Lithography-free fabrication of high quality substrate-supported and freestanding graphene devices

Bao, Wenzhong; Liu, G.; Zhao, Zhenyu; Zhang, H.; Yan, Dong; Deshpande, Aparna; LeRoy, Brian J.; Lau, Chun Ning

doi:10.1007/s12274-010-1013-5

Cited by 89 publications

(85 citation statements)

References 19 publications

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“…Two different types of BLG devices are fabricated: (i) Graphene sheets are exfoliated on substrates or across predefined trenches that are 250 nm deep and approximately 3 μm wide, then coupled to 5-nm∕50-nm Ti∕Al metal electrodes through shadow mask evaporation (42). The typical back-gate coupling ratio of such devices is approximately 2.5 × 10 10 cm −2 V −1 .…”

Section: Methodsmentioning

confidence: 99%

Evidence for a spontaneous gapped state in ultraclean bilayer graphene

Bao

Velasco

Zhang

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

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115

151

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At the charge neutrality point, bilayer graphene (BLG) is strongly susceptible to electronic interactions and is expected to undergo a phase transition to a state with spontaneously broken symmetries. By systematically investigating a large number of single-and double-gated BLG devices, we observe a bimodal distribution of minimum conductivities at the charge neutrality point. Although σ min is often approximately 2-3 e 2 ∕h (where e is the electron charge and h is Planck's constant), it is several orders of magnitude smaller in BLG devices that have both high mobility and low extrinsic doping. The insulating state in the latter samples appears below a transition temperature T c of approximately 5 K and has a T ¼ 0 energy gap of approximately 3 meV. Transitions between these different states can be tuned by adjusting disorder or carrier density.topological states | anomalous hall | spontaneous quantum Hall states | electron-electron interactions | layer antiferromagnets B ilayer graphene (BLG) has provided a fascinating new platform for both post-silicon electronics and exotic many-body physics (1-23). Because its conduction and valence bands touch at two points in momentum space and have approximately quadratic dispersion accompanied by momentum-space pseudospin textures with vorticity J ¼ 2, charge-neutral BLG is likely to have a broken-symmetry ground state in the absence of disorder (6-11, 15-18, 24-26). Theoretical work on the character of the ground state in neutral BLG has examined a variety of distinct but related pseudospin ferromagnet states-including gapped anomalous Hall (5,6,19),[16][17][18]22), and current loop states (26)-that break time-reversal symmetry, and gapless nematic states, which alter Dirac point structure and reduce rotational symmetry (6-11, 15-18, 24, 25). The pseudospin degree of freedom reflects the presence of two low-energy carbon sites per unit cell that are localized in different layers. Experimental work has confirmed the strong role of interactions, but has been equivocal in specifying ground-state properties. In particular, both gapped and gapless states have been reported (19-23) in suspended BLG. The low-temperature minimum conductivity at the charge neutrality point (CNP), σ min , has ranged from approximately 0.05 to 250 μS. These orders-of-magnitude differences between σ min values measured in apparently similar samples have been baffling.In this paper we attempt to shed light on these ambiguous findings by systematically examining a large number of single-and double-gated BLG samples, with mobility values ranging from 500 to 2;000 cm 2 ∕V·s for substrate-supported samples and 6,000 to 350,000 for suspended samples. We find a surprisingly constant σ min value of approximately 2-3 e 2 ∕h for a majority of the devices (here, e is the electron charge and h is Planck's constant), independent of their mobility and of the presence or absence of substrates. However, for T below approximately 5 K, the best devices form an insulating state with an energy gap of approximately 2-3...

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Section: Methodsmentioning

confidence: 99%

Evidence for a spontaneous gapped state in ultraclean bilayer graphene

Bao

Velasco

Zhang

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

Self Cite

115

151

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“…We are the fi rst to propose and confi rm the battery performance improvement by such a new charging strategy in the fi rst cycle, which is possible based on the fundamental studies at the nanoscale. Cu electrodes (50-nm thickness) are then deposited on top of selected MoS 2 sheets using a shadow mask technique [ 35 ] in an electron beam evaporator. For resistance measurements, Hall bar electrodes are predeposited on glass substrates, and mechanically exfoliated MoS 2 crystals are then transferred onto the electrodes (Supporting Information Figure S3).…”

Section: Doi: 101002/aenm201401742mentioning

confidence: 99%

In Situ Investigations of Li‐MoS₂ with Planar Batteries

Wan

Bao

Liu

et al. 2014

Advanced Energy Materials

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109

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We have designed a planar microbattery that allows in situ electrical transport measurement during electrochemical charge and discharge in micron-sized individual crystallites of 2D-layered nanosheets, as well as optical studies (transmittance, Raman spectroscopy), which can give information about the electronic structure of the active material. To demonstrate the utility of our microbattery platform, we study the lithiation of MoS 2 crystallites. We observe that the electrical conductivity of the 2D MoS 2 crystallites is highly dependent on thickness and the rate of lithiation in the fi rst cycle. We use in situ TEM to confi rm that upon rapid fi rst-cycle lithiation the formation of a Mo conductive network imbedded in the Li 2 S matrix leads to an enhanced electrical conductivity compared to the pristine MoS 2 . We applied the results in Li-MoS 2 coin cells with composite electrodes, demonstrating that batteries with fast lithiation on the fi rst cycle showed signifi cantly higher specifi c capacity than batteries lithiated slowly. The microbattery platform can be generally applied to other energy storage materials and a wide range of characterization techniques, and is thus a powerful tool to uncover the properties of nanoscale materials undergoing electrochemical modifi cation. As in the example demonstrated here, we expect this platform will lead to new insights into the operation of battery materials at the nanoscale, leading to new strategies in improving the cell performance.A schematic crystal structure of 2H-MoS 2 is illustrated in Figure 1 a, with the lattice parameters a = 3.16 Å and c = 12.29 Å. Our microbattery platform developed for the in situ measurement of the optical transmittance and electrical transport of 2D nanomaterials is illustrated in Figure 1 b. A MoS 2 crystal and lithium metal are deposited on top of the Cu transport electrodes and current collector, respectively. The MoS 2 fl ake can be charged/discharged by connecting transport electrodes and current collector to an electrochemical workstation. Coupling the microbattery with a transmission optical microscope/probe station, we can then carry out in situ optical transmittance and electrical transport measurements on the same MoS 2 crystal. Optical images of uniform-thickness mechanically exfoliated MoS 2 crystals are shown in Figure 1 c,d, with dimensions as large as 100 µm × 100 µm. We used atomic force microscopy (AFM) to determine the morphology and thickness of exfoliated MoS 2 , in Figure 1 e,f. A photograph of the complete microbattery device is shown in Figure 1 g. The region of the transport electrodes is expanded in Figure 1 h, showing a large uniform area of MoS 2 crystal spanning the electrodes.To understand the intrinsic resistance change during lithiation, an in situ electrical transport measurement was carried out using our microbattery setup. Figure 2 a shows the simultaneously measured resistance and electrochemical potential at a small constant lithiation current at 0.5 µA for a single MoS 2 crystal of thickness 35 n...

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“…Single Lorentzian fitting of the 2D peak [see inset Fig.1 (c)] with I(2D)/I(G)= 3.4 confirms SLG 35 . Initially, to avoid contamination from wet chemical process and resist used in lithography steps, a mechanical shadow-masking method 36 was followed for electrical contacts. Cr(10nm)/Au(50nm) was deposited twice after masking the graphene flake using 50µm (or 25µm) diameter tungsten wires [see Fig.1(b)] under optical microscope.…”

Section: Methodsmentioning

confidence: 99%

Inhomogeneous screening of gate electric field by interface states in graphene FETs

Singh¹,

Gupta²

2017

J. Phys.: Condens. Matter

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The electronic states at graphene-SiO2 interface and their inhomogeneity is investigated using the back-gate-voltage dependence of local tunnel spectra acquired with a scanning tunneling microscope. The conductance spectra show two, or occasionally three, minima that evolve along the bias-voltage axis with the back gate voltage. This evolution is modeled using tip-gating and interface states. The energy dependent interface states' density, Dit(E), required to model the back-gate evolution of the minima, is found to have significant inhomogeneity in its energy-width. A broad Dit(E) leads to an effect similar to a reduction in the Fermi velocity while the narrow Dit(E) leads to the pinning of the Fermi energy close to the Dirac point, as observed in some places, due to enhanced screening of the gate electric field by the narrow Dit(E).

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Lithography-free fabrication of high quality substrate-supported and freestanding graphene devices

Cited by 89 publications

References 19 publications

Evidence for a spontaneous gapped state in ultraclean bilayer graphene

Evidence for a spontaneous gapped state in ultraclean bilayer graphene

In Situ Investigations of Li‐MoS₂ with Planar Batteries

Inhomogeneous screening of gate electric field by interface states in graphene FETs

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Lithography-free fabrication of high quality substrate-supported and freestanding graphene devices

Cited by 89 publications

References 19 publications

Evidence for a spontaneous gapped state in ultraclean bilayer graphene

Evidence for a spontaneous gapped state in ultraclean bilayer graphene

In Situ Investigations of Li‐MoS2 with Planar Batteries

Inhomogeneous screening of gate electric field by interface states in graphene FETs

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Product

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In Situ Investigations of Li‐MoS₂ with Planar Batteries