Visualizing Atomic-Scale Negative Differential Resistance in Bilayer Graphene

Abstract

We investigate the atomic-scale tunneling characteristics of bilayer graphene on silicon carbide using the scanning tunneling microscopy. The high-resolution tunneling spectroscopy reveals an unexpected negative differential resistance (NDR) at the Dirac energy, which spatially varies within the single unit cell of bilayer graphene. The origin of NDR is explained by two near-gap van Hove singularities emerging in the electronic spectrum of bilayer graphene under a transverse electric field, which are strongly localized on two sublattices in different layers. Furthermore, defects near the tunneling contact are found to strongly impact on NDR through the electron interference. Our result provides an atomic-level understanding of quantum tunneling in bilayer graphene, and constitutes a useful step towards graphene-based tunneling devices. PACS numbers: 73.22.Pr, 73.20.At, 74.55.+v, 68.37.Ef Understanding quantum tunneling at the atomic level is essential in the study of nanoscale materials and their applications in the tunneling devices [1,2]. One of the most interesting tunneling phenomena is negative differential resistance (NDR), characterized by the reversal of the standard current-voltage relationship, decreasing current with increasing voltage. NDR is the basic operating principle of Esaki and resonant-tunneling diodes, and has enabled various novel applications [3,4]. Motivated by its fundamental importance and potential applications, there have been efforts to study NDR in graphene [5-9], a prototypical two-dimensional material with tunable electronic properties [10][11][12][13]. However, tunneling-induced NDR has not yet been realized in graphene, while a recent study has proposed a new method for NDR in graphene that is not based on the quantum tunneling effect [9].The scanning tunneling microscope (STM) is a powerful tool, which can not only directly probe NDR, but also provide key information on the mechanism. STM has been widely employed to study various electronic properties of graphene at the atomic scale, such as scattering and interference [14,15], but investigation into the sub-unit cell regime has been limited. Here, we report the first observation of NDR based on quantum tunneling at the vertical junction of STM over bilayer graphene. The applied electric field across bilayer graphene induces two van Hove singularities in the electronic spectrum, which are strongly localized on two sublattices in different layers. Such a localization of electronic singularities leads to a novel atomic-scale variation of NDR, which is directly visualized by our high-resolution tunneling spectroscopy within the single unit cell of bilayer graphene. This result provides the atomic-level understanding of quantum tunneling in bilayer graphene.Bilayer graphene was prepared on 6H-SiC(0001) wafers (N-dopant concentration of 1 × 10 18 cm −3 ) by thermal graphitization in a flow of argon as described in Ref. [16]. The samples were transferred through the air to the ultrahigh vacuum chamber (5 × 10 −11 torr), and briefl...

Current flow in biased bilayer graphene: Role of sublattices

“…We see that although the charge on top layer keeps completely located over only one sublattice (nondimer B T ), the charge density over the bottom layer is mostly sublattice unpolarized, i.e., equally shared between the two sublattices. This interesting characteristic of these systems, which has already been point out in previous experimental [25], analytical [17,38], and numerical [42] works, can possibly explain why the current goes preferentially over the bottom layer.…”

“…Therefore many devices based on these systems have been proposed recently, which involve the ability to control this layer pseudospin (the charge density polarization between layers induced by the bias) for different bias layouts [20][21][22], such as the creation of electron highways [23] or pseudospin-valve devices [4,5,24]. Experimentally, charge localization over different layers and different sublattices due to a bias voltage has been observed in * Corresponding author: carlos.gonzalez@fca.unicamp.br these systems by STM images [25], indicating the possibility of controlling layer and sublattice pseudospins in real samples. Even though all the attention that has been given to the possibilities of controlling charge densities in BLG through the bias, the charge flow has been neglected.…”

Electronic structure of vacancy resonant states in graphene: A critical review of the single-vacancy case

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Spectroscopic characterization of Landau-level splitting and the intermediate v=0 phase in bilayer graphene

“…1(a)] combined with magnetic-field-varied STS spectra which is generated by a substrate-induced interlayer potential [31][32][33]. The two lowenergy states are localized in different layers, resulting in the large asymmetry of their intensities [34,35]. The full width at half maximum of P1 is only ~3.4 meV, meaning that the conduction-band edge is almost flat.…”