1. Introduction

Selenium (Se) is a natural part of the human diet and one of the most important essential trace minerals for all mammalian species. It has a widespread role in physiological and biomedical applications because of its bioavailability, low toxicity and important therapeutic properties. The fabrication and investigation of selenium nanoparticles (SeNPs) is an interesting and important field in biomedicine. Particularly, because of its low-dimensional nanostructures, the SeNP has unique chemical and physical properties, in terms of the size and quantum effect. For instance, SeNP is a good anti-tumoral and chemopreventive agent for that renders cancer cells susceptible to drugs [1–4]. The enzyme-like properties and intrinsic fluorescence of SeNPs are also applied in cellular imaging and biocatalysis [5]. Additionally, their high photoconductivity and optical nonlinearity [6] mean that SeNPs have been widely used in photovoltaic cells, high-power batteries, catalysis, sensors and electronics [7–10]. For these applications, various methods of synthesizing SeNPs have been developed, such as chemical reduction [11], chemical oxidation [12], biosynthesis [13,14], electrochemical reaction [15], and pulsed laser ablation in liquid [16–20]. However, a number of challenges remain, in terms of controlling the size and shape, production time and energy-consumption.

In this study, a vacuum-free, chemical-solution-free, template-free, liquid-free and rapid preparation method of femtosecond (fs) laser-induced plasma shock wave is used to prepare both amorphous and crystalline SeNPs simultaneously on the surface of topological insulator Bi₂Se₃ single crystals at room temperature and ambient pressure. SeNPs of various shapes and sizes are found at different radial distances from the ablation center on the surface of Bi₂Se₃ single crystals. These are reliably controlled using the kinetic energy of SeNPs obtained from the laser pulse.

2. Methods and results

The (0001) surface of the Bi₂Se₃ single crystal was irradiated by the Ti:sapphire laser at room temperature and atmospheric pressure. Figure 1(a) shows the experimental setup and a photograph of a Bi₂Se₃ single crystal after laser irradiation (the diameter of spot ∼ 104 µm) at a wavelength of 800 nm and a pulse energy of 49 nJ. A concentric ring pattern forms on the surface of the laser-irradiated Bi₂Se₃ single crystal. The rings are labeled by numbers 1-9 from the burning hole.

 

Fig. 1. Morphology and elemental content on the surface of a Bi₂Se₃ crystal that is irradiated by a femtosecond laser (femtosource XL, Femto lasers with central wavelength of 800 nm, pulse duration of 50 fs, and repetition rate of 5.1 MHz). (a) A SEM image of a Bi₂Se₃ crystal after laser irradiation for 30 seconds. Inset: the schematics of femtosecond laser-induced plasma shock wave; W: femtosecond laser-induced plasma shock wave, Air: air molecules and Se: Se ions. (b) The SEM (which is equipped with electron backscatter diffraction (EBSD) analysis with resolution of 10 nm, and operated under base pressure, P = 9.6×10⁻⁵ Pa) images of Se nanoparticles that are produced in various regions (marked by the #1-9, same as in (a)) on the surface of a Bi₂Se₃ crystal. (c) The distance-dependent (d: from the center of burning hole and also marked by the #1-9, same as in (a)) atomic concentration of each element, measured by SEM-energy dispersive spectroscopy (EDS).

Download Full Size | PDF

The surface morphology in the different concentric ring areas is determined using high-resolution scanning electron microscopy (SEM) images, which are shown in Fig. 1(b). In region 1, the surface is very rough and irregular, because it is seriously damaged by fs laser pulses. There are some micro- and nano-scale particles with a spherical or rectangular shape in regions 3-9. The shape of these particles depends on their position on the surface of the Bi₂Se₃ single crystal. The atomic composition for all regions is analyzed using SEM with energy dispersive spectroscopy (EDS) and the results are shown in Fig. 1(c). The particles in region 1 are dominated by Bi and O. The composition of particles in regions 3 and 4 is dominated by Se. However, from the region 5 to region 9, the ratio of Bi to Se is around 2/3, which indicates that the composition is dominated by the Bi₂Se₃ substrate. This is because the nano-scale Se particles on the Bi₂Se₃ substrates are not measured by SEM-EDS.

In order to identify the composition and the crystal structure of the nano-scale particles in regions 5-9, SEM-electron backscatter diffraction (EBSD) was used. In Fig. 2, the composition and crystal structure is shown in the dashed-yellow rectangle of Fig. 2(a). The phase color is purple in Fig. 2(b), so the particle in position I is mainly composed of trigonal Se^b (t-Se, see Fig. 5 in Appendix) crystals. Figure 2(c) shows that the orientation of the nanoparticles in region 4 is dominated by (01$\bar{1}$0). However, the rectangular shape of the Se nanoparticles in region 5 gradually evolves to a spherical shape as the distance from the laser-ablated hole increases, which implies that there is a change in the crystal structure in regions 7-9.

 

Fig. 2. Crystallinity and orientation of Se nanoparticles in region 4 of Fig. 1(b). (a) A SEM image of the Se nanoparticles with the 70°-tilt angle view that is required for the EBSD measurements. The area within the yellow dashed rectangle was analyzed by EBSD, as shown in (b) and (c). (b) The phase colors show two types of trigonal Se crystals: Se^a (dark red) and Se^b (purple). (c) The crystal orientation corresponding to the trigonal Se crystals in (b). (d)-(g) High-resolution transmission electron microscope (HRTEM) images of the Se nanoparticles in region 9 (see Fig. 1(b)) and their composition mapping. Red represents Carbon (C), blue represents Oxygen (O), brown represents Bismuth (Bi) and green represents Selenium (Se).

Download Full Size | PDF

Additionally, the different crystal phases in SeNPs were further quantitatively studied by the micro-Raman spectroscopy. As shown in Fig. 3(a), spectrum 0 was measured at the position of the burning hole and spectra 1 to 9 were measured in the regions 1 to 9, marked in Fig. 1(a). Because the substrate is Bi₂Se₃, all of the Raman scattering spectra show the characteristic peak of A²_1g (at ∼174 cm⁻¹) [21], except spectrum 1. The peak at 158 cm⁻¹, which is observed in regions 0-2, is caused by the vibration mode of α-Bi₂O₃ [22], which is consistent with the high concentration of Bi and O that is determined using SEM-EDS and shown in Fig. 1(c). Besides, a broad peak is clearly observed around 250 cm⁻¹ in spectra 3-9. The spectra in Fig. 3(b) show that the broad peak is actually composed of several vibrational modes of various phases: amorphous-Se (a-Se), monoclinic-Se (m-Se, see Fig. 5 in Appendix) and t-Se [23,24]. Therefore, four Lorentzian peaks with central frequencies of 233, 237, 250 and 259 cm⁻¹ are used to fit this broad peak, as shown in Fig. 3(b). The main peak at ∼250 cm⁻¹ is attributed to the A₁ mode of m-Se or a-Se. The shoulder at ∼259 cm⁻¹ is caused by the intra-ring stretching of the Se₈ rings that are present in amorphous Se. Obviously, a sharp peak (with a width of ∼5 cm⁻¹) appears at ∼233 cm⁻¹. This is attributed to the asymmetric bond stretching E mode of bulk t-Se, as clearly seen in spectrum 5 from region 5 in Fig. 1(b). One more Raman signal from the symmetry stretching A₁ mode of the Se chain in t-Se (see Fig. 5(b) in Appendix) is also clearly observed at 237 cm⁻¹. These results indicate that Se nanoparticles that are fabricated using fs laser-induced plasma shock wave can form different types of crystalline structures.

 

Fig. 3. Lattice structures of Se nanoparticles in various regions of Fig. 1(b). (a) The micro-Raman spectra (homemade Raman system with excitation light wavelength of 632.9 nm, excited laser power of 0.5 mW to prevent local laser heating during the measurements. The resolution of detector is 0.5 cm⁻¹) for Se nanoparticles in regions 0 to 9 (see Fig. 1(b): #0: is the center of the burning hole that is generated by femtosecond laser pulses). (b) The enlarged Raman spectra in (a) with Lorentzian peak fitting. The black solid line represents raw data, the red dotted line represents the E mode for bulk t-Se (at 233 cm⁻¹), the red solid line represents the A₁ mode for t-Se (at 237 cm⁻¹), the blue dashed line represents the A₁ mode for m-Se/a-Se (at 250 cm⁻¹). And the green dashed-dot line represents the intra-ring stretching of Se₈ rings in a-Se (at 259 cm⁻¹).

Download Full Size | PDF

3. Discussion

To determine the ratio of crystalline to amorphous SeNPs, the area (A₂₃₃, A₂₃₇, A₂₅₀, and A₂₅₉) of each fitting peak in Raman spectra of Fig. 3(b) is estimated (see section A.3 in Appendix). Additionally, the high-resolution transmission electron microscope (HRTEM) images in Fig. 2 further demonstrate the position-dependent distribution of crystalline and amorphous SeNPs. These results clearly show that the Se can exist in different allotropic forms, which are a mixture of disordered chains and rings (i.e., the amorphous state), and three crystalline states, including the most stable trigonal and monoclinic forms. Because the phase transformation energy of 6.63 kJ/mol is small [6], the thermodynamically unstable a-Se easily transforms to the stable t-Se at room temperature.

The results from region 3 to region 9 can be explained by the expansion of plasma during fs laser ablation. When the fs laser irradiates on the Bi₂Se₃ crystal surface, the Bi-Se bonds (the σ bonds in the bulk of Bi₂Se₃ with the energy of 17 eV [25]) are broken by the strong electric field that the fs laser pulse produces and the remainder of the laser energy is transferred to Bi and Se ions as the kinetic energy. The SEM-EDS (in Fig. 1) and Raman spectroscopy (in Fig. 3) results demonstrate that Bi ions bond with oxygen and then form Bi₂O₃ on substrates. This is because Bi ions have larger atomic mass (atomic mass of Bi: 209.0; Se: 79.0). The Bi ions would drop with a shorter time and nucleation in the region close to the burning hole with a homogenous distribution on the target surface. Generally, O₂ has larger electronegativity compared with N₂, and the oxidation reaction of Bi is spontaneous reaction with moderate activation energy at room temperature [26,27]. Consequently, the main products after laser irradiation would be Bi₂O₃ near the burning hole.

During laser ablation, Se ions also splash from the burning hole and then some Se ions are deposited on the surface of the Bi₂Se₃ substrates. Assuming that Se ions receive some energy from the laser pulses (E_pulse) after ablation and that they have a kinetic energy of E_k,_Se, the energy transfer equation for Se ions can be written as:

 

Fig. 4. The relationship between the laser pulse energy and the shortest distance at which Se ions are deposited and the distance-dependent number density of SeNPs (number of Se ions). (a) The shortest distance d_shortest at which Se ions are deposited increases as the laser pulse energy E_pulse increases. The solid line is the linear fit with the slope of 2490 ± 80 (m/J). (b) The number density (ρ_Se) for SeNPs at various distances (from region 3 to region 9 in Figs. 1(a) and 1(b)). The number (N_Se) of Se ions at various distances (from region 3 to region 9 in Figs. 1(a) and 1(b)). The solid line is the fit with the Maxwell–Boltzmann velocity distribution function [29].

Download Full Size | PDF

The number of SeNPs significantly increases as d increases, as shown in Fig. 1(b). In region 3, the number density for Se nanoparticles is only ∼0.065 particles/µm². As d increases, the number density for SeNPs gradually increases to ∼35 particles/µm² in region 9, as shown in Fig. 4(b). During nucleation and growth, the amorphous Se particles form into spherical shapes (in regions 6-9) to minimize the interfacial free energy, and the trigonal Se particles in regions 4-6 assume an anisotropic crystalline structure (see Fig. 5 in Appendix) to produce a rectangular (quasi-1D) nanostructure. It is worth noting that the size of SeNPs decreases as d increases, so the total volume of Se that is deposited in each region is estimated as shown in Fig. 4(b). The total volume of Se gradually decreases when d increases. This relationship is described well by the Maxwell–Boltzmann velocity distribution function [29] (see the solid line in Fig. 4(b)). This fitting also determines the deceleration factor, a = 1.83×10⁹ m/s², whose order of magnitude is consistent with the results (∼4.4×10⁹ m/s²) that are calculated using the velocity distribution for a femtosecond laser-induced plasma shock wave over 0.5 mm in [30]. Since m_Se = 1.31×10⁻²⁵ kg and d_shortest / E_pulse ${\approx} $ 2490 ± 80 (m/J) obtained from the slope of the solid line in Fig. 4(a), the energy transfer efficiency (=1/T_Se) from laser pulses to kinetic energy of Se ions is only around 0.01%. This means that most of the energy of the laser pulses is used to break the Bi-Se bonds in Bi₂Se₃ crystals. Even though the Se ions gain energy from laser pulses, most are expelled during the laser-induced plasma expansion and are not deposited on the surface of the Bi₂Se₃ crystals to form the SeNPs because the Se has been shown to have a high vapor pressure on crystal growth [31]. Moreover, SeO₂ may be also produced after the laser irradiation due to the larger electronegativity of O₂ (compared with N₂). However, the lower boiling point (350℃) would result in the vaporization of SeO₂. In addition, SeO₂ can also react with H₂O and produce H₂SeO₃ in ambient [32]. Both SeO₂ and H₂SeO₃ are very soluble in water (H₂O), which can further reduce to Se [33,34]. Hence, we did not observe the production of SeO₂ as shown by the Raman spectra in Fig. 3.

 

Fig. 5. Schematics of various crystalline structures of Se: [35] (a) Trigonal Se crystals, (b) Se chains in the trigonal Se crystals and (c) Se₈ rings in the β-monoclinic Se crystal.

Download Full Size | PDF

Finally, all SeNPs in Fig. 1 (from d = 150 µm to 340 µm) include $1.57 \times {10^{15}}$ Se atoms (or $2.60 \times {10^{ - 8}}$ moles), which can be estimated through integrating the Se distribution function in Fig. 4. By multiplying 79.0 (the atomic mass of Se), the total weights of all SeNPs are 2.05 µg with 30 seconds (laser irradiation time). Therefore, the production rate of SeNPs is 1.48 mg/hr (or 24.6 µg/min) in this study, which can be further enhanced by increasing laser power with parallel processing. Besides, the crystal phases and sizes of the SeNPs can be simply obtained at different distance from the laser irradiation point or by controlling the laser power.

4. Summary

In summary, this study demonstrates the vacuum-free, chemical-solution-free, template-free, liquid-free and rapid preparation of Se nanoparticles with an amorphous and crystalline structure at room temperature and ambient pressure using femtosecond laser-induced plasma shock wave and kinetic energy-selective growth. Various shapes (spherical and rectangular) and sizes (100-900 nm) of Se nanoparticles are present on the surface of Bi₂Se₃ substrates, so these are applicable as active components in nanoscale applications. This novel preparation method could also be extended to other systems.

Appendix

A.1 The crystalline structures of Se

Figure 5 shows the various crystalline structures of Se. The trigonal Se (t-Se) crystals in Fig. 5(a) are formed by A-B-C stacking of three hexagonal layers along the c-axis [35]. There are also two types of trigonal Se crystals that have different c-axis length: trigonal Se^a with a c-axis of 4.89 Å and trigonal Se^b with a c-axis of 4.95 Å. Figure 5(b) shows the Se chains in the trigonal Se crystals. The Se chain that is marked in green in Fig. 5(b) is the same as the Se atoms that are linked by green lines in Fig. 5(a). The crown structure of the Se₈ ring in β-monoclinic Se crystals is shown in Fig. 5(c).

A.2 The energy transfer efficiency from laser pulses to kinetic energy for Se ions

According to Eq. (3), the energy transfer coefficient T_Se is calculated as:

(4)$${T_{Se}} = \frac{{3{E_{pulse}}}}{{a\; {d_{shortest}}\; m_{Se}^{\prime}}}$$

where the values of E_pulse, m’_Se, a and d_Shortest are shown in Table 1. Using the values that are listed in Table 1 gives T_Se= 9 $\times {10^3}$. Therefore, the energy transfer efficiency (=1/T_Se) from laser pulses to kinetic energy for Se ions is ∼0.01%.

Table 1. The values of the parameters for Eq. (4).

View Table | View all tables in this article

A.3 The position-dependent intensity, width and area of the Raman peaks

Figures 6(a) and 6(b) respectively show the intensity and width of the Raman peaks in Fig. 3(b) as a function of their position in Fig. 1(b) (the E mode for bulk t-Se is at 233 cm⁻¹, the A₁ mode for t-Se is at 237 cm⁻¹, the A₁ mode for m-Se/a-Se is at 250 cm⁻¹ and the intra-ring stretching of Se₈ rings in a-Se is at 259 cm⁻¹). Table 2 shows the area of each fitted peak in Fig. 3(b).

 

Fig. 6. (a) The intensity and (b) the width of the Raman peaks in Fig. 3(b) as a function of positions (i.e., the curve number for the horizontal-axis in the Figures) in Fig. 1(b). Note: The E mode for bulk t-Se is at 233 cm⁻¹, the A₁ mode for t-Se is at 237 cm⁻¹, the A₁ mode for m-Se/a-Se is at 250 cm⁻¹ and intra-ring stretching for Se₈ rings in a-Se is at 259 cm⁻¹.

Download Full Size | PDF

Table 2. The area of each fitted peak in Fig. 3(b): A₂₃₃ is the area of the Raman peak at 233 cm⁻¹: A₂₃₇ is the area of the Raman peak at 237 cm⁻¹, A₂₅₀ is the area of the Raman peak at 250 cm⁻¹, A₂₅₉ is the area of the Raman peak at 259 cm⁻¹ and R_n (n is region number) = (A₂₃₃+A₂₃₇)/(A₂₃₃+A₂₃₇+A₂₅₀ + A₂₅₉).

View Table | View all tables in this article

The ratio of crystalline to amorphous Se nanoparticles was determined by the area A₂₃₃, A₂₃₇, A₂₅₀, and A₂₅₉ (see Table 2). However, the main peak of 250 cm⁻¹ cannot be used to distinguish the phases of m-Se and a-Se. The percentage of crystallization is defined by R_n (n is region number), which is equal to (A₂₃₃+A₂₃₇)/(A₂₃₃+A₂₃₇+A₂₅₀ + A₂₅₉) for spectra 5, 6 and 7 and A₂₃₃/(A₂₃₃+A₂₅₀ + A₂₅₉) for spectra 3/4, 8 and 9. In region 3/4, the value of R_3/4 is estimated to be 12.3%, which indicates that the Se structure in region 3/4 is dominated by a-Se. The value for R₅ increases to 67.4% in region 5, which is consistent with the observations using SEM-EBSD, for which the results are shown in Fig. 2. As the number of square-shaped Se nanoparticle decreases in region 6, the value of R₆ shrinks to 16.5%. In the regions 7, 8 and 9, the respective values for R₇, R₈, and R₉ are 19.1%, 19.1% and 20.3%, which indicates that t-Se crystals and a-Se co-exist but the structure is dominated by a-Se.

Funding

Ministry of Science and Technology, Taiwan (106-2119-M-009-013-FS, 106-2628-M-009-003-MY3, 107-2119-M-009-010-MY2); Center for Emergent Functional Matter Science of National Chiao Tung University from The Featured Areas Research Center Program and the Research Team of Photonic Technologies; Ministry of Education.

References

1. P. A. Tran, L. Sarin, R. H. Hurt, and T. J. Webster, “Opportunities for nanotechnology-enabled bioactive bone implants,” J. Mater. Chem. 19(18), 2653–2659 (2009). [CrossRef]  

2. P. Bao, Z. Chen, R. Z. Tai, H. M. Shen, F. L. Martin, and Y. G. Zhu, “Selenite-induced toxicity in cancer cells is mediated by metabolic generation of endogenous selenium nanoparticles,” J. Proteome Res. 14(2), 1127–1136 (2015). [CrossRef]  

3. M. Stolzoff and T. J. Webster, “Reducing bone cancer cell functions using selenium nanocomposites,” J. Biomed. Mater. Res., Part A 104(2), 476–482 (2016). [CrossRef]  

4. B. Yu, T. Liu, Y. Du, Z. Luo, W. Zheng, and T. Chen, “X-ray-responsive selenium nanoparticles for enhanced cancer chemoradiotherapy,” Colloids Surf., B 139, 180–189 (2016). [CrossRef]  

5. A. Khalid, P. A. Tran, R. Norello, D. A. Simpson, A. J. O’Connor, and S. Tomljenovic-Hanic, “Intrinsic fluorescence of selenium nanoparticles for cellular imaging applications,” Nanoscale 8(6), 3376–3385 (2016). [CrossRef]  

6. K. A. A. Mary, N. V. Unnikrishnan, and R. Philip, “Ultrafast optical nonlinearity in nanostructured selenium allotropes,” Chem. Phys. Lett. 588, 136–140 (2013). [CrossRef]  

7. C. Luo, Y. Xu, Y. Zhu, Y. Liu, S. Zheng, Y. Liu, A. Langrock, and C. Wang, “Selenium@mesoporous carbon composite with superior lithium and sodium storage capacity,” ACS Nano 7(9), 8003–8010 (2013). [CrossRef]  

8. S. Chaudhary and S. K. Mehta, “Selenium nanomaterials: applications in electronics, catalysis and sensors,” J. Nanosci. Nanotechnol. 14(2), 1658–1674 (2014). [CrossRef]  

9. H. Dong, A. Quintilla, M. Cemernjak, R. Popescu, D. Gerthsen, E. Ahlswede, and C. Feldmann, “Colloidally stable selenium@copper selenide core@shell nanoparticles as selenium source for manufacturing of copper–indium–selenide solar cells,” J. Colloid Interface Sci. 415, 103–110 (2014). [CrossRef]  

10. X. Peng, L. Wang, X. Zhang, B. Gao, J. Fu, S. Xiao, K. Huo, and P. K. Chu, “Reduced graphene oxide encapsulated selenium nanoparticles for high-power lithium–selenium battery cathode,” J. Power Sources 288, 214–220 (2015). [CrossRef]  

11. S. Ahmed, J. Brockgreitens, K. Xu, and A. Abbas, “Sponge-supported synthesis of colloidal selenium nanospheres,” Nanotechnology 27(46), 465601 (2016). [CrossRef]  

12. C. P. Shah, C. Dwivedi, K. K. Singh, M. Kumar, and P. N. Bajaj, “Riley oxidation: A forgotten name reaction for synthesis of selenium nanoparticles,” Mater. Res. Bull. 45(9), 1213–1217 (2010). [CrossRef]  

13. S. Dhanjal and S. S. Cameotra, “Aerobic biogenesis of selenium nanospheres by Bacillus cereus isolated from coalmine soil,” Microb. Cell Fact. 9(1), 52 (2010). [CrossRef]  

14. A. A. Kamnev, P. V. Mamchenkova, Y. A. Dyatlova, and A. V. Tugarova, “FTIR spectroscopic studies of selenite reduction by cells of the rhizobacterium Azospirillum brasilense Sp7 and the formation of selenium nanoparticles,” J. Mol. Struct. 1140, 106–112 (2017). [CrossRef]  

15. X. T. Ye, L. Chen, L. Liu, and Y. Bai, “Electrochemical synthesis of selenium nanoparticles and formation of sea urchin-like selenium nanoparticles by electrostatic assembly,” Mater. Lett. 196, 381–384 (2017). [CrossRef]  

16. M. Quintana, E. Haro-Poniatowski, J. Morales, and N. Batina, “Synthesis of selenium nanoparticles by pulsed laser ablation,” Appl. Surf. Sci. 195(1-4), 175–186 (2002). [CrossRef]  

17. S. C. Singh, S. K. Mishra, R. K. Srivastava, and R. Gopal, “Optical properties of selenium quantum dots produced with laser irradiation of water suspended Se nanoparticles,” J. Phys. Chem. C 114(41), 17374–17384 (2010). [CrossRef]  

18. P. G. E. Kuzmin, G. A. Shafeev, V. V. Voronov, R. V. Raspopov, E. A. Arianova, E. N. Trushina, I. V. Gmoshinskii, and S. A. E. Khotimchenko, “Bioavailable nanoparticles obtained in laser ablation of a selenium target in water,” Quantum Electron. 42(11), 1042–1044 (2012). [CrossRef]  

19. O. Van Overschelde, G. Guisbiers, and R. Snyders, “Green synthesis of selenium nanoparticles by excimer pulsed laser ablation in water,” APL Mater. 1(4), 042114 (2013). [CrossRef]  

20. A. Ionin, A. Ivanova, R. Khmel’nitskii, Y. Klevkov, S. Kudryashov, N. Mel’nik, A. Nastulyavichus, A. Rudenko, I. Saraeva, N. Smirnov, D. Zayarny, A. Baranov, D. Kirilenko, P. Brunkov, and A. Shakhmin, “Milligram-per-second femtosecond laser production of Se nanoparticle inks and ink-jet printing of nanophotonic 2D-patterns,” Appl. Surf. Sci. 436, 662–669 (2018). [CrossRef]  

21. W. Richter and C. R. Becker, “A raman and far-infrared investigation of phonons in the rhombohedra1 V2-V13 Compounds Bi₂Te₃, Bi₂Se₃, Sb₂Te₃ and Bi₂(Te_1-xSe_x)₃ (0 <|> x <|> l), (Bi_1-ySb_y)₂Te₃ (0 <|> y <|> 1),” Phys. Status Solidi B 84(2), 619–628 (1977). [CrossRef]  

22. V. N. Denisovy, A. N. Ivlev, A. S. Lipin, B. N. Mavrin, and V. G. Orlovd, “Raman spectra and lattice dynamics of single-crystal-Bi₂O₃,” J. Phys.: Condens. Matter 9(23), 4967–4978 (1997). [CrossRef]  

23. A. E. Raevskaya, A. L. Stroyuk, S. Y. Kuchmiy, V. M. Dzhagan, D. R. T. Zahn, and S. Schulze, “Annealing-induced structural transformation of gelatin-capped Se nanoparticles,” Solid State Commun. 145(5-6), 288–292 (2008). [CrossRef]  

24. G. Lucovsky, A. Mooradian, W. Taylor, G. B. R. Wright, and C. Keezer, “Identification of the fundamental vibrational modes of trigonal, α-monoclinic and amorphous selenium,” Solid State Commun. 5(2), 113–117 (1967). [CrossRef]  

25. G. J. Shu, S. C. Liou, S. K. Karna, R. Sankar, M. Hayashi, and F. C. Chou, “Dynamic surface electronic reconstruction as symmetry-protected topological orders in topological insulator Bi₂Se₃,” Phys. Rev. Mater. 2(4), 044201 (2018). [CrossRef]  

26. C.-S. Tu, P.-Y. Chen, C.-S. Chen, C.-Y. Lin, and V. H. Schmidt, “Tailoring microstructure and photovoltaic effect in multiferroic Nd-substituted BiFeO3 ceramics by processing atmosphere modification,” J. Eur. Ceram. Soc. 38(4), 1389–1398 (2018). [CrossRef]  

27. C. Machado, S. Aidel, M. Elkhatib, H. Delalu, and R. Metz, “Validation of a kinetic model of diffusion for complete oxidation of bismuth powder: influence of granulometry and temperature,” Solid State Ionics 149(1-2), 147–152 (2002). [CrossRef]  

28. R. Hergenröder, O. Samek, and V. Hommes, “Femtosecond laser ablation elemental mass spectrometry,” Mass Spectrom. Rev. 25(4), 551–572 (2006). [CrossRef]  

29. R. A. Serway and J. W. Jewett, Physics for Scientists and Engineers with Modern Physics (Brooks Cole, 2013), Chap. 21.

30. X. Chen, R.-Q. Xu, J.-P. Chen, Z.-H. Shen, L. Jian, and X.-W. Ni, “Shock-wave propagation and cavitation bubble oscillation by Nd:YAG laser ablation of a metal in water,” Appl. Opt. 43(16), 3251–3257 (2004). [CrossRef]  

31. F.-T. Huang, M.-W. Chu, H. H. Kung, W. L. Lee, R. Sankar, S.-C. Liou, K. K. Wu, Y. K. Kuo, and F. C. Chou, “Nonstoichiometric doping and Bi antisite defect in single crystal Bi₂Se₃,” Phys. Rev. B 86(8), 081104 (2012). [CrossRef]  

32. Åke Olin, Bengt Noläng, Evgeniy G. Osadchii, Lars-Olof Öhman, and Erik Rosén, Chemical Thermodynamics of Selenium, volume 7 (Elsevier Science, 2005), Chap. 5.

33. M. S. Kazacos and B. Miller, “Studies in Selenious Acid Reduction and CdSe Film Deposition,” J. Electrochem. Soc. 127(4), 869–873 (1980). [CrossRef]  

34. F. Séby, M. Potin-Gautier, E. Giffaut, and O. F. X. Donard, “Assessing the speciation and the biogeochemical processes affecting the mobility of selenium from a geological repository of radioactive wastesto the biosphere,” Analusis 26(5), 193–198 (1998). [CrossRef]  

35. K. E. Murphy, M. B. Altman, and B. Wunderlich, “The monoclinic-to-trigonal transformation in selenium,” J. Appl. Phys. 48(10), 4122–4131 (1977). [CrossRef]