Shuo Sun

Ultrasensitive and Broadband MoS2 Photodetector Driven by Ferroelectrics

A few-layer MoS2 photodetector driven by poly(vinylidene fluoride-trifluoroethylene) ferroelectrics is achieved. The detectivity and responsitivity are up to 2.2 × 10(12) Jones and 2570 A W(-1), respectively, at 635 nm with ZERO gate bias. E(g) of MoS2 is tuned by the ultrahigh electrostatic field from the ferroelectric polarization. The photoresponse wavelengths of the photodetector are extended into the near-infrared (0.85-1.55 μm).

Synthetically controlling the optoelectronic properties of dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene-alt-diketopyrrolopyrrole-conjugated polymers for efficient solar cells

We have demonstrated that, by changing the substituent groups on dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT), one could effectively and rationally tune the energy levels, optical band gaps and charge transporting properties, etc. of the DTDBT derivatives (DTBDTs) and their conjugated polymers (CPs) and diketopyrrolopyrrole (DPP) derivatives.

Visible to short wavelength infrared In2Se3-nanoflake photodetector gated by a ferroelectric polymer

Photodetectors based on two-dimensional (2D) transition-metal dichalcogenides have been studied extensively in recent years. However, the detective spectral ranges, dark current and response time are still unsatisfactory, even under high gate and source-drain bias. In this work, the photodetectors of In2Se3 have been fabricated on a ferroelectric field effect transistor structure. Based on this structure, high performance photodetectors have been achieved with a broad photoresponse spectrum (visible to 1550 nm) and quick response (200 μs). Most importantly, with the intrinsic huge electric field derived from the polarization of ferroelectric polymer (P(VDF-TrFE)) gating, a low dark current of the photodetector can be achieved without additional gate bias. These studies present a crucial step for further practical applications for 2D semiconductors.

Two-dimensional negative capacitance transistor with polyvinylidene fluoride-based ferroelectric polymer gating

Conventional field-effect transistors (FETs) are not expected to satisfy the requirements of future large integrated nanoelectronic circuits because of these circuits’ ultra-high power dissipation and because the conventional FETs cannot overcome the subthreshold swing (SS) limit of 60 mV/decade. In this work, the ordinary oxide of the FET is replaced only by a ferroelectric (Fe) polymer, poly(vinylidene difluoride-trifluoroethylene) (P(VDF-TrFE)). Additionally, we employ a two-dimensional (2D) semiconductor, such as MoS2 and MoSe2, as the channel. This 2D Fe-FET achieves an ultralow SS of 24.2 mV/dec over four orders of magnitude in drain current at room temperature; this sub-60 mV/dec switching is derived from the Fe negative capacitance (NC) effect during the polarization of ferroelectric domain switching. Such 2D NC-FETs, realized by integrating of 2D semiconductors and organic ferroelectrics, provide a new approach to satisfy the requirements of next-generation low-energy-consumption integrated nanoelectronic circuits as well as the requirements of future flexible electronics.Nanoelectronics: ferroelectric polymers enable ultra-low subthreshold slope in MoS 2 transistorsReplacing the conventional oxide with a ferroelectric polymer in 2D MoS2 field-effect transistors allows sub-60 mV/dec operation. A team led by Jianlu Wang at the Chinese Academy of Sciences fabricated a negative capacitance field-effect transistor based on a metal-ferroelectric-semiconductor structure, with a 2D semiconductor (MoS2 or MoSe2) as the channel. Notably, when the oxide commonly used in field-effect transistors was replaced by a ferroelectric poly(vinylidene difluoride-trifluoroethylene) polymer, the resulting device achieved a subthreshold slope of 24.2 mV/dec at a drain voltage of 0.1 V, at room temperature. Further reduction of the polymer thickness to 50 nm resulted in a 51.2 mV/dec subthreshold slope. These results show promise for overcoming the 60 mV/decade subthreshold slope limit which plagues conventional transistors.

Space-charge Effect on Electroresistance in Metal-Ferroelectric-Metal capacitors

Resistive switching through electroresistance (ER) effect in metal-ferroelectric-metal (MFM) capacitors has attracted increasing interest due to its potential applications as memories and logic devices. However, the detailed electronic mechanisms resulting in large ER when polarisation switching occurs in the ferroelectric barrier are still not well understood. Here, ER effect up to 1000% at room temperature is demonstrated in C-MOS compatible MFM nanocapacitors with a 8.8 nm-thick poly(vinylidene fluoride) (PVDF) homopolymer ferroelectric, which is very promising for silicon industry integration. Most remarkably, using theory developed for metal-semiconductor rectifying contacts, we derive an analytical expression for the variation of interfacial barrier heights due to space-charge effect that can interpret the observed ER response. We extend this space-charge model, related to the release of trapped charges by defects, to MFM structures made of ferroelectric oxides. This space-charge model provides a simple and straightforward tool to understand recent unusual reports. Finally, this work suggests that defect-engineering could be an original and efficient route for tuning the space-charge effect and thus the ER performances in future electronic devices.

Effect of alkylthiophene spacers and fluorine on the optoelectronic properties of 5,10-bis(dialkylthien-2-yl)dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene-alt-benzothiadiazole derivative copolymers

Alternating conjugated copolymers based on 5,10-bis(dialkylthien-2-yl)dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT) and 2,1,3-benzothiadiazole (BT) or 5,6-difluoro-2,1,3-benzothiadiazole (FBT) with alkylthiopene spacers were synthesized, and the effect of insertion of alkylthiophene spacers and fluorine atoms on the characteristics of the copolymers, such as the energy levels, intrachain π–π interaction, dielectric constants, photovoltaic properties, etc., were systematically investigated. It has been found that: (i) the introduction of alkylthiophene spacers not only led to an increase in the intrachain interaction of the copolymers, but also resulted in an increase in the highest occupied molecular orbital (HOMO) levels and the lowest unoccupied molecular orbital (LUMO) levels, and (ii) the inclusion of fluorine atoms resulted in a decrease in both HOMO and LUMO energy levels with enhancement of the planarity and hole mobility. However, the inclusion of fluorine atoms had little effect on the LUMO levels relative to the decrease in the HOMO levels, and almost did not affect the dielectric constant of the copolymers. Use of the materials in polymeric photovoltaic cells led to high performance photovoltaic cells (PVCs) with power conversion efficiencies of 6.04–7.12%. The results demonstrated that the optoelectronic and aggregation properties of the 5,10-bis(alkylthien-2-yl)dithieno-[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene-alt-benzothiadiazole derivative copolymers can be effectively regulated by the introduction of alkylthiophene spacers and/or fluorine atoms into the backbone.

High-Performance Photovoltaic Detector Based on MoTe2/MoS2 Van der Waals Heterostructure

Van der Waals heterostructures based on 2D layered materials have received wide attention for their multiple applications in optoelectronic devices, such as solar cells, light-emitting devices, and photodiodes. In this work, high-performance photovoltaic photodetectors based on MoTe2 /MoS2 vertical heterojunctions are demonstrated by exfoliating-restacking approach. The fundamental electric properties and band structures of the junction are revealed and analyzed. It is shown that this kind of photodetectors can operate under zero bias with high on/off ratio (>105 ) and ultralow dark current (≈3 pA). Moreover, a fast response time of 60 µs and high photoresponsivity of 46 mA W-1 are also attained at room temperature. The junctions based on 2D materials are expected to constitute the ultimate functional elements of nanoscale electronic and optoelectronic applications.

The ambipolar evolution of a high-performance WSe2 transistor assisted by a ferroelectric polymer

In recent years, the electrical characteristics of WSe2 field effect transistors (FETs) have been widely investigated with various dielectrics. Among them, being able to perfectly tune the polarity of WSe2 is a meaningful and promising work. In this work, we systematically study the electrical properties of bilayer WSe2 FETs modulated by ferroelectric polymer poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)). Compared to traditional gate dielectric SiO2, the P(VDF-TrFE) not only can tune both electron and hole concentrations to the same high level, but also improve the hole mobility of bilayer WSe2 to 265.96 cm2V-1s-1 under SiO2 gating. Its drain current on/off ratio also has been improved to 2 × 105 for p-type and 4 × 105 for n-type driven by P(VDF-TrFE). More importantly, the ambipolar behaviors of bilayer WSe2 can be effectively achieved and maintained because of the remnant polarization field of P(VDF-TrFE). This work indicates that WSe2 FETs with P(VDF-TrFE) gating have huge potential for complementary logic transistor applications, and paves an effective way to achieve in-plane p-n junction.

Photodetectors: Ultrasensitive and Broadband MoS2 Photodetector Driven by Ferroelectrics (Adv. Mater. 42/2015)

Ultrasensitive and broadband MoS2 photodetectors driven by ferroelectrics are developed by J. Wang, W. Hu, and co-workers on page 6575. A high detectivity (≈2.2 × 10(12) Jones), photoresponsitivity (2570 A W(-1) ), and photoresponse wavelength (visible to 1.55 μm) detector is achieved under an ultra-high electrostatic field from ferroelectric polarization.

Pyromellitic Diimide-Benzodithiophene Copolymer for Polymer Solar Cells: Effect of Side Chain Length and Thiophene π-Bridge on Optical and Electronic Properties

Three donor-acceptor conjugated polymers were synthesized by combining electron donating benzodithiophene and electron accepting pyromellitic diimide. The relationship between structure and optical, electrochemical properties of the polymers were investigated. The results indicate that replacing the 2-ethylhexyl side chain with the longer 2-octyldodecyl side chain stabilized both the HOMO and LUMO level, and the insertion of thiophene π-bridge between pyromellitic diimide and benzodithiophene led to bathochromic-shifted and broader absorption. The polymers were used as donor materials to fabricate polymer solar cells. The highest open-circuit voltages reached 0.69 V and power conversion efficiencies reached 0.11%.