Ángel Pérez del Pino

Shaping Supramolecular Nanofibers with Nanoparticles Forming Complementary Hydrogen Bonds

Functionalized gold nanoparticles with complementary H-bonding groups can control the secondary structure of xerogel fibers formed by a molecular conductor thanks to their incorporation into the nanowires, which show metal-like conductivity once doped without the need for annealing. The picture shows a photograph of the xerogel, TEM images of Au particles in the gel and a single fiber, and an AFM image revealing the texture of the gel.

Supramolecular electroactive organogel and conducting nanofibers with C3-symmetrical architectures

Two C3 symmetric tris(TTF) derivatives, based on a central unit containing the rigid core 1,3,5-tricarbonyl-benzene substituted with three 3,3′-diamino-2,2′-bipyridines, have been synthesized by a convergent strategy. Single crystal X-ray analysis of the precursor 3′-[(ethylenedithio-tetrathiafulvalenyl)formylamino]-2,2′-bipyridine-3-amine shows a planar transoid conformation for the bipyridine unit, favored by intramolecular hydrogen bonding. The compound N,N′,N″-tris{3[3′-[bis(ethylthio)-tetrathiafulvalenyl]formylamino]-2,2′-bipyridyl}-benzene-1,3,5-tricarboxamide, having C3 symmetry, presents gelator properties in chlorinated solvents. The gel formed in ortho-dichlorobenzene provided—after evaporation of the solvent—a xerogel constituted by a complex network of thick and thinner fibers as demonstrated by TEM and AFM microscopies. The thick fibers were about 100 nm wide and between 1 and 5 µm long, and the thinner ones between 12 and 18 nm wide and 50 to 500 nm long. Iodine doping of the material induced the formation of a mixed valence system with charge transfer, as indicated by IR-NIR spectroscopic measurements. The doped material has a TTF:I3 ratio of 2.4:1 ten minutes after doping, but slowly loses iodine over days. The morphology of the gel did not change after the doping process, as revealed by SEM and AFM experiments. Current sensing AFM measurements showed that the thicker fibers are more conducting than the thinner ones, a likely consequence of the better ordering and/or more effective interfiber contacts in the former.

Solvent effect on the morphology and function of novel gel-derived molecular materials

The gelation of two distinct hydrocarbon solvents by a new π-functional molecule, followed by doping and measurement of conducting properties of the derived xerogel, reveals an important effect of the main gel component on the shape and organisation of the supramolecular fibres formed by the aromatic moieties. The gelator—a tetrathiafulvalene (TTF) derivative with two hydrophobic chains incorporating amide groups near the aromatic group—was also cast onto hydrophobic and hydrophilic surfaces from homogeneous solution and shows the dramatic influence of the concentration and surface on the aggregate formation, as revealed by atomic force microscopy (AFM). This observation underlines the advantage of using the gel route to prepare films of these materials. The doped xerogels show the effect of the solvent at the microscopic and macroscopic levels, as revealed by current sensing AFM and bulk four point conductivity measurements. The real polymorphism of the xerogels was confirmed by electron paramagnetic resonance (EPR) spectroscopy. In both materials, prepared from gels in (S)-limonene and n-hexane, and in contrast to a related compound with one hydrogen bonding group, the double hydrogen bond motif leads to materials which do not show structural phase transitions when heated. This feature shows the potential benefit of incorporating several hydrogen bonding groups on the phase stability of gel derived materials; to stabilise the metastable states to produce materials with different properties from a single compound by processing in different solvents.

Use of unnatural β-peptides as a self-assembling component in functional organic fibres

A homochiral synthetic dipeptide incorporating two cyclobutyl rings has been used as an assembling unit for the pi-electron-rich tetrathiafulvalene (TTF) moiety. The molecule was prepared and characterised to show all the features of the two components, whereby chirality and pi-function are incorporated in the same species. Supramolecular fibres are formed by the compound, as proven by atomic force microscopy (AFM) and transmission electron microscopy. The dimensions of the nanostructures suggest that the molecules pack into dimeric tapes with the peptide head groups at the centre. Current-sensing AFM shows that once doped, films of the material are capable of conducting electricity.

Varied nanostructures from a single multifunctional molecular material

The control of the morphology of nanostructures formed from a single component molecular material incorporating electron accepting and donating moieties is shown, from both solution and gel states. The compound comprises one tetrathiafulvalene (TTF) and two pyrene units which act as the π-electron rich and deficient units, respectively, and which are united by amide-containing linkers whose additional role is to aide aggregation by hydrogen bonding. This role was demonstrated by IR and NMR spectroscopy. The gels were deposited onto surfaces and the solvent allowed to evaporate, leaving films formed by meshes of fibres with different morphologies in accord with the different solvents used to form the materials. Doping of these xerogels with iodine vapour afforded conducting films whose characteristics were probed with current sensing atomic force microscopy (CS-AFM), providing current maps and I–V curves which show how dramatically the processing solvent can influence the electronic properties of these xerogel-derived materials.

Ultraviolet pulsed laser irradiation of multi-walled carbon nanotubes in nitrogen atmosphere

Laser irradiation of randomly oriented multi-walled carbon nanotube (MWCNT) networks has been carried out using a pulsed Nd:YAG UV laser in nitrogen gas environment. The evolution of the MWCNT morphology and structure as a function of laser fluence and number of accumulated laser pulses has been studied using electron microscopies and Raman spectroscopy. The observed changes are discussed and correlated with thermal simulations. The obtained results indicate that laser irradiation induces very fast, high temperature thermal cycles in MWCNTs which produce the formation of different nanocarbon forms, such as nanodiamonds. Premelting processes have been observed in localized sites by irradiation at low number of laser pulses and low fluence values. The accumulation of laser pulses and the increase in the fluence cause the full melting and amorphization of MWCNTs. The observed structural changes differ from that of conventional high temperature annealing treatments of MWCNTs.

Boosting electrical conductivity in a gel-derived material by nanostructuring with trace carbon nanotubes

An organogelator with two distinct π-functional units is able to incorporate carbon nanotubes into its mesh of fibres in the gel state. The morphology of the material derived from this nanocomposite after evaporation of the solvent is a complex mesh of fibres which is clearly different from the pure gelator. This feature indicates a role of the nanotubes in assisting the formation of a fibre structure in the gel thanks to their interaction with the pyrene units in the organogelator. The nanocomposite conducts electricity once the p-type gelator is doped with iodine vapour. The change in morphology caused by the carbon material increases the conductivity of the material compared with the purely organic conducting system. It is remarkable that this improvement in the physical property is caused by an extremely small proportion of the carbon material (only present at a ratio of 0.1% w/w). The practically unique properties of TTF unit allow measurements with both doped and undoped materials with conducting atomic force microscopy which have demonstrated that the carbon nanotubes are not directly responsible for the increased conductivity.

Localized, stepwise template growth of functional nanowires from an amino acid-supported framework in a microfluidic chip

A spatially controlled synthesis of nanowire bundles of the functional crystalline coordination polymer (CP) Ag(I)TCNQ (tetracyanoquinodimethane) from previously fabricated and trapped monovalent silver CP (Ag(I)Cys (cysteine)) using a room-temperature microfluidic-assisted templated growth method is demonstrated. The incorporation of microengineered pneumatic clamps in a two-layer polydimethylsiloxane-based (PDMS) microfluidic platform was used. Apart from guiding the formation of the Ag(I)Cys coordination polymer, this microfluidic approach enables a local trapping of the in situ synthesized structures with a simple pneumatic clamp actuation. This method not only enables continuous and multiple chemical events to be conducted upon the trapped structures, but the excellent fluid handling ensures a precise chemical activation of the amino acid-supported framework in a position controlled by interface and clamp location that leads to a site-specific growth of Ag(I)TCNQ nanowire bundles. The synthesis is conducted stepwise starting with Ag(I)Cys CPs, going through silver metal, and back to a functional CP (Ag(I)TCNQ); that is, a novel microfluidic controlled ligand exchange (CP → NP → CP) is presented. Additionally, the pneumatic clamps can be employed further to integrate the conductive Ag(I)TCNQ nanowire bundles onto electrode arrays located on a surface, hence facilitating the construction of the final functional interfaced systems from solution specifically with no need for postassembly manipulation. This localized self-supported growth of functional matter from an amino acid-based CP shows how sequential localized chemistry in a fluid cell can be used to integrate molecular systems onto device platforms using a chip incorporating microengineered pneumatic tools. The control of clamp pressure and in parallel the variation of relative flow rates of source solutions permit deposition of materials at different locations on a chip that could be useful for device array preparation. The in situ reaction and washing procedures make this approach a powerful one for the fabrication of multicomponent complex nanomaterials using a soft bottom-up approach.

Study of the deposition of graphene oxide by matrix-assisted pulsed laser evaporation

Thin films composed of graphene-based materials exhibit promising functional properties for the development of high-performance devices in a wealth of applications. However, there are significant technological challenges which force one to search for alternative pathways of materials production and deposition. This paper reports the deposition of graphene oxide (GO) flakes on quartz substrates by using the ultraviolet matrix-assisted pulsed laser evaporation (MAPLE) technique in vacuum or controlled nitrogen gas environment. Water, which is highly transparent to UV radiation, was used as matrix solvent for the preparation of the MAPLE targets. The results reveal that GO platelets can be successfully transferred by MAPLE technique. Besides, the GO material experiences a significant deoxygenating process during deposition, leading to the formation of reduced GO. Numerical simulations also show that the thickness of GO platelets highly influences the deposition process and the structure of the immobilized material. Thick enough aggregates can reach temperatures of thousands of degrees and undergo a large degradation in their structure.

Nanocomposites combining conducting and superparamagnetic components prepared via an organogel

A nanocomposite material combining an organic molecular gelator and oleate-coated iron oxide nanoparticles in proportions which range from one to fifty weight percent of the inorganic material has been prepared via the gel state. The proportion of nanoparticles and organic gelator in this mixed colloidal system gives very different characteristics to the final hybrid xerogel. Characterisation of the xerogels by transmission electron microscopy shows that at low loadings of the inorganic material a uniform distribution is observed, while above ten weight percent of nanoparticles a clear phase separation of the components (organic and inorganic) is revealed. Doping of the organic component of the xerogels by chemical oxidation results in the formation of conducting composites, whose electrical characteristics—probed by current sensing atomic force microscopy and spectroscopy—vary importantly with the amount of iron oxide colloid. The best conductors are found at low loadings of inorganic particles, at which an interesting alignment of the organic fibres is observed. The work shows that conducting materials incorporating magnetic particles can be prepared simply through the organogel route, and raises possibilities for the discovery of new properties that could come from the combination of these or related systems.