| 1. |
- Chua, C., et al.
(författare)
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Quantum Hall Effect and Quantum Point Contact in Bilayer-Patched Epitaxial Graphene
- 2014
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 14:6, s. 3369-3373
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Tidskriftsartikel (refereegranskat)abstract
- We study an epitaxial graphene monolayer with bilayer inclusions via magnetotransport measurements and scanning gate microscopy at low temperatures. We find that bilayer inclusions can be metallic or insulating depending on the initial and gated carrier density. The metallic bilayers act as equipotential shorts for edge currents, while closely spaced insulating bilayers guide the flow of electrons in the monolayer constriction, which was locally gated using a scanning gate probe.
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| 2. |
- Lartsev, Arseniy, 1987, et al.
(författare)
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Tuning carrier density across Dirac point in epitaxial graphene on SiC by corona discharge
- 2014
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:6
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate reversible carrier density control across the Dirac point (Δ n∼ 1013cm-2) in epitaxial graphene on SiC (SiC/G) via high electrostatic potential gating with ions produced by corona discharge. The method is attractive for applications where graphene with a fixed carrier density is needed, such as quantum metrology, and more generally as a simple method of gating 2DEGs formed at semiconductor interfaces and in topological insulators.
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| 3. |
- Yager, Thomas, 1987, et al.
(författare)
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Express Optical Analysis of Epitaxial Graphene on SiC: Impact of Morphology on Quantum Transport
- 2013
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 13:9, s. 4217-4223
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Tidskriftsartikel (refereegranskat)abstract
- We show that inspection with an optical microscope allows surprisingly simple and accurate identification of single and multilayer graphene domains in epitaxial graphene on silicon carbide (SiC/G) and is informative about nanoscopic details of the SiC topography, making it ideal for rapid and noninvasive quality control of as-grown SiC/G. As an illustration of the power of the method, we apply it to demonstrate the correlations between graphene morphology and its electronic properties by quantum magneto-transport.
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| 4. |
- 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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| 5. |
- Yager, Tom, et al.
(författare)
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Wafer-scale homogeneity of transport properties in epitaxial graphene on SiC
- 2015
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Ingår i: Carbon. - : Elsevier. - 0008-6223 .- 1873-3891. ; 87, s. 409-414
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Tidskriftsartikel (refereegranskat)abstract
- Magnetotransport measurements on Hall bar devices fabricated on purely monolayer epitaxial graphene on silicon carbide (SiC/G) show a very tight spread in carrier concentration and mobility across wafer-size dimensions. In contrast, SiC/G devices containing bilayer graphene domains display variations in their electronic properties linked to the amount of bilayer content. The spread in properties among devices patterned on the same SiC/G wafer can thus be understood by considering the inhomogeneous number of layers often grown on the surface of epitaxial graphene on SiC. (C) 2015 Elsevier Ltd. All rights reserved.
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| 6. |
- Chua, C., et al.
(författare)
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Observation of Coulomb blockade in nanostructured epitaxial bilayer graphene on SiC
- 2017
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 119, s. 426-430
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Tidskriftsartikel (refereegranskat)abstract
- We study electron transport in nanostructures patterned in bilayer graphene patches grown epitaxially on SiC as a function of doping, magnetic field, and temperature. Away from charge neutrality transport is only weakly modulated by changes in carrier concentration induced by a local side-gate. At low n-type doping close to charge neutrality, electron transport resembles that in exfoliated graphene nanoribbons and is well described by tunnelling of single electrons through a network of Coulomb-blockaded islands. Under the influence of an external magnetic field, Coulomb blockade resonances fluctuate around an average energy and the gap shrinks as a function of magnetic field. At charge neutrality, however, conduction is less insensitive to external magnetic fields. In this regime we also observe a stronger suppression of the conductance below T*, which we interpret as a sign of broken interlayer symmetry or strong fluctuations in the edge/potential disorder.
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| 7. |
- Hill-Pearce, R. E., et al.
(författare)
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The effect of bilayer regions on the response of epitaxial graphene devices to environmental gating
- 2015
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Ingår i: Carbon. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0008-6223 .- 1873-3891. ; 93, s. 896-902
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Tidskriftsartikel (refereegranskat)abstract
- The effect of a bilayer area on the electronic response to environmental gating of a monolayer graphene Hall bar device is investigated using room temperature magnetotransport and scanning Kelvin probe microscopy measurements in a controlled environment. The device is tuned through the charge neutrality point with n-p-n-junctions formed. Scanning Kelvin probe measurements show that the work function of the monolayer graphene decreases more than that of the bilayer area however magnetotransport measurements show a larger change in carrier concentration for bilayer graphene with environmental gating. Interface scattering at the boundary between the monolayer and bilayer regions also affects device response with field-dependent suppression of the conductivity observed near the charge neutrality point. Simultaneous electronic and environmental scanning Kelvin probe measurements are used to build nano-scale maps of the work function of the device surface revealing the areas of greatest work function change with environmental gating. Crown Copyright (C) 2015 Published by Elsevier Ltd. All rights reserved.
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| 8. |
- Lartsev, Arseniy, 1987, et al.
(författare)
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A prototype of RK/200 quantum Hall array resistance standard on epitaxial graphene
- 2015
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 118:4, s. art nr. 044506-
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Tidskriftsartikel (refereegranskat)abstract
- Epitaxial graphene on silicon carbide is a promising material for the next generation of quantum Hall re-sistance standards. Single Hall bars made of graphene have already surpassed their state-of-the-art GaAsbased counterparts as an RK/2 (RK = h/e^2) standard, showing at least the same precision and higher break-down current density. Compared to single devices, quantum Hall arrays using parallel or series connectionof multiple Hall bars can offer resistance values spanning several orders of magnitude and (in case of parallelconnection) significantly larger measurement currents, but impose strict requirements on uniformity of thematerial. To evaluate the quality of the available material, we have fabricated arrays of 100 Hall bars con-nected in parallel on epitaxial graphene. One out of four devices has shown quantized resistance that matchedthe correct value of RK/200 within the measurement precision of 1e-4 at magnetic fields between 7 and 9Tesla. The defective behaviour of other arrays is attributed mainly to non-uniform doping. This result con-firms the acceptable quality of epitaxial graphene, pointing towards the feasibility of well above 90% yieldof working Hall bars.
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| 9. |
- Lartsev, Arseniy, 1987
(författare)
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Quantum Hall devices on epitaxial graphene: towards large-scale integration
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Quantum Hall devices have been used as the primary standard of electrical resistance for over two decades, and they are unlikely to be replaced in this role any time soon. The work presented in this thesis was being done towards the goal of establishing epitaxial graphene on silicon carbide as a new material of choice for these devices. Experiments on individual devices have already demonstrated that due to unique electronic properties of graphene and peculiarities of its interaction with the SiC substrate, quantum resistance standards based on epitaxial graphene can operate at higher temperatures, lower magnetic fields, or higher current densities, as compared to their state-of-the-art gallium arsenide counterparts. Here, we were aiming at developing the technology for reliable mass-production of the devices.One of the issues that we address is the carrier density control. We have found that photochemical gating, a technique which has previously been used for this purpose, becomes unreliable when the electron density needs to be lowered by more than 1016 m2. Instead, corona discharge can be used for efficient electrostatic gating, enabling us to sweep the carrier density from 4*1016 electrons*m-2 to 5*1016 holes*m^-2 and to observe the quantum Hall effect at low doping.The presence of bilayer patches in majority-monolayer samples is another important problem. We have observed both metallic and insulating behaviour of these patches while driving the monolayer into the quantum Hall regime. When the bilayer is metallic, we show that a patch completely crossing the Hall bar will break down the quantum Hall effect in a way that agrees with theoretical expectations. Further, we propose imaging these patches by optical microscopy as a way of avoiding them, by selecting substrates where the patches are sufficiently small and sparse. We demonstrate that, despite the optical contrast being less than 2%, the bilayer areas can be imaged in real time using digital post-processing. Also, we show that optical microscopy can be used to detect the steps that form on the SiC surface during graphene growth, and even measure their height: steps as low as 1.5 nm could be clearly seen.Finally, we have fabricated arrays of 100 Hall bars connected in parallel, devices which provide a low-ohmic quantum standard if every single Hall bar works correctly. We have chosen a substrate with a sufficiently low bilayer content, and adapted the geometry of the Hall bar to the shape of the patches. One out of for devices has performed correctly within the relative measurement precision of 10-4 in magnetic fields above 7 tesla. We see this as a confirmation that the quality of graphene was sufficiently high to enable ≥99% yield of working Hall bars.
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| 10. |
- Melios, C., et al.
(författare)
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Detection of Ultralow Concentration NO2 in Complex Environment Using Epitaxial Graphene Sensors
- 2018
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Ingår i: ACS Sensors. - : American Chemical Society (ACS). - 2379-3694. ; 3:9, s. 1666-1674
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate proof-of-concept graphene sensors for environmental monitoring of ultralow concentration NO2 in complex environments. Robust detection in a wide range of NO2 concentrations, 10-154 ppb, was achieved, highlighting the great potential for graphene-based NO2 sensors, with applications in environmental pollution monitoring, portable monitors, automotive and mobile sensors for a global real-time monitoring network. The measurements were performed in a complex environment, combining NO2/synthetic air/water vapor, traces of other contaminants, and variable temperature in an attempt to fully replicate the environmental conditions of a working sensor. It is shown that the performance of the graphene-based sensor can be affected by coadsorption of NO2 and water on the surface at low temperatures (≤70 °C). However, the sensitivity to NO2 increases significantly when the sensor operates at 150 °C and the cross-selectivity to water, sulfur dioxide, and carbon monoxide is minimized. Additionally, it is demonstrated that single-layer graphene exhibits two times higher carrier concentration response upon exposure to NO2 than bilayer graphene.
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| 11. |
- Panchal, V., et al.
(författare)
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Atmospheric doping effects in epitaxial graphene: correlation of local and global electrical studies
- 2016
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Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 3:1
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Tidskriftsartikel (refereegranskat)abstract
- We directly correlate the local (20 nm scale) and global electronic properties of a device containing mono-, bi- and tri-layer epitaxial graphene (EG) domains on 6H-SiC (0001) by simultaneously performing local surface potential measurements using Kelvin probe force microscopy and global transport measurements. Using well-controlled environmental conditions we investigate the doping effects of N-2, O-2, water vapour and NO2 at concentrations representative of the ambient air. We show that presence of O-2, water vapour and NO2 leads to p-doping of all EG domains. However, the thicker layers of EG are significantly less affected. Furthermore, we demonstrate that the general consensus of O-2 and water vapour present in ambient air providing majority of the p-doping to graphene is a common misconception. We experimentally show that even the combined effect of O-2, water vapour, and NO2 at concentrations higher than typically present in the atmosphere does not fully replicate p-doping from ambient air. Thus, for EG gas sensors it is essential to consider naturally occurring environmental effects and properly separate them from those coming from targeted species.
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| 12. |
- Panchal, V., et al.
(författare)
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Local electric field screening in bi-layer graphene devices
- 2014
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Ingår i: Frontiers of Physics. - : Frontiers Media SA. - 2296-424X. ; 2, s. 1-10
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Tidskriftsartikel (refereegranskat)abstract
- © 2014 Panchal, Giusca, Lartsev, Yakimova and Kazakova. We present experimental studies of both local and macroscopic electrical effects in uniform single- (1LG) and bi-layer graphene (2LG) devices as well as in devices with non-uniform graphene coverage, under ambient conditions. DC transport measurements on sub-micron scale Hall bar devices were used to show a linear rise in carrier density with increasing 2LG coverage. Electrical scanning gate microscopy was used to locally top gate uniform and non-uniform devices in order to observe the effect of local electrical gating. We experimentally show a significant level of electric field screening by 2LG. We demonstrate that SGM technique is an extremely useful research tool for studies of local screening effects, which provides a complementary view on phenomena that are usually considered only within a macroscopic experimental scheme.
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| 13. |
- Panchal, V., et al.
(författare)
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Magnetic Scanning Probe Calibration Using Graphene Hall Sensor
- 2013
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Ingår i: IEEE Transactions on Magnetics. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9464 .- 1941-0069. ; 49:7, s. 3520-3523
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic force microscopy (MFM) offers a unique insight into the nanoscopic scale domain structures of magnetic materials. However, MFM is generally regarded as a qualitative technique and, therefore, requires meticulous calibration of the magnetic scanning probe stray field (B-probe) for quantitative measurements. We present a straightforward calibration of B-probe using scanning gate microscopy on epitaxial graphene Hall sensor in conjunction with Kelvin probe force microscopy feedback loop to eliminate sample-probe parasitic electric field interactions. Using this technique, we determined B-probe similar to 70 mT and similar to 76 mT for probes with nominal magnetic moment similar to 1 x 10(-13) and > 3 x 10(-13) emu, respectively, at a probe-sample distance of 20 nm.
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| 14. |
- Panchal, V., et al.
(författare)
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Visualisation of edge effects in side-gated graphene nanodevices
- 2014
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 4
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Tidskriftsartikel (refereegranskat)abstract
- Using local scanning electrical techniques we study edge effects in side-gated Hall bar nanodevices made of epitaxial graphene. We demonstrate that lithographically defined edges of the graphene channel exhibit hole conduction within the narrow band of similar to 60-125 nm width, whereas the bulk of the material is electron doped. The effect is the most pronounced when the influence of atmospheric contamination is minimal. We also show that the electronic properties at the edges can be precisely tuned from hole to electron conduction by using moderate strength electrical fields created by side-gates. However, the central part of the channel remains relatively unaffected by the side-gates and retains the bulk properties of graphene.
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