Broadband nonlinear absorption properties of two-dimensional hexagonal tellurene nanosheets

Multivalency-induced structural variations of 2D selenium nanosheets: facile solution-phase synthesis and optical properties

The structural stability of two-dimensional (2D) phases derived from bulk selenium (Se) is intrinsically rooted in the multivalent nature of the material. The emergence of 2D Se, as its morphology evolves from 1D to 2D, was initially inspired by theoretical predictions of various quasi-stable structural phases of 2D Se. Here, we report a facile liquid-phase synthesis of free-standing few-layer selenium nanosheets (SeNS) employing a simple magnetic stirring of their bulk counterpart in N-methyl pyrrolidone (NMP). The synthesized SeNS possess lateral dimensions ranging from several hundreds of nanometers to a few microns, with a minimum thickness of ∼1 nm. High-resolution transmission electron microscopy reveals the existence of α- and β-selenene. Fourier transform infrared analysis suggests that the inherent surface/edge functionalization of 2D SeNS by NMP enhances their dispersion stability. The UV-vis-NIR absorption spectrum of SeNS exhibits a shoulder peak at 330 nm, attributed to surface/edge functionalization, and multiple peaks across the vis-NIR region, stemming from size quantization effects. The functionalized selenium nanosheets generate photoluminescence that spans the blue-green range, while the size quantization of SeNS leads to green-orange luminescence. The non-linear optical studies following Z-scan experiments with an open aperture revealed reverse saturable absorption (RSA) and strong optical limiting in 2D SeNS under 532 nm, 10 ns laser pulses. Notably, a transition from RSA to saturable absorption (SA) has also been observed in samples stirred over an extended period. In this perspective, the results illustrate the first experimental realization of free-standing multivalent 2D selenium in allotropic forms with unique optical properties.

Controllable Synthesis, Formation Mechanism, and Photocatalytic Activity of Tellurium with Various Nanostructures

Telluriums (Te) with various nanostructures, including particles, wires, and sheets, are controllably synthesized by adjusting the content of polyvinylpyrrolidone (PVP) in a facile solvothermal reaction. Te nanostructures all have complete grain sizes with excellent crystallinity and mesopore structures. Further, the formation mechanisms of Te nanostructures are proposed to be that the primary nuclei of Te are released from the reduction of TeO32- using N2H4·H2O, and then grow into various nanostructures depending on the different content of PVP. These nanostructures of Te all exhibit the photocatalytic activities for the degradation of MB and H2 production under visible light irradiation, especially Te nanosheets, which have the highest efficiencies of degradation (99.8%) and mineralization (65.5%) at 120 min. In addition, compared with pure Te nanosheets, the rate of H2 production increases from 412 to 795 μmol∙h-1∙g-1 after the introduction of Pt, which increases the output by nearly two times. The above investigations indicate that Te with various nanostructures is a potential photocatalyst in the field of degradation of organic pollutants and H2 fuel cells.

Hydrothermal Synthesis of Te Nanosheets: Growth Mechanism and Electrical Property

Hydrothermal synthesis is a highly efficient way to yield multiform Te nanosheets. However, the growth mechanisms and property discrepancies between different types of Te nanosheets are still unclear. In this paper, we perform an investigation on this issue by monitoring the hydrothermally synthesized Te nanosheets at different growth stages with transmission electron microscopy and electrical tests. Three main types of Te nanosheets and their variants are revealed including trapezoidal and "V"-shaped configurations. It is found that the different types of Te nanosheets dominate at different reaction stages, indicating a sequential growth scenario. Surfactants and surface energy co-determine the growth kinetics, while the crystallographic attachments lead to specifically included angles of 74° and 41° in the "V"-shaped Te nanosheets. The fractions of the three main types of Te nanosheets as a function of reaction time are statistically tracked, and their crystalline structures, interfaces, and preferential growth orientations are uncovered. Moreover, the electrical properties of the Te nanosheets are tested, and the results show an interface-related feature. These findings provide some new insights into the synthesis and property of low-dimensional Te functional materials.

High performance 1.9 µm passively Q-switched bulk laser with germanene as a saturable absorber

Germanene is an analog of graphene, and its independent novel low-bending honeycomb structure gives outstanding advantages such as environmental stability and significant low-frequency optical absorbance. In this paper, the few-layer germanene was successfully prepared by the liquid phase exfoliation method. The saturable absorption characteristics of germanene in the infrared waveband were detected by the open-aperture Z-scan method. With germanene as a saturable absorber, a high-performance passively Q-switched bulk laser was realized at 1.9 µm. The shortest pulse width of 60.5 ns was obtained from continuous-wave pumping, corresponding to a single pulse energy of 6.7 µJ and peak power of 110 W. By utilizing the pulse pumping style with a repletion rate of 10 Hz, the single pulse energy and peak power increased to 45.8 µJ and 328 W, respectively, which exceeded all two-dimensional SA materials reported before. This research manifests that germanene is an excellent SA material for mid-infrared solid-state lasers.

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.

Elemental Two-Dimensional Materials for Li/Na-Ion Battery Anode Applications

Two-dimensional (2D) nanostructure is currently the subject in the fields of new energy storage and devices. During the past years, a broad range of 2D materials represented by graphene have been developed and endow with excellent electrochemical properties. Among them, elemental 2D materials (Xenes) are an emerged material family for Li/Na-ion battery (LIB/SIB) anodes. Compared with other 2D materials and bulk materials, Xenes may exhibit some great superiorities for Li/Na storage, including excellent conductivity, fast ion diffusion and large active sites exposure. In this review, we provide a systematic summary of the recent progress and achievements of Xenes as well as their applications in LIBs/SIBs. The broad categorization of Xenes from group IIIA to VIA has been concisely outlined, and the related details in syntheses, structures and Li/Na-ion storage properties are reviewed. Further, the latest research progress of Xenes in Li/Na ion batteries are summarized, together with mechanism discussions. Finally, the challenges and prospects of Xenes applied to Li/Na ion battery are proposed based on its current developments.

Chemistry, Functionalization, and Applications of Recent Monoelemental Two-Dimensional Materials and Their Heterostructures

The past decades have witnessed a rapid expansion in investigations of two-dimensional (2D) monoelemental materials (Xenes), which are promising materials in various fields, including applications in optoelectronic devices, biomedicine, catalysis, and energy storage. Apart from graphene and phosphorene, recently emerging 2D Xenes, specifically graphdiyne, borophene, arsenene, antimonene, bismuthene, and tellurene, have attracted considerable interest due to their unique optical, electrical, and catalytic properties, endowing them a broader range of intriguing applications. In this review, the structures and properties of these emerging Xenes are summarized based on theoretical and experimental results. The synthetic approaches for their fabrication, mainly bottom-up and top-down, are presented. Surface modification strategies are also shown. The wide applications of these emerging Xenes in nonlinear optical devices, optoelectronics, catalysis, biomedicine, and energy application are further discussed. Finally, this review concludes with an assessment of the current status, a description of existing scientific and application challenges, and a discussion of possible directions to advance this fertile field.

Unveiling the dimension-dependence of femtosecond nonlinear optical properties of tellurium nanostructures

Low dimensional tellurium is currently of great interest for potential electronic applications due to the experimentally observed Weyl fermions and the excellent carrier mobility, on/off ratios and current-carrying capacity in devices. However, the optical properties of Te nanostructures are not well explored, especially in the field of nonlinear optics. Here, we prepared a series of Te nanostructures by electrochemical exfoliation and liquid phase exfoliation methods, including one-dimensional (1D) Te nanowires (NWs), quasi-1D Te nanorods (NRs), zero-dimensional (0D) Te nanodots (NDs) and two-dimensional (2D) Te nanosheets (NSs). Femtosecond Z-scan measurements reveal unique dimension-dependent nonlinear optical (NLO) properties. 1D Te NWs and quasi-1D Te NRs exhibited higher saturable absorption behavior than 0D Te nanostructures, while the 2D Te NSs are a high performance optical limiting material. Ultrafast transient absorption spectroscopy revealed the dimension-dependent exciton dynamics. The reverse saturable absorption of 2D Te NSs is derived from faster exciton relaxation and stronger excited state absorption. This work paves the way for the design of saturable absorbers with high performance and broadens the application of 2D Te in the field of laser protection and other novel ultrafast photonics.

The nonlinear optical transition bleaching in tellurene

To date, outstanding linear and nonlinear optical properties of tellurene, caused by multiple two-dimensional (2D) phases and optical anisotropy, have attracted considerable interest for potential nanophotonics applications. In this work, the ultrafast nonlinear optical (NLO) properties of α-tellurene have been studied via Z-scan and pump-probe techniques at a broadband spectral region. Typical saturable absorption and band filling effects are observed in tellurene due to the Pauli exclusion principle. Analysis using density functional theory (DFT) computation shows the enhancements in NLO response within the ultraviolet-visible absorption spectral region are owing to the increased optical intraband transition in tellurene. Moreover, the effects of varying the photon energy of the probe pulse were explored. Our results indicated that probe pulses with higher photon energies can make smaller differential transmission signal, this effect is found to be negatively correlated with calculated joint density of states (JDOS). These results offer insights into the intrinsic photophysics of 2D tellurene, driving its applications in photonic and optoelectronic fields.

Synthesis and low-order optical nonlinearities of colloidal HgSe quantum dots in the visible and near infrared ranges

We synthesize colloidal HgSe quantum dots and characterize their nonlinear refraction and nonlinear absorption using a Nd:YAG laser and its second harmonic. The 7.5 nm quantum dots were synthesized using the hot-injection method. The nonlinear absorption (β = 9×10^-7 cm W^-1) and negative nonlinear refraction (γ = -5×10^-12 cm² W^-1) coefficients of colloidal quantum dots were determined using the 10 ns, 532 nm laser radiation. The joint influence of above processes was realized at a higher intensity of probe pulses. In the case of 10 ns, 1064 nm radiation, only negative nonlinear refraction dominated during z-scans of these quantum dots. The studies of optical limiting using two laser sources demonstrated the effectiveness of this process at 532 nm. The role of nonlinear scattering is analyzed. We discuss the mechanisms responsible for the nonlinear refraction processes in colloidal HgSe quantum dots.

1D to 2D Transition in Tellurium Observed by 4D Electron Microscopy

A new microwave-enhanced synthesis method for the production of tellurium nanostructures is reported-with control over products from the 1D regime (sub-5 nm diameter nanowires), to nanoribbons, to the 2D tellurene regime-along with a new methodology for local statistical quantification of the crystallographic parameters of these materials at the nanometer scale. Using a direct electron detector and image-corrected microscope, large and robust 4D scanning transmission electron microscopy datasets for accurate structural analysis are obtained. These datasets allow the adaptation of quantitative techniques originally developed for X-ray diffraction (XRD) refinement analyses to transmission electron microscopy, enabling the first demonstration of sub-picometer accuracy lattice parameter extraction while also obtaining both the size of the coherent crystallite domains and the nanostrain, which is observed to decrease as nanowires transition to tellurene. This new local analysis is commensurate with global powder XRD results, indicating the robustness of both the new synthesis approach and new structural analysis methodology for future scalable production of 2D tellurene and characterization of nanomaterials.

Passively Q-switched and mode-locked erbium-doped fiber lasers based on tellurene nanosheets as saturable absorber

Various two-dimensional (2D) materials show unique optical properties and excellent performance in acting as saturable absorber (SA) for demonstrating all-fiber ultra-fast lasers. Tellurene, as a new-fashioned few-layer 2D monoelemental material, was designed as an excellent saturable absorber to achieve Q-switched and mode-locked operations within erbium-doped fiber (EDF) lasers in our experiment. High-quality tellurene-based SA with a modulation depth of 0.97% was obtained by blending few-layer tellurene nanosheet solution prepared by liquid phase exfoliation method and the polyvinyl alcohol (PVA) solution. Inserting the SA into the EDF laser cavity by sandwiching the tellurene-PVA film between two fiber ferrules, either the passively Q-switched or the passively mode-locked operations can be obtained. The repetition rate varies from 15.92 to 47.61 kHz, and the pulse duration decreases from 8.915 to 5.196 µs in the passively Q-switched operation. To the best of our knowledge, this is the first demonstration focusing on the modulation application of tellurene in designing Q-switched pulsed laser operations. Additionally, mode-locked operations were also achieved by adjusting the polarization state. The obtained results fully indicate that tellurene can be developed as an efficient SA for pulsed fiber lasers.

2D graphdiyne: an excellent ultraviolet nonlinear absorption material

With an sp2-hybridized carbon atom structure, graphene is recognized as a nonlinear absorption (NLA) material, which has motivated scientists to explore new allotropes of carbon. Different from graphene, graphdiyne (GDY) consists of sp- and sp2-hybridized carbon atoms. An sp-hybridized carbon-carbon triple bond structure will bring in novel nonlinear optical properties, which are different from other allotropes of carbon. In this study, we investigated the broadband NLA properties (ultraviolet-infrared waveband) of GDY nanosheets, exfoliated using a liquid-phase exfoliation (LPE) method. The short ultraviolet cut-off wavelength (around 200 nm-220 nm) forebodes the potential application of GDY as an ultraviolet optical material. The outstanding NLA resulting in an ultraviolet waveband attests that the GDY nanosheets are veritable ultraviolet NLA materials, which have potential applications in ultraviolet optics. Our study broadens the application scopes of nanomaterials.