Hunting for Monolayer Boron Nitride: Optical and Raman Signatures

Abstract

We describe the identification of single-and few-layer boron nitride. Its optical contrast is much smaller than that of graphene but even monolayers are discernable by optimizing viewing conditions. Raman spectroscopy can be used to confirm BN monolayers. They exhibit an upshift in the fundamental Raman mode by up to 4 cm -1 . The number of layers in thicker crystals can be counted by exploiting an integer-step increase in the Raman intensity and optical contrast.Properties of few-nanometer-thick BN sheets (often referred to as few-layer BN) have been attracting steady interest over the last several years 1 . Although individual atomic planes of BN were also isolated 2 and investigated by transmission electron microscopy (TEM) 3-5 and atomic force microscopy (AFM) 6 , interest in BN monolayers has been rather limited, especially, if compared with the interest generated by its "sister" material, graphene 7 . This can be attributed to 1) the lack of hexagonal boron nitride (hBN) crystals suitable for the mechanical cleavage approach 7 and 2) difficulties in isolating and finding sufficiently large BN monolayers. The situation is now changing rapidly due to the availability of hBN single crystals, which allow the cleavage of relatively large (~100 μm) and relatively thin (several nm) BN samples with an atomically flat surface. 6,8,9 Such crystals have been used as a thin top dielectric to gate graphene 9 and as an inert substrate for graphene devices, which allowed a significant improvement of their electronic quality, 8 unlike the earlier attempts with highly-oriented pyrolytic boron nitride (HOPBN) 10 . Most recently, it has been demonstrated that BN films with 2 to 5 layer thickness can also be obtained by epitaxial growth on copper and subsequent transfer onto a chosen substrate. 11 Particularly motivating is the emerging possibility to use BN as an ultra-thin insulator separating graphene layers. The layers could then be isolated electrically but would remain coupled electronically via Coulomb interaction, similar to the case of narrow-spaced quantum well heterostructures. 12 Such atomically thin BN-graphene heterostructures may allow a variety of new interaction phenomena including, for example, exciton condensation 13 .In the case of graphene, its mono-, bi-and few-layers are often identified by their optical contrast 14 and Raman signatures 15 . Little is known about these characteristics for the case of BN and, in the previous AFM and TEM studies, 2,5,6 one had to rely on finding atomically thin BN regions either randomly or close to edges of thick BN flakes. In this Letter, we report optical and Raman properties of mono-and few-layer BN obtained by micromechanical cleavage of high-quality hBN. Because of its zero opacity (the band gap is larger than 5eV), 1 atomically-thin BN exhibits little optical contrast, even if the interference enhancement using oxidized Si wafers is employed. 14,16 For the standard oxide thickness of ~300 nm SiO 2 , 6,7 BN monolayers show white-light contrast of <1.5%, which makes ...

Direct chemical conversion of graphene to boron- and nitrogen- and carbon-containing atomic layers

“…A shoulder of the G peak at 1,610 cm À 1 is also observed 20 . For fully converted h-BN films, only the E 2g peak is detected at B1,370 cm À 1 and no 2D or G peaks are observed, demonstrating nearly complete substitution of carbon atoms 15,28 .…”

“…The full width at half maximum of the E 2g peak in the h-BN is B12 cm À 1 , suggesting that the overall quality of the converted h-BN films is better than that of films made by direct CVD growth on copper foils (B16 cm À 1 ) 15 and is comparable to that of mechanically exfoliated h-BN films (10B12 cm À 1 ) 28 . X-ray photoelectron spectroscopy (XPS) was employed for chemical analysis of the pristine graphene template and the converted h-BNC and h-BN as illustrated in Fig.…”

Electron Tunneling through Ultrathin Boron Nitride Crystalline Barriers

“…The combination of graphene and h-BN opens up the exciting possibility of creating a new class of atomically-thin multilayered heterostructures (5,6). Graphene and h-BN share the same crystal structure and have very similar lattice constants but, unlike graphene, h-BN is an insulator with a large energy bandgap of 6 eV (7,8). Most previous studies have focused on the use of thick layers of BN as a substrate for graphene electronics (7,9,10) or as a dielectric in experiments on coupled 2D electron gases (11).…”

Layer-by-Layer Dielectric Breakdown of Hexagonal Boron Nitride

“…10 Therefore, BN is considered to be an ideal insulator for layered channel materials. Several studies of the optical, 11 mechanical, 12 phonon, 13 electrical tunneling, 14,15 oxidation resistance, 16 and hydrophobic 17 properties of BN have also been reported.…”