Nanoscale chemical imaging by photoinduced force microscopy

Detecting stimulated Raman responses of molecules in plasmonic gap using photon induced forces

We demonstrate the stimulated Raman nanoscopy of a small number of molecules in a plasmonic gap, excited without resonant electronic enhancement, measured using near-field photon-induced forces, eliminating the need for far-field optical detection. We imaged 30 nm diameter gold nanoparticles functionalized with a self-assembled monolayer (SAM) of 4-nitrobenzenethiol (4-NBT) molecules. The maximum number of molecules detected by the gold-coated nano-probe at the position of maximum field enhancement could be fewer than about 42 molecules. The molecules were imaged by vibrating an Atomic Force Microscope (AFM) cantilever on its second flexural eigenmode enabling the tip to be controlled much closer to the sample, thereby improving the detected signal-to-noise ratio when compared to vibrating the cantilever on its first flexural eigenmode. We also demonstrate the implementation of stimulated Raman nanoscopy measured using photon-induced force with non-collinear pump and stimulating beams which could have applications in polarization dependent Raman nanoscopy and spectroscopy and pump-probe nano-spectroscopy particularly involving infrared beam/s. We also discuss using photon induced forces as a technique to sort and select best performing metal coated tips for further use in tip-enhanced experiments.

Substructure imaging of heterogeneous nanomaterials with enhanced refractive index contrast by using a functionalized tip in photoinduced force microscopy

The opto-mechanical force response from light-illuminated nanoscale materials has been exploited in many tip-based imaging applications to characterize various heterogeneous nanostructures. Such a force can have two origins: thermal expansion and induced dipoles. The thermal expansion reflects the absorption of the material, which enables one to chemically characterize a material at the absorption resonance. The induced dipole interaction reflects the local refractive indices of the material underneath the tip, which is useful to characterize a material in the spectral region where no absorption resonance occurs, as in the infrared (IR)-inactive region. Unfortunately, the dipole force is relatively small, and the contrast is rarely discernible for most organic materials and biomaterials, which only show a small difference in refractive indices for their components. In this letter, we demonstrate that refractive index contrast can be greatly enhanced with the assistance of a functionalized tip. With the enhanced contrast, we can visualize the substructure of heterogeneous biomaterials, such as a polyacrylonitrile-nanocrystalline cellulose (PAN-NCC) nanofiber. From substructural visualization, we address the issue of the tensile strength of PAN-NCC fibers fabricated by several different mixing methods. Our understanding from the present study will open up a new opportunity to provide enhanced sensitivity for substructure mapping of nanobiomaterials, as well as local field mapping of photonic devices, such as surface polaritons on semiconductors, metals and van der Waals materials.

Near-field nanoprobing using Si tip-Au nanoparticle photoinduced force microscopy with 120:1 signal-to-noise ratio, sub-6-nm resolution

We propose using a Si tip-Au nanoparticle (NP) combination system in photoinduced force microscopy (PiFM) to fundamentally improve its accuracy in the nanoscale characterization of light-matter interaction. Compared to conventional PiFM with Au-coated tips, such Si tip and Au NP combination enables superior photo-induced force detection while overcoming the tip-induced anisotropy by Au-coating. We map the near-field distribution of Au NPs in different arrangements achieving 120 signal-to-noise ratio and sub-6-nm resolution, even surpassing the tip-curvature limitation; we also map the azimuthally polarized beam profile showing an excellent symmetry. The proposed approach is essential to the promising single molecule spectroscopy.

Photoinduced Tip-Sample Forces for Chemical Nanoimaging and Spectroscopy

Control of photoinduced forces allows nanoparticle manipulation, atom trapping, and fundamental studies of light-matter interactions. Scanning probe microscopy enables the local detection of photoinduced effects with nano-optical imaging and spectroscopy modalities being used for chemical analysis and the study of physical effects. Recently, the development of a novel scanning probe technique has been reported with local chemical sensitivity attributed to the localization and detection of the optical gradient force between a probe tip and sample surface via infrared vibrationally resonant coupling. However, the magnitude and spectral line shape of the observed signals disagree with theoretical predictions of optical gradient forces. Here, we clarify this controversy by resolving and analyzing the interplay of several photoinduced effects between scanning probe tips and infrared resonant materials through spectral and spatial force measurements. Force spectra obtained on IR-active vibrational modes of polymer thin films are symmetric and match the material absorption spectra in contrast to the dispersive spectral line shape expected for the optical gradient force response. Sample thickness dependence shows continuous increase in force signal beyond the thickness where the optical dipole force would saturate. Our results illustrate that photoinduced force interactions between scanning probe tips and infrared-resonant materials are dominated by short-range thermal expansion and possibly long-range thermally induced photoacoustic effects. At the same time, we provide a guideline to detect and discriminate optical gradient forces from other photoinduced effects, which opens a new perspective for the development of new scanning probe modalities exploiting ultrastrong opto-mechanical coupling effects in tip-sample cavities.

Tip-Enhanced Thermal Expansion Force for Nanoscale Chemical Imaging and Spectroscopy in Photoinduced Force Microscopy

We investigate the tip-enhanced thermal expansion force for nanoscale chemical imaging and spectroscopy in the tip-sample junction. It is found, both theoretically and experimentally, that the tip-enhanced absorption of the near-field at the tip followed by sample expansion shows characteristic behaviors with respect to the sample thickness and the incident laser pulse width. The van der Waals interaction plays a major role in exerting a force on the tip from the thermally expanded sample. The force behavior of the photoinduced force microscope (PiFM) is compared with that of the existing photothermal-induced resonance technique (PTIR) to unravel the ambiguous thermal expansion force mechanism. The present study opens up new opportunities for enhancing the performance of optical nanoscopy and spectroscopy such as chemical imaging of nanobiomaterials and the local field mapping of photonic devices, including surface polaritons on van der Waals materials with the assistance of the thermal expansion of a functionalized tip.

Selective excitation and imaging of ultraslow phonon polaritons in thin hexagonal boron nitride crystals

We selectively excite and study two new types of phonon-polariton guided modes that are found in hexagonal boron nitride thin flakes on a gold substrate. Such modes show substantially improved confinement and a group velocity that is hundreds of times slower than the speed of light, thereby providing a new way to create slow light in the mid-infrared range with a simple structure that does not require nano-patterning. One mode is the fundamental mode in the first Restrahlen band of hexagonal boron nitride thin crystals on a gold substrate; the other mode is equivalent to the second mode of the second Restrahlen band of hexagonal boron nitride flakes that are suspended in vacuum. The new modes also couple efficiently with incident light at the hexagonal boron nitride edges, as we demonstrate experimentally using photo-induced force microscopy and scanning near-field optical microscopy. The high confinement of these modes allows for Purcell factors that are on the order of tens of thousands directly above boron nitride and a wide band, with new perspectives for enhanced light-matter interaction. Our findings demonstrate a new approach to engineering the dispersion of polaritons in 2D materials to improve confinement and light-matter interaction, thereby paving the way for new applications in mid-infrared nano-optics.

Identifying Nanoscale Structure-Function Relationships Using Multimodal Atomic Force Microscopy, Dimensionality Reduction, and Regression Techniques

Correlating nanoscale chemical specificity with operational physics is a long-standing goal of functional scanning probe microscopy (SPM). We employ a data analytic approach combining multiple microscopy modes using compositional information in infrared vibrational excitation maps acquired via photoinduced force microscopy (PiFM) with electrical information from conductive atomic force microscopy. We study a model polymer blend comprising insulating poly(methyl methacrylate) (PMMA) and semiconducting poly(3-hexylthiophene) (P3HT). We show that PiFM spectra are different from FTIR spectra but can still be used to identify local composition. We use principal component analysis to extract statistically significant principal components and principal component regression to predict local current and identify local polymer composition. In doing so, we observe evidence of semiconducting P3HT within PMMA aggregates. These methods are generalizable to correlated SPM data and provide a meaningful technique for extracting complex compositional information that is impossible to measure from any one technique.

Ultra-confined mid-infrared resonant phonon polaritons in van der Waals nanostructures

Hexagonal boron nitride has been proposed as an excellent candidate to achieve subwavelength infrared light manipulation owing to its polar lattice structure, enabling excitation of low-loss phonon polaritons with hyperbolic dispersion. We show that strongly subwavelength hexagonal boron nitride planar nanostructures can exhibit ultra-confined resonances and local field enhancement. We investigate strong light-matter interaction in these nanoscale structures via photo-induced force microscopy, scattering-type scanning near-field optical microscopy, and Fourier transform infrared spectroscopy, with excellent agreement with numerical simulations. We design optical nano-dipole antennas and directly image the fields when bright- or dark-mode resonances are excited. These modes are deep subwavelength, and strikingly, they can be supported by arbitrarily small structures. We believe that phonon polaritons in hexagonal boron nitride can play for infrared light a role similar to that of plasmons in noble metals at visible frequency, paving the way for a new class of efficient and highly miniaturized nanophotonic devices.

Time-Resolved In Situ Liquid-Phase Atomic Force Microscopy and Infrared Nanospectroscopy during the Formation of Metal-Organic Framework Thin Films

Metal-organic framework (MOF) thin films show unmatched promise as smart membranes and photocatalytic coatings. However, their nucleation and growth resulting from intricate molecular assembly processes are not well understood yet are crucial to control the thin film properties. Here, we directly observe the nucleation and growth behavior of HKUST-1 thin films by real-time in situ AFM at different temperatures in a Cu-BTC solution. In combination with ex situ infrared (nano)spectroscopy, synthesis at 25 °C reveals initial nucleation of rapidly growing HKUST-1 islands surrounded by a continuously nucleating but slowly growing HKUST-1 carpet. Monitoring at 13 and 50 °C shows the strong impact of temperature on thin film formation, resulting in (partial) nucleation and growth inhibition. The nucleation and growth mechanisms as well as their kinetics provide insights to aid in future rational design of MOF thin films.

Surface-Enhanced Raman Spectroscopy for Bioanalysis: Reliability and Challenges

Surface-enhanced Raman spectroscopy (SERS) inherits the rich chemical fingerprint information on Raman spectroscopy and gains sensitivity by plasmon-enhanced excitation and scattering. In particular, most Raman peaks have a narrow width suitable for multiplex analysis, and the measurements can be conveniently made under ambient and aqueous conditions. These merits make SERS a very promising technique for studying complex biological systems, and SERS has attracted increasing interest in biorelated analysis. However, there are still great challenges that need to be addressed until it can be widely accepted by the biorelated communities, answer interesting biological questions, and solve fatal clinical problems. SERS applications in bioanalysis involve the complex interactions of plasmonic nanomaterials with biological systems and their environments. The reliability becomes the key issue of bioanalytical SERS in order to extract meaningful information from SERS data. This review provides a comprehensive overview of bioanalytical SERS with the main focus on the reliability issue. We first introduce the mechanism of SERS to guide the design of reliable SERS experiments with high detection sensitivity. We then introduce the current understanding of the interaction of nanomaterials with biological systems, mainly living cells, to guide the design of functionalized SERS nanoparticles for target detection. We further introduce the current status of label-free (direct) and labeled (indirect) SERS detections, for systems from biomolecules, to pathogens, to living cells, and we discuss the potential interferences from experimental design, measurement conditions, and data analysis. In the end, we give an outlook of the key challenges in bioanalytical SERS, including reproducibility, sensitivity, and spatial and time resolution.

Wavelength-Dependent Optical Force Imaging of Bimetallic Al-Au Heterodimers

Many important applications of nanometer-scale metallic complexes arise from the light-induced, near-field interactions between their component structures. Here we examine the near-field interactions in bimetallic Al-Au plasmonic nanodisk heterodimers, where the coupling between the primitive plasmons of nanostructures composed of two different metals is studied. Understanding the correlations between nanoparticle morphology and near-field optical properties, particularly for nanostructures composed of two different metals, requires spectrally resolved near-field spatial information. An ideal tool for such investigations is the recently developed photoinduced force microscopy, where the electromagnetic forces between an optically excited plasmonic nanostructure and an adjacent scanning nanoscale tip are measured. Using this approach, we visualize the wavelength-dependent near-field interactions in these bimetallic heterodimers. This system provides a prime example of the diabatic, antenna-reactor picture of plasmon coupling where for a given wavelength the more resonant primitive "driving" plasmon induces a response, the "forced" plasmon, in the off-resonant component. We critically examine spectrally resolved tip-nanostructure forces, comparing experiment with theory, for tips and nanoscale structures of realistic dimensions relative to frequently used approximations for tip geometries. The contrasting effects of dielectric versus metallic tips on acquired spectral force profiles are also examined.

Nanoscale Domain Imaging of All-Polymer Organic Solar Cells by Photo-Induced Force Microscopy

Rapid nanoscale imaging of the bulk heterojunction layer in organic solar cells is essential to the continued development of high-performance devices. Unfortunately, commonly used imaging techniques such as tunneling electron microscopy (TEM) and atomic force microscopy (AFM) suffer from significant drawbacks. For instance, assuming domain identity from phase contrast or topographical features can lead to inaccurate morphological conclusions. Here we demonstrate a technique known as photo-induced force microscopy (PiFM) for imaging organic solar cell bulk heterojunctions with nanoscale chemical specificity. PiFM is a relatively recent scanning probe microscopy technique that combines an AFM tip with a tunable infrared laser to induce a dipole for chemical imaging. Coupling the nanometer resolution of AFM with the chemical specificity of a tuned IR laser, we are able to spatially map the donor and acceptor domains in a model all-polymer bulk heterojunction with resolution approaching 10 nm. Domain size from PiFM images is compared to bulk-averaged results from resonant soft X-ray scattering, indicating excellent quantitative agreement. Further, we demonstrate that in our all-polymer system, the AFM topography, AFM phase, and PiFM show poor correlation, highlighting the need to move beyond standard AFM for morphology characterization of bulk heterojunctions.

Mechanistic Insights into Growth of Surface-Mounted Metal-Organic Framework Films Resolved by Infrared (Nano-) Spectroscopy

Control over assembly, orientation, and defect-free growth of metal-organic framework (MOF) films is crucial for their future applications. A layer-by-layer approach is considered a suitable method to synthesize highly oriented films of numerous MOF topologies, but the initial stages of the film growth remain poorly understood. Here we use a combination of infrared (IR) reflection absorption spectroscopy and atomic force microscopy (AFM)-IR imaging to investigate the assembly and growth of a surface mounted MOF (SURMOF) film, specifically HKUST-1. IR spectra of the films were measured with monolayer sensitivity and <10 nm spatial resolution. In contrast to the common knowledge of LbL SURMOF synthesis, we find evidence for the surface-hindered growth and large presence of copper acetate precursor species in the produced MOF thin-films. The growth proceeds via a solution-mediated mechanism where the presence of weakly adsorbed copper acetate species leads to the formation of crystalline agglomerates with a size that largely exceeds theoretical growth limits. We report the spectroscopic characterization of physisorbed copper acetate surface species and find evidence for the large presence of unexchanged and mixed copper-paddle-wheels. Based on these insights, we were able to optimize and automatize synthesis methods and produce (100) oriented HKUST-1 thin-films with significantly shorter synthesis times, and additionally use copper nitrate as an effective synthesis precursor.

Quantitative Chemical Analysis at the Nanoscale Using the Photothermal Induced Resonance Technique

Photothermal induced resonance (PTIR), also known as AFM-IR, is a scanning probe technique that provides sample composition information with a lateral resolution down to 20 nm. Interest in PTIR stems from its ability to identify unknown samples at the nanoscale thanks, in first approximation, to the direct comparability of PTIR spectra with far-field infrared databases. The development of rapidly tuning quantum cascade lasers has increased the PTIR throughput considerably, making nanoscale hyperspectral imaging within a reasonable time frame possible. Consequently, a better understanding of PTIR signal generation and of the fine details of PTIR analysis has become of paramount importance for extending complex IR analysis methods developed in the far-field, e.g., for classification and hyperspectral imaging, to nanoscale PTIR spectra. Here we calculate PTIR spectra via thin-film optics, to identify subtle changes (band shifts, deviation from linear approximation, etc.) for common sample parameters in the case of PTIR with total internal reflection illumination. Results show signal intensity linearity and small band shifts as long as the sample is prepared correctly, with band shifts typically smaller than macroscale attenuated total reflection (ATR) spectroscopy. Finally, a generally applicable algorithm to retrieve the pure imaginary component of the refractive index (i.e., the chemically specific information) is provided to overcome the PTIR spectra nonlinearity.

Correlative imaging of biological tissues with apertureless scanning near-field optical microscopy and confocal laser scanning microscopy

Apertureless scanning near-field optical microscopy (ASNOM) has attracted considerable interest over the past years as a result of its valuable contrast mechanisms and capabilities for optical resolutions in the nanoscale range. However, at this moment the intersections between ASNOM and the realm of bioimaging are scarce, mainly due to data interpretation difficulties linked to the limited body of work performed so far in this field and hence the reduced volume of supporting information. We propose an imaging approach that holds significant potential for alleviating this issue, consisting of correlative imaging of biological specimens using a multimodal system that incorporates ASNOM and confocal laser scanning microscopy (CLSM), which allows placing near-field data into a well understood context of anatomical relevance. We demonstrate this approach on zebrafish retinal tissue. The proposed method holds important implications for the in-depth understanding of biological items through the prism of ASNOM and CLSM data complementarity.

Macroscopic Alignment and Assembly of π-Conjugated Oligopeptides Using Colloidal Microchannels

One-dimensional (1-D) supramolecular self-assembly offers a powerful strategy to achieve long-range unidirectional ordering of organic semiconducting materials via noncovalent interactions. Using a hierarchical assembly, electronic and optoelectronic materials can be constructed for applications including organic conducting nanowires, organic field-effect transistors (OFETs), and organic light-emitting devices (OLEDs). Despite recent progress, it remains challenging to precisely align and assemble 1-D structures over large areas in a rapid and straightforward manner. In this work, we demonstrate a facile strategy to macroscopically align supramolecular fibers using a templating method based on sacrificial colloidal microchannels. Through use of this approach, colloidal microchannels are generated on a solid surface using a simple fabrication method, followed by the spontaneous self-assembly of π-conjugated oligopeptides inside large arrays of microchannels triggered by solvent evaporation. Following oligopeptide assembly and removal of sacrificial microchannels, the structural properties of oligopeptide fibers were characterized using atomic force microscopy (AFM), atomic force microscope-infrared spectroscopy (AFM-IR), photoinduced force microscopy (PiFM), fluorescence polarization microscopy, and electron microscopy. These results reveal the macroscopic alignment of oligopeptide fibers into ordered structures over millimeter length scales, facilitated by colloidal microchannel templating. In addition, the charge transport properties (I-V curves) of π-conjugated oligopeptides assembled using this method were determined under a wide range of applied voltages using interdigitated array electrodes and conductive AFM. Overall, this work illustrates a simple yet robust strategy to pattern 1-D supramolecular fibers over large areas, thereby offering new routes for assembling materials for organic electronics.

Nanoscopic control and quantification of enantioselective optical forces

Circularly polarized light (CPL) exerts a force of different magnitude on left- and right-handed enantiomers, an effect that could be exploited for chiral resolution of chemical compounds as well as controlled assembly of chiral nanostructures. However, enantioselective optical forces are challenging to control and quantify because their magnitude is extremely small (sub-piconewton) and varies in space with sub-micrometre resolution. Here, we report a technique to both strengthen and visualize these forces, using a chiral atomic force microscope probe coupled to a plasmonic optical tweezer. Illumination of the plasmonic tweezer with CPL exerts a force on the microscope tip that depends on the handedness of the light and the tip. In particular, for a left-handed chiral tip, transverse forces are attractive with left-CPL and repulsive with right-CPL. Additionally, total force differences between opposite-handed specimens exceed 10 pN. The microscope tip can map chiral forces with 2 nm lateral resolution, revealing a distinct spatial distribution of forces for each handedness.

Controlling Molecular Doping in Organic Semiconductors

The field of organic electronics thrives on the hope of enabling low-cost, solution-processed electronic devices with mechanical, optoelectronic, and chemical properties not available from inorganic semiconductors. A key to the success of these aspirations is the ability to controllably dope organic semiconductors with high spatial resolution. Here, recent progress in molecular doping of organic semiconductors is summarized, with an emphasis on solution-processed p-type doped polymeric semiconductors. Highlighted topics include how solution-processing techniques can control the distribution, diffusion, and density of dopants within the organic semiconductor, and, in turn, affect the electronic properties of the material. Research in these areas has recently intensified, thanks to advances in chemical synthesis, improved understanding of charged states in organic materials, and a focus on relating fabrication techniques to morphology. Significant disorder in these systems, along with complex interactions between doping and film morphology, is often responsible for charge trapping and low doping efficiency. However, the strong coupling between doping, solubility, and morphology can be harnessed to control crystallinity, create doping gradients, and pattern polymers. These breakthroughs suggest a role for molecular doping not only in device function but also in fabrication-applications beyond those directly analogous to inorganic doping.

Side Chain Engineering on Medium Bandgap Copolymers to Suppress Triplet Formation for High-Efficiency Polymer Solar Cells

Suppression of carrier recombination is critically important in realizing high-efficiency polymer solar cells. Herein, it is demonstrated difluoro-substitution of thiophene conjugated side chain on donor polymer can suppress triplet formation for reducing carrier recombination. A new medium bandgap 2D-conjugated D-A copolymer J91 is designed and synthesized with bi(alkyl-difluorothienyl)-benzodithiophene as donor unit and fluorobenzotriazole as acceptor unit, for taking the advantages of the synergistic fluorination on the backbone and thiophene side chain. J91 demonstrates enhanced absorption, low-lying highest occupied molecular orbital energy level, and higher hole mobility, in comparison with its control polymer J52 without fluorination on the thiophene side chains. The transient absorption spectra indicate that J91 can suppress the triplet formation in its blend film with n-type organic semiconductor acceptor m-ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(3-hexylphenyl)-dithieno[2,3-d:2,3'-d']-s-indaceno[1,2-b:5,6-b']-dithiophene). With these favorable properties, a higher power conversion efficiency of 11.63% with high V_OC of 0.984 V and high J_SC of 18.03 mA cm^-2 is obtained for the polymer solar cells based on J91/m-ITIC with thermal annealing. The improved photovoltaic performance by thermal annealing is explained from the morphology change upon thermal annealing as revealed by photoinduced force microscopy. The results indicate that side chain engineering can provide a new solution to suppress carrier recombination toward high efficiency, thus deserves further attention.

Mechanical Detection and Imaging of Hyperbolic Phonon Polaritons in Hexagonal Boron Nitride

Mid-infrared nanoimaging and spectroscopy of two-dimensional (2D) materials have been limited so far to scattering-type scanning near-field optical microscopy (s-SNOM) experiments, where light from the sample is scattered by a metallic-coated atomic force microscope (AFM) tip interacting with the material at the nanoscale. These experiments have recently allowed imaging of plasmon polaritons in graphene as well as hyperbolic phonon polaritons in hexagonal boron nitride (hBN). Here we show that the high mechanical sensitivity of an AFM cantilever can be exploited for imaging hyperbolic phonon polaritons in hBN. In our imaging process, the lattice vibrations of hBN micrometer-sized flakes are locally enhanced by the launched phonon polaritons. These enhanced vibrations are coupled to the AFM tip in contact with the sample surface and recorded during scanning. Imaging resolution of Δ/20 is shown (Δ being the polaritonic fringes' separation distance), comparable to the best resolution in s-SNOM. Importantly, this detection mechanism is free from light background, and it is in fact the first photonless detection of phonon polaritons.

Nanoscale simultaneous chemical and mechanical imaging via peak force infrared microscopy

Nondestructive chemical and mechanical measurements of materials with ~10-nm spatial resolution together with topography provide rich information on the compositions and organizations of heterogeneous materials and nanoscale objects. However, multimodal nanoscale correlations are difficult to achieve because of the limitation on spatial resolution of optical microscopy and constraints from instrumental complexities. We report a novel noninvasive spectroscopic scanning probe microscopy method-peak force infrared (PFIR) microscopy-that allows chemical imaging, collection of broadband infrared spectra, and mechanical mapping at a spatial resolution of 10 nm. In our technique, chemical absorption information is directly encoded in the withdraw curve of the peak force tapping cycle after illumination with synchronized infrared laser pulses in a simple apparatus. Nanoscale phase separation in block copolymers and inhomogeneity in CH₃NH₃PbBr₃ perovskite crystals are studied with correlative infrared/mechanical nanoimaging. Furthermore, we show that the PFIR method is sensitive to the presence of surface phonon polaritons in boron nitride nanotubes. PFIR microscopy will provide a powerful analytical tool for explorations at the nanoscale across wide disciplines.

Hyperspectral infrared nanoimaging of organic samples based on Fourier transform infrared nanospectroscopy

Infrared nanospectroscopy enables novel possibilities for chemical and structural analysis of nanocomposites, biomaterials or optoelectronic devices. Here we introduce hyperspectral infrared nanoimaging based on Fourier transform infrared nanospectroscopy with a tunable bandwidth-limited laser continuum. We describe the technical implementations and present hyperspectral infrared near-field images of about 5,000 pixel, each one covering the spectral range from 1,000 to 1,900 cm⁻¹. To verify the technique and to demonstrate its application potential, we imaged a three-component polymer blend and a melanin granule in a human hair cross-section, and demonstrate that multivariate data analysis can be applied for extracting spatially resolved chemical information. Particularly, we demonstrate that distribution and chemical interaction between the polymer components can be mapped with a spatial resolution of about 30 nm. We foresee wide application potential of hyperspectral infrared nanoimaging for valuable chemical materials characterization and quality control in various fields ranging from materials sciences to biomedicine.

In hyperspectral imaging a broadband spectrum is recorded at each pixel, which creates information-rich images. Here, the authors combine this concept with Fourier transform infrared nanospectroscopy to achieve 5,000-pixel, nanoscale-resolution images at wavelengths between 5 and 10 micrometres.

Direct-Write Optical Patterning of P3HT Films Beyond the Diffraction Limit

Doping-induced solubility control is a patterning technique for semiconducting polymers, which utilizes the reduction in polymer solubility upon p-type doping to provide direct, optical control of film topography and doping level. In situ direct-write patterning and imaging are demonstrated, revealing sub-diffraction-limited topographic features. Photoinduced force microscopy shows that doping level can be optically modulated with similar resolution.

Recent advances in hybrid measurement methods based on atomic force microscopy and surface sensitive measurement techniques

This review summaries the recent progress of the combination of optical and non-optical surface sensitive techniques with the atomic force microscopy.

Nanoscale infrared imaging of zeolites using photoinduced force microscopy

Photoinduced force microscopy (PiFM) can probe the structural and catalytic information of functional porous materials at nanometer scale resolution.

Photoinduced Force Mapping of Plasmonic Nanostructures

The ability to image the optical near-fields of nanoscale structures, map their morphology, and concurrently obtain spectroscopic information, all with high spatiotemporal resolution, is a highly sought-after technique in nanophotonics. As a step toward this goal, we demonstrate the mapping of electromagnetic forces between a nanoscale tip and an optically excited sample consisting of plasmonic nanostructures with an imaging platform based on atomic force microscopy. We present the first detailed joint experimental-theoretical study of this type of photoinduced force microscopy. We show that the enhancement of near-field optical forces in gold disk dimers and nanorods follows the expected plasmonic field enhancements with strong polarization sensitivity. We then introduce a new way to evaluate optically induced tip-sample forces by simulating realistic geometries of the tip and sample. We decompose the calculated forces into in-plane and out-of-plane components and compare the calculated and measured force enhancements in the fabricated plasmonic structures. Finally, we show the usefulness of photoinduced force mapping for characterizing the heterogeneity of near-field enhancements in precisely e-beam fabricated nominally alike nanostructures - a capability of widespread interest for precise nanomanufacturing, SERS, and photocatalysis applications.

Octave-spanning mid-infrared pulses by plasma generation in air pumped with an Yb:KGW source

Femtosecond mid-infrared (IR) supercontinuum generation in gas media provides a broadband source suited for time-domain spectroscopies and microscopies. This technology has largely utilized <100  fs Ti:sapphire pump lasers. In this Letter, we describe the first plasma generation mid-IR source based on a 1030 nm, 171 fs Yb:KGW laser system; when its first three harmonics are focused in air, a conical mode supercontinuum is generated that spans <1000 to 2700  cm^-1 with a 190 pJ pulse energy and 0.5% RMS stability.