Quantum chemical perspective of nanoscale Raman spectroscopy with the three dimensional phonon confinement model
Raman spectroscopy of crystalline/molecular systems is well backed with quantum chemical calculations and group theory, making it a unique characterization tool. For the "intermediate" case of nanoscale systems, however, the use of Raman spectroscopy is limited by the lack of such theoretical bases. Here, we suggest to couple a scaled quantum mechanical (SQM) calculation with the phonon confinement model (PCM) to construct a universal and physically consistent basis for nanoscale Raman spectroscopy. Unlike the commonly used one dimensional dispersion PCM, we take into account the confinement along all the three dimensions of the k space. We apply it to diamond nanoparticles of sub 50nm size, a system with pronounced anisotropy of dispersion for which the use of three dimensional dispersion is a requisite. The model excellently reproduces size sensitive spectral features, including the peak position, bandwidth and asymmetry of the sp3 C C Raman band. This fundamental approach can be easily generalized to other nanocrystalline solids to hopefully contribute the future development of quantitative nanoscale Raman spectroscopy.