In some molecular-scale experiments, yes—but the result comes from tip-enhanced Raman spectroscopy (TERS), not ordinary Raman microscopy. TERS confines light near a sharp metallic tip, enabling highly localized Raman measurements. A below-1-nm result is specific to the sample, setup and method used; it is not a general resolution specification for Raman instruments.
What does “sub-nanometer resolution” mean in Raman imaging?
Raman spectroscopy identifies vibrational information from a sample. In a spatial map, researchers measure Raman signals at different positions and use their changes to distinguish features. In TERS, the tip is moved across the sample while the signal is measured, so the map can reveal spatial variation at the molecular scale. The resolution describes how finely the map distinguishes spatial features; it is not simply the diameter of the focused laser spot.
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That distinction matters: a claim about TERS does not mean conventional, far-field Raman microscopes generally resolve structures smaller than a nanometer. The sub-nanometer claim concerns particular near-field experiments and analyses.
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TERS brings a sharp metallic tip close to a sample. The optical field is concentrated at the tip–sample junction, creating a highly localized region from which enhanced Raman signals can be collected. Scanning the tip and recording the Raman response at each position builds a map.
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Several mechanisms have been proposed to explain the finest spatial detail, including near-field confinement, electric-field gradients and interactions between a molecule and the plasmonic nanogap. A 2015 conference abstract proposed that near-field self-interaction alters Raman excitation and radiation, forming a localized “super-hot spot” (Zhang, Chen and Li, 2015). In a 2019 study, Chen and colleagues interpreted single-molecule TERS images in terms of local sub-molecular density changes induced by the confined field during the Raman process; they also connected image symmetry to combinations of degenerate vibrations localized in molecular side groups (Chen et al., 2019). These accounts describe related explanations, not one universal mechanism established for every TERS experiment.
What evidence supports resolution below 1 nm?
A model-specific theoretical result
A 2015 theoretical analysis modeled a tip, substrate and flat-lying H2TBPP molecule. It defined spatial resolution as the full width at half maximum of lateral signal profiles. For that configuration, it calculated 1 nm resolution from each of two separate signal contributions—electric-field and electric-field-gradient effects—and proposed that their combined effects could yield resolution below 1 nm. The authors also noted that the result depends on tip–substrate distance. This is a calculation for a specified model, not a general experimental benchmark (Meng et al., 2015).
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Experimental TERS results vary
TERS resolution is not invariably sub-nanometer. A 2016 resonant TERS study of few-layer MoS2 reported approximately 20 nm spatial resolution while investigating imaging across a sub-nanometer tip–sample gap and quantum effects in signal enhancement (Zhang et al., 2016). The gap size and the reported spatial resolution are different quantities. This result does not invalidate molecular-scale results obtained with other samples and approaches; it shows why a resolution figure must be tied to its specific experiment.
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How should you evaluate a TERS resolution claim?
Before comparing two reported values, check what was measured and how the authors defined resolution. In particular, look for:
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- Evidence type: Is the value experimentally measured or inferred from a theoretical model?
- Sample and vibrational mode: Results can depend on the molecule or material and on the signal being mapped.
- Tip–sample configuration: Note the tip, substrate and junction geometry, including any stated gap conditions.
- Resolution definition: Check how the value was extracted; Meng et al., for example, used the full width at half maximum of lateral signal profiles for their model.
- Scope of the conclusion: Treat a result from one configuration as evidence for that configuration, not as a guaranteed capability of all Raman microscopes or even all TERS setups.
For readers asking whether Raman spectroscopy itself reaches below a nanometer, the careful answer is: particular TERS experiments and models support molecular-scale—and, in some cases, proposed sub-nanometer—spatial resolution. That finding should not be generalized to ordinary Raman microscopy.
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