Structure-Property Relationships for the Electronic Applications of Bis-Adduct Isomers of Phenyl-C61 Butyric Acid Methyl Ester

Higher adducts of a fullerene, such as the bis-adduct of PCBM (bis-PCBM), can be used to achieve shallower molecular orbital energy levels than, for example, PCBM or C60. Substituting the bis-adduct for the parent fullerene is useful to increase the open-circuit voltage of organic solar cells or achieve better energy alignment as electron transport layers in, for example, perovskite solar cells. However, bis-PCBM is usually synthesized as a mixture of structural isomers, which can lead to both energetic and morphological disorder, negatively affecting device performance. Here, we present a comprehensive study on the molecular properties of 19 pure bis-isomers of PCBM using a variety of characterization methods, including ultraviolet photoelectron spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, single crystal structure, and (time-dependent) density functional theory calculation. We find that the lowest unoccupied molecular orbital of such bis-isomers can be tuned to be up to 170 meV shallower than PCBM and up to 100 meV shallower than the mixture of unseparated isomers. The isolated bis-isomers also show an electron mobility in organic field-effect transistors of up to 4.5 × 10-2 cm2/(V s), which is an order of magnitude higher than that of the mixture of bis-isomers. These properties enable the fabrication of the highest performing bis-PCBM organic solar cell to date, with the best device showing a power conversion efficiency of 7.2%. Interestingly, we find that the crystallinity of bis-isomers correlates negatively with electron mobility and organic solar cell device performance, which we relate to their molecular symmetry, with a lower symmetry leading to more amorphous bis-isomers, less energetic disorder, and higher dimensional electron transport. This work demonstrates the potential of side chain engineering for optimizing the performance of fullerene-based organic electronic devices.

Fully 3D Printed Tin Selenide (SnSe) Thermoelectric Generators with Alternating n-Type and p-Type Legs

Tin selenide (SnSe) has attracted much attention in the field of thermoelectrics since the discovery of the record figure of merit (zT) of 2.6 ± 0.3. While there have been many publications on p-type SnSe, to manufacture efficient SnSe thermoelectric generators, ann-type is also required. Publications on n-type SnSe, however, are limited. This paper reports a pseudo-3D-printing technique to fabricate bulk n-type SnSe elements, by utilizing Bi as a dopant. Various Bi doping levels are investigated and characterized over a wide range of temperatures and through multiple thermal cycles. Stable n-type SnSe elements are then combined with printed p-type SnSe elements to fabricate a fully printed alternating n- and p-type thermoelectric generator, which is shown to produce 145 μW at 774 K.

Photocatalytic H2 production and degradation of aqueous 2-chlorophenol over B/N-graphene-coated Cu0/TiO2: A DFT, experimental and mechanistic investigation

Energy and environmental challenges are global concerns that scientists are interested in alleviating. It is on this premise that we prepared boron/nitrogen graphene-coated Cu0/TiO2 (B/N-graphene-coated Cu/TiO2) photocatalyst of varying B:N ratios with dual functionality of H2 production and 2-Chlorophenol (2-CP) degradation. In-situ coating of Cu0 with B/N-graphene is achieved via solvothermal synthesis and calcination under an inert atmosphere. All B/N-graphene-coated Cu/TiO2 exhibit higher photonic efficiencies (5.68%-7.06% at 300 < λ < 400 nm) towards H2 production than bare TiO2 (0.25% at 300 < λ < 400 nm). Varying the B:N ratio in graphene influences the efficiency of H2 generation. A B:N ratio of 0.08 yields the most active composite exhibiting a photonic efficiency of 7.06% towards H2 evolution and a degradation rate of 4.07 × 10-2 min-1 towards 2-chlorophenol (2-CP). Density functional theory (DFT) investigations determine that B-doping (p-type) enhances graphene stability on Cu0 while N-doping (n-type) increases the reduction potential of Cu0 relative to H+ reduction potential. X-ray photoelectron spectroscopy reveals that increasing the B:N ratio increases p-type BC2O while decreasing n-type pyridinic-N in graphene thus altering the interlayer electron density. Isotopic labelling experiments determine water reduction as the main mechanism by which H2 is produced over B/N-graphene-coated Cu/TiO2. The reactive species involved in the degradation of 2-CP are holes (h+), hydroxyl radical (OH•), and O2•-, of which superoxide (O2•-) plays the major role. This work displays B/N -graphene-coated Cu/TiO2 as a potential photocatalyst for large-scale H2 production and 2-CP degradation.

Interfacial water morphology in hydrated melanin

The importance of electrically functional biomaterials is increasing as researchers explore ways to utilise them in novel sensing capacities. It has been recognised that for many of these materials the state of hydration is a key parameter that can heavily affect the conductivity, particularly those that rely upon ionic or proton transport as a key mechanism. However, thus far little attention has been paid to the nature of the water morphology in the hydrated state and the concomitant ionic conductivity. Presented here is an inelastic neutron scattering (INS) experiment on hydrated eumelanin, a model bioelectronic material, in order to investigate its 'water morphology'. We develop a rigorous new methodology for performing hydration dependent INS experiments. We also model the eumelanin dry spectra with a minimalist approach whereas for higher hydration levels we are able to obtain difference spectra to extract out the water scattering signal. A key result is that the physi-sorbed water structure within eumelanin is dominated by interfacial water with the number of water layers between 3-5, and no bulk water. We also detect for the first time, the potential signatures for proton cations, most likely the Zundel ion, within a biopolymer/water system. These new signatures may be general for soft proton ionomer systems, if the systems are comprised of only interfacial water within their structure. The nature of the water morphology opens up new questions about the potential ionic charge transport mechanisms within hydrated bioelectronics materials.

Inducing upwards band bending by surface stripping ZnO nanowires with argon bombardment

Metal oxide semiconductors such as ZnO have attracted much scientific attention due their material and electrical properties and their ability to form nanostructures that can be used in numerous devices. However, ZnO is naturally n-type and tailoring its electrical properties towards intrinsic or p-type in order to optimise device operation have proved difficult. Here, we present an x-ray photon-electron spectroscopy and photoluminescence study of ZnO nanowires that have been treated with different argon bombardment treatments including with monoatomic beams and cluster beams of 500 atoms and 2000 atoms with acceleration volte of 0.5 keV-20 keV. We observed that argon bombardment can remove surface contamination which will improve contact resistance and consistency. We also observed that using higher intensity argon bombardment stripped the surface for nanowires causing a reduction in defects and surface OH^- groups both of which are possible causes of the n-type nature and observed a shift in the valance band edge suggest a shift to a more p-type nature. These results indicate a simple method for tailoring the electrical characteristic of ZnO.

Nitrogen/Carbon-Coated Zero-Valent Copper as Highly Efficient Co-catalysts for TiO2 Applied in Photocatalytic and Photoelectrocatalytic Hydrogen Production

Zero-valent copper (Cu⁰) is a promising co-catalyst in semiconductor-based photocatalysis as it is inexpensive and exhibits electronic properties similar to those of Ag and Au. However, its practical application in photocatalytic hydrogen production is limited by its susceptibility to oxidation, forming less active Cu species. Herein, we have carried out in situ encapsulation of Cu⁰ nanoparticles with N-graphitic carbon layers (14.4% N) to stabilize Cu⁰ nanoparticles (N/C-coated Cu) and improve the electronic communication with a TiO₂ photocatalyst. A facile solvothermal procedure is used to coat the Cu⁰ nanoparticles at 200 °C, while graphitization is achieved by calcination at 550 °C under an inert atmosphere. The resultant N/C-coated Cu/TiO₂ composites outperform the uncoated Cu counterparts, exhibiting a 27-fold enhancement of the hydrogen evolution rate compared to TiO₂ and achieving a rate of 19.03 mmol g^-1 h^-1 under UV-vis irradiation. Likewise, the N/C-coated Cu co-catalyst exhibits a less negative onset potential of -0.05 V toward hydrogen evolution compared to uncoated Cu (ca. -0.30 V). This superior activity is attributed to coating Cu⁰ with N/C, which enhances the stability, electronic communication with TiO₂, conductivity, and interfacial charge transfer processes. The reported synthetic approach is simple and scalable, yielding an efficient and affordable Cu⁰ co-catalyst for TiO₂.

Influences of Non-fullerene Acceptor Fluorination on Three-Dimensional Morphology and Photovoltaic Properties of Organic Solar Cells

Fluorination of conjugated molecules has been established as an effective structural modification strategy to influence properties and has attracted extensive attention in organic solar cells (OSCs). Here, we have investigated optoelectronic and photovoltaic property changes of OSCs made of polymer donors with the non-fullerene acceptors (NFAs) ITIC and IEICO and their fluorinated counterparts IT-4F and IEICO-4F. Device studies show that fluorinated NFAs lead to reduced V_oc but increased J_sc and fill-factor (FF), and therefore, the ultimate influence to efficiency depends on the compensation of V_oc loss and gains of J_sc and FF. Fluorination lowers energy levels of NFAs, reduces their electronic band gaps, and red-shifts the absorption spectra. The impact of fluorination on the molecular order depends on the specific NFA, and the conversion of ITIC to IT-4F reduces the structural order, which can be reversed after blending with the donor PBDB-T. Contrastingly, IEICO-4F presents stronger π-π stacking after fluorination from IEICO, and this is further strengthened after blending with the donor PTB7-Th. The photovoltaic blends universally present a donor-rich surface region which can promote charge transport and collection toward the anode in inverted OSCs. The fluorination of NFAs, however, reduces the fraction of donors in this donor-rich region, consequently encouraging the intermixing of donor/acceptor for efficient charge generation.

Voltammetric Detection of Caffeine in Beverages at Nafion/Graphite Nanoplatelets Layer-by-Layer Films

We report for the first time a procedure in which Nafion/Graphite nanoplatelets (GNPs) thin films are fabricated using a modified layer-by-layer (LbL) method. The method consists of dipping a substrate (quartz and/or glassy carbon electrodes) into a composite solution made of Nafion and GNPs dissolved together in ethanol, followed by washing steps in water. This procedure allowed the fabrication of multilayer films of (Nafion/GNPs)n by means of hydrogen bonding and hydrophobic‒hydrophobic interactions between Nafion, GNPs, and the corresponding solid substrate. The average thickness of each layer evaluated using profilometer corresponds to ca. 50 nm. The as-prepared Nafion/GNPs LbL films were characterized using various spectroscopic techniques such as X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), FTIR, and optical microscopy. This characterization highlights the presence of oxygen functionalities that support a mechanism of self-assembly via hydrogen bonding interactions, along with hydrophobic interactions between the carbon groups of GNPs and the Teflon-like (carbon‒fluorine backbone) of Nafion. We showed that Nafion/GNPs LbL films can be deposited onto glassy carbon electrodes and utilized for the voltammetric detection of caffeine in beverages. The results showed that Nafion/GNPs LbL films can achieve a limit of detection for caffeine (LoD) of 0.032 μM and linear range between 20‒250 μM using differential pulse voltammetry, whereas, using cyclic voltammetry LoD and linear range were found to be 24 μM and 50‒5000 μM, respectively. Voltammetric detection of caffeine in beverages showed good agreement between the values found experimentally and those reported by the beverage producers. The values found are also in agreement with those obtained using a standard spectrophotometric method. The proposed method is appealing because it allows the fabrication of Nafion/GNPs thin films in a simple fashion using a single-step procedure, rather than using composite solutions with opposite electrostatic charge, and also allows the detection of caffeine in beverages without any pre-treatment or dilution of the real samples. The proposed method is characterized by a fast response time without apparent interference, and the results were competitive with those obtained with other materials reported in the literature.

On-Demand Electrical Switching of Antibody-Antigen Binding on Surfaces

The development of stimuli-responsive interfaces between synthetic materials and biological systems is providing the unprecedented ability to modulate biomolecular interactions for a diverse range of biotechnological and biomedical applications. Antibody-antigen binding interactions are at the heart of many biosensing platforms, but no attempts have been made yet to control antibody-antigen binding in an on-demand fashion. Herein, a molecular surface was designed and developed that utilizes an electric potential to drive a conformational change in surface bound peptide moiety, to give on-demand control over antigen-antibody interactions on sensor chips. The molecularly engineered surfaces allow for propagation of conformational changes from the molecular switching unit to a distal progesterone antigen, resulting in promotion (ON state) or inhibition (OFF state) of progesterone antibody binding. The approach presented here can be generally applicable to other antigen-antibody systems and meets the technological needs for in situ long-term assessment of biological processes and disease monitoring on-demand.

Investigation into the effects of surface stripping ZnO nanosheets

ZnO nanosheets are polycrystalline nanostructures that are used in devices including solar cells and gas sensors. However, for efficient and reproducible device operation and contact behaviour the conductivity characteristics must be controlled and surface contaminants removed. Here we use low doses of argon bombardment to remove surface contamination and make reproducible lower resistance contacts. Higher doses strip the surface of the nanosheets altering the contact type from near-ohmic to rectifying by removing the donor-type defects, which photoluminescence shows to be concentrated in the near-surface. Controlled doses of argon treatments allow nanosheets to be customised for device formation.

Impact of Aggregation on the Photochemistry of Fullerene Films: Correlating Stability to Triplet Exciton Kinetics

The photochemistry and stability of fullerene films is found to be strongly dependent upon film nanomorphology. In particular, PC₆₁BM blend films, dispersed with polystyrene, are found to be more susceptible to photobleaching in air than the more aggregated neat films. This enhanced photobleaching correlated with increased oxygen quenching of PC₆₁BM triplet states and the appearance of a carbonyl FTIR absorption band indicative of fullerene oxidation, suggesting PC₆₁BM photo-oxidation is primarily due to triplet-mediated singlet oxygen generation. PC₆₁BM films were observed to undergo photo-oxidation in air for even modest (≤40 min) irradiation times, degrading electron mobility substantially, indicative of electron trap formation. This conclusion is supported by observation of red shifts in photo- and electro-luminescence with photo-oxidation, shown to be in agreement with time-dependent density functional theory calculations of defect generation. These results provide important implications on the environmental stability of PC₆₁BM-based films and devices.

Active removal of waste dye pollutants using Ta3N5/W18O49 nanocomposite fibres

A scalable solvothermal technique is reported for the synthesis of a photocatalytic composite material consisting of orthorhombic Ta₃N₅ nanoparticles and WO_x≤3 nanowires. Through X-ray diffraction and X-ray photoelectron spectroscopy, the as-grown tungsten(VI) sub-oxide was identified as monoclinic W₁₈O₄₉. The composite material catalysed the degradation of Rhodamine B at over double the rate of the Ta₃N₅ nanoparticles alone under illumination by white light, and continued to exhibit superior catalytic properties following recycling of the catalysts. Moreover, strong molecular adsorption of the dye to the W₁₈O₄₉ component of the composite resulted in near-complete decolourisation of the solution prior to light exposure. The radical species involved within the photocatalytic mechanisms were also explored through use of scavenger reagents. Our research demonstrates the exciting potential of this novel photocatalyst for the degradation of organic contaminants, and to the authors' knowledge the material has not been investigated previously. In addition, the simplicity of the synthesis process indicates that the material is a viable candidate for the scale-up and removal of dye pollutants on a wider scale.

Bi-phasic titanium dioxide nanoparticles doped with nitrogen and neodymium for enhanced photocatalysis

Bi-phasic or multi-phasic composite nanoparticles for use in photocatalysis have been produced by a new synthetic approach. Sol-gel methods are used to deposit multiple layers of active material onto soluble substrates. In this work, a layer of rutile (TiO2) was deposited onto sodium chloride pellets followed by an annealing step and a layer of anatase. After dissolving the substrate, bi-phasic nanoparticles containing half anatase and half rutile TiO2; with "Janus-like" characteristics are obtained. Nitrogen and neodymium doping of the materials were observed to enhance the photocatalytic properties both under UV and white light irradiation. The unique advantage of this synthetic method is the ability to systematically dope separate sides of the nanoparticles. Nitrogen doping was found to be most effective on the anatase side of the nanoparticle while neodymium was found to be most effective on the rutile side. Rhodamine B dye was effectively photodegraded by co-doped particles under white light.

A Substrate-Independent Approach for Cyclodextrin Functionalized Surfaces

6-Amino-6-deoxy-beta-cyclodextrin can be immobilized onto a range of solid surfaces via reaction with a predeposited pulsed plasma poly(glycidyl methacrylate) layer. X-ray photoelectron spectroscopy, infrared spectroscopy, and quartz crystal microbalance measurements have been employed to monitor guest-host interactions between N,N-dimethylformamide or cholic acid and surface-tethered 6-amino-6-deoxy-beta-cyclodextrin barrels.