Mengxing Sun

Lateral multilayer/monolayer MoS 2 heterojunction for high performance photodetector applications

Inspired by the unique, thickness-dependent energy band structure of 2D materials, we study the electronic and optical properties of the photodetector based on the as-exfoliated lateral multilayer/monolayer MoS2 heterojunction. Good gate-tunable current-rectifying characteristics are observed with a rectification ratio of 103 at Vgs = 10 V, which may offer an evidence on the existence of the heterojunction. Upon illumination from ultraviolet to visible light, the multilayer/monolayer MoS2 heterojunction shows outstanding photodetective performance, with a photoresponsivity of 103 A/W, a photosensitivity of 1.7 × 105 and a detectivity of 7 × 1010 Jones at 470 nm light illumination. Abnormal photoresponse under positive gate voltage is observed and analyzed, which indicates the important role of the heterojunction in the photocurrent generation process. We believe that these results contribute to a better understanding on the fundamental physics of band alignment for multilayer/monolayer MoS2 heterojunction and provide us a feasible solution for novel electronic and optoelectronic devices.

Adjustable hydrazine modulation of single-wall carbon nanotube network field effect transistors from p-type to n-type

Single-wall carbon nanotube (SWCNT) network field effect transistors (FETs), which show decent p-type electronic properties, have been fabricated. The use of hydrazine as an aqueous solution and a strong n-type dopant for the SWCNTs is demonstrated in this paper. The electrical properties are obviously tuned by hydrazine treatment at different concentrations on the surface of the SWCNT network FETs. The transport behavior of SWCNTs can be modulated from p-type to n-type, demonstrating the controllable and adjustable doping effect of hydrazine. With a higher concentration of hydrazine, more electrons can be transferred from the hydrazine molecules to the SWCNT network films, thus resulting in a change of threshold voltage, carrier mobility and on-current. By cleaning the device, the hydrazine doping effects vanish, which indicates that the doping effects of hydrazine are reversible. Through x-ray photoelectron spectroscopy (XPS) characterization, the doping effects of hydrazine have also been studied.

Poly (ethylene imine)-modulated transport behaviors of graphene field effect transistors with double Dirac points

The bottom-gated graphene field-effect transistors (GFETs) with HfO2 as the dielectric layer are fabricated and the transport behaviors with double Dirac points defined as the right conductance minima (VDirac+) and the left one (VDirac-) are obtained, which exhibit the unique W-shaped transform characteristics rather than the typical V-shape. This observation indicates that the graphene in the channel region shows the different doping behaviors from that underlying the metal contact region. The W-shaped characteristics of GFETs are affected by the ambient environment and the time-dependence of double Dirac points has been also found. After being treated by Poly (ethylene imine) (PEI), the transport behaviors of GFETs could be modulated, especially for VDirac+ showing more obvious negative shift, indicating that PEI has a remarkable n-doping effect on the graphene in the channel region. PEI also screens the environmental influence and could protect the graphene from being p-doped to some extent. At the sam...

All-Inorganic Perovskite Nanowires–InGaZnO Heterojunction for High-Performance Ultraviolet–Visible Photodetectors

Inorganic cesium lead halide perovskites have attracted intense interest in optoelectronic applications due to the relatively stable performance in air. However, most reported inorganic perovskite-based optoelectronic devices exhibit low photosensitivity, which greatly hinders their further applications. Here, we first demonstrate a hybrid optical structure, which combines the n-type thin-film InGaZnO and the all-inorganic perovskite nanowires of CsPbBr3 together. By this way, excellent optical and electrical properties such as a low dark current of 10-10 A (at -5 V), a high Iph/Idark of 1.2 × 104, a response time of 2 ms and photoresponsivity of 3.794 A/W have been obtained. It is also found that the photodetector shows good stability in air ambient for 2 months with little reduction in performance. Moreover, such hybrid photodetectors exhibit enhanced photocurrent and Iph/Idark in high-temperature environment. This work paves a new way for high-performance photodetectors and points out the possible application of the inorganic cesium lead halide perovskites in harsh environment.

Hybrid graphene/cadmium-free ZnSe/ZnS quantum dots phototransistors for UV detection

Graphene-based optoelectronic devices have attracted much attention due to their broadband photon responsivity and fast response time. However, the performance of such graphene-based photodetectors is greatly limited by weak light absorption and low responsivity induced by the gapless nature of graphene. Here, we achieved a high responsivity above 103 AW−1 for Ultraviolet (UV) light in a hybrid structure based phototransistor, which consists of CVD-grown monolayer graphene and ZnSe/ZnS core/shell quantum dots. The photodetectors exhibit a selective photo responsivity for the UV light with the wavelength of 405 nm, confirming the main light absorption from QDs. The photo-generated charges have been found to transfer from QDs to graphene channel, leading to a gate-tunable photo responsivity with the maximum value obtained at VG about 15V. A recirculate 100 times behavior with a good stability of 21 days is demonstrated for our devices and another flexible graphene/QDs based photoconductors have been found to be functional after 1000 bending cycles. Such UV photodetectors based on graphene decorated with cadmium-free ZnSe/ZnS quantum dots offer a new way to build environmental friendly optoelectronics.

Tunable transfer behaviors of single-layer WSe2 field effect transistors by hydrazine

Polarity modulation of single-layer WSe2 field effect transistor is investigated by using hydrazine as a solution-processable and effective n-type dopant for WSe2. Compared to the intrinsic hole-dominant ambipolar behaviors, highly effective n-type doping characteristics are achieved after hydrazine treatment. It is found that the on-current improves obviously by one order of magnitude and the metal-WSe2 contact resistance decreases at the same time. After hydrazine being removed, the electron doping effect disappears which indicates hydrazine treatment is a reversible doping process. This work provides a possibility for transition metal dichalcogenides-based logic device applications in the future.

Sucrose-templated nanoporous BiFeO3 for promising magnetically recoverable multifunctional environment-purifying applications: adsorption and photocatalysis

Bismuth ferrite (BiFeO3), which acts as a significant multiferroic material, exhibits unique magnetic and ferroelectric properties. Here, we adopted a sol–gel method to synthesize disordered nanoporous BFO with a sucrose template. By different characterization methods such as powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and N2 physisorption measurements, the large BET (Brunauer–Emmett–Teller) surface area (34.25 m2 g−1) and disordered pores (20–40 nm) of the sucrose-templated nanoporous BFO (SN-BFO) materials were determined. These SN-BFO materials exhibited high magnetic performance (Ms = 6.37 emu g−1) and a favorable magnetic recycling ratio. In addition, SN-BFO displayed appropriate adsorption characteristics and good photocatalytic stability, which demonstrates that SN-BFO is a candidate for promising recoverable multifunctional environment-purifying applications in the future.

Heterostructured graphene quantum dot/WSe2/Si photodetector with suppressed dark current and improved detectivity

A high-performance heterojunction photodetector is formed by combining an n-type Si substrate with p-type monolayer WSe2 obtained using physical vapor deposition. The high quality of the WSe2/Si heterojunction is demonstrated by the suppressed dark current of 1 nA and the extremely high rectification ratio of 107. Under illumination, the heterojunction exhibits a wide photoresponse range from ultraviolet to near-infrared radiation. The introduction of graphene quantum dots (GQDs) greatly elevates the photodetective capabilities of the heterojunction with strong light absorption and long carrier lifetimes. The GQDs/WSe2/Si heterojunction exhibits a high responsivity of ∼ 707 mA·W–1, short response time of 0.2 ms, and good specific detectivity of ∼ 4.51 × 109 Jones. These properties suggest that the GQDs/WSe2/Si heterojunction holds great potential for application in future high-performance photodetectors.

A Sprayed, Scalable, Wearable and Portable NO2 Sensor Array Using Fully Flexible AgNPs-All-Carbon Nanostructures

Flexible chemical sensors usually require transfer of prepared layers or whole device onto special flexible substrates and further attachment to target objects, limiting the practical applications. Herein, a sprayed gas sensor array utilizing silver nanoparticles (AgNPs)-all-carbon hybrid nanostructures is introduced to enable direct device preparation on various target objects. The fully flexible device is formed using metallic single-walled carbon nanotubes as conductive electrodes and AgNPs-decorated reduced graphene oxide as sensing layers. The sensor presents sensitive response ( Ra/ Rg) of 6.0-20 ppm NO2, great mechanical robustness (3000 bending cycles), and obvious sensing ability as low as 0.2 ppm NO2 at room temperature. The sensitivity is about 3.3 and 13 times as that of the sample based on metal electrodes and the sample without AgNP decoration. The fabrication method demonstrates good scalability and suitability on the planar and nonplanar supports. The devices attached on a lab coat or the human body perform stable performance, indicating practicability in wearable and portable fields. The flexible and scalable sensor provides a new choice for real-time monitoring of toxic gases in personal mobile electronics and human-machine interactions.

High-performance heterogeneous complementary inverters based on n-channel MoS2 and p-channel SWCNT transistors

Heterogeneous complementary inverters composed of bi-layer molybdenum disulfide (MoS2) and single-walled carbon-nanotube (SWCNT) networks are designed, and n-type MoS2/p-type SWCNT inverters are fabricated with a backgated structure. Field-effect transistors (FETs) based on the MoS2 and SWCNT networks show high electrical performance with large ON/OFF ratios up to 106 and 105 for MoS2 and SWCNT, respectively. The MoS2/SWCNT complementary inverters exhibit Vin-Vout signal matching and achieve excellent performances with a high peak voltage gain of 15, a low static-power consumption of a few nanowatts, and a high noise margin of 0.45VDD, which are suitable for future logic-circuit applications. The inverter performances are affected by the channel width-to-length ratios (W/L) of the MoS2-FETs and SWCNT-FETs. Therefore, W/L should be optimized to achieve a tradeoff between the gain and the power consumption.