Surface Chemistry Of Atomic Layer Deposition Research Articles

Using Atomic Layer Deposition (ALD) to Immobilize Molecular Catalysts on Solid Powder Supports

While homogeneous molecular catalysts are valued for their high reaction specificity, they are not as easily recoverable at industrial scales as heterogeneous catalyst powders that can be rapidly separated from the reaction solution. Here, we will discuss a new approach using atomic layer deposition (ALD) to immobilize molecular catalysts onto heterogeneous powder supports to achieve this combination of selectivity and recoverability. In this presentation, we demonstrate applicability using a non-noble metal (nickel) molecular catalyst to do Suzuki carbon-carbon cross-coupling reactions. Due to extremely short catalyst lifetimes caused by dimerization, this catalyst exhibits limited catalytic reactivity under homogeneous conditions. However, when heterogenized and immobilized, product yields of over 90% can be achieved in aqueous conditions, and the catalytic activity is preserved through over five hundred recovery and wash cycles. Following this work, we will also report on how modification of the ALD surface chemistry and number of ALD cycles affect catalytic performance.

Atomic Layer Deposition onto Thermoplastic Polymeric Nanofibrous Aerogel Templates for Tailored Surface Properties.

Poly(vinyl alcohol-co-ethylene) (EVOH) nanofibrous aerogel (NFA) templates were fabricated through vacuum freeze-drying from EVOH nanofibrous suspensions. Aluminum oxide (Al2O3) layers were deposited onto highly porous templates to form organic-inorganic hybrid aerogels by the atomic layer deposition (ALD) technique. Chemical and physical measurements showed that mechanical properties were improved through ALD. In addition, the surface chemistry of ALD modified aerogels showed a fascinating cyclic change based on the number of ALD deposition cycles. A transition from hydrophilicity to hydrophobicity was observed after a few cycles of ALD coating; however, additional deposition cycles changed the wettability characteristics back to hydrophilicity. This hydrophilic-hydrophobic-hydrophilic variation is shown to be governed by a combination of geometrical and chemical surface properties. Furthermore, the deposited Al2O3 could substantially improve aerogels strength and reduce permanent deformation after cyclic compression. The Young's modulus of aerogels increased from 5.54 to 33.27 kPa, and the maximum stress at 80% strain went up from 31.13 to 176.11 kPa, after 100 cycles of trimethyl-aluminum (TMA)/water ALD. Thermogravimetric analysis (TGA) results confirm that ALD can effectively improve the heat resistance characteristics of polymeric aerogel. The onset temperature and the residual mass increased with increasing numbers of ALD cycles. During pyrolysis, the nanofiber cores were decomposed, and the brittle pure Al2O3 self-supporting nanotube aerogels with the continuous hollow nanotubular network were formed. A coating of continuous thickness Al2O3 layer on individual nanofiber was achieved after 100 ALD cycles. In additional to mechanical strength and physical property changes, the ALD modified aerogel also shows a superhydrophobic and oleophilic surface chemistry, which could potentially be used to remove oils/organic solvents from water. The resultant aerogels exhibit excellent absorption capacity (31-73 g/g) for various liquids, and the material could be reused after distillation or squeezing. A successful scale-up of such materials could provide some insights into the design and development of thermoplastic polymeric NFAs with substantial industrial applications.

Inhibiting Metal Oxide Atomic Layer Deposition: Beyond Zinc Oxide.

Atomic layer deposition (ALD) of several metal oxides is selectivity inhibited on alkanethiol self-assembled monolayers (SAMs) on Au, and the eventual nucleation mechanism is investigated. The inhibition ability of the SAM is significantly improved by the in situ H2-plasma pretreatment of the Au substrate prior to the gas-phase deposition of a long-chain alkanethiol, 1-dodecanethiol (DDT). This more rigorous surface preparation inhibits even aggressive oxide ALD precursors, including trimethylaluminum and water, for at least 20 cycles. We study the effect that the ALD precursor purge times, growth temperature, alkanethiol chain length, alkanethiol deposition time, and plasma treatment time have on Al2O3 ALD inhibition. This is the first example of Al2O3 ALD inhibition from a vapor-deposited SAM. The inhibitions of Al2O3, ZnO, and MnO ALD processes are compared, revealing the versatility of this selective surface treatment. Atomic force microscopy and grazing-incidence X-ray fluorescence further reveal insight into the mechanism by which the well-defined surface chemistry of ALD may eventually be circumvented to allow metal oxide nucleation and growth on SAM-modified surfaces.

High-Resolution Distance Dependence Study of Surface-Enhanced Raman Scattering Enabled by Atomic Layer Deposition.

We present a high-resolution distance dependence study of surface-enhanced Raman scattering (SERS) enabled by atomic layer deposition (ALD) at 55 and 100 °C. ALD is used to deposit monolayers of Al2O3 on bare silver film over nanospheres (AgFONs) and AgFONs functionalized with self-assembled monolayers. Operando SERS is used to measure the intensities of the Al-CH3 and C-H stretches from trimethylaluminum (TMA) as a function of distance from the AgFON surface. This study clearly demonstrates that SERS on AgFON substrates displays both a short- and long-range nanometer scale distance dependence. Excellent agreement is obtained between these experiments and theory that incorporates both short-range and long-range terms. This is a high-resolution operando SERS distance dependence study performed in one integrated experiment using ALD Al2O3 as the spacer layer and Raman label simultaneously. The long-range SERS distance dependence should make it possible to detect chemisorbed surface species located as far as ∼3 nm from the AgFON substrate and will provide new insight into the surface chemistry of ALD and catalytic reactions.

Epitaxial Atomic Layer Deposition of Sn-Doped Indium Oxide

Coherently strained, epitaxial Sn-doped In2O3 (ITO) thin films were fabricated at temperatures as low as 250 °C using atomic layer deposition (ALD) on (001)-, (011)-, and (111)-oriented single-crystal Y-stabilized ZrO2 (YSZ) substrates. Resultant films possess cube-on-cube epitaxial relationships with the underlying YSZ substrates and are smooth, highly conductive, and optically transparent. This epitaxial ALD approach is favorable compared to many conventional growth techniques as it is a large-scale synthesis method that does not necessitate the use of high temperatures or ultrahigh vacuum. These films may prove valuable as a conductive growth template in areas where high-quality crystalline thin film substrates are important, such as solar energy materials, light-emitting diodes, or wide bandgap semiconductors. Furthermore, we discuss the applicability of this ALD system as an excellent model system for the study of ALD surface chemistry, nucleation, and film growth.

(Invited) Atomic Layer Deposition of Core-Shell Nanowires for Solar Energy Conversion Devices

Recently, there has been a dramatic increase in research of nanoscale materials for energy conversion and storage applications due to several advantageous features such as high surface areas, short transport distances, novel optical phenomena, and tunable material properties. However, with these benefits come challenges. In particular, the ability to precisely control the properties of surfaces and heterogeneous interfaces at the nanoscale limits the performance of many of these devices, and requires novel approaches. This problem becomes increasingly important as dimensions decrease, as the surface-to-volume ratios contiunally increase with decreasing feature size. Semiconductor nanowires (NWs) posses several advantages for energy conversion devices owing to their high surface areas, tunable compositions, and efficient charge transport properties [1]. However, precise control of surfaces and interfacial properties is crucial to maximize charge-transfer efficiency across the interfaces between NWs and the surrounding device components, while minimizing undesirable side reactions and recombination losses. One technique that has been increasingly explored for surface and interfacial engineering of nanostructured energy conversion and storage devices is Atomic Layer Deposition (ALD). This gas-phase process allows for highly conformal deposition of a wide variety of materials with sub-nm precision in material thickness and tunable chemical composition. By coating the surfaces of NWs with ALD, core-shell structures can be fabricated with atomic-scale control. This combination of conformality and thickness control facilitates precise tuning of the electronic, optical, thermal, and chemical properties of nanowire surface to optimize their interfaces in energy conversion devices. This talk will present several examples of using ALD to fabricate highly-controlled core-shell for energy conversion and storage devices, with an emphasis on solar energy applications. Examples include quantum-dot sensitized nanowires [2], earth abundant core-shell NW photovoltaics at the singe nanowire and array level [3], and coating of nanowire surfaces with passivation layers and co-catalysts for photoelectrochemical water splitting [4-5]. Additionally, the use of ALD for precise control of heterogeneous interfaces in hierarchical nanomaterial synthesis will be presented. The importance of fully understanding ALD surface chemistry will be discussed from a theoretical and experimental perspective. The talk will conclude with a perspective on future directions and challenges for widespread commercial adaption of these technologies.

Atomic Layer Deposition of Platinum Nanoparticles on Titanium Oxide and Tungsten Oxide Using Platinum(II) Hexafluoroacetylacetonate and Formalin as the Reactants

Pt nanoparticles were grown on titanium oxide and tungsten oxide at 200 °C by Pt atomic layer deposition (ALD) using platinum(II) hexafluoroacetylacetonate [Pt(hfac)2] and formalin as the reactants. The Pt ALD surface chemistry and Pt nanoparticles were examined using in situ Fourier transform infrared (FTIR) vibrational spectroscopy and ex situ transmission electron microscopy (TEM). The FTIR spectra identified the surface species after the Pt(hfac)2 and formalin exposures on TiO2. An infrared feature at ∼2100 cm–1 in the FTIR spectrum after Pt(hfac)2 and formalin exposures on TiO2 was consistent with CO on Pt, revealing that Pt(hfac)2 and formalin exposures led to the formation of Pt nanoparticles. The FTIR spectrum of Pt(hfac)2 on TiO2 was very similar to the FTIR spectrum of hexafluoroacetylacetone (hfacH) on TiO2. The FTIR spectra also revealed that hfacH blocked the adsorption of Pt(hfac)2 on TiO2. The coverage of the Pt nanoparticles could be reduced by preadsorbing hfacH on TiO2 prior to Pt(hfac)2 adsorption. Time-dependent FTIR spectra showed that the coverage of hfacH and its adsorption products were reduced versus time following hfacH exposure. Pt ALD on WOx at 200 °C led to the growth of Pt nanoparticles that were fairly similar to the Pt nanoparticles from Pt ALD on TiO2. The TEM images revealed that the size of the Pt nanoparticles on WOx could be adjusted by varying the number of Pt ALD cycles. Because of site-blocking by the hfac ligands, the Pt(hfac)2 and formalin reactants required many more ALD cycles for nucleation and growth compared with other Pt ALD surface chemistries.

Analytic expressions for atomic layer deposition: Coverage, throughput, and materials utilization in cross-flow, particle coating, and spatial atomic layer deposition

In this work, the authors present analytic models for atomic layer deposition (ALD) in three common experimental configurations: cross-flow, particle coating, and spatial ALD. These models, based on the plug-flow and well-mixed approximations, allow us to determine the minimum dose times and materials utilization for all three configurations. A comparison between the three models shows that throughput and precursor utilization can each be expressed by universal equations, in which the particularity of the experimental system is contained in a single parameter related to the residence time of the precursor in the reactor. For the case of cross-flow reactors, the authors show how simple analytic expressions for the reactor saturation profiles agree well with experimental results. Consequently, the analytic model can be used to extract information about the ALD surface chemistry (e.g., the reaction probability) by comparing the analytic and experimental saturation profiles, providing a useful tool for characterizing new and existing ALD processes.

Analysis of Al2O3 Atomic Layer Deposition on ZrO2 Nanoparticles in a Rotary Reactor

AbstractAl2O3 atomic layer deposition (ALD) is analyzed on ZrO2 nanoparticles in a rotary reactor. This rotary reactor allows for static exposures and efficiently utilizes the reactants for ALD on high surface area nanoparticles. The Al2O3 ALD is performed using exposures to Al(CH3)3 and H2O reactants. The pressure transients during these exposures are examined using a sequence of reactant micropulses. These micropulses are less than the required exposures for the ALD surface chemistry to reach completion. The pressure transients during identical sequential Al(CH3)3 and H2O micropulses change as the surface chemistry progresses to completion. These pressure transients allow the required saturation reactant exposure to be determined to maximize reactant usage. The ZrO2 nanoparticles are coated using various numbers of Al(CH3)3 and H2O reactant exposures. The Al2O3 ALD‐coated ZrO2 nanoparticles are subsequently analyzed using a number of techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), Auger electron spectroscopy (AES), scanning AES (SAES), and X‐ray photoelectron spectroscopy (XPS). The TEM images reveal very conformal Al2O3 ALD on the ZrO2 nanoparticles. The Al2O3 ALD thicknesses versus number of Al(CH3)3 and H2O reactant exposures yielded an Al2O3 ALD growth rate of 2.0 Å per reactant cycle. The AES and XPS results are consistent with an Al2O3 ALD film that completely and conformally covered the underlying ZrO2 nanoparticle. The SAES measurements show that the Al2O3 ALD films are continuous and homogeneous on the ZrO2 nanoparticles. These results demonstrate that a rotary reactor can successfully perform ALD with high reactant efficiency on high surface area nanoparticles.

Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process

Atomic layer deposition (ALD), a chemical vapor deposition technique based on sequential self-terminating gas–solid reactions, has for about four decades been applied for manufacturing conformal inorganic material layers with thickness down to the nanometer range. Despite the numerous successful applications of material growth by ALD, many physicochemical processes that control ALD growth are not yet sufficiently understood. To increase understanding of ALD processes, overviews are needed not only of the existing ALD processes and their applications, but also of the knowledge of the surface chemistry of specific ALD processes. This work aims to start the overviews on specific ALD processes by reviewing the experimental information available on the surface chemistry of the trimethylaluminum/water process. This process is generally known as a rather ideal ALD process, and plenty of information is available on its surface chemistry. This in-depth summary of the surface chemistry of one representative ALD process aims also to provide a view on the current status of understanding the surface chemistry of ALD, in general. The review starts by describing the basic characteristics of ALD, discussing the history of ALD—including the question who made the first ALD experiments—and giving an overview of the two-reactant ALD processes investigated to date. Second, the basic concepts related to the surface chemistry of ALD are described from a generic viewpoint applicable to all ALD processes based on compound reactants. This description includes physicochemical requirements for self-terminating reactions, reaction kinetics, typical chemisorption mechanisms, factors causing saturation, reasons for growth of less than a monolayer per cycle, effect of the temperature and number of cycles on the growth per cycle (GPC), and the growth mode. A comparison is made of three models available for estimating the sterically allowed value of GPC in ALD. Third, the experimental information on the surface chemistry in the trimethylaluminum/water ALD process are reviewed using the concepts developed in the second part of this review. The results are reviewed critically, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials. Although the surface chemistry of the trimethylaluminum/water ALD process is rather well understood, systematic investigations of the reaction kinetics and the growth mode on different substrates are still missing. The last part of the review is devoted to discussing issues which may hamper surface chemistry investigations of ALD, such as problematic historical assumptions, nonstandard terminology, and the effect of experimental conditions on the surface chemistry of ALD. I hope that this review can help the newcomer get acquainted with the exciting and challenging field of surface chemistry of ALD and can serve as a useful guide for the specialist towards the fifth decade of ALD research.

Conformal hydrophobic coatings prepared using atomic layer deposition seed layers and non-chlorinated hydrophobic precursors

Ultrathin and conformal films deposited using atomic layer deposition (ALD) can enhance the reliability and performance of micro-electro-mechanical systems (MEMS) devices. Al2O3 ALD films are particularly useful because the Al2O3 ALD surface chemistry is very favorable and amenable to growth on a wide variety of substrates. Al2O3 ALD can be utilized to deposit robust and reliable hydrophobic coatings. A thin Al2O3 ALD film is deposited and is used as a seed layer to prepare and optimize the MEMS surface for the subsequent attachment of the hydrophobic precursors. Once the Al2O3-coated surface is prepared, non-chlorinated alkylsilanes are chemically bonded to the surface hydroxyl groups on the ALD seed layer. The film growth was monitored using an in situ quartz crystal microbalance, Fourier transform infrared spectroscopy and ex situ Auger electron spectroscopy. This deposition technique results in a dense and conformal hydrophobic film with a water contact angle of 108 ± 2°. When annealed in air to 300 °C for 10 min, the hydrophobic ALD films remained hydrophobic with a contact angle greater than 90°. Using MEMS cantilever beam arrays, hydrophobic ALD-coated beams were determined to have an adhesion energy of 0.11 ± 0.03 mJ m−2 at 100% humidity as compared with an adhesion energy of 12 ± 1 mJ m−2 for the same beams without any coating.

Nucleation and growth during the atomic layer deposition of W on Al 2O 3 and Al 2O 3 on W

Nucleation and growth are critical during the atomic layer deposition (ALD) of ultra thin films and nanolaminates. This study examined the nucleation and growth during tungsten (W) ALD on aluminum oxide (Al 2O 3) surfaces and Al 2O 3 ALD on W surfaces using Auger electron spectroscopy (AES). W ALD was performed using alternating exposures of WF 6 and Si 2H 6. Al 2O 3 ALD was performed using alternating exposures of Al(CH 3) 3 and H 2O. AES signals were measured after each WF 6 and Si 2H 6 exposure during W ALD on Al 2O 3 and after each Al(CH 3) 3 and H 2O exposure during Al 2O 3 ALD on W. The AES measurements revealed that 3 WF 6/Si 2H 6 reaction cycles were required to nucleate the W ALD film on Al 2O 3 surfaces at 473 K. Subsequently, the W ALD film grew linearly at a rate of 2.6–3.5 Å per WF 6/Si 2H 6 reaction cycle. The AES measurements also revealed that only one H 2O/Al(CH 3) 3 cycle was needed to nucleate Al 2O 3 ALD on W at 450 K. Subsequently, the Al 2O 3 ALD film grew linearly at the rate of 1.0 Å per Al(CH 3) 3/H 2O reaction cycle. As expected from the W ALD surface chemistry, the W and Si AES signals oscillated dramatically during the sequential WF 6 and Si 2H 6 exposures. Many parameters were varied to determine their effect on the W ALD nucleation period. The WF 6 surface reaction was surprisingly insensitive to the Al 2O 3 substrate temperature and the initial hydroxyl coverage on the Al 2O 3 surface. These results for the nucleation and growth during W ALD on Al 2O 3 and Al 2O 3 ALD on W are relevant to the growth of W/Al 2O 3 nanolaminates that have potential as X-ray mirrors, thermal barrier coatings and tribological films.

Temperature-Induced Apparent Mass Changes Observed during Quartz Crystal Microbalance Measurements of Atomic Layer Deposition

The quartz crystal microbalance (QCM) is a valuable in situ probe of surface chemistry and film growth during atomic layer deposition (ALD). Unfortunately, the QCM is sensitive to both mass and temperature effects that complicate the interpretation of QCM measurements. To characterize the temperature effects, QCM measurements are performed at 170 °C in a hot-wall, ALD flow reactor using pulses of inert and other unreactive probe gases that simulate reactant dosing during ALD. The difference between the probe gas temperature and the QCM sensor temperature is shown to cause instantaneous positive or negative apparent mass transients during the gas pulse. In addition, there is a net apparent mass drift after the gas pulse. The apparent mass transients and apparent mass drifting can lead to misinterpretation of ALD surface chemistry and produce error in measured ALD growth rates. Temperature-induced apparent mass changes are shown to be affected by the temperature profile in the ALD flow reactor before the QCM sensor. Changes in the gas flux during the ALD dosing sequence, changes in the type of dosing gas, and adiabatic cooling of the dosing gas can also produce temperature-induced apparent mass changes. The temperature effects on the QCM sensor are also demonstrated during Al2O3 ALD growth using Al(CH3)3 and H2O. Experimental methods using unreactive probe gases, such as SF6, are developed to tune the temperature profile of the ALD flow reactor to minimize the temperature-induced apparent mass changes.