We present recent developments in Raman probe of confined optical and acoustic phonons in nonpolar semiconducting nanowires, with emphasis on Si and Ge. First, a review of the theoretical spatial correlation phenomenological model widely used to explain the downshift and asymmetric broadening to lower energies observed in the Raman profile is given. Second, we discuss the influence of local inhomogeneous laser heating and its interplay with phonon confinement on Si and Ge Raman line shape. Finally, acoustic phonon confinement, its effect on thermal conductivity, and factors that lead to phonon damping are discussed in light of their broad implications on nanodevice fabrication. 1. Introduction Since the discovery of the Raman scattering process by Raman and Krishnan in 1928 [1] and the invention of the laser in 1960 by Maiman [2–4], Raman spectroscopy has morphed from standard macroprobe of bulk materials to single molecular detection [5–11]. The innovations in Raman spectroscopy have made it one of the most widely used techniques to probe nanostructures, beside transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and atomic force microscopy (AFM) as shown in Figure 1. The data in Figure 1 was collected from the ISI web of science site for the period of January 1996 to July 2010 on nanoscale characterization tools used in probing nanostructures [12]. Even though probing individual nanostructures by Raman spectroscopy at nanoscale is limited by its low spatial resolution (diffraction limit of the objectives), recent developments in nano-Raman spectroscopy, including scanning near-field optical microscopy (NSOM), tip enhanced Raman spectroscopy (TERS), and surface enhanced Raman spectroscopy (SERS), have made single-molecule detection possible [5–11, 13–17], especially using SERS and TERS. While NSOM is based on the principle of optical tunneling of evanescent waves, TERS and SERS are based on excitation of surface plasmons on the surface of nanometals. The process arises when free electrons on the surface of a metal oscillate collectively in resonance with the oscillating electric field of the incident light wave. The interaction between the surface charge and the electromagnetic field helps concentrate the light on the surface of the metal, leading to electric field enhancement. Irrespective of such great innovative techniques in nano-Raman spectroscopy, nearly 84% (see Figure 1) of Raman characterization use conventional Raman spectroscopy to probe nanoscale samples [12]; that is, probing ensembles of
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