‘Old Chemistries’ for new applications: Perspectives for development of precursors for MOCVD and ALD applications

Unveiling surface reactivity: the crucial role of auxiliary ligands in gallium amidinate-based precursors for atomic layer deposition

Two novel gallium precursors for Atomic Layer Deposition (ALD), LGaMe2 and LGa(NMe2)2 with L = N,N'-di-tert-butylacetamidinato, were successfully synthesised from a carbodiimide and gallium trichloride. The compounds were characterised by NMR spectroscopy and HR-mass spectrometry, confirming their monomeric nature. Their surface reactivity under ALD conditions with H2O and H2S co-reactants was explored using in situ quartz crystal microbalance (QCM) measurements. LGaMe2, bearing methyl ligands, was found to inhibit film growth, with deposition halting after three cycles. In contrast, LGa(NMe2)2 facilitated the successful growth of films using both H2O and H2S leading to Ga2O3 and Ga2S3 respectively, as confirmed by additional thin film ex situ characterisation. This study underscores the critical role of auxiliary X ligands (here Me or NMe2) in determining ALD process efficiency, and emphasises the complexity and unique nature of surface chemistry compared to solution-phase behaviour.

Integrating Data Mining and Natural Language Processing to Construct a Melting Point Database for Organometallic Compounds

As semiconductor devices are miniaturized, the importance of atomic layer deposition (ALD) technology is growing. When designing ALD precursors, it is important to consider the melting point, because the precursors should have melting points lower than the process temperature. However, obtaining melting point data is challenging due to experimental sensitivity and high computational costs. As a result, a comprehensive and well-organized database for the melting point of the OMCs has not been fully reported yet. Therefore, in this study, we constructed a database of melting points for 1,845 OMCs, including 58 metal and 6 metalloid elements. The database contains CAS numbers, molecular formulas, and structural information and was constructed through automatic extraction and systematic curation. The melting point information was extracted using two methods: 1) 1,434 materials from 11 chemical vendor databases and 2) 411 materials identified through natural language processing (NLP) techniques with an accuracy of 86.3%, based on 2,096 scientific papers published over the past 29 years. In our database, the OMCs contain up to around 250 atoms and have melting points that range from -170 to 1610 °C. The main source is the Chemsrc database, accounting for 607 materials (32.9%), and Fe is the most common central metal or metalloid element (15.0%), followed by Si (11.6%) and B (6.7%). To validate the utilization of the constructed database, a multimodal neural network model was developed integrating graph-based and feature-based information as descriptors to predict the melting points of the OMCs but moderate performance. We believe the current approach reduces the time and cost associated with hand-operated data collection and processing, contributing to effective screening of potentially promising ALD precursors and providing crucial information for the advancement of the semiconductor industry.

A sustainable CVD approach for ZrN as a potential catalyst for nitrogen reduction reaction

In pursuit of developing alternatives for the highly polluting Haber-Bosch process for ammonia synthesis, the electrocatalytic nitrogen reduction reaction (NRR) using transition metal nitrides such as zirconium mononitride (ZrN) has been identified as a potential pathway for ammonia synthesis. In particular, specific facets of ZrN have been theoretically described as potentially active and selective for NRR. Major obstacles that need to be addressed include the synthesis of tailored catalyst materials that can activate the inert dinitrogen bond while suppressing hydrogen evolution reaction (HER) and not degrading during electrocatalysis. To tackle these challenges, a comprehensive understanding of the influence of the catalyst's structure, composition, and morphology on the NRR activity is required. This motivates the use of metal-organic chemical vapor deposition (MOCVD) as the material synthesis route as it enables catalyst nanoengineering by tailoring the process parameters. Herein, we report the fabrication of oriented and facetted crystalline ZrN thin films employing a single source precursor (SSP) MOCVD approach on silicon and glassy carbon (GC) substrates. First principles density functional theory (DFT) simulations elucidated the preferred decomposition pathway of SSP, whereas ab initio molecular dynamics simulations show that ZrN at room temperature undergoes surface oxidation with ambient O2, yielding a Zr-O-N film, which is consistent with compositional analysis using Rutherford backscattering spectrometry (RBS) in combination with nuclear reaction analysis (NRA) and X-ray photoelectron spectroscopy (XPS) depth profiling. Proof-of-principle electrochemical experiments demonstrated the applicability of the developed ZrN films on GC for NRR and qualitatively hint towards a possible activity for the electrochemical NRR in the sulfuric acid electrolyte.

The surface chemistry of the atomic layer deposition of metal thin films

In this perspective we discuss the progress made in the mechanistic studies of the surface chemistry associated with the atomic layer deposition (ALD) of metal films and the usefulness of that knowledge for the optimization of existing film growth processes and for the design of new ones. Our focus is on the deposition of late transition metals. We start by introducing some of the main surface-sensitive techniques and approaches used in this research. We comment on the general nature of the metallorganic complexes used as precursors for these depositions, and the uniqueness that solid surfaces and the absence of liquid solvents bring to the ALD chemistry and differentiate it from what is known from metalorganic chemistry in solution. We then delve into the adsorption and thermal chemistry of those precursors, highlighting the complex and stepwise nature of the decomposition of the organic ligands that usually ensued upon their thermal activation. We discuss the criteria relevant for the selection of co-reactants to be used on the second half of the ALD cycle, with emphasis on the redox chemistry often associated with the growth of metallic films starting from complexes with metal cations. Additional considerations include the nature of the substrate and the final structural and chemical properties of the growing films, which we indicate rarely retain the homogeneous 2D structure often aimed for. We end with some general conclusions and personal thoughts about the future of this field.

Highly stable CsPbBr3 perovskite phases from new lead β-diketonate glyme adducts

Lead is one of the key metals of the all-inorganic lead halide perovskites. This work tailors novel architectures of lead's coordination sphere using a β-diketone (H-hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione) and a glyme (monoglyme, diglyme, triglyme, or tetraglyme) ligand. The coordination chemistry and thermal behaviour of these "Pb(hfa)2·glyme" adducts have been analysed through FT-IR spectroscopy, 1H and 13C NMR analyses, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). Single-crystal X-ray diffraction studies provide evidence of the formation of a monomeric Pb(hfa)2·monoglyme structure. In order to validate the potentiality of these "Pb(hfa)2·glyme" precursors for the fabrication of Pb-based halide perovskites, a facile, one-step and low-temperature solution approach has been applied to prepare CsPbBr3 microcrystals with a process carried out in air under atmospheric pressure. Pure stoichiometric CsPbBr3 powders, obtained using Cs(hfa) and Br2 as cesium and bromide sources, respectively, show excellent stability under atmospheric conditions. Better results are obtained in terms of yield and stability from the diglyme and tetraglyme lead adducts. Field emission scanning electron microscopy (FE-SEM) indicates a good uniform morphology of cubic grains, while the structure and the 1 : 1 : 3 stoichiometry of Cs : Pb : Br are confirmed by X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX), respectively. Tauc plots derived from absorption spectroscopy point to optical energy band-gaps (Eg) in the 2.21-2.27 eV range, in agreement with literature data. The present research elucidates the potential of these novel "Pb(hfa)2·glyme" adducts as promising lead precursors for CsPbBr3 perovskite synthesis, paving the way for their implementation in various technological applications.

Chemical synthesis of complex oxide thin films and freestanding membranes

Oxides offer unique physical and chemical properties that inspire rapid advances in materials chemistry to design and nanoengineer materials compositions and implement them in devices for a myriad of applications. Chemical deposition methods are gaining attention as a versatile approach to develop complex oxide thin films and nanostructures by properly selecting compatible chemical precursors and designing an accurate cost-effective thermal treatment. Here, upon describing the basics of chemical solution deposition (CSD) and atomic layer deposition (ALD), some examples of the growth of chemically-deposited functional complex oxide films that can have applications in energy and electronics are discussed. To go one step further, the suitability of these techniques is presented to prepare freestanding complex oxides which can notably broaden their applications. Finally, perspectives on the use of chemical methods to prepare future materials are given.

Potassium and Cesium Fluorinated β-Diketonates: Effect of a Cation and Terminal Substituent on Structural and Thermal Properties

As potential precursors for the synthesis of fluoroperovskites, a family of heavy alkali metal (MI = K, Cs) fluorinated β-diketonates were prepared and characterized by elemental analysis, IR, and powder-XRD. The crystal structures of the new six complexes, MI(β-dikF)(H2O)X, X = 0 or 1, were also determined. The structural diversity of this poorly explored class of complexes was discussed, including the preferred types of cation polyhedra and the ligand coordination modes, and the thermal properties of the metal β-diketonates were studied by TG-DTA in an inert (He) atmosphere. The data obtained allowed us to reveal the effect of the metal cation and the terminal substituent on the structural and thermal features of this family of complexes.

Molybdenum(III) Amidinate: Synthesis, Characterization, and Vapor Phase Growth of Mo-Based Materials

The synthesis, characterization, and thermogravimetric analysis of tris(N,N'-di-isopropylacetamidinate)molybdenum(III), Mo(iPr-AMD)₃, are reported. Mo(iPr-AMD)₃ is a rare example of a homoleptic mononuclear complex of molybdenum(III) and fills a longstanding gap in the literature of transition metal(III) trisamidinate complexes. Thermogravimetric analysis (TGA) reveals excellent volatilization at elevated temperatures, pointing to potential applications as a vapor phase precursor for higher temperature atomic layer deposition (ALD), or chemical vapor deposition (CVD) growth of Mo-based materials. The measured TGA temperature window was 200-314 °C for samples in the 3-20 mg range. To validate the utility of Mo(iPr-AMD)₃, we demonstrate aerosol-assisted CVD growth of MoO₃ from benzonitrile solutions of Mo(iPr-AMD)₃ at 500 °C using compressed air as the carrier gas. The resulting films are characterized by X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy. We further demonstrate the potential for ALD growth at 200 °C with a Mo(iPr-AMD)₃/Ar purge/300 W O₂ plasma/Ar purge sequence, yielding ultrathin films which retain a nitride/oxynitride component. Our results highlight the broad scope utility and potential of Mo(iPr-AMD)₃ as a stable, high-temperature precursor for both CVD and ALD processes.

Liquid injection field desorption/ionization as a powerful tool to characterize volatile, labile, and reactive metal-organic complexes

Electron ionization mass spectrometry (EI-MS) is often used to characterize volatile and thermally stable organometallic complexes relevant for chemical vapor deposition (CVD) processes. However, this method has limitations for thermally unstable and labile organometallic complexes. In this context, EI-MS is not the preferred method of choice for characterizing such compounds. With three different representative organometallic complexes based on the transition metals yttrium, iridium, and silver, relevant as precursors for CVD of different materials, the significance of liquid injection field desorption/ionization mass spectrometry (LIFDI-MS) as an important precursor characterization tool is exemplified. The precursors are not only reactive toward ambient air, but also thermally labile especially in the case of iridium and silver complexes. As a promising alternative, LIFDI-MS is used to overcome the limitations of EI-MS. For the first time, these complexes were successfully analyzed using LIFDI-MS. The comparison between EI-MS and LIFDI-MS highlights that LIFDI-MS is superior for the mass spectrometric analysis of sensitive and labile complexes. In terms of precursor characterization, LIFDI-MS can be fully exploited to gain valuable insights into the decomposition mechanisms and identifying the nuclearity of organometallic precursors used for CVD applications.

CVD Grown Tungsten Oxide for Low Temperature Hydrogen Sensing: Tuning Surface Characteristics via Materials Processing for Sensing Applications

The intrinsic properties of semiconducting oxides having nanostructured morphology are highly appealing for gas sensing. In this study, the fabrication of nanostructured WO₃ thin films with promising surface characteristics for hydrogen (H₂ ) gas sensing applications is accomplished. This is enabled by developing a chemical vapor deposition (CVD) process employing a new and volatile tungsten precursor bis(diisopropylamido)-bis(tert-butylimido)-tungsten(VI), [W(N^t Bu)₂ (Nⁱ Pr₂ )₂ ]. The as-grown nanostructured WO₃ layers are thoroughly analyzed. Particular attention is paid to stoichiometry, surface characteristics, and morphology, all of which strongly influence the gas-sensing potential of WO₃ . Synchrotron-based ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), X-ray photoelectron emission microscopy (XPEEM), low-energy electron microscopy (LEEM) and 4-point van der Pauw (vdP) technique made it possible to analyze the surface chemistry and structural uniformity with a spatially resolved insight into the chemical, electronic and electrical properties. The WO₃ layer is employed as a hydrogen (H₂ ) sensor within interdigitated mini-mobile sensor architecture capable of working using a standard computer's 5 V 1-wirebus connection. The sensor shows remarkable sensitivity toward H₂ . The high, robust, and repeatable sensor response (S) is attributed to the homogenous distribution of the W⁵⁺ oxidation state and associated oxygen vacancies, as shown by synchrotron-based UPS, XPS, and XPEEM analysis.

Yttrium and Lithium Complexes with Diamidophosphane Ligand Bearing 2,1,3-Benzothiazolyl Substituent: Polydentate Complexation and Reversible NH–PH Tautomery

Deprotonation of a bis(amino)phosphane H2L = PhP(HNBtd)2 bearing a heterocyclic Btd = 2,1,3-benzothiadiazol-4-yl substituents at nitrogen atoms by silylamides LiNTms2 and Y(NTms2)3 (Tms = trimethylsilylamide) results in lithium and yttrium complexes with the deprotonated HL– and L2– forms as κ2-N and κ4-N chelating ligands. A binuclear complex [LiHL]2 was crystallized from Et2O, and was shown to reversibly dissociate in thf (tetrahydrofuran) with the NH(soln)–PH(crystal) tautomeric shift; the compound [Li2L] was spectroscopically characterized. Yttrium readily forms stable bis-ligand complexes [YL2]– and [YL(HL)]. In the latter, the H atom in HL resides on phosphorus; the coordination sphere remains accessible to another ligands, and it was crystallized as [{YL(HL)}2(µ-dioxane)] species (YN8O coordination). In the former complex, the coordination sphere was saturated (YN8) by closer bound ligands; it was crystallized as a salt with [Li(thf)4]+. The monoligand complex could not be cleanly obtained in a 1:1 reaction of H2L and Y(NTms2)3, and was only crystallographically characterized as a dimer [YL(NTms)2]2. Partial oxidation of the central P atom with the formation of phosphine-oxide ligands PhP(O)(NBtd)2– was observed. They co-crystallize in the same position as non-oxidized ligands in [YL2]– and [YL(NTms2)]2 species and participate in bonding between two units in the latter. TD-DFT calculations reveal that main transitions in the visible region of electronic spectra correspond to the charge transfer bands mostly associated with the orbitals located on Btd fragments.

High-Performance Iridium Thin Films for Water Splitting by CVD Using New Ir(I) Precursors

Thin films of iridium can be utilized in a wide range of applications and are particularly interesting for catalytic transformations. For the scalable deposition of functional Ir thin films, metalorganic chemical vapor deposition (MOCVD) is the method of choice, for which organometallic precursors that embody a high volatility and thermal stability need to be specifically tailored. Herein, we report the synthesis, analysis, and evaluation of new volatile Ir(I)-1,5-cyclooctadiene complexes bearing all-nitrogen coordinating guanidinate (N,N'-diisopropyl-2-dimethylamido-guanidinate (DPDMG)), amidinate (N,N'-diisopropyl-amidinate (DPAMD)), and formamidinate (N,N'-diisopropyl-formamidinate (DPfAMD)) ligands. The amidinate-based Ir complex [Ir(COD)(DPAMD)] together with O₂ was implemented in MOCVD experiments resulting in highly crystalline, dense, and conductive Ir films on a variety of substrate materials. The Ir deposits achieved outstanding electrochemical performance with overpotentials in the range of 50 mV at -10 mA·cm^-2 for catalytic hydrogen evolution reaction (HER) in acidic solution. The ability to deposit Ir layers via MOCVD exhibiting promising functional properties is a significant step toward large-scale applications.

Heterometallic Molecular Architectures Based on Fluorinated β-Diketone Ligands

This review summarizes the data on the synthesis of coordination compounds containing two or more different metal ions based on fluorinated β-diketonates. Heterometallic systems are of high interest in terms of their potential use in catalysis, medicine and diagnostics, as well as in the development of effective sensor devices and functional materials. Having a rich history in coordination chemistry, fluorinated β-diketones are well-known ligands generating a wide variety of heterometallic complexes. In this context, we focused on both the synthetic approaches to β-dicarbonyl ligands with additional coordination centers and their possible transformations in complexation reactions. The review describes bi- and polynuclear structures in which β-diketones are the key building blocks in the formation of a heterometallic framework, including the examples of both homo- and heteroleptic complexes.

Sodium β-Diketonate Glyme Adducts as Precursors for Fluoride Phases: Synthesis, Characterization and Functional Validation

Very few sodium complexes are available as precursors for the syntheses of sodium-based nanostructured materials. Herein, the diglyme, triglyme, and tetraglyme (CH₃O(CH₂CH₂O)_nCH₃, n = 2–4) adducts of sodium hexafluoroacetylacetonate were synthesized in a single-step reaction and characterized by IR spectroscopy, ¹H, and ¹³C NMR. Single-crystal X-ray diffraction studies provide evidence of the formation of the ionic oligomeric structure [Na₄(hfa)₆]²⁻•2[Na(diglyme₂]⁺ when the diglyme is coordinated, while a mononuclear seven-coordinated complex Na(hfa)•tetraglyme is formed with the tetraglyme. Reaction with the monoglyme (CH₃OCH₂CH₂OCH₃) does not occur, and the unadducted polymeric structure [Na(hfa)]_n forms, while the triglyme gives rise to a liquid adduct, Na(hfa)•triglyme•H₂O. Thermal analysis data reveal great potentialities for their applications as precursors in metalorganic chemical vapor deposition (MOCVD) and sol-gel processes. As a proof-of-concept, the Na(hfa)•tetraglyme adduct was successfully applied to both the low-pressure MOCVD and the sol-gel/spin-coating synthesis of NaF films.

Influence of different ester side groups in polymers on the vapor phase infiltration with trimethyl aluminum

The vapor phase infiltration (VPI) process of trimethyl aluminum (TMA) into poly(4-acetoxystyrene) (POAcSt), poly(nonyl methacrylate) (PNMA) and poly(tert-butyl methacrylate) (PtBMA) is reported. Depth-profiling X-ray photoelectron spectroscopy (XPS) measurements are used for the first time for VPI based hybrid materials to determine the aluminum content over the polymer film thickness. An understanding of the reaction mechanism on the interaction of TMA infiltrating into the different polymers was obtained through infrared (IR) spectroscopy supported by density functional theory (DFT) studies. It is shown that the loading with aluminum is highly dependent on the respective ester side group of the used polymer, which is observed to be the reactive site for TMA during the infiltration. IR spectroscopy hints that the infiltration is incomplete for POAcSt and PNMA, as indicated by the characteristic vibration bands of the aluminum coordination to the carbonyl groups within the polymers. In this context, two different reaction pathways are discussed. One deals with the formation of an acetal, the other is characterized by the release of a leaving group. Both were found to be in direct concurrence dependent on the polymer side group as revealed by DFT calculations of the IR spectra, as well as the reaction energies of two possible reaction paths. From this study, one can infer that the degree of infiltration in a VPI process strongly depends on the polymer side groups, which facilitates the choice of the polymer for targeted applications.

Synthesis and photophysical properties of rare earth complexes bearing silanediamido ligands Me2Si(NAryl)22− (Aryl = Dipp, Mes)

Luminescence of the newly obtained silanediamides of rare earths was investigated. The triplet states of the ligands {Me2Si(NAryl)2}2− (Aryl = Dpp, Mes) were determined by the emission of Gd complexes, and the bright emission of Tb3+ was observed.

Formation of Mononuclear N,O-chelate Zirconium Complexes by Direct Insertion of Epoxide into Tetrakis(dimethylamido)zirconium: Highly Promising Approach for Developing ALD Precursor of ZrO2 Thin Film

A zirconium complex containing an N,O-chelate and alkylamide ligand has great potential for application in atomic layer deposition (ALD). However, the synthesis of this mononuclear Zr complex remains a major...

Techniques of Preparation of Thin Films: Catalytic Combustion

Over the past several decades, an increasing amount of attention has been given to catalytic combustion as an environmentally friendly process. However, major impediments to large-scale application still arise on the materials side. Here, we review catalytic combustion on thin film catalysts in view of highlighting some interesting features. Catalytic films open the way for new designs of structured catalysts and the construction of catalysts for catalytic combustion. A special place is occupied by materials in the form of very thin films that reveal catalytic activity for various chemical reactions. In this review, we demonstrate the high catalytic activity of thin film catalysts in these oxidation reactions.

Tin(II) Ureide Complexes: Synthesis, Structural Chemistry, and Evaluation as SnO Precursors

In an attempt to tailor precursors for application in the deposition of phase pure SnO, we have evaluated a series of tin (1-6) ureide complexes. The complexes were successfully synthesized by employing N,N'-trialkyl-functionalized ureide ligands, in which features such as stability, volatility, and decomposition could be modified with variation of the substituents on the ureide ligand in an attempt to find the complex with the ideal electronic, steric, or coordinative properties, which determine the fate of the final products. The tin(II) ureide complexes 1-6 were synthesized by direct reaction [Sn{NMe₂}₂] with aryl and alkyl isocyanates in a 1:2 molar ratio. All the complexes were characterized by NMR spectroscopy as well as elemental analysis and, where applicable, thermogravimetric (TG) analysis. The single-crystal X-ray diffraction studies of 2, 3, 4, and 6 revealed that the complexes crystallize in the monoclinic space group P2(1)/n (2 and 4) or in the triclinic space group P-1 (3 and 6) as monomers. Reaction with phenyl isocyanate results in the formation of the bimetallic species 5, which crystallizes in the triclinic space group P-1, a consequence of incomplete insertion into the Sn-NMe₂ bonds, versus mesityl isocyanate, which produces a monomeric double insertion product, 6, under the same conditions, indicating a difference in reactivity between phenyl isocyanate and mesityl isocyanate with respect to insertion into Sn-NMe₂ bonds. The metal centers in these complexes are all four-coordinate, displaying either distorted trigonal bipyramidal or trigonal bipyramidal geometries. The steric influence of the imido-ligand substituent has a clear effect on the coordination mode of the ureide ligands, with complexes 2 and 6, which contain the cyclohexyl and mesityl ligands, displaying κ²-O,N coordination modes, whereas κ²-N,N' coordination modes are observed for the sterically bulkier tert-butyl and adamantyl derivatives, 3 and 4. The thermogravimetric analysis of the complexes 3 and 4 exhibited excellent physicochemical properties with clean single-step curves and low residual masses in their TG analyses suggesting their potential utility of these systems as MOCVD and ALD precursors.

Charged Particle-Induced Surface Reactions of Organometallic Complexes as a Guide to Precursor Design for Electron- and Ion-Induced Deposition of Nanostructures

Focused electron beam-induced deposition (FEBID) and focused ion beam-induced deposition (FIBID) are direct-write fabrication techniques that use focused beams of charged particles (electrons or ions) to create 3D metal-containing nanostructures by decomposing organometallic precursors onto substrates in a low-pressure environment. For many applications, it is important to minimize contamination of these nanostructures by impurities from incomplete ligand dissociation and desorption. This spotlight on applications describes the use of ultra high vacuum surface science studies to obtain mechanistic information on electron- and ion-induced processes in organometallic precursor candidates. The results are used for the mechanism-based design of custom precursors for FEBID and FIBID.

Catalytic membrane-based oxidation-filtration systems for organic wastewater purification: A review

Catalytic membranes can simultaneously realize physical separation and chemical oxidation in one integrated system, which is the frontier technology for effective removal of organic containments in wastewater treatment. The catalytic membrane coupled with advanced oxidation processes (AOPs) not only significantly enhances the pollutant removal efficiency but also inhibits the fouling of the membrane via self-cleaning. In this review, the preparation approaches of catalytic membranes including blending, surface coating, and bottom-up synthesis are comprehensively summarized. The different integrated catalytic membrane systems coupled with photocatalysis, Fenton oxidation, persulfate activations, ozonation and electrocatalytic oxidation are discussed in terms of mechanisms and performance. Besides, the principles, influencing factors, advantages and issues of the different catalytic membrane/oxidation systems are outlined comparatively. Finally, the future challenges, and research directions are suggested, which is conducive to the design and development of catalytic membrane-oxidation systems for practical remediation of organic containing wastewater.

Vapor-phase production of nanomaterials

The synthesis of nanomaterials, with characteristic dimensions of 1 to 100 nm, is a key component of nanotechnology. Vapor-phase synthesis of nanomaterials has numerous advantages such as high product purity, high-throughput continuous operation, and scalability that have made it the dominant approach for the commercial synthesis of nanomaterials. At the same time, this class of methods has great potential for expanded use in research and development. Here, we present a broad review of progress in vapor-phase nanomaterial synthesis. We describe physically-based vapor-phase synthesis methods including inert gas condensation, spark discharge generation, and pulsed laser ablation; plasma processing methods including thermal- and non-thermal plasma processing; and chemically-based vapor-phase synthesis methods including chemical vapor condensation, flame-based aerosol synthesis, spray pyrolysis, and laser pyrolysis. In addition, we summarize the nanomaterials produced by each method, along with representative applications, and describe the synthesis of the most important materials produced by each method in greater detail.

Liquid atomic layer deposition as emergent technology for the fabrication of thin films

Atomic layer deposition (ALD) is widely recognized as a unique chemical vapor deposition technique for the fabrication of thin films with high conformality and precise thickness control down to the Ångstrom level, thereby allowing surface and interface nanoengineering. However, several challenges such as the availability of chemical precursors for ALD and the use of vacuum conditions have hampered its widespread adoption and scalability for mass production. In recent years, the liquid phase homolog of ALD, liquid atomic layer deposition (LALD), has emerged as a much simpler and versatile strategy to overcome some of the current constraints of ALD. This perspective describes the different strategies that have been explored to achieve conformality and sub-nanometer thickness control with LALD, as well as the current challenges it faces to become a part of the thin-film community toolbox, in particular its automation and compatibility with different types of substrates. In this regard, the important role of LALD as complementary technology to ALD is emphasized by comparing the different pathways to deposit the same material and the precursors used to do so.

Thorium amidates function as single-source molecular precursors for thorium dioxide

We report the synthesis of four homoleptic thorium(iv) amidate complexes as single-source molecular precursors for thorium dioxide. Each can be sublimed at atmospheric pressure, with the substituents on the amidate ligands significantly impacting their volatility and thermal stability. These complexes decompose via alkene elimination to give ThO2 without need for a secondary oxygen source. ThO2 samples formed from pyrolysis of C-alkyl amidates were found to have higher purity and crystallinity than ThO2 samples formed from C-aryl amidates.

Stability Evaluation of Candidate Precursors for Chemical Vapor Deposition of Hafnium Diboride (HfB2)

Alternative candidate precursors to [Hf(BH₄)₄] for low-temperature chemical vapor deposition of hafnium diboride (HfB₂) films were identified using density functional theory simulations of molecules with the composition [Hf(BH₄)₂L₂], where L = -OH, -OMe, -O-t-Bu, -NH₂, -N=C=O, -N(Me)₂, and -N(CH₂)₅NH₂ (1-piperidin-2-amine referred to as Pip2A). Disassociation energies (E _D), potential energy surface (PES) scans, ionization potentials, and electron affinities were all calculated to identify the strength of the Hf-L bond and the potential reactivity of the candidate precursor. Ultimately, the low E _D (2.07 eV) of the BH₄ ligand removal from the Hf atom in [Hf(BH₄)₄] was partially attributed to an intermediate state where [Hf(BH₄)₃(H)] and BH₃ is formed. Of the candidate precursors investigated, three exhibited a similar mechanism, but only -Pip2A had a PES scan that indicated binding competitive with [Hf(BH₄)₄], making it a viable candidate for further study.

Amidinate Supporting Ligands Influence Molecularity in Formation of Uranium Nitrides

Uranium nitride complexes are attractive targets for chemists as molecular models for the bonding, reactivity, and magnetic properties of next-generation nuclear fuels, but these molecules are uncommon and can be difficult to isolate due to their high reactivity. Here, we describe the synthesis of three new multinuclear uranium nitride complexes, [U(BCMA)₂]₂(μ-N)(μ-κ¹:κ¹-BCMA) (7), [(U(BIMA)₂)₂(μ-N)(μ-NⁱPr)(K₂(μ-η³:η³-CH₂CHNⁱPr)]₂ (8), and [U(BIMA)₂]₂(μ-N)(μ-κ¹:κ¹-BIMA) (9) (BCMA = N,N-bis(cyclohexyl)methylamidinate, BIMA = N,N-bis(iso-propyl)methylamidinate), from U(III) and U(IV) amidinate precursors. By varying the amidinate ligand substituents and azide source, we were able to influence the composition and size of these nitride complexes. ¹⁵N isotopic labeling experiments confirmed the bridging nitride moieties in 7-9 were formed via two-electron reduction of azide. The tetra-uranium cluster 8 was isolated in 99% yield via reductive cleavage of the amidinate ligands; this unusual molecule contains nitrogen-based ligands with formal 1-, 2-, and 3- charges. Additionally, chemical oxidation of the U(IV) precursor U(N₃)(BCMA)₃ yielded the cationic U(V) species [U(N₃)(BCMA)₃][OTf]. Magnetic susceptibility measurements confirmed a U(IV) oxidation state for the uranium centers in the three nitride-bridged complexes and provided a comparison of magnetic behavior in the structurally related U(III)-U(IV)-U(V) series U(BCMA)₃, U(N₃)(BCMA)₃, and [U(N₃)(BCMA)₃][OTf]. At 240 K, the magnetic moments in this series decreased with increasing oxidation state, i.e., U(III) > U(IV) > U(V); this trend follows the decreasing number of 5f valence electrons along this series.

The Early Steps of Molecule-to-Material Conversion in Chemical Vapor Deposition (CVD): A Case Study

Transition metal complexes with β-diketonate and diamine ligands are valuable precursors for chemical vapor deposition (CVD) of metal oxide nanomaterials, but the metal-ligand bond dissociation mechanism on the growth surface is not yet clarified in detail. We address this question by density functional theory (DFT) and ab initio molecular dynamics (AIMD) in combination with the Blue Moon (BM) statistical sampling approach. AIMD simulations of the Zn β-diketonate-diamine complex Zn(hfa)₂TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N′,N′-tetramethylethylenediamine), an amenable precursor for the CVD of ZnO nanosystems, show that rolling diffusion of this precursor at 500 K on a hydroxylated silica slab leads to an octahedral-to-square pyramidal rearrangement of its molecular geometry. The free energy profile of the octahedral-to-square pyramidal conversion indicates that the process barrier (5.8 kcal/mol) is of the order of magnitude of the thermal energy at the operating temperature. The formation of hydrogen bonds with surface hydroxyl groups plays a key role in aiding the dissociation of a Zn-O bond. In the square-pyramidal complex, the Zn center has a free coordination position, which might promote the interaction with incoming reagents on the deposition surface. These results provide a valuable atomistic insight on the molecule-to-material conversion process which, in perspective, might help to tailor by design the first nucleation stages of the target ZnO-based nanostructures.

Rational Development of Guanidinate and Amidinate Based Cerium and Ytterbium Complexes as Atomic Layer Deposition Precursors: Synthesis, Modeling, and Application

Owing to the limited availability of suitable precursors for vapor phase deposition of rare-earth containing thin-film materials, new or improved precursors are sought after. In this study, we explored new precursors for atomic layer deposition (ALD) of cerium (Ce) and ytterbium (Yb) containing thin films. A series of homoleptic tris-guanidinate and tris-amidinate complexes of cerium (Ce) and ytterbium (Yb) were synthesized and thoroughly characterized. The C-substituents on the N-C-N backbone (Me, NMe2 , NEt2 , where Me=methyl, Et=ethyl) and the N-substituents from symmetrical iso-propyl (iPr) to asymmetrical tertiary-butyl (tBu) and Et were systematically varied to study the influence of the substituents on the physicochemical properties of the resulting compounds. Single crystal structures of [Ce(dpdmg)3 ] 1 and [Yb(dpdmg)3 ] 6 (dpdmg=N,N'-diisopropyl-2-dimethylamido-guanidinate) highlight a monomeric nature in the solid-state with a distorted trigonal prismatic geometry. The thermogravimetric analysis shows that the complexes are volatile and emphasize that increasing asymmetry in the complexes lowers their melting points while reducing their thermal stability. Density functional theory (DFT) was used to study the reactivity of amidinates and guanidinates of Ce and Yb complexes towards oxygen (O2 ) and water (H2 O). Signified by the DFT calculations, the guanidinates show an increased reactivity toward water compared to the amidinate complexes. Furthermore, the Ce complexes are more reactive compared to the Yb complexes, indicating even a reactivity towards oxygen potentially exploitable for ALD purposes. As a representative precursor, the highly reactive [Ce(dpdmg)3 ] 1 was used for proof-of-principle ALD depositions of CeO2 thin films using water as co-reactant. The self-limited ALD growth process could be confirmed at 160 °C with polycrystalline cubic CeO2 films formed on Si(100) substrates. This study confirms that moving towards nitrogen-coordinated rare-earth complexes bearing the guanidinate and amidinate ligands can indeed be very appealing in terms of new precursors for ALD of rare earth based materials.

Nanoparticles functionalized ceramic membranes: fabrication, surface modification, and performance

Membrane technologies are used intensively for desalination and wastewater treatment. Water filtration using ceramic membranes exhibited high performance compared with polymeric membranes due to various properties such as high resistance to fouling, permeability, rejection rate, and chemical stability. Recently, the performance of nanocomposite ceramic membranes was improved due to the development in nanotechnology. This article focusses on the development of porous ceramic membranes and nanomaterial functionalized ceramic membranes for water filtration applications. At the beginning, various fabrication methods of ceramic membranes were described, and the effect of surface modification techniques on the membrane intrinsic properties was reviewed. Then, the performance of nanoparticles functionalized ceramic membranes was evaluated in terms of physicochemical properties, rejection rate, and water permeability. This work can help new entrants and established researchers to become familiar with the current challenges and developments of nanoparticle-incorporated ceramic membranes for water filtration applications.

Atomic layer deposition of dielectric Y2O3 thin films from a homoleptic yttrium formamidinate precursor and water

We report the application of tris(N,N'-diisopropyl-formamidinato)yttrium(iii) [Y(DPfAMD)3] as a promising precursor in a water-assisted thermal atomic layer deposition (ALD) process for the fabrication of high quality Y2O3 thin films in a wide temperature range of 150 °C to 325 °C. This precursor exhibits distinct advantages such as improved chemical and thermal stability over the existing Y2O3 ALD precursors including the homoleptic and closely related yttrium tris-amidinate [Y(DPAMD)3] and tris-guanidinate [Y(DPDMG)3], leading to excellent thin film characteristics. Smooth, homogeneous, and polycrystalline (fcc) Y2O3 thin films were deposited at 300 °C with a growth rate of 1.36 Å per cycle. At this temperature, contamination levels of C and N were under the detectable limits of nuclear reaction analysis (NRA), while X-ray photoelectron spectroscopy (XPS) measurements confirmed the high purity and stoichiometry of the thin films. From the electrical characterization of metal-insulator-semiconductor (MIS) devices, a permittivity of 13.9 at 1 MHz could be obtained, while the electric breakdown field is in the range of 4.2 and 6.1 MV cm-1. Furthermore, an interface trap density of 1.25 × 1011 cm-2 and low leakage current density around 10-7 A cm-2 at 2 MV cm-1 are determined, which satisfies the requirements of gate oxides for complementary metal-oxide-semiconductor (CMOS) based applications.

Fabrication of GdxFeyOz films using an atomic layer deposition-type approach

The growth of complex oxide thin films with atomic precision offers bright prospects to study improved properties and novel functionalities.

A carbene stabilized precursor for the spatial atomic layer deposition of copper thin films

This paper demonstrates a carbene stabilized precursor [Cu(^tBuNHC)(hmds)] with suitable volatility, reactivity and thermal stability, that enables the spatial plasma-enhanced atomic layer deposition (APP-ALD) of copper thin films at atmospheric pressure. The resulting conductive and pure copper layers were thoroughly analysed and a comparison of precursor and process with the previously reported silver analogue [Ag(^tBuNHC)(hmds)] revealed interesting similarities and notable differences in precursor chemistry and growth characteristics. This first report of APP-ALD grown copper layers is an important starting point for high throughput, low-cost manufacturing of copper films for nano- and optoelectronic devices.

Error or exemption to the rule? Development of a diagnostic check for thermochemistry of metal-organic compounds

Volatile metal β-diketonates are well-known precursors used in Metal-Organic Chemical Vapour Deposition (MOCVD) for manufacturing film materials. Knowledge of vapour pressures and sublimation/vaporization thermodynamics of the MOCVD precursors is indispensable for optimization of deposition. However, the spread of available data could be unacceptably large for the same precursor for several reasons related to its chemical nature or incorrectly configured conditions of tensimetric investigation. In this work, we have developed an algorithm for a general diagnostic check, based on principles of group-additivity, for thermochemistry on solid-gas, liquid-gas, and solid-liquid phase transitions of metal-organic compounds and applied it to tris(beta-diketonato)iron complexes. The diagnostic tool helps to localize general "healthy" thermochemical interconnected data, and, subsequently, isolate molecules with definitely "ill" properties from the data pool. This diagnostic tool could be expanded and adapted for β-diketonate complexes with metals other than iron.

Additive-free spin coating of tin oxide thin films: synthesis, characterization and evaluation of tin β-ketoiminates as a new precursor class for solution deposition processes

The fabrication of SnOx in thin film form via chemical solution deposition (CSD) processes is favored over vacuum based techniques as it is cost effective and simpler. The precursor employed plays a central role in defining the process conditions for CSD. Particularly for processing SnO2 layers that are appealing for sensor or electronic applications, there are limited precursors available for CSD. Thus the focus of this work was to develop metalorganic precursors for tin, based on the ketoiminate ligand class. By systematic molecular engineering of the ligand periphery, a series of new homoleptic Sn(ii) β-ketoiminate complexes was synthesized, namely bis[4-(2-methoxyethylimino)-3-pentanonato] tin, [Sn(MEKI)2] (1), bis[4-(2-ethoxyethylimino)-2-pentanonato] tin, [Sn(EEKI)2] (2), bis[4-(3-methoxypropylimino)-2-pentanonato] tin, [Sn(MPKI)2] (3), bis[4-(3-ethoxypropylimino)-2-pentanonato] tin, [Sn(EPKI)2] (4) and bis[4-(3-isopropoxypropylimino)-2-pentanonato] tin, [Sn(iPPKI)2] (5). All these N-side-chain ether functionalized compounds were analyzed by nuclear magnetic resonance (NMR) spectroscopy, electron impact mass spectrometry (EI-MS), elemental analysis (EA) and thermogravimetric analysis (TGA). The solid state molecular structure of [Sn(MPKI)2] (3) was eludicated by means of single crystal X-ray diffraction (SCXRD). Interestingly, this class of compounds features excellent solubility and stability in common organic solvents alongside good reactivity towards H2O and low decomposition temperatures, thus fulfilling the desired requirements for CSD of tin oxides. With compound 3 as a representative example, we have demonstrated the possibility to directly deposit SnOx layers via hydrolysis upon exposure to air followed by heat treatment under oxygen at moderate temperatures and most importantly without the need for any additive that is generally used in CSD. A range of complementary analytical methods were employed, namely X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) to analyse the structure, morphology and composition of the SnOx layers.

Functional catalytic membrane development: A review of catalyst coating techniques

Catalytic membranes combine catalytic activity with conventional filtration membranes, thus enabling diverse attractive benefits into the conventional membrane filtration processes, such as easy catalyst reuse, antifouling, anti-microbial, and enhancing process efficiency. Up to date, tremendous progresses have been made on functional catalytic membrane preparation and applications, which significantly advances the competitiveness of membrane technologies in process industries. The present article provides a critical and holistic overview of the current state of knowledge on existing catalyst coating techniques for functional catalytic membrane development. Based on coating mechanisms, the techniques are generally categorized into physical and chemical surface coating routes. For each technique, we first introduce fundamental principle, followed by a critical discussion of their applications with representative case studies. Advantages and drawbacks are also emphasized for different surface coating technologies. Finally, future perspectives are highlighted to provide deep insights into their future developments.

Molecular Engineering of Metal Alkoxides for Solution Phase Synthesis of High-Tech Metal Oxide Nanomaterials

The 'bottom-up' synthesis of inorganic nanomaterials with precision at the atomic/molecular level offers many opportunities for the design and improvement of the nanomaterials for various applications. Molecular engineering during soft chemical processing for the synthesis of functional nanomaterials enables the desired chemical and physical properties of the precursors, such as solubility or volatility, clean decomposition, control of stoichiometry for multimetallic species to name a few, and leads to easy control of uniform particle size distribution, stoichiometry…. This Minireview illustrates some important aspects of the molecular engineering in light of some recent developments from the molecular synthesis of nanomaterials involving non-silicon metal alkoxide systems for high-tech applications.

Trickle Flow Aided Atomic Layer Deposition (ALD) Strategy for Ultrathin Molybdenum Disulfide (MoS2) Synthesis

The synthesis of large-area and high-quality two-dimensional (2D) MoS₂ is undoubtedly a significant challenge till now. In this work, an effective strategy for achieving 2D MoS₂ with enlarged grain size by Ni-foam-based trickle flow atomic layer deposition (ALD) was suggested, by which MoS₂ grain sizes up to 420 nm (monolayer sample) and 400 nm (five-layer sample) were obtained under the covering of 1 mm-thick Ni foam with a 2 mm gap from the substrate at 460 °C. In terms of specific ALD experiments, the Ni foam with a certain thickness placed on top of the substrate made the original precursor flow into a trickle-fluidization source flow, which decreased the nucleation density effectively. Thus, MoS₂ with enlarged grain sizes were obtained based on the typical ALD mechanism of large-scale vertical growth under the action of steric hindrance after a planar parallel growth around the crystal nucleus. In addition, the Ni foam also achieved a stable temperature field by enhancing the heat transfer around the substrate and thus improved its crystallinity.