Tip-enhanced Raman spectroscopy (TERS) is an advanced Raman spectroscopic technique that provides nanoscale resolution and chemical characterization for nanoanalysis.
In conventional Raman spectroscopy, the spatial resolution is limited by the diffraction limit of light to about 200-300 nm. This poses challenges for studying samples and processes at the nanometer scale. TERS overcomes this limitation by using a non-porous metallic nanoparticle tip to locally enhance the Raman scattering, achieving spatial resolution down to 10-20 nm, far beyond the diffraction limit of light.
Brief History and Timeline of TERS
- 1982 – The development of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) enable nanoscale imaging of surfaces.
- 1997 – Studies show that metallic STM tips can enhance Raman signals from molecules on surfaces. This lays the foundation for TERS.
- 2000 – First experimental demonstration of TERS independently by the groups of Pettinger et al. and Stöckle et al. using silver and gold STM tips.
- 2002-2004 – Further improvements in spatial resolution down to 20-30 nm and first TERS chemical imaging.
- 2005 – First gap-mode TERS using AFM tips coated with silver nanoparticles, improving stability.
- 2006-2010 – Commercial TERS systems introduced. Wider adoption of TERS in nanoscience. Ongoing advances in tip fabrication.
- 2010- Present – Further developments towards more reliable quantification, and instrumentation standardization through advanced tip designs and data analysis.
The Basic Principle of TERS
TERS combines Raman spectroscopy with scanning probe microscopy techniques like Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM). As shown in Figure 1, it uses a non-porous nanoparticle tip (usually made of noble metals like gold or silver) positioned very close to the sample surface.
When the tip-sample distance is smaller than the tip radius (<10 nm), an intense localized electromagnetic field is generated in the gap between the tip apex and the sample. This leads to the enhancement of Raman signals from molecules directly under the tip by a factor of 104 to 108 but can reach as high as 1011 in optimal cases. This is called the electromagnetic enhancement mechanism.
TERS Enhancement Mechanism (TERS EM)
The electric field at the tip of the sharp probe is significantly enhanced compared to the incident field. This near-field enhancement is highly localized and is responsible for the increased Raman scattering signal from molecules near the tip. [mathajax]
Enhancement Factor: The TERS enhancement factor (\(E_F\)) quantifies the enhancement in the Raman signal intensity. It is the square of the ratio of the electric field at the tip (\(|E_{tip}|^2\)) to the incident electric field (\(|E_{incident}|^2\)),
$$E_F = \frac{|E_{tip}|^2}{|E_{incident}|^2}$$
The high \(E_F\) values are crucial for achieving significant signal enhancement in TERS.
Additionally, a chemical enhancement mechanism based on charge transfer between the tip and sample molecules can provide further signal enhancement. By raster scanning the tip over the sample surface, high-resolution Raman spectral maps can be obtained.
Common Applications of TERS
Studying Carbon Nanomaterials
TERS has been extensively used to study various carbon nanomaterials like graphene, carbon nanotubes, and fullerenes. It can probe defects, functional groups, chirality, and other properties at the nanoscale. This helps understand growth mechanisms and structure-property relationships in carbon nanostructures.
Imaging Solid-Liquid Interfaces
TERS is suited to study interfacial phenomena at solid-liquid interfaces like electrochemical and catalytic processes. It can provide chemical imaging of surface adsorbates, reaction intermediates, diffusion, and interface structure at nanometer resolution under operando conditions.
Probing Biological Interactions
TERS has emerged as a bioanalytical technique to probe protein and DNA structures, enzyme function, protein aggregation, nucleic acid interactions, and cell nanoscale organization. It can study biological processes without labels or damage.
Analyzing Nanoscale Electronic Materials
TERS has been used to characterize nanoelectronics materials like transistors, photovoltaics, and lithium-ion batteries. It gives chemical maps of dopants, defects, interfaces, and failure mechanisms in devices with ~10 nm resolution.
Polymer Characterization
TERS enables investigating polymer morphology, phase separation, crystallization, orientation of molecular chains, and additives at the nanoscale – key for understanding structure-property relationships.
Common Challenges While Performing TERS
While TERS is a powerful technique, there are some challenges:
- Maintaining a stable tip-sample nanogap is tricky. This affects reproducibility and quantification.
- The signal enhancement is highly dependent on tip shape, material, and proximity to the sample. Tip degradation over time affects performance.
- Specialized tips, scanners, and optics are required, increasing system complexity.
- Imaging artifacts can arise from factors like sample tilt and orientation.
- Biological and liquid samples can be hard to image due to laser heating and interference from solvent molecules.
FAQs
What type of tips are used in TERS?
TERS tips are typically made from gold or silver nanoparticles fabricated on the apex of AFM tips by techniques like electron beam lithography. The shape, size, and material of the nanoparticle affect the enhancement factor.
How does TERS complement other microscopy techniques?
TERS complements other techniques like electron microscopy (high-resolution topological imaging) and fluorescence microscopy (high-sensitivity molecular imaging) by providing chemical information at nanometer resolution,
What types of samples can be studied with TERS?
TERS is broadly applicable to “hard” and “soft” matter samples including polymers, nanotubes, biomolecules, cells, solid-liquid interfaces, semiconductors, and nanomaterials like graphene, perovskites, etc.
What spectroscopic information is obtained in TERS?
TERS can deliver information about molecular structure, chemical bonds, functional groups, and molecular interactions/complexation at specific locations on the sample by providing vibrational Raman spectra.
How does TERS imaging work?
TERS imaging works by scanning the TERS tip over the sample surface and acquiring spectra at each point, Raman spectral images are generated where contrast comes from spatial variations in molecular composition and chemical bonding.