In a beautiful example of work that approaches the ultimate limits of nano-fabrication, a TERS tip has been used to reversibly tailor a graphene surface by inducing defects.  Graphene is 2D hexagonal lattice of carbon atoms with unique electronic transport properties, high mobility and stability with applications for novel extraordinary fast microelectronic devices. Despite the fact that  graphene is fabricated by mechanical methods, it has a very low density of lattice defects.  The  crystal deformations, corrugations, and extrinsic rippled can produce structure distortion as lattice defects.    Defects in graphene can break its hexagonal symmetry and affect its properties, so any ability to control them is important to improving graphene properties.  Also, controlling graphene defects has application potential in phase transformation, recording information, and nanoscale switching.  Thus the ability to control and even reverse defects in graphene has important practical applications.  

In this paper Ag particles were deposited on the exfoliated graphene prepared on SiO2/Si substrate. The obtained Raman results show that the metal nanoparticles strongly interact with graphene and induce artificial defects (through appearance of D band). In spite of the advantages of the SERS, this process was irreversible and uncontrolled.  Tip enhanced Raman spectroscopy (TERS) is an excellent alternative to SERS where the Au or Ag metallized  probes approach the sample and create enhancement near the tip apex due to surface plasmons excitation near the metal apex.  The proces is reversible and controllable by careful approach and retract of the TERS probe.

In this work exploring the origin of graphene defects, the TERS tip performed a double function both by artificially inducing the defect and detecting it by enhancing the Raman signal.  A MultiView 4000 equipped with nanoparticle-tipped TERS and AFM-TERS feedback induce a reversible defect in the graphene through interaction with the TERS tip; the defect went away when the TERS tip was retracted.  The Nanonics instrumentation has several distinct advantages for these kinds of experimental setups:

 

1. Open optical axis to scanning head, enabling integration with up-right and inverted optical microscopes for TERS measurements on the opaque and transparent samples.   

2. Laser-free, tuning fork based feedback resulting in no optical interference between the feedback and Raman signal.

3. Tip and sample scanning in the one scanning head. Tip scanner enables the accurate positioning of the TERS probe relatively to the focused laser for the maximum Raman enhancement.

 

TERS spectra are shown below of the tip on (red) and off (black) the graphene surface show new bands (D,D+G) induced by the TERS probe.  These bands are associated with the creation of the extrinsic deformation on the graphene surface.
 

                           

 
These results show that TERS is an effective, non invasive, reversible and controllable technique to study 2D nanomaterial on the nanoscale.

Access full publication here  [P. Wang et al, Plasmonics, vol 7, p. 555, 2012]
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