Pressure-assisted tip-enhanced Raman imaging at a resolution of a few nanometres

Scanning probe microscopy methods1,2 can image samples with extremely high resolutions, opening up a wide range of applications in physics3, chemistry4 and biology5. However, these passive techniques, which trace the sample surface softly, give indirect topographic information. Here, we show an active imaging technique that has the potential to achieve optically a molecular resolution by directly interacting and perturbing the sample molecules. This technique makes use of an external pressure, applied selectively on a nanometric volume of the sample through the apex of a sharp nanotip, to obtain a local distortion of only those molecules that are pressurized. The vibrational frequencies of these molecules are distinctly different from those of unpressurized molecules. By sensing this difference, our active microscopic technique can achieve extremely high resolution. Using an isolated single-walled carbon nanotube and a two-dimensional adenine nanocrystal, we demonstrate a spatial resolution of 4 nm. Optical microscopy is a well-established technique for the study of the basic properties of materials. However, owing to the diffraction limit imposed by the wave nature of propagating light, spatial resolution in optical imaging is restricted to approximately half the probing light wavelength (Fig. 1a). Although recent advances6–8 in far-field optical microscopy have improved spatial resolution by a small amount, optical imaging still remains far from providing true nanoscale resolution. Near-field scanning optical microscopy (NSOM)9, on the other hand, has emerged as a powerful step forward in optical microscopy towards a breakthrough in subwavelength imaging. Recent developments have been based on using nanosized conical metallic tips10,11 as the NSOM probe. The imaging mechanism in this technique relies on confining the incident light in close proximity to the tip apex owing to the localized modes of surface plasmon polaritons (SPPs). When the tip is scanned close to the sample during the imaging process, the size of the confined light (here ‘size’ refers to the volume within which the confined light has considerable intensity, but where a sharp boundary cannot be defined), which is comparable to the size of the tip apex, determines the spatial resolution (Fig. 1b). As well as the confinement, resonant excitation of localized SPPs also provides strong enhancement of the light field at the tip apex. This enhancement makes it possible to observe optical signals, such as Raman scattering12–14 and fluorescence15, that would otherwise be quite weak due to the nanometric volume of the sample. Recent advances in tip-enhanced Raman spectroscopy and microscopy have demonstrated high sensitivity down to the single molecular level16–18 and ultrahigh spatial resolution better than 15 nm (refs 19–23). Because the resolution in this technique is determined by the size of the tip apex, it would seem reasonable to assume that the resolution could be improved simply by decreasing the size of the tip apex. The current value of spatial resolution ( 10–15 nm) could not, however, be improved further, indicating that the spatial resolution has already reached a plateau where no further improvement seems to be possible. This is probably because the field enhancement, which arises from the collective oscillations of free charge carriers, requires large numbers of metal atoms at the tip apex. By reducing the size of the tip apex, we therefore also reduce the possibility of observing the optical signal. This puts a practical limit on improving the resolution by decreasing the size of the tip apex. It is, therefore, necessary to think beyond plasmonic effects, which are predominantly responsible for the high resolution in NSOM measurements, so that spatial resolution in optical imaging can be further improved. One of the possible ways to do this is by combining a completely different phenomenon in the usual NSOM technique in such a way that the combination improves the resultant resolution. We have recently shown that combining nonlinearity19 or chemical effects24 in regular NSOM techniques can have an obvious impact on improving resolution. Here, we discuss an entirely different phenomenon, which, when combined with NSOM techniques, provides a dramatic improvement in spatial resolution. Using the same tip that is used for confining the light in NSOM, we apply a controlled amount of pressure on the sample so that the sample molecules directly under the physical contact with the tip apex are mechanically perturbed, and hence have a different optical response compared to the other unpressurized molecules of the sample25,26. It has been recently predicted27 that by adopting an advanced experimental technique, it could be possible to distinctly sense the optical signal originating from the pressurized part of the sample and hence obtain a much higher spatial resolution in a locally pressurized sample. Indeed, by precisely controlling the contact area between the sample and the tip, it is possible to ensure that a single or at most a very few molecules in the sample undergo a mechanical perturbation. By sensing the modified optical response of these sample molecules, a resolution as high as the molecular level can potentially be achieved (Fig. 1c). The optical response of the sample in our proposed technique was measured by means of tip-enhanced Raman scattering (TERS). Because the vibrational frequencies of Raman modes depend on the mechanical distortion of the sample, pressure-dependent shifts of Raman modes provide a perfect tool to detect selectively the distorted sample molecules. We term this new technique ‘tip-pressurized Raman microscopy’. Isolated single-walled carbon nanotubes (SWNTs) and two-dimensional adenine nanocrystals were used as the samples to demonstrate high spatial resolution in the tip-pressurized Raman experiments. After setting the silver-coated tip (apex diameter, 35 nm) on an isolated SWNT, Raman spectra (Fig. 2a) were measured in situ while increasing the tip-applied force, gradually, up to 3.3 nN. The two peaks observed in spectra (i) originate from the tangential C–C stretching vibrational modes and constitute the so-called G-band. The spectral shape of the G-band in this experiment suggests that