We present a systematic study based on first principles molecular dynamics simulations of lead iodide perovskites with three different cations, including methylammonium (MA), formamidinium (FA) and cesium. Using the high temperature perovskite structure as a reference, we investigate the instabilities that develop as the material is cooled down to 370 K. All three perovskites display anharmonicity in the motion of the iodine atoms, with the stronger effect observed for the MAPbI 3 and CsPbI 3. At high temperature, this behavior can be traced back to the reduced effective size of the Cs + and MA + cations. MAPbI 3 undergoes a spontaneous phase transition within our simulation model driven by the dipolar interaction between neighboring MA cations as the temperature is decreased from 450 K. The reverse transformation from tetragonal to cubic is also monitored through the large distribution of the octahedral tilting angles accompanied by an increase in the anharmonicity of the iodine atoms motion. Both MA and FA hybrid perovskites show a strong coupling between the molecular orientations and the local lattice deformations, suggesting mixed order-disorder/displacive characters of the high temperature phase transitions.
The formamidinium lead iodide hybrid perovskite is studied using first principles molecular dynamics simulations and further analyzed using group theory. The simulations are performed on large supercells containing 768 atoms under isothermal and fully anisotropic isobaric conditions. Two trajectories, one at 300 K and another at 450 K, were extended for over 50 ps in order to perform a detailed assessment of the rotational dynamics of organic cations. The characteristic rotations of the cation are analyzed by defining two rotation axes. It is found that the formamidinium molecules rotate preferentially around the direction parallel to the line connecting the two nitrogen atoms. The rotational dynamics shows some characteristics already observed in methylammonium lead iodide, like the heterogeneous dynamics at room temperature that disappears at 450 K. The orientational probability of the molecules is explored in terms of an expansion in cubic harmonics up to the 12th order. It reveals a strong directionality at room temperature that relaxes when increasing the temperature. These findings are further rationalized using Landau and group theories suggesting a mixed displacive/order-disorder structural instability at lower temperatures.
2D molybdenum disulfide (MoS ) possesses excellent optoelectronic properties that make it a promising candidate for use in high-performance photodetectors. Yet, to meet the growing demand for practical and reliable MoS photodetectors, the critical issue of defect introduction to the interface between the exfoliated MoS and the electrode metal during fabrication must be addressed, because defects deteriorate the device performance. To achieve this objective, the use of an atomic layer-deposited TiO interlayer (between exfoliated MoS and electrode) is reported in this work, for the first time, to enhance the performance of MoS photodetectors. The TiO interlayer is inserted through 20 atomic layer deposition cycles before depositing the electrode metal on MoS /SiO substrate, leading to significantly enhanced photoresponsivity and response speed. These results pave the way for practical applications and provide a novel direction for optimizing the interlayer material.
The explosive demand for a wide range of data processing has sparked interest towards a new logic gate platform as the existing electronic logic gates face limitations in accurate and fast computing. Accordingly, optoelectronic logic gates (OELGs) using photodiodes are of significant interest due to their broad bandwidth and fast data transmission, but complex configuration, power consumption, and low reliability issues are still inherent in these systems. Herein, we present a novel all-in-one OELG based on the bipolar spectral photoresponse characteristics of a self-powered perovskite photodetector (SPPD) having a back-to-back p+-i-n-p-p+ diode structure. Five representative logic gates (“AND”, “OR”, “NAND”, “NOR”, and “NOT”) are demonstrated with only a single SPPD via the photocurrent polarity control. For practical applications, we propose a universal OELG platform of integrated 8 × 8 SPPD pixels, demonstrating the 100% accuracy in five logic gate operations irrelevant to current variation between pixels.
CitationFlemban TH, Singaravelu V, Sasikala Devi AA, Roqan IS (2015 We demonstrate a novel, one-step, catalyst-free method for the production of size-controlled vertical highly conductive ZnO NR arrays with highly desirable characteristics by pulsed laser deposition using a Gd-doped ZnO target. Our study shows that an in situ transparent and conductive Gd nanolayer (with a uniform thickness of ~1 nm) at the interface between a latticematched (11-20) a-sapphire substrate and ZnO is formed during the deposition. This nanolayer significantly induces relaxation mechanism that controls the dislocation distribution along the growth direction; which consequently improves the formation of homogeneous vertically aligned ZnO NRs. We demonstrate that both the lattice orientation of the substrate and the Gd characteristics are important in enhancing the NR synthesis, and we report precise control of the NR density by changing the oxygen partial pressure. We show that these NRs possess high optical and electrical quality, with a mobility of 177 cm 2 (V.s), which is comparable to the best-reported mobility of ZnO NRs. Therefore, this new and simple method has significant potential for improving the performance of materials used in a wide range of electronic and optoelectronic applications.
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