Visualizing Metal Distribution in Plants Using Synchrotron X-Ray Fluorescence Microscopy Techniques.

Recent improvements in synchrotron-based X-ray fluorescence (SXRF) microscopy established it as an advanced analytical tool for analyzing 2D- and 3D distribution of mineral elements in plants. Among existing imaging techniques, SXRF microscopy offers several unique capabilities, including in situ metal quantification in plant tissues and high sensitivity, as low as 1 mg kg-1, at the nanoscale spatial resolution. SXRF is increasingly utilized in different plant science disciplines to provide a fundamental understanding of metal homeostasis, and the function of trace elements in plant metabolism and development. Here, we describe methods for SXRF imaging, including sample preparation, the optimization of conventional SXRF for analyzing trace elements, and the development of confocal SXRF (C-SXRF).

YSL3-mediated copper distribution is required for fertility, seed size and protein accumulation in Brachypodium.

Addressing the looming global food security crisis requires the development of high-yielding crops. In agricultural soils, deficiency in the micronutrient copper significantly decreases grain yield in wheat (Triticum aestivum), a globally important crop. In cereals, grain yield is determined by inflorescence architecture, flower fertility, grain size, and weight. Whether copper is involved in these processes, and how it is delivered to the reproductive organs is not well understood. We show that copper deficiency alters not only the grain set but also flower development in both wheat and its recognized model, Brachypodium distachyon. We then show that the Brachypodium yellow stripe-like 3 (YSL3) transporter localizes to the phloem, transports copper in frog (Xenopus laevis) oocytes, and facilitates copper delivery to reproductive organs and grains. Failure to deliver copper, but not iron, zinc, or manganese to these structures in the ysl3 CRISPR-Cas9 mutant results in delayed flowering, altered inflorescence architecture, reduced floret fertility, grain size, weight, and protein accumulation. These defects are rescued by copper supplementation and are complemented by YSL3 cDNA. This knowledge will help to devise sustainable approaches for improving grain yield in regions where soil quality is a major obstacle for crop production. Copper distribution by a phloem-localized transporter is essential for the transition to flowering, inflorescence architecture, floret fertility, size, weight, and protein accumulation in seeds.

Time multiplexed deep reactive ion etching of germanium and silicon-A comparison of mechanisms and application to x-ray optics.

Although the mechanisms of deep reactive ion etching (DRIE) of silicon have been reported extensively, very little by comparison has been discussed concerning DRIE of germanium. By directly comparing silicon and germanium etching in a time multiplexed DRIE process, the authors extract significant differences in etch mechanisms from a design of experiment and discuss how these differences are relevant to the design and fabrication of silicon and germanium collimating channel array x-ray optics. The differences are illuminated by characteristics such as reactive ion etching (RIE)-lag, aspect ratio dependent etching, and sidewall passivation. Specifically, the authors demonstrate the more severe nature of RIE-lag in germanium, especially at aspect ratios exceeding 13:1. In addition, the differences in the profile evolution between silicon and germanium are shown to be a result of differences in sidewall passivation. There is also a correlation between the different sidewall passivation and the inherent lack of scalloping in the case of germanium DRIE.

Characterization of Arsenic in dried baby shrimp (Acetes sp.) using synchrotron-based X-Ray Spectrometry and LC coupled to ICP-MS/MS.

The arsenic content of dried baby shrimp (Acetes sp.) was investigated as part of an independent field study of human exposure to toxic metals/metalloids among the ethnic Chinese community located in Upstate New York. The dried baby shrimp were analyzed in a home environment using a portable X-ray Fluorescence (XRF) instrument based on monochromatic excitation. Study participants had obtained their dried baby shrimp either from a local Chinese market or prepared them at home. The shrimp are typically between 10-20 mm in size and are consumed whole, without separating the tail from the head. Elevated levels of As were detected using portable XRF, ranging between 5-30 µg/g. Shrimp samples were taken to the Cornell High Energy Synchrotron Source (CHESS) for Synchrotron Radiation µXRF (SR-µXRF) elemental mapping using a 384-pixel Maia detector system. The Maia detector provided high resolution trace element images for As, Ca, and Br, (among others) and showed localized accumulation of As within the shrimp's cephalothorax (head), and various abdominal segments. As quantification by SR-µXRF was performed using a Lobster hepatopancreas reference material pellet (NRC-CNRC TORT-2), with results in good agreement with both portable XRF and ICP-MS. Additional As characterization using µX-ray Absorption Near Edge Spectroscopy (µXANES) with the Maia XRF detector at CHESS identified arsenobetaine and/or arsenocholine as the possible As species present. Further arsenic speciation analysis by LC-ICP-MS/MS confirmed that the majority of As (>95%) is present as the largely non-toxic arsenobetaine species with trace amounts of arsenocholine, methylated As and inorganic As species detected.

Macroscopic Molecular Ordering and Exciton Delocalization in Crystalline Phthalocyanine Thin Films.

We present spatially-, temporally- and polarization-resolved dual photoluminescence/linear dichroism microscopy experiments that investigate the correlation between long-range order and the nature of exciton states in solution-processed phthalocyanine thin films. The influence of grain boundaries and disorder is absent in these films because typical grain sizes are 3 orders of magnitude larger than focused excitation beam diameters. These experiments reveal the existence of a delocalized singlet exciton, polarized along the high mobility axis in this quasi-1D electronic system. The strong delocalized π orbitals overlap, controlled by the molecular stacking along the high mobility axis, is responsible for breaking the radiative recombination selection rules. Using our linear dichroism scanning microscopy setup, we further established that a rotation of molecules (i.e., a structural phase transition) that occurs above 100 K prevents the observation of this exciton at room temperature.

Tuning polymorphism and orientation in organic semiconductor thin films via post-deposition processing.

Though both the crystal structure and molecular orientation of organic semiconductors are known to impact charge transport in thin-film devices, separately accessing different polymorphs and varying the out-of-plane molecular orientation is challenging, typically requiring stringent control over film deposition conditions, film thickness, and substrate chemistry. Here we demonstrate independent tuning of the crystalline polymorph and molecular orientation in thin films of contorted hexabenzocoronene, c-HBC, during post-deposition processing without the need to adjust deposition conditions. Three polymorphs are observed, two of which have not been previously reported. Using our ability to independently tune the crystal structure and out-of-plane molecular orientation in thin films of c-HBC, we have decoupled and evaluated the effects that molecular packing and orientation have on device performance in thin-film transistors (TFTs). In the case of TFTs comprising c-HBC, polymorphism and molecular orientation are equally important; independently changing either one affects the field-effect mobility by an order of magnitude.

Post-deposition processing methods to induce preferential orientation in contorted hexabenzocoronene thin films.

The structuring in organic electrically active thin films critically influences the performance of devices comprising them. Controlling film structure, however, remains challenging and generally requires stringent deposition conditions or modification of the substrate. To this end, we have developed post-deposition processing methods that are decoupled from the initial deposition conditions to induce different out-of-plane molecular orientations in contorted hexabenzocoronene (HBC) thin films. As-deposited HBC thin films lack any long-range order; subjecting them to post-deposition processing, such as hexanes-vapor annealing, thermal annealing, and physical contact with elastomeric poly(dimethyl siloxane), induces crystallization with increasing extents of preferential edge-on orientation, corresponding to greater degrees of in-plane π-stacking. Accordingly, transistors comprising HBC thin films that have been processed under these conditions exhibit field-effect mobilities that increase by as much as 2 orders of magnitude with increasing extents of molecular orientation. The ability to decouple HBC deposition from its subsequent structuring through post-deposition processing affords us the unique opportunity to tune competing molecule-molecule and molecule-solvent interactions, which ultimately leads to control over the structure and electrical function of HBC films.

van der Waals epitaxial growth of graphene on sapphire by chemical vapor deposition without a metal catalyst.

van der Waals epitaxial growth of graphene on c-plane (0001) sapphire by CVD without a metal catalyst is presented. The effects of CH(4) partial pressure, growth temperature, and H(2)/CH(4) ratio were investigated and growth conditions optimized. The formation of monolayer graphene was shown by Raman spectroscopy, optical transmission, grazing incidence X-ray diffraction (GIXRD), and low voltage transmission electron microscopy (LVTEM). Electrical analysis revealed that a room temperature Hall mobility above 2000 cm(2)/V·s was achieved, and the mobility and carrier type were correlated to growth conditions. Both GIXRD and LVTEM studies confirm a dominant crystal orientation (principally graphene [10-10] || sapphire [11-20]) for about 80-90% of the material concomitant with epitaxial growth. The initial phase of the nucleation and the lateral growth from the nucleation seeds were observed using atomic force microscopy. The initial nuclei density was ~24 µm(-2), and a lateral growth rate of ~82 nm/min was determined. Density functional theory calculations reveal that the binding between graphene and sapphire is dominated by weak dispersion interactions and indicate that the epitaxial relation as observed by GIXRD is due to preferential binding of small molecules on sapphire during early stages of graphene formation.

High-speed in situ X-ray scattering of carbon nanotube film nucleation and self-organization.

The production of high-performance carbon nanotube (CNT) materials demands understanding of the growth behavior of individual CNTs as well as collective effects among CNTs. We demonstrate the first use of grazing incidence small-angle X-ray scattering to monitor in real time the synthesis of CNT films by chemical vapor deposition. We use a custom-built cold-wall reactor along with a high-speed pixel array detector resulting in a time resolution of 10 msec. Quantitative models applied to time-resolved X-ray scattering patterns reveal that the Fe catalyst film first rapidly dewets into well-defined hemispherical particles during heating in a reducing atmosphere, and then the particles coarsen slowly upon continued annealing. After introduction of the carbon source, the initial CNT diameter distribution closely matches that of the catalyst particles. However, significant changes in CNT diameter can occur quickly during the subsequent CNT self-organization process. Correlation of time-resolved orientation data to X-ray scattering intensity and height kinetics suggests that the rate of self-organization is driven by both the CNT growth rate and density, and vertical CNT growth begins abruptly when CNT alignment reaches a critical threshold. The dynamics of CNT size evolution and self-organization vary according to the catalyst annealing conditions and substrate temperature. Knowledge of these intrinsically rapid processes is vital to improve control of CNT structure and to enable efficient manufacturing of high-density arrays of long, straight CNTs.

Guiding crystallization around bends and sharp corners.

Control over the molecular orientation in organic thin films is demonstrated with precise in-plane spatial resolution over large areas. By exploiting the differential crystallization rates on substrates with different surface energies, the radial symmetry of spherulitic growth can be disrupted by preferentially selecting the molecular orientations that promote growth along the paths of the underlying patterns.

Lattice expansion of highly oriented 2D phthalocyanine covalent organic framework films.

Expanding into application: covalent organic framework (COF) films are ideally suited for vertical charge transport and serve as precursors of ordered heterojunctions. Their pores, however, were previously too small to accommodate continuous networks of complementary electron acceptors. Four phthalocyanine COFs with increased pore size well into the mesoporous regime are now described.

Oriented 2D covalent organic framework thin films on single-layer graphene.

Covalent organic frameworks (COFs), in which molecular building blocks form robust microporous networks, are usually synthesized as insoluble and unprocessable powders. We have grown two-dimensional (2D) COF films on single-layer graphene (SLG) under operationally simple solvothermal conditions. The layered films stack normal to the SLG surface and show improved crystallinity compared with COF powders. We used SLG surfaces supported on copper, silicon carbide, and transparent fused silica (SiO(2)) substrates, enabling optical spectroscopy of COFs in transmission mode. Three chemically distinct COF films grown on SLG exhibit similar vertical alignment and long-range order, and two of these are of interest for organic electronic devices for which thin-film formation is a prerequisite for characterizing their optoelectronic properties.

Epitaxial growth of graphitic carbon on C-face SiC and Sapphire by chemical vapor deposition (CVD).

Epitaxial, graphitic carbon thin films were directly grown on C-face/ (0001̄) SiC and (0001) sapphire by chemical vapor deposition (CVD), using propane as a carbon source and without any catalytic metal on the substrate surface. Raman spectroscopy shows the signature of multilayer graphene/graphite growth on both the SiC and sapphire. Raman 2D-peaks have Lorentzian lineshapes with FWHM of ~60 cm(-1) and the ratio of the D-peak to G-peak intensity (I(D)/I(G)) linearly decreases (down to 0.06) as growth temperature is increased. The epitaxial relationship between film and substrates were determined by x-ray diffraction. On both substrates, graphitic layers are oriented parallel to the substrate, but exhibit significant rotational disorder about the surface normal, and predominantly rhombohedral stacking. Film thicknesses were determined to be a function of growth time, growth temperature, and propane flow rate.

Measuring the lengthening kinetics of aligned nanostructures by spatiotemporal correlation of height and orientation.

Owing to their inherent tortuosity, the collective height of vertically aligned nanostructures does not equal the average length of the individual constituent nanostructures, and therefore temporal height measurement is not an accurate measure of the genuine growth kinetics. We use high-resolution spatial mapping of alignment by small-angle X-ray scattering (SAXS) to transform real-time measurements of array height to the average length of the nanostructures. Applying this approach to carbon nanotube (CNT) forest growth transforms the kinetics from a sub-linear to a linear relationship with time, highlighting the potential for insights into the limiting growth mechanisms of CNTs and other one-dimensional nanostructures.