DFT Simulation and Vibrational Analysis of the IR and Raman Spectra of a CdSe Quantum Dot Capped by Methylamine and Trimethylphosphine Oxide Ligands

Surface effects and interactions with capping ligancls strongly influence the properties of semiconductor quantum dots (QDs), opening up possibilities for new technologies, e.g. molecule sensing, and limiting the efficiencies of other applications, e.g. based on QD luminescence. By computing and analyzing in detail the infrared and Raman spectra of two ligands showing qualitatively different bonding to the QD surface, we demonstrate that vibrational spectroscopy constitutes a powerful tool for studying surface-ligand interactions due to its high sensitivity to changes in molecular structure. By forming a covalent bond with a CdSe QD, a single molecule of trimethylphosphine oxide (TMPO) exhibits a strong red-shift in the frequency of P=O that transforms from a double to a single bond. In addition, interaction with the QD breaks TMPO's C-3v symmetry, splitting the signals arising from the three methyl groups: The interaction of a methylamine molecule with the CdSe QD is weaker and occurs via a coordination bond. Nevertheless, a strong blue shift is seen in the frequency of the NH2 wagging mode, arising due to steric hindrance of this motion induced by the QD proximity. The theoretical predictions agree available experimental data, establish the mechanisms for QD-ligand interactions, and provide specific guidelines analysis of QD surfaces. well with the for vibrational