Raman spectroscopy as a method for structural characterization of ZnO-based systems at the nanoscale

Abstract

We present a straightforward method for determining the crystalline coherence length (D_c) of ZnO-based systems with long-range order in the scale of tens of nanometers. The proposed equation enables calculating D_c by simply utilizing the intensities of two peaks of a Raman measurement, namely: D_c = A (I_E1(LO)/I_E2^high) + 66.5, where I_E1(LO) and I_E2^high are the intensities of E₁(LO) and E₂^high Raman peaks, respectively, and the coefficient A depends on the laser wavelength used as excitation. Such methodology can be applied to measurements taken with most of the visible lasers available for Raman experiments. Based on the results of photoluminescence analyses, it can be inferred that the relative intensities of these Raman peaks are influenced by both D_c and the exciting laser wavelength, owing to resonance processes that selectively involve phonons out of the Brillouin Zone center. A significant competitive advantage of this method stands out in the fact that Raman spectra are very sensitive even to slight structural modifications that are below the detection limit of conventional characterization techniques, such as X-ray diffraction, and the versatile and easy way of performing in-situ analyses, in addition to the possibility to take measurements with microscopic spatial resolution without the demand for large X-ray sources or synchrotron environments.