Atomic Layer Deposition of Crystalline MoS2 Thin Films : New Molybdenum Precursor for Low-Temperature Film Growth

Molybdenum disulfide (MoS2) is a semiconducting 2D material, which has evoked wide interest due to its unique properties. However, the lack of controlled and scalable methods for the production of MoS2 films at low temperatures remains a major hindrance on its way to applications. In this work, atomic layer deposition (ALD) is used to deposit crystalline MoS2 thin films at a relatively low temperature of 300 degrees C. A new molybdenum precursor, Mo(thd)(3) (thd = 2,2,6,6-tetramethylheptane-3,5-dionato), is synthesized, characterized, and used for film deposition with H2S as the sulfur precursor. Self-limiting growth with a low growth rate of approximate to 0.025 angstrom cycle(-1), straightforward thickness control, and large-area uniformity are demonstrated. Film crystallinity is found to be relatively good considering the low deposition temperature, but the films have significant surface roughness. Additionally, chemical composition as well as optical and wetting properties are evaluated. MoS2 films are deposited on a variety of substrates, which reveal notable differences in growth rate, surface morphology, and crystallinity. The growth of crystalline MoS2 films at comparably low temperatures by ALD contributes toward the use of MoS2 for applications with a limited thermal budget.Peer reviewe

Highly conductive and stable Co9S8 thin films by atomic layer deposition : from process development and film characterization to selective and epitaxial growth

Co9S8 is an interesting sulfide material with metallic conductivity that has shown promise for various energy applications. Herein, we report a new atomic layer deposition process producing crystalline, pure, and highly conductive Co9S8 thin films using CoCl2(TMEDA) (TMEDA = N,N,N ',N '-tetramethylethylenediamine) and H2S as precursors at 180-300 degrees C. The lowest resistivity of 80 mu omega cm, best uniformity, and highest growth rate are achieved at 275 degrees C. Area-selective deposition is enabled by inherent substrate-dependency of film nucleation. We show that a continuous and conductive Co9S8 film can be prepared on oxide-covered silicon without any growth on Si-H. Besides silicon, Co9S8 films can be grown on a variety of substrates. The first example of an epitaxial Co9S8 film is shown using a GaN substrate. The Co9S8 films are stable up to 750 degrees C in N-2, 400 degrees C in forming gas, and 225 degrees C in O-2 atmosphere. The reported ALD process offers a scalable and cost-effective route to high-quality Co9S8 films, which are of interest for applications ranging from electrocatalysis and rechargeable batteries to metal barrier and liner layers in microelectronics and beyond.Peer reviewe

Titanium alkylphosphate functionalised mesoporous silica for enhanced uptake of rare-earth ions

The separation of rare-earth elements (REEs) is usually carried out by a multi-stage solvent extraction process utilising organophosphorus extractants. Inspired by the structure of the solvating extractant tri-n-butyl phosphate (TBP), new sorbents were designed by covalently attaching short n-alkyl chains (ethyl, n-propyl and n-butyl) to titanium(IV) phosphate functionalised mesoporous MCM-41 silica by a layer-by-layer grafting route. Mesoporous MCM-41 silica served as a versatile porous support and the grafted titanium(IV) derivatives provided enhanced acid stability and solvating extraction capability. Various characterisation methods including solid-state 13C, 29Si and 31P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) and simultaneous thermogravimetry and differential scanning calorimetry-mass spectroscopy (TG/DSC-MS) were used to confirm the ligand attachment. The hybrid materials showed a better uptake of rare-earth ions from nitrate feed solution than the unmodified inorganic material. The optimal separation factor (SF) obtained for scandium–lanthanum separation is in excess of 100 000 at pH 2.1. The SFs calculated for dysprosium–neodymium are approximately 3, which is comparable to that of TBP in a typical solvent extraction setup. This study provides a new strategy to design inorganic–organic hybrid sorbents based on the structure of organophosphorus extractants via metal(IV)–O–P bonds.status: publishe

Molecular Layer Deposition of Thermally Stable Polybenzimidazole-Like Thin Films and Nanostructures

The deposition of polybenzimidazole (PBI)-like thin films by molecular layer deposition is reported here for the first time using isophthalic acid (IPA) and 3,3 '-diaminobenzidine (DAB) as monomers and trimethylaluminum (TMA) as a linker precursor. Two precursor pulsing sequences are tested, the ABCB (TMA + IPA + DAB + IPA) and ABC (TMA + IPA + DAB) type MLD processes result in different types of PBI-like films. With the ABCB sequence thin film growth per cycle (GPC) of 6.0 angstrom is obtained at 225-280 degrees C, whereas GPC of 7.0 angstrom is obtained with the ABC sequence. Films are characterized in detail by Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, time-of-flight elastic recoil detection analysis, and atomic force microscopy. The films have good thermal stability and withstand annealing at 400 degrees C in both air and nitrogen. PBI nanostructures are prepared by depositing PBI-like film on electroblown polyvinylpyrrolidone fibers and removing the template fibers by annealing or dissolution into ethanol.Peer reviewe