The Crystalline Structure of ZnSe Nanoparticles

The crystalline structure could be unambiguously observed through x-ray diffraction patterns. From the X-ray diffraction θ-2θ scans, all the diffraction peaks in this pattern (Figure 4.8) can be indexed to the hexagonal structure according to the JCPDS card no.80-0008 for ZnSe (a = b = 3.974 Å, c = 6.506 Å). It clearly shows that the crystalline structure transforms from the ZnSe cubic phase with (200) and (400) into ZnSe hexagonal phase with (100), (002), (101), (102), (110), (103), and (112).

Figure 4.7: The X-ray diffraction pattern of ZnSe wafers.

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Cubic (311)

Cubic (220)

H Cubic (111)

ZnSe particles at F =290.52mJ/cm2

Hexagonal (112)

The structural phase transition takes place is not only at specific processing laser fluence. As shown in Figure 4.10 and Figure 4.11, the nanoparticles synthesized under various laser fluences have the same hexagonal structure.

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2θ (degree)

F=185 mJ/cm² F=172 mJ/cm² F=156 mJ/cm² F=135 mJ/cm² F=109 mJ/cm²

Figure 4.9: The XRD patterns of ZnSe nanoparticles under various laser fluences.

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2

(degree)

F=264 mJ/cm² F=249 mJ/cm² F=235 mJ/cm² F=220 mJ/cm² F=198 mJ/cm²

Figure 4.10: The XRD patterns of ZnSe nanoparticles under various laser fluences.

To obtain more information about the structural phase transition on ZnSe, the crystal growth database of ZnSe [32] reveals some important physical properties.

(a) (b)

Figure 4.11: The (a) cubic and (b) hexagonal structures of ZnSe.

The cubic structure is the stable form of ZnSe at room temperature and atmospheric pressure. The stacking sequence of this structure is aαbβcγaα (3C) where the Greek and Roman letters denote the close-packed planes of Zn and Se atoms, respectively, as shown in Figure 4.9 (a). At high temperatures near the melting point, the hexagonal phase does exist (space group P6₃mc) with the aαbβaα (2H) stacking sequence as shown in Figure 4.9 (b). And it is important to note that there is only a small energy difference between hexagonal and cubic ZnSe (~5.3 meV/atom) [34].

Indeed, there are few researches on synthesized the pure hexagonal ZnSe nanostructures [35, 36]. Owing to the hexagonal ZnSe is a metastable phase at ambient condition, it is only fabricated under very strict growth conditions. However, the hexagonal ZnSe can be easily and reliably achieved by femtosecond laser ablation which has been

demonstrated in this study.

It is interesting to note that the ZnSe nanoparticles fabricated in this study are the metastable hexagonal phase. A physical picture for the growth mechanism of the metastable hexagonal phase will be proposed through thermodynamic point of view. From following phase diagram (Figure 4.12), there are two paths to induce structural phase transition from cubic (zinc-blende) to hexagonal (wurtzite) structures by increasing the temperatures or the pressures.

Figure 4.12: Part of phase diagram for ZnSe [33]. The solid line denoted by s defined the solid-vapor equilibrium. The dashed lines denoted by m give the equilibrium between the vapor phase and metastable state. The arrowheads describe the final product.

It is important to note that the discussions of the nanostructure thermodynamics and stability can be applied to understand the growth mechanism of other nanostructures which are critical for optimizing the nanostructure.

According to the early researches, the cubic structure transforms to the hexagonal structure when the temperature is above the transition

on the ZnSe crystals, it will heat the ZnSe crystals and cause the crystal temperature rise. Due to the laser pulses, the transient increase in temperature, ΔT, for materials can be estimated by using the following relation. ΔT is far below the structural transition temperature, 1698 K. Therefore, the structural transition should not be triggered by the temperature increase here.

This indicates that when nanoparticles are produced through femtosecond laser ablation method, the rapid injection of high energy might induce ultrahigh pressure and cause ZnSe transformation from cubic to hexagonal.

Here we introduce the concept of “ablation pressure (or the so-called shock pressure)” [37]. Many theoretical and experimental studies have been reported over the past few decades [38-42]. While the laser was irradiated on solids, a high-density plasma is formed around the surface of samples. The compression of plasma in laser driven implosions has been characterized as the ablative or exploding pusher according to the relative importance of the surface ablation pressure and bulk pressure due to preheating through electrons.

In 1982, P. Mora derived that the shock pressure is related to laser

and target parameters [43]. wavelength in µm, A and Z are, respectively, the mass number and the atomic number of the target, and the time t is in ns. In Figure 4.13 (a), we simulated the effective pressure on the ablation region in the laser peak power density range of 0 ~ 4.5×10¹² W/cm² and the maximum laser driven pressure is about 2.6 Mbars. From the average power density point of view, the Figure 4.13 (b) shows the ablation pressure below 1.25×10^-9 Mbars.

(a)

0.0 9.0x10¹¹ 1.8x10¹² 2.7x10¹² 3.6x10¹² 4.5x10¹² 0.0

Ablation pressure under

femtosecond laser peak power density

Ablation pressure (Mbar)

Power density (W/cm²)

(b)

Power density (W/cm²)

Ablation pressure under

femtosecond laser average power density

Figure 4.13: The simulated ablation pressure as a function of power density. (a) for the laser peak power density case and (b) for the average power density case.

In fact, the phenomena of pressure induced phase transition have been found numbering many II-VI compounds by Samara’s group. For ZnSe case, the solid-solid transition point is around 165 kbars as shown in Figure 4.14 [44].

Figure 4.14: Resistance as a function of pressure in ZnSe [44].

In conclusion, the structural phase transition takes place during the ZnSe nanoparticles fabrication by femtosecond laser ablation because of the great ablation pressure.