Exciton storage in CdSe/CdS tetrapod semiconductor nanocrystals: Electric field effects on exciton and multiexciton states

CdSe/CdS nanocrystal tetrapods are interesting building blocks for excitonic circuits, where the flow of excitation energy is gated by an external stimulus. The physical morphology of the nanoparticle, along with the electronic structure, which favors electron delocalization between the two semiconductors, suggests that all orientations of a particle relative to an external electric field will allow for excitons to be dissociated, stored, and released at a later time. While this approach, in principle, works, and fluorescence quenching of over 95$\%$ can be achieved electrically, we find that discrete trap states within the CdS are required to dissociate and store the exciton. These states are rapidly filled up with increasing excitation density, leading to a dramatic reduction in quenching efficiency. Charge separation is not instantaneous on the CdS excitonic antennae in which light absorption occurs, but arises from the relaxed exciton following hole localization in the core. Consequently, whereas strong electromodulation of the core exciton is observed, the core multiexciton and the CdS arm exciton are not affected by an external electric field.

Figure 1

Separation and storage of excitons in an electric field. (a) Transmission electron micrograph and schematic of the electronic structure of CdSe/CdS core-shell tetrapods in and out of an external electric field, where the dashed lines represent localized states, while the solid lines depict the quantum-confined band states (CB, conduction band, VB, valence band). (b) Structure of the device enabling time-resolved electromodulation of the fluorescence. (c) The PL quenching efficiency as a function of applied external electric field (i.e., device bias) shows an initial linear dependence followed by saturation. The solid pink line serves as a guide to the eye, showing a linear dependence. (d) The time-resolved PL within the electric field shows substantial quenching but the same power-law decay dynamics, followed by an intensity overshoot upon removal of the external field. (e), (f) Corresponding emission spectra of quenching and overshoot (solid black: unmodulated; dashed red: modulated).

Figure 2

Dependence of prompt PL intensity and quenching efficiency on excitation density at different external electric field strengths. (a) PL intensity dependence of the tetrapods on excitation density in the presence of different external fields, where the black line depicts a linear relationship. (b) The PL quenching efficiency of the same device in corresponding external electric fields shows a logarithmic decrease with increasing excitation density to a uniform saturation point. The blue (solid) line is a guide to the eye.

Figure 3

Transient luminescence of a device for which the electrical pulse is applied after excitation by a laser pulse. (a) The recombination rate of intrinsically generated trapped excitations is initially increased by the electric field pulse, which promotes charge detrapping of carrier pairs with dipoles parallel to the electric field. Each data point corresponds to an integral measurement using the gated ICCD camera for a particular gate width at a certain delay time after the exciting laser pulse. (b) The relative intensity of the electrically induced PL burst decreases with increasing excitation density due to the limited population of trap states in the individual nanoparticle. The inset in (b) shows the absolute area of the PL burst, which grows logarithmically with excitation density until it appears to saturate at large excitation densities. The PL burst was measured in a 60 ns integration window, delayed 1040 ns after the laser trigger.

Figure 4

Multiexciton emission from tetrapods at high excitation densities. (a) Simultaneous absorption of multiple photons in an individual nanoparticle can lead to emission either from the arm ($X_A$), the core ($X$), or multiexciton states in the core such as the biexciton ($X_B$) and triexciton ($X_T$). (b) Comparison of gated picosecond luminescence spectra of a solution of nanocrystals in toluene (excitation with a 140 fs laser pulse, detection in a 0–40 ps time window) at high (solid black curve) and low (dashed blue curve) excitation densities clearly reveals the spectral signatures of the multiexciton states. (c) Electric field quenching in devices is only observed in the core exciton channel, not for the arm exciton, biexciton, or triexciton. (d) Consequently, the stored excitation energy after removal of the electric field pulse returns solely as single exciton core emission.