Changjiu Teng

Ligand-directed rapid formation of ultralong ZnO nanowires by oriented attachment for UV photodetectors

Ultralong ZnO nanowires with lengths of 20–80 μm and aspect ratios of 200–500 were synthesized within 15 minutes via a low-temperature hydrothermal method. With the assistance of sodium dodecyl sulfonate (SDSN) as the capping ligand, ZnO nanowires were formed by the initial nucleation of nanocrystals followed by the ligand-directed oriented attachment. Head-to-head attachment, side-by-side coalescence and nanocrystals attached to the surfaces were observed at different growth stages. ZnO microrods (lengths: 2–10 μm, diameters: 0.5 to 5 μm) were formed in the absence of SDSN. FT-IR spectra, XPS analysis and molecular dynamics simulations revealed that SDSN molecules were preferentially adsorbed onto the (100) planes rather than polar (001) planes, with their sulfonate groups coordinating with the surface zinc ions and possibly forming Zn–SO3 complexes. Such selective adsorption not only protected the initially nucleated ZnO nanocrystals from rapid aggregation, but also directed their subsequent self-assembly into highly-anisotropic nanowires. The as-prepared ZnO nanowires exhibited improved photoluminescence properties compared to the microrods. I–V characteristics indicated that the ZnO nanowires exhibited a much lower dark current, while an enhanced photocurrent upon 360 nm light illumination compared to the microrods. In addition, UV photodetectors using ZnO nanowires showed 27 times higher photo-sensitivity and 15.4/13.8 times higher rise/decay rates compared to those using ZnO microrods, which were attributed to the morphological effects in addition to the improved optical properties.

Enhancement in magnetic properties of magnesium substituted bismuth ferrite nanoparticles

We report a potential way to effectively improve the magnetic properties of BiFeO3 (BFO) nanoparticles through Mg2+ ion substitution at the Fe-sites of BFO lattice. The high purity and structural changes induced by Mg doping are confirmed by X-ray powder diffractometer and Raman spectra. Enhanced magnetic properties are observed in Mg substituted samples, which simultaneously exhibit ferromagnetic and superparamagnetic properties at room temperature. A physical model is proposed to support the observed ferromagnetism of Mg doped samples, and the superparamagnetic properties are revealed by the temperature dependent magnetization measurements. The improved magnetic properties and soft nature obtained by Mg doping in BFO nanoparticles demonstrate the possibility of BFO nanoparticles to practical applications.

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.

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...

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.

Tunable hysteresis behaviors in perovskite transistors

We reported the perovskite based field effect transistors (CH3NH3PbI3-FETs) using HfO2 as the bottom gating dielectrics. A typical hysteresis behavior has been observed in the transfer characteristics of CH3NH3PbI3-FETs with a large hysteresis window. The modulation of gate voltage on the hysteresis behaviors has been demonstrated by applying different gate voltage sweeping ranges. The shift of hysteresis window in the transfer characteristics of CH3NH3PbI3 based transistor has been also investigated under the illumination, which may contribute to a fast photoresponse.