Gold, silver, platinum and palladium typically crystallize with the face-centred cubic structure. Here we report the high-yield solution synthesis of gold nanoribbons in the 4H hexagonal polytype, a previously unreported metastable phase of gold. These gold nanoribbons undergo a phase transition from the original 4H hexagonal to face-centred cubic structure on ligand exchange under ambient conditions. Using monochromated electron energy-loss spectroscopy, the strong infrared plasmon absorption of single 4H gold nanoribbons is observed. Furthermore, the 4H hexagonal phases of silver, palladium and platinum can be readily stabilized through direct epitaxial growth of these metals on the 4H gold nanoribbon surface. Our findings may open up new strategies for the crystal phase-controlled synthesis of advanced noble metal nanomaterials.

Ferroelectricity has been believed to be an important but controversial origin of the excellent photovoltaic performance of organometal trihalide perovskites (OTPs). Here we investigate the ferroelectricity of a prototype OTP, CH3NH3PbI3 (MAPbI3), both theoretically and experimentally. Our first-principles calculations based on 3-D periodic boundary conditions reveal that a ferroelectric structure with polarization of ∼8 μC/cm(2) is the globally stable one among all possible tetragonal structures; however, experimentally no room-temperature ferroelectricity is observed by using polarization-electric field hysteresis measurements and piezoresponse force microscopy. The discrepancy between our theoretical and experimental results is attributed to the dynamic orientational disorder of MA(+) groups and the semiconducting nature of MAPbI3 at room temperature. Therefore, we conclude that MAPbI3 is not ferroelectric at room temperature; however, it is possible to induce and experimentally observe apparent ferroelectric behavior through our proposed ways. Our results clarify the controversy of the ferroelectricity in MAPbI3 and also provide valuable guidance for future studies on this active topic.

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We report calculations of the band gaps and optical spectra of perovskite BaSnO3 and SrSnO3 as a function of strain. We find that the behavior of these compounds is controlled almost entirely by the volumetric strain and in particular that unlike commonly studied transition metal based perovskites, there is little sensitivity to strains other than volumetric. The most effective tuning parameters for the gap and optical properties of stannate perovskites are composition (A-site alloying) and volumetric strain.

We show, using Boltzmann transport calculations and analysis of experimental data, that hexagonal PdCoO2 and PtCoO2 have a highly unusual metallic transport. The in-plane transport is typical of a very good metal, with high conductivity and low positive thermopower. The c-axis transport is completely different, with 2 orders of magnitude lower, but still coherent, conductivity and remarkably a very large negative thermopower. This large anisotropy of the thermopower provides an opportunity for investigating transport in a highly unusual regime using bulk materials.

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High-performance piezoelectric materials are critical components for electromechanical sensors and actuators. For more than 60 years, the main strategy for obtaining large piezoelectric response has been to construct multiphase boundaries, where nanoscale domains with local structural and polar heterogeneity are formed, by tuning complex chemical compositions. We used a different strategy to emulate such local heterogeneity by forming nanopillar regions in perovskite oxide thin films. We obtained a giant effective piezoelectric coefficient d33,f* of ~1098 picometers per volt with a high Curie temperature of ~450°C. Our lead-free composition of sodium-deficient sodium niobate contains only three elements (Na, Nb, and O). The formation of local heterogeneity with nanopillars in the perovskite structure could be the basis for a general approach to designing and optimizing various functional materials.

The methylammonium lead iodide perovskite (MAPbI3) is presently a desirable material for photovoltaic application. Its structure is orthorhombic at low temperature and tetragonal at room temperature. Most theoretical works have focused on either tetragonal or orthorhombic phase alone leaving a gap in the understanding of the structural phase transition in between. In this work, by ab initio calculations, we elucidate the origin of structural phase transition between these two phases. We show that there exists a critical ratio of out-of-plane to in-plane lattice constants, c/a ∼ 1.45, where at low c/a the orthorhombic Pnma phase is stable while the tetragonal I4/mcm phase is stable at high c/a. Varying the c/a ratio leads to a change of PbI6 octahedral tilting with the rotation of CH3NH3(+) cations about the NH3 component in and out of the Oxy plane. The origin of this rotation is identified. We propose that under epitaxial conditions a gradual change in structural phase of the MAPbI3 perovskite may exist and understanding its electronic properties will be beneficial toward the solar cell community.