57 research outputs found
Local probing of the field emission stability of vertically aligned multiwalled carbon nanotubes
Metallic cantilever in high vacuum atomic force microscope has been used as
anode for field emission experiments from densely packed vertically aligned
multi-walled carbon nanotubes. The high spatial resolution provided by the
scanning probe technique allowed precise setting of the tip-sample distance in
the submicron region. The dimension of the probe (curvature radius below 50nm)
allowed to measure current contribution from sample areas smaller than 1um^2.
The study of long-term stability evidenced that on these small areas the field
emission current remains stable (within 10% fluctuations) several hours (at
least up to 72 hours) at current intensities between 10-5A and 10-8A.
Improvement of the current stability has been observed after performing
long-time Joule heating conditioning to completely remove possible adsorbates
on the nanotubes.Comment: 15 pages, 7 figure
Rare-earth monosulfides as durable and efficient cold cathodes
In their rocksalt structure, rare-earth monosulfides offer a more stable
alternative to alkali metals to attain low or negative electron affinity when
deposited on various III-V and II-VI semiconductor surfaces. In this article,
we first describe the successful deposition of Lanthanum Monosulfide via pulsed
laser deposition on Si and MgO substrates and alumina templates. These thin
films have been characterized by X-ray diffraction, atomic force microscopy,
high resolution transmission electron microscopy, ellipsometry, Raman
spectroscopy, ultraviolet photoelectron spectroscopy and Kelvin probe
measurements. For both LaS/Si and LaS/MgO thin films, the effective work
function of the submicron thick thin films was determined to be about 1 eV from
field emission measurements using the Scanning Anode Field Emission Microscopy
technique. The physical reasons for these highly desirable low work function
properties were explained using a patchwork field emission model of the
emitting surface. In this model, nanocrystals of low work function materials
having a orientation perpendicular to the surface and outcropping it are
surrounded by a matrix of amorphous materials with higher work function. To
date, LaS thin films have been used successfully as cold cathode emitters with
measured emitted current densities as high as 50 A/cm2. Finally, we describe
the successful growth of LaS thin films on InP substrates and, more recently,
the production of LaS nanoballs and nanoclusters using Pulsed Laser Ablation.Comment: 61 pages, 24 figure
NEW FORMULATIONS FOR THE TRAVELING REPAIRMAN PROBLEM
Given a complete directed graph G = (V, E), the traveling repairman problem consists of determining a Hamiltonian circuit minimizing the sum of the waiting times of customers located at the nodes of the graph. In this paper, we propose two new linear integer formulations for TRP and computationally evaluate the strength of their linear programming relaxations. Computational results show the efficiency of the second new formulation considering the LP relaxations gaps and CPU times. The larger instance solved to optimality with the proposed second formulation involves 29 vertices. This compares favorably with a previously published formulation
The Black and White Traveling Salesman Problem
The black and white traveling salesman problem (BWTSP) is defined on a graph G whose vertex set is partitioned into
black and white vertices. The aim is to design a shortest Hamiltonian tour on G subject to cardinality and length constraints:
both the number of white vertices as well as the length of the tour between two consecutive black vertices are bounded
above. The BWTSP has applications in airline scheduling and in telecommunications. This paper proposes an integer linear
formulation for the undirected BWTSP, as well as several classes of valid inequalities. An exact branch-and-cut algorithm
is then developed. Extensive tests show that it can solve exactly instances involving up to 100 vertices. The algorithm can
also be applied directly to solve unit demand vehicle routing problems of similar sizes
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