Structure–property–processing relationships of single-wall carbon nanotube thin film piezoresistive sensors

In an investigation of structure–property–processing relationships for SWCNT thin film piezoresistive sensors, the gauge factor of the sensors for a small tensile deformation (less than 2% strain) was found to be close to unity and showed negligible dependence on the film thickness and SWCNT bundle length (L) and diameter (d). However, for a large tensile deformation (20–30% strain), the film thickness and the microstructure of SWCNTs had a compounding effect on the piezoresistive behavior. A gauge factor of ∼5 was obtained for the sensors fabricated with SWCNT bundles of short length and thin diameter (L = 549 nm and d = 3.7 nm) with thicker films. Furthermore, the gauge factor of the sensors was found inversely proportional to the excluded volume V_ex of SWCNT bundles (V_ex ∝ 1/L² d).     

Enhancement of memory margins for stable organic bistable devices based on graphene-oxide layers due to embedded CuInS2 quantum dots

Current–voltage (I–V) curves for Al/CuInS₂ (CIS) quantum dots (QDs) embedded in graphene-oxide layer/indium-tin-oxide devices at 300 K showed a current bistability with a maximum high conductivity (ON)/low conductivity (OFF) ratio of 1 × 10⁴, which was 100 times larger than the ON/OFF ratio of the device without CIS QDs. I–V curves and write-read-erase-read voltage cycles demonstrated the rewritable nonvolatile memory properties of the organic bistable devices (OBDs) with ON and OFF current states at the same voltage. The retention time was above 1 × 10⁵ s, indicative of the memory stability of the OBDs. I–V curve at lower voltages up to 0.05 V was attributed to the thermionic emission mechanism, and the curve in the applied voltage range from 0.06 to 0.17 V was related to an ohmic mechanism. The I–V characteristics in the applied voltage above 0.18 V dominantly followed the space-charge-limited-current behaviors.     

Y2O3:Yb,Tm and Y2O3:Yb,Ho powders for low-temperature thermometry based on up-conversion fluorescence

Recently, trivalent rare earth doped materials have received significant attention due to the strong temperature dependence of the fluorescence emission of these materials, which can be useful in temperature sensing. Here, we investigated Y₂O₃ ceramic powders doped with Yb³⁺ and co-doped with either Tm³⁺ or Ho³⁺. The powders were obtained via spray pyrolysis at 900 °C and additionally thermally treated at 1100 °C for 24 h. Structural characterization using X-ray powder diffraction confirmed the cubic bixbyte structure. Scanning electron microscopy (SEM) revealed that the particles exhibit a uniform spherical morphology. The up-conversion emissions were measured using laser excitation at 978 nm, resulting in the following transitions: blue emission in the range of 450–500 nm, weak red emission in the range of 650–680 nm and near infrared emission in the range of 765–840 nm for Tm³⁺, as well as green emission centered at 550 nm and weak near infrared emission at 755 nm for the Ho³⁺ ions. In addition, the temperature dependence of the fluorescence intensity ratios of different Stark components was analyzed in the range of 10–300 K. Significant temperature sensitivity was detected for several components, with the largest value of 0.097 K^?1 related to the intensity ratio of I₅₃₆ and I₇₇₂ emissions observed for the Y₂O₃:Yb,Ho powder.     

Characterization of luminescent properties of ZnO:Er thin films prepared by rf magnetron sputtering

ZnO:Er thin films were deposited on c-plane sapphire substrates by rf magnetron sputtering and annealed at 700 °C under air and H₂ atmospheres for the luminescent improvement. The effects of sputtering parameters and the annealing conditions on visible and 1.54 μm IR emissions were investigated. Structural and luminescent properties strongly depended on the deposition conditions and annealing atmospheres. By tuning the excitation wavelength, ZnO:Er thin films exhibited a strong emission band at around 465 nm and a weak emission at 525 nm originated from the energy transition of ⁴I_15/2–⁴F_5/2 and ⁴I_15/2–²H_11/2, respectively, while 1.54 μm IR emissions due to ⁴I_15/2–⁴I_13/2 transition.     

Single-walled carbon nanotube/silicone rubber composites for compliant electrodes

Single-walled carbon nanotube (SWCNT)/silicone rubber composites that can be used in fabricating compliant electrodes are prepared by spraying a mixed solution of ionic-liquid-based SWCNT gel and silicone rubber onto an elastic substrate. Subsequently, the composites are exposed to nitric acid vapor. Scanning electron microscopy and atomic force microscopy images of the composites show that the SWCNTs are finely dispersed in the polymer matrix due to the addition of the ionic liquid. Doping of the SWCNTs by nitric acid can significantly lower the sheet resistance (R_s) of the composites; samples with 4 wt% of SWCNT content exhibit the lowest R_s value (50 Ω sq^?1). This sheet resistance corresponds to a conductivity value of 63 S cm^?1. In addition, the composites retain a high conductivity after several tensile strains are applied. Stretching the composite sample to 300% of the original length increased the R_s value to 320 Ω sq^?1 (19 S cm^?1). Even after 20th stretch/release/stretch cycle, the conductivity remains constant at a value of 18 S cm^?1. These results provide a scalable route for preparing highly stretchable and conductive SWCNT composites with relatively low SWCNT concentrations.     

Surface immobilization of Mo6I8 octahedral cluster cores on functionalized amorphous carbon using a pyridine complexation strategy

Tetrahedral amorphous carbon (a-C) films have been grown by pulsed laser deposition to investigate a liquid phase process for surface immobilization of electroactive [Mo₆Iⁱ₈]^4 + transition metal cluster cores using a complexation reaction with a pyridine-terminated alkyl monolayer covalently bonded to the a-C surface (Py^S–alkyl/a-C). These films are stable against thermally-assisted grafting of alkene molecules and the covalent CC interface provides a robust monolayer/a-C assembly. Octahedral [Mo₆Iⁱ₈]^4 + cluster cores with iodine inner ligands and labile triflate apical ligands [Mo₆Iⁱ₈(CF₃SO₃)^a₆]^2 − have been immobilized through partial complexation in apical positions by surface pyridine groups (Py^S). The remaining CF₃SO₃⁻ apical ligands of [Mo₆Iⁱ₈ (Py^S)^a_y(CF₃SO₃)^a_6 − y] cluster units were further substituted with bromopyridine (Py-Br) to obtain air stable surface with expected final composition [Mo₆Iⁱ₈ (Py^S)^a_y(Py-Br)^a_6 − y]. The yield of the different reaction steps is followed by X-ray photoelectron spectroscopy, providing cluster coverage Σ_Mo6I8 = 9 × 10¹² cm^− 2. Each [Mo₆I₈]^4 + cluster is bound to the carbon surface through multiple anchoring metal sites (N_PYR = 3 or 4), indicating that pyridine-terminated alkyl chains are flexible enough to accommodate four bonds. Electrical transport through Hg//Mo₆I₈–Py^S–alkyl/a-C/p-Si(111) junctions shows rectifying current–voltage characteristics but does not reveal any signature of cluster immobilization.     

Absorption spectra and CT transitions of compounds containing ReI(CO)3Cl,S2+ and Se2+ as donors and o-benzoquinone diimine as acceptor

The electronic spectra of the cations [(BQDI)Se^II]²⁺ and [(BQDI)S^II]²⁺ are compared to that of [(BQDI)Re^I(CO)₃Cl] with BQDI = o-benzoquinone diimine. While these species show a BQDI-localized ππ¹ absorption between 300 and 360 nm, a longer-wavelength band extends to the visible region. This absorption is attributed to a CT transition from Re^I (λ_max = 538), Se^II (λ_max = 432) and S^II (λ_max = 360 nm) to the π¹ orbitals of BQDI. The analogy of MLCT and related CT transitions involving non-metals as CT donors is emphasized.     

Synthesis of bimetallic PtPd nanocubes on graphene with N,N-dimethylformamide and their direct use for methanol electrocatalytic oxidation

Bimetallic PtPd nanocubes supported on graphene nanosheets (PtPdNCs/GNs) were prepared by a rapid, one-pot and surfactant-free method, in which N,N-dimethylformamide (DMF) was used as a bi-functional solvent for the reduction of both metal precursors and graphene oxide (GO) and for the surface confining growth of PtPdNCs. The morphology, structure and composition of the thus-prepared PtPdNCs/GNs were characterized by transmission electron microscopy (TEM), high resolution TEM, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. Because no surfactant or halide ions were involved in the proposed synthesis, the prepared PtPdNCs/GNs were directly modified onto a glassy carbon electrode and showed high electrocatalytic activity for methanol oxidation in cyclic voltammetry without any pretreatments. Moreover, with the synergetic effects of Pt and Pd and the enhanced electron transfer by graphene, the PtPdNCs/GNs composites exhibited higher electrocatalytic activity (j_p = 0.48 A mg⁻¹) and better tolerance to carbon monoxide poisoning (I_f/I_b = 1.27) compared with PtPd nanoparticles supported on carbon black (PtPdNPs/C) (j_p = 0.28 A mg⁻¹; I_f/I_b = 1.01) and PtNPs/GNs (j_p = 0.33 A mg⁻¹; I_f/I_b = 0.95). This approach demonstrates that the use of DMF as a solvent with heating is really useful for reducing GO and metal precursors concurrently for preparing clean metal–graphene composites.     

Microstructure and corrosion properties of Ni-TiN nanocoatings prepared by jet pulse electrodeposition

Ni–TiN nanocoatings were successfully prefabricated by jet pulse electrodeposition. The effect of jet rate on cross-sectional composition, microstructure, microhardness, and corrosion properties of nanocoatings was examined by X-ray photoelectron spectroscopy, high-resolution transmission electron microscope, atomic force microscopy, microhardness tester and electrochemical workstation. Results illustrated that Ni–TiN nanocoatings deposited at jet rate of 3 m/s exhibited high concentration of Ni and Ti with average concentrations of Ni and Ti of 54.5 at% and 19.8 at%, respectively. Average diameters of Ni grains and TiN nanoparticles in Ni–TiN nanocoatings prepared at 3 m/s were 47.8 nm and 30.5 nm, respectively. Nanocoatings deposited at 1 m/s, 3 m/s and 5 m/s showed surface root-mean-square roughness value of 95.431, 30.091 and 58.454 nm, respectively, and presented maximum microhardness of 789.5, 876.2, and 849.9 HV, respectively. Ni–TiN nanocoating obtained at 3 m/s demonstrated minimum I_corr and E_corr values of 1.02 × 10⁻³ mA/cm² and − 0.551 V, respectively, signifying to offer the best corrosion resistance.     

Combustion synthesis and characterization of LSCF-based materials as cathode of intermediate temperature solid oxide fuel cells

Nanocrystalline SOFC cathode materials of perovskite family, La_1?xSr_xM_1?yCo_yO₃, where 0 < x ≤ 0.5, 0 < y ≤ 0.8 (M is transitional metal = Mn or Fe), have been synthesized at a relatively low temperature by combustion synthesis using alanine as a novel fuel. Detailed X-ray powder diffraction analyses show 47–96% phase purity in the as-synthesized powder and upon calcination at ～825 °C single-phase material is obtained wherein the nanocrystallinity (crystallite size ～19–24 nm) is retained. Densification studies of the materials are carried out within 900–1100 °C. The coefficient of thermal expansion (CTE) of these cathodes is measured. Electrical conductivity of the cathodes sintered at different temperatures are measured in the temperature range 700–900 °C and correlated with the density of the sintered materials. The electrochemical performances of Ni-YSZ anode-supported SOFC having YSZ electrolyte (～10 μm) with CGO interlayer (～15 μm) are studied with the developed cathodes in the temperature range 700–800 °C using H₂ as fuel and oxygen as oxidant. Highest current density of ～1.7 A/cm² is achieved during testing at 800 °C measured at 0.7 V with a cathode composition of La_0.5Sr_0.5Co_0.8Fe_0.2O₃. Precipitation of nanocrystalline grains over the core grains in porous microstructure of this cathode might be one of the reasons for such high cell performance.     

Graphene quantum dots directly generated from graphite via magnetron sputtering and the application in thin-film transistors

This work presents a novel method to prepare graphene quantum dots (GQDs) directly from graphite. A composite film of GQDs and ZnO was first prepared using the composite target of graphite and ZnO via magnetron sputtering, followed with hydrochloric acid treatment and dialysis. Morphology and optical properties of the GQDs were investigated using a number of techniques. The as-prepared GQDs are 4–12 nm in size and 1–2 nm in thickness. They also exhibited typical excitation-dependent properties as expected in carbon-based quantum dots. To demonstrate the potential applications of GQDs in electronic devices, pure ZnO and GQD–ZnO thin-film transistors (TFTs) using ZrO_x dielectric were fabricated and examined. The ZnO TFT incorporating the GQDs exhibited enhanced performance: an on/off current ratio of 1.7 × 10⁷, a field-effect mobility of 17.7 cm²/Vs, a subthreshold swing voltage of 90 mV/decade. This paper provides an efficient, reproducible and eco-friendly approach for the preparation of monodisperse GQDs directly from graphite. Our results suggest that GQDs fabricated using magnetron sputtering method may envision promising applications in electronic devices.     

The structure of 1D and 3D CuI nanocrystals grown within 1.5–2.5 nm single wall carbon nanotubes obtained by catalyzed chemical vapor deposition

Catalyzed chemical vapor deposition (CCVD) grown single wall carbon nanotubes (SWCNT) with diameter of D_m = 1.5–2.5 nm were used as templates to host one-dimensional nanocrystals of CuI. The CuI@SWCNT nanocomposite was obtained using capillary filling of preopened SWCNTs by CuI melt at 650 °C. Nanocomposite structural studies were performed on a FEI Titan 60–300 at 80 kV. According to the model and image simulation CuI crystallizes within 1.5–2.0 nm SWCNTs in the form of one-dimensional crystals with zinc blende or rock salt type unit cell connected by [0 0 1] edges and translated along 〈1 1 0〉. Copper cations occupy tetrahedral or octahedral sites in the lattice. In SWCNTs with D_m > 2.0 nm 3DCuI@SWCNTs were generated. The crystals of copper halides exhibit acceptor behavior as supported by Raman spectroscopy.     

Synthesis of fluorene- and anthracene-based π-conjugated polymers and dependence of emission range and luminous efficiency on molecular weight

In the present work poly[9,9-dioctylfluorene-co-2-pentyl-9,10-bis(4-vinylphenyl)anthracene], a fluorene- and anthracene-based copolymer, is synthesized through a Heck coupling reaction. In order to synthesize polymers with high-molecular weight, DMF (P1), DMF/p-Xylene = 1/1 (P2), p-Xylene (P3), and 1,4-Dioxane (P4) are used as solvents, which are an important factor in the synthesis process. The number of average molecular weights (M_n) of the synthesized polymers P1–P4 do not differ significantly, standing at 22,309, 12,369, 29,192, and 39,464, respectively, while their weight average molecular weights (M_w) show considerable differences (i.e. 50,055; 24,042; 125,406; and 231,053). Polymers P1–P4 demonstrate little difference in the results of a thermal analysis, electrochemical analysis, UV–vis analysis, and photoluminescence (PL) spectrum measurement. With regard to electroluminescence (EL) spectrum measurement, however, P1 and P2 show main luminous peaks at 508 nm, while P3 and P4's luminous peaks are seen at 516 nm. Moreover, luminous shoulder peaks were red-shifted with increase of molecular weight of polymers from 460 to 544 nm. In this process, the luminous area is red-shifted from greenish-blue to yellowish-green. The I–V–L measurement results show that the maximum brightness of P1, P2, and P3 ranges from 164 to 303 cd/m² and their luminous efficiency is low at 0.031–0.054 cd/A. Meanwhile, the turn-on voltage of P4, having greater molecular weight, is 9.5 V, and its maximum brightness and corresponding luminous efficiency are 736 cd/m² and 0.08 cd/A, respectively, implying that the luminous efficiency of devices improves as the molecular weight becomes greater.     

Structural,optical, and spectroscopic properties of Er3+-doped TeO2–ZnO–ZnF2 glass-ceramics

Transparent fluorotellurite glass-ceramics have been obtained by heat treatment of precursor Er-doped TeO₂–ZnO–ZnF₂ glasses. ErF₃ nanocrystals nucleated in the glass-ceramics have a typical size of 45 ± 10 nm. Based on the Judd-Ofelt theory, the main radiative parameters for the ⁴I_13/2 → ⁴I_15/2 transition have been obtained. The split of the absorption and emission bands and the reduction of the Ω₂ parameter, as compared to the glass, confirm the presence of Er³⁺ ions in a crystalline environment in glass-ceramic samples. The analysis of the ⁴I_13/2 decays suggests that a fraction of Er³⁺ ions remains in a glass environment while the rest forms nanocrystals. For the glass-ceramics, intense red and green upconversion emissions were observed with an enhancement of the ⁴F_9/2 → ⁴I_15/2 red one compared to the glass sample. The temporal evolution of the red emission together with the excitation upconversion spectra suggests that energy transfer processes are responsible for the enhancement of the red emission.     

Huge volume expansion and structural transformation of carbon nanotube aligned arrays during electrical breakdown in vacuum

We observed a huge volume expansion of aligned single walled carbon nanotube (SWCNT) arrays accompanied by structural transformation during electrical breakdown in vacuum. The SWCNT arrays were assembled between prefabricated palladium source and drain electrodes of 2 μm separation on a silicon/silicon dioxide substrate by dielectrophoresis. At high electrical field, the SWCNT arrays erupt into a large mushroom-like structure. Systematic studies with controlled electrical bias show that above a certain field the SWCNTs swell and transform to nanoparticles and flower-like structures with a small volume increase. Further increases in electrical bias and repeated sweeping results in their transformation into amorphous carbon as determined from scanning electron microscopy and transmission electron microscopy (TEM). Cross-sectional studies using a focused ion beam and TEM show the height of a 2–3 nm SWCNT array increased to about 1 μm with a volume increase of ～400 times. Electron energy loss spectroscopy reveals that graphitic sp² networks of SWCNT are transformed predominantly to sp³. The current–voltage measurements also show an increase in the resistance of the transformed structure.     

Synthesis and cytotoxicity of a ruthenium nitrosyl nitric oxide donor with isonicotinic acid and a cell penetrating peptide

The syntheses and properties of trans-[Ru(NH₃)₄(L)(NO)](BF₄)₃ (L = isonicotinic acid (inaH) (I) or ina-Tat48–60 (II)) are described. Tat48–60, a cell penetrating peptide fragment of the Tat regulatory protein of the HIV virus, was linked to the ruthenium nitrosyl through inaH. I and II release NO after reduction forming trans-[Ru(NH₃)₄(L)(H₂O)]^3 +. The IC₅₀ values against B16-F10 melanoma cells of I and II (21 μmol L^− 1 and 23 μmol L^− 1, respectively) are close to that of the commercially available cisplatin (33 μmol L^− 1) and smaller than similar complexes. The cytotoxicity is assigned to the NO released from I and II.     

Fluidized-bed synthesis of sub-millimeter-long single walled carbon nanotube arrays

The rapid growth method for vertically aligned, single walled carbon nanotube (SWCNT) arrays on flat substrates was applied to a fluidized-bed, using ceramic beads as catalyst supports as a means to mass produce sub-millimeter-long SWCNT arrays. Fe/Al₂O_x catalysts were deposited on the surface of Al₂O₃ beads by sputtering and SWCNTs were grown on the beads by chemical vapor deposition (CVD) using C₂H₂ as a feedstock. Scanning electron microscopy and transmission electron microscopy showed that SWCNTs of 2–4 nm in diameter grew and formed vertically aligned arrays of 0.5 mm in height. Thermogravimetric analysis showed that the SWCNTs had a catalyst impurity level below 1 wt.%. Furthermore, they were synthesized at a carbon yield as high as 65 at.% with a gas residence time as short as <0.2 s. Our fluidized-bed CVD, which efficiently utilizes the three-dimensional space of the reactor volume while retaining the characteristics of SWCNTs on substrates, is a promising option for mass-production of high-purity, sub-millimeter-long SWCNT arrays.     

Optimization,synthesis and characterization of vanadium-substituted thick-walled three-dimensional SBA-16

A systematic study was carried out to examine the viable synthesis condition at high acidic condition for the direct incorporation of a high level of vanadium into the 3D cubic arrangement of SBA-16 molecular sieves. The extent of mesopore structural ordering was determined by X-ray diffraction, N₂ physisorption, SEM and TEM analysis. The coordination and nature of the V sites were characterized by FT-Raman, electron spin resonance, ²⁹Si nuclear magnetic resonance spectroscopy, UV–visible diffuse reflectance spectroscopy and NH₃–TPD analysis. The surface area of calcined V-SBA-16 was ～1000 m²/g, while the mesopore volume of the SBA-16 cage ranged from 0.3 to 0.6 cm³g^?1. The pore size distribution was ～3.0 nm with a wall thickness of ～9.7 nm at the maximum. The Si/V ratios as determined by ICP-OES technique was varied from 438 to 8, respectively. The calcined samples exhibited promising catalytic activity in the oxidation of styrene with tert-butyl hydroperoxide as an oxidant.     

Electrochromic properties of Al doped B-subsituted NiO films prepared by sol–gel

In this paper, Al doped B-substituted NiO films were prepared by sol–gel method. The effect of the Al content on the structure of the Al_xB_0.15NiO films were studied with X-ray diffraction (XRD) and transmission electron microscopy (TEM). The electrochemical and EC properties were examined by cyclic voltammetric (CV) measurements and UV–Vis spectrophotometry, respectively. Al doping could prevent the crystallization of the films, which exhibited much better electrochemical and electrochromic properties than undoped samples. The bleached state absorbance could be significantly lowered when the Al added. EC efficiencies measured at λ = 500 nm of the films with different Al doping content reach ～30 cm² C^?1, with a change in transmittance up to 70%.     

Low-temperature synthesis and microwave dielectric properties of trirutile-structure MgTa2O6 ceramics by aqueous sol–gel process

Trirutile-structure MgTa₂O₆ ceramics were prepared by aqueous sol–gel method and microwave dielectric properties were investigated. Highly reactive nanosized MgTa₂O₆ powders were successfully synthesized at 500 °C in oxygen atmosphere with particle sizes of 20–40 nm. The evolution of phase formation was detected by DTA–TG and XRD. Sintering characteristic and microwave dielectric properties of MgTa₂O₆ ceramics were studied at different temperatures ranging from 1100 to 1300 °C. With the increase of sintering temperature, density, ?_r and Q · f values increased and saturated at 1200 °C with excellent microwave properties of ?_r ～ 30.1, Q · f ～ 57,300 GHz and τ_f ～ 29 ppm/°C. The sintering temperature of MgTa₂O₆ ceramics was significantly reduced by aqueous sol–gel process compared to conventional solid-state method.