Trimethylaluminum Research Articles

Surface chemical mechanisms of trimethyl aluminum in atomic layer deposition of AlN

Two different pathways for decomposition of adsorbed trimethyl aluminum in the ALD of AlN have been investigated by computation chemistry. The two pathways give different saturation coverage and thus growth per cycle.

Molecular layer deposition of alucone in high aspect ratio trenches: The effect of TMA outgassing on step-coverage

Infiltration of trimethylaluminum (TMA) in molecular layer deposition-enabled alucone thin films on planar substrates is a common observation reported in the literature. An insufficient TMA purge time in such cases is often found to lead to a CVD component in the overall film growth due to the reactions between the outgassing TMA and the co-reactant. In this work, the effect of the CVD component on the step coverage of alucone films when grown in high-aspect ratio trenches is studied. The thickness was initially found to increase with increasing distance from the trench aperture before starting to decrease up to the film’s maximum penetration depth. With the support of modeling, the reason behind the observed thickness profile was investigated and attributed to the combination of an increasing outgassing rate of TMA and a decreasing ethylene glycol (EG) concentration along the trench depth. Furthermore, the maximum thickness and the position where it is obtained in the trench are found to depend on TMA and EG doses, TMA purge time, the trench height, and the trench depth. Finally, the possibility of adopting the additional CVD component in film growth for void-less gap fill of 3D trenches is discussed, as well as the suppression of the same CVD component, without compromising the growth rate, by using dimethylaluminum isopropoxide as a substitute for TMA is evaluated.

Atomic layer deposition of diethylzinc/zinc oxide on InAs surface quantum dots: Self-clean-up and passivation processes

Passivation capping of molecular beam epitaxy (MBE)-grown InAs surface quantum dots (SQDs) is achieved by ex situ atomic layer deposition (ALD)-grown ZnO using diethylzinc (DEZ) as the zinc precursor, the main passivation agent, and oxygen plasma. Photoluminescence (PL) intensity is enhanced by 2-fold as the DEZ/ZnO passivation cap thickness reached 30 nm. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) show that contrary to conventional wet chemistry passivation or MBE capping, in which InAs SQDs can be damaged or be shrunken by intermixing of In and Ga, DEZ/ZnO passivation capping almost preserves the shape of the underlying SQDs. The cross-sectional analysis with transmission electron microscopy (TEM) reveals the formation of the ZnO grains on top of the sample accompanied by slight SQD height and width reduction, which are presumably related to the removal of the InAs native oxides. The DEZ “self-clean-up” passivation mechanism, where zinc precursor is responsible for reduction of the surface non-radiative recombination sites, is studied by X-ray photoelectron spectroscopy (XPS). The factors that control the DEZ “self-clean-up” efficiency such as the chain of chemical reactions, steric hindrance, decomposition activation energy, Gibbs reactivity, or Lewis acidity, are evaluated. The results are discussed in comparison with trimethylaluminum (TMA), a precursor used for Al2O3 deposition. We find that the “self-cleaning” by DEZ and TMA occurs through processes of different chemical nature.

Dipole formation to modulate flatband voltage using ALD Al2O3 and La2O3 at the interface between HfO2 and Si or Ge substrates

In this study, we have studied the flatband voltage (VFB) modulation by the formation of Al and La-induced dipoles at the interface between Si or Ge substrates and HfO2 gate dielectric in the metal–oxidesemiconductor (MOS) devices. The Al2O3 and La2O3 layers deposited on the HfO2 thin film serve to form dipoles. These Al2O3 and La2O3 thin film layers were atomically deposited using trimethyl-aluminum (TMA) and La(iPrCp)3 for the metal precursor and H2O and O3 for the oxidant, respectively. The dipole, an enabler that modulates the VFB in MOS devices, is formed at the interface between the high-k thin film and substrates upon subsequent annealing. As the thickness of the Al2O3 increases, VFB shifts in the positive direction on both substrates. The VFB shifted by about 230 mV with a 1.5 nm thick Al2O3 capping layer on Si substrate and 610 mV on the Ge substrate. On the other hand, VFB shifted in a negative direction of about 181 mV in the 1.5 nm thick-La2O3 capping layer. Our results indicate that electronegative atoms using Al2O3 and La2O3 are an efficient way to modulate VFB for Si or Ge-based 3D device structures without sufficient space margin for gate stack film formation.

Unveiling Hidden Chemistry between an Aluminum ALD Precursor and Li Metal Anodes

Trimethylaluminum (TMA) is a widely utilized aluminum precursor for atomic layer deposition (ALD) in various aluminum-based coatings. For example, TMA produces Al2O3 with H2O,[1] AlF3 with HF-pyridine,[2] and Al2S3 with H2S [3] with well-defined ALD mechanisms. These ALD processes have been widely applied into various materials to introduce ultra-thin Al-based coating layers, including energy storage materials such as battery cathodes, anodes, and solid electrolytes. Our group demonstrated that the Al2O3 ALD process using TMA can be used for Li-metal anode coating to improve electrochemical stability and reduce Li-dendrite formation during battery cycling.[4]Although many studies about ALD on Li metal anodes have been reported using TMA, most of them focus on Li-metal battery performance, instead of ALD chemistry. During deep investigation on ALD chemistry of TMA, we recently discovered an interesting chemistry between TMA and reactive metal substrate (i.e. Li metal). Based on our observation, TMA reacts with Li-metal during ALD cycles then producing a carbon composite layer on the top of Li metal, not following conventional ALD mechanisms. It may come from highly reactive nature both of TMA and Li metal. Despite of wide use of TMA, this phenomenon has not been reported to date because Li metal is passivated by several layers such as Li2CO3, LiOH and Li2O even stored in glovebox environment. In this presentation, we will discuss the chemistry between TMA and Li metal found by in-situ QCM, high-resolution XPS, SEM, and Raman studies. Then we will also demonstrate how this unexpected ‘side reaction’ can be utilized in Li-metal battery applications using liquid and solid electrolytes.[1] 1. A. C. Kozen et al., ACS Nano, 9, 5884–5892 (2015).[2] Y. Lee, H. Sun, M. J. Young, and S. M. George, Chemistry of Materials, 28, 2022–2032 (2016).[3] X. Meng, Y. Cao, J. A. Libera, and J. W. Elam, Chemistry of Materials, 29, 9043–9052 (2017).[4] L. Chen et al., Journal of Materials Chemistry A, 5, 12297–12309 (2017). Figure 1

Conversion Reactions and Redox Changes on the Surface of Lithium-Ion Battery Cathode Materials during Chemical Vapor Treatment for ALD

Atomic layer deposition (ALD) has emerged as a promising technology for applying ultrathin protective coatings on lithium-ion battery (LIB) cathode surfaces to improve their cycling stability. While there have been numerous reports evaluating the electrochemical performance of these surface-modified cathode materials, the chemical changes induced on the surface of the cathode materials by the ALD coatings and the individual ALD precursors are not fully studied. We performed a systematic investigation to understand the interfacial changes of 12 different cathode materials upon coating with aluminum oxide (Al2O3) using trimethyl aluminum (TMA) and H2O, and aluminum fluoride (AlF3) using TMA and hydrogen fluoride pyridine (HFPy). We also explored the effects of the individual TMA and HFPy precursors on the cathode surfaces. The surface composition and microstructure of these cathode materials, which range from simple transition metal oxides (e.g., NiO and MnO) to complex multi-element cathode materials (e.g., LiNixMn1-x-yCoyO2, NMC), were studied via X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM). The XPS measurements reveal that the transition metals in the cathode materials undergo selective oxidation/reduction depending upon the nature of the precursor, the coating, and the cathode material. Furthermore, XPS and STEM measurements show the conversion of surface carbonate species to the corresponding metal fluorides upon HF exposure. This conversion reaction is self-limiting but extends hundreds of nanometers below the surface in the case of Li2CO3. ALD and chemical vapor treatment provide new avenues to systematically control the interface of the cathode materials in LIBs that are not possible by conventional coating methods.

Improved Properties of Li-Ion Battery Electrodes Protected By Al2O3 and ZnO Ultrathin Layers Prepared By Atomic Layer Deposition

Surface modification using thin layers grown by atomic layer deposition (ALD) is an effective strategy for performance improvement of Li-ion batteries. Ultrathin Al2O3 layers are the most studied coating material. In our contribution we compare properties of Al2O3 and ZnO ultrathin layers prepared by ALD on lithium-iron-phosphate (LFP) cathodes and Si-graphite anodes.Ultrathin Al2O3 and ZnO films were grown at 100 °C using trimethylaluminum (TMA) and diethylzinc (DEZ) precursors, respectively, with water vapors as reactant. The growth rates of the Al2O3 and ZnO films were about 0.1 nm/cycle on control flat Si substrates. ALD layers thickness in this study ranged from 5 to 20 cycles. A modified deposition recipe utilizing a prolonged precursor dwell time in the reactor chamber for coverage of the highly porous battery electrodes has been utilized. As TMA, DEZ and H2O all have rather high vapor pressures, we assume that significant part of the electrodes were coated.Porous LFP cathodes (NANOMYTE BE-60E, NEI Corp.) with the thickness of 70 μm were used as a substrate. The average particle size of the LFP substrate was ~ 2 μm with the specific surface area of 15 m2/g and active material loading of 7.3 mg/cm2. Experimental capacity C of the LFP electrode is 170 mAh/g in the voltage range 2.5 - 4.1 V using 0.1 charging/discharging c-rate.As an anode material porous silicon-graphite composite electrode sheets NANOMYTE BE-150E (NEI Corp) with the thickness of 65 μm and composition of active material 20% Si and 65 % graphite were used. The nominal capacity C of the silicon-graphite anode is 750mAh/g in the voltage range 0.05 - 1V using 0.05 charging/discharging c-rate.In our contribution we present specific discharge capacity of the electrodes as a function of charging/discharging cycles. For both pristine and Al2O3 protected LFP cathode sheets specific discharge capacity of 160 mAh/g was achieved at the c-rate of 0.2. The specific capacity of the pristine LFP electrode dropped from 160 mAh/g at the c-rate 0.2 to about 90 mAh/g for the c-rate 1, while the electrode protected by 0.5 nm of Al2O3 saturated at 120 mAh/g. Properties of ZnO- protected LFP cathodes are compared to that covered by Al2O3.Electrochemical properties of the silicon-graphite anode were studied using low c-rate of 0.05. During the first cycles the specific capacity is relatively low due to the formation of the solid-electrolyte interface layer at the surface. After several cycles the capacity increased to the nominal value of 750 mAh/g. In our study we compare the charging/discharging capability of the pristine silicon-graphite anode and that of the electrodes covered by 5 -20 ALD cycles of Al2O3 and ZnO films during the first charging/discharging cycles. We discuss the effect of ALD protecting layers on the rate capability of the silicon-graphite anode. Finally, the results of the electrochemical measurements are compared to those obtained by X-ray photoelectron spectroscopy.

Application of Parametric Modeling in Atomic Layer Deposition

Atomic Layer Deposition (ALD) is sequential and conformal process that provides precise thickness control due to its self-limiting nature even in high aspect ratio structures. The exponential growth in the use of ALD is due the availability of hundreds of precursor chemistries for diverse technological applications beyond just semiconductors and microelectronics. However, one of recurring challenges is the problem of results disparity, and errors that underline the causes of disparity, primarily may be due to lack of consistency in process optimization and reproducibility. In this research, a Design of Experiment (DoE) approach is adopted for parametric optimization for synthesis of Al2O3 thin films using ALD. The objective of this work is to adopt DoE for reproducible ALD synthesis processes within statistical reliability.Briefly, the precursors used for Al2O3 ALD (Ultratech Veeco Thermal ALD) were Trimethylaluminum (TMA) and H2O with Ozone. The effect of four parameters, namely pulse duration of TMA and water, number of cycles, and temperature were evaluated for the response parameter of Al2O3 thickness on Si with 300 nm of SiO2. E-Chip DoE software was chosen for data modeling. E-Chip considers multivariate analysis to provide the parametric relationship for Analysis of Variances (ANOVA) and predictive surface response analysis, thus eliminating the need for extensive process optimization experiments that are required for full factorial design. Initially, screening experiments were conducted to determine the levels of input parameters. The pulse duration for TMA and H2O were 0.01-0.02 sec, the number of cycles were varied between 50 and 100, the temperature range was 100-200 0C. For each sample, five thickness and roughness readings were measured on an ex-situ ellipsometer and Atomic Force Microscope (AFM). The stoichiometry of Al2O3 thin films were evaluated using XPS.The data analysis reveals that the influential parameters are number of cycles, pulse duration, and temperature in the order of their significance. The direct relationship between number of cycles and Al2O3 thickness infers that flux available is effectively used for layer growth. The consistency of layer thickness is related not just to the conformality but also to the adsorption of reactive species effectively. Uneven pulse duration also contributes to the linear growth of layer thickness, which supports the effectiveness of reactive species saturation and purging of un-reactive elements. The temperature variation was also an influential parameter as it affected the vapor pressure for TMA and H2O. This mechanistic model based on DoE is a suitable tool to predict the ALD’s growth per cycle, which can further help in understanding of any chemisorption with substrate treatment or variation in film roughness characteristics. Study of other DoE parameters such as type of substrate, substrate functionalization, reactor size in relation to the size of substrate, and purge duration are ongoing. Figure 1 describes the ANOVA chart estimates and surface response graphs, which identifies the bounded parametric region along with the ability to predict the influence of change in parameters based on developed mechanistic model. Figure 1

(Invited) Investigating Surface Reaction Thermodynamics: In Situ Calorimetry for Atomic Layer Deposition

Atomic layer deposition (ALD) is comprised of sequential self-limiting surface reactions that are inherently sensitive to the surface chemistry of the substrate and the reactivity of the chosen precursors. In order to better understand ALD processes and direct their future development we must investigate the fundamental mechanisms of ALD reactions. This is commonly done using a combination of computational modeling and experimental investigations using techniques like spectroscopic ellipsometry and IR spectroscopy. We present pyroelectric calorimetry for ALD as an in situ technique to measure the time resolved heat generation from ALD reactions. This provides a new dimension of thermodynamic and kinetic reaction data.As an experimental technique, pyroelectric calorimetry measures ALD reactions on realistic surfaces as opposed to idealized models. We have designed and constructed custom calorimeters that are compatible with typical ALD process parameters. These calorimeters have been calibrated with a thermal resolution down to 0.1 μJ/cm2 and a temporal resolution of 50ns. To put this in perspective for an ALD reaction, this corresponds to about 0.1% of the heat evolved in the trimethylaluminum (TMA) and water ALD process and about 104 faster than our measurement of the TMA half reaction. The temporal resolution of our calorimeters is orders of magnitude faster than most complementary in situ analysis techniques including ellipsometry, IR spectroscopy, quartz crystal microgravimetry, and mass spectrometry. This time resolution provides information on precursor flow dynamics and reaction kinetics.Pyroelectric calorimetry was used to investigate the TMA and water half reactions for ALD Al2O3 along with in situ spectroscopic ellipsometry thickness measurements and Rutherford backscattering spectrometry (RBS) composition analysis. This enabled comparisons of the change in thickness with the amount of heat generated as each half reaction saturates. Atomic growth rates calculated form RBS were used to calculate the heat generated on a per-atom basis. These results were then compared with proposed reaction mechanisms and energy changes from density functional theory (DFT) computational modeling.Measurements of reaction and adsorption heats can also be used to compare and evaluate different precursors and inhibitor molecules for ALD growth inhibition in area-selective ALD. We investigated how the thermodynamics of adsorption can guide our choice of precursor-inhibitor pairs when using small molecule inhibitors that rely on competitive adsorption and chemical passivation to prevent ALD growth. Pyroelectric calorimetry offers new insight into what drives and limits ALD reactions and provides many opportunities for future investigations.

Investigating the Thermo-Oxidative Degradation and Operational Limits for Hybrid AlOX–PET Fabrics Created Via Vapor Phase Infiltration (VPI)

In vapor phase infiltration (VPI), vapor phase metalorganic precursors are diffused into polymers and further reacted with water vapor to form hybrid organic-inorganic materials. Hybrid materials created through VPI have shown utility in industrially relevant applications including membranes for chemical separation, photovoltaics, and catalysis. VPI can be conducted at different temperatures, with each processing temperature resulting in different chemical states that alter the properties of the hybrid material. In this project, polyethylene terephthalate (PET) fabrics are infiltrated with a trimethylaluminum (TMA) precursor and co-reacted with water vapor at a range of infiltration temperatures (60-140 ˚C) to create hybrid AlOX - PET fabrics. The purpose of this project is to develop a set of thermal operational limits for these hybrid organic-inorganic materials and translate techniques for thermal characterization from the textile community to the VPI community. Thermo-oxidative degradation for the hybrid fabrics was quantified through changes in degradation temperature and activation energies. Thermogravimetric analysis (TGA) was employed to quantify inorganic loadings (~20 weight percent inorganic), degradation temperatures, and the activation energy for degradation using the Flynn/Wall/Ozawa (FWO) method for decomposition kinetics. Most hybrid fabrics demonstrated lower degradation temperatures than neat PET, except for the fabric infiltrated at 140˚C which had increased degradation temperatures. Activation energies for the first thermal-oxidative degradation event for the 80˚C, 100˚C, and 140˚C fabrics decreased by a range of 62˚C to 84˚C compared to neat PET, while the 120˚C fabric’s activation energy slightly increased by 10˚C. Additionally, a new degradation event occurred for fabrics infiltrated at low temperatures which was explored through FTIR characterization and attributed to structural rearrangements of the inorganic component. Overall, this work demonstrates how VPI process conditions alter both the organic-to-inorganic chemical binding as well as the inorganic cluster structure in these hybrids and how these structural features affect thermophysical operational limits of the material.

Characterization of Novel ALD Process for the Synthesis of CaO for 12CaO·7Al2O3 electride Applications

In this work we report on a novel CaO ALD process development with a little used solid ALD precursor for Calcium. This forms part of a larger effort to synthesize 12CaO·7Al2O3 [C12A7], which is a new electronic oxide compound with very low work function and applications potentials in displays and Field-Emission (FE) Cathodes. Currently there are no existing thin film technologies for the conformal deposition of C12A7 electride films on complex device surfaces with nanoscale precision. Use of ALD to obtain precise nanoscale interleaving of CaO and Al2O3 layers over Micro-Hollow Cathode Discharge devices MHCD or FE devices allows conversion to correct alloying of C12A7 in the desired nano-layers. Success in reproducibly obtaining these layers will significantly benefit applications using cathodes, such as light sources, nano-sat thrusters, vacuum electronics and small-format microwave amplifiers. For the final C12A7 film, the challenge is to develop two ALD processes for two binary films in this case CaO and Al2O3 which must be synthesized as alternating nanolaminates in the correct stoichiometric ratio and are subsequently reacted under high temperature annealing. Since ALD of Al2O3 is already a well-known established ALD process, we had to focus on developing an ALD process for CaO. For this study we decided to utilize the relatively novel solid ALD precursor Bis(N,N-di-iso-propylformamidinato)calcium(II) dimer [Ca(famd)2], which is a low vapor pressure solid Ca amidinate precursor and which has to be reacted with ozone or DI H2O vapors. The chemical structure of the Calcium precursor is shown schematically in Fig. 1. The Al2O3 binary compound will be synthesized with trimethylaluminum (TMA) and DI water vapor. The low vapor pressure of Ca amidinate precursor Ca(famd)2 in solid form constitutes a major technical challenge and cannot be reacted by ALD in vapor draw mode but necessitated as new hardware a 150mL booster bubbler with dip tube for bubbling the molten precursor and the installation of a Low Vapor Pressure Delivery (LVPD) kit with bypass. For this presentation we will report the physical characterization of the ALD CaO film with FE-SEM surface morphology studies, SEM EDS elemental analysis of the final composition, X-ray diffraction XRD analysis and atomic force microscopy AFM. Fig. 1: Schematic of chemical structure of the novel solid ALD Ca amidinate precursor Bis(N,N-di-iso-propylformamidinato)calcium(II) dimer [Ca(famd)2] Figure 1

Exploring the Vapor Phase Infiltration of Trimethylaluminum into Polyacrylonitrile Fabrics

Vapor phase infiltration (VPI) creates a hybrid organic-inorganic material by modifying the bulk of a polymer substrate with a metalorganic vapor phase precursor. In VPI, the metalorganic precursor sorbs to the polymer, then diffuses through the bulk and becomes entrapped—either by a chemical bond between the polymer and the inorganic or by a co-reaction with a second species—yielding a hybrid composed of the polymer and, often, a metal oxide. Some of the polymers that have been studied under VPI include poly(methyl methacrylate) (PMMA), polyethylene terephthalate (PET), polydimethyl siloxane (PDMS), and poly(3-hexylthiophene) (P3HT). There is a notable exception in this list: polyacrylonitrile (PAN). The high reactivity of the nitrile functional group on the backbone of the PAN polymer chain suggests PAN would perform well in VPI. PAN is used in textiles and filtration, but one of its most common applications is as a precursor to carbon fiber. Previous hybrid materials created via VPI have demonstrated new optical and thermal properties that differ from the neat polymer. If infiltrated successfully, these properties in PAN could be of use in its application spaces. In this work, we expand the VPI material space with the first study of infiltration of PAN. To explore the bounds of this system in terms of achievable inorganic loadings and hybrid material chemical structure, PAN is infiltrated with the metalorganic precursor trimethylaluminum (TMA) and co-reacted with water vapor under variable precursor exposure times, precursor desorption times, and processing temperatures. PAN is found to infiltrate with high quantities of inorganic loading (as measured by thermogravimetric analysis) that can be tuned from 1% to 17% inorganic by weight. Inorganic loading depends strongly on precursor exposure time with a sample processed at 140℃ showing a 25% increase when the exposure time is increased from 5 hours to 25 hours. Infiltration temperature also plays a significant role with hybrid materials demonstrating a 78% increase in inorganic loading when the PAN processing temperature is increased from 80℃ to 140℃. The apparent increase in inorganic loading with temperature may be related to the mass transport and reaction kinetics of the system under VPI conditions. Precursor desorption time, on the other hand, has minimal impact on inorganic loading suggesting most sorbed precursor participates in a chemical interaction with the polymer during infiltration. In exploring the resulting AlOx/PAN hybrid chemical structure with FTIR, this mechanism was supported by an observed decrease of the PAN nitrile peak at ~2240 cm-1 that scaled with inorganic loading. These structural changes in inorganic loading and chemical state are found to directly correspond to the optical (UV-Vis) and thermal (TGA) properties of the hybrid materials. The results of these characterization techniques imply that the VPI process causes a significant chemical change to the PAN fabric and suggests the process could possibly induce PAN cyclization, which may be advantageous for carbon fiber applications. Overall, this work establishes the significant and tunable inorganic loading of AlOx/PAN hybrid fabrics and expands VPI to a new polymer system.

Fabrication of Block Copolymer Templated Extended Surface Model Electrocatalysts By Atomic Layer Deposition and Physical Vapor Deposition

Templating platinum group metal (PGM) nanostructures from microphase separated block copolymers (BCPs) allows for electrocatalytic activity to be studied on highly ordered, model surfaces with precisely controlled feature sizes and morphologies, thereby allowing catalyst electroactivity and stability to be probed as a function of nanoscale geometry. In our previous work [1], BCP templated electrocatalysts fabricated via incipient wetness impregnation of PGM precursors were tested for hydrogen oxidation/evolution and water-splitting activity on interdigitated electrodes using liquid and solid polymer electrolytes. This talk presents the fabrication of such electrocatalysts via Atomic Layer Deposition (ALD) and Physical Vapor Deposition (PVD) techniques (e-beam evaporation and sputtering). Self-assembled lamellae of poly-(styrene-block-methylmethacrylate) (PSbPMMA) thin films on Si wafer and glassy carbon substrates were subjected to 5 - 15 ALD cycles with trimethylaluminium (TMA) precursor to induce selective growth of alumina in the PMMA domains via Sequential Infiltration Synthesis. Thereafter, the polymer template was removed via Reactive Ion Etching (RIE) with oxygen plasma. The resultant alumina nanostructures templated from self-assembled PSbPMMA was then used as a substrate either for Platinum ALD with (trimethyl)methylcyclopentadienylplatinum(IV) precursor atop a Titanium Nitride adhesive layer, or for sputtering of Pt atop a Ti adhesive layer to obtain conformal Pt coatings of up to 20 nm in thickness. When e-beam evaporation was used, The PMMA domains were selectively removed by successive wet and dry etch steps, followed by deposition and lift-off. The PGM electrocatalyst nanostructures were characterized via a combination of electron microcopy, X-ray diffraction, and X-ray photoelectron spectroscopy, and the Pt content of the catalysts were determined via microwave leaching and digestion of the electrocatalyst thin film followed by Inductively Coupled Plasma Mass Spectroscopy. The nanostructured electrocatalysts were tested for oxygen reduction (ORR) activity in a conventional rotating disk electrode setup using 0.1 M perchloric acid electrolyte, and the ORR mass activities and active surface area were obtained.[1] Bhattacharya, D., Arges, C. G., et al., Small, 2021, 17, 2100437

Chemical Stability of Atomic Layer Deposited (ALD) Alumina Thin Films in Aqueous Solutions

Aluminum oxide (alumina) thin films deposited through atomic layer deposition (ALD) are of great interest in chemical barrier and corrosion protection applications. However, the stability of ALD alumina in aqueous solutions is still not fully understood. Due to its metastable amorphous phase, the hydration and degradation behavior of ALD alumina films behaves differently from its crystalline Al2O3 counterpart. A full understanding of why these films hydrate and/or dissolve requires the exploration of different deposition conditions and ion content in solutions used. This talk will discuss efforts to further elucidate the hydration and dissolution behavior of ALD alumina films. For this study, alumina thin films were ALD deposited onto silicon substrate at 150 °C using trimethylaluminum (TMA) and H2O. These films were then studied in Type 1 DI water, 2M NaCl, .5M NaCl, and .1M NaCl solutions at room temperature. Films were gently dried using a nitrogen gun and thickness was measured using a Cauchy ellipsometry model. After 15 days of immersion in Type 1 DI water, significant thickness growth is observed at twice (27 days) and 2.5 times (33 days) the normalized thickness. Similar hydration is not observed in salt-containing aqueous solutions nor upon exposure to air. We will discuss the possible effects of CO2 dissolution and carbonate formation as well as ionic species on the hydration and dissolution processes of these alumina films. Finally, we will show how a post-ALD plasma treatment to the film’s surface alters the hydration and dissolution behavior, improving stability in several aqueous conditions.

Optimizing Aluminum Oxyhydroxide Vapor Phase Infiltration for the Vapor Phase Mordanting of Natural Dyes to Polyester Fabrics

Industrial textile dyeing and finishing processes are a significant environmental concern producing large quantities of wastewater that require significant treatment to remove unused synthetic chemicals. Polyester fabrics in particular utilize high temperatures and chemical carrier agents in their dyeing processes. In this work, we present a method for attaching natural dye materials derived from madder root to polyethylene terephthalate fabrics via the introduction of aluminum oxyhydroxides through vapor phase infiltration. Vapor phase infiltration (VPI) is an emerging polymer modification technique that exposes a polymer to metalorganic precursors that diffuse into the polymer and become entrapped as inorganic clusters. The resulting hybrids have unique properties of both organic and inorganic materials. The VPI process has found significant use in modifying the properties of textile materials including their mechanical and optical properties. Here, without VPI treatment, the PET fabric cannot be dyed with this natural dye. This talk will discuss how controlling the inorganic loading of hybrid AlOx-PET fabrics infiltrated with trimethylaluminum (TMA) and co-reacted with water vapor at 80 ˚C and 140 ˚C can optimize fabrics for dyeability while minimizing impact on key fabric mechanical properties. At low temperatures (80 ˚C) the resulting hybrid contains inorganic clusters not strongly bound to the polymer chain while at high temperatures (140 ˚C) the inorganic is chemically bound via the carbonyl group. Inorganic loading was controlled by varying the dose pressure of TMA (moles of TMA) and fabric mass (moles of carbonyl functional group) during VPI and quantified using thermogravimetric analysis (TGA). The resulting hybrid AlOx-PET fabrics had various inorganic loadings from one weight percent aluminum oxyhydroxide to above twenty weight percent. The hybrid fabrics were then dyed with Alizarin (derived from madder root) and the dye absorbance was quantified with UV-Vis spectroscopy. At low inorganic loadings for both temperatures, the strength of color varied with inorganic content, but a steady-state absorbance was reached at around 1.8 percent inorganic loading. At these low inorganic loadings, the hybrid fabrics maintain key mechanical behaviors such as stiffness (as measured by drape) which is seen to increase significantly with additional inorganic loading. In addition to optimizing strength of color and fabric stiffness, the time to dye saturation was also explored by varying dye times and quantifying with UV-Vis. By exploring dye saturation curves for different inorganic loadings and infiltration temperatures, kinetic information was gathered to further optimize this dyeing process for industrialization. Overall, using VPI as a vapor phase mordanting process to fix natural dyes to PET fabrics illustrates the impact that even small quantities of the vapor deposited inorganic can create enough of a base for dye to adhere to, highlighting the practical use of these fabrics in the field of textile sustainability.

Using Density Functional Theory and Machine Learning to Predict the Binding Energies of Metal-Organics to Organic Functional Groups for Hybrid Material Creation

Understanding chemical interactions between organic and metal-organic molecules has wide-ranging interest to the vapor deposition community for creating hybrid organic-inorganic materials via techniques such as molecular layer deposition and vapor phase infiltration (VPI). In the case of VPI, a vapor-phase metal-organic precursor is infused into the bulk of a polymer and becomes incorporated at the nanoscale through either chemical interaction with the polymer or the formation of a non-volatile species via the introduction of a co-reactant. VPI has applicability in a number of industrially relevant fields including the creation of novel organic-inorganic hybrid membranes which have shown enhanced stability in organic solvents, while retaining high permeance and selectivity. Motivated by this application, this work uses density functional theory (DFT) to explore chemical interactions occurring during the VPI of polymer of intrinsic microporosity (PIM-1, a polymeric membrane material) with trimethylaluminum (TMA) and its co-reaction with water. These computations revealed that the coordination between the polymer and metal-organic is a critical mechanism for the formation of the hybrid and its resultant solvent stability. To expand understanding of this critical characteristic and accelerate the design of organic-inorganic hybrid materials, a DFT dataset of computed binding energies was generated from suitable and representative atomic-level models of several common polymer functional groups and over 100 metal-organic precursors. From this dataset, a predictive machine learning model for the binding energy of metal-organic molecules to polymers has been developed. This predictive model, along with the chemical guidelines obtained from feature analysis, will aid the selection of potential candidates for novel organic-inorganic hybrid membranes as well as hybrid material creation as a whole.

Kinetics of TiCl4 Vapor Phase Infiltration (VPI) into PMMA and the Resulting Thermophysical and Optical Properties of the TiOx -PMMA Hybrids

Vapor phase infiltration (VPI) is a post-polymerization modification technique that imbues inorganic materials into polymers to create organic-inorganic hybrid materials with new properties distinct from the parent polymer. While several VPI precursor-polymer chemistries have been explored, a lack of chemical intuition remains for fully understanding the thermodynamics and kinetics that govern the VPI process. This study seeks to continue to build this knowledge by examining the VPI process kinetics for TiO2 infiltration into PMMA via the use of TiCl4 and H2O precursors. In this research, polymethylmethacrylate / TiO2 hybrid materials are prepared using VPI. The depth of infiltration of the TiO2species into ~200 nm PMMA thin films is studied using x-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FTIR) spectroscopy and spectroscopic ellipsometry. The kinetics for TiCl4 infiltration increases with both VPI process temperature and TiCl4 exposure time. However, the rates of infiltration are considerably slower than those observed in the more commonly studied trimethylaluminum (TMA) / PMMA system. Even at 150 °C, process times of at least 12 hours are required to fully infiltrate a 200 nm PMMA film whereas using TMA similar films are fully infiltrated within 1 hour at the same process temperature. Films that we believe to be fully infiltrated at 150 °C and 24 hours of TiCl4 exposure have a 6 at% Ti in the innermost bulk, as determined by XPS. Interestingly, unlike AlOx-PMMA hybrids, these TiOx – PMMA hybrids exhibit significant changes in their optical properties. Increased titanium loading leads to a 4% increase in refractive index and increased UV absorbance in the UV range of 270-350nm. Furthermore, films infiltrated at 150 °C and 24 hours of TiCl4 exposure had a 50 % and 70 % reduction in coefficients of thermal expansion (CTE) below and above Tg respectively. Reduced CTE trends with increased titanium loading. We will discuss possible opportunities to use these new properties for various applications.This project is supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Enhanced Performance of LiFePO4 Cathodes Protected By Atomic Layer Deposited Ultrathin Alumina Films

The LiFePO4 (LFP) cathode has a theoretical specific capacity of 170 mAh/g. This material is stable, safe, and environmentally benign. Li-ion batteries with LFP cathodes have a long cycle life with excellent charging/discharging performance. In our contribution, we show that the surface modification of the LFP cathode using ultrathin alumina films grown by atomic layer deposition (ALD) can improve Li-ions charge transfer and rate performance of the Li-ion in half-cell configuration with the LFP cathode.We used two types of LFP cathodes in our study with Li-metal as an anode in a half cell configuration. The first LFP electrode had a thickness of 25 μm and consisted of particles of 0.5 - 1 μm in size [1], while the second one (commercial NANOMYTE BE-60E (NEI Corp.)) had a thickness of 70 μm and the average LFP particle size was 2 μm.Ultrathin Al2O3 films were grown on LFP cathodes by ALD at 100 °C using trimethylaluminum (TMA) and water vapors. Due to the self-saturation surface-limited nature of the ALD reaction, even substrates of highly complex morphology can be uniformly coated, where the limitation is the reach of the precursor vapor molecules. The thickness of the ALD layers depends linearly on the number of ALD cycles and ranges from 0.5 to 2 nm for 5 to 20 cycles.Galvanostatic charging/discharging experiments were performed to evaluate the rate capability of the electrodes. Pristine and Al2O3 coated LFP electrodes were laser cut to 18 mm diameter circular electrodes, inserted in the electrochemical test cells PAT-Cell (EL-CELL), and tested using PAT-Tester-x-8 analyzer. LFP with Al2O3 protecting layers was compared to the pristine LFP electrode.For the LFP cathode with the thickness of 25 μm the pristine sample saturates at the specific capacity of 32 mAh/g at 1.8 c-rate, while for the electrode treated with 5 and 20 cycles of Al2O3 the specific capacity dropped to 80 and 60 mAh/g, respectively. The electrodes protected by Al2O3 film always exhibited better rate capability than the pristine sample.A more detailed analysis of the performance was accomplished for the NANOMYTE electrodes. A specific discharge capacity of 160 mAh/g at a c-rate of 0.2 was achieved for both pristine and Al2O3-protected electrodes. Two supercycles with the c-rate 0.2, 0.5, and 1 were applied to the samples. The specific capacity of the pristine LFP electrode dropped from 160 mAh/g for the c-rate 0.2 to about 90 mAh/g at the c-rate 1, while the electrode which was protected by 5 cycles of Al2O3 exhibited a capacity of 120 mAh/g.Investigation of the rate capability fading using electrochemical impedance spectroscopy revealed a gradual increase of the Nyquist plot semicircle diameter as a function of cycles. According to the equivalent circuit model, the semicircle diameter corresponds to the charge transfer at the solid/electrolyte interface. The charge transfer resistance at the solid/electrolyte interface increases with charge/discharge cycling. The charge transfer resistance of the pristine electrode increased more steeply than that of the electrode with 5 cycles of Al2O3. Improvement of the charge transfer resistance of the LFP electrode covered by thin Al2O3 film can be explained by the formation of the Li3.4Al2O3 phase upon lithiation. The diffusivity of the Li ions in the Li3.4Al2O3 phase is several orders of magnitude higher than that of in the Al2O3 film [2]. The results obtained by electrochemical characterization are compared to the X-ray photoelectron spectroscopy performed on samples before and after galvanostatic charging/discharging measurements.[1] A. Fedorkova et al., J. Power Sources 195 (2010) 3907.[2] S. C. Jung et al., J. Phys. Chem. Lett. 4 (2013) 2681.

(Invited) Ligand Addition for Thermal Atomic Layer Etching of Metals

Thermal atomic layer etching (ALE) is typically defined by sequential surface reactions. Thermal ALE can be viewed as the reverse of atomic layer deposition (ALD). Thermal ALE is usually performed with two steps: modification of the surface and removal of the modified surface. The modification can be oxidation, fluorination, or chlorination. The removal step is often a ligand-exchange reaction, creating stable volatile species that leave the surface, leading to a net removal of material. For example, aluminum oxide can be etched using thermal ALE with HF to fluorinate the surface and trimethylaluminum (TMA) to exchange methyl ligands for surface fluoride ligands, resulting in volatile Al(CH3)xFy species.Thermal ALE of metals is particularly challenging because the oxidation state of the metal must be changed to match the oxidation state of the volatile metal etch product. This oxidation is performed in the modification step of thermal ALE. After changing the oxidation state, the metal then produces a stable complex during the volatile release reaction. In this work, Ni and Co ALE are developed by modifying the metal via chlorination using SO2Cl2. Subsequently, the metal is removed by addition of an L ligand to the metal center to create a stable, volatile etch product, MClxLy (Figure 1). The L ligands explored in this work were PMe3 and tetramethyl ethylenediamine (TMEDA).The chlorination and ligand-addition approach for metal ALE is based on the Covalent Bond Classification (CBC) method. X ligands are one-electron donors like Cl and other halogens. L ligands are two-electron donors like PMe3. TMEDA is a bidentate ligand. One TMEDA ligand acts as two L ligands since TMEDA binds to the metal center through the lone electron pairs on the two nitrogen groups. According to the CBC method, nickel complexes typically have NiX2L3 or NiX2L2 configurations. Likewise, cobalt complexes are typically found in the CoX2L4, CoX2L2, or CoX3L3 configurations. The goal for Ni or Co ALE is to create these volatile metal compounds using sequential SO2Cl2 and L ligand surface reactions.Both Ni and Co ALE have been studied with in situ quartz crystal microbalance (QCM) and X-ray reflectivity (XRR) to determine the etch rates. Quadrupole mass spectrometry (QMS) was also employed to determine the volatile metal-containing etch species. Using SO2Cl2 and PMe3, Ni etch rates varied from 0.1-3 Å/cycle at temperatures from 75-200 °C. QMS measurements also identified NiCl2(PMe3)2 (NiX2L2) as the stable volatile etch product. Cobalt ALE using SO2Cl2 and TMEDA displayed etch rates from 1-12 Å/cycle at temperatures from 175-300 °C. QMS measurements also detected CoCl2(TMEDA) (CoX2L2) as the stable volatile etch product. In addition, cobalt can be etched using SO2Cl2 and PMe3 from 70-200 °C, with etch rates between 2-4 Å/cycle. These two Co ALE pathways with TMEDA and PMe3 are complementary with TMEDA useful at higher temperatures and PMe3 useful at lower temperatures.Many other X and L ligands could be used for the halogenation and ligand-addition reactions during metal thermal ALE. The CBC method can be used as a guide to develop the X ligands for surface modification and the L ligands for volatile release. This approach should lead to various other metal ALE processes. Selective metal thermal ALE should also be possible using different X and L ligands. Figure 1

Sequential Infiltration Synthesis of Al2O3 in Biodegradable Polybutylene Succinate: Characterization of the Infiltration Mechanism.

The introduction of inorganic materials into biopolymers has been envisioned as a viable option to modify the optical and structural properties of these polymers and promote their exploitation in different application fields. In this work, the growth of Al2O3 in freestanding ∼30-μm-thick poly(butylene succinate) (PBS) films by sequential infiltration (SIS) at 70 °C via trimethylaluminum (TMA) and H2O precursors was investigated for the first time. The incorporation of Al2O3 into the PBS matrix was clearly demonstrated by XPS analysis and SEM-EDX cross-sectional images showing a homogeneous Al2O3 distribution inside the PBS films. Raman measurements on infiltrated freestanding PBS show a reduction of the signal related to the ester carbonyl group as compared to pristine freestanding PBS films. Accordingly, FTIR and NMR characterization highlighted that the ester group is involved in polymer–precursor interaction, leading to the formation of an aliphatic group and the concomitant rupture of the main polymeric chain. Al2O3 mass uptake as a function of the number of SIS cycles was studied by infiltration in thin PBS films spin-coated on Si substrates ranging from 30 to 70 nm. Mass uptake in the PBS films was found to be much higher than in standard poly(methyl methacrylate) (PMMA) films, under the same process conditions. Considering that the density of reactive sites in the two polymers is roughly the same, the observed difference in Al2O3 mass uptake is explained based on the different free volume of these polymers and the specific reaction mechanism proposed for PBS. These results assessed the possibility to use SIS as a tool for the growth of metal oxides into biopolymers, paving the way to the synthesis of organic–inorganic hybrid materials with tailored characteristics.