Ultrafast Exciton Dynamics in Silicon Nanowires

Abstract

Ultrafast exciton dynamics in one-dimensional (1D) silicon nanowires (SiNWs) have been investigated using femtosecond transient absorption techniques. A strong transient bleach feature was observed from 500 to 770 nm following excitation at 470 nm. The bleach recovery was dominated by an extremely fast feature that can be fit to a triple exponential with time constants of 0.3, 5.4, and ∼75 ps, which are independent of probe wavelength. The amplitude and lifetime of the fast component were excitation intensity-dependent, with the amplitude increasing more than linearly and the lifetime decreasing with increasing excitation intensity. The fast decay is attributed to exciton-exciton annihilation upon trap state saturation. The threshold for observing this nonlinear process is sensitive to the porosity and surface properties of the sample. To help gain insight into the relaxation pathways, a four-state kinetic model was developed to explain the main features of the experimental dynamics data. The model suggests that after initial photoexcitation, conduction band (CB) electrons become trapped in the shallow trap (ST) states within 0.5 ps and are further trapped into deep trap (DT) states within 4 ps. The DT electrons finally recombine with the hole with a time constant of ∼500 ps, confirming the photophysical processes to which we assigned the decays.

Choice of stent and outcomes after treatment of drug‐eluting stent restenosis in highly complex lesions

“…In Figure 8, we analyze energy dissipation rates for different initial excitations. The rates of energy dissipation of undoped quantum dot change from 0.4 to 2 inverse picoseconds, in agreement with computational32 and experimental data 33. There we see that first three points a, a′ and b, b′ and c. c′ for codoped and undoped models follow the same trends and are of similar values.…”

Exciton Dynamics in Semiconductor Nanocrystals

“…Other particles have generally been found to be weakly luminescent or even non‐luminescent at room temperature, e.g. , PbS,127, 128 PbI 2, 129 CuS,130 Ag 2 S,131 and silica‐passivated Si nanowires 132. Two reasons account for the weak luminescence: an indirect nature of the semiconductor bandgap,133 which entails nonradiative recombination, or a high density of internal and/or surface trap states which act to quench the luminescence 132.…”

“…, PbS,127, 128 PbI 2, 129 CuS,130 Ag 2 S,131 and silica‐passivated Si nanowires 132. Two reasons account for the weak luminescence: an indirect nature of the semiconductor bandgap,133 which entails nonradiative recombination, or a high density of internal and/or surface trap states which act to quench the luminescence 132. Controlling the surface by removal of surface trap states can lead to significant enhancement of luminescence as well as the ratio of bandedge‐over‐trap state emission 134–139.…”

Accelerating electrostatic interaction calculations with graphical processing units based on new developments of ewald method using non‐uniform fast fourier transform

“…During the past decades, a large variety of computational schemes have been proposed with varied computational efficiencies. [1][2][3][4][5] Among them, the Ewald summation method [6] does a remarkable job by splitting the very slowly converged Coulomb potential into two terms which converge exponentially fast. However, the traditional Ewald summation method suffers from its drawbacks-it is computationally demanding since one part is solved in reciprocal space with several Fourier transforms [7][8][9][10] ; the algorithm scales as N 2 (with N being the number of charged particles in the simulation box) or as N 3/2 with optimized splitting parameters and the corresponding cutoffs.…”