Defect-assisted saturable absorption characteristics in Mn doped ZnO nano-rods

Abstract

We have investigated the effect that manganese (Mn)-doping in ZnO sub-wavelength rods (or nanorods) has on nonlinear optical properties, namely two-photon absorption (TPA) and nonlinear refraction using the single-beam Z-scan technique. Mn-doped ZnO nanorods (NRs) were prepared by a low temperature aqueous growth technique. The results show that the Mn-doping concentration primarily determines whether ZnO NRs will exhibit saturable absorption (SA) or two-photon-absorption (TPA) characteristics in an open-aperture experiment. At high Mn-doping concentrations, ZnO NRs exhibit SA behaviour which can be attributed to a high occupation probability of defect states as well as the saturation of linear absorption of sub-wavelength rod aggregates at high optical fluence. In contrast to high Mn-doping concentration in ZnO NRs, we observed TPA features in 0.5% Mn-doped ZnO NRs. The employability of such structures in the area of optical limiting and switching is essentially derived from the possibility to tune the nonlinear optical absorption which could be realized by appropriate Mn-doping in ZnO NR architecture.

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Introduction

Amongst many semiconductor materials, ZnO has received considerable attention in the area of nonlinear optics primarily due to its great potential for a variety of applications.¹ In comparison to bulk ZnO, ZnO-based sub-wavelength structures exhibit enhanced optical nonlinearities and a fast response time as a consequence of material as well as structural resonance effects.^2,3 For example, one-dimensional (1D) ZnO sub-wavelength sized crystals, or nanocrystals, exhibit a wide range of promising applications in UV photonic devices,⁴ field emission devices,^5,6 sensors,⁷ and piezoelectric nanogenerators.⁸ It is worthwhile to note that ZnO is a wide band gap (3.4 eV) semiconductor which can be grown into subwavelength structures that vary in size from a few tens of nanometers to a few hundred nanometers using physical as well as chemical methods. However, the recipe adopted to grow such ZnO based aggregates primarily determines their physical and chemical properties.¹ This happens due to different surface morphology and varying spatial arrangements of ZnO subwavelength clusters. For example, optical confinement of aligned ZnO-nanorod (NR) arrays results in improved optical emission properties which could be useful for UV lasing along the long-axis of aligned NRs.^9,10 Due to unequal atomic dimensions of Zn as well as O along with high inherent polar potentials of the ZnO crystal,^11,12 ZnO exhibits enhanced optical nonlinearity. The nonlinear optical processes such as second-harmonic-generation (SHG) and self-phase modulation (SPM) etc. exhibit remarkable enhancement by virtue of resonant transitions to exciton levels in ZnO NRs as well as in ZnO thin films.¹³

It is worthwhile to note that the doping of ZnO can significantly tune the bandgap which can be employed for favorably altering optical as well as electronic properties of sub-wavelength aggregates.^14,15 For example, it was observed that the spintronic properties of ZnO films could be enhanced through Mn, Co, Ni, or Fe doping as a consequence of strong coupling between the magnetic moments of the dopants in the aggregates of the host (ZnO). However the structural and physical properties of such aggregates must be thoroughly understood to be used in devices operating at or above room temperature.^16,17 Among all of the magnetic ion-doped ZnO architectures, Mn-doping has received considerable attention essentially due to its high thermal solubility and excellent lattice-matching with the ZnO matrix.¹⁸ It is to be noted that Mn is an isovalent impurity for Zn, and the ionic radius (0.066 nm) of Mn²⁺ is comparable to that of Zn²⁺ (0.060 nm) which results in a theoretical solubility limit of 35% for Mn, while maintaining the wurtzite structure of ZnO thin films.^2,19 Also, it has been proposed that the observation of room-temperature ferromagnetism in Mn-doped ZnO thin films could be due to the stabilization of an oxygen-deficient Mn_xO_y secondary phase which results in an exchange interaction between magnetic moments localized at Mn sites mediated by free charge carriers.²⁰ It is interesting to note that this ferromagnetic ordering results in magnetization-induced modifications in nonlinear optical susceptibilities (χ⁽²⁾_ijk, χ⁽³⁾_ijkl etc.) which result in enhanced efficiencies for second-harmonic-generation (SHG), self-focusing/defocusing, four-wave mixing etc.²¹ Therefore, it is expected that the nonlinear optical (NLO) properties of Mn-doping in ZnO based sub-wavelength aggregates could substantially differ from bulk ZnO or thin-films essentially due to additional electric-dipole contribution from geometrical effects as well as magnetization-induced effects. However, a comprehensive investigation of third-order nonlinear optical properties of Mn-doped ZnO based sub-wavelength architectures is yet to be carried out for ascertaining the role of geometry as well as magnetization (due to Mn-doping). In this work, we present a detailed study on nonlinear refractive-index (NLR) and two-photon absorption (TPA) coefficient for different fractions of Mn-doping in ZnO nanorods which have been fabricated using a hydrothermal chemical wet synthesis at a low temperature.²² The NLO behaviour of the NRs was ascertained using a single-beam Z-scan technique with frequency doubled mode-locked Nd:YVO₄ laser pulses in the sub-nanosecond regime.²³

Experimental details

Commercially available 500 nm thick sputter coated indium tin oxide (ITO) layers were used as substrates for ZnO NR growth. Analytical grade zinc nitrate and hexamethylenetetramine (HMTA) were obtained from Sigma Aldrich. The substrates were ultrasonically cleaned in acetone, ethanol and de-ionized (DI) water for 10 minutes each. Subsequently, the substrates were cleaned in freshly prepared aqua regia (3HCl + HNO₃) for 5 minutes in an ultrasonic bath followed by thorough rinsing in DI water. The cleaned substrates were vertically immersed in aqueous solutions of 0.5 M zinc nitrate hexahydrate, 0.5 M HMTA and 0.5%, 1.5%, 2.0%, 2.5% (molar percentage) manganese chloride tetrahydrate respectively and were kept in Borosil bottles. Thereafter the mixture was refluxed at 90 °C in a regular laboratory oven for 6 hours for all samples. After that the bottles were eventually allowed to cool down to room temperature, and the samples were thoroughly rinsed in DI water to dissolve the residual salt and surfactant.²⁴ Morphological changes in the NR samples were investigated by using a field emission scanning electron microscope (FESEM) (Carl-Zeiss). The elemental analysis was carried out in plane-view mode by the energy dispersive X-ray spectroscopy (EDS) system (Oxford Instruments), attached to a FESEM, using 20 kV electron beam. The crystallinity and orientation of the ZnO NRs were investigated using X-ray diffraction spectra (Bruker, D8-Discover). The linear optical properties of the samples were studied using UV-visible absorption spectroscopy and photoluminescence (PL) spectroscopy. The variation in the absorption-edges of the ZnO nanorods due to the introduction of Mn²⁺ ions was estimated from the UV-visible spectra. The room temperature photoluminescence (PL) spectra were recorded using a FS920 Edinburgh setup which used a 325 nm, 15 mW He–Cd laser as the excitation source.

For the nonlinear optical measurements, the experimental set up was based on the Z-scan technique, as shown in Fig. 1. The single-beam Z-scan experiment was carried out using a Q-switched diode-pumped solid-state (DPSS) laser emitting linearly-polarized, sub-nanosecond pulses in a TEM₀₀ mode-intensity profile. The pulse energy varied from 1 μJ to 125 μJ for the measurements with pulse width t_p ≈ 0.7 ns at 40 Hz repetition rate so as to minimize the impact of thermally induced optical nonlinearities. The beam was focused to a spot size of w₀ ≈ 45 μm using a converging lens (f = 150 mm) which resulted in a Rayleigh length (z₀ = πw₀²/λ) of ≈12 mm. This ensured negligibly small ‘sample + substrate’ thicknesses compared to z₀. The samples were translated a distance of 100 mm through the beam focal point and the transmitted power in an open-aperture (OA) as well as in a closed-aperture (CA) configuration was measured using a fast photo-diode sensor (Model S120C; Thorlabs Inc.).

Fig. 1 Schematic of the Z scan experimental set up. HWP: half wave plate, PBS: polarizing beam splitter, L: lens (150 mm), S: sample, A: aperture, PD1: reference detector, PD2: signal photo detector.

Results and discussion

Fig. 2(a)–(j) show FESEM micrographs of 0.5, 1.5, 2.0, and 2.5 (mol%) aqueous grown Mn doped ZnO NRs respectively at different magnifications. In all cases, the NRs were grown at 90 °C temperature for 6 hours. The FESEM micrographs reveal that the Mn doping does not affect the hexagonal morphology of the ZnO aggregates, because Mn normally substitutes the Zn²⁺ sites of ZnO forming Zn_1−xMn_xO. The NRs exhibit lengths ranging from ≈2–4 μm with ≈400–600 nm diameters.

Fig. 2 Panels (a), (c), (e), (g), and (i) show the FESEM micrographs of as grown, 0.5%, 1.5%, 2%, and 2.5% Mn-doped ZnO NRs respectively and panels (b), (d), (f), (h), and (j) show magnified views of panels (a), (c), (e), (g), and (i) respectively.

In order to confirm the Mn-doping in ZnO NRs, the EDS spectra were collected from several areas of the samples. The EDS mapping of 1.5% Mn-doped ZnO NRs was recorded from the selected area of the typical electron image as shown in Fig. 3(a). Panels (b)–(d) show the elemental mapping of Zn, O and Mn respectively, which demonstrate the presence of Mn in ZnO NRs. The actual doping of Mn in the ZnO NRs was estimated from EDS in terms of atomic% and was plotted as a function of the molar% of Mn and is shown in Fig. 3(e). From the EDS analysis the average atomic% of Mn, calculated from different areas of the samples, were found to be 0.4, 1.2, 1.6, and 1.9 corresponding to the 0.5, 1.5, 2.0, and 2.5% molar concentrations of the Mn precursor respectively.

Fig. 3 (a) FESEM image of Mn doped ZnO NRs, (b)–(d) EDS elemental mapping images of Zn, O and Mn respectively, (e) calculated atomic percentage of the Mn K_α edge with respect to the molar percentage of Mn in the solution (f) X-ray diffraction pattern of pure ZnO NRs and Mn doped ZnO NRs on an ITO (*) substrate respectively, the inset shows a magnified view of the (100), (002), and (101) phases of ZnO and Mn doped ZnO NRs.

The glancing angle X-ray diffraction patterns of un-doped and 2.5 mol% Mn-doped ZnO NRs are shown in Fig. 3(f). The sharp and intense diffraction peak indexed as (002) indicates its most preferential growth direction and confirms the good crystalline quality of the wurtzite structure of the ZnO NRs. This could also be verified from FESEM micrographs where ZnO NRs are erected vertically. After Mn doping the prominent (100), (002), and (101) XRD peaks show a shift towards lower theta values as compared to the undoped NRs, which reveal the development of stress in the matrix due to the expansion in the unit cell. The developed stress is primarily a consequence of the substitution of Zn²⁺ (0.74 Å) by the larger ionic radii of Mn²⁺ ions (0.8 Å). This again confirms that Mn²⁺ ions have replaced a few Zn²⁺ from the ZnO NR matrix.

Fig. 4(a) shows the room temperature (RT) cumulative PL spectra of 0.5%, 1.5%, 2.0% and 2.5% Zn_1−xMn_xO NRs respectively, the inset shows the RT PL spectrum of the un-doped ZnO NRs. The RT PL spectra show that the Mn-doped ZnO NRs give a strong UV emission as a consequence of near-band-edge (NBE) emission and this is consistent with bulk ZnO.²⁵ In addition to NBE emission; there are emission peaks at longer wavelengths due to defect states. PL peaks at 470 nm for all of the samples (see Fig. 4(a)) is a distinct signature of defect induced emission. In order to appreciate this feature, we measured the UV-visible absorption spectra of Zn_1−xMn_xO NRs for various Mn-doping concentrations (0.5%, 1.5%, 2.0% and 2.5% respectively) as shown in Fig. 4(b). The absorption spectra for all Mn-doping concentrations (in ZnO NRs) are characterized by peaks around the 380–400 nm wavelengths (3.26–3.10 eV) corresponding to NBE absorptions and there is a discernible asymmetry in each spectrum. The asymmetry in absorption with a long tail extending deeper into the visible band could be understood by noting the existence of closely-spaced multiple defect states close to band-edges which are primarily due to Zn vacancies (V_Zn) or O vacancies (V_O). The absorption spectrum of un-doped ZnO NRs markedly differs from the Mn doped spectra (Fig. 4(b)) and it is characterized by the appearance of new absorption edges corresponding to the defect states. Using the relation , where α, β, hϑ and E_g are absorption coefficient, constant, photon energy and band gap respectively, we have converted the absorption spectra into Tauc plots²⁶ as shown in Fig. 4(c). It has been observed that the band gaps of un-doped ZnO NRs are 3.18 eV, shown in the inset of Fig. 4(c). After the Mn-doping has been carried out, the absorption edge begins to appear at lower energy values which suppresses the bandgap absorption edge. The absorption edges estimated from Tauc plots for Zn_1−xMn_xO NRs are in the range of 2.48–2.52 eV as shown in Fig. 4(d). The band gaps in Mn doped samples remain the same as in un-doped ZnO NRs and a typical UV emission of around 380 nm is observed in the PL spectra along with 470 nm peaks from defect related states (Fig. 4(a)). Similar observations are also reported by Prabhakar et al.²⁷ for CVD grown Mn doped ZnO NRs. They observed the lower absorption edge of 2.18 eV for 2% Mn doping and explained that the reduced band edge is due to lattice expansion and substitution of Mn ions into the ZnO lattice. We observed the absorption edge at 2.52 eV for 2% Mn doping whereas we observed 2.53 eV for 0.5% doping. The mismatch in absorption edge reported values²⁷ may arise due to the method of preparation, experimental parameters, and doping procedure.

Fig. 4 (a) Room temperature (RT) photoluminescence spectra for various doping% of Zn_1−xMn_xO ZnO NRs, RT PL spectrum of ZnO NRs is shown in the inset; (b) RT absorption spectra of pure ZnO NRs and Zn_1−xMn_xO NRs; (c) Tauc plots of Zn_1−xMn_xO NRs, ZnO NRs shown in inset and (d) band gap calculated from (c) of Zn_1−xMn_xO NRs as a function of the Mn doping concentrations.

It has been observed that the band gap remains almost same with Mn doping concentration. The measured absorption edges of defect states lie substantially close to that for Zn-vacancies or O-vacancies (≈2.2 eV) which have been reported earlier.²⁸ Therefore, a defect induced TPA behavior is expected in such ZnO NR architectures when irradiated with frequency-doubled near-infrared lasers emitting at 532 nm. Also, the existence of defect states could also bring about enhancement in efficiency of non-resonant nonlinear optical processes.

Fig. 5(a–d) and 6(a–d) show representative traces for CA and OA Z-scan measurements of different Mn-doping concentrations in ZnO NRs at incident on-axis laser intensities of ≈1–3 GW cm⁻². Pre-focal maxima and post-focal minima in the CA transmittance (Fig. 5a–d) is a signature of a self-defocusing effect or a negative value for the nonlinear refractive index (n₂). As expected, 0.5% Mn-doped ZnO NRs exhibit a dip in OA transmittance at the focal plane (z = 0) in Fig. 6(a), thereby indicating a positive value for the TPA coefficient (β). However, it is interesting to observe a transmittance peak at the focal plane (z = 0) in OA measurements for higher concentrations of Mn-doping in ZnO NRs which can be seen in Fig. 5(b)–(d). Therefore, enhancement in Mn-doping concentration brings in saturable absorption (SA) characteristics in ZnO NRs which have not been observed before. For comparison, the CA and OA Z-scan transmissions of ZnO nanorods at 532 nm are plotted in Fig. 7. The pre-focal peak followed by a post-focal variation of the CA transmission in Fig. 7(a) indicates a self-defocusing effect (n₂ is negative). On the other hand, the OA measurements for ZnO nanorods exhibit a drop in transmission at the focal point (see Fig. 7(b)) which is a distinct signature of positive nonlinear absorption. In order to ascertain the interference of the substrates on our NLO measurements, we carried out CA and OA Z-scan transmissions of ITO coated quartz substrates at similar laser fluences. We observe that the CA as well as OA normalized transmittance exhibited negligibly weak variations which rules out any contribution from the substrate.

Fig. 5 Closed aperture (CA) Z-scan curves for Zn_1−xMn_xO nanorods for different Mn-doping concentration (a) x = 0.005 (b) x = 0.015 (c) x = 0.02 and (d) x = 0.025.

Fig. 6 The open aperture (OA) Z-scan curves for Zn_1−xMn_xO nanorods for different Mn-doping concentration (a) x = 0.005 (b) x = 0.015 (c) x = 0.02 and (d) x = 0.025.

Fig. 7 (a) Closed Aperture (CA) and (b) Open Aperture (OA) Z-scan traces for pure ZnO nanorods.

The normalized transmittance for CA (eqn (1)) and OA (eqn (2)) Z-scan measurements are given by^23,29

where x = z/z₀, Δφ₀ = kn₂I₀L_eff and ΔΨ₀ = βI₀L_eff/2 are the respective phase changes due to nonlinear refraction and nonlinear absorption, I₀ is the on-axis irradiance at focus (z = 0), L_eff is the effective sample thickness and S = 1 corresponds to the OA configuration. In order to obtain the macroscopic NLO parameters (n₂ and β), we theoretically fit the experimental Z-scan traces shown in Fig. 5(a)–(d), 6(a)–(d) and 7(a and b) for different I₀ values using eqn (1) and (2) respectively. The results are summarized in Table 1 for pure ZnO and different Mn-doping concentrations in ZnO NRs. For comparison, we have also tabulated the values of n₂ as well as β for ZnO thin films and un-doped NRs from the available literature.^30–33 It is evident that the pure ZnO nanorods as well as Mn-doped ZnO nanorods exhibit self-defocusing effects for all Mn-doping concentrations with at least an order of magnitude higher n₂ values than compared to ZnO thin films. Although, there is a significant degree of inconsistency with regard to the sign of nonlinear absorption (β) of ZnO thin-films, it is apparent that β values for Mn-doped ZnO NRs exhibit significantly higher values in comparison with ZnO thin-films. Such enhancement in n₂ and β could be primarily attributed to Mn-doping as well as the geometrical arrangement of Mn-doped ZnO NRs.³⁴ It is worthwhile to note that the FESEM micrographs in Fig. 2 show vertically erected Mn doped ZnO NRs with uniform axial dimensions. Even though, the distribution of NRs is random, there exists a Fabry–Perot-like localized resonance due to small-scale ordering within the architecture which results in the local enhancement of the field.³⁵ In addition to the creation of defect states (within the band gap of ZnO), this local field enhancement leads to an increase in NLO interactions and hence, higher values for n₂ and β.

Table 1 Summary of NLO properties of ZnO thin films and ZnO nanorods

Sample	n2 (cm^2 W^−1)	β (cm W^−1)	Parameters	References
ZnO thin films	−0.90 × 10^−14	4.20 × 10^−9	λ = 532 nm; pulse width = 25 ps	30
ZnO thin films	2.57 × 10^−11	−1.53 × 10^−7	λ = 830 nm; pulse width = 175 fs	31
ZnO nanorods	3.11 × 10^−10	5.61 × 10^−6	λ = 815 nm; pulse width = 85 fs	32
ZnO thin films	5.57 × 10^−11	−0.61 × 10^−6	λ = 815 nm; pulse width = 85 fs	32
ZnO nanorods	—	5.90 × 10^−7	λ = 800 nm; pulse width = 130 fs	33
ZnO nanorods	2.1 × 10^−10	3.5 × 10^−5	λ = 532 nm; pulse width = 0.7 ns	This work
0.5% Mn doped ZnO nanorods	−1.88 × 10^−10	1.32 × 10^−5	λ = 532 nm; pulse width = 0.7 ns	This work
1.5% Mn doped ZnO nanorods	−1.59 × 10^−10	−1.03 × 10^−5	λ = 532 nm; pulse width = 0.7 ns	This work
2% Mn doped ZnO nanorods	−1.25 × 10^−10	−4.17 × 10^−6	λ = 532 nm; pulse width = 0.7 ns	This work
2.5% Mn doped ZnO nanorods	−1.34 × 10^−10	−5.45 × 10^−6	λ = 532 nm; pulse width = 0.7 ns	This work

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On a similar note, the variation in NLO characteristics for different Mn-doping concentrations could also be appreciated by analyzing the modifications in band structure as well as the geometric arrangement of doped ZnO NRs. The peaks in Z-scan OA transmittance (Fig. 5(b–d)) which indicate a SA behaviour could be attributed to the non-availability of unoccupied defect states at high laser intensities. As a consequence, the probability of absorption of incident laser radiation by the population of molecules in the ground-state of ZnO NRs (Mn-doping greater than 0.5%) reduces which manifests into transmission maxima at the focal point. In other words, the defect states are close to being completely occupied at high laser intensities (close to focus) and therefore, the ground state absorption reduces.^31,36 This brings about an increase in transmission as we come closer to the focal point. Due to this mechanism, the expected TPA behaviour of Mn-doped (>0.5%) ZnO NRs is completely suppressed and taken over by a feature which characterizes SA behaviour. This point could be substantiated by noting the fact that the charge carriers are trapped more efficiently at higher concentrations of Mn²⁺ ions at defect sites which result in elongated lifetimes for the defect states.^37,38 Therefore, the defect state population rapidly saturates at higher laser intensities for higher concentrations of Mn-doping in ZNO NRs. It is also worthwhile to observe that the OA normalized transmittance in Fig. 5(a) shows a drop close to the focal point (z = 0), which is a distinct signature of TPA, in 0.5% Mn-doping concentration in ZnO NRs. A plausible reason for this could be ascertained by observing the fact that the energy of two-photons at 532 nm is 4.67 eV which is almost 1.9 (≈2) times the difference between the ground-state and the defect-state in the case of 0.5% Mn-doped ZnO nanorods. Assuming band-to-band transition takes place via the simultaneous absorption of two-photons, the defect-state essentially mediates the transition from lower energy band to a higher band. In addition, the relaxation lifetimes from defect-state to ground-state are smaller for lower Mn-doping concentrations. This reduces the possibility of saturation for the defect state and enhances the probability of absorption of photons at higher intensities. Therefore, TPA behaviour is expected which is actually being observed in Fig. 6(a) and 7(b).

Conclusions

In conclusion, Mn-doped ZnO NRs were prepared by a low temperature aqueous growth technique. The effects of Mn-doping on the geometrical structural arrangement, band structure and NLO properties were investigated. For studying the NLO characteristics, the single-beam Z-scan technique was employed using a sub-nanosecond excitation source at 532 nm. The UV-visible absorption measurements of ZnO NRs showed a marginal change in bandgap as a function of Mn-doping concentrations. In NLO measurements, all the samples with varying concentrations of Mn-doping exhibited a self-defocusing effect with significantly higher n₂ values as compared to ZnO thin films. The significantly large third-order NLO behavior could be appreciated by observing the modifications in the geometrical arrangement and band structure of ZnO NRs as a function of Mn-doping. Un-doped ZnO NRs and 0.5% Mn doped ZnO NRs exhibited TPA features while strong SA behavior was observed for 1.5%, 2%, 2.5% Mn-doping concentrations. In addition to the difference in NLA response for OA measurements, the magnitude of β for ZnO NRs varies negligibly as Mn-doping concentration in ZnO NRs changes. From this investigation, it could be suggested that Mn-doping in ZnO NR architecture facilitates improved NLO interactions which can be employed for designing ultrafast photonic switches and efficient optical phase-shifters. The MPA observed in pure ZnO NRs and 0.5% Mn doped ZnO NRs suggests their utilization in nonlinear optical devices such as optical limiters while higher Mn-doped ZnO NRs could be suitable candidates for saturable absorber materials.

Acknowledgements

The authors acknowledge the funding from National Institute of Science Education and Research (NISER), Department of Atomic Energy (DAE), India.

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Footnote

† A. Singh and S. Kumar contributed equally to this work.

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