Stabilization of Metastable Thermoelectric Crystalline Phases by Tuning the Glass Composition in the Cu-As-Te System.

Recrystallization of amorphous compounds can lead to the stabilization of metastable crystalline phases, which offers an interesting way to unveil novel binary or ternary compounds and control the transport properties of the obtained glass ceramics. Here, we report on a systematic study of the Cu-As-Te glassy system and show that under specific synthesis conditions using the spark-plasma-sintering technique, the α-As2Te3 and ß-As2Te3 binary phases and the previously unreported AsTe3 phase can be selectively crystallized within an amorphous matrix. The microstructures and transport properties of three different glass ceramics, each of them containing one of these phases with roughly the same crystalline fraction (∼30% in volume), were investigated in detail by means of X-ray diffraction, scanning electron microscopy, neutron thermodiffraction, Raman scattering (experimental and lattice-dynamics calculations), and transport-property measurements. The physical properties of the glass ceramics are compared with those of both the parent glasses and the pure crystalline phases that could be successfully synthesized. SEM images coupled with Raman spectroscopy evidence a "coast-to-island" or dendriticlike microstructure with microsized crystallites. The presence of the crystallized phase results in a significant decrease in the electrical resistivity while maintaining the thermal conductivity to low values. This study demonstrates that new compounds with interesting transport properties can be obtained by recrystallization, which in turn provides a tuning parameter for the transport properties of the parent glasses.

Gold and Nickel Extended Thiophenic-TTF Bisdithiolene Complexes.

Gold and nickel bisdithiolene complexes with methyl and tert-butyl substituted thiophenetetrathiafulavalenedithiolate ligands (α-mtdt and α-tbtdt) were prepared and characterized. These complexes were obtained, under anaerobic conditions, as tetrabutylammonium salts. The diamagnetic gold monoanion (n-Bu4N)[Au(α-mtdt)2] (3) and nickel dianionic species (n-Bu4N)x[Ni(α-mtdt)2] (x = 1,2) (4) were similar to the related non-substituted extended thiophenic-TTF (TTF = tetrathiafulvalene) bisdithiolenes. However the introduction of the large, bulky substituent tert-butyl, led to the formation of a Au (I) dinuclear complex, (n-Bu4N)2[Au2(α-tbtdt)2] (5). The neutral methyl substituted gold and nickel complexes were easily obtained through air or iodine exposure as polycrystalline or amorphous fine powder. [Au(α-mtdt)2] (6) and [Ni(α-mtdt)2] (7) polycrystalline samples display properties of a metallic system with a room temperature electrical conductivity of 0.32 S/cm and ≈4 S/cm and a thermoelectric power of ≈5 µV/K and ≈32 µV/K, respectively. While [Au(α-mtdt)2] (6) presented a Pauli-like magnetic susceptibility typical of conducting systems, in [Ni(α-mtdt)2] (7) large magnetic susceptibilities indicative of high spin states were observed. Both electric transport properties and magnetic properties for gold and nickel [M(α-mtdt)2] are indicative that these compounds are single component molecular conductors.

Polymorphism and Superconductivity in Bilayer Molecular Metals (CNB-EDT-TTF)4I3.

Electrocrystallization from solutions of the dissymmetrical ET derivative cyanobenzene-ethylenedithio-tetrathiafulvalene (CNB-EDT-TTF) in the presence of triiodide I3- affords two different polymorphs (ß″ and κ) with the composition (CNB-EDT-TTF)4I3, both with a bilayer structure of the donors. These polymorphs differ in the packing patterns (ß″- and κ-type) of the donor molecules in each layer, in both cases with bifurcated C-N···H interactions effectively coupling head-to-head donor molecules between layer pairs. Two ß″ polymorphs can be obtained with different degrees of anionic ordering. In one disordered phase, ß″d, with a smaller unit cell, the triiodide anions are disordered over two possible positions in a channel between the donor bilayers, while in the ordered phase, ß″o, the triiodide anions occupy only one of those positions in this channel, leading to the doubling of the unit cell in the layer plane. These results for ß″ phases contrast with the κ polymorph previously reported, for which weaker disorder of the triiodide anions, over two possible orientations with 94 and 6% occupation factors, was observed. While the ß″ polymorphs remains metallic down to 1.5 K with a ρ300K/ρ4K resistivity ratio of 250, the κ polymorph presents a much smaller resistivity ratio in the range of 4-10 and superconductivity with an onset temperature of 3.5 K.

Bilayer Molecular Metals Based on Dissymmetrical Electron Donors.

The electrocrystallization from solutions of cyanobenzene-ethylenedithio-tetrathiafulvalene (CNB-EDT-TTF) in the presence of different anions X = ClO4(-), PF6(-), and I3(-), affords a new type of 2D molecular metals with composition (CNB-EDT-TTF)4X based on an unprecedented bilayer structure of the donors induced by effective head to head interdonor interactions through the nitrile groups, which is responsible for 2D metallic systems with unusual properties such as the higher band filling, larger effective mass of carriers, and almost degenerated double Fermi surfaces.

Polymorphism in Thermoelectric As2Te3.

Metastable ß-As2Te3 (R3̅m, a = 4.047 Å and c = 29.492 Å at 300 K) is isostructural to layered Bi2Te3 and is known for similarly displaying good thermoelectric properties around 400 K. Crystallizing glassy-As2Te3 leads to multiphase samples, while ß-As2Te3 could indeed be synthesized with good phase purity (97%) by melt quenching. As expected, ß-As2Te3 reconstructively transforms into stable α-As2Te3 (C2/m, a = 14.337 Å, b = 4.015 Å, c = 9.887 Å, and ß = 95.06°) at 480 K. This ß â†’ α transformation can be seen as the displacement of part of the As atoms from their As2Te3 layers into the van der Waals bonding interspace. Upon cooling, ß-As2Te3 displacively transforms in two steps below T(S1) = 205-210 K and T(S2) = 193-197 K into a new ß'-As2Te3 allotrope. These reversible and first-order phase transitions give rise to anomalies in the resistance and in the calorimetry measurements. The new monoclinic ß'-As2Te3 crystal structure (P2(1)/m, a = 6.982 Å, b = 16.187 Å, c = 10.232 Å, ß = 103.46° at 20 K) was solved from Rietveld refinements of X-ray and neutron powder patterns collected at low temperatures. These analyses showed that the distortion undergone by ß-As2Te3 is accompanied by a 4-fold modulation along its b axis. In agreement with our experimental results, electronic structure calculations indicate that all three structures are semiconducting with the α-phase being the most stable one and the ß'-phase being more stable than the ß-phase. These calculations also confirm the occurrence of a van der Waals interspace between covalently bonded As2Te3 layers in all three structures.

Dithiophene-TTF Salts; New Ladder Structures and Spin-Ladder Behavior.

(α-DT-TTF)2[Au(i-mnt)2] and (α-DT-TTF)2[Co(mnt)2] are two new salts of the donor α-dithiophene-tetrathiafulvalene with stable diamagnetic anions, both presenting a ladder structure of the donors organized in paired segregated stacks. The first one is isostructural with previously reported closely related compounds and presents a magnetic spin-ladder behavior with J∥= 83.5 K and J⊥ = 110.3 K as estimated from spin susceptibility data in single crystals. (α-DT-TTF)2[Co(mnt)2] presents a new structural type with a different arrangement of pairs of donor stacks, alternating with stacks of dimerized [Co(mnt)2] anions which are however arranged in an uncorrelated fashion perpendicular to the stacking axis. Due to the strong coupling between the disordered anion chains and the donor chains, this compound does not present a magnetic spin-ladder behavior. The low temperature superstructure of (DT-TTF)2[Cu(mnt)2] below the transition at 235 K, previously known to be associated with a lattice doubling along the stacking axis, was solved by synchrotron radiation diffraction in small single crystals. It is found that this dimerization is due to donor charge localization with the spin carriers being associated with fully oxidized donor species alternating with neutral donors.

(α-DT-TTF)2[Au(mnt)2]: a weakly disordered molecular spin-ladder system.

The synthesis and characterization of (α-DT-TTF)2[Au(mnt)2] is reported. The magnetic properties of this new salt show that it is still a rare example of an organic spin-ladder. (α-DT-TTF)2[Au(mnt)2] shares the same ladder structure of the DT-TTF and ETT-TTF analogues, and its room temperature conductivity is ∼2 S/cm. Despite the observed donor orientation disorder associated with the thiophenic sulfur atoms, the intermolecular interactions between donor units, calculated using the extended Hückel approximation and a double-ξ basis set, show that the interaction values do not depend on the configuration of the sulfur atom on the thiophenic ring. The insensitivity of the spin-ladder magnetic properties to the donor molecular disorder in (α-DT-TTF)2[Au(mnt)2] is a direct consequence of the negligible contribution of the disordered thiophenic sulfur atom to the HOMO. In the related donor ETT-TTF, this contribution is significant and destroys the magnetic interactions, and no spin-ladder is observed. This compound not only enlarges the number of organic spin-ladder systems in this series of closely related compounds but also provides an interesting example of weakly disordered molecular spin-ladder system.

Thermoelectric Properties of Nickel and Selenium Co-Doped Tetrahedrite.

As the search continues for novel, cheaper, more sustainable, and environmentally friendly thermoelectric materials in order to expand the range of applications of thermoelectric devices, the tetrahedrite mineral (Cu12Sb4S13) stands out as a potential candidate due to its high abundance, low toxicity, and good thermoelectric performance. Unfortunately, as most current thermoelectric materials achieve zTs above 1.0, ternary tetrahedrite is not a suitable alternative. Still, improvement of its thermoelectric performance has been achieved to zTs ≈ 1 via isovalent doping and composition tuning, but most studies were limited to a single doping element. This project explores the effects of simultaneous doping with nickel and selenium in the thermoelectric properties of tetrahedrite. Simulated properties for different stoichiometric contents of these dopants, as well as the measured thermoelectric properties of the correspondent materials, are reported. One of the samples, Cu11.5Ni0.5Sb4S12.5Se0.5, stands out with a high power factor = 1279.99 µW/m·K2 at 300 K. After estimating the thermal conductivity, a zT = 0.325 at 300 K was obtained for this composition, which is the highest for tetrahedrites for this temperature. However, analysis of the weighted mobility shows the presence of detrimental factors, such as grain boundaries, disorder, or ionized impurity scattering, pointing to the possibility of further improvements.

An Electrical Contacts Study for Tetrahedrite-Based Thermoelectric Generators.

High electrical and thermal contact resistances can ruin a thermoelectric device's performance, and thus, the use of effective diffusion barriers and optimization of joining methods are crucial to implement them. In this work, the use of carbon as a Cu11Mn1Sb4S13 tetrahedrite diffusion barrier, and the effectiveness of different fixation techniques for the preparation of tetrahedrite/copper electrical contacts were investigated. Contacts were prepared using as jointing materials Ni and Ag conductive paints and resins, and a Zn-5wt% Al solder. Manual, cold- and hot-pressing fixation techniques were explored. The contact resistance was measured using a custom-made system based on the three points pulsed-current method. The legs interfaces (Cu/graphite/tetrahedrite) were investigated by optical and scanning electron microscopies, complemented with energy-dispersive X-ray spectroscopy, and X-ray diffraction. No interfacial phases were formed between the graphite and the tetrahedrite or Cu, pointing to graphite as a good diffusion barrier. Ag water-based paint was the best jointing material, but the use of hot pressing without jointing materials proves to be the most reliable technique, presenting the lowest contact resistance values. Computer simulations using the COMSOL software were performed to complement this study, indicating that high contact resistances strongly reduce the power output of thermoelectric devices.

Computer Simulations of Silicide-Tetrahedrite Thermoelectric Generators.

With global warming and rising energy demands, it is important now than ever to transit to renewable energy systems. Thermoelectric (TE) devices can present a feasible alternative to generate clean energy from waste heat. However, to become attractive for large-scale applications, such devices must be cheap, efficient, and based on ecofriendly materials. In this study, the potential of novel silicide-tetrahedrite modules for energy generation was examined. Computer simulations based on the finite element method (FEM) and implicit finite difference method (IFDM) were performed. The developed computational models were validated against data measured on a customized system working with commercial TE devices. The models were capable of predicting the TEGs' behavior with low deviations (≤10%). IFDM was used to study the power produced by the silicide-tetrahedrite TEGs for different ΔT between the sinks, whereas FEM was used to study the temperature distributions across the testing system in detail. To complement these results, the influence of the electrical and thermal contact resistances was evaluated. High thermal resistances were found to affect the devices ΔT up to ~15%, whereas high electrical contact resistances reduced the power output of the silicide-tetrahedrite TEGs by more than ~85%.

Physical characterization of functionalized spider silk: electronic and sensing properties.

This work explores functional, fundamental and applied aspects of naturally harvested spider silk fibers. Natural silk is a protein polymer where different amino acids control the physical properties of fibroin bundles, producing, for example, combinations of ß-sheet (crystalline) and amorphous (helical) structural regions. This complexity presents opportunities for functional modification to obtain new types of material properties. Electrical conductivity is the starting point of this investigation, where the insulating nature of neat silk under ambient conditions is described first. Modification of the conductivity by humidity, exposure to polar solvents, iodine doping, pyrolization and deposition of a thin metallic film are explored next. The conductivity increases exponentially with relative humidity and/or solvent, whereas only an incremental increase occurs after iodine doping. In contrast, iodine doping, optimal at 70 °C, has a strong effect on the morphology of silk bundles (increasing their size), on the process of pyrolization (suppressing mass loss rates) and on the resulting carbonized fiber structure (that becomes more robust against bending and strain). The effects of iodine doping and other functional parameters (vacuum and thin film coating) motivated an investigation with magic angle spinning nuclear magnetic resonance (MAS-NMR) to monitor doping-induced changes in the amino acid-protein backbone signature. MAS-NMR revealed a moderate effect of iodine on the helical and ß-sheet structures, and a lesser effect of gold sputtering. The effects of iodine doping were further probed by Fourier transform infrared (FTIR) spectroscopy, revealing a partial transformation of ß-sheet-to-amorphous constituency. A model is proposed, based on the findings from the MAS-NMR and FTIR, which involves iodine-induced changes in the silk fibroin bundle environment that can account for the altered physical properties. Finally, proof-of-concept applications of functionalized spider silk are presented for thermoelectric (Seebeck) effects and incandescence in iodine-doped pyrolized silk fibers, and metallic conductivity and flexibility of micron-sized gold-sputtered silk fibers. In the latter case, we demonstrate the application of gold-sputtered neat spider silk to make four-terminal, flexible, ohmic contacts to organic superconductor samples.

Extraction of Aloesin from Aloe vera Rind Using Alternative Green Solvents: Process Optimization and Biological Activity Assessment.

Aloesin is an aromatic chromone with increasing applications in the cosmetic and health food industries. To optimize its extraction from the Aloe vera leaf rind, the independent variables time (10-210 min), temperature (25-95 °C) and organic solvent composition (0-100%, w/w) were combined in a central composite design coupled with response surface methodology. The solvents consisted of binary mixtures of water with ethanol, propylene glycol, or glycerol. The aloesin levels quantified in each extract were used as response for optimization. The theoretical models were fitted to the experimental data, statistically validated, and used to obtain the optimal extraction conditions. Then, a dose-response analysis of the solid/liquid ratio (S/L) was performed under the optimal conditions determined for each alcohol-water system and revealed that a linear improvement in extraction efficiency can be achieved by increasing the S/L ratio by up to 40 g/L. This analysis also allowed to experimentally validate the predictive models. Furthermore, the aloesin-rich extracts revealed antioxidant activity through thiobarbituric acid reactive substances (TBARS) formation inhibition, antimicrobial effects against bacterial and fungal strains, and no toxicity for PLP2 cells. Overall, this study provided optimal extraction conditions for the recovery of aloesin from Aloe vera rind through an eco-friendly extraction process and highlighted its bioactive potential.

Order versus disorder in chiral tetrathiafulvalene-oxazoline radical-cation salts: structural and theoretical investigations and physical properties.

Electrocrystallization experiments with the chiral ethylenedithio-tetrathiafulvalene-methyl-oxazoline (EDT-TTF-OX) donors (R)-, (S)-, and (rac)-1 have provided two series of mixed-valence salts with the PF(6) (-) and [Au(CN)(2)](-) anions. Within each series the cell parameters are the same for the three R, S, and rac compounds, except for the space group, which is centrosymmetric triclinic P$\bar 1$ for the racemic forms and noncentrosymmetric P1 for the enantiopure salts. In the racemic salt [(rac)-1](2)PF(6) the two enantiomers crystallize disordered on the same crystallographic site with a site occupational factor of 0.6:0.4, whereas this type of disorder is not possible in the enantiopure salts. Both s-cis and s-trans conformations, when taking into account the mutual orientation of the TTF and oxazoline moieties, are present in this first series. In sharp contrast, in the series of salts [1](2)[Au(CN)(2)], only the s-trans conformation is observed with no structural disorder. Theoretical calculations at the DFT level of theory revealed a very small energy difference between the two stable planar s-cis and s-trans conformations, which are both energy minima in either neutral or oxidized states. Single-crystal conductivity measurements showed metallic-like behavior for all the salts down to 220-250 K with a smooth increase in resistivity at lower temperatures. The conductivity at room temperature is 5 S cm(-1) for [(rac)-1](2)PF(6), in which disorder was observed, whereas for [(R)-1](2)PF(6) and [(S)-1](2)PF(6) the average value is around 100 S cm(-1). In the second series of salts the conductivity at room temperature is 125-130 S cm(-1) for [(rac)-1](2)[Au(CN)(2)] and [(R)-1](2)[Au(CN)(2)]. Extended Hückel band structure calculations revealed identical features for the three salts of the [1](2)[Au(CN)(2)] series and are consistent with the electronic structures of quasi-one-dimensional conductors.

Conducting neutral gold bisdithiolene complex [Au(dspdt)2]˙.

[Au(dspdt)2] (dspdt = 2,3-dihydro-5,6-selenophenedithiolate) is an unprecedented example of a neutral gold bisdithiolene complex with a unique structure composed of interacting dimer and trimer chains displaying relatively high electrical conductivity (0.1 S cm-1 at room temperature).

Hydrogen bonded anion ribbons, networks and clusters and sulfur-anion interactions in novel radical cation salts of BEDT-TTF with sulfamate, pentaborate and bromide.

The novel radical cation salt (BEDT-TTF)3(sulfamate)2·2H2O (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) is semiconducting with donor stacks comprised of pairs of partially oxidized molecules and a single more highly oxidized molecule which is twisted out of the stack by ca. 30°. Hydrogen bonded pairs of sulfamate ions are linked into parallel ribbons by further hydrogen bonding between sulfamates and bridging water molecules. In contrast, the BEDT-TTF salt with pentaborate contains infinite layers formed of a network of hydrogen bonded pentaborate anions. Two new bromide salts of BEDT-TTF are reported, one is a semiconducting 1 : 1 salt in which the bromide is integrated among the BEDT-TTF donors, while the other contain a square of four bromide ions linked together by hydrogen bonding to a centrally located H5O2(+) cation for every five BEDT-TTF molecules.

Transition metal bisdithiolene complexes based on extended ligands with fused tetrathiafulvalene and thiophene moieties: new single-component molecular metals.

The gold and nickel bisdithiolene complexes based on new highly extended ligands incorporating fused tetrathiafulvalene and thiophene moieties (alpha-tdt=thiophenetetrathiafulvalenedithiolate and dtdt=dihydro- thiophenetetrathiafulvalenedithiolate), were prepared and characterised by using cyclic voltammetry, single crystal X-ray diffraction, EPR, magnetic susceptibility and electrical transport measurements. These complexes, initially obtained under anaerobic conditions as diamagnetic gold monoanic [nBu(4)N][Au(alpha-tdt)(2)] (4), [nBu(4)N][Au(dtdt)(2)] (3) and nickel dianionic species [(nBu(4)N)(2)][Ni(alpha-tdt)(2)] (8), [(nBu(4)N)(2)][Ni(dtdt)(2)] (7), can be easily oxidised to the stable neutral state just by air or iodine exposure. The monoanionic complexes crystallise in at least two polymorphs, all of which have good cation and anion segregation in alternated layers, the anion layers making a dense 2D network of short SS contacts. All of the neutral complexes, obtained as microcrystalline or quasi amorphous fine powder, present relatively large magnetic susceptibilities that correspond to effective magnetic moments in the range 1-3 mu(B) indicative of high spin states and very high electrical conductivity that in case of the Ni compound can reach sigma(RT) approximately 250 S cm(-1) with a clear metallic behaviour. These compounds are new examples of the still rare single-component molecular metals.

Structural, magnetic, and electrical characterization of new polycrystalline phases of nickel- and platinum-doped [(DT-TTF)n][Au(mnt)2] (n = 1, 2).

Doping of spin-ladder systems by isostructural paramagnetic complexes was attempted. Despite the close isostructural nature of the pure (DT-TTF)2[M(mnt)2] (M = Au, Ni, Pt) end-members, which present a ladder structure, doping of the spin-ladder (DT-TTF)2[Au(mnt)2] with either 5% or 25% [M(mnt)2]- (M = Ni, Pt) generates two (metrically) new phases. Their markedly different crystal structures have been determined using laboratory X-ray powder diffraction data. (DT-TTF)2[Au0.75Ni0.25(mnt)2] consists of a mixed-valence compound (of triclinic symmetry), which was only detected, pure or in a mixture of phases, when [Ni(mnt)2]- was used as a dopant. Differently, the stoichiometric 1:1 [DT-TTF][Au0.75Pt0.25(mnt)2] monoclinic phase was found when [Pt(mnt)2]- (in 5% and 25%) was employed as the doping agent. Remarkably, only in the 5% Pt doping experiment, the major component of the mixture was the ladder structure compound (DT-TTF)2[Au(mnt)2] doped with minor amounts of Pt. This 5% Pt-doped specimen shows an EPR signal (g = 2.0115, DeltaHpp = 114 G at 300 K) wider than the pure compound (DT-TTF)2[Au(mnt)2], denoting exchange between the donor spins and Pt(mnt)2- centers. The electrical transport properties of the 5% Pt-doped composition at high temperatures are comparable to those of (DT-TTF)2[Au(mnt)2] with room-temperature conductivity sigma300K = 13 S/cm and thermopower S300K = 46 microV/K, with a sharp transition at 223 K similar to that previously observed in the Cu analogue at 235 K.