Hollow ZnO microspheres self-assembled from rod-like nanostructures: morphology-dependent linear and Kerr-type nonlinear optical properties

Kerr-type nonlinear optics (bound-electronic nonlinear refraction) has been an intensive area of research since the observation of birefringence in 1875 by Scottish physicist John Kerr [1]. Exotic optical phenomenon, e.g., self-phase modulation for the generation of optical solitons, and self-lensing of laser beams, offers application opportunities for quantum non-demolition measurements, quantum-bit regeneration, quantum-state teleportation, optical solitons, all-optical switching, etc., which are commonly realized through optical Kerr effects [2, 3]. The improved Kerr nonlinearities along with reduced linear and nonlinear absorption characteristics provide new possibilities to control nonlinear interactions of optical systems for low light powers showing high sensitivities [4‐8]. Such notable achievements have been the inspiration for the development of optical materials garnering the advantages of enhanced Kerr nonlinearities. With the progress in manufacturing technologies, a variety of materials including semiconductors [9, 10], polymers [11], organic liquids [1], plasmonic materials [12], and YAG crystals [3] have been explored showing considerable Kerr nonlinearities. Thakur et al. analyzed the fast, tunable and broadband optical Kerr nonlinearity of graphene in the near-infrared spectral region [11]. Qian et al. presented giant Kerr response of ultrathin gold films showing quantum size effects [12]. Li et al. demonstrated picosecond Z-scan study of bound electronic Kerr effect in LiNbO₃ crystal associated with two-photon absorption [13]. In another interesting study, Wang and colleagues measured enhanced Kerr nonlinearity via atomic coherence in a three-level atomic system [14]. According to previously published research articles, the optical bi-stability/multi-stability and its plausible applications in all-optical switching nanodevices can be realized through Kerr nonlinearities by changing the optical anisotropy and geometric parameters [15, 16].

Nano-sized metal-oxide semiconductors with desirable optical functions such as large nonlinearities and short response time have drawn significant attention for the realization of new-generation nonlinear photonic devices in the past few decades [17]. Several research studies have attempted to provide cohesive understanding of the third-order nonlinearity, in particular, the Kerr type nonlinearity in metal-oxide-related semiconducting materials. For instance, Yan and co-workers performed a systematic study of ultrafast nonlinear optical properties and optical Kerr effect in doped zinc oxide (ZnO) nanowires at 800 nm [10]. Merikhi et al. examined magneto-optical Kerr effect in ZnTMO₂ chalcopyrite, where TM was chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co) and nickel (Ni) by considering that polar Kerr angles range from the infrared (IR) to ultraviolet (UV) radiation [18]. In an earlier work, we have reported enhanced Kerr-type third-order optical susceptibility for silver/cerium oxide (Ag/CeO₂) Schottky-type nanostructures with near infrared irradiation [19, 20].

Zinc Oxide (ZnO) is a II–VI group n-type compound semiconductor that has long been of interest for a variety of applications ranging from blue and ultraviolet optical devices to chemical sensors as it possesses unique features such as wide optical band-gap, high linear refractive index, large excitonic binding energy, high electron mobility nature, low electrical resistance, and high chemical activity, amongst others [21‐25]. Since the report of ZnO thin films in 1960, various physical and chemical synthesis routes have been developed and applied to gas sensors, transducers, solar cells, photocatalysis, and photonic instruments [26]. Compared to other semiconductor materials, ZnO enjoys the advantage of being relatively facile to form into complicated geometrical morphologies using solution synthesis techniques due to the polar nature of hexagonal wurtzitic lattice [26]. Morphology-controlled synthesis by various approaches including chemical vapor deposition [27, 28], hydrothermal solution synthesis [29‐31] and sol–gel processing [32] leads to a wide range of distinct ZnO nanostructures such as rods [33, 34], pyramids [35, 36], stars [37], flowers [38], etc. There are many published work describing the fabrication and performance of ZnO-based nanorods or nanowires due to low dimensionalities leading to quantum confinement effects [26]. For example, Baruah et al. studied the effect of pH variation on the dimension and morphology of ZnO nanorods grown through hydrothermal process on pre-seeded glass substrates at temperature ~ 100 °C [39]. The synthesis of three-dimensional nanostructures consisting of a mass of organized one-dimensional or two-dimensional building blocks have shown enhanced optoelectronic properties due to the functioning as both the building blocks as well as the interconnections [40‐45]. Several investigations have demonstrated hollow ZnO microspheres assembled by one-dimensional building units exhibiting unique properties such as low densities, enlarged surface areas, and shortened transport distances presenting highly efficient surface permeability [46]. Many scientists consider that ZnO semiconductor nanostructures with improved nonlinearities (e.g., large value of the third-order optical susceptibility) can be utilized as potential materials for the assembly of instruments based on nonlinear optics [47‐49]. Consequently, numerous studies have attempted to explore nonlinear optical characteristics in ZnO-based micro/nanostructures. For example, Loiko et al. has shown that ZnO nanorods grown on various optical surfaces are promising broadband saturable absorbers for nanosecond near-IR lasers in bulk and waveguide geometries [50]. Second and third harmonic generation measurements in amorphous ZnO thin films has been performed by rotational Maker fringe technique using Nd:YAG laser at 1064 nm in picosecond range for the verification of the nonlinear optical properties [51]. In addition, it has been found that the restricted motion of excitons and phonons can impact nonlinear functionalities of the nanomaterials depending on their shapes and sizes [52‐54]. Indeed, many research articles have illustrated diverse methodologies to understand the morphology-dependent optical nonlinearities. For example, Maki et al. described morphology-related third-order nonlinear optical responses of polydiacetylene nanofibers owing to the long effective π-conjugation lengths [55]. In another study, Oikawa reported hybridized organic nanocrystals as optically functional materials with enhanced optical nonlinearities depending upon crystal size and shape due to the quantum confinement effects [56].

In a study by Li et al., a series of mesoporous hollow ZnO microspheres were synthesized via a hydrothermal method to investigate the influence of reagent ratio on morphologies and gas sensing properties [57]. It is believed that in addition to the experimental parameters, e.g., the metal precursor concentration and stabilizing agents, the morphology can be influenced by reaction conditions such as growth temperature and time [58]. Therefore, it is desirable to exploit the role of different synthetic conditions for modifying the morphological characteristics of ZnO nanostructures and the subsequent optical nonlinearities upon exposure to high-intensity light beams. In this study we have developed morphological characteristics of ZnO microspheres synthesized by modifying a procedure introduced Li et al. [57] and evaluated the linear and nonlinear optical properties of the three-dimensional ZnO nanostructures. Three important issues are focused in the current research: (i) effect of different hydrothermal temperatures (150, 200, and 250 °C) on morphological characteristics, (ii) investigating morphology dependent spectral optics and (iii) examining Kerr-type nonlinear parameters using single-beam Z-scan technique. To the best of our knowledge, this is the first report on the optical nonlinearities of such three-dimensional ZnO microstructures in the near infra-red region (1064 nm). In general, the synthesized hollow ZnO microstructures can offer several advantages originating from their unique properties, e.g., low density, high surface area, and shortened transport distances, providing efficient surface permeability [46], which can improve practical applications in photonic devices.

Zinc acetate dihydrate [C₄H₆O₄Zn⋅2H₂O, molecular weight: 219.49 g/mol, 98%], Glycerol [C₃H₈O₃, molecular weight: 92.09 g/mol, 85%], and absolute Ethanol [C₂H₆O, molecular weight: 46.07 g/mol, 96%] were purchased from Merck and employed as received without further purifications. Both distilled and deionized water were used in all experiments.

ZnO samples were synthesized by modifying a previously reported method via a hydrothermal treatment route using zinc acetate dihydrate precursor and glycerol as stabilizing agent [57]. Figure 1 schematically describes the overall synthetic procedure for the synthesis of ZnO microspheres. During a typical synthesis, 1 g of zinc acetate dihydrate was dissolved in 10 mL of deionized water under vigorous stirring for 30 min and followed by the addition of 75 mL of glycerol. The reaction solution was mixed for 1 h in a magnetic stirring bath and then transferred into a 150 mL Teflon lined autoclave, where it was maintained at different temperatures of 150, 200, and 250 °C for 24 h. After gradual cooling to the room temperature, the resultant precipitate was filtered and washed with distilled water and ethanol several times to remove any residual ions. The wet product was dried at 60 °C for 24 h and calcined at 400 °C for 4 h. The resultant ZnO samples were then labeled as S150, S200, and S250 depending on the applied hydrothermal growth temperatures of 150, 200 and 250 °C, respectively.

UV–Vis diffuse reflectance spectra (DRS) were measured in Avaspec-2048-TEC spectrophotometer. Photoluminescence (PL) spectra were obtained using a Perkin Elmer LS55 fluorescence spectrophotometer equipped with xenon lamp to detect emission characteristics under an excitation wavelength fixed at 250 nm. Nonlinear characteristics of third-order optics were examined using standard single-beam Z-scan technique according to the experimental optical setup reported in a previously published study [59]. ZnO samples (0.01 g of powder dispersed in 20 mL of ethanol) were poured in a 1 mm thickness quartz cell, and irradiated using a pulsed Nd-YVO₄ laser system emitting 1064 nm wavelength with a radiance power of 20 mW, pulse duration of 50 ns, repetition rate of 10 kHz. The beam intensity was determined to be 3.98 × 10⁶ W/m² for beam waist of ω₀ = 32 μm and the diffraction length z₀ = 3 mm at the foci of a 300 mm focal length lens. The samples were mounted along the optical axis in a motorized transition stage with 0.65 mm steps. The morphology and microstructure of the samples were investigated in a field emission scanning electron microscope (FESEM, Zeiss SIGMA VP-500) and a transmission electron microscope (TEM, Philips-CM120-120 kV). All the micrographs were analyzed using ImageJ software for estimating average particle sizes from 50 to 70 particles in each micrograph. Phase characterizations and surface functional group identification are provided in the supporting information.

We attempt to deduce information about the effect of synthesis temperature on linear and nonlinear optical characteristics of hydrothermally grown ZnO samples. The PL analysis revealed an enhancement in the intensity of SW emission with increasing synthesis temperature during the hydrothermal growth process. This can be attributed to the higher crystallinity of ZnO nanostructures obtained at higher synthesis temperatures, as confirmed by XRD analysis (Fig. S1) and the dimensionless parameter \({{I_{{{\text{LW}}}} } \mathord{\left/ {\vphantom {{I_{{{\text{LW}}}} } {I_{{{\text{SW}}}} }}} \right. \kern-\nulldelimiterspace} {I_{{{\text{SW}}}} }}\) (Table 1). In addition, better crystallinity leads to reduced absorption intensity in the samples prepared at higher growth temperatures (Fig. S1). Amorphous materials are known to absorb light due to transitions in the defects that are prevalent in such structures.

Kerr-type nonlinear optical performance of ZnO samples prepared at different hydrothermal temperatures can originate from two distinct possibilities. The emergence of higher third-order optical characteristics in samples S200 and S250 than those obtained for sample S150 implies the possible influence of crystalline structure on Kerr-type optical nonlinearities. Samples treated at higher hydrothermal temperatures exhibit larger nonlinearities, which may be attributed to crystalline quality improvement in which non-stoichiometry is reduced as the oxygen vacancies are filled with increasing growth temperature (Table 1). However, the highest Kerr-type optical parameters were found for sample S250, suggesting the synergistic effect of surface morphology on optical nonlinearities. The increased nonlinearities can be affected by increasing the photon-exposure sites originating from hollow porous microspheres of sample S250 compared to solid structures formed in samples S200 and S150. The obtained results confirm that controlling of hydrothermal temperature is essential for nonlinear optical performance. In addition, it needs to be emphasized that although the relatively low absorption of ZnO samples at 1064 nm can minimize the possibility of the contribution of thermal effects, the conversion of the absorbed energy into phonons from non-radiative absorption processes may not be negligible, when excitation occurs under high-repetition rate laser sources with the nano-second pulse width regime. Actually, the Gaussian laser beams during their long-term interactions with ZnO samples can be partially adsorbed and then converted to heat to form a lens-like optical element due to cumulative thermal effects [72].

Indeed, it is rational to compare the obtained results with those found for structures reported in the literature for approximately similar experimental conditions. The comparative study reveals considerable differences in both magnitude and the sign of Kerr-type nonlinearities estimated in current work with those listed in Table 3. The Z-scan measurements indicate positive Kerr-type nonlinear indices in order of 10^–11 W/m² for hydrothermally grown ZnO samples (Table 2), which are about 10 times higher than negative nonlinearities given in Table 3. It should be noted that although all the samples were illuminated by a similar laser system, there is a difference in the acquired radiation intensity. As it was noted, hydrothermally grown samples have been exposed to laser beams with the intensity of 3.98 × 10⁶ W/m², which is ~ 10 times lower than the irradiation intensity on the nanostructures presented in Table 3 (1.25 × 10⁷ W/m²). The current data could also be comparable with that measured for erbium-doped zinc oxide thin films at wavelength of 1064 nm using the third harmonic generation (THG), showing a third-order nonlinear susceptibility of ~ 10³ times higher than those estimated in the current study [73]. Many reports have been published focusing on nonlinearities of ZnO nanostructures. In Table 4, we have summarized examples of numerical values of nonlinear refractions measured using Z-scan technique of different ZnO structures.

In summary, a simple hydrothermal procedure using zinc acetate dihydrate/glycerol aqueous solution was successfully utilized to fabricate hollow ZnO microspheres self-assembled by rod-like nanostructures. The morphology of ZnO microspheres could be adjusted by changing the hydrothermal treatment temperatures (150, 200, and 250 °C) during reaction process. A plausible formation mechanism was suggested to describe self-assembly of rod-like ZnO nanostructures into hollow microspheres. Morphology-dependent linear optical characteristics were justified using photoluminescence and UV–Vis absorbance spectra, showing red-shifts and improved crystalline qualities with increasing hydrothermal growth temperature. Kerr-type optical nonlinearities were studied using the Z-scan method under a pulsed laser system operating at 1064 nm. The Z-scan analysis exhibited the positive lens effect presenting self-focusing behavior in all the tested samples. The highest Kerr-type optical susceptibility was found to be 2.31 × 10^–6 esu for the sample treated at a hydrothermal temperature of 250 °C. The obtained results reveal the synergistic effect of surface morphologies to provide promising routes for developing nonlinear optical materials in future technologies.