Measuring the local quantum capacitance of graphene using a strongly coupled graphene nanoribbon

Abstract

We present electrical transport measurements of a van-der-Waals heterostructure consisting of a graphene nanoribbon separated by a thin boron nitride layer from a micron-sized graphene sheet. The interplay between the two layers is discussed in terms of screening or, alternatively, quantum capacitance. The ribbon can be tuned into the transport gap by applying gate voltages. Multiple sites of localized charge leading to Coulomb blockade are observed in agreement with previous experiments. Due to the strong capacitive coupling between the ribbon and the graphene top layer sheet, the evolution of the Coulomb blockade peaks in gate voltages can be used to obtain the local density of states and therefore the quantum capacitance of the graphene top layer. Spatially varying density and doping are found which are attributed to a spatial variation of the dielectric due to fabrication imperfections.PACS numbers: 71.15. Mb, 81.05.ue, 72.80.Vp One of the advantages of layered materials such as graphene, hexagonal boron nitride (hBN) or various dichalcogenides, is the possibility to fabricate stacks in order to obtain a heterostructure material with new properties. This was recently exploited to fabricate a variety of different device structures [1][2][3][4][5][6][7][8] and to investigate a number of novel effects: localization in super clean systems [9, 10], current drag [11][12][13] and the emergence of superlattices [14][15][16]. Most of these experiments were performed with so-called "double-layer graphene", consisting of two graphene layers separated by a thin insulator layer. In contrast to Bernal-stacked bilayer graphene, charge transfer between the two layers is not possible.Above experiments were conducted with micron-sized graphene sheets. In this paper we develop this idea one step further: we pattern the lower graphene sheet laterally into a nanoribbon (e.g. Refs. [17][18][19][20][21]) and stack an insulator plus a micron-sized graphene sheet on top. This experiment is a crucial step towards fabricating and understanding stacked graphene nanostructures. If the nanoribbon is tuned into the so-called transport gap [17,18], Coulomb blockade can be observed [17,18]. The localized charge in the ribbon serves as a sensitive and local detector for its electrostatic surrounding and allows us to probe properties of the graphene top layer. This is conceptually similar to an SET above a 2DEG [22] and scanning SET measurements [23,24]. Despite using a comparably thick insulator layer (13 nm), we can still reach the regime where the spacing between the layers is smaller than the average distance between two charge carriers within one layer [12].In this paper we present simultaneous electrical transport measurements through a graphene nanoribbon and a graphene sheet (top layer) that are separated by a layer of hBN, and therefore strongly capacitively coupled. By * dominikb@phys.ethz.ch applying gate voltages, we can independently tune both the graphene top layer and the graphene ribbon to their respective Dirac point. If t...

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