Alicia de Andrés

Raman spectroscopy for the study of reduction mechanisms and optimization of conductivity in graphene oxide thin films

Highly reduced few-layer graphene oxide films with conductivities of up to 500 S cm−1 are obtained. The thin films with an optimized compromise between sheet resistance (3.1 kΩ sq−1) and transparency (around 80% to 90%) are suitable for touch screens and transparent electrodes in OLEDs. We discuss the effects of low temperature annealing and chemical reduction on the properties of the films and present an optimized reduction process that allows the original 2D/G Raman intensity ratio of few-layer graphene to be recovered. The Raman spectrum of graphene oxide is found to be only related to oxygen-free graphene-like regions with Raman bands at 1130 and 3155 cm−1 that probably involve C–H vibrations of rings and edges, while a band at 1700 cm−1 is assigned to irregular rings such as Stone Wales defects. All the bands involve resonant Raman processes and disappear in highly reduced samples. Clear correlations of the D band width with the sp2 content in thin films and resistivity have been observed, indicating that this is a good Raman parameter for evaluating the quality of the samples. The structural defects produced by the release of embedded water and some of the oxygen functional groups during annealing are detrimental for intra-grain conductivity but greatly enhance inter-grain connectivity.

Multimetal rare earth MOFs for lighting and thermometry: tailoring color and optimal temperature range through enhanced disulfobenzoic triplet phosphorescence

New isostructural compounds [Ln7(3,5-DSB)4(OH)9(H2O)15]·4H2O (Ln = Eu, Gd and Tb) (DSB = 3,5-disulfobenzoate), with excellent optical properties, have been obtained and studied. By combining the lanthanide cations with the DSB ligand the formation of a singular heptanuclear [Ln7(OH)9]+12 metallic core secondary building unit (SBU) has been promoted. This core is defined as a bi-capped dicubane SBU, and is a 4-connected node in a bidimensional net with (44·62) topology. The highly efficient emission from the triplet state of the disulfobenzoic ligand in the Gd-MOF and the exponential temperature dependence of ITriplet/ILn = Io exp(−EA/kBT) allow the design of thermometers with different color ranges through the adequate multimetal combination that cover almost the whole visible spectrum and are useful in a wide temperature range (10–300 K) with unprecedented sensitivities up to 32% K−1. The Eu-DSB and Tb-DSB MOFs present an optimum antenna effect, a temperature independent f–f emission temporal decay and, in spite of the close packing of the Ln ions inside the MOF, no concentration quenching. The external efficiency is drastically increased in MOFs with a combination of a few percent of Eu and/or Tb with Gd by means of radiative and non-radiative energy transfers from the Gd ions and Gd connected ligands.

Breakdown into nanoscale of graphene oxide: Confined hot spot atomic reduction and fragmentation

Nano-graphene oxide (nano-GO) is a new class of carbon based materials being proposed for biomedical applications due to its small size, intrinsic optical properties, large specific surface area, and easy to functionalize. To fully exploit nano-GO properties, a reproducible method for its production is of utmost importance. Herein we report, the study of the sequential fracture of GO sheets onto nano-GO with controllable lateral width, by a simple, and reproducible method based on a mechanism that we describe as a confined hot spot atomic fragmentation/reduction of GO promoted by ultrasonication. The chemical and structural changes on GO structure during the breakage were monitored by XPS, FTIR, Raman and HRTEM. We found that GO sheets starts breaking from the defects region and in a second phase through the disruption of carbon bonds while still maintaining crystalline carbon domains. The breaking of GO is accompanied by its own reduction, essentially by the elimination of carboxylic and carbonyl functional groups. Photoluminescence and photothermal studies using this nano-GO are also presented highlighting the potential of this nanomaterial as a unique imaging/therapy platform.

Enhancement of the curie temperature along the perovskite series RCu3Mn4O12 driven by chemical pressure of R3+ cations (R = Rare Earths)

The compounds of the title series have been prepared from citrate precursors under moderate pressure conditions (P = 2 GPa) and 1000 degrees C in the presence of KClO(4) as oxidizing agent. The crystal structures are cubic, space group Im3 (No. 204); the unit cell parameters linearly vary from a = 7.3272(4) A (R = La) to a = 7.2409(1) A (R = Lu) at room temperature. A neutron or synchrotron X-ray diffraction study of all the members of the series reveals an interesting correlation between some structural parameters and the magnetic properties. The electron injection effect upon replacement of Ca(2+) with R(3+) cations in the parent CaCu(3)Mn(4)O(12) oxide leads to a substantial increment of the ferrimagnetic Curie temperature (T(C)). An essential ingredient is supplied by the internal pressure of the R(3+) cations upon a decrease in size along the rare-earth series, from La to Lu: the concomitant compression of the MnO(6) octahedral units for the small rare earths provides progressively shorter Mn-O distances and improves the overlapping between Mn and O orbitals, thereby promoting superexchange and enhancing T(C) by 50 K along the series. This interaction is also reinforced by a ferromagnetic component that depends on the local distortion of the MnO(6) octahedra, which also increases along the series, constituting an additional factor, via intersite virtual charge transfer t-e orbital hybridization, for the observed increment of T(C).

Stable organic radical stacked by in situ coordination to rare earth cations in MOF materials

With the correct choice of the solvothermal conditions, we have achieved the unprecedented in situ formation of the free radical form of the anthraquinone-1,5-disulfonate molecule, and its favorable organization. The semiquinone radicals are coordinated to rare-earth cations to produce a 2D framework with a very high charge mobility and electric conductivity through the π–π-interactions. The existence of AQDS3−˙ anion radicals is proven on the base of: i) the electrical neutrality: elemental analyses for the different lanthanide RPF-8 bulks, the maximum residual electron densities in the structure, rule out the existence of any other neutralizing ion, ii) the geometrical modifications in the antraquinone molecules, and iii) although less definitive, due to the low magnetic moment μ = 0.39 μB, the exhibited paramagnetism for the La (3+) with no unpaired electrons.

Carbon nanotubes—mesoporous silica composites as controllable biomaterials

A composite based on mesoporous ordered silica (SBA-15) and carbon nanotubes (CNTs), has been synthesized to be used in biomedical applications as a conductor composite for cell stimulation and regeneration. The influence of the CNT addition time on the SBA-15–CNTs composite synthesis has been studied and optimized attending to morphology and structure, to produce a homogeneous material with both structures preserved. With the addition of the CNTs, the electrical resistance has decreased 8 orders in magnitude in comparison to the SBA-15 itself. The potential of these materials to be used as bioimplants has been evaluated in vitro by soaking the composite in simulated body fluid (SBF) at 37 °C.

Quality of graphene on sapphire: long-range order from helium diffraction versus lattice defects from Raman spectroscopy

We report a new method to produce high-quality, transparent graphene/sapphire samples, using Cu as a catalyst. The starting point is a high-quality graphene layer prepared by CVD on Cu(111)/Al2O3. Graphene on sapphire is obtained in situ by evaporation of the Cu film in UHV. He-diffraction, atomic force microscopy (AFM), Raman spectroscopy and optical transmission have been used to assess the quality of graphene in a metal free area. We used helium atom scattering as a sensitive probe of the crystallinity of the graphene on sapphire. The observation of high reflectivity and clear diffraction peaks demonstrates the presence of flat and homogeneous graphene domains over lateral scales of microns, consistent with the AFM results. Surprisingly, putting graphene on sapphire improves the quality of the He-diffraction spectra. Graphene forms a moire pattern with a (11 × 11) periodicity, aligned with the (1 × 1) sapphire unit cell. The lattice constant of graphene on sapphire is a = (2.44 ± 0.02) A. The phonon dispersion of the graphene flexural mode has been measured. This allowed the determination of the bending rigidity k = 0.61 ± 0.15 eV, and the graphene–sapphire coupling strength g = (5.8 ± 0.4) × 1019 N m−3. The uniformity of the graphene has also been investigated by Raman mapping. Judging by the ratio of the 2D to G peaks, the quality of the graphene is not degraded by Cu removal. The high transparency (80%) measured in the visible range makes this system suitable for many applications that require hybrid properties commonly associated with metals (conductivity) and insulators (transparency). Our study shows that He-diffraction and Raman provide crucial information on quite different, complementary aspects of the same samples.

SrMnO3 Thermochromic Behavior Governed by Size-Dependent Structural Distortions

The influence of particle size in both the structure and thermochromic behavior of 4H-SrMnO3 related perovskite is described. Microsized SrMnO3 suffers a structural transition from hexagonal (P63/mmc) to orthorhombic (C2221) symmetry at temperature close to 340 K. The orthorhombic distortion is due to the tilting of the corner-sharing Mn2O9 units building the 4H structural type. When temperature decreases, the distortion becomes sharper reaching its maximal degree at ∼125 K. These structural changes promote the modification of the electronic structure of orthorhombic SrMnO3 phase originating the observed color change. nano-SrMnO3 adopts the ideal 4H hexagonal structure at room temperature, the orthorhombic distortion being only detected at temperature below 170 K. A decrease in the orthorhombic distortion degree, compared to that observed in the microsample, may be the reason why a color change is not observed at low temperature (77 K).

Influence of the substrate on the bulk properties of hybrid lead halide perovskite films

In addition to the known effect of the substrate on the interfacial properties of perovskite films, here we show that the bulk properties of hybrid lead halide perovskite films depend on the type of substrate used for film growth. Despite the relative large film thickness, ∼600 nm, the roughness and nature of the substrate layer (glass, FTO, TiO2 and PEDOT:PSS) affect not just the degree of preferential orientation and crystal grain size but also the lattice parameters of CH3NH3PbI3 films synthesized from the PbCl2 precursor. The obtained changes in lattice parameters indicate that the Pb–Pb distance varies by around 0.7%. We suggest that the substrate roughness and chemical nature determine the concentration of defects mainly by varying the chlorine content and probably by the incorporation of oxygen and iodine vacancies during film nucleation and growth. These differences also have consequences in the observed light induced transformations. Upon laser illumination, the formation of additional defects, most probably related to oxygen, is revealed by 110 and 165 cm−1 Raman peaks. With increasing laser power the chemical transformation into PbOx is clearly identified by the 140 and 275 cm−1 Raman peaks. The irreversible photoluminescence enhancement observed at low power with illumination time, also dependent on the substrate nature, is proposed to be due to the localization of the electron–hole excitons created in the vicinity of the light generated defects. These results shed light on the performance of the perovskite layer and help to understand how bulk processes, where ion migration is a conspicuous example, are severely affected by interfacial properties such as those imposed by the substrate.

Elastic constants of graphene oxide few-layer films: correlations with interlayer stacking and bonding

We propose a strategy to study the elastic properties of extremely thin graphene oxide (GO) films using Brillouin spectroscopy. The dependence of the surface acoustic wave of a gold capping layer on the structural, chemical and morphological changes occurring to the underneath GO film with temperature is reported and analyzed. At room temperature the shear constant c44 is ∼17 GPa and hardens up to 28 GPa at 100 °C due to the partial elimination of embedded water layers and to interlayer distance shrinking. At 200 °C the almost complete elimination of water induces layer stacking disorder, further GO–GO distance reduction and a significant increase of all elastic constants. The in-plane constants harden due to the partial restoration of the sp2 C network (c11: from 268 to 620 GPa) and the out of plane constants harden due to the H bonds that now directly connect the neighbouring GO layers (c44 ≈ 80 GPa). The obtained Youngs moduli are significantly higher than those reported for GO paper because the ultra-thin GO films are highly ordered and there is no macroscopic applied strain during the measurement. The results obtained here are associated with the intrinsic properties of GO as in-plane and inter-layer bonding.