| 1. |
- Fu, Yifeng, 1984, et al.
(författare)
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Templated Growth of Covalently Bonded Three-Dimensional Carbon Nanotube Networks Originated from Graphene
- 2012
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Ingår i: Advanced Materials. - : Wiley. - 0935-9648 .- 1521-4095. ; 24:12, s. 1576-1581
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Tidskriftsartikel (refereegranskat)abstract
- A template-assisted method that enables the growth of covalently bonded three-dimensional carbon nanotubes (CNTs) originating from graphene at a large scale is demonstrated. Atomic force microscopy-based mechanical tests show that the covalently bonded CNT structure can effectively distribute external loading throughout the network to improve the mechanical strength of the material.
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| 2. |
- Lindahl, Niklas, 1981, et al.
(författare)
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Determination of the Bending Rigidity of Graphene via Electrostatic Actuation of Buckled Membranes
- 2012
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 12:7, s. 3526-3531
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Tidskriftsartikel (refereegranskat)abstract
- Classical continuum mechanics is used extensively to predict the properties of nanoscale materials such as graphene. The bending rigidity, kappa, is an important parameter that is used, for example, to predict the performance of graphene nanoelectromechanical devices and also ripple formation. Despite its importance, there is a large spread in the theoretical predictions of kappa for few-layer graphene. We have used the snap-through behavior of convex buckled graphene membranes under the application of electrostatic pressure to determine experimentally values of kappa for double-layer graphene membranes. We demonstrate how to prepare convex-buckled suspended graphene ribbons and fully clamped suspended membranes and show how the determination of the curvature of the membranes and the critical snap-through voltage, using AFM, allows us to extract kappa. The bending rigidity of bilayer graphene membranes under ambient conditions was determined to be 35.5(-15.0)(+20.0) eV. Monolayers are shown to have significantly lower kappa than bilayers.
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| 3. |
- Lindvall, Niclas, 1985, et al.
(författare)
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Cleaning graphene using atomic force microscope
- 2012
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 111:6, s. Article Number: 064904-
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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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| 4. |
- Lindvall, Niclas, 1985
(författare)
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Towards graphene-based devices: Fabrication and characterization
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Graphene is a new material with a large set of impressive properties, interesting both for fundamental studies and applications. Reliable synthesis of large-scale, high-quality graphene is key to its future success. This thesis is focused on the development of such fabrication techniques and experimental studies on three different graphene-based devices.The highest quality graphene is produced by mechanical exfoliation of graphite. While this technique is not scalable, it provides high quality graphene for scientific purposes and proof of principle devices. Catalytic chemical vapor deposition of graphene on copper is the most promising scalable method for graphene synthesis. Techniques for high temperature growth of graphene from methane as a precursor gas on high-purity copper foils are developed. Also, techniques for transferring the as‑grown graphene to insulating substrates are presented. Large-scale graphene with high uniformity and a mobility ~3000 cm2/Vs at room temperature is obtained.The transfer process needed for catalytically grown graphene on copper introduces issues with process reliability and future integration in semiconductor manufacturing. A non-catalytic chemical vapor deposition technique is shown to give uniform large-area graphene directly on insulating substrates like SiO2 and Si3N4, avoiding transfer. Raman spectroscopy, transmission electron microscopy, and transport measurements show that graphene grown this way is nanocrystalline and its electronic properties are inferior to those of catalytically grown graphene.Contamination and unintentional doping are difficult to avoid in graphene processing. A mechanical cleaning technique using an atomic force microscope is shown to efficiently remove contaminants and improve the electronic properties of graphene. The technique is easy and can be applied to substrates that cannot sustain standard graphene cleaning procedures.Graphene devices are realized on ferroelectric barium strontium titanate. The strong field effect in graphene is utilized as a read-out of the ferroelectric state, thus realizing a hybrid graphene-ferroelectric memory device.A graphene-based cold-electron bolometer is fabricated and characterized at cryogenic temperature. The low volume of graphene makes it an interesting absorber material for high-sensitivity bolometers.The Aharonov-Bohm effect is studied in graphene rings having metallic mirrors. The mirrors confine electrons to the Aharonov-Bohm ring, improving the visibility of higher-order Aharonov-Bohm oscillations.
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| 5. |
- Lindvall, Niclas, 1985, et al.
(författare)
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Towards transfer-free fabrication of graphene NEMS grown by chemical vapour deposition
- 2012
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Ingår i: Micro and Nano Letters. - : Institution of Engineering and Technology (IET). - 1750-0443. ; 7:8, s. 749-752
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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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| 6. |
- Lindvall, Niclas, 1985, et al.
(författare)
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Transfer-free fabrication of suspended graphene grown by chemical vapor deposition
- 2012
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Ingår i: 7th IEEE International Conference on Nano/Micro Engineered and Molecular Systems. NEMS 2012, Kyoto, 5 - 8 March 2012. - 9781467311243 ; , s. 19-22
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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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| 7. |
- Nam, Youngwoo, 1983, et al.
(författare)
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Graphene p-n-p junctions controlled by local gates made of naturally oxidized thin aluminium films
- 2012
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223. ; 50:5, s. 1987-1992
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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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| 8. |
- Nam, Youngwoo, 1983, et al.
(författare)
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The Aharonov-Bohm effect in graphene rings with metal mirrors
- 2012
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223. ; 50:15, s. 5562-5568
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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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| 9. |
- Sun, Jie, 1977, et al.
(författare)
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Chemical vapor deposition of nanocrystalline graphene directly on arbitrary high-temperature insulating substrates
- 2012
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Ingår i: 7th IEEE International Conference on Nano/Micro Engineered and Molecular Systems. NEMS 2012, Kyoto, 5 - 8 March 2012. - 9781467311243 ; , s. 11-14
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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.
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| 10. |
- Sun, Jie, 1977, et al.
(författare)
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Controllable chemical vapor deposition of large area uniform nanocrystalline graphene directly on silicon dioxide
- 2012
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 111:4
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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.
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