355 research outputs found
Determination of substrate pinning in epitaxial and supported graphene layers via Raman scattering
The temperature-induced shift of the Raman G line in epitaxial graphene on
SiC and Ni surfaces, as well as in graphene supported on SiO2, is investigated
with Raman spectroscopy. The thermal shift rate of epitaxial graphene on
6H-SiC(0001) is found to be about three times that of freestanding graphene.
This result is explained quantitatively as a consequence of pinning by the
substrate. In contrast, graphene grown on polycrystalline Ni films is shown to
be unpinned, i.e., to behave elastically as freestanding, despite the
relatively strong interaction with the metal substrate. Moreover, it is shown
that the transfer of exfoliated graphene layers onto a supporting substrate can
result in pinned or unpinned layers, depending on the transfer protocol.Comment: 4 pages, 5 figures. To appear in Physical Review B, Brief
Communicatio
Evolution in Surface Morphology of Epitaxial Graphene Layers on SiC Induced by Controlled Structural Strain
The evolution in the surface morphology of epitaxial graphene films and
6H-SiC(0001) substrates is studied by electron channeling contrast imaging.
Whereas film thickness is determined by growth temperature only, increasing
growth times at constant temperature affect both internal stress and film
morphology. Annealing times in excess of 8-10 minutes lead to an increase in
the mean square roughness of SiC step edges to which graphene films are pinned,
resulting in compressively stressed films at room temperature. Shorter
annealing times produce minimal changes in the morphology of the terrace edges
and result in nearly stress-free films upon cooling to room temperature.Comment: 3 pages, 2 figures. Applied Physics Letters 93 (2008), 19191
Experiments on Quantum and Thermal Desorption from ^4He Films
Desorption of He atoms from thin films may be resolved experimentally into quantum and thermal components. We show that quantum desorption becomes the dominant part of the signal in submonolayer films. We also show that, when all effects of collisions between desorbed atoms are eliminated, quantum desorption is not focused normal to the surface of optically polished sapphire crystals
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Adhesion characteristics of PDMS surfaces during repeated pull-off force measurements
To mimic the adhesive effects of gecko toes, artificial surfaces have been manufactured recently using polydimethylsiloxanes (PDMS). However, the effects of repeated contacts on the adhesive properties remain largely unexplored. In this paper we report on the effect of repeated pull-off force measurements on the adhesion behavior of PDMS (polymer kit Sylgard 184, Dow Corning) tested with a borosilicate glass probe. A decrease in pull-off force with increase in number of test cycles is found until a plateau is reached. The initial value and the rate of change in pull-off force strongly depend on the sample preparation procedure, including curing time and cross-linking. It is proposed that the behavior is due to steady coverage of the probe with free oligomers. The results are crucial for developing reusable, durable, and residue-free bioinspired adhesives
In-Situ Observation of Surface and Near-Surface Modification Using Scattering of Ballistic Phonons
We have investigated the feasibility of phonon-reflection techniques as non-destructive means to probe surface and/or near-surface damage in otherwise highly perfect crystals. A UHV liquid-helium stage, suitable for phonon-reflection measurements, was installed on a beam line of a tandem van de Graaff accelerator which was used to implant MeV ions into the substrate in order to modify the subsurface region in situ. Here, we report our investigation on the effects of 1 MeV Arâș implantation in AlâOâ single crystals by monitoring the reflection of terahertz (THz) phonons (50 Ă
wavelength) from the implanted region. The results are supported by x-ray rocking measurements and Monte Carlo simulations.
Using a 15 kV ion gun on the same beam line we have also bombarded AlâOâ crystals coated with thin films of gold. The effects of a 7.5 keV Arâș irradiation on this Au - AlâOâ system are also discussed in this thesis.
The relevance of this work is discussed in connection to the observations made by other groups and also to our previous work (reported in Appendix 3) on phonon-induced desorption of He atoms as well as the Kapitza anomaly.</p
3D Stretchable Arch Ribbon Array Fabricated via Grayscale Lithography.
Microstructures with flexible and stretchable properties display tremendous potential applications including integrated systems, wearable devices and bio-sensor electronics. Hence, it is essential to develop an effective method for fabricating curvilinear and flexural microstructures. Despite significant advances in 2D stretchable inorganic structures, large scale fabrication of unique 3D microstructures at a low cost remains challenging. Here, we demonstrate that the 3D microstructures can be achieved by grayscale lithography to produce a curved photoresist (PR) template, where the PR acts as sacrificial layer to form wavelike arched structures. Using plasma-enhanced chemical vapor deposition (PECVD) process at low temperature, the curved PR topography can be transferred to the silicon dioxide layer. Subsequently, plasma etching can be used to fabricate the arched stripe arrays. The wavelike silicon dioxide arch microstructure exhibits Young modulus and fracture strength of 52âGPa and 300âMPa, respectively. The model of stress distribution inside the microstructure was also established, which compares well with the experimental results. This approach of fabricating a wavelike arch structure may become a promising route to produce a variety of stretchable sensors, actuators and circuits, thus providing unique opportunities for emerging classes of robust 3D integrated systems
Observation of near-surface damage by phonon scattering
We have investigated the feasibility of phonon-reflection techniques as nondestructive means to probe surface and/or near-surface damage in otherwise highly perfect crystals. An UHV liquid-helium stage, suitable for phonon-reflection measurements, was installed on a beam line of a tandem van de Graaff accelerator which was used to implant MeV ions into the substrate in order to modify the surface region in situ. Here, we report our investigation on the effects of 1-MeV Ar+ implantation in Al2O3 single crystals by monitoring the reflection of terahertz (THz) phonons (50-AÌ wavelength) from the implanted region. The results are compared to other surface techniques. Using a 15-kV ion gun on the same beam line, we have also bombarded Al2O3 crystals coated with thin films of gold. The effects of a 7.5-keV Ar+ beam on this Au-Al2O3 system are also discussed in this paper
Nanofriction mechanisms derived from the dependence of friction on load and sliding velocity from air to UHV on hydrophilic silicon
This paper examines friction as a function of the sliding velocity and
applied normal load from air to UHV in a scanning force microscope (SFM)
experiment in which a sharp silicon tip slides against a flat Si(100) sample.
Under ambient conditions, both surfaces are covered by a native oxide, which is
hydrophilic. During pump-down in the vacuum chamber housing the SFM, the
behavior of friction as a function of the applied normal load and the sliding
velocity undergoes a change. By analyzing these changes it is possible to
identify three distinct friction regimes with corresponding contact properties:
(a) friction dominated by the additional normal forces induced by capillarity
due to the presence of thick water films, (b) higher drag force from ordering
effects present in thin water layers and (c) low friction due to direct
solid-solid contact for the sample with the counterbody. Depending on
environmental conditions and the applied normal load, all three mechanisms may
be present at one time. Their individual contributions can be identified by
investigating the dependence of friction on the applied normal load as well as
on the sliding velocity in different pressure regimes, thus providing
information about nanoscale friction mechanisms
Strong interlayer coupling in van der Waals heterostructures built from single-layer chalcogenides
Semiconductor heterostructures are the fundamental platform for many
important device applications such as lasers, light-emitting diodes, solar
cells and high-electron-mobility transistors. Analogous to traditional
heterostructures, layered transition metal dichalcogenide (TMDC)
heterostructures can be designed and built by assembling individual
single-layers into functional multilayer structures, but in principle with
atomically sharp interfaces, no interdiffusion of atoms, digitally controlled
layered components and no lattice parameter constraints. Nonetheless, the
optoelectronic behavior of this new type of van der Waals (vdW) semiconductor
heterostructure is unknown at the single-layer limit. Specifically, it is
experimentally unknown whether the optical transitions will be spatially direct
or indirect in such hetero-bilayers. Here, we investigate artificial
semiconductor heterostructures built from single layer WSe2 and MoS2 building
blocks. We observe a large Stokes-like shift of ~100 meV between the
photoluminescence peak and the lowest absorption peak that is consistent with a
type II band alignment with spatially direct absorption but spatially indirect
emission. Notably, the photoluminescence intensity of this spatially indirect
transition is strong, suggesting strong interlayer coupling of charge carriers.
The coupling at the hetero-interface can be readily tuned by inserting
hexagonal BN (h-BN) dielectric layers into the vdW gap. The generic nature of
this interlayer coupling consequently provides a new degree of freedom in band
engineering and is expected to yield a new family of semiconductor
heterostructures having tunable optoelectronic properties with customized
composite layers.Comment: http://www.pnas.org/content/early/2014/04/10/1405435111.abstrac
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