Novel highly conductive and transparent graphene based conductors

Future wearable electronics, displays and photovoltaic devices rely on highly conductive, transparent and yet mechanically flexible materials. Nowadays indium tin oxide (ITO) is the most wide spread transparent conductor in optoelectronic applications, however the mechanical rigidity of this material limits its use for future flexible devices. Here we report novel transparent conductors based on few layer graphene (FLG) intercalated with ferric chloride (FeCl3) with an outstandingly high electrical conductivity and optical transparency. We show that upon intercalation a record low sheet resistance of 8.8 Ohm/square is attained together with an optical transmittance higher than 84% in the visible range. These parameters outperform the best values of ITO and of other carbon-based materials, making these novel transparent conductors the best candidates for future flexible optoelectronics

Monitoring of Cell Layer Integrity with a Current-Driven Organic Electrochemical Transistor

The integrity of CaCo-2 cell barriers is investigated by organic electrochemical transistors (OECTs) in a current-driven configuration. Ion transport through cellular barriers via the paracellular pathway is modulated by tight junctions between adjacent cells. Rupturing its integrity by H2O2 is monitored by the change of the output voltage in the transfer characteristics. It is demonstrated that by operating the OECT in a current-driven configuration, the sensitive and temporal resolution for monitoring the cell barrier integrity is strongly enhanced as compared to the OECT transient response measurement. As a result, current-driven OECTs are useful tools to assess dynamic and critical changes in tight junctions, relevant for clinical applications as drug targeting and screening

Investigation of infrared phonon modes in multiferroic single-crystal FeTe2_{2}O5_{5}Br

Reflection and transmission as a function of temperature (5--300 K) have been measured on single crystals of the multiferroic compound FeTe$_{2}$O$_{5}$Br utilizing light spanning the far infrared to the visible portions of the electromagnetic spectrum. The complex dielectric function and optical properties were obtained via Kramers-Kronig analysis and by fits to a Drude-Lortentz model. Analysis of the anisotropic excitation spectra via Drude-Lorentz fitting and lattice dynamical calculations have lead to the observation of all 52 IR-active modes predicted in the $ac$ plane and 43 or the 53 modes predicted along the b axis of the monoclinic cell. Assignments to groups (clusters) of phonons have been made and trends within them are discussed in light of our calculated displacement patterns.Comment: 9 pages, 7 figure

Sign patterns for chemical reaction networks

Most differential equations found in chemical reaction networks (CRNs) have the form $dx/dt=f(x)= Sv(x)$, where $x$ lies in the nonnegative orthant, where $S$ is a real matrix (the stoichiometric matrix) and $v$ is a column vector consisting of real-valued functions having a special relationship to $S$. Our main interest will be in the Jacobian matrix, $f'(x)$, of $f(x)$, in particular in whether or not each entry $f'(x)_{ij}$ has the same sign for all $x$ in the orthant, i.e., the Jacobian respects a sign pattern. In other words species $x_j$ always acts on species $x_i$ in an inhibitory way or its action is always excitatory. In Helton, Klep, Gomez we gave necessary and sufficient conditions on the species-reaction graph naturally associated to $S$ which guarantee that the Jacobian of the associated CRN has a sign pattern. In this paper, given $S$ we give a construction which adds certain rows and columns to $S$, thereby producing a stoichiometric matrix $\widehat S$ corresponding to a new CRN with some added species and reactions. The Jacobian for this CRN based on $\hat S$ has a sign pattern. The equilibria for the $S$ and the $\hat S$ based CRN are in exact one to one correspondence with each equilibrium $e$ for the original CRN gotten from an equilibrium $\hat e$ for the new CRN by removing its added species. In our construction of a new CRN we are allowed to choose rate constants for the added reactions and if we choose them large enough the equilibrium $\hat e$ is locally asymptotically stable if and only if the equilibrium $e$ is locally asymptotically stable. Further properties of the construction are shown, such as those pertaining to conserved quantities and to how the deficiencies of the two CRNs compare.Comment: 23 page

Molybdenum targets for production of 99mTc by a medical cyclotron

Introduction Alternative methods for producing the medical imaging isotope 99mTc are actively being developed around the world in anticipation of the imminent shutdown of the National Research Universal (NRU) reactor in Chalk River, Ontario, Canada and the high flux reactor (HFR) in Petten, Holland that together currently produce up to 80 % of the world’s supply through fission. The most promising alternative methods involve accelerators that focus Bremsstrahlung radiation or protons on metallic targets comprised of 100Mo and a supporting material used to conduct heat away during irradiation. As an example, the reaction 100Mo(p,2n)99mTc provides a direct route that can be incorporated into routine production in regional nuclear medicine centers that possess medical cyclotrons for production of other isotopes, such as those used for Positron Emission Tomography (PET). The targets used to produce 99mTc are subject to a number of operational constraints. They must withstand the temperatures generated by the irradiation and be fashioned to accommodate temperature gradients from in situ cooling. The targets must be resilient, which means they cannot disintegrate during irradiation or post processing, because of the radioactive nature of the products. Yet, the targets must be easily post-processed to separate the 99mTc. In addition, the method used to manufacture the targets must not be wasteful of the 100Mo, because of its cost (~$2/mg). Any manufacturing process should be able to function remotely in a shielded space to accommodate the possibility of radioactive recycled target feedstock. There are a number of methods that have been proposed for large-scale target manufacturing including electrophoretic deposition, pressing and sinter-ing, electroplating and carburization [1]. How to develop these methods for routine production is an active business [2,3]. From the industrial perspective, plasma spraying showed promising results initially [4], but the process became very expensive requiring customized equipment in order to reduce losses because of overspray,which also required a large inventory of expen-sive feedstock. In this paper we report the ex-perimental validation of an industrial process for production of targets comprising a Mo layer and a copper support. Materials and methods Target Design Targets have been manufactured for irradiation at 15 MeV. Two targets are shown in FIG. 1: one as-manufactured and another after irradiation; no visible changes were observed following irradiation. The supporting circular copper (C101) disks have diameters of 24 mm and thickness of 1.6 mm. The molybdenum in the center of the target is fully dense with thickness 230 μm determined from SEM cross-sections.Targets have also been manufactured for irradi-ation in a general-purpose target holder designed to be attached to all makes of cyclotrons found in regional nuclear medicine centers. The elliptical targets were designed for high-volume production of 99mTc with 15 MeV protons at currents of 400 µA with 15% collimation [4]. The elliptical shape reduces the heat flux associated with high current sources. The cooling channels on the back of the target are designed to with-stand the high temperature generated during Irradiation. A thermal simulation of expected temperatures during irradiation is shown in FIG. 3. The center of the target is expected to reach 260 oC during irradiation. The elliptical targets were formed from a 27 mm C101 copper plate with width 22 mm and length 55 mm. The molybdenum in the center of the target is fully dense with thickness 60 m de-termined from SEM cross-sections. FIG. 4 shows the molybdenum deposition in the center of the target in a form of an ellipse (38×10 mm). Results and Conclusions Circular targets have been produced and suc-cessfully irradiated for up to 5 h with a proton beam with energy 15 MeV and current 50 µA. (FIG. 1). The targets were resilient. Before irradi-ation the targets were subjected to mechanical shock tests and thermal gradients with no ob-servable effect. After irradiation there was no indication of any degradation. The manufacturing process produced 20 consistently reproducible targets within an hour with a molybdenum loss of less than 2 %. After irradiation the targets were chemically processed and the products characterized by Ge-HP gamma spectrometry. Only Tc isotopes were found. No other contami-nants were identified after processing. The de-tails of the separation and purification are de-scribed elsewhere [5]. Circular targets suitable for low-volume produc-tion of 99mTc have been manufactured and test-ed. The targets have been shown to meet the required operation constraints: the targets are resilient withstanding mechanical shock and irradiation conditions; they are readily produced with minimal losses; and post-processing after irradiation for 5 h has been shown to produce 99mTc. Elliptical targets suitable for high-volume pro-duction of 99mTc with high power cyclotrons have been manufactured (FIG. 4). Like the circular targets, the elliptical targets are readily pro-duced with minimal losses and are able to with-stand mechanical shock and thermal gradients; however, they have yet to be irradiated

Deposition temperature influence on the wear behaviour of carbon-based coatings deposited on hardened steel

An evaluation regarding the influence of substrate material characteristics and deposition parameters on the tribological behaviour of carbon-based is presented. Chromium nitride interlayers and carbon-based thin films were deposited on hardened AISI 5115 case hardening steel, by magnetron sputtering. The physical vapour deposition (PVD) deposition was performed at three different temperatures: 180 °C, 200 °C and 250 °C. The chemical composition of the samples was assessed by Rutherford Backscattering Spectroscopy (RBS), the structure by X-ray Diffraction (XRD), and the surface morphology by Atomic Force Microscopy (AFM). The surface chemistry was analysed by X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy. The coatings are homogeneous, amorphous, with a smooth surface. The mechanical behaviour has been assessed on a pin-on disk rotational tribometer (wear characteristics), on a micro scratch tester (adhesion to the substrate), by ball-cratering (film thickness); by nanoindentation (hardness and the modulus of elasticity). A strong correlation between the substrate characteristics, but more importantly, of the deposition temperature, on one hand, and the mechanical characteristics, on the other hand, has been observed. The fracture toughness is positively influenced by the presence of the ceramic interlayer (chromium nitride). The modulus of elasticity and friction coefficient (both in dry conditions and lubricated) are decreased for higher deposition temperatures, however the higher deposition temperature negatively affects the hardness of the steel substrate.We hereby acknowledge the structural funds project PRO-DD (POSCCE, O.2.2.1., ID 123, SMIS 2637, ctr. no 11/2009) for providing some of the infrastructure used in this work

Computational Complexity of Atomic Chemical Reaction Networks

Informally, a chemical reaction network is "atomic" if each reaction may be interpreted as the rearrangement of indivisible units of matter. There are several reasonable definitions formalizing this idea. We investigate the computational complexity of deciding whether a given network is atomic according to each of these definitions. Our first definition, primitive atomic, which requires each reaction to preserve the total number of atoms, is to shown to be equivalent to mass conservation. Since it is known that it can be decided in polynomial time whether a given chemical reaction network is mass-conserving, the equivalence gives an efficient algorithm to decide primitive atomicity. Another definition, subset atomic, further requires that all atoms are species. We show that deciding whether a given network is subset atomic is in $\textsf{NP}$, and the problem "is a network subset atomic with respect to a given atom set" is strongly $\textsf{NP}$-$\textsf{Complete}$. A third definition, reachably atomic, studied by Adleman, Gopalkrishnan et al., further requires that each species has a sequence of reactions splitting it into its constituent atoms. We show that there is a $\textbf{polynomial-time algorithm}$ to decide whether a given network is reachably atomic, improving upon the result of Adleman et al. that the problem is $\textbf{decidable}$. We show that the reachability problem for reachably atomic networks is $\textsf{Pspace}$-$\textsf{Complete}$. Finally, we demonstrate equivalence relationships between our definitions and some special cases of another existing definition of atomicity due to Gnacadja

Extraordinary linear dynamic range in laser-defined functionalized graphene photodetectors

Graphene-based photodetectors have demonstrated mechanical flexibility, large operating bandwidth, and broadband spectral response. However, their linear dynamic range (LDR) is limited by graphene's intrinsichot-carrier dynamics, which causes deviation from a linear photoresponse at low incident powers. At the same time, multiplication of hot carriers causes the photoactive region to be smeared over distances of a few micro-meters, limiting the use of graphene in high-resolution applications. We present a novel method for engineer-ing photoactive junctions in FeCl3-intercalated graphene using laser irradiation. Photocurrent measured at these planar junctions shows an extraordinary linear response with an LDR value at least 4500 times larger than that of other graphene devices (44 dB) while maintaining high stability against environmental contamination without the need for encapsulation. The observed photoresponse is purely photovoltaic, demonstrating complete quenching of hot-carrier effects. These results pave the way toward the design of ultrathin photode-tectors with unprecedented LDR for high-definition imaging and sensing.Comment: 44 pages, includes supplementar

Transport properties of copper phthalocyanine based organic electronic devices

Ambipolar charge carrier transport in Copper phthalocyanine (CuPc) is studied experimentally in field-effect transistors and metal-insulator-semiconductor diodes at various temperatures. The electronic structure and the transport properties of CuPc attached to leads are calculated using density functional theory and scattering theory at the non-equilibrium Green's function level. We discuss, in particular, the electronic structure of CuPc molecules attached to gold chains in different geometries to mimic the different experimental setups. The combined experimental and theoretical analysis explains the dependence of the mobilityand the transmission coefficient on the charge carrier type (electrons or holes) and on the contact geometry. We demonstrate the correspondence between our experimental results on thick films and our theoretical studies of single molecule contacts. Preliminary results for fluorinated CuPc are discussed.Comment: 18 pages, 16 figures; to be published in Eur. Phys. J. Special Topic