| 1. |
- Hong, S. J., et al.
(författare)
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Verification of electron doping in single-layer graphene due to H-2 exposure with thermoelectric power
- 2015
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 106:14
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Tidskriftsartikel (refereegranskat)abstract
- We report the electron doping of single-layer graphene (SLG) grown by chemical vapor deposition (CVD) by means of dissociative hydrogen adsorption. The transfer characteristic showed n-type doping behavior similar to that of mechanically exfoliated graphene. Furthermore, we studied the thermoelectric power (TEP) of CVD-grown SLG before and after exposure to high-pressure H-2 molecules. From the TEP results, which indicate the intrinsic electrical properties, we observed that the CVD-grown SLG is n-type doped without degradation of the quality after hydrogen adsorption. Finally, the electron doping was also verified by Raman spectroscopy.
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| 2. |
- Park, M., et al.
(författare)
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Electrical and thermoelectric transport by variable range hopping in reduced graphene oxide
- 2017
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 111:17
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Tidskriftsartikel (refereegranskat)abstract
- This study investigated the transport properties of single-layer reduced graphene oxides (rGOs). The rGOs were prepared by the bubble deposition method followed by thermal reduction. The crossover of the transport mechanism from Efros-Shklovskii (ES) variable range hopping (VRH) between the localized states to Mott-VRH was observed near 70K using the temperature-dependent conductance. The ES-VRH conduction below 70K is apparent in the electric field dependence of the field-driven hopping transport in the high-electric field regime. We also figure out that the thermoelectric power is consistent with the 2D Mott VRH above 70K. We argue that the VRH conduction results from the topological disorders of rGO as confirmed by Raman spectroscopy. This infers that the average distance between defects is approximately 2.0 nm.
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| 3. |
- Boschi, Alex, et al.
(författare)
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Mesoscopic 3D Charge Transport in Solution-Processed Graphene-Based Thin Films: A Multiscale Analysis
- 2023
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Ingår i: Small. - 1613-6810 .- 1613-6829. ; 19:42
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Tidskriftsartikel (refereegranskat)abstract
- Graphene and related 2D material (GRM) thin films consist of 3D assembly of billions of 2D nanosheets randomly distributed and interacting via van der Waals forces. Their complexity and the multiscale nature yield a wide variety of electrical characteristics ranging from doped semiconductor to glassy metals depending on the crystalline quality of the nanosheets, their specific structural organization ant the operating temperature. Here, the charge transport (CT) mechanisms are studied that are occurring in GRM thin films near the metal-insulator transition (MIT) highlighting the role of defect density and local arrangement of the nanosheets. Two prototypical nanosheet types are compared, i.e., 2D reduced graphene oxide and few-layer-thick electrochemically exfoliated graphene flakes, forming thin films with comparable composition, morphology and room temperature conductivity, but different defect density and crystallinity. By investigating their structure, morphology, and the dependence of their electrical conductivity on temperature, noise and magnetic-field, a general model is developed describing the multiscale nature of CT in GRM thin films in terms of hopping among mesoscopic bricks, i.e., grains. The results suggest a general approach to describe disordered van der Waals thin films.
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| 4. |
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| 5. |
- He, Hans, et al.
(författare)
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Polymer-encapsulated molecular doped epigraphene for quantum resistance metrology
- 2019
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Ingår i: Metrologia. - : Institute of Physics Publishing. - 0026-1394 .- 1681-7575. ; 56:4
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Tidskriftsartikel (refereegranskat)abstract
- One of the aspirations of quantum metrology is to deliver primary standards directly to end-users thereby significantly shortening the traceability chains and enabling more accurate products. Epitaxial graphene grown on silicon carbide (epigraphene) is known to be a viable candidate for a primary realisation of a quantum Hall resistance standard, surpassing conventional semiconductor two-dimensional electron gases, such as those based on GaAs, in terms of performance at higher temperatures and lower magnetic fields. The bottleneck in the realisation of a turn-key quantum resistance standard requiring minimum user intervention has so far been the need to fine-tune the carrier density in this material to fit the constraints imposed by a simple cryo-magnetic system. Previously demonstrated methods, such as via photo-chemistry or corona discharge, require application prior to every cool-down as well as specialist knowledge and equipment. To this end we perform metrological evaluation of epigraphene with carrier density tuned by a recently reported permanent molecular doping technique. Measurements at two National Metrology Institutes confirm accurate resistance quantisation below 5n-1. Furthermore, samples show no significant drift in carrier concentration and performance on multiple thermal cycles over three years. This development paves the way for dissemination of primary resistance standards based on epigraphene
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| 6. |
- He, Hans, et al.
(författare)
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Stable and Tunable Charge Carrier Control of Graphene for Quantum Resistance Metrology
- 2018
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Ingår i: 2018 Conference on Precision Electromagnetic Measurements (CPEM 2018). - : IEEE. - 9781538609736 - 9781538609743
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Konferensbidrag (refereegranskat)abstract
- Here we demonstrate a stable and tunable method to alter the carrier concentration of epitaxial graphene grown on silicon carbide. This technique relies on chemical doping by an acceptor molecule. Through careful tuning one can produce chemically doped graphene quantum resistance devices which show long-term stability in ambient conditions and have performance comparable to that of GaAs quantum resistance standards. This development paves the way for controlled device fabrication of graphene quantum hall resistance standards, which can be reliably tailored to operate below 5 T and above 4 K out-of-the-box, without further adjustments from the end-user.
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| 7. |
- He, Hans, 1989, et al.
(författare)
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Uniform doping of graphene close to the Dirac point by polymer-assisted assembly of molecular dopants
- 2018
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- Tuning the charge carrier density of two-dimensional (2D) materials by incorporating dopants into the crystal lattice is a challenging task. An attractive alternative is the surface transfer doping by adsorption of molecules on 2D crystals, which can lead to ordered molecular arrays. However, such systems, demonstrated in ultra-high vacuum conditions (UHV), are often unstable in ambient conditions. Here we show that air-stable doping of epitaxial graphene on SiC—achieved by spin-coating deposition of 2,3,5,6-tetrafluoro-tetracyano-quino-dimethane (F4TCNQ) incorporated in poly(methyl-methacrylate)—proceeds via the spontaneous accumulation of dopants at the graphene-polymer interface and by the formation of a charge-transfer complex that yields low-disorder, charge-neutral, large-area graphene with carrier mobilities ~70 000 cm2V−1s−1at cryogenic temperatures. The assembly of dopants on 2D materials assisted by a polymer matrix, demonstrated by spin-coating wafer-scale substrates in ambient conditions, opens up a scalable technological route toward expanding the functionality of 2D materials.
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| 8. |
- Kim, Kyung Ho, 1984, et al.
(författare)
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Ambipolar charge transport in quasi-free-standing monolayer graphene on SiC obtained by gold intercalation
- 2020
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 102:16
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Tidskriftsartikel (refereegranskat)abstract
- We present a study of quasi-free-standing monolayer graphene obtained by intercalation of Au atoms at the interface between the carbon buffer layer (Bu-L) and the silicon-terminated face (0001) of 4H-silicon carbide. Au intercalation is achieved by deposition of an atomically thin Au layer on the Bu-L followed by annealing at 850 °C in an argon atmosphere. We explore the intercalation of Au and decoupling of the Bu-L into quasi-free-standing monolayer graphene by surface science characterization and electron transport in top-gated electronic devices. By gate-dependent magnetotransport we find that the Au-intercalated buffer layer displays all properties of monolayer graphene, namely gate-tunable ambipolar transport across the Dirac point, but we find no observable enhancement of spin-orbit effects in the graphene layer, despite its proximity to the intercalated Au layer.
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| 9. |
- Kim, Kyung Ho, 1984, et al.
(författare)
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Apparent Power Law Scaling of Variable Range Hopping Conduction in Carbonized Polymer Nanofibers
- 2016
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- We induce dramatic changes in the structure of conducting polymer nanofibers by carbonization at 800 degrees C and compare charge transport properties between carbonized and pristine nanofibers. Despite the profound structural differences, both types of systems display power law dependence of current with voltage and temperature, and all measurements can be scaled into a single universal curve. We analyze our experimental data in the framework of variable range hopping and argue that this mechanism can explain transport properties of pristine polymer nanofibers as well.
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| 10. |
- Kim, Kyung Ho, 1984, et al.
(författare)
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Chemical Sensing with Atomically Thin Platinum Templated by a 2D Insulator
- 2020
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Ingår i: Advanced Materials Interfaces. - : Wiley. - 2196-7350. ; 7:12
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Tidskriftsartikel (refereegranskat)abstract
- Boosting the sensitivity of solid‐state gas sensors by incorporating nanostructured materials as the active sensing element can be complicated by interfacial effects. Interfaces at nanoparticles, grains, or contacts may result in nonlinear current–voltage response, high electrical resistance, and ultimately, electric noise that limits the sensor read‐out. This work reports the possibility to prepare nominally one atom thin, electrically continuous platinum layers by physical vapor deposition on the carbon zero layer (also known as the buffer layer) grown epitaxially on silicon carbide. With a 3–4 Å thin Pt layer, the electrical conductivity of the metal is strongly modulated when interacting with chemical analytes, due to charges being transferred to/from Pt. The strong interaction with chemical species, together with the scalability of the material, enables the fabrication of chemiresistor devices for electrical read‐out of chemical species with sub part‐per‐billion (ppb) detection limits. The 2D system formed by atomically thin Pt on the carbon zero layer on SiC opens up a route for resilient and high sensitivity chemical detection, and can be the path for designing new heterogenous catalysts with superior activity and selectivity.
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| 11. |
- Kim, Kyung Ho, 1984, et al.
(författare)
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Probing variable range hopping lengths by magneto conductance in carbonized polymer nanofibers
- 2018
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 8:1
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Tidskriftsartikel (refereegranskat)abstract
- Using magneto transport, we probe hopping length scales in the variable range hopping conduction of carbonized polyacetylene and polyaniline nanofibers. In contrast to pristine polyacetylene nanofibers that show vanishing magneto conductance at large electric fields, carbonized polymer nanofibers display a negative magneto conductance that decreases in magnitude but remains finite with respect to the electric field. We show that this behavior of magneto conductance is an indicator of the electric field and temperature dependence of hopping length in the gradual transition from the thermally activated to the activation-less electric field driven variable range hopping transport. This reveals magneto transport as a useful tool to probe hopping lengths in the non-linear hopping regime.
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| 12. |
- Kim, Kyung Ho, 1984, et al.
(författare)
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Thermal stability of epitaxial graphene electrodes for conductive polymer nanofiber devices
- 2017
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Ingår i: Crystals. - : MDPI AG. - 2073-4352. ; 7:12
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Tidskriftsartikel (refereegranskat)abstract
- We used large area, monolayer graphene epitaxially grown on SiC (0001) as contact electrodes for polymer nanofiber devices. Our fabrication process, which avoids polymer resist residues on the graphene surface, results in graphene-polyaniline nanofiber devices with Ohmic contacts and electrical conductivity comparable to that of Au-nanofiber devices. We further checked the thermal stability of the graphene contacts to polyaniline devices by annealing up to T = 800 °C, the temperature at which polyaniline nanofibers are carbonized but the graphene electrode remains intact. The thermal stability and Ohmic contact of polymer nanofibers are demonstrated here, which together with the chemical stability and atomic flatness of graphene, make epitaxial graphene on SiC an attractive contact material for future all-carbon electronic devices.
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| 13. |
- Kovtun, Alessandro, et al.
(författare)
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Multiscale Charge Transport in van der Waals Thin Films: Reduced Graphene Oxide as a Case Study
- 2021
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 15:2, s. 2654-2667
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Tidskriftsartikel (refereegranskat)abstract
- Large area van der Waals (vdW) thin films are assembled materials consisting of a network of randomly stacked nanosheets. The multiscale structure and the two-dimensional (2D) nature of the building block mean that interfaces naturally play a crucial role in the charge transport of such thin films. While single or few stacked nanosheets (i.e., vdW heterostructures) have been the subject of intensive works, little is known about how charges travel through multilayered, more disordered networks. Here, we report a comprehensive study of a prototypical system given by networks of randomly stacked reduced graphene oxide 2D nanosheets, whose chemical and geometrical properties can be controlled independently, permitting to explore percolated networks ranging from a single nanosheet to some billions with room-temperature resistivity spanning from 10-5 to 10-1 ω·m. We systematically observe a clear transition between two different regimes at a critical temperature T*: Efros-Shklovskii variable-range hopping (ES-VRH) below T∗ and power law behavior above. First, we demonstrate that the two regimes are strongly correlated with each other, both depending on the charge localization length ζ, calculated by the ES-VRH model, which corresponds to the characteristic size of overlapping sp2 domains belonging to different nanosheets. Thus, we propose a microscopic model describing the charge transport as a geometrical phase transition, given by the metal-insulator transition associated with the percolation of quasi-one-dimensional nanofillers with length ζ, showing that the charge transport behavior of the networks is valid for all geometries and defects of the nanosheets, ultimately suggesting a generalized description on vdW and disordered thin films.
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| 14. |
- Lara Avila, Samuel, 1983, et al.
(författare)
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Towards quantum-limited coherent detection of terahertz waves in charge-neutral graphene
- 2019
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Ingår i: Nature Astronomy. - : Springer Science and Business Media LLC. - 2397-3366. ; 3:11, s. 983-988
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Spectacular advances in heterodyne astronomy1,2 have been largely due to breakthroughs in detector technology3. To exploit the full capacity of future terahertz (∼300 GHz–5 THz) telescope space missions4, new concepts of terahertz coherent receivers are needed, providing larger bandwidths and imaging capabilities with multipixel focal plane heterodyne arrays5. Here we show that graphene uniformly doped to the Dirac point, with material resistance dominated by quantum localization and thermal relaxation governed by electron diffusion, enables highly sensitive and wideband coherent detection of signals from 90 to 700 GHz and, prospectively, across the entire terahertz range. We measure on proof-of-concept graphene bolometric mixers an electron diffusion-limited gain bandwidth of 8 GHz (corresponding to a Doppler shift of 480 km s−1 at 5 THz) and intrinsic mixer noise temperature of 475 K (which would be equivalent to ~2 hν/kB at ν = 5 THz), limited by the residual thermal background in our setup. An optimized device will result in a mixer noise temperature as low as 36 K, with the gain bandwidth exceeding 20 GHz, and a local oscillator power of <100 pW. In conjunction with the emerging quantum-limited amplifiers at the intermediate frequency6,7, our approach promises quantum-limited sensing in the terahertz domain, potentially surpassing superconducting technologies, particularly for large heterodyne arrays
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| 15. |
- Sheng, C. X., et al.
(författare)
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Ultrafast Transient Spectroscopy of Trans-Polyacetylene in the Midinfrared Spectral Range
- 2020
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 124:1
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Tidskriftsartikel (refereegranskat)abstract
- Trans-polyacetylene [t-(CH)(x)] possesses twofold ground state degeneracy. Using the Su-Schrieffer-Heeger Hamiltonian, scientists predicted charged solitons to be the primary photoexcitations in t-(CH)(x); this prediction, however, has led to sharp debate. To resolve this saga, we use subpicosecond transient photomodulation spectroscopy in the mid-IR spectral range (0.1-1.5 eV) in neat t-(CH)(x) thin films. We show that odd-parity singlet excitons are the primary photoexcitations in t-(CH)(x), similar to many other nondegenerate pi-conjugated polymers. The exciton transitions are characterized by two photoinduced absorption (PA) bands at 0.38 and 0.6 eV, and an associated photoluminescence band at similar to 1.5 eV having similar polarization memory. The primary excitons undergo internal conversion within similar to 100 fs to an even parity (dark) singlet exciton with a PA band at similar to 1.4 eV. We also find ultrafast photogeneration of charge polarons when pumping deep into the polymer continuum band, which are characterized by two other PA bands in the mid-IR and associated photoinduced IR vibrational modes.
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| 16. |
- Shtepliuk, Ivan, et al.
(författare)
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Clustering and Morphology Evolution of Gold on Nanostructured Surfaces of Silicon Carbide: Implications for Catalysis and Sensing
- 2021
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Ingår i: ACS Applied Nano Materials. - : American Chemical Society (ACS). - 2574-0970. ; 4:2, s. 1282-1293
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Tidskriftsartikel (refereegranskat)abstract
- A fundamental understanding of the behavior of gold (Au) nanostructures deposited on functional surfaces is imperative to discover and leverage interface-related phenomena that can boost the efficiency of existing electronic devices in sensorics, catalysis, and spintronics. In the present work, Au layers with nominal thickness of 2 nm were sputter-deposited on graphenized SiC substrates represented by buffer layer (BuL)/4H-SiC and monolayer epitaxial graphene (MLG)/4H-SiC. Morphometric analysis by means of scanning electron microscopy shows that Au on BuL self-assembles in nearly round-shaped plasmonically active islands, while on MLG, a fractal growth of considerably larger and ramified islands is observed. To correlate the experimentally established differences in surface morphology on the two types of graphenized substrates with energetics and kinetics of Au nanostructure growth, the deposit-substrate interaction strength was studied using density functional theory (DFT) calculations, molecular dynamics simulations, and optical measurements. The theoretical considerations involve participation of Au clusters with different sizes and energetics at the initial stages of the metal nanostructure formation. The results indicate that gold exhibits a considerably stronger interaction with BuL than with MLG, which can be considered as a key aspect for explaining the experimentally observed morphological differences. From the statistical analysis of Raman spectra, indications of Au intercalation of MLG are discussed. The current research shows that, due to its unique surface chemistry, buffer layer has peculiar affinity to gold when compared to other atomically flat surfaces, which is beneficial for boosting high-performance catalytic and sensing technologies based on low-dimensional materials.
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| 17. |
- Yager, Thomas, 1987, et al.
(författare)
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Low contact resistance in epitaxial graphene devices for quantum metrology
- 2015
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Ingår i: AIP Advances. - : AIP Publishing. - 2158-3226 .- 2158-3226. ; 5:8, s. 087134-
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Tidskriftsartikel (refereegranskat)abstract
- We investigate Ti/Au contacts to monolayer epitaxial graphene on SiC (0001) for applications in quantum resistance metrology. Using three-terminal measurements in the quantum Hall regime we observed variations in contact resistances ranging from a minimal value of 0.6 Ω up to 11 kΩ. We identify a major source of high-resistance contacts to be due bilayer graphene interruptions to the quantum Hall current, whilst discarding the effects of interface cleanliness and contact geometry for our fabricated devices. Moreover, we experimentally demonstrate methods to improve the reproducibility of low resistance contacts (
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