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Abstract
Understanding the dispersion state of 2D black phosphorus (BP) nanosheets is of great significance for the application of BP based materials. However, the dispersion state of BP colloids cannot be analyzed by the conventional methods based on the optical signal detecting due to the intensive light absorption of BP in the ultraviolet-visible region and the high turbidity caused by the aggregation of pristine BP nanosheets. In this work, 3D light scattering has been proven to be an effective technique to characterize the dispersion characteristics of nanocolloids with intensive light absorption and limited dispersity by suppressing multiple scattering. The effect of the concentration, the temperature and the solvent on the dispersion stability of BP nanosheets has been systematically studied by modulated 3D cross-correlation. It has been shown that measurements at smaller angles can obtain better autocorrelation functions. The BP colloids exhibit excellent colloidal stability when the concentration is below 250 μgmL−1, above which aggregation tends to form. Increasing the temperature has shown to result in the formation of aggregation in the colloids. BP nanosheets possess better dispersion state in polar solvents than in nonpolar solvents, and the aggregation process in nonpolar solvents can be monitored by the slow relaxation mode of 3D light scattering. This study indicates that 3D light scattering paves way to analyze the dispersion dynamics of various 2D nanoparticle colloids with intensive light absorption and poor dispensability.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Black phosphorus (BP) is a unique and important member of the 2D material family due to its direct electronic bandgap. Individual phosphorus atoms in BP are covalently bound to three neighbouring atoms and form a puckerred structure, while individual layers are held together by Van der Waals forces [1]. BP was first synthesized under high-pressure heating conditions by Bridgman in 1914 [2]. Park and Sohn exploited a new method of high-energy ball milling in 2007 [3]. Most recently, bulk BP crystals were successfully fabricated in large batches by Nilges and associates using chemical vapour transportation [4–6], which triggered extensive exploration in the theoretical development and engineering applications of BP. Compared to the bulk BP crystals, few-layer BP is more suitable for the applications in the fields of high-performance field effect transistors [7], photodetectors [8], energy storage materials [9], gas sensors [10] and lasers [11,12] as a result of its direct band gap, higher specific surface area, better dispersibility in solvents and easier processing. Thus, bulk BP crystals are typically exfoliated into 2D atomically thin BP nanosheets to form colloids.
It is noted that when the concentration exceeds the dispersion limit, the exfoliated pristine BP nanosheets tend to aggregate and form bundles in colloids because of their large surface area and the electrostatic screening effect of BP nanosheets [13]. Similar to other 2D materials, the formation of aggregation results in BP colloids with various dispersion states. The dispersion and aggregation of nanoparticles in the micro-scale are closely related to the macroscopic properties and application [14–16]. Therefore, understanding the dispersion states of 2D BP colloids in different conditions is of great significance. Previously, the dispersion state of nanocolloids is investigated by detecting the transmission or backscattering signals with special dispersion stability analyzers [17–20]. However, the dispersion stability of BP colloids cannot be analysed by the traditional methods based on the optical signal detecting for the following two reasons. One is that the intensive light absorption of 2D BP nanosheets in the ultraviolet-visible region makes the light transmission very low, the other is that the high turbidity caused by the aggregation of pristine BP nanosheets makes the colloids exhibit intensive multiple scattering and amplified background signals. Hence, reliable analysis of the dispersion states of BP colloids still remains a challenge.
Light scattering is one of the most powerful experimental techniques in the characterization of colloidal dispersions [21–23]. This technique is suitable for the characterization of colloidal particle sizes ranging from a few nanometers to several micrometers based on the Stokes-Einstein equation. Light scattering can be used to characterize the physical properties of the colloids, such as the local density fluctuations and the thermophysical properties [24]. Light scattering is also frequently used to investigate the dynamic aggregation of colloidal suspensions, and it allows researchers to follow the phase transition from colloid to gel based on the light intensity fluctuations caused by the diffusive motion of the particles in solvents [25]. However, a major drawback of conventional light scattering is its restriction to transparent samples with slight absorption and singly-scattered light only. In case of samples with intensive light absorption, the slight transmitted light results in a low signal-to-noise ratio. In case of turbid or opaque samples, the intensive multiple scattering in the colloid strongly influences the signal-to-noise ratio and leads to results that can be wrong by orders of magnitude [26]. A well-established technique for overcoming the problem of multiple scattering in the classic light scattering is 3D light scattering with a cross-correlation mode. This technique uses two simultaneous light scattering experiments performed at the same scattering vector on the same sample volume. The two scattering experiments are temporally separated by modulating the incident laser beams and gating the detector outputs at frequencies exceeding the timescale of the system dynamics. This robust modulation scheme eliminates the cross-talk between the two beam-detector pairs and leads to a four-fold improvement in the cross-correlation intercept, which can ensure the measurement accuracy and precision [25–27].
In this study, bulk BP crystals are synthesized through a facile low-pressure chemical transport route. BP colloids are prepared by a two-step exfoliation using tip-sonication and bath-sonication. The structural characteristics of BP nanosheets are identified by transmission electron microscopy (TEM), Raman spectroscopy and atomic force microscopy (AFM). The light absorption properties of BP colloids are analysed by UV-Vis-NIR spectrometry. 3D light scattering with two identical beams focusing on the same spot of the sample is exploited to investigate the dispersion dynamics of BP colloids while retaining a high signal-to-noise ratio. Issues related to multiple scattering in dense BP colloids can be overcome by this technique. The challenge of low light-transmission of BP colloids caused by the light absorption can be resolved as the optical path length in the sample is significantly reduced to 0.2 mm to increase the light transmission. The viscosity of the samples is measured to support the results from 3D light scattering.
2. Experimental section
2.1. Materials
Red phosphorus, tin, SnI4, toluene, acetone, CH2Cl2, CCl4, N-Methyl pyrrolidone (NMP), ethanol, and sodium hydroxide (NaOH) were purchased from Aladdin reagent. All the chemicals were directly used without further purification.
2.2. Synthesis of bulk BP crystals
Bulk BP crystals were synthesized through a facile low-pressure chemical transport route. In detail, red phosphorus, tin, and SnI4 were heated under vacuum at 923 K for 5 h with a heating ramp rate of about 1.35 K per min and then the temperature was reduced to 773 K with a cooling rate of 0.33 K per min, followed by a natural cooling process. The black product was purified using toluene and acetone.
2.3. Preparation BP colloids
BP colloids were prepared by the liquid exfoliation of bulk BP crystals. Briefly, 20 mg of bulk BP crystals was mixed with 40 mg of NaOH and 45 mL of NMP, and sonicated with a sonic tip for 6h at 650 W. The ultrasonic frequency was varied from 19 to 25 kHz and the ultrasound probe was used for 0.5 s with an interval of 2 s. The dispersion was then agitated in an ultrasonic ice bath for 24 h at 500 W. The resulting dispersion was centrifuged for 10 min at 2000 revolutions per minute and the supernatant containing the BP nanosheets was decanted gently. Afterwards, the supernatant was centrifuged at 12000 revolutions per minute for 30 min and the precipitate was re-dispersed in NMP.
2.4 Instruments and characterization
TEM and high resolution TEM images were taken to observe the micro structure of exfoliated BP on the JEOL JEM-2100 TEM at an acceleration voltage of 200 kV. AFM (SPA400) image was used to measure the thickness of BP nanosheets in the non-tapping mode under ambient conditions. Raman testing sample was prepared by drying BP colloids on a silicon wafer. Laser Raman spectrometer (HORIBA Jobin Yvon, LabRAM HR800) was applied to analyse the Raman scattering signals of BP nanosheets. The measurement was carried out at room temperature by sample excitation at 488 nm with a He-Ne laser. UV-Vis-NIR spectrometer (Lengguang, UV-9000) was used to detect the light absorption properties of BP colloids. The concentrations of BP nanosheets in the colloids (NMP as the solvent) were 50 μgmL−1, 37.5 μgmL−1, 25 μgmL−1, 12.5 μgmL−1, 6.25 μgmL−1, 4.167 μgmL−1 and 3.125 μgmL−1, respectively. The rheology of BP colloids was performed on a stress controlled advanced rheometer (Malvern, Kinexus Pro, UK) in steady mode. All measurements were conducted with cone-plate configuration (radius: 44 mm, cone angle: 4°) with truncated gap of 150 μm. In the steady-state measurements, shear viscosity was monitored as function of shear rate (from 0.01 s−1 to 100 s−1). The shear rate was increased continuously with the 30 s integration time for each shear rate. 3D light scattering measurements were performed using the 3D cross-correlation technique, on a 3D single-mode fiber goniometer system (LS Instruments) equipped with a Flex correlator and a diode laser with a wavelength of 680.4 nm. The temperature was regulated by a water bath. Angular scans covering 30°-150° were performed.
3. Results and discussion
The Bulk BP crystals prior to exfoliation are kept in a glovebox to avoid oxidation. After subjected to strong tip ultrasonication, mild bath ultrasonication and centrifugation, a stable brown BP colloid in NMP is obtained as shown in Fig. 1(a). When the BP dispersion is irradiated by a laser with a wavelength of 632 nm, intensive light scattering results in the Tyndall effect, thereby confirming its colloidal nature in Fig. 1(b). TEM, high-resolution TEM, AFM and Raman spectroscopy are performed to examine the morphology and structure of the exfoliated BP. As shown in Fig. 1(c), bulk BP crystals are exfoliated into BP nanosheets, the HR-TEM image in Fig. 1(d) shows the clear lattice fringes of about 0.23 nm matching the x direction of monolayer BP structure. Moreover, the uniform lattice parameter acquired by HR-TEM verifies that the exfoliated BP nanosheets maintain the unique crystalline nature, suggesting that the BP nanosheets produced by liquid exfoliation retain the original crystalline structure. Thickness is one of the most significant data in measuring exfoliation performance. AFM is introduced to investigate the topographic morphology (see Fig. 1(e), 1(f)) of ultrathin BP layers. The AFM results show that the thickness is about 0.8 nm (see Fig. 1(g), verifying that bulk BP crystals are successfully exfoliated into monolayer BP nanosheets. Raman spectroscopy is an effective technique for the identification of crystallographic orientation and chemical composition of materials. Herein, Raman spectrum is measured to study the structural transformation of BP after liquid exfoliation process. The BP colloids (CH2Cl2 as solvent) are drop-cast onto a silicon wafer, the Raman spectrum recorded from the red pentagram in Fig. 1(h) is listed in Fig. 1(i). BP nanosheets exhibit characteristic peaks at 363.9, 441.2, and 469.0 cm−1, which corresponds to Ag1, B2g, and Ag2 phonon vibration modes originating from in-plane (B2g, and Ag2) and out-of-plane (Ag1) vibrational modes, respectively (Fig. 1(j)) [28].
Fig. 1 Characterization of BP colloids and BP nanosheets. Photograph of the atomically thin BP colloid in NMP (a), the Tyndall effect of BP colloid (b), typical low-magnification TEM image of BP nanosheets (c), magnified high-resolution TEM image taken from a selective area in c (d), AFM image of BP nanosheets and the corresponding height profile along the drawn lines (e, f), statistics of the thickness of BP nanosheets (g), microscope image of dry nanosheets on a silicon wafer (h), the in situ Raman spectrum of exfoliated BP nanosheets located in the marked area in h and the illustrations of the vibrational motions in BP nanosheets (i, j).
As the light absorption of BP colloids is a significant factor in the aspect of characterizing the dispersion stability. UV-Vis spectroscopy is used to investigate the light absorption properties of BP colloids. To ensure the good linear function between concentration and absorbance that obeys the Lambert-Beer law, seven low concentrated BP colloids (from 3.125 μg mL−1 to 50 μg mL−1) are prepared by dispersing BP nanosheets of different weights into NMP. Their spectra (wavelength range 250-900 nm) are recorded immediately after sonication during 30 minutes. Results show that BP colloids possess weak light absorption from 550 nm to 900 nm, mild light absorption from 450 nm to 550 nm and strong light absorption from 250 nm to 450 nm (Fig. 2(a)). In order to quantify the dependence of absorption on the concentration of BP colloids and to calculate the molar absorptivity of BP nanosheets at different wavelengths, the calibration curves for the BP colloids at 300 nm, 400 nm, 500 nm, 600 nm and 808 nm are obtained (Fig. 2(b)). Results in Fig. 2(c) show that absorbance versus concentration obeys a linear correlation at 300 nm, 400 nm, 500 nm, 600 nm and 808 nm, whereby the molar absorptivity of BP colloids at corresponding wavelengths are derived to be 708.38, 370.14, 66.284, 22.921 and 7.4338 Lcm−1mol−1 based on the Lambert-Beer law. Meanwhile, the coefficients of determination (R2) of these fitting equations at 300 nm, 400 nm, 500 nm, 600 nm and 808 nm are 0.999, 0.999, 0.994, 0.990 and 0.980, respectively, indicating that there is highly reliable linear relation between the absorbance and the concentration of well-dispersed BP colloids. Based on the Tauc approach of BP [29,30], the band gap energy (Eg) obtained from BP colloids with concentrations of 3.125 μg mL−1, 4.167 μg mL−1, 6.250 μg mL−1, 12.50 μg mL−1, 25.00 μg mL−1 37.50 μg mL−1 and 50 μg mL−1 are 2.96, 2.94, 2.95, 2.98, 2.97, 2.88 and 2.95 eV, and the Eg probably corresponds to the high-energy band-to-band transition of BP nanosheets [31].
Fig. 2 Optical absorption properties of BP colloids. UV/Vis absorption spectra of BP colloids with different concentrations at 25 °C (a), calibration curves for the BP colloids at 300 nm, 400 nm, 500 nm, 600 nm and 808 nm at 25 °C (b), the fitting equation, coefficients of determination (R2) of fitting equations and molar absorptivity of BP colloids at 300 nm, 400 nm, 500 nm, 600 nm and 808 nm at 25 °C (c), (Ahν)2 versus hν curves of BP nanosheets, BP is a direct bandgap materials [30], thus r = 2 for Tauc plot (Ahν)r (d), Eg of BP nanosheets obtained from BP colloids with different concentrations (e).
An increase in the concentration of BP colloids results in intensive light absorption and turbidity; two factors which make traditional methods to analyze the dispersion states unreliable. We, for the first time, use 3D light scattering with a cross-correlation scheme to solve this problem. This is achieved by simultaneously performing two scattering experiments on the same scattering volume such that both experiments probe the same scattering vector q at different scattering geometries as shown in Fig. 3. The switch on/off frequency of each laser beam is 106 Hz, the multiple scattering originating from each beam can be deduced by the cross-correlation operation and the single scattering signal can be reserved. The fluctuating single scattering intensities of the two experiments, scattered light intensity I1(t) and I2(t), are cross-correlated to yield the normalized intensity cross-correlation Eq. (1). g(2)(q, τ) is related to the amplitude correlation function g(1)(q, τ) by the relation as shown in Eq. (1). g(1)(q, τ) is theoretically represented as an exponential decay function 2. q is experimentally determined by the refractive index of the solvent (n), the scattering angle (θ) and the wavelength of the laser light (λ) as shown in Eq. (3), which provides a quantitative measure for the length scale of the light scattering experiment. The self-diffusion coefficient D is expressed as Eq. (4) according to the Stokes-Einstein-equation. Finally, the normalized intensity cross-correlation equation g(2)(q, τ) can be expressed as Eq. (5) [21,32].
Fig. 3 Schematic illustration of the setup of 3D light scattering. This technique uses two simultaneous light scattering experiments performed at the same scattering vector on the same sample volume. The two scattering experiments are temporally separated by modulating the incident laser beams and gating the detector outputs at frequencies exceeding the timescale of the system dynamics.
In order to investigate the dynamic dispersion states of BP colloids at various concentrations, temperatures, and in different solvents, the modulated dynamic light scattering setup equipped with two detectors which cross-correlate the scattered signal from the same volume and at the same scattering vector is used. Figure 4 shows the intensity autocorrelation functions measured at different scattering angles from 30° to 150°. Results indicate that the scattering angles have significant effect on the autocorrelation functions for a given BP colloid. The intercept β of autocorrelation function tends to decrease when increasing the scattering angle, and the decrease of β usually leads to the increase of background signal and the decrease of correlation of the single scattering. Therefore, measurement at smaller scattering angles can obtain better autocorrelation functions. The relaxation process of BP colloids also exhibits obvious angle dependence. As soon as the scattering angles gradually increase, the autocorrelation functions shift to shorter decay time and the relaxation decay times exhibit decreasing trend. The scattering intensities before the equilibrium state decrease dramatically with the increase of the scattering angles, this variation trend is in accordance with Eq. (5). The above results prove that the classic theory and methodology of 3D light scattering are applicable for the characterization of nanocolloids with intensive light absorption and high turbidity. In order to study the correlation between the dispersion state and the 3D light scattering signal, the autocorrelation functions of BP colloids with different concentrations from 31.25 μgmL−1 to 1000 μgmL−1 are measured as shown in Fig. 5. We note that the intercept β of autocorrelation function exhibits a decrease trend with the increase of BP concentration, the reason for this phenomenon is that the additional misalignment or stray light caused by the BP aggregation particles in the colloids inevitably deteriorates the well-defined correlation of single scattering. Results also indicate that the BP concentrations have significant effect on the autocorrelation functions, the values of autocorrelation function (g(2)(θ, τ)-1) before the equilibrium state decrease dramatically with the increase of the BP concentrations. The reason for the reduction includes two aspects, one is that BP colloids can absorb the incident light of 680.4 nm as shown in Fig. 2, the other is that the higher concentration can lead to larger size of BP aggregation and smaller β. Therefore, we can qualitatively figure out the relationship between auto-correlation function and the dispersion state of BP colloids based on the variation of g(2)(θ, τ)-1 and β when changing the BP concentration. That is to say, larger BP concentration leads to lager BP aggregation size, worse dispersion state and smaller g(2)(θ, τ)-1 and β. Furthermore, as the intercept β begins to dramatically decrease when the BP concentration exceeds 250 μgmL−1, it can be deduced that the dispersion states of BP colloids begin to dramatically deteriorate when the BP concentration exceeds 250 μgmL−1. However, as soon as the concentrations gradually increase, the relaxation decay times show no obvious change, the data quality almost deteriorates at the same stage of the relaxation process between 2 × 10−4 and 2 × 10−2 s, which means the multiple scattering in dense BP colloids is effectively suppressed. As the diffusion, dispersion and aggregation of colloids are sensitive to temperature, the effect of temperature on the dispersion state of BP colloids is studied. Figure 6(a) shows the intensity autocorrelation functions of a low-concentration BP colloid measured at different temperatures from 297 K to 327 K. Results indicate that the temperature significantly affect the autocorrelation functions, the intercept β of autocorrelation function exhibits a decrease trend with the increase of temperature, which is similar with the trend in Fig. 5 caused by the increase of concentration, indicating that increasing the temperature can also accelerate the aggregation of BP nanosheets. However, the data quality almost deteriorates at the same stage of the relaxation process between 3 × 10−5 and 1 × 10−2 s, which means the multiple scattering caused by the heat-triggered aggregation has also been suppressed. Figures 6(b) and 6(c) shows the solvation effect on the dispersion state of BP colloids by recording the intensity autocorrelation functions of a low-concentration BP colloid in NMP, CH2Cl2, CH3CH2OH and CCl4. Results indicate that the BP nanosheets possess better dispersivity in NMP and CH3CH2OH as the scattering intensity and the intercept β in NMP and CH3CH2OH are obviously higher than those in CH2Cl2 and CCl4. It is worth noting the relaxation processes of BP nanosheets in CH2Cl2 and CCl4 is much slower than in NMP and CH3CH2OH as a result of the worse dispersion of BP nanosheets in CH2Cl2 and CCl4. The slow relaxation mode corresponding to the aggregation process has also been reported for BP colloids in CH2Cl2 and CCl4 (see the insert in Figs. 6(b) and 6(c)), suggesting the weak interaction of neighboring BP nanosheets in good solvents of NMP and CH3CH2OH has been evolved into strong interaction in poor solvents of CH2Cl2 and CCl4. It is worth noting that NMP is a universally good solvent to disperse various nanoparticles, such as BP, transition metal dichalcogenides, carbon nanotubes and graphene [33–35].
Fig. 4 The normalized intensity cross-correlation curves as a function of delay time τ for BP colloids in NMP at T = 297 K for scattering angles from 30° to 150° and the concentrations of BP nanosheets are 31.25, 62.5, 125, 250, 500 and 1000 μgmL−1, from a to f.
Fig. 5 The normalized intensity cross-correlation curves as a function of delay time τ for BP colloids with the concentrations ranging from 31.25 μgmL−1 to 1000 μgmL−1 at T = 297 K and the scattering angles is 30°.
Fig. 6 The normalized intensity cross-correlation curves as a function of delay time τ for BP colloids (scattering angle 30, 31.25μgmL−1) with increasing the temperature from 297 K to 327 K (a). The normalized intensity cross-correlation curves as a function of delay time τ for BP colloids in NMP and CH2Cl2 at a fixed concentration of 31.25μgmL−1 at 297 K and the scattering angle is 30° (b). The normalized intensity cross-correlation curves as a function of delay time τ for BP colloids in CH3CH2OH and CCl4 at a fixed concentration of 31.25μgmL−1 at 297 K and the scattering angle is 30° (c). The insert in Fig. 6(b) and Fig. 6(c) shows the signal from 0.01s to 1s.
Furthermore, to understand the shear behavior and the relationship to dispersion states, a detailed viscoelastic study in the linear-viscoelastic regime has been conducted for single-layer BP nanosheet colloids. Figure 7(a) shows the measured viscosity as a function of shear rate for colloids of single-layer BP at different concentrations. The lowest tested concentration (31.25 μgmL−1) shows a low shear rate (γ = 0.01 s−1) viscosity of 0.01437 Pa•s, which is approximately 8 times the measured high shear rate (γ = 100 s−1) viscosity of 0.00173 Pa•s. When the concentration of BP nanosheets is increased to 62.5 μgmL−1, the low shear rate (γ = 0.01 s−1) viscosity is 0.1352 Pa•s, the corresponding high shear rate (γ = 100 s−1) viscosity is 0.00143 Pa•s. As the BP concentration increases to 125.0 μgmL−1, the low shear rate (γ = 0.01 s−1) viscosity is 1.383 Pa•s, the corresponding high shear rate (γ = 100 s−1) viscosity is 0.000785 Pa•s. The above results prove that all the tested BP colloids demonstrate shear thinning behavior, which is a typical characteristic of colloidal solutions. At the low shear rate end of the viscosity range (γ = 0.01 s−1), it is observed that 4 times increase in BP concentration results in almost 100 times increase in the viscosity. However, at high shear rates (γ = 100 s−1) the viscosities are basically the same when the BP concentrations increasing from 31.25 to 500.0 μgmL−1 as a result of shear thinning behaviour. When the BP concentrations exceed 250.0 μgmL−1, there is viscosity fluctuation in the process of shear thinning. This irregular phenomenon may be caused by the poor dispersity of BP nanosheets colloids, leading to the large aggregation of BP nanosheets, this result is in accordance with that in Fig. 5. The temperature dependence of the viscosities of BP colloids with different concentrations is also assessed in Figs. 7(b) and 7(f), when the temperature is at 297 K, the colloids exhibit classic shear thinning behaviour. When the temperature is increased to 307 K and 317 K, there is irregular fluctuation at lower shear rate (<10 s−1). The shear fluctuation at higher temperatures also indicates that there is the heat-triggered aggregation of BP colloids.
Fig. 7 Viscosities of BP colloids with various concentrations and at different temperatures. Measured viscosity versus shear rate of single-layer BP colloids with concentrations ranging from 31.25 to 500.0 μgmL−1 at 297K (a). Images b-f correspond to the temperature dependence of the viscosities of BP colloids with concentrations of 31.25, 62.5, 125, 250 and 500 μgmL−1.
4. Conclusions
In summary, a series of BP colloids with concentration ranging from 31.25 μgmL−1 to 1000 μgmL−1 have been prepared by two-step exfoliation in tip-sonication and bath-sonication. UV/Vis absorption measurement shows the low-concentration BP colloids possess the linear light absorption in the UV/Vis region, which obeys the Lambert-Beer law, and the molar absorptivities of BP nanosheets increase with the decrease of absorption wavelength. The Eg of exfoliated BP nanosheets is around 2.95 ev based on the Tauc fitting. 3D light scattering has been proved to be an effective technique to characterize the dispersion characteristics of nanocolloids with intensive light absorption and limited dispersity by suppressing the multiple scattering. Results indicate that measurement at smaller angles can obtain better autocorrelation function. The BP colloids exhibit excellent dispersion state when the concentration is below 250 μgmL−1 and begin to form aggregation when exceeding 250 μgmL−1. Increasing the temperature of BP colloids can result in the formation of BP aggregation in the colloids. BP nanosheets possess better dispersion state in polar solvent than in nonpolar solvent, and the aggregation process in nonpolar solvent can be monitored by the slow relaxation mode of the 3D light scattering. It is worth noting that 2D nano-semiconductors generally exhibit intensive light absorption and poor dispersivity in colloids, which makes these colloids possess low light transmittance, therefore 3D light scattering also provides an effective way to analyze the dispersion dynamics of various 2D semiconductor colloids.
Funding
National Natural Science Foundation of China (No. 61805047); the Guangzhou Science Technology and Innovation Commission (No. 201807010108); Foshan Municipal Science and Technology Bureau (2015IT100162); the Innovative project of College Students (201811845151, 201811845154 and xj201811845162).
Acknowledgment
The authors gratefully acknowledge Qi Yan and Zhan Wei for helpful discussions and thank Yufeng Fan for his contribution to the construction of the early-stage experimental setup.
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