External Electric Field Effects on State Energy and Photoexcitation Dynamics of Water-Soluble CdTe Nanoparticles

Abstract

External electric field effects on absorption and photoluminescence (PL) spectra of colloidal CdTe nanoparticles have been measured in a poly(vinyl alcohol) (PVA) film. The electroabsorption spectra across the first exciton band are similar in shape to the second derivative of the absorption spectra, indicating the enhancement of the electric dipole moment following the optical transition to the first exciton state. The magnitude of the enhancement has been evaluated as a function of the size of the CdTe nanoparticles. The electrophotoluminescence (E-PL) spectra show a significant quenching of PL in the presence of electric fields. The direct measurements of the field-induced change in PL decay show that the field-induced quenching of PL arises from the field-induced decreases both in lifetime and in initial population of the exciton-emitting state. The E-PL spectra also show that the application of electric fields induces a red-shift or blue-shift of the PL spectra, depending on the size of the nanoparticles. It is also shown that the PL intensity of CdTe nanoparticles in PVA increases under photoirradiation at an atmospheric condition and decreases in a vacuum condition. The present results show that the emission properties of CdTe nanoparticles can be controlled by application of external perturbation such as electric field or photoirradiation.

Electric Field Effects on Photoluminescence of CdSe Nanoparticles in a PMMA Film

“…The blue shift of the E-PL spectrum in comparison with the corresponding exciton PL band (see Figure 5) means that the polarizability in the exciton-emitting state is larger than that in the ground state. Note that E-PL spectra having inverted scale of intensity are compared with the PL spectra in Figures 5 and 6 and that a similar shift was observed in CdTe nanoparticles [32]. The size dependence of the field-induced quenching is shown in Figure 8.…”

“…If a well-passivated CdSe having a much longer lifetime is used, there may be a possibility that the electric field effect in the fluorescence lifetime is much larger than the one observed in the present experiments. The decrease in average PL lifetime observed in the present experiments mainly results from the change in the ratio among the pre-exponential factors in the presence of F. The slight field-induced decrease in τ 2 and τ 4 suggests that the nonradiative process at the exciton-emitting state is accelerated by F. The non-radiative process from the exciton-emitting state is considered to be the formation of surface trap states undergoing further non-radiative decays [30,32,40], suggesting the field-induced enhancement of the trap state formation process. Since the population of the exciton state from the photoexcited state is markedly reduced by the application of electric fields, the population of the trap state also becomes smaller in the presence of an electric field, even when the formation yield of the trap state from the exciton state is a little enhanced by electric fields.…”

“…The significant increase in |Δμ| with increasing particle size was also observed for CdS nanoparticles in a PMMA film [30]. On the other hand, the CdTe nanoparticles exhibited different behavior in a poly(vinyl alcohol) (PVA) film: the |Δμ| value remains unchanged with particle size in the region of 2-4 nm, and slight increase in |Δμ| with increasing particle size was observed with diameters larger than 4 nm [32]. It has also been reported that the first exciton band gives the second derivative shape in the E-A spectra of PbS and PbSe [33,34].…”

“…We applied external electric fields up to 0.5 MV cm −1 on absorption, PL, and PL decay profile of CdSe nanoparticles. We compared the present results with those of CdS and CdTe nanoparticles which we had previously reported [29][30][31][32]. Hereafter, electroabsorption and electrophotoluminescence spectra are abbreviated as E-A and E-PL spectra, respectively, and applied electric field is denoted by F.…”

“…As shown in Figure 1, all the E-A spectra could be fitted by the second derivative of the first exciton band, but the small contribution of the first derivative spectrum was necessary to reproduce the E-A spectra of CdSe with a diameter of 2.1 and 2.5 nm. It should be also noted that the E-A spectra were fitted when the absorption profile of the first exciton band was assumed to be a Gaussian [30,32]. The coefficient A χ of Equation (1) is negligible in all the E-A spectra, indicating that the field-induced change in transition dipole moment to the first exciton state is negligibly small in the CdSe nanoparticles.…”