Nanoscale chemical imaging by photoinduced force microscopy

Dual-Color Peak Force Infrared Microscopy

Peak force infrared (PFIR) microscopy achieves nanoscale infrared imaging at sub-10 nm spatial resolution through photothermal mechanical detection of atomic force microscopy (AFM). However, it suffers from a major limitation that only one infrared frequency can be scanned for an AFM frame at a time. To overcome this limitation, we report here dual-color PFIR microscopy that enables simultaneous imaging at two infrared frequencies. This dual-color PFIR microscopy bypasses the limitations of frame drift and distortion of AFM when comparing two images of different infrared frequencies. We benchmark the performance and spatial resolution of this method using structured polymers exhibiting phase separation. We further demonstrate the application of this technique in imaging biological samples by mapping the cell wall of Escherichia coli (E. coli) bacteria. The presence of a bacterial outer membrane was detected without extrinsic labels. This dual-color PFIR microscopy enables simultaneous nondestructive chemical nanoimaging of multiple chemical components and will be useful for potential applications such as in situ dual-channel monitoring of chemical reactions.

Operando Characterization of Organic Mixed Ionic/Electronic Conducting Materials

Operando characterization plays an important role in revealing the structure-property relationships of organic mixed ionic/electronic conductors (OMIECs), enabling the direct observation of dynamic changes during device operation and thus guiding the development of new materials. This review focuses on the application of different operando characterization techniques in the study of OMIECs, highlighting the time-dependent and bias-dependent structure, composition, and morphology information extracted from these techniques. We first illustrate the needs, requirements, and challenges of operando characterization then provide an overview of relevant experimental techniques, including spectroscopy, scattering, microbalance, microprobe, and electron microscopy. We also compare different in silico methods and discuss the interplay of these computational methods with experimental techniques. Finally, we provide an outlook on the future development of operando for OMIEC-based devices and look toward multimodal operando techniques for more comprehensive and accurate description of OMIECs.

Super-resolution mid-infrared spectro-microscopy of biological applications through tapping mode and peak force tapping mode atomic force microscope

Small biomolecules at the subcellular level are building blocks for the manifestation of complex biological activities. However, non-intrusive in situ investigation of biological systems has been long daunted by the low spatial resolution and poor sensitivity of conventional light microscopies. Traditional infrared (IR) spectro-microscopy can enable label-free visualization of chemical bonds without extrinsic labeling but is still bound by Abbe's diffraction limit. This review article introduces a way to bypass the optical diffraction limit and improve the sensitivity for mid-IR methods - using tip-enhanced light nearfield in atomic force microscopy (AFM) operated in tapping and peak force tapping modes. Working principles of well-established scattering-type scanning near-field optical microscopy (s-SNOM) and two relatively new techniques, namely, photo-induced force microscopy (PiFM) and peak force infrared (PFIR) microscopy, will be briefly presented. With ∼ 10-20 nm spatial resolution and monolayer sensitivity, their recent applications in revealing nanoscale chemical heterogeneities in a wide range of biological systems, including biomolecules, cells, tissues, and biomaterials, will be reviewed and discussed. We also envision several future improvements of AFM-based tapping and peak force tapping mode nano-IR methods that permit them to better serve as a versatile platform for uncovering biological mechanisms at the fundamental level.

Quantifying molecular- to cellular-level forces in living cells

Mechanical cues have been suggested to play an important role in cell functions and cell fate determination, however, such physical quantities are challenging to directly measure in living cells with single molecule sensitivity and resolution. In this review, we focus on two main technologies that are promising in probing forces at the single molecule level. We review their theoretical fundamentals, recent technical advancements, and future directions, tailored specifically for interrogating mechanosensitive molecules in live cells.

Enhancement of Photoresponse on Narrow-Bandgap Mott Insulator α-RuCl3 via Intercalation

Charge doping to Mott insulators is critical to realize high-temperature superconductivity, quantum spin liquid state, and Majorana fermion, which would contribute to quantum computation. Mott insulators also have a great potential for optoelectronic applications; however, they showed insufficient photoresponse in previous reports. To enhance the photoresponse of Mott insulators, charge doping is a promising strategy since it leads to effective modification of electronic structure near the Fermi level. Intercalation, which is the ion insertion into the van der Waals gap of layered materials, is an effective charge-doping method without defect generation. Herein, we showed significant enhancement of optoelectronic properties of a layered Mott insulator, α-RuCl₃, through electron doping by organic cation intercalation. The electron-doping results in substantial electronic structure change, leading to the bandgap shrinkage from 1.2 eV to 0.7 eV. Due to localized excessive electrons in RuCl₃, distinct density of states is generated in the valence band, leading to the optical absorption change rather than metallic transition even in substantial doping concentration. The stable near-infrared photodetector using electronic modulated RuCl₃ showed 50 times higher photoresponsivity and 3 times faster response time compared to those of pristine RuCl₃, which contributes to overcoming the disadvantage of a Mott insulator as a promising optoelectronic device and expanding the material libraries.

Correlating the Morphological Evolution of Individual Catalyst Particles to the Kinetic Behavior of Metallocene-Based Ethylene Polymerization Catalysts

Kinetics-based differences in the early stage fragmentation of two structurally analogous silica-supported hafnocene- and zirconocene-based catalysts were observed during gas-phase ethylene polymerization at low pressures. A combination of focused ion beam-scanning electron microscopy (FIB-SEM) and nanoscale infrared photoinduced force microscopy (IR PiFM) revealed notable differences in the distribution of the support, polymer, and composite phases between the two catalyst materials. By means of time-resolved probe molecule infrared spectroscopy, correlations between this divergence in morphology and the kinetic behavior of the catalysts' active sites were established. The rate of polymer formation, a property that is inherently related to a catalyst's kinetics and the applied reaction conditions, ultimately governs mass transfer and thus the degree of homogeneity achieved during support fragmentation. In the absence of strong mass transfer limitations, a layer-by-layer mechanism dominates at the level of the individual catalyst support domains under the given experimental conditions.

Chemical Composition and Strain at Interfaces between Different Morphologies in Block Copolymer Thin Films

Transitional composition between two thin-film morphologies of the block copolymer, polystyrene-block-poly(tert-butyl acrylate) (PS-b-PtBuA), was investigated using near-field infrared spectroscopy and atomic force microscopy mechanical measurements. These techniques allowed block identification with nanoscale spatial resolution and elucidated the material's sub-surface composition. PS was found to form coronae around the PtBuA block in spherical valleys on flat areas of the film, and coronae of PtBuA surrounding the PS lamellae were observed at the edge of the polymer film, where parallel lamellae are formed. Furthermore, we found that the peak position and width varied by location, which may be a result of block composition, chain tension, or substrate interaction.

Polymerized small molecular acceptor based all-polymer solar cells with an efficiency of 16.16% via tuning polymer blend morphology by molecular design

All-polymer solar cells (all-PSCs) based on polymerized small molecular acceptors (PSMAs) have made significant progress recently. Here, we synthesize two A-DA’D-A small molecule acceptor based PSMAs of PS-Se with benzo[c][1,2,5]thiadiazole A’-core and PN-Se with benzotriazole A’-core, for the studies of the effect of molecular structure on the photovoltaic performance of the PSMAs. The two PSMAs possess broad absorption with PN-Se showing more red-shifted absorption than PS-Se and suitable electronic energy levels for the application as polymer acceptors in the all-PSCs with PBDB-T as polymer donor. Cryogenic transmission electron microscopy visualizes the aggregation behavior of the PBDB-T donor and the PSMA in their solutions. In addition, a bicontinuous-interpenetrating network in the PBDB-T:PN-Se blend film with aggregation size of 10~20 nm is clearly observed by the photoinduced force microscopy. The desirable morphology of the PBDB-T:PN-Se active layer leads its all-PSC showing higher power conversion efficiency of 16.16%.

Through development of non-fullerene acceptors, OPVs have reached efficiencies of 18%, yet the inadequate operational lifetime still poses a challenge for the commercialisation. Here, the authors investigate the origin of instability of NFA solar cells, and propose some strategies to mitigate this issue.

Integrated Tapping Mode Kelvin Probe Force Microscopy with Photoinduced Force Microscopy for Correlative Chemical and Surface Potential Mapping

Kelvin probe force microscopy (KPFM) is a popular technique for mapping the surface potential at the nanoscale through measurement of the Coulombic force between an atomic force microscopy (AFM) tip and sample. The lateral resolution of conventional KPFM variants is limited to between ≈35 and 100 nm in ambient conditions due to the long-range nature of the Coulombic force. In this article, a novel way of generating the Coulombic force in tapping mode KPFM without the need for an external AC driving voltage is presented. A field-effect transistor (FET) is used to directly switch the electrical connectivity of the tip and sample on and off periodically. The resulting Coulomb force induced by Fermi level alignment of the tip and sample results in a detectable change of the cantilever oscillation at the FET-switching frequency. The resulting FET-switched KPFM delivers a spatial resolution of ≈25 nm and inherits the high operational speed of the AFM tapping mode. Moreover, the FET-switched KPFM is integrated with photoinduced force microscopy (PiFM), enabling simultaneous acquisitions of high spatial resolution chemical distributions and surface potential maps. The integrated FET-switched KPFM with PiFM is expected to facilitate characterizations of nanoscale electrical properties of photoactive materials, semiconductors, and ferroelectric materials.

Dual-pulse photoactivated atomic force microscopy

Photoactivated atomic force microscopy (pAFM), which integrates light excitation and mechanical detection of the deflections of a cantilever tip, has become a widely used tool for probing nanoscale structures. Raising the illuminating laser power is an obvious way to boost the signal-to-noise ratio of pAFM, but strong laser power can damage both the sample and cantilever tip. Here, we demonstrate a dual-pulse pAFM (DP-pAFM) that avoids this problem by using two laser pulses with a time delay. The first laser heats the light absorber and alters the local Grüneisen parameter value, and the second laser boosts the mechanical vibration within the thermal relaxation time. Using this technique, we successfully mapped the optical structures of small-molecule semiconductor films. Of particular interest, DP-pAFM clearly visualized nanoscale cracks in organic semiconductor films, which create crucial problems for small-molecule semiconductors. DP-pAFM opens a promising new optical avenue for studying complex nanoscale phenomena in various research fields.

Fabrication of Multilayered Composite Nanofibers Using Continuous Chaotic Printing and Electrospinning: Chaotic Electrospinning

Multi-material and multilayered micro- and nanostructures are prominently featured in nature and engineering and are recognized by their remarkable properties. Unfortunately, the fabrication of micro- and nanostructured materials through conventional processes is challenging and costly. Herein, we introduce a high-throughput, continuous, and versatile strategy for the fabrication of polymer fibers with complex multilayered nanostructures. Chaotic electrospinning (ChE) is based on the coupling of continuous chaotic printing (CCP) and electrospinning, which produces fibers with an internal multi-material microstructure. When a CCP printhead is used as an electrospinning nozzle, the diameter of the fibers is further scaled down by 3 orders of magnitude while preserving their internal structure. ChE enables the use of various polymer inks for the creation of nanofibers with a customizable number of internal nanolayers. Our results showcase the versatility and tunability of ChE to fabricate multilayered structures at the nanoscale at high throughput. We apply ChE to the synthesis of unique carbon textile electrodes composed of nanofibers with striations carved into their surface at regular intervals. These striated carbon electrodes with high surface areas exhibit 3- to 4-fold increases in specific capacitance compared to regular carbon nanofibers; ChE holds great promise for the cost-effective fabrication of electrodes for supercapacitors and other applications.

Liquid-Phase Peak Force Infrared Microscopy for Chemical Nanoimaging and Spectroscopy

Peak force infrared (PFIR) microscopy is an emerging atomic force microscopy that bypasses Abbe's diffraction limit in achieving chemical nanoimaging and spectroscopy. The PFIR microscopy mechanically detects the infrared photothermal responses in the dynamic tip-sample contact of peak force tapping mode and has been applied for a variety of samples, ranging from soft matters, photovoltaic heterojunctions, to polaritonic materials under the air conditions. In this article, we develop and demonstrate the PFIR microscopy in the liquid phase for soft matters and biological samples. With the capability of controlling fluid compositions on demand, the liquid-phase peak force infrared (LiPFIR) microscopy enables in situ tracking of the polymer surface reorganization in fluids and detecting the product of click chemical reaction in the aqueous phase. Both broadband spectroscopy and infrared imaging with ∼10 nm spatial resolution are benchmarked in the fluid phase, together with complementary mechanical information. We also demonstrate the LiPFIR microscopy on revealing the chemical composition of a budding site of yeast cell wall particles in water as an application on biological structures. The label-free, nondestructive chemical nanoimaging and spectroscopic capabilities of the LiPFIR microscopy will facilitate the investigations of soft matters and their transformations at the solid/liquid interface.

Substrate-mediated hyperbolic phonon polaritons in MoO3

Hyperbolic phonon polaritons (HPhPs) are hybrid excitations of light and coherent lattice vibrations that exist in strongly optically anisotropic media, including two-dimensional materials (e.g., MoO₃). These polaritons propagate through the material's volume with long lifetimes, enabling novel mid-infrared nanophotonic applications by compressing light to sub-diffractional dimensions. Here, the dispersion relations and HPhP lifetimes (up to ≈12 ps) in single-crystalline α-MoO₃ are determined by Fourier analysis of real-space, nanoscale-resolution polariton images obtained with the photothermal induced resonance (PTIR) technique. Measurements of MoO₃ crystals deposited on periodic gratings show longer HPhPs propagation lengths and lifetimes (≈2×), and lower optical compressions, in suspended regions compared with regions in direct contact with the substrate. Additionally, PTIR data reveal MoO₃ subsurface defects, which have a negligible effect on HPhP propagation, as well as polymeric contaminants localized under parts of the MoO₃ crystals, which are derived from sample preparation. This work highlights the ability to engineer substrate-defined nanophotonic structures from layered anisotropic materials.

Determining the water content of nominally anhydrous minerals at the nanometre scale

The amount and distribution of water in nominally anhydrous minerals (NAMs) are usually determined by Fourier-transform infrared spectroscopy. This method is limited by the spot size of the beam to the study of samples with dimensions greater than a few micrometers. Here, we demonstrate the potential of using photoinduced force microscopy for the measurement of water in NAMs with samples sizes down to the nanometer scale with a study of water concentration across grain boundaries in forsterite. This development will enable the study of water speciation and diffusion in small-grained rock matrixes and allow a determination of the influence of nanoscale heterogeneity on the incorporation of water to NAMs.

Tuning the structure of cerium phosphate nanorods

The one-pot, shape selective synthesis of cerium phosphate nanorods has been explored and developed to give nanoparticles with aspect ratios between 3–24.8.

Photonic Excitation of a Micromechanical Cantilever in Electrostatic Fields

We present a specific near-field configuration where an electrostatic force gradient is found to strongly enhance the optomechanical driving of an atomic force microscope cantilever sensor. It is shown that incident photons generate a photothermal effect that couples with electrostatic fields even at tip-surface separations as large as several wavelengths, dominating the cantilever dynamics. The effect is the result of resonant phenomena where the photothermal-induced parametric driving acts conjointly (or against, depending on electric field direction) with a photovoltage generation in the cantilever. The results are achieved experimentally in an atomic force microscope operating in vacuum and explained theoretically through numerical simulations of the equation of motion of the cantilever. Intrinsic electrostatic effects arising from the electronic work-function difference of tip and surface are also highlighted. The findings are readily relevant for other optomicromechanical systems where electrostatic force gradients can be implemented.

SSNOMBACTER: A collection of scattering-type scanning near-field optical microscopy and atomic force microscopy images of bacterial cells

BACKGROUND

In recent years, a variety of imaging techniques operating at nanoscale resolution have been reported. These techniques have the potential to enrich our understanding of bacterial species relevant to human health, such as antibiotic-resistant pathogens. However, owing to the novelty of these techniques, their use is still confined to addressing very particular applications, and their availability is limited owing to associated costs and required expertise. Among these, scattering-type scanning near field optical microscopy (s-SNOM) has been demonstrated as a powerful tool for exploring important optical properties at nanoscale resolution, depending only on the size of a sharp tip. Despite its huge potential to resolve aspects that cannot be tackled otherwise, the penetration of s-SNOM into the life sciences is still proceeding at a slow pace for the aforementioned reasons.

RESULTS

In this work we introduce SSNOMBACTER, a set of s-SNOM images collected on 15 bacterial species. These come accompanied by registered Atomic Force Microscopy images, which are useful for placing nanoscale optical information in a relevant topographic context.

CONCLUSIONS

The proposed dataset aims to augment the popularity of s-SNOM and for accelerating its penetration in life sciences. Furthermore, we consider this dataset to be useful for the development and benchmarking of image analysis tools dedicated to s-SNOM imaging, which are scarce, despite the high need. In this latter context we discuss a series of image processing and analysis applications where SSNOMBACTER could be of help.

In situ Nanoscale Infrared Spectroscopy of Water Adsorption on Nanoislands of Surface-Anchored Metal-Organic Frameworks

Despite technological advancements, probing gas‐solid interfaces at the nanoscale is still a formidable challenge. New nano‐spectroscopic methods are needed to understand the guest–host interactions of functional materials during gas sorption, separation, and conversion. Herein, we introduce in situ Photoinduced Force Microscopy (PiFM) to evidence site‐specific interaction between Metal‐Organic Frameworks (MOFs) and water. To this end, we developed amphiphilic Surface‐anchored MOF (SURMOF) model systems using self‐assembly for the side‐by‐side hetero‐growth of nanodomains of hydrophilic HKUST‐1 and hydrophobic ZIF‐8. PiFM was used to probe local uptake kinetics and to show D₂O sorption isotherms on (defective) HKUST‐1 paddlewheels. By monitoring defect vibrations, we visualized in real‐time the saturation of existing defects and the creation of D₂O‐induced defects. This work shows the potential of in situ PiFM to unravel gas sorption mechanisms and map active sites on functional (MOF) materials.

Observation of nanoscale opto-mechanical molecular damping as the origin of spectroscopic contrast in photo induced force microscopy

Infrared photoinduced force microscopy (IR-PiFM) is a scanning probe spectroscopic technique that maps sample morphology and chemical properties on the nanometer (nm)-scale. Fabricated samples with nm periodicity such as self-assembly of block copolymer films can be chemically characterized by IR-PiFM with relative ease. Despite the success of IR-PiFM, the origin of spectroscopic contrast remains unclear, preventing the scientific community from conducting quantitative measurements. Here we experimentally investigate the contrast mechanism of IR-PiFM for recording vibrational resonances. We show that the measured spectroscopic information of a sample is directly related to the energy lost in the oscillating cantilever, which is a direct consequence of a molecule excited at its vibrational optical resonance—coined as opto-mechanical damping. The quality factor of the cantilever and the local sample polarizability can be mathematically correlated, enabling quantitative analysis. The basic theory for dissipative tip-sample interactions is introduced to model the observed opto-mechanical damping.

Existing high-dimensional optical imaging techniques that record space and polarization cannot detect the photon’s time of arrival due to the limited speeds of electronic sensors. Here, the authors develop a single-shot ultrafast imaging modality to record light-speed high-dimensional events with picosecond resolution.

An Optimized Fibril Network Morphology Enables High-Efficiency and Ambient-Stable Polymer Solar Cells

Morphological stability is crucially important for the long-term stability of polymer solar cells (PSCs). Many high-efficiency PSCs suffer from metastable morphology, resulting in severe device degradation. Here, a series of copolymers is developed by manipulating the content of chlorinated benzodithiophene-4,8-dione (T1-Cl) via a random copolymerization approach. It is found that all the copolymers can self-assemble into a fibril nanostructure in films. By altering the T1-Cl content, the polymer crystallinity and fibril width can be effectively controlled. When blended with several nonfullerene acceptors, such as TTPTT-4F, O-INIC3, EH-INIC3, and Y6, the optimized fibril interpenetrating morphology can not only favor charge transport, but also inhibit the unfavorable molecular diffusion and aggregation in active layers, leading to excellent morphological stability. The work demonstrates the importance of optimization of fibril network morphology in realizing high-efficiency and ambient-stable PSCs, and also provides new insights into the effect of chemical structure on the fibril network morphology and photovoltaic performance of PSCs.

Direct Chemical Imaging of Ligand-Functionalized Single Nanoparticles by Photoinduced Force Microscopy

Chemical characterizations of biochemically functionalized single nanoparticles are necessary to optimize the nanoparticle surface functionality in recently advanced nanobiological applications but have not yet been fully explored because of technical difficulties. Exploiting the photoinduced force exerted on a light-illuminated nanoscale tip, nanoscale mid-infrared hyperspectral images with a 10 nm spatial resolution of a monolayer ligand-functionalized single gold nanoparticle under ambient and environmental conditions are presented. We extend our study to the diagnosis of nanoscale heterogeneous chemical contaminants which come from a particle functionalization process but are undetectable in conventional ensemble-averaged imaging technique. High sensitivity and high spatial resolution are achieved via the strongly localized tip-enhanced force at the junction between the gold-coated tip and the functionalized nanoparticle in photoinduced force microscopy, which far exceeds the capability of the conventional methods. The present study paves a new way to directly detect heterogeneous nanochemicals at the single-component level, which is necessary to evaluate nanomaterial safety in biomedical applications.

Nanomechanical mapping of soft materials with the atomic force microscope: methods, theory and applications

Fast, high-resolution, non-destructive and quantitative characterization methods are needed to develop materials with tailored properties at the nanoscale or to understand the relationship between mechanical properties and cell physiology. This review introduces the state-of-the-art force microscope-based methods to map at high-spatial resolution the elastic and viscoelastic properties of soft materials. The experimental methods are explained in terms of the theories that enable the transformation of observables into material properties. Several applications in materials science, molecular biology and mechanobiology illustrate the scope, impact and potential of nanomechanical mapping methods.

Closed-loop atomic force microscopy-infrared spectroscopic imaging for nanoscale molecular characterization

Atomic force microscopy-infrared (AFM-IR) spectroscopic imaging offers non-perturbative, molecular contrast for nanoscale characterization. The need to mitigate measurement artifacts and enhance sensitivity, however, requires narrowly-defined and strict sample preparation protocols. This limits reliable and facile characterization; for example, when using common substrates such as Silicon or glass. Here, we demonstrate a closed-loop (CL) piezo controller design for responsivity-corrected AFM-IR imaging. Instead of the usual mode of recording cantilever deflection driven by sample expansion, the principle of our approach is to maintain a zero amplitude harmonic cantilever deflection by CL control of a subsample piezo. We show that the piezo voltage used to maintain a null deflection provides a reliable measure of the local IR absorption with significantly reduced noise. A complete analytical description of the CL operation and characterization of the controller for achieving robust performance are presented. Accurate measurement of IR absorption of nanothin PMMA films on glass and Silicon validates the robust capability of CL AFM-IR in routine mapping of nanoscale molecular information.

Simultaneous Optical Photothermal Infrared (O-PTIR) and Raman Spectroscopy of Submicrometer Atmospheric Particles

Physicochemical analysis of individual atmospheric aerosols at the most abundant sizes in the atmosphere (<1 μm) is analytically challenging, as hundreds to thousands of species are often present in femtoliter volumes. Vibrational spectroscopies, such as infrared (IR) and Raman, have great potential for probing functional groups in single particles at ambient pressure and temperature. However, the diffraction limit of IR radiation limits traditional IR microscopy to particles > ∼10 μm, which have less relevance to aerosol health and climate impacts. Optical photothermal infrared (O-PTIR) spectroscopy is a contactless method that circumvents diffraction limitations by using changes in the scattering intensity of a continuous wave visible laser (532 nm) to detect the photothermal expansion when a vibrational mode is excited by a tunable IR laser (QCL: 800-1800 cm^-1 or OPO: 2600-3600 cm^-1). Herein, we simultaneously collect O-PTIR spectra with Raman spectra at a single point for individual particles with aerodynamic diameters <400 nm (prior to impaction and spreading) at ambient temperature and pressure, by also collecting the inelastically scattered visible photons for Raman spectra. O-PTIR and Raman spectra were collected for submicrometer particles with different substrates, particle chemical compositions, and morphologies (i.e., core-shell), as well as IR mapping with submicron spatial resolution. Initial O-PTIR analysis of ambient atmospheric particles identified both inorganic and organic modes in individual sub- and supermicrometer particles. The simultaneous IR and Raman microscopy with submicrometer spatial resolution described herein has considerable potential both in atmospheric chemistry and numerous others fields (e.g., materials and biological research).

Near Field Differential Interference Contrast Microscopy

Near field scanning optical microscopy exploiting differential interference contrast enhancement is demonstrated. Beam splitting in the near field region is implemented using a dual color probe based on plasmonic color sorter idea. This provides the ability to illuminate two neighboring points on the sample simultaneously. It is shown that by modulating the two wavelengths employed in exciting such a probe, phase difference information can be retrieved through measuring the near field photoinduced force at the difference of the two modulation frequencies. This difference in frequency is engineered to correspond to the first resonant frequency of the cantilever, resulting in improved SNR, and sensitivity. The effect of both topographical and material changes in the proposed near field differential interference (NFDIC) technique are investigated for CNT and silica samples. This method is a promising technique for high contrast and high spatial resolution microscopy.

Using chaotic advection for facile high-throughput fabrication of ordered multilayer micro- and nanostructures: continuous chaotic printing

This paper introduces the concept of continuous chaotic printing, i.e. the use of chaotic flows for deterministic and continuous extrusion of fibers with internal multilayered micro- or nanostructures. Two free-flowing materials are coextruded through a printhead containing a miniaturized Kenics static mixer (KSM) composed of multiple helicoidal elements. This produces a fiber with a well-defined internal multilayer microarchitecture at high-throughput (>1.0 m min^-1). The number of mixing elements and the printhead diameter determine the number and thickness of the internal lamellae, which are generated according to successive bifurcations that yield a vast amount of inter-material surface area (∼10² cm² cm^-3) at high resolution (∼10 µm). This creates structures with extremely high surface area to volume ratio (SAV). Comparison of experimental and computational results demonstrates that continuous chaotic 3D printing is a robust process with predictable output. In an exciting new development, we demonstrate a method for scaling down these microstructures by 3 orders of magnitude, to the nanoscale level (∼150 nm), by feeding the output of a continuous chaotic 3D printhead into an electrospinner. The simplicity and high resolution of continuous chaotic printing strongly supports its potential use in novel applications, including-but not limited to-bioprinting of multi-scale layered biological structures such as bacterial communities, living tissues composed of organized multiple mammalian cell types, and fabrication of smart multi-material and multilayered constructs for biomedical applications.

Highly Efficient All-Small-Molecule Organic Solar Cells with Appropriate Active Layer Morphology by Side Chain Engineering of Donor Molecules and Thermal Annealing

It is very important to fine-tune the nanoscale morphology of donor:acceptor blend active layers for improving the photovoltaic performance of all-small-molecule organic solar cells (SM-OSCs). In this work, two new small molecule donor materials are synthesized with different substituents on their thiophene conjugated side chains, including SM1-S with alkylthio and SM1-F with fluorine and alkyl substituents, and the previously reported donor molecule SM1 with an alkyl substituent, for investigating the effect of different conjugated side chains on the molecular aggregation and the photophysical, and photovoltaic properties of the donor molecules. As a result, an SM1-F-based SM-OSC with Y6 as the acceptor, and with thermal annealing (TA) at 120 °C for 10 min, demonstrates the highest power conversion efficiency value of 14.07%, which is one of the best values for SM-OSCs reported so far. Besides, these results also reveal that different side chains of the small molecules can distinctly influence the crystallinity characteristics and aggregation features, and TA treatment can effectively fine-tune the phase separation to form suitable donor-acceptor interpenetrating networks, which is beneficial for exciton dissociation and charge transportation, leading to highly efficient photovoltaic performance.

Excited-State Imaging of Single Particles on the Subnanometer Scale

At the intersection of spectroscopy and microscopy lie techniques that are capable of providing subnanometer imaging of excited states of individual molecules or nanoparticles. Such approaches are particularly important for imaging macromolecules or nanoparticles large enough to have a high probability of containing a defect. These inevitable defects often control properties and function despite an otherwise ideal structure. We discuss real-space imaging techniques such as using scanning tunneling microscopy tips to enhance optical measurements and electron energy-loss spectroscopy in a scanning transmission electron microscope, which is based on focused electron beams to obtain high-resolution spatial information on excited states. The outlook for these methods is bright, as they will provide critical information for the characterization and improvement of energy-switching, electron-switching, and energy-harvesting materials.

Tip-Enhanced Infrared Imaging with Sub-10 nm Resolution and Hypersensitivity

Here we demonstrate sub-10 nm spatial resolution sampling of a volume of ∼360 molecules with a strong field enhancement at the sample-tip junction by implementing noble metal substrates (Au, Ag, Pt) in photoinduced force microscopy (PiFM). This technique shows the versatility and robustness of PiFM and is promising for application in interfacial studies with hypersensitivity and super spatial resolution.

All-digital histopathology by infrared-optical hybrid microscopy

Optical microscopy for biomedical samples requires expertise in staining to visualize structure and composition. Midinfrared (mid-IR) spectroscopic imaging offers label-free molecular recording and virtual staining by probing fundamental vibrational modes of molecular components. This quantitative signal can be combined with machine learning to enable microscopy in diverse fields from cancer diagnoses to forensics. However, absorption of IR light by common optical imaging components makes mid-IR light incompatible with modern optical microscopy and almost all biomedical research and clinical workflows. Here we conceptualize an IR-optical hybrid (IR-OH) approach that sensitively measures molecular composition based on an optical microscope with wide-field interferometric detection of absorption-induced sample expansion. We demonstrate that IR-OH exceeds state-of-the-art IR microscopy in coverage (10-fold), spatial resolution (fourfold), and spectral consistency (by mitigating the effects of scattering). The combined impact of these advances allows full slide infrared absorption images of unstained breast tissue sections on a visible microscope platform. We further show that automated histopathologic segmentation and generation of computationally stained (stainless) images is possible, resolving morphological features in both color and spatial detail comparable to current pathology protocols but without stains or human interpretation. IR-OH is compatible with clinical and research pathology practice and could make for a cost-effective alternative to conventional stain-based protocols for stainless, all-digital pathology.

The Influence of Additives on the Interfacial Width and Line Edge Roughness in Block Copolymer Lithography

The challenges of patterning next generation integrated circuits have driven the semiconductor industry to look outside of traditional lithographic methods in order to continue cost effective size scaling. The directed self-assembly (DSA) of block copolymers (BCPs) is a nanofabrication technique used to reduce the periodicity of patterns prepared with traditional optical methods. BCPs with large interaction parameters (χ eff), provide access to smaller pitches and reduced interface widths. Larger χ eff is also expected to be correlated with reduced line edge roughness (LER), a critical performance parameter in integrated circuits. One approach to increasing χ eff is blending the BCP with a phase selective additive, such as an Ionic liquid (IL). The IL does not impact the etching rates of either phase, and this enables a direct interrogation of whether the change in interface width driven by higher χ eff translates into lower LER. The effect of the IL on the layer thickness and interface width of a BCP are examined, along with the corresponding changes in LER in a DSA patterned sample. The results demonstrate that increased χ eff through additive blending will not necessarily translate to a lower LER, clarifying an important design criterion for future material systems.

High-Q dark hyperbolic phonon-polaritons in hexagonal boron nitride nanostructures

The anisotropy of hexagonal boron nitride (hBN) gives rise to hyperbolic phonon-polaritons (HPhPs), notable for their volumetric frequency-dependent propagation and strong confinement. For frustum (truncated nanocone) structures, theory predicts five, high-order HPhPs, sets, but only one set was observed previously with far-field reflectance and scattering-type scanning near-field optical microscopy. In contrast, the photothermal induced resonance (PTIR) technique has recently permitted sampling of the full HPhP dispersion and observing such elusive predicted modes; however, the mechanism underlying PTIR sensitivity to these weakly-scattering modes, while critical to their understanding, has not yet been clarified. Here, by comparing conventional contact- and newly developed tapping-mode PTIR, we show that the PTIR sensitivity to those weakly-scattering, high-Q (up to ≈280) modes is, contrary to a previous hypothesis, unrelated to the probe operation (contact or tapping) and is instead linked to PTIR ability to detect tip-launched dark, volumetrically-confined polaritons, rather than nanostructure-launched HPhPs modes observed by other techniques. Furthermore, we show that in contrast with plasmons and surface phonon-polaritons, whose Q-factors and optical cross-sections are typically degraded by the proximity of other nanostructures, the high-Q HPhP resonances are preserved even in high-density hBN frustum arrays, which is useful in sensing and quantum emission applications.

Nanoscale spectroscopic origins of photoinduced tip-sample force in the midinfrared

Photoinduced force at tip–sample junction provides nanoscale spectroscopic information with label-free and far-field background-free manner. This approach, spectronanoscopy through force detection, shows higher sensitivity and 1,000 times better spatial resolution than conventional ensemble averaged infrared microscopy, even under ambient and environmental conditions. Unfortunately, the origin of this promising photoinduced force effect is sometimes unclear because the force has 2 independent physical aspects: One is the electromagnetic effect related to induced dipoles in tip and sample, and the other one is the thermodynamic effect related to thermal heating of sample. Here, we reveal how the light illumination results in the 2 kinds of photoinduced forces at the tip–sample junction and provide quantitative interpretation of nanoscale spectroscopic measurements.

When light illuminates the junction formed between a sharp metal tip and a sample, different mechanisms can contribute to the measured photoinduced force simultaneously. Of particular interest are the instantaneous force between the induced dipoles in the tip and in the sample, and the force related to thermal heating of the junction. A key difference between these 2 force mechanisms is their spectral behavior. The magnitude of the thermal response follows a dissipative (absorptive) Lorentzian line shape, which measures the heat exchange between light and matter, while the induced dipole response exhibits a dispersive spectrum and relates to the real part of the material polarizability. Because the 2 interactions are sometimes comparable in magnitude, the origin of the chemical selectivity in nanoscale spectroscopic imaging through force detection is often unclear. Here, we demonstrate theoretically and experimentally how the light illumination gives rise to the 2 kinds of photoinduced forces at the tip–sample junction in the midinfrared. We comprehensively address the origin of the spectroscopic forces by discussing cases where the 2 spectrally dependent forces are entwined. The analysis presented here provides a clear and quantitative interpretation of nanoscale chemical measurements of heterogeneous materials and sheds light on the nature of light–matter coupling in optomechanical force-based spectronanoscopy.

Substrate Matters: Surface-Polariton Enhanced Infrared Nanospectroscopy of Molecular Vibrations

Infrared nanospectroscopy based on Fourier transform infrared near-field spectroscopy (nano-FTIR) is an emerging nanoanalytical tool with large application potential for label-free mapping and identification of organic and inorganic materials with nanoscale spatial resolution. However, the detection of thin molecular layers and nanostructures on standard substrates is still challenged by weak signals. Here, we demonstrate a significant enhancement of nano-FTIR signals of a thin organic layer by exploiting polariton-resonant tip-substrate coupling and surface polariton illumination of the probing tip. When the molecular vibration matches the tip-substrate resonance, we achieve up to nearly one order of magnitude signal enhancement on a phonon-polaritonic quartz (c-SiO₂) substrate, as compared to nano-FTIR spectra obtained on metal (Au) substrates, and up to two orders of magnitude when compared to the standard infrared spectroscopy substrate CaF₂. Our results will be of critical importance for boosting nano-FTIR spectroscopy toward the routine detection of monolayers and single molecules.

Antenna array-enhanced attenuated total reflection IR analysis in an aqueous solution

Attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) is a powerful technique that provides structural and functional information during dynamic reactions in aqueous solutions. One existing limitation is the sensitivity to extract the signals of trace-level analytes from the background water in situ and in real time. Here, we proposed a novel ATR-SEIRAS platform that integrated a large-scale triangle gold antenna array onto a conventional ATR-IR platform to increase the sensitivity of this analytical technique. A square centimeter level well-ordered gold antenna array was fabricated onto an Si prism via nanosphere lithography. The size-dependent antenna array resonance had weak correlation with the incident polarization and antenna orientation, allowing antenna array-enhanced IR detection without the requirement of a microscope. In addition, the antenna resonance shift that occurred due to analyte adsorption-induced refractive index variation could be minimized benefiting from the high refractive index of Si (3.4). As a demonstration, we dynamically monitored the adsorption of the trace levels of proteins on top of the antenna array with a real signal enhancement factor larger than 300. Our platform opens an avenue to apply antenna array-enhanced IR spectroscopy in an aqueous environment measured via commercial IR instruments, which is extremely promising for the interfacial applications that require signal enhancement.

Generalized Heterodyne Configurations for Photoinduced Force Microscopy

Infrared chemical microscopy through mechanical probing of light-matter interactions by atomic force microscopy (AFM) bypasses the diffraction limit. One increasingly popular technique is photoinduced force microscopy (PiFM), which utilizes the mechanical heterodyne signal detection between cantilever mechanical resonant oscillations and the photoinduced force from the light-matter interaction. So far, PiFM has been operated in only one heterodyne configuration. In this Article, we generalize heterodyne configurations of PiFM by introducing two new schemes: harmonic heterodyne detection and sequential heterodyne detection. In harmonic heterodyne detection, the laser repetition rate matches integer fractions of the difference between the two mechanical resonant modes of the AFM cantilever. The high harmonic of the beating from the photothermal expansion mixes with the AFM cantilever oscillation to provide the PiFM signal. In sequential heterodyne detection, the combination of the repetition rate of laser pulses and the polarization modulation frequency matches the difference between two AFM mechanical modes, leading to detectable PiFM signals. These two generalized heterodyne configurations for PiFM deliver new avenues for chemical imaging and broadband spectroscopy at ∼10 nm spatial resolution. They are suitable for a wide range of heterogeneous materials across various disciplines: from structured polymer film, to polaritonic boron nitride materials, to isolated bacterial peptidoglycan cell walls. The generalized heterodyne configurations introduce flexibility for the implementation of PiFM and the related tapping-mode AFM-IR and provide possibilities for an additional modulation channel in PiFM for targeted signal extraction with nanoscale spatial resolution.

Nanoscale mapping of chemical composition in organic-inorganic hybrid perovskite films

Lead-based organic-inorganic hybrid perovskite (OIHP) solar cells can attain efficiencies over 20%. However, the impact of ion mobility and/or organic depletion, structural changes, and segregation under operating conditions urge for decisive and more accurate investigations. Hence, the development of analytical tools for accessing the grain-to-grain OIHP chemistry is of great relevance. Here, we used synchrotron infrared nanospectroscopy (nano-FTIR) to map individual nanograins in OIHP films. Our results reveal a spatial heterogeneity of the vibrational activity associated to the nanoscale chemical diversity of isolated grains. It was possible to map the chemistry of individual grains in CsFAMA [Cs_0.05FA_0.79MA_0.16Pb(I_0.83Br_0.17)₃] and FAMA [FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃] films, with information on their local composition. Nanograins with stronger nano-FTIR activity in CsFAMA and FAMA films can be assigned to PbI₂ and hexagonal polytype phases, respectively. The analysis herein can be extended to any OIHP films where organic cation depletion/accumulation can be used as a chemical label to study composition.

High-resolution, high-contrast mid-infrared imaging of fresh biological samples with ultraviolet-localized photoacoustic microscopy

Mid-infrared (MIR) microscopy provides rich chemical and structural information about biological samples, without staining. Conventionally, the long MIR wavelength severely limits the lateral resolution owing to optical diffraction; moreover, the strong MIR absorption of water ubiquitous in fresh biological samples results in high background and low contrast. To overcome these limitations, we propose a method that employs photoacoustic detection highly localized with a pulsed ultraviolet (UV) laser on the basis of the Grüneisen relaxation effect. For cultured cells, our method achieves water-background suppressed MIR imaging of lipids and proteins at UV resolution, at least an order of magnitude finer than the MIR diffraction limits. Label-free histology using this method is also demonstrated in thick brain slices. Our approach provides convenient high-resolution and high-contrast MIR imaging, which can benefit diagnosis of fresh biological samples.

Ultrafast Dynamic Microscopy of Carrier and Exciton Transport

We highlight the recent progress in ultrafast dynamic microscopy that combines ultrafast optical spectroscopy with microscopy approaches, focusing on the application transient absorption microscopy (TAM) to directly image energy and charge transport in solar energy harvesting and conversion systems. We discuss the principles, instrumentation, and resolutions of TAM. The simultaneous spatial, temporal, and excited-state-specific resolutions of TAM unraveled exciton and charge transport mechanisms that were previously obscured in conventional ultrafast spectroscopy measurements for systems such as organic solar cells, hybrid perovskite thin films, and molecular aggregates. We also discuss future directions to improve resolutions and to develop other ultrafast imaging contrasts beyond transient absorption.

Coiled-Coil-Mediated Assembly of an Icosahedral Protein Cage with Extremely High Thermal and Chemical Stability

The organization of protein molecules into higher-order nanoscale architectures is ubiquitous in Nature and represents an important goal in synthetic biology. Furthermore, the stabilization of enzyme activity has many practical applications in biotechnology and medicine. Here we describe the symmetry-directed design of an extremely stable, enzymatically active, hollow protein cage of M_r ≈ 2.1 MDa with dimensions similar to those of a small icosahedral virus. The cage was constructed based on icosahedral symmetry by genetically fusing a trimeric protein (TriEst) to a small pentameric de novo-designed coiled coil domain, separated by a flexible oligo-glycine linker sequence. Screening a small library of designs in which the linker length varied from 2 to 12 residues identified a construct containing 8 glycine residues (Ico8) that formed well-defined cages. Characterization by dynamic light scattering, negative stain, and cryo-EM and by atomic force and IR-photoinduced force microscopy established that Ico8 assembles into a flexible hollow cage comprising 20 copies of the esterase trimer, 60 protein subunits in total, with overall icosahedral geometry. Notably, the cages formed by Ico8 proved to be extremely stable toward thermal and chemical denaturation: whereas TriEst was unfolded by heating ( T_m ≈ 75 °C) or denatured by 1.5 M guanidine hydrochloride, the Ico8 cages remained folded even at 120 °C or in 8 M guanidine hydrochloride. The increased stability of the cages is a new property that emerges from the higher-order structure of the protein cage, rather than being intrinsic to the components from which it is constructed.

Photo-Induced Force Microscopy by Using Quartz Tuning-Fork Sensor

We present the photo-induced force microscopy (PiFM) studies of various nano-materials by implementing a quartz tuning fork (QTF), a self-sensing sensor that does not require complex optics to detect the motion of a force probe and thus helps to compactly configure the nanoscale optical mapping tool. The bimodal atomic force microscopy technique combined with a sideband coupling scheme is exploited for the high-sensitivity imaging of the QTF-PiFM. We measured the photo-induced force images of nano-clusters of Silicon 2,3-naphthalocyanine bis dye and thin graphene film and found that the QTF-PiFM is capable of high-spatial-resolution nano-optical imaging with a good signal-to-noise ratio. Applying the QTF-PiFM to various experimental conditions will open new opportunities for the spectroscopic visualization and substructure characterization of a vast variety of nano-materials from semiconducting devices to polymer thin films to sensitive measurements of single molecules.

Formation Mechanism of Skin-Core Chemical Structure within Stabilized Polyacrylonitrile Monofilaments

Although it has been half a century since polyacrylonitrile (PAN)-based carbon fibers were first developed, the exact formation mechanism of skin-core structure of PAN-based carbon fibers, especially the stabilized PAN fibers, was still not well clarified from the viewpoint of the chemical structure. In order to address this aforementioned challenge, a powerful tool with nanoscale resolution named photo-induced force microscopy was applied to map the chemical group distribution in the cross section of stabilized PAN fibers and reveal the evolution mechanism of skin-core structure throughout the whole stabilization process. The results indicated that the formation of skin-core structure of stabilized PAN fiber was attributed to the complex and overlapped chemical reactions caused by gradient of oxygen along radial direction and the formation of dense crystal layer at the interface between the skin and core part. Finally, the crystal layer was destroyed and the monofilaments tended to be homogeneous with further oxidation.