Liquid-phase Exfoliation Method Research Articles

A facile and clean process for exfoliating MoS2 nanosheets assisted by a surface active agent in aqueous solution

A facile, efficient and environmentally friendly process to exfoliate MoS2 is essentially critical to apply the obtained mono- and few-layer nanosheets in various electronic devices and sensors. Here we report a liquid phase exfoliation method for exfoliation of MoS2, which employs a surfactant of sodium dodecyl benzene sulfonate (SDBS) in water. The nonpolar benzene rings in SDBS can firmly bind to the MoS2 layer, facilitating the effective exfoliation of nanosheets in aqueous solution. It is found that the exfoliation efficiency and thickness of MoS2 nanosheets are related to the concentration of SDBS, and the mechanism was investigated. Defect free mono- and few-layer MoS2 nanosheets are obtained by controlling the amount of SDBS in solution, which exhibit stable dispersion in water over months, and it renders them as having great potential for solution-based device fabrication.

Solution-Based Processing of Optoelectronically Active Indium Selenide.

Layered indium selenide (InSe) presents unique properties for high-performance electronic and optoelectronic device applications. However, efforts to process InSe using traditional liquid phase exfoliation methods based on surfactant-assisted aqueous dispersions or organic solvents with high boiling points compromise electronic properties due to residual surface contamination and chemical degradation. Here, these limitations are overcome by utilizing a surfactant-free, low boiling point, deoxygenated cosolvent system. The resulting InSe flakes and thin films possess minimal processing residues and are structurally and chemically pristine. When employed in photodetectors, individual InSe nanosheets exhibit a maximum photoresponsivity of ≈5 × 107 A W-1 , which is the highest value of any solution-processed monolithic semiconductor to date. Furthermore, the surfactant-free cosolvent system not only stabilizes InSe dispersions but is also amenable to the assembly of electronically percolating InSe flake arrays without posttreatment, which enables the realization of ultrahigh performance thin-film photodetectors. This surfactant-free, deoxygenated cosolvent approach can be generalized to other layered materials, thereby presenting additional opportunities for solution-processed thin-film electronic and optoelectronic technologies.

Two-Dimensional Nanosheets by Rapid and Efficient Microwave Exfoliation of Layered Materials

Layered materials beyond graphene have generated renewed interests in numerous fields. Liquid-phase exfoliation methods face essential challenges in their universal application toward various two-dimensional materials (2DMs), short processing time, high yield, chemical stability, ultrathin thickness, and large lateral size of the nanosheets. To date, few reported methods are satisfactory in these requirements. We develop a general microwave-assisted, rapid (30 min), efficient (up to 50% yield), and potentially scalable approach to exfoliate 2DMs into mono- and few-layer nanosheets of superior chemical stability and large lateral size. 2DMs including h-BN, g-C3N4, BP, TMDs (MoS2, WS2, MoSe2), Zn2(bim)4, and Ti3C2Tx are tested for exfoliation in different fluid media including organic solvents and PF6–-containing ionic liquids (ILs). The nanosheets (e.g., BP) are surprisingly stable, probably attributed to solvation shells preventing the exfoliated sheets from reacting with water and oxygen. Theoretical simulations reveal that the dielectric constant of the fluid medium is a key factor determining the exfoliation efficiency. The preferred fluid media should be strongly polar (e.g., organic solvents with a high dielectric constant), which indicates materials’ ability to store electromagnetic energy via polarization. Finally, we demonstrate the 3D printing of nanosheet-based hybrids for potential applications. This general strategy paves a new promising pathway for the efficient liquid exfoliation of various 2DMs.

Preparation of graphene quantum dots through liquid phase exfoliation method

Here we report the preparation of low defect blue fluorescent graphene quantum dots (GQDs) through microwave expansion and liquid phase exfoliation (LPE) of graphite flakes with a high preparation yield of 0.63 g of GQDs per 1 g of original graphite substrate. The maximum emission in the prepared GQDs occurs at 467 nm for 370 nm excitation. The photoluminescence quantum yield of the prepared GQDs was measured as 3.4%. Prepared GQDs are composed of carbon, oxygen and nitrogen elements. They have high percentage (86%) of C-C/C=C, indicating few structural defects of the graphitic core, due to lesser number of edge- or surface-located oxygenated functional groups. This preparation technique, unlike others, does not require additional chemical components and is energetically undemanding. Furthermore, the reported method is also suitable for scalable preparation of quantum structures from other two-dimensional layered materials.

1.3 μm Q-switched solid-state laser based on few-layer ReS2 saturable absorber

A novel two-dimensional saturable absorber, few-layered rhenium disulfide (ReS2), was prepared by liquid phase exfoliation method successfully. Moreover, for the first time, it was successfully employed in passively Q-switched bulk Nd:YAG lasers at 1.3 μm in this paper. The maximum average output power of 78 mW was obtained with the shortest pulse width of 403 ns and the repetition rate of 214 kHz, corresponding to single pulse energy and pulse peak power of 0.42 µJ and 0.9 W, respectively. The results indicate that ReS2 has the potential to be used for passive Q-switching laser generation at 1.3 μm wavelength.

Fabrication of highly efficient carbon coated exfoliated tungsten disulfide nanosheets core-shell nanostructure as a promising solar-light driven photocatalyst

Exfoliated Tungsten disulfide (e-WS2) nanosheets were obtained from bulk Tungsten disulfide (WS2) in a liquid-phase ultrasonic exfoliation method using a DMF solvent. Core-shell mannered carbon coating was developed by hydrothermal process on exfoliated Tungsten disulfide (e-WS2) to enhance the efficient charge separation, specific surface area with mesoporosity and stability for chemical processes and mechanical strains. The carbon coated e-WS2 nanosheets (C@e-WS2) was performed by series of characterization techniques and explored to the photocatalytic degradation of malachite green (MG) dye by comparing with bulk WS2 and e-WS2. Among all the photocatalysts highest performance was observed for the C@e-WS2. C@e-WS2 catalyst displays more than 11 fold enhancements in the photocomposition rate of MG over bulk WS2 and nearly 8 fold improvements over e-WS2 for 90 min irradiation. In addition, for complete degradation of MG dye over the bulk WS2, e-WS2, and C@e-WS2 catalysts were observed at 150, 130, and 90 min under simulated solar light irradiation. The results obtained using core-shell mannered C@e-WS2 are stepping stone for developing cost-effective and noble metal-free catalysts for photodegradation.

Liquid Phase Acoustic Wave Exfoliation of Layered MoS2: Critical Impact of Electric Field in Efficiency

Liquid phase exfoliation techniques of layered crystals establish the basis for high yield production of two-dimensional (2D) flakes suspension. However, such techniques generally require a long processing time. The recent demonstration of the piezoelectric phenomenon in noncentrosymmetric layered transition metal dichalcogenides, such as molybdenum disulfide (MoS2), leads to new opportunities for fast and efficient exfoliation processes. Here we use concomitant electric field and mechanical shear force for producing a suspension of MoS2 nanoflakes from exfoliation of their layered bulk powder particles. The electrical and mechanical fields are applied by a surface acoustic wave (SAW) microcentrifugation device. We show that the overall yield per unit of time of 3.816%/h can be achieved, which is at least an order of magnitude larger than previously reported liquid phase exfoliation methods. Simultaneously, the impressive monolayer yield is 58% in an excellent agreement with the computational estimation b...

Few-layer TiSe2 as a saturable absorber for nanosecond pulse generation in 2.95 μm bulk laser.

1T-phase titanium diselenide (1T-TiSe2), a model two-dimensional (2D) transition metal dichalcogenide, has attracted much attention due to its intriguing electrical and optical properties. In this work, a 1T-TiSe2-based high-quality large-area saturable absorber (SA) (1T-TiSe2-SA) was successfully fabricated with the liquid-phase exfoliation method. With the as-prepared 1T-TiSe2-SA, a stable, passively Q-switched laser operating at 2.95μm was first realized. Under an absorbed pump power of 3.35W, the maximum average output power was 130mW with a slope efficiency of 5%. A pulse width of 160.5ns was obtained, which is the shortest among 3.0μm passively Q-switched lasers ever achieved with 2D materials as SAs, to the best of our knowledge. The results indicate that 1T-TiSe2 is a promising alternative as a nonlinear optical modulator for short-pulse laser generation near the 3.0μm mid-infrared region.

Application of ionic liquids on green exfoliation preparation of graphene in large scale

Graphene has attracted much attention since it was discoveried for its excellent physical and chemical properties. Thus, it is vital to prepare high-quality graphene in large scale, especially in green methods. Among them, liquid phase exfoliation is a promising method for large-scale preparation of graphene. Green solvent ionic liquid-assisted graphite exfoliation is worthy of much attention, which can obtain monolayer and few-layer graphene, and the yield is nearly two orders of magnitude higher than that of the earlier used organic solvent N -methylpyrrolidone. Here, we firstly classified and summarized the various solvents used in liquid phase exfoliation, then based on this, we discussed the application of ionic liquids for the actual exfoliation preparation of graphene. And it provides a reference for seeking strategies which can overcome many of the challenges faced by current liquid phase exfoliation methods in order to develop more efficient green exfoliation systems.

Liquid-phase exfoliated semiconducting single-walled carbon nanotubes as a saturable absorber for passively Q-switched laser

An economical way to extract semiconducting single-walled carbon nanotubes from common single-walled carbon nanotubes by the liquid-phase exfoliation method was demonstrated. Using the extracted samples as a saturable absorber, a passively Q-switched Nd : YVO4 laser was realized, delivering a pulse duration of 83 ns at a repetition rate of 290 kHz, resulting in a maximum peak power of 1.3 W. Such an outstanding saturable absorption might be caused by high diameter concordance and semi-conducting property.

High-efficiency, high-energy ytterbium-doped Q-switched fibre laser with graphene oxide-COOH saturable absorber

Employing graphene oxide (GO)–COOH film as a new saturable absorber (SA), we demonstrate a high-efficiency, high-energy Q-switched ytterbium-doped fiber (YDF) laser operating at a wavelength of 1.03 μm for the first time. GO–COOH film was prepared using a liquid-phase exfoliation method. Stable Q-switched pulses ranging from 4.5 μs to 1.35 μs with repetition rate in the range of 25 kHz–135 kHz were achieved. The maximum total output power was 82 mW, slope efficiency was 61.1% and the highest single pulse energy was 685 nJ at the repetition rate of 74 kHz—which is, to the best of our knowledge, the highest slope efficiency and largest pulse energy yet obtained from a Q-switched single-mode YDF laser with SA. The Q-switched YDF laser demonstrated in this paper confirms that GO–COOH is a cost-effective SA suitable for practical application in ∼1.0 μm Q-switched YDF lasers.

Development of polyimide films reinforced with boron nitride and boron nitride nanosheets for transparent flexible device applications

The benefits of reinforcing polyimide (PI) films with boron nitride (BN) particles and boron nitride nanosheets (BNNSs) were assessed with the aim of enhancing their thermal, optical, and mechanical properties for flexible device applications. BNNSs were prepared from BN particles using a liquid-phase exfoliation method assisted by an ultrasonic probe-type sonicator and centrifugator. PI-based composite films blended with BNNSs and BN particles were fabricated at various concentrations via mechanical stirring and spin coating. The transparency of the PI/BNNS composite films remained almost the same as that of pure PI films up to 3 wt.% whereas the transparency of the PI/BN composite films decreased with increasing concentration of the BN fillers at 550 nm. The thermal stability improved significantly with increasing concentrations of both BN and BNNS relative to that of pure PI films. The temperature for 5% weight loss of the PI/BNNS composite film was higher than that of the PI/BN composite film at the same filler concentration. The composite films with 2 wt.% BN or BNNS showed the lowest wear rate, and the PI/BNNS composite films showed more stable frictional behavior compared to the PI/BN composite films. In addition, bending tests showed that the PI/BNNS composite films exhibited excellent flexibility compared to the PI/BN composite films. Overall, the results indicate that the BNNS can be effectively used as a filler that can enhance the thermal and mechanical properties of polymer materials for flexible device applications.

Broadband rhenium disulfide optical modulator for solid-state lasers

Rhenium disulfide (ReS2), a member of group VII transition metal dichalcogenides (TMDs), has attracted increasing attention because of its unique distorted 1T structure and electronic and optical properties, which are much different from those of group VI TMDs (MoS2, WS2, MoSe2, WSe2, etc.). It has been proved that bulk ReS2 behaves as a stack of electronically and vibrationally decoupled monolayers, which offers remarkable possibilities to prepare a monolayer ReS2 facilely and offers a novel platform to study photonic properties of TMDs. However, due to the large and layer-independent bandgap, the nonlinear optical properties of ReS2 from the visible to mid-infrared spectral range have not yet been investigated. Here, the band structure of ReS2 with the introduction of defects is simulated by the ab initio method, and the results indicate that the bandgap can be reduced from 1.38 to 0.54 eV with the introduction of defects in a suitable range. In the experiment, using a bulk ReS2 with suitable defects as the raw material, a few-layered broadband ReS2 saturable absorber (SA) is prepared by the liquid phase exfoliation method. Using the as-prepared ReS2 SA, passively Q-switched solid-state lasers at wavelengths of 0.64, 1.064, and 1.991 μm are investigated systematically. Moreover, with cavity design, a femtosecond passively mode-locked laser at 1.06 μm is successfully realized based on the as-prepared ReS2 SA for the first time. The results present a promising alternative for a rare broadband optical modulator and indicate the potential of ReS2 in generating Q-switched and mode-locked pulsed lasers. It is further anticipated that this work may be helpful for the design of 2D optoelectronic devices with variable bandgaps.

Investigation of thermophysical properties of nanofluids containing poly(vinyl alcohol)-functionalized graphene

An experimental study was performed to evaluate the colloidal stability of water-based polyvinyl alcohol-functionalized few-layer graphene (water-based PVA–Gr) nanofluids and ethylene glycol-based polyvinyl alcohol-functionalized few-layer graphene (EG-based PVA–Gr) nanofluids. To this end, a liquid-phase exfoliation method was employed for mass production of graphene sheets (Gr). Then, a simple and novel method was introduced to do a direct functionalization of Gr with PVA. Surface functionality groups and morphology of PVA–Gr were analyzed by infrared spectroscopy, Raman spectroscopy and transmission electron microscopy. The results consistently confirmed the formation of PVA functionalities on Gr, while the structure of GNP has remained relatively intact. Then, UV–Vis was employed to investigate the stability of PVA–Gr in water and EG. The easily miscible PVA functionalities formed a great colloidal stability for Gr sheets. As a second criterion for having a promising coolant, thermophysical properties were measured experimentally. The thermal conductivity, density and viscosity of the nanofluids at concentrations of 0.025, 0.05 and 0.1 mass% were experimentally measured. As compared to the base fluid, the water-based PVA–Gr nanofluids show a significant enhancement at different conditions, like representing ~40% enhancement for 0.1 mass% at 40 °C. This simple and efficient procedure may play an important role for mass production of hydrophilic Gr, which be able to disperse in different solvents.

WSe2 as a saturable absorber for a passively Q-switched Ho,Pr:LLF laser at 295 µm

Multilayer WSe2 films were prepared by the liquid-phase exfoliation method and dispersed onto a YAG substrate. The obtained WSe2-SA was used to investigate a passively Q-switched Ho,Pr:LLF laser at 2.95 µm. Under an absorbed pump power of 4.51 W, the shortest pulse duration of 571 ns was realized with an average output power of 147 mW and a repetition rate of 89.3 kHz, corresponding to a single pulse energy of 1.65 µJ and a peak power of 2.88 W. The results indicate that WSe2 is a promising candidate for application as a saturable absorber to generate mid-IR pulsed lasers at ~3 µm.

Low-temperature synthesis and characteristics of fractal graphene layers

Large scale fractal graphene layers are obtained by complex method of liquid phase exfoliation and self-organization. Atomic force microscopy (AFM) is used to study the surface properties of formed layers and to assess their thickness. Surface potential of graphene and potential transition between the graphene and substrate is measured by Kelvin probe method. The influence of the effect of dielectric confinement on the optical properties of graphene is discussed in this work. Raman scattering spectra were used for structural analysis and assessment of the level of defects. Current-voltage characteristics of graphene ribbons were measured and discussed for different number of layers.

Nonlinear absorption properties of silicene nanosheets

As the cousins of graphene, i.e. same group IVA element, the nonlinear absorption (NLA) properties of silicene nanosheets were rarely studied. In this paper, we successfully exfoliated the two-dimensional silicene nanosheets from bulk silicon crystal using liquid phase exfoliation method. The NLA properties of silicene nanosheets were systemically investigated for the first time, as we have known. Silicene performed exciting saturable absorption and two photon absorption (2PA) behavior. The lower saturable intensity and larger 2PA coefficient at 532 nm excitation indicates that silicene has potential application in ultrafast lasers and optical limiting devices, especially in visible waveband.

High-quality 2-μm Q-switched pulsed solid-state lasers using spin-coating-coreduction approach synthesized Bi2Te3 topological insulators

In this paper, the fabrication process and characterization of Bi2Te3 topological insulators (TIs) synthesized by the spin-coating-coreduction approach (SCCA) is reported. With this approach, high-uniformity nano-crystalline TI saturable absorbers (TISAs) with large-area uniformity and controllable thickness are prepared. By employing these prepared TIs with different thicknesses as SAs in 2-μm solid-state Q-switched lasers, thickness-dependent output powers and pulse durations of the laser pulses are obtained, and the result also exhibits stability and reliability. The shortest pulse duration is as short as 233 ns, and the corresponding clock amplitude jitter is around 2.1%, which is the shortest pulse duration in TISA-based Q-switched 2-μm lasers to the best of our knowledge. Moreover, in comparison with the TISA synthesized by the ultrasound-assisted liquid phase exfoliation (UALPE) method, the experimental results show that lasers with SCCA synthesized TISAs have higher output powers, shorter pulse durations, and higher pulse peak powers. Our work suggests that the SCCA synthesized TISAs could be used as potential SAs in pulsed lasers.

B, N, S, Cl doped graphene quantum dots and their effects on gas-sensing properties of Ag-LaFeO3

P-type Ag-LaFeO3 (AL) is a promising gas-sensing material owing to its large surface area, rich active oxygen lattice, good thermostability, controllable structure and strong reducibility. While the operating temperature of which is high. In this study, B, N, S and Cl doped graphene quantum dots (GQD) were prepared with a simple liquid-phase exfoliation method and with which decorated AL in order to low the operating temperature of AL. The sensors based on the doped GQD operated at room temperature and showed affinity toward different volatile organic compounds (VOCs). B-GQD revealed p-type semiconductor property and reduced the operating temperature of AL markedly form 90 °C to 55 °C when composite with it because of the synergy effect. Undoped GQD and N, S, Cl doped GQD showed n-type semiconductor property and oppositely increased the operating temperature of AL to different extent when composite with AL. That is, the more electrons in the outer layer of the doped element, the higher the temperature of GQD/AL composite rise. Thus the gas-sensing properties of B-GQD/AL composite was further researched, the results showed that the composite possessed high sensitivity, good selectivity, good stability and low operating temperature to low concentration of formaldehyde.

Passively Q-switched Yb:YAG laser based on a MoSe2 saturable absorber.

A MoSe2 nanosheets-based saturable absorber (SA) was fabricated successfully by the liquid-phase exfoliation method. A passively Q-switched crystalline Yb:YAG laser was realized with the MoSe2 SA inserted inside the cavity. The shortest pulses at a wavelength of 1049nm with a duration of 250ns, maximum repetition rate of 181kHz, and an average output power of 158mW were emitted, corresponding to a maximum pulse energy of 0.87μJ. To the best of our knowledge, this is the first experimental demonstration of the nonlinear absorption property of MoSe2 nanoplatelets in a crystalline Yb-doped solid-state laser, which also proves the great potential of the MoSe2 SA as an optical modulator in the 1μm spectral region.