Extended photo-response of ZnO/CdS core/shell nanorods fabricated by hydrothermal reaction and pulsed laser deposition
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Heterogenous nanostructures shaped with CdS covered ZnO (ZnO/CdS) core/shell nanorods (NRs) are fabricated on indium-tin-oxide by pulsed laser deposition of CdS on hydrothermally grown ZnO NRs and characterized through morphology examination, structure characterization, photoluminescence and optical absorption measurements. Both the ZnO cores and the CdS shells are hexagonal wurtzite in structure. Compared with bare ZnO NRs, the fabricated ZnO/CdS core/shell NRs present an extended photo-response and have optical properties corresponding to the two excitonic band-gaps of ZnO and CdS as well as the effective band-gap formed between the conduction band minimum of ZnO and the valence band maximum of CdS.Keywords:
Nanorod
Wurtzite crystal structure
Pulsed Laser Deposition
Indium tin oxide
Cadmium sulfide
Wide-bandgap semiconductor
GaN nanorods with a wurtzite/zinc-blende (WZ/ZB) heterostructure are synthesized by chemical vapor deposition. They have a triangular cross section and grow along the WZ [011¯0] direction. The WZ and ZB phases appear alternately along the nanorod’s transverse direction, forming a type-II superlattice structure. Two ultraviolet emission peaks dominate the photoluminescence spectra of the GaN nanorods. One originates from excitonic transitions within the WZ regions. The other shows an anomalous “S-shaped” energy shift with increasing temperature, and is attributed to radiative recombinations of carriers localized at potential fluctuations in ZB regions. The carrier localization also results in high luminescent efficiency of the GaN nanorods.
Nanorod
Wurtzite crystal structure
Wide-bandgap semiconductor
Ultraviolet
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Density functional theory calculations and electron energy loss spectroscopy indicate that the electronic structure of ordered orthorhombic MgSiN2 is similar to that of wurtzite AlN. A band gap of 5.7 eV was calculated for both MgSiN2 (indirect) and AlN (direct) using the Heyd-Scuseria-Ernzerhof approximation. Correction with respect to the experimental room-temperature band gap of AlN indicates that the true band gap of MgSiN2 is 6.2 eV. MgSiN2 has an additional direct gap of 6.3 eV at the Γ point.
Wurtzite crystal structure
Wide-bandgap semiconductor
Orthorhombic crystal system
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The direct to indirect band gap transition in ultrathin [0001] ZnO nanowires with the structural transformation from the regular wurtzite structure to a more close-packed hexagonal structure during uniaxial compression is studied by using the first-principles calculations. The results show that all ZnO nanowires exhibit direct band gap in wurtzite structure and indirect band gap in hexagonal structure. For the same wire the band gap in hexagonal structure is smaller than that in wurtzite structure. The origin of the band gap transition from direct to indirect one is discussed.
Wurtzite crystal structure
Wide-bandgap semiconductor
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In this study, electronic properties such as the band structure, total density of state (TDOS) and partial density of states (PDOS) of zinc-blende GaN, wurtzite GaN and pnma-GaN crystals are explored through first-principles calculations within the generalized gradient approximation (GGA), and the influence of hydrostatic pressures on the electronic properties are also researched. Results show that the three GaN compounds are all semiconductors with a direct band gap. Although the band gap increases monotonically with the increase of hydrostatic pressure, the hydrostatic pressure has limited effect on the hybridization of valence band and conduction band. In addition, under the same pressure, the band gap of wurtzite GaN is slightly larger than that of the others. Further, calculation results also show that TDOSs of the three compounds are obviously different, and the increase in pressure reduces the peaks of both TDOSs and PDOSs.
Wurtzite crystal structure
Hydrostatic pressure
Wide-bandgap semiconductor
Density of states
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We report a dramatic increase in the light extraction efficiency of GaN-based blue light-emitting diodes (LEDs) by ZnO nanorod arrays on a planar indium tin oxide (ITO) transparent electrode. ZnO nanorods were grown into aqueous solution at the low temperature of 90 °C. With 20 mA current injection, the light output efficiency of the LED with ZnO nanorod arrays on ITO was increased by about 57% with no increase in a forward voltage over the conventional LEDs with planar ITO. The increased light extraction by the ZnO nanorod arrays is due to the formation of sidewalls and a rough surface, resulting in a multiple photon scattering at the LED surface.
Nanorod
Indium tin oxide
Wide-bandgap semiconductor
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The main challenge for light-emitting diodes is to increase the efficiency in the green part of the spectrum. Gallium phosphide (GaP) with the normal cubic crystal structure has an indirect band gap, which severely limits the green emission efficiency. Band structure calculations have predicted a direct band gap for wurtzite GaP. Here, we report the fabrication of GaP nanowires with pure hexagonal crystal structure and demonstrate the direct nature of the band gap. We observe strong photoluminescence at a wavelength of 594 nm with short lifetime, typical for a direct band gap. Furthermore, by incorporation of aluminum or arsenic in the GaP nanowires, the emitted wavelength is tuned across an important range of the visible light spectrum (555-690 nm). This approach of crystal structure engineering enables new pathways to tailor materials properties enhancing the functionality.
Wurtzite crystal structure
Gallium phosphide
Wide-bandgap semiconductor
Crystal (programming language)
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With a recently developed unique deep ultraviolet picoseconds time-resolved photoluminescence (PL) spectroscopy system and improved growth technique, we are able to determine the detailed band structure near the Γ point of wurtzite (WZ) AlN with a direct band gap of 6.12 eV. Combined with first-principles band structure calculations we show that the fundamental optical properties of AlN differ drastically from that of GaN and other WZ semiconductors. The discrepancy in energy band gap values of AlN obtained previously by different methods is explained in terms of the optical selection rules in AlN and is confirmed by measurement of the polarization dependence of the excitonic PL spectra.
Wurtzite crystal structure
Wide-bandgap semiconductor
Picosecond
Ultraviolet
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Numerical simulation based on first-principles calculations is applied to study the energy band structural characteristics and the band-gap properties of wurtzite InGaN. The results show that the direct band gap, the band gap bowing parameter, the width of valence band, and the width of top valence band increase with compressive strain and decrease with tensile strain. The biaxial strain effect on the indirect band gap is little. In general, there is a larger band gap bowing parameter and larger strain-induced band gap bowing variation in Ga-rich alloys. In addition, the direct band gap, the indirect band gap, the width of valence band, and the width of top valence band decrease with increase of indium composition. Wurtzite InGaN remains the characteristic of a direct band gap material under biaxial stress.
Wurtzite crystal structure
Bowing
Wide-bandgap semiconductor
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We report on the spectral dynamics of the reflectivity, site-selectively excited photoluminescence, photoluminescence excitation, and time-resolved luminescence in quaternary AlInGaN epitaxial layers grown on GaN templates. The incorporation of a few percents of In into AlGaN causes significant smoothening of the band-bottom potential profile in AlInGaN layers owing to improved crystal quality. An abrupt optical bandgap indicates that a nearly lattice-matched AlInGaN/GaN heterostructure with large energy band offsets can be grown for high-efficiency light-emitting devices.
Wide-bandgap semiconductor
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This paper describes, Synthesis of zinc oxide nanorods (ZnO NRs) using hydrothermal technique at different growth time. The structural and morphological properties were characterized by X-ray diffraction (XRD), Energy Dispersive X-Ray (EDX) and Field Emission Scanning Electron Microscope (FE-SEM). The ZnO NRs were obvious hexangular wurtzite structure and preferentially oriented along the c-axis (002) and growth vertically to the substrates. The optical properties were studied. From UV-Visible spectrophotometer and Photoluminescence (PL), the optical band gap energy of all ZnO NRs samples (S1, S2 and S3) were calculated to be (3.425 eV, 3.4 eV, 3.425 eV) respectively. Also, the effect of growth time on ZnO nanorods was studied.
Wurtzite crystal structure
Nanorod
Field emission microscopy
Hydrothermal Synthesis
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