Publications

In search of different chemical mechanisms for electro-freezing of supercooled water (SCW), we describe here the differences between the inert electrodes of Co and Ni and their ability to be converted into ice makers. The formation of crystalline domains by anodic electrochemical treatment of cobalt in 10 μM KOH solution triggers ice nucleation of supercooled water at ∼−3 °C without the application of voltage. X-ray diffraction and XPS demonstrate that these crystalline domains are composed of oxyhydroxide molecules. As predicted by their Moiré fringes, crystals of hexagonal ice grow epitaxially on top of these domains along their [002] direction. In contrast, Ni anodes under similar electrochemical treatment do not form crystalline domains of the oxyhydroxides and therefore do not yield ice-nucleating surfaces.

The local environments of Sc and Y in predominantly ⟨002⟩ textured, Al_1-xDo_xN (Do = Sc, x = 0.25, 0.30 or Y, x = 0.25) sputtered thin films with wurtzite symmetry were investigated using X-ray absorption (XAS) and photoelectron (XPS) spectroscopies. We present evidence from the X-ray absorption fine structure (XAFS) spectra that, when x = 0.25, both Sc³⁺ and Y³⁺ ions are able to substitute for Al³⁺, thereby acquiring four tetrahedrally coordinated nitrogen ligands, i.e., coordination number (CN) of 4. On this basis, the crystal radius of the dopant species in the wurtzite lattice, not available heretofore, could be calculated. By modeling the scandium local environment, extended XAFS (EXAFS) analysis suggests that when x increases from 0.25 to 0.30, CN for a fraction of the Sc ions increases from 4 to 6, signaling octahedral coordination. This change occurs at a dopant concentration significantly lower than the reported maximum concentration of Sc (42 mol % Sc) in wurtzite (Al, Sc)N. XPS spectra provide support for our observation that the local environment of Sc in (Al, Sc)N may include more than one type of coordination.

Biological and synthetic molecular motors, fueled by various physical and chemical means, can perform asymmetric linear and rotary motions that are inherently related to their asymmetric shapes. Here, we describe silver-organic micro-complexes of random shapes that exhibit macroscopic unidirectional rotation on water surface through the asymmetric release of cinchonine or cinchonidine chiral molecules from their crystallites asymmetrically adsorbed on the complex surfaces. Computational modeling indicates that the motor rotation is driven by a pH-controlled asymmetric jet-like Coulombic ejection of chiral molecules upon their protonation in water. The motor is capable of towing very large cargo, and its rotation can be accelerated by adding reducing agents to the water.

Metal-free halide perovskites (MFHaPs) have garnered significant attention in recent years due to their desirable properties, such as low toxicity, light weight, chemical versatility, and potential for optoelectronics. MFHaPs with the formula A2+B+X−3 (where A is a large organic divalent cation, B+ is typically NH4+, and X is a halide) have been studied extensively, but few studies have examined alternative cations at the B position. This paper reports the synthesis of three MFHaP-related single crystals, DABCO-N2H5-X3 (DABCO = N-N-diazabicyclo[2.2.2]octonium, X = Br and I) and (DABCO)3-N2H5(NH4)2Cl9, which feature hydrazinium (N2H5) at the B position. The crystals have a perovskite-like, one-dimensional, edge-connected structure and exhibit optical and band structure properties. The crystals were then tested as X-ray detectors, where they showed excellent photoresponsivity, stability, and low background noise, owing to the large semi-gap that dictates long lifetimes. The detectors exhibited sensitivity as high as 1143 ± 10 µC Gyair−1 cm−2 and a low detection limit of 2.68 µGy s−1 at 10 V. The researchers suggest that the stronger hydrogen bonding in N2H5+ compounds compared to NH4+ MFHaPs may contribute to the detectors enhanced stability.

A measurement of the top-quark mass (m_t) in the tt¯ → lepton + jets channel is presented, with an experimental technique which exploits semileptonic decays of b-hadrons produced in the top-quark decay chain. The distribution of the invariant mass m_ℓμ of the lepton, ℓ (with ℓ = e, μ), from the W-boson decay and the muon, μ, originating from the b-hadron decay is reconstructed, and a binned-template profile likelihood fit is performed to extract m_t. The measurement is based on data corresponding to an integrated luminosity of 36.1 fb⁻¹ of √s = 13 TeV pp collisions provided by the Large Hadron Collider and recorded by the ATLAS detector. The measured value of the top-quark mass is m_t = 174.41 ± 0.39 (stat.) ± 0.66 (syst.) ± 0.25 (recoil) GeV, where the third uncertainty arises from changing the Pythia8 parton shower gluon-recoil scheme, used in top-quark decays, to a recently developed setup. [Figure not available: see fulltext.].

A search for the pair production of heavy leptons as predicted by the type-III seesaw mechanism is presented. The search uses protonproton collision data at a centre-of-mass energy of 13 TeV, corresponding to 139fb^-1 of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on final states with three or four electrons or muons from the possible decays of new heavy leptons via intermediate electroweak bosons. No significant deviations above the Standard Model expectation are observed; upper and lower limits on the heavy lepton production cross-section and masses are derived respectively. These results are then combined for the first time with the ones already published by ATLAS using the channel with two leptons in the final state. The observed lower limit on the mass of the type-III seesaw heavy leptons combining two, three and four lepton channels together is 910 GeV at the 95% confidence level.

A protocol for successfully depositing [001] textured, 23 µm thick films of Al0.75Sc0.25N, is proposed. The procedure relies on the fact that sputtered Ti is [001]-textured α-phase (hcp). Diffusion of nitrogen ions into the α-Ti film during reactive sputtering of Al0.75,Sc0.25N likely forms a [111]-oriented TiN intermediate layer. The lattice mismatch of this very thin film with Al0.75Sc0.25N is ~3.7%, providing excellent conditions for epitaxial growth. In contrast to earlier reports, the Al0.75Sc0.25N films prepared in the current study are Al-terminated. Low growth stress (

A search for the Higgs boson decaying into a pair of charm quarks is presented. The analysis uses protonproton collisions to target the production of a Higgs boson in association with a leptonically decaying W or Z boson. The dataset delivered by the LHC at a centre-of-mass energy of √s=13TeV and recorded by the ATLAS detector corresponds to an integrated luminosity of 139 fb^-1. Flavour-tagging algorithms are used to identify jets originating from the hadronisation of charm quarks. The analysis method is validated with the simultaneous measurement of WW, WZ and ZZ production, with observed (expected) significances of 2.6 (2.2) standard deviations above the background-only prediction for the (W/ Z) Z(→ cc¯) process and 3.8 (4.6) standard deviations for the (W/ Z) W(→ cq) process. The (W/ Z) H(→ cc¯) search yields an observed (expected) upper limit of 26 (31) times the predicted Standard Model cross-section times branching fraction for a Higgs boson with a mass of 125GeV, corresponding to an observed (expected) constraint on the charm Yukawa coupling modifier |κ_c|

A search for invisible decays of the Higgs boson as well as searches for dark matter candidates, produced together with a leptonically decaying Z boson, are presented. The analysis is performed using proton−proton collisions at a centre-of-mass energy of 13 TeV, delivered by the LHC, corresponding to an integrated luminosity of 139 fb⁻¹ and recorded by the ATLAS experiment. Assuming Standard Model cross-sections for ZH production, the observed (expected) upper limit on the branching ratio of the Higgs boson to invisible particles is found to be 19% (19%) at the 95% confidence level. Exclusion limits are also set for simplified dark matter models and two-Higgs-doublet models with an additional pseudoscalar mediator.

Research on the photoreduction of CO2 often has been dominated by the use of sacrificial reducing agents. A pathway that avoids this problem would be the development of photocathodes for CO2 reduction that could then be coupled to a photoanodic oxygen evolution reaction. Here, we present the use of coppersubstituted graphitic carbon nitride (Cu−CN) on a fluorinated tin oxide (FTO) electrode for the photoelectrochemical twoelectron reduction of CO2 to CO as a major product (>95%) and formic acid (

Symmetry-imposed restrictions on the number of available pyroelectric and piezoelectric materials remain a major limitation as 22 out of 32 crystallographic material classes exhibit neither pyroelectricity nor piezoelectricity. Yet, by breaking the lattice symmetry it is possible to circumvent this limitation. Here, using a unique technique for measuring transient currents upon rapid heating, direct experimental evidence is provided that despite the fact that bulk SrTiO3 is not pyroelectric, the (100) surface of TiO2-terminated SrTiO3 is intrinsically pyroelectric at room temperature. The pyroelectric layer is found to be ≈1 nm thick and, surprisingly, its polarization is comparable with that of strongly polar materials such as BaTiO3. The pyroelectric effect can be tuned ON/OFF by the formation or removal of a nanometric SiO2 layer. Using density functional theory, the pyroelectricity is found to be a result of polar surface relaxation, which can be suppressed by varying the lattice symmetry breaking using a SiO2 capping layer. The observation of pyroelectricity emerging at the SrTiO3 surface also implies that it is intrinsically piezoelectric. These findings may pave the way for observing and tailoring piezo- and pyroelectricity in any material through appropriate breaking of symmetry at surfaces and artificial nanostructures such as heterointerfaces and superlattices.

Chemically resolved electrical measurements of zinc oxysulfide over-layers on gold show very poor conductance under either electrical or optical input signals, whereas simultaneous application of the two yields extremely high sample currents. The effect and its dependence on the wavelength and electrical parameters are explained by the in-situ derived band diagram, in which a buffer level of charge traps cannot contribute directly to conductance, while yet amplifying the photoconductance by orders of magnitudes under sub-bandgap illumination. This AND-type doubly triggered response proposes interesting applications and an answer to problems encountered in related optoelectronic devices.

The pyroelectricity of AgI crystals strongly affects the icing temperature of super-cooled water, as disentangled from that of epitaxy. This deduction was achieved by the design of polar crystalline ceramic pellets of AgI, with experimentally determined sense of polarity. These pellets are suitable for measuring both their pyroelectric properties as well as the icing temperature of super-cooled water, separately on each of the expressed Ag+ and I- hemihedral surfaces. The positive pyroelectric charge at the silver-enriched side elevates the icing temperature, whereas the negative charge at the iodide side decreases that temperature. Moreover, the effect of pyroelectric charge remains dominant despite the presence of contaminants on both the silver and the iodide-enriched surfaces. Consequently an electrochemical process for ice nucleation is suggested, which might be of relevance for understanding the role played by electric charges in heterogeneous icing processes in general.

Direct exploration of the mutually interfering morphological and charge-transport characteristics in self-assembled monolayers (SAMs) is reported. These strongly coupled properties are addressed by means of surface spectroscopy techniques, combined so as to consistently account at high sensitivity for a broad range of surface properties without any use of a top contact. Applied to doped bovine serum albumin (BSA) SAMs, we show how the BSA conformation, its dehydration, and monolayer assembly are all correlated. Moreover, the electrical properties, transport, and charge trapping are highly affected by the SAM compositional and structural state, with a specific role of water molecules. Our results reveal and further demonstrate a useful approach to the complex challenges presented by (bio) molecular electronics.

A simple method for preparation of hybrid of graphene oxide (GO) and Laponite (Lap), obtained by solvent evaporation from their highly stable aqueous dispersions is reported. The dispersion up to ~ 1 mg/ml of GO in 1% Lap dispersion, i.e., 10:1 of Lap:GO was found to be stable without flocculation for several months; lower mass ratios of Lap to GO than this showed marginal flocculation with time. The electrostatic interaction between cations present in the interlayers of Lap and the functional groups of GO is envisaged to be the cause for the stable dispersion, which was confirmed by the presence of cations; viz., Na⁺ and small amounts of K⁺ and Mg^2 + in the aqueous filtrate of the hybrid. Their interaction was further confirmed by higher absorption of GO in aqueous Lap dispersion than that in water using UVvis spectroscopy. The resulting hybrid material was found to be partially reduced and self-assembled to form layered structure in its dry state. The hybrids further showed improved electrical conductivity (~ 0.01 S/cm) upon chemical reduction. The present study demonstrates a facile method for preparation of a new hybrid material and greener pathway for GO reduction; though partially. This hybrid has potential as multifunctional filler for clay polymer nanocomposites.

Effective control of chemistry at interfaces is of fundamental importance for the advancement of methods of surface functionalization and patterning that are at the basis of many scientific and technological applications. A conceptually new type of interfacial chemical transformations has been discovered, confined to the contact surface between two solid materials, which may be induced by exposure to X-rays, electrons or UV light, or by the application of electrical bias. One of the reacting solids is a removable thin film coating that acts as a reagent/catalyst in the chemical modification of the solid surface on which it is applied. Given the diversity of thin film coatings that may be used as solid reagents/catalysts and the lateral confinement options provided by the use of irradiation masks, conductive AFM probes or stamps, and electron beams in such solid-phase reactions, this approach is suitable for precise targeting of different desired chemical modifications to predefined surface sites spanning the macro- to nanoscale.

The excitation of cavity standing waves in double-slit structures in thin gold films, with slit lengths between 400 and 2560 nm, was probed with a strongly focused electron beam in a transmission electron microscope. The energies and wavelengths of cavity modes up to the 11th mode order were measured with electron energy loss spectroscopy to derive the corresponding dispersion relation. For all orders, a significant redshift of mode energies accompanied by a wavelength elongation relative to the expected resonator energies and wavelengths is observed. The resultant dispersion relation is found to closely follow the well-known dispersion law of surface-plasmon polaritons (SPPs) propagating on a gold/air interface, thus providing direct evidence for the hybridized nature of the detected cavity modes with SPPs.

The combination of photonics and spintronics opens new ways to transfer and process information. It is shown here that in systems in which organic molecules and semiconductor nanoparticles are combined, matching these technologies results in interesting new phenomena. We report on light induced and spin-dependent charge transfer process through helical oligopeptide-CdSe nanoparticles' (NPs) architectures deposited on ferromagnetic substrates with small coercive force (∼100-200 Oe). The spin control is achieved by the application of the chirality-induced spin-dependent electron transfer effect and is probed by two different methods: spin-controlled electrochemichemistry and photoluminescence (PL) at room temperature. The injected spin could be controlled by excitation of the nanoparticles. By switching the direction of the magnetic field of the substrate, the PL intensity could be alternated.

Vibrational spectroscopy in the electron microscope would be transformative in the study of biological samples, provided that radiation damage could be prevented. However, electron beams typically create high-energy excitations that severely accelerate sample degradation. Here this major difficulty is overcome using an 'aloof' electron beam, positioned tens of nanometres away from the sample: high-energy excitations are suppressed, while vibrational modes of energies

Palladium nanoparticles were deposited on multiwall WS₂ nanotubes. The composite nanoparticles were characterized by a variety of techniques. The Pd nanoparticles were deposited uniformly on the surface of WS₂ nanotubes. An epitaxial relationship between the (111) plane of Pd and the (013) plane of WS₂ was mostly observed. The composite nanoparticles were found to perform efficiently as catalysts for cross-coupling (Heck and Suzuki) reactions. The role of the nanotubes support in the catalytic process is briefly discussed.

Manganese(IV,V)-hydroxo and oxo complexes are often implicated in both catalytic oxygenation and water oxidation reactions. Much of the research in this area is designed to structurally and/or functionally mimic enzymes. On the other hand, the tendency of such mimics to decompose under strong oxidizing conditions makes the use of molecular inorganic oxide clusters an enticing alternative for practical applications. In this context it is important to understand the reactivity of conceivable reactive intermediates in such an oxide-based chemical environment. Herein, a polyfluoroxometalate (PFOM) monosubstituted with manganese, [NaH₂(Mn-L)W₁₇F₆O₅₅]^q-, has allowed the isolation of a series of compounds, Mn(II, III, IV and V), within the PFOM framework. Magnetic susceptibility measurements show that all the compounds are high spin. XPS and XANES measurements confirmed the assigned oxidation states. EXAFS measurements indicate that Mn(II)PFOM and Mn(III)PFOM have terminal aqua ligands and Mn(V)PFOM has a terminal hydroxo ligand. The data are more ambiguous for Mn(IV)PFOM where both terminal aqua and hydroxo ligands can be rationalized, but the reactivity observed more likely supports a formulation of Mn(IV)PFOM as having a terminal hydroxo ligand. Reactivity studies in water showed unexpectedly that both Mn(IV)-OH-PFOM and Mn(V)-OH-PFOM are very poor oxygen-atom donors; however, both are highly reactive in electron transfer oxidations such as the oxidation of 3-mercaptopropionic acid to the corresponding disulfide. The Mn(IV)-OH-PFOM compound reacted in water to form O₂, while Mn(V)-OH-PFOM was surprisingly indefinitely stable. It was observed that addition of alkali cations (K⁺, Rb⁺, and Cs⁺) led to the aggregation of Mn(IV)-OH-PFOM as analyzed by electron microscopy and DOSY NMR, while addition of Li⁺ and Na⁺ did not lead to aggregates. Aggregation leads to a lowering of the entropic barrier of the reaction without changing the free energy barrier. The observation that O₂ formation is fastest in the presence of Cs⁺ and ∼fourth order in Mn(IV)-OH-PFOM supports a notion of a tetramolecular Mn(IV)-hydroxo intermediate that is viable for O₂ formation in an oxide-based chemical environment. A bimolecular reaction mechanism involving a Mn(IV)-hydroxo based intermediate appears to be slower for O₂ formation.

This article reports on a facile and fast strategy for the self-assembled monolayer (SAM) functionalization of nickel surfaces, employing cyclic voltammetry (CV) cycling of a suitable tailored solution containing the species to be adsorbed. Results are presented for ultrathin films formed on Ni by 1-hexadecanethiol (C16), l-cysteine (l-cys), and the poly{methyl (2R)-3-(2,2'-bithiophen-4-ylsulfanyl)-2-[(tert-butoxycarbonyl)amino]propanoate} (PCT-L) thiophene-based chiral polymer. The effective formation of high-quality ultrathin organic films on the nickel was verified both electrochemically and by exploiting typical surface characterization techniques such as contact angle, ellipsometry, atomic force microscopy (AFM), polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS).

Solid-state semiconductor-sensitized solar cells require a thin, dense hole-blocking layer at the conducting glass substrate (F-doped tin oxide (FTO)) to prevent shorting beween the FTO and hole conductor. We found that by adding a small amount of Sb ions to a ZnO chemical deposition bath a thin (few tens of nanometers thick) dense and uniform layer of Sb-incorporated ZnO forms. Here we investigate the electronic properties of this layer in comparison to the continuous ZnO layer at the base of the ZnO rods formed in the standard preparation. Devices incorporating the Sb-incorporated dense layer followed by a standard ZnO nanorod growth, onto which CdS or CdSe was grown followed by a CuSCN hole conductor, showed 100-200 mV higher photovoltage together with occasional improvement in the short-circuit current. Electrochemical and electrical measurements indicated complete coverage of the FTO substrate by both preparations; however, the shunt resistance (resistance to a reverse leakage current) in the cells (and films) made using the Sb-incorporated ZnO layer is dramatically increased. Using bias-dependent incident photon-to-electron conversion efficiency studies, we found that an increased dark or leakage current develops in the cell on illumination with UV light together with application of a forward bias. This can be explained by the presence of a "Schottky junction" at the FTO\ZnO interface. This increased leakage current is significantly larger in cells without the Sb-incorporated ZnO compact layer.

The work function (WF) of ZnO is modified by two types of dipole-bearing phenylphosphonate layers, yielding a maximum WF span of 1.2 eV. H ₃CO-phenyl phosphonate, with a positive dipole (positive pole pointing outwards from the surface), lowers the WF by ∼350 meV. NC-phenyl phosphonate, with a negative dipole, increases the WF by ∼750 meV. The WF shift is found to be independent of the type of ZnO surface. XPS data show strong molecular dipoles between the phenyl and the functionalizing (CN and OMe) tail groups, while an opposite dipole evolves in each molecular layer between the surface and the phenyl rings. The molecular modification is found to be invariant to supra-bandgap illumination, which indicates that the substrate's space charge-induced built-in potential is unlikely to be the reason for the WF difference. ZnO, grown by several different methods, with different degrees of crystalline perfection and various morphologies and crystallite dimensions, could all be modified to the same extent. Furthermore, a mixture of opposite dipoles allows gradual and continuous tuning of the WF, varying linearly with the partial concentration of the CN-terminated phosphonate in the solution. Exposure to the phosphonic acids during the molecular layer deposition process erodes a few atomic layers of the ZnO. The general validity of the treatment and the fine-tuning of the WF of treated interfaces are of interest for solar cells and LED applications.

The interface level alignment of alkyl and alkenyl monolayers, covalently bound to oxide-free Si substrates of various doping levels, is studied using X-ray photoelectron spectroscopy. Using shifts in the C 1s and Si 2p photoelectron peaks as a sensitive probe, we find that charge distribution around the covalent Si-C bond dipole changes according to the initial position of the Fermi level within the Si substrate. This shows that the interface dipole is not fixed but rather changes with the doping level. These results set limits to the applicability of simple models to describe level alignment at interfaces and show that the interface bond and dipole may change according to the electrostatic potential at the interface.

Non-contact pyroelectricity measurements based on x-ray photoelectron spectroscopy (XPS) are presented. Applied to Lithium Tantalate crystals, we demonstrate how the XPS-derived surface potential provides a simple probe of the desired property, free of all top-contact related difficulties. In particular, the increase in Lithium Tantalate spontaneous polarization under cooling, an experimentally challenging feature, is evaluated. We further inspect the roll of surface contaminants and the control over trapped surface charge in the XPS vacuum environment. Our approach can be extended to other non-contact probes, as well as to measuring additional electrical properties, such as piezoelectricity and ferroelectricity.

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, V_oc values lower than expected are achieved. In fact, ETA cells suffer from a very low V_oc/E _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-V 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.

Dr Williams (AIP Adv., 2012, 2, 010701) suggested that cleaning Teflon by high pressure oxygen plasma may have affected our result that Cu ²⁺ and Pd ²⁺ ions can be absorbed but not chemically reduced by a Teflon surface rubbed by PMMA (Phys. Chem. Chem. Phys., 2012, 14, 5551). In response, we show that this treatment does not affect the adsorption of Cu ²⁺ and Pd ²⁺. We reaffirm our statement that Cu ²⁺ and Pd ²⁺ ions can be adsorbed by a Teflon surface only after rubbing with the organic polymers, not before rubbing.

Control over molecular scale electrical properties within nano junctions is demonstrated, utilizing site-directed C₆₀ targeting into protein macromolecules as a doping means. The protein molecules, self-assembled in a miniaturized transistor device, yield robust and reproducible operation. Their device signal is dominated by an active center that inverts affinity upon guest incorporation and thus controls the properties of the entire macromolecule. We show how the leading routes of electron transport can be drawn, spatially and energetically, on the molecular level and, in particular, how the dopant effect is dictated by its "strategic" binding site. Our findings propose the extension of microelectronic methodologies to the nanometer scale and further present a promising platform for ex situ studies of biochemical processes.

It has recently been reported that Teflon and polyethylene (PE) if rubbed by polymethylmethacrylate (PMMA) or Nylon as well as non-rubbed PMMA and Nylon induce "redox" reactions, including those of the reduction of Pd ⁺² and Cu ⁺² ions. On this basis, it was deduced that these dielectric materials may hold ≅10 ¹³-10 ¹⁴ of "hidden" electrons cm ^-2, a value at least three orders of magnitude higher than the charge that a dielectric surface can accumulate without being discharged in air. The "hidden" electrons were termed "cryptoelectrons". In variance to these reports, we offer here an alternative interpretation. Our model is supported by X-ray photoelectron spectroscopy, contact angle and vibrating electrode (modified Kelvin probe) measurements performed on representative examples. Rubbing of the polymers was found to transfer polymer fragments between the rubbed surfaces altering their physical properties. The transferred polymer fragments promote adsorption of Cu ²⁺ and Pd ²⁺ ions. It was found that Teflon and PE rubbed with PMMA and Nylon, and non-rubbed PMMA and non-rubbed Nylon do not induce "redox" reactions of Cu ²⁺ and Pd ²⁺ ions but adsorb these ions on their surfaces. Furthermore, the earlier reported reduction of Pd ²⁺ to Pd ⁰ by electrons, as detected by catalytic activity of Pd ⁰ in a Cu-plating bath, can be alternatively explained by reduction of adsorbed Pd ²⁺ by the reducing agents of the bath itself. Based on these findings, we support the hypothesis that charging of dielectric polymers is due to ions or free radicals rather than electrons and there is no evidence to invoke a hypothesis of "cryptoelectrons".

We report near-perfect transfer of the electrical properties of oxide-free Si surface, modified by a molecular monolayer, to the interface of a junction made with that modified Si surface. Such behavior is highly unusual for a covalent, narrow bandgap semiconductor, such as Si. Short, ambient atmosphere, room temperature treatment of oxide-free Si(100) in hydroquinone (HQ)/alkyl alcohol solutions, fully passivates the Si surface, while allowing controlled change of the resulting surface potential. The junctions formed, upon contacting such surfaces with Hg, a metal that does not chemically interact with Si, follow the Schottky-Mott model for metal-semiconductor junctions closer than ever for Si-based junctions. Two examples of such ideal behavior are demonstrated: a) Tuning the molecular surface dipole over 400 mV, with only negligible band bending, by changing the alkyl chain length. Because of the excellent passivation this yields junctions with Hg with barrier heights that follow the change in the Si effective electron affinity nearly ideally. b) HQ/methanol passivation of Si is accompanied by a large surface dipole, which suffices, as interface dipole, to drive the Si into strong inversion as shown experimentally via its photovoltaic effect. With only ∼0.3 nm molecular interlayer between the metal and the Si, our results proves that it is passivation and prevention of metal-semiconductor interactions that allow ideal metal-semiconductor junction behavior, rather than an insulating transport barrier.

A procedure for the production of metal-coated quasi-amorphous pyroelectric thin SrTiO ₃ films is described. The films are grown in a modified 306 Edwards magnetron sputtering setup under controlled thermal conditions and stabilized high-accuracy vacuum and gas pressure conditions. Three-layer 200 nm metal-100 nm SrTiO ₃-100 nm metal films are studied. The pyroelectric nature of the electric response of these films to heating is directly established, since metallic contacts made from the same material do not distort the measurement results, which excludes the effect of a contact potential difference on the measurement results.

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.

The presence of water at the surface of sputtered thin films of Ce _0.8Gd_0.2O_1.9, which display elastic anomalies as a function of thermal treatment, and of stoichiometric CeO₂ films, which do not, was monitored using X-ray photoelectron spectroscopy. We find that considerably more water is strongly bound at the surface of the Ce _0.8Gd_0.2O_1.9 films than of the CeO₂ films. This supports the theoretical prediction that water binds preferentially at the oxygen vacancy sites. In addition, all films were treated according to the protocol which has been shown to produce inelastic behavior in the doped films: annealed at 500 °C, exposed to ambient atmosphere for one month and then heated to 250 °C in ultra-high vacuum. Neither the Ce _0.8Gd_0.2O_1.9 nor the CeO₂ films show any change in the amount of adsorbed water. We therefore conclude that changes in the amount of surface adsorbed water do not play a role in the elastic anomalies observed as a function of thermal treatment of Ce_0.8Gd _0.2O_1.9 films.

Polymers offer the advantage that they may independently combine desirable supramolecular structure with useful local monomeric properties to yield optimal performance of different tasks. Here we utilise the remarkable lubricating properties both of dense polymer brushes, and of hydration sheaths about charges via the emerging paradigm of hydration lubrication, to design a grafted-from polyzwitterionic brush system, where each of the monomers has a structure similar to the highly-hydrated phosphorylcholine headgroups of phosphatidylcholine lipids. Such polyzwitterions are grown from a macroinitiator coating the substrate (mica) surface using atom transfer radical polymerisation (ATRP) of 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) to form exceptionally robust poly(MPC) brushes. We have characterized these brush layers via X-ray reflectometry, X-ray photoelectron spectroscopy, surface forces measurements and atomic force microscopy. Such brushes, designed to optimise their lubrication properties, are indeed found to provide state of the art boundary lubrication, achieving friction coefficients as low as 0.0004 at pressures up to 75 atmospheres over a wide range of sliding velocities. Such low friction is comparable with that of articular cartilage in healthy mammalian joints, which represents nature's benchmark for boundary lubrication in living organisms, and suggests that hydration lubrication plays a major role in reducing friction in biological systems.

A nanostructure, being either an Inorganic Fullerene-like (IF) nanostructure or an Inorganic Nanotube (INT), A1-x-Bx-chalcogenide are described. A being a metal or transition metal or an alloy of metals and/or transition metals, B being a metal or transition metal B different from that of A and x being ≤0.3. A process for their manufacture and their use for modifying the electronic character of A-chalcogenide are described.

ZnO nanorod films are increasingly used as an alternative to nanoporous TiO₂ as the electron conductor in semiconductor-sensitized nanoporous solar cells (SSSCs). Semiconductor light absorbers deposited onto ZnO by chemical bath deposition (CBD) tend to form poorly covering deposits of isolated clusters. We show how a short solution pretreatment of the ZnO nanorod films with a sulfide solution, which forms a thin ZnS film at the ZnO surface, results in a very large improvement of the coverage and uniformity of the CBD semiconductor coatings. These composite films give large improvements in solar cell efficiency. Several factors can be responsible for this improvement: better coverage of the ZnO nanorods, less shorting between hole conductor and conducting substrate, and the formation of a ZnS buffer layer.

WS₂ belongs to a family of layered metal dichalcogenide compounds that are known to form cylindrical (inorganic nanotubes-INT) and polyhedral nanostructures - onion or nested fullerene-like (IF) particles. The outermost layers of these IF nanoparticles can be peeled under shear stress, thus IF nanoparticles have been studied for their use as solid lubricants. However, the IF nanoparticles tend to agglomerate, presumably because of surface structural defects induced by elastic strain and curvature, a fact that has a deleterious effect on their tribological properties. In the present work, chemical modification of the IF-WS₂ surface with alkyl-silane molecules is reported. The surface-modified IF nanoparticles display improved dispersion in oil-based suspensions. The alkyl-silane coating reduces the IF-WS₂ nanoparticles' tendency to agglomerate and consequently improves the long-term tribological behavior of oil formulated with the IF additive.

In this paper we present a new problem, the fast set intersection problem, which is to preprocess a collection of sets in order to efficiently report the intersection of any two sets in the collection. In addition we suggest new solutions for the two-dimensional substring indexing problem and the document listing problem for two patterns by reduction to the fast set intersection problem.

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.

We report on electronic transport measurements through dense monolayers of CH ₃(CH ₂) _nPO ₃H ₂ molecules of varying chain lengths, with a strong and stable bond through the phosphonic acid end group to a GaAs surface and a Hg top contact. The monolayers maintain their high quality during and after the electrical measurements. Analyses of the electronic transport measurements of junctions, and of UV and inverse photoemission spectroscopy data on band alignments of free surfaces, yield insight about the electrical transport mechanism. Transport characteristics for n-GaAs junctions at low forward bias are identical for different chain lengths, a strong indication of high-quality monolayers. Tunneling barrier and carrier effective mass values for n- and p-GaAs samples were deduced from the transport data. In this way we find a tunneling barrier for n-GaAs of 1.3 eV, while UPS data for the lowest unoccupied system orbital (LUSO) point to a 2.4 eV barrier. This discrepancy can be understood by invoking states, closer to the Fermi level than the LUSO state, that contribute to charge transport. Such states lead to a manifold of transitions, each having a different probability, both because of differences in the tunnel barrier and because of differences in density of these interface-induced states; i.e., the single barrier, deduced from J-V measurements, is an effective value only.

We point to the ability of noncontact measurements of electron transport via self-assembled monolayers to provide chemical, Å-resolved information about the underlying molecule. A conceptual framework is presented to model a current flow of soft electrons through a molecular monolayer to a substrate and explore the information content of this and other emerging noncontact measurements. A numerical scheme is developed where advantage is taken of the split-operator formalism to propagate the incident electronic wave function over a suitable periodic potential energy surface representing the self-assembled monolayer. The (experimentally observable) potential difference introduced by the transmitted electrons is extracted from the time-averaged electron density using the Poisson equation of classical electrostatics.

The preparation of alkoxy monolayers on oxide-free Si (100) and on electronic current transport measurements through junctions that are made up of these monolayers, sandwiched between Si as one electrode and a Hg drop as the other electrode, was reported. Based on the polarized IR spectra, the average tilt angles of the alkoxy and alkyl chains with respect to the surface normal are found to be very similar at 28° and 27° respectively. The monolayer thickness value for the OC12 layer, calculated using an inelastic mean free path of 3.3 nm, is 15∓2Å. The atomic force microscopy (AFM) and contact angle measurements indicate that neither binding chemistry nor molecular length alter the monlayer density. The X-ray photoelectron spectroscopy (XPS) binding energy of the C atom closets to the Si is lower than that of the C atom of the alkyl backbone.

The temporal chemical changes occurring at the surface of fullerene-like (IF) nanoparticles of WS₂ were investigated using X-ray photo-electron spectroscopy (XPS) and compared to those of bulk powder (2H) of the same material. It is possible to follow the long term (surface oxidation and carbonization) occurring at defects on the outermost surface (0001) layer of the fullerene-like nanoparticles. Similar but perhaps more distinctive changes are observed on the prismatic (hk0) surfaces of the 2H powder. Vacuum annealing is shown to remove most of these changes and bring the surface close to its stoichiometric composition. In accordance with previous measurements, further evidence is obtained for the existence of water molecules which are entrapped in the hollow core and interstitial defects of the fullerene-like nanoparticles during the synthesis. They are also shown to be removed by the vacuum annealing process. Chemically resolved electrical measurements (CREM) in the XPS show that the vacuum annealed IF samples become more intrinsic. Finally, tribological measurements show that the vacuum annealed IF samples perform better as an additive to oil than the non-annealed IF samples and the bulk (2H) platelets powder.

A quantum-mechanical scattering theory for relativistic, highly focused electron beams in the vacuum near nanoscopic platelets is presented, revealing an excitation mechanism due to the electron wave scattering from the platelet edges. Radiative electromagnetic excitations within the light cone are shown to arise, allowed by the breakdown of momentum conservation along the beam axis in the inelastic-scattering process. Calculated for metallic (silver and gold) and insulating (SiO2 and MgO) nanoplatelets, radiative features are revealed above the main surface-plasmon-polariton peak, and dramatic enhancements in the electron-energy-loss probability at gaps of the "classical" spectra are found. The corresponding radiation should be detectable in the vacuum far-field zone, with e beams exploited as sensitive "tip detectors" of electronically excited nanostructures.

WS₂ inorganic fullerene-like (IF) nanoparticles were subjected to intercalation with potassium, sodium, and rubidium atoms in heated sealed ampules. The product of the intercalation process was not pure and was composed of both intercalated and nonintercalated phases. X-ray diffraction measurements under inert conditions of the intercalated powders showed that the interlayer expansion was correlated with the alkali metal radius. Small increase of the a-axis was observed as well and was explained on the grounds of the WS₂ band structure. The XPS analysis of the rubidium intercalated material showed a rise in the Fermi energy as a result of the intercalation, endowing the originally p-type nanoparticles an n-type character.

Investigations of charge transport mechanisms in thin CdSe nanoparticles films deposited on silicon, using surface photovoltage and chemically resolved electrical measurements, reveal a strongly nonlinear optical response and a negative differential resistance. Here, both phenomena are rationalized within a phenomenological model consisting of two spatially separated types of traps, one related to the CdSe nanoparticles and the other to the nanoparticle/ substrate interface. The model successfully explains a broad range of both old and new experimental observations and is used as a numerical framework for showing how the interplay between hole and electron processes dominates the photoelectrical properties of nanoparticle films.

Secondary electron emission (SEE) is a major player in surface charging during X-ray photoelectron spectroscopy (XPS); its characteristics and applicability as a current source for electrical measurements are studied. We employ sample biasing and a top retarding grid to control the photoelectron current, and further compare their I-V characteristics with direct spectroscopy of the secondary electrons. Using silica-coated gold substrates, the effect of sample work function on the emitted secondary electrons is shown and fine control over the surface potential gradients, in the range of 10-100 meV, is achieved. XPS-based chemically resolved electrical measurements (CREM) can thus be extended to the positive current regime.

IF-MO_1-xNb_xS₂ nanoparticles have been synthesized by a vapor-phase reaction involving the respective metal halides with H₂S. The IF-MO_1-xNb_xS₂ nanoparticles, containing up to 25% Nb, were characterized by a variety of experimental techniques. Analysis of the powder X-ray powder diffraction, X-ray photoelectron spectroscopy, and different electron microscopy techniques shows that the majority of the Nb atoms are organized as nanosheets of NbS₂ within the MoS₂ host lattice. Most of the remaining Nb atoms (3%) are interspersed individually and randomly in the MoS₂ host lattice. Very few Nb atoms, if any, are intercalated between the MoS₂ layers. A sub-nanometer film of niobium oxide seems to encoat the majority of the nanoparticles. X-ray photoelectron spectroscopy in the chemically resolved electrical measurement mode (CREM) and scanning probe microscopy measurements of individual nanoparticles show that the mixed IF nanoparticles are metallic independent of the substitution pattern of the Nb atoms in the lattice of MoS₂ (whereas unsubstituted IF-MoS₂ nanoparticles are semiconducting). Furthermore the IF-MO_1-xNb_xS₂ nanoparticles are found to exhibit interesting single electron tunneling effects at low temperatures.

Electron irradiation can alter electronic charge transport through Si-CH₂(CH₂)₁₂-CH₃//Hg molecular junctions. Applying UPS, XPS, Auger, NEXAFS, and electrical transport measurements, we show that irradiation induces defects, most likely C=C bonds and C-C cross-links, which introduce new electronic states into the HOMO-LUMO gap of the alkyl chains, and, hence, effectively dope these layers. We demonstrate a 1-2 order of magnitude enhancement in current, clearly distinguishable from that of defects in as-prepared layers.

A model of structural transformations of amorphous into quasi-amorphous BaTiO₃ is suggested. The model is based on previously published data and on X-ray photoelectron spectroscopy data presented in the current report Both amorphous and quasi-amorphous phases of BaTiO₃ are made up of a network of slightly distorted TiO₆ octahedra connected in three different ways: by apices (akin to perovskite), edges, and faces. Ba ions in these phases are located in the voids between the octahedra, which is a nonperovskite environment. These data also suggest that Ba ions compensate electrical-charge imbalance incurred by randomly connected octahedra and, thereby, stabilize the TiO₆ network. Upon heating, the edge-to-edge and face-to-face connections between TiO₆ octahedra are severed and then reconnected via apices. Severing the connections between TiO₆ octahedra requires a volume increase, suppression of which keeps some of the edge-to-edge and face-to-face connections intact. Transformation of the amorphous thin films into the quasi-amorphous phase occurs during pulling through a steep temperature gradient. During this process, the volume increase is inhomogeneous and causes both highly anisotropic strain and a strain gradient. The strain gradient favors breaking those connections, which aligns the distorted TiO₆ octahedra along the direction of the gradient. As a result, the structure becomes not only anisotropic and non-centrosymmetric, but also acquires macroscopic polarization. Other compounds may also form a quasi-amorphous phase, providing that they satisfy the set of conditions derived from the suggested model.

Chemical bath deposited PbSe films were subjected to postdeposition treatment with aqueous (typically 0.25-0.5 M) KOH. For films deposited using a citrate complex, this treatment resulted in dissolution of surface lead oxides (seen from XPS and EXAFS measurements) and growth of the nanocrystals (from ca. 5 to as much as 20 nm, measured by XRD and TEM) by an Ostwald ripening mechanism and formation of a porous network. For films deposited using KOH-complexed Pb, this growth did not occur. The latter films are made up of PbSe crystals (ca. 4 nm) embedded in an amorphous matrix of lead oxide. Successful etching of the crystallite surface passivation is found to be critical for the growth progress. While the KOH treatment removed most of this matrix, the individual crystals of PbSe still remained passivated with a surface where Pb was apparently bonded to both O and Se. With use of a concentrated KOH solution (3 M) for long periods of time (> 1 h), this surface could be removed and crystal growth occurred to give a network of PbSe crystals several tens of nanometers in size. This study, besides explaining the very different chemical behaviors of the two types of PbSe films, demonstrates the important role of what appear to be small differences in surface chemistries in determining the chemical properties of nanocrystals.

Light-induced chemically resolved electrical measurements (CREM) under controlled electrical conditions are used to study photovoltaic effects at selected regions in nanocrystalline CdSe-based films. The method, based on X-ray photoelectron spectroscopy (XPS), possesses unique capabilities for exploring charge trapping and charge transport mechanisms, combining spectrally filtered input signals with photocurrent detection and with a powerful, site-selective, photovoltage probe.

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.

A method and device are presented for measuring the electrical properties of a specimen. The specimen is excited with high energy radiation to cause emission of internal charged particles from the specimen. Electrical power is supplied to a circuit, that is formed by the specimen and any added component connected to a back contact of the specimen. The electric power supply includes at least one of the following: irradiating the circuit with low energy charged particles; subjecting the circuit to an external field of the kind affecting the flux of emitted internal charged particles, and supplying a bias voltage to the back contact of the specimen. During the power supply to the specimen, at least one of the following is carried out: an electric current through the specimen is measured, and the emitted charged particles are analyzed versus their energy (using a contactless voltmeter) which provides local potential values at chemical entities of the specimen. This technique enables determination of rich, chemically resolved, electrical properties of a specimen, such as IV characteristic, and/or evaluation of a work function characteristic, and/or characterization of electric leakage or breakdown conditions of the sample, and/or characterization of accumulation of charge within at least one region of the sample, and/or chemically resolved photovoltaic characteristics (photovoltage and/or photocurrent) of the sample.

A C₃-symmetric tridentate hexahydroxamate ligand molecule was specially synthesized and used for coordination self-assembly of branched multilayers on Au surfaces precoated with a self-assembled monolayer (SAM) of ligand anchors. Layer-by-layer (LbL) growth of multilayers via metal-organic coordination using Zr⁴⁺ ions proceeds with high regularity, adding one molecular layer in each step, as shown by ellipsometry, wettability, UV-vis spectroscopy, and atomic force microscopy (AFM). The branched multilayer films display improved stiffness, as well as a unique defect self-repair capability, attributed to cross-linking in the layers and lateral expansion over defects during multilayer growth. Transmetalation, i.e., exposure of Zr ⁴⁺-based assemblies to Hf⁴⁺ ions, was used to evaluate the cross-linking. Conductive atomic force microscopy (AFM) was used to probe the electrical properties of the multilayers, revealing excellent dielectric behavior. The special properties of the branched layers were emphasized by comparison with analogous multilayers prepared similarly using linear (tetrahydroxamate) ligand molecules. The process of defect annihilation by bridging over defective areas, attributed to lateral expansion via the excess bishydroxamate groups, was demonstrated by introduction of artificial defects in the anchor monolayer, followed by assembly of two layers of either the linear or the branched molecule. Analysis of selective binding of Au nanoparticles (NPs) to unblocked defects emphasized the superior repair mechanism in the branched layers with respect to the linear ones.

The charge redistribution that occurs within dipolar molecules as they self-assemble into organized organic monolayer films has been studied. The extent of charge transfer is probed by work function measurements, using low-energy photoelectron spectroscopy (LEPS), contact potential difference (CPD), and X-ray photoelectron spectroscopy (XPS), with the latter providing fine details about the internal charge distribution along the molecule. In addition, two-photon photoelectron spectroscopy is applied to investigate the electronic structure of the adsorbed layers. We show that charge transfer acts to reduce the dipole-dipole interaction between the molecules but may either decrease or increase the molecule-to-surface dipole moment.

Diodes made by (indirectly) evaporating Au on a monolayer of molecules that are adsorbed chemically onto GaAs, via either disulfide or dicarboxylate groups, show roughly linear but opposite dependence of their effective barrier height on the dipole moment of the molecules. We explain this by Au-molecule (electrical) interactions not only with the exposed end groups of the molecule but also with its binding groups. We arrive at this conclusion by characterizing the interface by in situ UPS-XPS, ex situ XPS, TOF-SIMS, and Kelvin probe measurements, by scanning microscopy of the surfaces, and by current-voltage measurements of the devices. While there is a very limited interaction of Au with the dicarboxylic binding groups, there is a much stronger interaction with the disulfide groups. We suggest that these very different interactions lead to different (growth) morphologies of the evaporated gold layer, resulting in opposite effects of the molecular dipole on the junction barrier height.

If a thin film (tens of nm) of CdSe quantum dots (4 nm diameter) is deposited by chemical bath deposition onto various substrates, the films, although essentially intrinsic, behave as if they were n-type with respect to charge separation. However, films deposited under certain deposition conditions on Si (both n⁺- and p⁺-type) behave as if they were p-type. In this case, we show that it is possible to switch this p-type photoresponse by either light illumination intensity or injection of electrons from an external filament. Using both surface photovoltage spectroscopy and a novel adaptation of X-ray photoelectron spectroscopy, we show how this behavior results from a Cd(OH)₂ layer adsorbed at the Si surface at the beginning of the deposition. This response is explained by a competition between a high concentration of relatively shallow hole traps in the CdSe and a lower concentration of deeper electron traps in the Cd(OH)₂. The relative occupancies of these traps determine the fields in the film and their response to external parameters.

A new series of stilbene-based chromophores have been used to prepare structurally related siloxane-based monolayers in order to determine which factors control the intermolecular chromophore-chromophore interactions in the solid state. The reaction of chromophore precursors 4-styrylpyridine (1), 4-[2-(4-bromophenyl)-vinyl]-pyridine (2), 4-(2-naphthalen-1-ylvinyl)-pyridine (3), 4-(2-anthracen-9-ylvinyl)-pyridine (4), and 4- (2-pyren-2-ylvinyl)-pyridine (5) with excess 3-iodo-n-propyl-1-trimethoxysilane resulted in the corresponding salts 6-10 in quantitative yield. The assembly of chromophores 6-10 on hydrophilic substrates from solution resulted in the formation of densely packed monolayers with a film thickness of similar to1 nm. The average chromophore density (similar to1 chromophore/50 Angstrom(2)) is well within the range that allows pi-pi stacking to occur. Transmission UV-vis spectroscopy of the siloxane-based films shows that the intermolecular interactions are a function of the aryl groups (e.g., phenyl, bromophenyl, naphthalene, anthracene, and pyrene). Relatively weak electronic interactions occur between the surface-bound chromophores 6, 7, and 10, whereas strong electronic interactions occur between surface-bound chromophores 8 and 9. The series of monolayers on sodium lime glass and polished silicon is characterized by a combination of physicochemical methods including X-ray photoelectron spectroscopy, advancing aqueous contact angle measurements, optical spectroscopy, atomic force microscopy, and synchrotron X-ray reflectivity.

Various metallabenzene complexes, analogues of benzene where one CH unit has been replaced by an organometallic fragment, have been reported in the literature. A detailed theoretical investigation on the chemistry of these complexes is presented here. This includes an evaluation of their aromaticity, the mechanisms of formation of osmium, iridium, and platinum metallabenzene complexes, and one intriguing aspect of their chemistry, the formation of cyclopentadienyl (Cp) complexes. X-ray photoelectron spectroscopy (XPS) measurements on two osmabenzene examples are also presented. In addition, diffuse functions for use with the SDD and SDB-cc-pVDZ basis set-RECP combinations are presented for the transition metals.

Pronounced surface diffusion is observed during x-ray photoelectron spectroscopy measurements of 2H platelets and inorganic fullerene-like (IF) MS2 (M=WMo) powders, intercalated with alkaline (A=K,Na) elements. Using controlled surface charging the intercalants migrate towards the surface, where they oxidize. This dry deintercalation is controllable via external charging parameters, yet showing that internal chemical and structural parameters play an important role in the process. Diffusion rates out of 214 matrixes are generally higher than in corresponding IF samples. Clear differences are also found between Mo and W-based systems. Application of this approach into surface modification and processing is, proposed. (C) 2003 American Vacuum Society.

A composite surface comprising evenly distributed silicon oxide islands on a gold substrate is described. The composite surface is prepared by evaporation of a thin (50 nm) gold layer on oxide-free (H-passivated) silicon, followed by thermal diffusion of Si through the Au layer, gradually forming islands of SiO₂ on the Au surface. The rate of Si diffusion through the Au, and hence, the rate of SiO₂ island formation, is controlled by the annealing temperature, the Si crystallographic face, the Au film thickness, and the contacting atmosphere. The Au-SiO₂ composite surfaces can be used in applications requiring substrates patterned on a mesoscopic scale, while exposing two chemically dissimilar phases. One such application is shown here, namely, the formation of thiol-silane monolayers, for which the distribution of the different molecules in the resultant monolayer is determined by the substrate composition. The XPS controlled surface charging (CSC) method is used to establish a site-selective adsorption. The SiO₂ islands are found to be rather labile, shifting and aggregating upon self-assembly of alkanethiol molecules on the Au exposed areas. Pretreatment of the islands with a long-chain silane stabilizes the morphology.

CdS quantum particles (QPs) assembled at predetermined distances from a gold substrate are prepared within a Langmuir-Blodgett film that forms an organic host matrix. The system is characterized by controlled surface charging (CSC) in X-ray photoelectron spectroscopy (XPS) and complementary methods, successfully resolving the discrete QP layers. A quantitative study of substrate-QPs charge-transfer channels is provided by laser-intensity dependent contact potential difference (CPD) measurements. The extracted electron-transfer rate constants exhibit marked differences in electron transfer from the film toward the substrate versus the backward process. The charging of the hybrid film was found to be either negative or positive depending on the intensity of the laser that photoexcites the QPs.

Pronounced electromagnetic quantum-size effects are observed in directional near-field electron energy loss spectroscopy of rectangular nanocrystals. A theoretical analysis, exploiting a unique coincidence of all relevant length parameters in this spectroscopy and the momentum space filtering imposed by the exponentially decaying electron-plasmon interaction in the vacuum, reveals strong sensitivity of the EEL signal to the size and shape of the nanocrystal. The breakdown of momentum conservation along the beam direction, associated with the finite size of the nanocrystal, drastically alters the EEL signal pattern.

Elliptically shaped (Pb_1-xCd_x)S nanoparticles (NPs) of average size 2.3 × 2.9 nm (minor axis × major axis) have been prepared via reaction of a solid [oligo(p-phenylene-ethynylene) dicarboxylate]Pb_0.9Cd_0.1 salt matrix, with gaseous H₂S. A significantly long emission lifetime, with multi-exponential behavior, is detected in time-resolved photoluminescence measurements, substantially different from the decay patterns of pure PbS and CdS NPs within the same organic matrix. Evidence for the co-existence of Cd and Pb within the same particle is provided by light-induced X-ray photoelectron spectroscopy.

Layered metal disulfides-MS₂ (M = Mo, W) in the form of fullerene-like nanoparticles and in the form of platelets (crystallites of the 2H polytype) have been intercalated by exposure to alkali metal (potassium and sodium) vapor using a two-zone transport method. The composition of the intercalated systems was established using X-ray energy dispersive spectrometer and X-ray photoelectron spectroscopy (XPS). The alkali metal concentration in the host lattice was found to depend on the kind of sample and the experimental conditions. Furthermore, an inhomogeneity of the intercalated samples was observed. The product consisted of both nonintercalated and intercalated phases. X-ray diffraction analysis and transmission electron microscopy of the samples, which were not exposed to the ambient atmosphere, showed that they suffered little change in their lattice parameters. On the other hand, after exposure to ambient atmosphere, substantial increase in the interplanar spacing (3-5 Å) was observed for the intercalated phases. Insertion of one to two water molecules per intercalated metal atom was suggested as a possible explanation for this large expansion along the c-axis. Deintercalation of the hydrated alkali atoms and restacking of the MS₂ layers was observed in all the samples after prolonged exposure to the atmosphere. Electric field induced deintercalation of the alkali metal atoms from the host lattice was also observed by means of the XPS technique. Magnetic moment measurements for all the samples indicate a diamagnetic to paramagnetic transition after intercalation. Measurements of the transport properties reveal a semiconductor to metal transition for the heavily K intercalated 2H-MOS₂. Other samples show several orders of magnitude decrease in resistivity and two- to five-fold decrease in activation energies upon intercalation. These modifications are believed to occur via charge transfer from the alkali metal to the conduction band of the host lattice. Recovery of the pristine compound properties (diamagnetism and semiconductivity) was observed as a result of deintercalation.

Composite materials of quantum particles (Q-particles) arranged in layers within crystalline powders of π-conjugated, rodlike dicarboxylic acids are reported. The synthesis of the composites, either as three-dimensional crystals or as thin films at the air-water interface, comprises a two-step process: 1) The preparation of the Cd salts 6(Cd), 8(Cd) or Pb salts 6(Pb), 8(Pb) of the oligo(p-phenyleneethynylene)dicarboxylic acids 6(H), 8(H), in which the metal ions are arranged in ribbons and are separated by the long axis of the organic molecules, as demonstrated by X-ray powder diffraction analysis of the solids and grazing incidence X-ray diffraction analysis of the films on water. 2) Topotactic solid/gas reaction of these salts with H₂S to convert the metal ions into Q-particles of CdS or PbS embedded in the organic matrix that consists of the acids 6 (H) and 8 (H). These hybrid materials have been characterized by X-ray photoelectron spectroscopy and transmission electron microscopy.

Probing the structure of material layers just a few nanometres thick requires analytical techniques with high depth sensitivity. X-ray photoelectron spectroscopy(1) (XPS) provides one such method, but obtaining vertically resolved structural information from the raw data is not straightforward. There are several XPS depth-profiling methods, including ion etching(2), angle-resolved XPS (ref. 2) and Tougaard's approach(3), but all suffer various limitations(2-5). Here we report a simple, non-destructive XPS depth-profiling method that yields accurate depth information with nanometre resolution. We demonstrate the technique using self-assembled multilayers on gold surfaces; the former contain 'marker' monolayers that have been inserted at predetermined depths. A controllable potential gradient is established vertically through the sample by charging the surface of the dielectric overlayer with an electron flood gun. The local potential is probed by measuring XPS line shifts, which correlate directly with the vertical position of atoms. We term the method 'controlled surface charging', and expect it to be generally applicable to a large variety of mesoscopic heterostructures.

Oriented crystalline monolayers, ~ 14 Å thick, of a 2 x 2 Ag⁺ grid complex, self-assembled at the air-solution interface starting from an waterinsoluble ligand 3,6-bis[2-(6-phenylpyridine)]pyridazine spread on silver-ion-containing solutions, were examined by grazing-incidence X-ray diffraction and specular X-ray reflectivity using synchrotron radiation. The monolayer structure was refined, including a determination of the positions of the counterions, with the SHELX-97 computer program. The monolayers were transferred from the interface onto various solid supports and visualized by scanning force microscopy, and characterized by X-ray photoelectron spectroscopy in terms of molecular structure. On surface compression, the initial selfassembled monolayer undergoes a transition to a crystalline bilayer in which the two layers, almost retaining the original arrangement, are in registry. Such a phase transition is of relevance to the understanding of crystal nucleation.

A method is described for the preparation of hybrid organic/inorganic structures where the inorganic component comprises semiconductor nanoparticles aligned in periodic layers within three-dimensional (3-D) crystalline powders and Langmuir-Blodgett (LB) films. The preparation process comprises the organization of metal ions in the form of periodic arrays within 3-D crystals or the LB films, followed by a topotactic gas/solid reaction. The method is illustrated for the organization of CdS nanoparticles within alkanoic acids. The order of the nanoparticles is achieved by introducing site directing nucleation centers of Cd thioalkanoates within Cd alkanoates, in the form of solid solutions. The formed particles are attached to the organic matrix via -C(O)S-Cd-S- bonds. The structure of those supramolecular architectures has been characterized by a variety of complementary methods, including transmission electron microscopy (TEM) and electron diffraction (ED), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and other spectroscopic measurements.

Recently, new structural formulations of metal dichalcogenides have resulted in greatly enhanced lubricant properties, placing the mechanisms of lubrication for this class of materials under closer scrutiny. In this work, scanned probe microscope techniques have been used to probe the surface on a nanoscale, comparing friction and adhesion at specific surface sites. In addition, the role of interlayer shear and subsurface defects in moderating friction are examined. (C) 1999 Elsevier Science B.V. All rights reserved.

Li samples were freshly prepared (shearing) and stored (2 days) in dimethyl carbonate (DMC), and ethyl carbonate-diethyl carbonate (EC-DEC), and dry (20 ppm of H₂O) and wet (500 ppm of H₂O) EC-DMC solutions of LiAsF₆ (1 M), and were then studied by X-ray photoelectron spectroscopy (XPS). The XPS analysis, including depth profiling of these surface films, appears to be reliable on the qualitative level only, because both the X-ray beam and sputtering should be suspected as being partially destructive to the surface films on lithium. These studies basically confirm previous conclusions obtained by Fourier transform infrared spectroscopy spectroscopic studies of Li surfaces. Surface films formed on Li in alkyl carbonate solutions of LiAsF₆ are comprised of ROCO₂Li, Li₂CO₃, LiF, Li_xAsF_y, and Li oxides. XPS could also detect surface species with Li-C bonds (e.g., LiCH₂CH₂OCO₂Li). When EC is present, its reduction dominates the surface film formation. The presence of water suppresses both solvent and salt anion reduction and enriches the surface films with Li₂CO₃ (because of secondary reactions of water with surface species), LiOH, and Li₂O. These studies also confirm that the surface films formed on Li have a multilayer structure.

A Comment on the Letter by H. Cohen, et al. Phys. Rev. Lett. 80, 782 (1998). The authors of the Letter offer a Reply.

The surface (0.2-0.5 nm) chemical characteristics of coal fly ash (CFA) before and after interaction with Mediterranean deep seawater was studied by X-ray photoelectron spectroscopy (XPS). Significantly lower values of Si, Ca, and S and higher values of Mg and CI were found in the retrieved CFA as compared to fresh CFA. It is suggested that hydrolysis of the oxide matrixes results in an alkaline environment which rapidly leads to several chemical reactions. The two most important are (a) dissolution of the amorphous silicate and the calcium phases and (b) precipitation of Mg(OH)₂-brucite. A depth profile of the retrieved CFA was measured by both line-shape analysis of the XPS spectra and by consecutive cycle of sputtering. The thickness of the brucite layer is estimated to be 1.3 nm.

Self-assembly (SA) of long-chain alkanethiols on copper was studied. Two factors were found to have substantial influence on the SA process: (i) the chemical reactivity of Cu toward substances present in the adsorption solution, particularly the solvent, and (ii) surface pretreatment, which influences the amount of oxide and the surface morphology. Both factors are less important in the case of SA onto gold because of its chemical inertness. Monolayers of octadecanethiol (C18SH) were adsorbed from different solvents (ethanol, toluene, and bicyclohexyl) at various thiol concentrations onto Cu surfaces pretreated in several different ways. The monolayers were characterized by contact-angle measurements, grazing-incidence Fourier transform infrared spectroscopy, and scanning force microscopy. Ethanol, the most common solvent for alkanethiol SA, is found to have a negative effect on monolayer SA apparently because of its chemical reactivity toward copper. With toluene as a solvent, better oriented and more crystalline monolayers are obtained provided that a higher thiol concentration is used to compensate for the higher solubility of thiols in toluene. Treatment of the Cu surface prior to SA is shown to significantly improve the SA by reducing the amount of surface oxide and the surface corrugation. The effect of the solvent is more critical than surface oxidation; hence, high-quality monolayers are formed in the presence of thin oxide layers on Cu surfaces. Superior C18SH monolayers, in terms of organization and crystallinity, are obtained by SA from toluene onto Cu surfaces sputtered-annealed in high vacuum, even when the Cu surface is subjected to short exposure to air before SA.

The tribological properties of textured WS₂, MoS₂ and WSe₂ films, which were prepared using an ultra-thin interlayer of Ni/Cr (van der Waals rheotaxy technique) on quartz substrate, were determined in ambient conditions. Using scanning force microscope adapted for tribological measurements, very low (0.04 and below) friction coefficients and little wear were measured on flat areas of the films. Macroscopic (engineering) tribological measurements, using the reciprocating ball on flat tribometer, exhibit somewhat higher friction coefficients. Compactization of the films under the load and little wear were observed for the films even after a few hundred cycles. X-ray photoelectron spectroscopy of the WS₂ film after the wear experiment confirmed that some oxidation took place within the wear track, but the overall integrity of the film was preserved. These measurements indicate that highly textured films of this kind are promising candidates for tribological coatings, where oil-free lubrication is required.

A novel type of bilayer on a gold surface, based upon metal-ion coordination to hydroxamate moieties, is described, Tailor-made bifunctional ligands containing hydroxamate groups (for metal coordination) and a cyclic disulfide residue (for surface attachment) have been prepared. The bishydroxamate binding site forms 2:1 ligand/metal complexes with octacoordinating metal ions such as Zr-IV Ce-IV, and Ti-IV:the cyclic disulfide moiety anchors the complex to the gold surface, Two routes to bilayer formation are demonstrated: i) a one-step process from preformed 2:1 complexes, and ii) a stepwise process including formation of the ligand monolayers followed by binding of a guest ion and a second layer of ligand molecules. The former approach allows full characterization of the complexes before bilayer assembly, whereas the latter enables construction of either symmetric (identical) or asymmetric (nonidentical) bilayers. Both types of bilayers were characterized by ellipsometry, contact angle, and XPS measurements. Symmetric bilayers obtained by the two processes have similar properties.

A new method for the synthesis of sol-gel films of zirconia-containing nanoparticles of CdS is demonstrated. The method consists of dip-coating of the substrate from a preprepared mother solution containing zirconia tetrapropoxide, cadmium acetate, and ammonium thiocyanate and subsequent annealing of the dried film at elevated temperatures. The films were characterized by optical absorption spectra, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The satellite spectrum of the Cd core levels was analyzed, and thereby the plasmonic structure of the film was resolved. The potential of the process for the fabrication of a class of sol-gel films with semiconductor nanoparticles having optical quality is discussed.

Nanostructured particles of amorphous carbon-activated palladium metallic clusters have been prepared (in situ) at room temperature by ultrasound irradiation of an organometallic precursor, tris-μ-[dibenzylideneacetone]-dipalladium [(φ-CH=CH-CO-CH=CH-φ)₃Pd₂] in mesitylene. Characterization by elemental analysis, transmission electron microscopy, differential scanning calorimetry, X-ray diffraction, X-ray photoelectron spectroscopy, and BET surface area measurements shows that the product powder consists of nanosize particles, agglomerated in clusters of approximately 800 Å. Each particle is found to have a metallic core, covered by a carbonic shell that plays an important role in the stability of the nanoparticles. The catalytic activity in a Heck reaction, in the absence of phosphine ligands, has been demonstrated.

Thin films of metal dichalcogenide compounds with a layered structure, such as MoS₂ (WSe₂), play an important role in a number of technologies, like solid lubrication, experimental photovoltaic cells, etc. Such films usually adopt a type-I texture, in which case the c-axis of the crystallites is parallel to the substrate plane. However, for the aforementioned applications, type-II texture, where the c-axis of the crystallite is perpendicular to the substrate, is required. We have recently demonstrated a novel growth technique (Van der Waals rheotaxy, VdWR) which yields a crystalline film having exclusively type-II texture on amorphous, (quartz) substrate. In the present work superior crystalline, optical and electronic properties of the overlying WSe₂ (WS₂) film together with an improved adhesion of the film to the quartz substrate are obtained by replacing the ultra thin Ni film with a Ni/Cr film.

Electronic excitations of solid C60 have been studied using the reflection EELS technique with a 0.2 eV energy resolution. A comparison with graphite revealed an overall similarity of the spectral gross features, while significant differences were observed in the fine details, mainly at low loss energies. A similar dominant plasmon was observed in C60 at a slightly shifted energy of 23.2 eV. For C60, in contrast to graphite, a clear energy loss onset was found at 1.7 eV and up to 6 eV six loss peaks were clearly observed. It was found that minima in the loss spectrum fit well with reported optical transitions. This observation supports an interpretation which attributes a significant collective character to the loss peaks.