- Post Doctoral
MIT Unit Affiliation:
- Electrical Engineering & Computer Science
Post Doc Sponsor / Advisor:
Date PhD Completed:
Top 3 Areas of Expertise:
Expected End Date of Post Doctoral Position:
1. Light-matter interactions in anisotropic 2D materials
2. Chemical vapor deposition of 2D materials and heterostructures
3. Integration and circuits of nanodevices
Surface-enhanced Raman scattering (SERS), which is considered as one of the most promising techniques for practical applications, has attracted many scientists’ attention. However, even though SERS was discovered in the 1970s and single molecule sensitive detection was realized in 1997, there is still a long way to practical applications since it still has many challenges. Two of the most important, but complex issues are: (1) the availability of an ideal SERS substrate. It is difficult to obtain a satisfying SERS substrate, which is cheap, easy to produce and can be used repeatedly, meanwhile, the Raman signals obtained on which is intrinsic, uniform, and stable; (2) the understanding of the enhancement mechanism. There are two widely accepted mechanisms, i.e., electromagnetic mechanism (EM) and chemical mechanism (CM). They usually coexist, and difficult to be investigated separately. Especially for the CM, in theory, it is difficult to build a perfect theoretical model since it involves in too many influencing factors; while in experiment, it is difficult to find a perfect system where there is only CM without the disturbance from the huge enhancement of the EM.
Graphene is a perfect 2D material with a monolayer of carbon atoms packed into a honeycomb crystal plane. This thesis based on a Raman enhancement phenomenon on graphene substrate, which was first reported by us, referred to as graphene enhanced Raman scattering (GERS). There seems to be many advantages by taking graphene as a SERS substrate. It can overcome many disadvantages in traditional SERS substrate which are based on a rough metal surface, such as the non-uniform and unstable Raman signals, the background interference from the photo-carbonization, and the complex, expensive, and low reproduced processes of the substrate fabrication. In addition, GERS was attributed to be a system with only CM. Towards to the controversies and puzzle between EM and CM, we proposed GERS supplied a new chance for investigating CM, and the dependence of GERS on graphene layers, the distance between the probe molecules and graphene, the excitation energy of the laser, the molecular orientation, was investigated for deeper understanding of the CM effect. This thesis included the following parts:
1. Graphene enhanced Raman scattering (GERS) has been discovered. Taking graphene as a substrate, the dye molecules (such as phthalocyanine (Pc), rhodamine 6G (R6G), protoporphyin IX (PPP) and crystal violet (CV)) as probe molecules, graphene was found can enhance the Raman signals of the molecules of several or several tens of times, by comparing the Raman signals of the equal number of the molecules on the area with and without graphene, This phenomenon was named as graphene enhanced Raman scattering (GERS) effect. GERS owned many advantages comparing to traditional SERS in spectra obtaining. Moreover, the enhancement mechanism of GERS was contributed to only CM, which opened a new way for investigating CM.
2. The number of graphene layers dependence was investigated in GERS. Not only monolayer graphene, but also few-layer graphene (N=1--6) was found having the almost equally remarkable GERS effect. As the further increasing of the number of the graphene layers, GERS intensity decreased due to the damping of the lightwave during the propagation in graphite. In addition, by change the Fermi level of monolayer and bilayer graphene by doping, the abnormal dependence of GERS on the number of graphene layers was observed, which is corresponding to the common understanding of CM as an “electronic enhancement”.
3. The first layer effect of the CM was investigated in GERS. With the assistant of Langmuir-Blodgett (LB) technique, multilayered molecular structure was constructed on graphene, and the GERS effect was found can work only for the first layer molecule in contact with graphene (first layer effect). Also, the distance of the chemical groups in one molecule away from graphene can influence the enhancement. The closer it is, the stronger the enhancement will be.
4. The excitation wavelength dependence was investigated in GERS. Using a triple spectrometer Raman system with a tunable dye laser, the Raman excitation profiles of GERS were obtained. The charge transfer in GERS was proved to be a ground state charge transfer mechanism since the profiles were fitting well with the function of the normal Raman intensity under resonant condition, but no excited state charge transfer resonant was observed in the profiles.
5. The molecular orientation dependence was investigated in GERS. With the help of LB technique and annealing, upstanding and lying-down molecular orientations were obtained controllably, characterized by the UV-visible absorption spectra and atomic force microscopy (AFM). GERS was found highly dependent on the molecular orientation, with a much larger enhancement for the lying-down molecular orientation. The chemical enhancement factors of the different vibrational modes in Pc derivatives are calculated by considering both the change of the molecular orientation and the number of the molecules. The results are promising to be used as a standard to evaluate the interaction between graphene and the molecules.