Multiplexed orientation and structure analysis by imaging near-edge X-ray absorption fine structure (MOSAIX) for combinatorial surface science

Model Asphaltenes Adsorbed onto Methyl- and COOH-Terminated SAMs on Gold

Petroleum asphaltenes are surface-active compounds found in crude oils, and their interactions with surfaces and interfaces have huge implications for many facets of reservoir exploitation, including production, transportation, and oil-water separation. The asphaltene fraction in oil, found in the highest boiling-point range, is composed of many different molecules that vary in size, functionality, and polarity. Studies done on asphaltene fractions have suggested that they interact via polyaromatic and heteroaromatic ring structures and functional groups containing nitrogen, sulfur, and oxygen. However, isolating a single pure chemical structure of asphaltene in abundance is challenging and often not possible, which impairs the molecular-level study of asphaltenes of various architectures on surfaces. Thus, to further the molecular fundamental understanding, we chose to use functionalized model asphaltenes (AcChol-Th, AcChol-Ph, and 1,6-DiEtPy[Bu-Carb]) and model self-assembled monolayer (SAM) surfaces with precisely known chemical structures, whereby the hydrophobicity of the model surface is controlled. We applied solutions of asphaltenes to these SAM surfaces and then analyzed them with surface-sensitive techniques of near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). We observe no adsorption of asphaltenes to the hydrophobic surface. On the hydrophilic surface, AcChol-Ph penetrates into the SAM with a preferential orientation parallel to the surface; AcChol-Th adsorbs in a similar manner, and 1,6-DiEtPy[Bu-Carb] binds the surface with a bent binding geometry. Overall, this study demonstrates the need for studying pure and fractionated asphaltenes at the molecular level, as even within a family of asphaltene congeners, very different surface interactions can occur.

Surface analysis tools for characterizing biological materials

Surfaces represent a unique state of matter that typically have significantly different compositions and structures from the bulk of a material. Since surfaces are the interface between a material and its environment, they play an important role in how a material interacts with its environment. Thus, it is essential to characterize, in as much detail as possible, the surface structure and composition of a material. However, this can be challenging since the surface region typically is only minute portion of the entire material, requiring specialized techniques to selectively probe the surface region. This tutorial will provide a brief review of several techniques used to characterize the surface and interface regions of biological materials. For each technique we provide a description of the key underlying physics and chemistry principles, the information provided, strengths and weaknesses, the types of samples that can be analyzed, and an example application. Given the surface analysis challenges for biological materials, typically there is never just one technique that can provide a complete surface characterization. Thus, a multi-technique approach to biological surface analysis is always required.

NEXAFS imaging to characterize the physio-chemical composition of cuticle from African Flower Scarab Eudicella gralli

The outermost surface of insect cuticle is a high-performance interface that provides wear protection, hydration, camouflage and sensing. The complex and inhomogeneous structure of insect cuticle imposes stringent requirements on approaches to elucidate its molecular structure and surface chemistry. Therefore, a molecular understanding and possible mimicry of the surface of insect cuticle has been a challenge. Conventional optical and electron microscopies as well as biochemical techniques provide information about morphology and chemistry but lack surface specificity. We here show that a near edge X-ray absorption fine structure microscope at the National Synchrotron Light Source can probe the surface chemistry of the curved and inhomogeneous cuticle of the African flower scarab. The analysis shows the distribution of organic and inorganic surface species while also hinting at the presence of aragonite at the dorsal protrusion region of the Eudicella gralli head, in line with its biological function.

Biology serves as inspiration in materials development; this requires improved understanding of the surface chemistry responsible for processes which are being mimicked. Here, the authors report on the use of near edge X-ray absorption fine structure (NEXAFS) imaging to analyze the surface chemistry of insect cuticle.

Strain and Bond Length Dynamics upon Growth and Transfer of Graphene by NEXAFS Spectroscopy from First-Principles and Experiment

As the quest toward novel materials proceeds, improved characterization technologies are needed. In particular, the atomic thickness in graphene and other 2D materials renders some conventional technologies obsolete. Characterization technologies at wafer level are needed with enough sensitivity to detect strain in order to inform fabrication. In this work, NEXAFS spectroscopy was combined with simulations to predict lattice parameters of graphene grown on copper and further transferred to a variety of substrates. The strains associated with the predicted lattice parameters are in agreement with experimental findings. The approach presented here holds promise to effectively measure strain in graphene and other 2D systems at wafer levels to inform manufacturing environments.

Magnetic Field Landscapes Guiding the Chemisorption of Diamagnetic Molecules

It is shown that the self-assembly of diamagnetic molecule submonolayers on a surface can be influenced by magnetic stray field landscapes emerging from artificially fabricated magnetic domains and domain walls. The directed local chemisorption of diamagnetic subphthalocyaninatoboron molecules in relation to the artificially created domain pattern is proved by a combination of surface analytical methods: ToF-SIMS, X-PEEM, and NEXAFS imaging. Thereby, a new method to influence self-assembly processes and to produce patterned submonolayers is presented.

Evidence of a molecular boundary lubricant at snakeskin surfaces

During slithering locomotion the ventral scales at a snake's belly are in direct mechanical interaction with the environment, while the dorsal scales provide optical camouflage and thermoregulation. Recent work has demonstrated that compared to dorsal scales, ventral scales provide improved lubrication and wear protection. While biomechanic adaption of snake motion is of growing interest in the fields of material science and robotics, the mechanism for how ventral scales influence the friction between the snake and substrate, at the molecular level, is unknown. In this study, we characterize the outermost surface of snake scales using sum frequency generation (SFG) spectra and near-edge X-ray absorption fine structure (NEXAFS) images collected from recently shed California kingsnake (Lampropeltis californiae) epidermis. SFG's nonlinear optical selection rules provide information about the outermost surface of materials; NEXAFS takes advantage of the shallow escape depth of the electrons to probe the molecular structure of surfaces. Our analysis of the data revealed the existence of a previously unknown lipid coating on both the ventral and dorsal scales. Additionally, the molecular structure of this lipid coating closely aligns to the biological function: lipids on ventral scales form a highly ordered layer which provides both lubrication and wear protection at the snake's ventral surface.

High-throughput analysis of molecular orientation on surfaces by NEXAFS imaging of curved sample arrays

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy provides detailed information about the orientation and alignment of thin films. NEXAFS is a synchrotron-based technique-the availability of beam-time per user is typically limited to no more than a few weeks per year. The limited availability is currently a true barrier for using NEXAFS in combinatorial studies of molecular alignment. We have recently demonstrated how large area full field NEXAFS imaging allows users to pursue combinatorial studies of surface chemistry. Now we report an extension of this approach which allows the acquisition of orientation information from a single NEXAFS image. An array with 80 elements (samples), containing eight series of different surface modifications, was mounted on a curved substrate allowing the collection of NEXAFS spectra with a range of orientations with respect to the X-ray beam. Images collected from this array show how hyperspectral NEXAFS data collected from curved surfaces can be used for high-throughput molecular orientation analysis.