TERS Imaging of Twisted Bilayer Graphene

Graphene observed with nanometer resolution

Graphene is famous for its gapless band structure called Dirac cones. This unique band structure makes electrons in graphene behave like massless Dirac fermions, and gives graphene some special properties such as extraordinarily high mobility and ballistic transport. These extraordinary features make graphene an ideal material for nano electronics, for example, thin-film transistors, transparent and conductive composites and electrodes, flexible and printable electronics[1].

The electronic properties of these nano electronics strongly depend on the integrity of a designed graphene sheet. Any change of its intrinsic structure, such as local strain, defect or contaminants will modify the electronic properties, results a favorable feature or an unexpected defect.

Tip-enhanced Raman spectroscopy (TERS) is able to break the diffraction limit and take a Raman image with a resolution of 10 ~ a few tens of nanometers[2]- [4]. Figures below show TERS images of 2D/G ratio, 2D band, G band and D band of graphene. For comparison, an AFM image is attached at the bottom. From 2D/G ratio image, single-layer, twisted bilayer (explained below) and multilayer are distinguished as indicated by different colors. 2D band and G band images indicate the layer difference as well. At the edges and sheet ripples of graphene, D band image distinctly indicates defects and local strain respectively. Such details of edges and sheet ripples is confirmed from the AFM image.

 
TERS imaging of twisted bilayer graphene



: Single-layer graphene
: Twisted bilayer graphene
: Multi-layer graphene


Excitation
488 nm
Objective
100x, 1.40NA
Number of Spectra
4,950 (99 x 50), 20 nm/pixel
Measurement time
0.3 sec/pixel

TERSsense, a tip-enhanced Raman microscope manufactured by Nanophoton, has the great characteristic of ready provision of probes. Our probes are originally developed from design and 100% performance guaranteed. Every probe itself can give strong enhancement with high reproducibility. In contrast to gap-mode arrangement, there is no need to have a sample sandwiched between a metal probe and a metal-coated substrate. And, the adoption of transmission configuration of TERSsense makes it possible implement extremely sensitive measurement, with the utilization of a high NA(1.40) objective lens.

Raman spectrum of twisted bilayer graphene

Several new phenomena appear when two graphene layers are superposed with a mismatched rotation angle. Such a superposed two layers graphene is named as twisted bilayer graphene[5]. It exhibits very similar monolayer characteristics in Raman spectrum, but is very different from bilayer graphene. Twisted bilayer graphene demonstrates relatively large blue shift and broad width at 2D band with comparison to monolayer graphene, and its intensity is sometimes high and sometimes low. Meanwhile, a small red shift occurs at G band. All these phenomena are the mismatched rotation angle dependent. The reason is that the mismatched rotation angle adds a new degree of freedom to the graphene system, modifying the electronic structure[6]. Figures below show TERS images of 2D shift, 2D width and G shift.

TERS images of 2D shift, 2D width and G shift.





Furthermore, mismatched rotation gives rise to a new Raman feature, called R' peak (R stands for rotation). R' can be considered as a proof of the twisted bilayer graphene. It is also rotation angle dependent[7]. Figure below shows the R' TERS image, indicating the twisted bilayer graphene domain.

TERS image of intensity distribution of R’ peak


 
Reference

[1]Ferrari, A. C. & Basko, D. M. Raman spectroscopy as a versatile tool for studying the properties of graphene. Nat. Nanotechnol. 8, 235–46 (2013).


[2]Verma, P., Ichimura, T., Yano, T., Saito, Y. & Kawata, S. Nano-imaging through tip-enhanced Raman spectroscopy: Stepping beyond the classical limits. Laser Photon. Rev. 4, 548–561 (2009).


[1]Ferrari, A. C. & Basko, D. M. Raman spectroscopy as a versatile tool for studying the properties of graphene. Nat. Nanotechnol. 8, 235–46 (2013).


[3]Yano, T. et al. Tip-enhanced nano-Raman analytical imaging of locally induced strain distribution in carbon nanotubes. Nat. Commun. 4, 2592 (2013).


[4]Chen, C., Hayazawa, N. & Kawata, S. A 1.7nm resolution chemical analysis of carbon nanotubes by tip-enhanced Raman imaging in the ambient. Nat. Commun. 5, 3312 (2014).


[5] Kim, K. et al. Raman spectroscopy study of rotated double-layer graphene: Misorientation-angle dependence of electronic structure. Phys. Rev. Lett. 108, 1–6 (2012).


[6]Jorio, A. & Cançado, L. G. Raman spectroscopy of twisted bilayer graphene. Solid State Commun. 175-176, 3–12 (2013).


[7]Carozo, V. et al. Raman signature of graphene superlattices. Nano Lett. 11, 4527–4534 (2011).