Diamond surface functionalization via visible light-driven C-H activation for nanoscale quantum sensing

Nitrogen-vacancy (NV) centers in diamond are a promising platform for nanoscale NMR sensing. Despite significant progress toward using NV centers to detect and localize nuclear spins down to the single spin level, NV-based spectroscopy of individual, intact, arbitrary target molecules remains elusive. Such sensing requires that target molecules are immobilized within nanometers of NV centers with long spin coherence. The inert nature of diamond typically requires harsh functionalization techniques such as thermal annealing or plasma processing, limiting the scope of functional groups that can be attached to the surface. Solution-phase chemical methods can be readily generalized to install diverse functional groups, but they have not been widely explored for single-crystal diamond surfaces. Moreover, realizing shallow NV centers with long spin coherence times requires highly ordered single-crystal surfaces, and solution-phase functionalization has not yet been shown with such demanding conditions. In this work, we report a versatile strategy to directly functionalize C-H bonds on single-crystal diamond surfaces under ambient conditions using visible light, forming C-F, C-Cl, C-S, and C-N bonds at the surface. This method is compatible with NV centers within 10 nm of the surface with spin coherence times comparable to the state of the art. As a proof-of-principle demonstration, we use shallow ensembles of NV centers to detect nuclear spins from surface-bound functional groups. Our approach to surface functionalization opens the door to deploying NV centers as a tool for chemical sensing and single-molecule spectroscopy.

Elucidation of Surface Functional Groups Deposited by Electrochemical Surface Treatment of Discontinuous Carbon Fiber by NEXAFS and XPS

Control of carbon fiber heteroatom (oxygen and nitrogen) functionalization using electrochemical oxidation is explored in a variety of electrolyte solutions. Results of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy indicate that most electrolytes in aqueous and anodic conditions are limited to heteroatom surface content of no more than 13 atomic percent (at %) with a majority C-O and/or C-N moieties; the remaining moieties include an oxidative sequence of carbon (alcohol to ketone to carboxylate) and more complex O- and N-containing groups. The pH of the electrolyte solution was found to be crucial in controlling the ratio of the amount of oxygen to nitrogen functionalities, with the increased basicity of solution resulting in higher nitrogen deposition. The oxidative (and/or thermal) decomposition of many electrolytes during electrochemical treatment can have a major impact on functionalization through changes to pH. Oxidation of carbon fiber in some electrolyte solutions showed higher surface concentrations of heteroatoms (25-30 at %) than most electrolytes (13 at %). Mechanisms were proposed to explain how some electrolytes can exceed 13 at % of heteroatom deposition. Specifically, we hypothesized that electrolytes that contain organic ions with chelation capabilities and moieties that produce additional sites of functionalization can overcome that threshold.

Toggling Stereochemical Activity through Interstitial Positioning of Cations between 2D V2O5 Double Layers

The 5/6s2 lone-pair electrons of p-block cations in their lower oxidation states are a versatile electronic and geometric structure motif that can underpin lattice anharmonicity and often engender electronic and structural instabilities that underpin the function of active elements in nonlinear optics, thermochromics, thermoelectrics, neuromorphic computing, and photocatalysis. In contrast to periodic solids where lone-pair-bearing cations are part of the structural framework, installing lone-pair-bearing cations in the interstitial sites of intercalation hosts provides a means of a systematically modulating electronic structure through the choice of the group and the period of the inserted cation while preserving the overall framework connectivity. The extent of stereochemical activity and the energy positioning of lone-pair-derived mid-gap states depend on the cation identity, stoichiometry, and strength of anion hybridization. V2O5 polymorphs are versatile insertion hosts that can accommodate a broad range of s-, p-, and d-block cations. However, the insertion of lone-pair-bearing cations remains largely underexplored. In this article, we examine the implications of varying the 6s2 cations situated in interlayer sites between condensed [V4O10]n double layers. Systematic modulations of lattice distortions, electronic structure, and magnetic ordering are observed with increasing strength of stereochemical activity from group 12 to group 14 cations. We compare and contrast p-block-layered MxV2O5 (M = Hg, Tl, and Pb) compounds and map the significance of local off-centering arising from the stereochemical activity of lone-pair cations to the emergence of filled antibonding lone-pair 6s2-O 2p-hybridized mid-gap states mediated by second-order Jahn-Teller distortions. Crystallographic studies of cation coordination environments and the resulting modulation of V-V interactions have been used in conjunction with variable-energy hard X-ray photoelectron spectroscopy measurements, first-principles electronic structure calculations, and crystal orbital Hamilton population analyses to decipher the origins of stereochemical activity. Magnetic susceptibility measurements reveal antiferromagnetic signatures for all the three compounds. However, the differences in V-V interactions significantly affect the energy balance of the superexchange interactions, resulting in an ordering temperature of 160 and 260 K for Hg0.5V2O5 and δ-Tl0.5V2O5, respectively, as compared to 7 K for δ-Pb0.5V2O5. In δ-Pb0.5V2O5, the strong stereochemical activity of electron lone pairs and the resulting electrostatic repulsions enforce superlattice ordering, which strongly modifies the electronic localization patterns along the [V4O10] slabs, resulting in disrupted magnetic ordering and an anomalously low ordering temperature. The results demonstrate a versatile strategy for toggling the stereochemical activity of electron lone pairs to modify the electronic structure near the Fermi level and to mediate superexchange interactions.

Effect of Stereochemically Active Electron Lone Pairs on Magnetic Ordering in Trivanadates

Stereoactive electron lone pairs derived from filled 5/6s2 states of p-block cations are an intriguing electronic and geometric structure motif that have been exploited for diverse applications such as thermoelectrics, thermochromics, photocatalysis, and nonlinear optics. Layered trivanadates are dynamic intercalation hosts, where the insertion of cations can be used to tune electron correlation, charge localization, and magnetic ordering. However, the interaction of 5/6s2 stereoactive electron lone pairs with layered trivanadates remains unexplored. In this study, we contrast s- and p-block trivanadates and map off-centering in the coordination environment and reduction in symmetry arising from the stereochemical activity of lone pair cations to the emergence of filled antibonding lone-pair 6s2-O 2p hybridized states. The former is studied by high-resolution single-crystal X-ray diffraction studies of TlV3O8 and isostructural RbV3O8 to probe distinct differences in Tl and Rb coordination environments and the resulting modulation of V-V interactions in V3O8 slabs. The latter has been probed by variable-energy hard X-ray photoelectron spectroscopy (HAXPES) measurements, which manifest orbital-specific contributions from bonding and antibonding interactions of stereoactive Tl 6s2 electron lone pairs in TlV3O8. The spectroscopic assignment of valence band states to stereoactive lone pairs is further corroborated by first-principles electronic structure calculations, crystal orbital Hamilton population analyses, and electron localization function maps. The presence of the Tl 6s2 electron lone pair in TlV3O8 brings about the off-centering of Tl+ cations, which leads to anisotropy in Tl-O bonds. The off-centering of Tl ions weakens V-O bonds in one direction, which subsequently strengthens directional V-V coupling. Magnetic measurements reveal ferromagnetic signatures for both RbV3O8 and TlV3O8. However, the differences in V···V interactions significantly affect the energy balance of the superexchange interactions, resulting in an ordering temperature of 140 K for TlV3O8 as compared to 125 K for RbV3O8. The results demonstrate the distinctive effects of stereochemically active lone pairs in modifying electronic structure near the Fermi level and for mediating superexchange interactions.

Moral distress in rural veterinarians as an outcome of the Mycoplasma bovis incursion in southern New Zealand

AIMS

To gain insight into the world of rural veterinarians during the Mycoplasma bovis incursion within southern Aotearoa New Zealand by exploring their experiences during the incursion, and to understand the consequences, positive and negative, of these experiences.

METHODS

A qualitative social science research methodology, guided by the philosophical paradigm of pragmatism, was used to collect data from an information-rich sample (n = 6) of rural veterinarians from Otago and Southland. Interview and focus group techniques were used, both guided by a semi-structured interview guide. Veterinarians were asked a range of questions, including their role within the incursion; whether their involvement had any positive or negative impact for them; and their experience of conflicting demands. Analysis of the narrative data collected was guided by Braun and Clarke's approach to reflexive thematic analysis.

RESULTS AND FINDINGS

All six participants approached agreed to participate. Analysis of the data provided an understanding of the trauma they experienced during the incursion. An overarching theme of psychological distress was underpinned by four sub-themes, with epistemic injustice and bearing witness the two sub-themes reported to be associated with the greatest experience of psychological distress. These, along with the other two identified stressors, led to the experience of moral distress, with moral residue and moral injury also experienced by some participants.

CONCLUSIONS

Eradication programmes for exotic diseases in production animals inevitably have an impact on rural veterinarians, in their role working closely with farmers. Potentially, these impacts could be positive, recognising and utilising veterinarians' experience, skills and knowledge base. This study, however, illustrates the significant negative impacts for some rural veterinarians exposed to the recent M. bovis eradication programme in New Zealand, including experiences of moral distress and moral injury. Consequently, this eradication programme resulted in increased stress for study participants. There is a need to consider how the system addresses future exotic disease incursions to better incorporate and utilise the knowledge and skills of the expert workforce of rural veterinarians and to minimise the negative impacts on them.

CLINICAL RELEVANCE

To date, the experience of moral distress by rural veterinarians during exotic disease incursions has been under-reported globally and unexplored in New Zealand. The findings from this study contribute further insights to the existing limited literature and provide guidance on how to reduce the adverse experiences on rural veterinarians during future incursions.

ABBREVIATIONS

MPI: Ministry for Primary Industries; PITS: Perpetration-induced traumatic stress; PTSD: Post-traumatic stress disorder.

Identifying stressor criteria that hinder or challenge junior clinical medical student learning

INTRODUCTION

What makes something a stressor within clinical students' education is unclear. Medical students moving from a predominantly protected classroom environment to a situated-work environment provided an ideal transition point to explore the criteria that might make a learning experience a stressor and whether these stressors hinder or challenge learning.

METHOD

Data on the stressors associated with learning experiences in clinical education were collected from New Zealand undergraduate medical students. Free text comments, in a survey-based questionnaire were supplemented by focus group data. Using inductive thematic analysis with grounded theory, themes were generated about the characteristics of stressors; referred to here as stressor criteria. These stressor criteria were then classified according to their impact on perceived learning.

RESULTS

Under the broad headings of the nature of the learning task, external factors, internal factors, and social interaction; 12 stressor criteria groupings were defined. Some of these criteria were a positive challenge to learning (e.g. legitimacy of the task, novelty of the learning, social interactions) and others a hindrance.

DISCUSSION

Not all stressors hinder learning. Instead, and depending on their nature, many result in perceived assistance to learning. Stressors hindering learning need to be recognised by the teacher, especially those that can be converted from a hindrance to an assistance.

Evidence that gecko setae are coated with an ordered nanometre-thin lipid film

The fascinating adhesion of gecko to virtually any material has been related to surface interactions of myriads of spatula at the tips of gecko feet. Surprisingly, the molecular details of the surface chemistry of gecko adhesion are still largely unknown. Lipids have been identified within gecko adhesive pads. However, the location of the lipids, the extent to which spatula are coated with lipids, and how the lipids are structured are still open questions. Lipids can modulate adhesion properties and surface hydrophobicity and may play an important role in adhesion. We have therefore studied the molecular structure of lipids at spatula surfaces using near-edge X-ray absorption fine structure imaging. We provide evidence that a nanometre-thin layer of lipids is present at the spatula surfaces of the tokay gecko (Gekko gecko) and that the lipids form ordered, densely packed layers. Such dense, thin lipid layers can effectively protect the spatula proteins from dehydration by forming a barrier against water evaporation. Lipids can also render surfaces hydrophobic and thereby support the gecko adhesive system by enhancement of hydrophobic-hydrophobic interactions with surfaces.

Multimodal electrochemistry coupled microcalorimetric and X-ray probing of the capacity fade mechanisms of Nickel rich NMC - progress and outlook

Lithium nickel manganese cobalt oxide (NMC) is a commercially successful Li-ion battery cathode due to its high energy density; however, its delivered capacity must be intentionally limited to achieve capacity retention over extended cycling. To design next-generation NMC batteries with longer life and higher capacity the origins of high potential capacity fade must be understood. Operando hard X-ray characterization techniques are critical for this endeavor as they allow the acquisition of information about the evolution of structure, oxidation state, and coordination environment of NMC as the material (de)lithiates in a functional battery. This perspective outlines recent developments in the elucidation of capacity fade mechanisms in NMC through hard X-ray probes, surface sensitive soft X-ray characterization, and isothermal microcalorimetry. A case study on the effect of charging potential on NMC811 over extended cycling is presented to illustrate the benefits of these approaches. The results showed that charging to 4.7 V leads to higher delivered capacity, but much greater fade as compared to charging to 4.3 V. Operando XRD and SEM results indicated that particle fracture from increased structural distortions at >4.3 V was a contributor to capacity fade. Operando hard XAS revealed significant Ni and Co redox during cycling as well as a Jahn-Teller distortion at the discharged state (Ni³⁺); however, minimal differences were observed between the cells charged to 4.3 and 4.7 V. Additional XAS analyses using soft X-rays revealed significant surface reconstruction after cycling to 4.7 V, revealing another contribution to fade. Operando isothermal microcalorimetry (IMC) indicated that the high voltage charge to 4.7 V resulted in a doubling of the heat dissipation when compared to charging to 4.3 V. A lowered chemical-to-electrical energy conversion efficiency due to thermal energy waste was observed, providing a complementary characterization of electrochemical degradation. The work demonstrates the utility of multi-modal X-ray and microcalorimetric approaches to understand the causes of capacity fade in lithium-ion batteries with Ni-rich NMC.

Carbon-free high-performance cathode for solid-state Li-O2 battery

The development of a cathode for solid-state lithium-oxygen batteries has been hindered in practice by a low capacity and limited cycle life despite their potential for high energy density. Here, a previously unexplored strategy is proposed wherein the cathode delivers a specific capacity of 200 milliampere hour per gram over 665 discharge/charge cycles, while existing cathodes achieve only ~50 milliampere hour per gram and ~100 cycles. A highly conductive ruthenium-based composite is designed as a carbon-free cathode by first-principles calculations to avoid the degradation associated with carbonaceous materials, implying an improvement in stability during the electrochemical cycling. In addition, water vapor is added into the main oxygen gas as an additive to change the discharge product from growth-restricted lithium peroxide to easily grown lithium hydroxide, resulting in a notable increase in capacity. Thus, the proposed strategy is effective for developing reversible solid-state lithium-oxygen batteries with high energy density.

Structure of Keratins in Adhesive Gecko Setae Determined by Near-Edge X-ray Absorption Fine Structure Spectromicroscopy

Geckos have the astonishing ability to climb on vertical surfaces due to the adhesive properties of fibrous setae at the tips of their toe pads. While the adhesion mechanism principle, based on van der Waals interactions of myriads of spatula located at the outermost end of the setal arrays, has been studied extensively, there are still open questions about the chemistry of gecko setae. The gecko adhesive system is based on keratin fibrils assembled to support the entire setal structure. At the same time, the structure and alignment of keratin molecules within the ultrafine spatula tissue, which can support the enormous mechanical strain, still remain unknown. We have studied the molecular structure of gecko spatula using near-edge X-ray absorption fine structure (NEXAFS) imaging. We indeed found that the setae consist of a β-sheet structure aligned with the adhesion direction of the setae. Such alignment may provide mechanical stability to the setae and resistance to wear across different length scales.

Relationships among perceived learning, challenge and affect in a clinical context

BACKGROUND

Challenge, sometimes perceived as stress, may be beneficial or detrimental to learning but the circumstances when it may be beneficial are not clear. This study looks at the association of challenge with perceived learning and how this might be influenced by affect, context or the type of learning.

METHOD

The participants, medical students in their first years of experiential clinical exposure, rated specified learning episodes (LEs) on the perceived learning (low to high), challenge (low to high) and affect (feeling positive to negative). Such learning episodes were self-identified or identified by course organisers. Correlations, using Kendall's tau-b test, were conducted to explore the associations among learning, challenge and affect. In the second stage the types of LEs were then thematically classified in order to determine those that were positive for learning and challenging and/or associated with positive affect.

RESULT

There were positive correlations between perceived learning and challenge, and between perceived learning and affect for both types of LEs. The circumstances in which challenge (stress) promoted learning were authentic environments, authentic tasks and simulated clinical activities; most requiring a degree of social interaction.

CONCLUSION

Challenge and positive affect are beneficial in the perception of discrete learning, but are two separate constructs. Ideally both challenge and affect need to operate alongside authentic supportive clinical activities, that by their nature involve others, to maximise perceived learning.

Charge-transfer satellites and chemical bonding in photoemission and x-ray absorption of SrTiO3 and rutile TiO2: Experiment and first-principles theory with general application to spectroscopic analysis

First-principles, real-time-cumulant, and Bethe-Salpeter-equation calculations fully capture the detailed satellite structure that occurs in response to the sudden creation of the core hole in both photoemission and x-ray absorption spectra of the transition-metal compounds SrTiO3 and rutile TiO2. Analysis of the excited-state, real-space charge-density fluctuations betrays the physical nature of these many electron excitations that are shown to reflect the materials' solid-state electronic structure and chemical bonding. This first-principles development of the cumulant-based core hole spectral function is generally applicable to other systems and should become a standard tool for all similar spectroscopic analysis going beyond the quasiparticle physics of the photoelectric effect.

Core level spectroscopies locate hydrogen in the proton transfer pathway - identifying quasi-symmetrical hydrogen bonds in the solid state

Short, strong hydrogen bonds (SSHBs) have been a source of interest and considerable speculation over recent years, culminating with those where hydrogen resides around the midpoint between the donor and acceptor atoms, leading to quasi-covalent nature. We demonstrate that X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy provide deep insight into the electronic structure of the short OHN hydrogen bond of 3,5-pyridinedicarboxylic acid, revealing for the first time distinctive spectroscopic identifiers for these quasi-symmetrical hydrogen bonds. An intermediate nitrogen (core level) chemical shift occurs for the almost centrally located hydrogen compared to protonated (ionic) and non-ionic analogues, and it reveals the absence of two-site disorder. This type of bonding is also evident through broadening of the nitrogen 1s photoemission and 1s → 1π* peaks in XPS and NEXAFS, respectively, arising from the femtosecond lifetimes of hydrogen in the potential wells slightly offset to either side of the centre. The line-shape of the core level excitations are thus related to the population occupancies, reflecting the temperature-dependent shape of the hydrogen potential energy well. Both XPS and NEXAFS provide a distinctive identifier for these quasi-symmetrical hydrogen bonds, paving the way for detailed studies into their prevalence and potentially unique physical and chemical properties.

Reversible epitaxial electrodeposition of metals in battery anodes

The propensity of metals to form irregular and nonplanar electrodeposits at liquid-solid interfaces has emerged as a fundamental barrier to high-energy, rechargeable batteries that use metal anodes. We report an epitaxial mechanism to regulate nucleation, growth, and reversibility of metal anodes. The crystallographic, surface texturing, and electrochemical criteria for reversible epitaxial electrodeposition of metals are defined and their effectiveness demonstrated by using zinc (Zn), a safe, low-cost, and energy-dense battery anode material. Graphene, with a low lattice mismatch for Zn, is shown to be effective in driving deposition of Zn with a locked crystallographic orientation relation. The resultant epitaxial Zn anodes achieve exceptional reversibility over thousands of cycles at moderate and high rates. Reversible electrochemical epitaxy of metals provides a general pathway toward energy-dense batteries with high reversibility.

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.

Student belief about the value of challenge

BACKGROUND

The undergraduate curriculum tends to focus on how individuals can cope with stress especially when transitioning from the classroom to the clinical workplace environment. Often this carries the message that stress is bad, yet little attention has been paid to the influence of one's belief regarding the value of stress for learning. Because stress is often perceived as bad, we chose to use the term 'challenge' in exploring the associations amongst belief of the value of challenge, the challenge experienced, the perceived learning, affect and staff support.

METHODS

At the end of each clinical module within a medical curriculum, medical students rated the perceived learning, degree of challenge, affect, support and the value of challenge for learning. The value and associations amongst these variables were analysed.

RESULTS

The challenge for students varied according to the type of module. Students generally considered that challenge promoted rather than hindered learning. The level of challenge experienced may influence the perception of the value of challenge for learning. However, when challenge was regarded as beneficial, this was strongly, positively associated with perceived learning, positive affect and support.

DISCUSSION

Students who believe challenge is positive also perceive that such challenges promote learning. Likewise students who regard challenge as negative are less likely to learn from such challenges. The positive relationship between the belief of the value of challenge with affect and support may have positive implications for well-being. It is contended that curriculum planners should acknowledge the potential positive influence of stressors in clinical education and that challenge can be seen as valuable when there is student support and measures associated with maintaining a positive affect.

Comparison of soil organic carbon speciation using C NEXAFS and CPMAS 13C NMR spectroscopy

We compared synchrotron-based C near-edge X-ray absorption fine structure (NEXAFS) and CPMAS ¹³C nuclear magnetic resonance (NMR) spectroscopy with respect to their precision and accuracy to quantify different organic carbon (OC) species in defined mixtures of soil organic matter source compounds. We also used both methods to quantify different OC species in organic surface horizons of a Histic Leptosol as well as in mineral topsoil and subsoil horizons of two soils with different parent material, stage of pedogenesis, and OC content (Cambisol: 15-30 OC mgg^-1, Podzol: 0.9-7 OC mgg^-1). CPMAS ¹³C NMR spectroscopy was more accurate and precise (mean recovery of different C functional groups 96-103%) than C NEXAFS spectroscopy (mean recovery 92-113%). For organic surface and topsoil samples, NMR spectroscopy consistently yielded larger O-alkyl C percentages and smaller alkyl C percentages than C NEXAFS spectroscopy. For the Cambisol subsoil samples both methods performed well and showed similar C speciation results. NEXAFS spectroscopy yielded excellent spectra with a high signal-to-noise ratio also for OC-poor Podzol subsoil samples, whereas this was not the case for CPMAS ¹³C NMR spectroscopy even after sample treatment with HF. Our results confirm the analytical power of CPMAS ¹³C NMR spectroscopy for a reliable quantitative OC speciation in soils with >10mgOCg^-1. Moreover, they highlight the potential of synchrotron-based C NEXAFS spectroscopy as fast, non-invasive method to semi-quantify different C functional groups in soils with low C content (0.9-10mgg^-1).

An all-diamond X-ray position and flux monitor using nitrogen-incorporated ultra-nanocrystalline diamond contacts

Diamond X-ray detectors with conducting nitrogen-incorporated ultra-nanocrystalline diamond (N-UNCD) films as electrodes were fabricated to measure X-ray beam flux and position. Structural characterization and functionality tests were performed for these devices. The N-UNCD films grown on unseeded diamond substrates were compared with N-UNCD films grown on a seeded silicon substrate. The feasibility of the N-UNCD films acting as electrodes for X-ray detectors was confirmed by the stable performance in a monochromatic X-ray beam. The fabrication process is able to change the surface status which may influence the signal uniformity under low bias, but this effect can be neglected under full collection bias.

A practical superconducting-microcalorimeter X-ray spectrometer for beamline and laboratory science

We describe a series of microcalorimeter X-ray spectrometers designed for a broad suite of measurement applications. The chief advantage of this type of spectrometer is that it can be orders of magnitude more efficient at collecting X-rays than more traditional high-resolution spectrometers that rely on wavelength-dispersive techniques. This advantage is most useful in applications that are traditionally photon-starved and/or involve radiation-sensitive samples. Each energy-dispersive spectrometer is built around an array of several hundred transition-edge sensors (TESs). TESs are superconducting thin films that are biased into their superconducting-to-normal-metal transitions. The spectrometers share a common readout architecture and many design elements, such as a compact, 65 mK detector package, 8-column time-division-multiplexed superconducting quantum-interference device readout, and a liquid-cryogen-free cryogenic system that is a two-stage adiabatic-demagnetization refrigerator backed by a pulse-tube cryocooler. We have adapted this flexible architecture to mate to a variety of sample chambers and measurement systems that encompass a range of observing geometries. There are two different types of TES pixels employed. The first, designed for X-ray energies below 10 keV, has a best demonstrated energy resolution of 2.1 eV (full-width-at-half-maximum or FWHM) at 5.9 keV. The second, designed for X-ray energies below 2 keV, has a best demonstrated resolution of 1.0 eV (FWHM) at 500 eV. Our team has now deployed seven of these X-ray spectrometers to a variety of light sources, accelerator facilities, and laboratory-scale experiments; these seven spectrometers have already performed measurements related to their applications. Another five of these spectrometers will come online in the near future. We have applied our TES spectrometers to the following measurement applications: synchrotron-based absorption and emission spectroscopy and energy-resolved scattering; accelerator-based spectroscopy of hadronic atoms and particle-induced-emission spectroscopy; laboratory-based time-resolved absorption and emission spectroscopy with a tabletop, broadband source; and laboratory-based metrology of X-ray-emission lines. Here, we discuss the design, construction, and operation of our TES spectrometers and show first-light measurements from the various systems. Finally, because X-ray-TES technology continues to mature, we discuss improvements to array size, energy resolution, and counting speed that we anticipate in our next generation of TES-X-ray spectrometers and beyond.

Isotropic thin PTCDA films on GaN(0 0 0 1)

The growth of 3, 4, 9, 10-perylene tetracarboxylic dianhydride (PTCDA) on the Ga-polar GaN(0 0 0 1) surface has been studied by x-ray photoelectron spectroscopy (XPS), spot profile analysis low-energy electron diffraction (SPA-LEED), near edge x-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). The stoichiometric ratios derived from XPS indicate that the molecules remain intact upon adsorption on the surface. Furthermore, no chemical shifts can be observed in the C 1s and O 1s core levels with progressing deposition of PTCDA, suggesting none or only weak interactions between the molecules and the substrate. NEXAFS data indicate the PTCDA molecules being oriented with their molecular plane parallel to the surface. High-resolution STM shows PTCDA islands of irregular shape on the sub-micron scale, and together with corresponding SPA-LEED data reveals a lateral ordering of the molecules that is compatible with the presence of (1 0 2) oriented PTCDA nano-crystals. SPA-LEED moreover clearly shows the presence of homogeneously distributed rotational domains of two-dimensionally isotropic PTCDA.

In Situ Solid-State Reactions Monitored by X-ray Absorption Spectroscopy: Temperature-Induced Proton Transfer Leads to Chemical Shifts

The dramatic colour and phase alteration with the solid-state, temperature-dependent reaction between squaric acid and 4,4'-bipyridine has been probed in situ with X-ray absorption spectroscopy. The electronic and chemical sensitivity to the local atomic environment through chemical shifts in the near-edge X-ray absorption fine structure (NEXAFS) revealed proton transfer from the acid to the bipyridine base through the change in nitrogen protonation state in the high-temperature form. Direct detection of proton transfer coupled with structural analysis elucidates the nature of the solid-state process, with intermolecular proton transfer occurring along an acid-base chain followed by a domino effect to the subsequent acid-base chains, leading to the rapid migration along the length of the crystal. NEXAFS thereby conveys the ability to monitor the nature of solid-state chemical reactions in situ, without the need for a priori information or long-range order.

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.

Ligand-induced dependence of charge transfer in nanotube-quantum dot heterostructures

As a model system to probe ligand-dependent charge transfer in complex composite heterostructures, we fabricated double-walled carbon nanotube (DWNT)-CdSe quantum dot (QD) composites. Whereas the average diameter of the QDs probed was kept fixed at ∼4.1 nm and the nanotubes analyzed were similarly oxidatively processed, by contrast, the ligands used to mediate the covalent attachment between the QDs and DWNTs were systematically varied to include p-phenylenediamine (PPD), 2-aminoethanethiol (AET), and 4-aminothiophenol (ATP). Herein, we have put forth a unique compilation of complementary data from experiment and theory, including results from transmission electron microscopy (TEM), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, Raman spectroscopy, electrical transport measurements, and theoretical modeling studies, in order to fundamentally assess the nature of the charge transfer between CdSe QDs and DWNTs, as a function of the structure of various, intervening bridging ligand molecules. Specifically, we correlated evidence of charge transfer as manifested by changes and shifts associated with NEXAFS intensities, Raman peak positions, and threshold voltages both before and after CdSe QD deposition onto the underlying DWNT surface. Importantly, for the first time ever in these types of nanoscale composite systems, we have sought to use theoretical modeling to justify and account for our experimental results. Our overall data suggest that (i) QD coverage density on the DWNTs varies, based upon the different ligand pendant groups used and that (ii) the presence of a π-conjugated carbon framework within the ligands themselves coupled with the electron affinity of their pendant groups collectively play important roles in the resulting charge transfer from QDs to the underlying CNTs.

Atomistic Interrogation of B-N Co-dopant Structures and Their Electronic Effects in Graphene

Chemical doping has been demonstrated to be an effective method for producing high-quality, large-area graphene with controlled carrier concentrations and an atomically tailored work function. The emergent optoelectronic properties and surface reactivity of carbon nanostructures are dictated by the microstructure of atomic dopants. Co-doping of graphene with boron and nitrogen offers the possibility to further tune the electronic properties of graphene at the atomic level, potentially creating p- and n-type domains in a single carbon sheet, opening a gap between valence and conduction bands in the 2-D semimetal. Using a suite of high-resolution synchrotron-based X-ray techniques, scanning tunneling microscopy, and density functional theory based computation we visualize and characterize B-N dopant bond structures and their electronic effects at the atomic level in single-layer graphene grown on a copper substrate. We find there is a thermodynamic driving force for B and N atoms to cluster into BNC structures in graphene, rather than randomly distribute into isolated B and N graphitic dopants, although under the present growth conditions, kinetics limit segregation of large B-N domains. We observe that the doping effect of these BNC structures, which open a small band gap in graphene, follows the B:N ratio (B > N, p-type; B < N, n-type; B═N, neutral). We attribute this to the comparable electron-withdrawing and -donating effects, respectively, of individual graphitic B and N dopants, although local electrostatics also play a role in the work function change.

Mapping polaronic states and lithiation gradients in individual V2O5 nanowires

The rapid insertion and extraction of Li ions from a cathode material is imperative for the functioning of a Li-ion battery. In many cathode materials such as LiCoO2, lithiation proceeds through solid-solution formation, whereas in other materials such as LiFePO4 lithiation/delithiation is accompanied by a phase transition between Li-rich and Li-poor phases. We demonstrate using scanning transmission X-ray microscopy (STXM) that in individual nanowires of layered V2O5, lithiation gradients observed on Li-ion intercalation arise from electron localization and local structural polarization. Electrons localized on the V2O5 framework couple to local structural distortions, giving rise to small polarons that serves as a bottleneck for further Li-ion insertion. The stabilization of this polaron impedes equilibration of charge density across the nanowire and gives rise to distinctive domains. The enhancement in charge/discharge rates for this material on nanostructuring can be attributed to circumventing challenges with charge transport from polaron formation.

Synthesis and Characterization of Aminopropyltriethoxysilane-Polydopamine Coatings

Polydopamine coatings are of interest due to the fact that they can promote adhesion to a broad range of materials and can enable a variety of applications. However, the polydopamine-substrate interaction is often noncovalent. To broaden the potential applications of polydopamine, we show the incorporation of 3-aminopropyltriethoxysilane (APTES), a traditional coupling agent capable of covalent bonding to a broad range of organic and inorganic surfaces, into polydopamine coatings. High energy X-ray photoelectron spectroscopy (HE-XPS), conventional XPS, near-edge X-ray absorption fine structure (NEXAFS), Fourier transform infrared-attenuated total reflectance (FTIR-ATR), and ellipsometry measurements were used to investigate changes in coating chemistry and thickness, which suggest covalent incorporation of APTES into polydopamine. These coatings can be deposited either in Tris buffer or by using an aqueous APTES solution as a buffer without Tris. APTES-dopamine hydrochloride deposition from solutions with molar ratios between 0:1 and 10:1 allowed us to control the coating composition across a broad range.

NEXAFS Sensitivity to Bond Lengths in Complex Molecular Materials: A Study of Crystalline Saccharides

Detailed analysis of the C K near-edge X-ray absorption fine structure (NEXAFS) spectra of a series of saccharides (fructose, xylose, glucose, galactose, maltose monohydrate, α-lactose monohydrate, anhydrous β-lactose, cellulose) indicates that the precise determination of IPs and σ* shape resonance energies is sensitive enough to distinguish different crystalline saccharides through the variations in their average C-OH bond lengths. Experimental data as well as FEFF8 calculations confirm that bond length variations in the organic solid state of 10(-2) Å can be experimentally detected, opening up the possibility to use NEXAFS for obtaining incisive structural information for molecular materials, including noncrystalline systems without long-range order such as dissolved species in solutions, colloids, melts, and similar amorphous phases. The observed bond length sensitivity is as good as that originally reported for gas-phase and adsorbed molecular species. NEXAFS-derived molecular structure data for the condensed phase may therefore be used to guide molecular modeling as well as to validate computationally derived structure models for such systems. Some results indicate further analytical value in that the σ* shape resonance analysis may distinguish hemiketals from hemiacetals (i.e., derived from ketoses and aldoses) as well as α from β forms of otherwise identical saccharides.

High-Surface-Area Nitrogen-Doped Reduced Graphene Oxide for Electric Double-Layer Capacitors

A two-step method consisting of solid-state microwave irradiation and heat treatment under NH3 gas was used to prepare nitrogen-doped reduced graphene oxide (N-RGO) with a high specific surface area (1007 m(2)  g(-1) ), high electrical conductivity (1532 S m(-1) ), and low oxygen content (1.5 wt %) for electrical double-layer capacitor applications. The specific capacitance of N-RGO was 291 F g(-1) at a current density of 1 A g(-1) , and a capacitance of 261 F g(-1) was retained at 50 A g(-1) , which indicated a very good rate capability. N-RGO also showed excellent cycling stability and preserved 96 % of the initial specific capacitance after 100 000 cycles. Near-edge X-ray absorption fine-structure spectroscopy results provided evidenced for the recovery of π conjugation in the carbon networks with the removal of oxygenated groups and revealed chemical bonding of the nitrogen atoms in N-RGO. The good electrochemical performance of N-RGO is attributed to its high surface area, high electrical conductivity, and low oxygen content.

High-resolution X-ray emission spectroscopy with transition-edge sensors: present performance and future potential

X-ray emission spectroscopy (XES) is a powerful element-selective tool to analyze the oxidation states of atoms in complex compounds, determine their electronic configuration, and identify unknown compounds in challenging environments. Until now the low efficiency of wavelength-dispersive X-ray spectrometer technology has limited the use of XES, especially in combination with weaker laboratory X-ray sources. More efficient energy-dispersive detectors have either insufficient energy resolution because of the statistical limits described by Fano or too low counting rates to be of practical use. This paper updates an approach to high-resolution X-ray emission spectroscopy that uses a microcalorimeter detector array of superconducting transition-edge sensors (TESs). TES arrays are discussed and compared with conventional methods, and shown under which circumstances they are superior. It is also shown that a TES array can be integrated into a table-top time-resolved X-ray source and a soft X-ray synchrotron beamline to perform emission spectroscopy with good chemical sensitivity over a very wide range of energies.

Interrogating the Carbon and Oxygen K-Edge NEXAFS of a CO2-Dosed Hyperbranched Aminosilica

Using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, we shed light on the nature of the interaction between CO2 and the amine moieties in a hyperbranched aminosilica (HAS) material, a porous aminosilica composite with great potential for postcombustion carbon capture applications. We show that after dosing a pristine (annealed) HAS sample with CO2, the C K-edge NEXAFS spectrum presents a new π* resonance at 289.9 eV, which can be attributed to the formation of a C═O (carbonyl) bond. Additional analyses of the O K-edge using model samples containing carbamate, carbonate, and bicarbonate functional groups as reference demonstrate a carbamate bonding mechanism for the chemical adsorption of CO2 by the HAS material under the conditions employed. These findings show the capability of the C and O K-edge NEXAFS technique to identify CO2-adsorbate species despite the high concentration of C and O atoms inherently present in the sample (prior to CO2 dosing) and the significant similarities between the possible adsorbates.

Role of pentahedrally coordinated titanium in titanium silicalite-1 in propene epoxidation

A new pentahedrally coordinated Ti was discovered in TS-1 and showed a higher catalytic activity than the tetrahedrally coordinated one.

Incisive probing of intermolecular interactions in molecular crystals: core level spectroscopy combined with density functional theory

The α-form of crystalline para-aminobenzoic acid (PABA) has been examined as a model system for demonstrating how the core level spectroscopies X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) can be combined with CASTEP density functional theory (DFT) to provide reliable modeling of intermolecular bonding in organic molecular crystals. Through its dependence on unoccupied valence states NEXAFS is an extremely sensitive probe of variations in intermolecular bonding. Prediction of NEXAFS spectra by CASTEP, in combination with core level shifts predicted by WIEN2K, reproduced experimentally observed data very well when all significant intermolecular interactions were correctly taken into account. CASTEP-predicted NEXAFS spectra for the crystalline state were compared with those for an isolated PABA monomer to examine the impact of intermolecular interactions and local environment in the solid state. The effects of the loss of hydrogen-bonding in carboxylic acid dimers and intermolecular hydrogen bonding between amino and carboxylic acid moieties are evident, with energy shifts and intensity variations of NEXAFS features arising from the associated differences in electronic structure and bonding.

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.

Core level crystallography: Probing H-bonding through nitrogen XPS and NEXAFS

Determining the location of hydrogen is not always straightforward, despite its potential for wide-reaching effects, such as altering physicochemical properties and biological/chemical processes. Proton transfer can be considered a simple chemical reaction, with a continuum from neutral to protonated states, and short, strong H-bonds (SSHB) and disordered systems between the two extremes. X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) intrinsically probe the local environment, with sensitivity to the chemical state of the atom and, importantly, nature of the local chemical and bonding environment. Organic molecular crystals have been studied by nitrogen XPS and NEXAFS, offering an alternative to X-ray and neutron diffraction. Strong chemical shifts occur with proton transfer to nitrogen (+N-H---O vs. N---H-O), unambiguously characterizing protonated and H-bonded systems,[1] leading to direct observation of an unusual solid-state colour change for 4,4'-bipyridine/squaric acid with heating[2] involving proton transfer to nitrogen with temperature-dependent measurements. Correlation between H-bond lengths and chemical shifts indicates potential for predicting H-bond lengths. SSHBs provide an interesting case, as hydrogen can reside midway between donor and acceptor, having a 3-centre, 4-electron bond with quasi-covalent character and atypical properties. Intermediate chemical shifts are found with hydrogen midway between donor and acceptor in 3,5-pyridinedicarboxylic acid, with increased peak width representative of hydrogen's broadened single minimum potential well.[3] This contrasts with conventional 2-site hydrogen disorder, in which signals from both donor and acceptor environments result in 2 peaks reflecting the % occupancy. Valuable electronic and structural information is obtained from the variety of organic systems investigated, with XPS clearly distinguishing different types of crystallographic materials (Fig 1).

Thermo-Active Behavior of Ethylene-Vinyl Acetate | Multiwall Carbon Nanotube Composites Examined by in Situ near-Edge X-ray Absorption Fine-Structure Spectroscopy

NEXAFS spectroscopy was used to investigate the temperature dependence of thermally active ethylene-vinyl acetate | multiwall carbon nanotube (EVA|MWCNT) films. The data shows systematic variations of intensities with increasing temperature. Molecular orbital assignment of interplaying intensities identified the 1s → π*_C=C and 1s → π*_C=O transitions as the main actors during temperature variation. Furthermore, enhanced near-edge interplay was observed in prestrained composites. Because macroscopic observations confirmed enhanced thermal-mechanical actuation in prestrained composites, our findings suggest that the interplay of C=C and C=O π orbitals may be instrumental to actuation.

Conservation of artists' acrylic emulsion paints: XPS, NEXAFS and ATR-FTIR studies of wet cleaning methods

Works of art prepared with acrylic emulsion paints became commercially available in the 1960s. It is increasingly necessary to undertake and optimise cleaning and preventative conservation treatments to ensure their longevity. Model artists' acrylic paint films covered with artificial soiling were thus prepared on a canvas support and exposed to a variety of wet cleaning treatments based on aqueous or hydrocarbon solvent systems. This included some with additives such as chelating agents and/or surfactants, and microemulsion systems made specifically for conservation practice. The impact of cleaning (soiling removal) on the paint film surface was examined visually and correlated with results of attenuated total reflection Fourier transform infrared, XPS and near-edge X-ray absorption fine structure analyses – three spectroscopic techniques with increasing surface sensitivity ranging from approximately − 1000, 10 and 5 nm, respectively. Visual analysis established the relative cleaning efficacy of the wet cleaning treatments in line with previous results. X-ray spectroscopy analysis provided significant additional findings, including evidence for (i) surfactant extraction following aqueous swabbing, (ii) modifications to pigment following cleaning and (iii) cleaning system residues. © 2014 The Authors. Surface and Interface Analysis published by John Wiley & Sons, Ltd.

Near-edge X-ray absorption fine structure studies of electrospun poly(dimethylsiloxane)/poly(methyl methacrylate)/multiwall carbon nanotube composites

This work describes the near conduction band edge structure of electrospun mats of multiwalled carbon nanotube (MWCNT)-polydimethylsiloxane-poly(methyl methacrylate) by near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Effects of adding nanofillers of different sizes were addressed. Despite observed morphological variations and inhomogeneous carbon nanotube distribution, spun mats appeared homogeneous under NEXAFS analysis. Spectra revealed differences in emissions from glancing and normal spectra, which may evidence phase separation within the bulk of the micrometer-size fibers. Further, dichroic ratios show polymer chains did not align, even in the presence of nanofillers. Addition of nanofillers affected emissions in the C-H, C═O, and C-C regimes, suggesting their involvement in interfacial matrix-carbon nanotube bonding. Spectral differences at glancing angles between pristine and composite mats suggest that geometric conformational configurations are taking place between polymeric chains and carbon nanotubes. These differences appear to be carbon nanotube-dimension dependent and are promoted upon room temperature mixing and shear flow during electrospinning. CH-π bonding between polymer chains and graphitic walls, as well as H-bonds between impurities in the as-grown MWCNTs and polymer pendant groups are proposed bonding mechanisms promoting matrix conformation.

Local atomic and electronic structure of boron chemical doping in monolayer graphene

We use scanning tunneling microscopy and X-ray spectroscopy to characterize the atomic and electronic structure of boron-doped and nitrogen-doped graphene created by chemical vapor deposition on copper substrates. Microscopic measurements show that boron, like nitrogen, incorporates into the carbon lattice primarily in the graphitic form and contributes ~0.5 carriers into the graphene sheet per dopant. Density functional theory calculations indicate that boron dopants interact strongly with the underlying copper substrate while nitrogen dopants do not. The local bonding differences between graphitic boron and nitrogen dopants lead to large scale differences in dopant distribution. The distribution of dopants is observed to be completely random in the case of boron, while nitrogen displays strong sublattice clustering. Structurally, nitrogen-doped graphene is relatively defect-free while boron-doped graphene films show a large number of Stone-Wales defects. These defects create local electronic resonances and cause electronic scattering, but do not electronically dope the graphene film.

Inside and Outside: X-ray Absorption Spectroscopy Mapping of Chemical Domains in Graphene Oxide

The oxidative chemistry of graphite has been investigated for over 150 years and has attracted renewed interest given the importance of exfoliated graphene oxide as a precursor to chemically derived graphene. However, the bond connectivities, steric orientations, and spatial distribution of functional groups remain to be unequivocally determined for this highly inhomogeneous nonstoichiometric material. Here, we demonstrate the application of principal component analysis to scanning transmission X-ray microscopy data for the construction of detailed real space chemical maps of graphene oxide. These chemical maps indicate very distinct functionalization motifs at the edges and interiors and, in conjunction with angle-resolved near-edge X-ray absorption fine structure spectroscopy, enable determination of the spatial location and orientations of functional groups. Chemical imaging of graphene oxide provides experimental validation of the modified Lerf-Klinowski structural model. Specifically, we note increased contributions from carboxylic acid moieties at edge sites with epoxide and hydroxyl species dominant within the interior domains.

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

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, as a technique, offers detailed information about the bonding environment of molecules at a surface. However, because it is a synchrotron based method, beam-time is limited and users must typically prioritize and narrowly define the scopes of experiments. In this study, we demonstrate a novel method that opens up the possibility of the use of large area NEXAFS imaging to pursue combinatorial studies. To explore the capabilities of the NIST full field NEXAFS microscope available at the National Synchrotron Light Source as a high throughput imaging instrument, we collected NEXAFS images from a sample array consisting of 144 different elements with a periodic sequence of different surface modifications. NEXAFS images collected from this model system illustrate how hyperspectral NEXAFS data can be used for parallel analysis of large numbers of samples either directly from the overall image or by extracting spectra from regions of interest.