In recent years, perovskite solar cells have been considerably developed, however the lead in the absorber MAPbI3 is a potential threat to the environment. To explore potential alternatives, the structural and electronic properties of MAGeX3 (X = Cl, Br, I) were investigated using different density functional theory methods, including GGA-PBE, PBE-SOC, HSE06 and HSE-SOC. The results implied that MAGeI3 exhibits an analogous band gap, substantial stability, remarkable optical properties, and significant hole and electron conductive behavior compared with the so far widely used absorber MAPbI3. Moreover, the calculations revealed that the energy splitting resulting from the spin-orbit coupling is evident on Pb, moderate on Ge, I and Br, and negligible on Cl. Our work not only sheds some light on screening novel absorbers for perovskite solar cells but also deepens the understanding of these functional materials.

Thermoelectric material provides a high hope for converting harmful and useless heat into useful energy -electricity. It can also be used as solid-state Peltier coolers in integrated circuits -an outstanding challenge for electronic engineers. The desire for a high efficient thermoelectric material has never been so keen. Although many works have been done, we are still far from having a recipe for thermoelectric materials. Nano-structuring provides an effective way to increase figure of merit (ZT) by reducing the thermal conductivity without affecting electronic property. 1 Here, we propose a novel nanoscale three-dimensional (3D) Si phononic crystal (PnC) with spherical pores, which can reduce the thermal conductivity of bulk Si by a factor up to 10,000 times at room temperature. The extreme-low thermal conductivity could lead to a larger value of ZT than unity. The thermal conductivity changes little when temperature increases from room temperature to 1100 K. The phonon participation ratio spectra show there are more phonon localizations as the porosity of PnC increases.Compared with other thermoelectric materials, Si nanostructures are low-cost, environment friendly and widely used in semiconductor industry. 2,3-5 Similar to photonic crystals, PnCs are constructed by a periodic array of scattering inclusions distributed in a host material.Since the period length decreases to nanometers, PnCs can affect the transport of terahertz lattice vibrations -phonons. Due to its periodic change of the density and /or elastic constants, PnCs exhibit phononic band gaps. 5This remarkable property is very different from those of traditional materials and can be engineered to achieve new functionalities, such as modulating thermal conductivity.A successful methodology has been reported to implement two-dimensional (2D) PnC geometry in Si,6,7 which is compatible with standard CMOS fabrication and mass produced. can also significantly reduce lattice thermal conductivity with specific choices of the pore size and spacing. On the other hand, Si PnCs and nanoporous Si could preserve their electrical properties with little degradation. 6,10 Consequently, the reduction of thermal conductivity in nanoscale Si PnCs and nanoporous Si could lead to a larger ZT than unity.The 3D Si PnC is constructed by periodic arrangement of nanoscale supercell constructed from a cubic cell with a spherical pore (shown in Fig.1). The centers of cubic and spherical pore are overlapped. The period length of PnC is the distance between centers of two nearest supercell.The porosity is defined as the ratio of number of removed atoms in pore to the total number of atoms in a cubic Si cell.To compare the thermal conductivity of 3D PnC with bulk Si, we calculate the thermal conductivity of bulk Si at 300 K as 170±16 W/m-K by EMD method, where the volume of the simulation cell of bulk Si is 12×12×12 units The experimental value of thermal conductivity of bulk Si at 300 K 13 is around 156 W/m-K, which means the thermal conductivity obtained from molecular dyna...

We studied the thermal conductivity of graphene phononic crystal (GPnC), also named as graphene nanomesh, by molecular dynamics simulations. The dependences of thermal conductivity of GPnCs (κ GPnC ) on both length and temperature are investigated. It is found that the thermal conductivity of GPnCs is significantly lower than that of graphene (κ G ) and can be efficiently tuned by changing the porosity and period length.For example, the ratio κ GPnC / κ G can be changed from 0.1 to 0.01 when the porosity is changed from about 21% to 65%. The phonon participation ratio spectra reveal that more phonon modes are localized in GPnCs with larger porosity. Our results suggest that creating GPnCs is a valuable method to efficiently manipulate the thermal conductivity of graphene.

Porous materials provide a large surface-to-volume ratio, thereby providing a knob to alter fundamental properties in unprecedented ways. In thermal transport, porous nanomaterials can reduce thermal conductivity by not only enhancing phonon scattering from the boundaries of the pores and therefore decreasing the phonon mean free path, but also by reducing the phonon group velocity. Herein, a structure-property relationship is established by measuring the porosity and thermal conductivity of individual electrolessly etched single-crystalline silicon nanowires using a novel electron-beam heating technique. Such porous silicon nanowires exhibit extremely low diffusive thermal conductivity (as low as 0.33 W m −1 K −1 at 300 K for 43% porosity), even lower than that of amorphous silicon. The origin of such ultralow thermal conductivity is understood as a reduction in the phonon group velocity, experimentally verified by measuring the Young's modulus, as well as the smallest structural size ever reported in crystalline silicon (<5 nm). Molecular dynamics simulations support the observation of a drastic reduction in thermal conductivity of silicon nanowires as a function of porosity. Such porous materials provide an intriguing platform to tune phonon transport, which can be useful in the design of functional materials toward electronics and nanoelectromechanical systems.

Investigating noble-metal-free and earth-abundant cocatalysts has emerged as a promising and challenging issue for improving the photocatalytic hydrogen production efficiency. In this work, a novel CoNi-ZnIn2S4 (CoNi-ZIS) photocatalyst composed of ZnIn2S4 (ZIS) nanosheets (2D) and CoNi bimetal nanosheets (2D) has been designed and fabricated via a simple hydrothermal and chemical reduction process. The detailed characterization results suggest that the introduction of CoNi bimetal is the major driving factor for the improved photocatalytic performance of CoNi-ZIS photocatalyst, which can accelerate the charge migration and inhibit the photoinduced charge carrier (electrons and holes) recombination. The photocatalytic hydrogen production rate of CoNi-ZIS prepared under the optimized condition reaches 100.1 μmol·h–1 irradiated by visible light (λ > 420 nm) in the 0.1 M ascorbic acid (AA), which is approximately 4.3 times higher than that of ZIS. In addition, the potential mechanism over boosted photocatalytic activity was presented according to the characterization results. This study not only develops a specific photocatalyst with an outstanding H2 production property but also provides a favorable inspiration for designing efficient ZnIn2S4-based photocatalyst systems.

Silicon membranes patterned by nanometer-scale pillars standing on the surface provide a practical platform for thermal conductivity reduction by resonance hybridization. Using molecular simulations, we investigate the effect of nanopillar size, unit-cell size, and finite-structure size on the net capacity of the local resonators in reducing the thermal conductivity of the base membrane. The results indicate that the thermal conductivity reduction increases as the ratio of the volumetric size of a unit nanopillar to that of the base membrane is increased, and the intensity of this reduction varies with unit-cell size at a rate dependent on the volumetric ratio. Considering sample size, the resonance-induced thermal conductivity drop is shown to increase slightly with the number of unit cells until it would eventually level off.In semiconducting materials, heat is carried mostly by phonons which are quanta of lattice vibrations [1]. This provides an opportunity to introduce significant changes to the thermal transport properties by direct engineering of the phonon characteristics−which are shaped primarily by the phonon band structure and the nature of the underlying scattering mechanisms [2]. Recent reviews survey developments in theory, computation, and experiment pertaining to nanoscale thermal transport in a variety of materials and point to the remarkable possibilities for using nanostructuring as a means for phonon engineering [3].Thermoelectric energy conversion stands to benefit profoundly from the ability to alter the phonon properties by nanostructuring [4], as well as by reducing the material dimensionality [5]. Thermoelectric materials, which generate electricity from heat and vice versa, are characterized by a figure of merit defined as ZT = σT S 2 /k, where S is the Seebeck coefficient, σ is the electrical conductivity, k is the thermal conductivity (consisting of a lattice component and an electrical component), and T is absolute temperature [6]. One strategy to improve the value of ZT , particularly in semiconductors, is to reduce the lattice thermal conductivity and attempt to do so without negatively affecting S and σ. A promising approach for achieving this goal is to introduce nanoscale local resonators as intrinsic substructures within, or attached to, a host crystalline material [7,8]. The emerging system, called nanophononic metamaterial (NPM), exhibits unique properties that are not attainable in conventional nanostructured media such as nanocomposites [9] or nanophononic crystals [10]. The substructure resonances, which could be numerous for relatively large substructures, may be tuned to couple with all or most of the heat-carrying phonon modes of the underlying host medium. This atomic-scale coupling mechanism is essentially a resonance hybridization between the wavenumberdependent wave modes of the host medium (phonons) and the wavenumber-independent vibration modes of the local substructure (vibrons). The outcome is significant reductions in the phonon group velocities across roughly ...

We studied how the period length and the mass ratio affect the thermal conductivity of isotopic nanoscale three-dimensional (3D) phononic crystal of Si. Simulation results by equilibrium molecular dynamics show isotopic nanoscale 3D phononic crystals can significantly reduce the thermal conductivity of bulk Si at high temperature (1000 K), which leads to a larger ZT than unity. The thermal conductivity decreases as the period length and mass ratio increases. The phonon dispersion curves show an obvious decrease of group velocities in 3D phononic crystals. The phonon's localization and band gap is also clearly observed in spectra of normalized inverse participation ratio in nanoscale 3D phononic crystal.

A surface-enhanced Raman scattering (SERS) measurement of 3,3',4,4'-tetrachlorobiphenyl (PCB77) with aptamer capturing in a microfluidic device was demonstrated. To construct the microfluidic chip, an ordered Ag nanocrown array was fabricated over a patterned polydimethylsiloxane (PDMS) that was achieved by replicating an anodic aluminum oxide (AAO) template. The patterned PDMS sheet was covered with another PDMS sheet having two input channel grooves to form a close chip. The Ag nanocrown array was used for the SERS enhancement area and the detection zone. PCB 77 aptamers were injected into one channel and the other allows for analytes (PCBs). The mercapto aptamers captured the targets in the mixed zone and were immobilized to the SERS detection zone via S-Ag bonds so as to further improve both the SERS sensitivity and selectivity of PCB77. Such an aptamer-based microfluidic chip realized a rapid SERS detection. The lowest detectable concentration of 1.0 × 10(-8) M was achieved for PCB77. This work demonstrates that the aptamer-modified SERS microfluidic sensor can be utilized for selective detections of organic pollutants in the environment.