The influence of surface trapping and dark states on the fluorescence emission efficiency and lifetime of CdSe and CdSe/ZnS quantum dots

Surface Sensitive Photoluminescence of Carbon Nanodots: Coupling between the Carbonyl Group and π-Electron System

The functional groups and π-electron system of carbon dots (C-dots) were carefully controlled by several innovative chemical methods, without any changes in size, to unravel the relationship between the surface structure and photoluminescence (PL). The results of experiments and theoretical calculations reveal that the PL of C-dots is related to the surface state. The energy gap is determined by the coupling of the π-electron system and carbonyl group, and the quantum yield (QY) is dependent on the carbonyl group. The carbonyl group is the main factor increasing the ratio of nonradiation to radiation recombination, thereby leading to the low QY of C-dots. This work provides a strategy for effectively tuning the structure of C-dots, giving rise to the tunable PL emission wavelength and highly desirable QY, which enables us to further unravel the PL mechanism.

Band-edge oscillator strength of colloidal CdSe/CdS dot-in-rods: comparison of absorption and time-resolved fluorescence spectroscopy

We studied the oscillator strength f_gap of the band gap transition in heteronanocrystals (hNCs) with a spherical CdSe core embedded in an elongated CdS shell. A comparison with f_gap of core-only CdSe NCs confirmed a reduction of the electron-hole overlap in hNCs with a band gap larger than 2.05 eV or smaller than 1.98 eV. However, the decrease in f_gap is limited to about 50% when compared to CdSe NCs, suggesting that residual confinement still localizes the electron near the core. We correlated f_gap with the radiative lifetime obtained from multiexponential photoluminescence (PL) decay traces. The different components were attributed to radiative decay, or deep and shallow carrier trapping, respectively, using the PL quantum efficiency (QE) as a guideline. Our data highlight the challenges associated when extracting the radiative decay, and demonstrate the added value of absorption spectroscopy to obtain the band-edge oscillator strength and the associated radiative recombination rate in colloidal hNCs.

Fluorescence-Converging Carbon Nanodots-Hybridized Silica Nanosphere

Ultrabright carbon nanodots-hybridized silica nanospheres (CSNs) are synthesized through the Stöber process of silane functionalized C-dots. The fluorescence of carbon nanodots is converged intensely. A CSN is about 3800 times brighter than a single-carbon nanodot. Along with their high brightness and low cytotoxicity, CSNs also indicate their potential application in cellular labeling.

Symmetric and asymmetric Au-AgCdSe hybrid nanorods

This paper describes a facile method for synthesis of Au-AgCdSe hybrid nanorods with controlled morphologies and spatial distributions. The synthesis involved deposition of Ag tips at the ends of Au nanorod seeds, followed by selenization of the Ag tips and overgrowth of CdSe on these sites. By simply manipulating the pH value of the system, the AgCdSe could selectively grow at one end, at both the ends or on the side surface of a Au nanorod, generating a mike-like, dumbbell-like, or toothbrush-like hybrid nanorod, respectively. These three types of Au-AgCdSe hybrid nanorods displayed distinct localized surface plasmon resonance and photoluminescence properties, demonstrating an effective pathway for maneuvering the optical properties of nanocrystals.

Hole surface trapping in CdSe nanocrystals: dynamics, rate fluctuations, and implications for blinking

Carrier trapping is one of the main sources of performance degradation in nanocrystal-based devices. Yet the dynamics of this process is still unclear. We present a comprehensive investigation into the efficiency of hole transfer to a variety of trap sites located on the surface of the core or the shell or at the core/shell interface in CdSe nanocrystals with both organic and inorganic passivation, using the atomistic semiempirical pseudopotential approach. We separate the contribution of coupling strength and energetics in different systems and trap configurations, obtaining useful general guidelines for trapping rate engineering. We find that trapping can be extremely efficient in core-only systems, with trapping times orders of magnitude faster than radiative recombination. The presence of an inorganic shell can instead bring the trapping rates well below the typical radiative recombination rates observed in these systems.

CdSe/ZnS core shell quantum dot-based FRET binary oligonucleotide probes for detection of nucleic acids

We report the design, synthesis, and characterization of a binary oligonucleotideprobe for selective DNA or RNA detection. The probe is based on fluorescence resonance energy transfer (FRET) from quantum dot (CdSe/ZnS core shell) DNA conjugates to organic dye (cyanine-5) DNA conjugates. Selective hybridization of the donor/acceptor DNA conjugates to target DNA enhances FRET and a change in fluorescence signature was observed.