Far-field midinfrared superresolution imaging and spectroscopy of single high aspect ratio gold nanowires.

Limited approaches exist for imaging and recording spectra of individual nanostructures in the midinfrared region. Here we use infrared photothermal heterodyne imaging (IR-PHI) to interrogate single, high aspect ratio Au nanowires (NWs). Spectra recorded between 2,800 and 4,000 cm-1 for 2.5-3.9-µm-long NWs reveal a series of resonances due to the Fabry-Pérot modes of the NWs. Crucially, IR-PHI images show structure that reflects the spatial distribution of the NW absorption, and allow the resonances to be assigned to the m = 3 and m = 4 Fabry-Pérot modes. This far-field optical measurement has been used to image the mode structure of plasmon resonances in metal nanostructures, and is made possible by the superresolution capabilities of IR-PHI. The linewidths in the NW spectra range from 35 to 75 meV and, in several cases, are significantly below the limiting values predicted by the bulk Au Drude damping parameter. These linewidths imply long dephasing times, and are attributed to reduction in both radiation damping and resistive heating effects in the NWs. Compared to previous imaging studies of NW Fabry-Pérot modes using electron microscopy or near-field optical scanning techniques, IR-PHI experiments are performed under ambient conditions, enabling detailed studies of how the environment affects mid-IR plasmons.

Using Infrared Photothermal Heterodyne Imaging to Characterize Micro- and Nanoplastics in Complex Environmental Matrices.

A key challenge for addressing micro- and nanoplastics (MNPs) in the environment is being able to characterize their chemical properties, morphologies, and quantities in complex matrices. Current techniques, such as Fourier transform infrared spectroscopy, provide these broad characterizations but are unsuitable for studying MNPs in spectrally congested or complex chemical environments. Here, we introduce a new, super-resolution infrared absorption technique to characterize MNPs, called infrared photothermal heterodyne imaging (IR-PHI). IR-PHI has a spatial resolution of ∼300 nm and can determine the chemical identity, morphology, and quantity of MNPs in a single analysis with high sensitivity. Specimens are supported on CaF2 coverslips under ambient conditions from where we (1) quantify MNPs from nylon tea bags after steeping in ultrapure water at 25 and 95 °C, (2) identify MNP chemical or morphological changes after steeping at 95 °C, and (3) chemically identify MNPs in sieved road dust. In all cases, no special sample preparation was required. MNPs released from nylon tea bags at 25 °C were fiber-like and had characteristic IR frequencies corresponding to thermally extruded nylon. At 95 °C, degradation of the nylon chemical structure was observed via the disappearance of amide group IR frequencies, indicating chain scission of the nylon backbone. This degradation was also observed through morphological changes, where MNPs altered shape from fiber-like to quasi-spherical. In road dust, IR-PHI analysis reveals the presence of numerous aggregate and single-particle (<3 µm) MNPs composed of rubber and nylon.

Deep image restoration for infrared photothermal heterodyne imaging.

Infrared photothermal heterodyne imaging (IR-PHI) is an all-optical table top approach that enables super-resolution mid-infrared microscopy and spectroscopy. The underlying principle behind IR-PHI is the detection of photothermal changes to specimens induced by their absorption of infrared radiation. Because detection of resulting refractive index and scattering cross section changes is done using a visible (probe) laser, IR-PHI exhibits a spatial resolution of ∼300 nm. This is significantly below the mid-infrared diffraction limit and is unlike conventional infrared absorption microscopy where spatial resolution is of order ∼5µm. Despite having achieved mid-infrared super-resolution, IR-PHI's spatial resolution is ultimately limited by the visible probe laser's diffraction limit. This hinders immediate application to studying samples residing in spatially congested environments. To circumvent this, we demonstrate further enhancements to IR-PHI's spatial resolution using a deep learning network that addresses the Abbe diffraction limit as well as background artifacts, introduced by experimental raster scanning. What results is a twofold improvement in feature resolution from 300 to ∼150 nm.

Approaches to mid-infrared, super-resolution imaging and spectroscopy.

This perspective highlights recent advances in super-resolution, mid-infrared imaging and spectroscopy. It provides an overview of the different near field microscopy techniques developed to address the problem of chemically imaging specimens in the mid-infrared "fingerprint" region of the spectrum with high spatial resolution. We focus on a recently developed far-field optical technique, called infrared photothermal heterodyne imaging (IR-PHI), and discusses the technique in detail. Its practical implementation in terms of equipment used, optical geometries employed, and underlying contrast mechanism are described. Milestones where IR-PHI has led to notable advances in bioscience and materials science are summarized. The perspective concludes with a future outlook for robust and readily accessible high spatial resolution, mid-infrared imaging and spectroscopy techniques.

Crystal structure of 1,3-dimethyl-3-phenyl-pyrrolidine-2,5-dione: a clinically used anti-convulsant.

In the title compound, C12H13NO2, the five-membered ring has an envelope conformation; the disubstituted C atom lies out of the mean plane through the four other ring atoms (r.m.s. deviation = 0.0038 Å) by 0.1877 (18) Å. The plane of the phenyl substituent is practically perpendicular to that of the planar part of the five-membered ring, with a dihedral angle of 87.01 (5)°. In the crystal, mol-ecules are linked by weak C-H⋯O hydrogen bonds, forming inversion dimers. The dimers are linked by further C-H⋯O hydrogen bonds, as well as carbon-yl-carbonyl attractive inter-actions [O⋯C = 3.2879 (19) Å], forming a three-dimensional framework structure.

Excitation Energy Dependence of Semiconductor Nanocrystal Emission Quantum Yields.

Accurate measurements of semiconductor nanocrystal (NC) emission quantum yields (QYs) are critical to condensed phase optical refrigeration. Of particular relevance to measuring NC QYs is a longstanding debate as to whether an excitation energy-dependent (EED) QY exists. Various reports indicate existence of NC EED QYs, suggesting that the phenomenon is linked to specific ensemble properties. We therefore investigate here the existence of EED QYs in two NC systems (CsPbBr3 and CdSe) that are possible candidates for use in optical refrigeration. The influence of NC size, size-distribution, surface ligand, and as-made emission QYs are investigated. Existence of EED QYs is assessed using two approaches (an absolute approach using an integrating sphere and a relative approach involving excitation spectroscopy). Altogether, our results show no evidence of EED QYs across samples. This suggests that parameters beyond those mentioned above are responsible for observations of NC EED QYs.