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.

Uranium is a high-value energy element, yet also poses an appreciable environmental burden. The demand for a straightforward, low energy, and environmentally friendly method for encapsulating uranium species can be beneficial for long-term storage of spent uranium fuel and a host of other applications. Leveraging on the low melting point (60 °C) of uranyl nitrate hexahydrate and nanocapillary effect, a uranium compound is entrapped in the hollow core of WS₂ nanotubes. Followingly, the product is reduced at elevated temperatures in a hydrogen atmosphere. Nanocrystalline UO₂ nanoparticles anchor within the WS₂ nanotube lumen are obtained through this procedure. Such methodology can find utilization in the processing of spent nuclear fuel or other highly active radionuclides as well as a fuel for deep space missions. Moreover, the low melting temperatures of different heavy metal-nitrate hydrates, pave the way for their encapsulation within the hollow core of the WS₂ nanotubes, as demonstrated herein.

Recent research on two-dimensional (2D) transition metal dichalcogenides (TMDCs) has led to remarkable discoveries of fundamental phenomena and to device applications with technological potential. Large-scale TMDCs grown by chemical vapor deposition (CVD) are now available at continuously improving quality, but native defects and natural degradation in these materials still present significant challenges. Spectral hysteresis in gate-biased photoluminescence (PL) measurements of WSe₂ further revealed long-term trapping issues of charge carriers in intrinsic defect states. To address these issues, we apply here a two-step treatment with organic molecules, demonstrating the \u201chealing\u201d of native defects in CVD-grown WSe₂ and WS₂ by substituting atomic sulfur into chalcogen vacancies. We uncover that the adsorption of thiols provides only partial defect passivation, even for high adsorption quality, and that thiol adsorption is fundamentally limited in eliminating charge traps. However, as soon as the molecular backbone is trimmed and atomic sulfur is released to the crystal, both bonds of the sulfur are recruited to passivate the divalent defect and the semiconductor quality improves drastically. Time-dependent X-ray photoelectron spectroscopy (XPS) is applied here together with other methods for the characterization of defects, their healing, leading energies and occupation. First-principles calculations support a unified picture of the electronic passivation of sulfur-healed WSe₂ and WS₂. This work provides a simple and efficient method for improving the quality of 2D semiconductors and has the potential to impact device performance even after natural degradation.

Well-defined Zn2GeO4/g-C3N4 nanocomposites with a band alignment of type-I were prepared by the ultrasound-assisted solvent method, starting from g-C3N4 nanosheets and incorporating 0, 10, 20, and 40 wt% of Zn2GeO4. In this study, we have investigated in-depth the photoluminescence emission and photocatalytic activity of these nanocomposites. Our experimental results showed that an increased mass ratio of Zn2GeO4 to g-C3N4 can significantly improve their photoluminescence and photocatalytic responses. Additionally, we have noted that the broadband photoluminescence (PL) emission for these nanocomposites reveals three electronic transitions; the first two well-defined transitions (at ca. 450 nm and 488 nm) can be attributed to π*→ lone pair (LP) and π*→π transitions of g-C3N4, while the single shoulder at ca. 532 nm is due to the oxygen vacancy (Vo) as well as the hybridization of 4s and 4p orbital states in the Zn and Ge belonging to Zn2GeO4. These experimental findings are also supported by theoretical calculations performed under periodic conditions based on the density functional theory (DFT) fragment. The theoretical findings for these nanocomposites suggest a possible strain-induced increase in the Zn-O bond length, as well as a shortening of the Ge-O bond of both tetrahedral [ZnO4] and [GeO4] clusters, respectively. Thus, this disordered structure promotes local polarization and a charge gradient in the Zn2GeO4/g-C3N4 interface that enable an efficient separation and transfer of the photoexcited charges. Finally, theoretical results show a good correlation with our experimental data.

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 (

Metal-free halide perovskites, as a specific category of the perovskite family, have recently emerged as novel semiconductors for organic ferroelectrics and promise the wide chemical diversity of the ABX(3)perovskite structure with mechanical flexibility, light weight, and eco-friendly processing. However, after the initial discovery 17 years ago, there has been no experimental information about their charge transport properties and only one brief mention of their optoelectronic properties. Here, growth of large single crystals of metal-free halide perovskite DABCO-NH4Br3(DABCO =N-N '-diazabicyclo[2.2.2]octonium) is reported together with characterization of their instrinsic optical and electronic properties and demonstration, of metal-free halide perovskite optoelectronics. The results reveal that the crystals have an unusually large semigap of approximate to 16 eV and a specific band nature with the valence band maximum and the conduction band minimum mainly dominated by the halide and DABCO(2+), respectively. The unusually large semigap rationalizes extremely long lifetimes approaching the millisecond regime, leading to very high charge diffusion lengths (tens of mu m). The crystals also exhibit high X-ray attenuation as well as being lightweight. All these properties translate to high-performance X-ray imaging with sensitivity up to 173 mu C Gy(air)(-1)cm(-2). This makes metal-free perovskites novel candidates for the next generation of optoelectronics.

This guide deals with methods to control surface charging during XPS analysis of insulating samples and approaches to extracting useful binding energy information. The guide summarizes the causes of surface charging, how to recognize when it occurs, approaches to minimize charge buildup, and methods used to adjust or correct XPS photoelectron binding energies when charge control systems are used. There are multiple ways to control surface charge buildup during XPS measurements, and examples of systems on advanced XPS instruments are described. There is no single, simple, and foolproof way to extract binding energies on insulating material, but advantages and limitations of several approaches are described. Because of the variety of approaches and limitations of each, it is critical for researchers to accurately describe the procedures that have been applied in research reports and publications.

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.

Carbon-based suspension electrodes are currently intensively investigated for emerging electrochemical systems, such as flow batteries, flow capacitors, and capacitive deionization cells. The main limitation of such electrodes is their low electric conductivity, which is typically orders of magnitude lower than that of traditional static carbon electrodes. Two main categories of suspension electrodes exist: 1) slurry electrodes where particles are not significantly affected by gravity, and 2) fluidized bed electrodes where particles are affected by gravity. We introduce a novel category that we term \u201ccombined\u201d suspension electrodes, which combine dilute slurries and dense fluidized beds. We present experimental measurements of the electrochemical impedance and electric conductivity of two combined electrodes. For one set of materials, the measured electric conductivity of the combined electrode is at least an order of magnitude above the fluidized bed and slurry components alone, demonstrating that a synergetic effect can be achieved when adding dilute slurry to dense fluidized bed. For a second set of materials, results show that the combined electrode conductivity is lower than the slurry component alone, a counter-intuitive result, demonstrating that increasing electrode carbon loading does not always enhance the electric conductivity.

The riddle of anomalous polar behavior of the centrosymmetric crystal of α-glycine is resolved by the discovery of a polar, several hundred nanometer thick hydrated layer, created at the {010} faces during crystal growth. This layer was detected by two independent pyroelectric analytical methods: (i) periodic temperature change technique (Chynoweth) at ambient conditions and (ii) contactless X-ray photoelectron spectroscopy under ultrahigh vacuum. The total polarization of the surface layer is extremely large, yielding ≈1 μC·cm^-2, and is preserved in ultrahigh vacuum, but disappears upon heating to 100 °C. Molecular dynamics simulations corroborate the formation of polar hydrated layers at the sub-microsecond time scale, however with a thickness of only several nanometers, not several hundred. This inconsistency might be reconciled by invoking a three-step nonclassical crystal growth mechanism comprising (i) docking of clusters from the supersaturated solution onto the evolving crystal, (ii) surface recognition and polar induction, and (iii) annealing and dehydration, followed by site-selective recrystallization.

The metal-insulator transition (MIT) properties of correlated oxides thin films, such as VO₂, are dramatically affected by strain induced at the interface with the substrate, which usually changes with deposition thickness. For VO₂ grown on r-cut sapphire, there is a minimum deposition thickness required for a significant MIT to appear, around 60 nm. We show that in these thicker films an interface layer develops, which accompanies the relaxation of film strain and enhanced electronic transition. If these interface dislocations are stable at room temperature, we conjectured, a new route opens to control thickness of VO₂ films by postdeposition thinning of relaxed films, overcoming the need for thickness-dependent strain-engineered substrates. This is possible only if thinning does not alter the films' electronic properties. We find that wet etching in a dilute NaOH solution can effectively thin the VO₂ films, which continue to show a significant MIT, even when etched to 10 nm, for which directly deposited films show nearly no transition. The structural and chemical composition were not modified by the etching, but the grain size and film roughness were, which modified the hysteresis width and magnitude of the MIT resistance change.

Electron energy loss spectroscopy (EELS) in a monochromated transmission electron microscope is applied to probe standing-wave-like cavity modes hybridized with surface plasmon polaritons (SPP) in rectangular submicron slits in a thin gold film. Coupling of hybridized SPP-cavity modes between two adjacent slits is studied by systematically varying the width of the metal bar d that separates the identical slits in a two-slit system. Measurements on two-slit systems with different slit lengths L and fixed width reveal energy shifts and mode splitting of the fundamental SPP cavity mode which can be generally described as a function of a dimensionless scaling parameter L/d. Numerical simulations with the Discontinuous Galerkin Time-Domain (DGTD) method confirm the experimental data and reveal insights into the underlying complex coupling mechanisms.

The present work reports a simple and direct sputtering deposition to form solid state TiO2 vertical bar Ag independent plasmonic solar cells. The independent plasmonic solar cells are based on a Schottky barrier between two materials, TiO2 and Ag. The Ag functions as the absorber generating "hot" electrons, as well as the contact for the solar cell. The Ag sputtering is performed for different durations, to form Ag nanoparticles with a wide size distribution on the surface of rough spray pyrolysis deposited TiO2. Incident photon to current efficiency (IPCE) measurements show photovoltaic activity below the TiO2 bandgap, which is caused by the silver nanoparticles that have a wide plasmonic band, leading to the generation of "hot" electrons. X-ray photoelectron spectroscopy analysis supports the "hot" electron injection mechanism by following the Ag plasmon band and detecting local photovoltages. The measurements show that electrons are formed in the Ag upon illumination and are injected into the TiO2, producing photovoltaic activity. J-V measurements show photocurrents up to 1.18 mA cm(-2) and photovoltages up to 430 mV are achieved, with overall effi ciencies of 0.2%. This is, to our knowledge, the highest performance reported for such independent plasmonic solar cells.

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.

We report on the passivation properties of molecularly modified, oxide-free Si(111) surfaces. The reaction of 1-alcohol with the H-passivated Si(111) surface can follow two possible paths, nucleophilic substitution (S_N) and radical chain reaction (RCR), depending on adsorption conditions. Moderate heating leads to the S_N reaction, whereas with UV irradiation RCR dominates, with S_N as a secondary path. We show that the site-sensitive S_N reaction leads to better electrical passivation, as indicated by smaller surface band bending and a longer lifetime of minority carriers. However, the surface-insensitive RCR reaction leads to more dense monolayers and, therefore, to much better chemical stability, with lasting protection of the Si surface against oxidation. Thus, our study reveals an inherent dissonance between electrical and chemical passivation. Alkoxy monolayers, formed under UV irradiation, benefit, though, from both chemical and electronic passivation because under these conditions both S_N and RCR occur. This is reflected in longer minority carrier lifetimes, lower reverse currents in the dark, and improved photovoltaic performance, over what is obtained if only one of the mechanisms operates. These results show how chemical kinetics and reaction paths impact electronic properties at the device level. It further suggests an approach for effective passivation of other semiconductors.

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.

Electrical transport studies across nm-thick dielectric films can be complicated, and datasets compromised, by local electrical breakdown enhanced by nm-sized features. To avoid this problem we need to know the minimal voltage that causes the enhanced electrical breakdown, a task that usually requires numerous measurements and simulation of which is not trivial. Here we describe and use a model system, using a "floating" gold pad to contact Au nanoparticles, NPs, to simultaneously measure numerous junctions with high aspect ratio NP contacts, with a dielectric film, thus revealing the lowest electrical breakdown voltage of a specific dielectric-nanocontact combination. For a 48 ± 1.5 Å SiO ₂ layer and a ∼7 Å monolayer of organic molecules (to link the Au NPs) we show how the breakdown voltage decreases from 4.5 ± 0.4 V for a flat contact, to 2.4 ± 0.4 V if 5 nm Au NPs are introduced on the surface. The fact that larger Au NPs on the surface do not necessarily result in significantly higher breakdown voltages illustrates the need for combining experiments with model calculations. This combination shows two opposite effects of increasing the particle size, i.e., increase in defect density in the insulator and decrease in electric field strength. Understanding the process then explains why these systems are vulnerable to electrical breakdown as a result of spikes in regular electrical grids. Finally we use XPS-based chemically resolved electrical measurements to confirm that breakdown occurs indeed right below the nm-sized features.

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.

The effect of the surface treatments on the transport properties of a two-dimensional electron gas was studied at the quantum limit. The surface of the Al0.36 Ga0.64 As/GaAs heterostructure was either coated with gold or etched with HCl solution, or etched and then coated by a self-assembled monolayer (SAM) of either phosphonated (ODP-C18 H39 PO3) or thiolated (ODT-C18 H37 S) molecules. The etching process was found to reduce significantly both the mobility and the charge density. This effect was reversed upon sequential adsorption of the phosphonated SAM. We propose fine tuning of the device performance by the flexible chemistry of the assembled molecules, two of them demonstrated here. The results indicate that the surface oxidation does not necessarily play the dominant role in this respect and, in particular, that octadecane phosphonic acid (ODP) can protect the substrate from both oxidation and the formation of a passivating carbon layer. In contrast, octadecanethiol (ODT) is not stable enough and is not effective in eliminating surface states, as a result devices covered with ODT behave like those with etched surfaces.

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.

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.

We use the recently developed chemically resolved electrical measurements (CREM) to sensitively measure hot-electron transport characteristics in thin dielectric layers. By comparing bare gate-oxide layers, SiO2 and SiON, pronounced differences are revealed that are absent from standard contact measurements and from CREM conducted on top metallic pads. The "on pad" and standard measurements obey a similar defect-assisted "Poole-Frenkel" transport, whereas I∼ Vα characterizes the hot-electron transport through the bare overlayer, with a clear thickness dependence of α. These unique CREM features offer useful advantages in gate-oxide characterization.

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 time dependent chemical changes occurring at the surface of inorganic fullerene-like (IF) nanoparticles of WS₂ were investigated using X-ray photoelectron spectroscopy (XPS) and compared to those of bulk powder, 2H-WS₂. It was possible to follow the long term surface oxidation and carbonization occurring at defects on the outermost surface (0001) molecular layers of the inorganic fullerene-like nanoparticles. Vacuum annealing was shown to remove most of these contaminants and bring the surface close to its pristine stoichiometric composition. In accordance with previous measurements, further evidence was obtained for the existence of water molecules, which were entrapped in the hollow core and interstitial defects of the fullerene-like nanoparticles during the synthesis. These water molecules were also shown to be removable by the vacuum annealing process. Chemically resolved electrical measurements (CREM) in the XPS showed that the IF samples had become less p-type after the vacuum annealing. Finally, tribological measurements showed that the vacuum annealed IF samples performed better as an oil additive than the non-annealed IF samples and the 2H-WS₂ 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.

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.

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.

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.

A method for work-function evaluation is proposed, based on recording the shift of x-ray photoelectron signals from a surface irradiated by low-energy electrons. The method is capable of measuring samples with very low conductivity, poor back contacts, and high dielectric constants. The method is also applicable to magnetic materials and can be particularly effective for studies of multilayer and heterogeneous systems.

Coordination self-assembly of bishydroxamate-based metal-organic multilayers on gold employing a layer-by-layer (LbL) approach was investigated. It is shown that the solution chemistry of the participating metal ion has a marked influence on the composition and properties of the multilayers. Use of Ce4+ and particularly zirconium(IV) acetylacetonate (Zr(acac)(4)) solutions in the ion-binding step of multilayer construction leads to multilayers with a near-stoichiometric metal ion-to-ligand ratio, suggesting a structure close to that predicted by a simple coordination self-assembly scheme. On the other hand use of a ZrCl4 solution as the source of metal ions in the multilayer construction leads to a multilayer with greater thickness and a large excess of Zr(IV), evenly distributed between the organic layers. In the latter case, a ratio of ca. 1:2 between the excess Zr and oxygen, as well as long-term Zr4+ binding experiments showing deposition of ZrO2, suggest the formation of a zirconia-type nanophase between the bishydroxamate organic repeat units during multilayer self-assembly. Hence, while the multilayer prepared using Zr(acaC)(4) solution appears to represent a "true" coordination-based structure, the one prepared using ZrCl4 is best described as a composite organic-ceramic multilayer. Composite multilayers prepared in this way display different properties from those of the stoichiometric ones, such as improved dielectric behavior and higher stiffness. Even greater mechanical stability is obtained with multilayers constructed using alternate binding of ZrCl4 and Ce4+. The concept of LbL formation of coordination-based composite organic-ceramic structures may be useful in obtaining nanometer-scale structures with tunable properties.

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.

Noncontact chemically resolved electrical measurements, which are capable of probing selected regions within fine heterostructures, using x-ray photoelectron spectroscopy (XPS) were analyzed. A modified x-ray photoelectron spectrometer was used to measure I-V curves of molecular layers, free of substrate and contact contributions. The use of a high-quality self-assembled monolayer (SAM), NH₂C₂₀Si, on p-Si, for the improvement in the performance of this technique was also discussed. The results indicate the efficient applications of this approach in conductance measurements through molecular layers.

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.

An electromagnetic theory of valence electron excitations by an external electron beam in the near field regions of uniaxial anisotropic nanoplatelets is presented. It is shown that in an interface of high symmetry (i.e., perpendicular to the symmetry axis) only extraordinary waves can be excited by the electron beam, usually as surface-plasmon-polariton modes. However, the anisotropy also allows excited extraordinary waves to propagate as waveguide modes. In an interface of low symmetry a mixture of both waves is inseparably excited. Application is made to directional near-field electron energy loss spectroscopy of uniaxial dielectric nanoplatelets. It is found that all relevant length parameters in this spectroscopy happen to fall in the same range, giving rise to enhanced sensitivity of the electron energy loss (EEL) signal to the size and geometry of the detected nanoparticle. The breakdown of momentum conservation in the electron-plasmon scattering event, associated with the finite size of the platelet along the beam direction, strongly changes the EEL signal pattern.

The transport of low-energy (

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.

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.

A Reply to the Comment by P. M. Echenique, A. Howie, and R. H. Ritchie.

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.

Self-assembled coatings of long chain thiols on nanoparticles of amorphous iron as well as on amorphous Fe₂O₃ were formed. The thermal stability of these coatings was investigated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the particles and by mass spectrometric analysis of the molecules desorbed from the surface by heating the amorphous substrates. The results showed a sharp weight loss centered at about 200 °C for the thiol-coated particles. The mass spectrometric study of the molecules removed from the surface of the amorphous iron revealed the formation of a dialkyl thioether. For the amorphous Fe₂O₃ surfaces, a dialkyl disulfide was removed from the surface. The desorptions from the Fe₂O₃ surface occurred at higher temperatures than those for the amorphous iron. The different mechanisms responsible for these desorption reactions are 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.

This work entails a comparative study of both Li and synthetic graphite electrodes in electrolyte solutions based on ethylene and dimethyl carbonates (EC-DMC) and the impact of the salt used [from the LiAsF₆, LiClO₄, LiPF₆, LiBF₄, and LiN(SO₂CF₃)₂ list]. The presence of some additives in solutions (e.g., Li₂CO₃, CO₂, tributylamine) and the effect of the particle size of the carbon on the electrode's behavior were investigated. The correlation between the surface chemistry, the morphology, and the performance of Li and graphite electrodes was explored using surface sensitive Fourier transform infrared and x-ray and photoelectron spectroscopies, impedance spectroscopy, x-ray diffraction and scanning electron microscopy in conjunction with standard electrochemical techniques. Synthetic graphite anodes could be cycled (Li intercalation-deintercalation) hundreds of times at a capacity close to the optimal (x → 1 in Li_xC₆) in EC-DMC solutions due to the formation of highly stable and passivating surface films in which EC reduction products such as (CH₂OCO₂Li)₂ are the major constituents. The cycling efficiency of Li metal anodes in these solutions, however, is lower than that obtained in ethereal solutions and seems to be too low for Li-metal liquid electrolyte, rechargeable battery application. The connection between the solution composition and the electrode's performance is discussed.

736 Intercalation Compounds MÖ Utilizing equations 1 and 2 with the following parameters (in eV): шр1 = 16.6, шр2 = 6.7, шп1 = 3.0, wn2 = 10.2, ¿i = 6.0, ¿s = 1.0, and D = 4.0 ; a fit to the
E0 = 2KeV spectrum has been obtained (see Fig. 2). Both, N(E) and the second derivative graphs
are shown, in order to account for absolute intensities, as well as fine details. A comparison with
the N(E) curve is made after a background subtraction. NIB > : \ \ 4" (b) E\u201e=2KeV EXPERIMENTAL
CALCULATED E0 = 0.5KeV EXPERIMENTAL CALCULATED 10 20 ENERGY leV) 0 10 20 30