Vibrational Spectroscopy and Modeling of Quantum Dot-Molecule Interfaces
We are using electronic structure theory to study how the vibrations of the ligands on the quantum dots participate in thermal dissipation from the excited state. In radiationless decay, the energy of the quantum dot’s excited state is used to excite vibrations in the system. We’re interested in how we can change the chemical structure of the interface to control the competition between radiative and radiationless decay rates.
In Quantum Dot-Ligand systems that undergo radiationless decay, the energy from the electronic transition is used to excite vibrational modes.
The direct measurement and control of energy dissipation pathways in a nano-material is perhaps the single biggest challenge for maximizing efficiency and minimizing losses in photochemical or photophysical processes. Changes to molecular vibrations provide a method for tracking the structural changes a molecule-QD complex undergoes as it interacts with a photoexcited electron or hole.
Here, we utilize a mid-IR detector and an ultrafast laser to track vibrations in the molecule as well as the photoexcited electron and hole as a function of time to observe energy dissipation mechanisms.
Molecules such as octylphosphonate and PTC are traps for photogenerated electrons and holes and measuring changes to the molecular vibrations as a charge carrier interacts with it may give insight into the particular loss mechanism that occurs. The mid-IR detector is capable of measuring electron trapping, hole trapping, and hole relaxation pathways.
Transient Absorption measures the change in the absorbance of a particular wavelength over time as the result of a photoexcitation caused by a pump laser. We can utilize a mid-IR detector to look for changes in molecular vibrations to track electron and hole migration within the nanostructure.
Transient Absorption starts with an initial 100 femtosecond, 795 nm pulse generated by a Ti: Sapphire oscillator that is amplified with a regenerative amplifier. Once amplified, the pulse is split in two and fed into 2 Optical Parametric Amplifiers (OPAs) which change the wavelengths into a visible pump and a mid-IR probe. A ZnSe wedge splits the mid-IR probe into a signal and reference and all three lasers converge on the sample. The signal and the pump beams are overlapped in space and time so that the molecules in that region are excited, while the reference measures molecules in their steady state. A spectrometer determines the amount of mid-IR light at each wavelength and the difference between the signal and reference indicates changes to molecular vibrations caused by the pump. A delay stage enables the time dynamics of vibrational changes to be monitored.
Sponsored by the Army Research Office through a Presidential Early Career Award for Scientists and Engineers (PECASE)