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Novel flower-like hyperbranched ZnTe nanostructures prepared via catalyst-assisted vacuum thermal evaporation
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Novel flower-like hyperbranched ZnTe nanostructures prepared via catalyst-assisted vacuum thermal evaporation ⁎
Jin Li , Yan Ma College of Physics Science and Technology, Xinjiang University, Urumqi 830046, Xinjiang, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Zinc telluride Hyperbranched nanostructure Vapor-liquid-solid (VLS) growth mechanism Screw-dislocation-driven mechanism
Novel flower-like hyperbranched ZnTe nanostructures were prepared by catalyst-assisted vacuum thermal evaporation method. Various analysis techniques including X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), highresolution transmission electron microscopy (HRTEM) and selected area electronic diffraction (SAED) were conducted on the as-prepared products. The XRD analysis demonstrates the ZnTe nanostructures are high pure single crystalline of zinc-blende structure. These novel ZnTe nanostructures are grown by a combination of the vapor–liquid–solid (VLS) growth mechanism and screw-dislocation-driven mechanism. The nanostructures formed by VLS mechanism have smaller sizes of several to ten micrometers and secondary branches with diameters of 100 nm. The nanostructures combined VLS and screw-dislocation-driven growth mechanism have a diagonal size of 40 µm and they consist of quartic branches, among which the secondary branches are spiral. The stems of the nanostructures with no-spiral secondary branches and their branches of the two kinds of nanostructures are all capped by a spherical catalyst particle, which is an indication of VLS growth mechanism. These as-prepared ZnTe nanostructures perhaps have potential applications in optoelectronics due to their unique geometric configurations.
1. Introduction Nowadays, one-dimentional (1D) nanoscale semiconductors are attracting more and more attention due to their unique properties and their potential applications in nanoelectronics, optoelectronics and photovoltaic devices. It is well-known that the electronic and optical properties of semiconductor nanocrystals are greatly influenced by their crystal structure as well as shape and size. Recently, it has been thought that hierarchical nanostructures with complex morphology may have novel properties due to their unique geometric configurations. Furthermore, these hierarchical nanostructures have micrometer-scale and they can be used as building units in functional nanodevices. Therefore, many material researchers have devoted themselves to design and synthesize of hierarchical nanostructures. Zhang et al. [1] have fabricated hierarchical ZnO nanostructure through a thermal evaporation of Zn and ZnO powders. Leu et al. [2] have prepared Zn/ZnO core shelled hierarchical structure by direct annealing. Wang et al. [3] have obtained six-fold-symmetrical hierarchical ZnO nanostructure arrays via a two-step vapor-phase transport method. Wu and Hu et al. [4] have obtained six-fold-symmetrical AlN hierarchical nanostructures through the chemical reaction between AlCl3 and NH3.
⁎
ZnTe as an important Ⅱ-Ⅵ group semiconductor material with direct and wide band gap (Eg=2.28 eV, 300 K) has important applications in electronics and optoelectronics. Many ZnTe nanostructures have been obtained via various methods. Jiang et al. [5] have grown zinc-blendestructured ZnTe twinned nanowires and uniform nanoribbons by hydrogen assisted thermal evaporation method employing Au as catalyst. Geng et al. [6] have prepared multilayer superstructures of singlecrystalline ZnTe nanowire films on silicon (100) wafers through placing Te and Zn powder in different location in a horizontal quartz tube. Li et al. [7] have synthesized single-crystalline ZnTe nanowire arrays by the pulsed electrochemical deposition. Sadowski et al. [8] have grown ZnTe nanowires on GaAs (100) substrate by Au-catalyzed molecular beam epitaxy. Park et al. [9] have obtained ZnTe nanoflowers, nanodots, and nanorods via a solvothermal method. Prasad et al. [10] have prepared ZnTe nanowires via a solvothermal method. Gilad Reut et al. [11] have demonstrated the vapor−liquid−solid growth of guided horizontal ZnTe nanowires displaying p-type behavior on four different planes of sapphire. However, these nanostructures are simple wires, belts or rods rather than ZnTe nanostructures with complex morphology. The same method has been prepared some nanostructures, such as nanosheets and nanowires of otherⅡ-Ⅵ group semiconductors [12,13].
Corresponding author. E-mail address: [email protected] (J. Li).
http://dx.doi.org/10.1016/j.ceramint.2017.06.001 Received 18 April 2017; Received in revised form 24 May 2017; Accepted 2 June 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Li, J., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.06.001
Ceramics International xxx (xxxx) xxx–xxx
J. Li, Y. Ma
Table 1 The different experimental conditions of sample A and B. sample
Mole ratio of Bi and ZnTe
Distance of the source and substrate
The maximal deposition current
Sample A Sample B
1:0.05 1:0.06
12 mm 10 mm
120 A 125 A
As reported in this article, we have successfully prepared novel sixfold symmetrical spiral flower-like hyperbranched hierarchical ZnTe nanostructures via a simple one-step thermal evaporation of a mixture of ZnTe and Bi powder. 2. Experimental section The preparation of the novel spiral flower-like hyperbranched ZnTe nanostructures was carried out in a conventional vacuum thermal evaporation system. A mixture of ZnTe (99.99﹪) and Bi (99.999﹪) powder by a certain mole ratio was put in a home-made molybdenum boat, which was fixed on the electrode in order to be heated to vapor the source material. The indium tin oxide (ITO) glass substrates were placed above the molybdenum boat at a distance of about 10 or 12 mm away from the source material. When the system was evacuated down to 5×10−3 Pa, the heating current was turned on and increased with the rate of 3.33 A/min. When the heating current increased to 110 A, it can be seen from the observation window that transparent glass substrates became opaque, indicating the growth started. The maximal deposition current was 120A-125A. The deposition time was 10 min. The black red product was collected on the substrate.(Table 1). The products were characterized by X-ray diffraction (XRD, Mac Science M18XHF22-SRA with Cu Ka radiation) and scanning electron microscopy (SEM, LEO1430VP) equipped with an energy-dispersive Xray spectroscopy (EDS) analyzer attachment. The detailed microstructure analysis was performed using high-resolution transmission electron microscopy (HRTEM, JEOLJEM-2100), transmission electron microscopy (TEM, Hitachi H-600) and selected area electronic diffraction (SAED).
Fig. 1. XRD spectrum of the products.
rotating angles of petals may be different, and rotating direction is anticlockwise or clockwise. Also, some structure is not symmetrical to centre of flower. The diagonal size of the as-prepared nanostructures is around 40 µm. Fig. 2(c) shows SEM image of a typical flower-like structure, which displays that the ZnTe nanostructures have windmilllike outer shape. In fact, it can be seen that it is a hyperbranched structure with six-fold symmetry after careful observation. Fig. 2(d) is a high-magnification SEM image of the boxed region 1 in Fig. 2(c), which reveals the central area of the flower-like structure. Fig. 2(e) is a highmagnification SEM image of the boxed region 2 in Fig. 2(c), clearly revealing the hyperbranched structure. Fig. 2(f) is a high-magnification SEM image of the boxed region 3 in Fig. 2(c), showing the top of secondary branches. Fig. 3(a) and (b) are the SEM images of the boxed region 4 in Fig. 2(b) with higher magnification. Fig. 3(a) shows the structures that have not grown completely and are in the process of growth. Fig. 3(b) displays they have a three longer and three shorter secondary branches. Fig. 3(c) is an EDX spectrum taken from the spherical particle of the boxed region 1 in Fig. 3(b). It is obvious that the composition of spherical particle is mainly Bi catalyst. Fig. 3(d) is an EDX spectrum of the boxed region 2 in Fig. 3(b), indicating that the product is mainly composed of Zn and Te with a molar ratio of about 1:1. A trace amount impurity of O was detected in Fig. 3(c) and Fig. 3(d), which probably comes from oxidization of products. Fig. 4 displays the SEM images of ZnTe sample B. Fig. 4(a) shows a low-magnification SEM image, from which we can see several snowflake-like nanostructures. The rest is some smaller structures that can not be seen clearly. Fig. 4(b) is a top-view image of a single snowflake-like nanostructure, which has six branches. The diagonal size of the as-prepared nanostructures is around 40 µm. Fig. 4(c) and Fig. 4(d) shows are the images of nanostructures that lie down so that the image that we can see is the side-looking picture. Through this picture, we can estimate the rotating angle is about 6.67 degree. Fig. 4(e) and Fig. 4(f) are structures of sample that is in the process of growth. It has six symmetrical secondary branches with a spherical particle on the top. Fig. 5(a) and Fig. 5(b) show the TEM images of ZnTe hyperbranched nanostructures, revealing the branches are very dense. The spherical catalyst particles capped on the branches can be seen very clearly in Fig. 5(b). Fig. 5(c) is a HRTEM image of ZnTe samples,
3. Results and discussion 3.1. Crystal structure analysis The phase structure of the synthesized product was analyzed by Xray diffraction. Fig. 1 shows the XRD spectrum of sample A. Besides the peaks from the catalyst Bi, other peaks are all indexed to zincblende-structured ZnTe with lattice constants a = 0.3253 nm and c = 0.5209 nm (JCPDS 65-5730), which indicates that the product is zincblende-structured ZnTe. The relatively strong (111) peak implies that the ZnTe nanostructures have a preferred orientation along the < 111 > direction. 3.2. Morphology, composition and microstructure analysis The SEM images of windmill-like ZnTe nanostructures, namely sample A are shown in Fig. 2. Fig. 2(a) is a low-magnification SEM image, giving an overall view of the product. It can be seen that the product consists of more than twenty flower-like nanostructures and a large amount of structures that have not grown completely and may be in the process of growth. From Fig. 2(b), we can see the flower-like nanostructures more clearly. They all have six petals, however, the
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Fig. 2. SEM images of windmill-like ZnTe nanostructures.
then they reach the substrate and recombine to form ZnTe wires under the induction of Bi. With the process of experiment going on, three branches grow from the stem prior and another three secondary branches which are shorter grow then, as indicated in Fig. 6(e). Another case is that six secondary branches have the same length, as indicated in Fig. 6(h). It is worthy noting that the stems and branches of these nanostructures are capped with spherical catalyst particles, which is an indication of the so-called VLS growth mechanism. Fig. 7 is the growth process schematic diagram of ZnTe hyperbranched nanostructures driven by combination of VLS and screw-
which shows clear lattice strings with spacings of 0.43 nm, which can be assigned to zinc-blende ZnTe {220} planes, indicating the growth direction of this ZnTe branch is < 110 > . The SAED images, as shown in Fig. 5(d), reveals the high crystallinity nature of the as-prepared ZnTe samples. 3.3. Discussion of growth mechanism Fig. 6 depicts the VLS growth process schematic diagram of ZnTe hyperbranched nanostructures. First, bismuth (Bi) deposits on the substrate. Then, ZnTe vapor were decomposed to Zn and Te vapor
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Fig. 3. (a)The structures that have not grown completely and are in the process of growth and (b) SEM image of high magnification; (c) and (d) EDX spectrum taken from the boxed region 1 and region 2 in (b).
−
dislocation-driven mechanism [14–19]. On the area of substrate having no Bi catalysts, a screw-dislocation is introduced and when the temperature reached the sublimation point of ZnTe, ZnTe nanowires grow with the assistance of screw-dislocation. With the process of experiment going on, after Bi catalysts stick to the nanowires, secondary branches grow from the stem. Here, three kinds of situation are divided. First case is that the secondary branches rotate clockwise around stems from the end to the top of the stems and their lengths become shorter gradually, as indicated in Fig. 7(f). Second case is that the secondary branches rotate clockwise around stem and their lengths become shorter gradually from the end to the top of the stems, which is indicated in Fig. 7(i). Third case is that the secondary branches rotate clockwise around stem from the end to the top of the stems and their lengths are constant, which is indicated in Fig. 7(l). Fig. 8 describes detailed schematic diagram of a secondary branch. Several third and fourth branches grow up from the secondary branch. According to the symmetry of crystal growth, combination of SEM images and XRD spectra of ZnTe samples, we regards that the main stem grows along [111] direction. The secondary branches
grows [1 10] direction, while the growth direction of the third − branches becomes [01 1] or [111]. The fourth branches have preferred − orientation along [111] or [10 1] direction. The results are consistent with the XRD spectra and SAED images. We think that the stems grow followed by a screw dislocation-driven mechanism. The growth of quaternary branches follows the as-known catalyst-assisted VLS growth mechanism. The difference of the nanostructures with spiral secondary branches and the nanostructures with no-spiral secondary branches nanostructures in morphology lies in that their different growth mechanism. For the spiral nanostructures, screw dislocation rather than VLS mechanism leads to the growth of stems, which can be confirmed by following three points: First, there is no catalyst particle on the top of stems of spiral nanostructures. Second, their secondary branches are spiral, because the stems driven by screw dislocation are warped. Third, their sizes are bigger than that of no-spiral nanostructures driven by VLS mechanism. These ZnTe nanostructures perhaps have potential applications in optoelectronics due to their unique geometric configurations.
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Fig. 4. SEM images of another ZnTe sample.
4. Conclusions
among which secondary branches grow spirally around stems. These novel ZnTe nanostructures were addressed by the vapor–liquid–solid (VLS) and screw-dislocation-driven growth mechanism. They perhaps have potential applications in optoelectronics due to their unique geometric configurations.
In conclusion, novel flower-like hyperbranched ZnTe nanostructures were prepared by catalyst-assisted vacuum thermal evaporation method. The results demonstrate the ZnTe nanostructure are single crystalline with zinc-blende structure. Their morphologies are six-fold symmetric hierarchical nanostructures consisted of quartic branches,
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Fig. 5. (a) and (b) show the TEM images of ZnTe hyperbranched nanostructures; (c) HRTEM image of ZnTe samples; (d) SAED images.
Fig. 6. The VLS growth process schematic diagram of ZnTe hyperbranched nanostructures.
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Fig. 7. The growth process schematic diagram of ZnTe hyperbranched nanostructures driven by combination of VLS and screw-dislocation-driven mechanism.
[5] [6]
[7] [8]
[9] [10] [11]
Fig. 8. The detailed schematic diagram of a secondary branch.
[12]
Acknowledgements
[13]
This work was supported by the University Scientific Research Plan Key Projects Foundation of Xinjiang Uygur Autonomous Region (No. XJEDU2016I018).
[14]
[15]
References
[16] [1] Y. Wu, Z.H. Xi, G.M. Zhang, J.L. Zhang, D.Z. Guo, Fabrication of hierarchical zinc oxide nanostructures through multistage gas-phase reaction, Cryst. Growth Des. 8 (2008) 2646. [2] C.Y. Kuan, M.H. Hon, J.M. Chou, I.C. Leu, Growth behavior, microstructure characterization, and field-emission property of 6-fold hierarchical Zn/ZnO structures prepared by direct annealing, Cryst. Growth Des. 9 (2009) 813. [3] Z.Q. Wang, J.F. Gong, Y. Su, Y.W. Jiang, S.G. Yang, Six-fold-symmetrical hierarchical ZnO nanostructure arrays: synthesis, characterization, and field emission properties, Cryst. Growth Des. 10 (2010) 2455. [4] F. Zhang, Q. Wu, X.B. Wang, N. Liu, J. Yang, Y.M. Hu, L.S. Yu, X.Z. Wang, Z. Hu,
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J.M. Zhu, 6-fold-symmetrical AlN hierarchical nanostructures: synthesis and fieldemission properties, J. Phys. Chem. C 113 (2009) 4053. Q.F. Menga, C.B. Jianga, S.X. Maob, Temperature-dependent growth of zincblende-structured ZnTe nanostructures, J. Cryst. Grow. (2008). Y.J. Guo, B.Y. Geng, L. Zhang, F.M. Zhan, J.H. You, Fabrication, characterization, and strong exciton emission of multilayer ZnTe nanowire superstructures, J. Phys. Chem. C 112 (2008) 20307. L. Li, Y.W. Yang, X.H. Huang, G.H. Li, L. Zhang, Fabrication and characterization of single-crystalline ZnTe nanowire arrays, J. Phys. Chem. B 109 (2005) 12394. E. Janik, J. Sadowski, P. Dłużewski, S. Kret, L.T. Baczewski, A. Petroutchik, E. Łusakowska, J. Wróbel, W. Zaleszczyk, G. Karczewski, T. Wojtowicz, ZnTe nanowires grown on GaAs substrates by molecular beam epitaxy, Appl. Phys. Lett. 89 (2006) 133114. S.H. Lee, Y.J. Kim, J. Park, Shape evolution of znte nanocrystals: nanoflowers, nanodots, and nanorods, Chem. Mater. 19 (2007) 4670. K.T. Yong, Y. Sahoo, H. Zeng, M.T. Swihart, J.R. Minter, P.N. Prasad, Formation of ZnTe nanowires by oriented attachment, Chem. Mater. 19 (2007) 4108. Gilad Reut, Eitan Oksenberg, Ronit Popovitz-Biro, Katya Rechav, Ernesto Joselevich, Guided growth of horizontal p-Type ZnTe nanowires, J. Phys. Chem. C. 120 (2016) 17087–17100. L.Y. Yang, R. Wu, J. Li, Y.F. Sun, J.K. Jian, CdTe nanosheets and pine-like hyperbranched nanostructures prepared by a modified film technique: catalystassisted vacuum thermal evaporation, Mater. Lett. 65 (2011) 17–20. L.Y. Yang, W.J. Wang, B. Song, R. Wu, J. Li, Y.F. Sun, F. Shang, X.L. Chen, J.K. Jian, Novel route to scalable synthesis of II-VI semiconductor nanowires: catalyst-assisted vacuum thermal evaporation, J. Cryst. Growth 312 (2010) 2852–2856. S. Jin, M.J. Bierman, S.A. Morin, A new twist on nanowire formation: screwdislocation-driven growth of nanowires and nanotubes, J. Phys. Chem. Lett. 1 (2010) 1472. J. Zhu, H.L. Peng, A.F. Marshall, D.M. Barnett, W.D. Nix, Y. Cui, Formation of chiral branched nanowires by the Eshelby twist, Nat. Nanotechnol. 3 (2008) 477. H.M. Cuppen, W.S. Graswinckel, H. Meekes, Screw dislocations on polycenes: a requirement for crystallization, Cryst. Growth Des. 4 (2004) 1351. Y.K. Albert Lau, D.J. Chernak, M.J. Bierman, S. Jin, Formation of PbS nanowire pine trees driven by screw dislocations, J. Am. Chem. Soc. 131 (2009) 16461. S.A. Morin, S. Jin, Screw dislocation-driven epitaxial solution growth of ZnO nanowires seeded by dislocations in GaN substrates, Nano Lett. 10 (2010) 3459. Fei Meng, Marc Estruga, Audrey Forticaux, Stephen A. Morin, Qiang Wu, Zheng Hu, Song Jin, Formation of stacking faults and the screw dislocation - driven growth: a case study of aluminum nitride nanowires, ACS Nano 7 (12) (2015) 11369.
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Novel flower-like hyperbranched ZnTe nanostructures prepared via catalyst-assisted vacuum thermal evaporation ⁎
Jin Li , Yan Ma College of Physics Science and Technology, Xinjiang University, Urumqi 830046, Xinjiang, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Zinc telluride Hyperbranched nanostructure Vapor-liquid-solid (VLS) growth mechanism Screw-dislocation-driven mechanism
Novel flower-like hyperbranched ZnTe nanostructures were prepared by catalyst-assisted vacuum thermal evaporation method. Various analysis techniques including X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), highresolution transmission electron microscopy (HRTEM) and selected area electronic diffraction (SAED) were conducted on the as-prepared products. The XRD analysis demonstrates the ZnTe nanostructures are high pure single crystalline of zinc-blende structure. These novel ZnTe nanostructures are grown by a combination of the vapor–liquid–solid (VLS) growth mechanism and screw-dislocation-driven mechanism. The nanostructures formed by VLS mechanism have smaller sizes of several to ten micrometers and secondary branches with diameters of 100 nm. The nanostructures combined VLS and screw-dislocation-driven growth mechanism have a diagonal size of 40 µm and they consist of quartic branches, among which the secondary branches are spiral. The stems of the nanostructures with no-spiral secondary branches and their branches of the two kinds of nanostructures are all capped by a spherical catalyst particle, which is an indication of VLS growth mechanism. These as-prepared ZnTe nanostructures perhaps have potential applications in optoelectronics due to their unique geometric configurations.
1. Introduction Nowadays, one-dimentional (1D) nanoscale semiconductors are attracting more and more attention due to their unique properties and their potential applications in nanoelectronics, optoelectronics and photovoltaic devices. It is well-known that the electronic and optical properties of semiconductor nanocrystals are greatly influenced by their crystal structure as well as shape and size. Recently, it has been thought that hierarchical nanostructures with complex morphology may have novel properties due to their unique geometric configurations. Furthermore, these hierarchical nanostructures have micrometer-scale and they can be used as building units in functional nanodevices. Therefore, many material researchers have devoted themselves to design and synthesize of hierarchical nanostructures. Zhang et al. [1] have fabricated hierarchical ZnO nanostructure through a thermal evaporation of Zn and ZnO powders. Leu et al. [2] have prepared Zn/ZnO core shelled hierarchical structure by direct annealing. Wang et al. [3] have obtained six-fold-symmetrical hierarchical ZnO nanostructure arrays via a two-step vapor-phase transport method. Wu and Hu et al. [4] have obtained six-fold-symmetrical AlN hierarchical nanostructures through the chemical reaction between AlCl3 and NH3.
⁎
ZnTe as an important Ⅱ-Ⅵ group semiconductor material with direct and wide band gap (Eg=2.28 eV, 300 K) has important applications in electronics and optoelectronics. Many ZnTe nanostructures have been obtained via various methods. Jiang et al. [5] have grown zinc-blendestructured ZnTe twinned nanowires and uniform nanoribbons by hydrogen assisted thermal evaporation method employing Au as catalyst. Geng et al. [6] have prepared multilayer superstructures of singlecrystalline ZnTe nanowire films on silicon (100) wafers through placing Te and Zn powder in different location in a horizontal quartz tube. Li et al. [7] have synthesized single-crystalline ZnTe nanowire arrays by the pulsed electrochemical deposition. Sadowski et al. [8] have grown ZnTe nanowires on GaAs (100) substrate by Au-catalyzed molecular beam epitaxy. Park et al. [9] have obtained ZnTe nanoflowers, nanodots, and nanorods via a solvothermal method. Prasad et al. [10] have prepared ZnTe nanowires via a solvothermal method. Gilad Reut et al. [11] have demonstrated the vapor−liquid−solid growth of guided horizontal ZnTe nanowires displaying p-type behavior on four different planes of sapphire. However, these nanostructures are simple wires, belts or rods rather than ZnTe nanostructures with complex morphology. The same method has been prepared some nanostructures, such as nanosheets and nanowires of otherⅡ-Ⅵ group semiconductors [12,13].
Corresponding author. E-mail address: [email protected] (J. Li).
http://dx.doi.org/10.1016/j.ceramint.2017.06.001 Received 18 April 2017; Received in revised form 24 May 2017; Accepted 2 June 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Li, J., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.06.001
Ceramics International xxx (xxxx) xxx–xxx
J. Li, Y. Ma
Table 1 The different experimental conditions of sample A and B. sample
Mole ratio of Bi and ZnTe
Distance of the source and substrate
The maximal deposition current
Sample A Sample B
1:0.05 1:0.06
12 mm 10 mm
120 A 125 A
As reported in this article, we have successfully prepared novel sixfold symmetrical spiral flower-like hyperbranched hierarchical ZnTe nanostructures via a simple one-step thermal evaporation of a mixture of ZnTe and Bi powder. 2. Experimental section The preparation of the novel spiral flower-like hyperbranched ZnTe nanostructures was carried out in a conventional vacuum thermal evaporation system. A mixture of ZnTe (99.99﹪) and Bi (99.999﹪) powder by a certain mole ratio was put in a home-made molybdenum boat, which was fixed on the electrode in order to be heated to vapor the source material. The indium tin oxide (ITO) glass substrates were placed above the molybdenum boat at a distance of about 10 or 12 mm away from the source material. When the system was evacuated down to 5×10−3 Pa, the heating current was turned on and increased with the rate of 3.33 A/min. When the heating current increased to 110 A, it can be seen from the observation window that transparent glass substrates became opaque, indicating the growth started. The maximal deposition current was 120A-125A. The deposition time was 10 min. The black red product was collected on the substrate.(Table 1). The products were characterized by X-ray diffraction (XRD, Mac Science M18XHF22-SRA with Cu Ka radiation) and scanning electron microscopy (SEM, LEO1430VP) equipped with an energy-dispersive Xray spectroscopy (EDS) analyzer attachment. The detailed microstructure analysis was performed using high-resolution transmission electron microscopy (HRTEM, JEOLJEM-2100), transmission electron microscopy (TEM, Hitachi H-600) and selected area electronic diffraction (SAED).
Fig. 1. XRD spectrum of the products.
rotating angles of petals may be different, and rotating direction is anticlockwise or clockwise. Also, some structure is not symmetrical to centre of flower. The diagonal size of the as-prepared nanostructures is around 40 µm. Fig. 2(c) shows SEM image of a typical flower-like structure, which displays that the ZnTe nanostructures have windmilllike outer shape. In fact, it can be seen that it is a hyperbranched structure with six-fold symmetry after careful observation. Fig. 2(d) is a high-magnification SEM image of the boxed region 1 in Fig. 2(c), which reveals the central area of the flower-like structure. Fig. 2(e) is a highmagnification SEM image of the boxed region 2 in Fig. 2(c), clearly revealing the hyperbranched structure. Fig. 2(f) is a high-magnification SEM image of the boxed region 3 in Fig. 2(c), showing the top of secondary branches. Fig. 3(a) and (b) are the SEM images of the boxed region 4 in Fig. 2(b) with higher magnification. Fig. 3(a) shows the structures that have not grown completely and are in the process of growth. Fig. 3(b) displays they have a three longer and three shorter secondary branches. Fig. 3(c) is an EDX spectrum taken from the spherical particle of the boxed region 1 in Fig. 3(b). It is obvious that the composition of spherical particle is mainly Bi catalyst. Fig. 3(d) is an EDX spectrum of the boxed region 2 in Fig. 3(b), indicating that the product is mainly composed of Zn and Te with a molar ratio of about 1:1. A trace amount impurity of O was detected in Fig. 3(c) and Fig. 3(d), which probably comes from oxidization of products. Fig. 4 displays the SEM images of ZnTe sample B. Fig. 4(a) shows a low-magnification SEM image, from which we can see several snowflake-like nanostructures. The rest is some smaller structures that can not be seen clearly. Fig. 4(b) is a top-view image of a single snowflake-like nanostructure, which has six branches. The diagonal size of the as-prepared nanostructures is around 40 µm. Fig. 4(c) and Fig. 4(d) shows are the images of nanostructures that lie down so that the image that we can see is the side-looking picture. Through this picture, we can estimate the rotating angle is about 6.67 degree. Fig. 4(e) and Fig. 4(f) are structures of sample that is in the process of growth. It has six symmetrical secondary branches with a spherical particle on the top. Fig. 5(a) and Fig. 5(b) show the TEM images of ZnTe hyperbranched nanostructures, revealing the branches are very dense. The spherical catalyst particles capped on the branches can be seen very clearly in Fig. 5(b). Fig. 5(c) is a HRTEM image of ZnTe samples,
3. Results and discussion 3.1. Crystal structure analysis The phase structure of the synthesized product was analyzed by Xray diffraction. Fig. 1 shows the XRD spectrum of sample A. Besides the peaks from the catalyst Bi, other peaks are all indexed to zincblende-structured ZnTe with lattice constants a = 0.3253 nm and c = 0.5209 nm (JCPDS 65-5730), which indicates that the product is zincblende-structured ZnTe. The relatively strong (111) peak implies that the ZnTe nanostructures have a preferred orientation along the < 111 > direction. 3.2. Morphology, composition and microstructure analysis The SEM images of windmill-like ZnTe nanostructures, namely sample A are shown in Fig. 2. Fig. 2(a) is a low-magnification SEM image, giving an overall view of the product. It can be seen that the product consists of more than twenty flower-like nanostructures and a large amount of structures that have not grown completely and may be in the process of growth. From Fig. 2(b), we can see the flower-like nanostructures more clearly. They all have six petals, however, the
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Fig. 2. SEM images of windmill-like ZnTe nanostructures.
then they reach the substrate and recombine to form ZnTe wires under the induction of Bi. With the process of experiment going on, three branches grow from the stem prior and another three secondary branches which are shorter grow then, as indicated in Fig. 6(e). Another case is that six secondary branches have the same length, as indicated in Fig. 6(h). It is worthy noting that the stems and branches of these nanostructures are capped with spherical catalyst particles, which is an indication of the so-called VLS growth mechanism. Fig. 7 is the growth process schematic diagram of ZnTe hyperbranched nanostructures driven by combination of VLS and screw-
which shows clear lattice strings with spacings of 0.43 nm, which can be assigned to zinc-blende ZnTe {220} planes, indicating the growth direction of this ZnTe branch is < 110 > . The SAED images, as shown in Fig. 5(d), reveals the high crystallinity nature of the as-prepared ZnTe samples. 3.3. Discussion of growth mechanism Fig. 6 depicts the VLS growth process schematic diagram of ZnTe hyperbranched nanostructures. First, bismuth (Bi) deposits on the substrate. Then, ZnTe vapor were decomposed to Zn and Te vapor
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Fig. 3. (a)The structures that have not grown completely and are in the process of growth and (b) SEM image of high magnification; (c) and (d) EDX spectrum taken from the boxed region 1 and region 2 in (b).
−
dislocation-driven mechanism [14–19]. On the area of substrate having no Bi catalysts, a screw-dislocation is introduced and when the temperature reached the sublimation point of ZnTe, ZnTe nanowires grow with the assistance of screw-dislocation. With the process of experiment going on, after Bi catalysts stick to the nanowires, secondary branches grow from the stem. Here, three kinds of situation are divided. First case is that the secondary branches rotate clockwise around stems from the end to the top of the stems and their lengths become shorter gradually, as indicated in Fig. 7(f). Second case is that the secondary branches rotate clockwise around stem and their lengths become shorter gradually from the end to the top of the stems, which is indicated in Fig. 7(i). Third case is that the secondary branches rotate clockwise around stem from the end to the top of the stems and their lengths are constant, which is indicated in Fig. 7(l). Fig. 8 describes detailed schematic diagram of a secondary branch. Several third and fourth branches grow up from the secondary branch. According to the symmetry of crystal growth, combination of SEM images and XRD spectra of ZnTe samples, we regards that the main stem grows along [111] direction. The secondary branches
grows [1 10] direction, while the growth direction of the third − branches becomes [01 1] or [111]. The fourth branches have preferred − orientation along [111] or [10 1] direction. The results are consistent with the XRD spectra and SAED images. We think that the stems grow followed by a screw dislocation-driven mechanism. The growth of quaternary branches follows the as-known catalyst-assisted VLS growth mechanism. The difference of the nanostructures with spiral secondary branches and the nanostructures with no-spiral secondary branches nanostructures in morphology lies in that their different growth mechanism. For the spiral nanostructures, screw dislocation rather than VLS mechanism leads to the growth of stems, which can be confirmed by following three points: First, there is no catalyst particle on the top of stems of spiral nanostructures. Second, their secondary branches are spiral, because the stems driven by screw dislocation are warped. Third, their sizes are bigger than that of no-spiral nanostructures driven by VLS mechanism. These ZnTe nanostructures perhaps have potential applications in optoelectronics due to their unique geometric configurations.
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Fig. 4. SEM images of another ZnTe sample.
4. Conclusions
among which secondary branches grow spirally around stems. These novel ZnTe nanostructures were addressed by the vapor–liquid–solid (VLS) and screw-dislocation-driven growth mechanism. They perhaps have potential applications in optoelectronics due to their unique geometric configurations.
In conclusion, novel flower-like hyperbranched ZnTe nanostructures were prepared by catalyst-assisted vacuum thermal evaporation method. The results demonstrate the ZnTe nanostructure are single crystalline with zinc-blende structure. Their morphologies are six-fold symmetric hierarchical nanostructures consisted of quartic branches,
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Fig. 5. (a) and (b) show the TEM images of ZnTe hyperbranched nanostructures; (c) HRTEM image of ZnTe samples; (d) SAED images.
Fig. 6. The VLS growth process schematic diagram of ZnTe hyperbranched nanostructures.
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Fig. 7. The growth process schematic diagram of ZnTe hyperbranched nanostructures driven by combination of VLS and screw-dislocation-driven mechanism.
[5] [6]
[7] [8]
[9] [10] [11]
Fig. 8. The detailed schematic diagram of a secondary branch.
[12]
Acknowledgements
[13]
This work was supported by the University Scientific Research Plan Key Projects Foundation of Xinjiang Uygur Autonomous Region (No. XJEDU2016I018).
[14]
[15]
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