We report a novel technique to synthesize fluorescent carbon dots decorated graphene oxide (CDs-GO) sheets by microwave irradiation using citric acid as the precursor and ethylenediamine as the passivation agent. The samples are characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and ultra violet-visible spectroscopy (UV-Vis). Fluorescence studies reveal strong excitation wavelength dependent fluorescence behaviour for these nanocomposites dispersed in water. Third order nonlinear optical properties of these composites are studied using Z scan technique with Q-switched nanosecond pulsed Nd: YAG laser and the measured highest nonlinear optical susceptibility is 4.624x10-10esu. The optical limiting studies of these nanocomposites yield a lower limiting threshold of 104MW/cm2 enabling them as a strong candidate for optical limiting applications.

Topics
Transmission electron microscopy, X-ray diffraction, Nanocomposites, Nanomaterials, Nonlinear optics, Z-scan technique, Lasers, Oxides

The material graphite oxide was first discovered by the oxford chemist Benjamin Brodie in 1859.1 Graphene oxide (GO) can be considered as the insulating and disordered analogue of the highly conducting crystalline graphene. Graphene based materials have been of great interest to the scientific community for the last three decades due to several potential applications such as energy storage materials, sensors, paper like materials, optoelectronics and biomedical applications.2–4 GO is a graphene sheet reformed with oxygen functional groups in the form of epoxy and hydroxyl groups on the basal plane and various other types at the edges. The GO is currently the most common precursor used for the synthesis of graphene materials.5 Carbon nanoparticles with size less than 10 nm are termed as carbon dots (CDs) which are used in various photonics related applications like light emitting diodes, imaging and sensing.6–8 CDs can be made highly fluorescent by making use of different passivation agents.9,10 Mostly the graphene oxide are prepared by Hummers method and its modified methods.11–14 Here we report synthesis of graphene oxide nano sheets attached with carbon dots from citric acid under microwave irradiation.

Materials with nonlinear optical (NLO) response are considered to be promising candidates for photonic applications such as optical communication, optical limiting and optical data storage applications.15 Centre of inversion symmetry of graphene enables strong second order nonlinear response which arises from regularity breaking induced by the presence of a strong external electric field. There are few reports available on the nonlinear properties of GO in nano seconds and picosecond regimes.16 Enhanced nonlinear response of graphene based materials are also reported with different hybrid materials.17 Recently, enhancement of nonlinear optical properties have been achieved for GO-Au nanocrystal composites.18 

CDs-GO composite are reported to enhance the photocurrent response of solid state devices and also the sensing mechanism of dopamine,19,20 which involves the synthesis of the material by a two-step process. In the present work, we report a novel one step synthesis of CDs decorated GO using microwave assisted pyrolysis. There have been no report on the NLO properties of CDs- GO composites. Recently there are report based on the enhanced two photon absorption properties of GO–Ag nanocomposite by S. Biswas et al.18 In this manuscript, NLO properties of CDs- GO composites have been thoroughly investigated and its optical limiting behaviour is reported.

Citric acid (CA) monohydrous and ethylenediamine (EDA) were purchased from Merc India chemicals. Other chemicals were analytically pure and used as received.

The CDs-GO were prepared by simple microwave assisted synthesis. For this 1g CA monohydrous was taken in a beaker and 1 ml of ethylenediamine was added and the mixture was heated using a domestic microwave oven of power 800 Watts for 3 minutes. The dark brown solid thus obtained were grounded and the fine powder were used for further measurements. The resulting powder dispersed in water with different concentrations for fluorescence and NLO measurements were labelled as CDs-GO1= 3 mg/ml, CDs-GO2 = 4 mg/ml, CDs-GO3 = 5 mg/ml and CDs-GO4 = 6 mg/ml.

Optical absorption spectra of the prepared CDs were recorded using UV–Visible Spectrophotometer (Jasco V-570 UV/VIS/NIR) and fluorescence spectra using Cary Eclipse fluorescence spectrophotometer (Varian). Functional group present in the samples were analysed using Fourier transform infra-red spectrometer (FT-IR) (Thermo Nicolet model Avatar 370). High resolution transmission electron microscope (HR-TEM) images of CDs-GO were recorded using JEOL (model JEM 2100). Elemental analysis were carried out using Elementar Vario EL III.

Typical procedure of the preparation of CDs-GO is shown in Figure 1. The degree of carbonisation is obtained using CHN measurement (Table I) which shows that most of the precursor are carbonised and the passivation was confirmed by the presence of nitrogen and hydrogen.

FIG. 1.
FIG. 1. Procedure for the preparation of carbon dots decorated GO.
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Procedure for the preparation of carbon dots decorated GO.

FIG. 1.
FIG. 1. Procedure for the preparation of carbon dots decorated GO.
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Procedure for the preparation of carbon dots decorated GO.

Close modal
TABLE I.

Elemental analysis of carbon dots decorated GO.

Sample NameN%C%H%O%
GO 14.39 47.83 6.72 31.06 
Sample NameN%C%H%O%
GO 14.39 47.83 6.72 31.06 
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Fig 2a shows the XRD pattern of the sample with a characteristic peak at 2θ=12.60 corresponding to GO (001) similar to the reported results.13 The peak at 2θ=26.50 confirms the presence of CDs (002) in the sample.

FIG. 2.
FIG. 2. a) XRD and b) Absorption and emission spectra of CDs decorated GO.
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a) XRD and b) Absorption and emission spectra of CDs decorated GO.

FIG. 2.
FIG. 2. a) XRD and b) Absorption and emission spectra of CDs decorated GO.
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a) XRD and b) Absorption and emission spectra of CDs decorated GO.

Close modal

The optical absorption of CDs-GO is shown in Fig 2b. Absorption spectra indicate two dominant peaks centred at 238 nm and 350 nm. The main peak at 238 nm is from the p-p* transition of C-C and C=C bonds in sp2 hybrid regions and a shoulder peak at 350nm is due to n-π* transition of the C=O bond in sp3 hybrid region. A blue fluorescence emission is observed in material with an excitation of 365nm. There are very few reports on the emission of graphene oxide in the literature. Previous reports indicate that the fluorescence of carbon dots decorated GO may originate from the optical transitions from structural disorder-induced localized states in the π-π* gap of sp2 sites.12,21 GO-Polyvinyl alcohol hybrid shows a strong emission in the observed wavelength.22 Prepared sample shows sp2 clusters and also oxygen containing functional groups as verified in FT-IR measurement (Fig 3a). The hydrophilic nature of the present sample is due to the OH group at 3425cm-1. The stretching vibrations of amine functional groups at 1654 cm-1 is observed.

FIG. 3.
FIG. 3. a) FT-IR and b) Tauc Plot of CDs decorated GO.
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a) FT-IR and b) Tauc Plot of CDs decorated GO.

FIG. 3.
FIG. 3. a) FT-IR and b) Tauc Plot of CDs decorated GO.
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a) FT-IR and b) Tauc Plot of CDs decorated GO.

Close modal

Optical band gap of CDs-GO were calculated using Tauc plot which is shown in figure 4b.

∝hv=A(hv−Eg)m
(1)

Where ∝ represents the absorption coefficient, hv is the energy in eV, Eg is the bandgap energy and A is the constant. The parameter m is indicative of the nature of transition and here is equal to ½. The obtained value of band gap is 3.2 eV, which shows an intrinsic semiconductor like absorption in the blue optical region. This value of band gap is in agreement with previous report.23 

FIG. 4.
FIG. 4. a) Excitation dependent emission b) Life time of CDs decorated GO.
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a) Excitation dependent emission b) Life time of CDs decorated GO.

FIG. 4.
FIG. 4. a) Excitation dependent emission b) Life time of CDs decorated GO.
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a) Excitation dependent emission b) Life time of CDs decorated GO.

Close modal

Fluorescence studies of these nanocomposites are carried out and the obtained fluorescence spectra are shown in Fig 4a. Here we have observed an excitation wave length dependent emission red shift from 450nm to 550nm as shown in Fig 4a. Surface functional groups attached to the surface of CDs-GO composite will create multiple discrete electronic states leading to the excitation dependent emission shown in fig 5a.24 Few reports on excitation dependent fluorescence of carbon nanoparticles and GO are present in the literature.25,26 Fig 4b shows the time-resolved fluorescence lifetime of CDs-GO with an excitation wavelength of 367nm. An average fluorescence lifetime of 4.4 nano seconds was measured by fitting the decay trace using triexponential functions as follows

It=A1exp(−tτ1)+A2exp(−tτ2)+A3exp(−tτ3)
(2)

Where A1, A2 and A3 are the fractional contributions of time-resolved decay lifetimes of and τ1, τ2 and τ3 respectively. This triexponential behaviour of CDs-GO composite indicate that the presence of three systems of discrete energy levels. These fluorescence centres provide the excitation dependent emission, due to the conjugated structures and emissive traps.27 

FIG. 5.
FIG. 5. a) Energy level diagram b) FE-SEM images of CDs decorated GO.
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a) Energy level diagram b) FE-SEM images of CDs decorated GO.

FIG. 5.
FIG. 5. a) Energy level diagram b) FE-SEM images of CDs decorated GO.
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a) Energy level diagram b) FE-SEM images of CDs decorated GO.

Close modal

The synthesized CDs-GO composite analyzed using FE-SEM shows flake like morphology. The FE-SEM images obtained with different resolutions are shown in Fig 5b. High resolution TEM images of the CDs-GO composite are shown in fig 6. Figure 6b clearly show the multi layer structure of GO which is perfectly crystalline in nature. Selected area diffraction (SAED) pattern show the high crystalline nature of the sample Fig. 6(d).

FIG. 6.
FIG. 6. The HR-TEM image of carbon dots decorated with GO of different resolutions a) 100nm, b) 200 nm, c) 20 nm, d) SAED pattern of CDs decorated GO.
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The HR-TEM image of carbon dots decorated with GO of different resolutions a) 100nm, b) 200 nm, c) 20 nm, d) SAED pattern of CDs decorated GO.

FIG. 6.
FIG. 6. The HR-TEM image of carbon dots decorated with GO of different resolutions a) 100nm, b) 200 nm, c) 20 nm, d) SAED pattern of CDs decorated GO.
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The HR-TEM image of carbon dots decorated with GO of different resolutions a) 100nm, b) 200 nm, c) 20 nm, d) SAED pattern of CDs decorated GO.

Close modal

The Z-scan technique using an Nd:YAG laser having 7 ns pulses at a repetition rate of 10 Hz giving a second harmonic output at 532 nm was used to measure the optical nonlinearity. The sample is moved along the beam axis of light focused with a lens of focal length 20 cm. The radius of the beam waist w0 is calculated to be 42 μm. The rayleigh length, z0 = πw02/λ, is estimated to be 10.7 mm, which is much greater than the thickness of the sample cuvette (1 mm), which is an essential prerequisite for Z-scan experiments. The transmitted beam energy, reference beam energy and their ratio are measured simultaneously by an energy ratio meter having two identical pyroelectric detector heads. The Z-scan experimental setup are shown in figure 7.

FIG. 7.
FIG. 7. Z-scan experimental set up.
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Z-scan experimental set up.

FIG. 7.
FIG. 7. Z-scan experimental set up.
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Z-scan experimental set up.

Close modal

The normalized transmittance for two photon absorption in the open aperture condition is given by

Tz=1πqo(z,0)∫∞−∞ln1+qo(z,0)e−t2dτ
(3)

Where

qo(z,0)=βIoLeff/(1+Z2/Z02).
(4)

Here, I0 is the laser intensity in the focal plane, ß is the nonlinear optical absorption coefficient, Leff is the effective thickness with linear absorption coefficient α. Leff is given by

Leff=(1−e−αL)/α
(5)

The real and imaginary parts of the third-order nonlinear susceptibility (χ3) were deduced according to the parameters β and γ as shown in Eqs 6 and 7

lmχ3(esu)=εoc2no2240π2wβ (m/W)
(6)
Reχ3(esu)=no3πγ (m2/W)
(7)

Where λ, εo, c and n0 are the wavelength of the laser light, permittivity of free space, speed of light and linear refractive index of the glass, respectively. The total optical third-order nonlinear susceptibility χ3 is given by Eq 8.

χ(3)esu=Reχ32+lmχ(3)21/2
(8)

Recently, there were reports on nonlinear optical properties of GO16,28 but to the best of our knowledge there are no reports on nonlinear optical properties of CDs - GO composites. Open aperture (OA) Z-scans measurement of the carbon dots decorated GO are shown in Fig 8 which indicates a transmission maximum at low laser intensity and a transmission minimum at higher excitation intensity characteristic of reverse saturable absorption (RSA) behaviour. The values of the NLO parameters and the third-order nonlinear susceptibility (χ3) determined are shown in Table II. Bandgap (Eg) is greater than incident photon energy (hv⁠), therefore it can be concluded that the optical nonlinearity is arising from the two photon absorption (TPA). So that the optical limiting properties of CDs-GO composite attribute to the TPA process.29 

FIG. 8.
FIG. 8. a) Open aperture Z-scan of CDs-GO with different power b) CDs-GO with different concentrations.
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a) Open aperture Z-scan of CDs-GO with different power b) CDs-GO with different concentrations.

FIG. 8.
FIG. 8. a) Open aperture Z-scan of CDs-GO with different power b) CDs-GO with different concentrations.
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a) Open aperture Z-scan of CDs-GO with different power b) CDs-GO with different concentrations.

Close modal
TABLE II.

The Third-order nonlinear optical parameters of carbon dots decorated GO with varying concentrations.

LinearNonlinear AbsorptionNonlinear RefractiveNonlinear SusceptibilityFigure
SampleAbsorption (a.u)(β)× 10-9 m/WIndex n2× 10-9 m2/Wχ(3) × 10-10 esuof merit
CDs-GO (3 mg/ml) 0.89 0.272 -1.874 3.001 3.66 
CDs-GO (4 mg/ml) 0.94 0.291 -2.026 3.202 3.41 
CDs-GO (5 mg/ml) 2.37 0.639 -2.346 3.707 1.95 
CDs-GO (6 mg/ml) 3.30 0.897 -2.885 4.624 1.71 
LinearNonlinear AbsorptionNonlinear RefractiveNonlinear SusceptibilityFigure
SampleAbsorption (a.u)(β)× 10-9 m/WIndex n2× 10-9 m2/Wχ(3) × 10-10 esuof merit
CDs-GO (3 mg/ml) 0.89 0.272 -1.874 3.001 3.66 
CDs-GO (4 mg/ml) 0.94 0.291 -2.026 3.202 3.41 
CDs-GO (5 mg/ml) 2.37 0.639 -2.346 3.707 1.95 
CDs-GO (6 mg/ml) 3.30 0.897 -2.885 4.624 1.71 
View Large

RSA characteristics of materials are useful for optical limiting applications. Concentration dependent optical limiting behaviour of water dispersed CDs-GO is shown in Fig 9. It is observed that optical limiting threshold is minimum at high concentration (106 MW/cm2) and maximum at low concentration (116 MW/cm2). Better optical limiting behaviour is observed in comparison with pure carbon source.30 

FIG. 9.
FIG. 9. Optical limiting curve of CDs-GO.
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Optical limiting curve of CDs-GO.

FIG. 9.
FIG. 9. Optical limiting curve of CDs-GO.
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Optical limiting curve of CDs-GO.

Close modal

Figure 10 shows the closed aperture (CA) Z-scan measurements of CDs-GO. The sign of n2 of all the samples are negative as the curves indicate a valley after a peak which can be attributed to the effect of self-defocusing in these materials.31 

FIG. 10.
FIG. 10. CA Z scan of CDs- GO a) Concentration depended CA Z scan, b) Power dependent CA Z-scan of power 175 MW/cm2, c) 251 MW/cm2, d) 313 MW/cm2.
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CA Z scan of CDs- GO a) Concentration depended CA Z scan, b) Power dependent CA Z-scan of power 175 MW/cm2, c) 251 MW/cm2, d) 313 MW/cm2.

FIG. 10.
FIG. 10. CA Z scan of CDs- GO a) Concentration depended CA Z scan, b) Power dependent CA Z-scan of power 175 MW/cm2, c) 251 MW/cm2, d) 313 MW/cm2.
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CA Z scan of CDs- GO a) Concentration depended CA Z scan, b) Power dependent CA Z-scan of power 175 MW/cm2, c) 251 MW/cm2, d) 313 MW/cm2.

Close modal

Nonlinear optical properties of CDs-GO composite are compared with previous reports in Table III, which indicate that the present samples have better nonlinear properties than the others. The experimental values of χ(3) of CDs-GO composite can be compared with GO and GO-Ag nanocomposites are 5.7×10-13 esu and 16×10-13 esu by Biswas et al.18 Bourlinos et al has reported the χ(3) value of 0.48×10-13 esu with an optical limiting threshold of 0.5 J/cm2.32 A low value of χ(3) reported by Kumar et al in graphene of 5×10-15 esu, which is dispersed in DMF and 5×10-15 esu in water.33 All the comparative studies are carried out with nanosecond regime by Nd: YAG laser at 532 nm excitation.

TABLE III.

Comparison of Optical nonlinearity of Graphene oxide based materials.

NonlinearNonlinearOptical limiting
SampleSusceptibility χ(3)Absorption(β)thresholdreference
GO 5.7×10-13 esu 17 cmGW-1 10 J/cm2 18  
GO-Ag 16×10-13 esu 45 cmGW-1 6.4 J/cm2 18  
NDs 0.36×10-13 esu – 0.3 J/cm2 32  
CDs 0.48×10-13 esu – 0.5 J/cm2 32  
G-DMF 5×10-15 esu 2×10-8 cm/W – 33  
G-H2O 5×10-15 esu 9×10-8 cm/W – 33  
GO-CDs 4.624×10-10 esu 0.89 ×10-9 m/W 0.8 J/cm2 Our results 
NonlinearNonlinearOptical limiting
SampleSusceptibility χ(3)Absorption(β)thresholdreference
GO 5.7×10-13 esu 17 cmGW-1 10 J/cm2 18  
GO-Ag 16×10-13 esu 45 cmGW-1 6.4 J/cm2 18  
NDs 0.36×10-13 esu – 0.3 J/cm2 32  
CDs 0.48×10-13 esu – 0.5 J/cm2 32  
G-DMF 5×10-15 esu 2×10-8 cm/W – 33  
G-H2O 5×10-15 esu 9×10-8 cm/W – 33  
GO-CDs 4.624×10-10 esu 0.89 ×10-9 m/W 0.8 J/cm2 Our results 
View Large

Third order nonlinearity in materials finds applications in optical switching and super continuum generation. Figure of merit (F= γ/βλ) is used to estimate the third-order nonlinear performance of materials at a certain wavelength.34 Figure of merit is found to change with the concentration of these nanocomposites as illustrated in Table II. A maximum figure of merit of 3.66 is observed at lower concentration.

Carbon dots decorated graphene oxide composites were synthesized through a novel one pot microwave assisted pyrolysis technique for nonlinear optical applications. The morphological and structural characterizations of the samples reveal the presence of CDs in GO sheets. XRD, FE-SEM and HR-TEM analysis confirmed formation of multilayer GO sheet with the presence of CDs. Surface functional groups were identified and quantified through CHN and FT-IR analysis. Both OA and CA Z-scan studies reveal the nonlinear optical performance of the CDs-GO composite. Strong fluorescence of the sample makes them potential candidates for Bio-imaging and LED applications.

Financial support from university grants commission (UGC), Government of India and Kerala state council for science technology and environment (KSCSTE), Government of Kerala is gratefully acknowledged.

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2019
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