Introduction

In recent years, research has focused on nonlinear optical properties that occur as a result light interaction with matter. The NLO effect does not occur under normal light irradiation or relatively low light intensities. Typically, only laser light has sufficient intensity to change the optical properties of a material1,2i.e. the NLO effects arise due in the high intense light interaction with the matter. The optical property of materials depends on the exposure time and intensity of incident light. Even though the high intense lasers are of great importance, the high fluency can damage the sensitive optical components and human tissues as well eyes3,4. The enhanced use of high intense laser in medical, defense and for scientific purposes, In this regard, the need of devolving materials and devices that protect or controls laser fluency arises. The enhanced use of lasers is simultaneously increasing the need for optical limiters, which are essential to protect human eye and optical devices from laser damage5,6. The right materials for developing optical limiter have become challenging. Over the last few years significant research progress has been made for the designing and developing of nonlinear active material with quick reaction times, higher threshold, stability and linear transmittance7.

Various semiconductor materials have been widely explored such as tungstates, nitrides, metal oxides, borates and phosphates8,9,10,11,12,13of these semiconducting materials, molybdates have attained a great deal of interest due to their unique nonlinear properties such as two photon absorption 2PA, multi-photon absorption (MPA), reverse saturable absorption (RSA) and Saturable absorption (SA) also focusing/defocusing effects due to nonlinear refraction14,15,16. Especially, transition metal molybdates have received significant attention due to their remarkable structural, optical, electrical and luminescent properties having wide applications, such as laser materials, scintillation detectors, and photoluminescence devices17. Among transition metal molybdates, Silver molybdate is an excellent OL material but has not been explored. The NLO properties of the host silver molybdate can be varied by tuning band gap of the material by adding appropriate dopant. Previous findings suggests that adding rare earth ions such as Er or Eu can enhance the NLO properties of the host material because the incorporation of trivalent ions into the MoO4cluster can occupy the divalent site of the host lattice18. Thus incorporation of Er into the host lattice generates new transition states originated from the completely occupied 5 s and 5p, shielded by unoccupied 4f. shells19. So herein we report the OL and 2PA property of silver molybdate and exhibited good OL performance. For modifying the OL threshold different concentration of Er was doped into the Ag2-xMoO4 lattice which was successful and 2PA as well as OL efficiency was found to enhance with Er concentration. So this Er doped Ag2-xMoO4 nanostructure can be used photonic device applications.

Experimental

Synthesis of Er doped Ag2MoO4

Sodium Molybdate Dihydrate (Na2MoO4.2H2O), Erbium (III) Nitrate Hexahydrate extrapure (Er(NO3)3. 6H2O) and Silver Nitrate Extrapure AR (AgNO3) was used as precursors. The precursors was purchased from SRL with analytical grade and used without further purification. A typical chemical precipitation method was followed for the synthesis of Ag2-xMoO4 (X = 0, 0.01, 0.03, 0.05) nanostructures. Initially, 0.3 M of silver Nitrate was dissolved in 20 ml of DI water under stirring at 500 RPM for 30 min. To the above Ag+solution with different molar concentration of Erbium Nitrate (0.1, 0.3, 0.5%) was added dropwise under vigorous stirring. After 30 min of stirring 0.3 M of Sodium molybdate were added dropwise until the precipitate was formed. The obtained white precipitate were centrifuged with ethanol and distilled water until the pH becomes neutral. This was followed by filtering and drying at 70 °C for 24 h, later calcinating at 600 °C20.

Z Scan experimental setup

The NLO response of the as prepared NPs was examined using Q switched Pulsed laser Z scan technique with a laser excitation of 532 nm, frequency of 50 Hz, beam waist of 16.9 µm. The laser beam is splited into two using a beam splitter; the reflected beam is collected by the detector for reference while the transmitted beam is directed towards the sample using a convex lens of focal length 15 cm. The thermal effects were avoided by performing the experiment with low repetition rate (10 Hz) nano pulsed green laser. Also the Z-scan experiment was performed in single shot mode to avoid the thermal effects in the sample.Equally weighted sample were dispersed in diethelyene glycol and the linear transmittance were fixed to be around 70%.

Result and discussion

X ray diffraction analysis

The long-range structural order–disorder, and lattice periodicity of the synthesized nanostructures were studied using XRD. Figure 1 shows the XRD pattern of pristine silver molybdate (Ag2MoO4) and Erbium doped silver molybdate Ag2-xMoO4 (X = 0, 0.01, 0.03, 0.05) nanostructures obtained by co-precipitation technique. The Ag2-xMoO4 (X = 0, 0.01, 0.03, 0.05) nanostructures show sharp and intense peaks that are indexable to a cubic structure and 141/a space group, which is in concurrence with the JCPDS card number 01–075-025021. The sharp XRD patterns suggest the highly crystalline and structurally ordered nature of the samples. The slight shift to lower angle and reduction in the peak intensity with the incorporation of Er ions in the crystal lattice suggests that addition of Er ions can induce only slight structural deformation and does not influence the cubic periodicity of the Ag2MoO4 lattice. This can be due to the smaller ionic radii of Er (1.00 Å) compared to Ag (1.15 Å) so the Er ions can smoothly substitute the Ag ions in the lattice without altering the Ag2MoO4crystal structure. The slight shift in the position of peak around 32° towards lower 2ϴ values can be due the homogenous defects formed by structural relaxations22,23. Here the relaxation can be due to various types of sites and No of vacancies occupied by Er ions. Moreover no additional traces of Er related peaks were found for Ag2-xMoO4 (X = 0, 0.01, 0.03, 0.05) nanostructures. The crystallite size found using Scherrer formula for Ag2MoO4 was 38.5 nm and the crystallite size is found to increase with Er concentration which might be due to the impurities such as Er3+ in the Ag2MoO4 crystal structure. Hence, the addition of Er ions in small amount can substitute Ag ions easily without altering the crystal structure therefore the obtained results are long range order and matches well with the previous reports.

Fig. 1
figure 1

XRD Pattern of Er doped Ag2MoO4.

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Optical studies

The UV–Vis spectroscopy was employed for analyzing the optical absorption and energy bandgap (Eg) of the as prepared Ag2-xMoO4 (X = 0, 0.01, 0.03, 0.05) nanostructures as depicted in Fig. 2 (A). The optical absorption of the nanostructures has a significant influence in altering the nonlinear optical limiting properties. The pristine silver molybdate have optical absorption band edge over the UV region i.e. around 200- 380 nm. Further it is observed from Fig. 2 (A) that with the addition of Er ions into the Ag2-xMoO4 lattice, the Er doped samples exhibited similar absorption edges having blue shift compared to the pristine sample further it is found that the absorption edges in the visible region increases with Er concentration. The absorption in the UV region is due to the electron charge transfer from the completely occupied oxygen 2p orbital to empty Molybdate 4d orbital in the MoO4cluster. These charge transfer is not only due to the impurity or intermediate doped energy levels but also due to the intrinsic transitions in the semiconductor material23. The optical energy bandgap (Eg) of the Ag2-xMoO4 (X = 0, 0.01, 0.03, 0.05) nanostructures was estimated using the relation proposed by wood and Tauc method. Literature reports, based on experimental and theoretical calculations suggest that optical absorption in Ag2-xMoO4 nanostructures governs indirect type of electronic transition. The Eg for pristine Ag2-xMoO4is calculated to be 3.10 eV also from Fig. 0.2 (B) it is clear that the energy bandgap increases with increase in Er concentration from 3.12 eV to 3.38 eV24. The enhancement in the band gap upon Er addition can be due the quantum confinement effects where the electron within the CB and hole within the VB confines spatially the surface potential barrier. Therefore, an increase in the energy transfer from the VB to CB leads to the confinement of electron hole pair thereby the bandgap increases. Moreover, the energy bandgap of the material depends on various other parameters such as synthesizing technique, size and shape of the particle, calcinating temperature etc.

Fig. 2
figure 2

(A) UV–Vis absorption spectra and (B) band gap using Tauc plot for Er doped Ag2MoO4.

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Fourier transform infrared spectroscopy

The FTIR spectra of ErxAg2-xMoO4 (where x = 0, 0.1, 0.3 and 0.5) nanostructues are shown in Fig. 3. The sharp transmittance band around 650 cm−1 ascribes to stretching Ag–O vibration mode. The weaker transmittance band around 1000 cm−1 attributes to the symmetric stretching vibration O–Mo-O bond. The broad transmission band around 1500 cm−1 is assigned to the O–H bending vibration of the water molecules on the surface of the Ag2MoO4lattice23,24.

Fig. 3
figure 3

FTIR Spectra of Er Doped Ag2MoO4.

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Scanning electron microscopy

Figure 4 (A and B) depicts the SEM image of Pure Ag2MoO4 and 0.5% Er- Ag2MoO4 nanostructures. The Ag2MoO4 samples exhibited tiny grain like morphology owing to the process of Ostwald ripening which is due to the growth mechanism and nucleation starting from the Gibbs free energy. The Gibbs free energy per unit volume can be controlled by tuning the solute concentration and higher Gibbes energy is possessed by the supersaturated solution leading to the nucleation mechanism. The addition of Er ions similar morphology was obtained with a reduction in the particle size indication the influence of Er ions in the Ag2MoO4lattice. The reduction the particle size can be due to the large surface area as well as higher surface energy and smaller ionic radii of Er ions25,26,27. Further the EDX spectra in Fig. 4 (C and D) depicts the controllability of elemental composition in the as prepared samples also it is observed that no additional elements were present in the samples confirms the purity of the samples.

Fig. 4
figure 4

SEM Image of (A) Ag2MoO4, (B) 0.5% Er- Ag2MoO4 EDX spectra of (C) Ag2MoO4 and (D) 0.5% Er- Ag2MoO4.

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X Ray photoelectron spectroscopy

The XPS analysis revealed the surface chemical composition of 0.3% Er doped Ag2MoO4 nanostructures. The typical high resolution survey spectra are shown in Fig. 5 No addition impurity peaks was identified expect the C 1 s peak which is generated by the hydrocarbons within the XPS instrument indicating the phase purity of the synthesized sample. The binding energy has been corrected by taking the C 1 s spectra as the charge reference. The XPS spectra of C1s envelope is deconvoluted into three peaks as depicted in Fig. 5F. The main peak at 284.5 eV attributes to the C–C bonds while the short peaks positioned at 285.9 eV is assigned to the C-H bond and the peak at 288.5 eV attributes to the double bond with carbon atom and oxygen C = O27,28,29,30. The core level XPS spectra of Ag are depicted in Fig. 5B having two peaks at 366.6 and 372.5 eV ascribing to Ag 3d3/2 and Ag 3d5/2 states attributing the Ag+state of silver in doped semiconducting materials31,32,33,34. The Mo 3d XPS peaks at 231.9 eV corresponds to Mo 3d5/2 and other at 235 eV attributing to Mo 3d3/2 confirming the Mo6+ state as shown in Fig. 5C. The core level XPS spectra of Oxygen are deconvoluted into two peaks at 530 eV and 532.2 eV as depicted in Fig. 5D33,34. These deconvoluted peaks attribute to the absorbed oxygen molecules or water molecules and the surface oxygen vacancies of the sample. The dominant peak at lower binding energy i.e. at 530 eV attributes to the lattice oxygen where as the predominant peak at 532.2 eV is assigned to the non lattice oxygen vacancy, which confirms the presence of oxygen vacancy within the crystal lattice. These oxygen vacancies have a significant impact in enhancing the nonlinear properties since, the vacancies acts as electron trapping centers. Finally the XPS spectra of Er are shown in the Fig. 5E. Exhibiting the doublet characteristics of Er 4d5/2 with peaks at 168.1 eV and 170.2 eV respectively. The the Er 4d5/2peak at 170.2 eV is corresponds to the Er- OH bond while, the peak at 168.1 eV attributes to Er-O bond and confirming the + 3 oxidation state of Er35,36,]37.

Fig. 5
figure 5

XPS spectra of 0.3% Er doped Ag2 MoO4 samples.

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Nonlinear studies

The nonlinear optical limiting properties of Erbium doped silver molybdate ErxAg2-xMoO4 (where x = 0, 0.1, 0.3 and 0.5) nanostructures was extensively investigated by Open apperture Z-scan technique using a 532 nm Q switched Nd: YAG laser with a repititation rate of 9 ns and pulse width of 5 ns. The beam waist (ω0) was estimated to be 16.9 μm at the focus and Rayleigh length (ZR) was 1.69 mm. equally, weighed samples were mixed with ethylene glycol in a 10 beaker and sonicated for 10 min to achieve homogeneously dispersed particles. The experiment was performed by moving the dispersed sample taken in 1 mm quartz cuvette linearly along the laser beam propagating direction and variation in the transmittance through the sample is measured where the sample exhibited a linear transmittance around 70%. The change in transmittance depends on the distortion of the Gaussian beam and the nonlinear absorptions. So as the sample nears the focus the beam intensity enhances owing to the light matter interaction resulting in the occurrence of nonlinear effects such as nonlinear absorption where the transmittances increases or decreases depending on the type of absorption occurring in the material. In the case of a semiconductor material if the energy bandgap if higher than the photon energy then for the transition of electrons to higher energy states occurs by the absorption of two or more photons which, can be due to genuine or sequential 2PA. In the case of sequential excited state absorption, generally free electrons excite to the conduction band by simultaneous absorption of two photons through actual intermediate state in the resonant regime. Where, in a non-resonant regime the simultaneous 2PA occurs through virtual intermediate states generally known as genuine 2PA38,39,40,41.

The open aperture Z-scan trance of the Erbium doped silver molybdate ErxAg2-xMoO4 where x = 0, 0.1, 0.3 and 0.5) nanostructures are depicted in Fig. 6. It is observed that as the sample moves towards the focus (Z = 0) the transmittance decreases and then increases as it moves away from the focus indicating the signature of reverse saturable absorption. Various mechanisms such as two, three or multi photon absorption are responsible for RSA, So the proper mechanism in the present case can be confirmed by performing theortical fit on the obtained experimental data using the relation

$${\text{T}}_{\text{OA}}=\frac{1}{{\left(1+\left(\text{n}-1\right)\upbeta {\text{L}}_{\text{eff}}{\left(\frac{{\text{I}}_{0}}{1+{\left(\frac{\text{Z}}{{\text{Z}}_{0}}\right)}^{2}}\right)}^{\text{n}-1}\right)}^{\frac{1}{\text{n}-1}}}$$
(1)

.

Fig. 6
figure 6

Open Aperture Traces of Er doped Ag2MoO4.

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Using the aforementioned relation the theoretical fit on the experimental data confirms that 2PA mechanism is responsible for the obtained nonlinear absorption. The nonlinear absorption is found to enhance with increase in the Er concentration. Since the band gap of Ag2MoO4 and Er doped Ag2MoO4samples is higher the input laser photon energy. So, simultaneously or sequentially two photons are absorbed for exciting to higher energy levels through real or virtual energy levels. Thus the real mechanism behind 2PA (i.e. genuine or sequential 2PA) can be confirmed by performing intensity dependent experiment. If the nonlinear absorption is independent of the incident energy or intensity genuine 2PA dominates whereas if the nonlinear absorption varies with input irradiance indicates the sequential ESA nature42,43,44,45,46.

The nonlinear absorption in the Ag2MoO4 nanostructures can be due to the surface plasmon resonance and local field effects produced. It is clear for the Table 1, the 2PA increases with the addition of Er ions which can be due the difference in the oxidation state of Ag0 and Er3+ and the defects oxygen vacancies created upon the Er doping which is confirmed by the XPS analysis. These induced oxygen vacancies acts as electron trapping centers which leads to the more absorption of the photo generated electrons in the excited state resulting in the decreased recombination rate and enhanced nonlinear absorption. When the Er3+ doped Ag2MoO4 is exposed to a laser excitation of 532 nm wavelengths initially the electron absorbs a photon and undergoes a transition from ground state to the first excited state. Due to the strong laser irradiation the electron absorbs a second photon and transit to the virtual state. but the electron cannot retain in the virtual state for more time so they undergoes transition to the excitionic trapped state through non radiative emission of energy. In addition, it is also observed that maximum absorption occurs in the UV region for Ag2MoO4 and the absorption in the visible region is found to enhance for the Er doped samples which avails the near resonant state for green laser excitation at 532 nm. Thus, these conditions fulfill the requirement for occurring excited state absorption and sequential 2PA in Ag2MoO4 and Er doped Ag2MoO4nanostructures. Here, one photon is absorbed and excited to nearest resonant state then sequentially another photon is absorbed and transits to higher excited state. Hence, the excited state absorption is responsible for the 2PA mechanism18,47,48,49,50,51. Further the occurrence of sequential 2PA was confirmed the intensity dependent study performed over 0.5% Er doped samples as shown in Fig. 7.

Table 1 The nonlinear optical parameters of Er- Ag2MoO4.
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Fig. 7
figure 7

Intensity dependent OA traces and Optical limiting threshold of 0.5% Er- Ag2MoO4.

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Material exhibiting nonlinear absorption originating from excited state absorption can be used for developing optical limiters. Optical limiters are devices that posses higher transmittance at low input fluency and attenuates the input light at higher power and acts as a opaque material. If a material shows strong nonlinear absorption beyond a fixed value of the input intensity it is known as onset optical limiting threshold. Lower the optical limiting threshold of the material more efficient will be the optical limiter52,53. For tailoring or enhancing the optical limiting efficiency of the material silver molybdate were doped with erbium ions, the defects and oxygen vacancies created upon erbium doping enhanced the absorption coefficient resulting in a reduction in the OLT value of the material. It is clear from the Table 1 and from the Fig. 8 that optical limiting threshold of the sample decreases with the addition of erbium ions into the host lattice. Also the obtained value is of the order of 10–13 with is comparable with best reported materials as shown in Table 2. Thus it can be concluded that the erbium doped silver molybdate can be used as a promising candidate for optical safety devices under nano-regime pulsed laser excitation54,55,56.

Fig. 8
figure 8

Optical Limiting Curves of Er- Ag2MoO4.

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Table 2 The comparison of nonlinear properties with recently reported materials.
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Conclusion

In summary, Ag2-xMoO4 nanostructures were synthesized by co-precipitation technique. The structural confirmation was done by XRD analysis and the crystallite size were calculated and found to increase with increase in Er concentration. The optical properties were studied by UV Vis spectroscopy exhibiting maximum absorption in the UV region and the absorption is found to enhance in the visible region with the addition of Er ions. The SEM analysis revealed the grain like morphology of the samples also with the addition of Er ions similar morphology. The oxidation states of the elements (Ag0, Mo6+, O2+ and Er3+) were confirmed with XPS analysis. Finally, the OA results indicates the RSA nature of the samples originated from 2PA also it is observed that the 2PA is found to increase with the addition of Er ions simultaneously reduction in the OL threshold making it a promising candidate for photonic devices.