All-Inorganic Hydrothermally Processed Semitransparent Sb2S3 Solar Cells with CuSCN as the Hole Transport Layer.

An inorganic wide-bandgap hole transport layer (HTL), copper(I) thiocyanate (CuSCN), is employed in inorganic planar hydrothermally deposited Sb2S3 solar cells. With excellent hole transport properties and uniform compact morphology, the solution-processed CuSCN layer suppresses the leakage current and improves charge selectivity in an n-i-p-type solar cell structure. The device without the HTL (FTO/CdS/Sb2S3/Au) delivers a modest power conversion efficiency (PCE) of 1.54%, which increases to 2.46% with the introduction of CuSCN (FTO/CdS/Sb2S3/CuSCN/Au). This PCE is a significant improvement compared with the previous reports of planar Sb2S3 solar cells employing CuSCN. CuSCN is therefore a promising alternative to expensive and inherently unstable organic HTLs. In addition, CuSCN makes an excellent optically transparent (with average transmittance >90% in the visible region) and shunt-blocking HTL layer in pinhole-prone ultrathin (<100 nm) semitransparent absorber layers grown by green and facile hydrothermal deposition. A semitransparent device is fabricated using an ultrathin Au layer (∼10 nm) with a PCE of 2.13% and an average visible transmittance of 13.7%.

Exploring the Origin of the Thermal Sensitivity of Near-Infrared-II Emitting Rare Earth Nanoparticles.

Rare-earth doped nanoparticles (RENPs) are attracting increasing interest in materials science due to their optical, magnetic, and chemical properties. RENPs can emit and absorb radiation in the second biological window (NIR-II, 1000-1400 nm) making them ideal optical probes for photoluminescence (PL) in vivo imaging. Their narrow emission bands and long PL lifetimes enable autofluorescence-free multiplexed imaging. Furthermore, the strong temperature dependence of the PL properties of some of these RENPs makes remote thermal imaging possible. This is the case of neodymium and ytterbium co-doped NPs that have been used as thermal reporters for in vivo diagnosis of, for instance, inflammatory processes. However, the lack of knowledge about how the chemical composition and architecture of these NPs influence their thermal sensitivity impedes further optimization. To shed light on this, we have systematically studied their emission intensity, PL decay time curves, absolute PL quantum yield, and thermal sensitivity as a function of the core chemical composition and size, active-shell, and outer-inert-shell thicknesses. The results revealed the crucial contribution of each of these factors in optimizing the NP thermal sensitivity. An optimal active shell thickness of around 2 nm and an outer inert shell of 3.5 nm maximize the PL lifetime and the thermal response of the NPs due to the competition between the temperature-dependent back energy transfer, the surface quenching effects, and the confinement of active ions in a thin layer. These findings pave the way for a rational design of RENPs with optimal thermal sensitivity.

Optimizing Upconversion Nanoparticles for FRET Biosensing.

Upconversion nanoparticles (UCNPs) are some of the most promising nanomaterials for bioanalytical and biomedical applications. One important challenge to be still solved is how UCNPs can be optimally implemented into Förster resonance energy transfer (FRET) biosensing and bioimaging for highly sensitive, wash-free, multiplexed, accurate, and precise quantitative analysis of biomolecules and biomolecular interactions. The many possible UCNP architectures composed of a core and multiple shells doped with different lanthanoid ions at different ratios, the interaction with FRET acceptors at different possible distances and orientations via biomolecular interaction, and the many and long-lasting energy transfer pathways from the initial UCNP excitation to the final FRET process and acceptor emission make the experimental determination of the ideal UCNP-FRET configuration for optimal analytical performance a real challenge. To overcome this issue, we have developed a fully analytical model that requires only a few experimental configurations to determine the ideal UCNP-FRET system within a few minutes. We verified our model via experiments using nine different Nd-, Yb-, and Er-doped core-shell-shell UCNP architectures within a prototypical DNA hybridization assay using Cy3.5 as an acceptor dye. Using the selected experimental input, the model determined the optimal UCNP out of all theoretically possible combinatorial configurations. An extreme economy of time, effort, and material was accompanied by a significant sensitivity increase, which demonstrated the powerful feat of combining a few selected experiments with sophisticated but rapid modeling to accomplish an ideal FRET biosensor.

Green Synthesis of Near-Infrared Plasmonic Gold Nanostructures by Pomegranate Extract and Their Supramolecular Assembling with Chemo- and Photo-Therapeutics.

Au nanostructures exhibiting a localized surface plasmon resonance in the near-infrared spectral window are obtained in a single, green step at room temperature by pomegranate extract in the presence of a highly biocompatible ß-cyclodextrin branched polymer, without the need of preformed seeds, external reducing and sacrificial agents, and conventional surfactants. The polymeric component makes the Au nanostructures dispersible in water, stable for weeks and permits their supramolecular assembling with the chemotherapeutic sorafenib and a nitric oxide (NO) photodonor (NOPD), chosen as representative for chemo- and photo-therapeutics. Irradiation of the plasmonic Au nanostructures in the therapeutic window with 808 nm laser light results in a good photothermal response, which (i) is not affected by the presence of either the chemo- or the phototherapeutic guests and (ii) does not lead to their photoinduced decomposition. Besides, irradiation of the hybrid Au nanoassembly with the highly biocompatible green light results in the NO release from the NOPD with efficiency similar to that observed for the free guest. Preliminary biological experiments against Hep-G2 hepatocarcinoma cell lines are also reported.

Large-Scale MOCVD Deposition of Nanostructured TiO2 on Stainless Steel Woven: A Systematic Investigation of Photoactivity as a Function of Film Thickness.

Heterogeneous photocatalysis is considered as one of the most appealing options for the treatment of organic pollutants in water. However, its definitive translation into industrial practice is still very limited because of both the complexity of large-scale production of catalysts and the problems involved in handling the powder-based photocatalysts in the industrial plants. Here, we demonstrate that the MOCVD approach can be successfully used to prepare large-scale supported catalysts with a good photocatalytic activity towards dye degradation. The photocatalyst consisted of nanostructured TiO2 thin film deposited on a stainless steel mesh substrate. The film thickness, the morphological features, and the crystallographic properties of the different portions of the sample were correlated to the position in the reactor chamber and the reaction conditions. The photocatalytic activity was evaluated according to the international standard test ISO 10678:2010 based on methylene blue degradation. The photocatalytic activity is essentially constant (PMB over 40 µmol·m-2·h-1) throughout the film, except for the portion of sample placed at the very end of the reactor chamber, where the TiO2 film is too thin to react properly. It was assessed that a minimum film thickness of 250-300 nm is necessary to reach the maximum photocatalytic performance.

Spatial and Temporal Resolution of Luminescence Quenching in Small Upconversion Nanocrystals.

Luminescent upconversion nanocrystals (UCNCs) have become one of the most promising nanomaterials for biosensing, imaging, and theranostics. However, their ultimate translation into robust luminescent probes for daily use in biological and medical laboratories requires comprehension and control of the many possible deactivation pathways that cause upconversion luminescence (UCL) quenching. Here, we demonstrate that thorough modeling of UCL rise and decay kinetics using a freely accessible software can identify the UCL quenching mechanisms in small (<40 nm) UCNCs with spatial and temporal resolution. Applied to the most relevant ß-NaYF4:Yb3+,Er3+ UCNCs, our model showed that only a few distinct nonradiative low-energy transitions were deactivated via specific solvent and ligand vibrations with a strong downstream effect on the population and depopulation dynamics of the emitting states. UCL quenching could penetrate ca. 4 nm inside the UCNC, which resulted in significant size-dependent changes of UCL intensities and spectra. Despite the large surface-to-volume ratios and UCL quenching via the UCNC surface, we found strong contributions of the outer layers to the overall UCL, which will be highly important for the design of UCNPs to investigate biomolecular interactions via distance-dependent energy transfer methods. Our advanced kinetic model is easily scalable to different UCNC architectures, environments, and energy transfer interactions such that relatively simple modeling of UCL kinetics can be used for efficiently optimizing UCNCs for their final application as practical luminescent probes.

PEG-Neridronate-Modified NaYF4:Gd3+,Yb3+,Tm3+/NaGdF4 Core-Shell Upconverting Nanoparticles for Bimodal Magnetic Resonance/Optical Luminescence Imaging.

Upconverting nanoparticles are attracting extensive interest as a multimodal imaging tool. In this work, we report on the synthesis and characterization of gadolinium-enriched upconverting nanoparticles for bimodal magnetic resonance and optical luminescence imaging. NaYF4:Gd3+,Yb3+,Tm3+ core upconverting nanoparticles were obtained by a thermal coprecipitation of lanthanide oleate precursors in the presence of oleic acid as a stabilizer. With the aim of improving the upconversion emission and increasing the amount of Gd3+ ions on the nanoparticle surface, a 2.5 nm NaGdF4 shell was grown by the epitaxial layer-by-layer strategy, resulting in the 26 nm core-shell nanoparticles. Both core and core-shell nanoparticles were coated with poly(ethylene glycol) (PEG)-neridronate (PEG-Ner) to have stable and well-dispersed upconverting nanoparticles in a biological medium. FTIR spectroscopy and thermogravimetric analysis indicated the presence of ∼20 wt % of PEG-Ner on the nanoparticle surface. The addition of inert NaGdF4 shell resulted in a total 26-fold enhancement of the emission under 980 nm excitation and also affected the T 1 and T 2 relaxation times. Both r 1 and r 2 relaxivities of PEG-Ner-modified nanoparticles were much higher compared to those of non-PEGylated particles, thus manifesting their potential as a diagnostic tool for magnetic resonance imaging. Together with the enhanced luminescence efficiency, upconverting nanoparticles might represent an efficient probe for bimodal in vitro and in vivo imaging of cells in regenerative medicine, drug delivery, and/or photodynamic therapy.

Synthesis and Mechanical Characterization of a CuMoTaWV High-Entropy Film by Magnetron Sputtering.

Development of high-entropy alloy (HEA) films is a promising and cost-effective way to incorporate these materials of superior properties in harsh environments. In this work, a refractory high-entropy alloy (RHEA) film of equimolar CuMoTaWV was deposited on silicon and 304 stainless-steel substrates using DC-magnetron sputtering. A sputtering target was developed by partial sintering of an equimolar powder mixture of Cu, Mo, Ta, W, and V using spark plasma sintering. The target was used to sputter a nanocrystalline RHEA film with a thickness of ∼900 nm and an average grain size of 18 nm. X-ray diffraction of the film revealed a body-centered cubic solid solution with preferred orientation in the (110) directional plane. The nanocrystalline nature of the RHEA film resulted in a hardness of 19 ± 2.3 GPa and an elastic modulus of 259 ± 19.2 GPa. A high compressive strength of 10 ± 0.8 GPa was obtained in nanopillar compression due to solid solution hardening and grain boundary strengthening. The adhesion between the RHEA film and 304 stainless-steel substrates was increased on annealing. For the wear test against the E52100 alloy steel (Grade 25, 700-880 HV) at 1 N load, the RHEA film showed an average coefficient of friction (COF) and wear rate of 0.25 (RT) and 1.5 (300 °C), and 6.4 × 10-6 mm3/N m (RT) and 2.5 × 10-5 mm3/N m (300 °C), respectively. The COF was found to be 2 times lower at RT and wear rate 102 times lower at RT and 300 °C than those of 304 stainless steel. This study may lead to the processing of high-entropy alloy films for large-scale industrial applications.

Er3+-to-dye energy transfer in DNA-coated core and core/shell/shell upconverting nanoparticles with 980 nm and 808 nm excitation of Yb3+ and Nd3.

The capability of upconverting nanoparticles (UCNPs) to convert near infrared (NIR) into visible light has become an important feature for biosensing, imaging, therapy, and their combination. While significant achievements have been accomplished during the last decade developing nanohybrids based on UCNPs as energy donors in Förster resonance energy transfer (FRET) systems, it is still challenging to understand and control FRET from UCNPs to dyes and to adapt the NIR excitation wavelength. Here, we describe the synthesis, characterization, and steady-state and time-resolved FRET analysis of UCNP-DNA nanohybrids, in which dye labelled single stranded (ss)DNA was attached to Yb-Er-co-doped core UCNPs (c-UCNPs) and c-UCNPs with a thin Nd-doped shell and a second thin undoped shell (css-UCNPs). Despite differences in sizes, compositions, donor-acceptor distances, brightness, and excitation wavelength (980 nm for Yb3+ and 808 nm for Nd3+), all UCNP-DNA nanohybrids showed very similar concentration dependent FRET-quenching of UCNP luminescence with efficiencies between 0 and ∼20%. We analyzed luminescence intensities, decay times, and rise times and could show the entanglement of excitation and emission kinetics by simply changing the excitation wavelength from 980 nm to 808 nm for the same css-UCNPs. Time-gated FRET-sensitized dye luminescence showed dye-ssDNA concentration dependence over four orders of magnitude (1 nM to 10 µM), which suggested a possible application to nucleic acid biosensing for both 808 and 980 nm excitation.

Ag2S/MoS2 Nanocomposites Anchored on Reduced Graphene Oxide: Fast Interfacial Charge Transfer for Hydrogen Evolution Reaction.

Hydrogen evolution reaction through electrolysis holds great potential as a clean, renewable, and sustainable energy source. Platinum-based catalysts are the most efficient to catalyze and convert water into molecular hydrogen; however, their large-scale application is prevented by scarcity and cost of Pt. In this work, we propose a new ternary composite of Ag2S, MoS2, and reduced graphene oxide (RGO) flakes via a one-pot synthesis. The RGO support assists the growth of two-dimensional MoS2 nanosheets partially covered by silver sulfides as revealed by high-resolution transmission electron microscopy. Compared with the bare MoS2 and MoS2/RGO, the Ag2S/MoS2 anchored on the RGO surface (the ternary system Ag2S/MoS2/RGO) demonstrated a high catalytic activity toward hydrogen evolution reaction (HER). Its superior electrochemical activity toward HER is evidenced by the positively shifted (-190 mV vs reversible hydrogen electrode (RHE)) overpotential at a current density of -10 mA/cm2 and a small Tafel slope (56 mV/dec) compared with a bare and binary system. The Ag2S/MoS2/RGO ternary catalyst at an overpotential of -200 mV demonstrated a turnover frequency equal to 0.38 s-1. Electrochemical impedance spectroscopy was applied to understand the charge-transfer resistance; the ternary sample shows a very small charge-transfer resistance (98 Ω) at -155 mV vs RHE. Such a large improvement can be attributed to the synergistic effect resulting from the enhanced active site density of both sulfides and to the improved electrical conductivity at the interfaces between MoS2 and Ag2S. This ternary catalyst opens up further optimization strategies to design a stable and cheap catalyst for hydrogen evolution reaction, which holds great promise for the development of a clean energy landscape.

Tin Dioxide Electrolyte-Gated Transistors Working in Depletion and Enhancement Modes.

Metal oxide semiconductors are interesting for next-generation flexible and transparent electronics because of their performance and reliability. Tin dioxide (SnO2) is a very promising material that has already found applications in sensing, photovoltaics, optoelectronics, and batteries. In this work, we report on electrolyte-gated, solution-processed polycrystalline SnO2 transistors on both rigid and flexible substrates. For the transistor channel, we used both unpatterned and patterned SnO2 films. Since decreasing the SnO2 area in contact with the electrolyte increases the charge-carrier density, patterned transistors operate in the depletion mode, whereas unpatterned ones operate in the enhancement mode. We also fabricated flexible SnO2 transistors that operate in the enhancement mode that can withstand moderate mechanical bending.

Energetics of CO oxidation on lanthanide-free perovskite systems: the case of Co-doped SrTiO3.

The energetics of the catalytic oxidation of CO on a complex metal oxide are investigated for the first time via density functional theory calculations. The catalyst, Co-doped SrTiO3, is modelled using periodically repeated slabs based on the SrTiO3(100) surface. The comparison of the energy profiles obtained for the pure host and the Co-doped material reveals the actual pathway followed by the reaction, and shows that Co doping enhances the catalytic properties of SrTiO3 by reducing the energy cost for the formation of oxygen vacancies.

A Player Often Neglected: Electrochemical Comprehensive Analysis of Counter Electrodes for Quantum Dot Solar Cells.

The role played by the counter electrode (CE) in quantum dot sensitized solar cells (QDSSCs) is crucial: it is indeed responsible for catalyzing the regeneration of the redox electrolyte after its action to take back the oxidized light harvesters to the ground state, thus keeping the device active and stable. The activity of CE is moreover directly related to the fill factor and short circuit current through the resistance of the interface electrode-electrolyte that affects the series resistance of the cell. Despite that, too few efforts have been devoted to a comprehensive analysis of this important device component. In this work we combine an extensive electrochemical characterization of the most common materials exploited as CEs in QDSSCs (namely, Pt, Au, Cu2S obtained by brass treatment, and Cu2S deposited on conducting glass via spray) with a detailed characterization of their surface composition and morphology, aimed at systematically defining the relationship between their nature and electrocatalytic activity.

Structural and photophysical properties of rare-earth complexes encapsulated into surface modified mesoporous silica nanoparticles.

The encapsulation of [Eu(dbm)3phen] into functionalized mesoporous silica nanoparticles (MSN) has been carried out to study the effect of chemical environments on the photoluminescence properties of the rare-earth complex. Surface functionalization was achieved by the reaction of the silanol groups on the surface of mesoporous silica with different organosilylating agents such as (3-aminopropyl)-triethoxysilane (APTES), (3-mercaptopropyl)-trimethoxysilane (MPTMS), and ethoxytrimethylsilane (ETMS). A change in the luminescence properties of the Eu(dbm)3phen complex has been observed on its encapsulation into surface modified mesoporous silica nanoparticles. The modification of photophysical properties is attributed to the interaction of Eu(dbm)3phen with the different chemical environments in the functionalized mesoporous silica nanoparticles (MSN). The luminescence properties of the rare-earth complex in surface-modified MSN increase in the order MSN < MSN-ETMS < MSN-MPTMS < MSN-APTES. The Eu(dbm)3phen complex encapsulated in the functionalized mesoporous silica nanoparticles shows an enhanced luminescence and an increased lifetime compared to the pure rare-earth complex in the solid state and that in unmodified MSN. This implies that some interactions of the lanthanide complexes take place during their incorporation process into the organically modified mesoporous silica nanoparticles. The organically modified mesoporous silica nanoparticles were characterized by Fourier transform infrared spectroscopy (FTIR) and N2 adsorption desorption measurements. The luminescence properties of the encapsulated Eu(dbm)3phen were studied in detail. Moreover, the effect of functionalized MSNs on the structural behaviour of the Eu(dbm)3phen was investigated by solid state nuclear magnetic resonance (SSNMR) techniques using an analogous diamagnetic model complex, Y(dbm)3phen, encapsulated into functionalized MSNs. These studies indicate that the encapsulated rare-earth complex shows some interactions with the functional groups anchored on the surface of MSNs.

Growth kinetics of CdSe quantum dots generated in polar polymers.

Growth kinetics of CdSe nanocrystals generated inside three selected polymers (polyvinylpyrrolidone--PVP, polyethyleneglycol--PEG and polyvinylalcohol--PVA) are demonstrated to follow a self-catalytic path, with growth rates depending on the nature of the polymer, i.e. on the capability to activate the cadmium species present in the solution of a metal precursor. A two-step process drives the size evolution of nanocrystals and a critical diameter value can be identified at which the growth regime changes. The medium-term stability evaluation of nanocomposites indicates that, after an initial rearrangement, polymers keep stable the embedded CdSe nanocrystals.

Fourier transform infrared spectroscopy and solid-state nuclear magnetic resonance studies of octadecyl modified metal oxides obtained from different silane precursors.

Octadecyl (C(18)) modified metal oxide substrates, including titania, zirconia, hafnia, and alumina, are prepared using two types of silylating reagents, n-octadecyltrihydridosilane and n-octadecyltrichlorosilane. Fourier transform infrared (FTIR) and solid-state (29)Si nuclear magnetic resonance (NMR) measurements are performed to examine the cross-linking of the silanes. Solid-state (13)C NMR spectroscopy provides information about the conformation and mobility of surface-immobilized alkyl chains. Variable temperature FTIR investigations are carried out to study the influence of the organosilane precursors and metal oxides on the conformational order of the alkyl modified systems. It is found that grafting by means of n-octadecyltrichlorosilane yields higher grafting densities than surface modification with n-octadecyltrihydridosilane. Combined pyridine adsorption and diffuse reflectance infrared Fourier transform (DRIFT) measurements are performed on the titania and hafnia substrates to evaluate potential surface heterogeneities, i.e. Lewis and Brønsted sites. Differences in the alkyl chain conformational order within the series of C(18) modified metal oxides are explained by the presence of island structures. The reduced C(18) conformational order for the samples grafted with n-octadecyltrihydridosilane is traced back to the lower grafting density which in turn points to a lower reactivity of this silylating reagent. The most striking result is the higher conformational order of the C(18) chains grafted in the present surface modified metal oxides when compared with silica-based systems. This finding is attributed to the lower porosity of the metal oxide supports along with more closely packed chains on the surface.