Charge Mobility and Strain Engineering in Two-Step MS-Grown MoS2/Seed Layer Heterointerface and Photo-Excitation Mechanism.

Two-dimensional (2D) materials have potential application and wide development prospects in photoelectron and spintronic devices. However, the properties of different growth conditions are challenging to study in the future. This, in turn, hinders further research into 2D materials and the manufacture of high-quality devices. A comprehensive understanding of the ultrafast laser spectroscopy and dynamics that take into account the substrate-transition metal dichalcogenide (TMD) interaction is lacking. Here, the strain effect is elucidated by systematically investigating the interfacial interaction between different substrates and MoS2. The strain and interface engineering of MoS2/seeds layer heterointerface and light-matter coupling are discussed in the Raman and photoluminescence spectra. The dramatic enhanced PL originates from the phase transition of MoS2 on different substrates and electron-hole pairs dissociated by exciton screening effect. Finite-difference time-domain simulation confirmed that the electric field, magnetic field, and polarization field of the heterojunction system changed after the strain was applied. In addition, based on the dependence of physical parameters of MoS2, the relative numerical changes of physical parameters of MoS2 films on different substrates as well as the photoelectric transfer, strain, and charge doping levels on the surface or interface will provide a direction for optimizing the selection of various devices.

Hydrothermal synthesis and controlled growth of group-VIB W metal compound nanostructures from tungsten oxide to tungsten disulphide.

Two-dimensional lateral group-VIB transition metal dichalcogenides (TMDs) have attracted much attention in the fast evolving field of advanced photoelectric functional materials, but their controllable fabrication is challenging. Herein, an emerging synthetic route for sulfurization of tungsten oxide was developed. During the hydrothermal reaction, the optimization of the precursor selection and synthesis parameters led to the tunable properties of WO3-WSxOy-WS2 nanostructures. The vulcanization was thermodynamically favorably at low temperatures and in an environment with a sufficient S source, wherein WO3 was reduced by H atoms to WO3-x, and S atoms were preferentially adsorbed on O vacancies. The WSxOy nanostructures have a narrow band-gap attributed to the effect of S on the valence band top and electronic density of states by density functional theory. The photocurrent response and charge transfer properties of WSxOy were improved due to the charge transport between WS2 and WO3. Understanding the formation and transformation of WS2 nanostructures in solution contributes to the discovery of the important structure-efficiency relationship, which may be extended to other TMDs systems. Hence, extensive research efforts are still needed to develop safer and more efficient synthesis and modification methods to fully utilize the distinctive advantageous properties of TMDs in the photoelectric field.

Composition engineering of AZO films for controlled photon-electron conversion and ultrafast nonlinear optical behavior.

Exploring micro-nano photonic crystals as nonlinear optical switching and optical limiting devices for Gaussian light fields with ultrashort pulse widths has attracted extensive research, mainly originating from its controllable modulation of the third/fifth-order nonlinear optical behavior and ultrafast carrier dynamics. In this work, Al-doped ZnO (AZO) films with controllable and excellent third-order nonlinear optical behavior have been uniformly deposited on quartz substrates by a single-step co-sputtering method. Al dopant-dependent ultrafast carrier dynamics and nonlinear optical properties in hexagonal ZnO films are discussed. The bonding mode of Al atoms in the ZnO lattice changed from substitutional to substitutional-decoration, which has been controllably achieved at different DC sputtering powers. The strain, crystallinity, grain size, dislocation density, and texture coefficient of the sample were quantitatively calculated by XRD and Raman spectroscopy, which confirmed that the phase parameters can be regulated by the sputtering power. In addition, Hall test and photoluminescence spectra showed the contribution of the donor level on the band structure and the electron transfer characteristics, which will provide a strategy for understanding multi-type carrier dynamics under strong light fields. The finite-difference time-domain method was used to simulate the linear optical absorption/transmittance of the sample under a plane-wave optical field, which proved that the light-matter interaction failed to be significantly suppressed by shading and scattering effects. The carrier relaxation process and nonlinear absorption/refractive effects were controllably optimized by dopant Al atoms, which were confirmed by Z-scan and transient absorption spectroscopy. Compared with pure ZnO films, the third-order nonlinear refraction and absorption coefficients of AZO-power films can reach -8.926 × 10-15 m2 W-1 and -0.634 × 10-7 m W-1, respectively. AZO films with ultrafast carrier dynamics and controllable excellent third-order nonlinear optical coefficients can be used as all-optical switches and optical limiting devices, which provide a reference for advanced micro-nano optical materials.

POMCPs with Novel Two Water-Assisted Proton Channels Accommodated by MXenes for Asymmetric Supercapacitors.

To develop high-performance supercapacitors, the negative electrode is at present viewed as one of the most challenging tasks for obtaining the next-generation of energy storage devices. Therefore, in this study, a polyoxometalate-based coordination polymer [Zn(itmb)3 H2 O][H2 SiW12 O40 ]·5H2 O (1) is designed and prepared by a simple hydrothermal method for constructing a high-capacity negative electrode. Polymer 1 has two water-assisted proton channels, which are conducive to enhancing the electrical conductivity and storage capacity. Then, MXene Ti3 C2 Tx is chosen to accommodate coordination polymer 1 as the interlayer spacers to improve the conductivity and cycling stability of 1, while preventing the restacking of MXene. Expectedly, the produced composite electrode 1@Ti3 C2 Tx shows an excellent specific capacitance (1480.1 F g-1 at 5 A g-1 ) and high rate performance (a capacity retention of 71.5% from 5 to 20 A g-1 ). Consequently, an asymmetric supercapacitor device is fabricated using 1@Ti3 C2 Tx as the negative electrode and celtuce leaves-derived carbon paper as the positive electrode, which demonstrates ultrahigh energy density of 32.2 Wh kg-1 , and power density 2397.5 W kg-1 , respectively. In addition, the ability to illuminate a red light-emitting diode for several minutes validates its feasibility for practical application.

Two-step strategy, growth mechanism and optical properties of plasmonic Ag-modified ZnO nanomaterials.

In this paper, plasma silver (Ag) modified zinc oxide (ZnO) (AZO) was used to form AZO nanomaterials (including AZO nanofilms (NFm), AZO nanowires (NWs) and AZO nanoflowers (NFw)) in a two-step-controlled manner to investigate the effect of compounding different contents of Ag on the linear optical aspects of ZnO materials. The growth mechanism of the AZO nanomaterials with different strategies is discussed. If Ag nanoparticles (NPs) grow on the ZnO NFm surface, they first grow with ZnO as the core and then self-core into islands, which are undoubtedly influenced by factors such as the growth mechanism of ZnO as well as Ag. If Ag is grown on the surface of the ZnO NWs and ZnO NFw, it is more likely to self-core owing to factors such as the roughness of the ZnO NWs and ZnO NFw surfaces. The AZO nanomaterials have excellent optical properties based on the surface plasmon resonance, local electromagnetic field and charge transfer mechanism between Ag and ZnO. With the increase in Ag content, the absorption edges of AZO NFm are red-shifted, and the absorption edges of AZO NWs and AZO NFw are first blue-shifted and then red-shifted. The results show that AZO nanomaterials prepared using different methods not only have different growth morphologies, but also have different optical properties with potential for the preparation of optical devices.

Effect of silver doping on ultrafast broadband nonlinear optical responses in polycrystalline Ag-doped InSe nanofilms at near-infrared.

There is great interest in transition metal-doped InSe because of its high nonlinearity and ultrafast response time at higher light fluence. Herein, Ag-doped InSe nanofilms were precisely manufactured using a direct current-radio frequency sputtering method, and their ultrafast broadband nonlinear optical responses in near-infrared were systematically researched. Ag-doped InSe nanofilm exhibited a broadband nonlinear optical response (800-1100 nm) and ultrafast carrier absorption (<1 ps), and can act as a potential semiconducting material for all-optical devices. Through precise control of the sputtering process parameters, Ag-doped InSe nanofilms were successfully prepared that were smooth, uniform, and exhibited no cracks. Nonlinear optical studies (femtosecond transient absorption spectroscopy and Z-scan measurement) indicated that nonlinear absorption behavior in Ag-doped InSe nanofilm withstands a transformation from saturation absorption to reverse saturation absorption arising from ground state bleaching, free-carrier absorption (FCA), and two-photon absorption (TPA). Additionally, nonlinear refraction behavior in Ag-doped InSe nanofilm was successfully detected near the intrinsic absorption edge, which arose from Kerr refraction and free-carrier refraction. More importantly, the broadband nonlinear response, ultrafast carrier absorption, and carrier recovery time of Ag-doped InSe nanofilm has the ability to controllably tune via Ag doping. Furthermore, Ag-doped InSe nanofilm possesses the nonlinear figure of merit (FOM) of 2.02, which indicates that Ag-doped InSe nanofilm is a promising semiconducting material for all-optical switching devices in near-infrared.

The photoluminescence, field emission and femtosecond nonlinear absorption properties of Al-doped ZnO nanowires, nanobelts, and nanoplane-cone morphologies.

Al-doped ZnO (AZO) nanowires, nanobelts and nanoplane-cone nanostructures have been successfully synthesized. The structural, photoluminescence (PL) and field emission (FE) properties of AZO nanowires have been characterized. The dependence of the PL properties of AZO nanostructures versus excitation laser power in the range from 1 to 12 mW and temperature in the range of 10-273 K was discussed. The PL measurement results demonstrated that the ultraviolet emission came from a near band edge emission, and two peaks in visible light region were due to deep-level emission. Moreover, the AZO nanowires have a relatively stronger ultraviolet emission than other kinds of samples. The FE measurements indicate that the turn-on field for the nanoplane-cone structure is 2.52 V µm-1, which is smaller than 4.42 V µm-1 for nanowires and 5.28 V µm-1 for nanobelts. In addition, the nonlinear absorption properties of AZO nanowires were measured using a femtosecond Z-scan technique. The effect of morphology on the nonlinear optical absorption properties of AZO nanowires was studied. From the results, the AZO nanowires show reverse saturable absorption (RSA) behavior. Furthermore, the results show that the order of magnitude of the nonlinear absorption coefficient for AZO nanowires is ∼10-2 cm3 GW-2. Our results show that AZO films are a promising candidate in further optoelectronic device applications.

Ultrafast nonlinear optical properties and carrier dynamics of silver nanoparticle-decorated ZnO nanowires.

Silver (Ag) nanoparticle-decorated zinc oxide (ZnO) nanowires (Ag-ZnO) have been successfully synthesized by chemical vapour deposition and the magnetron sputtering method. Scanning electron microscopy images indicate that Ag nanoparticles are distributed uniformly on the surface of the ZnO nanowires. The results of room temperature photoluminescence (RTPL) reveal two major emission peaks for the Ag-ZnO nanowires, and the emission peaks in the visible region are stronger than those of the unmodified ZnO nanowires. The mechanism of RTPL and low temperature photoluminescence (LTPL) emission is discussed in detail. Nonlinear optical properties and ultrafast dynamics have been investigated using the Z-scan and two color pump-probe (TCPP) techniques, respectively. The nonlinear absorption properties in the nano-, pico- and femto-second regime have been analyzed using the singlet state three-level and four-level models, respectively. The samples show self-focusing nonlinearity and good two-photon absorption (TPA)-induced ground state saturation absorption as well as excited state reverse saturable absorption behavior. For the nanosecond and picosecond pulses, the reverse saturated absorption in the excited state mainly originates from the absorption at low excited states or deep levels; however, for the femtosecond pulse, it is caused by the absorption at high excited states. The TCPP results show that the ground state or deep level light bleaching (for nano- and pico-second regime) and TPA-induced excited-state absorption (for femtosecond regime) behaviors range from 470 nm to 620 nm. The remarkable nonlinear optical properties reveal that Ag-ZnO nanowires are potential nanocomposite materials for the development of nonlinear optical devices.

Third-Order Nonlinear Optical Properties of InN Thin Films Under 532 nm Nanosecond Laser Pulses.

In order to investigate the third-order nonlinear optical properties of InN thin film, the sample was deposited on sapphire substrates by reactive RF magnetron sputtering. The prepared samples with a hexagonal wurtzite structure were confirmed by both X-ray diffraction (XRD) and scanning electron microscope (SEM). The optical absorption spectrum of the prepared samples was measured by a double beam UV/Visible spectrophotometer. The results show that the optical bandgap of deposited thin film is 2.06 eV. The third-order nonlinear optical coefficients of the film were measured by using the open and closed aperture transmission Z-scan (TZ-scan) technique under nanosecond laser pulses with a wavelength of 532 nm. The test results show that the prepared InN thin film performs strong saturation absorption, and the InN thin film with positive nonlinear refractive index coefficient is the self-focusing material under the conditions of the nanosecond laser pulses with the photon energy larger than the bandgap of prepared samples.

Plasmon resonance of silver micro-sphere in fiber taper.

We have experimentally studied the plasmon resonance phenomenon of a silver micro-sphere with a diameter of 2.3 µm in cone-shaped air cavity of a hollow fiber taper. To take insight into the plasmon resonance phenomenon, we move the micro-sphere along the fiber and observe the significant shift of the resonance peak. We also explore the light response in both infrared and visible wavelength band by finite difference time domain method. The significant variations of the magnetic and power field distribution are observed. The interesting results imply that the configuration has great potential in optical sensors and color filters.

The lifetime of the triplet excited state and modulation characteristics of all-optical switching in phenoxy-phthalocyanines liquid.

We present our experimental results on the measurements of excited state dynamics in 2, 9, 16, 23-phenoxy-phthalocyanine (Pc1) and 2, 9, 16, 23-phenoxy-phthalocyanine-zinc (Pc2) using the pump-probe experiment. The results show that the lifetime of the first triplet excited state of the Pc2 longer than Pc1. The lifetimes of the triplet excited state for Pc2 and Pc1 are 12.8 µs and 10.1 µs at the same intensity, respectively. Moreover, analysis of modulation characteristics of all-optical switching (A-OS) shows that the stronger the light intensity of the pump light is, the smaller the normalized transmittance is, and the lower the A-OS response time is. The consequences of such short lifetimes are also discussed in view of the strong A-OS properties of these molecules.

Nested fiber ring resonator enhanced Mach-Zehnder interferometer for temperature sensing.

We numerically investigate the properties of the nested fiber ring resonator coupled Mach-Zehnder interferometer as a sensor. By introducing the phase bias of 0.5π in the reference arm, the two output intensities exhibit sharp asymmetric line shapes around the resonance wavelength. Utilizing the intensity interrogation, we analyze the effect of parameters on the sensitivity and the detection limit. For the 30 dB signal-noise system, the sensitivity and the detection limit can achieve 4.0866/°C and 7.341×10(-3)°C, respectively; the results indicate that this structure is suitable for high-sensitivity measurements.