Wen-Peng Han

Recent advances in flexible and stretchable electronic devices via electrospinning

Due to a variety of outstanding properties, such as large surface area, high length-to-diameter ratio, flexible surface functionality, tunable surface morphologies and superior mechanical performance, electrospun ultrathin fibers are suitable for flexible and stretchable devices, which have aroused much attention nowadays. In this review, we aim to summarize recent developments in the fabrication of flexible/stretchable electronic devices via electrospinning, including strain and pressure sensors, supercapacitors, organic field-effect transistors, and transparent electrodes, which are the key components of flexible/stretchable devices. Moreover, in order to further improve the performance of these devices, some challenges facing electrospun fibers (e.g. production on large scale, precise deposition and flexibility improvement of electrospun inorganic fibers) and subsequent integration for flexible/stretchable electronic devices have also been discussed.

Eu2+/Eu3+-emission-ratio-tunable CaZr(PO4)2:Eu phosphors synthesized in air atmosphere for potential white light-emitting deep UV LEDs

Mixed-valence Eu-doped CaZr(PO4)2 was prepared in air atmosphere by a two-step solid-state reaction for the first time. The reduction of Eu3+ to Eu2+ in air atmosphere was observed in the second synthesis step via the use of NH4H2PO4 and the first-step product CaZrO3:Eu3+ to synthesize CaZr(PO4)2:Eu, which strengthens the evidence linking the tetrahedral anion groups PO4 with the Eu reduction. This reduction mechanism is explained by a charge compensation model. The luminescence measurements indicate that the obtained phosphors exhibit a broad bluish-green fluorescence of Eu2+ and a sharp orange-red emission of Eu3+ on 200–350 nm ultraviolet (UV) excitations. A tunable white luminescence covering the whole visible spectrum is realized based on the gradual reduction of Eu3+ to Eu2+ with increasing the sintering time. The optimal white luminescence excited at 247 nm presents the CIE color coordinates of (0.322, 0.305), a correlated color temperature of 6243 K, and a good thermal stability. Furthermore, energy transfer from Eu2+ to Eu3+ is found in CaZr(PO4)2:Eu. Our investigation of CaZr(PO4)2:Eu may provide a practical basis to design and fabricate novel phosphors for white light-emitting diodes (W-LEDs).

Fabrication of p-type ZnO nanofibers by electrospinning for field-effect and rectifying devices

Ce-doped p-type ZnO nanofibers were synthesized by electrospinning and followed calcinations. The surface morphology, elementary composition, and crystal structure of the nanofibers were investigated. The field effect curve confirms that the resultant Ce-doped ZnO nanofibers are p-type semiconductor. A p-n heterojunction device consisting of Ce-doped p-type ZnO nanofibers and n-type indium tin oxide (ITO) thin film was fabricated on a piece of quartz substrate. The current-voltage (I-V) characteristic of the p-n heterojunction device shows typical rectifying diode behavior. The turn-on voltage appears at about 7 V under the forward bias and the reverse current is impassable.

Electrospun anatase TiO2 nanorods for flexible optoelectronic devices

Titanium dioxide (TiO2) nanorods with anatase phase were successfully fabricated by electrospinning and followed calcination. The TiO2 nanorods were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and UV-visible spectroscopy. The diameter of the TiO2 nanorods was about 60–150 nm and the length was 200 nm–2 μm. Electrical properties under bending were investigated by fixing the device to a curved surface with different curvatures, and the device showed a fast and stable resistance response to curvature changing. Photoelectric properties were studied by irradiation with different light intensities. The device exhibited a short response time (∼10 s) and a high sensitivity (∼103) which increased with the light intensity increasing. These results indicate that electrospun anatase TiO2 nanorods have potential applications in flexible photodetectors and solar cells.

Assembly of oriented ultrafine polymer fibers by centrifugal electrospinning

Uniaxially aligned and cross-aligned arrays of ultrafine polymer fibers have been fabricated by a novel and effective centrifugal electrospinning setup with rotating polymer solution jets. Comparing with conventional electrospinning (10-30 kV) and centrifugal spinning (4,000-12,000 rpm), this technique only requires a lower working voltage (2.8-6.0 kV), a slower rotational speed (360- 540 rpm), and a shorter spinning distance (2.0-4.0 cm). In addition, the influences of experimental parameters such as working voltage, rotational speed, collecting distance, and solution concentration on the alignment of the as-spun fibers are investigated using image analysis techniques. It is found that the working voltage and rotational speed mainly influence the perpendicular and linear velocities of the fibers, respectively. The polymer fibers tend to show higher alignment degree when these two velocities are very close. Optimum conditions (working voltage 3.0 kV, rotational speed 420 rpm, collecting distance 2.5 cm, and solution concentration 18 wt%) to maximize alignment degree (∼97%) of polystyrene fibers are also obtained.

Elastico-mechanoluminescent enhancement with Gd 3+ codoping in diphase (Ba,Ca)TiO 3 :Pr 3+

Elastico-mechanoluminescence (EML) in diphase (Ba,Ca)TiO3:Pr3+ with 60 mol% Ca content was greatly enhanced by Gd3+ codoping. The optimal EML intensity of (Ba,Ca)TiO3:Pr3+,Gd3+ is higher by 235% than that of (Ba,Ca)TiO3:Pr3+. The decreases of both photoluminescent intensity and reflectivity induced by Gd3+ codoping suggest the introduction of new trap centers. The thermoluminescent (ThL) measurement has been performed to investigate the effect of codoping on the trap depth and concentration. The consistent dependency correlations of EML intensity and ThL integral intensity on the Gd3+ concentration illuminate that the improved EML originates from the increased concentration of traps with suitable depth. A possible EML mechanism for (Ba,Ca)TiO3:Pr3+,Gd3+ is proposed on the basis of these experimental observations.

Piezoluminescence from ferroelectric Ca 3 Ti 2 O 7 :Pr 3+ long-persistent phosphor

A variety of up-and-coming applications of piezoluminescence in artificial skins, structural health diagnosis, and mechano-driven lightings and displays recently have triggered an intense research effort to design and develop new piezoluminescent materials. In this work, we deduced and verified an efficient piezoluminescence in ferroelectric Ca3Ti2O7:Pr3+ long-persistent phosphor, in view of three fundamental elements forming piezoluminescence - piezoelectricity, luminescent centers and carrier traps. Under the stimulation of mechanical actions including compression and friction, Ca3Ti2O7:Pr3+ shows an intense red emission from 1D2-3H4 transition of Pr3+. On the basis of investigations on structural and optical characteristics especially photoluminescence, persistent luminescence and thermoluminescence, we finally proposed a possible piezoluminescent mechanism in Ca3Ti2O7:Pr3+. Our research is expected to expand the horizon of existing piezoluminescent materials, accelerating the development and application of new materials.

Electrospun PEDOT:PSS/PVP Nanofibers for CO Gas Sensing with Quartz Crystal Microbalance Technique

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/polyvinylpyrrolidone (PEDOT:PSS/PVP) composite nanofibers were successfully fabricated via electrospinning and used as a quartz crystal microbalance (QCM) sensor for detecting CO gas. The electrical property of individual PEDOT:PSS/PVP nanofibers was characterized and the room temperature resistivity was at the magnitude of 105 Ω·m. The QCM sensor based on PEDOT:PSS/PVP nanofibers was sensitive to low concentration (5–50 ppm) CO. In the range of 5–50 ppm CO, the relationship between the response of PEDOT:PSS nanofibers and the CO concentration was linear. Nevertheless, when the concentration exceeded 50 ppm, the adsorption of the nanofiber membrane for CO gas reached saturation and the resonant frequency range had no change. Therefore, the results open an approach to create electrospun PEDOT:PSS/PVP for gas sensing applications.

A highly stretchable humidity sensor based on spandex covered yarns and nanostructured polyaniline

Stretchable sensors, as the important components of flexible electronic devices, have achieved progress in a variety of applications for monitoring physical or environmental conditions, such as sound, temperature, vibration, and pressure. However, it still remains a challenge to fabricate high performance stretchable humidity sensors. Herein, we present a novel stretchable humidity sensor, which was fabricated based on an ultrastretchable polyaniline composite fiber. Because of the composite fiber with a “twining spring” configuration (cotton fibers twining spirally around a polyurethane fiber) it maintains a stable electrical conductivity up to a strain of 200%. In addition, the conductivity of the composite fiber remains perfectly stable after 5000 cyclic stretching events of 200% strain. Incorporating the humidity sensitive properties of nanostructured polyaniline, the stretchable humidity sensor based on the composite fiber effectively maintains its humidity sensitivity at different elongations.

Optical contrast spectra studies for determining thickness of stage-1 graphene-FeCl3 intercalation compounds

Because of novel features in their structural, electronic, magnetic and optical properties, especially potential applications in nanoelectronics, the few-layer graphene intercalation compounds (FLGICs) have been intensively studied recently. In this work, the dielectric constant of the doped graphene of stage-1 FeCl3-GIC is obtained by fitting the optical contrast spectra. And fully intercalated stage-1 FeCl3-FLGICs were prepared by micromechanical cleavage method from graphite intercalation compounds (GICs) for the first time. Finally, we demonstrated that the thickness of stage-1 FeCl3-GICs by micromechanical cleavage can be determined by optical contrast spectra. This method also can be used to other FLGICs, such as SbCl5-FLGICs and AuCl5-FLGICs, etc.