Lifetime and screening of the C 1s photoemission in graphite

Explosive percolation yields highly-conductive polymer nanocomposites

Explosive percolation is an experimentally-elusive phenomenon where network connectivity coincides with onset of an additional modification of the system; materials with correlated localisation of percolating particles and emergent conductive paths can realise sharp transitions and high conductivities characteristic of the explosively-grown network. Nanocomposites present a structurally- and chemically-varied playground to realise explosive percolation in practically-applicable systems but this is yet to be exploited by design. Herein, we demonstrate composites of graphene oxide and synthetic polymer latex which form segregated networks, leading to low percolation threshold and localisation of conductive pathways. In situ reduction of the graphene oxide at temperatures of <150 °C drives chemical modification of the polymer matrix to produce species with phenolic groups, which are known crosslinking agents. This leads to conductivities exceeding those of dense-packed networks of reduced graphene oxide, illustrating the potential of explosive percolation by design to realise low-loading composites with dramatically-enhanced electrical transport properties.

Role of Nanoscale Inhomogeneities in Co2FeO4 Catalysts during the Oxygen Evolution Reaction

Spinel-type catalysts are promising anode materials for the alkaline oxygen evolution reaction (OER), exhibiting low overpotentials and providing long-term stability. In this study, we compared two structurally equal Co₂FeO₄ spinels with nominally identical stoichiometry and substantially different OER activities. In particular, one of the samples, characterized by a metastable precatalyst state, was found to quickly achieve its steady-state optimum operation, while the other, which was initially closer to the ideal crystallographic spinel structure, never reached such a state and required 168 mV higher potential to achieve 1 mA/cm². In addition, the enhanced OER activity was accompanied by a larger resistance to corrosion. More specifically, using various ex situ, quasi in situ, and operando methods, we could identify a correlation between the catalytic activity and compositional inhomogeneities resulting in an X-ray amorphous Co²⁺-rich minority phase linking the crystalline spinel domains in the as-prepared state. Operando X-ray absorption spectroscopy revealed that these Co²⁺-rich domains transform during OER to structurally different Co³⁺-rich domains. These domains appear to be crucial for enhancing OER kinetics while exhibiting distinctly different redox properties. Our work emphasizes the necessity of the operando methodology to gain fundamental insight into the activity-determining properties of OER catalysts and presents a promising catalyst concept in which a stable, crystalline structure hosts the disordered and active catalyst phase.

Tracing the structural evolution of quasi-freestanding germanene on Ag(111)

In the last decade, research on 2D materials has expanded massively due to the popularity of graphene. Although the chemical engineering of two-dimensional elemental materials as well as heterostructures has been extensively pursued, the fundamental understanding of the synthesis of 2D materials is not yet complete. Structural parameters, such as buckling or the interface structure of a 2D material to the substrate directly affect its electronic characteristics. In order to proceed the understanding of the element-specific growth and the associated ability of tuning material properties of two-dimensional materials, we performed a study on the structural evolution of the promising 2D material germanene on Ag(111). This study provides a survey of germanium formations at different layer thicknesses right up to the arising of quasi-freestanding germanene. Using powerful surface analysis tools like low-energy electron diffraction, x-ray photoelectron spectroscopy, and x-ray photoelectron diffraction with synchrotron radiation, we will reveal the internal and interfacial structure of all discovered germanium phases. Moreover, we will present models of the atomic and chemical structure of a \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {Ag}_2\hbox {Ge}$$\end{document}Ag2Ge surface alloy and the quasi-freestanding germanene with special focus on the structural parameters and electronic interaction at the interface.

Potassium Difluorophosphate as an Electrolyte Additive for Potassium-Ion Batteries

The limited cyclability and inferior Coulombic efficiency of graphite negative electrodes have been major impediments to their practical utilization in potassium-ion batteries (PIBs). Herein, for the first time, potassium difluorophosphate (KDFP) electrolyte additive is demonstrated as a viable solution to these bottlenecks by facilitating the formation of a stable and K⁺-conducting solid-electrolyte interphase (SEI) on graphite. The addition of 0.2 wt % KDFP to the electrolyte results in significant improvements in the (de)potassiation kinetics, capacity retention (76.8% after 400 cycles with KDFP vs 27.4% after 100 cycles without KDFP), and average Coulombic efficiency (∼99.9% during 400 cycles) of the graphite electrode. Moreover, the KDFP-containing electrolyte also enables durable cycling of the K/K symmetric cell at higher efficiencies and lower interfacial resistance as opposed to the electrolyte without KDFP. X-ray diffraction and Raman spectroscopy analyses have confirmed the reversible formation of a phase-pure stage-1 potassium-graphite intercalation compound (KC₈) with the aid of KDFP. The enhanced electrochemical performance by the KDFP addition is discussed based on the analysis of the SEI layer on graphite and K metal electrodes by X-ray photoelectron spectroscopy.

Surface States of (100) O-Terminated Diamond: Towards Other 1 × 1:O Reconstruction Models

Diamond surface properties show a strong dependence on its chemical termination. Hydrogen-terminated and oxygen-terminated diamonds are the most studied terminations with many applications in the electronic and bioelectronic device field. One of the main techniques for the characterization of diamond surface terminations is X-ray photoelectron spectroscopy (XPS). In this sense, the use of angleresolved XPS (ARXPS) experiments allows obtaining depth-dependent information used here to evidence (100)Oterminated diamond surface atomic configuration when fabricated by acid treatment. The results were used to compare the chemistry changes occurring during the oxidation process using a sublayer XPS intensity model. The formation of nondiamond carbon phases at the subsurface and higher oxygen contents were shown to result from the oxygenation treatment. A new (100) 1 × 1:O surface reconstruction model is proposed to explain the XPS quantification results of Oterminated diamond.

Differential properties and effects of fluorescent carbon nanoparticles towards intestinal theranostics

Given the potential applications of fluorescent carbon nanoparticles in biomedicine, the relationship between their chemical structure, optical properties and biocompatibility has to be investigated in detail. In this work, different types of fluorescent carbon nanoparticles are synthesized by acid treatment, sonochemical treatment, electrochemical cleavage and polycondensation. The particle size ranges from 1 to 6 nm, depending on the synthesis method. Nanoparticles that were prepared by acid or sonochemical treatments from graphite keep a crystalline core and can be classified as graphene quantum dots. The electrochemically produced nanoparticles do not clearly show the graphene core, but it is made of heterogeneous aromatic structures with limited size. The polycondensation nanoparticles do not have CC double bonds. The type of functional groups on the carbon backbone and the optical properties, both absorbance and photoluminescence, strongly depend on the nanoparticle origin. The selected types of nanoparticles are compatible with human intestinal cells, while three of them also show activity against colon cancer cells. The widely different properties of the nanoparticle types need to be considered for their use as diagnosis markers and therapeutic vehicles, specifically in the digestive system.

Statistics of work and orthogonality catastrophe in discrete level systems: an application to fullerene molecules and ultra-cold trapped Fermi gases

The sudden introduction of a local impurity in a Fermi sea leads to an anomalous disturbance of its quantum state that represents a local quench, leaving the system out of equilibrium and giving rise to the Anderson orthogonality catastrophe. The statistics of the work done describe the energy fluctuations produced by the quench, providing an accurate and detailed insight into the fundamental physics of the process. We present here a numerical approach to the non-equilibrium work distribution, supported by applications to phenomena occurring at very diverse energy ranges. One of them is the valence electron shake-up induced by photo-ionization of a core state in a fullerene molecule. The other is the response of an ultra-cold gas of trapped fermions to an embedded two-level atom excited by a fast pulse. Working at low thermal energies, we detect the primary role played by many-particle states of the perturbed system with one or two excited fermions. We validate our approach through the comparison with some photoemission data on fullerene films and previous analytical calculations on harmonically trapped Fermi gases.

Evidence for a glassy state in strongly driven carbon

Here, we report results of an experiment creating a transient, highly correlated carbon state using a combination of optical and x-ray lasers. Scattered x-rays reveal a highly ordered state with an electrostatic energy significantly exceeding the thermal energy of the ions. Strong Coulomb forces are predicted to induce nucleation into a crystalline ion structure within a few picoseconds. However, we observe no evidence of such phase transition after several tens of picoseconds but strong indications for an over-correlated fluid state. The experiment suggests a much slower nucleation and points to an intermediate glassy state where the ions are frozen close to their original positions in the fluid.

Pyrolyzed carbon film diodes

We have previously reported pyrolyzed parylene C (PPC) as a conductive carbon electrode material for use with micropipets, atomic force microscopy probes, and planar electrodes. Advantages of carbon electrode fabrication from PPC include conformal coating of high-aspect ratio micro/nanoscale features and the benefits afforded by chemical vapor deposition of carbon polymers. In this work, we demonstrate chemical surface doping of PPC through the use of previously reported methods. Chemically treated PPC films are characterized by multiple spectroscopic and electronic measurements. Pyrolyzed parylene C and doped PPC are used to construct diodes that are examined as both p-n heterojunction and Schottky barrier diodes. Half-wave rectification is achieved with PPC diodes and demonstrates the applicability of PPC as a conductive and semiconductive material in device fabrication.

Biosensor based on laccase immobilized on plasma polymerized allylamine/carbon electrode

In this work, a simple and rapid method was used to functionalize carbon electrode in order to efficiently immobilize laccase for biosensor application. A stable allylamine coating was deposited using a low pressure inductively excited RF tubular plasma reactor under mild plasma conditions (low plasma power (10 W), few minutes) to generate high density amine groups (N/C ratio up to 0.18) on rough carbon surface electrodes. The longer was the allylamine plasma deposition time; the better was the surface coverage. Laccase from Trametes versicolor was physisorbed and covalently bound to these allylamine modified carbon surfaces. The laccase activities and current outputs measured in the presence of 2,2'-azinobis-(3-ethylbenzothiazole-6-sulfonic acid) (ABTS) showed that the best efficiency was obtained for electrode plasma coated during 30 min. They showed also that for all the tested electrodes, the activities and current outputs of the covalently immobilized laccases were twice higher than the physically adsorbed ones. The sensitivity of these biocompatible bioelectrodes was evaluated by measuring their catalytic efficiency for oxygen reduction in the presence of ABTS as non-phenolic redox substrate and 2,6-dimethoxyphenol (DMP) as phenolic one. Sensitivities of around 4.8 μA mg(-1)L and 2.7 μA mg(-1)L were attained for ABTS and DMP respectively. An excellent stability of this laccase biosensor was observed for over 6 months.

Core-hole effects in fullerene molecules and small-diameter conducting nanotubes: a density functional theory study

Core-hole induced electron excitations in fullerene molecules, and small-diameter conducting carbon nanotubes, are studied using density functional theory with minimal, split-valence, and triply-split-valence basis sets plus the generalized gradient approximation by Perdew-Burke-Ernzerhof for exchange and correlation. Finite-size computations are performed on the carbon atoms of a C(60) Bucky ball and a piece of (3, 3) armchair cylindrical network, terminated by hydrogen atoms, while periodically boundary conditions are imposed on a (3, 3) nanotube unit cell. Sudden creation of the core state is simulated by replacing a 1s electron pair, localized at a central site of the structures, with the effective pseudo-potentials of both neutral and ionized atomic carbon. Excited states are obtained from the ground-state (occupied and empty) electronic structure of the ionized systems, and their overlaps with the ground state of the neutral systems are computed. These overlaps enter Fermi's golden rule, which is corrected with lifetime and finite-temperature effects to simulate the many-electron response of the nanoobjects. A model based on the linked cluster expansion of the vacuum persistence amplitude of the neutral systems, in a parametric core-hole perturbation, is developed and found to be reasonably consistent with the density functional theory method. The simulated spectrum of the fullerene molecule is found to be in good agreement with x-ray photoemission experiments on thick C(60) films, reproducing the low energy satellites at excitation energies below 4 eV within a peak position error of ca. 0.3 eV. The nanotube spectra show some common features within the same experiments and describe well the measured x-ray photoelectron lineshape from nanotube bundles with an average diameter of 1.2 nm.

Nature of the decrease of the secondary-electron yield by electron bombardment and its energy dependence

We performed a combined secondary electron yield (SEY) and x-ray photoelectron spectroscopy study as a function of the electron dose and energy on a Cu technical surface representative of the LHC accelerator walls. The electron bombardment is accompanied by a clear chemical modification, indicating an increased graphitization as the SEY decreases. The decrease in the SEY is also found to depend significantly on the kinetic energy of the primary electrons. When low-energy primary electrons are employed (E≤20 eV), the reduction of the SEY is slower and smaller in magnitude than when higher-energy electrons are used. Consequences of this observation are discussed mainly for their relevance on the commissioning scenario for the LHC in operation at CERN (Geneva), but are expected to be of interest for other research fields.

Ambipolar to unipolar conversion in graphene field-effect transistors

Typical graphene field-effect transistors (GFETs) show ambipolar conduction that is unfavorable for some electronic applications. In this work, we report on the development of unipolar GFETs. We found that the titanium oxide situated on the graphene surface induced significant hole doping. The threshold voltage of the unipolar p-type GFET was tunable by varying the density of the attached titanium oxide through an etching process. An annealing process followed by silicon nitride passivation was found to convert the p-type GFETs to unipolar n-type GFETs. An air-stable complementary inverter integrated from the p- and n-GFETs was also successfully demonstrated. The simple fabrication processes are compatible with the conventional CMOS manufacturing technology.

Mechanically stacked 1-nm-thick carbon nanosheets: ultrathin layered materials with tunable optical, chemical, and electrical properties

Carbon nanosheets are mechanically stable, free-standing two-dimensional materials with a thickness of ≈1 nm and well defined physical and chemical properties. They are made by radiation-induced cross-linking of aromatic self-assembled monolayers. Herein, a route is presented to the scalable fabrication of multilayer nanosheets with tunable electrical, optical, and chemical properties on insulating substrates. Stacks of up to five nanosheets with sizes of ≈1 cm(2) on oxidized silicon are studied. Their optical characteristics are investigated by visual inspection, optical microscopy, UV-vis reflection spectroscopy, and model calculations. Their chemical composition is studied by X-ray photoelectron spectroscopy. The multilayer samples are then annealed in an ultrahigh vacuum at various temperatures up to 1100 K. A subsequent investigation by Raman, X-ray photoelectron, and UV-vis reflection spectroscopy, as well as by electrical four-point probe measurements, demonstrates that the layered nanosheets transform into nanocrystalline graphene. This structural and chemical transformation is accompanied by changes in the optical properties and electrical conductivity and opens up a new path for the fabrication of ultrathin functional conductive coatings.

Chlorine speciation in nuclear graphite: consequences on temperature release and on leaching

This paper aims to get information on pristine chlorine speciation and its evolution with temperature in nuclear graphite used as a moderator in the first generation UNGG French reactors. For this purpose, temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) have been carried out on samples annealed in the temperature range 200–1000 °C. Two chemical forms of different thermal stabilities have been identified. Around 70% of chlorine is assigned to stable organic chlorine bound to aromatic carbon whereas around 30% is assigned to inorganic thermally labile oxychlorinated groups. The consequences for direct disposal are discussed.

Charge transport in proteins probed by resonant photoemission

The degrees of charge localization in the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of the bacterial surface layer protein of Bacillus sphaericus NCTC 9602 were studied by resonant photoemission. In agreement with a charge transport hopping mechanism that involves torsional motions of the peptide backbone, the lifetime of electrons excited into the LUMO was found to be approximately 100 fs.

Evidence of the Interaction of Evaporated Pt Nanoparticles with Variously Treated Surfaces of Highly Oriented Pyrolytic Graphite

The interactions of Pt nanoparticles, deposited by evaporation onto highly oriented pyrolytic graphite surfaces modified by kiloelectronvolt Ar+ beam treatment, have been studied by X-ray photoelectron spectroscopy core-level line shape analysis. The C1s and Pt4f7/2 peaks were each considered to be composed of one asymmetric peak, and changes in their asymmetry parameters were used to study their interfacial interactions. In addition to these changes, strong signal intensity changes with time were found for both the C1s and Pt4f peaks, indicating an initial crystalline orientational instability of the Pt nanoparticles, which is supported by time-dependent high-resolution electron microscopy studies at elevated temperatures.

Carbon 1s X-ray photoemission line shape analysis of highly oriented pyrolytic graphite: the influence of structural damage on peak asymmetry

C 1s XPS spectra of various highly oriented pyrolytic graphite (HOPG) surfaces, untreated, as well as those treated by keV Ar+ beam bombardment and low-energy O2, N2, Ar, and H2O plasmas, have been systematically studied by comparing two XPS peak-fitting procedures. These procedures treat the spectrum as either (1) the overlap of several symmetric component peaks or (2) a single asymmetric peak. The results indicate that, in the case of HOPG, the asymmetry parameter defining the single peak is directly related to the extent of damage to the alternant hydrocarbon structure of the HOPG surface, as manifested by its correlation with the symmetric peak component due to the damaged HOPG structure.

Ab initio theory of dynamical core-hole screening in graphite from x-ray absorption spectra

We have implemented the effect of dynamical core-hole screening, as given by Mahan, Nozières, and De Dominicis, in a first-principles based method and applied the theory to the x-ray absorption (XA) spectrum of graphite. It turns out that two of the conspicuous peaks of graphite are well described, both regarding the position, shape, and relative intensity, whereas one peak is absent in the theory. Only by incorporation of both excitonic and delocalized processes can a full account of the experimental spectrum be obtained theoretically, and we interpret the XA spectrum in graphite to be the result of a well screened and a poor screened process, much in the same way as is done for core level x-ray photoelectron spectroscopy.