Elimination of Hole–Surface Overlap in Graded CdS_<i>x</i>Se_1–<i>x</i> Nanocrystals Revealed by Ultrafast Fluorescence Upconversion Spectroscopy

Interaction of charge carriers with the surface of semiconductor nanocrystals plays an integral role in determining the ultimate fate of the excited state. The surface contains a dynamic ensemble of trap states that can localize excited charges, preventing radiative recombination and reducing fluorescence quantum yields. Here we report quasi-type-II band alignment in graded alloy CdS_<i>x</i>Se_1–<i>x</i> nanocrystals revealed by femtosecond fluorescence upconversion spectroscopy. Graded alloy CdS_<i>x</i>Se_1–<i>x</i> quantum dots are a compositionally inhomogeneous nano-heterostructure designed to decouple the exciton from the nanocrystal surface. The large valence band offset between the CdSe-rich core and CdS-rich shell separates the excited hole from the surface by confining it to the core of the nanocrystal. The small conduction band offset, however, allows the electron to delocalize throughout the entire nanocrystal and maintain overlap with the surface. Indeed, the ultrafast charge carrier dynamics reveal that the fast 1–3 ps hole-trapping process is fully eliminated with increasing sulfur composition and the decay constant for electron trapping (∼20–25 ps) shows a slight increase. These findings demonstrate progress toward highly efficient nanocrystal fluorophores that are independent of their surface chemistry to ultimately enable their incorporation into a diverse range of applications without experiencing adverse effects arising from dissimilar environments

Plasmonic Cu_<i>x</i>In_<i>y</i>S₂ Quantum Dots Make Better Photovoltaics Than Their Nonplasmonic Counterparts

A synthetic approach has recently been developed which results in Cu_<i>x</i>In_<i>y</i>S₂ quantum dots (QDs) possessing localized surface plasmon resonance (LSPR) modes in the near-infrared (NIR) frequencies. In this study, we investigate the potential benefits of near-field plasmonic effects centered upon light absorbing nanoparticles in a photovoltaic system by developing and verifying nonplasmonic counterparts as an experimental control. Simple QD-sensitized solar cells (QD-SSCs) were assembled which show an 11.5% relative increase in incident photon conversion efficiency (IPCE) achieved in the plasmon-enhanced devices. We attribute this increase in IPCE to augmented charge excitation stemming from near-field “antenna” effects in the plasmonic Cu_<i>x</i>In_<i>y</i>S₂ QD-SSCs. This study represents the first of its kind; direct interrogation of the influence of plasmon-on-semiconductor architectures with respect to excitonic absorption in photovoltaic systems

Correlation of Atomic Structure and Photoluminescence of the Same Quantum Dot: Pinpointing Surface and Internal Defects That Inhibit Photoluminescence

In a size regime where every atom counts, rational design and synthesis of optimal nanostructures demands direct interrogation of the effects of structural divergence of individuals on the ensemble-averaged property. To this end, we have explored the structure–function relationship of single quantum dots (QDs) <i>via</i> precise observation of the impact of atomic arrangement on QD fluorescence. Utilizing wide-field fluorescence microscopy and atomic number contrast scanning transmission electron microscopy (Z-STEM), we have achieved correlation of photoluminescence (PL) data and atomic-level structural information from individual colloidal QDs. This investigation of CdSe/CdS core/shell QDs has enabled exploration of the fine structural factors necessary to control QD PL. Additionally, we have identified specific morphological and structural anomalies, in the form of internal and surface defects, that consistently vitiate QD PL

Quantum Yield Heterogeneity among Single Nonblinking Quantum Dots Revealed by Atomic Structure-Quantum Optics Correlation

Physical variations in colloidal nanostructures give rise to heterogeneity in expressed optical behavior. This correlation between nanoscale structure and function demands interrogation of both atomic structure and photophysics at the level of single nanostructures to be fully understood. Herein, by conducting detailed analyses of fine atomic structure, chemical composition, and time-resolved single-photon photoluminescence data for the same individual nanocrystals, we reveal inhomogeneity in the quantum yields of single nonblinking “giant” CdSe/CdS core/shell quantum dots (g-QDs). We find that each g-QD possesses distinctive single exciton and biexciton quantum yields that result mainly from variations in the degree of charging, rather than from volume or structure inhomogeneity. We further establish that there is a very limited nonemissive “dark” fraction (<2%) among the studied g-QDs and present direct evidence that the g-QD core must lack inorganic passivation for the g-QD to be “dark”. Therefore, in contrast to conventional QDs, ensemble photoluminescence quantum yield is principally defined by charging processes rather than the existence of dark g-QDs