Sai Jiang

Speed up Ferroelectric Organic Transistor Memories by Using Two-Dimensional Molecular Crystalline Semiconductors

Ferroelectric organic field-effect transistors (Fe-OFETs) have attracted intensive attention because of their promising potential in nonvolatile memory devices. The quick switching between binary states is a significant fundamental feature in evaluating Fe-OFET memories. Here, we employ 2D molecular crystals via a solution-based process as the conducting channels in transistor devices, in which ferroelectric polymer acts as the gate dielectric. A high carrier mobility of up to 5.6 cm2 V-1 s-1 and a high on/off ratio of 106 are obtained. In addition, the efficient charge injection by virtue of the ultrathin 2D molecular crystals is beneficial in achieving rapid operations in the Fe-OFETs; devices exhibit short switching time of ∼2.9 and ∼3.0 ms from the on- to the off-state and from the off- to the on-state, respectively. Consequently, the presented strategy is capable of speeding up Fe-OFET memory devices by using solution-processed 2D molecular crystals.

High-performance non-volatile field-effect transistor memories using an amorphous oxide semiconductor and ferroelectric polymer

Ferroelectric field-effect transistors (Fe-FETs) are of great interest for a variety of non-volatile memory device applications. High-performance top-gate Fe-FET memories using ferroelectric polymers of poly(vinylidene fluoride–trifluoroethylene) (P(VDF–TrFE)) and the inorganic oxide of InSiO were fabricated. The extracted electron mobility was as high as 84.1 cm2 V−1 s−1 in a low-frequency state. The interfacial charge transfer between the P(VDF–TrFE) and InSiO during annealing of the P(VDF–TrFE) layer benefits improvement in the device performance. The results show the potential of our Fe-FET memories for next-generation electronics.

Spin-Coated Crystalline Molecular Monolayers for Performance Enhancement in Organic Field-Effect Transistors

In organic field-effect transistors, the first few molecular layers at the semiconductor/dielectric interface are regarded as the active channel for charge transport; thus, great efforts have been devoted to the modification and optimization of molecular packing at such interfaces. Here, we report organic monolayers with large-area uniformity and high crystallinity deposited by an antisolvent-assisted spin-coating method acting as the templating layers between the dielectric and thermally evaporated semiconducting layers. The predeposited crystalline monolayers significantly enhance the film crystallinity of upper layers and the overall performance of transistors using these hybrid-deposited semiconducting films, showing a high carrier mobility up to 11.3 cm2 V-1 s-1. Additionally, patterned transistor arrays composed of the templating monolayers are fabricated, yielding an average mobility of 7.7 cm2 V-1 s-1. This work demonstrates a promising method for fabricating low-cost, high-performance, and large-area organic electronics.

Low-voltage, High-performance Organic Field-Effect Transistors Based on 2D Crystalline Molecular Semiconductors

Two dimensional (2D) molecular crystals have attracted considerable attention because of their promising potential in electrical device applications, such as high-performance field-effect transistors (FETs). However, such devices demand high voltages, thereby considerably increasing power consumption. This study demonstrates the fabrication of organic FETs based on 2D crystalline films as semiconducting channels. The application of high-κ oxide dielectrics allows the transistors run under a low operating voltage (−4 V). The devices exhibited a high electrical performance with a carrier mobility up to 9.8 cm2 V−1 s−1. Further results show that the AlOx layer is beneficial to the charge transport at the conducting channels of FETs. Thus, the device strategy presented in this work is favorable for 2D molecular crystal-based transistors that can operate under low voltages.

Growth of Black Phosphorus Nanobelts and Microbelts

Black phosphorus nanobelts are fabricated with a one-step solid-liquid-solid reaction method under ambient pressure, where red phosphorus is used as the precursor instead of white phosphorus. The thickness of the as-fabricated nanobelts ranges from micrometers to tens of nanometers as studied by scanning electron microscopy. Energy dispersive X-ray spectroscopy and X-ray diffraction indicate that the nanobelts have the composition and the structure of black phosphorus, transmission electron microscopy reveals a typical layered structure stacked along the b-axis, and scanning transmission electron microscopy with energy dispersive X-ray spectroscopy analysis demonstrates the doping of bismuth into the black phosphorus structure. The nanobelt can be directly measured in scanning tunneling microscopy in ambient conditions.

Directly writing 2D organic semiconducting crystals for high-performance field-effect transistors

Solution-processed 2D organic crystals are of significant interest because of their unique characteristics that ensure promising applications in electronics. In this study, a simple and efficient approach to directly write 2D organic crystals using a rollerball pen has been presented. The obtained crystals exhibit highly crystalline features with atomic smoothness and large size. Field-effect transistors composed of the obtained crystals yield an average and a maximum carrier mobility values of 3.1 and 5.9 cm2 V−1 s−1, respectively. This study presents significant potential of the writing technique via a rollerball pen for the solution-processed fabrication of 2D organic crystals for high-performance, large-area printed electronics.

Unveiling the piezoelectric nature of polar α-phase P(VDF-TrFE) at quasi-two-dimensional limit

Piezoelectric response of P(VDF-TrFE), which is modulated by the dipole density due to the polarization switching on applying an electric field, allows it act as the fundamental components for electromechanical systems. As proposed since the 1970s, its polar α-phase is supposed to yield an enhanced piezoelectric activity. However, its experimental verification has never been reported, hampered by a substantial challenge for the achievement of a smooth, neat α-phase film. Here, we prepare ultrathin crystalline α-phase P(VDF-TrFE) films on the AlOx/Al-coated SiO2/Si substrates via a solution-based approach at room temperature. Thus, we unveil the piezoelectric nature of the polar α-phase P(VDF-TrFE) at a quasi-two-dimensional limit. The obtained values of the relative morphological deformation, the local effective piezoelectric coefficient, and the electric field-induced strain reach up to 37 pm, −46.4 pm V−1, and 4.1%, respectively. Such a robust piezoelectric response is even higher than that of the β-phase. Besides, the evolution of piezoelectricity, which is related to the piezoelectric properties of two polarization states, is also studied. Our work can enable the exploration of the prospective applications of polar α-phase P(VDF-TrFE) films.

Temperature dependence of piezo- and ferroelectricity in ultrathin P(VDF–TrFE) films

The polymer poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF–TrFE)) is highly desirable for piezoelectric and ferroelectric functional applications owing to its considerable electromechanical activity and reliable electrical polarization. However, a clear understanding of the effect of the thermal annealing on the electromechanical behavior and polarization nature of ultrathin crystalline P(VDF–TrFE) films is severely lacking. Here we report the thermally induced structural reorganization, and piezo- and ferroelectric features in the ultrathin P(VDF–TrFE) films. On applying a 40 °C annealing treatment, the polarization-patterned electrostrictive strain reaches the highest value of ∼53.7 pm. Besides, the ultrathin film exhibits a highly ordered antiparallel dipole alignment, the highest local piezoelectric activity, and an improved polarization relaxation time. The optimum film properties are achieved owing to a high degree of polymer chains oriented parallel to the substrate plane. Our results can reveal a promising avenue for nano-electro-mechanical and nano-ferroelectric electronic applications using ultrathin P(VDF–TrFE) films.

Interfacial Flat-Lying Molecular Monolayers for Performance Enhancement in Organic Field-Effect Transistors

Organic field-effect transistors (OFETs) are the most fundamental device units in organic electronics. Interface engineering at the semiconductor/dielectric interface is an effective approach for improving device performance, particularly for enhancing charge transport in conducting channels. Here, we report flat-lying molecular monolayers that exhibit good uniformity and high crystallinity at the semiconductor/dielectric interface, deposited through slow thermal evaporation. Transistor devices achieve high carrier mobility up to 2.80 cm2 V-1 s-1, which represents a remarkably improvement in device performance compared with devices that are completely based on fast-evaporated films. Interfacial flat-lying monolayers benefit charge transport by suppressing the polarization of dipoles and narrowing the broadening of trap density of states. Our work provides a promising strategy for enhancing the performance of OFETs by using interfacial flat-lying molecular monolayers.