Temperature dependence of hot-carrier relaxation in PbSe nanocrystals: An ab initio study

Temperature-dependent dynamics of phonon-assisted relaxation of hot carriers, both electrons and holes, is studied in a PbSe nanocrystal using ab initio time-domain density-functional theory. The electronic structure is first calculated, showing that the hole states are denser than the electron states. Fourier transforms of the time-resolved energy levels show that the hot carriers couple to both acoustic and optical phonons. At higher temperature, more phonon modes in the high-frequency range participate in the relaxation process due to their increased occupation number. The phonon-assisted hot-carrier relaxation time is predicted using nonadiabatic molecular dynamics, and the results clearly show a temperature-activation behavior. The complex temperature dependence is attributed to the combined effects of the phonon occupation number and the thermal expansion. Comparing the simulation results with experiments, we suggest that the multiphonon relaxation channel is efficient at high temperature, while the Auger-type process may dominate the relaxation at low temperature. This combined mechanism can explain the weak temperature dependence at low temperature and stronger temperature dependence at higher temperature

Ab Initio Time-Domain Study of the Triplet State in a Semiconducting Carbon Nanotube: Intersystem Crossing, Phosphorescence Time, and Line Width

Motivated by recent experiments (J. Am. Chem. Soc. 2011, 133, 17156), we used nonadiabatic (NA) molecular dynamics implemented within ab initio time-domain density functional theory to investigate the evolution of the excited electronic singlet and triplet states in the (6,4) carbon nanotube (CNT). The simulation simultaneously included the NA electron–phonon interaction and the spin–orbit (SO) interaction and focused on the intersystem crossing (ISC) from the first excited singlet state (S₁) to the triplet state (T₁) and subsequent relaxation to the ground electronic state (S₀). For the first time, the state-of-the-art methodology (Phys. Rev. Lett. 2005, 95, 163001; Phys. Rev. Lett. 2008, 100, 197402) has been advanced to include triplet states. The S₁–T₁ ISC was calculated to occur within tens of picoseconds, in agreement with the experimental data. This time scale is on the same order as the S₁–S₀ nonradiative decay time obtained previously for the (6,4) CNT. The homogeneous phosphorescence line width, which can be measured in single-molecule experiments, was predicted to be on the order of 10 meV at room temperature. This value is similar to the fluorescence line widths of CNTs suspended in air. The NA electron–phonon and SO couplings were found to be on the order of 1 meV; however, the former fluctuates much more than the latter, causing the ISC rate to be limited by the SO interaction rather than NA interaction. The electronic energy lost nonradiatively during ISC is deposited into high-frequency optical phonons of the CNT arising from C–C stretching motions. The calculations indicate that ISC can contribute to the nonradiative energy losses and low photoluminescence quantum yields observed in semiconducting CNTs

Formation and stability of dense arrays of Au nanoclusters on hexagonal boron nitride/Rh(111)

We have studied the nucleation and growth of Au clusters at submonolayer and greater coverages on the h-BN nanomesh grown on Rh(111) by means of scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). STM reveals that submonolayer Au deposited at 115 K nucleates within the nanomesh pores and remains confined to the pores even after warming to room temperature. Whereas there is a propensity of monoatomic high islands at low temperature, upon annealing, bi- and multilayer Au clusters emerge. Deposition of higher coverages of Au similarly results in Au clusters primarily confined to the nanomesh pores at room temperature. XPS analysis of core-level electronic states in the deposited Au shows strong final-state effects induced by restricted particle size dominating for low Au coverage, with indications that larger Au clusters are negatively charged by interaction through the h-BN monolayer. DFT calculations suggest that the structure of the Au clusters transitions from monolayer to bilayer at a size between 30 and 37 atoms per cluster, in line with our experiment. Bader charge analysis supports the negative charge state of deposited Au. © 2014 American Physical Society