Tip Enhanced Raman Spectroscopy (TERS)

Nanonics produces systems capable of combining surfaced enhanced Raman spectroscopy with scanning probe microscopy. The technique is known as TERS- Tip Enhanced Raman Spectroscopy and is described in more detail below.

When light is scattered from a molecule most of the incident photons are elastically scattered. A small fraction of light (approximately 1 in 107 photons) is scattered at optical frequencies different from the frequency of the incident photons. The emitted photons usually have a lower energy than the incident photons knows as a “Stokes shift” although it is possible, but less probable for them to have higher energy known as an “anti-stokes shift” The process leading to this inelastic scatter is termed the Raman effect and can occur with a change in vibrational, rotational or electronic energy of a molecule. Raman spectroscopy measures molecular vibrations determined by the structure and chemical bonding as well as the masses of the constituent atoms/ions under investigation. Raman spectra are unique in chemical and structural identification.

Since the Raman effect is intrincally weak much attention has, in recent years, focused on surface enhanced Raman spectroscopy (SERS) in a quest to obtain stronger Raman signals. Surface enhanced techniques were discovered to provide large increases in intensity and sensitivity of the Raman signal. In SERS techniques metallic, usually coinage (Au, Ag or Cu), particles are adsorbed at a sample's surface and provide signal enhancement via two mechanisms.

The first is an enhanced electromagnetic field produced at the surface of the metal. When the wavelength of the incident light is close to the plasma wavelength of the metal, conduction electrons in the metal surface are excited into an extended surface electronic excited state called a surface plasmon resonance. Molecules adsorbed or in close proximity to the surface experience an exceptionally large electromagnetic field. Vibrational modes normal to the surface are most strongly enhanced.
The second is a chemical enhancement affect that involves changes to the adsorbate electronic states due to a chemisorption of the analyte.  This essentialy involves the formation of a charge transfer complex between adsorbate and sample but this effect is much less signifcant than the electromagnetic effect.SERS allows for surface sensitivity and selectivity which is impossible in conventional Raman where the bulk signal is dominant.

The combination of SPM techniques in general and AFM in particular, with Raman  gives one the uniqe ability to combine topographic mapping with spectroscopy.

Near-field scanning optical microscopy (NSOM) has been successfully integrated with Raman spectrometry using an apertureless configuration, in which the laser is focused onto the sample through a microscope objective and Raman signal is collected by the same objective. This is similar to the conventional micro-Raman except that a metal tip is brought into the laser spot on sample surface to enhance the Raman signal, allowing enhancement by several orders of magnitude (≈10⁴) and Raman mapping on, for example real silicon devices, with a one second exposure time.

In this case, the Raman enhancement is a result of local field enhancement by the use of a  probe that is coated with a metal (usually Ag or Au). This technique has come to be known as TERS- or tip enhanced Raman spectroscopy. In contrast to SERS where the sample must be able to be coated TERS works on any sample and does not require any sample preparation. It also allows for investigation of scarce samples where such preparation techniques may be undesirable.

A schematic showing a gold coated SPM probe in and out of contact with the sample. The green light represents the laser.

The Data in figures a) and b) below, were obtained using Nanonics’ specialised surface enhanced Raman Scattering probes on a sample of Silicon with a 15nm strained layer of Silicon. In this case the data was recorded using the MultiView 2000 System, which allows probe movement independent of sample scanning. This allows one to first position the tip relative to the Raman laser, at a point where maximum enhancementof the Raman scatter is obtained and then, keeping the tip position constant with nanometric precision, scan along the sample.

The graphs show intensity as a function of Raman Shift. The peak in a) was obtained with no tip at all, while the peak in b was obtained with a tip in contact with the sample surface obtained using a MultiView 200 system.

a) Raman spectrum of Si without tip	b) Raman Spectrum of Si with tip in place

A difference spectrum between the two is shown in c)

.

c) Graph Comparing Raman spectra with tip in place (blue) and without (red). The TERS effect is clearly shown.

This shows a very strong enhancement in the Si peak at 518 cm^-1. This corresponds to the strained silicon that is on the surface of the sample. This result is an elegant example of a SERS or more precisely TERS (tip enhanced Raman scattering) effect. This is shown below in a 2D rendering of the results. The left hand image d) is a far field image at the stressed silicon frequency while the image on the right e) is the difference spectrum between tip in place and without the tip at all.