150 research outputs found
A local field emission study of partially aligned carbon-nanotubes by AFM probe
We report on the application of Atomic Force Microscopy (AFM) for studying
the Field Emission (FE) properties of a dense array of long and vertically
quasi-aligned multi-walled carbon nanotubes grown by catalytic Chemical Vapor
Deposition on a silicon substrate. The use of nanometric probes enables local
field emission measurements allowing investigation of effects non detectable
with a conventional parallel plate setup, where the emission current is
averaged on a large sample area. The micrometric inter-electrode distance let
achieve high electric fields with a modest voltage source. Those features
allowed us to characterize field emission for macroscopic electric fields up to
250 V/m and attain current densities larger than 10 A/cm. FE
behaviour is analyzed in the framework of the Fowler-Nordheim theory. A field
enhancement factor 40-50 and a turn-on field 15 V/m at an inter-electrode distance of 1 m are estimated.
Current saturation observed at high voltages in the I-V characteristics is
explained in terms of a series resistance of the order of M. Additional
effects as electrical conditioning, CNT degradation, response to laser
irradiation and time stability are investigated and discussed
Field emission from single multi-wall carbon nanotubes
Electron field emission characteristics of individual multiwalled carbon
nanotubes have been investigated by a piezoelectric nanomanipulation system
operating inside a scanning electron microscopy chamber. The experimental setup
ensures a high control capability on the geometric parameters of the field
emission system (CNT length, diameter and anode-cathode distance). For several
multiwalled carbon nanotubes, reproducible and quite stable emission current
behaviour has been obtained with a dependence on the applied voltage well
described by a series resistance modified Fowler-Nordheim model. A turn-on
field of about 30 V/um and a field enhancement factor of around 100 at a
cathode-anode distance of the order of 1 um have been evaluated. Finally, the
effect of selective electron beam irradiation on the nanotube field emission
capabilities has been extensively investigated.Comment: 16 pages, 5 figure
Local Tunneling Study of Three-Dimensional Order Parameter in the -band of Al-doped MgB Single Crystals
We have performed local tunneling spectroscopy on high quality
MgAlB single crystals by means of Variable Temperature Scanning
Tunneling Spectroscopy (STS) in magnetic field up to 3 Tesla. Single gap
conductance spectra due to c-axis tunneling were extensively measured, probing
different amplitudes of the three-dimensional as a function of Al
content. Temperature and magnetic field dependences of the conductance spectra
were studied in S-I-N configuration: the effect of the doping resulted in a
monotonous reduction of the locally measured down to 24K for x=0.2. On
the other hand, we have found that the gap amplitude shows a maximum value
meV for x=0.1, while the ratio increases
monotonously with doping. The locally measured upper critical field was found
to be strongly related to the gap amplitude, showing the maximum value
for x=0.1 substituted samples. For this Al concentration the
data revealed some spatial inhomogeneity in the distribution of on
nanometer scale.Comment: 4 pages, 3 figure
Field emission properties of as-grown multiwalled carbon nanotube films
Multiwalled carbon nanotubes have been produced by ethylene catalytic
chemical vapor deposition and used to fabricate thick and dense freestanding
films ("buckypapers") by membrane filtering. Field emission properties of
buckypapers have been locally studied by means of high vacuum atomic force
microscopy with a standard metallic cantilever used as anode to collect
electrons emitted from the sample. Buckypapers showed an interesting linear
dependence in the Fowler-Nordheim plots demonstrating their suitability as
emitters. By precisely tuning the tip-sample distance in the submicron region
we found out that the field enhancement factor is not affected by distance
variations up to 2um. Finally, the study of current stability showed that the
field emission current with intensity of about 3,3*10-5A remains remarkably
stable (within 5% fluctuations) for several hours.Comment: 18 pages, 5 figure
Field Dependence of Electronic Specific Heat in Two-Band Superconductors
The vortex structure is studied in light of MgB theoretically based on a
two-band superconducting model by means of Bogoliubov-de Gennes framework. The
field dependence of the electronic specific heat coefficient is
focused. The exponent in is shown to
become smaller by adjusting the gap ratio of the two gaps on the major and
minor bands. The observed extremely small value could be
explained reasonably well in this two-band model with the gap ratio .Comment: 5 pages, 4 figures, to be published in J. Phys. Soc. Jp
Field emission from two-dimensional GeAs
GeAs is a layered material of the IV–V groups that is attracting growing attention for possible applications in electronic and optoelectronic devices. In this study, exfoliated multilayer GeAs nanoflakes are structurally characterized and used as the channel of back-gate field-effect transistors. It is shown that their gate-modulated p-type conduction is decreased by exposure to light or electron beam. Moreover, the observation of a field emission (FE) current demonstrates the suitability of GeAs nanoflakes as cold cathodes for electron emission and opens up new perspective applications of two-dimensional GeAs in vacuum electronics. FE occurs with a turn-on field of ~80 Vum-1 and attains a current density higher than 10 Acm-2, following the general Fowler–Nordheim model with high reproducibility
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