Article Cite This: J. Phys. Chem. C 2018, 122, 25573−25579
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Dramatic Enhancement of the Nonlinear Optical Response of Hydrogenated Fluorographene: The Effect of Midgap States Ioannis Papadakis,†,‡ Zoi Bouza,†,‡ Stelios Couris,*,†,‡ Vasileios Mouselimis,§ and Athanasios B. Bourlinos§ †
Physics Department, University of Patras, 26504 Patras, Greece Institute of Chemical Engineering Sciences (ICE-HT), Foundation for Research and Technology-Hellas (FORTH), P.O. Box 1414, Patras 26504, Greece § Physics Department, University of Ioannina, 45110 Ioannina, Greece Downloaded via KAOHSIUNG MEDICAL UNIV on November 16, 2018 at 22:56:18 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
ABSTRACT: The third-order nonlinear optical (NLO) response of hydrogenated fluorographene (CFH), a two-dimensional counterpart of graphane, was investigated in the visible and in the infrared, using nanosecond laser excitation, and compared to that of graphene fluoride (CF) and (unfunctionalized) graphene (G). All three graphenes were found to exhibit important NLO response, where CFH exhibiting the largest under visible and infrared excitations. In the visible, the response of CFH was determined to be two to three times larger than that of CF and G. However, in the case of infrared excitation, a dramatic enhancement that has not been previously observed for graphene derivatives was noticed, which is more than 2 orders of magnitude higher than that of CF and about 1 order of magnitude higher than that of G. This is attributed to the presence of midgap states which are formed upon functionalization of graphene and can enhance resonantly the NLO response of CFH. It is worth noting that the third-order nonlinear susceptibility χ(3) of hydrogenated fluorographene reached as high as 3 × 10−9 esu. To the best of our knowledge, this is a rather exceptional value for graphene derivatives.
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limiters4−6 and ultrafast lasers,7−9 to optical sensors10,11 and several others, because a continuously growing interest is observed, concerning the exciting optoelectronic properties of graphene and its derivatives. A relatively recent member of the family of graphene derivatives is graphene fluoride or fluorographene (CF).12−14 Regarding this graphene derivative, the stronger the fluorination is, the more the carbon bonds of graphene which are transformed from the initial sp2 hybridization of unfunctionalized graphene to sp3 hybridization, resulting in modification of its electrical and optical properties. As a result, fluorographenes hold great promises for applications in highperformance materials, such as high-tech batteries, dielectrics, sensors, and so forth, whereas recently, it has been shown that they exhibit important optical limiting performance as well.15,16 An even more recent graphene derivative is hydrogenated fluorographene (CFH) which is considered as the 2D counterpart of graphane (hydrogenated graphene).17 CFH is directly derived from CF by hydride reaction and contains both sp3 and sp2 carbon domains. The former is related to the newly inserted C−H groups and residual C−F groups after hydride substitution, whereas the latter is related to graphenic domains resulted from the partial hydride reduction of CF.
INTRODUCTION The recent discovery of graphene,1 being a one-atom thick layer of sp2 hybridized carbon atoms arranged in a honeycomb arrangement forming a two-dimensional (2D) lattice, has excited the scientific curiosity because of its unexpected and unusual structure. Shortly after its discovery, it was realized that graphene is characterized by strong covalent intralayer bonds and weak van der Waals interactions between its adjacent layers which lead to a unique band structure system, this last being the source of the remarkable properties of graphene and in particular of the optoelectronic ones. These extraordinary properties have further boosted the scientific interest about graphene and its derivatives in view of their various potential applications. Therefore, the existence of linear dispersion of the massless Dirac fermions implies a smooth and almost constant absorption profile over the entire optical spectrum, thus denoting the resonant character of any optical excitation of graphene. In addition, saturable absorption because of Pauli blocking, when carriers generated because of strong optical excitation, leads to the depletion of the valence band and the filling of the conduction band, preventing any further absorption and thus expressed as an increase of the material transmission at high-enough photon pump rates. These properties combined with the high carrier mobility and the mechanical and thermal properties of graphene make graphene an ideal candidate for a wide range of optoelectronic applications, ranging from saturable absorbers,2,3 to optical © 2018 American Chemical Society
Received: August 31, 2018 Revised: October 18, 2018 Published: October 18, 2018 25573
DOI: 10.1021/acs.jpcc.8b08491 J. Phys. Chem. C 2018, 122, 25573−25579
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The Journal of Physical Chemistry C The ability of tuning the sp3/sp2 ratio can be an efficient tool and a powerful strategy to continuously tuning the band gap of CF, thus modifying its behavior from conductor to insulator.14,16,18 In addition, the formation of midgap states induced by the insertion of the F and/or H adatoms on the graphenic sheet has also been suggested and confirmed for these derivatives and is also expected to affect the optical and electronic properties. Although these phenomena are not yet fully rationalized, they are expected to add more flexibility in tuning these 2D material properties accordingly.19,20 In that view, the third-order nonlinear optical (NLO) response of the hydrogenated fluorographenes is expected to be substantially different compared to that of their nonhydrogenated counterparts. In the present study, the effective third-order NLO response and the related NLO properties (i.e., refraction and absorption) of hydrogenated fluorographene (CFH) are investigated under nanosecond (ns) laser excitation, both in the visible (532 nm) and in the infrared (1064 nm). To facilitate direct comparison of the CFH effective NLO response with the corresponding responses of CF and of pristine graphene, all graphenes have been studied at the same time under similar experimental conditions. A dramatic enhancement of more than 2 orders of magnitude of the NLO response of hydrogenated fluorographene was observed under infrared laser excitation, making CFH the graphene derivative with the larger transient NLO response reported so far to the best of our knowledge. The observed large enhancement of the NLO response is discussed in view of the midgap states formed within the band gap of CFH.
the propagation direction of a focused laser beam, by two different experimental configurations, widely known as “openaperture” (OA) and “closed-aperture” (CA) Z-scans, respectively. In the case where NLO absorption is present, instead of the CA Z-scan, the so-called “divided” (D) Z-scan is used for the determination of γ′, resulted from the division of the CA Zscan recording by the corresponding OA one. In this way, the effect of nonlinear absorption is removed from the CA Z-scan and the NLO refraction can be determined from the D Z-scan in the absence of nonlinear absorption. In practice, this approximation is more accurate under low linear (e.g., absorbance less than about 0.25) absorption and weak NLO absorption at the excitation wavelength. Otherwise, the Z-scan assumptions and approximations (e.g., thin sample approximation, wave-front phase distortion |ΔΦ0| < π, etc.) are not fulfilled anymore.21−25 As a guideline, for an accurate determination of the NLO parameters by means of the Zscan technique,21,22 it is safer the OA and CA Z-scan transmittance variations not to be very large (e.g., no exceeding about 20% of their value at the linear regime). If not, different methodologies26 are to be employed for the analysis of the Zscan data instead of the usual Z-scan approximative formulas, as these are not valid any more. In general, the appearance of a transmission minimum (or maximum) in an OA Z-scan recording indicates the presence of reverse saturable absorptionRSA (or saturable absorptionSA), corresponding to positive (or negative) NLO absorption coefficient β. Correspondingly, the appearance of a transmission peak followed by a transmission minimum (i.e., a peak valley) in the CA or the D Z-scan suggests self-defocusing action, corresponding to negative NLO refractive index parameter γ′. The opposite transmission configuration, that is, a transmission minimum followed by a transmission peak (i.e., a valley-peak) in the CA or the D Z-scan suggests a sample exhibiting self-focusing, thus corresponding to positive γ′. From the analysis of the various OA and CA Z-scans, obtained under different incident laser intensities, on various concentration dispersions, the NLO absorption coefficient β and the NLO refractive index parameter γ′ can be accurately determined. The former parameter is related to the imaginary part, Imχ(3), of the third-order nonlinear susceptibility χ(3), whereas the latter parameter is associated with the real part, Reχ(3). From the real and imaginary parts of the third-order NLO susceptibility, its absolute magnitude can be easily calculated.
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EXPERIMENTAL SECTION The NLO response of hydrogenated fluorographene (and of the other graphenes) was investigated using the Z-scan technique, employing a 4 ns Q-switched Nd:YAG laser operated at a repetition rate up to 10 Hz. For most of the experiments, a low repetition rate of 1 or 2 Hz was chosen. For the experiments, the laser fundamental output at 1064 nm and its second harmonic (SHG) at 532 nm were employed. CFH was obtained by hydride substitution of fluorine atoms of fluorographene, using sodium borohydride NaBH4 as the hydride source according to the procedure described in detail elsewhere.17 For the measurements, dispersions with different concentrations of CFH in acetone were prepared. Graphene and fluorographene were dispersed in dimethylformamide (DMF). The dispersions were placed in 1 mm thick glass cells for the measurement of their absorption spectra and for the Zscan measurements. The UV−vis−NIR absorption spectra of the prepared dispersions were measured with a spectrophotometer and were regularly checked during the experiments to ensure their stability. The laser beam was focused into the cells using a 20 cm focal length quartz lens. The spot radii at the focus of the 532 and 1064 nm laser beams were determined to be 17.5 and 30 μm, respectively, using a charge-coupled device camera. A more detailed description of the experimental setup and the procedures followed for the collection and the analysis of the experimental data can be found elsewhere.21−23 Here, only a brief description will be presented, and some critical points concerning the precautions that must be considered when using Z-scan will be reminded. Therefore, briefly, the nonlinear absorption coefficient, β, and the nonlinear refractive index parameter, γ′, were determined from measurements of the variation of the transmittance of a sample, as it moves along
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RESULTS AND DISCUSSION Recently, the third-order NLO response of pristine graphene, CF, and CFH under 35 ps, 532, and 1064 nm laser excitation was reported.17 CFH was found to exhibit the largest NLO response compared to G and CF, both for visible and infrared excitation, the former being larger than the latter, in agreement with the relatively larger linear absorption of CFH in the visible than in the infrared. The NLO response resulting from picosecond (ps) [or femtosecond (fs)] laser excitation is arising from the instantaneous electronic response of the electrons to the externally applied electric field of the laser light and is usually stated as electronic NLO response. During excitation with ps/fs laser pulses, different physical mechanisms can be effective, such as the optical Kerr effect, the degenerate four wave mixing, the third harmonic generation, the intensity-dependent refractive index, and others, depending 25574
DOI: 10.1021/acs.jpcc.8b08491 J. Phys. Chem. C 2018, 122, 25573−25579
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investigating the dependence of the band gap of different graphene derivatives (graphane,30−32 fluorographene,13,33,34 etc.) on the degree of functionalization and the conformation of the adatoms on the graphenic sheet.35−37 Therefore, whereas pristine graphene is a (almost) zero band gap 2D semiconductor (i.e., semimetal), fully fluorinated graphene (CF) is considered as a wide gap semiconductor, despite any inconsistencies between the experimentally and theoretically calculated band gap values. Actually, the experimentally determined optical band gap of CF has been reported between 3 and 3.8 eV, whereas theoretical calculations suggested band gap values reaching up to ∼6.3 eV.13,34 Moreover, it has been shown that the degree of fluorination and the conformations of the F atoms on the graphenic sheet can influence substantially the band gap of partially fluorinated graphene.13,33−35 Therefore, low degree of fluorination results to lowering of the band gap value of the fluorinated graphene. In particular, ab initio calculations and large-scale tight-binding simulations have revealed that in the case of partially fluorinated graphene, different types of structural disorders can exist, such as C vacancies, F vacancies, fluorine vacancy clusters, and fluorine clusters with armchair and/or zigzag or other conformations.34 These structural disorders correspond to the defect states (or surface states) of the typical inorganic semiconductors and are responsible for the presence of different types of midgap states and other extra excitations occurring within the optical gap.34 These midgap states can assist the enhancement of the absorption, thus favoring excited-state absorption, as it has been found for instance for other systems (e.g., glasses, semiconducting thin films, etc.).35−38 Graphane, on the other hand, with all C atoms bonded to H atoms alternately from either side of the graphene plane, is also considered as a wide band gap semiconductor with a reported band gap up to ∼5 eV or even larger.39−41 In addition, its band gap can be tuned effectively with the degree of H coverage. Therefore, for instance, a hydrogenated monolayer graphene with an H atom coverage of ∼12% shows a band gap slightly larger than 1 eV.35 In addition, the H vacancies (defined as the C atoms not bonded to H atom) and their conformations (such as cluster formation and their morphology, zigzag or/ and armchair arrangements, etc.) can also affect graphane’s band gap. The synthesis of CFH required two steps. In the first step, liquid-phase exfoliation of fluorinated graphite (C/F = 1:1 ratio, Aldrich) in DMF by sonication resulted to fluorographene sheets (CF). In the second step, the CF sheets were hydrogenated by means of hydride nucleophilic substitution/ reduction of the CF layers as described in detail elsewhere.17 From the analysis of the X-ray photoelectron spectroscopy measurements of the CFH samples, a C 18 H 2.2 F 2.8 O 1.3 stoichiometry was derived, leading to an estimation of graphene’s total functionalization of ∼35%, with ∼12 and ∼15% of H atom and F atom coverages, respectively.17 From the synthetic protocol employed, it becomes evident that upon defluorination of the fully fluorinated CF, F vacancies are created on the graphitic sheet. During hydrogenation, H atoms can be attached to some of the vacancies, forming C−H bonds, whereas some F vacancies may remain. In all cases, different conformations and/or cluster formations can be formed as it has been discussed elsewhere.39−41 The estimation of the effect of each individual situation described above on the band gap value is not known so far to the best of our knowledge.
on the system studied and the experimental details (e.g., the laser pulse duration, the low/high repetition rate of the laser, etc.). These mechanisms can in principle contribute to the overall macroscopic NLO response each one with different strengths because of the different dispersion laws they obey and the time scales (dynamics) associated with each specific mechanism.27−29 In the case of ns laser excitation (where usually lower laser intensities are used), however, the NLO response is mostly due to mechanisms occurring at slower time scales, as for instance two- or multiphoton absorption, excited-state absorption, free carrier absorption, thermal effects, and so forth or a combination of these. In this case, the observed NLO response and the related parameters are characterized as effective quantities (e.g., effective third-order susceptibility χ(3)).23 The present experimental results concern the determination and the comparison of the effective third-order NLO response of CFH, CF, and G under 4 ns, visible (532 nm), and infrared (1064 nm) laser excitation. In order to check for any nonlinear scattering, a sensitive photodiode was used, placed after the sample, at a direction of about 30° with respect to the laser beam propagation axis. In front of the photodiode, a slit was placed in order to minimize parasite light from reflections arising from reflective surfaces of the experimental setup. For the range of laser energies used in these experiments, no significant scattered light has been detected. In Figure 1, the UV−vis−NIR absorption spectrum of a 0.1 mg/mL dispersion of CFH in acetone is shown together with
Figure 1. Absorption spectra of graphene (G), fluorographene (CF), and hydrogenated fluorographene (CFH) dispersions (all corresponding to a concentration of 0.1 mg/mL).
the spectra of pristine graphene and fluorographene (both dispersed in DMF), all spectra referring to the same concentration of 0.1 mg/mL for easy comparison. All samples’ absorption at 532 nm was relatively larger than that at 1064 nm, the smaller relative variation between the two wavelengths observed for G. In addition, as can be seen, the absorption spectra of CF and CFH were similar to that of graphene, that is, smooth and featureless, except of a steeper increase to the UV side. This is suggestive of a larger band gap for the two functionalized graphenes compared to G, with CFH’s band gap being lower than that of CF and closer to that of G. This finding is in agreement with the findings of other studies 25575
DOI: 10.1021/acs.jpcc.8b08491 J. Phys. Chem. C 2018, 122, 25573−25579
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Figure 2. OA (a,b) and D (c,d) Z-scans obtained under 532 and 1064 nm excitation.
ns excitation two-photon absorption and/or excited states, absorption and other mechanisms become operational, as it has been discussed in detail elsewhere.6,42−44 Concerning the magnitude of the NLO absorption coefficient β of CFH, it was found to be 5 and 10 times larger than that of G and CF, respectively, under visible excitation, whereas a remarkable increase, by 40 and 200 times, respectively, was observed under infrared excitation. Concerning the NLO refraction of CFH, as depicted by the D Z-scans of Figure 2c,d, CFH and G exhibited positive NLO refraction, that is, a valley-peak configuration, indicating selffocusing action and positive nonlinear index of refraction, n2, whereas CF exhibited the opposite behavior, that is, a peakvalley configuration, denoting self-defocusing action and negative n2. Moreover, CFH had the largest response similar to the trend found previously, about the NLO absorption of these graphenes. In particular, the NLO refraction parameter γ′ of CFH was found to be about 2 and 3 times larger than that of G and CF for visible excitation, whereas for infrared excitation, 50-fold and 100-fold enhancements, respectively, were found. From the comparison of the sign of the NLO refraction of the graphenes studied here with that of ref 17, it is worth to mention that it was found to remain unchanged (i.e., both for ps and ns excitation), oppositely to the sign of the NLO absorption discussed previously. Therefore, G and CFH display self-focusing (n2 > 0), whereas CF shows selfdefocusing (n2 < 0) under both ns and ps laser excitations, for visible and infrared excitation as well. In Figure 3, the variation of the NLO parameters (i.e., β, γ′, χ(3)) with the excitation wavelength is presented. As shown, G and CF exhibited clearly stronger NLO absorption and refraction for visible than for infrared excitation, the modulation of the former being clearly weaker than that of the latter. On the other hand, CFH NLO absorption and refraction were both found significantly larger for infrared than for visible excitation, the modulation of the former being
Therefore, on the basis of the above discussion, CFH can be considered, in a simplistic view, as a 2D graphenic sheet combining some of the morphological characteristics of partially fluorinated graphene (resulted from the defluorination of fully fluorinated graphene during the preparation of CFH) with those of a partially hydrogenated graphene. The combination of the two adatoms, that is, F and H, on the same graphenic sheet could eventually provide another way of selective tuning of the electronic and optoelectronic properties of CFH, thus, offering desirable electronic properties suitable for the engineering of graphene-based nanoelectronic devices. In Figure 2, some representative OA and D Z-scans obtained under visible and infrared excitations are presented. As can be seen, the OA Z-scans of all graphenes’ dispersions shown in Figure 2a,b exhibited a clear transmission minimum, suggesting RSA behavior. It should be added that because the solvents used (i.e., acetone and DMF) do not present any NLO response under the present experimental conditions, the Zscans recordings reveal directly the sign of the NLO response, allowing for the immediate diagnosis of the signs of the NLO response from simple inspection of the experimental curves without further analysis. The RSA behavior of the current samples, in contrast to the SA one found for ps excitation, constitutes a clear evidence of the effect of laser pulse duration on the NLO response.16,18 Therefore, although SA has to be expected because of the almost flat absorption spectra, indicating the resonant character of visible and near-infrared excitations, nevertheless RSA behavior was observed under ns excitation at both excitation wavelengths. RSA was observed for all concentrations studied and intensities used (i.e., from 0.001 to 0.1 mg/mL and 2.46 to 271 MW/cm2). The change of the shape of OA Z-scans indicates the change of the sign of the NLO absorption coefficient β, from negative (β < 0) to positive (β > 0). This sign alteration reveals the change of the operational mechanism responsible for the NLO response. Therefore, for 25576
DOI: 10.1021/acs.jpcc.8b08491 J. Phys. Chem. C 2018, 122, 25573−25579
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various laser excitation energies. In all cases, care was taken to satisfy the experimental conditions (e.g., low enough transmission variations in both OA and CA Z-scans, low absorption, low NLO absorption, etc.) ensuring the adequate use of Z-scan technique within the range of validity of its approximations. In all cases, the enhanced effective NLO response of CFH under infrared excitation was confirmed. Finally, from the nonlinear absorption coefficient β and the NLO refractive index parameter γ′, the effective third-order susceptibility χ(3) was calculated for the three graphenes. CFH’s χ(3) was found to be the largest one in all cases. Specifically, it was determined to be 2−3 times larger than that of G and CF under visible excitation, whereas being 40 and more than 100 times larger in the case of infrared excitation. In Figure 3c, the variation of the effective χ(3) under visible and infrared excitation is presented for G, CF, and CFH. The decrease of the χ(3) values for both G and CF and the dramatic increase for the case of CFH with the laser excitation wavelength are clearly seen. Actually, CFH’s effective χ(3) value was determined to be greater than 3 × 10−9 esu (referring to a concentration of 1 mg/mL), which is among the largest ones reported to the best of our knowledge for graphene and graphene derivatives under ns laser excitation. The present great improvement of the NLO response of CFH is a very encouraging result toward the practical use of graphene derivatives for photonic applications in the near infrared spectral region. The determined values of the NLO parameters (i.e., the NLO refractive index parameter γ′, the NLO absorption coefficient β, the magnitude of the third-order susceptibility χ(3), and the nonlinear refractive index n2) of CFH, CF, and pristine G are summarized in Table 1 (all values referring to a concentration of 1 mg/mL to facilitate comparison). The very large NLO response of CFH under infrared excitation, which is more than 2 orders of magnitude larger compared to that of G and CF, creates new expectations for the use of CFH for photonic and optoelectronic applications. The combination of the low band gap of partially hydrogenated graphene together with the presence of midgap states could be the possible sources of the observed enhancement in the case of infrared excitation. Nevertheless, the presence of midgap states was experimentally confirmed for partially hydrogenated graphene.45 Moreover, first-principles calculations employed to study the effect of the different configurations and concentrations of H adatoms on the density of states of partially hydrogenated graphene suggested the presence of several lowenergy peaks, delta-function like peaks, and other structures within the band gap.46 Furthermore, it is interesting that similar conclusions were reached, by another first-principles
Figure 3. (a) Variation of the nonlinear absorption (β), (b) nonlinear refraction (γ′) and (c) third-order NLO susceptibility χ(3) of G, CF, and CFH with the excitation wavelength.
double than the latter. In more detail, the ratio of the β values of the NLO absorption, βvis/βir, was found to be 0.31, whereas the corresponding ratio of the γ′ values of the NLO refraction, γ′vis/γ′ir was found to be 0.17. This finding suggests that the physical mechanism responsible for the enhancement of the NLO response of CFH in the infrared will be more reliant on the absorption processes taking place. To further confirm the present experimental results and the remarkable enhancement of the NLO response of CFH observed for infrared excitation, additional experiments were performed on different concentration dispersions, employing
Table 1. NLO Parameters of G, CF, and CFH (All Values Are Referring to a Concentration of 1 mg/mL)
a
sample
β (×10−11 m/W)
G (in DMF) CF (in DMF) CFH (in acetone)
1240 ± 149 601 ± 72 6238 ± 811
G (in DMF) CF (in DMF) CFH (in acetone)
526 ± 65 100 ± 17 20 136 ± 2048
n2 was calculated using the relation: n2 (esu) =
cn0 γ′ 40π
γ′ (×10−18 m2/W) 532 nm 1440 ± 173 −1265 ± 177 3411 ± 375 1064 nm 401 ± 54 −193 ± 28 19 614 ± 2602
|χ|(3) (×10−13 esu)
a
n2 (×10−12 esu)
2000 ± 313 1769 ± 230 5000 ± 598
4921 ± 414 4322 ± 423 11 072 ± 896
778 ± 96 270 ± 39 31 000 ± 3831
1370 ± 129 659 ± 67 63 667 ± 6219
(m 2/W). 25577
DOI: 10.1021/acs.jpcc.8b08491 J. Phys. Chem. C 2018, 122, 25573−25579
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Sector”, funded by the Operational Programme “Competitiveness, Entrepreneurship and Innovation” (NSRF 2014−2020) and co-financed by Greece and the European Union (European Regional Development Fund). All authors acknowledge O. Gazeli (MSc) for valuable assistance with the artworks.
investigation studying how the adsorption of H atoms on graphene was occurring, through opening of a substantial band gap between the occupied and unoccupied bands, with the simultaneous formation of a spin-polarized midgap state.47 Actually, the large increase of the NLO absorption, which is doubled compared to that found for visible excitation, could be due to the presence of midgap states, which result in some resonant-like enhancement under 1064 nm laser light. Correspondingly, in the case of visible, 532 nm laser excitation, resonant conditions are not met anymore; hence, the observed NLO response is significantly lower. Even in this case (i.e., under visible excitation), the NLO response of CFH is significantly larger than that of G and CF, with CF’s response being lower than that of G. In other words, it seems that fluorinated graphene, with a larger band gap than pristine graphene, exhibits lower NLO response at both excitation wavelengths. Furthermore, interestingly, hydrogenation of CF not only assisted recovery of the NLO response but also increased it dramatically. The investigation of the effect of the degree of hydrogenation and fluorination of CFH and the variation of the F/H ratio on the NLO response are among the critical parameters that need further study toward the full understanding of the enhanced NLO response of CFH in view of real-world applications.
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(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, Y. Z. D.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric field effect in atomically thin carbon films. Science 2004, 306, 666−669. (2) Bao, Q.; Zhang, H.; Wang, Y.; Ni, Z.; Yan, Y.; Shen, Z. X.; Loh, K. P.; Tang, D. Y. Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers. Adv. Funct. Mater. 2009, 19, 3077−3083. (3) Sun, Z.; Hasan, T.; Torrisi, F.; Popa, D.; Privitera, G.; Wang, F.; Bonaccorso, F.; Basko, D. M.; Ferrari, A. C. Graphene mode-locked ultrafast laser. ACS Nano 2010, 4, 803−810. (4) Wang, J.; Hernandez, Y.; Lotya, M.; Coleman, J. N.; Blau, W. J. Broadband nonlinear optical response of graphene dispersions. Adv. Mater. 2009, 21, 2430−2435. (5) Liaros, N.; Koudoumas, E.; Couris, S. Broadband near infrared optical power limiting of few layered graphene oxides. Appl. Phys. Lett. 2014, 104, 191112. (6) Liu, Z.; Wang, Y.; Zhang, X.; Xu, Y.; Chen, Y.; Tian, J. Nonlinear optical properties of graphene oxide in nanosecond and picosecond regimes. Appl. Phys. Lett. 2009, 94, 021902. (7) Popa, D.; Sun, Z.; Hasan, T.; Torrisi, F.; Wang, F.; Ferrari, A. C. Graphene Q-switched, tunable fiber laser. Appl. Phys. Lett. 2011, 98, 073106. (8) Mary, R.; Brown, G.; Beecher, S. J.; Torrisi, F.; Milana, S.; Popa, D.; Hasan, T.; Sun, Z.; Lidorikis, E.; Ohara, S.; et al. 15 GHz picosecond pulse generation from a monolithic waveguide laser with a graphene-film saturable output coupler. Opt. Express 2013, 21, 7943− 7950. (9) Cizmeciyan, M. N.; Kim, J. W.; Bae, S.; Hong, B. H.; Rotermund, F.; Sennaroglu, A. Graphene mode-locked femtosecond Cr:ZnSe laser at 2500 nm. Opt. Lett. 2013, 38, 341−343. (10) Xing, F.; Meng, G.-X.; Zhang, Q.; Pan, L.-T.; Wang, P.; Liu, Z.B.; Jiang, W.-S.; Chen, Y.; Tian, J.-G. Ultrasensitive flow sensing of a single cell using graphene-based optical sensors. Nano Lett. 2014, 14, 3563−3569. (11) Koppens, F. H. L.; Mueller, T.; Avouris, P.; Ferrari, A. C.; Vitiello, M. S.; Polini, M. Photodetectors based on graphene, other two-dimensional materials and hybrid systems. Nat. Nanotechnol. 2014, 9, 780−793. (12) Karlický, F.; Kumara Ramanatha Datta, K.; Otyepka, M.; Zbořil, R. Halogenated graphenes: rapidly growing family of graphene derivatives. ACS Nano 2013, 7, 6434−6464. (13) Pumera, M.; Sofer, Z. Towards stoichiometric analogues of graphene: graphane, fluorographene, graphol, graphene acid and others. Chem. Soc. Rev. 2017, 46, 4450−4463. (14) Chronopoulos, D. D.; Bakandritsos, A.; Pykal, M.; Zbořil, R.; Otyepka, M. Chemistry, properties, and applications of fluorographene. Appl. Mater. Today 2017, 9, 60−70. (15) Chantharasupawong, P.; Philip, R.; Narayanan, N. T.; Sudeep, P. M.; Mathkar, A.; Ajayan, P. M.; Thomas, J. Optical power limiting in fluorinated graphene oxide: an insight into the nonlinear optical properties. J. Phys. Chem. C 2012, 116, 25955−25961. (16) Liaros, N.; Bourlinos, A. B.; Zboril, R.; Couris, S. Fluorographene: nonlinear optical properties. Opt. Express 2013, 21, 21027− 21038. (17) Papadakis, I.; Bouza, Z.; Couris, S.; Bourlinos, A. B.; Mouselimis, V.; Kouloumpis, A.; Gournis, D.; Bakandritsos, A.; Ugolotti, J.; Zboril, R. Hydrogenated Fluorographene: A 2D Counterpart of Graphane with Enhanced Nonlinear Optical Properties. J. Phys. Chem. C 2017, 121, 22567−22575. (18) Leenaerts, O.; Peelaers, H.; Hernández-Nieves, A. D.; Partoens, B.; Peeters, F. M. First principles investigation of graphene fluoride
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CONCLUSIONS In conclusion, the NLO response of CF, CFH, and G was studied under both visible and infrared 4 ns laser excitation and their NLO parameters (nonlinear absorption coefficient β and nonlinear refractive index parameter γ′) were determined as well. Hydrogenated fluorographene (CFH) was found to possess the largest effective third-order nonlinearity χ(3), compared to CF and G. In particular, for infrared excitation, CFH χ(3) value was determined to be more than 100 times larger than that of CF possibly because of the presence of midgap states. All three graphenes studied here were found to exhibit strong RSA behavior. Among them, CFH and G displayed self-focusing behavior, whereas CF exhibited selfdefocusing. The present findings show how functionalization can effectively modify the NLO properties of graphene derivatives through engineering of the band gap and the insertion of midgap states, thus leading to novel 2D materials suitable for advanced photonic applications.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Stelios Couris: 0000-0002-8495-7082 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS I.P. acknowledges “Advancing Young Researchers’ Human Capital in Cutting Edge Technologies in the Preservation of Cultural Heritage and the Tackling of Societal Challenges ARCHERS” project of Stavros Niarchos Foundation for partial support. Z.B. acknowledges partial support from the “Investigation of the Non-Linear Optical Response of Graphene’s Derivatives with the Z-scan Technique” (MIS 5002556) which is implemented under the “Action for the Strategic Development on the Research and Technological 25578
DOI: 10.1021/acs.jpcc.8b08491 J. Phys. Chem. C 2018, 122, 25573−25579
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The Journal of Physical Chemistry C and graphene. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 195436−195441. (19) Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. The electronic properties of graphene. Rev. Mod. Phys. 2009, 81, 109−162. (20) Katsnelson, M. I. Carbon in Two Dimensions; Cambridge University Press: Cambridge, U.K., 2012. (21) Sheik-Bahae, M.; Said, A. A.; Wei, T.-H.; Hagan, D. J.; Van Stryland, E. W. Sensitive measurement of optical nonlinearities using a single beam. IEEE J. Quant. Electr. 1990, 26, 760−769. (22) Said, A. A.; Sheik-Bahae, M.; Hagan, D. J.; Wei, T. H.; Wang, J.; Young, J.; Van Stryland, E. W. Determination of bound-electronic and free-carrier nonlinearities in ZnSe, GaAs, CdTe, and ZnTe. J. Opt. Soc. B: Opt. Physics 1992, 9, 405−414. (23) Couris, S.; Koudoumas, E.; Ruth, A. A.; Leach, S. Concentrations and wavelength dependence of the effective thirdorder susceptibility and optical limiting of C60 in toluene solution. J. Phys. B: At. Mol. Opt. Phys. 1996, 28, 4537−4554. (24) Aloukos, P.; Couris, S. Influence of the experimental conditions on the determination of nonlinear optical parameters of a medium using the Z-scan technique. SPIE Proc., 2003; Vol. 5131, pp 270−273. (25) Cheng, J. L.; Vermeulen, N.; Sipe, J. E. Third order optical nonlinearity of graphene. New J. Phys. 2014, 16, 053014. (26) del Coso, R.; Solis, J. Relation between nonlinear refractive index and third-order susceptibility in absorbing media. J. Opt. Soc. Am. B 2004, 21, 640−644. (27) Sutherland, R. L. Handbook of Nonlinear Optics, 2nd ed.; CRC Press: New York, 2003. (28) Boyd, R. W. Nonlinear Optics, 3rd ed.; Academic Press: Orlando, FL, USA, 2008. (29) de la Torre, G.; Vázquez, P.; Agulló-López, F.; Torres, T. Role of structural factors in the nonlinear optical properties of phthalocyanines and related compounds. Chem. Rev. 2004, 104, 3723−3750. (30) Lebègue, S.; Klintenberg, M.; Eriksson, O.; Katsnelson, M. I. Accurate electronic band gap of pure and functionalized graphane from GW calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 245117−245121. (31) Haberer, D.; Vyalikh, D. V.; Taioli, S.; Dora, B.; Farjam, M.; Fink, J.; Marchenko, D.; Pichler, T.; Ziegler, K.; Simonucci, S.; et al. Tunable band gap in hydrogenated quasi-free-standing graphene. Nano Lett. 2010, 10, 3360−3366. (32) Gao, H.; Wang, L.; Zhao, J.; Ding, F.; Lu, J. Band gap tuning of hydrogenated graphene: H coverage and configuration dependence. J. Phys. Chem. C 2011, 115, 3236−3242. (33) Jeon, K.-J.; Lee, Z.; Pollak, E.; Moreschini, L.; Bostwick, A.; Park, C.-M.; Mendelsberg, R.; Radmilovic, V.; Kostecki, R.; Richardson, T. J.; et al. Fluorographene: a wide bandgap semiconductor with ultraviolet luminescence. ACS Nano 2011, 5, 1042− 1046. (34) Yuan, S.; Rösner, M.; Schulz, A.; Wehling, T. O.; Katsnelson, M. I. Electronic structures and optical properties of partially and fully fluorinated graphene. Phys. Rev. Lett. 2015, 114, 047403−047407. (35) Tanaka, K. Optical nonlinearity in photonic glasses. J. Mater. Sci. 2005, 16, 633−643. (36) Sebastian, I.; Divya, S.; Nampoori, V. P. N.; Radhakrishnan, P.; Thomas, S. Impact of intermediate localized states on nonlinear optical absorption of Ga-Ge-Se nanocolloidal solutions. Appl. Phys. Lett. 2013, 102, 031115. (37) Nagpal, P.; Klimov, V. I. Role of mid-gap states in charge transport and photoconductivity in semiconductor nanocrystal films. Nat. Commun. 2011, 2, 486. (38) Fidler, A. F.; Gao, J.; Klimov, V. I. Electron-hole exchange blockade and memory-less recombination in photoexcited films of colloidal quantum dots. Nat. Phys. 2017, 13, 604−610. (39) Son, J.; Lee, S.; Kim, S. J.; Park, B. C.; Lee, H.-K.; Kim, S.; Kim, J. H.; Hong, B. H.; Hong, J. Hydrogenated monolayer graphene with reversible and tunable wide band gap and its field-effect transistor. Nat. Commun. 2016, 7, 13261−13268.
(40) Wu, B.-R.; Yang, C.-K. Electronic structures of clusters of hydrogen vacancies on graphene. Sci. Rep. 2015, 5, 15310. (41) Liu, L.; Zhang, J.; Gao, H.; Wang, L.; Jiang, X.; Zhao, J. Tailoring physical properties of graphene: Effects of hydrogenation, oxidation, and grain boundaries by atomistic simulations. Comp. Mat. Sci. 2016, 112, 527−546. (42) Liaros, N.; Tucek, J.; Dimos, K.; Bakandritsos, A.; Andrikopoulos, K. S.; Gournis, D.; Zboril, R.; Couris, S. The effect of the degree of oxidation on broadband nonlinear absorption and ferromagnetic ordering in graphene oxide. Nanoscale 2016, 8, 2908− 2917. (43) Zhang, X.-L.; Liu, Z.-B.; Li, X.-C.; Ma, Q.; Chen, X.-D.; Tian, J.-G.; Xu, Y.-F.; Chen, Y.-S. Transient thermal effect, nonlinear refraction and nonlinear absorption properties of graphene oxide sheets in dispersion. Opt. Express 2013, 21, 7511−7520. (44) Liaros, N.; Couris, S.; Koudoumas, E.; Loukakos, P. A. Ultrafast processes in graphene oxide during femtosecond laser excitation. J. Phys. Chem. C 2016, 120, 4104−4111. (45) Haberer, D.; Petaccia, L.; Farjam, M.; Taioli, S.; Jafari, S. A.; Nefedov, A.; Zhang, W.; Calliari, L.; Scarduelli, G.; Dora, B.; et al. Direct observation of a dispersionless impurity band in hydrogenated graphene. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 165433. (46) Huang, H.-C.; Lin, S.-Y.; Wu, C.-L.; Lin, M.-F. Configurationand concentration-dependent electronic properties of hydrogenated graphene. Carbon 2016, 103, 84−93. (47) Duplock, E. J.; Scheffler, M.; Lindan, P. J. D. Hallmark of perfect graphene. Phys. Rev. Lett. 2004, 92, 225502−225506.
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DOI: 10.1021/acs.jpcc.8b08491 J. Phys. Chem. C 2018, 122, 25573−25579
