Role of the Deposition Precursor Molecules in Defining Oxidation State of Deposited Copper in Surface Reduction Reactions on H-Terminated Si(111) Surface

The Effect of Surface Terminations on the Initial Stages of TiO2 Deposition on Functionalized Silicon

As atomic layer deposition (ALD) emerges as a method to fabricate architectures with atomic precision, emphasis is placed on understanding surface reactions and nucleation mechanisms. ALD of titanium dioxide with TiCl₄ and water has been used to investigate deposition processes in general, but the effect of surface termination on the initial TiO₂ nucleation lacks needed mechanistic insights. This work examines the adsorption of TiCl₄ on Cl-, H-, and HO- terminated Si(100) and Si(111) surfaces to elucidate the general role of different surface structures and defect types in manipulating surface reactivity of growth and non-growth substrates. The surface sites and their role in the initial stages of deposition are examined by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Density functional theory (DFT) computations of the local functionalized silicon surfaces suggest oxygen-containing defects are primary drivers of selectivity loss on these surfaces.

Effect of Precursors on the Electrochemical Properties of Mixed RuOx/MnOx Electrodes Prepared by Thermal Decomposition

Growing thin layers of mixed-metal oxides on titanium supports allows for the preparation of versatile electrodes that can be used in many applications. In this work, electrodes coated with thin films of ruthenium (RuOx) and manganese oxide (MnOx) were fabricated via thermal decomposition of a precursor solution deposited on a titanium substrate by spin coating. In particular, we combined different Ru and Mn precursors, either organic or inorganic, and investigated their influence on the morphology and electrochemical properties of the materials. The tested salts were: Ruthenium(III) acetylacetonate (Ru(acac)₃), Ruthenium(III) chloride (RuCl₃·xH₂O), Manganese(II) nitrate (Mn(NO₃)₂·4H₂O), and Manganese(III) acetylacetonate (Mn(acac)₃). After fabrication, the films were subjected to different characterization techniques, including scanning electron microscopy (SEM), polarization analysis, open-circuit potential (OCP) measurements, electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), cyclic voltammetry (CV), and galvanostatic charge-discharge (GCD) experiments. The results indicate that compared to the others, the combination of RuCl₃ and Mn(acac) produces fewer compact films, which are more susceptible to corrosion, but have outstanding capacitive properties. In particular, this sample exhibits a capacitance of 8.3 mF cm^-2 and a coulombic efficiency of higher than 90% in the entire range of investigated current densities.

Molecular mechanisms of atomic layer etching of cobalt with sequential exposure to molecular chlorine and diketones

The mechanism of thermal dry etching of cobalt films is discussed for a thermal process utilizing sequential exposures to chlorine gas and a diketone [either 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (hexafluoroacetylacetone, hfacH) or 2,4-pentanedione (acetylacetone, acacH)]. The process can be optimized experimentally to approach atomic layer etching (ALE); a sequential exposure to Cl2 and hfacH dry etchants at 140 °C is shown to proceed efficiently. The use of acacH as a diketone does not result in ALE with chlorine even at 180 °C, but the decrease of surface chlorine concentration and chemical reduction of cobalt is noted. However, thermal desorption analysis suggests that the reaction of chlorinated cobalt surface exposed to the ambient conditions (oxidized) with hfacH does produce volatile Co-containing products within the desired temperature range and the products contain Co3+. The effect of adsorption of ligands on the energy required to remove surface cobalt atoms is evaluated using the density functional theory.

Surface Chemistry of Thermal Dry Etching of Cobalt Thin Films Using Hexafluoroacetylacetone (hfacH)

Amechanism of thermal dry etching process of cobalt thin films by using 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (hexafluoroacetylacetone, hfacH) was investigated. This process, relevant to atomic layer etching (ALE) technology directed towards oxidized cobalt films, requires adsorption of molecular organic precursor, such as hfacH, at moderate temperatures and is often thought of as releasing water and Co(hfac)₂ at elevated temperatures. The reaction was analyzed in situ by temperature-programmed desorption (TPD) and the resulting surface was investigated ex situ by X-ray photoelectron spectroscopy (XPS). The changes in surface morphology during the process were monitored by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The removal of Co(hfac)₂ from the surface was observed above 650 K, a temperature well above commercially desired etching conditions, suggesting that the thermal etching process is more complex than originally envisioned. In addition, the upper limit of thermal treatment is established at 800 K, as the microscopic techniques clearly indicated surface morphology changes above this temperature. In addition, the structure of the surface at the nanoscale is observed to be affected by the presence of surface bound organic ligands even at room temperature. Thus, further mechanistic studies should address the kinetic regime and surface morphology to make inroads into mechanistic understanding of the dry etching process.

Gas-Phase Electron-Impact Activation of Atomic Layer Deposition (ALD) Precursors: MeCpPtMe3

The use of gas-phase electron-impact activation of metalorganic complexes to facilitate atomic layer depositions (ALD) was tested for the case of (methylcyclopentadienyl)Pt(IV) trimethyl (MeCpPtMe₃) on silicon oxide films. Uptake enhancements of more than 1 order of magnitude were calculated from X-ray photoelectron spectroscopy (XPS) data. On the basis of the measured C:Pt ratios, the surface species were estimated to mainly consist of MeCpPt moieties, likely because of the prevalent formation of [MeCpPtMe _x- nH]⁺ ions after gas-phase ionization (as determined by mass spectrometry). Counterintuitively, more extensive adsorption was observed on thick SiO₂ films than on the native thin SiO₂ film that forms on Si(100) wafers, despite the former having virtually no surface OH groups. The adsorption of MeCpPt fragments on silicon oxide surfaces was determined by density functional theory (DFT) calculations to be highly exothermic and to favor attachment to Si-O-Si bridge sites.

Comparison of ZnO surface modification with gas-phase propiolic acid at high and medium vacuum conditions

Recent advances in preservation of the morphology of ZnO nanostructures during dye sensitization required the use of a two-step preparation procedure. The first step was the key for preserving ZnO materials morphology. It required exposing clean ZnO nanostructures to a gas-phase prop-2-ynoic acid (propiolic acid) in vacuum. This step resulted in the formation of a robust and stable surface-bound carboxylate with ethynyl groups available for further modification, for example, with click chemistry. This paper utilizes spectroscopic and microscopic investigations to answer several questions about this modification and to determine if the process can be performed under medium vacuum conditions instead of high vacuum procedures reported earlier. Comparing the results of the preparation process at medium vacuum of 0.5 Torr base pressure with the previously reported investigations of the same process in high vacuum of 10^-5Torr suggests that both processes lead to the formation of the same surface species, confirming that the proposed modification scheme can be widely applicable for ZnO sensitization procedures and does not require the use of high vacuum. Additional analysis comparing the computationally predicted surface structures with the results of spectroscopic investigations yields the more complete description of the surface species resulting from this approach.

Deposition of copper from Cu(i) and Cu(ii) precursors onto HOPG surface: Role of surface defects and choice of a precursor

The surface reactivity of two copper-containing precursors, (Cu(hfac)₂ and Cu(hfac)VTMS, where hfac is hexafluoroacetyloacetonate and VTMS is vinyltrimethylsilane), was investigated by dosing the precursors onto a surface of highly ordered pyrolytic graphite (HOPG) at room temperature. The behavior of these precursors on a pristine HOPG was compared to that on a surface activated by ion sputtering and subsequent oxidation to induce controlled surface defects. X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy were used to confirm copper deposition and its surface distribution, and to compare with the results of scanning electron microscopy and atomic force microscopy investigations. As expected, surface defects promote copper deposition; however, the specific structures deposited depend on the deposition precursor. Density functional theory was used to mimic the reactions of each precursor molecule on this surface and to determine the origins of this different reactivity.

The Chemistry of Inorganic Precursors during the Chemical Deposition of Films on Solid Surfaces

The deposition of thin solid films is central to many industrial applications, and chemical vapor deposition (CVD) methods are particularly useful for this task. For one, the isotropic nature of the adsorption of chemical species affords even coverages on surfaces with rough topographies, an increasingly common requirement in microelectronics. Furthermore, by splitting the overall film-depositing reactions into two or more complementary and self-limiting steps, as it is done in atomic layer depositions (ALD), film thicknesses can be controlled down to the sub-monolayer level. Thanks to the availability of a vast array of inorganic and metalorganic precursors, CVD and ALD are quite versatile and can be engineered to deposit virtually any type of solid material. On the negative side, the surface chemistry that takes place in these processes is often complex, and can include undesirable side reactions leading to the incorporation of impurities in the growing films. Appropriate precursors and deposition conditions need to be chosen to minimize these problems, and that requires a proper understanding of the underlying surface chemistry. The precursors for CVD and ALD are often designed and chosen based on their known thermal chemistry from inorganic chemistry studies, taking advantage of the vast knowledge developed in that field over the years. Although a good first approximation, however, this approach can lead to wrong choices, because the reactions of these precursors at gas-solid interfaces can be quite different from what is seen in solution. For one, solvents often aid in the displacement of ligands in metalorganic compounds, providing the right dielectric environment, temporarily coordinating to the metal, or facilitating multiple ligand-complex interactions to increase reaction probabilities; these options are not available in the gas-solid reactions associated with CVD and ALD. Moreover, solid surfaces act as unique "ligands", if these reactions are to be viewed from the point of view of the metalorganic complexes used as precursors: they are bulky and rigid, can provide multiple binding sites for a single reaction, and can promote unique bonding modes, especially on metals, which have delocalized electronic structures. The differences between the molecular and surface chemistry of CVD and ALD precursors can result in significant variations in their reactivity, ultimately leading to unpredictable properties in the newly grown films. In this Account, we discuss some of the main similarities and differences in chemistry that CVD/ALD precursors follow on surfaces when contrasted against their known behavior in solution, with emphasis on our own work but also referencing other key contributions. Our approach is unique in that it combines expertise from the inorganic, surface science, and quantum-mechanics fields to better understand the mechanistic details of the chemistry of CVD and ALD processes and to identify new criteria to consider when designing CVD/ALD precursors.

Investigation of the influence of oxygen plasma on supported silver nanoparticles

Silver deposition precursor molecule trimethylphosphine(hexafluoroacetylacetonato)silver(I) [(hfac)AgP(CH3)3] was used to deposit silver onto water-modified (hydroxyl-terminated) solid substrates. A silicon wafer was used as a model flat surface, and water-predosed ZnO nanopowder was investigated to expand the findings to a common substrate material for possible practical applications. Following the deposition, oxygen plasma was used to remove the remaining organic ligands on a surface and to investigate its effect on the morphology of chemically deposited silver nanoparticles and films. A combination of microscopic and spectroscopic techniques including electron microscopy and x-ray photoelectron spectroscopy was used to confirm the change in the morphology of the deposited material consistent with Ostwald ripening as a result of plasma treatment. Particle agglomeration was observed on the surfaces, and the deposited metallic silver was oxidized to Ag2O following plasma treatment. The fluorine-containing ligands were completely removed. This result suggests that chemical vapor deposition can be used to deposit silver in a very controlled manner onto a variety of substrates using different topography methods and that the post-treatment with oxygen plasma is effective in preparing materials deposited for potential practical applications.

Silver Deposition onto Modified Silicon Substrates

Trimethylphosphine(hexafluoroacetylacetonato)silver(I) was used as a precursor to deposit silver onto silicon surfaces. The deposition was performed on silicon-based substrates including silica, H-terminated Si(100), and OH-terminated (oxidized) Si(100). The deposition processes at room temperature and elevated temperature (350 °C) were compared. The successful deposition resulted in nanostructures or nanostructured films as confirmed by atomic force microscopy (AFM) and scanning electron microscopy (SEM) with metallic silver being the majority deposited species as confirmed by X-ray photoelectron spectroscopy (XPS). The reactivity of the precursor depends drastically not only on the temperature of the process but also on the type of substrate. Density functional theory (DFT) was used to explain these differences and to propose the mechanisms for the initial deposition steps.

Transmetalation Process as a Route for Preparation of Zinc-Oxide-Supported Copper Nanoparticles

Supported nanoparticulate materials have a variety of uses, from energy storage to catalysis. In preparing such materials, precision control can often be achieved by applying chemical deposition methods. However, ligand removal following the initial deposition presents a substantial challenge because of potential surface contamination. Traditional approaches normally include multistep processing and require a substantial thermal budget. Using transmetalation chemistry, it is possible to circumvent both disadvantages and prepare chemically reactive copper nanoparticles supported on a commercially available ZnO powder material by metalorganic vapor copper deposition followed by very mild annealing to 350 K. The self-limiting copper deposition reaction is used to demonstrate the utility of this approach for hexafluoroacetylacetonate-copper-vinyltrimethylsilane, Cu(hfac)VTMS, reacting with ZnO. The low-temperature transmetalation is confirmed by a combination of spectroscopic studies. Model density functional theory calculations are consistent with a thermodynamic driving force for the process.