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.

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Controlling the thermal conductivity of semiconductors is of practical interest in optimizing the performance of thermoelectric and phononic devices. The insertion of inclusions of nanometer size in a semiconductor is an effective means of achieving such control; it has been proposed that the thermal conductivity of silicon could be reduced to 1 W/m/K using this approach and that a minimum in the heat conductivity would be reached for some optimal size of the inclusions. Yet the experimental verification of this design rule has been limited. In this work, we address this question by studying the thermal properties of silicon metalattices that consist of a periodic distribution of spherical inclusions with radii from 7 to 30 nm, embedded into silicon. Experimental measurements confirm that the thermal conductivity of silicon metalattices is as low as 1 W/m/K for silica inclusions and that this value can be further reduced to 0.16 W/m/K for silicon metalattices with empty pores. A detailed model of ballistic phonon transport suggests that this thermal conductivity is close to the lowest achievable by tuning the radius and spacing of the periodic inhomogeneities. This study is a significant step in elucidating the scaling laws that dictate ballistic heat transport at the nanoscale in silicon and other semiconductors.

In view of the continued controversy concerning the polar/nonpolar nature of the hybrid perovskite system, CH3NH3PbI3, we report the first investigation of a time-resolved pump-probe measurement of the second harmonic generation efficiency as well as using its more traditional form as a sensitive probe of the absence/presence of the center of inversion in the system both in its excited and ground states, respectively. Our results clearly show that SHG efficiency, if nonzero, is below the limit of detection, strongly indicative of a nonpolar or centrosymmetric structure. Our results on the same samples, based on temperature dependent single crystal X-ray diffraction and P-E loop measurements, are entirely consistent with the above conclusion of a centrosymmetric structure for this compound in all three phases, namely the high temperature cubic phase, the intermediate temperature tetragonal phase and the low temperature orthorhombic phase. It is important to note that all our experimental probes are volume averaging and performed on bulk materials, suggesting that basic material properties of CH3NH3PbI3 are consistent with a centrosymmetric, nonpolar structure.

Li 2 MO 3 -based layered structures play an important role in lithium rich composites to realize high energy density cathode for lithiumion batteries. However, detailed storage mechanism in lithium rich composition is still not clear due to complicated redox chemistry and their synergy effect. In the present work, we report electrochemical property of Li 2 RuO 3 and Li 2 IrO 3 and their solid solution Li 2 Ru 1-x Ir x O 3 (0 ≤ x ≤ 1). Despite having the same structure they differ in space group (C2/c for Li 2 RuO 3 and C2/m for Li 2 IrO 3 ), however; show the similar electrochemistry against lithium. The solid solution of Li 2 Ru 1-x Ir x O 3 compositions show a mix behavior depending upon the concentration of the specific phase. Electrochemical performance clarified that specific Li/Li 2 Ru 0.90 Ir 0.10 O 3 composition shows better cyclic behavior with 198.30 mAh g −1 discharge capacity at 40 mA g −1 current rate. During this study, few underlying points observed here are 1) high voltage oxidation process is mainly due to O 2 removal during de-lithiation, 2) the amount of O 2 removal decreases as a function of Ir component in the solid solution increases and more importantly 3) we suspect there will be Ru-Ir bond formation along with Ru-Ru and Ir-Ir bonds. These strong metal-metal bonding in the metal oxide plane shifts the electronic energy levels, as a consequence, oxygen removal is somewhat restricted. Therefore, the objective of this project is to extract knowledge from these two layered materials and use further to design new excellent cathode materials.