Characterization of undoped μc-SiO:H films prepared from (SiH4+CO2+H2)-plasma in RF glow discharge

Undoped hydrogenated microcrystalline silicon oxygen alloy films (μc-SiO:H) have been prepared from (SiH₄+CO₂+H₂)-plasma in RF glow discharge at a high H₂ dilution, moderately high RF power and substrate temperature. A detailed characterization of the films has been done by electrical, optical as well as structural studies, e.g., IR absorption spectroscopy, Raman scattering and transmission electron microscopy. The presence of a very small amount of oxygen induces the crystallization process, which fails to sustain at a higher oxygen dilution. At higher deposition temperature and in improved μc-network H content reduces, however, O incorporation is favoured. Sharp crystallographic rings in the electron diffraction pattern identify several definite planes of c-Si and no such crystal planes from c-SiO_X is detected.     

Influence of hydrogen dilution on low-temperature polycrystalline silicon formation using RF excitation SiH4/H2 plasma

The effect of hydrogen dilution was investigated on polycrystalline silicon formation using radio frequency excitation SiH₄/ H₂ plasma. The hydrogen dilution reduces the growth rate of the a-Si : H films. The dark conductivity of the a-Si : H films increases with increasing H₂ dilution. The dark conductivity of the poly-Si films formed by recrystallization annealing of the a-Si : H film decrease with H₂ dilution. The grain size, with X-ray diffraction spectroscopy and scanning electron microscopy images of the poly-Si films, is in reverse ratio to the H₂ dilution.     

Total SiH4/H2 pressure effect on microcrystalline silicon thin films growth and structure

The effect of the total SiH₄/H₂ gas pressure (1–10 Torr) on the growth rate, the film crystallinity and the nature of hydrogen bonding of microcrystalline silicon thin films deposited by 13.56 MHz plasma-enhanced chemical vapour deposition (PECVD) was investigated under well-controlled discharge conditions. The deposition rate presents an optimum for 2.5 Torr, which does not follow the trend of silane consumption that increases with pressure and is attributed to an increase in plasma density. The film crystallinity increases with pressure from 1–2.5 Torr and then remains almost the same, whereas the films deposited at 1 Torr are highly stressed. On the other hand, hydrogen bonding is also drastically affected.     

High-pressure condition of SiH4+Ar+H2 plasma for deposition of hydrogenated nanocrystalline silicon film

The characteristics of 13.56-MHz discharged SiH₄+Ar+H₂ plasma at high pressure (2–8 Torr), used for the deposition of hydrogenated nanocrystalline silicon (nc-Si:H) films in a capacitively coupled symmetric PECVD system, has been investigated. Plasma parameters such as average electron density, sheath field and bulk field are extracted from equivalent circuit model of the plasma using outputs (current, voltage and phase) of RF V–I probe under different pressure conditions. The conditions of growth in terms of plasma parameters are correlated with properties of the hydrogenated nanocrystalline silicon films characterized by Raman, AFM and dc conductivity. The film deposited at 4 Torr of pressure, where relatively low sheath/bulk field ratio is observed, exhibits high crystallinity and conductivity. The crystalline volume fraction of the films estimated from the Raman spectra is found to vary from 23% to 79%, and the trend of variation is similar to the RF real plasma impedance data.     

Microcrystallisation in Si:H: the effect of gas pressure in Ar-diluted SiH4 plasma

Using noble gas argon as a diluent of SiH₄ in RF glow discharge, undoped μc-Si:H thin films have been developed at a low power density of 30 mW/cm². It has been found that the gas pressure is a critical factor for the growth of μc-Si:H films. Undoped μc-Si:H films having σ_D10⁻⁶ S/cm and ΔE<0.57 eV have been obtained at and above a critical pressure of 0.8 Torr. When the RF power density is increased to 90 mW/cm², a more crystalline as well as highly conducting (σ_D10⁻⁴ S/cm) μc-Si:H film has been achieved at a deposition rate of 30 Å/min, which is much higher than that attained from H₂-diluted SiH₄ plasma, by conventional approach. The crystallinity of the films has been identified by the sharp Raman peak at 520 cm⁻¹ and a large number of micrograins in the TEM micrographs. The metastable state of Ar, denoted as Ar^*, plays the crucial role in inducing microcrystallisation by transferring its de-excitation energy at the surface of the growing film. A mechanism has been proposed to explain the dependence of the formation of μc-Si:H film on the working gas pressure in the plasma.     

Studies on Cu2ZnSnS4 (CZTS) absorber layer using different stacking orders in precursor thin films

Cu₂ZnSnS₄ (CZTS) thin films were deposited by sputtering on glass substrates using stacked precursors. The stacked precursor thin films were prepared from Cu, SnS₂ and ZnS targets at room temperature with different stacking orders of Cu/SnS₂/ZnS/glass (A), ZnS/Cu/SnS₂/glass (B) and SnS₂/ZnS/Cu/glass (C). The stacked precursor thin films were sulfurized using a tubular rapid thermal annealing system in a mixed N₂ (95%)+H₂S (5%) atmosphere at 550 °C for 10 min. The effects of the stacking order in the precursor thin films on the structural, morphological, chemical, electrical and optical properties of the CZTS thin films were investigated. X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy studies showed that the annealed CZTS thin film using a stacking order A had a single kesterite crystal structure without secondary phases, whereas stacking orders B and C have a kesterite phase with secondary phases, such as Cu_2−xS, SnS₂ and SnS. The annealed CZTS thin film using stacking order A showed a very dense morphology without voids. On the other hand, the annealed CZTS thin films using stacking orders B and C contained the volcano shape voids (B) and Sn-based secondary phases (C) on the surface of the annealed thin films. The direct band gap energies of the CZTS thin films were approximately 1.45 eV (A), 1.35 eV (B) and 1.1 eV (C).     

Multi-hydride systems with enhanced hydrogen storage properties derived from Mg(BH4)2 and LiAlH4

A two-step ball-milling method has been provided to synthesize Mg(BH₄)₂ using NaBH₄ and MgCl₂ as starting materials. The method offers high yield and high purity (96%) of the compound. The as-synthesized Mg(BH₄)₂ is then combined with LiAlH₄ by ball-milling in order to form new multi-hydride systems with high hydrogen storage properties. The structure, the dehydrogenation and the reversibility of the combined systems are studied. Analyses show that a metathesis reaction takes place between Mg(BH₄)₂ and LiAlH₄ during milling, forming Mg(AlH₄)₂ and LiBH₄. Mg(BH₄)₂ is excessive and remains in the ball-milled product when the molar ratio of Mg(BH₄)₂ to LiAlH₄ is over 0.5. The onset dehydrogenation temperature of the combined systems is lowered to ca. 120 °C, which is much lower than that of either Mg(BH₄)₂ or LiAlH₄. The dehydrogenation capacities of the combined systems below 300 °C are all higher than that of both Mg(BH₄)₂ and LiAlH₄. The combined systems are reversible for hydrogen storage at moderate hydrogenation condition, and rapid hydrogenation occurred within the initial 30 min. Moreover, the remained Mg(BH₄)₂ in the combined systems is found also partially reversible. The mechanism of the enhancement of the hydrogen storage properties and the dehydrogenation/hydrogenation process of the combined systems were discussed.     

Effect of Al on the hydrogen storage properties of Mg(BH4)2

The various Mg–B–Al–H systems composed of Mg(BH₄)₂ and different Al-sources (metallic Al, LiAlH₄ and its decomposition products) have been investigated as potential hydrogen storage materials. The role of Al was studied on the dehydrogenation and the rehydrogenation of the systems. The results indicate that the different Al-sources exhibit a similar improving effect on the dehydrogenation properties of Mg(BH₄)₂. Taking the Mg(BH₄)₂ + LiAlH₄ system as an example, at first LiAlH₄ rapidly decomposes into LiH and Al, then Mg(BH₄)₂ decomposes into MgH₂ and B, finally the MgH₂ reacts with Al, LiH and B, which forms Mg₃Al₂ and MgAlB₄. The system starts to desorb H₂ at 140 °C and desorbs 3.6 wt.% H₂ below 300 °C, while individual Mg(BH₄)₂ starts to desorb H₂ at 250 °C and desorbs only 1.3 wt.% H₂ below 300 °C. The isothermal desorption kinetics of the Mg–B–Al–H systems is about 40% faster than that of Mg(BH₄)₂ at the hydrogen desorption ratio of 90%. In addition, the Mg–B–Al–H systems show partial reversibility at moderate temperature and pressure. For Al-added system, the product of rehydrogenation is MgH₂, while for LiAlH₄-added system the product is composed of LiBH₄ and MgH₂.     

Phase dependent growth of superficial nanowalls-like structure on TiO2 thin films in molecular hydrogen (H2) annealing environment

We report the surface modification and growth of nanostructures on the surface of titanium oxide thin films during post deposition annealing in molecular hydrogen ambient. Titanium oxide thin films of a thickness of 200 nm were deposited by electron-beam evaporation at a substrate temperature of 300 °C. Films were annealed in 50 and 100 sccm flow rates of hydrogen in the temperature range of 200 °C–600 °C for 4 h. X-ray diffraction analysis showed a polycrystalline structure of the films. Anatase-to-rutile phase transformation took place, and was influenced by the hydrogen flow rate. Atomic force microscopy indicated the growth of 4–6 μm domains enclosed by nanowalls-like boundaries on the surface when the rutile phase was formed. Spectrophotometer measurements indicated that the films were transparent and a red shift in absorption edge was observed due to annealing. The direct band gaps of anatase and rutile were found to be 3.5 eV and 3.2 eV, respectively.     

Theoretical studies on hydrogen adsorption properties of lithium decorated diborene (B2H4Li2) and diboryne (B2H2Li2)

Using ab initio based quantum chemical calculations, we have studied the structure, stability and hydrogen adsorption properties of different boron hydrides decorated with lithium, examples of the corresponding anions being dihydrodiborate dianion, B₂H₂²⁻ and tetrahydrodiborate dianion, B₂H₄²⁻ which can be considered to be analogues and isoelectronic to acetylene (C₂H₂) and ethelene (C₂H₄) respectively. It is shown that there exists a B-B double bond in B₂H₄Li₂ and a B-B triple bond in B₂H₂Li₂. In both the complexes, the lithium sites are found to be cationic in nature and the calculated lithium ion binding energies are found to be very high. The cationic sites in these complexes are found to interact with molecular hydrogen through ion-quadrupole and ion-induced dipole interactions. In both the complexes, each lithium site is found to bind a maximum of three hydrogen molecules which corresponds to a gravimetric density of ∼23 wt% in B₂H₄Li₂ and ∼24 wt% in B₂H₂Li₂. We have also studied the hydrogen adsorption in a model one-dimensional nanowire with C₆H₄B₂Li₂ as the repeating unit and found that it can adsorb hydrogen to the extent 9.68 wt% and the adsorption energy is found to be −2.34 kcal/mol per molecular hydrogen.     

Temperature effect on the electrical properties of pyrolytic MnO2 thin films prepared from Mn(C2H3O2)2·4H2O

Spray deposited MnO₂ thin films onto glass substrate were subjected to a post-deposition heat treatment and the effects of temperature on electrical transport properties were studied in details. The heating and cooling cycles of the samples are reversible after successive heat-treatments in air and vacuum. The films were polycrystalline in structure and the oxygen chemisorption–desorption process was found to play an important role in controlling the electronic properties. Various grain-boundary and energy band parameters were calculated by taking conventional extrinsic semiconductor theory and grain boundary trapping models into account. The samples were non-degenerate n-type semiconductors. The transport properties are interpreted in terms of Seto's model which was proposed for polycrystalline semiconducting films. The inter-crystallite boundaries of the thin films play an important role in the transport properties.     

Kinetic study on fermentation from CO2 and H2 using the acclimated-methanogen in batch culture

Methane was produced from H₂ and CO₂ using the acclimated-mixed methanogens in a 3.71 fermentor in batch culture at pH 7.2 and 37°C. The Fermentation kinetics parameter for the growth of methanogens, overall mass transfer coefficient of the reactor, and the conversion rate of H₂ and CO₂ to CH₄ by the acclimated-mixed culture were determined using the technique of Vega et al. The maximum specific growth rate (μ_max) and H₂ specific consumption rate (q_max) were found to be 0.064(h⁻¹) and 104.8 (mmol h⁻¹ g⁻¹) respectively. Monod saturation constants for growth (Kp) and for inhibition (K′p) were found to be 3.54 (kPa) and 0.57 (kPa), respectively. These findings indicate that without very low dissolved H₂ levels, the fermentations are carried out under μ_max, and the specific uptake rate (q) was almost not affected at any dissolved H₂ level in the range studied. The yield of CH₄ (Yp/s) was calculated to be 0.245 (mol CH₄ mol⁻¹ H₂), which is near the stoichiometric value of 0.25. DH₂ was also measured using the Teflon tubing method and was in good agreement with those estimated by kinetic calculations.     

Mechanistic investigations on significantly improved hydrogen storage performance of the Ca(BH4)2-added 2LiNH2/MgH2 system

The hydrogen storage properties and mechanisms of the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system were systematically investigated. The results showed that the addition of Ca(BH₄)₂ pronouncedly improved hydrogen storage properties of the 2LiNH₂–MgH₂ system. The onset temperature for dehydrogenation of the 2LiNH₂–MgH₂–0.3Ca(BH₄)₂ sample is only 80 °C, a ca. 40 °C decline with respect to the pristine sample. Further hydrogenation examination indicated that the dehydrogenated 2LiNH₂–MgH₂–0.1Ca(BH₄)₂ sample could absorb ca. 4.7 wt% of hydrogen at 160 °C and 100 atm while only 0.8 wt% of hydrogen was recharged into the dehydrogenated pristine sample under the same conditions. Structural analyses revealed that during ball milling, a metathesis reaction between Ca(BH₄)₂ and LiNH₂ firstly occurred to convert to Ca(NH₂)₂ and LiBH₄, and then, the newly developed LiBH₄ reacted with LiNH₂ to form Li₄(BH₄)(NH₂)₃. Upon heating, the in situ formed Ca(NH₂)₂ and Li₄(BH₄)(NH₂)₃ work together to significantly decrease the operating temperatures for hydrogen storage in the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system.     

Effect of Ar plasma treatment on the photo-electrical properties of nanocrystal TiO2 films

Nanocrystal TiO₂ films were prepared by radio frequency (RF) sputtering technique and coating method, respectively. The samples were treated by Ar RF plasma. The crystal structure, absorption spectra and morphology of the TiO₂ films were investigated by X-ray diffraction (XRD), UV–VIS spectrophotometry and atomic force microscopy (AFM). The voltages and photo-currents of the TiO₂ electrodes were also measured. By Ar plasma treatment, the photo-currents of the sputtered and coated TiO₂ electrodes increased by 80% and 60%, respectively.     

Vibrational spectra of Ti:C2H4(nH2) and Ti:C2H4(nD2) (n = 1-5) complexes and the equilibrium isotope effect: Calculations and experiment

Calculations of the ability of titanium-ethylene complexes of the type, Ti:C₂H₄, to absorb molecular hydrogen have been performed using density functional theory. A maximum of 5H₂ molecules can be adsorbed on Ti:C₂H₄ thereby giving an uptake capacity of 11.72 wt%, in excellent agreement with previous experimental results reported by two of us (Phys. Rev. Lett., 100, 105505, 2008). Calculations of the vibrational frequencies in such complexes with both H₂ and D₂, Ti:C₂H₄(nH₂) and Ti:C₂H₄(nD₂), n = 1-5, have also been performed and the values obtained used to find the Equilibrium Isotope Effect (EIE). Measurements of the EIE are also reported and these are in excellent agreement with the EIE calculated for 5H₂ molecules adsorbed in the complex.     

Influence of titanium and nickel dopants on the dehydrogenation properties of Mg(AlH4)2: Electronic structure mechanisms

The structures and dehydrogenation properties of pure and Ti/Ni-doped Mg(AlH₄)₂ were investigated using the first-principles calculations. The dopants mainly affect the geometric and electronic structures of their vicinal AlH₄ units. Ti and Ni dopants improve the dehydrogenation of Mg(AlH₄)₂ in different mechanisms. In the Ti-doped case, Ti prefers to occupy the 13-hedral interstice (Ti_iA) and substitute for the Al atom (Ti_Al), to form a high-coordination structure TiH_n (n = 6, 7). The Ti 3d electrons hybridize markedly with the H 1s electrons in Ti_Al and with the Al 3p electrons in Ti_iA, which weakens the Al–H bond of adjacent AlH₄ units and facilitates the hydrogen dissociation. A TiAl₃H₁₃ intermediate in Ti_iA is inferred as the precursor of Mg(AlH₄)₂ dehydrogenation. In contrast, Ni tends to occupy the octahedral interstice to form the NiH₄ tetrahedron. The tight bind of the Ni with its surrounding H atoms inhibits their dissociation though the nearby Al–H bond also becomes weak. Therefore, Ti is the better dopant candidate than Ni for improving the dehydrogenation properties of Mg(AlH₄)₂ because of its abundant activated hydrogen atoms and low hydrogen removal energy.     

Studies on the de/re-hydrogenation characteristic of Mg(NH2)2/LiH mixture admixed with carbon nanofibres

The effect of carbon nanofibres (CNFs) on the de/re-hydrogenation characteristics of 1:2 magnesium amide (Mg(NH₂)₂) and lithium hydride (LiH) mixture is investigated. It is found that the desorption as well as absorption characteristic of the 1:2 Mg(NH₂)₂/LiH mixture is improved with admixing of different shaped (planar and helical) CNFs separately. The different shaped CNFs were synthesized through catalytic decomposition of acetylene gas over LaNi₅ alloy. The synthesized CNFs contain Ni-metal nano particles. Among two different types of nanofibres namely planar carbon nanofibres (PCNFs) and helical carbon nanofibres (HCNFs), the later was found to act as a better catalyst. The decomposition temperature of the pristine Mg(NH₂)₂/LiH mixture is ∼250 °C, reduced to 150 and 140 °C for the PCNF and HCNF admixed Mg(NH₂)₂/LiH mixture respectively. The activation energy for dehydrogenation reaction was found to ∼97.2 kJ/mol, which is further reduced to ∼67 and ∼65 kJ/mol for the PCNF and HCNF admixed Mg(NH₂)₂/LiH mixture respectively. The lowering of decomposition temperature and enhancement in desorption kinetics, with admixing of different shaped CNFs are described and discussed.     

Effect of sodium diffusion on the structural and electrical properties of Cu2ZnSnS4 thin films

Cu₂ZnSnS₄ (CZTS) is a kesterite semiconductor consisting of abundantly available elements. It has a band gap of 1.5 eV and a large absorption coefficient. Hence, thin films made of this material can be used as absorber layers of a solar cell. CZTS films were deposited on soda lime and Na free borosilicate glass substrates through Ultrasonic Spray Pyrolysis. The diffusion of sodium from soda lime glass was found to have a profound effect on characteristics like grain size, crystal texture and conductivity of CZTS thin films. Copper ion concentration also varied during the deposition and it was observed that the carrier concentration was enhanced when there was a deficiency of copper in the films. The effect of sodium diffusion and copper deficiency in enhancing the structural and electrical properties of CZTS films are presented in this paper.     

Influence of hydrogen dilution on structural, electrical and optical properties of hydrogenated nanocrystalline silicon (nc-Si:H) thin films prepared by plasma enhanced chemical vapour deposition (PE-CVD)

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were deposited from pure silane (SiH₄) and hydrogen (H₂) gas mixture by conventional plasma enhanced chemical vapour deposition (PE-CVD) method at low temperature (200 °C) using high rf power. The structural, optical and electrical properties of these films are carefully and systematically investigated as a function of hydrogen dilution of silane (R). Characterization of these films with low angle X-ray diffraction and Raman spectroscopy revealed that the crystallite size in the films tends to decrease and at same time the volume fraction of crystallites increases with increase in R. The Fourier transform infrared (FTIR) spectroscopic analysis showed at low values of R, the hydrogen is predominantly incorporated in the nc-Si:H films in the mono-hydrogen (SiH) bonding configuration. However, with increasing R the hydrogen bonding in nc-Si:H films shifts from mono-hydrogen (SiH) to di-hydrogen (SiH₂) and (SiH₂)_n complexes. The hydrogen content in the nc-Si:H films decreases with increase in R and was found less than 10 at% over the entire studied range of R. On the other hand, the Tauc's optical band gap remains as high as 2 eV or much higher. The quantum size effect may responsible for higher band gap in nc-Si:H films. A correlation between electrical and structural properties has been found. For optimized deposition conditions, nc-Si:H films with crystallite size 7.67 nm having good degree of crystallinity (84% ) and high band gap (2.25 eV) were obtained with a low hydrogen content (6.5 at%). However, for these optimized conditions, the deposition rate was quite small (1.6 Å/s).