After more than 15 years since TERS had its first proof of concept, HORIBA Scientific and AIST-NT have created a solution that brings TERS as an analytical method to a completely new level. TERS is not only able to perform so-called point measurements, but now TERS spectra can be collected in every single pixel of a scan map. A TERS map is composed of more than tens of thousands Raman spectra, for a full image acquisition time in less than 10 min.
To illustrate this capability for chemical imaging at the nanoscale, the above pictures present some TERS highlights obtained in 2015 by the NanoRaman team of HORIBA & AIST. Descriptions on those figures are found below, as well as comments by Dr. Marc Chaigneau, the AFM-Raman product manager at HORIBA Scientific.
The XploRA Nano equipped, with Au AFM-TERS tips, showed nanoscale chemical imaging of a single carbon nanotube with a spatial resolution of 8 nm (figure 1). Close to the local lattice defects (green circle on the map), a single step gives rise to noticeable intensity of the D peak (the so-called defect band), showing a resolution down to 1.3 nm along the nanotube. 'This spectacular improvement in the TERS resolution can be obtained thanks to the great stability of the NanoRaman optical coupling and the high frequency scanners of our SmartSPM AFM for an operation far away from noises”.
Figure1: TERS mapping of a single carbon nanotube showing an optical spatial resolution down to 8 nm across the tube, 1.3 nm along the tube (75 x 75 pixels, 100 ms acquisition time per pixel).
The TERS map of a graphene oxide flake with Ag AFM-TERS tips shows an drastic response from the wrinkles and creases in the sheet (figure 2, in green: G band, in red: C-H stretching band from the polymer residue). The signal contrast between the Raman near-field and far-field signal is leading to a local enhanced factor of 2 x 106. A fine analysis of the distribution of the ration of the G to D band intensities is giving an insight of the localized changes of the defect concentration in the graphene oxide flake. 'Such Raman amplification is only possible thanks to the plasmon resonance in Ag tips; the great news today is that the life-time of those tips has been increased to several weeks thanks to a protective layer'.
Figure2: D-band TERS map of a graphene oxide flake and typical single pixel TERS spectra taken over the wrinkles (red and blue), the flat portion (green) and out of the flake.
“Pulsed force lithography” (a mode included in our NanoRaman system) was used with ultrasharp single crystal diamond probes to write high quality patterns in single layer graphene oxide sheets. A dramatic enhancement of the gap mode TERS response of patterned flakes of graphene and graphene oxide can be observed in this example of “TERS” letters imprinted with a 15 nm indentation displacement (figure 3). 'The SmartSPM AFM offers an outstanding reproducibility in the tip exchange thanks to its full tuning and landing automation; that's the key to not being lost on the sample surface after performing the nanolithography!'
Figure 3: TERS map of the D-band intensity of the pattern imprinted in the single layer flake of graphene oxide on gold, with an indentation displacement of 15 nm using Pulsed force lithography.
To extend the TERS technique to other 2-D materials, resonant gap mode TERS mapping on exfoliated MoS2 has been performed using AFM-TERS tips, giving a noticeable rise of the A1g and A2u modes (figure 4). TERS maps are taken in the DualSpec mode which allows the far-field signal subtraction. 'Here again, the accessibility of AFM-TERS tips now, especially with a silver coating, opens the door to the TERS characterization of new fashionable 2D-nanomaterials. The huge enhancement factor makes visible some Raman modes that were very difficult to observe before at the nanoscale, and the DualSpec mode helps to perform on every single pixel of the hyperspectral TERS image the tip-up/tip-down subtraction'.
Figure 4: TERS mapping of few-layer MoS2 flake (intensity of the 408 cm-1 Raman band (A1g mode)), and two typical TERS spectra- on the edge of the flake and immediately off the edge.
TERS has been performed on a mixture C60 and C70 fullerenes co-deposited on gold (figure 5). In subsequently collected high resolution TERS maps, pixels that had a clear Raman signature of only one type of fullerene - either C60 or C70 are observed. Comparison of the C70 TERS spectra observed in this sample with the TERS spectra of monolayer of the same fullerenes further confirmed single-molecule sensitivity achieved in ambient AFM-based gap-mode TERS. “Single molecule sensitivity is really the ultimate goal of every spectroscopists! The single molecule sensitivity of gap mode TERS in STM feedback has already been demonstrated in ultrahigh vacuum at low temperatures, but in order for TERS to become a broadly accepted analytical technique, the setup and operation must be relatively simple and cost-efficient. In contrast, this beautiful work from our Application team is a clear proof of the single molecule detection through TERS in ambient condition!”
Figure 5: Left - TERS map of graphene oxide deposited on gold with an overcoat of diluted mixture of C60 and C70 fullerenes (128 pixels per line, 80 ms acquisition time per pixel). Right - Single pixel spectra from the map showing both C60 and C70 characteristic peaks (green) and spectra from only individual components (blue - C60, red- C70).