Enhanced Thermoelectric Properties of a Semiconducting Two-Dimensional Metal-Organic Framework via Iodine Loading.

We report the first result of a study in which molecular iodine has been incorporated via incipient wetness impregnation into the two-dimensional semiconducting metal-organic framework (MOF) Cu3(2,3,6,7,10,11-hexahydroxytriphenylene)2 Cu3(HHTP)2 to enhance its thermoelectric properties. A power factor of 0.757 µW m-1 K-2 for this MOF was obtained which demonstrates that this provides an effective route for the preparation of moderate-performance thermoelectric MOFs.

Electroless deposition of tellurium nanowires in eutectic solvents using immobilised silver islands.

In this work we demonstrate a new approach towards the electroless deposition of tellurium nanowires in deep eutectic solvents. Unlike most electroless deposition where the substrate is sacrificed to drive the reduction, our process uses immobilised silver epoxy islands on gold films to give localised galvanic displacement of the silver, resulting in an even growth of wires across the entire gold electrode surface. We demonstrate the strong dependence of the nanostructure on the experimental conditions, with changes in bath temperature, tellurium concentration and the halide component of the solvent leading to sizeable alterations in the nanowire geometry. This demonstrates electroless deposition as a promising synthetic route towards low-dimensional tellurium nanostructures.

Tunable Carrier Type of a Semiconducting 2D Metal-Organic Framework Cu3(HHTP)2.

In this work, a switch from n-type to p-type conductivity in electrodeposited Cu3(2,3,6,7,10,11-hexahydroxytriphenylene)2 [Cu3(HHTP2)] has been observed, which is most likely due to oxygen molecular doping. The synthesis of electrically conductive 2D metal-organic frameworks (MOFs) has been achieved through the introduction of highly conjugated organic linkers coordinated to their constituent metal-ion centers. However, the porous structure and unsaturated metal sites in MOFs make them susceptible to ambient adsorbates, which can affect their charge transport properties. This phenomenon has been experimentally investigated by GIXRD, Hall effect and Seebeck measurements, and X-ray photoelectron spectroscopy.

Electronic transport descriptors for the rapid screening of thermoelectric materials.

The discovery of novel materials for thermoelectric energy conversion has potential to be accelerated by data-driven screening combined with high-throughput calculations. One way to increase the efficacy of successfully choosing a candidate material is through its evaluation using transport descriptors. Using a data-driven screening, we selected 12 potential candidates in the trigonal ABX2 family, followed by charge transport property simulations from first principles. The results suggest that carrier scattering processes in these materials are dominated by ionised impurities and polar optical phonons, contrary to the oft-assumed acoustic-phonon-dominated scattering. Using these data, we further derive ground-state transport descriptors for the carrier mobility and the thermoelectric powerfactor. In addition to low carrier mass, high dielectric constant was found to be an important factor towards high carrier mobility. A quadratic correlation between dielectric constant and transport performance was established and further validated with literature. Looking ahead, dielectric constant can potentially be exploited as an independent criterion towards improved thermoelectric performance. Combined with calculations of thermal conductivity including Peierls and inter-branch coherent contributions, we conclude that the trigonal ABX2 family has potential as high performance thermoelectrics in the intermediate temperature range for low grade waste heat harvesting.

Direct measurement of the thermoelectric properties of electrochemically deposited Bi2Te3 thin films.

The best known thermoelectric material for near room temperature heat-to-electricity conversion is bismuth telluride. Amongst the possible fabrication techniques, electrodeposition has attracted attention due to its simplicity and low cost. However, the measurement of the thermoelectric properties of electrodeposited films is challenging because of the conducting seed layer underneath the film. Here, we develop a method to directly measure the thermoelectric properties of electrodeposited bismuth telluride thin films, grown on indium tin oxide. Using this technique, the temperature dependent thermoelectric properties (Seebeck coefficient and electrical conductivity) of electrodeposited thin films have been measured down to 100 K. A parallel resistor model is employed to discern the signal of the film from the signal of the seed layer and the data are carefully analysed and contextualized with literature. Our analysis demonstrates that the thermoelectric properties of electrodeposited films can be accurately evaluated without inflicting any damage to the films.

Thermoelectric Properties of Substoichiometric Electron Beam Patterned Bismuth Sulfide.

Direct patterning of thermoelectric metal chalcogenides can be challenging and is normally constrained to certain geometries and sizes. Here we report the synthesis, characterization, and direct writing of sub-10 nm wide bismuth sulfide (Bi2S3) using a single-source, spin-coatable, and electron-beam-sensitive bismuth(III) ethylxanthate precursor. In order to increase the intrinsically low carrier concentration of pristine Bi2S3, we developed a self-doping methodology in which sulfur vacancies are manipulated by tuning the temperature during vacuum annealing, to produce an electron-rich thermoelectric material. We report a room-temperature electrical conductivity of 6 S m-1 and a Seebeck coefficient of -21.41 µV K-1 for a directly patterned, substoichiometric Bi2S3 thin film. We expect that our demonstration of directly writable thermoelectric films, with further optimization of structure and morphology, can be useful for on-chip applications.

Full Thermoelectric Characterization of Stoichiometric Electrodeposited Thin Film Tin Selenide (SnSe).

Tin selenide (SnSe) has attracted much attention in the thermoelectric community since the discovery of the record figure of merit (ZT) of 2.6 in single crystal tin selenide in 2014. There have been many reports since of the thermoelectric characterization of SnSe synthesized or manufactured by several methods, but so far none of these have concerned the electrodeposition of SnSe. In this work, stoichiometric SnSe was successfully electrodeposited at -0.50 V vs SCE as shown by EDX, XPS, UPS, and XRD. The full ZT of the electrodeposits were then measured. This was done by both a delamination technique to measure the Seebeck coefficient and electrical conductivity which showed a peak power factor of 4.2 and 5.8 µW m-1 K-2 for the as deposited and heat-treated films, respectively. A novel modified transient 3ω method was used to measure the thermal conductivity of the deposited films on the deposition substrate. This revealed the thermal conductivity to be similar to the ultralow thermal conductivity of single crystal SnSe, with a value of 0.34 W m-1 K-1 being observed at 313 K.

Conformational Evolution of Molecular Signatures during Amyloidogenic Protein Aggregation.

Aggregation is a pathological hallmark of proteinopathies such as Alzheimer's disease and results in the deposition of ß-sheet-rich amyloidogenic protein aggregates. Such proteinopathies can be classified by the identity of one or more aggregated proteins, with recent evidence also suggesting that distinct molecular conformers (strains) of the same protein can be observed in different diseases, as well is in subtypes of the same disease. Therefore, methods for the quantification of pathological changes in protein conformation are central to understanding and treating proteinopathies. In this work, the evolution of Raman spectroscopic molecular signatures of three conformationally distinct proteins, bovine serum albumin (α-helical-rich), ß2-microglobulin (ß-sheet-rich), and tau (natively disordered), was assessed during aggregation into oligomers and fibrils. The morphological evolution was tracked using atomic force microscopy and corresponding conformational changes were assessed by their Raman signatures acquired in both wet and dried conditions. A deconvolution model was developed which allowed us to quantify the conformation of the nonregular protein tau, as well as for the oligomeric and fibrillar species of each of the proteins. Principle component analysis of the fingerprint region allowed further identification of the distinguishing spectral features and unsupervised distinction. While an increase in ß-sheet is seen on aggregation, crucially, however, each protein also retains a significant proportion of its native monomeric structure after aggregation. Thus, spectral analysis of each aggregated species, oligomeric, as well as fibrillar, for each protein resulted in a unique and quantitative "conformational fingerprint". This approach allowed us to provide the first differential detection of both oligomers and fibrils of the three different amyloidogenic proteins, including tau, whose aggregates have never before been interrogated using spontaneous Raman spectroscopy. Quantitative "conformational fingerprinting" by Raman spectroscopy thus demonstrates its huge potential and utility in understanding proteinopathic disease mechanisms and for providing strain-specific early diagnostic markers and targets for disease-modifying therapies.

Light-Induced Reversible DNA Ligation of Gold Nanoparticle Superlattices.

DNA-mediated self-assembly of nanoparticles has been of great interest because it enables access to nanoparticle superstructures that cannot be synthesized otherwise. However, the programmability of higher order nanoparticle structures can be easily lost under DNA denaturing conditions. Here, we demonstrate that light can be employed as an external stimulus to master the stability of nanoparticle superlattices (SLs) via the promotion of a reversible photoligation of DNA in SLs. The oligonucleotides attached to the nanoparticles are encoded to ligate using 365 nm light, effectively locking the SLs and rendering them stable under DNA denaturing conditions. The reversible process of unlocking these structures is possible by irradiation with light at 315 nm, recovering the structures to their natural state. Our work inspires an alternative research direction toward postassembly manipulation of nanoparticle superstructures using external stimuli as a tool to enrich the library of additional material forms and their application in different media and environments.

A Single-Step Route to Single-Crystal Molybdenum Disulphide (MoS2) Monolayer domains.

We report a simple, single-cycle synthetic method for forming highly-crystalline, micron-sized monolayer domains of phase-pure MoS2. This method combines liquid chemistry with discrete, layer-by-layer deposition from a novel Mo precursor. Single-crystalline MoS2 with domain sizes up to 100 µm have been obtained and characterised by optical and electron microscopy as well as Raman and photoluminescence spectroscopy.

Three-Dimensional Nanostructured Palladium with Single Diamond Architecture for Enhanced Catalytic Activity.

Fuel cells are a key new green technology that have applications in both transport and portable power generation. Carbon-supported platinum (Pt) is used as an anode and cathode electrocatalyst in low-temperature fuel cells fueled with hydrogen or low-molecular-weight alcohols. The cost of Pt and the limited world supply are significant barriers to the widespread use of these types of fuel cells. Comparatively, palladium has a 3 times higher abundance in the Earth's crust. Here, a facile, low-temperature, and scalable synthetic route toward three-dimensional nanostructured palladium (Pd) employing electrochemical templating from inverse lyotropic lipid phases is presented. The obtained single diamond morphology Pd nanostructures exhibited excellent catalytic activity and stability toward methanol, ethanol, and glycerol oxidation compared to commercial Pd black, and the nanostructure was verified by small-angle X-ray scattering, scanning tunneling electron microscopy, and cyclic voltammetry.

3D semiconducting nanostructures via inverse lipid cubic phases.

Well-ordered and highly interconnected 3D semiconducting nanostructures of bismuth sulphide were prepared from inverse cubic lipid mesophases. This route offers significant advantages in terms of mild conditions, ease of use and electrode architecture over other routes to nanomaterials synthesis for device applications. The resulting 3D bicontinous nanowire network films exhibited a single diamond topology of symmetry Fd3m (Q227) which was verified by Small angle X-ray scattering (SAXS) and Transmission electron microscopy (TEM) and holds great promise for potential applications in optoelectronics, photovoltaics and thermoelectrics.

Understanding the selective etching of electrodeposited ZnO nanorods.

ZnO nanotubes were prepared by selective dissolution of electrodeposited nanorods. The effect of solution pH, rod morphology, and chloride ion concentration on the dissolution mechanism was studied. The selective etching was rationalized in terms of the surface energy of the different ZnO crystal faces and reactant diffusion. The nanorod diameter and chloride concentration are the most influential parameters on the dissolution mechanism because they control homogeneous dissolution or selective etching of the (110) and (002) surfaces. Bulk solution pH only has an effect on the rate of dissolution. By accurate control of the dissolution process, the nanomorphology can be tailored, and the formation of rods with a thin diameter (10-20 nm), cavity, or ultra-thin-walled tubes (2-5 nm) can be achieved.

Effect of molecular asymmetry on the charge transport physics of high mobility n-type molecular semiconductors investigated by scanning Kelvin probe microscopy.

We have investigated the influence of the symmetry of the side chain substituents in high-mobility, solution processable n-type molecular semiconductors on the performance of organic field-effect transistors (OFETs). We compare two molecules with the same conjugated core, but either symmetric or asymmetric side chain substituents, and investigate the transport properties and thin film growth mode using scanning Kelvin probe microscopy (SKPM) and atomic force microscopy (AFM). We find that asymmetric side chains can induce a favorable two-dimensional growth mode with a bilayer structure, which enables ultrathin films with a single bilayer to exhibit excellent transport properties, while the symmetric molecules adopt an unfavorable three-dimensional growth mode in which transport in the first monolayer at the interface is severely hindered by high-resistance grain boundaries.

A new modular approach to nanoassembly: stable and addressable DNA nanoconstructs via orthogonal click chemistries.

Thermodynamic instability is a problem when assembling and purifying complex DNA nanostructures formed by hybridization alone. To address this issue, we have used photochemical fixation and orthogonal copper-free, ring-strain-promoted, click chemistry for the synthesis of dimeric, trimeric, and oligomeric modular DNA scaffolds from cyclic, double-stranded, 80-mer DNA nanoconstructs. This particular combination of orthogonal click reactions was more effective for nanoassembly than others explored. The complex nanostructures are stable to heat and denaturation agents and can therefore be purified and characterized. They are addressable in a sequence-specific manner by triplex formation, and they can be reversibly and selectively deconstructed. Nanostructures utilizing this orthogonal, chemical fixation methodology can be used as building blocks for nanomachines and functional DNA nanoarchitectures.

Triplex-directed covalent cross-linking of a DNA nanostructure.

The triplex approach to DNA recognition is exploited to direct covalent inter-strand cross-links to unique locations within a pre-assembled DNA nanostructure. This approach can be used to improve the stability of DNA nanostructures and demonstrates the feasibility of directing other reactive groups to unique locations within these complexes.

Surfactant-mediated electrodeposition of bismuth telluride films and its effect on microstructural properties.

We report the synthesis of highly crystallographically textured films of stoichiometric bismuth telluride (Bi(2)Te(3)) in the presence of a surfactant, sodium lignosulfonate (SL), that resulted in the improved alignment of films in the (110) plane and offered good control over the morphology and roughness of the electrodeposited films. SL concentrations in the range 60-80 mg dm(-3) at a deposition potential of -0.1 V vs SCE (saturated calomel electrode) were found to yield the most improved crystallinity and similar or superior thermoelectric properties compared with results reported in the literature.

Triplex-directed recognition of a DNA nanostructure assembled by crossover strand exchange.

DNA has been widely exploited for the self-assembly of nanosized objects and arrays that offer the potential to act as scaffolds for the spatial positioning of molecular components with nanometer precision. Methods that allow the targeting of components to specific locations within these structures are therefore highly sought after. Here we report that the triplex approach to DNA recognition, which relies on the specific binding of an oligonucleotide within the major groove of double-helical DNA, can be exploited to recognize specific loci within a DNA double-crossover tile and array, a nanostructure assembled by crossover strand exchange. The oligonucleotide can be targeted to both crossover and non-crossover strands and, surprisingly, across the region spanning the crossover junction itself. Moreover, by attaching biotin to the end of the oligonucleotide, we show that streptavidin molecules can be recruited to precise locations within a DX array, with an average spacing of 31.9 (±1.3) nm. This is a promising approach that could be exploited to introduce other components compatible with oligonucleotide synthesis into the wide variety of DNA nanostructures assembled by crossover strand exchange, such as those generated by DNA origami.

Optimization of the electrodeposition process of high-performance bismuth antimony telluride compounds for thermoelectric applications.

High-quality films of bismuth antimony telluride were synthesized by electrodeposition from nitric acid electroplating baths. The influence of a surfactant, sodium ligninsulfonate, on the structure, morphology, stoichiometry, and homogeneity of the deposited films has been investigated. It was found that addition of this particular surfactant significantly improved the microstructural properties as well as homogeneity of the films with a significant improvement in the thermoelectric properties over those deposited in the absence of surfactant. A detailed microprobe analysis of the deposited films yielded a stoichiometric composition of Bi(0.35)Sb(1.33)Te(3) for the films electrodeposited in the absence of surfactant and a stoichiometry of Bi(0.32)Sb(1.33)Te(3) for films deposited in the presence of surfactant.

High density p-type Bi0.5Sb1.5Te3 nanowires by electrochemical templating through ion-track lithography.

High density p-type Bi0.5Sb1.5Te3 nanowire arrays are produced by a combination of electrodeposition and ion-track lithography technology. Initially, the electrodeposition of p-type Bi0.5Sb1.5Te3 films is investigated to find out the optimal conditions for the deposition of nanowires. Polyimide-based Kapton foils are chosen as a polymer for ion track irradiation and nanotemplating Bi0.5Sb1.5Te3 nanowires. The obtained nanowires have average diameters of 80 nm and lengths of 20 microm, which are equivalent to the pore size and thickness of Kapton foils. The nanowires exhibit a preferential orientation along the {110} plane with a composition of 11.26 at.% Bi, 26.23 at.% Sb, and 62.51 at.% Te. Temperature dependence studies of the electrical resistance show the semiconducting nature of the nanowires with a negative temperature coefficient of resistance and band gap energy of 0.089+/-0.006 eV.