Dielectric constants of MAPbX3 (X = Br, I) in the 1 kHz–1 MHz range show strong temperature dependence near room temperature, in contrast to the nearly temperature-independent dielectric constant of CsPbBr3. This strong temperature dependence for MAPbX3 in the tetragonal phase is attributed to the MA+ dipoles rotating freely within the probing time scale. This interpretation is supported by ab initio molecular dynamics simulations on MAPbI3 that establish these dipoles as randomly oriented with a rotational relaxation time scale of ∼7 ps at 300 K. Further, we probe the intriguing possibility of transient polarization of these dipoles following a photoexcitation process with important consequences on the photovoltaic efficiency, using a photoexcitation pump and second harmonic generation efficiency as a probe with delay times spanning 100 fs–1.8 ns. The absence of a second harmonic signal at any delay time rules out the possibility of any transient ferroelectric state under photoexcitation.

We report the synthesis of copper iron sulfide (CuFeS2) quantum dots (QDs). These materials exhibit a tunable band gap that spans the range 0.5-2 eV (600-2500 nm). Although the as-prepared material is nonemissive, CuFeS2/CdS core/shell structures are shown to exhibit quantum yields that exceed 80%. Like other members of the I-III-VI2 family QDs, CuFeS2 based nanoparticles exhibit a long-lived emission that is significantly red-shifted compared to the band gap. CuFeS2 QDs are unique in terms of their composition. In particular, these QDs are the only band-gap-tunable infrared chromophore composed entirely of elements with atomic numbers less than 30.

While several potential applications of CuFeS quantum dots have already been reported, doubts regarding their optical and physical properties persist. In particular, it is unclear if the quantum dot material is metallic, a degenerately doped semiconductor, or else an intrinsic semiconductor material. Here we examine the physical properties of CuFeS quantum dots in order to address this issue. Specifically, we study the bump that is observed in the optical spectra of these quantum dots at ∼500 nm. Using a combination of structural and optical characterization methods, ultrafast spectroscopy, as well as electronic structure calculations, we ascertain that the unusual purple color of CuFeS quantum dots as well the golden luster of CuFeS films arise from the existence of a plasmon resonance in these materials. While the presence of free carriers causes this material to resemble gold, surface treatments are also described to suppress the plasmon resonance altogether.

We synthesized CuAlS 2 /ZnS quantum dots (QDs) composed of biocompatible, earth-abundant elements that can reduce salts of carbon dioxide under visible light. The use of an asymmetric morphology at a type-II CuAlS 2 /ZnS heterointerface balances multiple requirements of a photoredox agent by providing a low optical bandgap (∼1.5 eV), a large optical cross section (>10 −16 cm 2 above 1.8 eV), spatial proximity of both semiconductor components to the surface, as well as photochemical stability. CuAlS 2 /ZnS QDs thus have an unprecedented photochemical activity in terms of reducing carbon dioxide in the form of aqueous sodium bicarbonate under visible light, without the need for a cocatalyst, promoter, or sacrificial reagent while maintaining large turnover numbers in excess of 7 × 10 4 per QD. Devices based on these QDs exhibit energy conversion efficiencies as high as 20.2 ± 0.2%. These observations are rationalized through our spectroscopic studies that show short 550 fs electron dwell times in these structures. The high energy efficiency and the environmentally friendly composition of these materials suggest a future role in solar light harvesting.

The collapse of carriers into polarons strongly impacts properties such as charge transport, separation and recombination that are of fundamental relevance to opto-electronic devices such as photovoltaics. Here we observe the real-time process of the collapse of a wavefunction using ultrafast spectroscopy. We develop a method to extract changes in spontaneous lifetimes of an emitter over the course of its emission lifetime. This method enables us to detect the wavefunction collapse of photogenerated holes in quantum dots (QDs) . In particular, we observe that the spontaneous emission lifetime of these QDs is ∼46 ns immediately after excitonic cooling but changes drastically to ∼294 ns over the first 15 ps. The evolution in emission lifetimes and the corresponding variation in emission energetics imply changes in the hole wavefunction even after usual excitonic cooling is complete, and is consistent in its migration into a phonon coupled state located within the semiconductor band gap.