Previous reports suggest that Raman peaks in uniaxially loaded nanowires with diamond cubic and zinc blende crystal structures shift at rates that are significantly different from bulk specimens. We have investigated the first order Raman scattering from individual, free-standing, [111] oriented GaP nanowires ranging from 75 to 180 nm in diameter at uniaxial tensile stresses up to 5 GPa. All of the phonon modes were shifted to frequencies lower than previously reported for bulk GaP, and significant splitting of the degenerate transverse optical mode was observed. A general analysis method using single and double Lorentzian fits of the Raman peaks is presented and used to report more accurate values of the phonon deformation potentials (PDPs) that relate uniaxial strains to Raman peak shifts in GaP. A new set of PDPs determined from the nanowires revealed that the they have elastic moduli and failure strains that are consistent with bulk GaP. The analysis method eliminated the anomalous, inconsistent deformation behavior commonly reported in Raman-based strain measurements of nanowires, and can be extended to other materials systems with degenerate phonons.
The experimentally measured elastic moduli and yield strengths of nanowires and nanofilaments vary widely in the literature and are often beyond the theoretical limits of the particular material. In this work, Si nanowires with very low defect densities were loaded in uniaxial tension to establish the origins of their apparently nonlinear constitutive behavior. The diameters of the nanowires ranged from 230 to 460 nm and the growth directions were primarily [112] with the exception of a [111] oriented nanowire. The resulting fracture strengths of the nanowires ranged from 3.88 to 10.1 GPa. The nonlinear constitutive behavior was accompanied by fracture surfaces with features that were not commonly observed in Si. A nonlinear continuum elasticity model and electron microscopy established that reports of unusual deformation behavior and fracture surface morphologies are a direct byproduct of the electron and ion beam deposited adhesives (Pt-based in this work) used to affix specimens in place for testing.
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