Two Dimensional Atomic Crystals

TL;DR

By using micromechanical cleavage, a variety of 2D crystals including single layers of boron nitride, graphite, several dichalcogenides, and complex oxides are prepared and studied.

Abstract

We report free-standing atomic crystals that are strictly 2D and can be viewed as individual atomic planes pulled out of bulk crystals or as unrolled single-wall nanotubes. By using micromechanical cleavage, we have prepared and studied a variety of 2D crystals, including single layers of boron nitride, graphite, several dichalcogenides and complex oxides. These atomically-thin sheets (essentially gigantic 2D molecules unprotected from the immediate environment) are stable under ambient conditions, exhibit high crystal quality and are continuous on a macroscopic scale.

Figures (3)

• Figure 1: Two dimensional crystal matter. Single-layer crystallites of (a)$\mathrm{NbSe}_{2}$, (b) graphite, (c) $\mathrm{Bi}_{2} \mathrm{Sr}_{2} \mathrm{CaCu}_{2} \mathrm{O}_{\mathrm{x}}$ and (d) $\mathrm{MoS}_{2}$ visualized by AFM (a,b), SEM (c) and in an optical microscope (d). All scale bars are $1 \mu \mathrm{~m}$. The 2D crystallites are on top of an oxidized Si wafer ( 300 nm of thermal $\mathrm{SiO}_{2}$ ) (a-c) and on top of a holey carbon film (d). Note that 2D crystallites were often raised by an extra few Å above the supporting surface, probably due to a layer of absorbed water. In such cases, the pleated and folded regions seen on many AFM images and having the differential height matching the interlayer distance in the corresponding 3D crystals help to distinguish between single- and double-layer crystals.
• Figure 2: Electric field effect in 2D crystals. Changes in electrical conductivity$\sigma$ of 2D $\mathrm{NbSe}_{2}$, 2D $\mathrm{MoS}_{2}$ and graphene as a function of gate voltage. The inset shows our typical devices used for such measurements: It is an optical image (in white light) of 2D $\mathrm{NbSe}_{2}$ on top of an oxidized Si wafer (used as a gate electrode) with a set of Au contacts. The crystal is seen as a bluer region in the centre (scale bar $-5 \mu \mathrm{~m}$ ).
• Figure 3: Electric field effect in 2D crystals. Changes in electrical conductivity$\sigma$ of 2D $\mathrm{NbSe}_{2}$, 2D $\mathrm{MoS}_{2}$ and graphene as a function of gate voltage. The inset shows our typical devices used for such measurements: It is an optical image (in white light) of 2D $\mathrm{NbSe}_{2}$ on top of an oxidized Si wafer (used as a gate electrode) with a set of Au contacts. The crystal is seen as a bluer region in the centre (scale bar $-5 \mu \mathrm{~m}$ ).