Multidimensional Nano-Imaging of Structure, Coupling, and Disorder in Molecular Materials

Single-molecule-level detection of interfacial molecular structures and ultrafast dynamics

Elucidating the ultrafast dynamics of interfacial molecules at the single-molecule level is pivotal for advancing our understanding of fundamental chemical and biological processes. Here, for the first time, we realized detection of ultrafast vibrational dynamics by a novel technique that integrates femtosecond sum frequency generation vibrational spectroscopy (SFG-VS) with nanoparticle-on-mirror (NPoM) nanocavities (NPoM-SFG-VS). Using a symmetric stretching vibrational mode of para-nitrothiophenol (ν NO2 ) as a probe, we have successfully identified signals from self-assembled monolayers (SAMs) comprising ∼60 molecules, demonstrating the single-molecule-level sensitivity of the NPoM-SFG-VS. The dephasing time and vibrational relaxation time of ν NO2 at the single-molecule level were determined to be 0.33 ± 0.01 ps and 2.2 ± 0.2 ps, respectively. By controlling the solution concentration used to prepare SAMs (C), a correlation between peak frequency of ν NO2 and C is established. It was found that single-molecule-level detection was achieved at C ≤ 10-10 M. With this protocol, microregion distribution of interfacial molecule number can be mapped using NPoM-SFG imaging. This work provides insights into the structures and vibrational dynamics of individual interfacial molecules, aiding in precise engineering of surface properties and reactivity.

Data-driven signal-to-noise enhancement in scattering near-field infrared microscopy

This study introduces a machine-learning approach to enhance signal-to-noise ratios in scattering-type scanning near-field optical microscopy (s-SNOM). While s-SNOM offers a high spatial resolution, its effectiveness is often hindered by low signal levels, particularly in weakly absorbing samples. To address these challenges, we utilize a data-driven "patch-based" machine learning reconstruction method, incorporating modern generative adversarial neural networks (CycleGANs) for denoising s-SNOM images. This method allows for flexible reconstruction of images of arbitrary sizes, a critical capability given the variable nature of scanned sample areas in point-scanning probe-based microscopies. The CycleGAN model is trained on unpaired sets of images captured at both rapid and extended acquisition times, thereby modeling instrument noise while preserving essential topographical and molecular information. The results show significant improvements in image quality, as indicated by higher structural similarity index and peak signal-to-noise ratio values, comparable to those obtained from images captured with four times the integration time. This method not only enhances image quality but also has the potential to reduce the overall data acquisition time, making high-resolution s-SNOM imaging more feasible for a wide range of biological and materials science applications.

Spatial Variations in Hydrogen Bonding Interaction within Polymer Blends Revealed by Infrared Nanoimaging

Hydrogen bonding plays a crucial role in enhancing the miscibility of polymer blends, allowing for the tailoring of their physicochemical properties to meet diverse application demands. However, nanoscale imaging of its impact on the phase-separation behavior of multicomponent polymeric materials remains largely unexplored. In this work, we introduce scattering-type scanning near-field optical microscopy (s-SNOM) equipped with a broadly tunable quantum cascade laser as a tool for investigating spatial variations in hydrogen-bonding interactions within blends of polyvinyl acetate (PVAc) and polyvinylphenol (PVPh), spin-coated from tetrahydrofuran solution. Our multiwavelength s-SNOM imaging approach reveals distinct features, namely, the hydrogen bonding mediated miscible PVAc/PVPh blend and the phase-separated PVAc domain. These results provide a more detailed understanding, indicating that hydrogen bonding may not lead to a completely uniform blend throughout the film, as previously believed, based on far-field spectroscopy. Furthermore, through comparisons between topography and near-field images, we find that the PVAc/PVPh hydrogen-bonded domain exhibits a strong affinity for the Si surface with its native oxide, while the free (non-hydrogen-bonded) PVAc film is vertically phase-separated atop the blend. Overall, our work demonstrates that s-SNOM is an effective and efficient tool for studying intermolecular interactions relevant to various chemical and biological phenomena.

Vibrational Coupling Infrared Nanocrystallography

Coupling between molecular vibrations leads to collective vibrational states with spectral features sensitive to local molecular order. This provides spectroscopic access to the low-frequency intermolecular energy landscape. In its nanospectroscopic implementation, this technique of vibrational coupling nanocrystallography (VCNC) offers information on molecular disorder and domain formation with nanometer spatial resolution. However, deriving local molecular order relies on prior knowledge of the transition dipole magnitude and crystal structure of the underlying ordered phase. Here we develop a quantitative model for VCNC by relating nano-FTIR collective vibrational spectra to the molecular crystal structure from X-ray crystallography. We experimentally validate our approach at the example of a metal organic porphyrin complex with a carbonyl ligand as the probe vibration. This framework establishes VCNC as a powerful tool for measuring low-energy molecular interactions, wave function delocalization, nanoscale disorder, and domain formation in a wide range of molecular systems.

Antenna-coupled infrared nanospectroscopy of intramolecular vibrational interaction

Many photonic and electronic molecular properties, as well as chemical and biochemical reactivities are controlled by fast intramolecular vibrational energy redistribution (IVR). This fundamental ultrafast process limits coherence time in applications from photochemistry to single quantum level control. While time-resolved multidimensional IR-spectroscopy can resolve the underlying vibrational interaction dynamics, as a nonlinear optical technique it has been challenging to extend its sensitivity to probe small molecular ensembles, achieve nanoscale spatial resolution, and control intramolecular dynamics. Here, we demonstrate a concept how mode-selective coupling of vibrational resonances to IR nanoantennas can reveal intramolecular vibrational energy transfer. In time-resolved infrared vibrational nanospectroscopy, we measure the Purcell-enhanced decrease of vibrational lifetimes of molecular vibrations while tuning the IR nanoantenna across coupled vibrations. At the example of a Re-carbonyl complex monolayer, we derive an IVR rate of (25±8) cm-1 corresponding to (450±150) fs, as is typical for the fast initial equilibration between symmetric and antisymmetric carbonyl vibrations. We model the enhancement of the cross-vibrational relaxation based on intrinsic intramolecular coupling and extrinsic antenna-enhanced vibrational energy relaxation. The model further suggests an anti-Purcell effect based on antenna and laser-field-driven vibrational mode interference which can counteract IVR-induced relaxation. Nanooptical spectroscopy of antenna-coupled vibrational dynamics thus provides for an approach to probe intramolecular vibrational dynamics with a perspective for vibrational coherent control of small molecular ensembles.

Collective Mid-Infrared Vibrations in Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) is typically assumed to occur at individual molecules neglecting intermolecular vibrational coupling. Here, we show instead how collective vibrations from infrared (IR) coupled dipoles are seen in SERS from molecular monolayers. Mixing IR-active molecules with IR-inactive spacer molecules controls the intermolecular separation. Intermolecular coupling leads to vibrational frequency upshifts up to 8 cm^-1, tuning with the mixing fraction and IR dipole strength, in excellent agreement with microscopic models and density functional theory. These cooperative frequency shifts can be used as a ruler to measure intermolecular distance and disorder with angstrom resolution. We demonstrate this for photochemical reactions of 4-nitrothiophenol, which depletes the number of neighboring IR-active molecules and breaks the collective vibration, enabling direct tracking of the reaction. Collective molecular vibrations reshape SERS spectra and need to be considered in the analysis of vibrational spectra throughout analytical chemistry and sensing.

Cryogenic 4D-STEM analysis of an amorphous-crystalline polymer blend: combined nanocrystalline and amorphous phase mapping

Understanding and visualizing the heterogeneous structure of immiscible semicrystalline polymer systems is critical for optimizing their morphology and microstructure. We demonstrate a cryogenic 4D-STEM technique using a combination of amorphous radial profile mapping and correlative crystalline growth processing methods to map both the crystalline and amorphous phase distribution in an isotactic polypropylene (iPP)/ethylene-octene copolymer (EO) multilayer film with 5-nm step size. The resulting map shows a very sharp interface between the amorphous iPP and EO with no preferential crystalline structure near or at the interface, reinforcing the expected incompatibility and immiscibility of iPP and EO, which is a short-chain branched polyethylene. This technique provides a method for direct observation of interfacial structure in an unstained semicrystalline complex multicomponent system with a single cryogenic 4D-STEM dataset.

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Cryogenic 4D-STEM can simultaneously map amorphous and crystalline structure

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Direct observation of beam-sensitive polymers reveals nanostructure near interfaces

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Cryogenic 4D-STEM enables mapping of chemically/structurally similar polymers in blend