Correlation between molecular orbitals and doping dependence of the electrical conductivity in electron-doped Metal-Phthalocyanine compounds

We have performed a comparative study of the electronic properties of six different electron-doped metal phthalocyanine (MPc) compounds (ZnPc, CuPc, NiPc, CoPc, FePc, and MnPc), in which the electron density is controlled by means of potassium intercalation. In spite of the complexity of these systems, we find that the nature of the underlying molecular orbitals produce observable effects in the doping dependence of the electrical conductivity of the materials. For all the MPc's in which the added electrons are expected to occupy orbitals centered on the ligands (ZnPc, CuPc, and NiPc), the doping dependence of the conductivity has an essentially identical shape. This shape is different from that observed in MPc materials in which electrons are also added to orbitals centered on the metal atom (CoPc, FePc, and MnPc). The observed relation between the macroscopic electronic properties of the MPc compounds and the properties of the molecular orbitals of the constituent molecules, clearly indicates the richness of the alkali-doped metal-phthalocyanines as a model class of compounds for the investigation of the electronic properties of molecular systems

Double-gated graphene-based devices

We discuss transport through double gated single and few layer graphene devices. This kind of device configuration has been used to investigate the modulation of the energy band structure through the application of an external perpendicular electric field, a unique property of few layer graphene systems. Here we discuss technological details that are important for the fabrication of top gated structures, based on electron-gun evaporation of SiO$_2$. We perform a statistical study that demonstrates how --contrary to expectations-- the breakdown field of electron-gun evaporated thin SiO$_2$ films is comparable to that of thermally grown oxide layers. We find that a high breakdown field can be achieved in evaporated SiO$_2$ only if the oxide deposition is directly followed by the metallization of the top electrodes, without exposure to air of the SiO$_2$ layer.Comment: Replaced with revised version. To appear on New Journal of Physic

Sheffer sequences, probability distributions and approximation operators

We present a new method to compute formulas for the action on monomials of a generalization of binomial approximation operators of Popoviciu type, or equivalently moments of associated discrete probability distributions with finite support. These quantities are necessary to check the assumptions of the Korovkin Theorem for approximation operators, or equivalently the Feller Theorem for convergence of the probability distributions. Our method unifies and simplifies computations of well-known special cases. It only requires a few basic facts from Umbral Calculus. We illustrate our method to well-known approximation operators and probability distributions, as well as to some recent q-generalizations of the Bernstein approximation operator introduced by Lewanowicz and Wo´zny, Lupa¸s, and Phillips

2D material liquid crystal nanocomposites for optoelectronic and photonic devices

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this recordWe synthesise, characterise and move toward application of 2D material liquid crystalline nanocomposites for optoelectronic and photonic devices, focussing on those produced using graphene oxide, tungsten disulfide and boron nitride

Phase transitions and phase diagram of the ferroelectric perovskite NBT-BT by anelastic and dielectric measurements

The complex elastic compliance and dielectric susceptibility of (Na_{0.5}Bi_{0.5})_{1-x}Ba_{x}TiO_{3} (NBT-BT) have been measured in the composition range between pure NBT and the morphotropic phase boundary included, 0 <= x <= 0.08. The compliance of NBT presents sharp peaks at the rhombohedral/tetragonal and tetragonal/cubic transitions, allowing the determination of the tetragonal region of the phase diagram, up to now impossible due to the strong lattice disorder and small distortions and polarizations involved. In spite of ample evidence of disorder and structural heterogeneity, the R-T transition remains sharp up to x = 0.06, whereas the T-C transition merges into the diffuse and relaxor-like transition associated with broad maxima of the dielectric and elastic susceptibilities. An attempt is made at relating the different features in the anelastic and dielectric curves to different modes of octahedral rotations and polar cation shifts. The possibility is also considered that the cation displacements locally have monoclinic symmetry, as for PZT near the morphotropic phase boundary.Comment: 11 pages, 9 figures, submitted to Phys. Rev.

Shot Noise in Ballistic Graphene

We have investigated shot noise in graphene field effect devices in the temperature range of 4.2--30 K at low frequency ($f$ = 600--850 MHz). We find that for our graphene samples with large width over length ratio $W/L$, the Fano factor $\mathfrak{F}$ reaches a maximum $\mathfrak{F} \sim$ 1/3 at the Dirac point and that it decreases strongly with increasing charge density. For smaller $W/L$, the Fano factor at Dirac point is significantly lower. Our results are in good agreement with the theory describing that transport at the Dirac point in clean graphene arises from evanescent electronic states.Comment: Phys. Rev. Lett. 100, 196802 (2008

Low-temperature phase transformations of PZT in the morphotropic phase-boundary region

We present anelastic and dielectric spectroscopy measurements of PbZr(1-x)Ti(x)O(3) with 0.455 < x < 0.53, which provide new information on the low temperature phase transitions. The tetragonal-to-monoclinic transformation is first-order for x < 0.48 and causes a softening of the polycrystal Young's modulus whose amplitude may exceed the one at the cubic-to-tetragonal transformation; this is explainable in terms of linear coupling between shear strain components and tilting angle of polarization in the monoclinic phase. The transition involving rotations of the octahedra below 200 K is visible both in the dielectric and anelastic losses, and it extends within the tetragonal phase, as predicted by recent first-principle calculations.Comment: 4 pages, 4 figure

Functionalised hexagonal-domain graphene for position-sensitive photodetectors

LetterThis is the author accepted manuscript. The final version is available from IOP Publishing via the DOI in this record.Graphene's unique photoresponse has been largely used in a multitude of optoelectronics applications ranging from broadband photodetectors to wave-guide modulators. In this work we extend the range of applications to position-sensitive photodetectors (PSDs) using FeCl3-intercalated hexagonal domains of graphene grown by atmospheric pressure chemical vapour deposition (APCVD). The FeCl3-based chemical functionalisation of APCVD graphene crystals is affected by the presence of wrinkles and results in a non-uniform doping of the graphene layers. This doping profile creates multiple p–p+ photoactive junctions which show a linear and bipolar photoresponse with respect to the position of a focused light spot, which is ideal for the realization of a PSD. Our study paves the way towards the fabrication of flexible and transparent PSDs that could be embedded in smart textile and wearable electronics.S Russo and M F Craciun acknowledge financial support from EPSRC (Grant no. EP/J000396/1, EP/K017160/1, EP/K010050/1, EPG036101/1, EP/M001024/1, EPM002438/1), from Royal Society international Exchanges Scheme 2016/R1, from European Commission (FP7-ICT-2013-613024-GRASP) and from the Leverhulme Trust (grant title 'Quantum Drums' and 'Room temperature quantum electronics'). I Amit received funding from the People Programme (Marie Curie Actions) of the European Union's Eighth Framework Programme Horizon 2020 under REA grant agreement number 701704