Abstract
Terahertz near-field microscopy (THz-NFM) could locally probe low-energy molecular vibration dynamics below diffraction limits, showing promise to decipher intermolecular interactions of biomolecules and quantum matters with unique THz vibrational fingerprints. However, its realization has been impeded by low spatial and spectral resolutions and lack of theoretical models to quantitatively analyze near-field imaging. Here, we show that THz scattering-type scanning near-field optical microscopy (THz s-SNOM) with a theoretical model can quantitatively measure and image such low-energy molecular interactions, permitting computed spectroscopic near-field mapping of THz molecular resonance spectra. Using crystalline-lactose stereo-isomer (anomer) mixtures (i.e., α-lactose (≥95%, w/w) and β-lactose (≤4%, w/w)), THz s-SNOM resolved local intermolecular vibrations of both anomers with enhanced spatial and spectral resolutions, yielding strong resonances to decipher conformational fingerprint of the trace β-anomer impurity. Its estimated sensitivity was ~0.147 attomoles in ~8 × 10−4 μm3 interaction volume. Our THz s-SNOM platform offers a new path for ultrasensitive molecular fingerprinting of complex mixtures of biomolecules or organic crystals with markedly enhanced spatio-spectral resolutions. This could open up significant possibilities of THz technology in many fields, including biology, chemistry and condensed matter physics as well as semiconductor industries where accurate quantitative mappings of trace isomer impurities are critical but still challenging.
Highlights
Terahertz near-field microscopy (THz-NFM) could locally probe low-energy molecular vibration dynamics below diffraction limits, showing promise to decipher intermolecular interactions of biomolecules and quantum matters with unique THz vibrational fingerprints
This is realized by uniquely combining a THz s-SNOM capable of near-field imaging with substantially enhanced resolutions and a rigorous theoretical model based on the line dipole image method (LDIM) with quasi-electrostatic boundary conditions[8,9] to compute the local permittivity mapping of THz molecular resonance spectra
By demonstrating highly sensitive quantitative mappings of a small amount of isomer impurities (i.e., β-lactose) in the anomer mixture, we show the promise of our THz s-SNOM technical platform for trace analysis of impurities in organic crystals, isomer and polymorphic impurities in organic semiconducting materials, which critically impact the performance of organic thin-film transistors[29]
Summary
Terahertz near-field microscopy (THz-NFM) could locally probe low-energy molecular vibration dynamics below diffraction limits, showing promise to decipher intermolecular interactions of biomolecules and quantum matters with unique THz vibrational fingerprints. THz scattering-type scanning near-field optical microscopy (THz s-SNOM), which combines atomic force microscope (AFM) and THz-TDS, showed promise for quantitative spectroscopic imaging and sensing with enhanced spatial resolutions to probe low-energy molecular vibration dynamics below diffraction limits[3,5,8,9,10]. We present a new platform for a quantitative coherent broadband THz pulse spectroscopic mapping of molecular conformational dynamics of biomolecules This is realized by uniquely combining a THz s-SNOM capable of near-field imaging with substantially enhanced resolutions and a rigorous theoretical model based on the line dipole image method (LDIM) with quasi-electrostatic boundary conditions[8,9] to compute the local permittivity mapping of THz molecular resonance spectra. By demonstrating highly sensitive quantitative mappings of a small amount of isomer impurities (i.e., β-lactose) in the anomer mixture, we show the promise of our THz s-SNOM technical platform for trace analysis of impurities in organic crystals, isomer and polymorphic impurities in organic semiconducting materials, which critically impact the performance of organic thin-film transistors[29]
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