In recent decades, the family of metal halide hybrid perovskites has attracted attention owing to record-breaking achievements in fields such as photovoltaics (PV), light-emitting diodes (LEDs), lasers, sensors, and many other electronic devices. [1][2][3][4][5][6][7] Perovskites, with the generic chemical formula ABX 3 , owe these properties to their flexible cage crystal structure where the A-site is occupied by a monovalent cation which could either be an organic molecule, such as methylammonium2 ) (FA), or an inorganic atom, such as cesium (Cs þ ); the B-site accommodates a divalent inorganic cation such as lead (Pb 2þ ) or tin (Sn 2þ ); and X position is occupied by a halide group, which could be chloride (Cl À ), bromide (Br À ), or iodide (I À ). [8] A critical issue hindering the commercialization of perovskite-based optoelectronic devices is instability and phase degradation. The desirable α-phase perovskite structure, which is ideal for photoelectric conversion, eventually degrades in ambient conditions into the δ-phase, which is a yellowish nonperovskite phase with an unwanted large bandgap and poor charge transport. [3,[9][10][11][12] Various approaches have been attempted to limit such degradation processes. Common is to use encapsulation approaches, including copolymer micellar shielding, [13][14][15] core-shell formation, [15,16] polymer coprecipitation, [17] solid polymer composite formation, [18][19][20][21] incorporation into metalorganic frameworks (MOFs), [22] or in situ stabilization in mesoporous templates. [23] Another approach is through substitution of suitable ions which stabilize the cage symmetry of optically active perovskites. [6,24,25] Importantly, the substitution of ions also induces changes in the emission and absorption spectrum of the parent composition, resulting in a tunable bandgap by tailoring of the ionic composition. [26] Substitution of halides in particular can be leveraged to tune the emission maxima of the perovskite between 400 ≤ λ ≤ 800 nm. [27][28][29] Therefore, halide substitution offers the best avenue for improving the stability of ABX 3 perovskites, while simultaneously allowing bandgap tunability.

Vertical nanowire (NW) arrays are the basis for a variety of nanoscale devices. Understanding heat transport in these devices is an important concern, especially for prospective thermoelectric applications. To facilitate thermal conductivity measurements on as-grown NW arrays, a common NW-composite device architecture was adapted for use with the 3ω method. We describe the application of this technique to obtain thermal conductivity measurements on two GaAs NW arrays featuring ~130 nm diameter NWs with a twinning superlattice (TSL) and a polytypic (zincblende/wurtzite) crystal structure, respectively. Our results indicate NW thermal conductivities of 5.2 ± 1.0 W/m-K and 8.4 ± 1.6 W/m-K in the two samples, respectively, showing a significant reduction in the former, which is the first such measurements on TSL NWs. Nearly an order of magnitude difference from the bulk thermal conductivity (~50 W/m-K) is observed for the TSL NW sample, one of the lowest values measured to date for GaAs NWs.

No abstract

No abstract