The Relevant Approaches for Aligning Carbon Nanotubes

Carbon-nanotube (CNT) is a promising material owing to its compelling mechanical, thermal and electrical properties and has been applied in a broad variety of fields such as composite, fiber, film and microelectronic. Although the introductions of CNT have brought huge improvement for many applications, these properties of macrostructures prepared by CNTs still cannot meet those of individual CNT. Disordered alignment of CNTs in the matrix results in degradation of performance and hinders further application. Nowadays, quantities of methods are being researched to realize alignments of CNTs. In this paper, we introduce the application of CNTs and review some typical pathways for vertical and horizontal alignment, including chemical vapor disposition, vertical self-assembly, external force, film assisted, electric field, magnetic field and printing. Besides that, advantages and disadvantages of specific methods are also discussed. We believe that these efforts will contribute to further understanding the nature of aligned CNT and generating more effective ideas to the relevant workers.

Mode-tunable, micro/nanoscale electrohydrodynamic deposition techniques for optoelectronic device fabrication

The rapid development of fascinating new optoelectronic materials and devices calls for the innovative production of micro/nanostructures in a high-resolution, large-scale, low-cost fashion, preferably compatible with flexible/wearable applications. Powerful electrohydrodynamic (EHD) deposition techniques, which generate micro/nanostructures using high electrical forces, exhibit unique advantages in high printing resolution (<1 μm), tunable printing modes (electrospray for films, electrospinning for fibers and EHD jet printing for dots), and wide material applicability (viscosity 1-10 000 cps), making them attractive in the fabrication of high-density and high-tech optoelectronic devices. This review highlights recent advances related to EHD-deposited optoelectronics, ranging from solar cells, photodetectors, and light-emitting diodes, to transparent electrodes, with detailed descriptions of the EHD-based jetting mechanism, ink formulation requirements and corresponding jetting modes to obtain functional micro/nanostructures. Finally, a brief summary and an outlook on the future perspectives are proposed.

Electronic properties of carbon nanotubes as detected by photoemission and inverse photoemission

The relation between morphology and energy level alignment in carbon nanotubes (CNT) is a crucial information for the optimization of applications in nanoelectronics, optics, mechanics and (bio)chemistry. Here we present a study of the relation between the electronic properties and the morphology of single wall CNT (SWCNT), aligned multi wall CNT (MWCNT) and unaligned MWCNT. The CNT were synthesized via catalytic chemical vapor deposition in ultra-high vacuum conditions. Combined ultraviolet photoemission and inverse photoemission (IPES) spectra reveal a high sensitivity to the nanotube morphology. In the case of unaligned SWCNT the distinctive unoccupied Van Hove singularities (vHs) features are observed in the high resolution IPES spectra. Those features are assigned to semiconducting and metallic SWCNT states, according to calculated vHs DOS. The two MWCNT samples are similar in the diameter of the tube (about 15 nm) and present similar filled and empty electronic states, although the measured features in the aligned MWCNT are more defined. Noteworthy, interlayer states are also revealed. Their intensities are directly related to the MWCNT alignment. Focussing and geometrical effects associated to the MWCNT alignment are discussed to account the spectral differences.

Direct Printing of Asymmetric Electrodes for Improving Charge Injection/Extraction in Organic Electronics

Engineering the energy levels of organic conducting materials can be useful for developing high-performance organic field-effect transistors (OFETs), whose electrodes must be well controlled to facilitate easy charge carrier transport from the source to drain through an active channel. However, symmetric source and drain electrodes that have the same energy levels are inevitably unfavorable for either charge injection or charge extraction. In this study, asymmetric source and drain electrodes are simply prepared using the electrohydrodynamic (EHD)-jet printing technique after the careful work function engineering of organic conducting material composites. Two types of additives effectively tune the energy levels of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate-based composites. These solutions are alternately patterned using the EHD-jet printing process, where the use of an electric field makes fine jet control that enables to directly print asymmetric electrodes. The asymmetric combination of EHD-printed electrodes helps in obtaining advanced charge transport properties in p-type and n-type OFETs, as well as their organic complementary inverters. This strategy is believed to provide useful guidelines for the facile patterning of asymmetric electrodes, enabling the desirable properties of charge injection and extraction to be achieved in organic electronic devices.

Direct Patterned Zinc-Tin-Oxide for Solution-Processed Thin-Film Transistors and Complementary Inverter through Electrohydrodynamic Jet Printing

The solution-processed deposition of metal-oxide semiconducting materials enables the fabrication of large-area and low-cost electronic devices by using printing technologies. Additionally, the simple patterning process of these types of materials become an important issue, as it can simplify the cost and process of fabricating electronics such as thin-film transistors (TFTs). In this study, using the electrohydrodynamic (EHD) jet printing technique, we fabricated directly patterned zinc-tin-oxide (ZTO) semiconductors as the active layers of TFTs. The straight lines of ZTO semiconductors were successfully drawn using a highly soluble and homogeneous solution that comprises zinc acrylate and tin-chloride precursors. Besides, we found the optimum condition for the fabrication of ZTO oxide layers by analyzing the thermal effect in processing. Using the optimized condition, the resulting devices exhibited satisfactory TFT characteristics with conventional electrodes and conducting materials. Furthermore, these metal-oxide TFTs were successfully applied to complementary inverter with conventional p-type organic semiconductor-based TFT, showing high quality of voltage transfer characteristics. Thus, these printed ZTO TFT results demonstrated that solution processable metal-oxide transistors are promising for the realization of a more sustainable and printable next-generation industrial technology.

Printed gas sensors

This review presents the recent development of printed gas sensors based on functional inks.

Electrohydrodynamic Direct-Writing Micropatterns with Assisted Airflow

Electrohydrodynamic direct-writing (EDW) is a developing technology for high-resolution printing. How to decrease the line width and improve the deposition accuracy of direct-written patterns has been the key to the promotion for the further application of EDW. In this paper, an airflow-assisted spinneret for electrohydrodynamic direct-writing was designed. An assisted laminar airflow was introduced to the EDW process, which provided an additional stretching and constraining force on the jet to reduce the surrounding interferences and enhance jet stability. The flow field and the electric field around the spinneret were simulated to direct the structure design of the airflow-assisted spinneret. Then, a series of experiments were conducted, and the results verified the spinneret design and demonstrated a stable ejection of jet in the EDW process. With assisted airflow, the uniformity of printed patterns and the deposition position accuracy of a charged jet can be improved. Complex patterns with positioning errors of less than 5% have been printed and characterized, which provide an effective way to promote the integration of micro/nanosystems.