Emission properties of sequentially deposited ultrathin CH3NH3PbI3/MoS2 heterostructures

Hybrid organic-inorganic perovskite materials have obtained considerable attention due to their exotic optoelectronic properties and extraordinarily high performance in photovoltaic devices. Herein, we successively converted the ultrathin PbI₂/MoS₂ into the CH₃NH₃PbI₃/MoS₂ heterostructures via CH₃NH₃I vapor processing. Atomic force microscopy (AFM)、Scanning electron microscopy (SEM) and X-ray photoemission spectroscopy (XPS) measurements prove the high-quality of the converted CH₃NH₃PbI₃/MoS₂. Both MoS₂ and CH₃NH₃PbI₃ related photoluminescence (PL) intensity quenching in CH₃NH₃PbI₃/MoS₂ implies a Type-II energy level alignment at the interface. Temperature-dependent PL measurements show that the emission peak position shifting trend of CH₃NH₃PbI₃ is opposite to that of MoS₂ (traditional semiconductors) due to the thermal expansion and electron-phonon coupling effects. The CH₃NH₃PbI₃/TMDC heterostructures are useful in fabricating innovative devices for wider optoelectronic applications.     

Hole blocking PbI2/CH3NH3PbI3 interface

Modulated charge separation across (MO)/CH₃NH₃PbI₃ and (MO)/PbI₂/CH₃NH₃PbI₃ (MO = TiO₂, MoO₃) interfaces was investigated by surface photovoltage (SPV) spectroscopy. Perovskite layers were deposited by solution‐based one‐step preparation and two‐step preparation methods. An unreacted PbI₂ layer remained at the interface between the metal oxide and CH₃NH₃PbI₃ for two‐step preparation. For the two‐step preparation on TiO₂, the SPV signal related to absorption in CH₃NH₃PbI₃ increased in comparison to the one‐step preparation due to electron transfer from CH₃NH₃PbI₃ via PbI₂ into TiO₂ whereas the SPV signal related to defect transitions decreased. For the one‐step preparation on MoO₃, holes photogenerated in CH₃NH₃PbI₃ recombined with electrons in MoO₃. In contrast, a hole transfer from CH₃NH₃PbI₃ towards MoO₃ was blocked by the PbI₂ interlayer for the two‐step preparation on MoO₃. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Density functional theory study of the adsorption of methanthiol on Au(1 1 1): Role of gold adatoms

By performing density functional theory calculations, this work clarifies the sites and energetics of both the non-dissociative and dissociated adsorptions of CH₃SH on clean Au(1 1 1) and Au(1 1 1) with intrinsic defects. It was found that the adsorption on defect-free Au(1 1 1) is most stable for non-dissociative CH₃SH. Its direct molecular dissociation to form CH₃S/Au and H/Au is barred by an activation barrier of 0.9 eV. However, the presence of neighboring Au_ad can assist the dissociation reaction to form CH₃S–Au_ad–H by lowering the energy barrier to 0.6 eV. As for the dissociated CH₃S, the surface geometry of two CH₃S joined by a Au_ad is the most favorable one.     

A facile hydrothermal route to synthesize novel PbI2 nanorods

We report the synthesis of PbI₂ nanorods by reacting lead acetate with elemental iodine at room temperature by virtue of a wet chemical method at mild reactions. The diameters of the obtained PbI₂ nanorods are about 54 nm. The present technique may open a new doorway to one-dimensional nanosized rods from the same kind of materials with irregular shape or large size. X-ray powder diffraction (XRD), Fourier transform infrared analysis (FTIR), ultraviolet–visible–near infrared analysis (UV–vis–NIR), photoluminescence measurements (PL) and scanning electron microscopy (SEM) were used to characterize PbI₂ nanorods. The as obtained products were analyzed by X-ray powder diffraction, which confirms the formation of solid PbI₂ nanorods. From the UV–vis–NIR studies the band gap of PbI₂ nanorod was estimated. The various functional groups present in the PbI₂ nanorods were identified by FTIR analysis. Intense photoluminescence was also observed with some spectral tuning possibly giving a range of emission photon energies approximately spanning from 2.1 to 3.5 eV.     

Effect of thickness on the structural,optical and electrical properties of thermally evaporated PbI2 thin films

This work describes the physical properties of lead iodide (PbI₂) thin films with different thicknesses that were deposited on ultrasonically cleaned glass substrates using a thermal evaporation technique at 5×10^-6 torr. The initial material was purified by the zone refining technique under an atmosphere of argon gas. XRD analysis of the material demonstrates that the thin films were preferably oriented along the (001) direction. The size of the crystallites was calculated from the Scherer relation and found to be in the range of ～5–10 nm, with higher values being observed for increasing film thicknesses. The optical energy band gaps were evaluated and determined to belong to direct transitions. Because the band gap increased with decreasing film thickness, a systematic blue shift was observed. The surface morphologies of PbI₂ films exhibited a clear increase in grain size with increasing film thickness. The photoluminescence and dc conductivity of the thin films are also discussed.     

Ionic conductivity of polymer electrolyte membranes based on polyphosphazene with oligo(propylene oxide) side chains

A polyphosphazene [NP(NHR)₂]_n with oligo[propylene oxide] side chains − R = –[CH(CH₃)–CH₂O]_m–CH₃ (m = 6 – 10) was synthesized by living cationic polymerisation and polymer-analogue substitution of chlorine from the intermediate precursor [NPCl₂]_n using the corresponding primary amine RNH₂. The polymer had an average molecular weight of 3.3 × 10⁵ D. Polymer electrolytes with different concentrations of dissolved lithium triflate (LiCF₃SO₃) were prepared. Mechanically stable polymer electrolyte membranes were formed using UV radiation induced crosslinking of the polymer salt mixture in the presence of benzophenone as photoinitiator. The glass transition temperature of the parent polymer was found to be − 75 °C before cross linking. It increases after crosslinking and with increasing amounts of salt to a maximum of − 55 °C for 20 wt.% LiCF₃SO₃. The ionic conductivity was determined by impedance spectroscopy in the temperature range 0–80 °C. The highest conductivity was found for a salt concentration of 20 wt.% LiCF₃SO₃: 6.5 × 10^− 6 S·cm^− 1 at 20 °C and 2.8 × 10^− 4 S cm^− 1 at 80 °C. The temperature dependence of the conductivities was well described by the MIGRATION concept.     

Photochemistry of methyl bromide on the α-Cr2O3(0001) surface

The photochemical properties of the Cr-terminated α-Cr₂O₃(0001) surface were explored using methyl bromide (CH₃Br) as a probe molecule. CH₃Br adsorbed and desorbed molecularly from the Cr-terminated α-Cr₂O₃(0001) surface without detectable thermal decomposition. Temperature programmed desorption (TPD) revealed a CH₃Br desorption state at 240 K for coverages up to 0.5 ML, followed by more weakly bound molecules desorbing at 175 K for coverages up to 1 ML. Multilayer exposures led to desorption at ~ 130 K. The CH₃Br sticking coefficient was unity at 105 K for coverages up to monolayer saturation, but decreased as the multilayer formed. In contrast, pre-oxidation of the surface (using an oxygen plasma source) led to capping of surface Cr³⁺ sites and near complete removal of CH₃Br TPD states above 150 K. The photochemistry of chemisorbed CH₃Br was explored on the Cr-terminated surface using post-irradiation TPD and photon stimulated desorption (PSD). Irradiation of adsorbed CH₃Br with broad band light from a Hg arc lamp resulted in both photodesorption and photodecomposition of the parent molecule at a combined cross section of ~ 10^? 22 cm². Photodissociation of the CH₃–Br bond was evidenced by both CH₃ detected in PSD and Br atoms left on the surface. Use of a 385 nm cut-off filter effectively shut down the photodissociation pathway but not the parent molecule photodesorption process. From these observations it is inferred that d-to-d transitions in α-Cr₂O₃, occurring at photon energies < 3 eV, do not significantly promote photodecomposition of adsorbed CH₃Br. It is unclear to what extent band-to-band versus direct CH₃Br photolysis play in CH₃–Br bond dissociation initiated by more energetic photons.     

Activation of methyl acetate on Pd(111)

The adsorption and activation of methyl acetate (CH₃COOCH₃), one of the simplest carboxylic esters, on Pd(111) have been studied using self-consistent periodic density functional theory calculations. Methyl acetate adsorbs weakly through the carbonyl oxygen. Its activation occurs via dehydrogenation, instead of direct C–O bond dissociation, on clean Pd(111): It is much more difficult to dissociate the C–O bonds (E_a ≈ 2.0 eV for the carbonyl and acetate–methyl bonds; E_a = 1.0 eV for the acetyl–methoxy bond) than to dissociate the C–H bonds to produce enolate (CH₂COOCH₃; E_a = 0.74 eV) or methylene acetate (CH₃COOCH₂; E_a = 0.82 eV). The barriers for C–H and C–O bond dissociation are directly calculated for enolate and methylene acetate, and estimated for further dehydrogenated derivatives (CH₃COOCH, CH₂COOCH₂, and CHCOOCH₃) based on the Brønsted–Evans–Polanyi linear energy relations formed by the calculated steps. The enolate pathway leads to successive dehydrogenation to CCOOCH₃, whereas methylene acetate readily dissociates to yield acetyl. The selectivity for dissociating the acyl–alkoxy C–O bond, which is desired for alcohol formation, is therefore fundamentally limited by the facility of dehydrogenation under vacuum/low-pressure conditions on Pd(111).     

The absorption spectrum of methane at 80 and 294 K in the icosad (6717–7589 cm−1): Improved empirical line lists,isotopologue identification and temperature dependence

Using a cryogenic cell and a series of Distributed Feed Back (DFB) diode lasers, new high resolution spectra of methane have been recorded at 80 K and room temperature by differential absorption spectroscopy (DAS) between 6717 and 7589 cm^?1 (1.49–1.32 μm). The investigated spectral region corresponds to the very congested icosad, which is not tractable by theory. Empirical lists of 19,940 and 24,001 lines were constructed from the 80 K and room temperature spectrum, respectively. The room temperature list adds about 8500 features to the empirical list of 15,375 lines at 296 K adopted in the HITRAN database from the original work of L. Brown (Brown, L. Empirical line parameters of methane from 1.1 to 2.1 μm. JQSRT 2005;96:251–70). A number of relatively strong CH₄ lines located near strong water lines were found missing in the HITRAN line list. The improved sensitivity allowed adding more than 7000 lines to our previous list of about 12,000 transitions at 80 K (Campargue A, Wang L, Kassi S, Ma?át M, Votava O. Temperature dependence of the absorption spectrum of CH₄ by high resolution spectroscopy at 81 K: (II) The Icosad region (1.49–1.30 μm). JQSRT 2010;111:1141–51). In order to facilitate identification of the transitions of the different methane isotopologues present in “natural” isotopic abundance, spectra of highly enriched CH₃D and ¹³CH₄ samples were recorded with the same experimental setup, both at room temperature and at 80 K.From the variation of the line strengths between 80 K and 294 K, the low energy values of about 12,000 transitions were determined. They allow accounting for the temperature dependence of 84 and 93% of the methane absorbance in the region, at room temperature and 80 K, respectively. As a result, we provide as supplementary material two complete line lists at 80 K and 294 K, including CH₃D and ¹³CH₄ identification and lower state energy values.     

Adsorption and reaction of glycine on the rutile TiO2(011) single crystal surface

The adsorption and reaction of glycine on the surface of a rutile TiO₂(011) single crystal has been studied by X-ray Photoelectron Spectroscopy (XPS) and Temperature Programmed Desorption (TPD) techniques. Special attention was given to the formation and stability of the zwitterion structure (⁺NH₃–CH₂–COO^?) in comparison to that of the dissociated structure (NH₂–CH₂–COO^?). Both species have been observed on the surface at 300 K. The zwitterion structure was found less stable than the dissociated structure. This is in line with other experimental results related to proline on rutile TiO₂(110) single crystal [13, 14], glycine on rutile TiO₂(110) single crystal [17, 24] and computational results related to glycine on rutile TiO₂(110) single crystal [25]. By 500 K most of the zwitterion structure has been converted to the dissociated one. TPD results indicated that glycine reacts in a similar way to carboxylic acids on this surface with the main decomposition products being ketene (CH₂=C=O). Other masses left unassigned for were also observed during TPD. The most intense being m/e 55 that might be due to =CH–C(O)N=or C(O)N=CH fragments.     

1H NMR study of proton dynamics in (NH4)3H(SO4)2

Proton dynamics in (NH₄)₃H(SO₄)₂ has been studied by means of ¹H solid-state NMR. The ¹H magic-angle-spinning (MAS) NMR spectra were traced at room temperature (RT) and at Larmor frequency of 400.13 MHz. ¹H static NMR spectra were measured at 200.13 MHz in the range of 135–490 K. ¹H spin-lattice relaxation times, T₁, were measured at 200.13 and 19.65 MHz in the ranges of 135–490 and 153–456 K, respectively. The ¹H chemical shift for the acidic proton (14.7 ppm) indicates strong hydrogen bonds. In phase III, NH₄⁺ reorientation takes place; one type of NH₄⁺ ions reorients with an activation energy (E_a) of 14 kJ mol^− 1 and the inverse of a frequency factor (τ₀) of 0.85 × 10^− 14 s. In phase II, a very fast local and anisotropic motion of the acidic protons takes place. NH₄⁺ ions start to diffuse translationally, and no proton exchange is observed between NH₄⁺ ions and the acidic protons. In phase I, both NH₄⁺ ions and the acidic protons diffuse translationally. The acidic protons diffuse with parameters of E_a = 27 kJ mol^− 1 and τ₀ = 4.2 × 10^− 13 s. The translational diffusion of the acidic protons is responsible for the macroscopic proton conductivity, as the NH₄⁺ translational diffusion is slow and proton exchange between NH₄⁺ ions and the acidic protons is negligible.     

Infrared spectroscopic and conductivity studies of poly(N-methylpropylenimine)/lithium triflate electrolytes

Poly(propylenimine), PPI, was methylated using Eschweiler-Clark conditions to produce poly(N-methylpropylenimine), PMPI. Differences may be seen in the IR spectra of the PMPI (–CH₂CH₂CH₂NCH₃–) and its homolog poly(N-methylethylenimine), PMEI, (–CH₂CH₂NCH₃–), especially in the conformation region (~ 1100 to ~ 1400 cm^− 1). The addition of lithium trifluoromethanesulfonate, (LiCF₃SO₃), sharpens the distinctions between these systems. Comparison of IR spectra of polymer:LiCF₃SO₃ complexes at compositions ranging from pure polymer to 5:1 N:Li⁺ (molar ratio) suggests significant differences in the nature of polymer salt interactions and the complex structure present in each system. These are further evidenced by differential scanning calorimetry data in which PMPI displays less variation in glass transition temperature, T_g, with the addition of salt than seen in PMEI. These observations may be interpreted in terms of local structural changes originating in cation–anion and cation–polymer interactions, particularly at mid to high salt concentrations.     

Atomic and molecular adsorption on Pd(111)

The adsorption properties of a variety of atomic species (H, O, N, S, and C), molecular species (N₂, HCN, CO, NO, and NH₃) and molecular fragments (CN, NH₂, NH, CH₃, CH₂, CH, HNO, NOH, and OH) are calculated on the (111) facet of palladium using periodic self-consistent density functional theory (DFT–GGA) calculations at ¼ ML coverage. For each species, we determine the optimal binding geometry and corresponding binding energy. The vibrational frequencies of these adsorbed species are calculated and are found to be in good agreement with experimental values that have been reported in literature. From the binding energies, we calculate potential energy surfaces for the decomposition of NO, CO, N₂, NH₃, and CH₄ on Pd(111), showing that only the decomposition of NO is thermochemically preferred to its molecular desorption.     

Ultrasonically sprayed-on perovskite solar cells-effects of organic cation on defect formation of CH3NH3PbI3 films

Methylammonium lead iodide (CH₃NH₃PbI₃) based perovskite having low degrees of the disorder is of great interest for optoelectronic and photovoltaic applications. In this work, a layer of CH₃NH₃PbI₃ was successfully prepared using an ultrasonically sprayed-nebulous method. Changes in structural and optical properties alongside with photo-induced charge separation and transportation behavior were systematically studied. The surface photovoltage spectra reveal a significant reduction of the density of deep defect states as the organic content was increased. It was observed that the measured values of Urbach energies decrease from 33.36 to 28.24 meV as the amount of organic content was increased to an optimum value. The best perovskite solar cells obtained using the sprayed-on approach exhibited a J_sc of 16.54 mA/cm², a V_oc of 0.99 V, and a FF of 62.4, resulting in an overall PCE of 10.09%.     

Colossal magnetoresistance effect in epitaxially grown La2/3Ca1/3MnO3 perovskite-like manganite thin films

We report in this work, study on colossal magnetoresistance (CMR) effect in epitaxial La_2/3Ca_1/3 MnO₃ thin films grown on SrTiO₃ (0 0 1) substrates by pulsed laser deposition (PLD) technique. The films were grown on as-received SrTiO₃ substrates and on SrTiO₃ substrates prepared by HF etching (Koster et al., Appl. Phys. Lett. 73 (1998) 2920; V. Leca et al., Wet etching methods for perovskite substrates, University of Twente, MESA+ Research Institute, Low Temperature Division). Two of the samples were annealed in different conditions to investigate the films heat treatment effect on electric and magnetic properties. Electrical resistance was done using the four-probe method at temperatures in the range of 2–375 K without a magnetic field and in an external field of 5 T applied in the film plane. Resistance-magnetic field (R vs. H) at 77 K for the two annealed samples was done in a 5 T sweep magnetic field. The surface morphology and structural information of the films were obtained using atomic force microscopy (AFM) and X-ray diffraction (XRD), respectively. Secondary ion mass spectroscopy (SIMS) analysis was performed on the annealed samples to investigate any possible chemical reaction between La_2/3Ca_1/3MnO₃ thin films and SrTiO₃ substrate.     

Polymer electrolytes based on cross-linked cyclotriphosphazenes

Substituted cyclotriphosphazenes were used to prepare lithium ion conducting polymer networks. Two types of compounds were synthesized starting with the precursor hexachlorocyclotriphosphazene (HCCP): Type I (CVEEP) in which all the chlorine atoms in HCCP were replaced by vinyloxyethoxyethoxy groups (VEE = –OCH₂CH₂OCH₂CH₂OCHCH₂), and type II (CVMEEP) in which half of the chlorine was replaced by VEE and the other half by methoxyethoxyethoxy groups (MEE = –OCH₂CH₂OCH₂CH₂OCH₃). The terminal vinyl groups were used to build up a network by a thermally initiated cross-linking of lithium salt containing membranes. Polymer electrolytes with dissolved LiSO₃CF₃ and LiN(SO₂CF₃)₂ were investigated by impedance measurements. The ionic conductivity of CVMEEP with 10 wt.% LiSO₃CF₃ was 3.2 × 10^− 5 S/cm at 30 °C and 4.1 × 10^− 4 S/cm at 90 °C. Lower conductivity values in the range 10^− 8–10^− 9 S/cm were obtained at 30 °C for the highly crosslinked CVEEP. An interesting polymer electrolyte with good mechanical properties and a good conductivity of 1.3 × 10^− 5 S/cm (30 °C) was obtained from a solution of MEEP (= poly[bis(methoxy–ethoxy–ethoxy)phosphazene]) and LiSO₃CF₃ in CVEEP as an interpenetrating network.     

Characterization of luminescent chromophore obtained from silica spheres

Blue light emitting chromophores have been separated from silica spheres by soaking them into acetone for 120 days. The luminescent chromophores were not obtained from other solvents, including ether, methanol, ethanol, 2-propanol, chloroform and tetrahydrofuran. According to the Fourier transform infrared spectrum, the luminescent material is composed of C–OH, –CH₂, –CH₃, C=O, and Si–O–Si. UV–visible absorption peak of the chromophore is at 5.17 eV (240 nm). Field emission scanning electron microscope images show small cracks on the surface of aged spheres. The luminescence peak was at 2.81 eV (441 nm) for excitation energy between 3.88 and 3.35 eV and slightly shifted toward lower energy for excitation energy lower than 3.35 eV. The deconvoluted luminescent spectrum shows two emission bands at 3.08 and 2.74 eV, which are well-matched the oxygen deficient center model. Compared to the absorption peak (5.17 eV) and the emission peak (2.81 eV), large Stokes shift (2.36 eV) is observed.     

Intercalation of [Pt(NH3)4]2+ ions into layered sodium silicate magadiite: a useful method to enhance their stabilisation in a highly dispersed state

Al-free layered sodium silicate magadiite has been used as the host material for the stabilisation of [Pt(NH₃)₄]²⁺ ions via intercalation and/or ion-exchange reactions. The stabilisation of Pt(NH₃)₄]²⁺ ions in between the layers of Na-magadiite was confirmed by thermogravimetric analysis (TGA), where increased decomposition temperatures were observed for the intercalated materials. The intercalation behaviour of Na-magadiite was evident from the significant uptake of Pt ions (22.2 wt%). When silica gel was used as the host matrix, negligible uptake of Pt ions (1.3 wt%) was noticed. The X-ray diffraction (XRD) measurements revealed no appreciable change in the basal spacing of the intercalated materials. Nevertheless, the decrease in the intensity of the 001 peak with increasing Pt loadings (from 13.0 to 22.2 wt%) substantiated the intercalation of [Pt(NH₃)₄]²⁺ ions within the interlayer spaces of Na-magadiite. The transmission electron microscopy (TEM) studies of the intercalated materials revealed that [Pt(NH₃)₄]²⁺ ions were homogeneously intercalated in the magadiite matrix, ranging from 2 to 3 nm. Subsequent calcination of the intercalated materials at 600 °C in air led to the formation of Pt nanoparticles supported on silica. The results of XRD and TEM indicated that Pt nanoparticles were highly dispersed on the silica support and were in the range of 5–12 nm. Moreover, chemical analyses confirmed the high loading of Pt on silica in agreement with the TGA results.     

Highly sensitive and selective trimethylamine sensors based on WO3 nanorods decorated with Au nanoparticles

One-dimensional tungsten oxide (WO₃) gas sensing materials have been widely used for the detection of trimethylamine (TMA) gas. Furthermore, it is believed that an effective method to improve the gas sensing performance is to introduce noble metals into sensing materials. In this work, a novel gas sensing material was prepared by decorating Au nanoparticles on WO₃ nanorods. Based on field emission scanning electron microscopy (FESEM/EDS), X-ray diffraction (XRD), and transmission electron microscopy (TEM), the morphology and microstructure of as-prepared samples were characterized. Results show that Au nanoparticles with diameter of 13–15 nm are loaded on the surface of WO₃ nanorods with length of about 1–2 µm and width of 50–80 nm. Gas sensing tests reveal that the Au@WO₃ sensor has remarkably enhanced response to TMA gas compared with pure WO₃ nanorods. In addition, and the gas sensing mechanism has been investigated based on the experimental results. The superior sensing features indicate the present Au@WO₃ nanocomposites are promising for gas sensors, which can be used in the detection of the trimethylamine gas and this work provides insights and strategies for the fabrication of sensing materials.     

High temperature protonic conduction in Sr-doped LaP3O9

Electrical conduction of Sr-doped LaP₃O₉ ([Sr]/{[La] + [Sr]} = 2–10 mol%) was investigated under 0.4–5 kPa of p(H₂O) and 0.01–100 kPa of p(O₂) or 0.3–3 kPa of p(H₂) at 573–973 K. Sr-doped LaP₃O₉ showed apparent H/D isotope effect on conductivity regardless of the Sr-doping level under both H₂O/O₂ oxidizing and H₂/H₂O reducing conditions at investigated temperatures. Conductivities of the material were almost independent of p(O₂) and p(H₂O). These results demonstrated that the Sr-doped LaP₃O₉ exhibited protonic conduction under wide ranges of p(O₂), p(H₂O) and temperature. The conductivity of the Sr-doped LaP₃O₉ increased with increasing Sr concentration up to its solubility limit, ca. 3 mol%, while the further Sr-doping slightly degraded the conductivity. These indicate that Sr²⁺ substitution for La³⁺ leads to proton dissolution into the material and induced protonic conduction. Conductivities of the 3 mol% Sr-doped sample were 2 × 10^- 6–5 × 10^− 4 S cm^− 1 at 573–973 K.