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A series of LEDs that have a green CdSe@ZnS quantum dot (QD) emitting layer (EML) in response to treatment with formic acid (FA), 1,3-benzenedithiol (BDT), and 1,2-ethanedithiol (EDT) were fabricated and investigated. The initial long-chained ligands of oleic acid-based CdSe@ZnS QD layers were partially replaced by short-chained and more conductive ligands of FA, BDT, and EDT that can offer to improve the performance of QD-LEDs (QLEDs). Comparison of the operation characteristics of QLEDs without and with various solution treatments revealed that the control of charge injection and recombination conditions in QD EMLs is the key step for the improvement of the efficiency of our devices. The best device performance was observed for EDT treatment with the maximum luminance and luminous efficiency of 3776 cd m−2 and 1.47 cd A−1, respectively, which are increased by 487% and 270% compared with those of the untreated QLED.

Oxide thermoelectric materials are considered promising for high-temperature thermoelectric applications in terms of low cost, temperature stability, reversible reaction, and so on. Oxide materials have been intensively studied to suppress the defects and electronic charge carriers for many electronic device applications, but the studies with a high concentration of defects are limited. It desires to improve thermoelectric performance by enhancing its charge transport and lowering its lattice thermal conductivity. For this purpose, here, we modified the stoichiometry of cation and anion vacancies in two different systems to regulate the carrier concentration and explored their thermoelectric properties. Both cation and anion vacancies act as a donor of charge carriers and act as phonon scattering centers, decoupling the electrical conductivity and thermal conductivity.

This manuscript reports the synthesis and piezoelectric properties of strontium titanate, SrTiO3-modified bismuth sodium titanate-barium titanate, 0.965Bi0.5Na0.5TiO3–0.035BaTiO3, (BNBT-xST, x = 0.00−0.30) ceramics produced by facile low temperature sol–gel and hydrothermal methods. Close inspection of the X-ray diffraction profile through Rietveld refinement indicates ST modification in BNBT leads to a phase transition from the dominant rhombohedral phase to a single tetragonal phase. This structural transition leads to enhancement of electromechanical and energy storage properties. A large dynamic piezoelectric coefficient (d*33 = 350 pC/V) was observed for BNBT-0.1ST sample. High energy storage density (Wrec = 0.37) and large energy storage efficiency (η = 75%) were observed at 75 °C for the BNBT-0.3ST sample. The energy storage response of the tunable ferrorelectric ceramics suggests their possible use for high-performance dielectric capacitor applications.

Vertical heterostructures of graphene and MoS2 are realized by successive growth of MoS2 and graphene, each by chemical vapor deposition. The MoS2 coverage on graphene is controlled by ozone treatment of graphene, which affects the absorption and aggregation of the nucleation promoter perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) for MoS2 seeds on the graphene surface. Mostly single or bilayer MoS2 flakes are grown on graphene with negligible defects even after the ozone treatment and the high-temperature growth process (up to 650 °C). Efficient charge transfer in the grown heterostructures is observed in the photoluminescence and photoresponses of devices based on the heterostructures.

Large-scale growth of mostly monolayer molybdenum disulfide (MoS2) on quartz, sapphire, SiO2/Si, and waveguide substrates is demonstrated by chemical vapor deposition with the same growth parameters. Centimeter-scale areas with large flakes and films of MoS2 on all the growth substrates are observed. The atomic force microscopy and Raman measurements indicate the synthesized MoS2 is monolayer with high quality and uniformity. The MoS2 field effect transistors based on the as-grown MoS2 exhibit carrier mobility of 1–2 cm2V−1s−1 and On/Off ratio of ~104 while showing large photoresponse. Our results provide a simple approach to realize MoS2 on various substrates for electronics and optoelectronics applications.

Point defect or doping in Strontium titanium oxide (STO) largely determines the thermoelectric (TE) properties. So far, insufficient knowledge exists on the impact of double Schottky barrier on the TE performance. Herein, we report a drastic effect of double Schottky barrier on the TE performance in undoped STO. It demonstrates that incorporation of Reduced Graphene Oxide (RGO) into undoped STO weakens the double Schottky barrier and thereby results in a simultaneous increase in both carrier concentration and mobility of undoped STO. The enhanced mobility exhibits single crystal-like behavior. This increase in the carrier concentration and mobility boosts the electrical conductivity and power factor of undoped STO, which is attributed to the reduction of the double Schottky barrier height and/or the band alignment of STO and RGO that allow the charge transfer through the interface at grain boundaries. Furthermore, this STO/RGO interface also enhances the phonon scattering, which results in low thermal conductivity. This strategy significantly increases the ratio of σ/κ, resulting in an enhancement in ZT as compared with pure undoped STO. This study opens a new window to optimize the TE properties of many candidate materials.

Binary solid solution of gallium (Ga3+)-modified 0.65Bi1.05FeO3–0.35BaTiO3 (BFGO–BTO with 0–2 mol% Ga3+ content) ceramics were synthesized by a solid-state reaction method, followed by wate...

Controlling oxygen vacancy in undoped SrTiO3 was used to realize the phonon-glass electron-crystal (PGEC) concept. The undoped SrTiO3 samples were synthesized through solid-state-reaction method in air and then reduced through annealing under low oxygen partial pressure conditions. We show that decreasing the oxygen partial pressure (Po2) results in increase of the electrical conductivity, and the oxygen vacancies act as phonon scattering centers, which lead to decrease in the lattice thermal conductivity. Both the enhanced electrical conductivity (σ) and reduced thermal conductivity (κ) were achieved simultaneously through oxygen vacancy in undoped SrTiO3. This significantly increases the ratio of σ/κ that results in an enormously enhanced ZT value of about 13 times larger as compared with the samples reduced under relatively high Po2. Density-functional theory calculations also reveal that oxygen vacancies in SrTiO3 reduce the band-gap, and consequently increase the electrical conductivity.

An alternative oxide anode material to surpass Ni electrode in the solid oxide fuel cell (SOFC) technology, used at high temperature range (500–1000 °C), is needed for the SOFC industry. We investigated an advanced perovskite oxide electrode, CaVxO3-δ system, for the alternative oxide anode material. The CaVxO3-δ powder and bulk were fabricated by using an atmosphere controlled solid-state reaction method and spark plasma sintering (SPS), respectively. The bulk specimen showed 3-3 type nonstoichiometric composite structure, and the electrical conductivity and Seebeck coefficient measurements revealed metallic n-type conductivity. Surprisingly, the high electrical conductivity (> 12,000 S/cm at room temperature; >1000 S/cm at 1000 K) was stable at different atmospheres between air and reduction atmospheres, attributing to a non-redoxable electron-cloud-channel phase protected by a redoxable protection phase. It is believed that the CaVxO3-δ system can be used as a promising and atmosphere stable oxide electrode in the fuel cell technology and other applications.

Electrical characteristics of ion gels prepared by loading different amounts of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) in Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) are investigated and compared with those of ion liquid, [EMIM][TFSI] for possible application as a gate stack for flexible electronic devices. Capacitance and impedance as a function of frequency are measured, which can be well accounted for by a simple circuit model identifying the local device components. The operation of a flexible field effect transistor based on graphene and the ion gel as a top gate stack is also demonstrated.

This paper reports a novel synthesis of iron (Fe) powders in which high-purity and controlled primary and secondary assembled particles can be produced at low cost with safe process. Iron hydroxide powder precursors with fine-sized and acicular morphology were prepared and then reduced at an optimum temperature under reducing atmosphere by using single-step and two-step reduction. In the case of two-step reduction, the precursor powders were reduced by using a hydrogen/argon mixed gas for 1.5 h with a subsequent second reduction using higher hydrogen/argon mixed gas for 2 h. The morphology of the Fe powders strongly depends on the reduction conditions and precursor's morphology. The two-step reduced powders had pure iron powders with large elongated shape and porous sponge-like interlocking structure. The synthesized Fe powders were mixed with KClO4 and the pyrotechnic performance was examined. It is found that metal powder prepared by using two-step reduction is very suitable for producing porous sponge-like microstructure and for pyrotechnic applications.

It has been a difficulty to form well-distributed nano- and mesosized inclusions in a Bi2Te3-based matrix and thereby realizing no degradation of carrier mobility at interfaces between matrix and inclusions for high thermoelectric performances. Herein, we successfully synthesize multistructured thermoelectric Bi0.4Sb1.6Te3 materials with Fe-rich nanoprecipitates and sub-micron FeTe2 inclusions by a conventional solid-state reaction followed by melt-spinning and spark plasma sintering that could be a facile preparation method for scale-up production. This study presents a bismuth antimony telluride based thermoelectric material with a multiscale structure whose lattice thermal conductivity is drastically reduced with minimal degradation on its carrier mobility. This is possible because a carefully chosen FeTe2 incorporated in the matrix allows its interfacial valence band with the matrix to be aligned, leading to a significantly improved p-type thermoelectric power factor. Consequently, an impressively hig...

•High capacitance on In0.53Ga0.47As epitaxial layers grown on Si substrates•Achievement of capacitance equivalent thickness lower than 1.5nm with low leakage current•Enhancement of electrical performances by a surface treatment using trimethylaluminum.

Strontium titanium oxide co-doped with yttrium and nickel (SrxY1-xTiyNi1-yO3-δ; hereafter, SYTN), was investigated as an alternative anode material for solid oxide fuel cells. To improve the ionic conductivity of the Sr0.92Y0.08TiO3-δ (SYT) anode, Ni2+ was substituted into the B-site (initially occupied by Ti4+), thereby forming oxygen vacancies. To analyze the effects of Ni-doping in the SYT anode, the electrochemical properties of the SYTN anode were compared with those of the Ni-infiltrated SYT(Ni@SYT) using H2 and CH4 as fuels. The electrochemical reactions at the SYTN anode in the presence of both H2 and CH4 were limited by relatively slow reactions, such as non-charged processes including oxygen surface exchange and solid surface diffusion. The high electrical conductivity and excellent catalytic activity of the Ni nanoparticles in the Ni@SYT anode led to improved cell performance. CH4 decomposition at the Ni@SYT anode occurred via thermal pyrolysis of CH4 rather than by steam methane reforming, resulting in carbon deposition. In comparison, the poor inherent catalytic activity for CH4 oxidation exhibited by the SYTN anode minimized carbon deposition on the anode surface.

SrTiO3 (STO) is one of the most promising oxide thermoelectric (TE) materials with high thermopower, but the low electrical conductivity and high thermal conductivity are still issues to be solved. Recently, grain boundary engineering using reduced graphene oxide (RGO) has been demonstrated to have capable of simultaneously improving both the electrical and thermal properties of STO. In this study, we investigated the combined effect of La/Nb doping and RGO coating in STO to improve further the electrical conductivity and the TE efficiency of STO materials. Two series of La- and Nb-doped Sr1-xLaxTi1-yNbyO3 (x, y = 0–0.02) samples incorporated with RGO were prepared by a combinatorial procedure of solid-state reaction, RGO coating, and the spark plasma sintering (SPS) process. The structural, morphological and temperature-dependent thermoelectric properties of the Sr1-xLaxTi1-yNbyO3/RGO were studied and as a result, the figure of merit was enhanced ∼6.5 times by the combined effect of donor doping and lowering the double Schottky barrier height due to RGO at grain boundaries.

We report an in situ characterization of transition-metal dichalcogenide (TMD) monolayers and twisted bilayers using a high-speed second-harmonic generation (SHG) imaging technique. High-frequency laser modulation and galvano scanning in the SHG imaging enabled a rapid identification of the crystallinity in the TMD, including the orientation and homogeneity with a speed of 1 frame/s. For a twisted bilayer MoS2, we studied the SHG peak intensity and angles as a function of the twist angle under a strong interlayer coupling. In addition, rapid SHG imaging can be used to visualize laser-induced ablation of monolayer and bilayer MoS2 in situ under illumination by a strong femtosecond laser. Importantly, we observed a characteristic threshold behavior, the ablation process occurred for a very short time duration once the preheating condition was reached. We investigated the laser thinning of the bilayer MoS2 with different twist angles. When the twist angle was 0°, the SHG decreased by approximately one-fourth of the initial intensity when one layer was removed. Conversely, when the twist angle was approximately 60° (the SHG intensity was suppressed), the SHG increased abruptly close to that of the nearby monolayer when one layer was removed. Precise layer-by-layer control was possible because of the unique threshold behavior of the laser-induced ablation.

We investigate the interaction of carbon nanotubes (CNTs) embedded in a polymer matrix [poly(methyl methacrylate) (PMMA)] with Ar plasma, which results in the formation of PMMA nanostructures, as CNTs act as an etching mask. Because of the large differences in the Ar ion sputtering yields between CNTs and PMMA, PMMA lines with the width comparable to that of CNTs and as high as 20 nm (for single-walled CNTs) or 80 nm (for multiwalled CNTs) can be obtained after repeated exposure of CNT/PMMA films to Ar plasma. We also follow the etching process by investigating changes in the IV characteristics and Raman spectra of CNTs after each exposure to Ar plasma, which shows progressive defect generations in CNTs while they maintain structural integrity long enough to act as the etching mask for PMMA underneath. We demonstrate that the PMMA nanostructure patterns can be transferred to a different polymer substrate using nanoimprinting.

We investigate the thermoelectric properties of reduced graphene oxide (RGO) incorporated Bi0.36Sb1.64Te3 composites. A melt spinning process enables RGO to be uniformly distributed with Bi0.36Sb1.64Te3 matrix and the incorporated RGO increases the carrier mobility of Bi0.36Sb1.64Te3 matrix with the enhancement of the power factor by 20%. The grain sizes of the Bi0.36Sb1.64Te3 materials decrease with the RGO amount, leading to the reduction in lattice thermal conductivity by enhancing grain boundary scattering. Owing to the above effects, the thermoelectric figure of merit (ZT) can be enhanced in the measured temperature range, where the optimum ZT value is reached to 1.16 at 393 K, ∼15% higher ZT value than pristine Bi0.36Sb1.64Te3. Consequently, this RGO incorporation method could be widely used to improve thermoelectric performances in other conventional thermoelectric materials.

This paper demonstrates a time-of-flight mass spectrometer (TOFMS) fabricated using simple micro electromechanical system technologies. It consists of two components: a triode-type field emitter for electron-impact ionization and ion separator with a repeller, an acceleration electrode, and a flight tube. As a cold-cathode, the field emitter with carbon nanotube is a promising electron source and offers several advantages over conventional thermionic electron sources, such as compact size, low power consumption because of the absence of heating elements, excellent durability, and high electron emission density. Micromachined TOFMS successfully detects signals of Ar and CH3I. This is practically proved with preliminary experiments on ionization and mass spectrum tests with Ar and CH3I, ions, including finite element modeling and theoretical analysis.

We investigated the thermoelectric (TE) properties of a p-type oxide material (Li1−x NbO2, with x = 0–0.6). The composition was synthesized via a solid-state reaction method under a reducing atmosphere. The charge transport properties were determined through the electrical conductivity and Seebeck coefficient measurements. The electrical conductivity was non-monotonically varied with x value and showed metallic behavior with increased temperature and above 650 K temperature independent behavior dominated by extrinsic conduction. On the other hand, the Seebeck coefficient was increased with an increase in the temperature, and decreased gradually with an increase in the Li vacancy concentration by both synthesis and gradual phase transition to a Li-rich Li3NbO4 phase with temperature and appeared as an n-type TE at x = 0.6 under high temperatures, which was attributed to an Nb substitution into the Li site. The thermal conductivity was monotonically reduced with the increase in temperature due to the cation disorder defects and second phases. The Li vacancy induced Li1−x NbO2-based compounds under low oxygen partial pressure show promise as a candidate p-type material for thermoelectric applications, particularly for co-processing with n-type oxide thermoelectric materials fabricated under conditions of low oxygen partial pressure.

We report systematic efficiency variations of green-emitting CdSe@ZnS quantum-dot (QD) LEDs (QLEDs) in response to in situ treatments with 1,2-ethanedithiol (EDT) solutions at various concentrations. The main effect of in situ EDT treatment on a QD layer spin-coated onto a ZnO layer was vacuum-level shift due to dipole moments on the surface of the QD layer and at the interface between QD and ZnO layers. Competing contributions of these dipole moments were responsible for changes in energy level configurations and, accordingly, electron and hole barriers that resulted in discrepancies in electron- and hole-current variations. QLED efficiency was best when treated with an EDT solution of 4 mM, attributable to the largest increase in the hole- to electron current ratio. The maximum luminous yield of the 4 mM EDT-treated QLED was 5.43 cd A−1, which is 10 times higher than that of an untreated device. Furthermore, the luminous yield of this treated device remained as high as 2.56 cd A−1 at a luminance of 500 cd m−2.

We report on a substantial increase in luminance and luminous efficiency of green-light emitting devices (LEDs) that use colloidal CdSe@ZnS quantum dots (QDs) as a light-emitting material in response to treatment with 1,2-ethanedithiol (EDT). The maximum luminance increased from 1146 to 8075 cd m-2, and luminous yield from 0.15 to 1.41 cd A-1 as a result of treating an incomplete device with drops of EDT right after spin-coating QDs onto a ZnO-nanoparticle layer. Based on systematic studies on substrate-dependent change in photoluminescence, and current-voltage and luminance-voltage characteristics, we propose that passivation of intra-gap defect states and relative shifts of energy levels relevant to the operation of QD LEDs are two main results of EDT treatment. In particular, we argue that energy-level shift without emission-color change can be attributed to surface-dipole effects.

We report the enhanced thermoelectric properties in Cu x Bi 0.38 Sb 1.62 Te 3 compounds through simultaneous achievement of improved charge transport and suppressed thermal transport by Cu incorporation. Incorporated Cu acted as an acceptor rather than donor, resulting in the significant improvement of p -type electric conduction. Density of state effective mass was almost linearly proportional to the Cu content in the compounds, and it led to the enhancement of power factor in Cu-doped compounds regardless of the increase in hole concentration. Furthermore, reduction of bipolar and lattice thermal conductivities could also be achieved by the Cu incorporation, consequently leading to the improved thermoelectric properties of the compounds at relatively high temperatures. Effects of Cu on the thermal and charge transport properties are discussed in detail, and the controllability of the thermoelectric performance for optimum operating temperature is also reported.

Bi2Te3−ySey:Im (y = 0.15–0.6 and m = 0.0025–0.01) solid solutions were prepared by mechanical alloying and hot pressing. The lattice constants that were measured from x-ray diffraction patterns decreased linearly with increasing Se content, but they were not changed remarkably by I doping. The average relative densities of the hot-pressed specimens are higher than 97%. All of the specimens exhibited n-type conductions in the measured temperature range from 323 K to 523 K, and their electrical conductivity decreased slightly with increasing temperature, indicating degenerate semiconductor behaviors. The electrical conductivity decreased with increasing Se content, whereas it was increased by I doping, and this is in contrast with the Seebeck coefficient; this resulted from the changes of the electron concentrations due to the Se substitution and the I doping. The thermal conductivity decreased with increasing Se content, and this is the result of both the decreased electronic thermal conductivity due to the decreased carrier concentration and the decreased lattice thermal conductivity due to the increased alloy scattering. The maximum dimensionless figure of merit for Bi2Te2.4Se0.6, ZTmax = 0.84 at 473 K, is due to its low thermal conductivity and high Seebeck coefficient.

We have successfully synthesized the composites of Ag-doped zinc antimonide (Ag-ZnSb) and few-layer-graphene (FLG) by high-energy-mechanical milling and spark plasma sintering using Ag-ZnSb ingot and FLG as raw materials. The FLG/Ag-ZnSb shows 22%-reduced lattice thermal conductivity by increased phonon boundary scattering due to embedded FLG. The electrical conductivity of the composites was decreased due to reduced carrier concentration, while the Seebeck coefficient of the composites was increased by electron donating effect of FLG. The maximum figure of merit, ZT , of 0.86 was observed at 630 K. The peak ZT temperature in the FLG/Ag-ZnSb cases (~630 K) were higher than the pristine Ag-ZnSb compound (~580 K) resulting in advantages for the higher temperature power generation.

Iron (Fe) powders with high-purity and controlled primary and secondary assembled particles were synthesized with reduction process to control the particle shape and thereby variation of its electromagnetic properties. Iron precursors, such as iron hydroxide and iron oxide powder precursors, with fine-sized and acicular or spherical/cubic morphology, were prepared by co-precipitation and hydrothermal processes and then reduced at different reducing atmospheres by using a hydrogen/argon mixed gas. The electromagnetic properties were evaluated by dielectric permittivity and magnetic permeability and their loss measurements. The reduced Fe powders showed different particle shapes and sizes with different reduction conditions, which affected both primary and secondary particles and thereby electromagnetic properties. The morphology of the Fe powders strongly depended on the reduction conditions and precursor's morphology. The electromagnetic properties were varied with different shapes and sizes of primary and secondary particles. It was found that the permeability is better for interconnected secondary particles with smaller primary particle size, and the dielectric permittivity is much more variable than the magnetic permeability with different parameters mentioned above.

We fabricated high efficiency yellow-color organic light-emitting diodes (OLEDs) by co-doping 2,8-di(t-butyl)-5,11-di[4-(t-butyl)phenyl]-6,12-diphenylnaphthacene (TBRb) and tris(8-hydroxyquinolinato)aluminum (Alq 3 ) guests in a N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) host. Co-doping of electron-transporting Alq 3 with yellow dopant TBRb in hole-dominant NPB layers resulted in substantial luminance-yield improvement compared to TBRb-only counterparts without color contamination from Alq 3 green emission. The luminance yield of 11.4 cd/A, corresponding to the co-doping of 8% TBRb and 2% Alq 3 , is larger than previously reported luminance-yield values of conventional TBRb-based yellow OLEDs. Another advantage of the TBRb-Alq 3 co-doped OLEDs is an insignificant roll-off of efficiency at high current-density and/or brightness levels.

Bi2Te3-ySey (y = 0 - 0.6) solid solutions were prepared by using mechanical alloying and hot pressing. The lattice constants decreased with increasing Se content, which revealed the successful formation of solid solutions by using a planetary mill. The relative densities of hot-pressed specimens were higher than 96%. All specimens indicated n-type conductions in the measuring temperature range from 323 K to 523 K, and the electrical conductivity slightly decreased with increasing temperature. The Seebeck coefficient increased with increasing Se content, and the electrical and the thermal conductivities decreased; thus, the dimensionless figure of merit was improved. A maximum dimensionless figure of merit of 0.76 was obtained at 473 K for Bi2Te2.55Se0.45.

A significant challenge in nano and biophotonics is to demonstrate fully biocompatible nano-optics devices that can perform biofunctions in vivo. Here we present a scalable, cost-effective, and large-area nanofabrication method for creating a quasi-3D plasmonic crystal using poly(ethylene glycol) (PEG) and gold (Au), both biocompatible materials. The plasmonic crystal was prepared by depositing an Au layer on the upper hemisphere of the replicated PEG nanospheres array. Additionally we demonstrated that the fabricated plasmonic crystal can behave as a label-free glucose sensor with sensitivity and figure-of-merit values comparable to other plasmonic crystal based sensors.

Adhesion control of various liquid droplets on a liquid-repellent surface is a fundamental technique in novel open channel microfluidic systems. Herein, we demonstrate reversible liquid droplet adhesion switching on superamphiphobic Pd-decorated Ag dendrites (Pd/Ag dendrites). Although adhesion between liquids and the superamphiphobic surfaces was extremely low under ambient air, high adhesion was instantly achieved by exposure of the dendrites to 8% hydrogen gas. Transition from low to high adhesion and the reverse case were successfully repeated more than 10 times by switching from atmospheric ambient air to 8% hydrogen gas. This is the first technique that allows real-time reversible adhesion change with various liquid droplets to a surface using gas-induced structural changes and can potentially be used to realize various functions for droplet-based microfluidics.

We used electrostatic force microscopy (EFM) to investigate local conducting states of atomically thin individual graphene oxide (GO) sheets and monitor the spatial evolution of their conducting properties during the reduction process. Because of the thinness of the GO sheets and finite carrier density, the electric field is partially screened in the reduced GO, which is manifested in the EFM phase signals. We found inhomogeneous oxidation states in as-prepared GO sheets and followed the evolution of reduction process in the individual GO sheets during both thermal and chemical reduction. We also compared the EFM measurement results with simultaneous IV characteristics to assess correlations between two measurements.

A multi-inlet-multi-outlet (MIMO) superhydrophobic droplet guiding track was demonstrated for water droplet manipulation using an electrostatic force-induced trajectory switching system. Without applying an external electrostatic field, the water droplet rolled along the superhydrophobic guiding track due to its extreme water repellent properties and gravitational force. By applying a DC bias to a capacitor above the guiding track, the trajectory of the water droplet can be easily controlled by the electrostatic attraction. Electrostatically-induced trajectory switching was successfully achieved when the electrostatic and gravitational forces exerted on the water droplet were properly balanced. On a MIMO superhydrophobic droplet guiding track with three inlets and four outlets, the water droplet was guided along the intended trajectory.

We demonstrate the development of an effective technique to remove the poly methyl methacrylate (PMMA) layer used for transferring graphene synthesized by a chemical vapor deposition (CVD). This was achieved utilizing electron-beam bombardment and following developing processes, prior to the use of conventional organic solvents. Field-effect transistors were fabricated on the transferred graphene in order to explore their Dirac points and carrier motilities in the ambient condition - the results were then compared with those from the conventional wet chemical treatment. It was found that the Dirac points were located close to the zero gate bias when compared to those from the acetone and the acetic acid treatments. Most significantly, the field-effect mobility reached as high as 6770 cm2/Vs and 7350 cm2/Vs on average for holes and electrons, respectively, which is more than seven times improvement in comparison to conventional acetone treatments for CVD-grown graphene devices.

Scientific Reports 6: Article number: 35994; published online: 27 October 2016; updated: 16 March 2017. Ujwal Thakur was omitted from the author list in the original version of this Article. This has been corrected in the PDF and HTML versions of the Article, as well as the Supplementary Informationfile.

The thermal stability of the La0.9Fe3CoSb12 skutterudite was examined by varying the aging conditions of temperature, time, and atmosphere (air and vacuum). Thermogravimetry, differential scanning calorimetry and x-ray diffraction analyses revealed that the La0.9Fe3CoSb12 was significantly oxidized at temperatures above 673 K in air. The Sb-based oxides were preferably formed when aged at high temperatures in air. The thickness of the oxide layers was found to increase with increasing aging temperature and time, while the activation energy for the oxidation of the Fe-Sb-based skutterudite was lower than that of the Co-Sb-based skutterudites. However, when the analysis was performed in a vacuum, the La0.9Fe3CoSb12 skutterudite was stable, and no oxide layers were observed after aging at 823 K for 100 h. These results suggest that a protective coating for the skutterudite materials or the encapsulated packaging of the skutterudite modules is required for high-temperature applications.

We report fully solution-processed fabrication of transparent conducting oxide-free counter electrodes (CEs) for dye-sensitized solar cells (DSSCs) by combining spray-coating of single-wall carbon nanotubes (SWCNTs) and chemical reduction of chloroplatinic acid precursor to platinum nanoparticles (Pt NPs) with formic acid. The power conversion efficiency of a semitransparent DSSC with such SWCNT-based CE loaded with Pt NPs is comparable to that of a control device with a conventional CE. Quantification of Pt loading shows that network morphology of entangled SWCNTs is efficient in forming and retaining chemically reduced Pt NPs. Moreover, electron microscopy and electrochemical impedance spectroscopy results show that mainly Pt NPs, which are tens of nanometers in diameter and reside at the surface of SWCNT CEs, contribute to electrocatalytic activity for triiodide reduction, to which we attribute strong correlation between power conversion efficiency of DSSCs and time constant deduced from equivalent-circuit analysis of impedance spectra.

We report the fabrication of efficient indium-tin-oxide-free organic solar cells based on poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM). All layers of the devices from the lowermost silver nanowire cathode to the uppermost conducting polymer anode are deposited from solution and processed at plastic-compatible temperatures

The ionic and electronic conductivities of nonstoichiometric BaTiO 3 (undoped, Ca-doped, and Zr-doped BaTiO 3–δ ) ceramics were investigated through high temperature equilibrium conductivity (HiTEC) and in-situ impedance measurements at various equilibrium conditions with different oxygen partial pressures over a temperature range of 950–1050 °C. Contribution of mobile oxygen vacancies on the electrical conductivity has been determined by HiTEC measurement as a function of oxygen partial pressure; the electrical conductivity with mobile oxygen vacancies shows a broad transition from p -type to n -type, and thereby there is an increase of the minimum conductivity at the n – p transition point. Through combining in-situ impedance spectroscopy measurements with the HiTEC measurements, it was confirmed clearly that the mobile oxygen vacancy contributes to the total conductivity, and the oxides become mixed conductors around the n – p transition regime (minimum electronic conductivity regime). It was found that Warburg impedance can be observed at the condition of t ion /t electronic ≳ 0.05 in the temperature range of 950–1050 °C and p O 2 range of 0.95–10 − 16 atm. The ionic conductivity varied with the concentration of extrinsic oxygen vacancies and dopants, and the activation energy for mobility of oxygen vacancy in Ca-doped BaTiO 3–δ was found to be 1.04 ± 0.05 eV using the two techniques in a very good agreement.

Bio-inspired superhydrophobic surfaces have attracted considerable attention due to their potential applications. Although various techniques to fabricate artificial superhydrophobic surfaces have been demonstrated, most of the methods lack water adherence or controllable wetting properties of the surfaces, which hinders their practical usage. In this paper, we present a simple approach to fabricate water-adhesive superhydrophobic silicon nanowire (Si NW) surfaces by applying a thermal annealing treatment in oxygen ambient. The Si NW arrays were fabricated using a metal assisted chemical etching method. After the cycled rapid thermal annealing (RTA) process at 1000 °C under oxygen ambient, the water contact angle of the Si NW surface changed dramatically from 0 to 154.3° with high water-adhesive properties. This drastic change of the wettability could be attributed to the formed siloxane groups (− Si–O–Si–) on the thermally-treated Si NW surfaces; H 2 O is released from two adjacent silanol groups (–Si–O–H) to form siloxane groups during the RTA process. When the annealed Si NW was exposed in air, the wettability of the superhydrophobic Si NW was reconverted due to the re-formation of silanol groups (–Si–O–H). The wettability conversion of Si NW between superhydrophilic and superhydrophobic was repeated with good reversibility.

Vertically aligned arrays of ultralong ZnO nanowires were synthesized on SiO2 substrates with carbothermal vapor phase transport method with Au seeding layer. High density of vertically aligned ZnO nanowires with lengths from a few to ∼300 μm could be grown by controlling growth conditions. Supply of high concentration of Zn vapor and control of the ratio between Zn vapor and oxygen are found to have the most significant effects on the growth of long ZnO nanowires in the vapor–solid growth mechanism. The nanowires are of high crystalline quality as confirmed by various structural, compositional, and luminescent measurements. Luminescent and electrical properties of ZnO nanowires with different growth conditions were also investigated.

We report the rapid synthesis of millimeter-long vertically-aligned carbon-nanotubes (VACNTs) by hot-filament chemical-vapor-deposition without the use of water vapor. The growth rate increased initially up to ~ 190 μm/min but decreased thereafter resulting in the growth of up to 2.2 ± 0.2 mm in 23 ± 2 min. A thermodynamic model driven by a carbon-concentration gradient can account for very rapid initial growth with Arrhenius-type exponential temperature dependence. Another model devised for the quantitative elucidation of the monotonic decrease in growth-rate and quasi saturation of VACNT growth confirmed that the growth kinetics of VACNTs are controlled by the concomitant contribution of a diffusion-limited precursor supply and reaction-driven catalyst deactivation.

Cadmium-Iron-Sulphide (Cd-Fe-S) thin films have prepared on indium doped tin oxide (ITO) coated con- ducting glass substrates by potentiostatic electrodeposition technique. X-ray diffraction pattern shows that the deposited films exhibit mixture of hexagonal CdS and hexagonal FeS phases. Surface morphology and film composition represent that films with smooth surface and better stoichiometry are obtained at 0.0375 M CdSO4 concentration. Optical parameters such as band gap, refractive index and extinction coefficient which are evaluated from optical absorption measurements. The experimental observations are discussed in detail.

Graphene, functionalized with oleylamine (OA) and soluble in non-polar organic solvents, was produced on a large scale with a high yield by combining the Hummers process for graphite oxidation, an amine-coupling process to make OA-functionalized graphite oxide (OA-GO), and a novel reduction process using trioctylphosphine (TOP). TOP acts as both a reducing agent and an aggregation-prevention surfactant in the reduction of OA-GO in 1,2-dichlorobenzene (DCB). The reduction of OA-GO is confirmed by X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, and Raman spectroscopy. The exfoliation of GO, OA-GO, and OA-functionalized graphene (OA-G) is verified by atomic force microscopy. The conductivity of TOP-reduced OA-G, which is deduced from the current–voltage characteristics of a vacuum-filtered thin film, shows that the reduction of functionalized GO by TOP is as effective as the reduction of GO by hydrazine.

Organic bulk-heterojunction solar cells using thin-film single-walled carbon-nanotube (SWCNT) anodes deposited on glass are reported. Two types of SWCNT films are investigated: spin-coated films from dichloroethane (DCE), and spray-coated films from deionized water using sodium dodecyl sulphate (SDS) or sodium dodecyl benzene sulphonate (SDBS) as the surfactant. All of the films are found to be mechanically robust, with no tendency to delaminate from the underlying substrate during handling. Acid treatment with HNO 3 yields high conductivities >1000 S cm ―1 for all of the films, with values of up to 7694 ± 800 S cm ―1 being obtained when using SDS as the surfactant. Sheet resistances of around 100 Ω sq ―1 are obtained at reasonable transmission, for example, 128 ± 2 Ω sq ―1 at 90% for DCE, 57 ± 3 Ω sq ―1 at 65% for H 2 O:SDS, and 68 ± 5 Ω sq ―1 at 70% for H 2 O:SDBS. Solar cells are fabricated by successively coating the SWCNT films with poly(3,4-ethylenedioxythiophene) : p o l y(st y rene sulphonate) (PEDOT:PSS), a blend of regioregular poly(3-hexylthiophene) (P3HT) and 1-(3-methoxy-carbonyl)-propyl-1-phenyl-(6,6)C 61 (PCBM), and LiF/Al. The resultant devices have respective power conversions of 2.3, 2.2 and 1.2% for DCE, H 2 O:SDS and H 2 O:SDBS, with the first two being at a virtual parity with reference devices using ITO-coated glass as the anode (2.3%).

We developed a room-temperature encapsulation process based on multi-stack of ultra thin Al 2 O 3 and polyurea layers for top-emission organic light-emitting devices (TEOLEDs). Device structure, including a capping layer for refractive-index matching and a thick polyurea buffer layer, was optimized to enhance light extraction without distorting electroluminescence spectrum. The efficiency of a TEOLED encapsulated with 5 pairs of Al 2 O 3 (50 nm)/polyurea(20 nm) layers was better than that of a glass-encapsulated TEOLED, whereas their color coordinates were almost identical. Moreover, the half-decay lifetime of a TEOLED encapsulated with 5 pairs of Al 2 O 3 /polyurea layers was 86% of that of a glass-encapsulated TEOLED. Water vapor transition rate of 5 pairs of Al 2 O 3 (50 nm)/polyurea(20 nm) layers on PET film was measured as low as 5 × 10 −4 g/m 2 day.

Nonlinear optical characteristics of single-walled carbon nanotubes (SWCNTs) dispersed in dichlorobenzene and imbedded in polymer were investigated at 800 nm using the time-resolved optical Kerr gate technique. For systematic study of the influence of SWCNT bundling on optical nonlinearities, SWCNT solutions with different concentrations and a series of SWCNT/polymer composites deposited on glass substrates with different concentrations and thicknesses were prepared. The nonlinear response was comparable to the pulse duration of the laser used (∼90 fs) both in SWCNT solutions and SWCNT/polymer composites. Over three orders of magnitude enhancement was observed in the third-order nonlinear susceptibility of SWCNT/polymer composite film compared with that of SWCNT solution. An appreciable reduction of microscopic and macroscopic nonlinearities was observed with increasing SWCNT concentrations due to stronger bundling of SWCNTs.

Growth of a dense forest of vertically-aligned carbon-nanotubes (VA-CNTs) by a hot filament chemical vapor deposition (HFCVD) method was investigated. The growth of very pure VA-CNTs via the bottom growth mechanism was extremely fast during the initial few minutes, but it slowed down and reached the terminal height of a couple of millimeters. The different temporal variations of the VA-CNT height and growth rates corresponding to different growth stages indicated that a temperature-mediated thermodynamic model, precursor diffusion, and catalyst deactivation were responsible for the VA-CNT growth in respective stages.

This paper demonstrates a number of features that can contribute to BaTiO3 being perturbed from an equilibrium condition. The BaTiO3 synthesis study was conducted with a modified citrate process, but the kinetic aspects limiting the equilibrium state are believed to be general to any processing route. The equilibrium conditions can be determined through a detailed analysis of the paraelectric–ferroelectric phase transition data measured by differential scanning calorimeter. Broadened and multiple latent heat peaks are found indicating a distribution of slightly different ferroelectric transition temperatures for nonequilibrated BaTiO3 materials. A number of factors have been found that limit the equilibrium conditions, and these include time at formation conditions of temperature and oxygen partial pressure, a second phase BaCO3, and the rapid nucleation of a BaTi2O5 phase on quenching to room temperature from temperatures in and around 1250°C. All of these nonequilibrium factors that occurs at high temperature lead to the ferroelectric phase transition having multiple phase transitions, owing to regions of the BaTiO3 having different partial Schottky concentrations.