Nanoscale Infrared Spectroscopic Characterization of Extended Defects in 4H-Silicon Carbide.

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.

Unraveling the Supramolecular Structure and Nanoscale Dislocations of Bacterial Cellulose Ribbons Using Correlative Super-Resolution Light and Electron Microscopy.

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.

Advanced characterisation of 3D structure and porosity of ordinary portland cement (OPC) mortar using plasma focused ion beam tomography and X-ray computed tomography.

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.

Unexpected Near-Infrared to Visible Nonlinear Optical Properties from 2-D Polar Metals.

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.

Spontaneous Relaxation of Heteroepitaxial Thin Films by van der Waals-Like Bonding on Te-Terminated Sapphire Substrates.

Here the novel direct heteroepitaxial growth method of a 3D heteroepitaxial system is demonstrated on a 3D substrate, CdTe (111)/Al2 O3 (0001), which forms a spontaneous vdW-like bond at the interface, instead of the two 3D crystals being strongly bound. Despite a large lattice mismatch, the thin films are single crystals and maintain high quality due the compliance of the interface which accommodates strain. This weak bonding interface is accomplished by the self-assembly of a pseudomorphic chalcogenide layer on the sapphire surface during growth. Since the vdW-like interface forms spontaneously in situ during growth, it is easily scalable to large wafer sizes, without the need to layer transfer 2D materials onto the growth substrate for remote epitaxy. Further, the weak adhesion of the films on the substrates allow for epitaxial film transfer to a variety of other substrates, leaving the original growth substrate for future reuse. This type of 3D/3D vdW-like interface is exploitable as a compliant interlayer for additional epitaxy, and may even be observable directly in other material systems grown on complex oxides, allowing for the production of large area high quality freestanding and layer transferred epitaxial devices for material systems not currently possible by conventional processing techniques.

Low-loss TeO2-coated Si3N4 waveguides for application in photonic integrated circuits.

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 Si3N4 strip height and width and TeO2 layer thickness, a small mode area, small bend radius and high optical intensity overlap with the TeO2 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.

Multi-Angle Plasma Focused Ion Beam (FIB) Curtaining Artifact Correction Using a Fourier-Based Linear Optimization Model.

We present a flexible linear optimization model for correcting multi-angle curtaining effects in plasma focused ion beam scanning electron microscopy (PFIB-SEM) images produced by rocking-polishing schemes. When PFIB-SEM is employed in a serial sectioning tomography workow, it is capable of imaging large three-dimensional volumes quickly, providing rich information in the critical 10-100 nm feature length scale. During tomogram acquisition, a "rocking polish" is often used to reduce straight-line "curtaining" gradations in the milled sample surface. While this mitigation scheme is effective for deep curtains, it leaves shallower line artifacts at two discretized angles. Segmentation and other automated processing of the image set requires that these artifacts be corrected for accurate microstructural quantification. Our work details a new Fourier-based linear optimization model for correcting curtaining artifacts by targeting curtains at two discrete angles. We demonstrate its capabilities by processing images from a tomogram from a multiphase, heterogeneous concrete sample. We present methods for selecting the parameters which meet the user's goals most appropriately. Compared to previous works, we show that our model provides effective multi-angle curtain correction without introducing artifacts into the image, modifying non-curtain structures or causing changes to the contrast of voids. Our algorithm can be easily parallelized to take advantage of multi-core hardware.

Transfer of Chemically Modified Graphene with Retention of Functionality for Surface Engineering.

Single-layer graphene chemically reduced by the Birch process delaminates from a Si/SiOx substrate when exposed to an ethanol/water mixture, enabling transfer of chemically functionalized graphene to arbitrary substrates such as metals, dielectrics, and polymers. Unlike in previous reports, the graphene retains hydrogen, methyl, and aryl functional groups during the transfer process. This enables one to functionalize the receiving substrate with the properties of the chemically modified graphene (CMG). For instance, magnetic force microscopy shows that the previously reported magnetic properties of partially hydrogenated graphene remain after transfer. We also transfer hydrogenated graphene from its copper growth substrate to a Si/SiOx wafer and thermally dehydrogenate it to demonstrate a polymer- and etchant-free graphene transfer for potential use in transmission electron microscopy. Finally, we show that the Birch reduction facilitates delamination of CMG by weakening van der Waals forces between graphene and its substrate.

Low-loss, extreme subdiffraction photon confinement via silicon carbide localized surface phonon polariton resonators.

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 (λ(res)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.

Elucidating the Mechanism of Large Phosphate Molecule Intercalation Through Graphene-Substrate Heterointerfaces.

Intercalation is the process of inserting chemical species into the heterointerfaces of two-dimensional (2D) layered materials. While much research has focused on the intercalation of metals and small gas molecules into graphene, the intercalation of larger molecules through the basal plane of graphene remains challenging. In this work, we present a new mechanism for intercalating large molecules through monolayer graphene to form confined oxide materials at the graphene-substrate heterointerface. We investigate the intercalation of phosphorus pentoxide (P2O5) molecules directly from the vapor phase and confirm the formation of confined P2O5 at the graphene-substrate heterointerface using various techniques. Density functional theory (DFT) corroborates the experimental results and reveals the intercalation mechanism, whereby P2O5 dissociates into small fragments catalyzed by defects in the graphene that then permeates through lattice defects and reacts at the heterointerface to form P2O5. This process can also be used to form new confined metal phosphates (e.g., 2D InPO4). While the focus of this study is on P2O5 intercalation, the possibility of intercalation from predissociated molecules catalyzed by defects in graphene may exist for other types of molecules as well. This in-depth study advances our understanding of intercalation routes of large molecules via the basal plane of graphene as well as heterointerface chemical reactions leading to the formation of distinctive confined complex oxide compounds.

Divalent-anion salt effects in polyelectrolyte multilayer depositions.

We systematically investigate the effects of divalent anions on the assembly of polyelectrolyte multilayers by fabricating polystyrene sulfonate (PSS)/polyallylamine hydrochloride (PAH) multilayer films from aqueous solutions containing SO(4)(2-), HPO(4)(2-), or organic dicarboxylate dianions. The chosen concentrations of these anions (i.e., ≤0.05 M) allow us to isolate their effects on the assembly process from those of the polyelectrolyte solubility or solution ionic strength (maintained constant at µ = 1.00 M by added NaCl). Compared to a control film prepared from solutions containing only Cl(-) anions, stratified multilayers deposited in the presence of dianions exhibit increased UV absorbance, thickness, and roughness. From the dependence of film properties on the solution concentration of SO(4)(2-) and number of polyelectrolyte layers deposited, we derive a generic model for the PSS/PAH multilayer formation that involves adsorption of PAH aggregates formed in solution via electrostatic interactions of PAH with bridging dianions. Experiments using HPO(4)(2-) and organic dicarboxylate species of varying structure indicate that the separation, rigidity, and angle between the discrete negatively charged sites in the dianion govern the formation of the PAH aggregates, and therefore also the properties of the multilayer film. A universal linear relationship between film UV absorbance and thickness is observed among all dianion types or concentrations, consistent with the model.

Scalable Characterization of 2D Gallium-Intercalated Epitaxial Graphene.

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.

Bacteriophage ZCSE2 is a Potent Antimicrobial Against Salmonella enterica Serovars: Ultrastructure, Genomics and Efficacy.

Developing novel antimicrobials capable of controlling multidrug-resistant bacterial pathogens is essential to restrict the use of antibiotics. Bacteriophages (phages) constitute a major resource that can be harnessed as an alternative to traditional antimicrobial therapies. Phage ZCSE2 was isolated among several others from raw sewage but was distinguished by broad-spectrum activity against Salmonella serovars considered pathogenic to humans and animals. Lytic profiles of ZCSE2 against a panel of Salmonella were determined together with low temperature activity and pH stability. The morphological features of the phage and host infection processes were characterized using a combination of transmission electron and atomic force microscopies. Whole genome sequencing of ZCSE2 produced a complete DNA sequence of 53,965 bp. No known virulence genes were identified in the sequence data, making ZCSE2 a good candidate for phage-mediated biological control purposes. ZCSE2 was further tested against S. Enteritidis in liquid culture and was observed to reduce the target bacterium to below the limits of detection from initial concentrations of 107-108 Colony Forming Units (CFU)/mL. With a broad host-range against pathogenic Salmonella serovars, phage ZCSE2 constitutes a potential tool against a major cause of human and animal disease.

Controlling the Infrared Dielectric Function through Atomic-Scale Heterostructures.

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.

Nanopatterning of GeTe phase change films via heated-probe lithography.

The crystallization of amorphous germanium telluride (GeTe) thin films is controlled with nanoscale resolution using the heat from a thermal AFM probe. The dramatic differences between the amorphous and crystalline GeTe phases yield embedded nanoscale features with strong topographic, electronic, and optical contrast. The flexibility of scanning probe lithography enables the width and depth of the features, as well as the extent of their crystallization, to be controlled by varying probe temperature and write speed. Together, these technologies suggest a new approach to nanoelectronic and opto-electronic device fabrication.

Plasmonic nanopillar arrays for large-area, high-enhancement surface-enhanced Raman scattering sensors.

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 × 10(8) 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.

Characterization of metal oxide nanofilm morphologies and composition by terahertz transmission spectroscopy.

An all-optical terahertz absorption technique for nondestructive characterization of nanometer-scale metal oxide thin films grown on silicon substrates is described. Example measurements of laser-deposited TiO2 and atomic layer-deposited films of HfO2 are presented to demonstrate applicability to pure Y2O3, Al2O3, and VOx and mixed combinatorial films as a function of deposition conditions and thickness. This technique is also found to be sensitive to HfO2 phonon modes in films with a nominal thickness of 5 nm.