870 research outputs found
Suitability of carbon nanotubes grown by chemical vapor deposition for electrical devices
Using carbon nanotubes (CNTs) produced by chemical vapor deposition, we have
explored different strategies for the preparation of carbon nanotube devices
suited for electrical and mechanical measurements. Though the target device is
a single small diameter CNT, there is compelling evidence for bundling, both
for CNTs grown over structured slits and on rigid supports. Whereas the
bundling is substantial in the former case, individual single-wall CNTs (SWNTs)
can be found in the latter. Our evidence stems from mechanical and electrical
measurements on contacted tubes. Furthermore, we report on the fabrication of
low-ohmic contacts to SWNTs. We compare Au, Ti and Pd contacts and find that Pd
yields the best results.Comment: pdf including figures, see:
http://www.unibas.ch/phys-meso/Research/Papers/2004/Suitability-CVD-tubes.pd
Current carrying capacity of carbon nanotubes
The current carrying capacity of ballistic electrons in carbon nanotubes that
are coupled to ideal contacts is analyzed. At small applied voltages, where
electrons are injected only into crossing subbands, the differential
conductance is . At applied voltages larger than
( is the energy level spacing of first non crossing subbands),
electrons are injected into non crossing subbands. The contribution of these
electrons to current is determined by the competing processes of Bragg
reflection and Zener type inter subband tunneling. In small diameter nanotubes,
Bragg reflection dominates, and the maximum differential conductance is
comparable to . Inter subband Zener tunneling can be non negligible as
the nanotube diameter increases because is inversely
proportional to the diameter. As a result, with increasing nanotube diameter,
the differential conductance becomes larger than , though not
comparable to the large number of subbands into which electrons are injected
from the contacts. These results may be relevant to recent experiments in large
diameter multi-wall nanotubes that observed conductances larger than .Comment: 12 pages, 4 figure
Determination of the Intershell Conductance in Multiwalled Carbon Nanotubes
We report on the intershell electron transport in multiwalled carbon
nanotubes (MWNT). To do this, local and nonlocal four-point measurements are
used to study the current path through the different shells of a MWNT. For
short electrode separations 1 m the current mainly flows
through the two outer shells, described by a resistive transmission line with
an intershell conductance per length of ~(10 k\Omega)^{-1}/m. The
intershell transport is tunnel-type and the transmission is consistent with the
estimate based on the overlap between -orbitals of neighboring shells.Comment: 5 pages, 4 figure
Recommended from our members
Evaluation of botanical diversity in Oregon vineyards
Three different ground cover management strategies were compared at the OSU research vineyard near Alpine, Oregon. Botanical diversity was actively increased in two diverse treatments. Another treatment was botanically uniform and contained creeping bentgrass (Agrostis stolonifera). The composition of the vineyard floor vegetation and grapevine performance as affected by the treatments was evaluated. Shoot length and average leaf size of the grapevines were increased (> 30%) in treatments with more diverse ground covers, the main-shoot leaf area per vine was larger but the lateral-shoot leaf area was not affected. Photosynthesis and transpiration rates were not different among the treatments except for two measurements, which showed lower photosynthesis rates in the bentgrass treatment. The water use efficiency of photosynthesis tended to be higher for grapevine leaves in more diverse treatments except at veraison 1997. The leaf chlorophyll content was higher in the more diverse treatments at bloom, but was similar in all treatments later in the season. The juice soluble solids (Brix) at harvest were higher (4 %) in the diverse treatments, and in one of the two investigated years, fruit yield was also higher. Percent fruit set, titratable acidity, and pH were not affected by the treatments. The experiment showed that the grapevines in botanically uniform ('grass') plots produced less vegetative growth and delayed fruit maturity, even with a lower crop load. In addition to the experiment, four commercial vineyards in the Willamette Valley in Oregon were surveyed to establish a list and number of resident (weedy) plant species. At the scale of the whole vineyards, 9, 10, 11, and 13 plant species were observed. All four sites were grass dominated and five broadleaf plant species occurred in all four sites. The data sets indicated that the number of plant species was not in all cases randomly distributed over the vineyard. The data showed a continuous trend to higher numbers of plant species from east to west in one vineyard. In another vineyard, the data showed a patch of lower numbers of plant species in a small part of the field. The data in the other two vineyards did not indicate patterns or trends
Sensing electric fields using single diamond spins
The ability to sensitively detect charges under ambient conditions would be a
fascinating new tool benefitting a wide range of researchers across
disciplines. However, most current techniques are limited to low-temperature
methods like single-electron transistors (SET), single-electron electrostatic
force microscopy and scanning tunnelling microscopy. Here we open up a new
quantum metrology technique demonstrating precision electric field measurement
using a single nitrogen-vacancy defect centre(NV) spin in diamond. An AC
electric field sensitivity reaching ~ 140V/cm/\surd Hz has been achieved. This
corresponds to the electric field produced by a single elementary charge
located at a distance of ~ 150 nm from our spin sensor with averaging for one
second. By careful analysis of the electronic structure of the defect centre,
we show how an applied magnetic field influences the electric field sensing
properties. By this we demonstrate that diamond defect centre spins can be
switched between electric and magnetic field sensing modes and identify
suitable parameter ranges for both detector schemes. By combining magnetic and
electric field sensitivity, nanoscale detection and ambient operation our study
opens up new frontiers in imaging and sensing applications ranging from
material science to bioimaging
Recommended from our members
Integrated Production: Increasing and Characterizing Biodiversity in Oregon Vineyards Evaluation of Botanical Diversity in Oregon Vineyards
Soil and vineyard vegetation management are tools that may be used to create stable growing conditions for the grapevine roots and to preserve soil fertility for high grape quality in future years. The use of cover crops is commonly practiced in viticultural areas where precipitation exceeds 600-700 min (24-28 in) a year, because cover crops reduce soil erosion and surface water runoff on hillsides, maintain soil organic matter and nutrient levels in the soil, and slow the soil compaction process and subsequent loss of pore space. Cover crop plants and management practices should be selected carefully and adapted to the specific site and local weather conditions to ensure adequate vine growth, yield and grape quality. The composition and botanical diversity of the plant community are key elements in the agro-ecosystem 4 vineyard'. Botanical diversity is positively related to faunistic diversity (e.g. Remund et al., 1989). The general link between diversity and ecological stability is subject of numerous studies in the present. The objective of this study is to demonstrate and contrast effects of high botanical diversity versus low botanical diversity on grapevine performance. Additionally, an extended botanical survey was carried out in order to get a realistic number of resident plant species and to define 'high' botanical diversity for vineyards in the Willamette Valley
Continental weathering and recovery from ocean nutrient stress during the Early Triassic Biotic Crisis
Following the latest Permian extinction ∼252 million years ago, normal marine and terrestrial
ecosystems did not recover for another 5-9 million years. The driver(s) for the Early Triassic
biotic crisis, marked by high atmospheric CO2 concentration, extreme ocean warming, and
marine anoxia, remains unclear. Here we constrain the timing of authigenic K-bearing mineral
formation extracted from supergene weathering profiles of NW-Pangea by Argon geochronology,
to demonstrate that an accelerated hydrological cycle causing intense chemical
alteration of the continents occurred between ∼254 and 248 Ma, and continued throughout
the Triassic period. We show that enhanced ocean nutrient supply from this intense continental
weathering did not trigger increased ocean productivity during the Early Triassic
biotic crisis, due to strong thermal ocean stratification off NW Pangea. Nitrogen isotope
constraints suggest, instead, that full recovery from ocean nutrient stress, despite some brief
amelioration ∼1.5 million years after the latest Permian extinction, did not commence until
climate cooling revitalized the global upwelling systems and ocean mixing ∼10 million years
after the mass extinction
Carbon nanotubes adhesion and nanomechanical behavior from peeling force spectroscopy
Applications based on Single Walled Carbon Nanotube (SWNT) are good example
of the great need to continuously develop metrology methods in the field of
nanotechnology. Contact and interface properties are key parameters that
determine the efficiency of SWNT functionalized nanomaterials and nanodevices.
In this work we have taken advantage of a good control of the SWNT growth
processes at an atomic force microscope (AFM) tip apex and the use of a low
noise (1E-13 m/rtHz) AFM to investigate the mechanical behavior of a SWNT
touching a surface. By simultaneously recording static and dynamic properties
of SWNT, we show that the contact corresponds to a peeling geometry, and
extract quantities such as adhesion energy per unit length, curvature and
bending rigidity of the nanotube. A complete picture of the local shape of the
SWNT and its mechanical behavior is provided
- …