Raman Enhancement Effect on Two-dimensional Layered Materials: Graphene, h-BN, and MoS2

Mildred Dresselhaus, Massachusetts Institute of Technology, DMR 1004147

The schematic illustration of the structure of graphene, h-BN, and MoS2, and the typical Raman spectra of CuPc molecules on these two-dimensional materials and on blank SiO2/Si substrate.

Graphene h-BN MoS2

800 1200 1600 2000

CuPc/SiO2/Si

CuPc/MoS2

CuPc/h-BN

Inte

nsity (a.u

)

Raman shift (cm-1)

CuPc/graphene

(CuPc)

Here, we study the Raman enhancement effect on graphene, hexagonal boron nitride (h-BN) and molybdenum disulfide (MoS2), by using the copper phthalocyanine (CuPc) molecule as a probe, which can sit on these layered materials in a face-on configuration. However, it is found that the Raman enhancement effect, which was observable on graphene, hBN, and MoS2, has different enhance-ment factors for the different vibrational modes of CuPc, depending strongly on the substrate. The higher frequency phonon modes (such as those at 1342, 1452, 1531 cm-1) of CuPc are enhanced more strongly on graphene than on h-BN, while the lower frequency phonon modes (such as those at 682, 749, 1142, 1185 cm-1) of CuPc are enhanced more strongly on h-BN than on graphene. MoS2 showed the weakest Raman enhancement effect as a substrate among the three 2D materials. These differences were attributed to the different enhancement mechanisms related to the different electronic properties and chemical bonds: 1) graphene is a zero-gap semiconductor and has a non-polar C-C bond, which induces charge transfer; 2) h-BN is insulating and has a highly ionic B-N bond; while 3) MoS2 is semiconducting with the sulfur atoms on the surface and has a polar covalent bond (Mo-S) with the polarity in the vertical direction to the surface. This work confirms the importance of charge transfer for the Raman enhancement effect with a thin conducting substrate. Furthermore, we also demonstrate that the strong inter-face dipole-dipole interaction can also induce an obvious Raman enhancement, such as for the CuPc molecule on a h-BN substrate. This study benefits future potential application of 2D materials, as well as their hybrid structures, for their possible use in the observation and utilization of the SERS effect.

News Media Take Interest in Using 2D Sheets as SERS Substrate

Mildred Dresselhaus, Massachusetts Institute of Technology, DMR 1004147

The research results that have been achieved by the work described here were recognized by Nanotechweb.org. This photo of postdoc Xi Ling and the PI appeared on May 9, 2014: http://nanotechweb.org/cws/article/tech/57170

The text broadly announced the novel generalized concept of doing SERS on a flat surface like graphene. The application to other 2D materials extends the Raman enhancement effect from electronics and optoelectronics more generally, since may applications require bandgaps in the optical range.

We consider such research announcement to advance public interest in scientific discovery, and to be a form of public outreach.