Covalent Functionalization of Silicon with Plasma-Grown "Fuzzy" Graphene: Robust Aqueous Photoelectrodes for CO2 Reduction by Molecular Catalysts.

Carbon electrodes are ideal for electrochemistry with molecular catalysts, exhibiting facile charge transfer and good stability. Yet for solar-driven catalysis with semiconductor light absorbers, stable semiconductor/carbon interfaces can be difficult to achieve, and carbon's high optical extinction means it can only be used in ultrathin layers. Here, we demonstrate a plasma-enhanced chemical vapor deposition process that achieves well-controlled deposition of out-of-plane "fuzzy" graphene (FG) on thermally oxidized Si substrates. The resulting Si|FG interfaces possess a silicon oxycarbide (SiOC) interfacial layer, implying covalent bonding between Si and the FG film that is consistent with the mechanical robustness observed from the films. The FG layer is uniform and tunable in thickness and optical transparency by deposition time. Using p-type Si|FG substrates, noncovalent immobilization of cobalt phthalocyanine (CoPc) molecular catalysts was employed for the photoelectrochemical reduction of CO2 in aqueous solution. The Si|FG|CoPc photocathodes exhibited good catalytic activity, yielding a current density of ∼1 mA/cm2, Faradaic efficiency for CO of ∼70% (balance H2), and stable photocurrent for at least 30 h at -1.5 V vs Ag/AgCl under 1-sun illumination. The results suggest that plasma-deposited FG is a robust carbon electrode for molecular catalysts and suitable for further development of aqueous-stable Si photocathodes for CO2 reduction.

Photoelectrochemical Proton-Coupled Electron Transfer of TiO2 Thin Films on Silicon.

TiO2 thin films are often used as protective layers on semiconductors for applications in photovoltaics, molecule-semiconductor hybrid photoelectrodes, and more. Experiments reported here show that TiO2 thin films on silicon are electrochemically and photoelectrochemically reduced in buffered acetonitrile at potentials relevant to photoelectrocatalysis of CO2 reduction, N2 reduction, and H2 evolution. On both n-type Si and irradiated p-type Si, TiO2 reduction is proton-coupled with a 1e-:1H+ stoichiometry, as demonstrated by the Nernstian dependence of the Ti4+/3+ E1/2 on the buffer pKa. Experiments were conducted with and without illumination, and a photovoltage of ∼0.6 V was observed across 20 orders of magnitude in proton activity. The 4 nm films are almost stoichiometrically reduced under mild conditions. The reduced films catalytically transfer protons and electrons to hydrogen atom acceptors, based on cyclic voltammogram, bulk electrolysis, and other mechanistic evidence. TiO2/Si thus has the potential to photoelectrochemically generate high-energy H atom carriers. Characterization of the TiO2 films after reduction reveals restructuring with the formation of islands, rendering TiO2 films as a potentially poor choice as protecting films or catalyst supports under reducing and protic conditions. Overall, this work demonstrates that atomic layer deposition TiO2 films on silicon photoelectrodes undergo both chemical and morphological changes upon application of potentials only modestly negative of RHE in these media. While the results should serve as a cautionary tale for researchers aiming to immobilize molecular monolayers on "protective" metal oxides, the robust proton-coupled electron transfer reactivity of the films introduces opportunities for the photoelectrochemical generation of reactive charge-carrying mediators.

Control of ZIF-62 and agZIF-62 Film Thickness within Asymmetric Tubular Supports through Pressure and Dose Time Variation of Atomic Layer Deposition.

Thin-films of metal-organic frameworks (MOFs) have widespread potential applications, especially with the emergence of glass-forming MOFs, which remove the inherent issue of grain boundaries and allow coherent amorphous films to be produced. Herein, it is established that atomic layer deposition (ALD) of zinc oxide lends excellent control over the thickness and localization of resultant polycrystalline and glass zeolitic imidazole framework-62 (ZIF-62) thin-films within tubular α-alumina supports. Through the reduction of the chamber pressure and dose times during zinc oxide deposition, the resultant ZIF-62 films are reduced from 38 µm to 16 µm, while the presence of sporadic ZIF-62 (previously forming as far as 280 µm into the support) is prevented. Furthermore, the glass transformation shows a secondary reduction in film thickness from 16 to 2 µm.

Multi-Electron Transfer at H-Terminated p-Si Electrolyte Interfaces: Large Photovoltages under Inversion Conditions.

Photovoltages for hydrogen-terminated p-Si(111) in an acetonitrile electrolyte were quantified with methyl viologen [1,1'-(CH3)2-4,4'-bipyridinium](PF6)2, abbreviated MV2+, and [Ru(bpy)3](PF6)2, where bpy is 2,2'-bipyridine, that respectively undergo two and three one-electron transfer reductions. The reduction potentials, E°, of the two MV2+ reductions occurred at energies within the forbidden bandgap, while the three [Ru(bpy)3]2+ reductions occurred within the continuum of conduction band states. Bandgap illumination resulted in reduction that was more positive than that measured with a degenerately doped n+-Si demonstrative of a photovoltage, Vph, that increased in the order MV2+/+ (260 mV) < MV+/0 (400 mV) < Ru2+/+ (530 mV) ∼ Ru+/0 (540 mV) ∼ Ru0/- (550 mV). Pulsed 532 nm excitation generated electron-hole pairs whose dynamics were nearly constant under depletion conditions and increased markedly as the potential was raised or lowered. A long wavelength absorption feature assigned to conduction band electrons provided additional evidence for the presence of an inversion layer. Collectively, the data reveal that the most optimal photovoltage, as well as the longest electron-hole pair lifetime and the highest surface electron concentration, occurs when E° lies energetically within the unfilled conduction band states where an inversion layer is present. The bell-shaped dependence for electron-hole pair recombination with the surface potential was predicted by the time-honored SRH model, providing a clear indication that this interface provides access to all four bias conditions, i.e., accumulation, flat band, depletion, and inversion. The implications of these findings for photocatalysis applications and solar energy conversion are discussed.

Reduced fatigue and leakage of ferroelectric TiN/Hf0.5Zr0.5O2/TiN capacitors by thin alumina interlayers at the top or bottom interface.

Hf0.5Zr0.5O2(HZO) thin films are promising candidates for non-volatile memory and other related applications due to their demonstrated ferroelectricity at the nanoscale and compatibility with Si processing. However, one reason that HZO has not been fully scaled into industrial applications is due to its deleterious wake-up and fatigue behavior which leads to an inconsistent remanent polarization during cycling. In this study, we explore an interfacial engineering strategy in which we insert 1 nm Al2O3interlayers at either the top or bottom HZO/TiN interface of sequentially deposited metal-ferroelectric-metal capacitors. By inserting an interfacial layer while limiting exposure to the ambient environment, we successfully introduce a protective passivating layer of Al2O3that provides excess oxygen to mitigate vacancy formation at the interface. We report that TiN/HZO/TiN capacitors with a 1 nm Al2O3at the top interface demonstrate a higher remanent polarization (2Pr∼ 42µC cm-2) and endurance limit beyond 108cycles at a cycling field amplitude of 3.5 MV cm-1. We use time-of-flight secondary ion mass spectrometry, energy dispersive spectroscopy, and grazing incidence x-ray diffraction to elucidate the origin of enhanced endurance and leakage properties in capacitors with an inserted 1 nm Al2O3layer. We demonstrate that the use of Al2O3as a passivating dielectric, coupled with sequential ALD fabrication, is an effective means of interfacial engineering and enhances the performance of ferroelectric HZO devices.

Toxin-Blocking Textiles: Rapid, Benign, Roll-to-Roll Production of Robust MOF-Fabric Composites for Organophosphate Separation and Hydrolysis.

Current approaches to create zirconium-based metal-organic framework (MOF) fabric composites for catalysis, water purification, wound healing, gas sorption, and other applications often rely on toxic solvents, long reaction/post processing times, and batch methods hindering process scalability. Here, a novel mechanism was reported for rapid UiO-66-NH2 synthesis in common low-boiling-point solvents (water, ethanol, and acetic acid) and revealed acid-base chemistry promoting full linker dissolution and vapor-based crystallization. The mechanism enabled scalable roll-to-roll production of mechanically resilient UiO-66-NH2 fabrics with superior chemical protective capability. Solvent choice and segregated spray delivery of organic linker and metal salt MOF precursor solutions allowed for rapid MOF nucleation on the fiber surface and decreased the energy and time needed for post-processing, producing an activated composite in less than 165 min, far outpacing conventional MOF-fabric synthesis approaches. The MOF-fabric hydrolyzed and blocked permeation of the chemical warfare agent soman, outperforming the protection-standard activated carbon cloth. This work presents both chemical insights into Zr-MOF powder and fabric composite formation by a rapid, industrially relevant approach and demonstrates its practicality and affordability for high-performing personal protective equipment.

Role of Oxygen Source on Buried Interfaces in Atomic-Layer-Deposited Ferroelectric Hafnia-Zirconia Thin Films.

Hafnia-zirconia (HfO2-ZrO2) solid solution thin films have emerged as viable candidates for electronic applications due to their compatibility with Si technology and demonstrated ferroelectricity at the nanoscale. The oxygen source in atomic layer deposition (ALD) plays a crucial role in determining the impurity concentration and phase composition of HfO2-ZrO2 within metal-ferroelectric-metal devices, notably at the Hf0.5Zr0.5O2 /TiN interface. The interface characteristics of HZO/TiN are fabricated via sequential no-atmosphere processing (SNAP) with either H2O or O2-plasma to study the influence of oxygen source on buried interfaces. Time-of-flight secondary ion mass spectrometry reveals that HZO films grown via O2-plasma promote the development of an interfacial TiOx layer at the bottom HZO/TiN interface. The presence of the TiOx layer leads to the development of 111-fiber texture in HZO as confirmed by two-dimensional X-ray diffraction (2D-XRD). Structural and chemical differences between HZO films grown via H2O or O2-plasma were found to strongly affect electrical characteristics such as permittivity, leakage current density, endurance, and switching kinetics. While HZO films grown via H2O yielded a higher remanent polarization value of 25 µC/cm2, HZO films grown via O2-plasma exhibited a comparable Pr of 21 µC/cm2 polarization and enhanced field cycling endurance limit by almost 2 orders of magnitude. Our study illustrates how oxygen sources (O2-plasma or H2O) in ALD can be a viable way to engineer the interface and properties in HZO thin films.

In situ analysis of growth rate evolution during molecular layer deposition of ultra-thin polyurea films using aliphatic and aromatic precursors.

Organic thin films formed by molecular layer deposition (MLD) are important for next-generation electronics, energy storage, photoresists, protective barriers and other applications. This study uses in situ ellipsometry and quartz crystal microbalance to explore growth initiation and growth rate evolution during MLD of polyurea using aromatic p-phenylene diisocyanate (PDIC) or aliphatic 1,6-hexamethylene diisocyanate (HDIC) combined with ethylenediamine (ED) or 1,6-hexanediamine (HD) co-reactants. During the first 10-20 cycles of growth, we show the growth rate can increase and/or decrease substantially depending on the substrate as well as the flexibility, length, and structure of the isocyanate and amine reactants used. The transition from initial to steady growth is attributed to a change in active surface site density as the growth proceeds, where the number of sites is determined by a balance between steric effects that block active sites, double reactions that consume multiple active sites, and precursor physisorption and sub-surface diffusion that create new active sites, where the extent of each mechanism depends on the precursors and deposition conditions. Results shown here provide useful insight into mechanisms needed to control growth of ultra-thin organic films for advanced applications.

Stretchable and Multi-Metal-Organic Framework Fabrics Via High-Yield Rapid Sorption-Vapor Synthesis and Their Application in Chemical Warfare Agent Hydrolysis.

Protocols to create metal-organic framework (MOF)/polymer composites for separation, chemical capture, and catalytic applications currently rely on relatively slow solution-based processing to form single MOF composites. Here, we report a rapid, high-yield sorption-vapor method for direct simultaneous growth of single and multiple MOF materials onto untreated flexible and stretchable polymer fibers and films. The synthesis utilizes favorable reactant absorption into polymers coupled with rapid vapor-driven MOF crystallization to form high surface area (>250 m2/gcomposite) composites, including UiO-66-NH2, HKUST-1, and MOF-525 on spandex, nylon, and other fabrics. The resulting composites are robust and maintain their functionality even after stretching. Stretchable MOF fabrics enable rapid solid-state hydrolysis of the highly toxic chemical warfare agent soman and paraoxon-methyl simulant. We show that this approach can readily be scaled by solution spray-coating of MOF precursors and to large area substrates.

Multimaterial Self-Aligned Nanopatterning by Simultaneous Adjacent Thin Film Deposition and Etching.

Printed component sizes in electronic circuits are approaching 10 nm, but inherent variability in feature alignment during photolithography poses a fundamental barrier for continued device scaling. Deposition-based self-aligned patterning is being introduced, but nuclei defects remain an overarching problem. This work introduces low-temperature chemically self-aligned film growth via simultaneous thin film deposition and etching in adjacent regions on a nanopatterned surface. During deposition, nucleation defects are avoided in nongrowth regions because deposition reactants are locally consumed via sacrificial etching. For a range of materials and process conditions, thermodynamic modeling confirms that deposition and etching are both energetically favorable. We demonstrate nanoscale patterning of tungsten at 220 °C with simultaneous etching of TiO2. Area selective deposition (ASD) of the sacrificial TiO2 layer produces an orthogonal sequence for self-aligned patterning of two materials on one starting pattern, i.e., TiO2 ASD on SiO2 followed by W ASD on Si-H. Experiments also show capacity for self-aligned dielectric patterning via favorable deposition of AlF3 on Al2O3 at 240 °C with simultaneous atomic layer etching of sacrificial ZnO. Simultaneous deposition and etching provides opportunities for low-temperature bottom-up self-aligned patterning for electronic and other nanoscale systems.

Doubly Protective MOF-Photo-Fabrics: Facile Template-Free Synthesis of PCN-222-Textiles Enables Rapid Hydrolysis, Photo-Hydrolysis and Selective Oxidation of Multiple Chemical Warfare Agents and Simulants.

New materials and chemical knowledge for improved personal protection are among the most pressing needs in the international community. Reported attacks using chemical warfare agents (CWAs,) including organophosphate soman (GD) and thioether mustard gas (HD) are driving research in field-deployable catalytic composites for rapid toxin degradation. In this work, we report simple template-free low temperature synthesis that enables for the first time, a deployable-structured catalytic metal-organic framework/polymer textile composite "MOF-fabric" showing rapid hydrolysis and oxidation of multiple active chemical warfare agents, GD and HD, respectively, and their simulants. Our method yields new zirconium-porphyrin based nano-crystalline PCN-222 MOF-fabrics with adjustable MOF loading and robust mechanical adhesion on low-cost nonwoven polypropylene fibers. Importantly, we describe quantitative kinetic analysis confirming that our MOF-fabrics are as effective as or better than analogous MOF powders for agent degradation, especially for oxidation. Faster oxidation using the MOF-fabrics is ascribed to the composite geometry, where active MOF catalysts are uniformly displayed on the MOF-textile enabling better reactant transport and reactive oxidant generation. Furthermore, we note the discovery of visible photo-activation of GD hydrolysis by a MOF-fabric, which is ascribed to oxidation at the active metal node site, significantly increasing the rate over that observed without illumination. These results provide important new insights into the design of future materials and chemical systems to protect military, first-responders, and civilians upon exposure to complex chemical toxins.

Toward understanding the phase-selective growth mechanism of films and geometrically-shaped flakes of 2D MoTe2.

Two-dimensional (2D) molybdenum ditelluride (MoTe2) is an interesting material for fundamental study and applications, due to its ability to exist in different polymorphs of 2H, 1T, and 1T', their phase change behavior, and unique electronic properties. Although much progress has been made in the growth of high-quality flakes and films of 2H and 1T'-MoTe2 phases, phase-selective growth of all three phases remains a huge challenge, due to the lack of enough information on their growth mechanism. Herein, we present a novel approach to growing films and geometrical-shaped few-layer flakes of 2D 2H-, 1T-, and 1T'-MoTe2 by atmospheric-pressure chemical vapor deposition (APCVD) and present a thorough understanding of the phase-selective growth mechanism by employing the concept of thermodynamics and chemical kinetics involved in the growth processes. Our approach involves optimization of growth parameters and understanding using thermodynamical software, HSC Chemistry. A lattice strain-mediated mechanism has been proposed to explain the phase selective growth of 2D MoTe2, and different chemical kinetics-guided strategies have been developed to grow MoTe2 flakes and films.

The Effect of Surface Hydroxylation on MOF Formation on ALD Metal Oxides: MOF-525 on TiO2/Polypropylene for Catalytic Hydrolysis of Chemical Warfare Agent Simulants.

Metal-organic framework (MOF) fibrous composites were synthesized in a variety of methods in attempt to incorporate the highly effective reactivity of MOFs into a more facile and applicable format. Recent advances have demonstrated incorporating a metal oxide nucleation surface or reactive layer promotes conformal, well-adhered MOF growth on substrates. These materials have demonstrated promising reactivity in capturing or degrading chemical warfare agents and simulants. Here, we examine the mechanisms for MOF nucleation from metal oxide thin films to explore why some metal oxide sources are better suited for one synthesis mechanism over another. We isolate metal oxide extent of hydroxylation as an indicative factor as to whether the film serves as a nucleation promoter or may be converted directly to the MOF thin films. MOF-525 growth on Al2O3, TiO2, and ZnO coated fibers is demonstrated to corroborate these findings and used to degrade chemical warfare agent simulant dimethyl-4-nitrophenyl phosphate.

Air, Water Vapor, and Aerosol Transport through Textiles with Surface Functional Coatings of Metal Oxides and Metal-Organic Frameworks.

Currently, air permeable chemical/biological (CB) protective garments are based on activated carbon technology, which reduces moisture vapor transport needed for evaporative cooling and has potential to absorb and concentrate toxic materials. Researchers are exploring classes of sorbent materials that can selectively accumulate and decompose target compounds for potential to enhance protective suits and allow for novel filtration devices. Here, the metal-organic frameworks (MOFs) UiO-66-NH2 and HKUST-1 have been identified as such materials. To better understand how MOFs can perform in future CB protective systems, atomic layer deposition (ALD) and solution deposition were used to modify nonwoven polypropylene and flame-resistant fabrics with HKUST-1 and UiO-66-NH2. Air permeation, water vapor transport, filtration efficiency, and chemical reactivity against chemical agent simulants were assessed in relation to ALD thickness and MOF crystal size. MOF deposition on substrates decreased both air and chemical permeation while increasing filtration efficiency and chemical sorption. Moisture vapor transport was not affected by MOF growth on substrates, which is promising when considering thermal properties of protective garments. Future work should continue to explore how MOF deposition onto fiber and textile substrates impacts transport properties and chemical absorbance.

Investigation of the Corrosion Behavior of Atomic Layer Deposited Al2O3/TiO2 Nanolaminate Thin Films on Copper in 0.1 M NaCl.

Fifty nanometers of Al2O3 and TiO2 nanolaminate thin films deposited by atomic layer deposition (ALD) were investigated for protection of copper in 0.1 M NaCl using electrochemical techniques. Coated samples showed increases in polarization resistance over uncoated copper, up to 12 MΩ-cm², as measured by impedance spectroscopy. Over a 72-h immersion period, impedance of the titania-heavy films was found to be the most stable, as the alumina films experienced degradation after less than 24 h, regardless of the presence of dissolved oxygen. A film comprised of alternating Al2O3 and TiO2 layers of 5 nm each (referenced as ATx5), was determined to be the best corrosion barrier of the films tested based on impedance spectroscopy measurements over 72 h and equivalent circuit modeling. Dissolved oxygen had a minimal effect on ALD film stability, and increasing the deposition temperature from 150 °C to 250 °C, although useful for increasing film quality, was found to be counterproductive for long-term corrosion protection. Implications of ALD film aging and copper-based surface film formation during immersion and testing are also discussed briefly. The results presented here demonstrate the potential for ultra-thin corrosion barrier coatings, especially for high aspect ratios and component interiors, for which ALD is uniquely suited.

Water-Stable Chemical-Protective Textiles via Euhedral Surface-Oriented 2D Cu-TCPP Metal-Organic Frameworks.

Abatement of chemical hazards using adsorptive metal-organic frameworks (MOFs) attracts substantial attention, but material stability and crystal integration into functional systems remain key challenges. Herein, water-stable, polymer fiber surface-oriented M-TCPP [M = Cu, Zn, and Co; H2 TCPP = 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin] 2D MOF crystals are fabricated using a facile hydroxy double salt (HDS) solid-source conversion strategy. For the first time, Cu-TCPP is formed from a solid source and confirmed to be highly adsorptive for NH3 and 2-chloroethyl ethyl sulfide (CEES), a blistering agent simulant, in humid (80% relative humidity (RH)) conditions. Moreover, the solid HDS source is found as a unique new approach to control MOF thin-film crystal orientation, thereby facilitating radially arranged MOF crystals on fibers. On a per unit mass of MOF basis in humid conditions, the MOF/fiber composite enhances NH3 adsorptive capacity by a factor of 3 compared to conventionally prepared MOF powders. The synthesis route extends to other MOF/fiber composite systems, therefore providing a new route for chemically protective materials.

Toxic Organophosphate Hydrolysis Using Nanofiber-Templated UiO-66-NH2 Metal-Organic Framework Polycrystalline Cylinders.

Metal organic frameworks (MOFs), the UiO series in particular, have attracted much attention because of the high surface area and ability to capture and decontaminate chemical warfare agents. Much work has been done on incorporating these MOFs into or onto textile materials while retaining the desirable properties of the MOF. Many different techniques have been explored to achieve this. Atomic layer deposition (ALD) of TiO2 followed by solvothermal synthesis of MOF has become one of the most adaptable techniques for growing MOFs on the surface of many different polymer fabric materials. However, little work has been done with using this technique on polymer composite materials. In this work, UiO-66-NH2 was grown onto the surface of poly(methyl methacrylate) (PMMA)/Ti(OH)4 and poly(vinylidene fluoride) (PVDF)/Ti(OH)4 composite fibers by first modifying the surface with ALD of TiO2 (@TiO2) followed by solvothermal synthesis of MOF (@MOF). The catalytic activity of these materials was then evaluated using the simulant paraoxon-methyl (DMNP). These new MOF-functionalized composite fabrics were compared to polyamide-6 (PA-6)@TiO2@MOF- and polypropylene (PP)@TiO2@MOF-functionalized fabrics. PMMA/Ti(OH)4@TiO2@MOF fibers resulted in unique hollowed fibers with high surface area of 264 m2/g and fast catalytic activity. The catalytic activity of these samples was found to be related to the active MOF mass fraction on the MOF-functionalized composite fabric, with the hollowed PMMA/Ti(OH)4@TiO2@MOF having the highest weight percent of active MOF and a DMNP t1/2 of 26 min followed by PA-6@TiO2@MOF with 45 min, PVDF/Ti(OH)4@TiO2@MOF with 61 min, and PP@TiO2@MOF with 83 min.

Solvothermal Synthesis of MIL-96 and UiO-66-NH2 on Atomic Layer Deposited Metal Oxide Coatings on Fiber Mats.

Metal-organic frameworks (MOFs), which contain reactive metal clusters and organic ligands allowing for large porosities and surface areas, have proven effective in gas adsorption, separations, and catalysis. MOFs are most commonly synthesized as bulk powder, requiring additional processes to adhere them to functional devices and fabrics that risk decreasing the powder porosity and adsorption capacity. Here, we demonstrate a method of first coating fabrics with metal oxide films using atomic layer deposition (ALD). This process creates conformal films of controllable thickness on each fiber, while providing a more reactive surface for MOF nucleation. By submerging the ALD coated fabric in solution during solvothermal MOF synthesis, the MOFs create a conformal, well-adhered coating on the fibers, resulting in a MOF-functionalized fabric, without additional adhesion materials that may block MOF pores and functional sites. Here we demonstrate two solvothermal synthesis methods. First, we form a MIL-96(Al) layer on polypropylene fibers using synthetic conditions that convert the metal oxide to MOF. Using initial inorganic films of varying thicknesses, diffusion of the organic linker into the inorganic allows us to control the extent of MOF loading on the fabric. Second, we perform a solvothermal synthesis of UiO-66-NH2 in which the MOF nucleates on the conformal metal oxide coating on polyamide-6 (PA-6) fibers, thereby producing a uniform and conformal thin film of MOF on the fabric. The resulting materials can be directly incorporated into filter devices or protective clothing and eliminate the maladroit qualities of loose powder.

Thermally Driven Self-Limiting Atomic Layer Etching of Metallic Tungsten Using WF6 and O2.

The semiconductor industry faces a tremendous challenge in the development of a transistor device with sub-10 nm complex features. Self-limiting atomic layer etching (ALE) is essential for enabling the manufacturing of complex transistor structures. In this study, we demonstrated a thermally driven ALE process for tungsten (W) using sequential exposures of O2 and WF6. Based on the insight gained from the previous study on TiO2 thermal ALE, we proposed that etching of W could proceed in two sequential reaction steps at 300 °C: (1) oxidation of metallic tungsten using O2 or O3 to form WO3(s) and (2) formation and removal of volatile WO2F2(g) during the reaction between WO3(s) and WF6(g). The O2/WF6 etch process was experimentally studied using a quartz crystal microbalance (QCM). We find that both the O2 and WF6 ALE half reactions are self-limiting, with an estimated steady-state etch rate of ∼6.3 Å/cycle at 300 °C. We also find that etching of W proceeds readily at 300 °C, but not at temperatures lower than 275 °C. Thermodynamic modeling reveals that the observed temperature dependence is likely due to the limited volatility of WO2F2. The use of WF6 with O3 in place of O2 also allows W etching, where the stronger oxidant leads to a larger mass removal rate per cycle. However, we find O2 to be more controllable for precise metal removal per cycle. In addition, etched W films were examined with ex situ analytical tools. Using spectroscopic ellipsometry (SE) and scanning electron microscopy (SEM), we confirm etching of tungsten film on silicon substrates. Surface analysis by X-ray photoelectron spectroscopy (XPS) revealed a minimal fluorine content on the W film after partial etching and on the silicon surface after full etching. This suggests that W ALE does not significantly alter the chemical composition of W films. This work serves to increase the understanding of ALE reactions and expand the base of available ALE processes for advanced material processing.

Synthesis of organic aerogels with tailorable morphology and strength by controlled solvent swelling following Hansen solubility.

We introduce a generalized approach to synthesize aerogels that allows remarkable control over its mechanical properties. The Hansen solubility parameters are used to predict and regulate the swelling properties of the precursor gels and, consequently, to achieve aerogels with tailored density and mechanical properties. As a demonstration, crosslinked organogels were synthesized from cellulose esters to generate aerogels. By determination of Hansen's Relative Energy Difference, it was possible to overcome the limitations of current approaches that solely rely on the choice of precursor polymer concentration to achieve a set of aerogel properties. Hence, from a given concentration, aerogels were produced in a range of mass densities, from 25 to 113 mg/cm3. Consequently, it was possible to tailor the stiffness, toughness and compressive strength of the aerogels, in the ranges between 14-340, 4-103 and 22-373 kPa, respectively. Additionally, unidirectional freeze-drying introduced pore alignment in aerogels with honeycomb morphologies and anisotropy. Interestingly, when the swelling of the polymeric gel was arrested in a non-equilibrium state, it was possible to gain additional control of the property space. The proposed method is a novel and generic solution to achieving full control of aerogel development, which up to now has been an intractable challenge.