1,721,003 research outputs found
Effect of relative humidity on the aerodynamic diameter of airborne bacterial aerosols
No full text학위논문(석사) - 한국과학기술원 : 기계공학전공, 2002.2, [ vii, 46 p. ]한국과학기술원 : 기계공학전공
Role of Charge-Trapping Iodine Frenkel Defects for Hysteresis in Organic-Inorganic Hybrid Perovskite from First-Principles Calculations
No full textFirst-principles density functional theory calculations are employed to provide insight into charge-trapping phenomena that lead to the I-V hysteresis in CH3NH3PbI3 (MAPbI(3)). Energetic studies, variation in electronic structure, and charge-density analyses show that two types of iodine Frenkel defects can be stabilized by trapping excess charge carriers in MAPbI(3). As a result, these defects cause the phenomena of slow charge trapping and detrapping. Based on this insight, we propose several possible ways to eliminate charge trapping in devices based on MAPbI(3).11Nsciescopu
The effect of compositional engineering of imidazolium lead iodide on the resistive switching properties
No full textWe report here resistive switching memory characteristics of imidazolium lead iodide depending on the molar ratio of PbI2 to imidazolium iodide (ImI), that is, PbI2 : ImI = 1 : 0, 1 : 0.5, 1 : 1, 1 : 2, 1 : 3 and 0 : 1. X-ray diffraction confirms that the stoichiometric composition results in a hexagonal structure of (Im)PbI3, showing a one-dimensional face-sharing [PbI3-] chain. Bipolar resistive switching characteristics are observed regardless of the mixing ratio, where the forming process is required prior to SET and RESET processes at around +0.2 V and -0.2 V, respectively. The ON/OFF ratio is increased from 10(6) to 10(9) as the ImI content is increased due to the increased HRS associated with the pronounced insulating characteristics by ImI, whereas, the stoichiometric (Im)PbI3 exhibits 5 times longer endurance (10(3)) and an order of magnitude longer retention time (10(4) s) as compared to other compositions. Multilevel data storage capability is confirmed by changing the compliance current. The low resistance state (LRS) and the high resistance state (HRS) are associated with Ohmic conduction and Schottky conduction, respectively. Density functional theory (DFT) calculation reveals that the defect formation energy of iodine vacancy is estimated to be low indicating that (Im)PbI3 has a sufficient concentration of iodide vacancy for filament formation. Further energy barrier calculations show that iodide migration preferentially occurs along the 1-dimensional [234] crystallographic direction rather than the interlayer [130] direction. A good performance of the (Im)PbI3-based memristor is thus related to the low defect formation energy of iodide vacancy and the preferential growth of the filament along the 1-dimensional chain.11Nsciescopu
Effect of oxygen vacancies on electrical conductivity of La0.5Sr0.5FeO3-delta from first-principles calculations
No full textWe use first-principles density functional theory calculations to understand how oxygen vacancies degrade the electrical conductivity of mixed ionic-electronic conductor (MIEC) at low oxygen partial pressure (P-O2). Analysis focused on La0.5Sr0.5FeO3-delta, which shows the highest mixed conductivity among cobalt-free iron-based perovskite oxides. Calculation results show that hole compensation by electrons released from oxygen vacancies lowers the electrical conductivity and eventually leads to metal-to-semiconductor transition at low P-O2. Analyses of effective mass change and charge-density show that holes are the major charge carrier of electrical conductivity, but the contribution of electrons to conductivity increases as temperature increases. We suggest several possible ways to reduce the degradation of electrical conductivity at low P-O2. Our results provide guidelines to design highly effective oxygen-selective membranes.11Nsciescopu
Designing zero-dimensional dimer-type all-inorganic perovskites for ultra-fast switching memory
No full textHalide perovskite has been applied for resistive switching memory devices, but there are challenges remained to achieve practical application. By using high-throughput screening based on first-principles calculations, the authors discover that lead-free dimer-Cs3Sb2I9 meets the requirements, which exhibits switching speed of 20 ns
A-Site Effect on the Oxidation Process of Sn-Halide Perovskite: First-Principles Calculations
No full textTin-halide perovskite solar cells (Sn-PSCs) are promising candidates as an alternative to toxic lead-halide PSCs. However, Sn2+ is easily oxidized to Sn4+, so Sn-PSCs are unstable in air. Here, we use first-principles density functional theory calculations to elucidate the oxidation process of Sn2+ at the surface of ASnBr(3) [A = Cs or CH3NH3 (MA)]. Regardless of the A-site cation, adsorption of O2 leads to the formation of SnO2, which creates a Sn vacancy at the surface. The A-site cation determines whether the created vacancies are stabilized in the bulk or at the surface. For CsSnBr3, the Sn vacancy is stabilized at the surface, so further oxidation is limited. For MASnBr(3), the Sn vacancy moves into bulk region, so additional Sn is supplied to the surface; as a result, a continuous oxidation process can occur. The stabilization of Sn vacancy is closely related to the polarization that the A-site cation causes in the system.11Nsciescopu
The origin of hysteresis in hybrid Perovskite solar cell from First-Principles calculations
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Strain-Mediated Phase Stabilization: A New Strategy for Ultrastable alpha-CsPbI3 Perovskite by Nanoconfined Growth
No full textAll-inorganic cesium lead triiodide (CsPbI3) perovskite is considered a promising solution-processable semiconductor for highly stable optoelectronic and photovoltaic applications. However, despite its excellent optoelectronic properties, the phase instability of CsPbI3 poses a critical hurdle for practical application. In this study, a novel stain-mediated phase stabilization strategy is demonstrated to significantly enhance the phase stability of cubic a-phase CsPbI3. Careful control of the degree of spatial confinement induced by anodized aluminum oxide (AAO) templates with varying pore sizes leads to effective manipulation of the phase stability of alpha-CsPbI3. The Williamson-Hall method in conjunction with density functional theory calculations clearly confirms that the strain imposed on the perovskite lattice when confined in vertically aligned nanopores can alter the formation energy of the system, stabilizing alpha-CsPbI3 at room temperature. Finally, the CsPbI3 grown inside nanoporous AAO templates exhibits exceptional phase stability over three months under ambient conditions, in which the resulting light-emitting diode reveals a natural red color emission with very narrow bandwidth (full width at half maximum of 33 nm) at 702 nm. The universally applicable template-based stabilization strategy can give in-depth insights on the strain-mediated phase transition mechanism in all-inorganic perovskites.11Nsciescopu
First-Principles Investigations to Evaluate the Spin-Polarized Metal-to-Insulator Transition of Halide Cuprite Perovskites for Smart Windows
No full textAlthough smart windows have received wide attention as energy-saving devices, conventional metal-to-insulator materials such as VO2 hinder their commercial usage because of their high transition temperature and low solar energy modulation. Further development can be achieved by finding a new material system that can effectively overcome these limitations. In this study, first-principles density functional theory calculations are used to investigate the possibility of exploiting a spin-polarized band gap material for smart window applications. Halide cuprite perovskites (A(2)CuX(4)) were chosen because they have a spin-polarized band gap that can be tuned by element selection at sites A and X. Our study shows that the optical transmittance of the insulating phase is increased by a violation of the selection rule. The spin-polarized band gap is closely related to the metal-to-insulator transition temperature and can be modulated by chemical engineering, strain engineering, or both. Therefore, A(2)CuX(4) is a promising candidate for smart windows.11Nsciescopu
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