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Novel insight into the mechanisms that govern nucleation during tungsten atomic layer deposition is presented through a detailed analysis using density functional theory. Using the calculated energetics, the authors suggest the most probable series of reactions that lead to monolayer formation on desired growth surfaces, Si(100) and W(110), during sequential doses of WF 6 and SiH 4. From this analysis, they conclude that a relatively high-energy barrier exists for initial nucleation of WF 6 on a silicon substrate; therefore, the system is limited to physical adsorption and is only capable of accessing nucleation pathways once the reaction barrier is energetically accessible. During early doses of WF 6, the initial silicon surface acts as the reductant. Results from this half-reaction provide support for the noncoalesced growth of initial W layers since nucleation is shown to require a 2:1 ratio of silicon to WF 6. In addition, the release of H 2 is significantly favored over HF production leading to the formation of fluorine-contaminated silicon sites; etching of these sites is heavily supported by the absence of fluorine observed in experimentally deposited films as well as the high volatility of silicon-subfluorides. In the second half-reaction, SiH 4 plays the multipurpose role of stripping fluorine atoms from W, displacing any adsorbed hydrogen atoms, and depositing a silicon-hydride layer. Saturation of the previously formed W layer with silicon-hydrides is a crucial step in depositing the consecutive layer since these surface species act as the reductants in the succeeding dose of WF 6. The SiH 4 half-reaction reaches a limit when all fluorine atoms are removed as silicon-subfluorides (SiF xH y) and tungsten sites are terminated with silicon-hydrides. The WF 6 dose reaches a limit in early doses when the reductant, i.e., the surface, becomes blocked due to the formation of a planar network of fluorine-containing tungsten intermediates and in later cycles when the reductant, i.e., adsorbed silicon-hydrides, is etched entirely from the surface. Overall, the calculated energetics indicate that WF xH y, SiF x, and H 2 molecules are the most probable by-products released during the ALD process. Results from this work contribute significantly to the fundamental understanding of atomic layer growth of tungsten using silicon species as reducing agents and may be used as a template for analyzing novel ALD processes.

This work explores a novel, thermal atomic layer deposition (ALD) process to deposit tin metal at a low temperature. The authors employ 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (DHP) to reduce SnCl4 on silicon substrates. The authors explored a range of temperatures between 130 and 210 °C to determine the ALD window, which was found to be 170–210 °C. The authors show that this process yields a growth rate of ∼0.3 A per cycle at 190 °C. Furthermore, X-ray photoelectron spectroscopy results showed that the film impurities are reduced for depositions within the ALD window. The reaction mechanism was explored using in situ mass spectrometry and in situ quartz crystal microbalance (QCM). Within the ALD temperature window, the QCM results showed a saturated mass gain during the SnCl4 exposure and a net mass loss during the DHP dose. Consistent with the QCM results, in situ mass spectroscopy data indicate that the DHP exposure step removes surface Cl via formation of volatile trimethylsilyl chloride and pyrazine by-products, effectively reducing the oxidation state of surface-bound Sn. This work is the first thermal Sn metal ALD process to be reported in literature and the oxidation/reduction chemistry presented here may be applied to other metal precursors, increasing the applicability of metal ALD use in industry.This work explores a novel, thermal atomic layer deposition (ALD) process to deposit tin metal at a low temperature. The authors employ 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (DHP) to reduce SnCl4 on silicon substrates. The authors explored a range of temperatures between 130 and 210 °C to determine the ALD window, which was found to be 170–210 °C. The authors show that this process yields a growth rate of ∼0.3 A per cycle at 190 °C. Furthermore, X-ray photoelectron spectroscopy results showed that the film impurities are reduced for depositions within the ALD window. The reaction mechanism was explored using in situ mass spectrometry and in situ quartz crystal microbalance (QCM). Within the ALD temperature window, the QCM results showed a saturated mass gain during the SnCl4 exposure and a net mass loss during the DHP dose. Consistent with the QCM results, in situ mass spectroscopy data indicate that the DHP exposure step removes surface Cl via formation of volatile trimethylsilyl chloride and pyraz...

Device lifetimes and commercial viability of dye-sensitized solar cells (DSSCs) and dye-sensitized photoelectrosynthesis cells (DSPECs) are dependent on the stability of the surface bound molecular chromophores and catalysts. Maintaining the integrity of the solution-metal oxide interface is especially challenging in DSPECs for water oxidation where it is necessary to perform high numbers of turnovers, under irradiation in an aqueous environment. In this study, we describe the atomic layer deposition (ALD) of TiO2 on nanocrystalline TiO2 prefunctionalized with the dye molecule [Ru(bpy)2(4,4'-(PO3H2)bpy)](2+) (RuP) as a strategy to stabilize surface bound molecules. The resulting films are over an order of magnitude more photostable than untreated films and the desorption rate constant exponentially decreases with increased thickness of ALD TiO2 overlayers. However, the injection yield for TiO2-RuP with ALD TiO2 also decreases with increasing overlayer thickness. The combination of decreased injection yield and 95% quenched emission suggests that the ALD TiO2 overlayer acts as a competitive electron acceptor from RuP*, effectively nonproductively quenching the excited state. The ALD TiO2 also increases back electron transfer rates, relative to the untreated film, but is independent of overlayer thickness. The results for TiO2-RuP with an ALD TiO2 overlayer are compared with similar films having ALD Al2O3 overlayers.

Low-temperature vapor-phase tungsten atomic layer deposition (ALD) using WF6 and dilute silane (SiH4, 2% in Ar) can yield highly conductive coatings on nylon-6 microfiber mats, producing flexible and supple nonwovens with conductivity of ∼1000 S/cm. We find that an alumina nucleation layer, reactant exposure, and deposition temperature all influence the rate of W mass uptake on 3D fibers, and film growth rate is calibrated using high surface area anodic aluminum oxide. Transmission electron microscopy (TEM) reveals highly conformal tungsten coatings on nylon fibers with complex "winged" cross-section. Using reactant gas "hold" sequences during the ALD process, we conclude that reactant species can transport readily to reactive sites throughout the fiber mat, consistent with conformal uniform coverage observed by TEM. The conductivity of 1000 S/cm for the W-coated nylon is much larger than found in other conductive nonwovens. We also find that the nylon mats maintain 90% of their conductivity after being flexed around cylinders with radii as small as 0.3 cm. Metal ALD coatings on nonwovens make possible the solvent-free functionalization of textiles for electronic applications.

Inherent chemical hazards in atomic layer deposition (ALD) processes can be mitigated significantly by careful selection of precursor materials. This work describes the effect of silane (SiH4) exposure on tungsten ALD growth when the silane is heavily diluted (2 at.-%) in argon. A wide ALD temperature window from 200 to ∼300°C is identified, exhibiting a growth rate of between 5 and 6 A per ALD cycle using SiH4 and tungsten hexafluoride (WF6) exposures of ∼6 × 105 and ∼5 × 105 Langmuirs (L), respectively. For deposition at lower temperature (150°C), growth rates of ∼4.5 A per cycle are obtained using a silane exposure of 30 s per cycle, where the partial pressure of silane at the inlet is controlled at 40 mTorr (corresponding to 1.2 × 106 L of silane). Compositional analysis by secondary ion mass spectroscopy (SIMS) and Auger electron spectroscopy (AES) show less than ∼5 at.-% Si in the W films, with the smallest Si content in films deposited at 300°C. We also describe effects of hot-wall reactor preconditioning on film growth. We conclude that the dilute silane co-reactant offers an alternative to the common disilane, borosilane, or undiluted silane precursors, allowing well-controlled W deposition at 150°C.

We report on the synthesis of tuned energy band gap Mg(x)Zn(1-x)O nanofibers (NFs) with different Mg(2+) content via the sol-gel electrospinning (ES) technique wherein the addition of the doping material affects not only the morphologies of as-spun ZnAc/PVA and MgAc/ZnAc/PVA nanofibers but also the crystal microstructure and optical properties of calcined ZnO and Mg(x)Zn(1-x)O nanofibers. Following an appropriate aqueous solution preparation of magnesium acetate (MgAc) and zinc acetate (ZnAc) with poly(vinyl alcohol) (PVA), electrospinning is performed and then as-spun nanofibers are calcined in an air atmosphere at 600 °C for 3 h. As-spun and calcined nanofiber diameters and morphologies are evaluated with scanning (SEM) and transmission (TEM) electron microscopies, whereas crystalline microstructural interpretations of ZnO and Mg(x)Zn(1-x)O are conducted with wide-angle X-ray diffraction spectra (XRD). Surface chemical composition and elemental evaluation of calcined nanofibers are examined with X-ray photoelectron spectroscopy (XPS), and optical properties and crystal defect analyses of the calcined nanofibers are conducted with photoluminescence spectra (PL). We observe a sharp reduction in fiber diameter upon calcination as a result of the removal of organic species from the fibers and conversion of ceramic precursors into ceramic nanofibers, and the appearance of a range of fiber morphologies from "bead in a string" to "sesame seed" coverage depending on fiber composition. Because Zn(2+) and Mg(2+) have similar ionicity and atomic radii, some Zn(2+) atoms are replaced by Mg(2+) atoms in the crystals, leading to a change in the properties of crystal lattices. The band gap energy of the calcined fibers increases significantly with addition of Mg(2+) along with an increase in the ultraviolet (UV) photoluminescence emission of the fibers.

Proton-exchange membrane fuel cells (PEMFCs) generally contain platinum-group-metal (PGM) electrocatalysts supported on a variety of different carbons. However, the long term durability of PGM electrocatalysts is decreased due to carbon support corrosion and loss of the electrochemically active surface area (ESA) as a result of nanoparticle agglomeration and/or Ostwald ripening. Moreover, the prohibitive expense of PGM materials has forced consideration of non-PGM alternatives. One way to mitigate carbon corrosion and increase long-term operation is by using metal oxide supports. For example, Huang et al. compared PEMFC performance of Pt/TiO2 to Pt/C (TKK) after subjecting both catalysts to an accelerated stress testing (AST) protocol, and found that Pt/TiO2 catalyst essentially retained its ESA and prevented Pt nanoparticle agglomeration. Wu et al. studied the oxygen reduction reaction (ORR) in 0.5 M H2SO4 on Fe-containing carbonized polyaniline supported on TiO2 (PANI-FeTiO2), and found an onset and half-wave (E1/2) potential of 0.98 and 0.83 V vs. RHE, respectively. The authors found that the PANI-Fe-TiO2 catalyst showed better results than Pt/C, and suggested that adding TiO2 not only decreased carbon corrosion but facilitated the ORR. In this study, we focus on developing durable non-PGM electrocatalysts for PEMFCs. We carbonize electrospun polyacrylonitrile (PAN) carbon nanofibers (CNFs) and coat them with TiO2 using atomic layer deposition (ALD). Furthermore, we enhance the surface area and reactive edges of the CNFs by adding nitrogendoped graphene platelets to the surface with a plasmaenhanced chemical vapor deposition (PECVD) process. PAN CNFs are attractive due to their inherent ORR activity and conductivity. Park et al. carbonized electrospun CNFs and reported a ~2x increase in bulk conductivity compared to XC-72R. The addition of TiO2 may act to alleviate carbon corrosion. We plan to study the ORR on TiO2-coated CNF/graphene electrocatalysts with rotating ring disk electrode (RRDE) voltammetry. Additionally, we attempt to compare the ORR activity for TiO2 coatings deposited with different ALD precursors. Physical characterizations such as XRD, XPS, TEM and SEM are performed to understand the electrocatalyst morphology, TiO2 loading and imaging, respectively. Figure 1a and 1b show carbonized PAN CNFs before and after growing graphene platelets onto the nanofiber walls, respectively.

Nanostructured surfaces are finding use in several medical applications, including tissue scaffolds and wound dressings. These surfaces are frequently manufactured from biocompatible polymers that are susceptible to ultraviolet (UV) damage. Polyethersulfone (PES) is a biocompatible polymer that undergoes oxidation and degradation when exposed to ultraviolet (UV) light. A uniform TiO2 coating can protect PES during exposure to UV sources (e.g., germicidal lamps and sunlight). The goal of this study was to determine whether atomic layer deposition (ALD) can successfully be used to grow TiO2 onto PES, protect it from UV irradiation, and reduce macrophage in vitro cytotoxicity. TiO2 was ALD-coated onto PES at 21 nm thickness. Uncoated PES exposed to UV for 30 min visibly changed color, whereas TiO2-coated PES showed no color change, indicating limited degradation. Macrophages exposed to UV-treated PES for 48 h showed reduced cell viability (via MTT assay) to 18% of control. In contrast, the cell viability for UV-treated TiO2-coated PES was 90% of control. Non-UV treated PES showed no decrease in cell viability. The results indicate that ALD of TiO2 thin films is a useful technique to protect polymers from UV damage and to retain low cytotoxicity to macrophages and other types of cells that are involved in wound healing. TiO2- coated PES membranes also have potential use in direct methanol fuel cells and in wastewater treatment membranes.

The characteristic processing behavior for growth of a conformal nanoscale hybrid organic-inorganic modification to polyamide 6 (PA6) by sequential vapor infiltration (SVI) is demonstrated. The SVI process is a materials growth technique by which exposure of organometallic vapors to a polymeric material promotes the formation of a hybrid organic-inorganic modification at the near surface region of the polymer. This work investigates the SVI exposure temperature and cycling times of sequential exposures of trimethylaluminum (TMA) on PA6 fiber mats. The result of TMA exposure is the preferential subsurface organic-inorganic growth by diffusion into the polymer and reaction with the carbonyl in PA6. Mass gain, infrared spectroscopy, and transmission electron microscopy analysis indicate enhanced materials growth and uniformity at lower processing temperatures. The inverse relationship between mass gain and exposure temperature is explained by the formation of a hybrid layer that prevents the diffusion of TMA into the polymer to react with the PA6 upon subsequent exposure cycles. As few as 10 SVI exposure cycles are observed to saturate the growth, yielding a modified thickness of ∼75 nm and mass increase of ∼14 wt %. Removal of the inherent PA6 moisture content reduces the mass gain by ∼4 wt % at low temperature exposures. The ability to understand the characteristic growth process is critical for the development of the hybrid materials fabrication and modification techniques.

The threat associated with chemical warfare agents (CWAs) motivates the development of new materials to provide enhanced protection with a reduced burden. Metal–organic frame-works (MOFs) have recently been shown as highly effective catalysts for detoxifying CWAs, but challenges still remain for integrating MOFs into functional filter media and/or protective garments. Herein, we report a series of MOF–nanofiber kebab structures for fast degradation of CWAs. We found TiO2 coatings deposited via atomic layer deposition (ALD) onto polyamide-6 nanofibers enable the formation of conformal Zr-based MOF thin films including UiO-66, UiO-66-NH2, and UiO-67. Cross-sectional TEM images show that these MOF crystals nucleate and grow directly on and around the nanofibers, with strong attachment to the substrates. These MOF-functionalized nanofibers exhibit excellent reactivity for detoxifying CWAs. The half-lives of a CWA simulant compound and nerve agent soman (GD) are as short as 7.3 min and 2.3 min, respectively. These results therefore provide the earliest report of MOF–nanofiber textile composites capable of ultra-fast degradation of CWAs.

Metal nanostructures have attractive electrical and thermal properties as well as structural stability, and are important for applications in flexible conductors. In this study, we have developed a method to fabricate and control novel complex platinum nanostructures with accordion-like profile using atomic layer deposition on lithographically patterned polymer templates. The template removal process results in unique structural transformation of the nanostructure profile, which has been studied and modeled. Using different template duty cycles and aspect ratios, we have demonstrated a wide variety of cross-sectional profiles from wavy geometry to pipe array patterns. These complex thin metal nanostructures can find applications in flexible/stretchable electronics, photonics and nanofluidics.

Nanocomposite organic/inorganic materials with spatially-controlled composition can be formed using vapor-phase atomic layer deposition (ALD) on bi-component polymer fibers. The ALD process promotes selective precursor infusion into the inner core of a core/shell polymer fiber, yielding nanoparticles encapsulated within the core. Likewise, choosing alternate precursors or reaction conditions yield particles or films on the outer polymer shell. In-situ infrared spectroscopy and transmission electron microscopy show that infusion yields selective dispersion of aluminum oxide in different polymer regions, forming fine nanoparticle dispersions or films. Selective inclusion of metal oxide materials during atomic layer deposition on polymers can create unique organic/inorganic composite structures for many advanced uses.

We report on the microstructural crystal phase transformation of electrospun TiO2 nanofibers generated via sol–gel electrospinning technique, and the incorporation of as-synthesized CdSe quantum dots (QDs) to different phases of TiO2 nanofibers (NFs) via bifunctional surface modification. The effect of different phases of TiO2 on photo-excited electron injection from CdSe QDs to TiO2 NFs, as measured by photoluminescence spectroscopy (PL) is also discussed. Nanofiber diameter and crystal structures are dramatically affected by different calcination temperatures due to removal of polymer carrier, conversion of ceramic precursor into ceramic nanofibers, and formation of different TiO2 phases in the fibers. At a low calcination temperature of 400 oC only anatase TiO2 nanofiber are obtained; with increasing calcination temperature (up to 500 oC) these anatase crystals became larger. Crystal transformation from the anatase to the rutile phase is observed above 500oC, with most of the crystals transforming into...

Background Atomic layer deposition (ALD) is a method for applying conformal nanoscale coatings on three-dimensional structures. We hypothesized that surface functionalization of multi-walled carbon nanotubes (MWCNTs) with polycrystalline ZnO by ALD would alter pro-inflammatory cytokine expression by human monocytes in vitro and modulate the lung and systemic immune response following oropharyngeal aspiration in mice. Methods Pristine (U-MWCNTs) were coated with alternating doses of diethyl zinc and water over increasing ALD cycles (10 to 100 ALD cycles) to yield conformal ZnO-coated MWCNTs (Z-MWCNTs). Human THP-1 monocytic cells were exposed to U-MWCNTs or Z-MWCNTs in vitro and cytokine mRNAs measured by Taqman real-time RT-PCR. Male C57BL6 mice were exposed to U- or Z-MWCNTs by oropharyngeal aspiration (OPA) and lung inflammation evaluated at one day post-exposure by histopathology, cytokine expression and differential counting of cells in bronchoalveolar lavage fluid (BALF) cells. Lung fibrosis was evaluated at 28 days. Cytokine mRNAs (IL-6, IL-1β, CXCL10, TNF-α) in lung, heart, spleen, and liver were quantified at one and 28 days. DNA synthesis in lung tissue was measured by bromodeoxyuridine (BrdU) uptake. Results ALD resulted in a conformal coating of MWCNTs with ZnO that increased proportionally to the number of coating cycles. Z-MWCNTs released Zn+2 ions in media and increased IL-6, IL-1β, CXCL10, and TNF-α mRNAs in THP-1 cells in vitro. Mice exposed to Z-MWCNTs by OPA had exaggerated lung inflammation and a 3-fold increase in monocytes and neutrophils in BALF compared to U-MWCNTs. Z-MWCNTs, but not U-MWCNTs, induced IL-6 and CXCL10 mRNA and protein in the lungs of mice and increased IL-6 mRNA in heart and liver. U-MWCNTs but not Z-MWCNTs stimulated airway epithelial DNA synthesis in vivo. Lung fibrosis at 28 days was not significantly different between mice treated with U-MWCNT or Z-MWCNT. Conclusions Pulmonary exposure to ZnO-coated MWCNTs produces a systemic acute phase response that involves the release of Zn+2, lung epithelial growth arrest, and increased IL-6. ALD functionalization with ZnO generates MWCNTs that possess increased risk for human exposure. Electronic supplementary material The online version of this article (doi:10.1186/s12989-016-0141-9) contains supplementary material, which is available to authorized users.

Rapid room-temperature synthesis of metal-organic frameworks (MOFs) is highly desired for industrial implementation and commercialization. Here we find that a (Zn,Cu) hydroxy double salt (HDS) intermediate formed in situ from ZnO particles or thin films enables rapid growth (1 min) of HKUST-1 (Cu3(BTC)2) at room temperature. The space-time-yield reaches3 × 10(4) kg·m(-3)·d(-1), at least 1 order of magnitude greater than any prior report. The high anion exchange rate of (Zn,Cu) hydroxy nitrate HDS drives the ultrafast MOF formation. Similarly, we obtained Cu-BDC, ZIF-8, and IRMOF-3 structures from HDSs, demonstrating synthetic generality. Using ZnO thin films deposited via atomic layer deposition, MOF patterns are obtained on pre-patterned surfaces, and dense HKUST-1 coatings are grown onto various form factors, including polymer spheres, silicon wafers, and fibers. Breakthrough tests show that the MOF-functionalized fibers have high adsorption capacity for toxic gases. This rapid synthesis route is also promising for new MOF-based composite materials and applications.

This article reviews and assesses recent progress in atomic layer deposition (ALD) and highlights how the field of ALD is expanding into new applications and inspiring new vapor-based chemical reaction methods. ALD is a unique chemical process that yields ultra-thin film coatings with exceptional conformality on highly non-uniform and non-planar surfaces, often with subnanometer scale control of the coating thickness. While industry uses ALD for high-κ dielectrics in the manufacturing of electronic devices, there is growing interest in low-temperature ALD and ALD-inspired processes for newer and more wide-ranging applications, including integration with biological and synthetic polymer structures. Moreover, the conformality and nanoscale control of ALD film thickness makes ALD ideal for encapsulation and nano-architectural engineering. Articles in this issue of MRS Bulletin present details of several growing interest areas, including the extension of ALD to new regions of the periodic table, and molecular layer deposition and vapor infiltration for synthesis of organic-based thin films. Articles also discuss ALD for nanostructure engineering and ALD for energy applications. A final article shows how the challenge of scaling ALD for high rate nanomanufacturing will push advances in plasma, roll-to-roll, and atmospheric pressure ALD.

Photocatalytically active zinc oxide nanocrystalline rods are grown on high surface area polybutylene terephthalate (PBT) polymer fiber mats using low temperature solution based methods, where the oxide crystal nucleation is facilitated using conformal thin films formed by low temperature vapor phase atomic layer deposition (ALD). Scanning electron microscopy (SEM) confirms that highly oriented single crystal ZnO nanorod crystals are directed normal to the starting fiber substrate surface, and the extent of nanocrystal growth within the fiber mat bulk is affected by the overall thickness of the ZnO nucleation layer. The high surface area of the nanocrystal-coated fibers is confirmed by nitrogen adsorption/desorption analysis. An organic dye in aqueous solution in contact with the coated fiber degraded rapidly upon ultraviolet light exposure, allowing quantitative analysis of the photocatalytic properties of fibers with and without nanorod crystals present. The dye degrades nearly twice as fast in contact with the ZnO nanorod crystals compared with samples with only an ALD ZnO layer. Additionally, the catalyst on the polymer fiber mat could be reused without need for a particle recovery step. This combination of ALD and hydrothermal processes could produce high surface area finishes on complex polymer substrates for reusable photocatalytic and other surface-reaction applications.

To synthesize long and uniform vertically aligned carbon nanotube (VACNT) arrays, it is essential to use catalytic nanoparticles (NPs) with monodisperse sizes and to avoid NP agglomeration at the growth temperature. In this work, VACNT arrays were grown on chemically synthesized Fe(3)O(4) NPs of diameter 6 nm by chemical vapor deposition. Coating the NPs with a thin layer of Al(2)O(3) prior to CNT growth preserves the monodisperse sizes, resulting in uniform, thick and dense VACNT arrays. Comparison with uncoated NPs shows that the Al(2)O(3) coating effectively prevents the catalyst NPs from sintering and coalescing, resulting in improved control over VACNT growth.

Although crystalline silicon is an industrially significant device substrate, its use in photoelectrochemical cells is complicated by the thin oxide film that forms at the electrode surface. We applied thin coatings of atomic layer deposited films on n- and p-type Si substrates to block the growth of SiO2 during solar light-driven water splitting reactions. Voltammetry accompanied by ellipsometric measurements show us that metal oxide coatings as thin as 1.6 nm are effective at stopping electrode oxidation during dark and photoelectrochemistry. We observed similar passivating behavior among TiO2, ZnO, and Al2O3 thin films. Titania and zinc oxide films enhanced photocurrent due to their absorption in the UV wavelengths. We also applied a common water oxidation catalyst, cobalt, to show the feasibility of catalyst integration on top of ALD passivating films.

The preparation of microporous and mesoporous metal oxide materials continues to attract considerable attention, because of their possible use in chemical separations, catalyst support, chemical sensors, optical and electronic devices, energy storage, and solar cells. While many methods are known for the synthesis of porous materials, researchers continue to seek new methods to control pore size distribution and macroscale morphology. In this work, we show that sequential vapor infiltration (SVI) can yield shape-controlled micro/mesoporous materials with tunable pore size, using polyesters as a sacrificial template. The reaction proceeds by exposing polymer fiber templates to a controlled sequence of metal organic and co-reactant vapor exposure cycles in an atomic layer deposition (ALD) reactor. The precursors infuse sequentially and thereby distribute and react uniformly within the polymer, to yield an organic–inorganic hybrid material that retains the physical dimensions of the original polymer template...

Growing interest in nanoscale organic-inorganic hybrid network polymer materials is driving exploration of new bulk and thin film synthesis reaction mechanisms. Molecular layer deposition (MLD) is a vapor-phase deposition process, based on atomic layer deposition (ALD) which proceeds by exposing a surface to an alternating sequence of two or more reactant species, where each surface half-reaction goes to completion before the next reactant exposure. This work describes film growth using trimethyl aluminum and heterobifunctional glycidol at moderate temperatures (90-150 °C), producing a relatively stable organic-inorganic network polymer of the form (-Al-O-(C(4)H(8))-O-)(n). Film growth rate and in situ reaction analysis indicate that film growth does not initially follow a steady-state rate, but increases rapidly during early film growth. The mechanism is consistent with subsurface species transport and trapping, previously documented during MLD and ALD on polymers. A water exposure step after the TMA produces a more linear growth rate, likely by blocking TMA subsurface diffusion. Uniform and conformal films are formed on complex nonplanar substrates. Upon postdeposition annealing, films transform into microporous metal oxides with ∼5 Å pore size and surface area as high as ∼327 m(2)/g, and the resulting structures duplicate the shape of the original substrate. These hybrid films and porous materials could find uses in several research fields including gas separations and diffusion barriers, biomedical scaffolds, high surface area coatings, and others.

Conductive coatings on complex fi brous systems are attracting interest for new electronic and other functional systems. Obtaining a quantitative conductivity value for complex surface coatings is often diffi cult. This work describes a procedure to quantify the effective electrical conductivity of conductive coatings on non-conductive fi brous networks. By applying a normal force orthogonal to the current and fi eld direction, fi ber/fi ber contact is improved and consistent conductance values can be measured. Nylon fi bers coated with an electroless silver plating shows effective conductivity up to 1950 S cm − 1 , and quartz fi bers coated with tungsten by atomic layer deposition (ALD) show values up to ∼ 1150 S cm − 1 . Cotton fi bers and paper coated with a range of ZnO fi lm thicknesses by ALD show effective conductivity of up to 24 S cm − 1 under applied normal force, and conductivity scaled as expected with fi lm coating thickness. Furthermore, we use the conductive coatings to produce an “all-fi ber” metal‐insulator‐metal capacitor that functions as a liquid chemical sensor. The ability to reliably analyze the effective conductivity of coatings on complex fi ber systems will be important to design and improve performance of similar devices and other electronic textiles structures.

A variety of biologically-derived hemostatic agents,including cellulose, bone wax, chitosan, collagen,and gelatin, have been utilized over the years forreducing blood loss associated with trauma [1,2].Ahemostatic agent should exhibit several properties,including (a) the ability to stop hemorrhage fromarterial and/or venous sources in a short time peri-od, (b) usability with minimal training, (b) lowweight, (c) high stability, (d) low cost, (e) and bio-compatibility (e.g., absence of cytotoxicity) [1–7].There are several benefits to the use of plant-derived materials such as cellulose in hemostasisapplications,including an absence of disease trans-mission from donor to recipient and an absence ofantibody formation to coagulation factors [8]. Cel-lulose is a wood pulp-derived material that hasbeen utilized in hemostasis for over 60 years;use ofoxidized cellulose, oxidized regenerated cellulose,carboxymethylcellulose, and cellulose acetate hasbeen described [1, 9–12]. In addition, these porous,water-absorbable materials do not need to be re-moved after use because they are degraded withinthe body over several weeks,increasing patient sat

The performance of polydimethylsiloxane (PDMS) elastomer in many of its applications, including surface molding and replication, microcontact lithography, and microfluidic device structures, is strongly influenced by its surface properties. While PDMS polymer is simple to use, the surface hydrophobicity and adsorptive properties of PDMS limit its functionality, for example, in aqueous microfluidic applications. Atomic layer deposition (ALD) is a low temperature vapor phase thin film coating technique that has recently been used to modify and encapsulate a wide range of polymer materials. In this work, the authors investigate reactions that proceed when PDMS polymer films are treated with cyclic gas exposure sequences commonly used to perform aluminum oxide ALD. Film growth is characterized by electron and infrared spectroscopy and by contact angle goniometry for a range of surface treatments and postdeposition air exposure times. The authors find that trimethylaluminum/water ALD can produce a smooth and u...

The demand for transistors and memory devices with smaller feature sizes and increasingly complex architectures furthers the need for advanced thin film patterning techniques. A pre-patterned, sacrificial layer can be used as a template for bottom-up fill of new materials which would otherwise be difficult to pattern using traditional top-down lithographic methods. This work investigates initial growth of TiN, TiO2, and HfO2 thin films during thermal atomic layer deposition (ALD) onto a high density, amorphous carbon (aC) sacrificial layer. ALD of TiN by TiCl4/NH3 at 390oC, TiO2 by Ti(OCH3)4/H2O at 250oC, and HfO2 by HfCl4/H2O at 300oC on as-deposited aC films resulted in uninhibited, continuous thin film growth. We find that carbon surface reduction and passivation using a H2 plasma resulted in delayed film coalescence for TiN, TiO2, and HfO2 on the aC. After 200 TiN cycles on H2 plasma-treated aC, Rutherford backscattering spectrometry shows Ti levels below the detection limit (8×1013 at/cm2), whereas SiO2 or Si3N4 substrates show TiN growth of ~6 nm, corresponding to a selectivity of ~200:1. Exposing plasma-treated aC to H2O induces nucleation for TiN ALD, consistent with favorable nucleation on hydroxyl sites. Therefore, the H2O co-reagent in TiO2 and HfO2 ALD contributes to loss of selectivity compared to TiN ALD using NH3. Furthermore, we demonstrate the use of the Avrami Equation to model the nucleation behavior on plasma-treated aC, providing useful insights into process improvements for increased selectivity. We confirm scaling of selectivity to sub 50 nanometer patterns using 45 nm aC/Si3N4 line/space patterns, where 3.5 nm TiO2 and 5.8 nm TiN films are deposited on Si3N4 with minimal particle formation on aC, with selectivity loss primarily on feature corners and edges. We conclude that improved scaling of selectivity to nanometer scale patterns can be achieved by optimizing surface loading and extent of plasma exposure, and by further understanding shape effects in nanoscale surface plasma modification.

Atomic layer deposition (ALD) shows promise for forming thin films on temperature-sensitive materials, such as polymers, for applications in filtration, sensing, etc. However, traditional batch ALD generally proceeds slowly and requires controlled, low-pressure equipment. One approach to combat this limitation is spatial ALD, which uses moving substrates through zones of reactant exposure. To date, studies of spatial ALD have primarily explored growth on planar and nonporous substrates. Here, the authors demonstrate a proof-of-concept atmospheric pressure flow-through spatial ALD reactor specifically designed for through-porous substrates, such as fiber webs. This paper describes detailed gas flow modeling and experimental analysis of their prototype reactor. Model results identify precursor gas flow rates, channel spacing, and the distance between the substrate and reactor surfaces as key factors to achieve uniform deposition. Using a previously developed surface wetting protocol, the authors experimentally verify operating conditions for uniform ALD alumina on polypropylene as a model fiber substrate. Under good ALD conditions, the spatial ALD reactor can complete ∼60 cycles/min over a large substrate area, which is 60 times faster than batch ALD. The authors quantify growth saturation conditions and find that under reduced gas flow rates or slow fiber translation speeds, a transition from ALD to chemical vapor deposition-like growth can be induced. Additionally, the authors demonstrate that fiber mat properties such as mat density and air permeability play important roles in the penetration depth of the precursors and, therefore, the conditions needed to achieve ALD. Overall, this work demonstrates a proof-of-concept reactor for high throughput ALD on porous substrates, and identifies important design challenges and considerations for future high-throughput ALD.

The high electrical and thermal conductivity of copper make it an irreplaceable material for many applications. Copper possesses adequate aqueous corrosion resistance through the formation of a semi-protective native oxide film, hence its use in residential and commercial plumbing. Still, it is susceptible to elevated corrosion rates and pitting in the presence of contaminants depending on pH and fluid temperature.1–7 Alloying to enhance corrosion resistance is common, though the corresponding degradation of electrical and thermal conductivity8 cannot be tolerated in applications such as high-powered RF systems. In these devices, copper is utilized in cooling channels and/or as electron collectors, where its high conductivities are paramount. Instead, very thin barrier films may provide enhanced corrosion resistance without a marked loss of desirable bulk properties. In this work, we grow thin films on copper for enhanced corrosion resistance using atomic layer deposition (ALD). ALD is a vapor phase chemical deposition method capable of growing conformal thin films with precise thickness control and no line-of-sight requirement, making it ideal for coating tube interiors and components with intricate geometries9. The films tested here are Al2O3, TiO2, and nanolaminate combinations of these two. The excellent sealing properties of ALD Al2O3 complements the high chemical and environmental stability of TiO2. Total film thickness of single layer and nanolaminate films is approximately 50nm. Corrosion resistance is quantified through potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 0.1M NaCl. Results show an increase in corrosion resistance for both single layer coatings and nanolaminate films over the bare copper. As expected, the charge transfer resistance of copper coated with a single layer Al2O3 film decreases by an order of magnitude over 72 hours, and energy dispersive x-ray spectroscopy (EDS) confirms Al and O depletion in the region exposed to electrolyte. Nanolaminate films exhibited as low as a 10% reduction in charge transfer resistance over that same period. Imaging reveals decreased coating degradation of nanolaminate film stacks compared to single layer films after exposure to saline solution over a 3-day period. References [1] Edwards, M., Ferguson, J. F., and Reiber, S. H., The Pitting Corrosion of Copper, J. Am. Water Work. Assoc. 86(7), 74–90 (1994). [2] McDougall, J. L., McCall, L., and Stevenson, M. E., Water Chemistry and Processing Effects on the Corrosion Degradation of Copper Tubing in Cooling Water Systems, Pract. Fail. Anal. 3(5), 81–88 (2003) doi:10.1361/152981503771816808. [3] Jeon, B., Sankaranarayanan, S. K. R. S., Duin, A. C. T. van, and Ramanathan, S., Atomistic Insights into Aqueous Corrosion of Copper, J. Chem. Phys. 134(23), 234706 (2011) doi:10.1063/1.3599090. [4] Brusic, V., Frisch, M. A., Eldridge, B. N., Novak, F. B., Kaufman, F. B., Rush, B. M., and Frankel, G. S., Copper Corrosion With and Without Inhibitors, J. Electrochem. Soc. 138(8), 2253 (1991) doi:10.1149/1.2085957. [5] Sobue, K., Sugahara, A., Nakata, T., Imai, H., and Magaino, S., Effect of Free Carbon Dioxide on Corrosion Behavior of Copper in Simulated Water, Surf. Coatings Technol. 169–170, 662–665 (2003) doi:10.1016/S0257-8972(03)00049-5. [6] Feng, Y., Siow, K.-S., Teo, W.-K., Tan, K.-L., and Hsieh, A.-K., Corrosion Mechanisms and Products of Copper in Aqueous Solutions at Various pH Values, Corrosion 53(5), 389–398 (1997). [7] Pehkonen, S. O., Palit, A., and Zhang, X., Effect of Specific Water Quality Parameters on Copper Corrosion, CORROSION 58(2), 156–165 (2002) doi:10.5006/1.3277316. [8] Lu, L., Shen, Y., Chen, X., Qian, L., and Lu, K., Ultrahigh Strength and High Electrical Conductivity in Copper, Science 304(5669), 422–426 (2004) doi:10.1126/science.1092905. [9] Parsons, G. N., George, S. M., and Knez, M., Progress and Future Directions for Atomic Layer Deposition and ALD-Based Chemistry, MRS Bull. 36, 865–871 (2011) doi:10.1557/mrs.2011.238.

Common thermoplastic polymers, such as poly(vinyl alcohol) and cellulose derivatives are abundant and inexpensive precursors for preparing carbon nanofibers. These polymers are soluble in common so...

In situ characterization of low temperature atomic layer deposition (ALD) of ZnO and aluminum-doped ZnO (ZnO:Al) establishes a relationship between species adsorption, mass uptake, and surface electrical conductance during deposition and dopant atom incorporation. Conductance measured in situ during ZnO ALD oscillates with species surface adsorption, consistent with surface potential modulation and charge transfer during surface reaction. Dopant introduction using trimethylaluminum impedes both surface potential modulation and film growth, and a reaction scheme involving surface proton exchange complexes is presented to understand the observed results. Electronically active doping is achieved only after Al species transition four to five monolayers into the film bulk, consistent with a nonuniform dopant atom distribution in the direction of film growth. Results have important implications in understanding relations between dopant incorporation, activation, and film growth mechanisms in atomic layer deposi...

Aluminum incorporation into ZnO films during atomic layer deposition (ALD) is investigated using in situ quartz crystal microbalance and electrical conductance analysis. Chemical interactions between Zn and Al species during ZnO:Al ALD depend on the order of metal precursor exposure. Exposing the growing ZnO surface to trimethyl aluminum (TMA) impedes the subsequent ∼4 monolayers of ZnO growth. However, the extent of interaction can be reduced by performing the TMA exposure immediately following a diethyl zinc step, without an intermediate water exposure step, consistent with increased surface mixing of Zn and Al species. Infrared spectroscopy analysis of heavily aluminum doped ZnO shows features consistent with the presence of amorphous ZnAl2O4 bonding units. For more lightly doped films, mass spectroscopic depth profiling confirms nonuniform aluminum distribution, even after annealing at 500 °C. Film conductance measured during growth shows complex trends that are highly repeatable over multiple doping ...

Three-dimensional nanoscale constructs are finding applications in many emerging fields, including energy generation and storage, advanced water and air purification, and filtration strategies, as well as photocatalytic and biochemical separation systems. Progress in these important technologies will benefit from improved understanding of fundamental principles underlying nanostructure integration and bottom-up growth processes. While previous work has identified hydrothermal synthesis conditions to produce nanoscale ZnO rods, sheets, and plates, strategies to systematically integrate these elements into more complex nano-architectures are not previously described. This article illustrates that amorphous nanoscale coatings formed by atomic layer deposition (ALD) are a viable means to modulate and screen the surface polarity of ZnO crystal faces and thereby regulate the growth morphology during successive hydrothermal nanocrystal synthesis. Using this new strategy, this work demonstrates direct integration and sequential assembly of nanocrystalline rods and sheets to produce complex three-dimensional geometric forms, where structure evolution is achieved by modifying the surface growth condition, keeping the hydrothermal growth chemistry unchanged. Therefore, rational planning of seed layer and feature spacing geometries may allow researchers to engineer, at the nanoscale, complex three-dimensional crystalline and semicrystalline constructs for a wide range of future applications.

The antifungal properties of two silver-coated natural cotton fiber structures prepared using a supercritical carbon dioxide (scCO2) solvent were examined. Scanning electron microscopy confirmed that the scCO2 process may be used to produce cotton fiber textiles with uniform silver nanoparticle coatings. A version of the Kirby-Bauer disk diffusion test was used to assess the ability of these textiles to inhibit fungal growth. Cotton fabric samples modified with Ag(hepta) and Ag(cod)(hfac) exhibited measurable zones of inhibition. On the other hand, the uncoated fabric had no zone of inhibition. Possible applications of antifungal textiles prepared using scCO2 processing include use in hospital uniforms and wound dressings.

Hybrid organic−inorganic polymer thin films of the form (−O−Zn−O−C2H4−)n have been deposited from diethyl zinc and ethylene glycol using molecular layer deposition (MLD) over a range of substrate temperatures between 100 and 170 °C. Infrared transmission confirms that the films consist of Zn−O and ethylene-oxide units. In analogy with known alucone polymers of the form (−O−Al−O−R−)n, the zinc-based hybrid material is an example of a “zincone” polymer. In situ quartz crystal microbalance analysis indicated that the sequential surface reactions of diethyl zinc and ethylene glycol are sufficiently self-limiting and saturating to enable well-controlled MLD growth. Quantitative analysis of in situ quartz crystal microbalance and film thickness results indicate that ethylene glycol molecules can undergo a “double reaction” where the OH groups on both ends of the diol react with available Zn−C2H5 surface sites to produce a relatively inert bridging alkane. The mass uptake per MLD cycle during Zn−hybrid film depo...

This work addresses a significant challenge in engineered molecular systems regarding both understanding and controlling the stability of molecule/nanoparticle nanostructures under ambient exposure. Results deal specifically with molecular electronic junctions, where electronic contacts and transport are known to be sensitive to sample history and ambient exposure. We demonstrate that low-temperature atomic layer deposition can gently encapsulate and stabilize molecular electronic junctions, making it feasible to handle and transport junctions in air for many days with minimal change in electronic conduction. These findings indicate the potential for long-term stability of advanced synthetic nanoparticle/molecule nanoconstructs. For this study, conductivity through nanoparticle/molecule/nanoparticle junctions is analyzed and found to be consistent with nonresonant charge tunneling through a single or a small number of oligomeric phenylene ethynylene molecules in the electrical junction. The conductivity w...

An effective method is developed for low temperature metal oxide deposition through thermal decomposition of metal diketonates in supercritical carbon dioxide (scCO2) solvent. The rates of Al(acac)3 (Aluminum acetyl acetonate) and Ga(acac)3 (Gallium acetyl acetonate) thermal decomposition in scCO2 to form conformal Al2O3 and Ga2O3 thin films on planar surfaces were investigated. The thermal decomposition reaction of Al(acac)3 and Ga(acac)3 was found to be initialized at ∼ 150 °C and 160 °C respectively in scCO2 solvent, compared to ∼ 250 °C and 360 °C in analogous vacuum-based processes. By measuring the temperature dependence of the growth rates of metal oxide thin films, the apparent activation energy for the thermal decomposition of Al(acac)3 in scCO2 is found to be 68 ± 6 kJ/mol, in comparison with 80–100 kJ/mol observed for the corresponding vacuum-based thermal decomposition reaction. The enhanced thermal decomposition rate in scCO2 is ascribed to the high density solvent which effectively reduces the energy of the polar transition states in the reaction pathway. Preliminary results of thin film deposition of other metal oxides including ZrOx, FeOx, Co2O3, Cr2O3, HfOx from thermal decomposition of metal diketonates or fluorinated diketonates in scCO2 are also presented.

Electrical conductance through conjugated biphenyl and saturated alkane molecules on gold electrodes is characterized using a molecule/nanoparticle/molecule electrical test-bed assembly, and comparisons are made between molecules containing isocyanide (−NC) and thiol (−S) terminal groups bound to the gold. Current versus voltage analysis is consistent with charge tunneling through all systems studied. For molecules containing biphenyl, diphenyl acetylene, and alkane bridges, those containing an isocyanide terminal group show an order of magnitude increase in conductance as compared to those containing a thiol terminal group. Various theoretical predictions of the effect of isocyanide terminal groups on charge transfer through conjugated molecular systems are discussed and related to the results observed for charge transfer through conjugated and saturated molecules. The similar trends in charge transport observed for the different metal/linker interfaces suggest that the metal/linker contact plays an impo...

Atomic layer deposition (ALD) of Al2O3 on electrospun poly(vinyl alcohol) microfiber templates is demonstrated as an effective and robust strategy by which to fabricate long and uniform metal-oxide microtubes. The wall thickness is shown to be precisely controlled within a molecular layer or so by adjusting the number of ALD cycles utilized. By judicious selection of the electrospinning and ALD parameters, designer tubes of various sizes and inorganic materials can be synthesized.

Charge transport through alkane monolayers on gold is measured as a function of molecule length in a controlled ambient using a metal/molecule/nanoparticle bridge structure and compared for both thiol and amine molecular end groups. The current through molecules with an amine/gold junction is observed to be more than a factor of 10 larger than that measured in similar molecules with thiol/gold linkages. Conducting probe atomic force microscopy is also used to characterize the same monolayer systems, and the results are quantitatively consistent with those found in the nanoparticle bridge geometry. Scaling of the current with contact area is used to estimate that approximately 100 molecules are probed in the nanoparticle bridge geometry. For both molecular end groups, the room-temperature conductivity at low bias as a function of molecule length shows a reasonable fit to models of coherent nonresonant charge tunneling. The different conductivity is ascribed to differences in charge transfer and wave function mixing at the metal/molecule contact, including possible effects of amine group oxidation and molecular conformation. For the amine/Au contact, the nitrogen lone pair interaction with the gold results in a hybrid wave function directed along the molecule bond axis, whereas the thiol/Au contact leads to a more localized wave function.

The fundamental chemical reaction conditions that define atomic layer deposition (ALD) can be achieved in an open environment on a macroscale surface too large and complex for typical laboratory reactor-based ALD. We describe the concept of in loco ALD using conventional modulated reactant flow through a surface-mounted “ALD delivery head” to form a precise nanoscale Al2O3 film on the window of a parked automobile. Analysis confirms that the processes eliminated ambient water contamination and met other conditions that define ALD growth. Using this tool, we demonstrate open-ambient patterned deposition, metal corrosion protection, and polymer surface modification.

Real time in situ conductance is collected continuously during atomic layer deposition (ALD) of zinc oxide films, and trends are used to study ALD nucleation on polypropylene, nylon-6, SiO2, TiO2, and Al2O3 substrates. The detailed conductance change during the ALD cycle is ascribed to changes in surface band bending upon precursor/reactant exposure. Conductive pathways form earlier on the inorganic surfaces than on the polymers, with Al2O3 substrates showing more rapid nucleation than SiO2 or TiO2, consistent with the expected density of nucleation sites (e.g., hydroxyl groups) on these different materials. The measured conductance is ohmic, and both two- and four-electrode configurations show the same data trends. Detailed analysis of conductivity at deposition temperatures between 100 and 175 °C shows faster conductivity decay at higher temperature during the water purge step, ascribed to thermally activated water desorption kinetics. Analysis of real-time conductivity during ALD of other material systems could provide further insight into key aspects of film nucleation and nuclei coalescence.

Oxidation of Hf metal films on Si appears to follow different charge generation rules than the traditional oxidation of Si described in detail by Deal et al. [B.E. Deal, M. Sklar, A.S. Grove, E.H. Snow, J. Electrochem. Soc. 114 (1967) 226]. Oxidation of thin Hf metal films on silicon in oxygen rich environments to form Hf-silicate results in rapid growth of silicon oxide interfacial layers and generation of significant charge concentration in the films. Oxidation of Hf in oxygen-deficient environment leads to improved control of the interface with much thinner interfacial layers and substantial reduction in the charge present in the films. Results from capacitance vs. voltage and X-ray photoelectron spectroscopy measurements are compared to correlate charge with chemical structure evolution during oxidation and dielectric layer formation. It is demonstrated that processing conditions may influence the quality of the Hf dielectric film significantly by generating positive charge that is not intrinsic to the material.

In the Phase I program, Calabazas Creek Research, Inc. (CCR), in collaboration with North Carolina State University (NCSU), fabricated copper accelerator cavities and used Atomic Layer Deposition (ALD) to apply thin metal coatings of tungsten and platinum. It was hypothesized that a tungsten coating would provide a robust surface more resistant to arcing and arc damage. The platinum coating was predicted to reduce processing time by inhibiting oxides that form on copper surfaces soon after machining. Two sets of cavity parts were fabricated. One was coated with 35 nm of tungsten, and the other with approximately 10 nm of platinum. Only the platinum cavity parts could be high power tested during the Phase I program due to schedule and funding constraints. The platinum coated cavity exhibit poor performance when compared with pure copper cavities. Not only did arcing occur at lower power levels, but the processing time was actually longer. There were several issues that contributed to the poor performance. First, machining of the base copper cavity parts failed to achieve the quality and cleanliness standards specified to SLAC National Accelerator Center. Secondly, the ALD facilities were not configured to provide the high levels of cleanliness required. Finally, the nanometermore » coating applied was likely far too thin to provide the performance required. The coating was ablated or peeled from the surface in regions of high fields. It was concluded that the current ALD process could not provide improved performance over cavities produced at national laboratories using dedicated facilities.« less

In-situ attenuated total internal reflection infrared spectroscopy has been used to examine initial adsorption and reaction steps in atomic layer deposition of HfO 2 from tetrakis(diethylamino) hafnium (TDEAHf) on SiO 2 and hydrogen-terminated Si(100) surfaces. At low deposition temperatures (25-250 °C), TDEAHf directly reacts with the Si-H surface, resulting in partial removal of Si-H bonds and formation of a four-membered Si-O-Hf-Si bonding structure that can rapidly oxidize. The hydrogen removal process is observed to continue through many cycles of TDEAHf/H 2 O exposure, signifying continued reactivity of the Hf precursor with the silicon surface. Density functional theory calculations have been performed for various reactions between tetrakis(dimethylamino) hafnium and Si-H surfaces, and several possible reaction pathways for hydrogen removal have been identified and analyzed. The calculations suggest that hydrogen removal proceeds by H abstraction by an amine ligand of the Hf precursor and that the abstraction reaction is made more facile by the presence of OH on the otherwise H-terminated Si surface.