Nonlinear Optical Properties and Temperature Dependent

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Nonlinear Optical Properties and Temperature Dependent Photoluminesecnce in hBN-GO Heterostructure 2D Material Subrata Biswas, Chandra Sekhar Tiwary, Soumya Vinod, Arup K Kole, Udit Chatterjee, Pathik Kumbhakar, and Pulickel M Ajayan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12834 • Publication Date (Web): 22 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017

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The Journal of Physical Chemistry

Nonlinear Optical Properties and Temperature Dependent Photoluminescence in hBN-GO Heterostructure 2D Material Subrata Biswasa, Chandra S. Tiwaryb*, Soumya Vinodb, Arup K. Kolea,†, Udit Chatterjeec, Pathik Kumbhakara,*, and Pulickel M. Ajayanb a

b

c

Nanoscience Laboratory, Dept. of Physics, National Institute of Technology Durgapur, 713209, West Bengal, India.

Department of Material Science and NanoEngineering, Rice University, Houston, Texas 77005, United States.

Laser Laboratory, Department of Physics, University of Burdwan, Burdwan, 713104, India. †

Presently at Department of Physics, School of Applied Sciences, KIIT University, Bhubaneswar, Odisha-751024

* Corresponding author E-mail: [email protected] (Prof. Pathik Kumbhakar), [email protected] (Dr. C. S. Tiwary) Abstract Recently, there is renaissance in theoretical and experimental studies on 2D heterostructures of two 2D wonder materials, namely graphene and hexagonal boron nitride (hBN) having plethora of application potentials in fundamental research as well as in developments of new technological devices. However, the nonlinear optical (NLO) property of hexagonal boron nitride nanosheets–graphene oxide (BNNS-GO) heterostructure has hitherto been remaining unexplored. Here, in this work NLO properties of BNNS-GO have been reported, for the first time, in the visible region by Z-scan technique in nanosecond regime. Nonlinear absorption coefficient (β2PA) and third order nonlinear susceptibility (χ3) of BNNS-GO have been found to be enhanced significantly by 13.4% and 21.7%, respectively in compared to those of bare BNNS. The synthesized heterostructure is showing a superior optical limiting property as compared to that of bare BNNS. The change in polarizabilities of GO sheets with the insertion of hBN in its framework, formation of donor-acceptor pair and bandgap narrowing effects have caused the enhancement in NLO properties of BNNS-GO heterostructure. Temperature dependent photoluminescence (PL) of BNNS-GO have been conducted in 278-303K temperature range and observed Arrhenius type variation of PL intensity with temperature. Thus, it is envisioned that this work will open new vistas of applications of 2D BNNS-GO heterostructure materials in developing photonic safety devices of eye to be used in military and in other industries.

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1. Introduction The families of 2D materials1-3 have attracted much attention in recent times due to their unique and fascinating linear and nonlinear optical (NLO) properties. Amongst all 2D materials, initially graphene family have attracted intense attention in the last decade and have provided unanticipated chances to manipulate on physical, chemical, linear and NLO properties. Due to the zero band-gap nature of graphene4 it shows outstanding mobility of charge carriers which behave as mass less Dirac fermions and absorb light in wide spread spectral range4. This property of graphene makes it suitable for ultrafast carrier dynamics5 and in fast saturable absorber 6 over a wide spectral range. The 2D П-conjugated electrons network of graphene get displaced in response to the externally applied strong electric field causing the large NLO response. Lim et al.7 have studied about NLO absorption properties of dispersed graphene single sheet and reported its broadband absorption properties. Liaros et al.8 have prepared GO dispersion in well defined condition and investigated its NLO response under the visible and infrared excitations with picoseconds and nanosecond pulses. GO colloids showed a large nonlinear absorption (NLA) but negligible nonlinear refraction (NLR) and broadband optical limiting. Many other researchers9,10 have successfully functionalized GO with zinc-phthalocyanine or fluorine and has reported broadband optical limiting properties. However, graphene has also suffered an inherent limitation for its practical applications in photonics and optoelectronics due to its zero band gap nature which delimiting it’s switched off capabilities. However, with the recent introduction of a new class of 2D materials, like insulating hBN11, molybdenum disulfide (MoS2)12, molybdenum diselenide (MoSe2)13, tungsten disulfide (WS2)14 and black phosphorous (BP)15 or heterostructure of hBN and MoS2 with graphene16-19, several new possibilities have opened up to fabricate 2D heterostructure NLO materials for different practical applications. It has been found that hBN is structurally analogous to graphene but unlike sp2 bonded carbon atom in graphene, hBN has alternating 2


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sp2 bonded B and N atoms20. Also hBN is electrically insulating with a room temperature large band-gap of 5.9 eV but exhibits numerous interesting physical properties, like high thermal conductivity, in-plane mechanical strength, and high chemical stability20,21. Wang et al. have investigated the effect of monolayer hBN to enhance the mobility of the charge carriers of graphene based field effect transistors.20 Song et al. have reported the large scale growth of a few layer hBN nanosheets by chemical vapor deposition method and their synthesized hBN layer shows a band-gap of 5.5 eV with the transparency in a wide spectral range, which can be used as dielectric in graphene electronics.21 Gruning et al. have demonstrated by means of first principle numerical simulations that electron-hole interaction can significantly contributes to the second-harmonic generation spectrum of hBN or MoS2.22 Although the NLO properties of graphene and its families have been widely investigated but there are a few reports on the NLO properties of the hBN. Recently temperature dependences of bandgap, linear and NLO properties of hBN nanosheet have been reported by our group.23 However, it has been recognized that by using only single component materials, like GO or hBN, the practical application of these materials in photonics or optoelectronics devices can not be achieved fully. Thus, efforts have been made to synthesize different heterostructured material.24-33 Decoration of graphene or GO with other classes of materials24,25 like quantum dots of CdS26 or PbS

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, large band gap semiconductors like

ZnO28, or with some layered materials like MoS229 have also gained significant research attention. These nanohybrids have been found to exhibit a large enhancement in NLO properties in compared with their naked counterparts. In case of graphene decorated with luminescent quantum dots the enhancement is attributed to the synergetic effect26,27 arises due to the charge transfer between the two components. In case ZnO decorated in graphene sheets the enhancement in NLO properties may occurs due to two photon absorption (TPA), reverse saturable absorption (RSA), and photoinduced electron transfer between ZnO and graphene

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sheets. For MoS2 graphene nanohybrid researchers have shown that they acts as donoracceptor pair

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when excited with a high intensity laser pulse. And the overall change in

electron relaxation dynamics occurs and it caused an enhancement in NLO properties. Recently, Wang et al.

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have investigated the NLO properties of covalently functionalized

reduced graphene oxide with porphyrin by Z-scan technique in both nanosecond and picosecond time domain at 532nm excitation. They have proposed that the improved NLO response for nanohybrids can be attributed to the synergistic effect between two compounds which promotes the electron or energy transfer between the covalently linked two components. Karamanis et al.31 have reported the quadratic NLO responses of graphene by confining hBN sections in its framework. But no reports have been made, so far on the measurement of NLO properties of hBN-GO heterostructures. In this study we have reported, for the first time, the NLO properties of BNNS-GO heterostructure, by Z-scan technique at visible laser excitation and in the nanosecond time domain. The NLA and NLR properties have been investigated by open aperture (OA) and closed aperture (CA) Z-scan techniques, respectively. Enhancements of 13.4% in nonlinear absorption coefficient (β2PA) and 21.7% in third order nonlinear susceptibility (χ3) in comparison with those of bare BNNS have been reported. The nano-sized section of hBN on the GO sheets has been clearly identified by HRTEM study as well as by XRD, RAMAN and FTIR analyses. Temperature dependent PL of BNNS-GO have been conducted in the temperature range of 278-303K and a quenching of PL is observed without any shift in peak position and an Arrhenius type dependence of PL intensity with temperature has been observed. The activation energy of thermal quenching of PL has been evaluated by numerical fitting and it is found out to be 227 meV which indicates its good thermal stability. The synthesized BNNS-GO heterostructure exhibits the optical limiting (OL) properties and may find application in military safety devices.

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2. Results and discussion Figure 1a depicts the UV-Vis absorption spectra of BNNS and BNNS-GO heterostructures. The absorption spectrum of BNNS shows absorption maxima at ~ 5.8-6.1 eV region corresponding to its inter-band absorption23. However, absorption spectra of BNNS-GO heterostructure, contain a π-plasmon (n-π* transition) absorption peak34 at ~ 4.5 eV due to GO and a slight blue shifted but intense inter-band absorption peak due to BNNS. The simultaneous presence of the characteristic peaks due to both BNNS and GO clearly indicates the presence of BNNS section in GO sheets. Also from the measured room temperature absorption characteristics we have calculated the bandgaps of both BNNS23 and BNNS-GO heterostructure. For calculation of bandgap, we have plotted (αhv)2 vs. hv at the band edge region having highest linear absorption (α) and it is found from Figure 1b that the linear relationship between (αhv)2 and hv [i.e. (αhv)2 ∝( hv − Eg)] is fulﬁlled. The bandgap of BNNS is 6.1 eV

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whereas the same for BNNS-GO is 5.6 eV. The black scattered points in

Figure 1b are the experimental data extracted from absorption spectra and the solid blue line has been drawn to find the point of intersection of it to the energy axis which gives the band gap of BNNS-GO. The inset of Figure 1b exhibits the Tyndall effect experiments with bare BNNS as well as with BNNS-GO heterostructures dispersed in water using a red continuous wave He-Ne laser. The cuvette in the left and right hand side contains BNNS and BNNS-GO heterostructures dispersed in DI water, respectively. The detectable scattering of laser light within the sample solution indicates the good colloidal nature of the prepared samples. BNNS dispersion in water appears milky white in color and it scttered the laser light much strongly than the BNNS-GO heterostructure, indicating the hydrophobic nature of the BNNS-GO heterostructure due to the presence of GO. However, from the figure we can read the letters written behind them showing that both samples are transparent to visible light. XRD pattern of the synthesized BNNS-GO heterostructure is demonstrated in Figure 2a in which the XRD

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appeared peak at 2θ = 10° corresponds to the reflection from (001) plane of GO

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and the

peak at 2θ = 27° due to the (002) plane of hBN 36. The presence of XRD peaks due to both hBN and GO in the synthesized heterostructure has confirmed that hBN are embedded into GO. From the analysis of XRD data of BNNS-GO heterostructure it is revealed that there is a huge low angle shift of 2.5° from usual position of (001) plane of GO reported in literature. This considerable shift of XRD peak of GO may be attributed to the insertion of BNNS layers within GO layers which is also confirmed by our HRTEM study as discussed later. Figure 2b shows the Fourier Transform Infrared (FTIR) spectra of BNNS-GO heterostructure. The presences of C=O and C-O of the stretching vibration of COOH group at 1730 and 1039 cm−1 are clearly evident from FTIR spectrum36, 37. The absorption peak at 1100 cm-1 is assigned due to the stretching vibration of C-OH alcohol and the broad peak between 3000-3800 cm-1 corresponds to the stretching and bending vibration of OH groups of water molecules36, 37. The presence of the oxygen containing groups as evident from FTIR spectra explains the hydrophilic nature of the heterostructures as we have observed in Tyndall effect experiments. The absorption bands appeared at a low frequency of ~ 785 cm-1 refer to the B-N-B bending vibration and the band at ~ 1385 cm-1 stands for in-plane B-N stretching of hBN36, 38 and also confirms the presence of hBN in BNNS-GO heterostructure. For further structural analysis we have used RAMAN spectroscopy which is often used as non-destructive technique for analysis of graphene based heterostructures. As hBN is structurally analogous to the graphene it exhibits a characteristics peak originates from E2g phonon vibration mode of B-N bond, like G peak of GO originates from E2g phonon vibration mode due to stretching of C-C bond sp2 graphitic structure36, 39. Figure 2c exhibits the RAMAN spectra of BNNS-GO heterostructure and the characteristic peak of hBN has appeared at 1360 cm-1 and the G band of GO has appeared at 1595 cm-1. The appearance of another broad peak at high frequency region indicates the presence of disorder (2D peak at ~ 3000 cm -1) in the crystal structure of BNNS-

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GO heterostructure. Thus RAMAN structural analysis has strongly confirmed the formation of the BNNS-GO heterostructures in the synthesized sample. The crystalline structure of synthesized heterostructure has also been characterized by the TEM study. Figures 2d-2f showed the SEM images of the heterostructure at different magnifications where the hBN sections on graphene sheets are clearly seen. Figures 2g and 2h exhibit the TEM images of the heterostructures on lacy carbon sheets and we can clearly observe the hBN sections embedded on to the GO sheets. The inset of the Figure 2g depicts the selected area electron diffraction (SAED) pattern taken on BNNS-GO sheet. We have observed the circular ring in SAED pattern which is an indication of the different crystallographic nature of the formed BNNS-GO heterostructure. In Figure 2i we have provided the HRTEM image showing the crystallographic planes of heterostructure where we have clearly identified the sp2 bonded B-N atoms in hBN in GO sheet.36, 40 Room temperature PL emission characteristics and the temperature dependences of the light emission property of the BNNS-GO heterostructure have been investigated and those results are shown in Figures 3a-3f. It has been reported earlier41, 42 by several researchers that hBN is a highly luminescent material and produce intense luminescence in UV region (3-5.7 eV) of the electromagnetic spectrum. Our group has also reported earlier broad UV PL emission in 3-5eV region in BNNS dispersion.23 The radiative recombination of deep level donor-acceptor pair (DAP)23, 43 is attributed to the PL emission in UV region. In case of GO the π- π* states of highly localized sp2 core of GO lies with the π- π* states of the functional group of GO which acts as radiative recombination centre and produce visible luminescence under excitations. The energy gap of this π-π* states generally varies from 1.7 eV to 2.4 eV but it depends strongly on the relative ratio of the sp2 and sp3 cluster size of GO.44 Figure 3a exhibits the excitation dependent PL emission spectra of the synthesized BNNS-GO heterostructures, being similar to that of BNNS, and we have observed broad emission

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spectra within 3.2-4.5 eV. However the intensity of the PL emission peak in BNNS-GO is considerably improved in comparison to those of BNNS or GO only as can be seen from Figure 3f. From Figure 3f it is observed that GO has almost negligible emission in 3-5 eV region. However, after the formation of the BNNS-GO heterostructure the emission intensity has been drastically improved and even it has become greater than that of BNNS only. This clearly indicates that hBN sections on the surface of the GO have provided immense light active sites, which has also contributed to the enhancement in NLO properties as observed later. Figure 3c shows the variation of peak intensity of the first PL peak with the excitation energy. Here we have observed that the peak PL intensity at first increases with the excitation energy and reaches to its maximum value at 6.06 eV but it is reduced with further increment in excitation energy. Figure 3d shows the PLE spectra (collected for the PL emission wavelength of 340nm), that can be used to explain the observed variation of peak PL intensity with excitation energy. It is found that the intensity of the PLE spectra increases after the excitation energy of 5.6 eV (220nm), reaches to its maximum value at excitation energy of ~ 6.0 eV and then it is decreased. A small peak at ~ 4.48 eV in PLE spectra is attributed to the π-plasmon (n-π* transition) absorption peak at ~ 4.50 eV due to GO as we have discussed earlier.44 The investigation of temperature dependent PL emssion

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is an

important tool to obtain information of the defect related emission. The temperature dependent PL emission of BNNS-GO in the temperature range of 278-318K has been presented in Figure 3b. The individual Gaussian fittings have revealed that position of the PL peak has not been altered with temperature however the intensity of PL peak has reduced with increasing temperature. Thus we have fitted the total integrated PL intensity of the sample at different temperatures with the Arrhenius relation47 of thermal quenching of PL intensity with temperature and are shown in the inset of Figure 3e. The activation energy is

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obtained by fitting is 227 meV which is greater than the activation energy of ZnO nanocrystal48 in the temperature range of 20-100K. The higher value of the activation energy indicates the good thermal stability of the synthesized BNNS-GO heterostructure and extends its possibilities to use in the laser driven devices as laser beam produce large local heating. To investigate the NLA process in BNNS and BNNS-GO heterostructure we have performed the OA Z-scan experiments.49,50 Figures 4a and 4b exhibit the intensity dependent OA Z-scan curves of both samples. The scattered points are the experimental OA Z-scan transmittance data (normalized). To reveal the multi-photon absorption (lPA) process50,51 responsible for NLA, we have fitted the experimental OA Z-scan points with the analytical equations reported elsewhere50,51, and also it is discussed below briefly. When a laser beam propagates through a thin sample having thickness = L, such that L

Nonlinear Optical Properties and Temperature Dependent

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