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Strategies for improving luminescence efficiencies of blue-emitting metal halide perovskites
Grandhi G. Krishnamurthy,김하준,Viswanath N. S. M.,조한빈,한주형,김성민,Im Won Bin 한국세라믹학회 2021 한국세라믹학회지 Vol.58 No.1
Lead halide perovskites (LHPs) are suitable as the emissive layers in light-emitting diodes (LEDs). The external quantum effi ciency of green LEDs based on LHPs is now over 20%. Nevertheless, the blue LHP LEDs lag behind the green ones in terms of effi ciency. Photoluminescence (PL) quantum yield (QY) and stability of the NCs under various operating conditions are two major factors that infl uence the LED performance. Therefore, to promote the eff orts towards achieving improved LED effi ciencies, herein, we summarize several synthetic methods that produce blue-emitting LHP NC, followed by several approaches devised to boost their PL QYs up to near unity. Light-induced anion segregation is one of the limitations of using blue-emitting mixed-halide LHPs, which triggers the attention to single halide, quantum-confi ned LHP nanoplatelets (NPLs). Syntheses, structure, and luminescent properties of organic–inorganic and all-inorganic blue-emitting LHP NPLs are discussed elaborately. In the last portion, the luminescent properties of lead-free metal halides, which are of current interest, are discussed, followed by an outlook and future directions. In conclusion, our review discusses various literature attempts to obtain stable blue-emitting LHP NCs, which can be helpful in a better design of the blue-emitting LHP NCs towards various light-emitting applications.
Grandhi, G. Krishnamurthy,Viswanath, N. S. M.,Cho, Han Bin,Kim, Seong Min,Im, Won Bin Royal Society of Chemistry 2019 Nanoscale Vol.11 No.44
<P>Green-emissive Cs4PbBr6 shows promise for light-emitting diode devices superior to that of CsPbBr3 NCs owing to their stability and high photoluminescence efficiency. Nevertheless, there is still no consensus regarding the basis of their green emission, which decelerates their advance in light-emitting applications. Herein, a systematic investigation on the concentration of capping ligands (oleylamine and oleic acid), which determines the predominant phase between CsPbBr3 and Cs4PbBr6 for a given Cs to Pb feed ratio, is conducted. This study deduces that oleylamine to oleic acid ratio plays a crucial role in obtaining either green-emissive or non-emissive Cs4PbBr6 NCs. Scrutiny of Cs4PbBr6 microscopic and optical data in addition to their emission quenching study with a hole-withdrawing molecule reveals that the green emission originates from the CsPbBr3 impurity phase. Furthermore, stable green emission is observed for CsPbBr3/Cs4PbBr6 nanocrystals when CsPbBr3 particles are well protected by the Cs4PbBr6 matrix. These CsPbBr3/Cs4PbBr6 films remained highly luminescent even after UV exposure for hours or annealing at ∼150 °C for days in addition to their long-term stability under an ambient atmosphere, which are the desirable properties for various practical applications.</P>
Residual stress behaviors induced by laser peening along the edge of curved models
Jongbin Im,Ramana V. Grandhi,Younghee Ro 대한기계학회 2012 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.26 No.12
Laser peening (LP) induces high-magnitude compressive residual stresses in a small region of a component. The compressive residual stresses cause plastic deformation that is resistant to fatigue fracture. Fatigue cracks are generally nucleated at critical areas, and LP is applied for those regions so as to delay the crack initiation. Many critical regions are located on the edge of the curved portion of structures because of stress concentration effects. Several investigations that are available for straight components may not give meaningful guidelines for peening curved components. Therefore, in this paper, we investigate residual stress behaviors induced by LP along the edge of curved models. Three curved models that have different curvatures are investigated for peening performance. Two types of peening configurations, which are simultaneous corner shot and sequential corner shots, are considered in order to obtain compressive residual stresses along an edge. LP simulations of multiple shots are performed to identify overlapping effects on the edge portion of a curved model. In addition, the uncertainty calculation of residual stress induced by LP considering laser pulse duration is performed.
Boundary Method for Shape Design Sensitivity Analysis in Solving Free-Surface Flow Problems
Choi Joo Ho,Kwak H. G.,Grandhi R. V. The Korean Society of Mechanical Engineers 2005 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.19 No.12
An efficient boundary-based optimization technique is applied in the numerical computation of free surface flow problems, by reformulating them into the equivalent optimal shape design problems. While the sensitivity in the boundary method has mainly been calculated using the boundary element method (BEM) as an analysis means, the finite element method (FEM) is used in this study because of its popularity and easy-to-use features. The advantage of boundary method is that the design velocity vectors are needed only on the boundary, not over the whole domain. As such, a determination of the complicated domain design velocity field, which is necessary in the domain method, is eliminated, thereby making the process easy to implement and efficient. Seepage and supercavitating flow problem are chosen to illustrate the accuracy and effectiveness of the proposed method.
Recep M. Gorguluarslan,Ramana V. Grandhi,최해진,최승겸 대한기계학회 2019 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.33 No.3
In the design of lattice structures fabricated by additive manufacturing, a multiscale modeling process is usually required to effectively account for fine scale uncertainties. The validation of the multiscale model predictions, on the other hand, is a challenging task. In this research, two prediction assessment approaches, namely the area validation metric and the Kolmogorov-Smirnov test, are presented in a systematic validation pyramid approach with u-pooling method to address this issue. The use of these two approaches are evaluated in terms of being an unbiased decision criterion for the prediction assessment and validation of the multiscale models. The fine scale material and geometry uncertainties are propagated onto homogenized properties using a stochastic upscaling method at each scale of interest. The homogenized model predictions are validated using the experimental data obtained for the lattice structure example fabricated by material extrusion process. The results indicate that the presented approach is capable of effectively validate the predictions of the multiscale models under uncertainty.
초공동(초공동)하의 수중 주행체 캐비테이터 형상 최적설계
최주호(JooHo Choi),Ramana V. Grandhi 대한기계학회 2003 대한기계학회 춘추학술대회 Vol.2003 No.11
When a projectile travels at high speed underwater, supercavitating flow arises, in which a huge cavity is<br/> generated behind the projectile so that only the nose, i.e., the cavitator, of the projectile is wetted, while the<br/> rest of it should be surrounded by the cavity. In that case, the projectile can achieve very high speed due to the<br/> reduced drag. Furthermore if the nose of the body is shaped properly, the attendant pressure drag can be<br/> maintained at a very low value, so that the overall drag is also reduced dramatically. In this study, shape<br/> optimization technique is employed to determine the optimum cavitator shape for minimum drag, given<br/> certain operating conditions. Shape optimization technique is also used to solve the potential flow problem for<br/> any given cavitator, which is a free boundary value problem having the cavity shape as unknown a priori.<br/> Analytical sensitivities are derived for various shape parameters in order to implement a gradient-based<br/> optimization algorithm. Simultaneous optimization technique is proposed for efficient cavitator shape<br/> optimization, in which the cavity and cavitator shape are determined in a single optimization routine.
Boundary Method for Shape Design Sensitivity Analysis in Solving Free - Surface Flow Problems
Joo Ho Choi,H. G. Kwak,R. V. Grandhi 대한기계학회 2005 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.19 No.12
An efficient boundary-based optimization technique is applied in the numerical computation of free surface flow problems, by reformulating them into the equivalent optimal shape design problems. While the sensitivity in the boundary method has mainly been calculated using the boundary element method (BEM) as an analysis means, the finite element method (FEM) is used in this study because of its popularity and easy-to-use features. The advantage of boundary method is that the design velocity vectors are needed only on the boundary, not over the whole domain. As such, a determination of the complicated domain design velocity field, which is necessary in the domain method, is eliminated, thereby making the process easy to implement and efficient. Seepage and supercavitating flow problem are chosen to illustrate the accuracy and effectiveness of the proposed method.