AVS2011 Session SS1-MoA: Selectivity and Reactivity of Chemisorbed Species

The reactions of C1-C4 aliphatic alcohols over cyclic (WO₃)₃ and (MoO )_n (n ranges from 3 to 6) clusters were studied experimentally and theoretically using temperature-programmed desorption, infrared reflection-absorption spectroscopy, and density functional theory. Three reaction channels, dehydration, dehydrogenation, and condensation, have been identified on (WO₃)₃ clusters while only dehydration and dehydrogenation have been observed on (MoO₃)_n. The desorption temperature of reaction products decreases with increasing alkyl chain length. The lack of a condensation channel on (MoO₃)_n is attributed to the lower reactivity of alcohols with (MoO₃)n as compared to (WO₃)₃ and consequently a negligible concentration of the Mo(VI) centers coordinated with two alkoxy species required for this reaction are formed. DFT calculations provide a detailed explanation for the reactivity and relative selectivity among the reaction channels and W(VI) and Mo(VI) metal centers.

This work was supported by the U.S. Department of Energy Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences, and was performed at EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the U.S. DOE by Battelle Memorial Institute under contract no. DE-AC06-76RLO 1830. Computational resources were provided at EMSL and the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory.

Electron beam induced deposition (EBID) is a direct-write lithographic technique where volatile organometallic precursors are decomposed by a focused electron beam in a low vacuum environment to create metallic nanostructures. As a tool for nanofabrication, EBID offers an attractive and unique combination of capabilities including high spatial resolution and the flexibility to deposit free-standing three-dimensional structures without the need for resist layers. However, a major limitation of EBID is that nanostructures deposited from organometallic precursors typically possess unacceptable levels of organic contamination. To overcome this limitation it is crucial to develop a more detailed and fundamental understanding of how adsorbed organometallics undergo electron stimulated decomposition. Using a selected suite of organometallic precursors used in EBID (CH₃CpPt(CH₃)₃), Pt(PF₃)₄ and W(CO)₆) I will describe how a surface science approach has been used to provide mechanistic and kinetic insights into EBID and to identify key structure-reactivity relationships. Central to our findings is the observation that for many organometallic precursors, EBID is initiated by the cleavage of a single metal-ligand bond and the release of the free ligand into the gas phase. However, subsequent electron stimulated reactions are characterized by decomposition rather than desorption of the residual ligands. Rationale design criteria for new organometallics which will decompose to produce metallic nanostructures with greater metallic purity have also been developed, such as the need to avoid using cyclopentadienyl ligands. In related studies we have also identified and rationalized the often significant effect that substrate temperature exerts on the composition of EBID materials created from organometallic precursors. Specifically, increased purity is expected for EBID films deposited at high substrate temperatures and low electron fluxes; the same conditions that reduce growth rates.

Direct attachment of organic molecules to semiconductor surfaces offers the ability precisely control interfacial properties through tailoring of the organic molecule. This study focuses specifically on organic functionalization of germanium, as the ability to control its interfacial properties may enable devices to take advantage of its favorable electronic properties, as compared to silicon. Past studies have shown that a number of isocyanate-containing molecules react with the Ge(100)-2×1 surface in ultra-high vacuum by [2+2] cycloaddition across the C=N bond of the isocyanate. In this study, we use in situ Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy to investigate further reaction following [2+2] cycloaddition. Density functional theory calculations are also used to corroborate and help understand these experimental results. We show that phenyl isocyanate pre-adsorbed on Ge(100)-2×1 is highly sensitive to subsequent exposure to water vapor. Experimental evidence suggests that water reacts with the adsorbed isocyanates to form a diphenyl urea compound, similar to what is expected for the reaction of phenyl isocyanate and water in solution. Extending the analogy with classic organic chemistry in solution, we find that addition of methyl or methoxy substituents to the phenyl ring of phenyl isocyanate can significantly decrease the adsorbed isocyanate’s reactivity towards water. Such ability to easily tune the reactivity of an adsorbate-covered surface using principles from organic chemistry demonstrates the flexibility of organic functionalization and could be of importance when using organic functionalization for various applications.

Surfactants are important compounds used in many industrial applications, with sodium dodecyl sulfate (SDS) being one of the most widely used surfactants. This study uses sum frequency generation (SFG) vibrational spectroscopy and surface plasmon resonance (SPR) sensing to investigate the structure of SDS films formed from the adsorption of SDS onto positively charged and hydrophilic surfaces. The surfaces studied included CaF₂ as well as RF glow discharge deposited films of allylamine. The SDS films were prepared by adsorption of SDS from water solutions ranging in concentration from 0.067 to 20 mM. Since the water molecules above the SDS layer interact with the films, peaks from both the SDS molecules and water molecules were studied. SFG spectra of SDS adsorbed onto the positively charged CaF₂ surface exhibits two well resolved CH₃ peaks at 2877 and 2942 cm^-1, and two OH peaks at ~3200 and ~3400 cm^-1. At the 0.2 mM SDS concentration on the CaF₂ surface the intensity of both the CH₃ and OH peaks decrease to close to background levels and then increase as the SDS concentration is raised. As the SDS solution concentration continues to increase the CH₃ and OH go through a second intensity minimum. This second intensity minimum occurs between 3-6 mM for the CH₃ peaks and near 8 mM for the OH peaks. Previous studies have suggested these SFG intensity minima are due to the neutralization of positively charged CaF₂ surfaces by the anionic charged head group of SDS (1). Since the shape and, thus, the phase of the SFG peaks are affected by the molecular environment, fits of the SFG data were used to quantify the orientation and alignment of the SDS layers across the wide range of SDS solution concentration. Since SFG is sensitive to both orientational order and the amount of material adsorbed we used SPR to determine the SDS coverage for the different solution concentrations in order to separate the two contributions. Combining SFG and SPR results provides a more detailed understanding of the structure and interactions of adsorbed SDS films.

(1) Becraft, K. A.; Moore, F. G.; Richmond, G. L. Journal of Physical Chemistry B 2003, 107, 3675.

Photon stimulated dynamics, such as desorption, dissociation, or chemical synthesis, at the water cluster which contains organic molecules have been studied in conjunction with the photochemistry at ice particles in cosmic space and in the atmosphere. In our experimental study, the clusters were prepared on the surface of a solid rare gas, which was condensed on the copper substrate cooled by liquid helium in an ultra high vacuum chamber. Our method has the advantage in controlling the cluster size and of high density of specimens in comparison with a molecular beam experiment. The clusters on the solid rare gas were excited by vacuum ultra-violet light with a photon energy between 12 and 108 eV with a pulse width of 10 ns, which was generated by a laser plasma light source [1]. The mass spectrum of photo-desorbed ions was measured by a time-of-flight (TOF) method. There are variety of species in the photo-desorbed ions from the co-adsorbed system of water and methane; protonated water clusters, (H₂O)_nH⁺, methane clusters, (CH₄)_nCH_k⁺, hetero-clusters of water and methane, (H₂O)_n(CH₄)_mCH_k⁺, and synthesized species, methanol, CH₃O⁺.

It was found that the presence of a water molecule in a cluster substantially enhanced, or was almost essential for, the desorption of any species, even for CH₃⁺ and CH₅⁺, observed in the spectrum. Dissociation of the water molecule plays a key role in the chemical reaction in the clusters. It was also found that the desorption yield of each species showed strong dependence on the composition and the size of the mother cluster on the substrate, which were controlled by the amount of adsorption of water and methane. Close and systematic investigation of their correlation has revealed the mother cluster which yields the each desorbed ions: (H₂O)(CH₄) clusters yield CH₃⁺, CH₄⁺, (H₂O)CH₃⁺, and CH₃O⁺ while (H₂O)(CH₄)₂ clusters yield CH₅⁺, (H₂O)CH₃⁺, and C₂H₄⁺. These specific behaviors were also the case for the clusters of water and ethylene.

[1] T. Tachibana et al, Surf. Sci., 593, 264-268 (2005).

β-crystobalite nanofilms, 2-nm thick, are nucleated on OH(1x1)Si(100) via the Herbots-Atluri (H-A) method [1,2] and form ultra-smooth, ordered, interphases that desorb at low temperatures (T </~ 200 °C) [3] These ordered oxide nanophases on OH (1x1)-Si(100) promote oxidation at low temperatures in ambient, when in contact with oxygen-deficient phases of SiO₂ used in electronics. They can nucleate and grow a cross-bonding interphase between two substrates and achieve “nanobonding” [4] between various combinations Si and silica.

Nanobonding means forming cross-bonding molecules which condense into a continuous macroscopic bonding interphase between 2 smooth surfaces put into mechanical contact. For this to occur, the surfaces need to exhibit wide flat atomic terraces (width >/10 nm), low atomic step density (< 500 steps/µm across atomic terraces direction) and very low particulate density (less than 1/100 µm2). This contrasts with the typical density of surface steps ( ~ 500 steps/µm a.a.t.d.) and particulate density >/~0.1 -1 µm^-2 in as received wafers or post-processing. A surface step density ≥ 500 step/µm a.a.t.d, typically found on Si(100) with miscuts < 0.025° and particulates densities ≥ 0.1 µm-2 particulates results in 3-dimensional isolated bonding points of contacts as opposed to more uniform, 2-dimensional interphases that grow laterally as well as across is shown to occur in nanobonding. Wet chemical processing and SEZ spin technology are compared and combined to smmoth susbtrates via the H-A chemistry [1,2] via Tapping Mode Atomic Force Microscopy, before and after nanobonding. Our results show nanobonding can result in bonding strength larger than 10 MPa/cm2 as measured by mechanical bond pull tests. Wafers fracture within the bulk of both Si and silicate substrates rather than interfacial delamination.

[1] US Patent 6,613,677, issued 9/2/03 "Long range ordered semiconductor interface phase and oxides." 6,613,677, Herbots, N; Atluri, V. P.; Bradley J.D.; Swati, Banerjee; Hurst, Q.B.; Xiang, J.

[2] US patent 7,851,365 issued 12/14/10, "Methods for preparing semiconductor substrates & interfacial oxides there on” Herbots N., Bradley J.D. , Shaw J.M. , Culbertson and Atluri V.P.

[3] Patent Filed: 4/30/09, “Low Temperature Wafer Bonding and for Nucleating Bonding Nanophases. N. Herbots, R. J. Culbertson, J.D. Bradley, M. A. Hart, D. A. Sell and S. D. Whaley

[4] N. Herbots, Q. Xing, M. Hart, J. D. Bradley, D. A. Sell, R. J. Culbertson, Barry J. Wilkens; "IBMM of OH Adsorbates and Interphases on Si-based Materials". Nucl. Instr. and Meth. in Phys. Res., B. IBMM 17^th International Conference Proceeds (Aug, 2010), accepted