29 research outputs found
First-principles study of the Young's modulus of Si <001> nanowires
We report the results of first-principles density functional theory
calculations of the Young's modulus and other mechanical properties of
hydrogen-passivated Si nanowires. The nanowires are taken to have
predominantly {100} surfaces, with small {110} facets. The Young's modulus, the
equilibrium length and the residual stress of a series of prismatic wires are
found to have a size dependence that scales like the surface area to volume
ratio for all but the smallest wires. We analyze the physical origin of the
size dependence, and compare the results to two existing models.Comment: 5 pages, 3 figure
Large area deep ultraviolet light of AlGaN/AlGaN multi quantum well with carbon nanotube electron beam pumping
Large area deep ultraviolet (DUV) light is generated by carbon nanotube (CNT) cold cathode electron beam (C-beam) irradiation on AlGaN/AlGaN multi quantum wells (MQWs) anode. We developed areal electron beam (EB) with CNT cold cathode emitters. The CNT emitters on silicon wafer were deposited with an area of 188 mm , and these were vertically aligned and had conical structures. We optimized the C-beam irradiation conditions to effectively excite AlGaN MQWs. When AlGaN MQWs were excited using an anode voltage of 3 kV and an anode current of 0.8 mA, DUV with a wavelength of 278.7 nm was generated in a large area of 303 mm . This DUV area is more than 11 times larger than the light emitting area of conventional EB pumped light sources and UV-LEDs
First-principles calculation of mechanical properties of Si <001> nanowires and comparison to nanomechanical theory
We report the results of first-principles density functional theory
calculations of the Young's modulus and other mechanical properties of
hydrogen-passivated Si nanowires. The nanowires are taken to have
predominantly {100} surfaces, with small {110} facets according to the Wulff
shape. The Young's modulus, the equilibrium length and the constrained residual
stress of a series of prismatic beams of differing sizes are found to have size
dependences that scale like the surface area to volume ratio for all but the
smallest beam. The results are compared with a continuum model and the results
of classical atomistic calculations based on an empirical potential. We
attribute the size dependence to specific physical structures and interactions.
In particular, the hydrogen interactions on the surface and the charge density
variations within the beam are quantified and used both to parameterize the
continuum model and to account for the discrepancies between the two models and
the first-principles results.Comment: 14 pages, 10 figure
Improved carrier injection of AlGaN-based deep ultraviolet light emitting diodes with graded superlattice electron blocking layers
A DUV-LED with a graded superlattice electron blocking layer (GSL-EBL) is demonstrated to show improved carrier injection into the multi-quantum well region. The structures of modified EBLs are designed via simulation. The simulation results show the carrier behavior mechanism of DUV-LEDs with a single EBL (S-EBL), graded EBL (G-EBL), and GSL-EBL. The variation in the energy band diagram around the EBL region indicates that the introduction of GSL-EBL is very effective in enhancing carrier injection. Besides, all DUV-LEDs emitting at 280 nm are grown in the high temperature metal organic chemical deposition system. It is confirmed that the optical power of the DUV-LED with the GSL-EBL is significantly higher than that of the DUV-LED with the S-EBL and G-EBL
Void containing AlN layer grown on AlN nanorods fabricated by polarity selective epitaxy and etching method
Creating voids between thin films is a very effective method to improve thin film crystal quality. However, for AlN material systems, the AlN layer growth, including voids, is challenging because of the very high Al atom sticking coefficient. In this study, we demonstrated an AlN template with many voids grown on AlN nanorods made by polarity selective epitaxy and etching methods. We introduced a low V/III ratio and NH pulsed growth method to demonstrate high-quality coalesced AlN templates grown on AlN nanorods in a metal organic chemical vapor deposition reactor. The crystal quality and residual strain of AlN were enhanced by the void formations. It is expected that this growth method can contribute to the demonstration of high-performance deep UV LEDs and transistors
Elastic constants and volume changes associated with two high-pressure rhombohedral phase transformations in vanadium
We present results from ab initio calculations of the mechanical properties
of the rhombohedral phase (beta) of vanadium metal reported in recent
experiments, and other predicted high-pressure phases (gamma and bcc), focusing
on properties relevant to dynamic experiments. We find that the volume change
associated with these transitions is small: no more than 0.15% (for beta -
gamma). Calculations of the single crystal and polycrystal elastic moduli
(stress-strain coefficients) reveal a remarkably small discontinuity in the
shear modulus and other elastic properties across the phase transitions even at
zero temperature where the transitions are first order.Comment: 6 pages, 3 figure
금속 유기 물질의 합성과 방향족 치환을 이용한 수정 및 응용
Metal-organic materials, metal-organic frameworks, metal-organic cages, post-synthetic modification, electrophilic aromatic substitutionDoctordCollectio