Siddharth Sampat

Influence of growth temperature on bulk and surface defects in hybrid lead halide perovskite films

The rapid development of perovskite solar cells has focused its attention on defects in perovskites, which are gradually realized to strongly control the device performance. A fundamental understanding is therefore needed for further improvement in this field. Recent efforts have mainly focused on minimizing the surface defects and grain boundaries in thin films. Using time-resolved photoluminescence spectroscopy, we show that bulk defects in perovskite samples prepared using vapor assisted solution process (VASP) play a key role in addition to surface and grain boundary defects. The defect state density of samples prepared at 150 °C (∼10(17) cm(-3)) increases by 5 fold at 175 °C even though the average grains size increases slightly, ruling out grain boundary defects as the main mechanism for the observed differences in PL properties upon annealing. Upon surface passivation using water molecules, the PL intensity and lifetime of samples prepared at 200 °C are only partially improved, remaining significantly lower than those prepared at 150 °C. Thus, the present study indicates that the majority of these defect states observed at elevated growth temperatures originates from bulk defects and underscores the importance to control the formation of bulk defects together with grain boundary and surface defects to further improve the optoelectronic properties of perovskites.

Order of magnitude enhancement of monolayer MoS2 photoluminescence due to near-field energy influx from nanocrystal films

Two-dimensional transition metal dichalcogenides (TMDCs) like MoS2 are promising candidates for various optoelectronic applications. The typical photoluminescence (PL) of monolayer MoS2 is however known to suffer very low quantum yields. We demonstrate a 10-fold increase of MoS2 excitonic PL enabled by nonradiative energy transfer (NRET) from adjacent nanocrystal quantum dot (NQD) films. The understanding of this effect is facilitated by our application of transient absorption (TA) spectroscopy to monitor the energy influx into the monolayer MoS2 in the process of ET from photoexcited CdSe/ZnS nanocrystals. In contrast to PL spectroscopy, TA can detect even non-emissive excitons, and we register an order of magnitude enhancement of the MoS2 excitonic TA signatures in hybrids with NQDs. The appearance of ET-induced nanosecond-scale kinetics in TA features is consistent with PL dynamics of energy-accepting MoS2 and PL quenching data of the energy-donating NQDs. The observed enhancement is attributed to the reduction of recombination losses for excitons gradually transferred into MoS2 under quasi-resonant conditions as compared with their direct photoproduction. The TA and PL data clearly illustrate the efficacy of MoS2 and likely other TMDC materials as energy acceptors and the possibility of their practical utilization in NRET-coupled hybrid nanostructures.

Plasmonic giant semiconductor nanocrystals with enhanced light output and suppressed blinking for biomedical applications

Hybrid semiconductor-metal nanoparticles are of both fundamental and practical interest. CdSe semiconductor nanocrystal quantum dots (NQDs) are emissive following direct absorption of photons, while nanosized metal structures exhibit plasmonic absorption and light scattering. In a hybrid nanoscale architecture, both optical phenomena can be integrated as metal-semiconductor coupling via non-radiative energy transfer and modifications of the radiative decay rates through Purcell effect, alter the emission mechanism resulting in a range of effects from photoluminescence (PL) quenching to enhancement.