Micro/Nanofabrication and Characterization of Advanced Materials and DevicesView this Special Issue
Tunable Piezophotonic Effect on Core-Shell Nanoparticles Prepared by Laser Ablation in Liquids under External Voltage
We report an experimental study on the piezophotonic effect of gold and lead zirconate titanate (PbZrTiO3) nanoparticles (NPs) and also their core-shell nanostructures prepared by the laser ablation in liquid method. To obtain these NPs and composite materials, the targets were immersed in deionized water and a polymeric solution of polyvinyl pyrrolidone (PVP) under Nd:YAG laser pulses irradiation. Linear and nonlinear properties of these NPs were studied by optical spectroscopy and the Z-scan technique. Furthermore, tunable nonlinear properties of the NPs were measured under an external electric field under illumination to investigate the piezophotonic effect. Our results show that, at the interface of PZT and Au, due to the Schottky barrier, we have electron/hole recombination prevention, which leads to efficient enhancement in the nonlinear properties.
The piezophotonic effect, which can be viewed as a two-way coupling impact among piezoelectricity and photoexcitation properties [1, 2], the conjugation of excitation by photons, piezoelectricity, and semiconductor properties (piezophototronic effect) of ZnO nanomaterials, was investigated by Wang et al. [1–4]. Their examination exhibited that the properties of the noncentro symmetry semiconductors could be tuned under piezoelectric ﬁelds. As a result of these findings, the materials received much attention in the electronic or photonic communities [5–7].
These materials are introduced as nanogenerators , high-power generators , electromechanical sensors , actuators , and energy converters  based on the coupling of piezoelectric and semiconducting properties. To achieve these applications, one of the most important arms is the material with the best piezoelectric effect such as PbZrTiO3 (PZT), which has low coercion, has large polarization, and acts as a high-Curie temperature electric insulator .
To get above-mentioned applications from these materials, they are a lot of reports in the bulk, thin film, or single nanoparticels of PZT structures . After that, to get more efficiency, composite, sandwich, or heterogeneous structures of these materials have been proposed. Consequently, the terminology “core/shell” of the semiconductor and metal was adopted to obtain the properties which arise from them, which are not the mix of individual properties, and also to obtain some tunable and synergetic properties emerging from the activity between metal and semiconductor components [15–17].
One may know that the materials made of noble-metal nanoparticles show efficient resonance due to the collective oscillation of electrons, named the surface plasmon resonance (SPR) phenomenon. This resonance in different propagative or localized forms can be used for amplification of the electric field in the vicinity of any other materials. The improvement of the local electric field is responsible for the amplification of the linear and also nonlinear properties [18–23].
According to the abovementioned properties, the semiconductor/metal interface can be used for controlling the optical properties of nanostructures by external or internal parameters such as electric or magnetic field and size of the core or shell part, respectively. These optical properties can be classified into two main parts: linear and nonlinear. For the linear part, there are a lot of papers which focus on the SPR wavelength and also the Fermi level or Fermi pinning in the interface of them and so on . For the nonlinear part, there are few works which focus on the change in the nonlinear properties of bare core-shell nanostructures by using the Z-scan approach, which is an outstanding and generally used procedure to characterize the nonlinear optical properties of materials [24, 25].
On the other hand, tuning and adjusting the optical and piezophotonic properties of nanostructures based on understanding of interfacial phenomena is a very important topic in the field of NPs. Until now, there is not any investigation about the linear and nonlinear properties under external electric and optical fields. In this study, we prepared Au, PZT, and Au@PZT and PZT@Au core/shell NPs by the laser ablation in liquids (LAL) method because of the various advantages of this technique, for example, simplicity of the experimental setup, low cost, and environmental friendly. This technique has become a promising method for the synthesis of NP systems such as single or core shell in liquid solutions [19, 26, 27]. This work focuses on the exact calculation of the effect of external electric field on the nonlinear refractive index, which was calculated by the closed-aperture z-scan technique to show the piezophotonic effect on samples. In addition, because the ferroelectric materials are sensitive to the external electric field, we use them in a transparent dielectric matrix under two internal (the local fields experienced around metal NPs) and external electric fields.
Nanomaterials with large third-order optical nonlinearities have recently become the important topic for researchers and are of wide scientific interest, mostly because of their possible application in rapid optical switching devices, which are becoming more and more important and are more commonly used . The nonlinearities of the third order are usually studied in centrosymmetric media, in which the second-order nonlinear susceptibility is zero .
In a linear dielectric medium, there is a linear relation arising from interaction between the induced electric polarization and the electric field for light as follows:where is the permittivity of vacuum and is the dielectric susceptibility of the medium.
The polarization P is a product of the individual dipole moment p, which is induced by the applied electric field E, and the number density of dipole moments. This relation between p and E becomes nonlinear as E enhanced enough high values. External electric fields are almost always smaller than the characteristic interatomic fields, even when laser beams are used, which means that the nonlinearity is usually weak [28, 29]. This is the basic representation for a nonlinear optical medium. By this fact and the combination with Maxwell’s equations, the refractive index, , and the optical absorption, , can be represented as functions of the intensity I of the incident laser beam . This nonlinear part of the refractive index can be written as follows:where indicates the intensity of the incident beam at the focal point as a function of the intensity of the laser, I, andwherewhere and are the maximum and minimum values of transmittance for closed-aperture Z-scan technique, respectively, is the nonlinear phase shift, and S is the aperture’s linear transmittance [30–32]. Also, Leff is the effective length of the material as a function of the sample length, L, and linear absorption coefficient, , given as [28–34]where is the sample length and is the linear absorption coefficient [28–34].
3. Experimental Method
The schematic diagram of the experimental setup is described in Figure 1 which shows the nanoparticle generation setup by the laser ablation in liquid method. Experiments were carried out by using Q-switched Nd:YAG lasers, operating at a wavelength of 1064 nm, pulse width of (5 ns), and repetition rate of (10 Hz), and 100 mJ energy per pulse was focused on the samples.
The laser beam was directed on the pure (99.9%) gold with a thickness of 1 mm and the PZT alloy with a thickness of 2 mm, which were used as the target and immersed in liquid for comparison (deionized water and polyvinyl pyrrolidone (PVP), a polymer with a mixture of 1% PVP and deionized water dissolved using a magnetic stirrer). In order to obtain more stable solution, in the container, we use a glass vessel under a high-reflectivity mirror and a lens with 150 mm focal length. The water was kept at the height above the target at 10 mm, and a quartz cover was used to avoid the splashing to the surroundings of the vessel.
The distance between the lens and the target was adjusted to obtain a best spot of laser beam focused on the target. For the preparation of bimetallic Au@PZT and PZT@Au core-shell NPs, a sequential two-step ablation method, shown schematically in Figure 2, was used. In this method, first Au NP colloidal solution was prepared by laser ablation of Au target in deionized water environment . Once, in a PVP again, irradiation of target endured for 3 min and was accompanied by a homogeneous liquid staining by pink color because of the plasmon resonance band of Au NPs in the visible region. During illumination by laser pulses, color change of the colloid was observed, and this process continued till the color of colloid became red, followed by ablation of the PZT target in the previously prepared Au NPs colloidal solution. The PZT NPs act as shell and form core for ablated hot Au species. The ablation duration was 2-3 min for PZT to obtain core-shell morphology with varying shell thickness. After preparation, all of the samples were stirred in an ultrasonic bath at room temperature (RT) for about 15 min.
In order to study the effects of an external electric field on the nonlinear refractive index of the NP samples, we employed the closed-aperture Z-scan technique, and measurements have been performed at λ = 532 nm, which is close to the SPR absorption band of the materials, using a CW diode-pumped solid-state laser as the light source.
The laser beam gave a Gaussian profile by vertical polarization and a power of 120 mW at normal incidence geometry. The sample was moved forward and backward along the Z-axis around the beam waist of the laser during the measurement, under the effect of an external electric field (used variable voltages (0–7 volt)), to analyze the nonlinear variation of the refractive index. The schematic of the setup for the Z-scan experiment is shown in Figure 3.
4. Results and Discussion
Optical absorption spectra of prepared NPs and core/shell samples were measured using a UV–visible spectrophotometer. The behavior is shown in Figure 4.
Figure 4(a) shows the UV-visible absorption of the products S1 to S4 NPs and core/shell samples in deionized water solution, and the UV-visible absorption of the products S5 to S8 NPs and core/shell samples in PVP solution is illustrated in Figure 4(b). These figures show that each sample has a surface plasmon resonance (SPR) absorption spectrum as indicated by their NPs sizes. The measured SPR peak of these samples indicates that the resonance peak become stronger in PVP solution because the matrix is more stable. Movability of the SPR wavelength of the samples occurs as shown in Table 1.
This red shift was because of the larger refractive index of core-shell structure when compared with PZT and Au NPs. Furthermore, the SPR depends unequivocally on the NPs shape and the size of the nanoparticles and also is sensitive to the refractive index of the surrounding medium due to the difference in shell thicknesses [9, 17, 35, 36].
Also, morphology of the samples was investigated by the use of a field emission scanning electron microscopy (FESEM) model (JSM-7610F), operating at an accelerating voltage of (10 kV), to identify and determine the shape and size of the produced particles. The formation of a metallic phase core/shell made up of particles of quasi-spherical morphology with a size range for core from 50 nm to 90 nm and shell from 20 nm to 34 nm for Au@PZT in water, and the sizes starting to increase to more than 100 nm for PZT@Au in the polymeric solution can be seen from Figure 5. It can be seen that there were no agglomeration effects present when the ablation was performed directly in the polymeric solution.
We realize that the dielectric properties of the material are extremely important and play the main role in the position and intensity of the peak of SPR of NPs. As we expected, coupling between the metallic and piezoelectric medium yields to the change in the refractive index of the samples.
It is evident from these figures that when Au NPs are ablated in water matrix, we have the main resonance peak in the refractive index as clarified by the absorption spectrum. But, when NPs are prepared in the PVP matrix, we can see slight change in the imaginary part of the refractive index in spite of decrease in the real part of the refractive index. This change can be described by the fact that we have PVP supporting larger gold NPs which is confirmed by FESEM image. This is clarified when we compare Au@PZT NPs in water matrix with those NPs in PVP (Figures 6(c) and 7(c)), in which we can see two distinct peaks in the PVP matrix in both real and imaginary parts of the refractive indices.
Measurements of the nonlinear refractive index in NPs and core/shell samples are revealed by using the Z-scan technique. The scan started from a distance near the focus to check the fact that the generating samples in a PVP polymer solution have a nonlinear refractive index value greater than that generated in deionized water. It can be seen that the peak-valley pattern of the normalized transmittance curve indicates the positive sign for samples produced in the water solution, except Au NPs. This is different from negative sign, in which the valley comes after the transmittance peak, indicating the self-defocusing phenomena in all applied voltages. Furthermore, the valley-peak configuration indicates the negative sign for samples produced in the PVP solution except Au@PZT NPs, as shown in Figure 8
One may know that the SPR wavelength in the nanocomposite materials can be actively modulated at regular intervals through the application of external parameters such as electric field. The effect of the external electric field on the nonlinear refractive index was studied with a direct variable voltage (0–7 volt) effect on the sample inside the cell during scanning using the Z-scan technique. We observed that an increase in the amount of voltage leads to increasing distance between the peak-valley transmittance (i.e., the difference of the normalized transmission between the peak and the valley). This means there is an increase in the quantity of nonlinear refractive index of all samples, as shown in Figure 9.
Figure 10 indicates that we have an increase in the amount of nonlinear refractive index in the nanoparticles in the polymer solution as compared to the water solution. Also, we have a clear increase in this amount when an external electric field is affected. This is an important result in the study of the effects on optical and electronic properties of devices.
A comparison of all samples with some plots was drawn, as shown in Figure 11, and we conclude that there are samples in which the nonlinear refractive index changes slightly, while other samples have a significant change in their magnitude. These changes depend on the boundary conditions between the core and the shell of the samples.
Our results show significant adjustability for the nonlinear refractive index for each set of NPs. When an electric field was applied (0–7 volt) to the NP samples, the enhancement can be seen for the nonlinear refractive index with rate 48% for metal NPs, while the proportion of improvement did not exceed 10% for the core/shell NPs.
There are various physical mechanisms that result in the nonlinear change of incorporate refractive index, electrostriction, electronic polarization, photorefractive effect, ultrafast electronic effects , and thermal effect , as the molecular reorientation collectively contributes to nonlinear change of indices [25, 37–39].
Our results show that the changes of the nonlinear indices are drastically because of the reorientation procedure of the induced polarization of the core/shell NPs and the electric field of the laser beam as changes are increased with increased applied voltage on core/shell NPs. Thermal effects can considerably contribute to the nonlinear refractive index under a CW laser for sample illumination which can occur because of thermal effects [40, 41]. In addition, we expect that thermal effect has more impact than the other effects. This result agrees with .
To get more sense about this manner, we use the energy diagram of the core and shell and investigate the interface physical phenomena such as Schottky barrier heights which are very important at the metal/semiconductor interfaces.
It, to a great extent, defines the current transport and the potential distribution through these interfaces and, consequently, the usefulness of the electronic devices . The energy band diagrams of the PZT@ Au core@ shell NPs interfaces are shown in Figure 12. The vacuum levels derived from the work functions for typical PZT and metal Au are about 5.8 and 5.1 eV, respectively, with an ionization potential of PZT of 6.8 eV [13, 42]. The possible band bowing in the PZT substrate is not shown in the graphs.
The electron affinity χ = 3.4 eV of the cleaned PZT thin films is derived from the XPS measurement, and an energy gap of 3.4 eV is considered from Reference . By considering the Schottky barrier heights for electrons at the interfaces and taking work function of 5.1 eV for Au into account, the dipole potential δ can be derived from the discontinuity of the vacuum level at the interface. Here, , , and are the conduction band, Fermi level, and valence band of PZT, respectively. The Fermi-level position at the PZT/Au interface has been determined utilizing a combination of metal work function and PZT electron affinity, = 1.8 eV, by the barriers for the injections of electrons () and holes () [13, 42]. Irradiating with the visible light, within the energy gap of semiconductors, such as the laser light of 532 nm, leads to absorption of photons and generates an electric field at the Schottky barrier which forces the photogenerated and to isolate in opposite directions. When the PZT@Au NPs are irradiated with light, the in the valence band of PZT comes to be excited to the conduction band. Since the EF of PZT@Au is at a lower level than the PZT conduction band, the Au interbands capture from PZT. Thus, the Au layer acts as the reservoir for the and prevents the fast recombination of photo-excited and in the semiconductor [42, 43]. This leads to increases in the absorption coefficient and thus nonlinear refractive index in the SPR sample quantities. This work will give some idea to those interested in scientific research for more advancement of piezophotonic effects in the optical drives.
In summary, we have synthesized Au NPs, PZT NPs, and core/shell NPs dispersed in various solvents by pulsed laser ablation with nanosecond laser pulses at 1064 nm and studied their optical-limiting properties. Our results show significant adjustability and tunability of the absorption band and SPR for each set of NPs that may be useful in the design of optical absorption devices. As we described how an external electric field affects the nonlinear refractive index of the samples, by using the closed-aperture Z-scan technique, a result suggests that the electric field modifies controlling of nonlinear refractive index values to samples which could be a perspective issue in piezophotonic materials development. Furthermore, combining the ferroelectric properties of PZT with the SPR wonders of Au NPs, we achieved tunability of the optical properties of the nanocomposite such as nonlinearity under external voltage.
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
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