Dramatic Enhancement of the Nonlinear Optical Response of

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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 J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08491 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 18, 2018

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The Journal of Physical Chemistry

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Dramatic Enhancement of the Nonlinear Optical Response of Hydrogenated

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Fluorographene: the Effect of Midgap States

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Ioannis Papadakis1,2, Zoi Bouza1,2, Stelios Couris1,2*, Vasileios Mouselimis3, Athanasios B.

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Bourlinos3

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1Physics

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2Institute

Department, University of Patras, 26504 Patras, Greece of Chemical Engineering Sciences (ICE-HT), Foundation for Research and

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Technology-Hellas (FORTH), P.O. Box 1414, Patras 26504, Greece

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3Physics

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*email: [email protected]

Department, University of Ioannina, 45110 Ioannina, Greece

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ACS Paragon Plus Environment

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Abstract

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The third-order nonlinear optical response of hydrogenated fluorographene (CFH), a 2D

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counterpart of graphane, was investigated in the visible and in the infrared, using ns laser

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excitation and compared to that of graphene fluoride (CF) and (unfunctionalized) graphene (G).

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All three graphenes were found to exhibit important nonlinear optical response, CFH exhibiting

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the largest under visible and infrared excitation. In the visible, the response of CFH was

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determined to be 2 to 3 times larger than that of CF and G. However, in the case of infrared

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excitation, a dramatic enhancement that has not been previously observed for graphene

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derivatives was noticed; more than two orders of magnitude higher than that of CF and about one

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order of magnitude higher than that of G. This is attributed to the presence of midgap states

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which are formed upon functionalization of graphene and can enhance resonantly the nonlinear

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optical response of CFH. It is worth noting that, the third-order nonlinear susceptibility χ(3) of

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hydrogenated fluorographene reached as high as 310-9 esu. To the best of our knowledge this is

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a rather exceptional value for graphene derivatives.

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Introduction

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The recent discovery of graphene1, being a one-atom thick layer of sp2 hybridized carbon

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atoms arranged in a honeycomb arrangement forming a two-dimensional lattice has excited the

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scientific curiosity because of its unexpected and unusual structure. Shortly after its discovery, it

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was realized that graphene is characterised by strong covalent intralayer bonds and weak Van der

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Waals interactions between its adjacent layers which lead to a unique band structure system, this

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last being the source of the remarkable properties of graphene and in particular of the

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optoelectronic ones. These extraordinary properties have further boosted the scientific interest

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about graphene and its derivatives in view of their various potential applications. So, the

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existence of linear dispersion of the massless Dirac fermions implies a smooth and almost

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constant absorption profile over the entire optical spectrum, thus denoting the resonant character

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of any optical excitation of graphene. In addition, saturable absorption because of Pauli blocking,

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when carriers generated because of strong optical excitation, leads to the depletion of the valence

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band and the filling of the conduction band, preventing any further absorption and thus expressed

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as an increase of the material transmission at high enough photon pump rates. These properties

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combined with the high carrier mobility and the mechanical and thermal properties of graphene

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make graphene an ideal candidate for a wide range of optoelectronic applications, ranging from

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saturable absorbers,2,3 to optical limiters4-6 and ultrafast lasers,7-9 to optical sensors10,11 and

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several others. Since, a continuously growing interest is observed concerning the exciting

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optoelectronic properties of graphene and its derivatives.

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A relatively recent member of the family of graphene derivatives, is graphene fluoride or

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fluorographene (CF).12-14 Regarding this graphene derivative, the stronger the fluorination is, the

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more the carbon bonds of graphene which are transformed from the initial sp2 hybridization of



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un-functionalized graphene to sp3 hybridization, resulting in modification of its electrical and

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optical properties. As a result, fluorographenes hold great promises for applications in high

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performance materials, such as high-tech batteries, dielectrics, sensors, etc., while recently it has

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been shown that they exhibit important optical limiting performance as well.15,16 An even more

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recent graphene derivative is hydrogenated fluorographene (CFH) which is considered as the 2D

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counterpart of graphane (hydrogenated graphene).17 CFH is directly derived from CF by hydride

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reaction and contains both sp3 and sp2 carbon domains. The former are related to the newly

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inserted C-H groups and residual C-F groups after hydride substitution, whereas the latter are

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related to graphenic domains resulted from the partial hydride reduction of CF. The ability of

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tuning the sp3/sp2 ratio can be an efficient tool and a powerful strategy to continuously tuning the

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band gap of CF, thus modifying its behaviour from conductor to insulator.14,16,18 In addition, the

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formation of midgap states induced by the insertion of the F and/or H adatoms on the graphenic

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sheet has been also suggested and confirmed for these derivatives and are expected to affect also

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the optical and electronic properties. Although these phenomena are not yet fully rationalised

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they are expected to add more flexibility in tuning the these 2D materials properties

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accordingly.19,20 In that view, the third-order nonlinear optical (NLO) response of the

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hydrogenated fluorographenes is expected to be substantially different compared to that of its

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non-hydrogenated counterparts. In the present study, the effective third-order NLO response and

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the related nonlinear optical properties (i.e. refraction and absorption) of hydrogenated

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fluorographene (CFH) are investigated under ns laser excitation, both in the visible (532 nm) and

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in the infrared (1064 nm). To facilitate direct comparison of the CFH effective NLO response

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with the corresponding responses of CF and of pristine graphene, all graphenes have been studied

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at the same time under similar experimental conditions. A dramatic enhancement of more than


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two orders of magnitude of the NLO response of hydrogenated fluorographene was observed

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under infrared laser excitation, making CFH the graphene derivative with the larger transient

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NLO response reported so far to the best of our knowledge. The observed large enhancement of

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the NLO response is discussed in view of the midgap states formed within the band gap of CFH.

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Experimental section

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The nonlinear optical response of hydrogenated fluorographene (and of the other

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graphenes) was investigated using the Z-scan technique, employing a 4 ns Q-switched Nd:YAG

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laser operated at a repetition rate up to 10 Hz. For most of the experiments, a low repetition rate

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of 1 or 2 Hz was chosen. For the experiments, the laser fundamental output at 1064 nm and its

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second harmonic (SHG) at 532 nm were employed.

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CFH was obtained by hydride substitution of fluorine atoms of fluorgraphene, using

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sodium borohydride NaBH4 as the hydride source according to the procedure described in detail

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elsewhere.17 For the measurements, dispersions with different concentrations of CFH in acetone

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were prepared. Graphene and fluorographene were dispersed in DMF. The dispersions were

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placed in 1 mm thick glass cells for the measurement of their absorption spectra and for the Z-

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scan measurements. The UV-Vis-NIR absorption spectra of the prepared dispersions were

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measured with a spectrophotometer and were regularly checked during the experiments to ensure

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their stability. The laser beam was focused into the cells using a 20 cm focal length quartz lens.

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The spot radii at the focus of the 532 and 1064 nm laser beams were determined to be 17.5 and

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30 μm respectively using a CCD camera.

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A more detailed description of the experimental setup and the procedures followed for the

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collection and the analysis of the experimental data can be found elsewhere.21-23 Here only a brief

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description will be presented and some critical points concerning the precautions that must be



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considered when using Z-scan will be reminded. So, briefly, the nonlinear absorption coefficient,

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β, and the nonlinear refractive index parameter, γ΄, were determined from measurements of the

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variation of the transmittance of a sample, as it moves along the propagation direction of a

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focused laser beam, by two different experimental configurations, widely known as “open-

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aperture” (OA) and “closed-aperture” (CA) Z-scans, respectively. In the case where NLO

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absorption is present, instead of the CA Z-scan the so-called “divided” (D) Z-scan is used for the

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determination of γ΄, resulted from the division of the CA Z-scan recording by the corresponding

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OA one. In this way, the effect of nonlinear absorption is removed from the CA Z-scan and the

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NLO refraction can be determined from the D Z-scan in the absence of nonlinear absorption. In

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practice, this approximation is more accurate under low linear (e.g., absorbance less than about

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0.25) absorption and weak NLO absorption at the excitation wavelength. Otherwise, the Z-scan

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assumptions and approximations (e.g., thin sample approximation, wave-front phase distortion

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ΔΦ00)



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while CF shows self-defocusing (n2

Dramatic Enhancement of the Nonlinear Optical Response of

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