Electrostatic layer-by-layer assembly of CdSe nanorod/polymer nanocomposite thin films

Effect of Spacer Length and Solvent on the Concentration-Driven Aggregation of Cationic Hydrogen-Bonding Donor Polythiophenes

Aggregation of cationic isothiouronium polythiophenes with alkoxy-spacers of different lengths at the 3-position of the thiophene ring was studied in solvents of different polarities. Hydrogen-bonding capacity was assessed by steady-state absorption and fluorescence spectroscopy, whereas the aggregation in aqueous solutions was studied by electron paramagnetic resonance spectroscopy, using paramagnetic probes of different polarities. The two polymers displayed similar features in respect to conformation, effect of cosolvents on aggregation, unstructured absorption-fluorescence spectra, Stokes shifts when aggregated, solvatochromic effect, and self-quenching concentration. However, these polymers also showed different specific interactions with water, Stokes shifts in water, effect of the solvent on the extent of dominant state of the S1 level, and also different inner cavities and hydrophobic-hydrophilic surface area in aqueous solution aggregates. Water maximized the difference between the polymers concerning the effect of specific increases in concentration, whereas the presence of 1,4-dioxane generated almost identical effects on both polymers.

Reduced graphene oxide–ZnO self-assembled films: tailoring the visible light photoconductivity by the intrinsic defect states in ZnO

ZnO is a wide direct bandgap semiconductor; its absorption can be tuned to the visible spectral region by controlling the intrinsic defect levels.

Strong and moldable cellulose magnets with high ferrite nanoparticle content

A major limitation in the development of highly functional hybrid nanocomposites is brittleness and low tensile strength at high inorganic nanoparticle content. Herein, cellulose nanofibers were extracted from wood and individually decorated with cobalt-ferrite nanoparticles and then for the first time molded at low temperature (<120 °C) into magnetic nanocomposites with up to 93 wt % inorganic content. The material structure was characterized by TEM and FE-SEM and mechanically tested as compression molded samples. The obtained porous magnetic sheets were further impregnated with a thermosetting epoxy resin, which improved the load-bearing functions of ferrite and cellulose material. A nanocomposite with 70 wt % ferrite, 20 wt % cellulose nanofibers, and 10 wt % epoxy showed a modulus of 12.6 GPa, a tensile strength of 97 MPa, and a strain at failure of ca. 4%. Magnetic characterization was performed in a vibrating sample magnetometer, which showed that the coercivity was unaffected and that the saturation magnetization was in proportion with the ferrite content. The used ferrite, CoFe2O4, is a magnetically hard material, demonstrated by that the composite material behaved as a traditional permanent magnet. The presented processing route is easily adaptable to prepare millimeter-thick and moldable magnetic objects. This suggests that the processing method has the potential to be scaled-up for industrial use for the preparation of a new subcategory of magnetic, low-cost, and moldable objects based on cellulose nanofibers.

In vitro evaluation of chondrosarcoma cells and canine chondrocytes on layer-by-layer (LbL) self-assembled multilayer nanofilms

Short-term cell-substrate interactions of two secondary chondrocyte cell lines (human chondrosarcoma cells, canine chondrocytes) with layer-by-layer self-assembled multilayer nanofilms were investigated for a better understanding of cellular-behaviour dependence on a number of nanofilm layers. Cell-substrate interactions were studied on polyelectrolyte multilayer nanofilms (PMNs) of eleven different biomaterials. Surface characterization of PMNs performed using AFM showed increasing surface roughness with increasing number of layers for most of the biomaterials. LDH-L and MTT assays were performed on chondrosarcoma cells and canine chondrocytes, respectively. A major observation was that 10-bilayer nanofilms exhibited lesser cytotoxicity towards human chondrosarcoma cells than their 5-bilayer counterparts. In the case of canine chondrocytes, BSA enhanced cell metabolic activity with increasing number of layers, underscoring the importance of the multilayer nanofilm architecture on cellular behaviour.

Ligand-controlled rates of photoinduced electron transfer in hybrid CdSe nanocrystal/poly(viologen) films

This paper describes a study of the rates of photoinduced electron transfer (PET) from CdSe quantum dots (QDs) to poly(viologen) within thin films, as a function of the length of the ligands passivating the QDs. Ultrafast (<10 ps), quantitative PET occurs from CdSe QDs coated with HS-(CH(2))(n)-COOH for n = 1, 2, 5, and 7 to viologen units. The observed decrease in the magnitude of the PET rate constant with n is weaker than that expected from the decay of the electron tunneling probability across extended all-trans mercaptocarboxylic acids but well-described by electron tunneling across a collapsed ligand shell. The PET rate constants for films with n = 10 and 15 are much slower than those expected based on the trend for n = 1-7; this deviation is ascribed to the formation of bundles of ligands on the surface of the QD that make the tunneling process prohibitively slow by limiting access of the viologen units to the surfaces of the QDs. This study highlights the importance of molecular-level morphology of donor and acceptor materials in determining the rate and yield of interfacial photoinduced electron transfer in thin films.

Finely tailored performance of inverted organic photovoltaics through layer-by-layer interfacial engineering

Control over interfacial properties in organic photovoltaics (OPVs) is critical for many aspects of their performance. Functionalization of the transparent conducting electrode, in this case, indium tin oxide (ITO), through an electrostatic layer by layer (eLbL) approach with cationic N,N'-bis[2-(trimethylammonium)ethylene] perylene-3,4,9,10-tetracarboxyldiimide (PTCDI(+)) and anionic poly(3,4-ethylenedioxythiophene):poly(p-styrenesulfonate) (PEDOT:PSS(-)), led to high control over the surface properties. The films were studied through a variety of surface and spectroscopic techniques, including X-ray photoelectron spectroscopy (XPS), UV-visible spectroscopy, atomic force microscopy (AFM), and ellipsometry. The work function of modified ITO was measured by UV photoelectron spectroscopy (UPS) and showed oscillating values with respect to odd-even layer numbers; the strong odd-even effect is due to the differing electronic characteristics of the top layer, either PTCDI(+) or PEDOT:PSS(-). The modified ITO electrodes were then used as the cathode in a series of inverted organic photovoltaic architectures. The performance of inverted OPVs was, in parallel to the UPS results, found to be highly dependent on the layer number of coated films and showed an obvious oscillation based on layer number. Inverted OPVs were retested after 128 days of storage in air, and almost all devices maintained over 70% of original power conversion efficiency (PCE).

C60 fullerene nanocolumns--polythiophene heterojunctions for inverted organic photovoltaic cells

Inverted organic photovoltaic cells have been fabricated based on vertical C(60) nanocolumns filled with spin-coated poly[3-(4-carboxybutyl)thiophene-2,5-diyl] (P3CBT). These C(60) nanocolumns were prepared via glancing angle deposition (GLAD), an efficient synthetic approach that controls the morphology of the resulting film, including intercolumn spacing, nanostructure shapes, and overall film thickness, among others. Intercolumn spacing was tuned to better match the expected P3CBT exciton diffusion length while simultaneously increasing heterointerface area. Due to observed in situ dissolution of the C(60) nanocolumns in solvents typically used to spin-coat polythiophene-based polymers (i.e., chloroform and chlorobenzene), the carboxylic acid-substituted polythiophene, P3CBT, was used as it is soluble in dimethyl sulfoxide (DMSO), a solvent that did not affect the structure of the GLAD-produced C(60) nanostructures. Preservation of the C(60) nanocolumnar structure in the presence of DMSO, with and without P3CBT, was verified by absorbance spectroscopy and SEM imaging. Incorporating these nanostructured C(60)/P3CBT films into photovoltaic devices on indium tin oxide (ITO) showed that the engineered nanomorphology yielded a 5-fold increase in short-circuit current and a power conversion efficiency (PCE) increase from (0.2 ± 0.03)% to (0.8 ± 0.2)% when compared to a planar device. When compared to a standard bulk heterojunction (BHJ) device based upon the same materials, the C(60)-GLAD device outperformed fully solution-processed bulk heterojunctions, which were observed to have PCEs of (0.49 ± 0.03)%.

Optimization of hybrid organic-inorganic interdigitated photovoltaic device structure using a 2D diffusion model

To improve the efficiency of organic photovoltaic devices the inclusion of semiconducting nanoparticles such as PbS has been used to enhance near-infrared absorption. Additionally the use of interdigitated heterojunctions has been explored as a means of improving charge extraction. In this paper we provide a two-dimensional model taking into account these approaches with the aim of predicting an optimized device geometry to maximize the efficiency. The steady-state exciton population has been calculated in each of the active regions taking into account the full optical response based on using a finite difference approach to obtain approximate numerical solutions to the 2D exciton diffusion equation. On the basis of this we calculate the contribution of each active material to the device short circuit current and power conversion efficiency. We show that optimized structures can lead to power conversions efficiencies of ∼50% compared to a maximum of ∼17% for planar heterojunction devices. To achieve this the interdigitated region thickness should be ∼800 nm with PbS and C(60) widths of ∼60 and 20 nm, respectively. Even modest nanopatterning using much thinner active regions provides improvements in efficiency and may be approached using a variety of methods including nanoimprinting lithography, nanotemplating, or the incorporation of presynthesized nanorod structures.

Improving nanoparticle dispersion and charge transfer in cadmium telluride tetrapod and conjugated polymer blends

The dispersion of CdTe tetrapods in a conducting polymer and the resulting charge transfer is studied using a combination of confocal fluorescence microscopy and atomic force microscopy (AFM). The results of this work show that both the tetrapod dispersion and charge transfer between the CdTe and conducting polymer (P3HT) are greatly enhanced by exchanging the ligands on the surface of the CdTe and by choosing proper solvent mixtures. The ability to experimentally probe the relationship between particle dispersion and charge transfer through the combination of AFM and fluorescence microscopy provides another avenue to assess the performance of polymer/semiconductor nanoparticle composites.

Indium tin oxide nanopillar electrodes in polymer/fullerene solar cells

Using high surface area nanostructured electrodes in organic photovoltaic (OPV) devices is a route to enhanced power conversion efficiency. In this paper, indium tin oxide (ITO) and hybrid ITO/SiO(2) nanopillars are employed as three-dimensional high surface area transparent electrodes in OPVs. The nanopillar arrays are fabricated via glancing angle deposition (GLAD) and electrochemically modified with nanofibrous PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(p-styrenesulfonate)). The structures are found to have increased surface area as characterized by porosimetry. When applied as anodes in polymer/fullerene OPVs (architecture: commercial ITO/GLAD ITO/PEDOT:PSS/P3HT:PCBM/Al, where P3HT is 2,5-diyl-poly(3-hexylthiophene) and PCBM is [6,6]-phenyl-C(61)-butyric acid methyl ester), the air-processed solar cells incorporating high surface area, PEDOT:PSS-modified ITO nanoelectrode arrays operate with improved performance relative to devices processed identically on unstructured, commercial ITO substrates. The resulting power conversion efficiency is 2.2% which is a third greater than for devices prepared on commercial ITO. To further refine the structure, insulating SiO(2) caps are added above the GLAD ITO nanopillars to produce a hybrid ITO/SiO(2) nanoelectrode. OPV devices based on this system show reduced electrical shorting and series resistance, and as a consequence, a further improved power conversion efficiency of 2.5% is recorded.

Epitaxial growth of nanostructured gold films on germanium via galvanic displacement

This work focuses on the synthesis and characterization of gold films grown via galvanic displacement on Ge(111) substrates. The synthetic approach uses galvanic displacement, a type of electroless deposition that takes place in an efficient manner under aqueous, room temperature conditions. Investigations involving X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques were performed to study the crystallinity and orientation of the resulting gold-on-germanium films. A profound effect of HF(aq) concentration was noted, and although the SEM images did not show significant differences in the resulting gold films, a host of X-ray diffraction studies demonstrated that higher concentrations of HF(aq) led to epitaxial gold-on-germanium, whereas in the absence of HF(aq), lower degrees of order (fiber texture) resulted. Cross-sectional nanobeam diffraction analyses of the Au-Ge interface confirmed the epitaxial nature of the gold-on-germanium film. This epitaxial behavior can be attributed to the simultaneous etching of the germanium oxides, formed during the galvanic displacement process, in the presence of HF. High-resolution TEM analyses showed the coincident site lattice (CSL) interface of gold-on-germanium, which results in a small 3.8% lattice mismatch due to the coincidence of four gold lattices with three of germanium.

Layer-by-layer assembled multilayer TiO(x) for efficient electron acceptor in polymer hybrid solar cells

We demonstrate that TiO(x) nanocomposite films fabricated using electrostatic layer-by-layer (LbL) assembly improve the power conversion efficiency of photovoltaic cells compared to conventional TiO(x) films fabricated via the sol-gel process. For this study, titanium precursor/poly(allylamine hydrochloride) (PAH) multilayer films were first deposited onto indium tin oxide-coated glass to produce TiO(x) nanocomposites (TiO(x)NC). The specific effect of the LbL processed TiO(x) on photovoltaic performance was investigated using the planar bilayer TiO(x)NC and highly regioregular poly(3-hexylthiophene) (P3HT) solar cells, and the P3HT/LbL TiO(x)NC solar cells showed a dramatic increase in power efficiency, particularly in terms of the short current density and fill factor. The improved efficiency of this device is mainly due to the difference in the chemical composition of the LbL TiO(x)NC films, including the much higher Ti(3+)/Ti(4+) ratio and the highly reactive facets of crystals as demonstrated by XPS and XRD measurement, thus enhancing the electron transfer between electron donors and acceptors. In addition, the grazing incidence wide-angle X-ray scattering (GIWAXS) study revealed the presence of more highly oriented P3HT stacks parallel to the substrate on the LbL TiO(x)NC film compared to those on the sol-gel TiO(x) films, possibly influencing the hole mobility of P3HT and the energy transfer near and at the interface between the P3HT and TiO(x) layers. The results of this study demonstrate that this approach is a promising one for the design of hybrid solar cells with improved efficiency.