Intense photoluminescence from Cu-doped CdSe nanotetrapods triggered by ultrafast hole capture

Introducing Ag Dopants into CdSe Nanoplatelets (NPLs) Leads to Effective Charge Separation for Better Photodetector Performance

Solution-processed colloidal cadmium chalcogenide nanoplatelets (NPLs)-based photodetectors (PD) are promising materials for next-generation optoelectronic devices due to their excellent optical properties. Here, we report on ultrafast carrier relaxation dynamics of four monolayer (4 ML) Ag-doped CdSe (Ag: CdSe) NPLs using ultrafast transient absorption spectroscopy and their photodetectors applications. A broad dopant emission is observed at around 650 nm with a large FWHM of ~431 meV and band edge emission at 515 nm. The intragap dopant state acts as a hole acceptor, which leads to better charge separation. The ultrafast transient absorption spectroscopy study shows faster carrier recombination dynamics with a hole transfer time scale of ~10 ps in Ag-doped CdSe NPLs. This supports the excited hole capture phenomenon at the dopant state. Ag-doped CdSe NPLs-based PD performed better than undoped CdSe NPLs with detectivity and responsivity values of 1.3×1010 Jones and 2.4 mA/W, respectively.

The effect of Cu(I)-doping on the photoinduced electron transfer from aqueous CdS quantum dots

The doping of CdS quantum dots (QDs) with Cu(I) disrupts electron-hole correlation due to hole trapping by the dopant ion, post-photoexcitation. The present paper examines the effect of such disruption on the rate of photoinduced electron transfer (PET) from the QDs to methyl viologen (MV2+), with implications in their photocatalytic activity. A significantly greater efficiency of PL quenching by MV2+ is observed for the doped QDs than for the undoped ones. Interestingly, the Stern-Volmer plots constructed using PL intensities exhibit an upward curvature for both the cases, while the PL lifetimes remain unaffected. This observation is rationalized by considering the adsorption of the quencher on the surface of the QDs and ultrafast PET post-photoexcitation. Ultrafast transient absorption experiments confirm a faster electron transfer for the doped QDs. It is also realized that the transient absorption experiment yields a more accurate estimate of the binding constant of the quencher with the QDs, than the PL experiment.

Effect of Lattice Disorder on Exciton Dynamics in Copper-Doped InP/ZnSexS1-x Core/Shell Quantum Dots

InP/ZnSexS1-x core/shell quantum dots (QDs) with varying Cu concentrations were synthesized by a one-pot hot-injection method. X-ray diffraction and high-resolution transmission electron microscopy results indicate that Cu doping did not alter the crystal structure or particle size of the QDs. The optical shifts in UV-visible absorption and photoluminescence (PL) suggest changes in the electronic structure and induction of lattice disorder due to Cu doping. Ultrafast transient absorption spectroscopy (TAS) reveled that a higher Cu-doping level leads to faster charge carrier recombination, likely due to increased nonradiative decay from defect states. Time-resolved PL (TRPL) studies show longer average lifetimes of charge carriers with increased Cu doping. These findings informed the development of a kinetic model to better understand how Cu-induced disorder affects charge carrier dynamics in the QDs, which is important for emerging applications of Cu-doped InP/ZnSexS1-x QDs in optoelectronics.

Mechanistic Study of Amphiphilic-Assisted Self-Assembled Cadmium Sulfide Quantum Dots into 3D Superstructures

Self-assembling of nanoparticles into complex superstructures is very challenging, which usually depends on postorganizing techniques or pre-existing templates such as polypeptide chains or DNA or external stimulus. Such self-assembled processes typically lead to close-packed structures. Here, it has been demonstrated that under carefully template-free reaction conditions CdS quantum dots (QDs) could be synthesized and simultaneously self-assembled into complex superstructures without compromising individual QD properties. The superstructures of CdS QDs attained by the chemical-based method demonstrate Stokes-shifted photoluminescence (PL) from trap states. Remarkably, the PL decay of superstructures exhibits a single-exponential feature. This behavior is unusual for the synthesized superstructures, indicating that the trap states are restricted to a narrow range. The growth mechanism of these superstructures is explained through the formation of liquid crystal phases (LCPs) with the help of a small-angle X-ray scattering (SAXS) analysis.

Single-Particle Dynamics of ZnS Shelling Induced Replenishment of Carrier Diffusion for Individual Emission Centers in CuInS2 Quantum Dots

Insights into blinking and photoactivation of aqueous copper indium sulfide (CIS) quantum dots have been obtained using fluorescence correlation spectroscopy (FCS) and fluorescence lifetime correlation spectroscopy (FLCS). An unusual excitation wavelength-dependence of photoactivation/photocorrosion is manifested in an increase in the initial correlation amplitude G(0) for λ_ex = 532 nm, but a decrease for λ_ex = 405 nm. This has been rationalized in terms of different contributions from surface-assisted recombination in the two cases. Blinking times obtained from the autocorrelation functions (ACFs) of the 100-200 ns lifetime component (core Cu-mediated recombination) are almost unaffected by shelling, but those from the ACF for the 10-30 ns lifetime (surface states) increase significantly. Absence of cross-correlation between the two recombinative states of bare CIS QDs and the emergence of an anticorrelation with the introduction of the ZnS shell are observed, indicating the diffusive nature of the two states for CIS-ZnS. The diffusion is inhibited in bare CIS QDs due to the preponderance of surface states.

Single particle level dynamics of photoactivation and suppression of Auger recombination in aqueous Cu-doped CdS quantum dots

Cu-doped CdS quantum dots (QDs) have been synthesized in water using 3-mercaptopropionic acid (3-MPA) as the capping agent. They exhibit intense photoluminescence and excellent color tunability, unlike most of the QDs synthesized/dispersed in water so far. Complete characterization of these aqueous doped CdS QDs has been performed for the first time, along with a single particle level elucidation of their exciton dynamics using fluorescence correlation spectroscopy. Photoactivation via dim/dark to bright particle conversion is observed at higher excitation powers. Dispersive blinking kinetics in undoped QDs reflects the involvement of a broad distribution of trap states. A lesser extent of dispersity is observed for doped QDs, in which hole-capture by Cu-defect states predominates. Excitation fluence dependence of the blinking rate highlights the role of Auger recombination in undoped QDs, which is suppressed significantly upon doping, due to disruption of the electron-hole correlation.

Exciton Dynamics in Colloidal CdS Quantum Dots with Intense and Stokes Shifted Photoluminescence in a Single Decay Channel

CdS quantum dots (QDs), synthesized by a sol-gel method, exhibit significantly Stokes shifted bright photoluminescence (PL), predominantly from the trap states. Surprisingly, the PL decay at the emission maximum is single-exponential. This is an unusual observation for as-prepared QDs and indicates a narrow distribution in the nature of trap states. A closer look reveals an additional fast component for the decays at shorter emission wavelengths, presumably due to the band edge emission, which remains elusive in the steady-state spectra. Indeed, a significantly narrower and blue-shifted emission band is observed in the decay-associated spectra. The contribution of this component to the steady-state PL intensity is shown to be overwhelmed by that of the significantly stronger trap emission. Exciton dynamics in the quantum dots is elucidated using transient absorption spectra, in which the stimulated emission is observed even at low pump power.

Role of Solvent in Electron-Phonon Relaxation Dynamics in Core-Shell Au-SiO2 Nanoparticles

Relaxation dynamics of plasmons in Au-SiO₂ core-shell nanoparticles have been followed by femtosecond pump-probe technique. The effect of excitation pump energy and surrounding medium on the time constants associated with the hot electron relaxation has been elucidated. A gradual increase in the electron-phonon relaxation time with pump energy is observed and can be attributed to the higher perturbation of the electron distribution in AuNPs at higher pump energy. Variation in time constants for the electron-phonon relaxation in different solvents is rationalized on the basis of their thermal conductivities, which govern the rate of dissipation of heat of photoexcited electrons in the nanoparticles. On the other hand, phonon-phonon relaxation is found to be much less effective than electron-phonon relaxation for the dissipation of energy of the excited electron and the time constants associated with it remain unaffected by thermal conductivity of the solvent.