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Creation of shuttle-shaped nanoparticles using the method of magnetic polarization pulse laser ablation
Applied Surface Science 494 (2019) 412–420
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Creation of shuttle-shaped nanoparticles using the method of magnetic polarization pulse laser ablation
T
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Yong Pan, Li Wang , Xueqiong Su, Dongwen Gao, Shufeng Li, Xiaowei Han College of Applied Sciences, Beijing University of Technology, Beijing 100124, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Chalcogenide Nanoparticles PLA Magnetic Polartzation
The GaxCo0.6-xZnS0.4 (x = 0.1, 0.3, 0.5) nanoparticles with shuttle-shaped are fabricated via a Magnetic Polarization Pulse Laser Ablation (MPPLA) method. Meanwhile, the circular-shaped nanoparticles also are prepared for comparative analysis. The particle size, structure, and valence state are systematically investigated by X-ray diffraction (XRD), Raman spectra and X-ray photoelectron spectroscopy (XPS). The morphology of the nanoparticles characterized by TEM indicating the high efficient of shuttle-shaped particles in preparation. A physical model of the effect of magnetic field on the plasma plume is established to explain the formation of shuttle-shaped nanoparticles. Then, we present an explosion-absorption model to demonstrate the growth mechanism of nanoparticles. We further discuss the cluster phenomena and summarized three types of clusters in our nanoparticle suspensions. Fluorescence properties are reflected by PL spectra, which demonstrates the best performance in concentration of GaxCo0.6-xZnS0.4 with shuttle-shaped. Therefore, all the results suggest that MPPLA is promising to prepare nanoparticles since it can adjust the shape, solve the cluster and modulate performance of NPs.
1. Introduction Chalcogenide materials such as ZnS has been extensively used in large number of mid-infrared materials such as satellite communication, medical diagnosis and treatment and sensing detection, which are a promising II–VI group semiconductor material [1–4]. Ga is very suitable for to emit or stimulate the spectrum of ultraviolet, visible. Its optoelectronic performance has long attracted people's attention [5]. Besides, Co as a transition metal doping ion has attracted much attention due to its abundant photon transitions energy levels and proven room temperature ferromagnetic property [6]. Thus, Co or Ga doped chalcogenide have been achieved a lot of research [7–9]. To date, nanomaterials have received extensive attention over the years because of their widely application [10]. Various nanomaterials such as nanoparticles [11], nanowire [12], nanoring [13], nanostars [14] and other nanostructure have begun to be systematically studied. Among them, the study of nanoparticles is very important because it is the most intuitive of micro-manifestation from macro-material. The interaction between laser and matter opens up a new way for people to understand the physical properties of materials, and promotes the progress of optical materials, photoelectric materials, photonics, optoelectronics and other fields. Pulse Laser Ablation (PLA) belongs to this scope, which is a physical process of preparing nanomaterials by bombarding materials ⁎
from the surface of bulk target with nanosecond, picosecond or femtosecond pulsed laser [15]. However, it is difficult to obtain regular nanoparticles by physical methods, especially for multi-doped composite materials. When laser interacts with matter, the plasma plume produced in solution is quite different from that in vacuum condition because PLA technology collects particles in solution. Herein, irregular shape and size are a major problem in PLA preparation, which has not been broken through for a long time. What's more, cluster problem is also the main difficulty in the preparation of nanoparticles, and the main challenge is considered to be effective in dispersing cluster nanoparticles. To obtain nanoparticles with different microstructures and to disperse nanoparticles of clusters, in this paper, we change the growth process of nanoparticles though the magnetic traction force which is generated between magnetic element Co incorporated into the material and the external magnetic field. The external magnetic field is composed of Fe3O4 magnetic powder. We report the preparation of shuttleshaped nanoparticles by Magnetic Polarization Pulse Laser Ablation (MPPLA) method for the first time. The shuttle-shaped GaxCo0.6-xZnS0.4 (x = 0.1, 0.3, 0.5) nanoparticles were achieved in water. Meanwhile, for comparative analysis, we also used the traditional PLA method to prepare nanoparticles. Then, the analysis of the material properties was characterized by a structure, morphology and optical measurement. We
Corresponding author. E-mail address: [email protected] (L. Wang).
https://doi.org/10.1016/j.apsusc.2019.07.187 Received 1 March 2019; Received in revised form 1 July 2019; Accepted 19 July 2019 Available online 22 July 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematics of the synthesis process for pulse laser ablation. Simultaneous laser ablation and magnetic traction treatments is shown in experiment A. The signal pulse laser ablation is displayed in the experiment B. The TEM image of diamond-shaped NPs in A and circular-shaped NPs in B are embedded in the left and right of picture. The plasma plume can be seen in the picture at the process of the preparation.
(400), (002), (111), (401), (−311), (600) and (510) are corresponded to Ga (JCPDS 43-1012). In addition, (111) at 44.7° is ascribed to α-Co (JCPDS 89-4307). The diffraction peaks of all nanoparticles are consistent with the target. The crystal structure is confirmed by the patterns of nanoparticles with different concentration. The peak positions of all elements have been detected indicating the success of element doping in the preparation process. What's important, many peaks intensity of round particles are weaker than that of shuttles, which indicates that shuttle-shaped particles have higher element concentration and a more complete structure. This phenomenon will lead to higher application performance of shuttle-shaped. From the XRD pattern we can find the difference between red (diamond-shaped in magnetic) and blue spectra (circular-shaped without magnetic). Thus, the reason for this difference is the magnetic field. Due to the external magnetic, the Co element with ferromagnetism is attracted by magnetic force, which results the part of the bond about Co breakage. Meanwhile, in the process of attraction, Co atoms collide with Ga atoms and cause them to overflow. When Co reaches magnetization saturation, the external magnetic field does not work on it. The overflow of Co and Ga will form polycrystalline structure, and the vacancies left by them will be filled with the rest of Co and Ga because the concentration of ZnS is constant (40%). The magnetic test of the material Fe3O4 shows that its coercivity is 123.57 Oe and its magnetic moment is 12.59 emu/g, which indicates that the material is a soft magnet (see Fig. 2b). Soft magnetic materials can achieve maximum magnetization with the smallest external magnetic field, especially for Co. In fact, the magnetic field produced by Fe3O4 will not be easily changed and disappeared, so it must exist in the process of preparation. The XPS spectra of doping elements Co and Ga in nanoparticles is presented in Fig. 2c-d. The peaks in 780–781 eV are corresponding to Co 2p3/2. According to the standard spectrum, Co is confirmed as +2 valence in nanoparticles. On the other hand, the peaks of Ga 2p3/2 at 1118 eV, and of Ga 2p1/2 at 1145 eV are simultaneously tested. In addition, binding energy of Ga 2p3/2 in metallic Ga is about 1116.6 eV, so Ga exists in the material with the +3 valence. Meanwhile, Ga exists in the impurity ZnCoGa2O4 produced during the preparation of target materials. In the preparation process of nanoparticles, it is very likely to overflow from the plasma plume. Ga2+ is also considered to exist in nanoparticles. Therefore, Ga is confirmed as +2 valence and +3 valence at the same time in the material. The Raman spectra of the nanoparticles with circular-shaped is shown in Fig. 3a. To gain more information of microstructure, multiple peak fit is used in the results of Raman. There is no phonon signal corresponding to wurtzite structure of ZnS. The low shift peaks at 168, 181 and 192 cm−1 is electrovalent bond of CoeS [18–20], which
have established a physical model to express the formation and growth mechanism of rhombic nanoparticles. 2. Experiment The target for PLA was prepared by the solid-state reaction method in our previously work [16,17]. Three concentration ratios of GaxCo0.6xZnS0.4 (x = 0.1, 0.3 and 0.5) targets can be used in this experiment. The process of PLA to fabricate nanoparticles is shown in Fig. 1. There are two ways to achieve the nanomaterials with a different shape. In the experiment A, the magnetic Fe3O4 powder is mixed to the deionized water. Then, a beaker containing the target is placed in the middle of the Fe3O4 powder solution. Magnetic Interaction will be generated between Fe3O4 powder and plasma plume due to Co+. In general, dispersiveness of nanoparticles collected in the ethyl alcohol is better than water. But for the PLA technology, ethyl alcohol is not allowed as a collection liquid because it is flammable in the process with laser. Therefore, all of the nanoparticles in our experiment are collected in deionized water. The laser power is set to 500 mW with Q-switched 355 nm,10 ns duration and 30 Hz repetition rate. The laser beam is focused through a quartz lens (f = 15 cm) to ~ 1 mm2 spot size on the target, and the produce time is controlled within 2 h. On the other hand, a traditional PLA technology is exploited without external magnetic field in the experiment B. But the laser power is set to a 500 mW and 700 mW for understanding the effect of different laser power on preparation of nanoparticles. The production time is adjusted to 2.5 and 3 h corresponding to different laser power. (see Fig. S1–5). The nanoparticle suspensions were ultrasonically dispersed for 15 min before testing. The structure of nanoparticles was characterized by X-Ray Diffraction (XRD) technique (Bruker D8 Advance), and Raman spectrum (Horiba Jobin Yvon T6400). The chemical composition of the nanoparticles was checked by X-ray photoelectron spectroscopy (XPS) technique (Escalab 250 Thermo). The morphology and the size were visualized by TEM (JEM 2100). The optical properties were measured by Photoluminescence spectra (NIR512). 3. Result and discussion 3.1. The structure of the nanoparticles Two shapes (three different concentrations for each shape) were tested by XRD, as seen in Fig. 2a. The patterns of shuttle-shaped and circular-shaped are represented by red and blue sign. Three diffraction peaks in the pattern can be indexed to the (111), (002) and (220) lattice planes of the sphalerite ZnS (JCPDS 65–9583), respectively. Then, 413
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Fig. 2. The structure properties of the Co-Ga-ZnS nanoparticles. (a) XRD. (b) The AGM spectrum of Fe3O4 nano-power. Inset: the value of magnetic moment and coercivity. (c)-(d) XPS spectra of the Co and Ga elements in ceramics.
30%–30%–40% and 50%–10%–40% due to the number of the CoeGa bonds, namely the more CoeGa bonds, the more Ga surrounds Co (Co is fixed by replacing Zn). Then Ga is also likely to replace Zn and to bond with Co and S. The reason of these two peaks not presenting in the concentration of 10%–50%–40% is the same to 219 and 229 cm−1. The Raman spectra of the nanoparticles with shuttle-shaped is presented in Fig. 3b. The peaks at 166, 171 and 193 cm−1 are still due to the phonon vibration of CoeS [18–20] in short-wave Raman drift region. As mentioned above, the peaks at 238 and 248 cm−1 also are described by the vibration information of CoeGa [19], which demonstrates that transitions in multi-level orbits occur more frequently at the closely concentration of Co and Ga. Raman peaks at 335, 340 and 360 cm−1 in the are attributed to longitudinal optical (LO) modes of ZnS [21,22]. Besides, the peaks at 432, 551 and 583 cm−1 are changed with the decreasing of Ga concentration. Thus, these peaks still can be explained by the characteristic peaks of the phonon vibration of the GaeS bones [23–25]. Furthermore, the information matched to covalent bands of GaeGa is located at 613 and 729 cm−1 [26], which is associated with the peak (111) corresponding to Ga in the XRD. This covalent bond is produced by the spilled Ga element, and it may combine with oxygen to form GaeO [27–29]. Because the circular Nps is not affected by the magnetism, so the peaks at 613 and 729 cm−1 is not generated. In addition, the peaks greater than 1300 (1337 to 1903 cm−1) belongs to the quartz glass substrates both in two shapes. Because CoeS and GaeS bonds are confirmed, we speculate the
illustrates part of Co+ replace the Zn+ successfully and not break the structure of ZnS. Meanwhile, the signal at 332, 340 and 360 cm−1 are known as the longitudinal optical (LO) mode similar to the Refs. [21, 22]. Combining the results of XRD, the sphalerite microstructure of circular-shaped nanoparticles is confirmed, which consist with the conclusion in Ref. [22]. The peaks around 230 cm−1, namely 219 and 229 cm−1, are considered as the heteropolar bond of CoeGa or CoeZn [19,20]. Due to the concentration of ZnS unchanged, this information is taken for CoeGa. Note that these peaks are not appeared in the concentration of 10%–50%–40%. The reason for this result can be explained by the size of the Ga atom bigger than that of Co and Zn. The vibration of Ga could crash the Co and make it far away in doping process. In the material with low Ga concentration (Ga-Co-ZnS, 50%–10%–40%), one part of CO is collided away by Ga, the other part is absorbed (ZnS has a high absorptivity for CO [16]). Thus, the probability of CoeGa bonding is very low in this moment. Then, with increasing of Ga and CO concentration, the bonding probability will naturally increase. At the same time, the vibration space of Ga is suppressed, and it is easier to bond with equal amount of Co (Co-Ga-ZnS, 30%–30%–40%). Furthermore, with the continuous increase of Ga content, GaeGa and GaeS (Replacement of Zn in ZnS) are more likely to bond. In addition, the high concentration of Ga means that the concentration of Co decreases (Co-Ga-ZnS, 50%–10%–40%), So the number of GaeCo bonds will abate. Additionally, the peaks at 480, 518 and 577 cm−1 can be explained by the GaeS [23–25] bond in the 414
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Fig. 3. (a) Raman Spectrum of the Co-Ga-ZnS nanoparticles with circular-shaped. (b) Raman Spectrum of the Co-Ga-ZnS nanoparticles with shuttle-shaped. (c)-(d) Crystal structure of Ga-Co-ZnS multi-doping materials. (e) The level band structure.
Special K Points of 9 × 9 × 6 method. Fast Fourier Transform (FFT) mesh extraction of 24 × 24 × 36 are applied. At the end of the selfconsistent process, the total energy of the structure converges to 510–6 eV/atom, the force acting on each atom is not more than 0.5 eV/ nm, and the internal stress is not more than 0.1 GPa. The Co and Ga position in the crystal matrix is presented in the Fig. 3c. According to the confirmed structure of sphalerite in ZnS and S-Ga/Co bonds, Ga and Co are doped mainly by substituting Zn. Considering the binding energy and atom size, the position of Co is to replace the zinc at the four
structure for these materials, which can be called mixed-sphalerite. In this structures, the element of Ga replace the Zn, bonding with the S and Co at the same time. We established the crystal structure of Ga-CoZnS according to the results of Raman and XRD, as shown in the Fig. 3ce. The method of Ultrasoft pseudopotential, in this paper, is selected by the base-function. Perdew-Burke-Ernzerhof (PPE) of Generalized Gradient Approximation (GGA) is used for exchange correlation potential (PBE is a gradient correction function of GGA). Plane wave truncation energy equals 380 eV. Total of Brillouin Sum used Monkorst-Park
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target.
corners, while Ga is mainly located in the face center of sphalerite structure because only the large-scale Ga doping here will not cause lattice distortion. Furthermore, the supercell is established to illustrate the bond information. The bonds of CoeS and GaeS is the mainly vibration information to confirm the successful doping fabrication in terms of nanoparticles. What we are more concerned about is the information of the CO-Ga bond confirmed in Raman. The number 1 is represents the Co-bond in the edge of a cell. As we discussed aforementioned, it is easier to bond with equal amount of Co as the increasing of Ga that still not bonded with other elements. Then, after bonding with the former Co atom, Ga can continue to attract other Co atoms to bond, which is similar to the crystal form of Ga2O3, as seen the region of number 2. Besides, the un-doping ions such as Ga and impurity could move to near Ga and further to generate GaeGa or GaeO. To prove the correctness of our model, the level structure was calculated. The highest band achieved by calculation in software is 3.8 eV in the point Z in brillouin zone, which is in good agreement with the experimental results 4.1 eV. The experimental results are obtained by tauc plot mapping according to the absorb spectrum data of Nps.
3.3. The physics modes of the shape formation, explosion-absorption and cluster growth of nanoparticles Fig. 5 explains in detail the formation of shuttle and circular nanoparticles, the formation of large-sized particles and cluster phenomena in TEM images. Firstly, the formation process of shuttle-shaped NPs is described by Fig. 5a. The obvious difference is reflected in the plasma plume in two processes. The obvious boundary line is exhibited by the plume in experiment B with respect to circular-shaped nanoparticles because it is not affected by other conditions. But for experiment A subjected to external magnetic field, the boundary ambiguity effect will be produced by plasma plume born from the interaction between laser and matter. Because the plasma in the plume is affected by the additional magnetic force, the nucleation, growth and cluster processes of the nanoparticles will undergo different transformations, which will further decide shape and size of NPs finally. Since Fe2O3 powder is evenly added in two sides of the target, so a uniform magnetic field is placed around the plasma plume. Two equal magnetic forces will be applied to the growth and polymerization of nanoparticles. In this process, the shape of nanoparticles and plasma plume will be gradually stretched by two sides (left and right), and finally shuttle-shaped particles will be formed. In order to explain the reasons for the formation of large-sized circular and other irregular nanoparticles in the process of high-power preparation, the explosive adsorption growth model of particles was established, as seen in Fig. 5b. The first stage is called the explosion stage. It means that when a strong laser interacts with matter, large of nuclei, particles and plasma may be bombarded directly from the surface (of course, these particles exist in the plasma plume during the preparation of PLA). Then, some unnucleated particles complete the nucleation process and enter the adsorption stage together with other particles spilled from the surface. At this stage, when other atoms, small clusters and nuclei in the outer region (more than 100 nm away from the core) drift close to the nucleating particles, these particles are most likely to be adsorbed by small nanoparticles, and eventually those small particles will slowly grow into large nanoparticles. According to the formula of F. Mafune [30], the radius of nanoparticles can be expressed by rp(t):
3.2. The morphology of nanoparticles Fig. 4a-h show TEM images of shuttle-shaped nanoparticles. The shuttle shape is formed by experiment A through external magnetic field generating from Fe3O4. Most of those nanoparticles existing with the shape of shuttle are revealed by Fig. 4a. But, a small number of the circular nanoparticles still are found (Fig. 4b) because of laser and solution on the magnetic field. The size of particles in this shape is larger than that of other types of nanoparticles, the big one approximating 430 nm (Fig. 4e). From the Fig. 4f we can clearly see that the morphology and lattice planes direction of shuttle-shaped nanoparticles. The obvious boundary and clear texture are shown in this enlarge picture due to the effect of magnetism, which gives us a clearer and more intuitive understanding of shuttle nanoparticles. Then, an obvious two-dimensional lattice image is shown in Fig. 4g. Two lattice planes marked are (111) with 0.332 nm and (220) with 0.212 nm, respectively, in sphalerite structure corresponding to the shuttle-shaped. The size distribution statistics are shown in Fig. 4h. The data show that the particle size distribution is the most in 200–300 nm, followed by 400–500 nm, and finally 300–400 nm. The average size of the diamondshaped nanoparticles is about 200–300 nm. In general, large-sized nanoparticles usually have better optical properties such as more likely to generate optical radiation. However, the experiment A successfully solves the cluster problem of nanoparticles, and their excellent dispersion can be seen from Fig. 4c-e. To illustrate, the Ga0.5Co0.1ZnS0.4, Ga0.3Co0.3ZnS0.4 and Ga0.1Co0.5ZnS0.4 are displayed in the (a, b), (c, d, f and g) and (e), respectively. The TEM images of circular-shaped multi-doping nanoparticles are shown in Fig. 4i-p. In the experiment B, all nanoparticles are circular in shape with clusters at 500 mW laser power, see Fig. 4i-l. The phenomena of severe cluster are a common problem in the preparation of traditional nanoparticles. All circular nanoparticles tested in TEM are concentration of Ga0.3Co0.3ZnS0.4. Of course, some particles with better dispersion are found, which have a more symmetrical size distribution. The Fig. 4n reveal a single circular-shaped NPs with 30 nm diameter, which is the average size for circular NPs according to the size value statistics (Fig. 4p). Statistical data in Fig. 4p show that the size of circular nanoparticles is mainly distributed in the 20–40 nm range. Unlike diamond-shaped particles, most of their sizes are below 50 nm. But there are also some abrupt changes in particle size, which are larger than 100 nm. The lattice spacing 0.283 nm is consistent with ZnS (111), see Fig. 4o. Compared with other single element nanoparticles or quantum dots, the size of composite multi-doped nanoparticles is larger. Then, large-sized circular and other irregular nanoparticles were found in suspensions prepared at 700 mW laser power (Fig. 4q-t). This phenomenon can be explained via explosion effect between laser and
3Np Va 1/3 ⎞ rp = ⎛ ⎝ 4π ⎠ ⎜
⎟
(1)
Here, Np is the number of particles after explosion. Va is the effective volume of multi-doped atoms. Then, we assume extravasation velocity of plasma is vp. So the volume growth rate of particle is the formula as follow:
Va dNp (t ) dt
= kπrp2 Va da vp
(2)
where da is the quantitative density of ions in plasma, which increases with time. k is the proportionality coefficient. kπrp2 is the geometric cross section area of particle. Therefore, the volume and radius of particles can be defined as
Va Np (t ) = ((Np Va )1/3 + (π /48)1/3kVa da vp t )3
(3)
rp (t ) = r0 + kVa da vp t /4
(4)
where r0 is the radius of particles after explosion. rp(t) is the radius of particles after a slow growth for t. In addition, many large particles are formed by two or three clusters. When the size of a single particle is large, the size of cluster particles can easily exceed 300 nm. There are three common cluster types including bead, rod/linear and regiment cluster, as shown in Fig. 5c. The generation of different cluster types can still be explained by the explosion-absorption theory. When the effect of explosion is greater than that of absorption, Beaded 416
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Fig. 4. TEM image of the GaxCo0.6-xZnS0.4 nanomaterials prepared by PLA. (a) and (c) The shuttle-shaped nanomaterials prepared via experiment A. (b) Zoom into the filament regions of (a). (d) Zoom into the filament regions of (d). (e) The single diamond-shaped nanoparticle. (f) The enlarged surface morphology of shuttleshaped nanoparticles. (g) Lattice image of shuttle-shaped nanoparticle. (h) The size distribution of shuttle-shaped nanoparticles. (f) The circular nanoparticles collected form experiment B. (g) Zoom into the filament regions of (f). (k) Clustering of nanoparticles. (l) Zoom into the filament regions of (k). (m) The circular nanoparticles with different sizes and well dispersion. (n) The single circular nanoparticle with 30 nm diameter. (p) The size distribution of circular-shaped nanoparticles. (q)-(t) The single and big nanoparticles under the laser power of 700 mW.
3.4. The analysis of optical properties
clusters are considered to be the dominant type. Then, rod/linear clusters are the main cluster types as an equal effect of explosion and absorption, which provides strong evidence to explain the formation of nanowires. When the effect of explosion is less than that of absorption, clusters appear more in the regiment.
Fig. 6 represent the PL spectra of Ga-Co-ZnS nanoparticles with different shape and concentration. The PL spectra of all shuttle-shaped nanoparticles are displayed in the Fig. 6a-c. In Fig. 6a, three peaks at 560, 670 and 860 nm in high Ga + concentration nanoparticles, namely 417
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Fig. 5. The modes of the shape formation, explosion-absorption and cluster growth of nanoparticles. (a) The formation of shuttle and circular nanoparticles. The insert picture is a real-time photograph of the plasma plume. (b) The formation of large-sized particles. (c) Three common cluster types including bead, rod/linear and regiment.
The reason for this phenomenon can be explained by the structural properties of different size particles. The nanoparticles size of shuttleshaped is larger than circular. So, they have complete structure, more energy levels and photon transitions. But for little size nanoparticles, they always tend to form single structure of energy level orbital so that the single peak is generated easier such as the Fig. 6d and f. However, Fig. 6e reveals multi-peaks at 560, 675 and 873 nm, which proves that the Co and the Ga energy level transition are moving up to the most as the concentration of them is close. Of course, the red and blue shift is still here. With the increasing of Ga concentration (10% → 30% → 50%), the vibration information of GaeS bond corresponding to the shift of 577 → 518 → 480 cm−1 in circular-shaped and 583 → 551 → 432 cm−1 in shuttle-shaped are supported by Raman spectrum, which illustrated the changing in bonding energy, position and density. These shifts will further lead to minor changes in the microstructures of materials. Therefore, the absorbed and emission properties in material could be directly changed. So the little change in PL peak has moved from
Ga0.5Co0.1ZnS0.4 (10%:50%:40%), are marked by green, red and black colour. Then, four mainly PL emissions at 560, 675, 751 and 870 nm in Ga0.3Co0.3ZnS0.4 (30%:30%:40%) are shown in Fig. 6b. For nanoparticles with high Co+ concentration (Ga0.1Co0.5ZnS0.4), the emission at 675 and 897 nm is detected. In short, the green light peak at 560 nm is attributed to the optical properties of the visible light range of Ga [31], and the red-light mission at 670–675 nm may be due to the transition between Co2+ and Ga2,3+ energy levels [32–34]. Meanwhile, the peak 751 nm confirmed this transition because it just existed in the material with the same concentration of Co and Ga element. The peaks of 860–897 nm are mainly put down to the infrared properties of CoZnS [32–34]. Besides, blue-shift is found in the changes of all peaks with the increasing of Ga concentration from 50% to 10%. Instead, redshift also existed in different Co concentration from 10% to 50%. This is consistent with the purpose of our design, which is the widely spectrum application. Then, the spectra of circular-shaped nanoparticles are illustrated in the Fig. 6d-f. Compared to the spectra of the shuttle-shaped, the number of fluorescent peaks of circular nanoparticles decreased. 418
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Fig. 6. Room temperature photoluminescence (PL) spectra of Ga-Co-ZnS nanoparticles with different concentration. (a)-(c) shuttle-shaped. (d)-(f) Circular-shaped. The inset picture is the TEM images and suspensions of nanoparticles corresponding to their different concentration.
proved by XPS. The morphology of the nanoparticles characterized by TEM indicating high efficient of our new method in terms of shuttleshaped. A physical model of the effect of magnetic field on the plasma plume was established to explain the formation of shuttle-shaped nanoparticles. Then, we established an explosion-absorption model to demonstrate the growth mechanism of nanoparticles. We further discussed the cluster phenomena and summarized three types of clusters in our nanoparticle suspensions. More abundant energy level and better optical properties in shuttle-shaped NPs were suggested by PL spectra, particularly in concentration of 30%–30%–40%. In conclusion, we have provided a new method for preparing nanoparticles with regular shape. This method can effectively solve the cluster problem of nanoparticles, and it has obvious modulation effect on the fluorescence properties of materials. Therefore, all the results suggest that MPPLA is promising to prepare nanoparticles since it can adjust the shape, solve the cluster and modulate performance of NPs.
675 nm to 670 nm as the increase of Ga content (see Fig. 6a and b). The direction of PL peaks shift is consistent with the Raman. Meanwhile, note that the PL peak at 560 nm is strong in Ga-rich material in terms of shuttle-shaped (Fig. 6a), which ascribed to the vibration at 613 and 729 cm-1 matched to GaeGa bond in Raman spectrum. Thus, the optical properties of Ga in the visible region are reflected in Ga-rich materials. When the concentration of Co and Ga is close in the material (30%–30%–40%)，all kinds of bonding information are detected by Raman, including the mode of CoeGa, GaeGa, GaeS, Lo mode of ZnS, CoeS. So this material has abundant energy level of multi-elements in which the transition increased. This multi-energy level structure will inevitably make its fluorescence spectrum showing a broad multi-peak shape (Fig. 6b and e), which reveals that our test results are relatively objective. Similarly, with the increasing of Co concentration (10% → 30% → 50%), the vibration information of CoeS bond corresponding to the shift of 168 → 181 → 192 cm-1 in circular-shaped and 166 → 171 → 193 cm-1 in shuttle-shaped are presented by Raman spectrum. These changes further illustrates that the changing of Co and Ga concentration has an influence on the micro-structure, which results the PL peaks red-shift with 860 → 870 → 897 nm in shuttle and 873 → 875 nm in circular.
Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC-11474014), Beijing Municipal Education Commission Science and Technology Program (KZ201610005001) and the open project of Beijing Engineering Researching Centre of Laser Technology (BG0046-2018-01).
4. Conclusion The shuttle-shaped GaxCo0.6-xZnS0.4 (x = 0.1, 0.3, 0.5) nanoparticles were fabricated via a Magnetic Polarization Pulse Laser Ablation (MPPLA) technology, which was first reported by this paper. Meanwhile, the formation mechanism and growth process of round and large size nanoparticles were deeply investigated by this experiment. Crystal structure of all nanoparticles was confirmed by XRD. The bonding vibration information of multi-doped elements was shown in Raman spectrum. It also illustrated that the transitions in multi-level orbits occur more frequently at the closely concentration of Co and Ga element. The +2 and +3 valence of Co and Ga in the materials were
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.07.187. References [1] X.F. Xu, W. Wang, D.L. Liu, D.D. Hu, T. Wu, X.H. Bu, P.Y. Feng, J. Am. Chem. Soc. 140 (2018) 888–891.
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Full length article
Creation of shuttle-shaped nanoparticles using the method of magnetic polarization pulse laser ablation
T
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Yong Pan, Li Wang , Xueqiong Su, Dongwen Gao, Shufeng Li, Xiaowei Han College of Applied Sciences, Beijing University of Technology, Beijing 100124, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Chalcogenide Nanoparticles PLA Magnetic Polartzation
The GaxCo0.6-xZnS0.4 (x = 0.1, 0.3, 0.5) nanoparticles with shuttle-shaped are fabricated via a Magnetic Polarization Pulse Laser Ablation (MPPLA) method. Meanwhile, the circular-shaped nanoparticles also are prepared for comparative analysis. The particle size, structure, and valence state are systematically investigated by X-ray diffraction (XRD), Raman spectra and X-ray photoelectron spectroscopy (XPS). The morphology of the nanoparticles characterized by TEM indicating the high efficient of shuttle-shaped particles in preparation. A physical model of the effect of magnetic field on the plasma plume is established to explain the formation of shuttle-shaped nanoparticles. Then, we present an explosion-absorption model to demonstrate the growth mechanism of nanoparticles. We further discuss the cluster phenomena and summarized three types of clusters in our nanoparticle suspensions. Fluorescence properties are reflected by PL spectra, which demonstrates the best performance in concentration of GaxCo0.6-xZnS0.4 with shuttle-shaped. Therefore, all the results suggest that MPPLA is promising to prepare nanoparticles since it can adjust the shape, solve the cluster and modulate performance of NPs.
1. Introduction Chalcogenide materials such as ZnS has been extensively used in large number of mid-infrared materials such as satellite communication, medical diagnosis and treatment and sensing detection, which are a promising II–VI group semiconductor material [1–4]. Ga is very suitable for to emit or stimulate the spectrum of ultraviolet, visible. Its optoelectronic performance has long attracted people's attention [5]. Besides, Co as a transition metal doping ion has attracted much attention due to its abundant photon transitions energy levels and proven room temperature ferromagnetic property [6]. Thus, Co or Ga doped chalcogenide have been achieved a lot of research [7–9]. To date, nanomaterials have received extensive attention over the years because of their widely application [10]. Various nanomaterials such as nanoparticles [11], nanowire [12], nanoring [13], nanostars [14] and other nanostructure have begun to be systematically studied. Among them, the study of nanoparticles is very important because it is the most intuitive of micro-manifestation from macro-material. The interaction between laser and matter opens up a new way for people to understand the physical properties of materials, and promotes the progress of optical materials, photoelectric materials, photonics, optoelectronics and other fields. Pulse Laser Ablation (PLA) belongs to this scope, which is a physical process of preparing nanomaterials by bombarding materials ⁎
from the surface of bulk target with nanosecond, picosecond or femtosecond pulsed laser [15]. However, it is difficult to obtain regular nanoparticles by physical methods, especially for multi-doped composite materials. When laser interacts with matter, the plasma plume produced in solution is quite different from that in vacuum condition because PLA technology collects particles in solution. Herein, irregular shape and size are a major problem in PLA preparation, which has not been broken through for a long time. What's more, cluster problem is also the main difficulty in the preparation of nanoparticles, and the main challenge is considered to be effective in dispersing cluster nanoparticles. To obtain nanoparticles with different microstructures and to disperse nanoparticles of clusters, in this paper, we change the growth process of nanoparticles though the magnetic traction force which is generated between magnetic element Co incorporated into the material and the external magnetic field. The external magnetic field is composed of Fe3O4 magnetic powder. We report the preparation of shuttleshaped nanoparticles by Magnetic Polarization Pulse Laser Ablation (MPPLA) method for the first time. The shuttle-shaped GaxCo0.6-xZnS0.4 (x = 0.1, 0.3, 0.5) nanoparticles were achieved in water. Meanwhile, for comparative analysis, we also used the traditional PLA method to prepare nanoparticles. Then, the analysis of the material properties was characterized by a structure, morphology and optical measurement. We
Corresponding author. E-mail address: [email protected] (L. Wang).
https://doi.org/10.1016/j.apsusc.2019.07.187 Received 1 March 2019; Received in revised form 1 July 2019; Accepted 19 July 2019 Available online 22 July 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematics of the synthesis process for pulse laser ablation. Simultaneous laser ablation and magnetic traction treatments is shown in experiment A. The signal pulse laser ablation is displayed in the experiment B. The TEM image of diamond-shaped NPs in A and circular-shaped NPs in B are embedded in the left and right of picture. The plasma plume can be seen in the picture at the process of the preparation.
(400), (002), (111), (401), (−311), (600) and (510) are corresponded to Ga (JCPDS 43-1012). In addition, (111) at 44.7° is ascribed to α-Co (JCPDS 89-4307). The diffraction peaks of all nanoparticles are consistent with the target. The crystal structure is confirmed by the patterns of nanoparticles with different concentration. The peak positions of all elements have been detected indicating the success of element doping in the preparation process. What's important, many peaks intensity of round particles are weaker than that of shuttles, which indicates that shuttle-shaped particles have higher element concentration and a more complete structure. This phenomenon will lead to higher application performance of shuttle-shaped. From the XRD pattern we can find the difference between red (diamond-shaped in magnetic) and blue spectra (circular-shaped without magnetic). Thus, the reason for this difference is the magnetic field. Due to the external magnetic, the Co element with ferromagnetism is attracted by magnetic force, which results the part of the bond about Co breakage. Meanwhile, in the process of attraction, Co atoms collide with Ga atoms and cause them to overflow. When Co reaches magnetization saturation, the external magnetic field does not work on it. The overflow of Co and Ga will form polycrystalline structure, and the vacancies left by them will be filled with the rest of Co and Ga because the concentration of ZnS is constant (40%). The magnetic test of the material Fe3O4 shows that its coercivity is 123.57 Oe and its magnetic moment is 12.59 emu/g, which indicates that the material is a soft magnet (see Fig. 2b). Soft magnetic materials can achieve maximum magnetization with the smallest external magnetic field, especially for Co. In fact, the magnetic field produced by Fe3O4 will not be easily changed and disappeared, so it must exist in the process of preparation. The XPS spectra of doping elements Co and Ga in nanoparticles is presented in Fig. 2c-d. The peaks in 780–781 eV are corresponding to Co 2p3/2. According to the standard spectrum, Co is confirmed as +2 valence in nanoparticles. On the other hand, the peaks of Ga 2p3/2 at 1118 eV, and of Ga 2p1/2 at 1145 eV are simultaneously tested. In addition, binding energy of Ga 2p3/2 in metallic Ga is about 1116.6 eV, so Ga exists in the material with the +3 valence. Meanwhile, Ga exists in the impurity ZnCoGa2O4 produced during the preparation of target materials. In the preparation process of nanoparticles, it is very likely to overflow from the plasma plume. Ga2+ is also considered to exist in nanoparticles. Therefore, Ga is confirmed as +2 valence and +3 valence at the same time in the material. The Raman spectra of the nanoparticles with circular-shaped is shown in Fig. 3a. To gain more information of microstructure, multiple peak fit is used in the results of Raman. There is no phonon signal corresponding to wurtzite structure of ZnS. The low shift peaks at 168, 181 and 192 cm−1 is electrovalent bond of CoeS [18–20], which
have established a physical model to express the formation and growth mechanism of rhombic nanoparticles. 2. Experiment The target for PLA was prepared by the solid-state reaction method in our previously work [16,17]. Three concentration ratios of GaxCo0.6xZnS0.4 (x = 0.1, 0.3 and 0.5) targets can be used in this experiment. The process of PLA to fabricate nanoparticles is shown in Fig. 1. There are two ways to achieve the nanomaterials with a different shape. In the experiment A, the magnetic Fe3O4 powder is mixed to the deionized water. Then, a beaker containing the target is placed in the middle of the Fe3O4 powder solution. Magnetic Interaction will be generated between Fe3O4 powder and plasma plume due to Co+. In general, dispersiveness of nanoparticles collected in the ethyl alcohol is better than water. But for the PLA technology, ethyl alcohol is not allowed as a collection liquid because it is flammable in the process with laser. Therefore, all of the nanoparticles in our experiment are collected in deionized water. The laser power is set to 500 mW with Q-switched 355 nm,10 ns duration and 30 Hz repetition rate. The laser beam is focused through a quartz lens (f = 15 cm) to ~ 1 mm2 spot size on the target, and the produce time is controlled within 2 h. On the other hand, a traditional PLA technology is exploited without external magnetic field in the experiment B. But the laser power is set to a 500 mW and 700 mW for understanding the effect of different laser power on preparation of nanoparticles. The production time is adjusted to 2.5 and 3 h corresponding to different laser power. (see Fig. S1–5). The nanoparticle suspensions were ultrasonically dispersed for 15 min before testing. The structure of nanoparticles was characterized by X-Ray Diffraction (XRD) technique (Bruker D8 Advance), and Raman spectrum (Horiba Jobin Yvon T6400). The chemical composition of the nanoparticles was checked by X-ray photoelectron spectroscopy (XPS) technique (Escalab 250 Thermo). The morphology and the size were visualized by TEM (JEM 2100). The optical properties were measured by Photoluminescence spectra (NIR512). 3. Result and discussion 3.1. The structure of the nanoparticles Two shapes (three different concentrations for each shape) were tested by XRD, as seen in Fig. 2a. The patterns of shuttle-shaped and circular-shaped are represented by red and blue sign. Three diffraction peaks in the pattern can be indexed to the (111), (002) and (220) lattice planes of the sphalerite ZnS (JCPDS 65–9583), respectively. Then, 413
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Fig. 2. The structure properties of the Co-Ga-ZnS nanoparticles. (a) XRD. (b) The AGM spectrum of Fe3O4 nano-power. Inset: the value of magnetic moment and coercivity. (c)-(d) XPS spectra of the Co and Ga elements in ceramics.
30%–30%–40% and 50%–10%–40% due to the number of the CoeGa bonds, namely the more CoeGa bonds, the more Ga surrounds Co (Co is fixed by replacing Zn). Then Ga is also likely to replace Zn and to bond with Co and S. The reason of these two peaks not presenting in the concentration of 10%–50%–40% is the same to 219 and 229 cm−1. The Raman spectra of the nanoparticles with shuttle-shaped is presented in Fig. 3b. The peaks at 166, 171 and 193 cm−1 are still due to the phonon vibration of CoeS [18–20] in short-wave Raman drift region. As mentioned above, the peaks at 238 and 248 cm−1 also are described by the vibration information of CoeGa [19], which demonstrates that transitions in multi-level orbits occur more frequently at the closely concentration of Co and Ga. Raman peaks at 335, 340 and 360 cm−1 in the are attributed to longitudinal optical (LO) modes of ZnS [21,22]. Besides, the peaks at 432, 551 and 583 cm−1 are changed with the decreasing of Ga concentration. Thus, these peaks still can be explained by the characteristic peaks of the phonon vibration of the GaeS bones [23–25]. Furthermore, the information matched to covalent bands of GaeGa is located at 613 and 729 cm−1 [26], which is associated with the peak (111) corresponding to Ga in the XRD. This covalent bond is produced by the spilled Ga element, and it may combine with oxygen to form GaeO [27–29]. Because the circular Nps is not affected by the magnetism, so the peaks at 613 and 729 cm−1 is not generated. In addition, the peaks greater than 1300 (1337 to 1903 cm−1) belongs to the quartz glass substrates both in two shapes. Because CoeS and GaeS bonds are confirmed, we speculate the
illustrates part of Co+ replace the Zn+ successfully and not break the structure of ZnS. Meanwhile, the signal at 332, 340 and 360 cm−1 are known as the longitudinal optical (LO) mode similar to the Refs. [21, 22]. Combining the results of XRD, the sphalerite microstructure of circular-shaped nanoparticles is confirmed, which consist with the conclusion in Ref. [22]. The peaks around 230 cm−1, namely 219 and 229 cm−1, are considered as the heteropolar bond of CoeGa or CoeZn [19,20]. Due to the concentration of ZnS unchanged, this information is taken for CoeGa. Note that these peaks are not appeared in the concentration of 10%–50%–40%. The reason for this result can be explained by the size of the Ga atom bigger than that of Co and Zn. The vibration of Ga could crash the Co and make it far away in doping process. In the material with low Ga concentration (Ga-Co-ZnS, 50%–10%–40%), one part of CO is collided away by Ga, the other part is absorbed (ZnS has a high absorptivity for CO [16]). Thus, the probability of CoeGa bonding is very low in this moment. Then, with increasing of Ga and CO concentration, the bonding probability will naturally increase. At the same time, the vibration space of Ga is suppressed, and it is easier to bond with equal amount of Co (Co-Ga-ZnS, 30%–30%–40%). Furthermore, with the continuous increase of Ga content, GaeGa and GaeS (Replacement of Zn in ZnS) are more likely to bond. In addition, the high concentration of Ga means that the concentration of Co decreases (Co-Ga-ZnS, 50%–10%–40%), So the number of GaeCo bonds will abate. Additionally, the peaks at 480, 518 and 577 cm−1 can be explained by the GaeS [23–25] bond in the 414
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Fig. 3. (a) Raman Spectrum of the Co-Ga-ZnS nanoparticles with circular-shaped. (b) Raman Spectrum of the Co-Ga-ZnS nanoparticles with shuttle-shaped. (c)-(d) Crystal structure of Ga-Co-ZnS multi-doping materials. (e) The level band structure.
Special K Points of 9 × 9 × 6 method. Fast Fourier Transform (FFT) mesh extraction of 24 × 24 × 36 are applied. At the end of the selfconsistent process, the total energy of the structure converges to 510–6 eV/atom, the force acting on each atom is not more than 0.5 eV/ nm, and the internal stress is not more than 0.1 GPa. The Co and Ga position in the crystal matrix is presented in the Fig. 3c. According to the confirmed structure of sphalerite in ZnS and S-Ga/Co bonds, Ga and Co are doped mainly by substituting Zn. Considering the binding energy and atom size, the position of Co is to replace the zinc at the four
structure for these materials, which can be called mixed-sphalerite. In this structures, the element of Ga replace the Zn, bonding with the S and Co at the same time. We established the crystal structure of Ga-CoZnS according to the results of Raman and XRD, as shown in the Fig. 3ce. The method of Ultrasoft pseudopotential, in this paper, is selected by the base-function. Perdew-Burke-Ernzerhof (PPE) of Generalized Gradient Approximation (GGA) is used for exchange correlation potential (PBE is a gradient correction function of GGA). Plane wave truncation energy equals 380 eV. Total of Brillouin Sum used Monkorst-Park
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target.
corners, while Ga is mainly located in the face center of sphalerite structure because only the large-scale Ga doping here will not cause lattice distortion. Furthermore, the supercell is established to illustrate the bond information. The bonds of CoeS and GaeS is the mainly vibration information to confirm the successful doping fabrication in terms of nanoparticles. What we are more concerned about is the information of the CO-Ga bond confirmed in Raman. The number 1 is represents the Co-bond in the edge of a cell. As we discussed aforementioned, it is easier to bond with equal amount of Co as the increasing of Ga that still not bonded with other elements. Then, after bonding with the former Co atom, Ga can continue to attract other Co atoms to bond, which is similar to the crystal form of Ga2O3, as seen the region of number 2. Besides, the un-doping ions such as Ga and impurity could move to near Ga and further to generate GaeGa or GaeO. To prove the correctness of our model, the level structure was calculated. The highest band achieved by calculation in software is 3.8 eV in the point Z in brillouin zone, which is in good agreement with the experimental results 4.1 eV. The experimental results are obtained by tauc plot mapping according to the absorb spectrum data of Nps.
3.3. The physics modes of the shape formation, explosion-absorption and cluster growth of nanoparticles Fig. 5 explains in detail the formation of shuttle and circular nanoparticles, the formation of large-sized particles and cluster phenomena in TEM images. Firstly, the formation process of shuttle-shaped NPs is described by Fig. 5a. The obvious difference is reflected in the plasma plume in two processes. The obvious boundary line is exhibited by the plume in experiment B with respect to circular-shaped nanoparticles because it is not affected by other conditions. But for experiment A subjected to external magnetic field, the boundary ambiguity effect will be produced by plasma plume born from the interaction between laser and matter. Because the plasma in the plume is affected by the additional magnetic force, the nucleation, growth and cluster processes of the nanoparticles will undergo different transformations, which will further decide shape and size of NPs finally. Since Fe2O3 powder is evenly added in two sides of the target, so a uniform magnetic field is placed around the plasma plume. Two equal magnetic forces will be applied to the growth and polymerization of nanoparticles. In this process, the shape of nanoparticles and plasma plume will be gradually stretched by two sides (left and right), and finally shuttle-shaped particles will be formed. In order to explain the reasons for the formation of large-sized circular and other irregular nanoparticles in the process of high-power preparation, the explosive adsorption growth model of particles was established, as seen in Fig. 5b. The first stage is called the explosion stage. It means that when a strong laser interacts with matter, large of nuclei, particles and plasma may be bombarded directly from the surface (of course, these particles exist in the plasma plume during the preparation of PLA). Then, some unnucleated particles complete the nucleation process and enter the adsorption stage together with other particles spilled from the surface. At this stage, when other atoms, small clusters and nuclei in the outer region (more than 100 nm away from the core) drift close to the nucleating particles, these particles are most likely to be adsorbed by small nanoparticles, and eventually those small particles will slowly grow into large nanoparticles. According to the formula of F. Mafune [30], the radius of nanoparticles can be expressed by rp(t):
3.2. The morphology of nanoparticles Fig. 4a-h show TEM images of shuttle-shaped nanoparticles. The shuttle shape is formed by experiment A through external magnetic field generating from Fe3O4. Most of those nanoparticles existing with the shape of shuttle are revealed by Fig. 4a. But, a small number of the circular nanoparticles still are found (Fig. 4b) because of laser and solution on the magnetic field. The size of particles in this shape is larger than that of other types of nanoparticles, the big one approximating 430 nm (Fig. 4e). From the Fig. 4f we can clearly see that the morphology and lattice planes direction of shuttle-shaped nanoparticles. The obvious boundary and clear texture are shown in this enlarge picture due to the effect of magnetism, which gives us a clearer and more intuitive understanding of shuttle nanoparticles. Then, an obvious two-dimensional lattice image is shown in Fig. 4g. Two lattice planes marked are (111) with 0.332 nm and (220) with 0.212 nm, respectively, in sphalerite structure corresponding to the shuttle-shaped. The size distribution statistics are shown in Fig. 4h. The data show that the particle size distribution is the most in 200–300 nm, followed by 400–500 nm, and finally 300–400 nm. The average size of the diamondshaped nanoparticles is about 200–300 nm. In general, large-sized nanoparticles usually have better optical properties such as more likely to generate optical radiation. However, the experiment A successfully solves the cluster problem of nanoparticles, and their excellent dispersion can be seen from Fig. 4c-e. To illustrate, the Ga0.5Co0.1ZnS0.4, Ga0.3Co0.3ZnS0.4 and Ga0.1Co0.5ZnS0.4 are displayed in the (a, b), (c, d, f and g) and (e), respectively. The TEM images of circular-shaped multi-doping nanoparticles are shown in Fig. 4i-p. In the experiment B, all nanoparticles are circular in shape with clusters at 500 mW laser power, see Fig. 4i-l. The phenomena of severe cluster are a common problem in the preparation of traditional nanoparticles. All circular nanoparticles tested in TEM are concentration of Ga0.3Co0.3ZnS0.4. Of course, some particles with better dispersion are found, which have a more symmetrical size distribution. The Fig. 4n reveal a single circular-shaped NPs with 30 nm diameter, which is the average size for circular NPs according to the size value statistics (Fig. 4p). Statistical data in Fig. 4p show that the size of circular nanoparticles is mainly distributed in the 20–40 nm range. Unlike diamond-shaped particles, most of their sizes are below 50 nm. But there are also some abrupt changes in particle size, which are larger than 100 nm. The lattice spacing 0.283 nm is consistent with ZnS (111), see Fig. 4o. Compared with other single element nanoparticles or quantum dots, the size of composite multi-doped nanoparticles is larger. Then, large-sized circular and other irregular nanoparticles were found in suspensions prepared at 700 mW laser power (Fig. 4q-t). This phenomenon can be explained via explosion effect between laser and
3Np Va 1/3 ⎞ rp = ⎛ ⎝ 4π ⎠ ⎜
⎟
(1)
Here, Np is the number of particles after explosion. Va is the effective volume of multi-doped atoms. Then, we assume extravasation velocity of plasma is vp. So the volume growth rate of particle is the formula as follow:
Va dNp (t ) dt
= kπrp2 Va da vp
(2)
where da is the quantitative density of ions in plasma, which increases with time. k is the proportionality coefficient. kπrp2 is the geometric cross section area of particle. Therefore, the volume and radius of particles can be defined as
Va Np (t ) = ((Np Va )1/3 + (π /48)1/3kVa da vp t )3
(3)
rp (t ) = r0 + kVa da vp t /4
(4)
where r0 is the radius of particles after explosion. rp(t) is the radius of particles after a slow growth for t. In addition, many large particles are formed by two or three clusters. When the size of a single particle is large, the size of cluster particles can easily exceed 300 nm. There are three common cluster types including bead, rod/linear and regiment cluster, as shown in Fig. 5c. The generation of different cluster types can still be explained by the explosion-absorption theory. When the effect of explosion is greater than that of absorption, Beaded 416
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Fig. 4. TEM image of the GaxCo0.6-xZnS0.4 nanomaterials prepared by PLA. (a) and (c) The shuttle-shaped nanomaterials prepared via experiment A. (b) Zoom into the filament regions of (a). (d) Zoom into the filament regions of (d). (e) The single diamond-shaped nanoparticle. (f) The enlarged surface morphology of shuttleshaped nanoparticles. (g) Lattice image of shuttle-shaped nanoparticle. (h) The size distribution of shuttle-shaped nanoparticles. (f) The circular nanoparticles collected form experiment B. (g) Zoom into the filament regions of (f). (k) Clustering of nanoparticles. (l) Zoom into the filament regions of (k). (m) The circular nanoparticles with different sizes and well dispersion. (n) The single circular nanoparticle with 30 nm diameter. (p) The size distribution of circular-shaped nanoparticles. (q)-(t) The single and big nanoparticles under the laser power of 700 mW.
3.4. The analysis of optical properties
clusters are considered to be the dominant type. Then, rod/linear clusters are the main cluster types as an equal effect of explosion and absorption, which provides strong evidence to explain the formation of nanowires. When the effect of explosion is less than that of absorption, clusters appear more in the regiment.
Fig. 6 represent the PL spectra of Ga-Co-ZnS nanoparticles with different shape and concentration. The PL spectra of all shuttle-shaped nanoparticles are displayed in the Fig. 6a-c. In Fig. 6a, three peaks at 560, 670 and 860 nm in high Ga + concentration nanoparticles, namely 417
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Fig. 5. The modes of the shape formation, explosion-absorption and cluster growth of nanoparticles. (a) The formation of shuttle and circular nanoparticles. The insert picture is a real-time photograph of the plasma plume. (b) The formation of large-sized particles. (c) Three common cluster types including bead, rod/linear and regiment.
The reason for this phenomenon can be explained by the structural properties of different size particles. The nanoparticles size of shuttleshaped is larger than circular. So, they have complete structure, more energy levels and photon transitions. But for little size nanoparticles, they always tend to form single structure of energy level orbital so that the single peak is generated easier such as the Fig. 6d and f. However, Fig. 6e reveals multi-peaks at 560, 675 and 873 nm, which proves that the Co and the Ga energy level transition are moving up to the most as the concentration of them is close. Of course, the red and blue shift is still here. With the increasing of Ga concentration (10% → 30% → 50%), the vibration information of GaeS bond corresponding to the shift of 577 → 518 → 480 cm−1 in circular-shaped and 583 → 551 → 432 cm−1 in shuttle-shaped are supported by Raman spectrum, which illustrated the changing in bonding energy, position and density. These shifts will further lead to minor changes in the microstructures of materials. Therefore, the absorbed and emission properties in material could be directly changed. So the little change in PL peak has moved from
Ga0.5Co0.1ZnS0.4 (10%:50%:40%), are marked by green, red and black colour. Then, four mainly PL emissions at 560, 675, 751 and 870 nm in Ga0.3Co0.3ZnS0.4 (30%:30%:40%) are shown in Fig. 6b. For nanoparticles with high Co+ concentration (Ga0.1Co0.5ZnS0.4), the emission at 675 and 897 nm is detected. In short, the green light peak at 560 nm is attributed to the optical properties of the visible light range of Ga [31], and the red-light mission at 670–675 nm may be due to the transition between Co2+ and Ga2,3+ energy levels [32–34]. Meanwhile, the peak 751 nm confirmed this transition because it just existed in the material with the same concentration of Co and Ga element. The peaks of 860–897 nm are mainly put down to the infrared properties of CoZnS [32–34]. Besides, blue-shift is found in the changes of all peaks with the increasing of Ga concentration from 50% to 10%. Instead, redshift also existed in different Co concentration from 10% to 50%. This is consistent with the purpose of our design, which is the widely spectrum application. Then, the spectra of circular-shaped nanoparticles are illustrated in the Fig. 6d-f. Compared to the spectra of the shuttle-shaped, the number of fluorescent peaks of circular nanoparticles decreased. 418
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Fig. 6. Room temperature photoluminescence (PL) spectra of Ga-Co-ZnS nanoparticles with different concentration. (a)-(c) shuttle-shaped. (d)-(f) Circular-shaped. The inset picture is the TEM images and suspensions of nanoparticles corresponding to their different concentration.
proved by XPS. The morphology of the nanoparticles characterized by TEM indicating high efficient of our new method in terms of shuttleshaped. A physical model of the effect of magnetic field on the plasma plume was established to explain the formation of shuttle-shaped nanoparticles. Then, we established an explosion-absorption model to demonstrate the growth mechanism of nanoparticles. We further discussed the cluster phenomena and summarized three types of clusters in our nanoparticle suspensions. More abundant energy level and better optical properties in shuttle-shaped NPs were suggested by PL spectra, particularly in concentration of 30%–30%–40%. In conclusion, we have provided a new method for preparing nanoparticles with regular shape. This method can effectively solve the cluster problem of nanoparticles, and it has obvious modulation effect on the fluorescence properties of materials. Therefore, all the results suggest that MPPLA is promising to prepare nanoparticles since it can adjust the shape, solve the cluster and modulate performance of NPs.
675 nm to 670 nm as the increase of Ga content (see Fig. 6a and b). The direction of PL peaks shift is consistent with the Raman. Meanwhile, note that the PL peak at 560 nm is strong in Ga-rich material in terms of shuttle-shaped (Fig. 6a), which ascribed to the vibration at 613 and 729 cm-1 matched to GaeGa bond in Raman spectrum. Thus, the optical properties of Ga in the visible region are reflected in Ga-rich materials. When the concentration of Co and Ga is close in the material (30%–30%–40%)，all kinds of bonding information are detected by Raman, including the mode of CoeGa, GaeGa, GaeS, Lo mode of ZnS, CoeS. So this material has abundant energy level of multi-elements in which the transition increased. This multi-energy level structure will inevitably make its fluorescence spectrum showing a broad multi-peak shape (Fig. 6b and e), which reveals that our test results are relatively objective. Similarly, with the increasing of Co concentration (10% → 30% → 50%), the vibration information of CoeS bond corresponding to the shift of 168 → 181 → 192 cm-1 in circular-shaped and 166 → 171 → 193 cm-1 in shuttle-shaped are presented by Raman spectrum. These changes further illustrates that the changing of Co and Ga concentration has an influence on the micro-structure, which results the PL peaks red-shift with 860 → 870 → 897 nm in shuttle and 873 → 875 nm in circular.
Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC-11474014), Beijing Municipal Education Commission Science and Technology Program (KZ201610005001) and the open project of Beijing Engineering Researching Centre of Laser Technology (BG0046-2018-01).
4. Conclusion The shuttle-shaped GaxCo0.6-xZnS0.4 (x = 0.1, 0.3, 0.5) nanoparticles were fabricated via a Magnetic Polarization Pulse Laser Ablation (MPPLA) technology, which was first reported by this paper. Meanwhile, the formation mechanism and growth process of round and large size nanoparticles were deeply investigated by this experiment. Crystal structure of all nanoparticles was confirmed by XRD. The bonding vibration information of multi-doped elements was shown in Raman spectrum. It also illustrated that the transitions in multi-level orbits occur more frequently at the closely concentration of Co and Ga element. The +2 and +3 valence of Co and Ga in the materials were
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