DSpace Community: KAIST Dept. of PhysicsKAIST Dept. of Physicshttp://hdl.handle.net/10203/5242020-07-10T02:16:24Z2020-07-10T02:16:24ZGrowth of a-axial GaN core nanowires, semi-polar (1(1)over-bar01) GaN/InGaN multiple quantum well co-axial nanowires on Si substrate, and their carrier dynamicsJohar, Muhammad AliWaseem, AadilSong, Hyun-GyuHassan, Mostafa AfifiBagal, Indrajit V.Cho, Yong-HoonRyu, Sang-Wanhttp://hdl.handle.net/10203/2751342020-07-02T06:20:08Z2020-07-01T00:00:00ZTitle: Growth of a-axial GaN core nanowires, semi-polar (1(1)over-bar01) GaN/InGaN multiple quantum well co-axial nanowires on Si substrate, and their carrier dynamics
Authors: Johar, Muhammad Ali; Waseem, Aadil; Song, Hyun-Gyu; Hassan, Mostafa Afifi; Bagal, Indrajit V.; Cho, Yong-Hoon; Ryu, Sang-Wan
Abstract: The quantum-confined Stark effect (QCSE) reduces the quantum efficiency of optical devices due to the reduced overlap of electron and hole wave functions because of the growth of GaN/InGaN multiple quantum wells (MQWs) on the polar facet. Here, we report the growth of non-polar [1 (1) over bar 20] GaN core nanowires on Si substrate by MOCVD. Subsequently, the active region of GaN/InGaN MQWs is grown on (1 (1) over bar 01) semipolar growth facet. The morphology of GaN core and GaN/InGaN MQWs is examined by SEM and TEM. The growth direction of GaN core and GaN/InGaN MQWs is confirmed by SAED patterns. The bandgap is tuned from 3.11 eV to 2.45 eV by increasing the InGaN QW thickness. Finally, time-resolved photoluminescence is conducted to evaluate the carrier dynamics. An ultrashort time constant between 51 ps and 74 ps is measured with increased QW thickness. The underlying reason for the very short carrier lifetime is the suppressed QCSE because the piezoelectric and polarization fields are reduced due to the semi-polar growth facet of QWs. The semi-polar growth of GaN/InGaN MQW co-axial nanowires on Si substrate underlines the potential of our fabrication technique for a variety of optical applications.2020-07-01T00:00:00ZPost-synthetic oriented attachment of CsPbBr3 perovskite nanocrystal building blocks: from first principle calculation to experimental demonstration of size and dimensionality (0D/1D/2D)Jeon, SanghyunJung, Myung-ChulAhn, JunhyukWoo, Ho KunBang, JunsungKim, DonggyuLee, Sang YeopWoo, Ho YoungJeon, JongchulHan, Myung JoonPaik, TaejongOh, Soong Juhttp://hdl.handle.net/10203/2751822020-07-03T03:20:06Z2020-06-01T00:00:00ZTitle: Post-synthetic oriented attachment of CsPbBr3 perovskite nanocrystal building blocks: from first principle calculation to experimental demonstration of size and dimensionality (0D/1D/2D)
Authors: Jeon, Sanghyun; Jung, Myung-Chul; Ahn, Junhyuk; Woo, Ho Kun; Bang, Junsung; Kim, Donggyu; Lee, Sang Yeop; Woo, Ho Young; Jeon, Jongchul; Han, Myung Joon; Paik, Taejong; Oh, Soong Ju
Abstract: Post-synthesis engineering methods that employ oriented attachment to precisely control the size and dimensionality (0D/1D/2D) of as-synthesized CsPbBr3 nanocrystals (NCs) are demonstrated. We investigated the chemical effects of the properties of polar solvents, including their immiscibility, polarity, and boiling point, on the surfaces of NCs, as well as their effect on the structural and optical properties of NCs. Appropriate exploitation of the solvent properties made it possible to use a polar solvent to mildly affect the NCs indirectly such that they discarded their ligands and became attached to proximal NCs without being destroyed. Based on our observations, we developed a method whereby a solution of the NCs in a non-polar solvent is mixed with a polar solvent to form an immiscible phase to induce epitaxial growth of CsPbBr3 NCs. The method enables the size of NCs to be easily regulated from 5 to 50 nm by controlling the engineering time. Taking advantage of the minimal effect of a mild solvent, we also developed a self-assembly method that operates at the liquid-air interface to systematically control the dimensionality. At this interface, the NCs self-assemble in the horizontal direction and grow into micron-sized, single-crystalline, defect-free nanowires (1D) and nanoplates (2D) via oriented attachment. Finally, we discuss the origin of the non-destructive oriented attachment phenomenon and the surface chemistry of a perovskite NC using density functional theory (DFT) simulations and a proposed model system.2020-06-01T00:00:00ZDriving a magnetized domain wall in an antiferromagnet by magnonsShen, PengtaoTserkovnyak, YaroslavKim, Se Kwonhttp://hdl.handle.net/10203/2754252020-07-10T01:55:08Z2020-06-01T00:00:00ZTitle: Driving a magnetized domain wall in an antiferromagnet by magnons
Authors: Shen, Pengtao; Tserkovnyak, Yaroslav; Kim, Se Kwon
Abstract: We theoretically study the interaction of magnons, quanta of spin waves, and a domain wall in a one-dimensional easy-axis antiferromagnet in the presence of an external magnetic field applied along the easy axis. To this end, we begin by obtaining the exact solution for spin waves in the background of a domain wall magnetized by an external field. The finite magnetization inside the domain wall is shown to give rise to reflection of magnons scattering off the domain wall, deviating from the well-known result of reflection-free magnons in the absence of a magnetic field. For practical applications of the predicted reflection of magnons, we show that the magnon reflection contributes to the thermally driven domain-wall motion. Our work leads us to envision that inducing a finite magnetization in antiferromagnetic solitons such as vortices and skyrmions can be used to engender phenomena that do not occur in the absence of magnetization.2020-06-01T00:00:00ZParallel propagating electromagnetic waves in magnetized quantum electron plasmas with finite temperatureWoo, ChanghoWoo, M. H.Choi, Cheong R.Min, Kyoung-Wookhttp://hdl.handle.net/10203/2745992020-06-10T09:20:21Z2020-05-01T00:00:00ZTitle: Parallel propagating electromagnetic waves in magnetized quantum electron plasmas with finite temperature
Authors: Woo, Changho; Woo, M. H.; Choi, Cheong R.; Min, Kyoung-Wook
Abstract: We studied parallel propagating electromagnetic waves in a magnetized quantum electron plasma of finite temperature, as an extension of our previous study on a zero temperature plasma. We obtained simple analytic dispersion relations in the long wavelength limit that included the thermal effect as correction terms to the zero temperature results. As in the zero temperature case, the lower branch of the R wave showed significant damping and became ill-defined at short wavelengths. Quantum effects seemed to give qualitative changes, such as the appearance of anomalous dispersion regions, to the classical dispersion relations when v(F)/v(th) <= 0.2 for a set of exemplary parameters of v(F) = 0.1c and omega(ce)/omega(pe) = 0.05 was used. We also noted that introduction of the Planck constant in the quantum Vlasov equation changed the shape of the anomalous dispersion region qualitatively, by forming a normal dispersion region in the middle of the original single broad anomalous dispersion region.2020-05-01T00:00:00Z