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Perovskite nanocrystals (NCs) have emerged as attractive gain materials for solution-processed microlasers. Despite the recent surge of reports in this field, it is still challenging to develop low-cost perovskite NC-based microlasers with high performance. Herein, we demonstrate low-threshold, spectrally tunable lasing from ensembles of CsPbBr3 NCs deposited on silica microspheres. Multiple whispering-gallery-mode lasing is achieved from individual NC/microspheres with a low threshold of ∼3.1 μJ cm−2 and cavity quality factor of ∼1193. Through time-resolved photoluminescence measurements, electron–hole plasma recombination is elucidated as the lasing mechanism. By tuning the microsphere diameter, the desirable single-mode lasing is successfully achieved. Remarkably, the CsPbBr3 NCs display durable room-temperature lasing under ∼107 shots of pulsed laser excitation, substantially exceeding the stability of conventional colloidal NCs. These CsPbBr3 NC-based microlasers can be potentially useful in photonic applications.

Upconversion micro/nanolasers are promising in fundamental physics research and practical applications. However, due to the limitation of gain medium and cavity quality, such lasers still suffer from a high lasing threshold (P th). Herein, upconverted whispering-gallery-mode lasing by two-photon absorption is achieved from CdS microplatelets with single-mode emission and low threshold (∼1.2 mJ cm−2). The threshold is three times lower than the best reported value in previous CdS upconversion lasers. Moreover, wavelength-tunable upconverted single-mode lasing is demonstrated from 510.4 to 518.9 nm with narrow linewidths around 0.85 nm, which is further verified through numerical simulations. In addition, the size-dependent lasing behavior is realized from single-mode to multimode oscillation; the corresponding lasing threshold decreases with increasing cavity edge length (L), following a P th ∝ 1/L 2 relationship. These results underscore the promise of CdS microplatelets for developing chip-level frequency upconversion lasers.

The Poisson’s ratios of graphene-copper nanolayered (GrCuNL) composites under tension and compression are investigated by molecular dynamics and theoretical analysis. The Poisson’s ratio of a GrCuNL composite can be tuned by tailoring its repeat layer spacing without changing the topological structures. The effect of constituent nanocrystalline Cu grain size on the Poisson’s ratio is negligible. There are remarkable in-plane anisotropy and tension-compression asymmetry in the Poisson’s ratio due to the chiral difference in compressive stress in graphene layers. A mechanical model considering the chirality and repeat layer spacing is proposed, which can accurately predict the Poisson’s ratio of a GrCuNL composite. For stable GrCuNL composites, the repeat layer spacing should be larger than 2 nm, and their tunable range of Poisson’s ratio is 0.1–0.35.

We systematically investigate the ultrafast hot carrier and phonon dynamics of PtSe2 with various van der Waals (vdW) layer numbers using femtosecond transient optical spectroscopy. The hot carrier dynamics of PtSe2 is found to be strongly dependent on its thickness. For semiconducting PtSe2 films (≤4 vdW layers), electron–phonon coupling, interlayer charge transfer and nonradiative recombination of electrons and holes contribute to the hot carrier decay, while electron–phonon coupling dominates the carrier decay in metallic PtSe2 films (with > 4 vdW layers). The PtSe2 films with 2 vdW layers show the most pronounced interlayer charge transfer decay process and are thus most suitable for photoelectric sensors and energy harvesting devices. The phonon dynamics of PtSe2 is also thickness dependent; its LB mode phonon frequency decreases as the number of vdW layers increases, as a result of the thickness-dependent dielectric screening of the long-range Coulomb interaction.

Vanadium dioxide (VO2) is a typical thermal induced phase transition material, exhibiting a transition from metallic phase at high temperature to insulating phase at low temperature, which is also accompanied by a conductivity change of over several orders of magnitude. The transition property makes VO2 prominent to achieve an effective degree of control of terahertz (THz) wave. In this paper, composite films consisting of metal grating with different periods and VO2 film were prepared by polymer assisted deposition method. Although the conductivity change of VO2 films deposited on gold grating structure across phase transition was declined to about two orders of magnitude, the amplitude modulation depth of THz of the composite films can still reach a high value. Furthermore, it was found that the THz modulation depth was related with the grating period. According to theoretical simulation, the fluctuation height of VO2 films, caused by metal grating structure during growth, can be used to regulate THz wave. These results demonstrate an economic and unsophisticated method to fabricate VO2 films with thickness fluctuation structure and then tune the THz waves.

PtSe2 is an emerging layered two-dimensional material of applied interest. Its monolayer shows promising properties for applications in electronic devices, while the bandgap of a multilayer PtSe2 film can be tuned via changing its thickness. In this work the bilayer PtSe2 thin films are investigated as an example of structural engineering with first-principles calculations. Various van der Waals corrections schemes are firstly discussed, and the optB86b scheme shows a better description of the semiconductor-metal transition for PtSe2 films. Six bilayer PtSe2 thin films in different stacking modes are constructed in order to structurally tune the electronic and transport properties. The bandgap can be effectively broadened with the structural engineering for wider potential applications. The carrier mobility, dynamical stability and Raman spectra are also calculated and discussed.

The construction of van der Waals heterostructures is deemed to be a novel scheme to circumvent the shortcomings of their components and bear potentials for applications in electronic devices. Here we systematically investigate the structural and electronic properties of a monolayer MnPSe3/MoTe2 heterostructure with the first principles calculations. The heterostructure stablizes in the antiferromagnetic state and possesses a typical type-II band alignment, with which the photoexcited electrons and holes can be effectively separated and their fast recombination can hence be suppressed. Meanwhile, an inherent electric field is observed at the interface between MnPSe3 and MoTe2. Interestingly, the band gap of the heterostructure shows a quasi-linear dependence on the external electric field applied, and is tunable within the semiconductor to semimetal range. The tunability with applied strain is also investigated and discussed.

Superplasticity can be achieved in nanoglasses but at the expense of strength, and such a loss can be mitigated via embedding stronger nanocrystals, i.e., forming nanoglass/nanocrystal composites. As an illustrative case, we investigate plastic deformation of Cu64Zr36 nanoglass/nanocrystalline Cu composites during uniaxial tension and nanoindentation tests with molecular dynamics simulations. With an increasing fraction of nanocrystalline grains, the tensile strength of the composite is enhanced, while its ductility decreases. The dominant interface type changes from a glass-glass interface to glass-crystal interface to grain boundary, corresponding to a failure mode transition from superplastic flow to shear banding to brittle intercrystal fracture, respectively. Accordingly, the indentation hardness increases continuously and strain localization beneath the indenter is more and more severe. For an appropriate fraction of nanocrystalline grains, a good balance among strength, hardness and ductility can be realized, which is useful for the synthesis of novel nanograined glass/crystalline composites with high strength, high hardness and superior ductility.

Molecular dynamics simulations are conducted to investigate disordering of stishovite (a high-pressure polymorph of silica) and the role of defects including grain boundaries, vacancies and free surfaces. It is shown that pre-existent defects initiate or facilitate solid-state disordering and melting. As illustrated in the case of vacancies, melting precedes solid-state disordering for stishovite with low defect concentrations, while defect-rich stishovite may transform into high-density vitreous silica which then undergoes a (quasi-)continuous transition into melt. In sharp contrast, melting of ordinary glass at the same heating rate is sluggish yet evidently first order. Disordering on free surfaces is anisotropic, being more pronounced on (101), (011) and (111) than on (100), (010), (001) and (110) crystallographic planes.

We present a general method to determine the solid?liquid interfacial energy (?sl) from the maximum supercooling (or superheating), and apply it to the water?ice system. For solid?liquid phase transitions, the nucleation-theory-based systematics of maximum superheating and supercooling relate a dimensionless nucleation barrier to the superheating (supercooling) and heating (cooling) rates. Given superheating (or supercooling) values from either experiments or simulations, ?sl can then be deduced from the dimensionless nucleation barrier, equilibrium melting temperature and enthalpy of fusion. We demonstrate the accuracy of this approach using molecular dynamics (MD) simulations of the Lennard?Jones system: our predictions of ?sl at various pressures are in excellent agreement with independent, direct MD simulations. With this approach, we predict ?sl for the water?ice (Ih and III) system using experimental supercooling values in the pressure range of 0?0.3?GPa. The predicted value (28 ? 0.8?mJ?m?2) agrees with measurements on H2O?Ih at ambient pressure.

A thermodynamically complete ab initio equation of state (EOS) for MgO was obtained using electron density functional theory and the quasiharmonic phonon approximation, and adjusted to match the ambient density. This EOS was demonstrated to be consistent with isotherm, thermal expansivity, heat capacity and melting curve measured in static experiments, and reproduced density and temperature measurements under shock wave loading of bulk and porous periclase. The Gruneisen parameter of periclase at a given density was shown to be weakly dependent on temperature. The B1–B2 phase change was calculated to occur near 320 GPa on the principal Hugoniot. The melting locus of periclase, relevant to the Earth's lower mantle pressures, was predicted to be accessible by shock wave loading of porous periclase, which could also put pressure and temperature bounds on B1–B2 transitions.

We investigate tensile deformation of metallic glass/crystalline interpenetrating phase nanocomposites as regards the effects of specific area of amorphous/crystalline phase interfaces, and grain boundaries. As an illustrative case, large-scale molecular dynamics simulations are performed on Cu64Zr36 metallic glass/Cu nanocomposites with different specific interface areas and grain boundary characteristics. Plastic deformation is achieved via shear bands, shear transformation zones, and crystal plasticity. Three-dimensional amorphous/crystalline interfaces serve as effective barriers to the propagation of shear transformation zones and shear bands if formed, diffuse strain localizations, and give rise to improved ductility. Ductility increases with increasing specific interface area. In addition, introducing grain boundaries into the second phase facilitates crystal plasticity, which helps reduce or eliminate mature shear bands in the glass matrix.

A comparative phase transition study of nanocrystalline and micro-TiO2 has been conducted under high pressure–temperature (P–T) conditions using energy-dispersive synchrotron x-ray diffraction (XRD). Our study reveals that on compression at room temperature, the micro-tetragonal anatase-type TiO2 started to transform to the orthorhombic columbite-type TiO2 near 1.6 GPa. In contrast, we did not observe this phase transition in nano-anatase at pressures of up to 8.5 GPa. At 8.5 GPa, by applying moderate heat, both samples were transformed completely to columbite-type TiO2 almost simultaneously, indicating that heat treatment could significantly expedite this phase transition. These columbite-type TiO2 phases were quenchable because after cooling them to room temperature and decompressing them to 2.0 GPa, the XRD patterns displayed no changes in comparison with those collected at 8.6 GPa and 1270 K. At 2 GPa, we heated the specimens again, and the rutile-type TiO2 started to emerge around 970 K. This phase was also quenchable after cooling and releasing pressure to ambient conditions. The grain size effects on the phase transition were discussed based on the kinetics mechanism. This study should be of considerable interest to the fields of materials science and condensed matter.

Molecular dynamics simulations are performed to investigate hydrostatic melting and shock-induced melting of single crystal Cu described by an embedded-atom method potential. The thermodynamic (equilibrium) melting curve obtained from our simulations agrees with static experiments and independent simulations. The planar solid–liquid interfacial energy is found to increase with pressure. The amount of maximum superheating or supercooling is independent of pressure, and is 1.24 ± 0.01 and 0.68 ± 0.01 at a heating or cooling rate of 1 K ps−1, respectively. We explore shock loading along three main crystallographic directions: , and . Melting along the principal Hugoniot differs considerably from and , possibly due to different extents of solid state disordering. Along , the solid is superheated by about 20%, before it melts with a pronounced temperature drop. In contrast, melting along and is quasi-continuous, and premelting (~7%) is observed.