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Träfflista för sökning "WFRF:(Yurgens Avgust 1959) "

Sökning: WFRF:(Yurgens Avgust 1959)

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1.
  • Biznárová, Janka, 1995, et al. (författare)
  • Mitigation of interfacial dielectric loss in aluminum-on-silicon superconducting qubits
  • 2024
  • Ingår i: npj Quantum Information. - 2056-6387. ; 10:1
  • Tidskriftsartikel (refereegranskat)abstract
    • We demonstrate aluminum-on-silicon planar transmon qubits with time-averaged T1 energy relaxation times of up to 270 μs, corresponding to Q = 5 million, and a highest observed value of 501 μs. Through materials analysis techniques and numerical simulations we investigate the dominant source of energy loss, and devise and demonstrate a strategy toward its mitigation. Growing aluminum films thicker than 300 nm reduces the presence of oxide, a known host of defects, near the substrate-metal interface, as confirmed by time-of-flight secondary ion mass spectrometry. A loss analysis of coplanar waveguide resonators shows that this results in a reduction of dielectric loss due to two-level system defects. The correlation between the enhanced performance of our devices and the film thickness is due to the aluminum growth in columnar structures of parallel grain boundaries: transmission electron microscopy shows larger grains in the thicker film, and consequently fewer grain boundaries containing oxide near the substrate-metal interface.
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2.
  • ANDERSSON, MICHAEL, 1988, et al. (författare)
  • Microwave characterization of Ti/Au-graphene contacts
  • 2013
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 103:17, s. 173111-
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper, we report on a microwave characterization of the interface between Ti/Au contacts and chemical vapor deposition graphene using structures of Corbino geometry, with primary focus on extracting and modeling the capacitance associated with the contact region. It is found that with the current contact resistivity, ρc∼10^−6 Ωcm2, the contact capacitance, on the order Cc∼1 μF/cm2, has a negligible effect on microwave transmission through the contact below ∼100 GHz. Finally, a parallel plate capacitance model for the contact is presented.
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3.
  • ANDERSSON, MICHAEL, 1988, et al. (författare)
  • Towards Graphene Electrodes for High Performance Acoustic Resonators
  • 2013
  • Ingår i: WOCSDICE. ; , s. 99-100
  • Konferensbidrag (refereegranskat)abstract
    • The tunable FBAR is a promising building block for versatile microwave systems. Utilizing graphene electrodes promises higher tunability and frequency. Increased parasitic resistance may hamper the Q-factor of the resonator. This paper reports the initiated study of graphene and contacts at DC and microwave frequencies for optimization of these parameters leading to graphene FBARs.
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4.
  • Bray, C., et al. (författare)
  • Temperature-dependent zero-field splittings in graphene
  • 2022
  • Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 106:24
  • Tidskriftsartikel (refereegranskat)abstract
    • Graphene is a quantum spin Hall insulator with a 45μeV-wide nontrivial topological gap induced by the intrinsic spin-orbit coupling. Even though this zero-field spin splitting is weak, it makes graphene an attractive candidate for applications in quantum technologies, given the resulting long spin-relaxation time. On the other side, the staggered sublattice potential, resulting from the coupling of graphene with its boron nitride substrate, compensates intrinsic spin-orbit coupling and decreases the nontrivial topological gap, which may lead to the phase transition into trivial band insulator state. In this work, we present extensive experimental studies of the zero-field splittings in monolayer and bilayer graphene in a temperature range 2-12 K by means of subterahertz photoconductivity-based electron spin-resonance technique. Surprisingly, we observe a decrease of the spin splittings with increasing temperature. We discuss the origin of this phenomenon by considering possible physical mechanisms likely to induce a temperature dependence of the spin-orbit coupling. These include the difference in the expansion coefficients between the graphene and the boron nitride substrate or the metal contacts, the electron-phonon interactions, and the presence of a magnetic order at low temperature. Our experimental observation expands knowledge about the nontrivial topological gap in graphene.
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5.
  • Buron, J.D., et al. (författare)
  • Correlation between THz AC and micro-four-point-probe DC conductivity mapping of graphene sheets
  • 2012
  • Ingår i: Laser and Tera-Hertz Science and Technology, LTST 2012. - Washington, D.C. : OSA.
  • Konferensbidrag (refereegranskat)abstract
    • We present quantitative correlation mapping of the sheet conductance of large areas of graphene. Terahertz time-domain spectroscopy (THz-TDS) maps the nanoscale conductance averaged over the beam spot size whereas micro four-point probe (M4PP) maps the micro-scale conductance.
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6.
  • Buron, J. D., et al. (författare)
  • Graphene Conductance Uniformity Mapping
  • 2012
  • Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 12:10, s. 5074-5081
  • Tidskriftsartikel (refereegranskat)abstract
    • We demonstrate a combination of micro four-point probe (M4PP) and non-contact terahertz time-domain spectroscopy (THz-TDS) measurements for centimeter scale quantitative mapping of the sheet conductance of large area chemical vapor deposited graphene films. Dual configuration M4PP measurements, demonstrated on graphene for the first time, provide valuable statistical insight into the influence of microscale defects on the conductance, while THz-TDS has potential as a fast, non-contact metrology method for mapping of the spatially averaged nanoscopic conductance on wafer-scale graphene with scan times of less than a minute for a 4-in. wafer. The combination of M4PP and THz-TDS conductance measurements, supported by micro Raman spectroscopy and optical imaging, reveals that the film is electrically continuous on the nanoscopic scale with microscopic defects likely originating from the transfer process, dominating the microscale conductance of the investigated graphene film.
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7.
  • Buron, J. D., et al. (författare)
  • Quantitative mapping of large area graphene conductance
  • 2012
  • Ingår i: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035. - 9781467315975
  • Konferensbidrag (refereegranskat)abstract
    • We present quantitative mapping of large area graphene conductance by terahertz time-domain spectroscopy and micro four point probe. We observe a clear correlation between the techniques and identify the observed systematic differences to be directly related to imperfections of the graphene sheet on the length scale of several micrometers.
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8.
  • Evelt, M., et al. (författare)
  • Chiral charge pumping in graphene deposited on a magnetic insulator
  • 2017
  • Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 95:2
  • Tidskriftsartikel (refereegranskat)abstract
    • We demonstrate experimentally that a sizable chiral charge pumping can be achieved at room temperature in graphene/yttrium iron garnet (YIG) bilayer systems. The effect, which cannot be attributed to the ordinary spin pumping, reveals itself in the creation of a dc electric field/voltage in graphene as a response to the dynamic magnetic excitations (spin waves) in an adjacent out-of-plane magnetized YIG film. We show that the induced voltage changes its sign when the orientation of the static magnetization is reversed, clearly indicating the broken mirror reflection symmetry about the planes normal to the graphene/YIG interface. The strength of effect shows a nonmonotonous dependence on the spin-wave frequency, in agreement with the proposed theoretical model.
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9.
  • Gaska, Karolina, 1986, et al. (författare)
  • Influence of manufacturing process on electrical properties of LDPE-GnP nanocomposites
  • 2017
  • Ingår i: 25th Nordic Insulation Symposium on Materials, Components and Diagnostics.
  • Konferensbidrag (refereegranskat)abstract
    • In this report electrical properties of the nanocomposite samples, prepared from graphene nanoplatelet (GnP) loaded low density polyethylene (LDPE) by extrusion and compression molding, were examined in order to elucidate the impacts of the nanoplatelets size and material’s manufacturing process. It is shown that the extrusion forces a strong anisotropy in material’s morphology. The graphene nanoplatelets become aligned along the flow direction. As compared to pure LDPE, a significant reductions of the through-plane low field electric conductivity is found in such samples. On the other hand, the samples produced by press molding exhibit slightly higher level of electric conductivity, which is connected to their less aligned microstructure and filler dispersion. For comparison results of measurements on LDPE-graphene monolayer sandwiches are also presented.
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10.
  • Guo, W., et al. (författare)
  • Rapid chemical vapor deposition of graphene on liquid copper
  • 2016
  • Ingår i: Synthetic Metals. - : Elsevier BV. - 0379-6779. ; 216, s. 93-97
  • Tidskriftsartikel (refereegranskat)abstract
    • Molten copper is used to catalyze the graphene synthesis by chemical vapor deposition. The Cu has no grains above melting temperature, which is favorable for graphene growth. Using a vertical cold wall system, the deposition rate is drastically increased as compared with common hot-wall tube furnaces, pushing the method one step forward towards applications. A molybdenum-graphite Joule heater is used to avoid mechanical deformation of the carrier foil for the catalyst to ease the subsequent processes. The rapid deposition makes it possible to observe graphene growth on liquid Cu even at low pressure, where severe Cu evaporation simultaneously occurs.
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11.
  • Indykiewicz, Kornelia, 1986, et al. (författare)
  • Current-induced enhancement of photo-response in graphene THz radiation detectors
  • 2022
  • Ingår i: AIP Advances. - : AIP Publishing. - 2158-3226 .- 2158-3226. ; 12:11
  • Tidskriftsartikel (refereegranskat)abstract
    • Thermoelectric readout in a graphene terahertz (THz) radiation detector requires a p-n junction across the graphene channel. Even without an intentional p-n junction, two latent junctions can exist in the vicinity of the electrodes/antennas through the proximity to the metal. In a symmetrical structure, these junctions are connected back-to-back and therefore counterbalance each other with regard to rectification of the ac signal. Because of the Peltier effect, a small dc current results in additional heating in one and cooling in another p-n junction, thereby breaking the symmetry. The p-n junctions then no longer cancel, resulting in a greatly enhanced rectified signal. This allows simplifying the design and controlling the sensitivity of THz radiation detectors.
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12.
  • Khan, Munis, 1991, et al. (författare)
  • High mobility graphene field effect transistors on flexible EVA/PET foils
  • 2024
  • Ingår i: 2D Materials. - 2053-1583. ; 11:3
  • Tidskriftsartikel (refereegranskat)abstract
    • Monolayer graphene is a promising material for a wide range of applications, including sensors, optoelectronics, antennas, EMR shielding, flexible electronics, and conducting electrodes. Chemical vapor deposition (CVD) of carbon atoms on a metal catalyst is the most scalable and cost-efficient method for synthesizing high-quality, large-area monolayer graphene. The usual method of transferring the CVD graphene from the catalyst to a target substrate involves a polymer carrier which is dissolved after the transfer process is completed. Due to often unavoidable damage to graphene, as well as contamination and residues, carrier mobilities are typically 1000–3000 cm2(Vs)−1, unless complex and elaborate measures are taken. Here, we report on a simple scalable fabrication method for flexible graphene field-effect transistors that eliminates the polymer interim carrier, by laminating the graphene directly onto office lamination foils, removing the catalyst, and depositing Parylene N as a gate dielectric and encapsulation layer. The fabricated transistors show field- and Hall-effect mobilities of 7000–10 000 cm2(Vs)−1 with a residual charge-carrier density of 2×1011 1 cm−2 at room temperature. We further validate the material quality by terahertz time-domain spectroscopy and observation of the quantum Hall effect at low temperatures in a moderate magnetic field of ∼5 T. The Parylene encapsulation provides long-term stability and protection against additional lithography steps, enabling vertical device integration in multilayer electronics on a flexible platform.
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13.
  • Khan, Munis, 1991, et al. (författare)
  • High mobility graphene on EVA/PET
  • 2022
  • Ingår i: Nanomaterials. - : MDPI AG. - 2079-4991. ; 12:3
  • Tidskriftsartikel (refereegranskat)abstract
    • Transparent conductive film on a plastic substrate is a critical component in low cost, flexible and lightweight optoelectronics. CVD graphene transferred from copper- to ethylene vinyl acetate (EVA)/polyethylene terephthalate (PET) foil by hot press lamination has been reported as a robust and affordable alternative to manufacture highly flexible and conductive films. Here, we demonstrate that annealing the samples at 60 ∘C under a flow of nitrogen, after wet etching of copper foil by nitric acid, significantly enhances the Hall mobility of such graphene films. Raman, Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used to evaluate the morphology and chemical composition of the graphene.
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14.
  • Li, Mengyue, 1985, et al. (författare)
  • Single-crystalline Bi2Sr2CaCu2O8+x detectors for direct detection of microwave radiation
  • 2015
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 106:15
  • Tidskriftsartikel (refereegranskat)abstract
    • We test radiation detectors made from single-crystalline Bi2Sr2CaCu2O8+x flakes put on oxidized Si substrates. The 100-nm-thick flakes are lithographically patterned into 4 x 12 mu m(2) large rectangles embedded in thin-film log-spiral antennas. The SiO2 layer weakens the thermal link between the flakes and the bath. Two modes of radiation detection have been observed. For a bolometric type of sensors a responsivity of similar to 300 V/W and a noise equivalent power of 30 nW/root Hz has been deduced at 70 K. Much more sensitive is the non-bolometric device showing characteristics similar to a Golay-type detector while being at least a thousand times faster. Making smaller (sub-mu m) structures is expected to significantly improve the performance of these devices and makes them very competitive among other microwave and terahertz detectors.
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15.
  • Lindvall, Niclas, 1985, et al. (författare)
  • Cleaning graphene using atomic force microscope
  • 2012
  • Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 111:6, s. Article Number: 064904-
  • Tidskriftsartikel (refereegranskat)abstract
    • We mechanically clean graphene devices using an atomic force microscope (AFM). By scanning an AFM tip in contact mode in a broom-like way over the sample, resist residues are pushed away from the desired area. We obtain atomically flat graphene with a root mean square (rms) roughness as low as 0.12 nm after this procedure. The cleaning also results in a shift of the charge-neutrality point toward zero gate voltage, as well as an increase in charge carrier mobility.
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16.
  • Lindvall, Niclas, 1985, et al. (författare)
  • Measurements of weak localization of graphene in inhomogeneous magnetic fields
  • 2015
  • Ingår i: JETP Letters. - : Pleiades Publishing Ltd. - 1090-6487 .- 0021-3640. ; 102:6, s. 367-371
  • Tidskriftsartikel (refereegranskat)abstract
    • Weak localization in graphene is studied in inhomogeneous magnetic fields. To generate the inhomogeneous field, a thin film of type-II superconducting niobium is put in close proximity to graphene. A deviation from the ordinary quadratic weak localization behavior is observed at low fields. We attribute this to the inhomogeneous field caused by vortices in the superconductor. The deviation, which depends on the carrier concentration in graphene, can be tuned by the gate voltage. In addition, collective vortex motion, known as vortex avalanches, is observed through magnetoresistance measurements of graphene.
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17.
  • Lindvall, Niclas, 1985, et al. (författare)
  • Towards transfer-free fabrication of graphene NEMS grown by chemical vapour deposition
  • 2012
  • Ingår i: Micro and Nano Letters. - : Institution of Engineering and Technology (IET). - 1750-0443. ; 7:8, s. 749-752
  • Tidskriftsartikel (refereegranskat)abstract
    • Graphene, an atomic monolayer of sp(2)-hybridised carbon atoms, is a promising material for future NEMS based on its remarkable electronic and mechanical properties. Through the rapid progress of chemical vapour deposition of large-scale, high-quality graphene, these applications seem to be close to reality. However, issues related to the graphene transfer process limit the reproducibility of such devices. In this Letter, the authors present two different approaches for fabricating suspended graphene devices without any transfer step, using both catalytically and non-catalytically grown graphene. The authors achieve high reproducibility in manufacturing flat and uniform suspended graphene beams. While good mechanical properties are observed, the electrical performance is still poor, requiring improvements.
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18.
  • Lindvall, Niclas, 1985, et al. (författare)
  • Transfer-free fabrication of suspended graphene grown by chemical vapor deposition
  • 2012
  • Ingår i: 7th IEEE International Conference on Nano/Micro Engineered and Molecular Systems. NEMS 2012, Kyoto, 5 - 8 March 2012. - 9781467311243 ; , s. 19-22
  • Konferensbidrag (refereegranskat)abstract
    • Graphene, a true two-dimensional material with extraordinary mechanical- and electronic properties, is thought to be ideal for nanoelectromechanical systems (NEMS), like mass- and force sensors. Here, we present two different ways to fabricate suspended graphene for the intended use in future NEMS applications. The fabrication schemes do not require transfer of graphene from a catalyst where the graphene is grown on to another supporting substrate. The transfer is a source of several issues causing irreproducibility in large-scale production of graphene devices. We obtain suspended graphene membranes by locally removing the copper thin film on top of which the graphene is catalytically grown. The membranes are uniform and exhibit mechanical properties similar to those of exfoliated graphene. Also, suspended graphene beams with electrical interconnects are fabricated from non-catalytically grown graphene on SiO 2. Both approaches represent the first steps towards transfer-free fabrication of suspended graphene for NEMS applications.
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19.
  • Liu, Lihui, 1985, et al. (författare)
  • A Mechanism for Highly Efficient Electrochemical Bubbling Delamination of CVD-Grown Graphene from Metal Substrates
  • 2016
  • Ingår i: Advanced Materials Interfaces. - : Wiley. - 2196-7350. ; 3:8
  • Tidskriftsartikel (refereegranskat)abstract
    • In most cases, transfer of chemical-vapor-deposited 2D materials from metallic foil catalysts onto a target substrate is the most necessary step for their promising fundamental studies and applications. Recently, a highly efficient and nondestructive electrochemical delamination method has been proposed as an alternative to the conventional etching transfer method, which alleviates the problem of cost and environment pollution because it eliminates the need to etch away the metals. Here, the mechanism of the electrochemical bubbling delamination process is elucidated by studying the effect of the various electrolytes on the delamination rate. A capacitor-based circuit model is proposed and confirmed by the electrochemical impedance spectroscopy results. A factor of 27 decrease in the time required for complete graphene delamination from the platinum cathodes is found when increasing the NaOH ratio in the electrolyte solution. The opposite trend is observed for delamination at the anode. The surface screening effect induced by nonreactive ions in the vicinity of the electrodes plays a key role in the delamination efficiency. The analysis is generic and can be used as a guideline to describe and design the electrochemical delamination of other 2D materials from their metal catalysts as well.
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20.
  • Lockhart de la Rosa, César Javier, 1987, et al. (författare)
  • Frame assisted H2O electrolysis induced H2 bubbling transfer of large area graphene grown by chemical vapor deposition on Cu
  • 2013
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 102:2
  • Tidskriftsartikel (refereegranskat)abstract
    • An improved technique for transferring large area graphene grown by chemical vapor deposition on copper is presented. It is based on mechanical separation of the graphene/copper by H-2 bubbles during H2O electrolysis, which only takes a few tens of seconds while leaving the copper cathode intact. A semi-rigid plastic frame in combination with thin polymer layer span on graphene gives a convenient way of handling-and avoiding wrinkles and holes in graphene. Optical and electrical characterizations prove the graphene quality is better than that obtained by traditional wet etching transfer. This technique appears to be highly reproducible and cost efficient.
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21.
  • Maussang, K., et al. (författare)
  • Temperature dependance of Intrinsic Spin Orbit Coupling Gap in Graphene probed by Terahertz photoconductivity
  • 2023
  • Ingår i: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035.
  • Konferensbidrag (refereegranskat)abstract
    • Graphene is a quantum spin Hall insulator, with a nontrivial topological gap induced by the spin-orbit coupling. Such splitting is weak (∼ 45 μ eV) in the absence of external magnetic field. However, due to rather long spin-relaxation time, graphene is an attractive candidate for applications in quantum technologies. When it is encapsulated in hexagonal boron nitride, the coupling between graphene and the substrate compensates intrinsic spin-orbit coupling and decreases the nontrivial topological gap, which may lead to phase transition into a trivial band insulator state. In this work, we have measured experimentally the zero-field splittings in monolayer and bilayer graphene by the means of subterahertz photoconductivity-based electron spin resonance technique. The dependance in temperature of such splittings have been also studied in the 2-12K range. We observed a decrease of the spin splittings with increasing temperature. Such behavior might be understood from several physical mechanisms that could induce a temperature dependence of the spin-orbit coupling. These includes the difference in the expansion coefficients between the graphene and the boron nitride substrate or the metal contacts, the electronphonon interactions, and the presence of a magnetic order at low temperature.
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22.
  • Nam, Youngwoo, 1983, et al. (författare)
  • Graphene p-n-p junctions controlled by local gates made of naturally oxidized thin aluminium films
  • 2012
  • Ingår i: Carbon. - : Elsevier BV. - 0008-6223. ; 50:5, s. 1987-1992
  • Tidskriftsartikel (refereegranskat)abstract
    • Graphene structures with both top- and bottom-electrostatic gates are studied. The top gate is made of thin aluminium (Al) film deposited directly onto graphene, with no prior dielectric layer in between. Natural oxidation of Al at the interface with graphene results in an insulating barrier proving useful in making top gates to graphene. For electrically disconnected top gate, graphene resistance as a function of the slowly-varying back-gate voltage shows hysteresis which reveals dielectric properties of the barrier. The estimated barrier thickness is only 2 nm allowing for very sharp profiles of the electric field in graphene devices. By applying voltages to both back- and top gates, effective p–n–p junctions with sharp interfaces can be created.
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23.
  • Nam, Youngwoo, 1983, et al. (författare)
  • Graphene thermocouple fabricated on a flexible and transparent substrate
  • 2024
  • Ingår i: AIP Advances. - 2158-3226 .- 2158-3226. ; 14:6
  • Tidskriftsartikel (refereegranskat)abstract
    • We demonstrate the realization of reliable, high-quality, micro-sized graphene-based field-effect devices on a flexible and transparent substrate, ethylene vinyl acetate (EVA)/polyethylene terephthalate (PET), using a convenient hot-press lamination transfer and employing parylene-N as a dielectric material for gating. Using this technique, we fabricate a graphene thermocouple on the EVA/PET substrate. Specifically, the graphene is patterned in a U-shape, and its legs are equipped with two independent top gates. Full control of the carrier density and type by electrostatic gating in the two graphene regions allow the formation of a thermocouple layout, exhibiting an enlarged thermovoltage signal when the two regions are doped with opposite types of carriers and leading to a maximum sensitivity with a thermopower of ∼73 µV/K. This agrees well with the working principle of thermocouple, and it proves the good compatibility and functionality of the graphene thermocouple on the EVA/PET substrate. Our findings suggest possible applications for producing scalable and reliable graphene-based electronic devices on flexible and transparent substrates in a simple way.
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24.
  • Nam, Youngwoo, 1983, et al. (författare)
  • Quantum Hall effect in graphene decorated with disordered multilayer patches
  • 2013
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 103:233, s. 233110-
  • Tidskriftsartikel (refereegranskat)abstract
    • Quantum Hall effect (QHE) is observed in graphene grown by chemical vapour deposition using platinum catalyst. The QHE is even seen in samples which are irregularly decorated with disordered multilayer graphene patches and have very low mobility (
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25.
  • Nam, Youngwoo, 1983, et al. (författare)
  • The Aharonov-Bohm effect in graphene rings with metal mirrors
  • 2012
  • Ingår i: Carbon. - : Elsevier BV. - 0008-6223. ; 50:15, s. 5562-5568
  • Tidskriftsartikel (refereegranskat)abstract
    • We measured the Aharonov-Bohm (AB) effect in graphene rings with superconducting-(Al) and normal-metal (Au) mirrors. The mirrors were deposited either on additional stubs connected to the rings in the transverse direction or on the ring bias lines. A significant enhancement of the visible phase coherence was observed in the latter case, in which we observed even the third harmonic of the AB oscillations. The superconductivity of the mirrors appears to be unimportant for the improved coherence in graphene. A large Fermi energy mismatch between graphene and the mirror material is sufficient for this effect. In addition, a transport gap was observed in our graphene structures at the gate voltage close to the Dirac point. The value of the gap can be reproduced by assuming the occurrence of Coulomb blockade effects in graphene.
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26.
  • Nam, Youngwoo, 1983, et al. (författare)
  • Unusual thermopower of inhomogeneous graphene grown by chemical vapor deposition
  • 2014
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 104:2, s. 021902 -
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on thermopower (TEP) and resistance measurements of inhomogeneous graphene grown by chemical vapor deposition (CVD). Unlike the conventional resistance of pristine graphene, the gate-dependent TEP shows a large electron-hole asymmetry. This can be accounted for by inhomogeneity of the CVD-graphene where individual graphene regions contribute with different TEPs. At the high magnetic field and low temperature, the TEP has large fluctuations near the Dirac point associated with the disorder in the CVD-graphene. TEP measurements reveal additional characteristics of CVD-graphene, which are difficult to obtain from the measurement of resistance alone.
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27.
  • Pandit, Santosh, 1987, et al. (författare)
  • Graphene-based sensor for detection of bacterial pathogens
  • 2021
  • Ingår i: Sensors. - : MDPI AG. - 1424-8220. ; 21:23
  • Tidskriftsartikel (refereegranskat)abstract
    • Microbial colonization to biomedical surfaces and biofilm formation is one of the key challenges in the medical field. Recalcitrant biofilms on such surfaces cause serious infections which are difficult to treat using antimicrobial agents, due to their complex structure. Early detection of microbial colonization and monitoring of biofilm growth could turn the tide by providing timely guidance for treatment or replacement of biomedical devices. Hence, there is a need for sensors, which could generate rapid signals upon bacterial colonization. In this study, we developed a simple prototype sensor based on pristine, non-functionalized graphene. The detection principle is a change in electrical resistance of graphene upon exposure to bacterial cells. Without functionalization with specific receptors, such sensors cannot be expected to be selective to certain bacteria. However, we demonstrated that two different bacterial species can be detected and differentiated by our sensor due to their different growth dynamics, adherence pattern, density of adhered bacteria and microcolonies formation. These distinct behaviors of tested bacteria depicted distinguishable pattern of resistance change, resistance versus gate voltage plot and hysteresis effect. This sensor is simple to fabricate, can easily be miniaturized, and can be effective in cases when precise identification of species is not needed.
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28.
  • Pandit, Santosh, 1987, et al. (författare)
  • Vertically Aligned Graphene Coating is Bactericidal and Prevents the Formation of Bacterial Biofilms
  • 2018
  • Ingår i: Advanced Materials Interfaces. - : Wiley. - 2196-7350. ; 5:7
  • Tidskriftsartikel (refereegranskat)abstract
    • The key first step in developing bacterial infections related to implants and medical devices is the attachment of planktonic bacterial cells, and subsequent formation of biofilms. Herein, it is reported that graphene, a 2D carbon-based material, can be effectively used to prevent bacterial attachment. The key parameter for this effect is the orientation of graphene with respect to the coated surface. Chemical vapor deposition (CVD) graphene, deposited horizontally on the surface, exhibits no antibacterial effect. By contrast, an array of graphene flakes grown perpendicularly to the surface by a plasma-enhanced CVD (PECVD) process prevent biofilm formation. Electron microscopy reveals that the exposed edges of vertically aligned graphene flakes penetrate the bacterial membrane and drain the cytosolic content. Bacteria are not able to develop resistance to this killing mechanism during multiple exposures. By keeping the height of the vertical graphene coating between 60 and 100 nm, the coating is able to effectively kill bacteria, while being completely harmless to mammalian cells.
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29.
  • Sellappan, Raja, 1981, et al. (författare)
  • Influence of graphene synthesizing techniques on the photocatalytic performance of Graphene/TiO2 nanocomposites
  • 2013
  • Ingår i: Physical Chemistry Chemical Physics. - 1463-9084 .- 1463-9076. ; 15:37, s. 15528-15537
  • Tidskriftsartikel (refereegranskat)abstract
    • Model photocatalysts composed of TiO2/graphene nanocomposites are prepared to address the role of graphene quality on their photocatalytic performance. Graphene is synthesized by catalyst-assisted chemical vapor deposition (CVD), catalyst-free CVD and solution processing methods. TiO2 is prepared by reactive magnetron sputtering and subsequent annealing. Fabricated model photocatalysts are with different morphology and physical properties, as revealed by spectrophotometry, atomic force microscopy, x-ray photoelectron spectroscopy, Raman spectroscopy, photoluminescence, and four-probe electrical measurements. All graphene–containing composites are with significantly higher photocatalytic activity compared to bare TiO2 films in the gas phase methanol photooxidation tests. The activity is proportional to the electrical conductivity and surface roughness of the respective carbon structure, which in turn depends on the preparation methods. The mechanisms of enhancement are further assessed by comparison with the performance of reference TiO2/graphitic-carbon and TiO2/Au thin films.
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30.
  • Skoblin, Grigory, 1991, et al. (författare)
  • A hybrid-type CVD system for graphene growth
  • 2015
  • Ingår i: Chemical Vapor Deposition. - : Wiley. - 1521-3862 .- 0948-1907. ; 21:7-9, s. 176-180
  • Tidskriftsartikel (refereegranskat)abstract
    • A cold-wall CVD system for graphene growth on metal foils is converted to an effectively hot-wall one by merely adding another heater, sandwiching the foil in between the heaters. This simple design demonstrates both an improved temperature uniformity characteristic for hot-wall systems, and a high responsivity distinctive for cold-wall ones. This beneficial combination allows for a much better control of graphene growth kinetics. An effectively hot-wall system for graphene growth is made from a cold-wall one by encapsulating the metal foil in between two heaters. This simple conversion demonstrates improved temperature control and high temperature ramping rate. It also allows reaching lower nucleation density and provides better control of growth kinetics.
  •  
31.
  • Skoblin, Grigory, 1991, et al. (författare)
  • Encapsulation of graphene in Parylene
  • 2017
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 110:5
  • Tidskriftsartikel (refereegranskat)abstract
    • Graphene encapsulated between flakes of hexagonal boron nitride (hBN) demonstrates the highest known mobility of charge carriers. However, the technology is not scalable to allow for arrays of devices. We are testing a potentially scalable technology for encapsulating graphene where we replace hBN with Parylene while still being able to make low-ohmic edge contacts. The resulting encapsulated devices show low parasitic doping and a robust Quantum Hall effect in relatively low magnetic fields
  •  
32.
  • Skoblin, Grigory, 1991, et al. (författare)
  • Graphene bolometer with thermoelectric readout and capacitive coupling to an antenna
  • 2018
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 112:6
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on a prototype graphene radiation detector based on the thermoelectric effect. We used a split top gate to create a p-n junction in the graphene, thereby making an effective thermocouple to read out the electronic temperature in the graphene. The electronic temperature is increased due to the AC currents induced in the graphene from the incoming radiation, which is first received by an antenna and then directed to the graphene via the top-gate capacitance. With the exception of the constant DC voltages applied to the gate, the detector does not need any bias and is therefore very simple to use. The measurements showed a clear response to microwaves at 94 GHz with the signal being almost temperature independent in the 4-100 K temperature range. The optical responsivity reached ∼700 V/W.
  •  
33.
  • Skoblin, Grigory, 1991, et al. (författare)
  • Thermoelectric effects in graphene at high bias current and under microwave irradiation
  • 2017
  • Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 7:1
  • Tidskriftsartikel (refereegranskat)abstract
    • We use a split top gate to induce doping of opposite signs in different parts of a graphene field-effect transistor, thereby effectively forming a graphene thermocouple. The thermocouple is sensitive to the electronic temperature in graphene, which can be several hundred kelvin higher than the ambient one at sufficiently high bias current. Combined with the high thermoelectric power of graphene, this allows for i) simple measurements of the electronic temperature and ii) building thermoelectric radiation detectors. A simple prototype graphene thermoelectric detector shows a temperature-independent optical responsivity of around 400 V/W at 94 GHz at temperatures of 4-50 K.
  •  
34.
  • Stattin, Martin, 1983, et al. (författare)
  • Graphene as transparent electrode for GaN-based VCSELs
  • 2012
  • Ingår i: International Workshop on Nitride Semiconductors 2012, October 14-19, 2012, Sapporo, Japan. ; , s. ThP-OD-34-
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)
  •  
35.
  • Stattin, Martin, 1983, et al. (författare)
  • Metal-Free Graphene as Transparent Electrode for GaN-Based Light-Emitters
  • 2013
  • Ingår i: Japanese Journal of Applied Physics. - 1347-4065 .- 0021-4922. ; 52:8 PART 2, s. 08JG05-
  • Tidskriftsartikel (refereegranskat)abstract
    • Graphene contacts to p-GaN are considered as an alternative to indium–tin-oxide transparent electrodes in GaN based vertical-cavity surface-emitting lasers (VCSELs). Contact properties were investigated on light-emitting diode and p-GaN test structures, where dielectric apertures were used to eliminate the influence of the metal pads used to bias the contacts. Using single layer graphene we were able to operate light emitting diodes with current densities of 300 A/cm2. Addition of a second layer of graphene increased the maximum bias current to 1 kA/cm2. However, the contacts are non-linear and cannot withstand high current densities for a long time. The results are promising but further investigation and improvement is needed for graphene to be a viable alternative to indium–tin-oxide for blue VCSELs.
  •  
36.
  • Sun, Jie, 1977, et al. (författare)
  • Chemical vapor deposition of nanocrystalline graphene directly on arbitrary high-temperature insulating substrates
  • 2012
  • Ingår i: 7th IEEE International Conference on Nano/Micro Engineered and Molecular Systems. NEMS 2012, Kyoto, 5 - 8 March 2012. - 9781467311243 ; , s. 11-14
  • Konferensbidrag (refereegranskat)abstract
    • Large area uniform nanocrystalline graphene is grown by chemical vapor deposition on arbitrary insulating substrates that can survive ∼1000°C. The as-synthesized graphene is nanocrystalline with a domain size in the order of ∼10 nm. The material possesses a transparency and conductivity similar to standard graphene fabricated by exfoliation or catalysis. A noncatalytic mechanism is proposed to explain the experimental phenomena. The developed technique is scalable and reproducible, compatible with the existing semiconductor technology, and thus can be very useful in nanoelectronic applications such as transparent electronics, nanoelectromechanical systems, as well as molecular electronics.
  •  
37.
  • Sun, Jie, 1977, et al. (författare)
  • Controllable chemical vapor deposition of large area uniform nanocrystalline graphene directly on silicon dioxide
  • 2012
  • Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 111:4
  • Tidskriftsartikel (refereegranskat)abstract
    • Metal-catalyst-free chemical vapor deposition (CVD) of large area uniform nanocrystalline graphene on oxidized silicon substrates is demonstrated. The material grows slowly, allowing for thickness control down to monolayer graphene. The as-grown thin films are continuous with no observable pinholes, and are smooth and uniform across whole wafers, as inspected by optical-, scanning electron-, and atomic force microscopy. The sp(2) hybridized carbon structure is confirmed by Raman spectroscopy. Room temperature electrical measurements show ohmic behavior (sheet resistance similar to exfoliated graphene) and up to 13% of electric-field effect. The Hall mobility is similar to 40 cm(2)/ Vs, which is an order of magnitude higher than previously reported values for nanocrystalline graphene. Transmission electron microscopy, Raman spectroscopy, and transport measurements indicate a graphene crystalline domain size similar to 10 nm. The absence of transfer to another substrate allows avoidance of wrinkles, holes, and etching residues which are usually detrimental to device performance. This work provides a broader perspective of graphene CVD and shows a viable route toward applications involving transparent electrodes.
  •  
38.
  • Sun, Jie, 1977, et al. (författare)
  • Direct Chemical Vapor Deposition of Large-Area Carbon Thin Films on Gallium Nitride for Transparent Electrodes: A First Attempt
  • 2012
  • Ingår i: IEEE Transactions on Semiconductor Manufacturing. - : Institute of Electrical and Electronics Engineers (IEEE). - 0894-6507 .- 1558-2345. ; 25:3, s. 494-501
  • Tidskriftsartikel (refereegranskat)abstract
    • Direct formation of large-area carbon thin films on gallium nitride by chemical vapor deposition without metallic catalysts is demonstrated. A high flow of ammonia is used to stabilize the surface of the GaN (0001)/sapphire substrate during the deposition at 950 degrees C. Various characterization methods verify that the synthesized thin films are largely sp(2) bonded, macroscopically uniform, and electrically conducting. The carbon thin films possess optical transparencies comparable to that of exfoliated graphene. This paper offers a viable route toward the use of carbon-based materials for future transparent electrodes in III-nitride optoelectronics, such as GaN-based light emitting diodes and laser diodes.
  •  
39.
  • Sun, Jie, 1977, et al. (författare)
  • Growth mechanism of graphene on platinum: Surface catalysis and carbon segregation
  • 2014
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 104:15
  • Tidskriftsartikel (refereegranskat)abstract
    • A model of the graphene growth mechanism of chemical vapor deposition on platinum is proposed and verified by experiments. Surface catalysis and carbon segregation occur, respectively, at high and low temperatures in the process, representing the so-called balance and segregation regimes. Catalysis leads to self-limiting formation of large area monolayer graphene, whereas segregation results in multilayers, which evidently "grow from below." By controlling kinetic factors, dominantly monolayer graphene whose high quality has been confirmed by quantum Hall measurement can be deposited on platinum with hydrogen-rich environment, quench cooling, tiny but continuous methane flow and about 1000 degrees C growth temperature. (C) 2014 AIP Publishing LLC.
  •  
40.
  • Sun, Jie, 1977, et al. (författare)
  • Large-area uniform graphene-like thin films grown by chemical vapor deposition directly on silicon nitride
  • 2011
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 98:25
  • Tidskriftsartikel (refereegranskat)abstract
    • Large-area uniform carbon films with graphene-like properties are synthesized by chemical vapor deposition directly on Si3N4/Si at 1000 degrees C without metal catalysts. The as deposited films are atomically thin and wrinkle- and pinhole-free. The film thickness can be controlled by modifying the growth conditions. Raman spectroscopy confirms the sp(2) graphitic structures. The films show ohmic behavior with a sheet resistance of similar to 2.3-10.5 k Omega/square at room temperature. An electric field effect of similar to 2-10% (V-G=-20 V) is observed. The growth is explained by the self-assembly of carbon clusters from hydrocarbon pyrolysis. The scalable and transfer-free technique favors the application of graphene as transparent electrodes.
  •  
41.
  • Sun, Jie, 1977, et al. (författare)
  • Low Partial Pressure Chemical Vapor Deposition of Graphene on Copper
  • 2012
  • Ingår i: IEEE Transactions on Nanotechnology. - 1941-0085 .- 1536-125X. ; 11:2, s. 255-260
  • Tidskriftsartikel (refereegranskat)abstract
    • A systematic study of the Cu-catalyzed chemical vapor deposition of graphene under extremely low partial pressure is carried out. A carbon precursor supply of just P-CH4 similar to 0.009 mbar during the deposition favors the formation of large-area uniform monolayer graphene verified by Raman spectra. A diluted HNO3 solution is used to remove Cu before transferring graphene onto SiO2/Si substrates or carbon grids. The graphene can be made suspended over a similar to 12 mu m distance, indicating its good mechanical properties. Electron transport measurements show the graphene sheet resistance of similar to 0.6 k Omega/square at zero gate voltage. The mobilities of electrons and holes are similar to 1800 cm(2)/Vs at 4.2 K and similar to 1200 cm(2)/Vs at room temperature.
  •  
42.
  • Sun, Jie, 1977, et al. (författare)
  • Noncatalytic chemical vapor deposition of graphene on high-temperature substrates for transparent electrodes
  • 2012
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 100:2
  • Tidskriftsartikel (refereegranskat)abstract
    • A noncatalytic chemical vapor deposition mechanism is proposed, where high precursor concentration, long deposition time, high temperature, and flat substrate are needed to grow large-area nanocrystalline graphene using hydrocarbon pyrolysis. The graphene is scalable, uniform, and with controlled thickness. It can be deposited on virtually any nonmetallic substrate that withstands similar to 1000 degrees C. For typical examples, graphene grown directly on quartz and sapphire shows transmittance and conductivity similar to exfoliated or metal-catalyzed graphene, as evidenced by transmission spectroscopy and transport measurements. Raman spectroscopy confirms the sp(2)-C structure. The model and results demonstrate a promising transfer-free technique for transparent electrode production.
  •  
43.
  • Tarasov, Mikhail, 1954, et al. (författare)
  • Electrical and optical properties of a bolometer with a suspended absorber and tunneling-current thermometers
  • 2017
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 110:24
  • Tidskriftsartikel (refereegranskat)abstract
    • We have developed a bolometer with a suspended normal-metal absorber connected to superconducting leads via tunneling barriers. Such an absorber has reduced heat losses to the substrate, which greatly increases the responsivity of the bolometer to over 10(9) V/W at 75 mK when measured by dc Joule heating of the absorber. For high-frequency experiments, the bolometers have been integrated in planar twin-slot and log-periodic antennas. At 300GHz and 100 mK, the bolometer demonstrates the voltage and current response of 3 x 10(8) V/W and 1.1 x 10(4) A/W, respectively, corresponding to the quantum efficiency of similar to 15 electrons per photon. An effective thermalization of electrons in the absorber favors the high quantum efficiency. We also report on how the in-plane-and transverse magnetic fields influence the device characteristics.
  •  
44.
  • Tarasov, Mikhail, 1954, et al. (författare)
  • Family of graphene-based superconducting devices
  • 2011
  • Ingår i: JETP Letters. - 1090-6487 .- 0021-3640. ; 94:4, s. 329-332
  • Tidskriftsartikel (refereegranskat)abstract
    • A family of highly sensitive devices based on a graphene nanobridge and superconducting electrodes has been developed, manufactured, and examined. These devices can be used to create a graphene-based integral receiver. A cold-electron bolometer prototype with superconductor-insulator-normal metal tunnel junctions has been studied. Its response to a change in the temperature and external microwave radiation has been measured. A superconducting quantum interferometer with a graphene strip as a weak coupling between superconducting electrodes has been examined. The corresponding modulation of the voltage by a magnetic field at a given current has been measured. The effect of the gate voltage on the resistance of graphene has been analyzed for these samples. To confirm that graphene is single-layer, measurements with the reference samples were performed in high magnetic fields, displaying the half-integer quantum Hall effect.
  •  
45.
  • Tarasov, Mikhail, 1954, et al. (författare)
  • SINIS bolometer with a suspended absorber
  • 2018
  • Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 969:1
  • Konferensbidrag (refereegranskat)abstract
    • We have developed a Superconductor-Insulator-Normal Metal-Insulator-Superconductor (SINIS) bolometer with a suspended normal metal bridge. The suspended bridge acts as a bolometric absorber with reduced heat losses to the substrate. Such bolometers were characterized at 100-350 mK bath temperatures and electrical responsivity of over 10 9 V/W was measured by dc heating the absorber through additional contacts. Suspended bolometers were also integrated in planar twin-slot and log-periodic antennas for operation in the submillimetre-band of radiation. The measured voltage response to radiation at 300 GHz and at 100 mK bath temperature is 3∗10 8 V/W and a current response is 1.1∗10 4 A/W which corresponds to a quantum efficiency of ∼15 electrons per photon. An important feature of such suspended bolometers is the thermalization of electrons in the absorber heated by optical radiation, which in turn provides better quantum efficiency. This has been confirmed by comparison of bolometric response to dc and rf heating. We investigate the performance of direct SN traps and NIS traps with a tunnel barrier between the superconductor and normal metal trap. Increasing the volume of superconducting electrode helps to reduce overheating of superconductor. Influence of Andreev reflection and Kapitza resistance, as well as electron-phonon heat conductivity and thermal conductivity of N-wiring are estimated for such SINIS devices.
  •  
46.
  • Verreet, B., et al. (författare)
  • Self-consistent estimations of heating in stacks of intrinsic Josephson junctions
  • 2007
  • Ingår i: Supercond. Sci. Technol.. ; 20:2, s. S48-53
  • Konferensbidrag (refereegranskat)abstract
    • We use the temperature dependent c axis critical current and characteristic gap voltage as temperature gauges to assess the self-heating in stacks of intrinsic Josephson junctions etched out from the ultrathin Bi2Sr2CaCu2O8+ δ(BSCCO) single crystals. The thermal resistance which characterizes the overall cooling efficiency is found to be ∼ 120–160 K mW-1 for the two stacks studied. This value is in agreement with earlier reports for stacks of similar geometry, while it is about two to three times higher than for simple mesas made on the surfaces of mm-size BSCCO single crystals. The suggested method can guarantee the validity of the temperature estimations inside the stacks of various geometries and different cooling efficiencies without the need for attaching additional thermometers.
  •  
47.
  • Warbinek, Jessica, et al. (författare)
  • A graphene-based neutral particle detector
  • 2019
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 114:6
  • Tidskriftsartikel (refereegranskat)abstract
    • A neutral particle detector is presented, in which the traditionally used target material, indium tin oxide (ITO), is replaced by graphene. The graphene-based detector enables collinear photodetachment measurements at a significantly shorter wavelength of light down to 230nm compared to ITO-based detectors, which are limited at 335 nm. Moreover, the background signal from the photoelectric effect is drastically reduced when using graphene. The graphene based detector, reaching 1.7 eV further into the UV energy range, allows increased possibilities for photodetachment studies of negatively charged atoms, molecules, and clusters. Published under license by AIP Publishing.
  •  
48.
  • Wiesner, M., et al. (författare)
  • Detection of graphene microelectromechanical system resonance
  • 2014
  • Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 116:22, s. 224510-
  • Tidskriftsartikel (refereegranskat)abstract
    • We present an experimental setup for fast detection of resonances of graphene microelectromechanical structures of different quality. The relatively simple technique used to read-out of the resonance frequency is the main advantage of the proposed system. The resolution is good enough to detect vibrations of the graphene resonator with the quality factor of ∼24 and resonance frequency of 104 MHz.
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