Structural and optical properties of epitaxial ZnO thin films on 4H–SiC (0 0 0 1) substrates prepared by pulsed laser deposition

Epitaxially grown ZnO thin films on 4H–SiC (0 0 0 1) substrates were prepared by using a pulsed laser deposition (PLD) technique at various substrate temperatures from room temperature to 600 °C. The crystallinity, in-plane relationship, surface morphology and optical properties of the ZnO films were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL) measurements, respectively. XRD analysis showed that highly c-axis oriented ZnO films were grown epitaxially on 4H–SiC (0 0 0 1) with no lattice rotation at all substrate temperatures, unlike on other hexagonal-structured substrates, due to the very small lattice mismatch between ZnO and 4H–SiC of ～5.49%. Further characterization showed that the substrate temperature has a great influence on the properties of the ZnO films on 4H–SiC substrates. The crystalline quality of the films was improved, and surfaces became denser and smoother as the substrate temperature increased. The temperature-dependent PL measurements revealed the strong near-band-edge (NBE) ultraviolet (UV) emission and the weak deep-level (DL) blue-green band emission at a substrate temperature of 400 °C.     

Low temperature solution-synthesis and photoluminescence properties of ZnO nanowires

A general and simple approach has been developed to prepare zinc oxide (ZnO) nanowires from commercially available zinc acetate precursors using a solution-phase reaction. The influence of post-annealing temperature on the morphology of the zinc oxides nanowires was also investigated. Using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) studies, we confirm that the ZnO nanowires are highly crystalline in nature with preferential orientation in the c-axis direction, with nanowire diameters of around 30–60 nm. The photoluminescence (PL) studies demonstrate a strong UV luminescence band around 382 nm, and three weak, broad bands at 454, 468, and 510 nm, respectively, in the blue and green range. After being annealed under an air atmosphere, the products show an apparent red-shift in the UV emission band. The intensity of the defect-related emission bands was significantly suppressed.     

High-density arrays of low-defect-concentration zinc oxide nanowire grown on transparent conducting oxide glass substrate by chemical vapor deposition

High-density ZnO nanowire arrays with low defect concentrations were directly grown on transparent conducting oxide glass substrates under catalyst-free and low temperature conditions by chemical vapor deposition (CVD). A possible growth mechanism of the nanowires is studied. The experiments indicate that correct levels of supersaturation and evaporation temperature are beneficial to the growth of ZnO nanowires. Photoluminescence exhibits a weak ultraviolet emission at 380 nm and a strong green emission at 495 nm. While using a double-tube growth system, the visible light emission diminishes and the 380 nm emission is the only emission, suggesting that ZnO nanowires with few defects can be prepared using the present CVD technique at low temperature.     

Finite size effect on the piezoelectric properties of ZnO nanobelts: A molecular dynamics approach

The size scale effect on the piezoelectric response of bulk ZnO and ZnO nanobelts has been studied using molecular dynamics simulation. Six molecular dynamics models of ZnO nanobelts are constructed and simulated with lengths of 150.97 Å and lateral dimensions ranging between 8.13 and 37.37 Å. A molecular dynamics model of bulk ZnO has also been constructed and simulated using periodic boundary conditions. The piezoelectric constants of the bulk ZnO and each of the ZnO nanobelts are predicted. The predicted piezoelectric coefficient of bulk ZnO is 1.4 C m^?2, while the piezoelectric coefficient of ZnO nanobelts increases from 1.639 to 2.322 C m^?2 when the lateral dimension of the ZnO NBs is reduced from 37.37 to 8.13 Å. The changes in the piezoelectric constants are explained in the context of surface charge redistribution. The results give a key insight into the field of nanopiezotronics and energy scavenging because the piezoelectric response and voltage output scale with the piezoelectric coefficient.     

Characterization of ZnO needle-shaped nanostructures grown on NiO catalyst-coated Si substrates

ZnO nanostructures were synthesized over NiO-coated Si substrate by a thermal evaporation of Zn powders in a vertical chemical vapor deposition (CVD) reactor. The ZnO nanostructures had a needle-like morphology and the diameter of the structures decreased linearly from the bottom to the top. The bottom diameters of the ZnO nano-needles normally ranged from 20 to 100 nm and the lengths were in the range of 2–3 μm. The clear lattice fringes in HRTEM image indicated the growth of good quality hexagonal single-crystal ZnO. Field emission (FE) characteristics of the ZnO nano-needles showed that the turn-on field was about 8.87 V/μm with a field enhancement factor of about 1099. The growth mechanism of the ZnO nano-needles was proposed on the basis of experimental data.     

Thin film epitaxy and structure property correlations for non-polar ZnO films

Heteroepitaxial growth and strain relaxation were investigated in non-polar a-plane (1 1 ?2 0)ZnO films grown on r-plane (1 0 ?1 2)sapphire substrates in the temperature range 200–700 °C by pulsed laser deposition. The lattice misfit in the plane of the film for this orientation varied from ?1.26% in [0 0 0 1] to ?18.52% in the [?1 1 0 0] direction. The alignment of (1 1 ?2 0)ZnO planes parallel to (1 0 ?1 2)sapphire planes was confirmed by X-ray diffraction θ?2θ scans over the entire temperature range. X-ray ?-scans revealed the epitaxial relationship:[0 0 0 1]ZnO6[?1 1 0 1]sap; [–1 1 0 0]ZnO6[?1 ?1 2 0]sap. Depending on the growth temperature, variations in the structural, optical and electrical properties were observed in the grown films. Room temperature photoluminescence for films grown at 700 °C shows a strong band-edge emission. The ratio of the band-edge emission to green band emission is 135:1, indicating reduced defects and excellent optical quality of the films. The resistivity data for the films grown at 700 °C shows semiconducting behavior with room temperature resistivity of 2.2 × 10^?3 Ω-cm.     

The photophysics of phenyl disubstituted polyacetylenes: picosecond photoluminescence and femtosecond photoinduced absorption studies of PDPA-n-Bu

We have obtained picosecond (ps) photoluminescence (PL) decays of phenyl disubstituted polyacetylenes (PDPA) with an attached n-butyl group (PDPA-n-Bu) in solution and as thin films. In solution, we observe only a single exponential decay at all emission wavelengths over the range from 2.15 to 2.50 eV. The PL lifetime in solution is 190 ps. The PL decays in thin films cannot be fit well using a single exponential, and are significantly longer in thin films than in solution. In addition, the PL decay times vary with emission wavelength in thin films with longer PL decay times observed at lower emission energies. We have also obtained the femtosecond (fs) photoinduced absorption (PA) spectra of PDPA-n-Bu in solution over the range from 1.70 to 2.95 eV. These data exhibit fast ground state bleaching at high energy, along with the growth of a broad PA feature at low energies that decays with a time constant consistent with the PL decay time in solution. At long times (>200 ps) we observe the growth of a long-lived PA feature peaked at 2.3 eV that persists for hundreds of nanoseconds. We also observe a broad feature in the infrared PA at 0.23 eV whose dynamics are the same as the band at 2.3 eV. We assign these two long-lived bands to polaron transitions.     

Synthesis and light-emitting properties of polyfluorene copolymers containing a hydrazone derivative as a comonomer

Poly[9,9-bis(2′-ethylhexyl)fluorene-2,7-diyl] (PBEHF) and a series of fluorene copolymers containing a hydrazone comonomer, poly{9,9-bis(2′-ethylhexyl)fluorene-2,7-diyl-co-[aza(2,7-fluorene-9-ylidene)methyl]diphenylamine} [poly(BEHF-co-FDPA)], were synthesized through Ni(0)-mediated polymerization. The synthesized polymers were characterized and their photophysical, electrochemical, and electroluminescent properties were investigated. Interestingly, the copolymer containing only 1% FDPA, poly(99BEHF-co-1FDPA), was found to exhibit photoluminescence (PL) emission that is significantly red-shifted with respect to that of the PBEHF homopolymer. All the copolymers exhibit PL emission maxima in the yellow region of the spectrum in the range from 530 to 540 nm, whereas the PBEHF homopolymer film produces maximum PL emission in the deep blue region at 424 nm. Light-emitting devices were fabricated in an ITO/PEDOT:PSS (50 nm)/polymers (80 nm)/Ca (50 nm)/Al (200 nm) configuration. The electroluminescence (EL) spectra of these devices consist of similar wavelengths to those found in the PL results; however, the EL devices constructed from the copolymers exhibited device performances that were significantly better than that of the device using the PBEHF homopolymer. This enhancement of performance is due to a more efficient charge carrier balance in the devices, which arises because of the effective exciton confinement (or charge carrier trapping) in the FDPA groups.     

Luminescent monomer of substituted tetrastyrylpyrene and poly(p-phenylenevinylene) derivative with pyrene segments: Synthesis and photophysics

1,3,6,8-Tetrabromopyrene reacted with 1-tert-butyl-4-vinylbenzene to afford the luminescent monomer M. In addition, the reaction of 1,6-dibromopyrene with 1,4-didodecyloxy-2,5-divinylbenzene yielded a poly(p-phenylenevinylene) (PPV) derivative P. M dissolved partially in chloroform and completely in THF. P showed an excellent solubility being readily soluble in common organic solvents. M emitted intense blue–green light in THF solution with photoluminescence (PL) maximum at 507 nm and green–yellow light in thin film with PL maximum at 570 nm. Polymer P displayed in THF a PL maximum at 464 nm and a shoulder at 513 nm. Thin film of this polymer exhibited PL maximum at 572 nm. Both M and P emitted from excimers in solid state. The PL emission quantum yields in THF were 0.35 for M and 0.32 for P. The emission maximum of M thin film was red shifted by 9 nm after annealing at 150 °C for 24 h as a result of conformational changes of the molecules with increase in temperature.     

Synthesis of polyaniline/NiO nanobelts by a self-assembly process

Polyaniline (PANI) nanotubes (∼180 nm in diameter) were synthesized in the presence of sodium dodecylbenzenesulfonate (SDBS) as a micellar template and dopant, whereas PANI/NiO nanobelts (300–700 nm in diameter) were obtained with the addition of NiO nanoparticles (∼10 nm in diameter). Results showed that the size of PANI/NiO composite nanobelts increased with an increase content of NiO nanoparticles. XRD, Fourier transform infrared (FTIR) and UV–vis spectroscopy were used to characterize the chemical structures of PANI nanotubes and PANI/NiO nanobelts. The thermal stability and conductivity of samples were affected by the content of NiO. The coordination bonds between NiO and aniline were the key factor that resulted in the morphological change of PANI.     

Field emission enhancement of ZnO nanorod arrays with hafnium nitride coating

Vertically well-aligned single crystal ZnO nanorod arrays were synthesized and enhanced field electron emission was achieved with hafnium nitride (HfN_x) coating under proper sputtering condition. HfN_x films with various composition have been coated on ZnO nanorod arrays using a reactive direct current (DC) magnetron sputtering system. Morphology and crystal configuration of the ZnO nanorod arrays were investigated by scanning electron microscopy and X-ray diffraction. The field emission properties of the coated and uncoated ZnO nanorod arrays were characterized. The as-grown ZnO nanorod arrays showed a turn-on electric field of 6.60 V μm^− 1 at a current density of 10 μA cm^− 2 and an emission current density of 1 mA cm^− 2 under the field of 9.32 V μm^− 1. While the turn-on electric field of the coated ZnO nanorod arrays sharply decreased to 2.42 V μm^− 1, an emission current density of 1 mA cm^− 2 under the field of only 4.30 V μm^− 1 can be obtained. A method to accurately measure the work function of the coated films was demonstrated.     

Synthesis and voltage-controlled multicolor electroluminescent property of copolyfluorenes containing a pendent anthraquinone-2,3-dicarboxylic imide

A new series of four fluorene-thiophene copolymers, PF-co-2%TAQI, PF-co-5%TAQI, PF-co-10%TAQI and PF-co-15%TAQI, were synthesized by the Suzuki cross-coupling polymerization of 9,9-dihexylfluorene-2,7-dibromofluorene and N-butyl 6-[3-(2,5-dibromothienyl)]anthraquinone-2,3-dicarboxylic imide. The polymers have good solubility in common organic solvents and can form thin films by spin coating or casting. The UV–vis and photoluminescence (PL) spectra for the copolymers in dilute solution are similar to those of polyfluorene, i.e. maximum absorption at 381 nm and 417 nm and PL at 439 nm. The UV–vis spectra of the polymer films also exhibit a single peak at 378 nm, while an extra emission peak was found at 612 nm, 618 nm, 628 nm and 626 nm for PF-co-2%TAQI, PF-co-5%TAQI, PF-co-10%TAQI and PF-co-15%TAQI, respectively, presumably resulting from the excimer of anthraquinone imide groups. The electroluminescent properties of the copolymers were investigated using single-layer (ITO/PEDOT/Polymer/LiF/Al) and double-layer (ITO/PEDOT/Polymer/TPBi/LiF/Al) (TPBi, 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene) LED devices. The double-layer devices based on copolymers emit multicolors under different voltages. The Commission Internationale de l’Eclairage (CIE) coordinates of PF-co-2%TAQI are (0.356, 0.249), (0.285, 0.210), (0.264, 0.277) and (0.240, 0.250) under 12 V, 14 V, 16 V and 18 V, respectively.     

Influence of annealing on the structural and optical properties of ZnO films grown by MOCVD

The properties of ZnO films grown on Si (1 1 1) substrates by Metal-Organic Chemical Vapor Deposition technique using diethylzinc and H₂O as reactant gases are reported. The primary focus is on understanding the origin of deep-level luminescence. As increasing the annealing temperature, a visible emission is observed both in samples annealing in oxygen atmosphere and nitrogen atmosphere. In addition, this broad defect emission becomes obviously asymmetric when the annealing temperature was increased to 1000 °C in oxygen atmosphere. Theoretical investigations have reported that the formation enthalpy of defects is varied under different conditions. With these results, it is suggested that the visible emission in ZnO films annealed in oxygen atmosphere is related to zinc vacancy and oxygen interstitial defects. While, the green emission in ZnO films which were annealed in nitrogen atmosphere is attributed to oxygen vacancy defects.     

Mechanism of polypyrrole and silver nanorod formation in lauric acid–cetyl trimethyl ammonium bromide coacervate gel template: Physical and conductivity properties

Polypyrrole (PPy) and silver (Ag) nanorods are synthesized in cetyl trimethylammonium bromide–lauric acid (CTAB–LA) complex coacervate gel template. When PPy–CTAB–LA system is polymerized with AgNO₃, Ag nanorods are produced while use of ammonium persulphate (APS) as initiator yields PPy nanorods. Ag-nanorods are produced from the initial stage while PPy nanorods take a longer time. The average diameter of Ag nanorods varies from 60 to 145 nm by increasing AgNO₃ concentration from 0.27 M to 1.08 M and that of PPy varies from 145 nm to 345 nm by changing pyrrole concentration from 1 × 10^?4 to 2 × 10^?4 M, respectively. Fourier transformed infrared (FTIR) spectra indicate stabilization of Ag nanorods through complexation of PPy with adsorbed Ag⁺ ions. PPy nanoparticles are stabilized by adsorbed sulphate ions and lauric acid, both are acting as dopant to it. FFT pattern and EDX spectra clearly indicate the presence of Ag nanocrystals and PPy on the surface of Ag nanorods, respectively. The mechanism of nanorod formation is attributed from UV–Vis spectra showing a red shift of surface plasmon band of Ag and π–π* transition band of PPy with time. The highest dc conductivity of PPy–Ag composite is found to be 414.2 S/cm, 7 orders higher than that of PPy nanorods (9.3 × 10^?4 S/cm). PPy–Ag systems show Ohmic behavior while PPy nanorods exhibit semi-conducting behavior. The preferential formation of Ag nanorod in AgNO₃ initiated polymerization is attributed to the higher cohesive force of Ag than that of PPy. With two times higher LA and CTAB concentration in the gel the Ag nanorod diameter decreases only 12% while that of PPy nanorod decreases by 50%. Possible reasons are discussed from the hard and soft nature of the two nanorods and from the elasticity of the gel template.     

The z value dependence of photoluminescence in Eu2+-doped β-SiAlON (Si6−zAlzOzN8−z) with 1 ≤ z ≤ 4

Eu²⁺-doped β-SiAlON (Si_6?zAl_zO_zN_8?z) phosphors with 1 ≤ z ≤ 4 were synthesized by gas pressure sintering. The emission spectra exhibit two broad bands with maxima at about 415 nm (violet) and 540 nm (green) under ultraviolet excitation. The green emission is dominant at z ≤ 2, while the violet emission becomes dominant at z > 2. The combination of two emission bands in a compound could lead to a white emission in Eu²⁺-doped β-SiAlON with 2 < z ≤ 3.     

Nonpolar ZnO film growth and mechanism for anisotropic in-plane strain relaxation

Using high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction, we investigated the strain relaxation mechanisms for nonpolar (1 1 ?2 0) a-plane ZnO epitaxy on (1 ?1 0 2) r-plane sapphire, where the in-plane misfit ranges from ?1.5% for the [0 0 0 1]ZnO6[1 ?1 0 ?1]sapphire to ?18.3% for the [?1 1 0 0]ZnO6[?1 ?1 2 0]sapphire direction. For the large misfit [?1 1 0 0]ZnO direction the misfit strains are fully relaxed at the growth temperature, and only thermal misfit and defect strains, which cannot be relaxed fully by slip dislocations, remain on cooling. For the small misfit direction, lattice misfit is not fully relaxed at the growth temperature. As a result, additive unrelaxed lattice and thermal misfit and defect strains contribute to the measured strain. Our X-ray diffraction measurements of lattice parameters show that the anisotropic in-plane biaxial strain leads to a distortion of the hexagonal symmetry of the ZnO basal plane. Based on the anisotropic strain relaxation observed along the orthogonal in-plane [?1 1 0 0] and [0 0 0 1]ZnO stress directions and our HRTEM investigations of the interface, we show that the plastic relaxation occurring in the small misfit direction [0 0 0 1]ZnO by dislocation nucleation is incomplete. These results are consistent with the domain-matching paradigm of a complete strain relaxation for large misfits and a difficulty in relaxing the film strain for small misfits.     

Enhanced ferroelectric properties of Pb(Zr0.52Ti0.48)O3 films on Pt/TiO2/SiO2/Si(001) using ZnO buffer layer

(0 0 1)-textured Pb(Zr_0.52Ti_0.48)O₃ films have been grown on Pt/TiO₂/SiO₂/Si(0 0 1) substrates by pulsed laser deposition using Al-doped ZnO as a buffer layer and bottom electrode. The as-fabricated ferroelectric capacitors exhibit large remanent polarization and excellent fatigue and retention properties.     

Mechanical characterization of hotplate synthesized vanadium oxide nanobelts

Vanadium oxide nanobelts have been synthesized on Si or SiN substrates by simply heating vanadium foils on a hotplate. As-grown nanobelts were characterized as V₂O₅·nH₂O (0.3 < n < 1.7) layer structures containing water molecular intercalation. Using an electromechanical resonance method, the Young’s modulus of the nanobelts was measured in situ in a scanning electron microscope. It was found that the Young’s modulus of as-grown nanobelts varied between 5.6 and 98 GPa, with a typical Q-factor of 120. Such scattered values were attributed to the different contents of water molecules in the nanobelts. After annealing at 450 °C in vacuum, the nanobelts were converted to α-V₂O₅ phase and a polycrystalline structure was observed. The Young’s modulus of the annealed nanobelts showed more consistent values at an average of 28.9 GPa, lower than the calculated modulus of bulk α-V₂O₅ at 68 GPa.     

Preparation and DSC application of the size-tuned ZnO nanoarrays

ZnO nanoarray was successfully prepared by a simple chemical bath deposition method on the glass slide deposited with a spin-coated ZnO seed crystal layer. With the measurements of SEM, XRD and UV–vis absorption spectra, the properties of the ZnO nanoarray were characterized in detail. The results show that the ZnO nanoarray is the hexagonal wurtzite structure and grow along the [0 0 1] axis. Furthermore, the energy-conversion property of the device was investigated in such ZnO nanoarray constructed dye-sensitized solar cells (DSCs), which has potential applications in the new-concept photovoltaic devices.     

Characterization,liquid crystalline behavior,optical and electrochemical study of new aliphatic–aromatic polyimide with naphthalene and perylene subunits

A new aliphatic–aromatic diamine with naphthalene diimide (DA) moiety and a polyimide with naphthalene and perylene diimide subunits (PI) were synthesized and their thermotropic liquid crystalline behavior was examined by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). Both compounds exhibited liquid crystalline properties. The temperatures of 5% weight loss (T_5%) of the diamine and polyimide range from 350 to 373 °C in nitrogen. The optical properties, that is, UV–vis absorption and photoluminescence (PL) of obtained monomer and polyimide were studied in solution and in solid state as a blend with inert poly(methyl methacrylate) (PMMA). Influence of solvent polarity, excitation wavelength and concentration on DA and PI emission was found. The investigated compounds emitted green light in chloroform solution. Relative PL intensity of the DA and PI was investigated in NMP in relation to rhodamine-B used as a standard. The electrochemical behavior of diamine and polyimide was studied (in solution) by differential pulse voltammetry (DPV). The HOMO and LUMO levels of these compounds were in the range of ?5.44 to ?6.03 eV and ?3.81 eV, respectively. Energy gap (E_g) values were calculated using cyclic voltammetry (CV) and UV–vis measurements. Introduction of a perylene moiety (PI) resulted in increase of E_g (2.22 eV) compared to DA which exhibited a E_g value of approximately 1.63 eV. The electrical properties of DA and PI were investigated by current–voltage (I–V) measurements on ITO/compound/Al devices.