Atomic Layer Deposition Chemistry Research Articles

Conformal coating of macroscopic nanoparticle compacts with ZnO via atomic layer deposition

Conformal atomic layer deposition (ALD) inside macroscopic nanoporous solids with aspect ratios greater than 103 can require ALD reactant exposures on the order of 103 Torr-s or greater. For some ALD chemistries, such large exposures raise the concern of non-self-limiting deposition. In the case of ZnO ALD from diethylzinc (DEZ) and H2O, exposures in the 10–103 Torr-s range have resulted in metallic Zn deposition at typical temperatures used for ZnO ALD on wafers (e.g., ∼180 °C). This Zn deposition can be suppressed by lowering the deposition temperature, but this slows H2O desorption and, thus, can necessitate impractically long purge times. In this work, we use static-dose ALD with DEZ and H2O exposures >104 Torr-s to deposit ZnO inside Al2O3 nanoparticle compacts (NPCs) with 50.5 ± 0.3% porosity, 100 nm NP diameter, 1.55 ± 0.05 mm thickness, and an aspect ratio of 7800 ± 200 (based on the half-thickness), and we explore a novel approach to the deposition temperature, T: T is cycled between 160 °C (for H2O purges) and 120 °C (for all other steps). For comparison, we also deposit ZnO with T held constant at 120 or 160 °C. Whereas the T = 160 °C process results in Zn metal deposition and nonuniform infiltration, the temperature-cycled process yields apparently self-limiting ZnO deposition at a growth per cycle (GPC) of ∼2.1 Å/cyc, forming an electrically conductive ZnO network that is uniform throughout the thickness of the NPC, with the exception of some ZnO depletion near the NPC surfaces, possibly due to the (unoptimized) long DEZ purge time. The T = 120 °C process produces similar results, although the GPC is slightly elevated, indicating diminished removal of H2O and/or OH during purges. We employ scanning electron microscopy with energy-dispersive x-ray spectroscopy, x-ray diffractometry, electrical resistivity measurements, and ALD chamber pressure analysis in our comparison of the three ALD processes.

Atomic layer deposition mechanism of hafnium dioxide using hafnium precursor with amino ligands and water

As a unique nanofabrication technology, atomic layer deposition (ALD) has been widely used for the preparation of various materials in the fields of microelectronics, energy and catalysis. Hafnium dioxide (HfO2), as an excellent high-κ oxide, is widely used as a substitute gate dielectric for conventional SiO2 in field effect transistor devices. Currently, HfO2 high-κ gate dielectric in MOSFET devices was prepared via ALD technology using tetrakis(dimethylamino)hafnium(IV) (Hf(NMe2)4, TDMAHf) and water (H2O) as the precursors. In this work, the complicated surface reaction mechanism of HfO2 H2O-based ALD using Hf(NMe2)4 was investigated using density functional theory calculations. In the TDMAHf reaction, the dimethylamino group of the precursor was eliminated by ligand exchange reaction with the hydroxyl group on the surface. Subsequently, a second dimethylamino group of TDMAHf could further be eliminated by another hydroxyl group on the surface. In the H2O reaction, ligand exchange reactions and coupling reactions could both occur. By the ligand exchange reactions, water molecule could react with the dimethylamino group on the surface. Simultaneously, the hydroxyl and amino groups could also react with neighboring hydroxyl groups to eliminate dimethylamine and water molecules via the coupling reactions. All these insights into the complicated surface reaction mechanism of H2O-based ALD could promote the further development of ALD chemistry and materials.

Time evolution of surface species during the ALD of high-k oxide on InAs

Understanding the reaction mechanisms involved during the early stage of atomic layer deposition (ALD) of HfO2 on InAs is a key requirement for improving interfaces in III-V semiconductor-based devices. InAs is an excellent candidate to outperform silicon regarding speed and power consumption, and combined with HfO2, it gives promise for a new generation of ultra-fast MOSFETs. However, an improved interface quality and in-depth understanding of the involved surface species are needed. Here, we use in situ and operando ambient pressure XPS to follow in real-time the reaction mechanisms which control the ALD chemistry. Besides the removal of all unwanted oxide from the III-V, the same oxygen atoms are found to form HfOx already from the first half-cycle. In contrast to the standard ALD model, no hydroxyl groups are needed on the InAs surface. Furthermore, we observe an insertion reaction forming unexpected surface species. The second ALD half-cycle allows the immediate removal of all organic species leaving behind a uniform HfO2 layer partially terminated by hydroxyl groups. We find that prolonged exposure times upon both half-cycles guarantee a sharp InAs/HfO2 interface. Such an improved interface is an important step towards fast and sustainable III-V semiconductor-based electronics.

Tunable Sulfur Incorporation into Atomic Layer Deposition Films Using Solution Anion Exchange

Metal sulfide and oxysulfide thin films deposited by atomic layer deposition (ALD) are important functional materials for a range of applications including solar cells, catalysts, and electronic devices. However, ALD of sulfur-containing films typically requires H2S, a toxic, corrosive, and flammable gas. To circumvent these challenges, we propose a method of tuning sulfur incorporation into ALD films by first growing a metal oxide, using CuOx as a model system, followed by a solution-phase sulfur anion-exchange process. By controlling the reaction time, solution molarity, and temperature, we demonstrate tunable sulfur incorporation into the films, which is described using a coupled reaction-diffusion model. The evolution of the film crystallinity, composition, and morphology was quantified as a function of the anion-exchange process parameters. Conformal anion exchange on a ZnO@TiO2@CuO core–shell–shell nanowire is shown, demonstrating the ability to maintain the conformality of the initial ALD process on high-aspect-ratio structures. Additionally, area-selective anion exchange was performed on micro-patterned substrates, illustrating a pathway toward device fabrication. Finally, the electrical properties of the converted films were evaluated, indicating a tunable reduction in sheet resistance of up to 4 orders of magnitude. In the future, the combination of ALD and anion-exchange chemistry can be used to incorporate sulfur without the need for H2S gas, while maintaining the atomically precise and conformal properties of the original ALD process.

Bimolecular Reaction Mechanism in the Amido Complex-Based Atomic Layer Deposition of HfO2.

The surface chemistry of the initial growth during the first or first few precursor cycles in atomic layer deposition is decisive for how the growth proceeds later on and thus for the quality of the thin films grown. Yet, although general schemes of the surface chemistry of atomic layer deposition have been developed for many processes and precursors, in many cases, knowledge of this surface chemistry remains far from complete. For the particular case of HfO2 atomic layer deposition on a SiO2 surface from an alkylamido-hafnium precursor and water, we address this lack by carrying out an operando atomic layer deposition experiment during the first cycle of atomic layer deposition. Ambient-pressure X-ray photoelectron spectroscopy and density functional theory together show that the decomposition of the metal precursor on the stoichiometric SiO2 surface in the first half-cycle of atomic layer deposition proceeds via a bimolecular reaction mechanism. The reaction leads to the formation of Hf-bonded methyl methylene imine and free dimethylamine. In addition, ligand exchange takes place involving the surface hydroxyls adsorbed at defect sites of the SiO2 surface.

Release Rate Studies of 5-Aminosalacylic Acid Coated with Atomic Layer-Deposited Al2O3 and ZnO in an Acidic Environment.

5-Aminosalicylic acid (5-ASA) is a first-line defense drug used to treat mild cases of inflammatory bowel disease. When administered orally, the active pharmaceutical ingredient is released throughout the gastrointestinal tract relieving chronic inflammation. However, delayed and targeted released systems for 5-ASA to achieve optimal dose volumes in acidic environments remain a challenge. Here, we demonstrate the application of atomic layer deposition (ALD) as a technique to synthesize nanoscale coatings on 5-ASA to control its release in acidic media. ALD Al2O3 (38.0 nm) and ZnO (24.7 nm) films were deposited on 1 g batch powders of 5-ASA in a rotatory thermal ALD system. Fourier transform infrared spectroscopy, scanning electron microscopy, and scanning/transmission electron microscopy establish the interfacial chemistry and conformal nature of ALD coating over the 5-ASA particles. While Al2O3 forms a sharp interface with 5-ASA, ZnO appears to diffuse inside 5-ASA. The release of 5-ASA is studied in a pH 4 solution via UV-vis spectroscopy. Dynamic stirring, mimicking gut peristalsis, causes mechanical attrition of the Al2O3-coated particles, thereby releasing 5-ASA. However, under static conditions lasting 5000 s, the Al2O3-coated particles release only 17.5% 5-ASA compared to 100% release with the ZnO coating. Quartz crystal microbalance-based etch studies confirm the stability of Al2O3 in pH 4 media, where the ZnO films etch 41× faster than Al2O3. Such results are significant in achieving a nanoscale coating-based drug delivery system for 5-ASA with controlled release in acidic environments.

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

Continuous, crystalline Sb2S3 ultrathin light absorber coatings in solar cells based on photonic concentric p-i-n heterojunctions

Many modern types of solar cells that rely exclusively on earth-abundant non-toxic materials include interfaces between a heavier metal chalcogenide and another type of semiconductor. Often, the chemical (adhesion) and physical (charge transfer) characteristics of those interfaces are the defining factors for the final device performance. Here, we describe that a ZnS adhesion layer is not sufficient to prevent the dewetting of Sb2S3 upon annealing a thin layer of it on an oxidic surface if the substrate is not planar and features highly curved surfaces. An ALD-coated sacrificial capping layer of ZnO prevents the morphological rearrangements of Sb2S3 during thermal crystallization and can be removed subsequently. When implemented towards a photovoltaic p-i-n heterojunction, this strategy furnishes perfect conformality of the layer stack but unsatisfactory performance. The correlation of interface chemistry with the electrical properties and the device performance identifies a reducing effect of ZnO atomic layer deposition chemistry on the Sb2S3 surface as the cause of Zn diffusion into the light absorbing semiconductor. This deleterious doping can be prevented by a preliminary oxidative treatment of the Sb2S3 surface with ozone. When applied to a structured substrate consisting of ordered arrays of nanospheres, this approach yields the first ever concentric p-i-n heterojunction solar cells with photonic light trapping effect—a geometry which in comparison with standard scattering layers’on top’ inherently generates a very large refractive index contrast. In the red part of the visible spectrum, light absorption amounts to the value expected with four passes through a planar layer of the thickness used here (35 nm Sb2S3). This effect allows us to demonstrate >5% overall solar energy conversion efficiency with only 35 nm of a simple light absorber phase that uses no toxic, rare materials.

Modification of LiMn2O4 surfaces by controlling the Acid–Base surface chemistry of atomic layer deposition

In this combined theoretical and experimental study, we report the effects of Al precursor selection on the growth chemistry, interfacial structure, and electrochemistry of Al2O3 coatings on spinel LiMn2O4 (LMO) surfaces by atomic layer deposition (ALD). Five Al precursors, exhibiting a range of Lewis acid–base properties, were used to establish trends in the ALD Al2O3 growth chemistry on LMO powders in comparison to redox-inactive planar silicon substrates. We show that the Lewis acid–base properties of the Al precursor ligands can be used to tune both the Mn oxidation states and the Al2O3 coverage on the LMO surface. Density functional theory calculations are used to examine the reaction mechanism of each precursor on LMO surfaces, elucidating a correlation between the Lewis acidity of the ligands and the decomposition thermochemistry. In-depth X-ray photoelectron spectroscopy and in situ Fourier transform infrared spectroscopy measurements support these theoretical predictions and further reveal how different Al precursors modify the atomic and electronic structure near the LMO surface during ALD. While the Mn oxidation state is strongly influenced by the Lewis acidity of the precursor ligand, the surface coverage and thickness of the Al2O3 coating are a more representative descriptor of the electrochemical performance measured in coin cell experiments. We show how the ligand acid–base properties of ALD precursors can be used to rationally tailor atomic layer growth mechanisms, which may enable atomic level control over the structure of functionalized interfaces for various applications related to catalysis, semiconductors, and energy storage.

Rational Modification of LiMn2O4 Surfaces By Controlling the Acid-base Surface Chemistry of Atomic Layer Deposition

Next-generation energy storage devices demand superb control over interfaces. Atomic layer deposition (ALD) coatings can suppress undesirable electrochemical reactions between lithium ion battery (LIB) cathode surfaces and the organic liquid electrolyte. However, our limited understanding of ALD cathode coatings restricts our ability to predict which coating/cathode combinations will succeed. In this study, we performed a combined theoretical-experimental study of a model coating/cathode system aimed at elucidating synthesis-structure-property relationships. We selected five ALD Al precursors exhibiting a range of Lewis acid-base properties to establish trends in the ALD Al2O3 chemistry on LiMn2O4. Density functional theory calculations deduced a correlation between the Lewis acidity of the ligands and the ALD thermochemistry. X-ray photoelectron spectroscopy and in situ infrared spectroscopy measurements supported these predictions and further revealed how the Al precursors modify the LiMn2O4 atomic and electronic surface structure. Finally, coin cell electrochemical measurements established the link between materials properties and device performance. We discovered that while the Mn oxidation state is strongly influenced by the Lewis acidity of the precursor ligand, the surface coverage and thickness of the Al2O3 coating are a more representative descriptor of the electrochemical performance measured in coin cell experiments.

Elucidating the Surface Chemistry of Atomic Layer Deposition on Complex Metal Oxide Cathode Materials

Current research in the field of lithium-ion batteries (LIBs) is geared towards stabilizing the electrode-electrolyte interfaces to improve the LIB cycling stability and storage capacity. In particular, irreversible reactions between non-aqueous electrolytes and metal oxide cathode surfaces can corrode the active material and reduce LIB performance. Protective coatings on the cathode surface can mitigate these issues, and atomic layer deposition (ALD) is well-suited to prepare ultrathin, conformal, corrosion-resistant barrier layers on the surface of the electrodes. While there are reports that some metal oxide and metal fluoride barrier coatings on select cathodes can improve LIB performance, there is no general understanding of why certain ALD coating/cathode combinations are beneficial while others are not. This problem is complicated by the move towards complex LIB cathodes such as lithium nickel manganese cobalt oxide (NMC) materials that may have multiple phases present on the surface including hydroxides and carbonates. Understanding how the ALD precursors react on these materials to create protective coatings is challenging and requires a systematic study to elucidate the surface chemistry of the ALD precursors with each component of the complex oxide surface. Therefore, this work aims at understanding the effects of ALD precursor exposures and ALD coatings on the surface electronic structure of LIB cathode materials.Al2O3 and AlF3 were selected as prototypical metal oxide and metal fluoride surface coatings as they are widely reported in literature due to their ability to act as barrier coatings on cathodes. Additionally, the ALD precursor vapors used for these coatings, i.e., trimethyl aluminium (TMA) and hydrogen fluoride pyridine (HFpy), were adsorbed on the surfaces of these cathode materials under typical ALD conditions to observe their effect on the surface composition and electronic state. The cathode materials chosen for this study included all of the component oxides for NMC cathodes (i.e. Li, Ni, Mn, and Co) in all common oxidation states (e.g., MnO, MnO2, and Mn2O3) as well as selected ternary, quaternary, and quinary forms. In each case, the substrate surface was characterized using X-ray photoelectron spectroscopy (XPS) before and after surface treatment to elucidate changes in the surface composition and charge state induced by the individual ALD precursors and the ultrathin coating. Overall, this fundamental study will assist in the design and synthesis of novel ALD barrier coatings that demonstrate the highest sensitivity on multi-element cathode materials, ultimately leading to robust electrode-electrolyte interfaces for high-performance LIBs.

Evaluation of silicon tetrahalide precursors for low-temperature thermal atomic layer deposition of silicon nitride

The development of atomic layer deposition (ALD) processes adaptable to a wide temperature range requires a detailed assessment of the chemical reactivity of the precursors. Theoretical approaches, such as density functional theory (DFT) calculations, can help accelerate and elaborate on the chemistry of ALD. In this paper, the reactivity of silicon tetrahalide precursors (SiCl4, SiBr4, and SiI4) toward a low-temperature thermal ALD of silicon nitride (SixNy) thin films using NH3 as the counter-reactant was evaluated using DFT calculations and supplementary experiments. The enthalpy of reaction, as well as the Gibbs free energy of the reaction, shows a periodic trend, such that the reactions of SiI4 are the most favorable at low deposition temperatures. Also, thermal ALD of SixNy using SiI4 and NH3 at a relatively low temperature of 350 °C was conducted, validating the computational results.

Atomic layer deposition of thin-film sodium manganese oxide cathode materials for sodium ion batteries.

To improve the performance of sodium ion batteries (NIBs), we need to better understand the materials chemistry occurring at the surface of NIB cathode materials. In this work, we aim to form thin films of sodium manganese oxide (NMO) cathode materials for NIBs using atomic layer deposition (ALD) with the vision to isolate and study these interfacial processes in the absence of bulk NMO. We combine established chemistries for ALD of manganese oxide (MnOx) using Mn(thd)3/O3 and sodium hydroxide (NaOH) using NaOtBu/H2O and adjust the sequence and ratios of these two chemistries to form NaxMnyO alloy films. We identify that increasing the O3 exposure during Mn(thd)3/O3 ALD beyond previously reported values increases the growth rate of MnOx from 0.23 to 0.62 Å per cycle and provides improved uniformity, yielding predominantly Mn5O8. Furthermore, alloying Mn(thd)3/O3 with NaOtBu/H2O mutually enhances the growth rate of both ALD chemistries, yielding a growth rate of ∼9 Å per supercycle for a 1 : 1 cycle ratio. This enhancement in growth arises from sub-surface reactions, including the reaction of NaOtBu to a depth of ∼1.3 nm into bulk MnOx to form Na2MnOx. By tuning cycle ratios and growth conditions, we demonstrate control over the NaxMnyO composition and measure different electrochemical properties depending on the composition. The formation of NMO thin films with controlled thickness and composition established in this work provides a means to systematically study interfacial processes occurring in NMO cathode materials for NIBs.

(Invited) Atomic Layer Deposition of Charge Transport Layers for Highly Efficient and Stable Perovskite Photovoltaics

Atomic layer deposition (ALD) is now being recognized as a powerful, general tool for modifying the surfaces of nanomaterials in applications for many renewable energy conversion devices. ALD techniques are now utilizing in charge transporting layers for solar cells. CH3NH3PbI3 with perovskite crystal structure has attracted considerable interest for high power conversion efficiency (PCE, now certified 25.2 %). ALD chemistry for TiO2, NiO, SnO2, and ZnO are well known and the process requires relatively low deposition temperature as low as ~ 100 °C, which is applicable to deposit onto the halide perovskite layer. In this presentation, highly efficient perovskite solar cells having a long-term stability that adapts uniform and dense inorganic charge transport layers grown by ALD are reported. The devices showed excellent water-resistant properties and long-term stability at 85 °C under illumination. Figure 1

(Invited) Elucidating the Mechanisms for Atomic Layer Growth through in Situ Studies

Atomic Layer Deposition (ALD) provides exquisite control over film thickness and composition and yields excellent conformality over large areas and within nanostructures. These desirable attributes derive from self-limiting surface chemistry, and can disappear if the self-limitation is removed. Understanding the surface chemical reactions, i.e. the ALD mechanism, can provide insight into the limits of self-limitation allowing better control, successful scale up, and the invention of new processes. In situ measurements are very effective for elucidating ALD growth mechanisms. In this presentation, I will describe investigations into the growth mechanisms of ALD nanocomposite films comprised of conducting (e.g. W, Mo and Re) and insulating (e.g. Al2O3, ZrO2 and TiO2) components using in situ measurements. These ALD nanocomposites have applications in particle detection, energy storage, and solar power. We have performed extensive in situ studies using quartz crystal microbalance (QCM), quadrupole mass spectrometry (QMS), Fourier transform infrared (FTIR) absorption spectroscopy, and current-voltage measurements. These measurements reveal unusual ALD chemistry occurring upon transitioning between the ALD processes for the two components. This results in unique reaction products that affect the properties of the films in beneficial ways. The knowledge gained from our in situ studies of the ALD nanocomposite films has helped us to overcome problems encountered when we scaled up the ALD processes to large area substrates. Beyond fundamental understanding, in situ measurements are extremely effective in ALD process development and process monitoring. I will end my talk by describing our recent work combining in situ measurements and machine learning to accelerate ALD process development.

Effect of Multilayer versus Monolayer Dodecanethiol on Selectivity and Pattern Integrity in Area-Selective Atomic Layer Deposition.

Monolayer and multilayer dodecanethiols (DDT) can be assembled onto a copper surface from the vapor phase depending on the initial oxidation state of the copper. The ability of the copper-bound dodecanethiolates to block atomic layer deposition (ALD) and the resulting behavior at the interfaces of Cu/SiO2 patterns during area-selective ALD (AS-ALD) are compared between mono- and multilayers. We show that multilayer DDT is ∼7 times more effective at blocking ZnO ALD from diethylzinc and water than is monolayer DDT. Conversely, monolayer DDT exhibits better performance than does multilayer DDT in blocking of Al2O3 ALD from trimethylaluminum and water. Investigation into interfacial effects at the interface between Cu and SiO2 on Cu/SiO2 patterns reveals both a gap at the SiO2 edges and a pitch size-dependent nucleation delay of ZnO ALD on SiO2 regions of multilayer DDT-coated patterns. In contrast, no impact on ZnO ALD is observed on the SiO2 regions of monolayer DDT-coated patterns. We also show that these interfacial effects depend on the ALD chemistry. Whereas an Al2O3 film grows on the TaN diffusion barrier of a DDT-treated Cu/SiO2 pattern, the ZnO film does not. These results indicate that the structure of the DDT layer and the ALD precursor chemistry both play an important role in achieving AS-ALD.

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.

Probing the Atomic-Scale Structure of Amorphous Aluminum Oxide Grown by Atomic Layer Deposition.

Atomic layer deposition (ALD) is a well-established technique for depositing nanoscale coatings with pristine control of film thickness and composition. The trimethylaluminum (TMA) and water (H2O) ALD chemistry is inarguably the most widely used and yet to date, we have little information about the atomic-scale structure of the amorphous aluminum oxide (AlOx) formed by this chemistry. This lack of understanding hinders our ability to establish process-structure-property relationships and ultimately limits technological advancements employing AlOx made via ALD. In this work, we employ synchrotron high-energy X-ray diffraction (HE-XRD) coupled with pair distribution function (PDF) analysis to characterize the atomic structure of amorphous AlOx ALD coatings. We combine ex situ and in operando HE-XRD measurements on ALD AlOx and fit these experimental data using stochastic structural modeling to reveal variations in the Al-O bond length, Al and O coordination environment, and extent of Al vacancies as a function of growth conditions. In particular, the local atomic structure of ALD AlOx is found to change with the substrate and number of ALD cycles. The observed trends are consistent with the formation of bulk Al2O3 surrounded by an O-rich surface layer. We deconvolute these data to reveal atomic-scale structural information for both the bulk and surface phases. Overall, this work demonstrates the usefulness of HE-XRD and PDF analysis in improving our understanding of the structure of amorphous ALD thin films and provides a pathway to evaluate how process changes impact the structure and properties of ALD films.

Enhanced operational stability through interfacial modification by active encapsulation of perovskite solar cells

Encapsulates are, in general, the passive components of any photovoltaic device that provides the required shielding from the externally stimulated degradation. Here we provide comprehensive physical insight depicting a rather non-trivial active nature, in contrast to the supposedly passive, atomic layer deposition (ALD) grown Al2O3 encapsulate layer on the hybrid perovskite [(FA0.83MA0.17)0.95Cs0.05PbI2.5Br0.5] photovoltaic device having the configuration: glass/FTO/SnO2/perovskite/spiro-OMeTAD/Au/(±) Al2O3. By combining various electrical characterization techniques, our experimental observations indicate that the ALD chemistry produces considerable enhancement of the electronic conductivity of the spiro-OMeTAD hole transport medium (HTM), resulting in electronic modification of the perovskite/HTM interface. Subsequently, the modified interface provides better hole extraction and lesser ionic accumulation at the interface, resulting in a significant lowering of the burn-in decay and nearly unchanged charge transport parameters explicitly under the course of continuous operation. Unlike the unencapsulated device, the modified electronic structure in the Al2O3 coated device is essentially the principal reason for better performance stability. Data presented in this communication suggest that the ionic accumulation at the spiro-OMeTAD/perovskite interface triggers the device degradation in the uncoated devices, which is eventually followed by material degradation, which can be avoided by active encapsulation.

Review of Atomic Layer Deposition of Nanostructured Solar Cells 4

This study reviews atomic layer deposition technique with a special interest on solar cells applications. Atomic layer deposition is a vapour phase deposition technique used for producing thin films for several applications. This review focuses on the chemistry of Atomic Layer Deposition of solar cells, merits and demerits of ALD on thin film solar cells. Solar cells have attracted a lot of interest due to their potential for affordable, clean and sustainable energy. Solar cells can be deposited using different deposition techniques but Atomic layer deposition currently attracts attention owing to the merits. ALD has functional merit to bulk materials, great processing flexibility and affordability. The review examined the merits of ALD and solar cells and areas for future study. It offers affordability, ease of control of film growth, conformal and improvement on the deposition of solar cells. Despite few demerits, ALD is poised to be the deposition technique of choice for modifying interfaces of the film for improved performance.