Excited State Properties and Spectroscopy of Semiconductor Nanoparticles
Final Report Abstract
Diamondoids are a class of saturated, C and H containing substances, well established within this research unit. They can be derived from diamond, by cutting out of the crystal small subunits and saturating with hydrogen. In general, diamondoids are relatively inert, hard compounds, and good monochromatic electron emitters with negative electron affinity. They are wide-gap semiconductors, transparent, and they absorb light significantly around and above about 6 eV. Pristine diamandoids have also been studied with respect to their photoluminescence, i.e., fluorescence properties. In project K, we were particularly interested in optical properties of diamondoids and molecular nanostructures derived from these. The hope is to arrive at tunable optical and electronic properties for possible applications in molecular electronics, nanophotonics, and materials science. In order to arrive at functional materials with tailored properties, various strategies can be / have been followed, among them: (i) The “doping” of diamondoids with other atoms, e.g., replacing C-H and CH2 units by N or O atoms, respectively. (ii) The “electronic blending”, e.g., replacing or connecting subunits with sp3-hybridized carbon atoms by unsaturated (sp2- or sp-hybridized) molecular fragments. (iii) Finally, the “functionalization” of diamondoids by attaching functional groups such as thiol, alcohol, amino groups, halogens etc. The focus of project K was (and is) on the theoretical modelling of optoelectronic properties of pristine and modified diamondoids, with the goal to tune them by chemical modification in silico. In particular, vibronic spectra involving electronic excitations (absorption, emission, resonance Raman) were computed with the help of time-dependent correlation function based methods and first principles quantum chemistry. Vibronic spectra give the most detailed insight into optical properties of these materials beyond simple vertical transitions. The time-dependent approach offers additional insight by relating optical signals directly to molecular dynamics in excited and ground electronic states. Further, it avoids the computation of Franck-Condon integrals which becomes cumbersome for larger molecules. Correlation-function methods were also used in this project to compute non-radiative transitions in photoexcited diamondoids, e.g. Inter System Crossing (ISC) and Internal Conversion (IC) by spin-orbit and non-Born-Oppenheimer (non-adiabatic) couplings, respectively. Computer programs and interfaces to existing quantum chemistry software to serve these purposes were developed during the project.
Publications
- Elucidation of optical properties of beta-carotene and diamondoids through vibrationally resolved spectroscopy: A time-dependent perspective, PhD thesis, University of Potsdam, 2014
S. Banerjee
- Vibrationally resolved absorption, emission and resonance Raman spectra of diamondoids: A study based on time-dependent correlation functions, Phys. Chem. Chem. Phys. 16, 144, 2014
S. Banerjee, P. Saalfrank
(See online at https://doi.org/10.1039/c3cp53535e) - Vibrationally resolved optical spectra of modified diamondoids obtained from time-dependent correlation function methods, Phys. Chem. Chem. Phys., 17, 19656, 2016
S. Banerjee, T. Stüker, P. Saalfrank
(See online at https://doi.org/10.1039/c5cp02615f)
