Tuning Acid-Base Properties Using Mg-Al Oxide Atomic Layer Deposition

Atomic-level chemical reaction promoting external quantum efficiency of organic photomultiplication photodetector exceeding 108% for weak-light detection

Low-cost, solution-processed photomultiplication organic photodetectors (PM-OPDs) with external quantum efficiency (EQE) above unity have attracted enormous attention. However, their weak-light detection is unpleasant because the anode Ohmic contact causes exacerbation in dark current. Here, we introduce atomic-level chemical reaction in PM-OPDs which can simultaneously suppress dark current and increase EQE via depositing a 0.8 nm thick Al₂O₃ by the atomic layer deposition. Suppression in dark current mainly originates from the built-in anode Schottky junction as a result of work function decrease of hole-transporting layer of which the chemical groups can react chemically with the bottom surface of Al₂O₃ layer at the atomic-level. Such strategy of suppressing dark current is not adverse to charge injection under illumination; instead, responsivity enhancement is realized because charge injection can shift from cathode to anode, of which the neighborhood possesses increased photogenerated carriers. Consequently, weak-light detection limit of the forwardly-biased PM-OPD with Al₂O₃ treatment reaches a remarkable level of 2.5 nW cm^-2, while that of the reversely-biased control is 25 times inferior. Meanwhile, the PM-OPD yields a record high EQE and responsivity of 4.31 × 10⁸% and 1.85 × 10⁶ A W^-1, respectively, outperforming all other polymer-based PM-OPDs.

Nonuniform-to-uniform structural transitions induced by ultrasonic vibrations

Recent millimetre-scale studies proposed that ultrasonic vibrations (UVs) promote material flow in welding joints via acting on dislocations. Here, we report atomic-scale results from molecular dynamics simulations of Mg-Al nanolayers joined by two means: only heat and heat accompanied by UVs (vibration amplitude, B = 0.1-10 nm and vibration frequency, f = 5.7-100 GHz) over the temperature range of 600-800 K. Comparative and quantitative analyses were performed on the structural evolution (including atomic diffusion, arrangements and distributions) of the joining Mg/Al interfaces and motions of dislocations, as well as on the influences of the vibration amplitude and vibration frequency on these two features. The results show that the applied UV with large vibration amplitudes (B ≥ 5 nm) and a low vibration frequency (f = 5.7 GHz) significantly facilitates atomic diffusion (10-1000 times as fast as that in the case free of UVs) and formations and motions of dislocations, resulting in nonuniform-to-uniform structural transitions and increases in the thicknesses of the joined Mg/Al interfaces. These results provide a way to understand how the applied UV acts on dislocations and atomic diffusion during the UV-assisted welding processes of Mg-Al and other systems.

Coherent-interface-induced second hardening deformation of Al-Mg-Al nanolayers by molecular dynamics simulations

Thermal diffusion plays an important role in the determination of the structures and properties of interfaces and nanolayers. Here we report results from molecular dynamics simulations of the tensile behavior of Al-Mg-Al nanolayers with their Al/Mg interfaces being joined by the thermal diffusion of atoms. We find that a different deformation mechanism applies in each case: low thermal diffusion temperatures (300 ≤ T1 < 664 K) and high thermal diffusion temperatures (664 ≤ T1 ≤ 846 K). The formation of coherent Al/Mg interfaces in the case of high T1 induces the second hardening deformation of Al-Mg-Al nanolayers before the stress reaching the tensile strength, significantly enhancing the tensile properties of Al-Mg-Al nanolayers in comparison to the case of low T1. This difference would provide guidance on the improvement of the mechanical properties of Al-Mg layered systems.

Melting at Mg/Al interface in Mg-Al-Mg nanolayer by molecular dynamics simulations

The melting at the magnesium/aluminum (Mg/Al) interface is an essential step during the fabrications of Mg-Al structural materials and biomaterials. We carried out molecular dynamics simulations on the melting at the Mg/Al interface in a Mg-Al-Mg nanolayer via analyzing the changes of average atomic potential energy, Lindemann index, heat capacity, atomic density distribution and radial distribution function with temperature. The melting temperatures (Tm) of the nanolayer and the slabs near the interface are significantly sensitive to the heating rate (vh) over the range ofvh ≤ 4.0 K ps^-1. The distance (d) range in which the interface affects the melting of the slabs is predicted to be (-98.2, 89.9) Å atvh→0,if the interface is put atd = 0 and Mg (Al) is located at the left (right) side of the interface. TheTmof the Mg (Al) slab just near the interface (e.g.d=4.0Å) is predicted to be 926.8 K (926.6 K) atvh→0,with 36.9 K (37.1 K) below 963.7 K for the nanolayer. These results highlight the importance of regional research on the melting at an interface in the nanolayers consisting of two different metals.