Effect of annealing temperature on silicon-based MoSx thin film solar cells

A suitable annealing temperature was found by adopting the sol–gel method to prepare silicon-based molybdenum sulfide film heterojunction solar cells. As shown by the results, a change in the efficiency of the solar cells, which was attributed to the fact that as the annealing temperature rises, the degree of crystallization of the film increases continuously, the degree of order of the crystal particles goes up first and then goes down, and the temperature change affects the proportion of Mo in different valence states. By comparison, it was found that when the temperature reached 500 °C, the degree of order of the film was raised and the film was in the initial zone from the amorphous to the microcrystal phase change and the proportion of Mo 6+ was relatively large, increasing the conversion efficiency of the device power to 7.55% and laying a good basis for preparing high-performance solar batteries made in the two-dimensional materials. When the annealing temperature continues to rise, the intergranular defects increase, and the overall degree of order of the film decreases. Furthermore, the highly crystalline thin films and the improvement in the device efficiency can be controlled if we obtained the relationship between the annealing temperature and the layers of the two-dimensional materials.

A suitable annealing temperature was found by adopting the sol–gel method to prepare silicon-based molybdenum sulfide film heterojunction solar cells.     

Effects of annealing temperature on properties of InSnZnO thin film transistors prepared by Co-sputtering

Indium-tin-zinc-oxide (ITZO) as the channel layer grown by co-sputtering of ZnO target and ITO target in the bottom gate thin-film transistors (TFTs) is proposed in this work. The microstructure and optical properties of ITZO thin films at different annealing temperatures were analyzed. The impact of various annealing temperatures on the ITZO TFT performance characteristics was systematically investigated as well. It was found that ITZO TFT with annealing temperature of 300 °C exhibits excellent electrical performance with a high saturation field-effect mobility (μ_sat) of 27.4 cm² V⁻¹ s⁻¹, a low threshold voltage (V_th) of −0.64 V, a small subthreshold swing (SS) value of 0.23 V per decade, and the high on-off current ratio (I_on/I_off) of 1.8 × 10⁷. In addition, it also shows good output curves including gate control capabilities and good electrode contact as well as extreme atmospheric stability. As shown by photoluminescence (PL) analysis and X-ray photoelectron spectroscopy (XPS) analysis, the beneficial effects of various annealing temperatures on device performance are attributed to the reorganization of the amorphous network and the control of defect chemistry in the films. The correlation between the post-deposition thermal treatment and the characteristics of a transistor was investigated and excellent performance of the transistor was demonstrated.

Indium-tin-zinc-oxide (ITZO) as the channel layer grown by co-sputtering of ZnO target and ITO target in the bottom gate thin-film transistors (TFTs) is proposed in this work.     

Influence of l-lysine on the permeation and antifouling performance of polyamide thin film composite reverse osmosis membranes

Polyamide thin film composite (TFC) reverse osmosis (RO) membranes were prepared in this study. l-Lysine is used as a type of aqueous additive during interfacial polymerization. As a result, the pure water flux (PWF) of the resulting membranes increased by around 18% and their salt rejection improved from 98.17% to 98.40% at an optimum l-lysine dosage of 0.1 wt%. Additionally, the anti-fouling properties of the resulting membranes were enhanced. The chemical structure of the membranes was investigated using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The morphologies of the top surface and cross-section of the membranes were revealed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). Furthermore, contact angle (CA) and zeta potential measurements were carried out to determine the surface properties of the membranes. The results showed that the TFC RO membrane became thinner, smoother, smaller in surface area, more hydrophilic and more negatively charged after the introduction of l-lysine. Accordingly, the reason for the enhancement in the PWF and anti-fouling properties of the TFC RO membranes with the introduction of l-lysine was analyzed. The thinner selective layer (increase in concentration gradient across the membrane) with carboxyl groups (hydrogen bond interactions) and loose structure (greater free volume and sub-nanometer pores) resulted in low hydraulic resistance to the permeability of the polyamide selective layer, which led to the enhancement in PWF. Also, the smoother and more hydrophilic top surface and the increase in negative charges in the selective layer contribute to the improvement in anti-fouling property.

Polyamide thin film composite (TFC) reverse osmosis (RO) membranes were prepared in this study.     

New aspects of improving the performance of WO3 thin films for photoelectrochemical water splitting by tuning the ultrathin depletion region

In this work, we explored a facile, scalable and effective method for substantially enhancing photocurrent and incident-photon-to-current efficiency of WO₃ thin-film photoanodes by a mild reduction treatment under low oxygen pressure. Experimental data from photoelectrochemical and electrochemical impedance spectroscopies have shown that such treatment can increase the charge carrier density on WO₃ photoanode surfaces resulting in improvements in hole collection efficiency and reduction in charge recombination. Despite a much thinner layer of WO₃ (about 500 nm) compared to those in other published studies, the electrodes exhibited an ultra-high photocurrent density of 1.81 mA cm⁻² at 1.23 V vs. RHE. This current density is one of the highest ones among WO₃-based photoanodes described in literature. The proposed surface modulation approach offers an effective and scalable method to prepare high-performance thin film photoanodes for photoelectrochemical water splitting.

Surface modulation approach offers an effective and scalable method for high-performance WO₃ photoanodes.     

Fully solution-induced high performance indium oxide thin film transistors with ZrOx high-k gate dielectrics

Solution based deposition has been recently considered as a viable option for low-cost flexible electronics. In this context, research efforts have been increasingly focused on the development of suitable solution-processed materials for oxide based transistors. In this work, we report a fully solution synthesis route, using 2-methoxyethanol as solvent, for the preparation of In₂O₃ thin films and ZrO_x gate dielectrics, as well as the fabrication of In₂O₃-based TFTs. To verify the possible applications of ZrO_x thin films as the gate dielectric in complementary metal oxide semiconductor (CMOS) electronics, fully solution-induced In₂O₃ TFTs based on ZrO₂ dielectrics have been integrated and investigated. The devices, with an optimized annealing temperature of 300 °C, have demonstrated high electrical performance and operational stability at a low voltage of 2 V, including a high μ_sat of 4.42 cm² V⁻¹ s⁻¹, low threshold voltage of 0.31 V, threshold voltage shift of 0.15 V under positive bias stress for 7200 s, and large I_on/I_off of 7.5 × 10⁷, respectively. The as-fabricated In₂O₃/ZrO_x TFTs enable fully solution-derived oxide TFTs for potential application in portable and low-power consumption electronics.

Solution based deposition has been recently considered as a viable option for low-cost flexible electronics.     

Medical nebulizer performance: effects of cascade impactor temperature

BACKGROUND: During operation of a jet nebulizer, the temperature of the nebulizer outlet could decrease by more than 10 degrees C, depending on the nebulizer type and operating conditions, such as driving flow rate and fill volume. The droplet size distribution generated from the nebulizer can be measured by a cascade impactor. However, when the cascade impactor is operated at ambient room temperature, the droplets could evaporate because of the temperature difference between the nebulizer outlet and the body of the impactor. METHODS: An 8-stage cascade impactor was used to measure the particle size distribution from 4 different types of jet nebulizer (LC Plus, Side-Stream, VixOne, and Micromist) in 2 temperature conditions: ambient (22 degrees C) and low (10 degrees C). Two different formulations, albuterol (aqueous solution) and budesonide (suspension), were used. RESULTS: There was a significantly larger (p < 0.05) mass median aerodynamic diameter and smaller respirable fraction for each nebulizer with the impactor at low temperature than with the impactor at ambient temperature. The mass median aerodynamic diameter of the nebulizers with the impactor operating at low temperature appeared 15-130% larger than with the impactor operating at ambient temperature, for both formulations. The respirable fraction also changed from 10% when the impactor was operated at low temperature to 65% when the impactor was operated at ambient temperature. CONCLUSION: The results provide important information for the use of a cascade impactor to measure the particle-size distribution of nebulizer aerosols.     

Effects of NaCl treatment on the performance and environmental stability of microporous SiO2-based thin film transistors

In this letter, we report the effects of NaCl treatment on the performance and environmental stability of microporous SiO₂-based thin film transistors (TFTs). It was found that appropriate amounts of NaCl treatment significantly improved the electric double layer (EDL) capacitance of such composite solid electrolytes from 1.9 to 4.7 μF cm⁻². A highest field effect mobility of 42.8 cm² V⁻¹ s⁻¹ was found for 1% NaCl treated microporous SiO₂-based TFTs. However, 10% and 26.5% NaCl treated microporous SiO₂-based TFTs showed good environmental stability of the I_on/I_off ratio with reasonable field effect mobility.

In this letter, we report the effects of NaCl treatment on the performance and environmental stability of microporous SiO₂-based thin film transistors (TFTs).     

SiO2 thin film growth through a pure atomic layer deposition technique at room temperature

In this study, less contaminated and porous SiO₂ films were grown via ALD at room temperature. In addition to the well-known catalytic effect of ammonia, the self-limitation of the reaction was demonstrated by tuning the exposure of SiCl₄, NH₃ and H₂O. This pure ALD approach generated porous oxide layers with very low chloride contamination in films. This optimized RT-ALD process could be applied to a wide range of substrates that need to be 3D-coated, similar to mesoporous structured membranes.

In this study, less contaminated and porous SiO₂ films were grown via ALD at room temperature.     

A thin film nanocomposite membrane with pre-immobilized UiO-66-NH2 toward enhanced nanofiltration performance

A facile controlled interfacial polymerization strategy was proposed for the synthesis of novel thin film nanocomposite (TFN) membranes for enhanced nanofiltration performance. UiO-66 nanoparticles were aminated and pre-immobilized onto a polymer substrate via polydopamine (PDA) coating to achieve a continuous and defect-free polyamide dense layer. The mediation of the PDA coating could not only enhance the structural stability of TFN nanofiltration membranes, but also improve the dispersion and anchorage of UiO-66-NH₂, thus closely fixing the position of UiO-66-NH₂ nanoparticles on the polymer substrate. Moreover, since the amino group (–NH₂) further reacted with PDA via Michael addition or Schiff base reaction, the in situ mutual reaction reduced the nanoparticle losses significantly during the draining off of the monomer solution in the fabrication process, which effectively cut down the actual dosage. The results showed that the PDA interlayer could induce the tight attachment of the PA layer to the support, enhancing the structural stability of TFN membranes. Furthermore, the dosage of UiO-66-NH₂ in the as-prepared TFN membranes could also be decreased to as low as 0.01 w/v%, which was nearly a 10–20-fold reduction in the required amount of UiO-66-NH₂ for the synthesis. The fabricated TFN/UiO-66-NH₂ membranes exhibited very high water permeance and competitive salt rejections in cross-flow nanofiltration, which shows the huge potential for the application of novel TFN membranes with controlled nanoparticle incorporation in industrial separation.

A facile controlled interfacial polymerization strategy was proposed for the synthesis of novel thin film nanocomposite (TFN) membranes for enhanced nanofiltration performance.     

Enhanced photoelectrochemical performance of LaFeO3 photocathode with Au buffer layer

Due to an appropriate band gap of 2.07 eV, perovskite LaFeO₃ (LFO) is an alternative candidate for high-efficiency photoelectrochemical (PEC) systems. However, the photocurrent of the LFO photocathode is too low to be practical. Herein, we prepared a LFO film with high crystal quality by inserting an Au thin layer between LFO and FTO in the LFO/FTO photocathode. Accordingly, an effective improvement PEC performance could be obtained and the photocurrent density of the FTO/Au/LFO electrode was increased to −19.60 μA cm⁻² at 0.6 V vs. RHE, which is 4.1 times higher than that of pristine FTO/LFO electrode. Based on the experimental and theoretical analysis, the enhancement of the photocurrent was attributed to the strong light harvesting, enhanced charge separation, and increased charge-collection efficiency of the Au/LFO structure. This work provides a promising strategy to develop high-efficiency PEC electrodes, and has potential to be applied in the visible-light water splitting area.

Au/LFO obtained by facile magnetron sputtering and sol–gel process presents a remarkable improvement in photocurrent up to −19.60 μA cm⁻².     

Ellipsometry study on gold-nanoparticle-coated gold thin film for biosensing application

The amplified plasmonic response from various distributions of gold nanoparticles (AuNPs) coated on top of gold thin film was studied via ellipsometry under total internal reflection mode. The surface plasmon resonance dip can be tuned from the visible to near infrared by simply varying the AuNP concentration. Theoretical modeling based on effective medium theory with a multi-slice model has been employed to fit the experimental results. Additionally, this experimental tool has been further extended to study bio-molecular interactions with metal surfaces as well as in studying protein-protein interaction without any labeling. Hence, this technique could provide a non-destructive way of designing tunable label-free optical biosensors with very high sensitivity.     

Thickness dependent thermal performance of a poly(3,4-ethylenedioxythiophene) thin film synthesized via an electrochemical approach

Polymer-based thermal interface materials (TIMs) have attracted wide attention in the field of thermal management because of their outstanding properties including light weight, low cost, corrosion resistance and easy processing. However, the low thermal conductivity (∼0.2 W m⁻¹ K⁻¹) of the intrinsic polymer matrix largely degrades the overall thermal performance of polymer-based TIMs even those containing highly thermal conductive fillers. Hence, enhancing the intrinsic thermal conductivity of the polymer matrix is one of the most critical problems needed to be solved. This paper studies the thermal conductivity of poly(3,4-ethylenedioxythiophene) (PEDOT) films fabricated via cyclic voltammetry. By controlling the number of cycles in the electrochemical synthesis, different thickness of PEDOT films could be obtained. A time-domain thermoreflectance (TDTR) system was employed to evaluate the thermal performance of such as-prepared PEDOT films. We have demonstrated that a PEDOT film with thickness of 40 nm achieves the highest out-of-plane thermal conductivity of ∼0.60 W m⁻¹ K⁻¹, which is almost three folds the thermal conductivity of commercially available pristine PEDOT:PSS film with similar thickness. The X-ray diffraction spectrum reveals that the PEDOT thin film with high crystallinity at the initial stage of electrochemical synthesis leads to enhanced thermal transportation. The findings in this work not only offer an opportunity to fabricate polymer materials exhibiting enhanced thermal conductivity, but also allow one to adjust the thermal performance of conducting polymers in practical applications.

Enhancing the intrinsic thermal conductivity of PEDOT films via a one-step template-less electrochemical synthesis.     

Room temperature two-photon-pumped random lasers in FAPbBr3/polyethylene oxide (PEO) composite perovskite thin film

Solution-processed organic–inorganic halide lead perovskites have attracted increasing attention due to their great potential in low-cost, effective, and versatile light emission applications and large-scale portable optoelectronic devices. In this paper, formamidinium lead tribromide perovskite thin films composited with polyethylene oxide (PEO) were fabricated by a solution processing method. Great enhancement of photoluminescence was observed and more attractively, two-photon-pumped random lasing action could be achieved at room temperature when pumped by a nanosecond pulse laser with excitation wavelength centered at 1064 nm. Evident transition from spontaneous upconversion emission to random lasing action was investigated by monitoring the log–log light emission slope and peak width at half height. The lasing threshold is at around 1.1 mJ cm⁻², which is comparable to that of other two-photon upconversion random lasers. The efficient random lasing emission originates from the multiple random scattering provided by the micrometer-scale rugged morphology and polycrystalline grain boundaries. Compared with conventional lasers that normally serve as a coherent light source, the perovskite random lasers show promise in fabricating low-cost thin-film lasing devices for flexible and speckle-free imaging applications.

Solution-processed organic–inorganic halide lead perovskites have attracted increasing attention due to their great potential in low-cost, effective, and versatile light emission applications and large-scale portable optoelectronic devices.     

Oxide removal and stabilization of bismuth thin films through chemically bound thiol layers

Bismuth has been identified as a material of interest for electronic applications due to its extremely high electron mobility and quantum confinement effects observed at nanoscale dimensions. However, it is also the case that Bi nanostructures are readily oxidised in ambient air, necessitating additional capping steps to prevent surface re-oxidation, thus limiting the processing potential of this material. This article describes an oxide removal and surface stabilization method performed on molecular beam epitaxy (MBE) grown bismuth thin-films using ambient air wet-chemistry. Alkanethiol molecules were used to dissolve the readily formed bismuth oxides through a catalytic reaction; the bare surface was then reacted with the free thiols to form an organic layer which showed resistance to complete reoxidation for up to 10 days.

Functionalisation of bismuth thin films with alkane thiols for oxide removal and passivation.

Bulk bismuth has been identified as a material of interest for electronic applications, due to its extremely high electron mobility (>10⁴ cm² V⁻¹ s at 300 K).¹ The lower Fermi velocity of the surface states of Bi (111) and (110), coupled with larger Fermi surface elements compared to the bulk, leads to a density of states at the Fermi level that is higher at the surface, such that the surface can be considered to be a pseudo-2 D material under certain orientations.^2,3 Bi nanowires have previously been investigated for their thermoelectric properties due to their small electron effective mass and highly anisotropic Fermi surface.⁴Thickness and crystallographic orientation are key parameters that determine the electronic structure of Bi thin films, the first as it affects the quantum confinement of the material⁵ and the second because of the strong electronic anisotropy⁶ of Bi. The relatively large Fermi wavelength of Bi (approximately 40 nm (ref. 7)) can potentially lead to quantum confinement effects at nanoscale dimensions, e.g. semimetal-to-semiconductor transitions have been both predicted⁵ and observed⁸ in Bi nanostructures. This semimetal-to-semiconductor transition in Bi has been utilised to form a semiconducting channel between semi-metallic source and drain regions, in what is defined as a confinement modulated gap transistor (CMGT).⁹ This is a dopant-free transistor that does not suffer from dopant fluctuation and activation problems. Recently a hetero-dimensional rectifier was formed on bismuth thin-films and consisted of a 3D semi-metallic region next to a thin 2D semiconducting region.¹⁰ Rectifying behaviour was observed in the Bi diode at room temperature, highlighting that a ‘junction’ can be created without dopant atoms.However, the fabrication of the Bi CMGTs relies on the formation of the metal-semimetal junction in a UHV system in order to prevent surface re-oxidation, as the surface has been shown to readily re-oxidise when in contact with air, leading to a decrease in resistance.⁸ While UHV oxide removal through RIE is a simple process, it does not allow for surface passivation and subsequent ambient air handling, requiring an additional capping step to prevent re-oxidation of the device.The oxidation of bismuth thin films was first reported by Hapase et al.¹¹ in 1967, who found that oxidation proceeds by diffusion of the Bi from the thin film through the oxide film, as previously hypothesised,¹²via a Wagner mechanism. Further studies then correlated the oxide thickness to a diffusion equation and showed that a metastable crystal phase of the oxide is present which evolves into a different stable phase upon heating applied.¹³ In 1990 Puckrin et al.¹⁴ compared the oxidation of bulk bismuth and thin films using different spectroscopy techniques and showed that, in both cases, BiO is the first oxide formed on the surface of the samples with a total thickness of approximately 8 monolayers (ca. 2 nm). Further results showed that prolonged air oxidation times lead to further oxidation of the film and to the formation of Bi₂O₃, which ties back with the results described previously by Kowalczyk et al.¹⁵ Despite these reports, the oxidation behavior of Bi thin films has not been studied extensively.Self-assembled monolayers (SAMs) formed by chemisorption of alkyl thiols on the surface of various inorganic substrates have been extensively reported in the literature; in particular, their use as corrosion inhibitors has been explored on various semiconductors^16–19 and metals.^20,21 There are two examples of alkyl thiol adsorption on bulk Bi electrode surfaces; Adamovski et al.²² reported the characterization of alkyl thiol adsorption thickness via the diffusion blocking properties of the layer, whilst Romann et al.²³ characterized the formation of bismuth thiolate at the surface of the electrode in the absence of a self-assembled monolayer.This article reports the first example of SAM formation on Bi thin films via a wet chemical process (Scheme 1). The SAM surfaces have been characterised via XPS, IR and AFM and show that, in a similar fashion as for Cu surfaces,²¹ the alkyl thiol plays an active role in removing the native bismuth oxide from the thin film surface and leads to subsequent stabilization of Bi surfaces preventing complete re-oxidation. Whilst complete oxide removal was not achieved for the films, likely due to a non-continuous passivating layer, the research highlights the ability to remove oxide and passivate Bi surfaces outside of a vacuum environment.Open in a separate windowScheme 1Depiction of the procedure for the removal of bismuth oxide and its subsequent passivation with thiols.Bi films used in this study (supplied by the research centre, Jülich, in Germany) were grown by molecular beam epitaxy (MBE) on undoped Si [111] substrates. Prior to bismuth deposition, the Si substrates were chemically cleaned by the HF-last RCA²⁴ procedure to remove the native oxide and to passivate the surface with hydrogen. The substrates were subsequently heated in situ to 700 °C for 20 min to desorb the hydrogen atoms from the Si surface. The Bi material flux was generated by an effusion cell operated at a temperature of 550 °C, which yields a growth rate of 17 nm h⁻¹.⁸A round bottom flask was preheated using a heating mantle up to 180 °C and left at this temperature for 30 min. The Bi thin films were then placed in the flask under N₂, annealed for 1 h and left to cool to room temperature. Solutions of 1-dodecanethiol (Sigma-Aldrich, ≥98%) in isopropanol (IPA) were prepared, with concentrations ranging from 1 to 100 mM. The solutions were degassed using three freeze–pump–thaw cycles and transferred in the sample flask using a cannula, thus preventing any contact with the outer atmosphere.The Bi samples were left immersed in solution at room temperature overnight after which time the solution was vacuum pumped out of the flask using a cannula. The samples were then dried by gently heating the flask using a heat gun in order to evaporate the remaining solvent. Clean IPA was pumped into the sample flask to rinse any adsorbed molecules. IPA was then removed, and the thin film dried once again. The same reaction procedure was also used on Bi powder (Fisher Scientific, 100 mesh, 99.5% metal basis) which was used as received. The samples were then transferred to a N₂ glovebox from the flask to a gel box which was subsequently transferred to a N₂ filled sample preserver. The gel box was kept in the sample preserver until X-ray photoelectron spectroscopy (XPS) measurements were carried out. The exposure time during the loading procedure into the XPS entry chamber was less than 30 s. The XPS powder samples were prepared by depositing the reacted powder on some carbon tape. The powder was degassed overnight in the XPS entry chamber before being analyzed. The exposure time to air of these samples was less than 1 h. XPS data were collected using an Oxford Applied Research Escabase® XPS system. Survey scans were recorded between 0–1400 eV with a step size of 0.7 eV, dwell time of 0.5 s and pass energy of 100 eV. Core level scans were acquired with a step size of 0.1 eV, dwell time of 0.5 s and pass energy of 20 eV averaged over 20 scans. A non-monochromated Al-kα X-ray source operating at 100 W power was used for all scans. All spectra were acquired at a take-off angle of 90° with respect to the analyser. Energy calibration for all spectra was obtained using the C 1s line at 284.8 eV as reference data was processed using CasaXPS software where a Shirley background correction was employed. Relative sensitivity factors used for quantification purposes are from the CasaXPS software library. XPS thickness measurements were performed following the methodology originally defined by Cumpson et al.,²⁵ (see eqn (1)).1where I_o and I_s represent the respective measured peak intensities of the overlayer (alkyl thiol molecules) and thin film peaks, S_o and S_s refer to the relative sensitivity factors for the overlayer and the thin film respectively. λ_o and λ_s are the attenuation lengths of electrons in the overlayer and the thin film. θ is the emission angle with respect to the surface normal. To minimise the effect of potential errors arising from surface roughness and inelastic scattering, a photon emission angle of 35° was used in conjunction with a 90° take-off angle with respect to the sample normal. The peak intensity of the overlayer peak, I_o, and the peak intensity of the thin film peak, I_s, were determined using CasaXPS software after a transmission correction. The relative sensitivity factors for the thin film peak S_s and the overlayer peak S_o were also obtained from the CasaXPS library and manually inputted into the data processing software to remove instrumental factors which may affect quantification. The local electron attenuation length (EAL) in the overlayer, λ_o, was estimated, using the NIST Electron Effective Attenuation Length database,²⁶ to be 2.1 ± 0.2 nm for the Bi 4f component, and 1.7 ± 0.2 nm for the Bi 4d_5/2 component. The surface was thoroughly cleaned by prolonged sonication in anhydrous solvents prior to characterization to remove all physisorbed species prior to analysis and to minimise contributions from contaminants to the overlayer thickness. ¹H (300 MHz) and ¹³C (75.5 MHz) Nuclear Magnetic Resonance (NMR) spectra were recorded on a Bruker Advance 300 MHz NMR spectrometer. All spectra were recorded at 300 K in deuterated chloroform (CDCl₃) using tetramethylsilane (TMS) as an internal standard. Attenuated total reflectance infrared (ATR-FTIR) spectra were recorded using a Nicolet 6700 Infrared Spectrometer with a VariGATR and a liquid cooled MgCdTe detector using 3000 scans at a resolution of 2 cm⁻¹. Spectra were collected under p-polarisation in an ambient atmosphere. Fig. 1 shows XPS scans of the Bi 4f core level peak for the thin films before and after reaction with different concentrations of thiol solutions. At 156.5 eV and 161.8 eV the Bi metal 4f components are present²⁷ and at 158.6 eV and 163.4 eV Bi–X 4f components²⁸ are present, where X is a heteroatom, in this case O. Treatment of the Bi films with a 1 mM 1-dodecanethiol solution did not reduce the intensity of the Bi–X peak considerably, as also shown in Table S1.^† However, XPS analysis of Bi films treated with 10 and 100 mM thiol solutions displayed a high Bi metal content, when the Bi peaks at 156.5 eV and 161.8 eV are compared to those of the as received sample. The shoulder peaks at 158.6 eV and 163.4 eV, whilst usually associated with Bi₂O₃, in this case are likely to be composed of both Bi–O and by Bi–S components. This hypothesis is based on the analysis of the thickness of the thiol-coated Bi thin films by XPS following the method outlined in the experimental section. The thickness of the overlayer, calculated using the Bi–X shoulder as the overlayer peak and the Bi 4f_7/2 peak as the thin film peak, was approximately 0.95 ± 0.10 nm. The thickness of the oxide calculated using the O 1s Bi₂O₃ component as the overlayer peak and the Bi 4d_5/2 peak as the thin film peak was found to be 0.43 ± 0.05 nm. This discrepancy in thickness is likely due to S also bound to surface Bi atoms, thus leading to a Bi 4f shoulder peak that was more intense due to contributions from both components, i.e. Bi–S and Bi–O.Open in a separate windowFig. 1Overlaid XPS spectra of Bi 4f core level after reaction with 1-dodecanethiol solutions at different concentrations. Graphs have been normalised to the maximum of the as-received sample to underline the oxide reduction effect. Fig. 2 shows an ATR-FTIR scan of the Bi thin film surface before and after the functionalisation process. The solvent-cleaned as-received Bi thin films presents vibrations only due to adventitious carbon adsorbed on the surface. After functionalisation and air exposure, multiple vibrations were observed at 2993 and 2909 cm⁻¹, arising from C–H stretches and vibrations from the aliphatic carbon chains of the thiol molecules. In the fingerprint region between 1500–750 cm⁻¹ there are the two peaks associated with the asymmetric stretching of SO₂ at 1367 cm⁻¹ and 1174 cm⁻¹ and one peak due to symmetric stretching at 1083 cm⁻¹.²⁹ This data suggests that the outer facing thiols from the second adsorbed layer, due to the oxidative environment created by the Bi₂O₃/Bi pair,^30,31 are readily oxidised to their sulfonic acid equivalents. Surface bound thiols are known to progressively oxidise to sulfonic acids in the presence of strong oxidative environments.^32,33Open in a separate windowFig. 2IR spectra of Bi surface before and after functionalisation.Bi(iii) likely catalyses the formation of a disulphide bond between two 1-dodecanethiol molecules, similarly to what has already been observed for copper³⁴ with water as a by-product.As the sample/solution system is kept under N₂, the oxide layer cannot form again and is consumed.³⁰ Once the bare Bi surface is exposed, the thiol passivates the surface as the Bi–S bond is thermodynamically more favourable than the Bi–O bond.^35,36 To confirm this mechanism, further experiments were undertaken on Bi powder. The powder samples, which presented a bismuth oxide : bismuth ratio similar to that of the films (see Table S1^†), were annealed at 180 °C for 1 h, under similar conditions to the Bi-coated Si substrates. The annealed powder samples were then immersed in 1-dodecanethiol solutions made in IPA and left in air in order to promote re-oxidation. After approximately 3 h, the solutions changed colour from colourless to yellow, and a precipitate was formed which was collected and dissolved in CDCl₃ for NMR analysis (See Fig. S1^†). NMR data for the as-received 1-dodecanethiol was also collected and used for comparison to check for the presence of starting material at the end of the reaction. Fig. 3(a) shows an NMR spectrum for the as-received 1-dodecanethiol; a quartet resonating at 2.51 ppm (J = 7.33 Hz), arising from the α-methylene group of the thiol group was identified. The β-methylene and the thiol proton resonate at 1.60 ppm (quintet, J = 7.58 Hz) and 1.39 ppm (unresolved multiplet) respectively, the methyl of the C12 carbon is found at 0.87 ppm (triplet, J = 6.90 Hz) and the remaining protons on C3–C11 form an unresolved multiplet at 1.26 ppm.³⁷Fig. 3(b) shows an NMR spectrum of the reaction product formed upon reacting 1-dodecanethiol with Bi powder; two α-methylene groups from a disulphide moiety resonate as a triplet at 2.68 ppm (J = 7.35 Hz); two β-methylene groups resonate at 1.67 ppm (quintet, J = 7.31 Hz) and between 1.26 and 1.39 ppm there is an unresolved multiplet formed by the protons on the two alkyl chains. The remaining two methyl groups at the end of the chains resonate at 0.88 ppm (triplet, J = 6.70 Hz).³⁸ No starting material was observed in the final NMR spectra, as the characteristic α-methylene quartet was not apparent. The reaction carried out with the Bi powder in air can proceed indefinitely until all the starting material is oxidized; the powder is in fact allowed to re-oxidize, unlike under N₂, thus catalysing the reaction. No precipitate or change in colour of the solution was observed when thiol passivation was undertaken on Bi thin films left in air, probably due to the much smaller surface area of Bi on a compared to the powder form, limiting the oxidation of a noticeable quantity of thiols. This might also explain why at low thiol concentrations only a small reduction of oxide was observed, as most of the thiols in solution are probably oxidized while removing the outer oxide layer, leaving only few molecules available to passivate the Bi films. Using concentrations higher than 100 mM led to visible corrosion of the surface of the sample; therefore, the optimum level was found to be 10 mM as it gave the highest oxide reduction without affecting surface quality.Open in a separate windowFig. 3NMR spectra of (a) the as-received 1-dodecanethiol used for the reaction and (b) the dodecyldisulfide product found after reacting 1-dodecanethiol solution with bismuth powder.Functionalised Bi thin film samples were then left in air and re-measured using XPS to check the extent of re-oxidation over time. Fig. 4(a) shows a comparison of the Bi 4f core level peaks of a 100 mM functionalised sample after 1 h, 2 days and 10 days of air exposure. The increase in intensity of the shoulder peaks at 159.3 eV and 164.6 eV arises from oxidation of exposed areas of the thin film, due to the non-continuity of the thiol film formed, as shown by AFM (see Fig. S2^†). Fig. 4(b) shows the O 1s peak of thiol functionalised samples before and after air exposure. At 530.3 eV the Bi₂O₃ component increases in intensity after air exposure,³⁹ further proof that part of the surface undergoes re-oxidation. This increase is accompanied by a general decrease in intensity in the two components at 532.6 eV (ref. 40 and 41) and 534.7 eV;⁴² likely belonging to O–H/S–O and H₂O respectively. The hydroxyl arising from H-termination of the oxidised areas of the sample, the sulfoxide from likely oxidation of the outer facing thiol groups and the water from adsorption on the layer of the reaction by-product due to hydrophilicity of the outer facing thiol group.^43,44 However, even after 10 days in air, there was no noticeable changes compared to a 1 h air exposure. Based on this data, an interdigitated “bilayer” of thiols is likely to be present; it has already previously been shown that these “bilayer” structures can be easily formed both on nanoparticles^45,46 and on 2-D films⁴⁷ when using a polar solvent. Moreover, a previous study on silver colloidal nanoparticles⁴⁸ has shown that an excess of precursor molecules can lead to bilayer structures which in turn lead to an increase in the ambient stability. In a similar fashion, Cu functionalisation with excess thiols, leads to multilayer formation of self-assembled copper thiolate layers.³⁴ This hypothesis is further corroborated by the thickness measurements which has quantified a total carbon layer thickness of 4.3 ± 0.5 nm, which equates to about 2.7 1-dodecanethiol monolayers. This configuration prevents further surface oxidation, thus allowing the bismuth to be more stable in air. In contrast, an unpassivated bismuth metal surface obtained by reactive ion etching (RIE), completely re-oxidizes in air after a 2 min exposure with oxidation levels comparable to the as received sample. Bi powder samples were also functionalized under N₂ in a flask under similar conditions. The XPS analysis showed that the total content of oxide was reduced, although not to the same extent as for the thin film samples (see Fig. S3^†); this is because of the higher oxide surface area of the powder, which in turn requires a higher number of thiol molecules to for oxide removal. In addition, as it was not possible to disperse the powder during the reaction, the thiol solution did not completely cover all the oxide surface available for the reaction.Open in a separate windowFig. 4(a) XPS spectra of Bi 4f core level at different times after functionalisation. Graphs have been normalised to the maximum of the sample after reaction to outline the reoxidation process. (b) O 1s core level XPS spectra before and after reoxidation with main component peaks being highlighted.     

The structure and photoelectrochemical activity of Cr-doped PbS thin films grown by chemical bath deposition

Nanocrystalline undoped and Cr-doped PbS thin films were prepared on glass substrates by a simple chemical bath deposition method. The X-ray diffraction analyses of the films showed their polycrystalline nature with cubic structure and preferential growth along the (111) orientation. Cr incorporation decreases the average PbS crystallite size from 59.97 to 37.59 nm, whereas the strain and dislocation density showed an increasing trend. The atomic ratio of doping for Cr is about 0.63, 1.75, and 4.70% according to energy-dispersive X-ray (EDX) spectroscopy. Morphological analysis showed that the average sizes of nanoclusters decreased from 73 to 41 nm as the Cr concentration increased. The optical band gap values are increased with increasing Cr doping. The photoelectrochemical (PEC) behaviors and the stability of the Cr doped PbS photoelectrodes were studied in 0.3 M Na₂SO₃ electrolyte solution. Also, the incident photon-to-current efficiency and applied bias photon-to-current efficiency are calculated and showed optimized values of 13.5% and 1.44% at 0.68 V and 390 nm. Moreover, the optimized electrode shows high chemical stability and a long lifetime. Finally, the effect of temperature on the PEC behaviors is evaluated and the different thermodynamic parameters are calculated.

Nanocrystalline undoped and Cr-doped PbS thin films were prepared on glass substrates by a simple chemical bath deposition method as photoelectrodes for solar water splitting.     

A carbon membrane-mediated CdSe and TiO2 nanofiber film for enhanced photoelectrochemical degradation of methylene blue

In this study, a carbon membrane-mediated CdSe and TiO₂ ternary film (CdSe/C/TiO₂) was prepared to degrade methylene blue (MB). Carbon membrane and CdSe were introduced to the surface of a TiO₂ nanofiber film via an in situ hydrothermal deposition process successively. The investigation shows that the carbon membrane not only provides a charge transfer channel to promote the transfer of electron from the conduction band of CdSe to that of TiO₂, but also improves the poor conduct between the TiO₂ film and electrolyte. The synergies between the carbon membrane and CdSe can make the ternary system harvest more visible light energy and facilitate the charge transfer property of TiO₂. The current density of CdSe/C/TiO₂ was about 9 folds higher compared with that of pure TiO₂ under UV and visible light irradiations. This ternary hybrid exhibits a superior activity during the photoelectrochemical (PEC) degradation of MB, and 92.43% can be removed after 120 min irradiation, which is improved by 21.14% than that of TiO₂.

A CdSe/C/TiO₂ nanofiber film was prepared for enhanced photoelectrochemical degradation ability, and carbon membrane as a carrier-transfer-channel could promote electron transfer.     

Effect of meals on resorption of bismuth from an oral bismuth gallate/bismuth nitrate preparation

In a cross-over study in ten healthy volunteers the effect of food on the absorption of bismuth following a single oral dose of 1200 mg (= 12 Bismofalk tablets) was evaluated by measuring its serum levels (0 to 24 h) and urinary excretion (for seven days). If this high dose was ingested one hour prior to the breakfast maximal serum concentrations (16.5 +/- 13.7 micrograms/l; mean +/- SD) were rapidly achieved (tmax = 0.7 +/- 0.5 h). These levels and the low urinary recovery of 0.32 +/- 0.25 mg (corresponding to 0.027% of the dose) indicated a minimal absorption. If the tablets were taken one hour after the breakfast absorption was slightly delayed (tmax = 1.9 +/- 2.4 h), however, the extent appeared to be unchanged. In spite of the high dosage as well as the large interindividual variability in the urinary recovery and the serum concentrations potential "toxic" serum levels of 100 micrograms/l were never reached. Thus, it can be concluded that the bismuth preparation used is suitable for topical action.     

Improving light absorption and photoelectrochemical performance of thin-film photoelectrode with a reflective substrate

The charge separation/transport efficiency is relatively high in thin-film hematite photoanodes in which the distance for charge transport is short, but simultaneously the high loss of light absorption due to transmission is confronted. To increase light absorption in thin-film Fe₂O₃:Ti, commercial substrates such as Cu foil, Ag foil, and a mirror are adopted acting as back-reflectors and individually integrated with the Fe₂O₃:Ti electrode. The promotion effect of the commercial back-reflectors on the light absorption efficiency and photoelectrochemical (PEC) performance of the hydrothermally prepared Fe₂O₃:Ti electrodes with a variety of film thicknesses is investigated. As a result, Ag foil and the mirror show favorable and equal efficacy while the promoting effect of Cu foil is limited. In addition, the photocurrent increment achieved by the Ag back-reflector decreases linearly along with the logarithmic of the film thickness and the optimized film thickness of the Fe₂O₃:Ti electrode is decreased from 520 to 290 nm. The high durability of Ag foil in the alkaline electrolyte during solar light irradiation is demonstrated. Furthermore, the reflective substrate also shows a promotion effect on the BiVO₄ photoanode and CuBi₂O₄ photocathode, as well as the unbiased photocurrent from a tandem cell constituted by TiO₂ and CuBi₂O₄.

The charge separation/transport efficiency is relatively high in thin-film hematite photoanodes in which the distance for charge transport is short, but simultaneously the high loss of light absorption due to transmission is confronted.     

Flexible electric-double-layer thin film transistors based on a vertical InGaZnO4 channel

Flexible electric-double-layer (EDL) thin film transistors (TFTs) based on a vertical InGaZnO₄ (IGZO) channel are fabricated at room temperature. Such TFTs show a low operation voltage of 1.0 V due to the large specific gate capacitance of 3.8 μF cm⁻² related to electric-double-layer formation. The threshold voltage, drain current on/off ratio and subthreshold swing are estimated to be −0.1 V, 1.2 × 10⁶ and 80 mV per decade, respectively. The combination of low voltage, high current on-to-off ratio and room temperature processing make the flexible vertical-IGZO-channel TFTs very promising for low-power portable flexible electronics applications.

Flexible electric-double-layer (EDL) thin film transistors (TFTs) based on a vertical InGaZnO₄ (IGZO) channel are fabricated at room temperature.     

Highly transparent copper iodide thin film thermoelectric generator on a flexible substrate

Simultaneously transparent and flexible conductive materials are in demand to follow the current trend in flexible technology. The search for materials with compliant optoelectronic properties, while simultaneously retaining their electric conductivity at high strain deformation, comprises promising opportunities in modern nanotechnology. Copper iodide (CuI) is not only the most transparent and highly conductive p-type material, but its optimization has contributed to improved ZT values in planar thin-film thermoelectrics. In this work, the readiness of CuI thin films to transparent, flexible technology is evidenced. A maximum ZT value of 0.29 for single CuI thin films of ca. 300 nm in thickness is reported. Values of open-circuit voltage V_oc, short circuit current I_sc and power output of p–n thermoelectric modules of Gallium-doped zinc oxide (GZO) and CuI thin films deposited on a transparent flexible Kapton® (type CS) substrate are reported, and a prototype of a flexible transparent thermoelectric generator based on 17 p–n modules was constructed. Bending analysis of CuI thin films reveals interesting, distinct results when submitted to compression and tension analysis – a behaviour not seen in conventional semiconducting thin films under equivalent strain conditions. A plausible account for such diversity is also included.

Simultaneously transparent and flexible conductive materials are in demand to follow the current trend in flexible technology. A highly transparent and flexible thermoelectric generator of 17 p–n modules was constructed based on copper iodide thin films.