10 research outputs found
Breakdown of the interlayer coherence in twisted bilayer graphene
Coherent motion of the electrons in the Bloch states is one of the
fundamental concepts of the charge conduction in solid state physics. In
layered materials, however, such a condition often breaks down for the
interlayer conduction, when the interlayer coupling is significantly reduced by
e.g. large interlayer separation. We report that complete suppression of
coherent conduction is realized even in an atomic length scale of layer
separation in twisted bilayer graphene. The interlayer resistivity of twisted
bilayer graphene is much higher than the c-axis resistivity of Bernal-stacked
graphite, and exhibits strong dependence on temperature as well as on external
electric fields. These results suggest that the graphene layers are
significantly decoupled by rotation and incoherent conduction is a main
transport channel between the layers of twisted bilayer graphene.Comment: 5 pages, 3 figure
Direct top-down fabrication of nanoscale electrodes for organic semiconductors using fluoropolymer resists
Additive Fabrication and Characterization of Biomimetic Composite Bone Scaffolds with High Hydroxyapatite Content
With the increased incidence of bone defects following trauma or diseases in recent years, three-dimensional porous scaffolds fabricated using bioprinting technologies have been widely explored as effective alternatives to conventional bone grafts, which provide cell-friendly microenvironments promoting bone repair and regeneration. However, the limited use of biomaterials poses a significant challenge to the robust and accurate fabrication of bioprinted bone scaffolds that enable effective regeneration of the target tissues. Although bioceramic/polymer composites can provide tunable biomimetic conditions, their effects on the bioprinting process are unclear. Thus, in this study, we fabricated hydroxyapatite (HA)/gelatin composite scaffolds containing large weight fractions of HA using extrusion-based bioprinting, with the aim to provide an adequate biomimetic environment for bone tissue regeneration with compositional and mechanical similarity to the natural bone matrix. The overall features of the bioprinted HA/gelatin composite scaffolds, including rheological, morphological, physicochemical, mechanical, and biological properties, were quantitatively assessed to determine the optimal conditions for both fabrication and therapeutic efficiency. The present results show that the bioprinted bioceramic/hydrogel scaffolds possess excellent shape fidelity; mechanical strength comparable to that of native bone; and enhanced bioactivity in terms of cell proliferation, attachment, and osteogenic differentiation. This study provides a suitable alternative direction for the fabrication of bioceramic/hydrogel-based scaffolds for bone repair based on bioprinting
Crystallographic orientation of early domains in CVD graphene studied by Raman spectroscopy
Crystallographic orientations of early multi-lobe graphene domains in CVD-grown graphene are investigated. Partially grown graphene domains were transferred onto flexible substrates and uniaxial tensile strain was applied. From the polarization dependence of the Raman G band, split due to strain, the crystallographic orientation was determined. It was found that there are some single crystal domains. However, lobes have different orientations in some other multi-lobe domains. Within a given lobe, the orientation does not change as the domain grows, indicating that the orientations present in a graphene domain are determined at an early stage of growth
Extraction of astaxanthin using ultrasound-assisted natural deep eutectic solvents from shrimp wastes and its application in bioactive films
Nanoelectromechanical systems from carbon nanotubes and graphene
Carbon nanotubes and graphene have many interesting properties. To exploit the properties in applications their synthesis and incorporation in devices has to be understood and controlled. This thesis is based on experimental studies on synthesis of carbon nanotubes and fabrication of nanoelectromechanical systems from carbon nanotubes and graphene.
Vertically aligned nanotube arrays with heights over 800 µm have been grown using acetylene with iron as catalyst on alumina support using thermal chemical vapor deposition. By varying the partial pressure of acetylene it was found that the addition-rate of carbon was proportional to the coverage of acetylene molecules on the catalyst nanoparticle.
In certain conditions the macroscopic pattern of the catalyst areas influenced the microscopic properties of the carbon nanotubes. It was shown that the initial carbon-precursor flow conditions could determine the number of walls produced. The amount of carbon incorporated into nanotubes was constant but regions that
experienced less carbon precursor gas flow due e.g. to depletion, produced longer but fewer-walled nanotubes.
Arrays of vertically aligned nanotubes were shown to deflect as a single unit under electrostatic actuation, making possible the fabrication of varactors. Measurements of deflection were used to determine an eff ective Young's modulus of 6(+- 4) MPa. The capacitance of such a device could be reproducibly changed by more than 20 %.
Devices based on the nanoelectromechanical properties of few-layer graphene were fabricated and characterized. Electrostatic actuation of buckled beams and membranes led to a "snap-through" switching at a critical applied voltage. By characterizing this behavior for diff erent sizes and geometries of membranes, it was possible to extract the bending rigidity of bilayered graphene, yielding a value of 35(+20,-15) eV.
CNTFETs with suspended graphene gates were fabricated. It was shown that a moveable graphene gate could control the conductance of the carbon nanotube and improve the switching characteristics. Inverse sub-threshold slope down to 53 mV per decade were measured at 100 K. The experimental data were compared
with theoretical simulations and it was inferred that the subthreshold slope could be improved beyond the thermal limit by improving the design of the device
Rectifying Single GaAsSb Nanowire Devices Based on Self-Induced Compositional Gradients
Device configurations that enable
a unidirectional propagation of carriers in a semiconductor are fundamental
components for electronic and optoelectronic applications. To realize
such devices, however, it is generally required to have complex processes
to make p–n or Schottky junctions. Here we report on a unidirectional
propagation effect due to a self-induced compositional variation in
GaAsSb nanowires (NWs). The individual GaAsSb NWs exhibit a highly
reproducible rectifying behavior, where the rectifying direction is
determined by the NW growth direction. Combining the results from
confocal micro-Raman spectroscopy, electron microscopy, and electrical
measurements, the origin of the rectifying behavior is found to be
associated with a self-induced variation of the Sb and the carrier
concentrations in the NW. To demonstrate the usefulness of these GaAsSb
NWs for device applications, NW-based photodetectors and logic circuits
have been made
Modification of Electrical Properties of Graphene by Substrate-Induced Nanomodulation
A periodically
modulated graphene (PMG) generated by nanopatterned
surfaces is reported to profoundly modify the intrinsic electronic
properties of graphene. The temperature dependence of the sheet resistivity
and gate response measurements clearly show a semiconductor-like behavior.
Raman spectroscopy reveals significant shifts of the G and the 2D
modes induced by the interaction with the underlying grid-like nanostructure.
The influence of the periodic, alternating contact with the substrate
surface was studied in terms of strain caused by bending of graphene
and doping through chemical interactions with underlying substrate
atoms. Electronic structure calculations performed on a model of PMG
reveals that it is possible to tune a band gap within 0.14–0.19
eV by considering both the periodic mechanical bending and the surface
coordination chemistry. Therefore, the PMG can be regarded as a further
step toward band gap engineering of graphene devices
Surface Modulation of Graphene Field Effect Transistors on Periodic Trench Structure
In this work, graphene field effect
transistors (FETs) were fabricated on a trench structure made by carbonized
poly(methylmethacrylate) to modify the graphene surface. The trench-structured
devices showed different characteristics depending on the channel
orientation and the pitch size of the trenches as well as channel
area in the FETs. Periodic corrugations and barriers of suspended
graphene on the trench structure were measured by atomic force microscopy
and electrostatic force microscopy. Regular barriers of 160 mV were
observed for the trench structure with graphene. To confirm the transfer
mechanism in the FETs depending on the channel orientation, the ratio
of experimental mobility (3.6–3.74) was extracted from the
current–voltage characteristics using equivalent circuit simulation.
It is shown that the number of barriers increases as the pitch size
decreases because the number of corrugations increases from different
trench pitches. The noise for the 140 nm pitch trench is 1 order of
magnitude higher than that for the 200 nm pitch trench