Modification of electronic properties during adsorption of conjugate organic molecules on the surface of polycrystalline SnO<Subscript>2</Subscript>

The modification of the electronic structure during adsorption of ultrathin copper phthalocyanine (CuPc) and 3, 4, 9, 10 perylene-tetracarboxylic-dianhydride (PTCDA) coatings on the surface of polycrystalline tin dioxide is traced. Auger electron spectroscopy is employed to find changes in the atomic composition of the surface. It is found with the help of low-energy electron total current spectroscopy using a testing beam of electrons with energies up to 30 eV that the total current spectra typical of organic films are formed when the thickness of the coating being deposited is 2–7 nm. The formation of an interface layer 1.5–2.0 nm in thickness is detected, in which the intensity of the structure of the total current spectra decreases and the effect of interaction of PTCDA molecules with the SnO₂ surface is manifested.     

Ca/Alq3 hybrid cathode buffer layer for the optimization of organic solar cells based on a planar heterojunction

Use of efficient anode cathode buffer layer (CBL) is crucial to improve the efficiency of organic photovoltaic cells. Here we show that using a double CBL, Ca/Alq₃, allows improving significantly cell performances. The insertion of Ca layer facilitates electron harvesting and blocks hole collection, leading to improved charge selectivity and reduced leakage current, whereas Alq₃ blocks excitons. After optimisation of this Ca/Alq₃ CBL using CuPc as electron donor, it is shown that it is also efficient when SubPc is substituted to CuPc in the cells. In that case we show that the morphology of the SubPc layer, and therefore the efficiency of the cells, strongly depends on the deposition rate of the SubPc film. It is necessary to deposit slowly (0.02 nm/s) the SubPc films because at higher deposition rate (0.06 nm/s) the films are porous, which induces leakage currents and deterioration of the cell performances. The SubPc layers whose formations are kinetically driven at low deposition rates are more uniform, whereas those deposited faster exhibit high densities of pinholes.     

Chemical bath deposition of SnO2 and Cd2SnO4 thin films

A new approach of chemical bath deposition (CBD) of SnO₂ thin films is reported. Films with a 0.2 μm thickness are obtained using the multi-dip deposition approach with a deposition time as little as 8–10 min for each dip. The possibility of fabricating a transparent conducting oxide layer of Cd₂SnO₄ thin films using CBD is investigated through successive layer deposition of CBD-SnO₂ and CBD-CdO films, followed by annealing at different temperatures. High quality films with transmittance exceeding 80% in the visible region are obtained. Annealed CBD-SnO₂ films are orthorhombic, highly stoichiometric, strongly adhesive, and transparent with an optical band gap of ～4.42 eV. Cd₂SnO₄ films with a band gap as high as 3.08 eV; a carrier density as high as 1.7 × 10²⁰ cm^?3; and a resistivity as low as 1.01 × 10^?2 Ω cm are achieved.     

Surface stoichiometry of La0.7Sr0.3MnO3 during in vacuo preparation; A synchrotron photoemission study

We present a study of the surface stoichiometry and contamination of La_0.7Sr_0.3MnO₃ thin films following exposure to air and subsequent in vacuo preparation. Samples were studied using both soft X-ray synchrotron photoemission (hν = 150 to 350 eV) and traditional Mg-Kα XPS (hν = 1253.6 eV) whilst annealing incrementally to ≈ 510°C in low pressures of O₂. In all cases, a Mn depleted and Sr rich surface oxide layer is observed, it is of reduced crystalline quality and is charge depleted. This surface layer is weakly affected by subsequent annealing, and is partially reversed by annealing in higher O₂ pressure. Surface carbon contamination is incrementally removed by annealing at increased temperatures, and at 270 °C, it is reduced to ≈ 0.4% of the topmost unit cell. The modification of the surface stoichiometry and electronic properties is consistent with the reported loss of magnetic properties in thin LSMO films.     

Modification of the electronic properties of the TiO2 (1 1 0) surface upon deposition of the ultrathin conjugated organic layers

A few nm thick 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA) and Cu-phthalocyanine (CuPc) overlayers were thermally deposited in situ in UHV onto TiO₂ (1 1 0) surface. Atomic composition of the surfaces under study was monitored using Auger electron spectroscopy (AES). The formation of the interfacial potential barrier and the structure of the unoccupied electronic states located 5-25 eV above the Fermi level (E_F) was monitored using a probing beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. The work function values upon the overlayer deposition changed from 4.6 to 4.9 eV at the PTCDA/TiO₂ (1 1 0) interface and from 4.6 to 4.3 eV at the CuPc/TiO₂ (1 1 0) interface. Band bending in the TiO₂ substrate, molecular polarization in the organic film and changes in the work function due to the change in the surface composition were found to contribute to the formation of the interfacial potential barriers. Oxygen admixture related peaks were observed in the AES and in the TCS spectra of the CuPc overlayers. A mechanism of the transformations in the PTCDA and CuPc overlayers on the TiO₂ (1 1 0) upon elevating temperature from 25 to 400 °C was suggested.     

Cathodoluminescence studies of un-doped and (Cu,Fe, and Co)-doped tin dioxide films deposited by spray pyrolysis

Un-doped and (Cu, Fe, and Co)-doped SnO₂ were studied using films deposited by spray pyrolysis. Room temperature cathodoluminescence (CL) was measured. Differences in CL spectra were observed as a function of deposition parameters (T_sub-350–550 °C), the nature and concentration of dopants (0–16 at.%), and the resulting high annealing temperature (T_an = 700–950 °C). A possible luminescence mechanism has been discussed. It was established that changes taking place in CL spectra were caused by the change of both the grain size and crystallinity (stoichiometry) of the surface layer. It was concluded that radiative recombination occurs through shallow donor levels associated with O-vacancies and trapped centers. It was assumed that in SnO₂ there are apparently three types of defects forming deep levels located at 0.8–0.9, 1.3–1.4, and ∼1.6 eV from the top of the valence band.     

Temperature dependent structural and optical properties of tin oxide nanoparticles

Nanocrystalline tin oxide (SnO₂) powders were synthesized through wet chemical route using tin metal as precursor. The morphology and optical properties, as well as the effect of sintering on the structural attributes of SnO₂ particles were analyzed using Transmission electron microscopy (TEM), UV–visible spectrophotometry (UV–vis) and X-ray diffraction (XRD), respectively. The data revealed that the lattice strain plays a significant role in determining the structural properties of sintered nanoparticles. The particle size was found to be 5.8 nm, 19.1 nm and 21.7 nm for samples sintered at 300 °C, 500 °C, and 700 °C, respectively. Also, the band gaps were substantially reduced from 4.1 eV to 3.8 eV with increasing sintering temperatures. The results elucidated that the structural and optical properties of the SnO₂ nanoparticles can be easily modulated by altering sintering temperature during de novo synthesis.     

Advanced ion beam technology for versatile oxide thin film formation

Various oxide films, such as SnO₂, In₂O₃, Al₂O₃, SiO₂, ZnO, and Sn-doped In₂O₃ (ITO) have been deposited on glass and polymer substrates by advanced ion beam technologies including ion-assisted deposition (IAD), hybrid ion beam, ion beam sputter deposition (IBSD), and ion-assisted reaction (IAR). Physical and chemical properties of the oxide films and adhesion between films and substrates were improved significantly by these technologies. By using the IAD method, non-stoichiometry, crystallinity, and microstructure of the films were controlled by changing assisted oxygen ion energy and arrival ratio of assisted oxygen ion to evaporated atoms. IBSD method has been carried out for understanding the growth mode of the films on glass and polymer substrate. Relationships between microstructure and electrical properties in ITO films on polymer and glass substrates were intensively investigated by changing ion energy, reactive gas environment, substrate temperature, etc. Smooth-surface ITO films (R_rms ⩽ 1 nm and R_p−v ⩽ 10 nm) for organic light-emitting diodes were developed with a combination of deposition conditions with controlling microstructure of a seed layer on glass. IAR surface treatment enormously enhanced the adhesion of oxide films to polymer substrate. In the case of Al₂O₃ and SiO₂ films, the oxygen and moisture barrier properties were also improved by IAR surface treatment. The experimental results of the oxide films prepared by the ion beam technologies and its applications will be represented in detail.     

Thin films of molecule-based charge transfer complex cobalt tetracyanoethylene: In situ X-ray photoemission study

Thin films of molecule-based charge transfer magnet, cobalt tetracyanoethylene [Co(TCNE)_x, x ~ 2] consisting of the transition metal Co, and an organic molecule viz. tetracyanoethylene (TCNE) have been deposited by using physical vapor deposition method under ultra-high vacuum conditions at room temperature. X-ray photoelectron spectroscopy (XPS) technique has been used extensively to investigate the electronic properties of the Co(TCNE)_x thin films. The XPS measurements show that the prepared Co(TCNE)_x films are clean, and oxygen free. The stoichiometries of the films, based on atomic sensitive factors, are obtained, and yields a ~ 1:2 ratio between metal Co and TCNE for all films. Interestingly, the positive shift of binding energy position for Co(2p), and negative shifts for C(1s) and N(1s) peaks suggest a charge-transfer from Co to TCNE, and cobalt is assigned to its Co(II) valence state. In the valence band investigation, the highest occupied molecular orbital (HOMO) of Co(TCNE)_x is found to be at ~ 2.4 eV with respect to the Fermi level, and it is derived either from the TCNE^? singly occupied molecular orbital (SOMO) or Co(3d) states. The peaks located at ~ 6.8 eV and ~ 8.8 eV are due to TCNE derived electronic states. The obtained core level and valence band results of Co(TCNE)_x, films are compared with those of V(TCNE)_x thin film magnet: a well known system of M(TCNE)_x type of organic magnet, and important points regarding their electronic properties have been brought out.     

Manganese doping influence on the plasmon energy of nickel films

Manganese doping in nickel films capped with copper have been prepared by evaporation in vacuum. The films are composed of grains with an average diameter of ~ 20 nm from scanning electron microscope scans. Optical absorption is measured over a wavelength range of 190–450 nm. Two plasmon peaks are observed at 3.30 eV and 4.45 eV for a range of concentrations of films. The 4.45 eV peak is a bulk plasmon peak that is enhanced by increasing the manganese in nickel. The 3.30 eV peak is a surface plasmon peak that increases in width or strength of plasmon resonance with increasing concentration of manganese. This may be a combination effect of charge carrier concentration and dielectric screening from the reformed electronic band structure caused by manganese doping. By adding manganese into nickel, the ferromagnetic order is further destroyed as a transition into a spin glass occurs. This spin glass behavior is seen in a coercivity measurement at 4 K where the coercivity drops precipitously as the doping concentration increases.     

Heterostructured organic interfaces probed by resonant photoemission

The application of the resonant photoemission spectroscopy (RPES) to various organic molecule based systems is reviewed. The chemical specificity and the possibility to conduct experiments in the energy domain that provides a time scale for charge dynamics, make the RPES a powerful tool to study organic heterojunctions and in particular to probe the charge transfer processes at organic interfaces. We briefly discuss the models used in RPES data analysis to extract the time scale of the excited charge delocalisation and the spatial correlation of core, valence occupied and unoccupied molecular states. As an example we report on 3,4,9,10-perylene tetracarboxylic acid dianhydride (PTCDA) on (1 × 2) Au(1 1 0) surface where organic layer metallicity is directly evidenced in RPES experiments. A particular attention is dedicated to bio-mimetic model molecules whose electronic structure at interfaces is the fundamental key for the design of real devices. In the last section we consider recent experiments that could open the way to new fields of applications regarding biological molecules and single molecule systems where RPES could elucidate the link between the quantum and the meso-scopic properties of such systems.     

Enhanced photosensitivity of InGaZnO-TFT with a CuPc light absorption layer

A copper phthalocyanine (CuPc) organic semiconductor is capped onto an amorphous indium–gallium–zinc-oxide (InGaZnO) thin film transistor (TFT) to enhance the photosensitivity of InGaZnO-TFT. The CuPc organic semiconductor is served as a light absorption layer and forms a p–n junction with the InGaZnO film. After 60 s white light illumination, light responsivity (R) of InGaZnO-TFT with a CuPc light absorption layer reaches a value of 148.5 A/W at a gate-source voltage (V_GS) of 20 V, which is much larger than that (31.2 A/W) of the conventional InGaZnO-TFT. The results are attributed to the following mechanism. First, a CuPc layer is employed as the light absorption layer. Second, CuPc/InGaZnO p–n junction enables the injection of electron into InGaZnO film. Our results indicate that using CuPc as light absorption layer is an effective approach to improve the photosensitivity of InGaZnO-TFT.     

Preparation of cadmium stannate films by spray pyrolysis technique

Cadmium stannate thin films were prepared by spray pyrolysis technique using cadmium acetate and tin(II) chloride precursors at substrate temperatures 450 °C and 500 °C. XRD pattern confirms the formation of orthorhombic (1 1 1) cadmium stannate phase for the film prepared at substrate temperature of 500 °C, whereas, films prepared at 450 °C are amorphous. Film formation does not occur at substrate temperature from 300 to 375 °C. SEM images reveal that the surface of the prepared Cd₂SnO₄ film is smooth. The average optical transmittance of ∼86% is obtained for the film prepared at substrate temperature of 500 °C with the film thickness of 400 nm. The optical band gap value of the films varies from 2.7 to 2.94 eV. The film prepared at 500 °C shows a minimum resistivity of 35.6 × 10⁻⁴ Ω cm.     

DFT-GGA study of NO adsorption on the LaO (001) surface of LaFeO3

In order to demonstrate the adsorption of the nitrogen monoxide molecule (NO) on the LaO (001) surface of LaFeO₃, we perform simulations based on density functional theory. The generalized gradient approximation (GGA) for the exchange-correlation energy functional indicates that the electronic state of the LaFeO₃ bulk is an anti-ferromagnetic insulator with a local magnetic moment of 4.1 μ_B at each Fe atom. Using the ultrasoft pseudo-potential method with spin-polarized GGA, fully optimized internal parameters as well as charge and spin density are determined for the NO-adsorbed structure prepared in a slab model. The calculated adsorption energy of NO is around ? 1.4 eV on the LaO (001) surface of LaFeO₃. This value decreases down to ? 4.46 eV at an oxygen vacancy site, where the nitrogen atom of NO is embedded in the 1st LaO layer forming a bond with Fe in the 2nd FeO layer.     

Preparation of YBCO film for microwave filter using a hybrid route

Pure (0 0 l)-textured CeO₂ buffer layers were deposited on single crystal r-plane Al₂O₃ (1–102) substrate by a hybrid process which was combined with magnetron sputtering for the seed layer and metal–organic deposition for the subsequent layer. Strongly c-axis oriented YBCO films were deposited on the CeO₂ buffered r-cut Al₂O₃ (1–102) substrates. Atomic force microscope and scanning electronic microscopy results show that the prepared buffers and YBCO films are relatively dense and smooth. The critical current of the YBCO films exceeds 1.5 MA/cm² at 77 K with the superconducting transition temperature of 90 K. The surface resistivity is as below as 14 μΩ at 1 GHz frequency. The results demonstrate that the hybrid route is a very promising method to prepare YBCO films for microwave application, which can combine the sputtering advantage for preparing of highly c-axis oriented CeO₂ buffer layers and the advantages of metal–organic deposition with rapid processing, low cost and easy preparation of large-area YBCO films.     

A facile thermal evaporation route for large-area synthesis of tin oxide nanowires: Characterizations and their use for liquid petroleum gas sensor

In this paper, a very simple procedure was presented for the reproducible synthesis of large-area SnO₂ nanowires (NWs) on a silicon substrate by evaporating Sn powders at temperatures of 700, 750, and 800 °C. As-obtained SnO₂ NWs were characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy. They revealed that the morphology of the NWs is affected by growth temperature and the SnO₂ NWs are single-crystalline tetragonal. The band gap of the NWs is in the range of 4.2–4.3 eV as determined from UV/visible absorption. The NWs show stable photoluminescence with an emission peak centered at around 620 nm at room-temperature. The sensors fabricated from the SnO₂ NWs synthesized at 700 °C exhibited good response to LPG (liquefied petroleum gas) at an operating temperature of 400 °C.     

Facile synthesis of low-dimensional SnO2 nanostructures: An investigation of their performance and mechanism of action as anode materials for lithium-ion batteries

Owing to high-energy density of rechargeable lithium-ion batteries (LIBs), they have been investigated as an efficient electrochemical power sources for various energy applications. High theoretical capacities of tin oxide (SnO₂) anodes have led us a path to meet the ever-growing demands in the development of high-performance electrode materials for LIBs. In this paper, a facile approach is described for the synthesis of porous low-dimensional nanoparticles and nanorods of SnO₂ for application in LIBs with the help of Tween-80 as a surfactant. The SnO₂ samples synthesized at different reaction temperatures produced porous nanoparticles and nanorods with average diameters of ~7–10 nm and ~70–110 nm, respectively. The SnO₂ nanoparticle electrodes exhibit a high reversible charge capacity of 641.1 mAh/g at 200 mA/g after 50 cycles, and a capacity of 340 mAh/g even at a high current density of 1000 mA/g during the rate tests, whereas the porous nanorod electrodes delivers only 526.3 mAh/g at 200 mA/g after 50 cycles and 309.4 mAh/g at 1000 mA/g. It is believed that finer sized SnO₂ nanoparticles are much more favorable to trap more Li⁺ ion during electrochemical cycling, resulting in a large irreversible capacity. In contrast, rapid capacity fading was observed for the porous nanorods, which is the result of their pulverization resulting from repeated cycling.     

Sunlight assisted photodegradation by tin oxide quantum dots

Rutile phase of SnO₂ quantum dots of average size of 2.5 nm were synthesized at a growth temperature of 70 °C and characterized with XRD, TEM, FTIR and Raman analysis. The effective strain within the lattice of SnO₂ quantum dots was calculated by Williamson–Hall method. The broad peaks in XRD as well as Raman spectra and the presence of Raman bands at 569 and 432 cm⁻¹ are due to lower crystallinity of nanoparticles. The optical band gap of SnO₂ quantum dots was increased to 3.75 eV attributed to the quantum size effect. SnO₂ quantum dots were annealed in air atmosphere and the crystallite size of the particles increased with annealing temperature. Sunlight assisted photodegration property of SnO₂ quantum dots was investigated with vanillin as a model system and it shows the photodegradation efficiency of 87%. The photoluminescence and photodegradation efficiency of nanocrystallite SnO₂ decreases with increase of crystallite size contributed to the reduction in population of defects and surface area.     

Electrical characteristics and efficiency of organic light-emitting diodes depending on hole-injection layer

In a device structure of ITO/hole-injection layer/N,N′-biphenyl-N,N′-bis-(1-naphenyl)-[1,1′-biphthyl]4,4′-diamine(NPB)/tris(8-hydroxyquinoline)aluminum(Alq₃)/Al, we investigated the effect of the hole-injection layer on the electrical characteristics and external quantum efficiency of organic light-emitting diodes. Thermal evaporation was performed to make a thickness of NPB layer with a rate of 0.5–1.0 Å/s at a base pressure of 5 × 10⁻⁶ Torr. We measured current–voltage characteristics and external quantum efficiency with a thickness variation of the hole-injection layer. CuPc and PVK buffer layers improve the performance of the device in several aspects, such as good mechanical junction, reducing the operating voltage, and energy band adjustment. Compared with devices without a hole-injection layer, we found that the optimal thickness of NPB was 20 nm in the device structure of ITO/NPB/Alq₃/Al. By using a CuPc or PVK buffer layer, the external quantum efficiencies of the devices were improved by 28.9% and 51.3%, respectively.     

Correlation between phosphorus concentration and opto-electrical properties of doped a-Si:H films

Phosphorus doped hydrogenated amorphous silicon (a-Si:H) films were prepared by decomposition of silane using RF plasma glow discharge. Both DC dark conductivity measurements, and spectrophotometric optical measurements through the range 200–3000 nm were recorded for the prepared films. The DC conductivity activation energy E_a decreased from 0.8 eV for the undoped sample to 0.34 eV for the highest used doping value. The optical energy gap E_g decreased ranging from 1.66 eV to 1.60 eV. The refractive index n, the density of charge carriers N/m* and the plasma frequency ω_p showed an opposite behavior, i.e. an increase in value with doping. Fitting the dispersion values to Sellmeier equation led to the determination of the material natural frequency of oscillating particles. A correlation between the changes in these parameters with the doping has been attempted.