Tatyana Bendikov

Cesium Enhances Long-Term Stability of Lead Bromide Perovskite-Based Solar Cells

Direct comparison between perovskite-structured hybrid organic-inorganic methylammonium lead bromide (MAPbBr3) and all-inorganic cesium lead bromide (CsPbBr3), allows identifying possible fundamental differences in their structural, thermal and electronic characteristics. Both materials possess a similar direct optical band gap, but CsPbBr3 demonstrates a higher thermal stability than MAPbBr3. In order to compare device properties, we fabricated solar cells, with similarly synthesized MAPbBr3 or CsPbBr3, over mesoporous titania scaffolds. Both cell types demonstrated comparable photovoltaic performances under AM1.5 illumination, reaching power conversion efficiencies of ∼6% with a poly aryl amine-based derivative as hole transport material. Further analysis shows that Cs-based devices are as efficient as, and more stable than methylammonium-based ones, after aging (storing the cells for 2 weeks in a dry (relative humidity 15-20%) air atmosphere in the dark) for 2 weeks, under constant illumination (at maximum power), and under electron beam irradiation.

Biological Sensing and Interface Design in Gold Island Film Based Localized Plasmon Transducers

Discontinuous, island-type gold films (typically < or = 10 nm nominal thickness) prepared by evaporation of the metal on transparent substrates show a localized surface plasmon resonance (LSPR) extinction in the visible-to-NIR range and can be used as optical transducers for monitoring local refractive index change. Such transducers, operated in the transmission configuration, provide an effective scheme for label-free biological sensing using basic spectrophotometric equipment. Optimization of the sensitivity of LPSR transducers requires consideration of the distance between the metal island surface and the bound analyte, strongly affecting the optical response due to the fast decay of the evanescent field of localized plasmons. In the present work Au island based LSPR transducers were used to monitor antibody-antigen interactions, demonstrating the effect of the biorecognition interface thickness. Evaporated Au island films derivatized with IgG or hCG antigens were used as biological recognition elements for selective sensing of antibody binding, distinguishing between specific and nonspecific interactions. The LSPR results are supported by XPS and ellipsometry data as well as by AFM and HRSEM imaging, the latter providing actual visualization of the two protein binding steps. Increase of the recognition interface thickness leads to a concomitant decrease in the extinction and wavelength sensitivity, generally conforming to a model of an exponentially decaying surface plasmon (SP) evanescent field.

How Innocent are Potentially Redox Non-Innocent Ligands? Electronic Structure and Metal Oxidation States in Iron-PNN Complexes as a Representative Case Study

Herein we present a series of new α-iminopyridine-based iron-PNN pincer complexes [FeBr2LPNN] (1), [Fe(CO)2LPNN] (2), [Fe(CO)2LPNN](BF4) (3), [Fe(F)(CO)2LPNN](BF4) (4), and [Fe(H)(CO)2LPNN](BF4) (5) with formal oxidation states ranging from Fe(0) to Fe(II) (LPNN = 2-[(di-tert-butylphosphino)methyl]-6-[1-(2,4,6-mesitylimino)ethyl]pyridine). The complexes were characterized by a variety of methods including (1)H, (13)C, (15)N, and (31)P NMR, IR, Mössbauer, and X-ray photoelectron spectroscopy (XPS) as well as electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) spectroscopy, SQUID magnetometry, and X-ray crystallography, focusing on the assignment of the metal oxidation states. Ligand structural features suggest that the α-iminopyridine ligand behaves as a redox non-innocent ligand in some of these complexes, particularly in [Fe(CO)2LPNN] (2), in which it appears to adopt the monoanionic form. In addition, the NMR spectroscopic features ((13)C, (15)N) indicate the accumulation of charge density on parts of the ligand for 2. However, a combination of spectroscopic measurements that more directly probe the iron oxidation state (e.g., XPS), density functional theory (DFT) calculations, and electronic absorption studies combined with time-dependent DFT calculations support the description of the metal atom in 2 as Fe(0). We conclude from our studies that ligand structural features, while useful in many assignments of ligand redox non-innocence, may not always accurately reflect the ligand charge state and, hence, the metal oxidation state. For complex 2, the ligand structural changes are interpreted in terms of strong back-donation from the metal center to the ligand as opposed to electron transfer.

The Absence of Redox Reactions for Palladium(II) and Copper(II) on Electrostatically Charged Teflon: Relevance to the Concept of “Cryptoelectrons”

Triboelectrification (electrification upon rubbing) of dielectric materials, one of the most ancient and commonly observed forms of electricity, has a number of useful and practical applications, such as triboelectrostatic separation of plastics, as well as numerous adverse effects related to uncontrolled charge accumulation and subsequent discharge. Nevertheless, the mechanism of charge transfer between two dielectric materials remains a matter of debate that is centered upon two different views: transfer of ions or transfer and trapping of electrons. To support the latter view, Liu and Bard recently reported that a high density of electrons, which they defined as “cryptoelectrons”, were present on a dielectric surface and could be detected by inducing “redox” reactions including reduction of Cu and Pd ions. On the basis of these reactions, they estimated the electron density on Teflon (polytetrafluoroethylene, PTFE) that was rubbed by PMMA (polymethylmethacrylate), and on pure PMMA to be 10 electrons cm . Our interest in charged dielectrics stems from the attempt to utilize them for nucleation of polar crystals as a possible alternative to pyroelectric surfaces that we used earlier to suppress or promote ice nucleation. As a part of this work, we provide direct evidence, based on X-ray photoelectron spectroscopy (XPS), that the surface charge created on these surfaces does not reduce copper and palladium ions. Instead, rubbing Teflon with PMMA causes material exchange and promotes adsorption of copper ions from aqueous solutions onto the rubbed Teflon; this process can be misinterpreted as an electrochemical reduction. These data challenge the role of cryptoelectrons in these reactions. One of the arguments supporting the existence of cryptoelectrons is their supposed ability to reduce Cu and Pd from aqueous solutions. 9] Copper reduction was deduced from two factors: a) the decrease in the intensity of the blue color of a 0.1 mmol solution of Cu ions that is brought into contact with Teflon rubbed by PMMA; this is not observed with Teflon not rubbed by PMMA; and b) the presence of metallic copper on the surface of the Teflon as detected by energy-dispersive X-ray fluorescence spectroscopy (EDS). The Pd reduction was inferred from Pdcatalyzed copper plating onto a PMMA-rubbed Teflon surface that is brought into contact with a saturated solution of PdCl2 and then immersed in a chemical copper plating bath 9,15] containing formaldehyde. We investigated the Teflon and PMMA surfaces before and after rubbing by using X-ray photoelectron spectroscopy (XPS), and found that the surface of PMMA acquires approximately 25–60 % of a monolayer of CF2 fragments and the surface of the Teflon acquires at least 25% of a nonfluorinated carbon monolayer, which is significantly more than any residue observed before rubbing (Figure 1a,b). Since both materials are polymers having a high molecular weight, this exchange 16] must be accompanied with the breaking of chemical bonds and the formation of a chemically polar active species. The transfer of polar (containing C=O groups) PMMA species to the Teflon surface should change the physical and chemical properties of the surface, including a decrease of the water-contact angle and ability to adsorb ions, such as Cu and Pd. Therefore, one can offer alternative interpretations of the experiments supporting the hypothesis of cryptoelectrons proposed by Liu and Bard. The oxidation states of copper and palladium were analyzed by XPS, which is a less aggressive probe compared to the electron beam in EDS. On the surface of the Teflon that was rubbed by PMMA, immersed in CuSO4 (1 mmol), and then rinsed thoroughly, copper was found in the + 2 oxidation state; for immersion of a similarly treated Teflon surface in PdCl2/HCl (0.2 mmol) the palladium was also in the + 2 oxidation state (Figure 2a,b). Similarly, only Cu and Pd are found on the Teflon, which was charged and then discharged with an antistatic gun. The quantity of copper and palladium corresponds to 10–30 % (ca. (1–3 10) cm ) and 1–5 % (ca. (1–5 10) cm ) respectively, of a monolayer. Furthermore, treating the surface containing Pd ions with a 1 % formaldehyde solution causes their reduction to Pd (Figure 2b). With another set of samples that served as a reference, we have found that the XPS probe (X-ray plus a low-energy electron flood gun) does not noticeably affect the oxidation state of copper. Although it does not cause oxidation of Pd, it does cause partial reduction of oxidized palladium after exposure to both factors of the XPS probe for at least an hour (Figure 2b). [*] Dr. S. Piperno, Prof. M. Lahav, Prof. I. Lubomirsky Department of Materials and Interfaces Weizmann Institute of Science, Rehovot 76100 (Israel) Igor.Lubomirsky@Weizmann Institute of Science Meir.Lahav@Weizmann Institute of Science

Substituent Variation Drives Metal/Monolayer/Semiconductor Junctions from Strongly Rectifying to Ohmic Behavior

An eight-orders of magnitude enhancement in current across Hg/X-styrene-Si junctions is caused by merely altering a substituent, X. Interface states are passivated and, depending on X, the Si Schottky junction encompasses the full range from Ohmic to strongly rectifying. This powerful electrostatic molecular effect has immediate implications for interface band alignment and sensing.

Odd–Even Effect in Molecular Electronic Transport via an Aromatic Ring

A distinct odd-even effect on the electrical properties, induced by monolayers of alkyl-phenyl molecules directly bound to Si(111), is reported. Monomers of H2C═CH-(CH2)n-phenyl, with n = 2-5, were adsorbed onto Si-H and formed high-quality monolayers with a binding density of 50-60% Si(111) surface atoms. Molecular dynamics simulations suggest that the binding proximity is close enough to allow efficient π-π interactions and therefore distinctly different packing and ring orientations for monomers with odd or even numbers of methylenes in their alkyl spacers. The odd-even alternation in molecular tilt was experimentally confirmed by contact angle, ellipsometry, FT-IR, and XPS with a close quantitative match to the simulation results. The orientations of both the ring plane and the long axis of the alkyl spacer are more perpendicular to the substrate plane for molecules with an even number of methylenes than for those with an odd number of methylenes. Interestingly, those with an even number conduct better than the effectively thinner monolayers of the molecules with the odd number of methylenes. We attribute this to a change in the orientation of the electron density on the aromatic rings with respect to the shortest tunneling path, which increases the barrier for electron transport through the odd monolayers. The high sensitivity of molecular charge transport to the orientation of an aromatic moiety might be relevant to better control over the electronic properties of interfaces in organic electronics.

Tuning electronic transport via hepta-alanine peptides junction by tryptophan doping.

Significance Charge transport is a process that is central to redox reactions, enzyme catalysis, and electronics. Integrating biomolecules into electronic devices provides a route to using natural evolution in artificial technology. Charge transport across biomolecules depends on the molecular structure and molecule–electrode contacts. We show that “doping” (7-alanine) peptide-based devices by one tryptophan enhances transport ≥10-fold, regardless of tryptophan position, a result that can be described by superexchange-mediated tunneling through multiple sites. Remarkably, close physical proximity of tryptophan to a gold electrode further enhances transport, showing how crucial electronic coupling to the electrode is. Our results provide insight into charge transfer through biomolecules and offer a strategy for tailored bioelectronics by mix-and-match amino acid building blocks. Charge migration for electron transfer via the polypeptide matrix of proteins is a key process in biological energy conversion and signaling systems. It is sensitive to the sequence of amino acids composing the protein and, therefore, offers a tool for chemical control of charge transport across biomaterial-based devices. We designed a series of linear oligoalanine peptides with a single tryptophan substitution that acts as a “dopant,” introducing an energy level closer to the electrodes’ Fermi level than that of the alanine homopeptide. We investigated the solid-state electron transport (ETp) across a self-assembled monolayer of these peptides between gold contacts. The single tryptophan “doping” markedly increased the conductance of the peptide chain, especially when its location in the sequence is close to the electrodes. Combining inelastic tunneling spectroscopy, UV photoelectron spectroscopy, electronic structure calculations by advanced density-functional theory, and dc current–voltage analysis, the role of tryptophan in ETp is rationalized by charge tunneling across a heterogeneous energy barrier, via electronic states of alanine and tryptophan, and by relatively efficient direct coupling of tryptophan to a Au electrode. These results reveal a controlled way of modulating the electrical properties of molecular junctions by tailor-made “building block” peptides.

Air-ozonolysis to generate contact active antimicrobial surfaces: activation of polyethylene and polystyrene followed by covalent graft of quaternary ammonium salts.

Air-ozonolysis was revealed as an accessible and effective approach for surface activation and further functionalization of hydrocarbon polymers. Antimicrobial contact active polyethylene (PE) and polystyrene (PS) were designed by generation on their surfaces OH-functional groups and covalent graft of dimethyloctadecyl [3-(trimethoxysilyl) propyl] ammonium chloride (C18-TSA) quaternary ammonium salt. The shortened analog, trimethyl [3-(trimethoxysilyl) propyl] ammonium chloride (C1-TSA), was also covalently attached to the activated PE and PS surfaces. X-ray photoelectron spectroscopy (XPS) and FTIR confirmed the surface modifications. Scanning electron (SEM) and confocal microscopy were utilized to monitor surface morphology and bacteria interactions. The antimicrobial effect of the C18-TSA grafted polymer surfaces was demonstrated on Gram-negative and Gram-positive bacteria species including human pathogen, Salmonella enterica. The shorter C1-TSA grafted polymers did not demonstrate bactericidal activity, suggesting the critical role of the alkyl chain length. The described strategy may establish a new general and safe platform for future development and application of contact active antimicrobial polymers.

Phosphonate-stabilized silver nanoparticles: one-step synthesis and monolayer assembly

The first case of phosphonic acid terminated, environmentally friendly silver nanoparticles (NPs) is described. The NPs are produced by a simple one-step synthesis using commercially available reagents, furnishing stable, rather monodisperse, size-controlled, aqueous-based Ag NPs, stabilized by aminomethylene phosphonic acid (AMP) molecules. In this synthesis the commercial reagent ethylenediamine tetra(methylene phosphonic acid) (EDTMP) serves as a reducing agent for Ag+ ions, while its oxidation product AMP is the stabilizer of the generated Ag NPs. The negatively charged, phosphonate-stabilized NPs were characterized by UV-vis spectroscopy, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Variation of the EDTMP/AgNO3 molar ratio enables simple and efficient control of the average particle diameter in the range ∼5.5 to 15 nm. The AMP-stabilized Ag NPs are stable in water under an inert atmosphere for at least 2 months with no observed aggregation. Self-assembled layers of phosphonate-coated Ag NPs were prepared on substrates primed with positively charged molecular self-assembled monolayers (SAMs) or with polyelectrolyte (PE) layers. The NP films were studied by UV-vis spectroscopy, polarization modulation Fourier-transform infrared reflection-absorption spectroscopy (PM-IRRAS), and high-resolution scanning electron microscopy (HRSEM). While NP monolayers commonly undergo extensive aggregation upon drying, the present phosphonate-stabilized Ag NP monolayers display homogeneously dispersed, mostly isolated NPs over large areas after adsorption and drying. The NP distribution and degree of aggregation can be modulated by the substrate type (gold, glass), the colloid solution pH, the nature of the primer layer (charged molecules, PEs), and the surface charge density. Phosphonate-terminated Ag NPs provide unique physical and chemical properties, including a negatively charged surface in a wide pH range, long-term stability, size control, and the possibility of participating in electrostatic and coordination binding processes.

CH3NH3PbBr3 is not pyroelectric, excluding ferroelectric-enhanced photovoltaic performance

To experimentally (dis)prove ferroelectric effects on the properties of lead-halide perovskites and of solar cells, based on them, we used second-harmonic-generation spectroscopy and the periodic temperature change (Chynoweth) technique to detect the polar nature of methylammonium lead bromide (MAPbBr3). We find that MAPbBr3 is probably centrosymmetric and definitely non-polar; thus, it cannot be ferroelectric. Whenever pyroelectric-like signals were detected, they could be shown to be due to trapped charges, likely at the interface between the metal electrode and the MAPbBr3 semiconductor. These results indicate that the ferroelectric effects do not affect steady-state performance of MAPbBr3 solar cells.