The thermal decomposition of hydroxyl-terminated generation-4 polyamidoamine dendrimer (G4OH) films deposited on Au surfaces has been compared with decomposition of the same dendrimer encapsulating an approximately 40-atom Pt particle (Pt-G4OH). Infrared absorption reflection spectroscopy studies showed that, when the films were heated in air to various temperatures up to 275 degrees C, the disappearance of the amide vibrational modes occurred at lower temperature for the Pt-G4OH film. Dendrimer decomposition was also investigated by thermogravimetric analysis (TGA) in both air and argon atmospheres. For the G4OH dendrimer, complete decomposition was achieved in air at 500 degrees C, while decomposition of the Pt-G4OH dendrimer was completed at 400 degrees C, leaving only platinum metal behind. In a nonoxidizing argon atmosphere, a greater fraction of the G4OH decomposed below 300 degrees C, but all of the dendrimer fragments were not removed until heating above 550 degrees C. In contrast, Pt-G4OH decomposition in argon was similar to that in air, except that decomposition occurred at temperatures approximately 15 degrees C higher. Thermal decomposition of the dendrimer films on Au surfaces was also studied by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) under ultrahigh vacuum conditions. Heating the G4OH films to 250 degrees C during the TPD experiment induced the desorption of large dendrimer fragments at 55, 72, 84, 97, 127, 146, and 261 amu. For the Pt-G4OH films, mass fragments above 98 amu were not observed at any temperature, but much greater intensities for H(2) desorption were detected compared to that of the G4OH film. XPS studies of the G4OH films demonstrated that significant bond breaking in the dendrimer did not occur until temperatures above 250 degrees C and heating to 450 degrees C caused dissociation of C=O, C-O, and C-N bonds. For the Pt-G4OH dendrimer films, carbon-oxygen and carbon-nitrogen bond scission was observed at room temperature, and further decomposition to atomic species occurred after heating to 450 degrees C. All of these results are consistent with the fact that the Pt particles inside the G4OH dendrimer catalyze thermal decomposition, allowing dendrimer decomposition to occur at lower temperatures. However, the Pt particles also catalyze bond scission within the dendrimer fragments so that decomposition of the dendrimer to gaseous hydrogen is the dominant reaction pathway compared to desorption of the larger dendrimer fragments observed in the absence of Pt particles.

As the cornerstone of multiple practical applications, silicon single crystal surfaces have attracted the interest of scientific and engineering communities for several decades. The most recent advances employ the surfaces precovered with a specific functionality to extend into the realm of organic and metal-organic films with well-defined interfaces, to protect the surfaces from oxidation and other contaminations, and to build the components of present and future molecular electronics and sensing devices. This critical review will focus on the reactivity of the selectively terminated Si(100) and Si(111) surfaces. The hydrogen and halogen-terminated surfaces are the most widely used and most heavily reviewed previously, thus only a brief summary will be given here with the emphasis of the most recent thermal approaches to functionalization of hydrogen-terminated silicon. The silicon surfaces precovered with NH(x) functionality are emerging as a very likely candidate both for the production of sharp interfaces and for coadsorption, co-assembly, and potential molecular templating of patterns on single crystalline surfaces. A brief overview of recent advances in achieving control over the hydroxyl-termination of silicon will be given. Some future directions for further development of chemistry, reactivity, and assembly on these surfaces, as well as potential applications, are highlighted in the last section (152 references).

Constant miniaturization of electronic devices and interfaces needed to make them functional requires an understanding of the initial stages of metal growth at the molecular level. The use of metal-organic precursors for metal deposition allows for some control of the deposition process, but the ligands of these precursor molecules often pose substantial contamination problems. One of the ways to alleviate the contamination problem with common copper deposition precursors, such as copper(I) (hexafluoroacetylacetonato) vinyltrimethylsilane, Cu(hfac)VTMS, is a gas-phase reduction with molecular hydrogen. Here we present an alternative method to copper film and nanostructure growth using the well-defined silicon surface. Nearly ideal hydrogen termination of silicon single-crystalline substrates achievable by modern surface modification methods provides a limited supply of a reducing agent at the surface during the initial stages of metal deposition. Spectroscopic evidence shows that the Cu(hfac) fragment is present upon room-temperature adsorption and reacts with H-terminated Si(100) and Si(111) surfaces to deposit metallic copper. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) are used to follow the initial stages of copper nucleation and the formation of copper nanoparticles, and X-ray energy dispersive spectroscopy (XEDS) confirms the presence of hfac fragments on the surfaces of nanoparticles. As the surface hydrogen is consumed, copper nanoparticles are formed; however, this growth stops as the accessible hydrogen is reacted away at room temperature. This reaction sets a reference for using other solid substrates that can act as reducing agents in nanoparticle growth and metal deposition.

In the work described here, the electronic structure of sulfuric acid in water is explored by liquid-jet photoelectron spectroscopy. From the S2p photoelectron spectra of H 2 SO 4 (aq), measured over a large concentration range and aided by previously reported HSO 4 − /SO 4 2and HSO 4 − /H 2 SO 4 concentration ratios in the bulk solution, we obtain detailed electronic structure information of each species. Comparing our results with previous studies on the dissociation of nitric acid, we argue that the solvation structure of H 2 SO 4 (aq) changes around 5−7 M concentration, at which point a dramatic change in both the HSO 4 − photoelectron peak width and binding energy occurs.

We report photoelectron spectroscopy measurements from binary acetonitrile−water solutions, for a wide range of acetonitrile mole fractions (x CH 3 CN = 0.011−0.90) using a liquid microjet. By detecting the nitrogen and carbon 1s photoelectron signal of CH 3 CN from aqueous surface and bulk solution, we quantify CH 3 CN's larger propensity for the solution surface as compared to bulk solution. Quantification of the strong surface adsorption is through determination of the surface mole fraction as a function of bulk solution, x CH 3 CN , from which we estimate the adsorption free energy using the Langmuir adsorption isotherm model. We also discuss alternative approaches to determine the CH 3 CN surface concentration, based on analysis of the relative amount of gas-versus liquid-phase CH 3 CN, obtained from the respective photoelectron signal intensities. Another approach is based on the core-level binding energy shifts between liquid-and gas-phase CH 3 CN, which is sensitive to the change in solution surface potential and thus sensitive to the surface concentration of CH 3 CN. Gibbs free energy of adsorption values are compared with previous literature estimates, and we consider the possibility of CH 3 CN bilayer formation. In addition, we use the observed changes in N 1s and C 1s peak positions to estimate the net molecular surface dipole associated with a complete CH 3 CN surface monolayer, and discuss the implications for orientation of CH 3 CN molecules relative to the solution surface. ■ INTRODUCTIONExperimental molecular-level investigations of the electronic structure of aqueous solutions have recently become possible by using photoelectron (PE) spectroscopy in combination with a liquid microjet either in vacuum 1−3 or at near ambient pressure conditions. 4−6 Studies reported to date are largely comprised of neat liquid water, aqueous solutions of common electrolytes, and low-concentration solutions containing common organic and inorganic solute molecules and ions. 7−21 Typically, PE spectroscopy accesses solute electron binding energies, both lowest ionization energies and core-level energies, the latter being most suited for interpreting differences in solvation configuration at the solution surface or in the bulk of solution. PE spectroscopy can also provide a quantitative measure of solute concentrations across the solution interface, or it can be used to characterize, for instance, chemical equilibria as a function of concentration or pH, both near the top surface region and more deeply into the solution. The possibility to make such a direct comparison between surface and bulk-solution properties is indeed a rather unique feature of PE spectroscopy. The method's variable information depth is due to the strongly energy-dependent electron mean free path, which can be adjusted experimentally by a suitable choice of applied ionization photon energies. 1,22,23 To our knowledge, the present work reports the first PE spectroscopy study of a binary highly volatile solution studied over a wide range of concent...