SPECTROSCOPYEUROPE 15 ARTICLE www.spectroscopyeurope.com VOL. 25 NO. 3 (2013) Introduction Single-walled carbon nanotubes (SWCNTs) 1 and graphene 2 have received tremendous research attention during the last few years. Both allotropes of carbon have matchless mechanical and electri- cal properties. The extraordinary tensile strength of SWCNTs ranges from 13 GPa to 126 GPa (much higher than of stain- less steel with 0.38 GPa to 1.55 GPa and of Kevlar ® with 3.6 GPa to 3.8 GPa); the strength of graphene is also about 100 times that of steel. This makes these materials very attractive to be used as fill- ers in polymer composites and thereby increase the polymer’s strength and elec- trical conductivity. 3,4 The excellent elec- trical conductivity of graphene makes a number of enhanced applications in electronics possible, for example, touch screens, field effect transistors and a large range of novel sensors of many kinds. Metallic SWCNTs can, theoretically, carry a current of 4 × 10 9 A cm –2 , which means that they conduct current about a thousand times better than copper. SWCNTs have a high thermal conductivity value of 3500 W m –1 K –1 along the axis of the tube, compared to 385 Wm –1 K –1 of copper. The thermal conductivity of graphene is even more impressive, with a measured value of 5000 Wm –1 K –1 . A SWCNT can be considered as a narrow sheet of graphene that has been rolled and fused. This is also appar- ent from the Raman spectrum: 5,6 most features can be found in the spectra of both allotropes. The G-line, attrib- uted to C–C stretching vibrations and ARTICLE Tip-enhanced Raman mapping (TERM) of single-walled carbon nanotubes and graphene Günter G. Hoffmann, a,* Marcos Ghislandi a,b and Gijsbertus de With a a Department of Chemical Engineering and Chemistry, Laboratory of Materials and Interface Chemistry (SMG), Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands. E-mail: g.g.hoffmann@tue.nl b CETENE–Center for Strategic Technologies of the Northeast, Av. Prof. Luiz Freire 1 (CDU), Recife–PE, 50740-540, Brazil named after its occurrence in graphite at 1582 cm –1 , is split into two compo- nents in CNTs: the G + -line at 1590 cm –1 , which is attributed to the stretching along the axis of the nanotube (longitu- dinal phonon mode) and the G – -line at 1570 cm –1 , which arises from the trans- versal mode of the C–C stretching. The D-line at ~1350 cm –1 is only found if there are defects in the sample, so it is used to judge the quality of the mate- rial. In graphene it can also be used to detect the edges of the sheets, as these are considered “defects” in an infinite graphene sheet. The G¢- or 2D-band at 2700 cm –1 is the first overtone of the D-band and can be used to analyse the number of graphene layers in a graphitic sample using its Raman shift and the 2D-/G-band intensity ratio. The radial breathing mode (RBM, at 100 cm –1 to 500 cm –1 ), which is a C–C bond-stretch- ing mode that moves all carbon atoms radially out, can, of course, only be observed from the tubes. As its wave- number is inversely proportional to the tube’s diameter, it can be advantageously used to analyse a sample’s content of tubes with different diameters. Tip-enhanced Raman spectroscopy (TERS) Raman spectroscopy and infrared spectro- scopy are complementary techniques for the qualitative and quantitative analysis of materials 6,7 which, in addition to chemi- cal composition, yield a lot of useful infor- mation including crystallinity and other properties of the sample under test. In the form of confocal Raman spectro- scopy and infrared microscopy, they can also be used to spatially map the sample; the former being the higher spatial resolu- tion technique. Due to physical limitations, the resolution is limited to 0.2–1 µm, depending upon the wavelength of the Raman spectroscopy excitation laser used. Using a “trick”, this limit may be overcome: in the case of Raman spectro- scopy, a noble metal tip is placed at the focus of a confocal Raman spectrome- ter. The resulting technique is therefore called tip-enhanced Raman spectroscopy (TERS) in the case of a stationary sample, or tip-enhanced Raman mapping (TERM), for the case of a scanned sample. Using TERM, carbon allotrope samples can be analysed with high spatial resolution. 8 The basic TERS experimental set-up for transparent samples is illustrated in Figure 1. Using a high numerical aper- ture (NA) microscope objective (of, for example, an inverted microscope), a laser beam is focused onto a sample. The sample can be moved in both the x- and y-directions by a high preci- sion piezo sample stage. A very sharp needle (radius at tip ~30 nm), etched electrochemically from a thin gold wire, is attached to a miniature quartz tuning fork. By means of a second x–y-scanner in an atomic force microscope (AFM) shear force head, the gold tip is placed with high accuracy at the exact centre of the laser focus. Feedback from the tuning fork, vibrating at its resonance frequency, is used to keep the gold tip at a constant distance to the sample with a piezo