Molecular precursor approach to metal oxide and pnictide thin films

AbstractMolecular precursors for the preparation of main group metal oxide and transition metal pnictide thin films have been developed. This work involves the design and synthesis of single-source precursors that contain all the elements required in the thin film. Design of the ideal precursor presents a significant challenge since they must be volatile, non-toxic and thermally stable. Therefore the precursors have been tailored to give clean, reproducible decomposition leading to high quality thin films with good coverage of the substrate. In this review key aspects of precursor synthesis and thin film deposition developed in our group are described. The range of precursors developed for main group oxides, in particular gallium and indium oxide, are discussed, with the most studied being the donor-functionalized alkoxides of the type [R2M(OR′)]2 (M=Ga, In; R=H, Me, Et; R′=CH2CH2NMe2, CH2CH2OMe etc.). Preliminary mechanistic studies suggest that monomers are formed in the gas phase via stabilization of the metal centre by the donor atom (N or O). Precursors to transition metal pnictides have also been developed, including guanidinates, imides, phosphine and arsine compounds and an overview of their use in film deposition is given

Plasmonic Gold Nanostars Incorporated into High-Efficiency Perovskite Solar Cells

Incorporating appropriate plasmonic nanostructures into photovoltaic (PV) systems is of great utility for enhancing photon absorption and thus improving device performance. Herein, the successful integration of plasmonic gold nanostars (AuNSs) into mesoporous TiO2 photoelectrodes for perovskite solar cells (PSCs) is reported. The PSCs fabricated with TiO2-AuNSs photoelectrodes exhibited a device efficiency of up to 17.72 %, whereas the control cells without AuNSs showed a maximum efficiency of 15.19 %. We attribute the origin of increased device performance to enhanced light absorption and suppressed charge recombination

Encapsulation of N-containing compounds in a new hydrophilic Cd-based crystalline sponge via coordinative alignment method

The crystalline sponge method (CSM) is a technology which allows precise molecular determination of non-crystalline compounds, without the need to crystallise them independently, by soaking them in a crystalline metal–organic framework (MOF). To expand the CSM to a wider range of guest molecules, the development of a new crystalline sponge is essential. In this study a new Cd-based MOF {[Cd7(4,4′4′′-[1,3,5-benzenetriyltris(carbonylimino)]-trisbenzoato)4(μ3-OH)2(H2O)4(DMF)4]·(solvent)x}n was synthesized and investigated as an alternative crystalline sponge (2). Sponge 2 demonstrated versatility in solvent stability compared to the well-studied [{(ZnI2)3(tris(4-pyridyl)-1,3,5-triazine)2·x(CHCl3)}n] (1) and was stable in the presence of polar aprotic, polar protic solvents and Lewis bases. Inclusion complexes with three solvents, acetonitrile, acetone, and isopropanol were prepared. These guest molecules were fixed in the pore via hydrogen bonding confirming the hydrophilic pore environment of sponge 2. Notably, sponge 2 also demonstrated the ability to accommodate N-containing compounds such as pyridine, 3,5-lutidine, and 4-aminopyridine via the coordinative alignment method (CAL). A study was conducted to compare the ability of sponge 2 and related pyridine containing sponge 3 by encapsulating the same pair of guests: N,N-dimethylaniline and propiophenone

n-Type conducting P doped ZnO thin films via chemical vapor deposition

Extrinsically doped ZnO thin films are of interest due to their high electrical conductivity and transparency to visible light. In this study, P doped ZnO thin films were grown on glass substrates via aerosol assisted chemical vapour deposition. The results show that P is a successful dopant for ZnO in the V+ oxidation state and is able to reduce resistivity to 6.0 × 10−3 Ω cm while maintaining visible light transmittance at ∼75%. The thins films were characterized by X-ray diffraction studies that showed only Bragg peaks for the wurtzite ZnO phase. Fitting of the diffraction data to a Le Bail model also showed a general expansion of the ZnO unit cell upon doping due to the substitution of Zn2+ ions with the larger P5+

Preparation and photocatalytic activity of ZnGa2O4-β-Ga2O3 thin films

Photocatalytic zinc gallate thin films were fabricated by aerosol-assisted chemical vapor deposition (AACVD). The photocatalytic enhancement of ZnGa2O4-β-Ga2O3 compared with ZnGa2O4 thin films results from heterojunction interface facilitating charge separation

Photocatalytic Oxygen Evolution from Cobalt-Modified Nanocrystalline BiFeO3 Films Grown via Low-Pressure Chemical Vapor Deposition from beta-Diketonate Precursors

BiFeO3 is an interesting multifunctional narrow band gap semiconductor that exhibits simultaneous multiferroic, photovoltaic, and photocatalytic behavior. Hence there is much interest in the growth of thin films of BiFeO3 via chemical vapor deposition (CVD); however, the number of suitable bismuth precursors is severely limited. A series of homoleptic bismuth(III) β-diketonate complexes were synthesized via simple room temperature ligand-exchange reactions from [Bi(N(SiMe3)2)3] and free diketonate ligands, which yielded the crystal structure of [Bi(acac)3] as a 1-D polymer. We attempted to use these complexes for low pressure CVD (LPCVD) growth of BiFeO3 films with [Fe(acac)3]; however, all bismuth complexes exhibited poor volatilities and decomposition characteristics, and as a result film growth was unsuccessful. Subsequently, the volatile alkoxide [Bi(OtBu)3], with [Fe(acac)3], was used to grow dense BiFeO3 films via low pressure CVD. The BiFeO3 films possessed multiferroic properties at room temperature and exhibited activity for visible light-driven water oxidation in the presence of a Ag+ electron scavenger, which improved significantly when modified with a cobalt surface cocatalyst. The increase in activity, probed by time-resolved photoluminescence spectroscopy, was attributed to improved charge carrier separation arising from the in-built internal electric field of BiFeO3 in addition to the presence of an efficient cobalt oxygen evolution catalyst

Ethyl Zinc β-Ketoiminates and β-Amidoenoates: Influence of Precursor Design on the Properties of Highly Conductive Zinc Oxide Thin Films from Aerosol-Assisted Chemical Vapour Deposition

Highly transparent (>85 %) and conductive (1.086×10-3  Ω cm) zinc oxide thin films have been deposited from specifically selected precursors allowing us to establish a direct correlation between their molecular structure and the optoelectronic properties of the deposited films. Mono-ligated ethyl zinc compounds of varying steric bulk: [EtZn(OC(Me)CH(Me)N(i Pr))]2 (1), [EtZn(OC(OEt)CH(Me)N(i Pr))]2 (2) and [EtZn(OC(OEt)CH(CH3 )N(Dipp))]2 (3) were compared with the related bis-ligated zinc complexes [Zn(OC(Me)CH(Me)N(i Pr))2 ] (4), [Zn(OC(OEt)CH(Me)N(i Pr))2 ] (5) and [Zn(OC(OEt)CH(Me)N(Dipp))2 ] (6). In all cases bulkier ligands resulted in poorer electronic properties of deposited films, whilst all mono-ligated compounds were shown as superior precursors. All complexes were characterised by 1 H and 13 C{1 H} NMR and elemental analysis, with the structure of 6 determined by single crystal X-ray diffraction. Zinc oxide films were deposited from single and dual source (with methanol) reactions of these precursors, and analysed via XRD, XPS and EDX. Optoelectronic properties were investigated through UV/vis spectroscopy and Hall effect measurements, and morphology was examined via SEM. Tauc plots from UV/vis data indicated that Film A showed the lowest band gap of 3.31 eV. Varying the elemental composition of the precursors led to changes in the elemental composition of the resultant films, as well as changes in their structural and optoelectronic properties. Using this approach of precursor design, we have been able to tune single source precursors towards zinc oxide to deposit films with specific properties

Visible-Light-Active Iodide-Doped BiOBr Coatings for Sustainable Infrastructure

The search for efficient materials for sustainable infrastructure is an urgent challenge toward potential negative emission technologies and the global environmental crisis. Pleasant, efficient sunlight-activated coatings for applications in self-cleaning windows are sought in the glass industry, particularly those produced from scalable technologies. The current work presents visible-light-active iodide-doped BiOBr thin films fabricated using aerosol-assisted chemical vapor deposition. The impact of dopant concentration on the structural, morphological, and optical properties was studied systematically. The photocatalytic properties of the parent materials and as-deposited doped films were evaluated using the smart ink test. An optimized material was identified as containing 2.7 atom % iodide dopant. Insight into the photocatalytic behavior of these coatings was gathered from photoluminescence and photoelectrochemical studies. The optimum photocatalytic performance could be explained from a balance between photon absorption, charge generation, carrier separation, and charge transport properties under 450 nm irradiation. This optimized iodide-doped BiOBr coating is an excellent candidate for the photodegradation of volatile organic pollutants, with potential applications in self-cleaning windows and other surfaces