in which the TSL is generated by the periodic 180° flipping of the < 112 > direction relative to the < 111 > growth direction of the nanowire. Superlattice coherence over at least ~ 500 nm has been observed. We suspect that our observations mean that a TSL structure can be grown in many compound semiconducting nanowire systems. Control of the TSL period should allow significant design possibilities for electronic, thermoelectric, thermal and electrooptic applications. Secondly, the first observation of a Raman antenna effect where the incident laser produces Raman scattering as if the individual nanowire was a dipole ? antenna. Interestingly, the effect is shown to require that the nanowire diameter d < , 4 where ? is the excitation wavelength. Thirdly, we demonstrate a new internally selfconsistent approach to measure the Young’s modulus E of a nanofilament, i.e., by the new Force-deflection spectroscopy (FDS). We find by FDS that ZnS nanowire exhibits a value for E 30% less than that of the bulk. This depression of E seems to be a general and surprising outcome for nanoscale mechanics. iv
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School:Pennsylvania State University

School Location:USA - Pennsylvania

Source Type:Master's Thesis

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