Formamidinium lead halide (FAPbX, X = Cl, Br, I) perovskite materials have recently drawn an increased amount of attention owing to their superior optoelectronic properties and enhanced material stability as compared with their methylammonium-based (MA-based) analogues. Herein, we report a study of the pressure-induced structural and optical evolutions of FAPbI hybrid organic-inorganic perovskite nanocrystals (NCs) using a synchrotron-based X-ray scattering technique coupled to in situ absorption and photoluminescence spectroscopies. As a result of their unique structural stability and soft nature, FAPbI NCs exhibit a wide range of band-gap tunability (1.44-2.17 eV) as a function of pressure (0-13.4 GPa). The study presented here not only provides an efficient and chemically orthogonal means to controllably engineer the band gap of FAPbI NCs using pressure but more importantly sheds light on how to strategically design the band gaps of FA-based hybrid organic-inorganic perovskites for various optoelectronic applications.

Understanding the underlying mechanisms that suppress thermal conduction in solids is of paramount importance for the targeted design of materials for thermal management and thermoelectric energy conversion applications. Bismuth copper oxychalcogenides, BiOCuQ (Q = Se, Te), are highly crystalline thermoelectric materials with an unusually low lattice thermal conductivity of ∼0.5 Wm(-1) K(-1), a value normally found in amorphous materials. Here we unveil the origin of the unusual thermal transport properties of these phases. First principles calculations of the vibrational properties combined with analysis of in-situ neutron diffraction data, demonstrate that weak bonding of copper atoms within the structure leads to an unexpected vibrational mode at low frequencies, which is likely to be a major contributor to the low thermal conductivity of these materials. In addition, we show that anharmonicity and the large Grüneisen parameter in these oxychalcogenides are mainly related to the low frequency copper vibrations, rather than to the Bi(3+) lone pairs.

Doped lead halide perovskite nanocrystals (NCs) have garnered significant attention due to their superior optoelectronic properties. Here, we report a synthesis of Cd-doped CsPbCl 3 NCs by decoupling Pb-and Cl-precursors in a hot injection method. The resulting Cd-doped perovskite NCs manifest a dual-wavelength emission profile with the first reported example of Cd-dopant emission. By controlling Cd-dopant concentration, the emission profile can be tuned with a dopant emission quantum yield of up to 8%. A new secondary emission (∼610 nm) is induced by an energy transfer process from photoexcited hosts to Cd-dopants and a subsequent electronic transition from the excited state ( 3 E g ) to the ground state ( 1 A 1g ) of [CdCl 6 ] 4− units. This electronic transition matches well with a first-principles density functional theory calculation. Further, the optical behavior of Cd-doped CsPbCl 3 NCs can be altered through postsynthetic anion-exchange reactions. Our studies present a new model system for doping chemistry studies in semiconductors for various optoelectronic applications.