Suppression of intrinsic roughness in encapsulated graphene

Abstract

Roughness in graphene is known to contribute to scattering effects which lower carrier mobility. Encapsulating graphene in hexagonal boron nitride (hBN) leads to a significant reduction in roughness and has become the de facto standard method for producing high-quality graphene devices. We have fabricated graphene samples encapsulated by hBN that are suspended over apertures in a substrate and used noncontact electron diffraction measurements in a transmission electron microscope to measure the roughness of encapsulated graphene inside such structures. We furthermore compare the roughness of these samples to suspended bare graphene and suspended graphene on hBN. The suspended heterostructures display a root mean square (rms) roughness down to 12 pm, considerably less than that previously reported for both suspended graphene and graphene on any substrate and identical within experimental error to the rms vibrational amplitudes of carbon atoms in bulk graphite. Our first-principles calculations of the phonon bands in graphene/hBN heterostructures show that the flexural acoustic phonon mode is localized predominantly in the hBN layer. Consequently, the flexural displacement of the atoms in the graphene layer is strongly suppressed when it is supported by hBN, and this effect increases when graphene is fully encapsulated.

In situ control of graphene ripples and strain in the electron microscope

“…Root-mean-square roughness R rms for the simulated structures varies between 75.2 pm for the least corrugated membrane (γ rms ≈ 5.0°) and 109.0 pm for the most corrugated one (γ rms ≈ 7.8°), which is close to the previously established values. 17,18 The diffraction spot size σ was estimated as the standard deviation of a (radially symmetric) 2D Gaussian peak fitted into the simulated diffraction spot. This way, the broadening of the diffraction spots can be directly related to the surface inclination.…”

“…This way, the broadening of the diffraction spots can be directly related to the surface inclination. Another way to estimate the out-of-plane shape of the material from the diffraction pattern is to measure the intensity of the diffraction spot as a function of α, 17,18 which however does not allow distinguishing corrugations in the different in-plane directions, as is done below.…”

“…16 The peak broadening in electron diffraction experiments provides a direct measure of the root mean square of the surface inclination, for which different experiments agree on values around 5°. 2,16,17 The amplitude of the out-of-plane corrugations of graphene was initially estimated to reach 1 nm, whereas recent works suggest a root-mean-square roughness of 170 17 or 114 pm 18 that can be further lowered down to 12 pm via encapsulation with hexagonal boron nitride layers. 18 In this article, we demonstrate in situ control of corrugations in freestanding graphene inside a transmission electron microscope.…”

“…2,16,17 The amplitude of the out-of-plane corrugations of graphene was initially estimated to reach 1 nm, whereas recent works suggest a root-mean-square roughness of 170 17 or 114 pm 18 that can be further lowered down to 12 pm via encapsulation with hexagonal boron nitride layers. 18 In this article, we demonstrate in situ control of corrugations in freestanding graphene inside a transmission electron microscope. Our experimental results are supported through atomistic simulations, and a simple analytical model that allows us to discuss the relationship between the elastic moduli for in-plane stretch and bending, and confirms that the optimal configuration for graphene under our experimental conditions is not flat.…”

Analog Circuit Applications Based on All‐2D Ambipolar ReSe2 Field‐Effect Transistors

“…The outstanding electrical properties of 2D material heterostructures have been demonstrated . In transistors, based on a 2D channel material with a hBN insulator the free dangling bonds of the channel surface were preserved, which prevented surface roughness scattering . Furthermore, when applied in multilayer structures with graphene as the contact of a transistor with a 2D channel material, layered channel‐based electronics have shown particularly low contact resistance and achieved good device performance .…”