New s-Block Metal Pyridinedicarboxylate Network Structures through Gas-Phase Thin-Film Synthesis

The combined atomic and molecular layer deposition (ALD/MLD) technique offers a unique way to build—both known and previously unknown—crystalline coordination polymer materials directly from gaseous precursors in a high-quality thin-film form. Here, we demonstrate the ALD/MLD of crystalline Li-, Na-, and K-based 3,5-pyridinedicarboxylate (3,5-PDC) thin films; the Li₂-3,5-PDC films are of the known Li-ULMOF-4 crystal structure whereas the other as-deposited crystalline films possess structures not previously reported. Another exciting possibility offered by ALD/MLD is the deposition of well-defined but amorphous metal–organic thin films, such as our Mg-, Ca-, Sr-, and Ba-based 3,5-PDC films, which can then be crystallized into water-containing structures through a post-deposition humidity treatment. All together, the new metal–organic structures realized in this study through ALD/MLD comprise a majority of the (anhydrous and water-containing) members of the s-block metal 3,5-pyridinedicarboxylate family.     

Rapid vapor-phase fabrication of organic-inorganic hybrid superlattices with monolayer precision

We report a new layer-by-layer growth method of self-assembled organic multilayer thin films based on gas-phase reactions. In the present molecular layer deposition (MLD) process, alkylsiloxane self-assembled multilayers (SAMs) were grown under vacuum by repeated sequential adsorptions of C=C-terminated alkylsilane and titanium hydroxide. The MLD method is a self- limiting layer-by-layer growth process, and is perfectly compatible with the atomic layer deposition (ALD) method. The SAMs films prepared exhibited good thermal and mechanical stability, and various unique electrical properties. The MLD method, combined with ALD, was applied to the preparation of organic-inorganic hybrid nanolaminate films in the ALD chamber. The organic-inorganic hybrid superlattices were then used as active mediums for two-terminal electrical bistable devices. The advantages of the MLD method with ALD include accurate control of film thickness, large-scale uniformity, highly conformal layering, sharp interfaces, and a vast library of possible materials. The MLD method with ALD is an ideal fabrication technique for various organic-inorganic hybrid superlattices.     

The importance of dye chemistry and TiCl(4) surface treatment in the behavior of Al(2)O(3) recombination barrier layers deposited by atomic layer deposition in solid-state dye-sensitized solar cells

Atomic layer deposition (ALD) was used to fabricate Al(2)O(3) recombination barriers in solid-state dye-sensitized solar cells (ss-DSSCs) employing an organic hole transport material (HTM) for the first time. Al(2)O(3) recombination barriers of varying thickness were incorporated into efficient ss-DSSCs utilizing the Z907 dye adsorbed onto a 2 μm-thick nanoporous TiO(2) active layer and the HTM spiro-OMeTAD. The impact of Al(2)O(3) barriers was also studied in devices employing different dyes, with increased active layer thicknesses, and with substrates that did not undergo the TiCl(4) surface treatment. In all instances, electron lifetimes (as determined by transient photovoltage measurements) increased and dark current was suppressed after Al(2)O(3) deposition. However, only when the TiCl(4) treatment was eliminated did device efficiency increase; in all other instances efficiency decreased due to a drop in short-circuit current. These results are attributed in the former case to the similar effects of Al(2)O(3) ALD and the TiCl(4) surface treatment whereas the insulating properties of Al(2)O(3) hinder charge injection and lead to current loss in TiCl(4)-treated devices. The impact of Al(2)O(3) barrier layers was unaffected by doubling the active layer thickness or using an alternative ruthenium dye, but a metal-free donor-π-acceptor dye exhibited a much smaller decrease in current due to its higher excited state energy. We develop a model employing prior research on Al(2)O(3) growth and dye kinetics that successfully predicts the reduction in device current as a function of ALD cycles and is extendable to different dye-barrier systems.     

Atomic layer deposition of TiO2 from TiI4 and H2O onto SiO2 surfaces: ab initio calculations of the initial reaction mechanisms

The initial surface reactions involved in the atomic layer deposition (ALD) of TiO2 from TiI4 and H2O onto a SiO2 substrate have been investigated using electronic structure calculations based on cluster models. The detailed atomic growth mechanisms on different types of functionalities on the SiO2 substrate have been proposed. The effects of quantum tunneling and hindered rotations of adsorbates on the rate of surface reactions have been investigated. The effects of tunneling were found to be negligible for all reactions, because typical ALD temperatures range from 150 to 450 degrees C. However, the rotational contributions to the rate constants must be taken into account in certain cases. All of the three surface functionalities investigated exhibit high chemical reactivity toward TiI4 precursors at typical ALD temperatures. The rate constants of the second half-reactions between Ti intermediates and H2O are 5-8 orders of magnitude smaller than the first half-reactions between TiI4 and the surface functionalities. Although the iodine release reaction has been used to explain previous experimental measurements, it is predicted to be unfavorable (kinetically and thermodynamically) and is unlikely to occur at typical ALD temperatures. Substitution of TiI4 with TiCl4 as the metal precursor can increase the binding energies of the absorbates onto the surface due to the high electronegativity of the Cl ligands. However, the activation barriers are not significantly different between these two metal precursors. More importantly, our calculations predict that TiI4 precursors tend to produce TiO2 films with fewer impurities than the TiCl4 precursors.     

Initial surface reactions of TiO2 atomic layer deposition onto SiO2 surfaces: density functional theory calculations

We present a density functional theory (DFT) study of the initial surface reactions of TiO2 deposition onto a SiO2 substrate using atomic layer deposition (ALD). The precursors for the deposition process were chosen to be TiCl4 and H2O, and several cluster models were used for the SiO2 substrate. We predict the activation barriers, transition states, and reaction pathways of the surface reactions, and we investigate the effect of surface heterogeneity (such as the presence of siloxane bridges) on the reactivity of the SiO2 surface. Our study suggests that the concentration and arrangement of different reactive groups on the substrate will strongly dictate the process of film growth during ALD, including the film morphology and the growth rate.     

Role of water in the atomic layer deposition of TiO(2) on SiO(2)

Atomic layer deposition (ALD) of TiO(2) on SiO(2) powder using sequential addition of TiCl(4) and H(2)O vapors has been investigated by infrared spectroscopy. In the first cycle, TiCl(4) reacts monofunctionally or bifunctionally with surface silanols forming (Si-O-)(n)Ti-Cl(4)(-)(n) (n = 1, 2) species. Subsequent addition of water vapor leads to the hydrolysis of the (Si-O-)(n)Ti-Cl(4)(-)(n) to form a Ti-O-Ti network, and at the same time, some cleavage of Si-O-Ti bonds occurs, regenerating Si-OH in the process. It is shown that the species formed on the surface in the first TiCl(4) dose are temperature dependent. However, after addition of H(2)O vapor, the amount of TiO(2) deposited in the first complete cycle is independent of reaction temperature. In the second and above cycles, the amount of TiO(2) deposited as a function of ALD cycles strongly correlates with the amount of water on the surface. This, in turn, led to a temperature dependence of the growth rate of the TiO(2) per cycle.     

A high-performing nanostructured TiO2 filter for volatile organic compounds using atomic layer deposition

Novel nanostructured gas filtering systems with TiO(2) thin films using atomic layer deposition (ALD) were developed for volatile organic compounds. A superior toluene adsorption efficiency was found for the nanostructured TiO(2) thin films.     

Atomic/Molecular Layer Deposited Iron–Azobenzene Framework Thin Films for Stimuli‐Induced Gas Molecule Capture/Release

The atomic/molecular layer deposition (ALD/MLD) technique provides an elegant way to grow crystalline metal–azobenzene thin films directly from gaseous precursors; the photoactive azobenzene linkers thus form an integral part of the crystal framework. Reversible water capture/release behavior for these thin films can be triggered through the trans–cis photoisomerization reaction of the azobenzene moieties in the structure. The ALD/MLD approach could open up new horizons for example, for the emerging fields of remotely controlled drug delivery and gas storage.     

Controlled synthesis of self-assembled 3D nanostructures using metastable atomic layer deposition

An important feature of atomic layer deposition (ALD) is the fact that the coating that has been deposited is conformal to the substrate surface. Therefore, prepatterned substrates are usually used for the fabrication of 3D nanostructures using ALD. This article presents a new method to generate 3D silver-silica nanostructures using plasma-enhanced atomic layer deposition of silica with tri(dimethylamino)silane (TDMAS) and oxygen plasma as precursors. For this method, silver nanoparticles are used as templates, and during the deposition of silica, the repeatable process of the formation of metastable silver oxides and their decomposition is involved, leading to strong side reactions and the formation of 3D silver-silica hybrid nanostructures. This method is known as metastable atomic layer deposition (MS-ALD). Unlike the conventional ALD, the coating of MS-ALD is not conformal to the substrate surface. Rather, the 3D nanostructures are self-assembled because of side reactions. The geometry of the formed nanostructures can be easily adjusted by tuning the deposition parameters, such as dose time of both precursors and cycle numbers. In our study, we observed nanosponges with features sizes of up to 4 for less than 45 MS-ALD cycles. Nanowire-like silver-silica hybrid nanostructures are generated at higher cycle numbers with feature sizes of up to 10 μm. A similar trend could be observed for changing the dose time of both precursors of TDMAS and oxygen plasma. The height of the nanostructures increases with dose time of both precursors. In contrast to this trend, the surface coverage declines when the investigated parameters (number of cycles, TDMAS, and oxygen plasma dose time) are increased.     

First principles simulation of reaction steps in the atomic layer deposition of titania: dependence of growth on Lewis acidity of titanocene precursor

It is a common finding that titanocene-derived precursors do not yield TiO(2) films in atomic layer deposition (ALD) with water. For instance, ALD with Ti(OMe)(4) and water gives 0.5 ?/cycle, while TiCp*(OMe)(3) does not show any growth (Me = CH(3), Cp* = C(5)(CH(3))(5)). From mass spectrometry we found that Ti(OMe)(4) occurs in the gas phase practically exclusively as a monomer. We then used first principles density functional theory (DFT) to model the ALD reaction sequence and find the reason for the difference in growth behaviour. Both precursors adsorb initially via hydrogen-bonding. The simulations reveal that the Cp* ligand of TiCp*(OMe)(3) lowers the Lewis acidity of the Ti centre and prevents its coordination to surface O ('densification') during both of the ALD pulses. The effect of Cp* on Ti seems to be both steric (full coordination sphere) and electronic (lower electrophilicity). This crucial step in the sequence of ALD reactions is therefore not possible in the case of TiCp*(OMe)(3) + H(2)O, which means that there is no deposition of TiO(2) films.     

Molecular layer deposition of poly(p-phenylene terephthalamide) films using terephthaloyl chloride and p-phenylenediamine

Ultrathin polymer films can be fabricated using the gas-phase method known as molecular layer deposition. This process typically uses bifunctional monomers in a sequential, self-limiting reaction sequence to grow conformal polymer films with molecular layer control. In this study, terephthaloyl chloride (TC) and p-phenylenediamine (PD) were used as the bifunctional monomers to deposit poly(p-phenylene terephthalamide) (PPTA) thin films. 3-Aminopropyl trimethoxysilane or ethanolamine was used to prepare amine-terminated surfaces prior to the PPTA MLD. The surface chemistry and growth rate during PPTA MLD at 145 degrees C were studied using in situ transmission Fourier transform infrared (FTIR) spectroscopy experiments on high surface area powders of SiO2 particles. PPTA MLD thin film growth at 145 degrees C was also examined using in situ transmission FTIR experiments on flat KBr substrates with an amine-terminated Al2O3 ALD overlayer. The integrated absorbances of the N-H and amide I stretching vibrations were measured and used to estimate the thin film thickness. X-ray reflectivity (XRR) experiments were also employed to measure the film thickness after PPTA MLD at 145 degrees C and 180 degrees C. The experiments revealed that the TC and PD reactions displayed self-limiting surface chemistry. The surface species alternated with sequential TC and PD exposures and the PPTA MLD films grew continuously. However, the growth rates per MLD cycle at 145 degrees C were less than expectations based on the size of the molecules involved in the reaction chemistry and were variable between 0.5 and 4.0 A per TC/PD reaction cycle. The lower growth rates are explained by the growth of a limited number of polymer chains on the substrate. The variability in the growth rate is attributed to the difficulties with the bifunctional monomer precursors. Alternative surface chemistries for polymer MLD are proposed that would avoid the use of bifunctional monomers.     

Photoactive Thin-Film Structures of Curcumin,TiO2 and ZnO

Curcumin is known as a biologically active compound and a possible antimicrobial agent. Here, we combine it with TiO₂ and ZnO semiconductors, known for their photocatalytic properties, with an eye towards synergistic photo-harvesting and/or antimicrobial effects. We deposit different nanoscale multi-layer structures of curcumin, TiO₂ and ZnO, by combining the solution-based spin-coating (S-C) technique and the gas-phase atomic layer deposition (ALD) and molecular layer deposition (MLD) thin-film techniques. As one of the highlights, we demonstrate for these multi-layer structures a red-shift in the absorbance maximum and an expansion of the absorbance edge as far as the longest visible wavelength region, which activates them for the visible light harvesting. The novel fabrication approaches introduced here should be compatible with, e.g., textile substrates, opening up new horizons for novel applications such as new types of protective masks with thin conformal antimicrobial coatings.     

Surface chemistry in the atomic layer deposition of TiN films from TiCl4 and ammonia

The surface chemistry of atomic layer depositions (ALD) of titanium nitride films using alternate doses of TiCl4 and NH3 was characterized by using X-ray photoelectron spectroscopy. The nature of the species deposited by each half-reaction was explored first. Evidence was obtained for the partial loss of chlorine atoms and the reduction of the metal during the adsorption of the TiCl4. Subsequent ammonia treatment removes most of the remaining chlorine and leads to the formation of a nitride. Both half-reactions were proven self-limited, stopping after the deposition of submonolayer quantities of the materials. Repeated ALD cycles were shown to lead to the buildup of thick films. However, those films display a Ti3N4 layer on top of the expected TiN. The data suggest that the reduction of the Ti4+ species may therefore occur during the TiCl4, not NH3, dosing step. The incorporation of impurities in the films was also investigated. Chlorine is only deposited on the surface, and in negligible quantities. This Cl appears to originate from readsorption of the HCl byproduct, and could be removed by light sputtering, heating, or further ammonia treatment. Oxygen incorporation, on the other hand, was unavoidable and was determined to possibly come from diffusion from the underlying substrate.     

Grazing incidence-X-ray fluorescence spectrometry for the compositional analysis of nanometer-thin high-kappa dielectric HfO2 layers.

In future microelectronic devices, SiO2 as a gate dielectric material will be replaced by materials with a higher dielectric constant. One such candidate material is HfO2. Thin layers are typically deposited from ligand-containing precursors in chemical vapor deposition (CVD) processes. In the atomic layer deposition (ALD) of HfO2, these precursors are often HfCl4 and H2O. Obviously, the material properties of the deposited films will be affected by residual ligands from the precursors. In this paper, we evaluate the use of grazing incidence--and total reflection-X-ray fluorescence spectrometry (GI-XRF and TXRF) for Cl trace analysis in nanometer-thin HfO2 films deposited using ALD. First, the results from different X-ray analysis approaches for the determination of Hf coverage are compared with the results from Rutherford backscattering spectrometry (RBS). Next, we discuss the selection of an appropriate X-ray excitation source for the analysis of traces within the high-kappa: layers. Finally, we combine both in a study on the accuracy of Cl determinations in HfO2 layers.     

In situ monitoring of atomic layer deposition in nanoporous thin films using ellipsometric porosimetry

Ellipsometric porosimetry (EP) is a handy technique to characterize the porosity and pore size distribution of porous thin films with pore diameters in the range from below 1 nm up to 50 nm and for the characterization of porous low-k films especially. Atomic layer deposition (ALD) can be used to functionalize porous films and membranes, e.g., for the development of filtration and sensor devices and catalytic surfaces. In this work we report on the implementation of the EP technique onto an ALD reactor. This combination allowed us to employ EP for monitoring the modification of a porous thin film through ALD without removing the sample from the deposition setup. The potential of in situ EP for providing information about the effect of ALD coating on the accessible porosity, the pore radius distribution, the thickness, and mechanical properties of a porous film is demonstrated in the ALD of TiO(2) in a mesoporous silica film.     

Probing the Structure and Chemistry of Perylenetetracarboxylic Dianhydride on Graphene Before and After Atomic Layer Deposition of Alumina

The superlative electronic properties of graphene suggest its use as the foundation of next generation integrated circuits. However, this application requires precise control of the interface between graphene and other materials, especially the metal oxides that are commonly used as gate dielectrics. Towards that end, organic seeding layers have been empirically shown to seed ultrathin dielectric growth on graphene via atomic layer deposition (ALD), although the underlying chemical mechanisms and structural details of the molecule/dielectric interface remain unknown. Here, confocal resonance Raman spectroscopy is employed to quantify the structure and chemistry of monolayers of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on graphene before and after deposition of alumina with the ALD precursors trimethyl aluminum (TMA) and water. Photoluminescence measurements provide further insight into the details of the growth mechanism, including the transition between layer-by-layer growth and island formation. Overall, these results reveal that PTCDA is not consumed during ALD, thereby preserving a well-defined and passivating organic interface between graphene and deposited dielectric thin films.     

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 (H₂O). 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.     

Study of interface properties in CuPc based hybrid inorganic-organic solar cells

Metal-substituted phthalocyanine thin films such as copper-phthalocyanine (CuPc) are often used as photo-active and hole transporting layers (HTLs) in fully organic photovoltaic devices. In this work, CuPc is vacuum sublimated on an electron acceptor layer of mesoporous titania (TiO(2)) for the formation of hybrid TiO(2):CuPc solar cell devices. The performance of these hybrid solar cell devices was demonstrated without and with dye sensitization at the TiO(2):CuPc interface. The charge separation and photocurrent contribution at the interfaces in these multilayer hybrid devices was studied by using a variety of optoelectrical and photophysical characterization techniques. It is important to understand the fundamental interface properties of these multilayer hybrid solar cell devices for optimized performance.     

Microwave-solvothermal synthesis of various polymorphs of nanostructured TiO2 in different alcohol media and their lithium ion storage properties

The various polymorphs (anatase, rutile, and brookite) of TiO(2) with different nanomorphologies have been synthesized by a facile microwave-assisted solvothermal process without surfactants, employing TiCl(4) or TiCl(3) as precursors in various alcohol (ethanol, propanol, butanol, and octanol) media. The samples have been characterized by X-ray diffraction (XRD), electron microscopy, and Brunauer-Emmett-Teller (BET) surface area analysis. The Ti/Cl ion concentration, reaction pH, and size of the alcohol molecule are found to control the morphology, crystal structure, and crystallite size of the TiO(2) particles. Among the various TiO(2) polymorphs synthesized, the rutile TiO(2) spheres built up of nanorods that were synthesized with TiCl(4) in octanol have an average pore size and surface area of, respectively, 5 nm and 404 m(2)/g and exhibit the best electrochemical performance with a capacity of >200 mAh/g after 100 cycles and high rate capability. The excellent electrochemical properties originate from the nanorod-building morphology and mesoporosity of TiO(2) spheres that provide good electrical contact, accommodates the strain smoothly, and facilitates facile lithium-ion diffusion.     

Deposition of thin films of organic-inorganic hybrid materials based on aromatic carboxylic acids by atomic layer deposition

Thin films of organic-inorganic hybrid materials have been grown by the atomic layer deposition (ALD) technique, using trimethylaluminium (TMA) and aromatic carboxylic acids such as 1,2-benzene dicarboxylic acid, 1,3-benzene dicarboxylic acid, 1,4-benzene dicarboxylic acid, 1,3,5-benzene tricarboxylic acid, 1,2,4,5-benzene tetracarboxylic acid as precursors. Growth rates as function of temperature show that all systems, with the exception of the benzoic acid-TMA system, possess ALD-windows and provides growth rates in the range of 0.25-1.34 nm/cycle. X-ray diffraction studies of the as-deposited films reveal their amorphous character, which is also supported by very low surface roughness as measured by atomic force microscopy. As-deposited films were investigated by Fourier Transform Infrared Spectroscopy proving that the deposited films are of a hybrid character.