Supplementary document for Origins and consequences of asymmetric nano-FTIR interferograms - 6933702.pdf

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Origins and consequences of asymmetric nano-FTIR interferograms

Infrared spectroscopy is essential in understanding the optical properties of materials and is a critical analytical technique for identifying and determining the properties of materials. One of its major shortcomings, the relatively low spatial resolution, was overcome by combining atomic force microscopes and infrared light sources in near-field IR microscopes. Scattering-type near-field optical microscopy, s-SNOM, has been a flagship technique in the infrared community for its ability to provide information on optical properties well below the far-field diffraction limit. When combined with broad-band light sources, the technique gives rise to the nano-FTIR technique, which enables IR spectroscopy at the nanoscale. While s-SNOM and nano-FTIR data analysis are understood, fundamental discussion of experimental data, especially in the context of previously developed frameworks is lacking. Here, we unambiguously describe the origins of asymmetric interferograms recorded with s-SNOM instruments, give detailed analysis of potential artifacts and recommendations on optimal instrument settings as well as data analysis parameters

Thermal Conductivity Measurements of Semitransparent Single-Walled Carbon Nanotube Films by a Bolometric Technique

We introduce a new technique for measurement of the thermal conductivity of ultrathin films of single-walled carbon nanotubes (SWNTs) utilizing IR radiation as heat source and the SWNT film as thermometer. The technique is applied to study the temperature dependence of the thermal conductivity of an as-prepared SWNT film obtained in the electric arc discharge process and a film of purified SWNTs prepared by vacuum filtration. The interplay between thermal and electrical transport in SWNT networks is analyzed in relation to the type of intertube junctions and the possibility of optimizing the thermal and electrical properties of SWNT networks for specific applications is discussed

Step-Scan IR Spectroelectrochemistry with Ultramicroelectrodes: Nonsurface Enhanced Detection of Near Femtomole Quantities Using Synchrotron Radiation

The result of interfacing step-scan spectroelectrochemistry with an IR microscope and synchrotron infrared (SIR) radiation is provided here. An external reflectance cell containing a 25 μm gold ultramicroelectrode is employed to achieve an electrochemical time constant less than one microsecond. The use of a prototypical electrochemical system, i.e., the mass-transport controlled reduction of ferricyanide, allows for a proof of principle evaluation of the viability of SIR for step-scan spectroelectrochemistry. An analysis of the importance of accounting for synchrotron source variation over the prolonged duration of a step-scan experiment is provided. Modeling of the material flux in the restricted diffusion space afforded by the external reflectance cell allows the quantitative IR results to be compared to theoretical predictions. The results indicate that only at very short times does linear diffusion within the cavity dominate the electrode response and the majority of the transient signal operates under conditions of quasi-hemispherical diffusion. The analytical information provided by the IR signal is found to be considerably less than that derived from the current response due the latter’s pronounced edge effects. The results provide a detection limit of 36 fmol for step-scan SIR measurements of ferrocyanide. Implications for future IR spectroelectrochemical studies in the microsecond domain are discussed

Super-Resolved 3D Mapping of Molecular Orientation Using Vibrational Techniques

When a sample has an anisotropic structure, it is possible to obtain additional information controlling the polarization of incident light. With their straightforward instrumentation approaches, infrared (IR) and Raman spectroscopies are widely popular in this area. Single-band-based determination of molecular in-plane orientation, typically used in materials science, is here extended by the concurrent use of two vibration bands, revealing the orientational ordering in three dimension. The concurrent analysis was applied to IR spectromicroscopic data to obtain orientation angles of a model polycaprolactone spherulite sample. The applicability of this method spans from high-resolution, diffraction-limited Fourier transform infrared (FT-IR) and Raman imaging to super-resolved optical photothermal infrared (O-PTIR) imaging. Due to the nontomographic experimental approach, no image distortion is visible and nanometer scale orientation domains can be observed. Three-dimensional (3D) bond orientation maps enable in-depth characterization and consequently precise control of the sample’s physicochemical properties and functions

Step-Scan IR Spectroelectrochemistry with Ultramicroelectrodes: Nonsurface Enhanced Detection of Near Femtomole Quantities Using Synchrotron Radiation

The result of interfacing step-scan spectroelectrochemistry with an IR microscope and synchrotron infrared (SIR) radiation is provided here. An external reflectance cell containing a 25 μm gold ultramicroelectrode is employed to achieve an electrochemical time constant less than one microsecond. The use of a prototypical electrochemical system, i.e., the mass-transport controlled reduction of ferricyanide, allows for a proof of principle evaluation of the viability of SIR for step-scan spectroelectrochemistry. An analysis of the importance of accounting for synchrotron source variation over the prolonged duration of a step-scan experiment is provided. Modeling of the material flux in the restricted diffusion space afforded by the external reflectance cell allows the quantitative IR results to be compared to theoretical predictions. The results indicate that only at very short times does linear diffusion within the cavity dominate the electrode response and the majority of the transient signal operates under conditions of quasi-hemispherical diffusion. The analytical information provided by the IR signal is found to be considerably less than that derived from the current response due the latter’s pronounced edge effects. The results provide a detection limit of 36 fmol for step-scan SIR measurements of ferrocyanide. Implications for future IR spectroelectrochemical studies in the microsecond domain are discussed

Direct Visualization of Ultrastrong Coupling between Luttinger-Liquid Plasmons and Phonon Polaritons

Ultrastrong coupling of light and matter creates new opportunities to modify chemical reactions or develop novel nanoscale devices. One-dimensional Luttinger-liquid plasmons in metallic carbon nanotubes are long-lived excitations with extreme electromagnetic field confinement. They are promising candidates to realize strong or even ultrastrong coupling at infrared frequencies. We applied near-field polariton interferometry to examine the interaction between propagating Luttinger-liquid plasmons in individual carbon nanotubes and surface phonon polaritons of silica and hexagonal boron nitride. We extracted the dispersion relation of the hybrid Luttinger-liquid plasmon–phonon polaritons (LPPhPs) and explained the observed phenomena by the coupled harmonic oscillator model. The dispersion shows pronounced mode splitting, and the obtained value for the normalized coupling strength shows we reached the ultrastrong coupling regime with both native silica and hBN phonons. Our findings predict future applications to exploit the extraordinary properties of carbon nanotube plasmons, ranging from nanoscale plasmonic circuits to ultrasensitive molecular sensing

Large-Area, Freestanding, Single-Layer Graphene–Gold: A Hybrid Plasmonic Nanostructure

Graphene-based plasmonic devices have recently drawn great attention. However, practical limitations in fabrication and device architectures prevent studies from being carried out on the intrinsic properties of graphene and their change by plasmonic structures. The influence of a quasi-infinite object (<i>i</i>.<i>e</i>., the substrate) on graphene, being a single sheet of carbon atoms, and the plasmonic device is overwhelming. To address this and put the intrinsic properties of the graphene–plasmonic nanostructures in focus, we fabricate large-area, freestanding, single-layer graphene–gold (LFG-Au) sandwich structures and Au nanoparticle decorated graphene (formed <i>via</i> thermal treatment) hybrid plasmonic nanostructures. We observed two distinct plasmonic enhancement routes of graphene unique to each structure <i>via</i> surface-enhanced Raman spectroscopy. The localized electronic structure variation in the LFG due to graphene–Au interaction at the nanoscale is mapped using scanning transmission X-ray microscopy. The measurements show an optical density of ∼0.007, which is the smallest experimentally determined for single-layer graphene thus far. Our results on freestanding graphene–Au plasmonic structures provide great insight for the rational design and future fabrication of graphene plasmonic hybrid nanostructures

Enhanced Stability of the Metal–Organic Framework MIL-101(Cr) by Embedding Pd Nanoparticles for Densification through Compression

Metal–organic frameworks (MOFs) are ideal platforms for new and original functionalization, as the confinement of metallic nanoparticles (NPs) within their pores. However, the insertion of NPs could impact the framework’s mechanical stability, thus affecting their performances in applications. Indeed, MOFs are usually loose powders that need to be compressed to increase the volumetric density before being employed as gas adsorbers. Here, we investigate the high-pressure behavior of the mesoporous MOF MIL-101 loaded with Pd NPs (20, 35 wt %) by synchrotron X-ray diffraction and infrared spectroscopy. The control of the metal content allows us to demonstrate that Pd NPs enhance the mechanical stability of MIL-101, with the bulk modulus and the crystalline–amorphous transition pressure increasing with the Pd loading. This is attributed to the NP steric hindrance, whereas the presence of host–guest chemical interactions is ruled out by infrared spectroscopy. We also define a spectroscopic quantity highlighting the framework amorphization that can be exploited from now on to characterize these materials when densified. Our results demonstrate that the incorporation of NPs makes MOFs not only more functional but also more mechanically stable and thus suitable for densification

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