Low-Temperature Energy Transfer via Self-Trapped Excitons in Mn2+-Doped 2D Organometal Halide Perovskites.

We investigate the mechanisms of energy transfer in Mn2+-doped ethylammonium lead bromide (EA2PbBr4:Mn2+), a two-dimensional layered perovskite (2DLP), using cryogenic optical spectroscopy. At temperature T > 120 K, photoluminescence (PL) is dominated by emission from Mn2+, with complete suppression of band edge (BE) emission and self-trapped exciton (STE) emission. However, for T < 120 K, in addition to Mn2+ emission, PL is observed from BE and STEs. Data further reveal that for 20 K < T < 120 K, STEs form the most dominant routes in assisting energy transfer (ET) from 2DLP to Mn2+ dopants. However, at higher Mn2+ concentration, higher activation energies indicate defect states come into play, successfully competing with STEs for ET both from BE to STE states and from STE to Mn2+. Finally, using polarization-resolved spectroscopy, we demonstrate optical spin orientation of the Mn2+ ions via ET from 2DLP excitons at zero magnetic field. Our results reveal fundamental insights on the interactions between quantum confined charge carriers and dopants in organometal halide perovskites.

Size and temperature dependence of photoluminescence of hybrid perovskite nanocrystals.

In this work, we studied the effects of particles' size and temperature on the photoluminescence (PL) of CH3NH3PbBr3 perovskite nanocrystals (PNCs), with the PNC size controlled by varying the surface passivating ligands. The structural and optical properties of the PNCs were investigated using UV-Vis and PL spectroscopy, revealing strong quantum confinement effects. Temperature dependent PL measurements showed the spectral blue shift of the PL peak for the small PNCs (3.1 ± 0.2 nm) with decreasing temperature from 300 K to 20 K, which is opposite to the red shift with decreasing temperature observed for large- (9.2 ± 0.5 nm) and middle-sized (5.1 ± 0.3 nm) PNCs. The PL lifetime also increased with increasing temperature for the larger PNCs, while it remained about the same for the small and middle-sized PNCs. This increase in lifetime with temperature is attributed to exciton dissociation to free carriers at higher temperatures and to the formation of polar domains in the PNCs. However, the small and middle-sized PNCs did not show such a trend, which may be due to efficient defect passivation as higher concentration of 3-aminopropyl trimethoxysilane (APTMS) was used and to the role of particle size in surface state delocalization. Cryo-X-ray diffraction showed no new peak formation or peak splitting as temperature was varied, which suggests efficient crystal phase stabilization in PNCs of all three sizes controlled by the concentration of APTMS. These results emphasize the importance of size and surface properties of PNCs in their optical properties such as PL quantum yield, PL lifetime, and crystal phase stability.

Plasmon-actuated nano-assembled microshells.

We present three-dimensional microshells formed by self-assembly of densely-packed 5 nm gold nanoparticles (AuNPs). Surface functionalization of the AuNPs with custom-designed mesogenic molecules drives the formation of a stable and rigid shell wall, and these unique structures allow encapsulation of cargo that can be contained, virtually leakage-free, over several months. Further, by leveraging the plasmonic response of AuNPs, we can rupture the microshells using optical excitation with ultralow power (<2 mW), controllably and rapidly releasing the encapsulated contents in less than 5 s. The optimal AuNP packing in the wall, moderated by the custom ligands and verified using small angle x-ray spectroscopy, allows us to calculate the heat released in this process, and to simulate the temperature increase originating from the photothermal heating, with great accuracy. Atypically, we find the local heating does not cause a rise of more than 50 °C, which addresses a major shortcoming in plasmon actuated cargo delivery systems. This combination of spectral selectivity, low power requirements, low heat production, and fast release times, along with the versatility in terms of identity of the enclosed cargo, makes these hierarchical microshells suitable for wide-ranging applications, including biological ones.

Stabilization of the Cubic Crystalline Phase in Organometal Halide Perovskite Quantum Dots via Surface Energy Manipulation.

Surface functionalization of nanoscale materials has a significant impact on their properties. We have demonstrated the effect of different passivating ligands on the crystal phase of organometal halide perovskite quantum dots (PQDs). Using static and dynamic spectroscopy, we studied phase transitions in CH3NH3PbBr3 PQDs ligated with either octylaminebromide (P-OABr) or 3-aminopropyl triethoxysilane (P-APTES). Around 140 K, P-OABr underwent a structural phase transition from tetragonal to orthorhombic, established by the emergence of a higher energy band in the photoluminescence (PL) spectrum. This was not observed in P-APTES, despite cooling down to 20 K. Additionally, time-resolved and excitation power-dependent PL, as well as Raman spectroscopy over a range of 300-20 K, revealed that recombination rates and types of charge carriers involved are significantly different in P-APTES and P-OABr. Our findings highlight how aspects of PQD phase stabilization are linked to nanoscale morphology and the crystal phase diagram.

Low temperature excitonic spectroscopy and dynamics as a probe of quality in hybrid perovskite thin films.

We have developed a framework for using temperature dependent static and dynamic photoluminescence (PL) of hybrid organic-inorganic perovskites (PVSKs) to characterize lattice defects in thin films, based on the presence of nanodomains at low temperature. Our high-stability PVSK films are fabricated using a novel continuous liquid interface propagation technique, and in the tetragonal phase (T > 120 K), they exhibit bi-exponential recombination from free charge carriers with an average PL lifetime of ∼200 ns. Below 120 K, the emergence of the orthorhombic phase is accompanied by a reduction in lifetimes by an order of magnitude, which we establish to be the result of a crossover from free carrier to exciton-dominated radiative recombination. Analysis of the PL as a function of excitation power at different temperatures provides direct evidence that the exciton binding energy is different in the two phases, and using these results, we present a theoretical approach to estimate this variable binding energy. Our findings explain this anomalous low temperature behavior for the first time, attributing it to an inherent fundamental property of the hybrid PVSKs that can be used as an effective probe of thin film quality.