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

Abstract

Surface effects and interactions with capping ligands 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 NH 2 wagging mode, arising due to steric hindrance of this motion induced by the QD proximity. The theoretical predictions agree well with the available experimental data, establish the mechanisms for QD−ligand interactions, and provide specific guidelines for vibrational analysis of QD surfaces.

Assignment of the Raman Spectrum of Benzylic Amide [2]Catenane: Raman Microscopy Experiments and First-Principles Calculations

“…So far, this kind of analysis at quantum level had been restricted to those systems with smaller unit cells, mainly inorganic compounds. [18][19][20][21][22] However, in the present work, we disclose the Raman spectrum of such a complex molecular solid, finding a very good agreement between the theoretical calculations and the experimental results.…”

Effect of Dot Size on Exciton Binding Energy and Electron–Hole Recombination Probability in CdSe Quantum Dots

“…23,24,49,50 The central challenge in the theoretical investigation of quantum dots is efficient computational treatment of large number of electrons in the system. For small clusters where all-electron treatment is feasible, ground state and excitedstate calculations have been performed using GW Bethe-Salpeter, [51][52][53] density functional theory (DFT) [54][55][56][57][58][59][60] , timedependent DFT (TDDFT) [61][62][63][64][65][66][67][68] , and MP2 69 . For bigger quantum dots where all-electron treatment is computationally prohibitive, atomistic semiemperical pseudopotential methods have been used extensively.…”

Branched Ligand Ethyl 2‐Mercaptopropionate as a Stabilizer for CdTe Quantum Dots and its use as a Cu2+ Ions Probe in Aqueous Medium

“…Also, C−H peaks observed for the ligand was noticed to be shifted to a slightly lower wavenumbers of 2967 and 2925 cm −1 . We are anticipating the conformational changes upon the interaction of QDs with ligands and the redistribution of electron density are responsible for this shift in vibrational position, as explained elsewhere . One of the significant information obtained from FTIR studies is the binding site of ligand with the QDs.…”

“…We are anticipating the conformational changes upon the interaction of QDs with ligands and the redistribution of electron density are responsible for this shift in vibrational position, as explained elsewhere. [23] One of the significant information obtained from FTIR studies is the binding site of ligand with the QDs. A small and sharp peak was observed at 2565 cm À 1 for E2MP ligand, which corresponds to the SÀ H vibrations.…”