Size-Dependent Nonlinear Optical Response of Black Phosphorus

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Size-Dependent Nonlinear Optical Response of Black Phosphorous Liquid Phase Exfoliated Nanosheets in Nanosecond Regime. Beata M. Szydlowska, Bartlomiej Tywoniuk, and Werner Josef Blau ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00469 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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Size Dependent Nonlinear Optical Response of Black Phosphorous Liquid Phase Exfoliated Nanosheets in Nanosecond Regime. Beata Maria Szydłowska,∗,†,‡,k Bartlomiej Tywoniuk,¶,§,k and Werner Josef Blau†,‡ †School of Physics, Trinity College Dublin, D02 PN40, Ireland ‡CRANN, Trinity College Dublin, D02 PN40, Ireland ¶School of Physics, University College Dublin, Dublin, D04 V1W8, Ireland §Institute for Discovery, University College Dublin, Dublin, D04 V1W8, Ireland kContributed equally to this work E-mail: [email protected] Abstract Atomically thin materials are exhibiting unique physical properties. This work investigates the size dependent nonlinear optical response of Black Phosphorous nanosheets for nanosecond laser pulses. Material is prepared with use of Liquid Phase Exfoliation. Experiments are performed using open aperture Z-Scan technique at 532 nm wavelength with ND: Yag, 10 Hz ns as laser source. We report that nonlinear absorption coefficient of Black Phosphorous is strongly dependent on flake lateral dimension as well as laser intensity.

Keywords 2D nanomaterials, black phosphorous, nonlinear optics, z-scan, nonlinear absorption 1

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Introduction Black Phosphorous (BP) is a novel 2-dimensional (2D) material often called as phosphorene. As the second elemental 2D material studied after graphene has brought lots of hopes for novel solutions in electronics applications 1–4 and generated huge excitement in a research environment. It is being believed that it can fill up the gap between no bandgap Graphene and transition-metal dichalcogenides 5 family known as TMDCs. 4 BP’s unique properties include a relatively large bandgap of about 0.3 - 2 eV varying between the bulk and monolayer. 6 Few layer BP is known as well from high charge-carrier mobility, 1 large current on/off ratios, anisotropy and as our recent work show broadband nonlinear optical response from visible to mid-infrared 4,5,7–9 range. Some of the most recent publications present BP as material possessing promising properties for use in optical modulators 10–13 and exhibiting better nonlinear response under nanosecond excitation than over femtosecond. While picosecond 14–16 and femtosecond 11,17 regimes were already investigated there is still a niche of knowledge to be filled in nanosecond regime. To the best of our knowledge, size dependent nonlinear optical responses have not been investigated in BP until now. In this manuscript, we report a comparative, size dependent investigation of nonlinear optical response of two different BP nanosheets flakes sizes in nanosecond regime.

Results and discussion BP nanosheets were prepared via versatile 18 Liquid Phase Exfoliation method and postprocessed as presented at Figure 1. Efficient and effective Black Phosphorous nanosheets exfoliation and high quality of 2D material were shown experimentally on liquid samples and results are presented at Figure 2. To confirm nanosheets high quality and size dependence of optical properties UV-Vis 2


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Figure 1: Schematic describing basic centrifugation cascade employed to prepare samples characterized in this study. Sediment discarded after the first centrifugation contained unexfoliated layered crystallites, Supernatant taken into further steeps contained broad distribution of exfoliated nanosheets of BP. Two final dispersions are small and big BP nanosheets and are called respectively BP-Small and BP-Big. measurements were carried out. As a result, three components: absorption, extinction and scattering were distinguished (Figure 2A). As expected thick and large flakes possess higher scattering contribution. It can be also noticed that extinction spectra shifts with a change of flakes dimensions. 19 The Raman spectrum (Figure 2B), clearly shows all expected characteristic peaks Ag1, B2g and Ag2 of few layers BP. 6 The position of observed Ag1, Bg2, and Ag2 modes blue shifts after exfoliation 5,6 and relative peak intensity changes related to nanosheets thickness are observed as in previous findings. BP nanosheets length and thickness were quantified based on nanosheets height profiles extracted from AFM images for both samples. 10 Lateral dimensions of BP-Small and BP-Big have been calculated statistically based on 200 nanosheets counts from AFM images for each sample (Figure S4). The Average BP-Small nanosheet is: 8-9 nm thick (=4 layers) and 114 nm long, while BP-Big: 22 nm is thick (=11 layers) and 338 nm long. Number of layers distribution for both dispersions are presented at histograms at Figure 2C and were calculated from the measured thickness of individual nanosheets based on step height analysis, 20 where =1

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Figure 2: A) Extinction, absorbance and scattering spectra of BP nanosheets for BP-Small and BP-Big dispersion, B) Raman Spectra’s of the BP nanosheets for both BP-Small and BP-Big dispersion, C) Statistical analysis of thickness dimension of nanosheets extracted from AFM images presented on histograms showing N (number of layers) distribution in BP-Small and BP-Big dispersion. equal to 2.06 nm. 10 Size dependence of nonlinear response 21 (NL) of two sizes BP nanosheets in CHP was studied in nanosecond regime with an open aperture z-scan system (Figure 3) with the total z distance of 120 nm employed.

Figure 3: Schematic diagram of the open-aperture z-scan setup. To acquire comparable results concentration of all samples was adjusted so the linear transmission was always at the level of 20 % at 532 nm. Open aperture z-scans were performed on both samples with multi incident energy set at 5 µJ, 10 µJ, 25 µJ. Each measurement has been repeated 50 times to eliminate errors and averaged to get reliable information on the NL response of the dispersions. Measured BP dispersions exhibits saturable absorption (SA) response, transmission increase as the intensity of the incident beam increases 4


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(Figure S1). Real upward facing curves acquired from the measurements were normalized and fitted according to a nonlinear beam propagation model (Eq. 1). dI(z 0 ) = −α(I)I dz 0

(1)

where z 0 is propagation distance and total absorption α consist as a sum of linear absorption coefficient α0 and nonlinear absorption coefficient βef f (Eq. 2).

α(I) = α0 + βef f I

(2)

The saturation intensity Isat was obtained by employing Saturable Absorption (SA) model (Eq. 3).

α(I) =

α0 I 1 + Isat

(3)

The imaginary part of the third order nonlinear optical susceptibility Imχ3 was calculated from relation that Imχ3 is in proportion to βef f (Eq. 4).

Imχ3 = [

10−7 cλn2 ]βef f 96π 2

(4)

where c is the velocity of light, λ is wavelength of incident light, and n is the refractive index. Last but not least, a figure of merit (F OM ) for the third order optical nonlinearity which removes the discrepancy caused by the different linear coefficients is quantified and defined in Eq. 5

F OM =

Imχ3 α0

(5)

The described standard open z-scan model shows excellent fitting for all the z-scan curves 5


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presented in this study. Fitted plots of experimental z-scan spectra are shown in Figure 4AB.

Figure 4: A, B) Open-aperture Z-scan fitted data. Normalized transmission plotted as a function of sample position Z (distance, mm) at 5µJ, 10µJ and 25µJ for BP-Big (A) and for BP-Small (B) C) NLO response of BP nanosheets with variable sizes BP-Big and BP-Small as a function of pulse fluence. They display upward-facing symmetric, characteristic nonlinear optical extinction (NLE) profiles for BP-Small and BP-Big dispersions. Further, NLO parameters were extracted during fitting experimental signal to referd theoretical equations. All calculated fitting and nonlinear parameters are displayed in Table 1. Figure 4A and Figure 4B present typical open-aperture z-scan curves of BP dispersions with different incident energy (5µJ, 10µJ, 25 µJ). Here, normalized transmission was plotted as a function of sample position along the z-axis. The peaks around z = 0 mm imply the obvious saturable absorption (SA) responses of BP nanosheets at 532 nm in nanosecond regime. As the pulse energy increases, the peak of z-scan trace also rises and get broader. Further analysis of data collected from open-aperture Z-scan measurements obtained from Black Phosphorous measurements revealed and emphasizes nanosheet size and energy dependent NLE. Normalized transmission is presented as a function of energy density (Jcm−2 ) for two different nanosheets sizes and three varying energies at Figure 4C. It is clearly visible that NLE response from Black Phosphorous nanosheets falls into two separate regimes. The weaker NLE responses come from BP-Big sample while stronger NLE response comes from BP-Small 6


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sample. That proves strong size dependence of NLE properties in Black Phosphorous. For nanosheets studied here at an energy density of 1 Jcm−2 , nonlinear optical dissipation was about 26 % for BP-Big and about 150 % for BP-Small. To show the magnitude of captured nonlinear optical response NLO coefficient βef f was estimated (Table 1), as described above based on theoretical fitting of experimental spectra. The NLO absorptive coefficients, βef f of BP dispersions, was estimated to be -16 cm/GW, -261 cm/GW at 5 µJ/pulse, -44 cm/GW, -188 cm/GW at 10 µJ/pulse, -30 cm/GW, -120 cm/GW at 25 µJ/pulse for BP-Small and BP-Big respectively for 5 µJ, 10 µJ and 25 µJ per pulse respectively and is plotted in Figure S2. There is a clear rise of βef f with energy increase for big flakes and no significant change for small flakes. This trend is also visible in Figure 4C, where the normalized transmission ∆T /T0 is plotted as a function of pulse fluence. As shown in Table 1, the nonlinear absorption coefficient βef f is larger for BPBig, which means that BP-Big nanosheets possess lower saturation intensity for saturable absorber devices in the visible range than BP-Small. (All calculated values for α0 and βef f , Imχ3 , F OM and ω0 at 532 nm and specific incident laser energies corresponding to both nanosheets sizes are presented in Table 1). Not only strong size dependence but also an input energy dependence was observed in both cases. The relation varies with the nanosheets thickness. It’s increasing together with incident energy for big flakes and marginally decreasing with energy increase for small nanosheets. On the other hand, ω0 (Figure S3) follows the same trend for both sizes of BP nanosheets. The open aperture z-scan data is presented in Figure S1. In order to carry out a comparative study, graphene and MoS2 dispersions were also characterized by z-scan under the same conditions. The linear optical transmission of graphene and MoS2 dispersions were adjusted to that of BP by concentration. The value of F OM for the BP samples is comparable with graphene 5 × 10−15 esu cm and MoS2 −1.56 × 10−14 esu cm.

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Conclusions In this study, Nonlinear Optical properties of Black Phosphorous nanosheets exfoliated with Liquid Phase Exfoliation method were investigated. AFM, Raman, and PL confirmed the successful exfoliation of BP crystal into a few and monolayered nanosheets. No oxidation was observed. NLO properties of two nanosheets dispersions with varying dimensions, were measured at three different incident laser pulse energies in interaction with 532 nm 6 ns 10 Hz laser pulses using z-scan open aperture technique. The open aperture z-scans directly confirmed the saturable absorption. Two main NLO Regimes were found, each corresponding to specific Black Phosphorous nanosheet size. The results showed that BP-Big exhibits a better NLO response than that BP-Small and stronger SA ability. Considering strong size related NLO responses such materials might be used as saturable absorbers in mod locked lenses and for optical switching

Experimental Material Preparation: Black Phosphorous Nanosheets dispersions were prepared with Liquid Phase Exfoliation method out of the Black Phosphorous crystals purchased from Smart Elements (purity: 99.998 %) and N-Cyclohexyl-2-pyrrolidone (Sigma Aldrich) was used as a solvent 10,20,22 method. Mentioned solvent is responsible for protecting flakes from air or moisture influence. 19,23 What is more it prevents also ageing and oxidation process. The 30 ml dispersion with an initial BP concentration of about 1 mg/ml was sonicated for 6 hours at 30 % amplitude with a horn-probe tip (VibraCell CVX, 750W) under a temperature of 0.5 ◦ C. 6,24 To keep the specified temperature for the duration of the whole experiment specialized cooling system was applied. Final dispersion achieved after sonication hereinafter referred to as: ‘Stock dispersion’ (STd). Was post processed via Liquid Centrifugation Cascade (LCC) according to the schema presented on Figure 1. STd was transferred into two equal aliquots of 15 ml each and centrifuged for 2 hours at a speed of 1 krpm. Sediment 8


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containing unexfoliated material was discarded. Further on we performed size selection. 25 Supernatant was subjected to subsequent centrifugation in 18 small vials (1.5 ml). 9 vials were centrifuged for 2 hours at a speed of 5 krpm, other 9 at speed 2 krpm also for 2 hours. 1,6,24 After that sediments and supernatants were separated and called accordingly: BP5ksup, BP5ksed, BP2ksup, BP2ksed. That gave 4 Black Phosphorous dispersions out of which 2 were taken for further experiments. Those are BP2ksed (relatively large flakes) and BP5ksup (relatively small flakes) called further in that manuscript as BP-Big and BP-Small respectively. UV-Vis Spectroscopy: Extinction and Absorption spectra were measured with a PerkinElmer 650 spectrometer in quartz cuvettes (Hellma) with 10 mm path length. Dispersions were measured in an integrating sphere to distinguish contributions from scattering. Raman spectroscopy: on individual flakes was performed using a Horiba Jobin Yvon LabRAM HR800 with 633 nm excitation laser in air under ambient conditions. Atomic Force Microscopy (AFM): was carried out on a Digital Instruments Veeco NanoscopeIIIa system equipped with an E-head (13 µm scanner) in tapping mode. Before material was deposited on a pre-heated (170 ◦ C silicon wafer (Si/SiO2, oxide layer of 300 nm), the dispersion was transferred into anhydrous isopropanol (IPA) with an oxide layer of 300 nm. Typical 8x8 µm2 images were taken at 0.75 Hz scan rate with 512 lines per image. For both dispersion, several height images were taken to allow proper statistical calculations and trustable numerical results of flakes dimensions (length and thickness). AFM images have been post-processed, and dimensional measurements were taken with the use of Gwydion software. Z-scan & Laser source: the open - aperture z-scan technique 24 was used to measure the total transmittance of samples. All z-scan experiments in this study were performed with 6-ns pulses from Q-switched Nd: YAG laser. The beam was spatially filtered to remove higher- order modes and tightly focused for all experiments. The laser was operated as second harmonic: 532 nm with pulse repetition rate of 10 Hz. Both Black Phosphorous

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dispersions samples were tested in quartz cuvettes with a path length of 1 mm. Linear transmission of samples was fixed at 20% for all z-scan measurements and energies. The energy of 5, 10 and 25 µJ of the incident beam was applied. Table 1: Values of Nonlinear Optical Coefficients for Black Phosphorous at various laser energies at 532 nm including βef f and ω0 and T % corresponding to both nanosheets sizes. βef f ω0 T Imχ3 FOM [cm/GW] [µm] [%] [esu, ×10−15 ] [esu cm, ×10−15 ] BP Small 5µJ -16 18 20 -6 -3 BP Small 10µJ -44 35 20 -17 -7 BP Small 25µJ -30 53 20 -11 -5 BP Big 5µJ -261 32 20 99 42 BP Big 10µJ -188 39 20 71 30 BP Big 25µJ -120 47 20 46 19 Sample

Isat [GW/cm2 ] 126 22 15 48 25 10

Acknowledgement The authors gratefully thank the financial supports from Science Foundation Ireland (SFI, 12/IA/1306). B.M.S and W.J.B. conceived the idea. B.M.S & B.T. wrote the paper. B.M.S & B.T. built the z-scan setup for this work and performed the optical experiment, B.T wrote software to run z-scan measurements and software to analyze z-scan measurements, B.M.S and B.T analyzed the NLO data B.M.S, and W.B interpreted data. B.M.S. prepared the samples, performed and analyzed characterizations. B.MS & B.T performed NLO measurements. Authors thank Kangpeng Wang for discussions.

Supporting Information Available The following files are available free of charge. • SupportingInformation.pdf: Detailed methods, Open-aperture Z-scan raw data, βef f and ω0 vs Energy, AFM images, lateral dimension of nanosheets

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Size-Dependent Nonlinear Optical Response of Black Phosphorus

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