Fabricating low loss waveguides over a large depth in glass by temperature gradient assisted femtosecond laser writing.

We propose a strategy of temperature gradient assisted femtosecond laser writing for elaboration of low loss waveguides (WGs) over a large depth in glass. The matter flow driven by the temperature distribution is responsible for forming a highly densified WG core with tunable size. Importantly, the unique position of the guiding core outside the focus allows for abating the influence of laser energy redistribution and inscribing low loss deep WGs. A low insertion loss (Li) of 0.6 dB at 1550 nm is achieved for WGs at the depth from 300 µm to 900 µm. Establishing strong dependence of Li on the WG size offers a unique route to improve WG performance. These findings highlight that the present method would provide new opportunities for creating low loss WG lattices at large depth.

Direct Synthesis of Colorful Single-Walled Carbon Nanotube Thin Films.

In floating catalyst chemical vapor deposition (FC-CVD), tuning chirality distribution and obtaining narrow chirality distribution of single-walled carbon nanotubes (SWCNTs) is challenging. Herein, by introducing various amount of CO2 in FC-CVD using CO as a carbon source, we have succeeded in directly synthesizing SWCNT films with tunable chirality distribution as well as tunable colors. In particular, with 0.25 and 0.37 volume percent of CO2, the SWCNT films display green and brown colors, respectively. We ascribed various colors to suitable diameter and narrow chirality distribution of SWCNTs. Additionally, by optimizing reactor temperature, we achieved much narrower ( n, m) distribution clustered around (11,9) with extremely narrow diameter range (>98% between 1.2 and 1.5 nm). We propose that CO2 may affect CO disproportionation and nucleation modes of SWCNTs, resulting in SWCNTs' various diameter ranges. Our work could provide a new route for high-yield and direct synthesis of SWCNTs with narrow chirality distribution and offer potential applications in electronics, such as touch sensors or transistors.

Well-dispersed CoS nanoparticles on a functionalized graphene nanosheet surface: a counter electrode of dye-sensitized solar cells.

With a facile electrophoretic deposition and chemical bath process, CoS nanoparticles have been uniformly dispersed on the surface of the functionalized graphene nanosheets (FGNS). The composite was employed as a counter electrode of dye-sensitized solar cells (DSSCs), which yielded a power conversion efficiency of 5.54 %. It is found that this efficiency is higher than those of DSSCs based on the non-uniform CoS nanoparticles on FGNS (4.45 %) and built on the naked CoS nanoparticles (4.79 %). The achieved efficiency of our cost-effective DSSC is also comparable to that of noble metal Pt-based DSSC (5.90 %). Our studies have revealed that both the exceptional electrical conductivity of the FGNS and the excellent catalytic activity of the CoS nanoparticles improve the conversion efficiency of the uniformly FGNS-CoS composite counter electrode. The electrochemical impedance spectra, cyclic voltammetry, and Tafel polarization have evidenced the best catalytic activity and the fastest electron transport. Additionally, the dispersion condition of CoS nanoparticles on FGNS plays an important role for catalytic reduction of I3 (-) .

A Fabric-Based Strain Sensor with a Microbridge Structure and the Supercapacitor-Powered Integrated Sensing System.

Developing fabric-based strain sensors with high sensitivity and stability is in high demand for wearable electronics. Herein, carbon nanotubes (CNTs) and polypyrrole (PPy) are coated on a thermoplastic polyurethane (TPU) fabric as strain sensors. A microbridge structure, in which CNT bridges the stretching-induced cracks, has been designed for the TPU-CNT-PPy strain sensor. The microbridge structure can significantly enhance the electrical resilience, ensuring the improved sensitivity and stability of strain sensors. As a result, our TPU-CNT-PPy strain sensors deliver high sensitivity (GF = 231.5) with a broad working range (150%) and fast response and recovery time (166/195 ms). In addition, our TPU-CNT-PPy could also be used as flexible electrodes of the microsupercapacitors (MSCs) as a power supplier for the integrated sensing system. The TPU-CNT-PPy-based MSCs exhibit a high specific capacitance (460.3 mF cm-2 at 0.5 mA cm-2) and excellent cycling stability (96.69% capacitance retention for 10,000 charge/discharge cycles). Finally, we demonstrated an integrated sensing system using TPU-CNT-PPy as both MSCs and strain sensors, where the current signals of the sensors could be well detected via Bluetooth. This study offers a microbridge strategy to fabricate strain sensors with high sensitivity and stability and develops an integrated sensing system for the actual applications of wearable electronics.

Electromagnetic Wave-Absorbing and Bending Properties of Three-Dimensional Gradient Woven Composites with Triangular Sections.

In order to solve defects such as poor integrity, delamination failure, and narrow absorption bandwidth, three-dimensional (3D) gradient honeycomb woven composites (GHWCs) with triangular sections were designed and prepared. Three-dimensional gradient honeycomb woven fabric was crafted with carbon fiber (CF) filaments and basalt fiber (BF) filaments as raw materials on an ordinary loom. Then, the 3D honeycomb woven fabric filled with rigid polyurethane foam was used as the reinforcement, and epoxy resin (EP) doped with carbon black (CB) and carbonyl iron powder (CIP) was conducted as the matrix. The 3D GHWC with triangular sections, which had both EM-absorbing and load-bearing functions, was prepared by the VARTM process. Through the macro test and micro characterization of 3D GHWCs with triangular sections, the overall absorbing properties and mechanical properties of the materials were analyzed. Moreover, the EM-absorbing mechanism and failure mode of the materials were clarified in this work. The results indicated that the CF filament reflective layer effectively improved the EM-absorbing and mechanical properties. Adding a CB/CIP-absorbing agent enhanced the overall EM-absorbing property but reduced the mechanical properties. The increasing number of gradient layers increased the maximum bending load, but the EM-absorbing performance first increased and then decreased. When the thickness was 15 mm, the maximum bending load was 3530 N, and the minimum reflection loss (RLmin) was -21.6 dB. The synergistic effects of EM-absorbing and mechanical properties were the best right now. In addition, this work provided a feasible strategy that adjusting the type of absorber and gradient aperture size ratio could meet the unique requirements of absorbing frequency and intensity, which has excellent application prospects in civil and military fields.

Colors of Single-Wall Carbon Nanotubes.

Although single-wall carbon nanotubes (SWCNTs) exhibit various colors in suspension, directly synthesized SWCNT films usually appear black. Recently, a unique one-step method for directly fabricating green and brown films has been developed. Such remarkable progress, however, has brought up several new questions. The coloration mechanism, potentially achievable colors, and color controllability of SWCNTs are unknown. Here, a quantitative model is reported that can predict the specific colors of SWCNT films and unambiguously identify the coloration mechanism. Using this model, colors of 466 different SWCNT species are calculated, which reveals a broad spectrum of potentially achievable colors of SWCNTs. The calculated colors are in excellent agreement with existing experimental data. Furthermore, the theory predicts the existence of many brilliantly colored SWCNT films, which are experimentally expected. This study shows that SWCNTs as a form of pure carbon, can display a full spectrum of vivid colors, which is expected to complement the general understanding of carbon materials.

Enhanced Tunneling in a Hybrid of Single-Walled Carbon Nanotubes and Graphene.

Transparent and conductive films (TCFs) are of great technological importance. Their high transmittance, electrical conductivity, and mechanical strength make single-walled carbon nanotubes (SWCNTs) a good candidate for the raw material for TCFs. Despite the ballistic transport in individual SWCNTs, electrical conductivity of SWCNT networks is limited by low efficiency of charge tunneling between the tube elements. Here, we demonstrate that the nanotube network sheet resistance at high optical transmittance is decreased by more than 50% when fabricated on graphene. This is a comparable improvement as that obtained through gold chloride (AuCl3) doping. However, while Raman spectroscopy reveals substantial changes in spectral features of AuCl3 doped nanotubes, this does not occur with graphene. Instead, temperature-dependent transport measurements indicate that a graphene substrate reduces the tunneling barrier heights, while its parallel conductivity contribution is almost negligible. Finally, we show that combining the graphene substrate and AuCl3 doping, brings the SWCNT thin film sheet resistance down to 36 Ω/□.

Wafer-Scale Thermophoretic Dry Deposition of Single-Walled Carbon Nanotube Thin Films.

We report the direct and dry deposition of transparent conducting films (TCFs) of aerosol-synthesized single-walled carbon nanotubes (SWNTs) using a thermophoretic precipitator (TP) designed for the uniform and efficient deposition of aerosol-synthesized nanomaterials on 50 mm wafers or similarly sized polymer substrates. The optical and electrical performance of the fabricated TCFs match or surpass the published results achieved using a filter-based collection of aerosol-synthesized SWNTs, and TCFs with sheet resistances of 60 Ω/sq. at 87.8% transmittance and 199 Ω/sq. at 96% transmittance on flexible polymer substrates are demonstrated. The precipitator design is immediately applicable in roll-to-roll fabrication of SWNT TCFs or other functional coatings of aerosol-synthesized nanomaterials.

Floating catalyst CVD synthesis of single walled carbon nanotubes from ethylene for high performance transparent electrodes.

We have developed the floating catalyst chemical vapor deposition (FCCVD) synthesis of single walled carbon nanotubes (SWCNTs) using C2H4 hydrocarbon as a carbon source and iron nanoparticles as the catalyst in an environmentally friendly and economical process. For the first time, ethylene was used as the only carbon source in FCCVD with N2 as the main carrier gas. No sulphur and less than 15% H2 in a N2 carrier gas were used. By varying the ferrocene concentration, the diameter of the SWCNTs was tuned in the range of 1.3-1.5 nm with the optimized control of ferrocene concentration. The process produced SWCNTs with an average length of 13 µm and with a low level of bundling, that is a high proportion (28%) of individual tubes. The electron diffraction (ED) pattern indicated a random chirality distribution of the tubes between armchair and zigzag structures. The ED analysis also revealed that 35-38% of tubes are metallic. As a result of having long SWCNTs with a low level of bundling and a high fraction of metallic tubes, we produced a highly conductive transparent film with a sheet resistance of 51 Ohm per sq. for 90% transmission at 550 nm after HNO3 treatment, this being one of the lowest sheet resistance values reported for SWCNT thin films.

Highly conductive and transparent single-walled carbon nanotube thin films from ethanol by floating catalyst chemical vapor deposition.

Single-walled carbon nanotube (SWCNT) films have great potential to replace indium tin oxide films for applications in transparent and conductive electronics. Here we report a high yield production of SWCNT transparent conducting films (TCFs) by the floating catalyst chemical vapor deposition method using ethanol as the carbon source. To the best of our knowledge, this is the first report regarding SWCNT TCFs using ethanol as the carbon source. The fabricated uniform SWCNT TCFs exhibit a competitive sheet resistance of 95 Ω sq-1 at 90% transmittance after doping with AuCl3. The SWCNT TCFs possess high quality and the mean length of SWCNT bundles is approximately 27.4 µm. Furthermore, the concentration of semiconducting SWCNTs is 75-77%. Additionally, the chirality maps obtained from electron diffraction analysis demonstrate that our SWCNTs are biased towards the armchair type.

In situ synthesis of a NiS/Ni3S2 nanorod composite array on Ni foil as a FTO-free counter electrode for dye-sensitized solar cells.

A NiS/Ni3S2 nanorod composite array that directly grows on Ni foil has been used as a counter electrode for dye-sensitized solar cells; these nickel sulfide nanorods exhibit excellent photo-electrical conversion efficiency when compared with conventional noble-metal Pt electrodes.

Fe3W3C/WC/graphitic carbon ternary nanojunction hybrids for dye-sensitized solar cells.

Fe3 W3 C/WC/graphitic carbon (GC) ternary nanojunction hybrids are synthesized through a solid-state pyrolysis process for dye-sensitized solar cells (DSSCs). First-principles calculations have been first employed to investigate the adsorption energy between I3 (-) and Fe3 W3 C and WC nanoclusters. Scanning Kelvin probe images indicate that the work function changes greatly due to the formation of ternary nanojunctions, which favor fast photoelectron transfer. A photoelectrical conversion efficiency of 7.1 % is achieved based on Fe3 W3 C/WC/GC hybrid counter electrodes, which is much higher than those of pure GC (5.02 %) and WC/GC hybrids (6.11 %). It has been further revealed that Fe3 W3 C/WC/GC hybrid counter electrodes exhibit the best catalytic performances according to relevant electrochemical measurements, which can be attributed to fast photoelectron transfer due to the ternary junctions and the addition of Fe3 W3 C with more catalytic metallic atoms.

Facile synthesis of high-crystallinity graphitic carbon/Fe3C nanocomposites as counter electrodes for high-efficiency dye-sensitized solar cells.

Because of the advantages of both rapid electron transport of graphitic carbon and high catalytic performance of Fe3C nanoparticle, highly crystalline graphitic carbon (GC)/Fe3C nanocomposites have been prepared by a facile solid-state pyrolysis approach and used as counter electrode materials for high-efficiency dye-sensitized solar cells (DSSCs). The content of Fe3C in the composites can be modified by different hydrochloric acid treatment time. In comparison with pure highly crystalline GC, the DSSC based on GC/Fe3C nanocomposite with 13.5 wt % Fe3C content shows higher conversion efficiency (6.04%), which indicates a comparable performance to the Pt-based DSSC (6.4%) as well. Moreover, not only does our DSSCs have comparable performance to that of the Pt-based DSSC (6.4%), but also is more cost-effective as well. To evaluate the chemical catalysis and stability of nanocomposite counter electrodes toward I3(-) reduction and the interfacial charge transfer properties, GC/Fe3C nanocomposites have been quantitatively characterized by cyclic voltammetry, electrochemical impedance spectra, and Tafel polarization curve. All the results have revealed that the GC/Fe3C nanocomposite counter electrodes can exhibit high catalytic performance and fast interfacial electron transfer, which can be acted as a very promising and high cost-effective materital for DSSCs.