Home Controllable nonlinear optical properties of different-sized iron phosphorus trichalcogenide (FePS3) nanosheets
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Controllable nonlinear optical properties of different-sized iron phosphorus trichalcogenide (FePS3) nanosheets

  • Danyun Xu , Zhe Guo , Yudi Tu , Xinzhe Li , Yu Chen , Zhesheng Chen , Bingbing Tian ORCID logo EMAIL logo , Shuqing Chen ORCID logo EMAIL logo , Yumeng Shi , Ying Li , Chenliang Su and Dianyuan Fan
Published/Copyright: September 2, 2020
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Nanophotonics
From the journal Nanophotonics Volume 9 Issue 15

Abstract

Two-dimensional iron phosphorus trichalcogenide (FePS3) has attracted significant attention for its use in electricity, magnetism and optical fields due to its outstanding physical and chemical properties. Herein, FePS3 was prepared using the chemical vapor transportation (CVT) method and then exfoliated by using fast electrochemical exfoliation. After gradient centrifugation, FePS3 nanosheets with thicknesses ranging from 1.5 to 20 nm and lateral dimensions of 0.5–3 μm were obtained. By utilizing the spatial self-phase modulation (SSPM) effect, the relationships between the nonlinear refractive index and the size of the FePS3 nanosheets were investigated in detail which revealed that the nonlinear refractive index can be effectively controlled by the size of the FePS3 nanosheets. It is worth noting that the optimal FePS3 nanosheets (3–5 layers thick and ∼1 μm in lateral dimensions) displayed the highest nonlinear refractive index of ∼10−5 cm2 W−1. This work demonstrates the potential that FePS3 nanosheets have for use in nonlinear optics or nonlinear optical devices.

Keywords: electrochemical exfoliation; FePS3 nanosheets; gradient centrifugation; nonlinear optical properties; spatial self-phase modulation

1 Introduction

Metal phosphorus trichalcogenides (MPTs) have drawn considerable interest in recent years due to their excellent magnetic, electronic, and optical properties for applications in catalysis, energy storage, and optoelectronic devices [1], [2], [3], [4], [5], [6], [7], [8]. MPTs have a general structure of MPX3, where M stands for a transition metal (Fe, Co, Ni, Cr, Mn, etc.) and X is S or Se. A monolayer sheet in the MPX3 structure consists of covalent- and ionic-bonded metal (M2+) and phosphorus atoms sandwiched between two layers of X atoms, forming a stable bipyramid structure of (P2X6)4−, similar to the superlattices of transition metal dichalcogenides (TMDs). The interlayers are held together by van der Waals forces with lower cleavage energies (0.29–0.54 J m−2) [9] relative to graphite (0.37 J m−2) [10], suggesting that MTP flakes can be exfoliated from their corresponding bulk crystals [11], [12]. Because of their ionic bonding, MTPs flakes show different physical and chemical properties and weaker interlayer cleavage energies relative to graphene and TMDs [13], [14], [15]. In addition, MPTs are a family of semiconductors with a tunable bandgap ranging from 1.3 to 3.5 eV that can be modified by changing the metal atoms and the number of layers [16]. Because of these properties, MPTs have shown higher absorption efficiency and broader absorption range compared to other two-dimensional (2D) materials [17], which is the cause of their excellent nonlinear optical properties.

For example, the nonlinear optical properties of NiPS3 nanosheets have been investigated by Z-scan measurements to generate picosecond laser pulses with a 1-µm band in the fast cavity [18]. Iron phosphorus trichalcogenide (FePS3) possesses a bandgap ranging from 1.60 (bulk) to 2.18 eV (monolayer) and shows substantially accelerated photocatalytic H2 production rates [6]. Moreover, FePS3 nanosheets have also been reported to have coexisting positive and negative photoconductivity for applications as ultraviolet photodetectors [19]. By taking advantage of these features, FePS3 nanosheets could have potential applications in the nonlinear optics field. Recently, the nonlinear refractive indexes for a series of 2D materials have been accurately acquired based on spatial self-phase modulation (SSPM), such as TMDs [20], black phosphorus [21] and graphdiyne [22]. However, there are few studies on the nonlinear optical properties of FePS3 nanosheets, especially studies on the size-dependent structural optical properties of FePS3 nanosheets and their application in field of optical devices.

Herein, we demonstrate the production of high-quality FePS3 nanosheets obtained by the rapid electrochemical exfoliation of chemical vapor transportation (CVT)–grown FePS3 crystals. With the aid of gradient centrifugation, FePS3 nanosheets with different sizes have been obtained with thicknesses ranging from 1.5 to 20 nm and lateral dimensions of 0.5–3 μm. The third-order nonlinear optical properties of these nanosheets were also explored using SSPM. The size of the FePS3 nanosheets was found to effectively control the nonlinear refractive indexes of FePS3.

2 Experimental section

2.1 Chemicals

All the starting chemicals were purchased commercially and used without further purification. Specifically, tetra-n-butylammonium tetrafluoroborate (TBAB, C16H36BF4N, 99%), iron powder (Fe, 99.99%) and red phosphorus powder (P, 99.99%) were purchased from Alfa Aesar (Beijing, China). Sulfur powder (S, 99.999%) was purchased from Aladdin (Shanghai, China). Propylene carbonate (PC, 99%), N,N-dimethylformamide (DMF, 99.5%), isopropyl alcohol (IPA, 99.5%) and polyvinyl pyrrolidone (PVP, 99%, molecular weight of 50,000) were purchased from Macklin (Shenzhen, China).

2.2 Preparation of bulk FePS3 crystals

The CVT method was used to prepare the bulk FePS3 crystals. Two grams of phosphorus, iron, and sulfur powders mixed in a stoichiometric molar ratio (1:1:3) were ground in an inert atmosphere glovebox. The mixed powders were sealed in a vacuum quartz tubes (length of 25 cm, internal diameter of 11 mm and wall thickness of 1 mm) under a pressure of 10−5 Pa via a Partulab device (MRVS-1002, China) equipped with a hydrogen/oxygen welding torch to obtain hermetically sealed tubes full of powder. The sealed tubes were placed in a two-zone furnace at a reaction temperature of 750 °C and growth temperature of 700 °C at a heating rate of 1 °C min−1 for nine days to create a temperature gradient for growing the bulk crystals. Finally, the bulk FePS3 crystals were successfully obtained after the furnace was cooled to room temperature.

2.3 Preparation of exfoliated FePS3 nanosheets

Compared with other exfoliation methods such as chemical vapor deposition, mechanical exfoliation and liquid-phase exfoliation, electrochemical exfoliation exhibits some merits of large-scale fabrication, high-quality, environmental-protection, time-saving and controllable-layer [23], [24], [25], [26]. Here, exfoliated FePS3 nanosheets were obtained by applying a direct current (DC) bias (−2.5 V) to the bulk FePS3 crystals with an electrochemical workstation (CHI 760E, Beijing Huake Putian, China) in a two-electrode system. Bulk FePS3 crystals were used as the working electrode by fixing the bulk crystals on a platinum electrode holder and a platinum wire was employed as the counter electrode. The electrolyte consisted of 60 mg of TBAB dissolved in 30 mL of PC. Similar to the previous work [26], TBAB salts were firstly driven to intercalate into the interlayer of bulk FePS3 crystals using a constant bias for seconds. And then lasting for a long time, some gaseous species were generated due to the electrochemical decomposition of TBAB salts, benefiting to the ultrafast volume expiation of FePS3 crystals. Finally, a suspension was harvested by manual-shaking for approximately 20 s. The suspension was then centrifuged to collect the expanded FePS3 which was then redispersed into DMF with 10 mg mL−1 PVP by using an ultrasonic water bath for 30 min. The dispersed suspension was then centrifuged to remove the unexfoliated FePS3, and the supernatant was washed with a large quantity of DMF and IPA by centrifugation. Finally, the FePS3 nanosheets were redispersed into IPA for further characterization and utilization. Additionally, we applied gradient centrifugation to obtain FePS3 nanosheets of different sizes, which is beneficial for identifying any size-related effects and the inherent nonlinear optical properties of FePS3.

2.4 SSPM of FePS3 nanosheets

This SSPM effect has been widely used to explore the nonlinear optical properties of low-dimensional materials [27]. In this experiment, 532 nm (MGL-S-300 mW, Shenzhen, China) and 633 nm (HNL210LB, Shenzhen, China) lasers were used as the light sources to investigate the SSPM response of FePS3 nanosheets dispersed in IPA in a 1-mm-thick cuvette. The light profile by the power meter (1919-R, Shenzhen, China) was recorded with a charge-coupled device (CCD) device (WinCamD-UCD 15, Shenzhen, China). All the samples were investigated at the same concentration of 0.28 mg/L. The formulas used to obtain the corresponding nonlinear optical coefficients are as follows.

From the basis of the nonlinear optical Kerr effect, the refractive indexes were determined by n=n0+n2I, where n0 is the linear refractive index, the nonlinear refractive index is n2 and I symbolizes the incident intensity. IPA was employed as the dispersing solvent, so the value of n0 is 1.42 [28]. n2 is defined by the following formula [29]:

(1)n2=λ2n0Leff⋅NI

where λ, Leff, and N stand for the incident laser wavelength, the effective optical length of the sample solution dispersion, and the ring numbers, respectively. Leff can be calculated by [28]:

(2)Leff=∫L2L1(1+z2z02)−1dz=z0 arctan(zz0)|L1L2

where L1 and L2 are the distance between the focal point and the two sides of cuvette and the thickness of the cuvette is defined as L(L=L1−L2). z0=ω02/λ (z0 is the diffraction length, while z is 0 and ω0 stands for the 1/e2 beam radius of the 0.03 cm incident laser). The corresponding third-order nonlinear susceptibility (χtotal(3)) can be obtained from the following equation [28]:

(3)χtotal(3)=cλn02.4×104π2Leff⋅dNdI

2.5 Characterization

The size distribution of the FePS3 nanosheets was analyzed in IPA by using a laser particle size analyzer (DelsaMax CORE, Beckman Coulter, USA). The morphology of the FePS3 nanosheets was determined by using atomic force microscopy (AFM, Dimension ICON system, Bruker, USA). A scanning electron microscope (SEM, TESCAN-MIRA3, Shanghai, China) was employed to characterize the cross section of the FePS3 crystals before and after intercalation. The morphology of the FePS3 nanosheets collected at different centrifugation speeds was observed using a high-resolution transmission electron microscope (HR-TEM, FEI Tecnai G20/JEM2010, FEI Company, USA). Raman spectra were recorded using a WITec (Alpha300 system, USA) with an excitation laser wavelength of λ = 532 nm. The absorbance spectrum of the FePS3 nanosheets dispersed in IPA was recorded from 200 to 1000 nm with a UV–vis spectrophotometer (Agilent cary5000, USA). The X-ray diffraction (XRD) spectra of FePS3 materials were characterized by XRD (Ultima IV, Japan). All of the characterization measurements were performed at room temperature.

3 Results and discussion

Bulk FePS3 single crystals were synthesized by CVT method (Figure 1a, see Supplementary material for details). Their XRD spectra show an intense c-axis orientation and the obtained peaks coincided well with the pattern of the FePS3 standard XRD data (JCPDS no. 30-0663) [30], suggesting the phase purity and high crystallinity of synthesized FePS3 single crystals (Figure S2). High quality FePS3 nanosheets were then obtained by electrochemical exfoliation of the bulk FePS3 single crystals using a two-electrode system with TBAB as the electrolyte (Figure 1b) [26], [31], [32]. After manual shaking, sonicating, removing the unexfoliated FePS3 and washing, the FePS3 nanosheets were finally dispersed in an IPA solution for the following application, which show an obvious Tyndall effect (Figure 1c). As shown in XRD spectra of exfoliated nanosheets (Figure S3), a strong peak is placed at 13.8° indexing to (0 0 1) and three small peaks of (0 0 2), (0 0 3) and (0 0 4) patterns are, respectively, located at 27.8°, 42.4° and 57.6°. Although the intensities of these peaks are weaker than that of the bulk counterpart, their position has no shifts, meaning that the crystallinity of nanosheets has been maintained. These results indicate the successful achievement of the exfoliation from the bulk crystals into thin nanosheets. The exfoliation mechanism is illustrated in Figure 1d, and the size-separation procedure for the FePS3 nanosheets using gradient centrifugation is shown in Figure S6. After screening by gradient centrifugation (Figure S6), FePS3 nanosheets of different sizes were obtained. These different sized FePS3 nanosheets suspensions were quite stable when stored under ambient conditions for more than 30 days (Figure S7). The size of the FePS3 nanosheets in the suspensions decreased as the rotation speed increased.

Figure 1: The electrochemical cathodic exfoliation process.(a) Pictures of bulk iron phosphorus trichalcogenide (FePS3) single crystals. (b) Diagrams of the equipment used for electrochemical exfoliation. (c) From left to right: pictures of the expanded FePS3, a dispersed FePS3 solution after manual shaking, a dispersed FePS3 solution after sonication and the final FePS3 dispersion produced via centrifugation showing the Tyndall effect. (d) Illustration of the mechanism of electrochemical exfoliation process for bulk FePS3 crystals using tetra-n-butylammonium as the electrolyte salt.
Figure 1:

The electrochemical cathodic exfoliation process.

(a) Pictures of bulk iron phosphorus trichalcogenide (FePS3) single crystals. (b) Diagrams of the equipment used for electrochemical exfoliation. (c) From left to right: pictures of the expanded FePS3, a dispersed FePS3 solution after manual shaking, a dispersed FePS3 solution after sonication and the final FePS3 dispersion produced via centrifugation showing the Tyndall effect. (d) Illustration of the mechanism of electrochemical exfoliation process for bulk FePS3 crystals using tetra-n-butylammonium as the electrolyte salt.

The sizes of the FePS3 nanosheets dispersed in IPA after centrifugation at different speeds are shown in the inset of Figure 2a. The FePS3 nanosheet suspensions are homogeneous and yellowish-brown and became darker with decreasing centrifugal speeds due to the increased size of the nanosheets. The absorbance of the suspensions decreased with increasing centrifugal speed and shows no prominent absorption peaks (Figure 2a), similar to previous reports [19]. Furthermore, AFM measurements were used to precisely analyze the size distribution of the FePS3 nanosheets. Typical AFM images of the FePS3 nanosheets acquired at different centrifugal speeds are shown in Figure 2b–g. FePS3 nanosheets of different sizes are all 2D in structure, verifying the successful exfoliation of the bulk FePS3 crystals. Some aggregated particles are also present in the AFM images, probably due to reaggregation occurring during the repeated dispersion and centrifugation steps. Comprehensive statistics of all nanosheets obtained at centrifugation speeds over or under 1000 rpm show that the FePS3 nanosheets have an average thickness of ∼5.5 nm and an average lateral dimension of ∼1 μm (Figures S8–S11a), the corresponding number of layers can be estimated as 3–4 layers [33]. As the centrifugation speed decreases, most of the collected nanosheets have lateral dimensions of 0.5–3 μm and a thickness of 1.5–20 nm (1–15 layers). However, some nanosheets with thicknesses of 5 nm were present under centrifugation speeds of 7000 rpm or less, most likely due to the large lateral dimension of these nanosheets. Therefore, the size distribution obtained by gradient centrifugation should consider both the lateral dimensions and the thickness of the nanosheets. The average size (lateral dimension and thickness) decreased as the centrifugation speed increased. The size distributions of the FePS3 nanosheets at different centrifugation speeds obtained from the AFM images matches well with the hydrodynamic size distributions measured by the particle-size description analyzer (Figure S11). Moreover, the results shown in the AFM images are also consistent with the TEM results (Figure S12). High-resolution TEM images and the corresponding elemental mapping analysis obtained for the nanosheets collected at a centrifugation speed of 5000 rpm are shown in Figure S12f, which demonstrates the uniform distribution of the Fe, P and S elements.

Figure 2: (a) UV–vis absorbance spectra of iron phosphorus trichalcogenide (FePS3) nanosheet dispersions from different centrifugation speeds with the corresponding pictures in the inset. Atomic force microscopy (AFM) images of FePS3 nanosheets with corresponding thickness and lateral size distributions from different centrifugation speeds of (b) 1000, (c) 3000, (d) 5000, (e) 7000 and (f) 9000 rpm with (g) the thickness step scale bar of −20 to 25 μm. (h) Raman spectra of bulk FePS3 crystals (black) and FePS3 nanosheets obtained from different centrifugation speeds of 1000 rpm (magenta), 3000 rpm (orange), 5000 rpm (green), 7000 rpm (cyan) and 9000 rpm (blue). It should be noted that all samples with different Raman intensities were characterized under the same conditions on a SiO2/Si substrate at a laser excitation wavelength of 532 nm. The spectral features at approximately 527 and 300 cm−1 represent the first-order and second-order Raman peak of silicon, respectively.
Figure 2:

(a) UV–vis absorbance spectra of iron phosphorus trichalcogenide (FePS3) nanosheet dispersions from different centrifugation speeds with the corresponding pictures in the inset. Atomic force microscopy (AFM) images of FePS3 nanosheets with corresponding thickness and lateral size distributions from different centrifugation speeds of (b) 1000, (c) 3000, (d) 5000, (e) 7000 and (f) 9000 rpm with (g) the thickness step scale bar of −20 to 25 μm. (h) Raman spectra of bulk FePS3 crystals (black) and FePS3 nanosheets obtained from different centrifugation speeds of 1000 rpm (magenta), 3000 rpm (orange), 5000 rpm (green), 7000 rpm (cyan) and 9000 rpm (blue). It should be noted that all samples with different Raman intensities were characterized under the same conditions on a SiO2/Si substrate at a laser excitation wavelength of 532 nm. The spectral features at approximately 527 and 300 cm−1 represent the first-order and second-order Raman peak of silicon, respectively.

Raman spectroscopy is a powerful and widely used technique for characterizing the structure of van der Waals–stacked materials and for revealing differences between bulk materials and the corresponding exfoliated nanosheets. The bulk FePS3 has a monoclinic structure with C2 symmetry which displays five Raman active modes attributed to vibrations of the iron cations (C Raman active modes) and (P2S6)4− bipyramid structures with D3d symmetry (A1g(1), A1g(2) and Eg(1), Eg(2) Raman active modes), as shown in Figure 2h. Near 157 cm−1, there is a highly Raman active counterpart of an infrared active Eu-type mode vibration originating from the (P2S6)4− unit. The A1g(1) and A1g(2) vibrations are at approximately 380 and 248 cm−1, respectively, which are caused by the out-of-plane phonon modes, while the two in-plane vibrations (Eg(1) and Eg(2)) are at approximately 279 and 227 cm−1, respectively. These results are consistent with previous studies on the Raman vibration modes of the FePS3 [34]. After exfoliation, Figure 2h shows there is an absence of the Eu-type mode, suggesting the presence of only few- and single-layer FePS3 nanosheets [6]. The sharp features of the A1g(1) and the other Raman vibration modes in the Raman spectra indict that the FePS3 nanosheets have retained a stable structure [30]. Furthermore, an obvious distinction between the bulk and exfoliated materials is the Raman shift of the iron cation vibration (C), which is also indicative of the success of the exfoliation process [16], [35]. Notably, compared to the MoS2, the interlayer Raman active modes display weakened Raman signal with decreased centrifugation speed, implying there are weaker interlayer van der Waals forces than those found in most other van der Waals–stacked materials [16].

To study the nonlinear optical behavior of the FePS3 nanosheets, typical measurements based on the SSPM effect were performed with 532 and 633 nm laser excitation sources as shown in Figure 3a. The laser beam traversed the FePS3 nanosheet dispersions and interacted with the FePS3 nanosheets. Once the power of the 532 nm incident light beam exceeded the excitation threshold of the nonlinear effect, a series of representative diffraction rings appeared from the center of the pattern with gradually increasing diameters (Figure S14) because of the non-uniform light intensity distribution on the beam cross-section, which is typical of the SSPM effect. To thoroughly investigate the influence that the size of the FePS3 nanosheets has on the nonlinear optical properties, different centrifugation speeds were used to obtain FePS3 nanosheets of different sizes. The corresponding relationship between the number of rings and incident intensity at different centrifugal speeds is shown in Figure 3b for systematic comparison. It can be seen in Figure 3b that the number of rings increases linearly in proportion to the incident intensity. Under different centrifugation speeds, the slopes (dN/dI) can be significantly different, which means that the nonlinear refractive index coefficient can be regulated and controlled. Most importantly, the FePS3 nanosheets centrifuged at 5000 rpm possess the highest slope (dN/dI) and the smallest threshold for the SSPM effect. These results are consistent with previous reports on gap-dependent SSPM [20], and can help further explain how the size of the SSPM diffraction rings are dependent on the intensity of the laser due to the intrinsic bandgap varying with changes in the size of the nanosheets. Notably, the FePS3 nanosheets obtained at 5000–3000 rpm display higher slopes, which makes sense because the FePS3 nanosheets from this centrifugation speed are approximately ∼7 nm in thickness and have lateral dimensions of ∼1 μm and show better nonlinear optical absorption. To further understand the value of highest slopes, the curve for the ring number versus the incident intensity at 5000 rpm is shown in Figure 3c. The highest slopes (dN/dI) are 0.4731 at the wavelength λ = 532 nm, while at 633 nm, the slope is 0.1219. Combining the linear fitting slopes and formulas (1), (2) and (3) introduced in the methods, the corresponding nonlinear optical coefficients were calculated, namely, the corresponding nonlinear refractive coefficients (n2), which are 8.86 × 10−5 cm2 W−1 (532 nm) and 2.72 × 10−5 cm2 W−1 (633 nm), respectively. The corresponding third-order nonlinear susceptibilities χtotal(3) for the FePS3 nanosheets are 4.56 × 10−5 (e.s.u.) (532 nm) and 1.40 × 10−5 (e.s.u.) (633 nm). Considering that Etotal=∑i=1MEi≈MEsingle layer, where M stands for the effective layer number of the FePS3 nanosheets, then χsingle layer(3) can be calculated using χtotal(3)=M2χsingle layer(3). In this experiment, M is in the order of ∼100 (Figure S5), so χsingle layer(3) is estimated to be ∼10−9 at wavelengths of 532 and 633 nm. Other typical 2D materials have been listed in Table S1, showing the variation of optical parameters with wavelength of laser lights.

Figure 3: (a) Spatial self-phase modulation (SSPM) experiments for the iron phosphorus trichalcogenide (FePS3) nanosheet solutions with experimental images. (b) Diagram of the relationship between the diffraction rings and the corresponding incident intensity for the FePS3 dispersion obtained from different centrifugation speeds at λ = 532 nm. (c) Diffraction rings of the FePS3 dispersion obtained at 5000 rpm as a function of the incident intensity with laser wavelengths of 532 nm (green) and 633 nm (red). (d) Schematic diagram showing the distortion of the FePS3 nanosheet dispersions. (e) The half-cone angle (θH) and the corresponding distortion angle (θD) as a function of the incident intensity at a laser wavelength of 532 nm. (f) Variation from the nonlinear refractive index of FePS3 before and after distortion using ExpDec1 fitting.
Figure 3:

(a) Spatial self-phase modulation (SSPM) experiments for the iron phosphorus trichalcogenide (FePS3) nanosheet solutions with experimental images. (b) Diagram of the relationship between the diffraction rings and the corresponding incident intensity for the FePS3 dispersion obtained from different centrifugation speeds at λ = 532 nm. (c) Diffraction rings of the FePS3 dispersion obtained at 5000 rpm as a function of the incident intensity with laser wavelengths of 532 nm (green) and 633 nm (red). (d) Schematic diagram showing the distortion of the FePS3 nanosheet dispersions. (e) The half-cone angle (θH) and the corresponding distortion angle (θD) as a function of the incident intensity at a laser wavelength of 532 nm. (f) Variation from the nonlinear refractive index of FePS3 before and after distortion using ExpDec1 fitting.

After prolonging the irradiation time, there was an expected phenomenon where the diffraction rings began to rapidly distort as shown in Figure 3d. At 1.1 s, a semicircular diffraction ring formed as the upper half of the diffraction rings collapsed toward the center. This phenomenon can be explained by thermal effects [29]. To further investigate the impact of this distortion on the measured nonlinear refractive index of the FePS3 nanosheets, the angles that formed between the sample and the pattern were used to analyze the variations of the collapse distortion in Figure 3d. When a laser travelled through the sample, a complete diffraction ring appeared on the screen with the corresponding maximum diffractive radius of RH and a distortion angle of θH. Following the distortion event, the upper half of the diffraction ring was changed and the new maximum diffractive radius and new distortion angle are named RH′ and θD′, respectively. The corresponding distorted diffraction radius and angle are marked with RD and θD. By learning from the previous equation [28], [36] (Equations (1)–(8) in Supplementary material) and parameters, the relation (Δn2/n2=θD/θH) is used to explain the relative variations of the nonlinear refractive index with the collapse angle under the condition of D (the distance between the sample and screen) ≫ RH or RH′. As shown in Figure 3e, the half-cone angle (θH) and distortion angle (θD) change linearly with the incident intensity. However, the ratio of θD/θH increases first and then tends toward saturation with increasing incident intensity, as shown in Figure 3f. Considering that θD/θH equates to Δn2/n2, the light intensity finally results in changes of Δn2  for the nonlinear refractive index of n2. Therefore, under strong light intensity or for extended laser irradiation times, a collapse will occur due to heating effect of the FePS3 nanosheets, resulting in measurement errors for n2.

To explore the interactions between light and the FePS3 nanosheets, an illumination experiment was performed as shown in Figure 4. The FePS3 nanosheets obtained at 5000 rpm have a bandgap of ∼2.22 eV (Figure 4a), close to the theoretical optical bandgap of monolayer FePS3 (2.54 eV) [9]. Light sources with wavelengths of 532 and 633 nm can be used to excite electrons during their interaction with the FePS3 nanosheets. In particular, the wavelength 532 nm is used to excite the electrons of materials with optical bandgaps smaller than 2.33 eV, while 633 nm is appropriate for materials with optical bandgaps below 1.96 eV. The electrons in the valence band are excited after absorbing incident photons and are promoted to the conduction band by overcoming the energy barrier of the interband transition (Figure 4b). The movements of these electrons are connected to oscillations of the electromagnetic field, resulting in the reorientation and alignment of the FePS3 nanosheets, which is the origin of their nonlinear optical properties [29].

Figure 4: (a) (i) UV–vis–NIR diffusive reflectance absorption spectra and (ii) the curve for the transformed Kubelka–Munk function with the photon energy for the FePS3 nanosheets collected at 5000 rpm. The FePS3 nanosheets are an indirect semiconductor with a corresponding bandgap of ∼2.22 eV. (b) The mechanism for the interactions between the light and the FePS3 nanosheets with the optical-bandgap structure in which the location of the conduction band minimum (CBM) is at the K point and the corresponding valence band maximum (VBM) is along the K → Γ path, showing the excitation spectrum of the free carriers of FePS3.
Figure 4:

(a) (i) UV–vis–NIR diffusive reflectance absorption spectra and (ii) the curve for the transformed Kubelka–Munk function with the photon energy for the FePS3 nanosheets collected at 5000 rpm. The FePS3 nanosheets are an indirect semiconductor with a corresponding bandgap of ∼2.22 eV. (b) The mechanism for the interactions between the light and the FePS3 nanosheets with the optical-bandgap structure in which the location of the conduction band minimum (CBM) is at the K point and the corresponding valence band maximum (VBM) is along the K → Γ path, showing the excitation spectrum of the free carriers of FePS3.

4 Conclusion

In conclusion, different sized FePS3 nanosheets were successfully obtained by combining electrochemical exfoliation of high-quality FePS3 crystals with gradient centrifugation. The nonlinear optical response of the FePS3 nanosheets based on SSPM was further developed to understand the nonlinear optical properties of the FePS3 nanosheets and any size-dependent effects. The obtained FePS3 nanosheets show obvious excitation wavelength SSPM, especially the nanosheets that were 3–5 layers thick with lateral dimensions of ∼1 μm. The superior nonlinear refractive index reached values of ∼10−5 cm2 W−1 and the corresponding third-order nonlinear susceptibility was in the order of ∼10−9. These experimental results indicate that FePS3 nanosheets have exceptional promise for applications in all kinds of nonlinear and optoelectronic devices and open the door for the use of other MTPs in the nonlinear optical field based on the SSPM effect.

Corresponding authors: Bingbing Tian and Shuqing Chen, International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen 518060, China, E-mail: (B. Tian), (S. Chen)

Funding source: Guangdong Natural Science

Award Identifier / Grant number: 2019A1515010675

Funding source: Science and Technology Project of Shenzhen

Award Identifier / Grant number: JCYJ20180305125106329

Award Identifier / Grant number: JCYJ20180305124927623

Award Identifier / Grant number: ZDSYS201707271014468

Funding source: Shenzhen Peacock Plan

Award Identifier / Grant number: 827-000273

Award Identifier / Grant number: KQJSCX20180328094001794

Award Identifier / Grant number: KQTD2016053112042971

Acknowledgements

The authors acknowledge the financial support from the Guangdong Natural Science Funds (2019A1515010675), the Science and Technology Project of Shenzhen (Grant Nos. JCYJ20180305125106329, JCYJ20180305124927623 and ZDSYS201707271014468), the Shenzhen Peacock Plan (Grant Nos. 827-000273, KQJSCX20180328094001794 and KQTD2016053112042971). Prof. C. Su thanks the support from the Guangdong Special Support Program, and the Pengcheng Scholar Program.

  1. Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

  2. Research funding: The authors acknowledge the financial support from the Guangdong Natural Science Funds (2019A1515010675), the Science and Technology Project of Shenzhen (Grant Nos. JCYJ20180305125106329, JCYJ20180305124927623 and ZDSYS201707271014468), the Shenzhen Peacock Plan (Grant Nos. 827-000273, KQJSCX20180328094001794 and KQTD2016053112042971).

  3. Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

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Supplementary Material

The online version of this article offers supplementary material (https://doi.org/10.1515/nanoph-2020-0336).

Received: 2020-06-18
Accepted: 2020-08-15
Published Online: 2020-09-02

© 2020 Danyun Xu et al., published by De Gruyter, Berlin/Boston

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/nanoph-2020-0336
Keywords for this article
electrochemical exfoliation; FePS3 nanosheets; gradient centrifugation; nonlinear optical properties; spatial self-phase modulation
Creative Commons
BY 4.0

Articles in the same Issue

  1. Reviews
  2. Electron-driven photon sources for correlative electron-photon spectroscopy with electron microscopes
  3. Tunable optical metasurfaces enabled by multiple modulation mechanisms
  4. Multipolar and bulk modes: fundamentals of single-particle plasmonics through the advances in electron and photon techniques
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  8. Broadband enhancement of second-harmonic generation at the domain walls of magnetic topological insulators
  9. Solar-blind ultraviolet photodetector based on vertically aligned single-crystalline β-Ga2O3 nanowire arrays
  10. Amplification by stimulated emission of nitrogen-vacancy centres in a diamond-loaded fibre cavity
  11. Graphene-coupled nanowire hybrid plasmonic gap mode–driven catalytic reaction revealed by surface-enhanced Raman scattering
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  14. Three-dimensional spatiotemporal tracking of nano-objects diffusing in water-filled optofluidic microstructured fiber
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