J. Fraxedas

Perspectives on Thin Molecular Organic Films

The engineering versatility of molecular erganic materials together with their attractive electronic, magnetic, and opitcal properties, have triggered their preparation as thin films in the last years. Although the interactions between molecules and inorganic substrates are rather weak (van der Wands hydrogen bonding, etc.), the crystallographic phase, the type and degree of orientation and the morphology of the resulting films critically depend on the interface and the kinetics of growth. The control of the structure and more phology of the films is thus essential when envisioning practical applications. The relevance of interfaces in the growth and doping of films is dicussed, and the phenomenology of polymorphism is explored. Additional issues such as the mechanical propertics are also addressed. The figure, showing a scanning tunneling microscopy image of a manolayer of bis(ethylene dithio)-tetrarhiofolvelene art a Au(III) substrate, illustrases the interface issue.

Nanoindentation: Toward the sensing of atomic interactions.

The mechanical properties of surfaces of layered materials (highly oriented pyrolytic graphite, InSe, and GaSe) and single-crystal ionic materials (NaCl, KBr, and KCl) have been investigated at the nanometer scale by using nanoindentations produced with an atomic force microscope with ultrasharp tips. Special attention has been devoted to the elastic response of the materials before the onset of plastic yield. A new model based on an equivalent spring constant that takes into account the changes in in-plane interactions on nanoindentation is proposed. The results of this model are well correlated with those obtained by using the Debye model of solid vibrations.

Charging and discharging of graphene in ambient conditions studied with scanning probe microscopy

By means of scanning probe microscopy we are able to inject charges in isolated graphene sheets deposited on SiO2/Si wafers and characterize the discharge induced by water in controlled ambient conditions. Contact potential differences between the graphene surface and the probe tip, measured by Kelvin probe microscopy, show a linear relationship with the tip bias during charge injection. The discharge depends on relative humidity and decays exponentially with time constants of the order of tens of minutes. We propose that graphene discharges through the water film adsorbed on the SiO2 surface.

In situ nanocalorimetry of thin glassy organic films

In this work, we describe the design and first experimental results of a new setup that combines evaporation of liquids in ultrahigh vacuum conditions with in situ high sensitivity thermal characterization of thin films. Organic compounds are deposited from the vapor directly onto a liquid nitrogen cooled substrate, permitting the preparation and characterization of glassy films. The substrate consists of a microfabricated, membrane-based nanocalorimeter that permits in situ measurements of heat capacity under ultrafast heating rates (up to 10(5) K/s) in the temperature range of 100-300 K. Three glass forming liquids-toluene, methanol, and acetic acid-are characterized. The spikes in heat capacity related to the glass-transition temperature, the fictive temperature and, in some cases, the onset temperature of crystallization are determined for several heating rates.

Thin Single Crystals of Organic Insulators, Metals, and Superconductors by Confined Electrocrystallization**

The confined electrocrystallization technique, first described by Thakur et al., involves forcing electrocrystallization to occur within the interface of two opposite substrates. This technique is further demonstrated and extended herein to the synthesis of a variety of thin crystalline films of conducting and insulating molecular materials alike, regardless of their structural dimensionality and shape, form, or habit of the corresponding bulk phases grown independently by standard electrocrystallization in solution. Given that in a confined environment the molecular species will progress much more slowly, or barely at all, to the vicinity of the active electrode, the deposition of neutral π-donor molecules in the form of a 1 μm thick layer on the surface of a glass slide, by sublimation under vacuum, proved to be a major experimental improvement, and also yielded better crystal quality in more reasonable amounts of time. The effect of temperature and the typically high current densities commonly achieved under these conditions, are discussed along with the experimental conditions used to grow thin crystals of (TMTTF)2ReO4 and (TMTSF)2ReO4, κ-(BEDT–TTF)2Cu(NCS)2 (25 °C), (BEDT–TTF)Cu2(NCS)3 (60 °C), (EDT–TTF)3[PW12O40]·(CH3CN)· (CH2Cl–CHCl2), β″-(BEDT–TTF)4·(guest)·[Re6Se6Cl8] and β″-(BEDT–TTF)4·(guest)·[Mo6Cl14]. It is concluded that the confined electrocrystallization technique appears to be suitable for growing thin crystals of radical ion salts of essentially all the phases previously obtained in bulk crystal form by classical electrocrystallization in solution.

Electrochemically induced reversible solid state transformations: electrosynthesis of Ag2Cu2O4 by room temperature oxidation of Ag2Cu2O3

Electrochemical oxidation at room temperature of a slurry of Ag2Cu2O3 yields a new silver copper oxide, formulated as Ag2Cu2O4, with one more atom of oxygen per unit formula, that can in turn revert to the original precursor. The resulting oxide presents a different electronic and crystal structure from its precursor, as shown by XPS, X-ray and electron diffraction. This phase transformation involves a radical structural change from a 3D to a 2D network, as well as electronic changes involving silver and oxygen. The potential of electrochemical techniques to induce crystal-chemical solid state transformations is analyzed.

Thin water films grown at ambient conditions on BaF2(111) studied by scanning polarization force microscopy

The interaction of water with freshly cleaved BaF(2)(111) surfaces has been studied using scanning force microscopy operated in different modes at room temperature and under controlled humidity. The Kelvin probe microscopy (KPM) mode has been used to study the evolution of the surface potential differences (SPDs). In the 20%-50% relative humidity (RH) range, adsorbed water forms two-dimensional solidlike bilayers (islands). The SPD between water islands and the bare substrate surface exhibits a sign crossover from negative ( approximately -30 mV) at low RHs to positive ( approximately +50 mV) at higher RHs, evidencing a cooperative and irreversible flipping of the preferential orientation of water dipoles, from pointing toward the surface evolving into the opposite direction. The KPM results suggest that the classical hexagonal (I(h)) bilayer configuration is not the most favorable structure.

Nanowires of molecule-based charge-transfer salts

Nanowires of molecule-based charge-transfer salts are prepared using two different processing techniques. Isolated [TTF][TCNQ] nanowires are grown by a simple adsorption in organic solution method on stainless steel conversion coatings, used as substrates. They are characterized by Raman spectroscopy and current–voltage measurements. Nanowire films of Per2[Au(mnt)2] and (EDT-TTFVO)4(FeCl4)2 are electrodeposited on (001)-oriented silicon wafers, used as anodes. In the second case, growth as nanowires occurs after functionalizing the Si electrode with a phospholipidic membrane. Electrodeposited nanowire films are studied by various techniques, including electron microscopy, vibrational spectroscopies, X-ray photoelectron spectroscopy and conductivity measurements.

Nanocrystalline N-doped ceria porous thin films as efficient visible-active photocatalysts

Nanocrystalline porous N-doped ceria films have been prepared by combining sol–gel chemistry, evaporation-induced self-assembly (EISA), flash crystallization and treatment in NH3 gas between 550 and 700 °C at flow rates of 270 and 670 cm3 min−1. The maximum nitrogen uptake is 0.06 moles per Ce, observed for the sample treated at 600 °C and NH3 flow rate of 670 cm3 min−1. Nitrogen bonding to cerium is proved by XPS, through the observation of the characteristic Ce–N feature at ∼397 eV. Microstructural characterization shows that the mesostructure of the films is lost for treatment temperatures in ammonia higher than 550 °C but the porous fraction remains between 35 and 40% after ammonolysis for all the samples. The nitrided films show photocatalytic activity under visible light in the acetaldehyde decomposition, with the best results obtained for the films treated at 550 °C under NH3 flow rate of 670 cm3 min−1.

Nanoparticles of organic conductors: synthesis and application as electrode material in organic field effect transistors

Stabilization of TTF·TCNQ nanoparticles is studied by varying the ionic liquid nature and solvent medium. The best dispersion is obtained in an acetonitrile/acetone mixture and the smaller size by using [BMIM][BF4], as a stabilizing ionic liquid. Applications of well-dispersed TTF·TCNQ nanoparticles (mean diameter of about 35 nm) as electrode material in organic field-effect transistors are also reported.