Megahertz-rate ultrafast X-ray scattering and holographic imaging at the European XFEL.

The advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, results from the first megahertz-repetition-rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL are presented. The experimental capabilities that the SCS instrument offers, resulting from the operation at megahertz repetition rates and the availability of the novel DSSC 2D imaging detector, are illustrated. Time-resolved magnetic X-ray scattering and holographic imaging experiments in solid state samples were chosen as representative, providing an ideal test-bed for operation at megahertz rates. Our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range.

Valence Band Structure Engineering in Graphene Derivatives.

Engineering of the 2D materials' electronic structure is at the forefront of nanomaterials research nowadays, giving an advance in the development of next-generation photonic devices, e-sensing technologies, and smart materials. Herein, employing core-level spectroscopy methods combined with density functional theory (DFT) modeling, the modification of the graphenes' valence band (VB) upon its derivatization by carboxyls and ketones is revealed. The appearance of a set of localized states in the VB of graphene related to molecular orbitals of the introduced functionalities is signified both experimentally and theoretically. Applying the DFT calculations of the density of states projected on the functional groups, their contributions to the VB structure are decomposed. An empirical approach, allowing one to analyze and predict the impact of a certain functional group on the graphenes' electronic structure in terms of examination of the model molecules, mimicking the introduced functionality, is proposed and validated. The interpretation of the arising states origin is made and their designation, pointing out their symmetry type, is proposed. Taken together, these results guide the band structure engineering of graphene derivatives and give a hint on the mechanisms underlying the alteration of the VB structure of 2D materials upon their derivatization.

Phase Transformation in UHMWPE Reactor Powders Synthesized on Various Catalysts in Mechanical and Thermal Fields.

Nowadays, a solvent-free method for production of high performance fibers directly from ultrahigh-molecular-weight polyethylene (UHMWPE) reactor powder is being actively developed. It causes the interest in the morphology of the reactor particles and their behavior in thermal and mechanical fields. Changes in the phase composition in virgin particles of ultra-high molecular-weight polyethylene reactor powders and in particles of powders compressed at room temperature under different pressures were studied in real time using synchrotron radiation with heating in the range of 300-370 K. It was found that the content of the monoclinic phase in reactor powders depends on the type of catalyst used for synthesis and on the applied pressure. It is shown that there are monoclinic phases of different nature: a structurally stabilized monoclinic phase formed during synthesis, and a monoclinic phase resulting from plastic deformation during compaction at room temperature. The behavior of these phases in temperature and mechanical fields is compared.

Graphene Amination towards Its Grafting by Antibodies for Biosensing Applications.

The facile synthesis of biografted 2D derivatives complemented by a nuanced understanding of their properties are keystones for advancements in biosensing technologies. Herein, we thoroughly examine the feasibility of aminated graphene as a platform for the covalent conjugation of monoclonal antibodies towards human IgG immunoglobulins. Applying core-level spectroscopy methods, namely X-ray photoelectron and absorption spectroscopies, we delve into the chemistry and its effect on the electronic structure of the aminated graphene prior to and after the immobilization of monoclonal antibodies. Furthermore, the alterations in the morphology of the graphene layers upon the applied derivatization protocols are assessed by electron microscopy techniques. Chemiresistive biosensors composed of the aerosol-deposited layers of the aminated graphene with the conjugated antibodies are fabricated and tested, demonstrating a selective response towards IgM immunoglobulins with a limit of detection as low as 10 pg/mL. Taken together, these findings advance and outline graphene derivatives' application in biosensing as well as hint at the features of the alterations of graphene morphology and physics upon its functionalization and further covalent grafting by biomolecules.

Toward On-Chip Multisensor Arrays for Selective Methanol and Ethanol Detection at Room Temperature: Capitalizing the Graphene Carbonylation.

The artificial olfaction units (or e-noses) capable of room-temperature operation are highly demanded to meet the requests of society in numerous vital applications and developing Internet-of-Things. Derivatized 2D crystals are considered as sensing elements of choice in this regard, unlocking the potential of the advanced e-nose technologies limited by the current semiconductor technologies. Herein, we consider fabrication and gas-sensing properties of On-chip multisensor arrays based on a hole-matrixed carbonylated (C-ny) graphene film with a gradually changed thickness and concentration of ketone groups of up to 12.5 at.%. The enhanced chemiresistive response of C-ny graphene toward methanol and ethanol, of hundred ppm concentration when mixing with air to match permissible exposure OSHA limits, at room-temperature operation is signified. Following thorough characterization via core-level techniques and density functional theory, the predominant role of the C-ny graphene-perforated structure and abundance of ketone groups in advancing the chemiresistive effect is established. Advancing practice applications, selective discrimination of the studied alcohols is approached by linear discriminant analysis employing a multisensor array's vector signal, and the fabricated chip's long-term performance is shown.

Template-Directed Polymerization Strategy for Producing rGO/UHMWPE Composite Aerogels with Tunable Properties.

In this paper, we suggest a previously unknown template-directed polymerization strategy for producing graphene/polymer aerogels with elevated mechanical properties, preservation of the nanoscale pore structure, an extraordinary crystallite structure, as well as tunable electrical and hydrophobic properties. The suggested approach is studied using the reduced graphene oxide (rGO)/ultrahigh molecular weight polyethylene (UHMWPE) system as an example. We also develop a novel method of ethylene polymerization with formation of UHMWPE directly on the surface of rGO sheets prestructured as the aerogel template. At a UHMWPE content smaller than 20 wt %, composite materials demonstrate completely reversible deformation and good conductivity. An ultrahigh polymer content (more than 80 wt %) results in materials with pronounced plasticity, improved hydrophobic properties, and a Young's modulus that is more than 200 times larger than that of pure rGO aerogel. Variation of the polymer content makes it possible to tune the electro-conductive properties of the aerogel in the range from 4.8 × 10-6 to 4.9 × 10-1 S/m and adjust its hydrophobic properties. The developed approach would make it possible to create composite materials with highly developed nanostructural morphology and advanced properties controlled by the thickness of the polymer layer on the surface of graphene sheets.

Manifesting Epoxide and Hydroxyl Groups in XPS Spectra and Valence Band of Graphene Derivatives.

The derivatization of graphene to engineer its band structure is a subject of significant attention nowadays, extending the frames of graphene material applications in the fields of catalysis, sensing, and energy harvesting. Yet, the accurate identification of a certain group and its effect on graphene's electronic structure is an intricate question. Herein, we propose the advanced fingerprinting of the epoxide and hydroxyl groups on the graphene layers via core-level methods and reveal the modification of their valence band (VB) upon the introduction of these oxygen functionalities. The distinctive contribution of epoxide and hydroxyl groups to the C 1s X-ray photoelectron spectra was indicated experimentally, allowing the quantitative characterization of each group, not just their sum. The appearance of a set of localized states in graphene's VB related to the molecular orbitals of the introduced functionalities was signified both experimentally and theoretically. Applying the density functional theory calculations, the impact of the localized states corresponding to the molecular orbitals of the hydroxyl and epoxide groups was decomposed. Altogether, these findings unveiled the particular contribution of the epoxide and hydroxyl groups to the core-level spectra and band structure of graphene derivatives, advancing graphene functionalization as a tool to engineer its physical properties.

Graphene Oxide Chemistry Management via the Use of KMnO4/K2Cr2O7 Oxidizing Agents.

In this paper, we propose a facile approach to the management of graphene oxide (GO) chemistry via its synthesis using KMnO4/K2Cr2O7 oxidizing agents at different ratios. Using Fourier Transformed Infrared Spectroscopy, X-ray Photoelectron Spectroscopy, and X-ray Absorption Spectroscopy, we show that the number of basal-plane and edge-located oxygenic groups can be controllably tuned by altering the KMnO4/K2Cr2O7 ratio. The linear two-fold reduction in the number of the hydroxyls and epoxides with the simultaneous three-fold rise in the content of carbonyls and carboxyls is indicated upon the transition from KMnO4 to K2Cr2O7 as a predominant oxidizing agent. The effect of the oxidation mixture's composition on the structure of the synthesized GOs is also comprehensively studied by means of X-ray diffraction, Raman spectroscopy, transmission electron microscopy, atomic-force microscopy, optical microscopy, and the laser diffraction method. The nanoscale corrugation of the GO platelets with the increase of the K2Cr2O7 content is signified, whereas the 10-100 µm lateral size, lamellar, and defect-free structure is demonstrated for all of the synthesized GOs regardless of the KMnO4/K2Cr2O7 ratio. The proposed method for the synthesis of GO with the desired chemistry opens up new horizons for the development of graphene-based materials with tunable functional properties.

From graphene oxide towards aminated graphene: facile synthesis, its structure and electronic properties.

In this paper we present a facile method for the synthesis of aminated graphene derivative through simultaneous reduction and amination of graphene oxide via two-step liquid phase treatment with hydrobromic acid and ammonia solution in mild conditions. The amination degree of the obtained aminated reduced graphene oxide is of about 4 at.%, whereas C/O ratio is up to 8.8 as determined by means of X-ray photoelectron spectroscopy. The chemical reactivity of the introduced amine groups is further verified by successful test covalent bonding of the obtained aminated graphene with 3-Chlorobenzoyl chloride. The morphological features and electronic properties, namely conductivity, valence band structure and work function are studied as well, illustrating the influence of amine groups on graphene structure and physical properties. Particularly, the increase of the electrical conductivity, reduction of the work function value and tendency to form wrinkled and corrugated graphene layers are observed in the aminated graphene derivative compared to the pristine reduced graphene oxide. As obtained aminated graphene could be used for photovoltaic, biosensing and catalysis application as well as a starting material for further chemical modifications.

Facile reduction of graphene oxide suspensions and films using glass wafers.

This paper reports a facile and green method for conversion of graphene oxide (GO) into graphene by low-temperature heating (80 °C) in the presence of a glass wafer. Compared to conventional GO chemical reduction methods, the presented approach is easy-scalable, operationally simple, and based on the use of a non-toxic recyclable deoxygenation agent. The efficiency of the proposed method is further expanded by the fact that it can be applied for reducing both GO suspensions and large-scale thin films formed on various substrates prior to the reduction process. The quality of the obtained reduced graphene oxide (rGO) strongly depends on the type of the used glass wafer, and, particularly, magnesium silicate glass can provide rGO with the C/O ratio of 7.4 and conductivity of up to 33000 S*cm-1. Based on the data obtained, we have suggested a mechanism of the observed reduction process in terms of the hydrolysis of the glass wafer with subsequent interaction of the leached alkali and alkali earth cations and silicate anions with graphene oxide, resulting in elimination of the oxygen-containing groups from the latter one. The proposed approach can be efficiently used for low-cost bulk-quantity production of graphene and graphene-based materials for a wide field of applications.

Controllable spherical aggregation of monodisperse carbon nanodots.

Monodisperse carbon nanodots (MCNDs) having an identical composition, structure, shape and size possess identical chemical and physical properties, making them highly promising for various technical and medical applications. Herein, we report a facile and effective route to obtain monodisperse carbon nanodots 3.5 ± 0.9 nm in size by thermal decomposition of organosilane within the pores of monodisperse mesoporous silica particles with subsequent removal of the silica template. Structural studies demonstrated that the MCNDs we synthesized consist of ∼7-10 defective graphene layers that are misoriented with respect to each other and contain various oxygen-containing functional groups. It was demonstrated that, owing to their identical size and chemical composition, the MCNDs are formed via coagulation primary aggregates ∼10-30 nm in size, which are, in turn, combined into secondary porous spherical aggregates ∼100-200 nm in diameter. The processes of coagulation of MCNDs and peptization of their hierarchical aggregates are fully reversible and can be controlled by varying the MCND concentration or the pH value of the hydrosols. Submicrometer spherical aggregates of MCNDs are not disintegrated as the hydrosol is dried. The thus obtained porous spherical aggregates of MCNDs are promising for drug delivery as a self-disassembling container for medicinal preparations.

Structure diagnostics of heterostructures and multi-layered systems by X-ray multiple diffraction.

This article presents the results of research on multi-layered heterostructures by a modified calculation technique of multiple X-ray diffraction. The Al x In1-x Sb heterostructure and a Zn(Mn)Se/GaAs(001) multi-layered system were used as models to specify conditions for cases of coincidental coplanar three-beam or coincidental noncoplanar four-beam X-ray diffraction. These conditions provide the means for a high-precision determination of lattice parameters and strain anisotropy in layers.

High-resolution X-ray diffraction and imaging.

This issue of Journal of Applied Crystallography includes some highlights of the 11th Biennial Conference on High-Resolution X-ray Diffraction and Imaging (XTOP), held in St Petersburg in 2012.

Nanographene and nanodiamond; new members in the nanocarbon family.

Nanographene and nanodiamond are new members of nanocarbons, which consist of nano-sized hexagonal and tetrahedral networks, respectively. The presence of edges and surfaces distinguishes nanographene and nanodiamond, respectively, from other nanocarbons owing to their structure dependent electronic features. Nanographene has an unconventional nonbonding pi-state (edge state) localized around its edge that is dependent on the edge geometry. The edge states, having localized spins, impart a nanographene-based molecular magnetic character. The structure and electronic/magnetic properties of nanodiamond vary depending on how the surface carbon atoms are terminated. Nanodiamond, with a naked surface, is subjected to structural reconstruction at the expense of sigma-dangling bonds. The hydrogenation of the surface is expected to give an electron reservoir function. The incompletely hydrogenated surface is magnetic with surface-induced spins.

Cobalt and magnesium ferrite nanoparticles: preparation using liquid foams as templates and their magnetic characteristics.

An easy and convenient method for the synthesis of cobalt and magnesium ferrite nanoparticles is demonstrated using liquid foams as templates. The foam is formed from an aqueous mixture of an anionic surfactant and the desired metal ions, where the metal ions are electrostatically entrapped by the surfactant at the thin borders between the foam bubbles and their junctions. The hydrolysis is carried out using alkali resulting in the formation of desired nanoparticles, with the foam playing the role of a template. However, in the formation of ferrites with the formula MFe(2)O(4), where the metal ion and iron possess oxidation states of +2 and +3, respectively, forming a foam from a 1:2 mixture of the desired ionic solutions would lead to a foam composition at variance with the original solution mixture because of greater electrostatic binding of ions possessing a greater charge with the surfactant. In our procedure, we circumvent this problem by preparing the foam from a 1:2 mixture of M(2+) and Fe(2+) ions and then utilizing the in situ conversion of Fe(2+) to Fe(3+) under basic conditions inside the foam matrix to get the desired composition of the metal ions with the required oxidation states. The fact that we could prepare both CoFe(2)O(4) and MgFe(2)O(4) particles shows the vast scope of this method for making even multicomponent oxides. The magnetic nanoparticles thus obtained exhibit a good crystalline nature and are characterized by superparamagnetic properties. The magnetic features observed for CoFe(2)O(4) and MgFe(2)O(4) nanoparticles are well in accordance with the expected behaviors, with CoFe(2)O(4) particles showing higher blocking temperatures and larger coercivities. These features can easily be explained by the contribution of Co(2+) sites to the magnetocrystalline anisotropy and the absence of the same from the Mg(2+) ions.