Organometallic−Polyoxometalate Hybrid Compounds: Metallosalen Compounds Modified by Keggin Type Polyoxometalates

Hybrid compounds with two functional centers consisting of a metallosalen moiety (M−salen; M = Mn, Co, Ni, and Pd) connected by an alkylene bridging group to a lacunary Keggin type polyoxometalate were synthesized and characterized. In these metallosalen−polyoxometalate compounds (M−salen−POM) it was shown by the use of a combination of UV−vis, 1H NMR, EPR, XPS, and cyclic voltammetry measurements that the polyoxometalate exerts a significant intramolecular electronic effect on the metallosalen moiety leading to formation of an oxidized metallosalen moiety. For the Mn−salen−POM, the metallosalen center is best described as a metal−salen cation radical species; that is, a localized “hole” is formed on the salen ligand. For the other M−salen−POM compounds, the metallosalen moiety can be described as a hybrid of a metal−salen cation radical species and an oxidized metal−salen species, that is, a delocalized “hole” is formed at the metallosalen center. It is proposed that these oxidized metallosalen centers are best characterized as stabilized charge transfer (metallosalen donor−polyoxometalate acceptor) complexes despite the relatively large distance between the two functional centers

Transient Charge Accumulation in a Capacitive Self-Assembled Monolayer

Charge accumulation in an organosilane monolayer self-assembled on silicon is studied using electron-spectroscopy-based chemically resolved electrical measurements (CREM). By resolving the net electrical response of the organic layer, a significant capability of holding extra charge is indicated. Quantum size effects at a molecularly thin layer and the role of competing discharge mechanisms, including defect-assisted leakage currents, are discussed

High-Resolution Lateral Differentiation Using a Macroscopic Probe: XPS of Organic Monolayers on Composite Au−SiO₂ Surfaces

X-ray photoelectron spectroscopy (XPS), an essentially macroscopic probe, is used to analyze mesoscopic systems at a lateral resolution given by the substrate structure. The method is based on controlled differential charging of multi-component surfaces, using a simple, commonly available XPS function, the electron flood gun. This new approach is applied here to a novel composite surface comprising SiO2 clusters on a {111} gold substrate, onto which different molecules are self-assembled to form a mixed organic monolayer. The method allows direct correlation of adsorbed molecules with surface sites, by analyzing XPS line shifts, which reflect local potential variations resulting from differential surface conductivity. This provides a powerful tool for resolving complex ultrathin films on heterogeneous substrates, on a length scale much smaller than the probe size

Band Alignment in Partial and Complete ZnO/ZnS/CdS/CuSCN Extremely Thin Absorber Cells: An X‑ray Photoelectron Spectroscopy Study

In all solar cells, and especially in extremely thin absorber (ETA) solar cells, proper energy band alignment is crucial for efficient photovoltaic conversion. However, available tabulated data usually do not agree with actual results, and in most cases, <i>V</i>_oc values lower than expected are achieved. In fact, ETA cells suffer from a very low <i>V</i>_oc/<i>E</i>_gap ratio, such as in ZnO/CdS/CuSCN cells. Here, we investigate limiting factors of ZnO/CdS/CuSCN ETA cells, applying X-ray photoelectron spectroscopy (XPS), chemically resolved electrical measurement (CREM), Kelvin probe, and <i>I</i>–<i>V</i> characterization. We show that electric fields are gradually developed in the cell upon increased absorber thickness. Moreover, an accumulation layer, unfavorable for the solar cell function, has been revealed at the oxide–absorber interface An effective chemical treatment to prevent formation of this accumulation layer is demonstrated

Band Alignment and Internal Field Mapping in Solar Cells

The internal fields and band offsets developing at individual interfaces, a critical aspect of device performance, are generally inaccessible by standard electrical tools. To address this problem, we propose chemically resolved electrical measurements (CREM) capable of resolving the internal details layer-by-layer. Applied to nanoporous photovoltaic cells, we thus extract a realistic band diagram for the multi-interfacial structure and, in particular, resolve the two p-n-like junction fields built spontaneously in the device. The lack of homogeneity common to many of these nanoporous cells is exploited here to “see” deep into the cell structure, beyond the typical depth limitations of the surface-sensitive technique. Further information on the cell operation under “real” working conditions is achieved by studying the charge trapping at each specific layer under optical and electrical stimuli. Our methodology overcomes a missing link in device characterization and in fundamental studies of nanoscale solid-state devices

Synthetic Mimic of Selective Transport Through the Nuclear Pore Complex

The nuclear pore complex is a large protein channel present universally in eukaryotic cells. It generates an essential macromolecular separation between the nucleus and cytoplasm. The transport mechanism relies on recognition of molecular cargos by receptor proteins, and on specific interaction between the receptors and the pores. We present a chemical mimic of this “receptor-mediated” transport using modified nanoporous membrane filters, polyisopropylacrylamide as the carrier molecule, or receptor, and single-stranded DNA as the cargo. We show that a complex of ssDNA and polyisopropylacrylamide diffuses faster through the modified pores than does the bare ssDNA, in spite of the larger size of the complex. The mobile polymer thus acts as a soluble receptor to usher a macromolecular cargo specifically through the pores

Long-Range Substrate Effects on the Stability and Reactivity of Thiolated Self-Assembled Monolayers

The reactivity of the tail group of molecules absorbed in a self-assembled monolayer is affected significantly by the substrate through long-range charge redistribution occurring during the adsorption. Alkyl dithiol monolayers on GaAs are highly stable as compared to monolayers of monothiols on GaAs or dithiols on gold. X-ray photoelectron spectroscopy (XPS) measurements reveal fairly weak binding of monothiol layers on GaAs, prone to rapid oxidation at the molecule−substrate interface. This is in contrast with the high stability of monothiols on gold. However, in the case of dithiols, the situation is reversed. When adsorbed on gold, the top thiol group tends to oxidize, whereas on GaAs, it does not. Furthermore, the monolayer was found to be stable in ambient for months. Contact potential difference (CPD) measurements showed a significant difference in charge distribution on the monolayers adsorbed on the two substrates, gold and GaAs. The change in reactivity and stability is attributed to the difference in the substrate-induced charge distribution across the adsorbed molecules

Anomalous Diffusion of High Molecular Weight Polyisopropylacrylamide in Nanopores

Passage of polymers through pores narrower than the hydrodynamic diameter is impeded by an entropic penalty for their confinement. This might be balanced by an attractive interaction with the pore walls. We found that the hydrogen-bonding polymer, poly(isopropylacrylamide) (pNIPAM), diffused readily through narrow pores in polycarbonate track-etched membranes. The trans side accumulation of pNIPAM followed a stretched exponential behavior. By contrast, a much smaller dextran diffused at a comparable or slower rate and showed ordinary Fick-like behavior. Comparison between the influence of pNIPAM surface adsorption and chemical grafting to the pores points to weak interpolymeric bonds as the source for the transport-accelerating surface interactions. We interpret the results as evidence for anomalous diffusion of pNIPAM inside the pores

Where is the Sodium in Self-Assembled Monolayers of Single-Stranded DNA?

Monolayers of single-stranded DNA (ssDNA) immobilized on surfaces form the basis of a number of important biotechnology applications, including DNA microarrays and biosensors. The organization of ssDNA as layer on a solid substrate allows one to investigate various properties of the DNA in a controlled manner and to use DNA for analytical applications as well as for exploring futuristic schemes for molecular electronics. It is commonly assumed that the adsorbed DNA layer contains some structural water and the cations. Here we show, based on XPS studies, that when monolayers of ssDNA are formed from sodium phosphate buffer and washed thoroughly, no Na+ signal is detected. A finite concentration of ions is observed when the DNA is made from a solution of Mg2+ ions, but it is still only a fifth of what it would be if all the phosphate ions were fully neutralized by the metal cations

Large-Scale Fabrication of 4-nm-Channel Vertical Protein-Based Ambipolar Transistors

We suggest a universal method for the mass production of nanometer-sized molecular transistors. This vertical-type device was fabricated using conventional photolithography and self-assembly methods and was processed in parallel fashion. We used this transistor to investigate the transport properties of a single layer of bovine serum albumin protein. This 4-nm-channel device exhibits low operating voltages, ambipolar behavior, and high gate sensitivity. The operation mechanism of this new device is suggested, and the charge transfer through the protein layer was explored