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Epitaxial Growth of GaN Films on Atomically Stepped (0001) Lithium-niobate (LiNbO_3) Substrates
Man Hoai Nam,양우철,김문덕,박인성 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.58 No.51
The structural properties of GaN films grown epitaxially on atomically-flat lithium niobate (LiNbO_3) substrates were investigated. Prior to GaN film growth, high-temperature treatment of the as-received LiNbO_3 removed surface damage and produced atomically stepped surfaces with an average roughness of ∼0.11 nm. The micro-steps were nearly parallel and periodic over the entire substrate surface. The step terrace width was ∼212 nm, and the average step height was ∼0.25 nm. GaN thin films were grown on flat surfaces with AlN buffer layers and GaN buffer layers grown at low temperature by using molecular beam epitaxy. Atomic force microscopy measurements showed a typical Ga-face GaN surface with spiral hillocks. The average roughness of the GaN film was ∼0.56 nm. X-ray diffraction (XRD) measurements indicated that the GaN (0001) plane was parallel to the (0001) plane of the LiNbO_3 substrate. The full width at half-maximum of the XRD rocking curve for GaN (0002) was ∼122.14 arcsec, which is comparable to that of highquality GaN films grown on other common substrates. Using transmission electron microscopy, we observed that the GaN epitaxial layers grown on LiNbO<SUB>3</SUB> had a crystalline relationship of (0001) GaN//(0001) LiNbO_3 with [10-10] GaN//[11-20] LiNbO_3. The successful growth of an epitaxial wide-band-gap GaN film on nonlinear optical LiNbO_3 is promising for the development of integrated multi-functional optoelectric devices on ferroelectrics.
Ultra-broadband and flexible metamaterial absorber based on MoS2 cuboids with Mie resonances
Ha Duong Thi,Nam Man Hoai,Tung Bui Son,Khuyen Bui Xuan,Lam Vu Dinh,Le-Van Quynh 한국물리학회 2023 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.82 No.11
In this work, we present a type of fexible and broadband metamaterial absorber operating in the GHz range. The proposed structure consists of three layers: a periodic square-shaped array made of molybdenum disulfde (MoS2) on the top, a continuous polyimide layer in the middle and a continuous copper layer at the bottom. For fat model, the proposed absorber exhibits a broadband absorption in the frequency range of 10.1–17.6 GHz with an absorption of more than 90% under normal incidence. Due to the symmetry of structure, the absorption feature is polarization-insensitive. The absorption remains above 80% in the frequency range of 11.1–15.6 GHz when the incident angle is up to 60° for TE-polarized wave, while the absorption is higher than 90% in the frequency range of 13.5–18 GHz for incident angles up to 60° for TM-polarized wave. For bending model, the absorption is signifcantly expanded when the bending radius decreases to under 100 mm. The physical mechanism of the absorption properties is explained in detail by the electric feld distribution, the magnetic feld distribution following the Mie resonance theory and infuence of the loss of MoS2. Obtained results in the work might contribute to the development of potential applications based on metamaterials in the microwave region such as imaging, protecting and light emitting devices.
Thanh-Tung Duonga,Phan Huy Hoang,Luu Thi Nhan,Luong Van Duong,Man Hoai Nam,Le Quoc Tuan 한국물리학회 2019 Current Applied Physics Vol.19 No.11
Large-grain-size and void-free CH3NH3PbI3 films with bilayer structure are fabricated by spin-coating a PbI2 layer onto a mesoporous TiO2 layer and sequentially spraying CH3NH3I (methylammonium iodide, MAI) multilayers. The sprayer is controlled by a homemade three-axis computer numerical control machine; thus, the substrates are coated by successive parallel passes achieved by moving the nozzle. Spray deposition at the optimal spray rate and substrate temperature produces a large-grain-size and void-free methylammonium lead iodide (MAPbI3) bilayer structure. The mesoporous TiO2 layer plays an important role in electron transport by preventing the return of electrons to the perovskite layer and decreasing the contact resistance at the perovskite/ compact TiO2/fluorine tin oxide interface. When the films are incorporated into a solar cell device with a conductive carbon counter electrode, a maximum power conversion efficiency of 10.58% is realised.
Temperature-Dependent Photoluminescence and Absoprtion of CdSe Quantum dots Embedded in PMMA
Tran Thi Kim Chi,Ung Thi Dieu Thuy,Nguyen Quang Liem,Man Hoai Nam,Do Xuan Thanh 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.52 No.5
Photoluminescence and absorption studies of CdSe quantum dots in polymethylmethacrylate (PMMA) were carried out in the temperature range 14 - 310 K. We found an anomalously discontinuous variation of the photoluminescence intensity and the peak position around 50 K. Two different kinds of states, whose populations are temperature-dependent, are proposed as the origins for the emissions at lower and higher temperatures. The absorption exhibited a temperature-dependent behavior similar to that of the photoluminescence.O Photoluminescence and absorption studies of CdSe quantum dots in polymethylmethacrylate (PMMA) were carried out in the temperature range 14 - 310 K. We found an anomalously discontinuous variation of the photoluminescence intensity and the peak position around 50 K. Two different kinds of states, whose populations are temperature-dependent, are proposed as the origins for the emissions at lower and higher temperatures. The absorption exhibited a temperature-dependent behavior similar to that of the photoluminescence.O
Thanh-Tung Duong,Ta Quoc Tuan,Dang Viet Anh Dung,Nguyen Van Quy,Dinh-Lam Vu,Man Hoai Nam,Nguyen Duc Chien,윤순길,Anh-Tuan Le 한국물리학회 2014 Current Applied Physics Vol.14 No.12
Polyaniline nanowires (PANI NWs) were deposited onto fluorine-doped tin oxide (FTO) glass substrate using the cyclic voltammetric method with aniline monomer precursor in HCl aqueous solution. The secondary oxidation peak plays an important role in polymerization of aniline monomer and the optimization of catalytic activity of PANI-based counter electrodes was achieved by controlling the number of cycles. The photovoltaic performance of the dye-sensitized solar cells (DSSCs) with PANI NWs counter electrodes (CEs) was optimized at 4th cycles, and then following parameters were obtained: Jsc ¼ 17.2 mA cm2, Voc ¼ 0.71 V, FF ¼ 59.3%, and efficiency (h) ¼ 7.24%. While, Jsc ¼ 14.7 mA cm2, Voc ¼ 0.77 V, FF ¼ 70.6%, and efficiency (h) ¼ 7.98% in cells with Pt CEs. The PANI NWs were attractive as an alternative CEs for the low-cost DSSCs instead of Pt.