Realizing reversible phase transformation of shape memory ceramics constrained in aluminum.

Small-scale shape memory ceramics exhibit superior shape memory or superelasticity properties, while their integration into a matrix material and the subsequent attainment of their reversible tetragonal-monoclinic phase transformations remains a challenge. Here, cerium-doped zirconia (CZ) reinforced aluminum (Al) matrix composite is fabricated, and both macroscopic and microscopic mechanical tests reveal more than doubled compressive strength and energy absorbance of the composites as compared with pure Al. Full austenitization in the CZ single-crystal clusters is achieved when they are constrained by the Al matrix, and reversible martensitic transformation triggered by thermal or stress stimuli is observed in the composite micro-pillars without causing fracture in the composite.

Simultaneous Enhancement of Polymerization Kinetics and Properties of Phthalonitrile Using Alumina Fillers.

Alumina particles are investigated as a potential catalyst for phthalonitrile polymerization and as a property enhancer. In this work, extensive characterizations were conducted on alumina-filled resorcinol-based phthalonitrile to differentiate between the catalytic effect and the filler effect. Thermal gravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy suggest the occurrence of chemical interaction between alumina fillers and phthalonitrile, which provides an insight into the better performance of alumina-filled phthalonitrile resins.

Bond engineering of molecular ferroelectrics renders soft and high-performance piezoelectric energy harvesting materials.

Piezoelectric materials convert mechanical stress to electrical energy and thus are widely used in energy harvesting and wearable devices. However, in the piezoelectric family, there are two pairs of properties that improving one of them will generally compromises the other, which limits their applications. The first pair is piezoelectric strain and voltage constant, and the second is piezoelectric performance and mechanical softness.

3D Printing of Transparent Spinel Ceramics with Transmittance Approaching the Theoretical Limit.

3D printing of transparent ceramics has attracted great attention recently but faces the challenges of low transparency and low printing resolution. Herein, magnesium aluminate spinel transparent ceramics with transmittance reaching 97% of the theoretical limit are successfully fabricated using a stereolithography-based 3D printing method assisted by hot isostatic pressing and the critical factors governing the transparency are revealed. Various transparent spinel lenses and microlattices are printed at a high resolution of ≈100-200 µm.

Unraveling the Mechanistic Origins of Epoxy Degradation in Acids.

Water diffusion into polymers like thermosetting epoxies is well-studied; however, comparably little has been reported thus far on the related but very different mechanism of acid diffusion and the corresponding influence on material degradation. The diffusion of hydrochloric acid into an amine-cured epoxy system was studied in this work using gravimetric analysis and dielectric monitoring concurrently, and the mass uptake behavior was observed to differ significantly compared with water diffusion, faster by an order of magnitude. A unique 3-stage diffusion of acid into epoxy was observed due to the influence of Coulombic interactions between oppositely charged ionic species diffusing at different rates.

High-Density 3D-Boron Nitride and 3D-Graphene for High-Performance Nano-Thermal Interface Material.

Compression studies on three-dimensional foam-like graphene and h-BN (3D-C and 3D-BN) revealed their high cross-plane thermal conductivity (62-86 W m K) and excellent surface conformity, characteristics essential for thermal management needs. Comparative studies to state-of-the-art materials and other materials currently under research for heat dissipation revealed 3D-foam's improved performance (20-30% improved cooling, temperature decrease by ΔT of 44-24 °C).

High brightness formamidinium lead bromide perovskite nanocrystal light emitting devices.

Formamidinium lead halide (FAPbX) has attracted greater attention and is more prominent recently in photovoltaic devices due to its broad absorption and higher thermal stability in comparison to more popular methylammonium lead halide MAPbX. Herein, a simple and highly reproducible room temperature synthesis of device grade high quality formamidinium lead bromide CH(NH)PbBr (FAPbBr) colloidal nanocrystals (NC) having high photoluminescence quantum efficiency (PLQE) of 55-65% is reported. In addition, we demonstrate high brightness perovskite light emitting device (Pe-LED) with these FAPbBr perovskite NC thin film using 2,2',2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) commonly known as TPBi and 4,6-Bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine (B3PYMPM) as electron transport layers (ETL).

Stabilization of 4H hexagonal phase in gold nanoribbons.

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.

Synthesis of ultrathin face-centered-cubic au@pt and au@pd core-shell nanoplates from hexagonal-close-packed au square sheets.

The synthesis of ultrathin face-centered-cubic (fcc) Au@Pt rhombic nanoplates is reported through the epitaxial growth of Pt on hexagonal-close-packed (hcp) Au square sheets (AuSSs). The Pt-layer growth results in a hcp-to-fcc phase transformation of the AuSSs under ambient conditions. Interestingly, the obtained fcc Au@Pt rhombic nanoplates demonstrate a unique (101)f orientation with the same atomic arrangement extending from the Au core to the Pt shell.

Surface modification-induced phase transformation of hexagonal close-packed gold square sheets.

Conventionally, the phase transformation of inorganic nanocrystals is realized under extreme conditions (for example, high temperature or high pressure). Here we report the complete phase transformation of Au square sheets (AuSSs) from hexagonal close-packed (hcp) to face-centered cubic (fcc) structures at ambient conditions via surface ligand exchange, resulting in the formation of (100)f-oriented fcc AuSSs. Importantly, the phase transformation can also be realized through the coating of a thin metal film (for example, Ag) on hcp AuSSs.

The role of metal layers in the formation of metal-silicon hybrid nanoneedle arrays.

We investigated nanoneedle arrays fabricated on a series of metal-silicon substrates using Ga(+) ion beam patterning. It is shown that the low sputtering rate of the metal is preserved on the tip of each nanoneedle in the form of a gallium alloy nanodot. The generated nanodot was found to greatly alleviate the ion sputtering of the underlying materials.

Shape memory and superelastic ceramics at small scales.

Shape memory materials are a class of smart materials able to convert heat into mechanical strain (or strain into heat) by virtue of a martensitic phase transformation. Some brittle materials such as intermetallics and ceramics exhibit a martensitic transformation but fail by cracking at low strains and after only a few applied strain cycles. Here we show that such failure can be suppressed in normally brittle martensitic ceramics by providing a fine-scale structure with few crystal grains.

Towards perfectly ordered novel ZnO/Si nano-heterojunction arrays.

The fabrication of a highly ordered novel ZnO/Si nano-heterojuntion array is introduced. ZnO seed layer is first deposited on the Si (P<111>) surface. The nucleation sites are then defined by patterning the surface through focused ion beam (FIB) system.

A hybrid nanostructure array for gas sensing with ultralow field ionization voltage.

We fabricate a unique hybrid nanostructure array for gas sensing based on the polarization mechanism at the nanoscale. It is shown that with platinum nanocrystallites on the top of each nanoneedle, this array can work at ultralow voltages (less than 10 V) as a field ionization gas sensor. We believe that the polarized platinum brings about a local enhanced electrical field, leading to the direct field ionization of gas molecules, which is confirmed by calculations of the charge accumulation and electrical field distribution.

Forest of gold nanowires: a new type of nanocrystal growth.

We report a nanowire growth that is highly unconventional: (1) nanowires can grow from substrate-bound seeds but cannot from colloidal seeds under otherwise the same conditions; (2) the nanowires grow from only one side of the seeds, with their diameter independent of the size of the seeds; and (3) vertically aligned ultrathin nanowires are obtained on substrates, using aqueous solution and ambient conditions. With carefully designed experiments, we propose and test a new mechanism that can explain these unusual phenonmena. It turns out that the strong binding of ligands in this system forces selective deposition of Au at the ligand-deficient interface between Au seeds and oxide substrates.

Joining copper oxide nanotube arrays driven by the nanoscale Kirkendall effect.

Various annealing conditions (environment, temperature, and duration) are applied to study the nanoscale Kirkendall effect of copper (Cu) nanowire (NW) arrays on a Si substrate. The results show that an appropriate amount of oxygen supply is crucial for uniform transformation from Cu NWs (average diameter ∼50 nm) into Cu oxide nanotube arrays. An annealing duration of 30 min at 200 °C in a low vacuum environment reveals that the voids are not uniformly distributed at the Cu/Cu oxide interface.

Vapor-liquid-solid growth of endotaxial semiconductor nanowires.

Free-standing and in-plane lateral nanowires (NWs) grown by the vapor-liquid-solid (VLS) process have been widely reported. Herein, we demonstrate that the VLS method can be extended to the synthesis of horizontally aligned semiconductor NWs embedded in substrates. Endotaxial SiGe NWs were grown in silicon substrates by tuning the directional movement of the catalyst in the substrates.

Self-organization of a hybrid nanostructure consisting of a nanoneedle and nanodot.

A special materials system that allows the self-organization of a unique hybrid nanonipple structure is developed. The system consists of a nanoneedle with a small nanodot sitting on top. Such hybrid nanonipples provide building blocks to assemble functional devices with significantly improved performance.

Graphene oxide-templated synthesis of ultrathin or tadpole-shaped au nanowires with alternating hcp and fcc domains.

Ultrathin Au nanowires (AuNWs) and tadpole-shaped nanowires are synthesized on graphene oxide (GO) sheet templates. For the first time, 1.6 nm-diameter AuNWs are shown to contain hexagonal close-packed (hcp) crystal domains, and the tadpole-shaped nanowires exhibit alternating sets of hcp and face-centered cubic (fcc) structures, associated with variation in wire thickness.

Synthesis of hexagonal close-packed gold nanostructures.

Solid gold is usually most stable as a face-centred cubic (fcc) structure. To date, no one has synthesized a colloidal form of Au that is exclusively hexagonal close-packed (hcp) and stable under ambient conditions. Here we report the first in situ synthesis of dispersible hcp Au square sheets on graphene oxide sheets, which exhibit an edge length of 200-500 nm and a thickness of ~ 2.

Surface-induced synthesis and self-assembly of metal suprastructures.

A systematic study on the synthesis of a series of self-assembled suprastructures, such as cubes, stars, belts, and microspheres, of Ag nanoparticles (AgNPs) in borosilicate glassware heavily cleaned with aqua regia is presented. These self-assembled structures are mostly formed from the crystallographically iso-oriented AgNPs, and exhibit well-defined shapes. In regular washed glassware, only Ag nanowires are synthesized.

Photochemically controlled synthesis of anisotropic Au nanostructures: platelet-like Au nanorods and six-star Au nanoparticles.

We report the shape-controlled synthesis of anisotropic Au nanostructures through TiO(2)-assisted photochemical reduction of HAuCl(4). By using this method, we have successfully synthesized the platelet-like Au nanorods and six-star Au nanoparticles. Importantly, the platelet Au nanorod exhibits the unique asymmetric five-twinned structure.

Network-enhanced photoresponse time of Ge nanowire photodetectors.

We demonstrated that the photoresponse time of Ge nanowire (NW) photodetectors could be greatly improved by using percolated NW networks (instead of single NW) as the active detection channels. Although the reset time for single-NW devices was >70 s, a fast reset time <1 s was observed for NW-network devices. The enhancement was attributed to the barrier-dominated conductance for network devices, which was not available in single-NW devices.