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Temperature-dependent variations in electric switching and transverse resistance of phase-change [(GeTe)2(Sb2Te3)]n (n=4 and 8) chalcogenide superlattice (CSL) films were studied using conductive scanning probe microscopy (SPM). Three temperature regions with different electric transport properties were recognized in point current-voltage (I-V) spectra and the surface potential maps measured with tantalum and platinum-coated SPM cantilevers. At around 80°C the switching voltage decreased abruptly from ∼2 V to 0.5 V and the thermal coefficient of resistance changes its sign, indicating different carrier transport mechanisms. The observed changes correlated with decrease in the surface potential by ∼150 meV from 25 to 150°C. The results were ascribed to an opening of the CSL electronic band gap near the Fermi energy caused by thermal stress, which led to the transition from a Dirac-like semimetal to a narrow-gap semiconductor.
Metal oxides like zinc oxide (ZnO) are promising materials for the active layer of thin-film transistors (TFTs) used in the drive circuit of next-generation large-area active matrix displays due to their high electronic mobility, high transmittance in the optical visible range and processability. Traditional deposition techniques employ RF sputtering or pulsed-laser deposition (PLD), which are relatively sophisticated techniques. The deposition of very thin (less than 50 nm thick) layers of ZnO using soluble organic precursors have been extensively investigated recently as an alternative to traditional deposition methods. Solution-based deposition processes include simple and affordable techniques like dip-coating, spin-coating, spray-pyrolysis and ink-jet printing. Spray-pyrolysis is particularly interesting due to the high film uniformity, low cost and high device performance. We carried out several experiments analyzing the performance of ZnO based devices using zinc acetate as organic precursor to confirm that spray pyrolysis deposition is a suitable technique for production of high-performance and reproducible TFTs. Moreover, we observed that device performance can significantly vary with little modifications on the deposition parameters, even for the same active layer composition and pyrolysis temperature. Electrical parameters, as the electrical mobility and the on/off ratio, varied several orders of magnitude, whereas the threshold voltage varied up to 20 V for the tested devices. Deposition parameters as the nozzle height during the deposition, nozzle air pressure and deposition time were varied until we obtained devices with optimum electrical performance. Optimized devices presented mobilities in the order of 1 cm2.V-1.s-1, on/off ratio of about 106 and relatively low operation voltages. A statistical analysis of a great number of devices manufactured using the same deposition parameters was also carried out to assure the reproducibility of the deposition technique.
Self-assembled TiO2 films deposited by aqueous-spray deposition were investigated to evaluate morphology, crystalline phase, and infrared optical constants. The Anatase nano-crystalline film had ∼10 nm characteristic surface roughness sparsely punctuated by defects of not more than 200 nm amplitude. The film is highly transparent throughout the visible to wavelengths of 12 μm. The indirect band gap was determined to be 3.2 eV. Important for long-wave infrared applications is that dispersion in this region is weak compared with the more commonly used dielectric SiO2 for planar structures. An example application to a metal-insulator-metal resonant absorber is presented. The low-cost, large-area, atmospheric-pressure, chemical spray deposition method allows conformal fabrication on flexible substrates for long-wave infrared photonics.
During decomposition of copper formate, a volatile intermediate is formed, that can be utilized to fabricate conductive copper lines for electrical interconnections. By the method called Reactive Transfer Printing (RTP), a pattern of copper (II) formate was printed, and placed adjacent to a second surface; decomposition of the printed pattern led to a transfer of copper to the second substrate. It was found that the yield of the transfer process improved due to presence of several carboxylic acids which are liquid with a high boiling point. Furthermore we found that the transport of copper starts at a lower temperature than previously reported, indicating that the first decomposition step of copper formate is related to the catalytic decomposition of formic acid on a copper surface. The findings enable printing of conductive copper patterns onto the interior surface of a glass vessel.
Sputtered lead-free piezoelectric materials like potassium sodium niobate (K1-xNaxNbO3 or KNN) have received significant technological interest in recent years in light of several reports of piezoelectric constants comparable to lead zirconium titanate (PZT). Potential applications include self-powered sensors, actuators, and low acoustic impedance transducers. For large area printed applications, it is vital to develop low-temperature solution processed deposition methods. In this work, sol-gel synthesis of K-rich (70:30) KNN was carried out under an argon atmosphere, using acetate precursors, followed by precipitation of white KNN powder upon careful drying. Powder X-ray diffraction (XRD) scans of the product with a Cu Kα source after calcination revealed a dominant (110) peak, accompanied by smaller (100) and (010) peaks, in agreement with published standard KNN data. The composition of K-rich phase was confirmed using energy dispersive X-ray spectroscopy (EDX). To produce thin films, the sol was spin coated on a surface-treated Au-coated Si substrate, followed by slow annealing to obtain low surface roughness films (RMS roughness ﹤∼10 nm) of thickness ∼200 nm after solvent removal. Atomic force microscopy (AFM) scans revealed an unremarkable amorphous film. However, deposition of the sol on the Au-coated backside of Si wafer under similar processing conditions revealed limited polycrystalline film formation observed using optical profilometry. Thin film XRD measurements of the deposited film reveal orthorhombic phase growth of KNN, though the unannealed film was more amorphous than the calcined KNN film. Preliminary piezoresponse force microscopy (PFM) scans were used to estimate a piezoelectric constant (d33) ∼ 2.7 pC/N, consistent with the general expectation of lower piezoelectric constants for thin sol-gel films. The highest processing temperature used at any step during the deposition process was 90°C, consistent with the applications involving flexible polyimide substrates. This low-temperature thin-film growth suggests a potential route towards integration of large area piezoelectric generators for environmentally-friendly autonomous flexible sensor applications, with better control of phase and composition during the solution-phase deposition of KNN.
We report on highly sensitive and flexible L-lactate enzymatic sensors. The sensing materials of this biosensor, two-dimensional (2D) zinc oxide nanoflakes (ZnO NFs), are synthesized on flexible gold(Au)-coated polyethylene terephthalate (PET) substrate using one step sonochemical approach for non-invasive lactate monitoring in human perspiration. ZnO NFs show high isoelectric points (IEP) and biocompatibility. Taking advantage of these unique properties, we immobilized Lactate oxidase (LOx) on the synthesized ZnO NFs. PET/Au/ZnO NFs sensors demonstrated detection of lactate in the range of 10 pM-10 µM for the electrode area of 0.5×0.5 cm2. The sensitivity of this linker free lactate sensor was found 2.23μA/M /cm2 and shows 4 times better response than conventional Au electrode with linker.
In the current work, we evaluate the influence of the processing parameters on the electrical properties of aluminium zinc oxide (AZO) thin films produced by airbrush spray-pyrolysis deposition technique. Spray-deposited AZO thin-films were produced with Al:Zn molar ratios varying from 0 % (pure ZnO) up to 30 %, using aluminium acetate and zinc acetate as organic precursors and water as solvent. Thermogravimetric analysis (TGA) and infrared spectroscopy (FTIR-ATR) were used to monitor the metal-oxide formation from the organic precursors as a function of the temperature. The results show that a temperature of 400 °C is necessary to completely degrade the organic phase and to obtain the desired inorganic metal-oxides films. The electrical properties of the TMOs were evaluated by d.c. current-voltage (I-V) analysis using planar thermally evaporated Al electrodes on top of the TMO layer, with different aspect ratios (1/18, 2/9, 5/13, 5/9 and 8/9). The lowest sheet resistance was obtained for AZO films at a molar Al concentration of 5 %. We also observed that, after carrying out a post-annealing treatment (30 mbar, 150 °C) the samples presented a decrease on the sheet resistance superior to 60 %, in comparison to the samples before the treatment.
Ternary lead chalcogenides, such as PbSxSe1-x, offer the possibility of room-temperature infrared detection with engineered cut-off wavelengths within the important 3-5 micron mid-wave infrared (MWIR) wavelength range. We present growth and characterization of aqueous spray-deposited thin films of PbSSe. Complexing agents in the aqueous medium suppress unwanted homogeneous reactions so that growth occurs only by the heterogeneous reaction on the hydrophilic substrate. The strongly-adherent films are smooth with a mirror-like finish. The films comprise densely packed grains with tens of nm dimensions and a total film thickness of ∼400-500 nm. Measured optical constants reveal absorption out to at least 4.5 μm wavelength and a ∼0.3 eV bandgap intermediate between those of PbS and PbSe. The semiconducting films are p-type with resistivity ∼1 and 85 Ohm-cm at 300 and 80 K, respectively. Sharp x-ray diffraction peaks identify the films as Clausthalite-Galena solid-state solution with a lattice constant that indicates an even mixture of PbS and PbSe. The photoconductive response is observed at both nitrogen and room temperature up to at least 2 kHz chopping frequency.