Nonlinear optical interactions of Co: ZnO nanoparticles in continuous and pulsed mode of operations

Abstract

Nanoparticles of Co: ZnO were synthesized by a co-precipitation method and characterized structurally by XRD (wurtzite), FTIR and TEM (spherical, 6–13 nm). The oxidation state of the Co ions is confirmed as +2 from electron paramagnetic resonance (EPR). UV-Vis absorption spectra represent a complete transparency in the visible region. A zinc vacancy is identified by PL measurements at 410 and 465 nm. The intensity dependent third order nonlinearity is studied using two frequency doubled (532 nm) pumping sources namely nanosecond pulsed Nd: YAG and CW Nd: YAG lasers. The nonlinear optical coefficients are obtained by analyzing the z-scan curve on the basis of a thermal lensing model for CW excitation whereas two photon absorption (2PA) along with a absorption saturation process for ns excitation.

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Introduction

Lasers of different wavelengths with a low divergence angle and small spot size can cause immediate damage to tissues. Notably, visible to near IR lasers penetrate the eyeball easily and affect the retina. Based on the maximum permissible exposure/accessible emission limits,¹ lasers have been categorized into 4 classes. Nd: YAG lasers (energy > 30 mJ and power > 5 mW) inevitably used in medicine, military sectors, communication technology and industries are in the hazardous and most dangerous laser class of 3b and 4. So, the search for protective materials to safeguard optical sensors and the human eye from laser induced damage has also increased. A typical limiting material should possess a high damage threshold, a low limiting threshold and a broadband limiting response. Nonlinear absorption and refraction are the two governing factors of the limiting behavior of a material and are explored from single beam z-scan measurements.²

Zinc oxide (ZnO) a multifunctional material, has vast applications including electronics, optoelectronics, solar cell, gas sensor, photocatalysis etc. Wide band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature make ZnO a good choice for all the applications.³ In addition, it has attracted the materials scientists much owing to its higher mechanical and chemical stability, environment friendliness, low cost and ease of growth. The inherent properties of ZnO can be tailored by the suitable dopants towards the desired applications. Bharati Panigrahy et al. have reported the enhancement in the ferromagnetic behavior of ZnO nanorods by Mn- and Co-doping which finds potential applications in spintronics.⁴ Recent study on Co-doped ZnO nanodisks and nanorods prepared by wet chemical method revealed the sunlight driven photocatalytic activity against methylene blue.⁵ Also, Guo et al. have demonstrated a 500 times larger non-resonant third order nonlinear susceptibility χ⁽³⁾ in PVP-capped ZnO nanoparticles than that of bulk ZnO.⁶ It is also reported that, ZnO–silica nanocomposites show random lasing action due to the formation and inversion of electron–hole plasma.⁷ However, its third order nonlinear optical behavior under short pulse and continuous wave laser illuminations have been of great importance because of optical switching and optical limiting applications. It is obvious from the literature that, third order nonlinearity of ZnO is resulting from either two photon absorption with induced absorption or thermal lensing. ZnO and Mn doped ZnO thin films show nonlinearity due to thermal lensing.⁸ Polymer/ZnO nanotops, ZnO–CdS nanocomposites and Er doped ZnO thin films possess optical nonlinearity through two photon absorption.^9–11

An effective three photon absorption due to sequential absorption of surface states were accounted by Ramakrishna et al. for Co doped ZnO nanostructures.¹² In the present study, the z-scan measurements and the optical limiting response of the pure and Co: ZnO nanoparticles are examined both at nanosecond pulsed (532 nm, 5 ns, 10 Hz) and CW (532 nm, 50 mW) Nd: YAG laser regime. The influencing factors, dopant Co and the defect states play a crucial role in the enhancement of limiting activity and so are analyzed systematically.

Experimental section

Synthesis

Nanoparticles of Co doped ZnO with different concentrations were synthesized by co-precipitation method at 325 K. Zinc acetate dihydrate, cobalt acetate hexahydrate and potassium hydroxide were taken as precursors. A solution containing 140 mmol of KOH was stirred for 2 hours at 325 K. In that, zinc acetate solution (24 mmol/100 ml) was mixed and continuously stirred for 2 hours. The resultant solution was aged for 2 days and centrifuged. The precipitate was washed with double distilled water and ethanol and dried at 400 K for 2 hours (ZC1). The procedure was repeated for Co doped ZnO with the addition of 5, 10, 15 and 20 at% of cobalt acetate hexahydrate and are hereafter coded as ZC2, ZC3, ZC4 and ZC5 respectively.

Characterization

ZnO product formation was initially verified with the X-ray diffraction (Rigaku Ultima III, 2θ: 20–80°) and FTIR (Jasco 460 plus, 4000–400 cm⁻¹) spectral analyses. Raman spectrum was recorded using a Witec confocal CRM200 laser Raman spectrometer with the 488 nm output of Ar⁺ laser excitation. The surface morphology and particle size were characterized by TEM (Philips CM200). Energy dispersive X-ray analysis was performed with an Oxford instruments for the elemental confirmation. Oxidation state of Co was examined using a JEOL JES – FAZOD EPR spectrometer in the X-band region (9.44 GHz) at 77 K and a X-ray photoelectron spectrometer (Specs, Germany). Electron energy loss spectrum (EELS) was recorded on a Carl Zeiss Libra 200 high resolution transmission electron microscope with field emission gun and in column Omega energy filter. Linear optical absorption and emission properties were analyzed using UV-Vis spectrophotometer (Perkin Elmer Lamda) and spectrofluorometer (Perkin Elmer LS 35). A 330 nm wavelength emitted from a Xe lamp was used for the excitation in photoluminescence (PL) for the defect identification.

Nonlinear optical measurements

The third order nonlinear optical characteristics like nonlinear absorption, nonlinear refraction and nonlinear susceptibility were determined for Co doped ZnO nanoparticles by z-scan technique. The standard z-scan experimental set-up used for the measurements is shown in Fig. 1. Z-scan experiments were executed with the excitation of second harmonics of diode pumped Nd: YAG laser emitting continuous wave form (50 mW) and a Q-switched Nd: YAG laser with nanosecond pulses (5 ns, 10 Hz, 100 μJ). The samples were taken in the form of dispersed solution in double distilled water with the linear transmittance of about 60%. The path length of the sample cell (quartz cuvette) is of 1 mm, lower than the estimated Rayleigh range (Z_R) of the beam which is a significant prerequisite in z-scan measurements. Closed and open z-scans were done with the presence and absence of aperture. During the exposure, the sample cell placed at the Z = 0 position (focal point of lens) was moved to and fro along the Z-axis. At each Z-position, the reference beam energy (input), transmitted beam energy (output) and their ratios were measured by the pyroelectric detectors and recorded in the PC.

Fig. 1 Experimental setup of z-scan measurements. BS-beam splitter, L-lens, S-sample, D1 and D2 – detectors, PC-computer.

Results and discussion

It is essential to study the structural changes of ZnO nanoparticles by Co addition for the better understanding of third order nonlinearity. Hence, the structural evolution of the samples has been investigated with the XRD spectra shown in Fig. 2. The existence of diffraction peaks of (100), (002), (101), (102), (110) and (103) planes clearly reveal the hexagonal crystal structure of ZnO for all the samples. Absence of separate impurity phase of Co in all the dopant concentrations indicate the inclusion of Co ions in ZnO and the solubility limit of Co is above 20 at%. In addition, due to the similar ionic radii of Zn²⁺ (0.74 Å) and Co²⁺ (0.72 Å) ions, Co ions can easily substitute Zn ions. The lattice parameters are calculated and the values are well agreed with the JCPDS 36-1451 (a = 3.253 and c = 5.209 Å). The broadening of the diffraction peaks indicates the decrease in the crystallite size of ZnO. It is also accompanied by the crystallite size (6–10 nm) calculated using the Debye–Scherrer formula (Table 1). It is found that, crystallite size decrease initially (<10 at%) and increase further but not in a linear manner. FTIR spectra shown in Fig. 3 provide the characteristic functional groups present in the prepared particles. The strong absorption band due to Zn–O stretching vibration at 449 cm⁻¹ confirms the formation of ZnO.¹³ There is a shift from 449 to 463 cm⁻¹ for the increase of Co concentration. Together with XRD, FTIR also authenticates the substitution of Co in ZnO. The two absorption bands of O–H at 3400–3500 cm⁻¹ and 1650 cm⁻¹ correspond to the stretching and bending vibrations respectively due to the presence of surface adsorbed H₂O molecules. The structural change by Co doping was also studied by Hooke's law.¹⁴ The increase (1.526–1.616) in force constant with Co doping illustrates the fluence of Co ion on Zn–O bonding.

Fig. 2 XRD patterns of pure and Co: ZnO nanoparticles.

Table 1 Parameters derived from XRD and FTIR

Sample	Crystallite size D (nm)	Wavenumber [small upsilon, Greek, macron] (cm^−1)	Force constant (N m^−1)
ZC1	9.8	449	1.526
ZC2	7.6	451	1.538
ZC3	6.9	452	1.543
ZC4	8.0	457	1.576
ZC5	9.1	463	1.616

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Fig. 3 FTIR transmittance spectra of pristine and Co doped ZnO nanoparticles.

Surface morphological changes due to Co doping were investigated by TEM and HRTEM micrographs shown in Fig. 4. It apparently shows the spherical shaped nanoparticles with the particle size of 6–13 nm. It evident the influence of Co on particle size but there is no change in morphology. The inset of Fig. 4b, the HRTEM image of ZnO, shows the well ordered crystalline planes with a interplanar ‘d’ spacing of 0.26 nm corresponding to the (002) plane. The SAED patterns of the ZC1 and ZC3 in Fig. 4c and f displays the seven concentric rings with bright spots indexed to (100), (002), (101), (102), (110), (103) and (200) planes. These planes are also in accordance with the XRD. Further, the substitution of Co was verified with the elemental composition done by the EDS analysis. EDS spectra (Fig. S1†) of ZC2 and ZC3 are composed of the elements Zn, O and Co. The XRD, FTIR and EDS corroborate the substitution of Co in the ZnO lattice and also the purity of the samples. The slight agglomerated state viewed in the TEM micrographs are due to the weak van der Wall's force between the nanoparticles.

Fig. 4 (a and d) TEM, (b and ​e) HRTEM and (c and ​f) SAED of pure and ZC3 respectively. Inset of (b) HRTEM image of a single ZnO nanoparticle.

Inorder to investigate the oxidation state of the Co, EPR spectra were recorded for pristine and Co: ZnO at 303 K and 77 K respectively (Fig. 5a and b). ZC1 exhibits a hyperfine signal at g ∼ 1.96 due to the singly ionized Zn vacancy defects. Though ZnO is a diamagnetic material, the defects alone show fine structure transitions. However, for ZC2 to ZC5 due to the short duration of spin–lattice relaxation time of Co at RT, the resonance lines are not observed.¹⁵ A strong peak at 142 mT (g_ǁ ∼ 4.6) and a weak shoulder at 255 mT (g_┴ ∼ 2.6) are clearly observed at 77 K in all Co doped ZnO samples indicating the substitution of Co²⁺ ions in ZnO. It is also noted that the signal of g_ǁ is intense than g_┴ due to the axial distortion of the crystal field.

Fig. 5 EPR spectrum of (a) pure ZnO at RT and (b) Co: ZnO at LNT.

Incorporation of Co ions in the hexagonal lattice and its oxidation state are further confirmed through the EELS and XPS spectra. In Fig. 6a, Co L₂ and L₃ edges are found in the range of 780–800 eV which clearly indicate the presence of Co ions in the host ZnO. The observed peaks arise due to the transition of Co 2p core electrons to the unoccupied 3d states which are in hybridization with 2p electrons of oxygen. XPS survey spectrum of ZC3 in Fig. 6b contains the peaks of Zn, O and Co. The peaks at 779.8 and 795.2 eV correspond to Co 2p_3/2 and 2p_1/2 core level transitions. The energy difference between these two states is found to be 15.4 eV and is well matched with the reported values of CoO having the Co ions with +2 oxidation state.¹⁶ So, it is inferred that, Co has the high-spin divalent oxidation state in the prepared sample and is furthermore evidences the substitution of Zn²⁺ ions by Co²⁺.

Fig. 6 (a) EELS spectra of ZC3 and ZC4 (b) XPS survey spectrum of ZC3 with enlarged Co 2p spectrum as inset.

Linear optical absorption coefficient (α), refractive index (n) and the linear transmittance of the materials evaluated from the UV-Vis spectra is of importance for the clear identification of the origin of nonlinearity. In Fig. 7, normalized optical absorption spectra of the pure and Co: ZnO nanoparticles is provided for the spectral range of 200–800 nm. An absorption edge due to the direct excitonic transition is observed at 365 nm for pure ZnO. The absorption edge of the Co doped ZnO nanoparticles get blue shifted from the pure ZnO. From the spectra of both, the blue shift of pure and doped ZnO samples compared to bulk ZnO is outlooked. This shift cannot be originated from the quantum confinement effect as the particle sizes are well above the exciton Bohr radius of ZnO (1.8 nm). The pronounced shift is attributed to Burstein–Moss effect in which the low energy transitions are screened due to the Fermi level shift to the conduction band.¹⁷ It is also found that, in the entire visible region the materials are being transparent.

Fig. 7 Normalized optical absorption spectra of pure and Co: ZnO NPs. The vertical arrow indicates the 532 nm laser excitation.

Impact of dopant ions on the crystal structure and the defect states were studied by the non-destructive methods like Raman and PL. Raman spectra (Fig. 8) confirm the phase purity and crystallinity. All the samples exhibit the characteristic phonon vibrations E_2H and E_2L of ZnO at 437 and 99 cm⁻¹ due to O and Zn sublattices respectively. The spectra also depict the first and second order multiphonon scattering vibrations at 205, 330, 660 and 1100 cm⁻¹ owing to 2E_2L, E2_H–L, A_1L and 2A_1L. Co doped ZnO samples show an additional phonon mode at 530 cm⁻¹. This peak arises possibly by the defect states¹⁸ and is further optimized by PL. PL spectra (Fig. 9) of all the nanoparticles possess a broad emission spectrum consists of three different emission peaks for the excitation of 330 nm. By Gaussian deconvolution, the peaks centered at 380, 410 and 465 nm are clearly seen (Fig. S2†). Generally, the radiative recombination of conduction band (CB) electrons with valence band (VB) holes occurs at 380 nm. This is also known as band edge emission. Intermediate states located below the CB minimum and above the VB maximum will emit in the region 400–800 nm. Sayan Bayan et al. reported that, the emission at 410 nm is due to the electron transition from CB to Zn vacancy site.¹⁹ The other emission at 465 nm is associated with the radiative emission of an electron from the CB to the ionized zinc vacancy level.²⁰ These deep levels in metal oxides are still under debate but in the present study it is verified with other spectral studies like EPR and EDS. Moreover, the decrease of defect to band edge emission ratio with increase of Co addition exemplifies the incorporation of Co²⁺ ions in the ZnO lattice.

Fig. 8 Micro-Raman spectra of pristine and Co doped ZnO nanoparticles.

Fig. 9 Normalized fluorescence emission spectra of pure and Co: ZnO NPs for the excitation of 330 nm.

Optical nonlinearity

To evaluate the nonlinear absorption coefficient β of Co: ZnO nanoparticles, open aperture z-scan testing was carried out at the excitation of 100 μJ energy pulses. All the samples exhibit a transmittance valley, showing the reverse saturable absorption (RSA) behavior (Fig. 10). The transmittance of the system decrease in RSA as the absorption cross-section of the excited state is larger than the ground-state. Since the RSA nature in the materials is induced by mechanisms like two/three photon absorption, free carrier absorption, excited state absorption, nonlinear scattering etc, the theoretical fit to the experimental data exact the origin. For Co: ZnO nanoparticles, the experimental curves are well fitted for two photon absorption (2PA) with saturable absorption and the nonlinearity is of third order. Hence, the intensity dependent effective nonlinear absorption coefficient α(I) has the form²¹where β represents the effective 2PA coefficient, I_s corresponds to the saturation intensity, α₀ is the linear absorption coefficient at the excitation wavelength, I is the intensity of the input laser beam. β in this scenario not only characterizes genuine nonlinear absorption occurring at the excitation wavelength but also the sequential interband and intraband transitions analogous to excited state absorption. To calculate the transmitted intensity as a function of the propagation distance, the intensity variation is numerically solved using the propagation equation:²²where Z′ is the propagation distance within the sample.

Fig. 10 Normalized open aperture z-scan curves of Co doped ZnO nanoparticles (5 ns, 100 μJ). Hollow circles: data points and the solid curves: numerical fits.

The nonlinear absorption coefficient β is a concentration dependent factor whereas the saturation intensity I_s is a concentration independent parameter (Table 2). The term β is found to be increase with Co concentration where I_s varies randomly. The 2PA coefficient estimated for Co: ZnO nanoparticles are comparable with the reported NLO materials (Table 3).

Table 2 Nonlinear optical parameters derived from z-scan measurements

Parameter	ZC1	ZC2	ZC3	ZC4	ZC5
Pulsed Nd: YAG (532 nm, 5 ns, 100 μJ)
2PA coefficient, β (× 10^−10 m W^−1)	0.31	0.39	0.40	0.52	0.48
Saturation intensity, Is (× 10^12 W m^−2)	4.79	3.89	4.29	2.79	1.79
Limiting threshold (J cm^−2)	—	3.91	3.73	2.87	3.15
CW Nd: YAG (532 nm, 50 mW)
Nonlinear absorption coefficient, β (× 10^−5 m W^−1)	6.74	6.71	6.60	6.62	6.88
Nonlinear refractive index, n2 (× 10^−8 cm^2 W^−1)	4.15	6.02	6.74	7.07	7.04
Limiting threshold (mW)	24.9	22.7	24.9	24.9	22.7
Clamping power (mW)	1.42	2.41	2.88	4.17	4.72

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Table 3 Two photon absorption coefficient of some semiconducting nanomaterials

Material	2PA coefficient (β) (m W^−1)	References
ZnO–CdS nanocomposites	10^−10 to 10^−11	23
GO–ZnO hybrids	0.9 to 15 × 10^−11	24
CdSe–CdS quantum dots	1.9 to 3.1 × 10^−10	25
Pt doped TiO2 nanoparticles	2.2 to 4 × 10^−11	26
Co: ZnO nanoparticles	0.31 to 0.48 × 10^−10	Present study

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Thermal nonlinearity

The effect of Co on the nonlinear optical properties of ZnO nanoparticles have also been investigated under CW laser excitation (532 nm, 50 mW). Both the open and closed aperture z-scans were performed to estimate the nonlinear absorption coefficient and the nonlinear refractive index of the materials. A lens is placed before the detector in open aperture z-scan to collect the laser beam transmitted by the sample whereas, in closed z-scan, the aperture (S-0.01) replaces the lens. To obtain the effective nonlinear refraction, closed z-scan data obtained is divided by the open z-scan data. The results of open aperture z-scan measurements show (Fig. 11) the maximum transmittance at high intensity. This is the indication of the saturable absorption behavior of the samples ZC1–ZC5. The nonlinear absorption coefficient β is estimated by fitting the experimental curve to the normalized transmission condition given by

Fig. 11 Normalized open aperture z-scan curves of Co doped ZnO nanoparticles (50 mW). Stars: data points and the solid curves with stars: theoretical fits.

The parameters q_o, L_eff, L, Z_R and ω_o are representing their usual phrases in z-scan measurements and it can be found elsewhere.²⁷

The nonlinear refractive index n₂ has been determined by closed aperture z-scan. The closed aperture z-scan ratio curves in Fig. 12 point out the self-defocusing property of the materials as the curve exhibit pre-focal peak followed by a post-focal valley nature. The curve itself states the negative sign of the nonlinear refractive index. The peak–valley difference (ΔT_p−v) extracted from the Fig. 12 is used in the following relation to calculate n₂.

Fig. 12 Normalized closed aperture z-scan ratio curves of Co doped ZnO nanoparticles (50 mW). Stars: data points and the solid curves with stars: theoretical fits.

The real and imaginary parts of the third order nonlinear optical susceptibility are related to β and n₂ by the following relations

where ε_o, the free space permittivity (8.854 × 10⁻¹² F m⁻¹) and C, the velocity of light at vacuum (3 × 10⁸ m s⁻¹). The calculated values of β and n₂ are found to be in the order of 10⁻⁵ m W⁻¹ and 10⁻⁸ cm² W⁻¹ respectively and are comparable with the previously reported NLO materials.

Origin of optical and thermal nonlinearities

The origin of nonlinearity under the short pulse and CW laser excitations is different. In the case of pulsed laser, nonlinearity arises due to absorption and its physical origin is purely electronic. In the present study, nonlinearity at pulsed regime arises by 2PA with saturable absorption. As the incident laser energy 2.33 eV is not enough to excite the electrons of ZnO with the band separation of 3.26 eV (PL), the sequential absorption of two photons one at ground state and the other at the intermediate state can excite the electrons. The enhancement of the β with Co concentration is caused by the increased rate of absorption of photons.

While at CW laser regime, nonlinear refraction happens during irradiation and the refractive nonlinearity is caused by nonradiative processes such as thermal, mechanical (or) electrostrictive etc. In the present study, thermal lensing model governs the nonlinearity and is confirmed by the peak–valley separation. When the condition peak–valley difference ΔT_p−v of 1.7 times the Rayleigh range Z_R is satisfied by the material, the nonlinearity is thermal in nature. The material exhibiting thermal nonlinearity will focus (or) defocus the incident photon energy by acting as a lens. In the present work, all the samples act as a negative lens with self-defocusing behavior. The nonlinearity and its increment with dopant concentration are caused by the refractive index change through the local heating. In other words, as the PL spectra infer the presence of defects in the pure and Co: ZnO, a small amount of energy is absorbed by the particles as well as the defect states during irradiation of laser. When the defects and Co concentration are high, the localized heating is also high due to the thermal agitation of the particles which results in the change in refraction. So, the enhancement in nonlinearity of Co doped samples is revealed.

Optical limiting

To examine the limiting behavior of the samples at pulsed and CW laser illuminations, the measurement was carried out using the z-scan experimental setup. Instead of moving the sample, the sample position was fixed at the focus and the input power is varied. A typical optical limiter has dual nature with respect to the input fluence. It completely transmits the low input fluence and becomes opaque at high input. In other words, the materials follow the Beer–Lambert's law of absorption at low incident energy and start to deviate from linearity at higher intensity. An important criterion for the material to become an optical limiter is the limiting threshold. The material with the lower optical limiting threshold is the best optical limiter. Fig. 13 corresponds to the optical limiting curves which are extracted from the open aperture z-scan results of ZC1–ZC5 under short pulse irradiation. The estimated limiting threshold value is in the order of 10¹³ W m⁻² and is comparable with the reported threshold of nanomaterials.^28,29 In contrast to the pulsed regime, limiting measurements were done in CW case by varying the input incident power. Fig. 14 depicts the optical limiting response of ZC1–ZC5 samples at CW laser excitation for the various input powers (0–40 mW). The clamping threshold of the samples is found to be less than 5 mW. In the prior case, the limiting nature arises by reverse saturable absorption induced by 2PA. Later case deals with the optical limiting caused by thermally induced refractive nonlinearity.

Fig. 13 Optical limiting curves of Co doped ZnO nanoparticles derived from the z-scan traces (5 ns, 100 μJ).

Fig. 14 Low power optical limiting response curves of Co doped ZnO nanoparticles.

Conclusion

The suitability of nanoparticles of Co doped ZnO synthesized by co-precipitation method as an optical limiting material is confirmed by z-scan method both at nanosecond and CW irradiation modes of Nd: YAG laser. Co substituted ZnO nanoparticles of hexagonal structure with spherical shapes are formed for all the concentrations. Photoluminescence measurements demonstrate the presence of zinc vacancy defects in the lattice. The optical band gap varies between 3.26 eV to 3.36 eV for ZC1–ZC5 leading to the 2PA. All the prepared materials exhibit third order nonlinearity at both pulsed and CW laser regimes owing to reverse saturable absorption and self-defocusing respectively. The increase of absorptive nonlinearity with Co concentration is more photon absorption by more number of Co ions while the refractive nonlinearity is achieved by the change of refractive index via the thermal gradient. The estimated β value for short pulse is in the order of 10⁻¹⁰ m W⁻¹ and 10⁻⁵ m W⁻¹ for CW excitation. The samples ZC4 (2.87 J cm⁻², 24.9 mW) and ZC5 (3.15 J cm⁻², 22.7 mW) show lowest optical limiting threshold for both nanosecond and CW laser excitations. Based on these, the synthesized materials are claimed as promising candidates in the design of optical devices like optical limiters for eye and sensor protection at both high and low power laser regimes.

Acknowledgements

The authors gratefully acknowledge Dr Shamima Hussain, UGC-DAE Consortium for Scientific Research, Kalpakkam node for XPS analysis, DST-FIST for the instrumentation facilities and the utilization of EPR facility at the Department of Chemistry, Bharathidasan University, Tiruchirappalli. P. R. sincerely thanks the University Grants Commission, Government of India for the award of research fellowship under the scheme of Research Fellowship in Sciences for Meritorious Students (Grant No. F. 4-1/2006 (BSR)/7-197/2007(BSR)).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra10756c

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