Changyong Lan

Ultrafast erbium-doped fiber laser mode-locked by a CVD-grown molybdenum disulfide (MoS2) saturable absorber.

We demonstrate an erbium-doped fiber laser passively mode-locked by a multilayer molybdenum disulfide (MoS(2)) saturable absorber (SA). The multilayer MoS(2) is prepared by the chemical vapor deposition (CVD) method and transferred onto the end-face of a fiber connector. Taking advantage of the excellent saturable absorption of the fabricated MoS(2)-based SA, stable mode locking is obtained at a pump threshold of 31 mW. Resultant output soliton pulses have central wavelength, spectral width, pulse duration, and repetition rate of 1568.9 nm, 2.6 nm, 1.28 ps, and 8.288 MHz, respectively. The experimental results show that multilayer MoS(2) is a promising material for ultrafast laser systems.

Passively

We demonstrate a passively Q-switched erbium-doped fiber laser (EDFL) based on few-layer MoS₂ as a saturable absorber (SA). Few-layer MoS₂ is prepared by the chemical vapor deposition method. The prepared MoS₂ is transferred onto the end face of a fiber connector to form a fiber-compatible MoS₂-based SA. The saturation intensity and modulation depth of the MoS₂ SA are measured to be 0.43 MW/cm² and 33.2%, respectively. The Q-switched EDFL has an all-fiber linear cavity with two fiber Bragg gratings as the end mirrors. By inserting the MoS₂ SA into the laser cavity, stable Q-switched operation is achieved at 1.55 μm. The laser has a pump threshold of 20.4 mW, a pulse repetition rate tunable from 10.6 to 173.1 kHz, and a minimum pulse duration of 1.66 μs. Our results show that few-layer MoS₂ is a promising SA for Q-switching laser operation.

Q

Single-crystalline GeS nanoribbons were synthesized by chemical vapor deposition for the first time. Structural characterization revealed that the nanoribbons grow along the [01] direction with a thickness of 20–50 nm, a width of several micrometers and a length of hundreds of micrometers. The GeS nanoribbons show a p-type behavior verified from the field effect transport measurement. The nanoribbon photodetectors respond to the entire visible incident light with a response edge at around 750 nm consistent with the band gap absorption of GeS. A strong nonlinear light-intensity-dependent response was observed between the measured illumination intensity from 0.25 to 212 μW cm−2. Under 530 nm light illumination, the maximum responsivity and external quantum efficiency are 139.9 A W−1 and 32730%, respectively. These results indicate that GeS nanoribbon is a promising semiconducting nanomaterial for high performance broadband visible-light sensing applications.

-Switched Erbium-Doped Fiber Laser Based on Few-Layer MoS 2 Saturable Absorber

Functioning both as electrochromic (EC) and transparent-conductive (TC) coatings, WO3/Ag/WO3 (WAW) trilayer film shows promising potential application for ITO-free electrochromic devices. Reports on thermal-evaporated WAW films revealed that these bifunctional WAW films have distinct EC characteristics; however, their poor adhesive property leads to rapid degradation of coloring-bleaching cycling. Here, we show that WAW film with improved EC durability can be prepared by reactive sputtering using metal targets. We find that, by introducing an ultrathin tungsten (W) sacrificial layer before the deposition of external WO3, the oxidation of silver, which leads to film insulation and apparent optical haze, can be effectively avoided. We also find that the luminous transmittance and sheet resistance were sensitive to the thicknesses of tungsten and silver layers. The optimized structure for TC coating was obtained to be WO3 (45 nm)/Ag (10 nm)/W (2 nm)/WO3 (45 nm) with a sheet resistance of 16.3 Ω/□ and a luminous transmittance of 73.7%. Such film exhibits compelling EC performance with decent luminous transmittance modulation ΔTlum of 29.5%, fast switching time (6.6 s for coloring and 15.9 s for bleaching time), and long-term cycling stability (2000 cycles) with an applied potential of ±1.2 V. Thicker external WO3 layer (45/10/2/100 nm) leads to larger modulation with maximum ΔTlum of 46.4%, but at the cost of significantly increasing the sheet resistance. The strategy of introducing ultrathin metal sacrificial layer to avoid silver oxidation could be extended to fabricating other oxide-Ag-oxide transparent electrodes via low-cost reactive sputtering.

Synthesis of single-crystalline GeS nanoribbons for high sensitivity visible-light photodetectors

van der Waals heterostructures built from two-dimensional (2D) materials have attracted wide attention because of their fascinating electrical and optoelectronic properties. Here, we report a highly responsive and broadband photodetector based on WS2–graphene van der Waals epitaxial heterostructures, which were fabricated by directly growing single crystalline few layer WS2 nanosheets on a polycrystalline graphene film. Upon light illumination, the current apparently reduces because of the transfer of photogenerated electrons from WS2 into the underlying p-type graphene and a photo-gating effect induced by the remaining holes, which is in stark contrast to ordinary semiconducting photoconductors. Thanks to the strong and broadband absorption of WS2, the WS2–graphene heterostructure photodetector exhibits a high responsivity with a maximum of 950 A W−1 at 405 nm and a wide spectrum response ranging from 340 to 680 nm. The high performance can be attributed to the internal built-in electric field at the WS2–graphene interface, which leads to the efficient separation of photogenerated electron–hole pairs. The WS2–graphene heterostructure photodetector may have potential applications in low cost, broadband, and flexible optoelectronics.

Reactive Sputter Deposition of WO3/Ag/WO3 Film for Indium Tin Oxide (ITO)-Free Electrochromic Devices

Van der Waals heterostructures built from two-dimensional materials on a conventional semiconductor offer novel electronic and optoelectronic properties for next-generation information devices. Here we report that by simply stacking a vapor-phase-synthesized multilayer n-type WS2 film onto a p-type Si substrate, a high-responsivity Zener photodiode can be achieved. We find that above a small reverse threshold voltage of 0.5 V, the fabricated heterojunction exhibits Zener tunneling behavior which was confirmed by its negative temperature coefficient of the breakdown voltage. The WS2/Si heterojunction working in the Zener breakdown regime shows a stable and linear photoresponse, a broadband photoresponse ranging from 340 to 1100 nm with a maximum photoresponsivity of 5.7 A/W at 660 nm and a fast response speed of 670 μs. Such high performance can be attributed to the ultrathin depletion layer involved in the WS2/Si p-n junction, on which a strong electric field can be created even with a small reverse voltage and thereby enabling an efficient separation of the photogenerated electron-hole pairs.

Highly responsive and broadband photodetectors based on WS2–graphene van der Waals epitaxial heterostructures

Two-dimensional (2D) materials have attracted wide attention due to their exotic properties. In particular, the lack of dangling bonds makes it possible to build highly lattice mismatched heterostructures composed of 2D materials and conventional semiconductors. Here, we report that by simply stacking a chemical vapor deposition grown monolayer WS2 film onto the surface of a room temperature sputtered ZnO film, significant enhanced ultra-violet (UV) photoresponse can be achieved. In this heterostructure of ZnO–WS2, the ZnO film acts as a light harvesting layer while the WS2 monolayer functions as a carrier transport layer which facilitates the photocarrier transport and reduces its recombination. Such a mechanism was confirmed by the observation of further photoresponsivity improvement of the ZnO–WS2 heterostructure under vacuum which removes the surface absorbates and thereby increases the carrier mobility of WS2. The strategy presented here can be applied to other wide band-gap semiconductors, shedding light on high sensitivity and flexible UV photodetectors based on van der Waals heterostructures.

Zener Tunneling and Photoresponse of a WS2/Si van der Waals Heterojunction

Two-dimensional (2D) nanomaterials have recently attracted considerable attention due to their promising applications in next-generation electronics and optoelectronics. In particular, the large-scale synthesis of high-quality 2D materials is an essential requirement for their practical applications. Herein, we demonstrate the wafer-scale synthesis of highly crystalline and homogeneous monolayer WS2 by an enhanced chemical vapor deposition (CVD) approach, in which precise control of the precursor vapor pressure can be effectively achieved in a multi-temperature zone horizontal furnace. In contrast to conventional synthesis methods, the obtained monolayer WS2 has excellent uniformity both in terms of crystallinity and morphology across the entire substrate wafer grown (e.g., 2 inches in diameter), as corroborated by the detailed characterization. When incorporated in typical rigid photodetectors, the monolayer WS2 leads to a respectable photodetection performance, with a responsivity of 0.52 mA/W, a detectivity of 4.9 × 109 Jones, and a fast response speed (< 560 μs). Moreover, once fabricated as flexible photodetectors on polyimide, the monolayer WS2 leads to a responsivity of up to 5 mA/W. Importantly, the photocurrent maintains 89% of its initial value even after 3,000 bending cycles. These results highlight the versatility of the present technique, which allows its applications in larger substrates, as well as the excellent mechanical flexibility and robustness of the CVD-grown, homogenous WS2 monolayers, which can promote the development of advanced flexible optoelectronic devices.

ZnO–WS2 heterostructures for enhanced ultra-violet photodetectors

Because of sluggish kinetics of the oxygen evolution reaction (OER), designing low-cost, highly active, and stable electrocatalysts for OER is important for the development of sustainable electrochemical water splitting. Here, {112} high-index facet exposed porous Co3O4 nanosheets with oxygen vacancies on the surface have been successfully synthesized via a simple hydrothermal method followed by NaBH4 reduction. As compared with the pristine and other faceted porous Co3O4 nanosheets (e.g., {110} and {111}), the as-prepared {112} faceted porous nanosheets exhibit a much lower overpotential of 318 mV at a current density of 10 mA cm-2. Importantly, these nanosheets also give excellent electrochemical stability, displaying an insignificant change in the required overpotential at a current density of 10 mA cm-2 even after a 14 h long-term chronoamperometric test. All these superior OER activity and stability could be attributed to their unique hierarchical structures assembled by ultrathin porous nanosheets, {112} high-index exposed facets with higher ratio of Co2+/Co3+ and oxygen vacancies on the surface, which can substantially enhance the charge transfer rate and increase the number of active sites. All these findings not only demonstrate the potency of our Co3O4 nanosheets for efficient water oxidation but also provide further insights into developing cost-effective and high-performance catalysts for electrochemical applications.

Wafer-scale synthesis of monolayer WS 2 for high-performance flexible photodetectors by enhanced chemical vapor deposition

We report on ultrafast-pulse generation in an erbium-doped fiber ring laser mode-locked by a graphene/WS2 van der Waals heterostructure saturable absorber. Atomic-layered WS2 is first synthesized on SiO2/Si substrate by the chemical vapor deposition (CVD) method, and then graphene/WS2 heterostructure is fabricated by transferring graphene onto the CVD-grown layered WS2. Taking advantage of excellent saturable absorption properties of the fabricated graphene/WS2 heterostructure, stable soliton pulses are successfully generated in the laser with a 3-dB spectral width of 2.3 nm and a pulse duration of 1.12 ps. Numerical simulations reproduce the mode-locked pulse emission in the experiment. Our research provides a new insight for tailoring versatile two-dimensional heterostructures so as to develop ultrafast photonic applications.