2,528 research outputs found
Metal-catalyst-free growth of carbon nanotubes and their application in field-effect transistors
The metal-catalyst-free growth of carbon nanotubes (CNTs) using chemical vapor deposition and the application in field-effect transistors (FETs) is demonstrated. The CNT growth process used a 3-nm-thick Ge layer on SiO2 that was subsequently annealed to produce Ge nanoparticles. Raman measurements show the presence of radial breathing mode peaks and the absence of the disorder induced D-band, indicating single walled CNTs with a low defect density. The synthesized CNTs are used to fabricate CNTFETs and the best device has a state-of-the-art on/off current ratio of 3×108 and a steep sub-threshold slope of 110 mV/dec
Pressure-Induced Anomalous Phase Transitions and Colossal Enhancement of Piezoelectricity in PbTiO
We find an unexpected tetragonal-to-monoclinic-to-rhombohedral-to-cubic phase
transition sequence induced by pressure, and a morphotropic phase boundary in a
pure compound using first-principles calculations. Huge dielectric and
piezoelectric coupling constants occur in the transition regions, comparable to
those observed in the new complex single-crystal solid-solution piezoelectrics
such as Pb(MgNb)O-PbTiO, which are expected to
revolutionize electromechanical applications. Our results show that
morphotropic phase boundaries and giant piezoelectric effects do not require
intrinsic disorder, and open the possibility of studying this effect in simple
systems.Comment: 4 pages, to appear in Phys. Rev. Let
Frustration of tilts and A-site driven ferroelectricity in KNbO_3-LiNbO_3 alloys
Density functional calculations for K_{0.5}Li_{0.5}NbO_3 show strong A-site
driven ferroelectricity, even though the average tolerance factor is
significantly smaller than unity and there is no stereochemically active A-site
ion. This is due to the frustration of tilt instabilities by A-site disorder.
There are very large off-centerings of the Li ions, which contribute strongly
to the anisotropy between the tetragonal and rhombohedral ferroelectric states,
yielding a tetragonal ground state even without strain coupling.Comment: 4 pages, 5 figure
Positive effective Q12 electrostrictive coefficient in perovskites
It is demonstrated that for classical perovskites such as BaTiO3, SrTiO3 and
PbTiO3 electrostrictive strain induced by an electric field may not obey
traditionally considered "extension along the field, contraction perpendicular
to it" behavior if a sample is cut obliquely to the cubic crystallographic
directions
Bogoliubov Theory and Lee-Huang-Yang Corrections in Spin-1 and Spin-2 Bose-Einstein Condensates in the Presence of the Quadratic Zeeman Effect
We develop Bogoliubov theory of spin-1 and spin-2 Bose-Einstein condensates
(BECs) in the presence of a quadratic Zeeman effect, and derive the
Lee-Huang-Yang (LHY) corrections to the ground-state energy, sound velocity,
and quantum depletion. We investigate all the phases of spin-1 and spin-2 BECs
that can be realized experimentally. We also examine the stability of each
phase against quantum fluctuations and the quadratic Zeeman effect.
Furthermore, we discuss a relationship between the number of symmetry
generators that are spontaneously broken and that of Nambu-Goldstone (NG)
modes. It is found that in the spin-2 nematic phase there are special
Bogoliubov modes that have gapless linear dispersion relations but do not
belong to the NG modes.Comment: v3: 62 pages, 18 figure
Metal-catalyst-free growth of silica nanowires and carbon nanotubes using Ge nanostructures
The use of Ge nanostructures is investigated for the metal-catalyst-free growth of silica nanowires and carbon nanotubes (CNTs). Silica nanowires with diameters of 10-50 nm and lengths of ? 1 ?m were grown from SiGe islands, Ge dots, and Ge nanoparticles. High-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS) reveal that the nanowires grow from oxide nanoparticles on the sample surface. We propose that the growth mechanism is thermal diffusion of oxide through the GeO2 nanostructures. CNTs with diameters 0.6-2.5 nm and lengths of less than a few ?m were similarly grown by chemical vapor deposition from different types of Ge nanostructures. Raman measurements show the presence of radial breathing mode peaks and the absence of the disorder induced D-band, indicating single walled CNTs with a low defect density. HRTEM images reveal that the CNTs also grow from oxide nanoparticles, comprising a mixture of GeO2 and SiO2
Growth of Carbon Nanotubes on HfO2 towards Highly Sensitive Nano-Sensors
Carbon nanotube (CNT) growth on HfO2 is reported for the first time. The process uses a combination of Ge and Fe nanoparticles and achieves an increase in CNT density from 0.15 to 6.2 mm length/mm2 compared with Fe nanoparticles alone. The synthesized CNTs are assessed by the fabrication of back-gate CNT field-effect transistors with Al source/drain contacts for nano-sensor applications. The devices exhibit excellent p-type behavior with an Ion=Ioff ratio of 105 and a steep sub-threshold slope of 130 mV/dec
Optimal configuration of microstructure in ferroelectric materials by stochastic optimization
An optimization procedure determining the ideal configuration at the
microstructural level of ferroelectric (FE) materials is applied to maximize
piezoelectricity. Piezoelectricity in ceramic FEs differ significantly from
that of single crystals because of the presence of crystallites (grains)
possessing crystallographic axes aligned imperfectly. The piezoelectric
properties of a polycrystalline (ceramic) FE is inextricably related to the
grain orientation distribution (texture). The set of combination of variables,
known as solution space, which dictates the texture of a ceramic is unlimited
and hence the choice of the optimal solution which maximizes the
piezoelectricity is complicated. Thus a stochastic global optimization combined
with homogenization is employed for the identification of the optimal granular
configuration of the FE ceramic microstructure with optimum piezoelectric
properties. The macroscopic equilibrium piezoelectric properties of
polycrystalline FE is calculated using mathematical homogenization at each
iteration step. The configuration of grains characterised by its orientations
at each iteration is generated using a randomly selected set of orientation
distribution parameters. Apparent enhancement of piezoelectric coefficient
is observed in an optimally oriented BaTiO single crystal. A
configuration of crystallites, simultaneously constraining the orientation
distribution of the c-axis (polar axis) while incorporating ab-plane
randomness, which would multiply the overall piezoelectricity in ceramic
BaTiO is also identified. The orientation distribution of the c-axes is
found to be a narrow Gaussian distribution centred around . The
piezoelectric coefficient in such a ceramic is found to be nearly three times
as that of the single crystal.Comment: 11 pages, 7 figure
The Persistence and Memory of Polar Nano-Regions in a Ferroelectric Relaxor Under an Electric Field
The response of polar nanoregions (PNR) in the relaxor compound
Pb[(ZnNb)Ti]O subject to a [111]-oriented
electric field has been studied by neutron diffuse scattering. Contrary to
classical expectations, the diffuse scattering associated with the PNR
persists, and is even partially enhanced by field cooling. The effect of the
external electric field is retained by the PNR after the field is removed. The
``memory'' of the applied field reappears even after heating the system above
, and cooling in zero field
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