Prediction of high thermoelectric performance in the lowdimensional metal halide Cs_(3)Cu_(2)I_(5)

Metal halides have emerged as a new generation of semiconductors with applications ranging from solar cells to chemical sensors.We assess the thermoelectric potential of Cs_(3)Cu_(2)I_(5),which has a crystal structure formed of zero-dimensional[Cu_(2)I_(5)]3−anionic clusters that are separated by Cs+counter cations.We find the compound exhibits the characteristics of a phonon-glass electroncrystal with a large imbalance in the conduction of heat and electrons predicted from first-principles transport theory.Strong anharmonic phonon–phonon scattering results in short-lived acoustic vibrations and an ultra-low lattice thermal conductivity(<0.1Wm^(−1)K^(−1)).The dispersive conduction band leads to a high electron mobility(>10 cm^(2)V^(−1)s^(−1)).For an n-type crystal at 600 K,a thermoelectric figure-of-merit ZT of 2.6 is found to be accessible,which for a cold source of 300 K corresponds to a thermodynamic heat-to-electricity conversion efficiency of 15%.

Building an optics and photonics research ecosystem in South Korea:Collaborative innovation between academia and industry

Photonics research is a rapidly growing field that has found applications in various aspects of our lives,ran-ging from communication to medical imaging1-5.Col-laborative innovation between academia and industry has been a driving force behind the significant advances in photonics research in South Korea.In this special issue,we present cutting-edge research in photonics,highlighting the importance of collaboration between academia and industry(Fig.1).

Capillary grip-induced stick-slip motion

We present capillary grip-induced stick-slip motion,a nanoscale tribological effect,where the role of a nanoscale confined water meniscus formed between a buckled sharp tip and a glass or mica surface is addressed by shear dynamic force measurement.We obtained the effective elasticity,viscosity,conservative(elastic)and non-conservative(viscous)forces,energy dissipation,and lateral force using small oscillation,amplitude-modulation,and shear-mode quartz tuning fork-atomic force microscopy(QTFAFM).We distinguished the conservative and non-conservative forces by investigating the dependence of normal load and relative humidity,slip length,and stick-slip frequency.We found that the confined nanoscale water enhances the lateral forces via capillary grip-induced stick-slip on a rough surface,resulting in an increase of static lateral force(3-fold for both substrates)and kinetic lateral force(6-fold for glass,3-fold for mica).This work provides quantitative and systematic understanding of nanoscale tribology properties in humid ambient conditions and is thus useful for control of friction as well as characterization of tribology in nanomaterials and nanodevices.

Exploiting the quantum mechanically derived force field for functional materials simulations

The computational design of functional materials relies heavily on large-scale atomistic simulations.Such simulations are often problematic for conventional classical force fields,which require tedious and time-consuming parameterization of interaction parameters.The problem can be solved using a quantum mechanically derived force field(QMDFF)—a system-specific force field derived directly from the first-principles calculations.We present a computational approach for atomistic simulations of complex molecular systems,which include the treatment of chemical reactions with the empirical valence bond approach.The accuracy of the QMDFF is verified by comparison with the experimental properties of liquid solvents.We illustrate the capabilities of our methodology to simulate functional materials in several case studies:chemical degradation of material in organic light-emitting diode(OLED),polymer chain packing,material morphology of organometallic photoresists.The presented methodology is fast,accurate,and highly automated,which allows its application in diverse areas of materials science.

Noise and detectivity limits in organic shortwave infrared photodiodes with low disorder

To achieve high detectivity in infrared detectors,it is critical to reduce the device noise.However,for non-crystalline semiconductors,an essential framework is missing to understand and predict the effects of disorder on the dark current.This report presents experimental and modeling studies on the noise current in exemplar organic bulk heterojunction photodiodes,with 10 donor-acceptor combinations spanning wavelength between 800 and 1600 nm.A significant reduction of the noise and higher detectivity were found in devices using non-fullerene acceptors(NFAs)in comparison to those using fullerene derivatives.The low noise in NFA blends was attributed to a sharp drop off in the distribution of bandtail states,as revealed by variable-temperature density-of-states measurements.Taking disorder into account,we developed a general physical model to explain the dependence of thermal noise on the effective bandgap and bandtail spread.The model provides theoretical targets for the maximum detectivity that can be obtained at different detection wavelengths in inherently disordered infrared photodiodes.