Plasmonics provides great promise for nanophotonic applications. However, the high optical losses inherent in metal-based plasmonic systems have limited progress. Thus, it is critical to identify ...alternative low-loss materials. One alternative is polar dielectrics that support surface phonon polariton (SPhP) modes, where the confinement of infrared light is aided by optical phonons. Using fabricated 6H-silicon carbide nanopillar antenna arrays, we report on the observation of subdiffraction, localized SPhP resonances. They exhibit a dipolar resonance transverse to the nanopillar axis and a monopolar resonance associated with the longitudinal axis dependent upon the SiC substrate. Both exhibit exceptionally narrow linewidths (7–24 cm–1), with quality factors of 40–135, which exceed the theoretical limit of plasmonic systems, with extreme subwavelength confinement of (λres 3/V eff)1/3 = 50–200. Under certain conditions, the modes are Raman-active, enabling their study in the visible spectral range. These observations promise to reinvigorate research in SPhP phenomena and their use for nanophotonic applications.
Efforts to create reproducible surface-enhanced Raman scattering (SERS)-based chemical and biological sensors has been hindered by difficulties in fabricating large-area SERS-active substrates with a ...uniform, reproducible SERS response that still provides sufficient enhancement for easy detection. Here we report on periodic arrays of Au-capped, vertically aligned silicon nanopillars that are embedded in a Au plane upon a Si substrate. We illustrate that these arrays are ideal for use as SERS sensor templates, in that they provide large, uniform and reproducible average enhancement factors up to ∼1.2 × 108 over the structure surface area. We discuss the impact of the overall geometry of the structures upon the SERS response at 532, 633, and 785 nm incident laser wavelengths. Calculations of the electromagnetic field distributions and intensities within such structures were performed and both the wavelength dependence of the predicted SERS response and the field distribution within the nanopillar structure are discussed and support the experimental results we report.
Zn-coated press hardened steels (PHS) are in high demand for automotive mass reduction and enhanced passenger safety applications while the Zn coating supplies robust cathodic corrosion protection. ...However, the mechanism of micro-crack formation during direct hot-press forming (DHPF) has not been adequately described. Thus, the objective of this work was to determine the mechanism for micro-crack formation in Zn-coated DHPF PHS that addressed the relationship between micro-cracking and the coating microstructure created during substrate austenitization. Zn-coated 22MnB5 steel sheets were annealed at 900 °C for annealing times ranging from 30 to 780 s and DHPF at 75 °C s−1 to obtain a fully martensitic substrate microstructure. The inward diffusion between the Zn coating and the substrate during annealing resulted in a dual phase coating microstructure initially comprising Γ-Fe3Zn10 + α-Fe(Zn), transitioning to a single phase α-Fe(Zn) coating after annealing for 240–420 s. Coincident coating α-Fe(Zn) and substrate Zn-enriched austenite (γ-Fe(Zn)) grain boundaries became Zn-enriched, forming a thin layer of α-Fe(Zn) along the γ-Fe(Zn) grain boundaries. It is proposed that coincident coating α-Fe(Zn) and substrate prior austenite grain boundaries (PAGBs) were weakened by this grain boundary α-Fe(Zn) layer. Upon the application of tensile stress, intergranular fracture occurred along the coincident coating α-Fe(Zn) and Zn-enriched PAGBs in the Zn-enriched martensite (M(Zn)) layer. It was further determined that crack propagation ceased and the crack tip was blunted when Zn-enrichment along the PAGBs in the M(Zn) layer was exhausted.
•Origin of microcracking in Zn-coated press-hardened steel was determined.•Microcracks occurred at coincident coating α-Fe(Zn) and substrate PAGBs.•Zn-enriched PAGBs had a thin layer of brittle α-Fe(Zn).•Crack propagation stopped when Zn-enrichment in PAGB was exhausted.
Cellulose is a structural linear polysaccharide that is naturally produced by plants and bacteria, making it the most abundant biopolymer on Earth. The hierarchical structure of cellulose from the ...nano- to microscale is intimately linked to its biosynthesis and the ability to process this sustainable resource for materials applications. Despite this, the morphology of bacterial cellulose microfibrils and their assembly into higher order structures, as well as the structural origins of the alternating crystalline and disordered supramolecular structure of cellulose, have remained elusive. In this work, we employed high-resolution transmission electron and atomic force microscopies to study the morphology of bacterial cellulose ribbons at different levels of its structural hierarchy and provide direct visualization of nanometer-wide microfibrils. The non-persistent twisting of cellulose ribbons was characterized in detail, and we found that twists are associated with nanostructural defects at the bundle and microfibril levels. To investigate the structural origins of the persistent disordered regions that are present along cellulose ribbons, we employed a correlative super-resolution light and electron microscopy workflow and observed that the disordered regions that can be seen in super-resolution fluorescence microscopy largely correlated with the ribbon twisting observed in electron microscopy. Unraveling the hierarchical assembly of bacterial cellulose and the ultrastructural basis of its disordered regions provides insights into its biosynthesis and susceptibility to hydrolysis. These findings are important to understand the cell-directed assembly of cellulose, develop new cellulose-based nanomaterials, and develop more efficient biomass conversion strategies.
Extended defects in wide-bandgap semiconductors have been widely investigated using techniques providing either spectroscopic or microscopic information. Nano-Fourier transform infrared spectroscopy ...(nano-FTIR) is a nondestructive characterization method combining FTIR with nanoscale spatial resolution (∼20 nm) and topographic information. Here, we demonstrate the capability of nano-FTIR for the characterization of extended defects in semiconductors by investigating an in-grown stacking fault (IGSF) present in a 4H-SiC epitaxial layer. We observe a local spectral shift of the mid-infrared near-field response, consistent with the identification of the defect stacking order as 3C-SiC (cubic) from comparative simulations based on the finite dipole model (FDM). This 3C-SiC IGSF contrasts with the more typical 8H-SiC IGSFs reported previously and is exemplary in showing that nanoscale spectroscopy with nano-FTIR can provide new insights into the properties of extended defects, the understanding of which is crucial for mitigating deleterious effects of such defects in alternative semiconductor materials and devices.
Scalable synthesis of two-dimensional gallium (2D-Ga) covered by graphene layers was recently realized through confinement heteroepitaxy using silicon carbide substrates. However, the thickness, ...uniformity, and area coverage of the 2D-Ga heterostructures have not previously been studied with high-spatial resolution techniques. In this work, we resolve and measure the 2D-Ga heterostructure thicknesses using scanning electron microscopy (SEM). Utilizing multiple correlative methods, we find that SEM image contrast is directly related to the presence of uniform bilayer Ga at the interface and a variation of the number of graphene layers. We also investigate the origin of SEM contrast using both experimental measurements and theoretical calculations of the surface potentials. We find that a carbon buffer layer is detached due to the gallium intercalation, which increases the surface potential as an indication of the 2D-Ga presence. We then scale up the heterostructure characterization over a few-square millimeter area by segmenting SEM images, each acquired with nanometer-scale in-plane resolution. This work leverages the spectroscopic imaging capabilities of SEM that allows high-spatial resolution imaging for tracking intercalants, identifying relative surface potentials, determining the number of 2D layers, and further characterizing scalability and uniformity of low-dimensional materials.
The visualisation and quantification of pore networks and main phases have been critical research topics in cementitious materials as many critical mechanical and chemical properties and ...infrastructure reliability rely on these 3D characteristics. In this study, we realised the mesoscale serial sectioning and analysis up to ∼80 μm by ∼90 μm by ∼60 μm on portland cement mortar using plasma focused ion beam (PFIB) for the first time. The workflow of working with mortar and PFIB was established applying a prepositioned hard silicon mask to reduce curtaining. Segmentation with minimal human interference was performed using a trained neural network, in which multiple types of segmentation models were compared. Combining PFIB analysis at microscale with X‐ray micro‐computed tomography, the analysis of capillary pores and air voids ranging from hundreds of nanometres (nm) to millimetres (mm) can be conducted. The volume fraction of large capillary pores and air voids are 11.5% and 12.7%, respectively. Moreover, the skeletonisation of connected capillary pores clearly shows fluid transport pathways, which is a key factor determining durability performance of concrete in aggressive environments. Another interesting aspect of the FIB tomography is the reconstruction of anhydrous phases, which could enable direct study of hydration kinetics of individual cement phases.
Surface phonon polaritons (SPhPs), the surface-bound electromagnetic modes of a polar material resulting from the coupling of light with optic phonons, offer immense technological opportunities for ...nanophotonics in the infrared (IR) spectral region. However, once a particular material is chosen, the SPhP characteristics are fixed by the spectral positions of the optic phonon frequencies. Here, we provide a demonstration of how the frequency of these optic phonons can be altered by employing atomic-scale superlattices (SLs) of polar semiconductors using AlN/GaN SLs as an example. Using second harmonic generation (SHG) spectroscopy, we show that the optic phonon frequencies of the SLs exhibit a strong dependence on the layer thicknesses of the constituent materials. Furthermore, new vibrational modes emerge that are confined to the layers, while others are centered at the AlN/GaN interfaces. As the IR dielectric function is governed by the optic phonon behavior in polar materials, controlling the optic phonons provides a means to induce and potentially design a dielectric function distinct from the constituent materials and from the effective-medium approximation of the SL. We show that atomic-scale AlN/GaN SLs instead have multiple Reststrahlen bands featuring spectral regions that exhibit either normal or extreme hyperbolic dispersion with both positive and negative permittivities dispersing rapidly with frequency. Apart from the ability to engineer the SPhP properties, SL structures may also lead to multifunctional devices that combine the mechanical, electrical, thermal, or optoelectronic functionality of the constituent layers. We propose that this effort is another step toward realizing user-defined, actively tunable IR optics and sources.
We report on high-quality tellurium oxide waveguides integrated on a low-loss silicon nitride wafer-scale platform. The waveguides consist of silicon nitride strip features, which are fabricated ...using a standard foundry process and a tellurium oxide coating layer that is deposited in a single post-processing step. We show that by adjusting the Si
N
strip height and width and TeO
layer thickness, a small mode area, small bend radius and high optical intensity overlap with the TeO
can be obtained. We investigate transmission at 635, 980, 1310, 1550 and 2000 nm wavelengths in paperclip waveguide structures and obtain low propagation losses down to 0.6 dB/cm at 2000 nm. These results illustrate the potential for compact linear, nonlinear and active tellurite glass devices in silicon nitride photonic integrated circuits operating from the visible to mid-infrared.
Near-infrared-to-visible second harmonic generation from air-stable two-dimensional polar gallium and indium metals is described. The photonic properties of 2D metals, including the largest ...second-order susceptibilities reported for metals (approaching 10 nm/V), are determined by the atomic-level structure and bonding of two-to-three-atom-thick crystalline films. The bond character evolved from covalent to metallic over a few atomic layers, changing the out-of-plane metal–metal bond distances by approximately ten percent (0.2 Å), resulting in symmetry breaking and an axial electrostatic dipole that mediated the large nonlinear response. Two different orientations of the crystalline metal atoms, corresponding to lateral displacements <2 Å, persisted in separate micrometer-scale terraces to generate distinct harmonic polarizations. This strong atomic-level structure–property interplay suggests metal photonic properties can be controlled with atomic precision.
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