Catalyst-free vapor phase growth of ultralong SnSe single-crystalline

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Catalyst-free vapor phase growth of ultralong SnSe single-crystalline nanowires Jiao Liu, Jikang Jian, Zhiqiang Yu, Zhihua Zhang, Binglei Cao, and Bingsheng Du Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01119 • Publication Date (Web): 20 Oct 2017 Downloaded from http://pubs.acs.org on October 22, 2017

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Catalyst-free vapor phase growth of ultralong SnSe single-crystalline nanowires Jiao Liu1, Jikang Jian1*, Zhiqiang Yu2, Zhihua Zhang3, Binglei Cao1, Bingsheng Du1 1

School of Physics and Optoelectronic Engineering, Guangdong University of Technology,

Guangzhou 510006, China 2

Department of Physics, Harbin Institute of Technology, Harbin 150001, China

3

Liaoning Key Materials Laboratory for Railway, School of Materials Science and Engineering,

Dalian Jiao tong University, Dalian 116028, China Abstract Herein, high quality ultralong tin selenide (SnSe) nanowires (NWs) have been synthesized via physical vapor deposition (PVD) without catalyst. The length of the synthesized SnSe NWs are hundreds of microns and even up to one millimeter while the mean diameter is about 270 nm and the aspect ratio of ultralong SnSe NWs can be over 3000. The microstructural characterizations indicate that the SnSe NWs are well-crystalized single crystals with growth direction along the normal of {011} planes. The formation of the SnSe NWs is addressed by an oriented onedimensional growth driven by the dynamic factors in the vapor-solid process. The near infra-red optical band gap of the SnSe NWs has been determined.

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Two-dimensional (2D) materials have attracted tremendous attention due to great importance in both fields of science and technology.1-3 Rationally, layered-crystal structures compounds are thought to be ideal systems to achieve 2D structure through breaking the weak van der Waals interaction between the layers.4 On the other hand, layered materials have intrinsic 2D growth habit in equilibrious or quasi-equilibrious conditions, which favorites the formation of 2D structures, too.5,6 Then, this will arise one interesting question: how to grow one-dimensional (1D) from a parent layered materials? It is well known that, 1D nanostructures, such as nanotubes (NTs), nanowires (NWs) and nanobelts (NBs),7,8 are important low-dimensional system with great scientific and technological significance. For most inorganic materials, vapor phase 1D growth has been well achieved via a catalyst-assisted vapor-liquid-solid (VLS) strategy that uses nanosized liquid droplets (i.e. melt catalyst particles) to confine the lateral growth along two directions and facilitates the longitudinal growth along one direction.9 Another extensively used strategy to 1D nanostructures is so-called vapor solid (VS) procedure in which the uniaxial growth of materials goes through a direct transition from vapor to solid phase without the assistant of catalyst.10,11 It is believed that the proper oriented growth factors of the vapor system including pressure, flow, temperature field distribution, etc., will play key roles to control the growth behavior of the materials. Besides of the growth parameters, researchers have demonstrated that the inherent crystalline structure is another important factor to affect the growth of nanostructures, as shown in the growth of ZnO nanostructures12 with diverse morphologies. To date, the mainstream semiconductors including Si,13 Ge,14 Ⅲ-Ⅳcompounds (GaN, AlN, GaAs, InAs, etc.)15,16 and Ⅱ-Ⅵ compounds (ZnO, ZnS, CdS, CdSe, etc.)17,18 have been successfully made into 1D form by VLS and/or VS route. However, well-controlled uniaxial growth of layer-structured compounds still challenged in vapor growth systems.

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Although some works reported VLS-grown 1D nanostructures (nanowires or nanoribbons) of Bi2Te3 and Bi2Se3 with typical layered crystalline structure, as seen in these results, the 1D morphology of the products was not properly defined by catalyst nanoparticles.19,20 SnSe is a narrow band-gap semiconductor21,22 with potential applications in infrared photodetectors and photovoltaic devices, but has been rarely focused for a long time compared with HgCdTe- and InAs-based infrared materials although it has obvious advantages from the point of view of environment and cost.23-25 Recently, SnSe has attracted increasing attention owing to its excellent thermoelectric performance.26,27 The study on the structure and temperature transformation of SnSe pointed out that room-temperature (300 K) SnSe has orthorhombic phase with a space group with Pnma (No. 62) and lattice constants of a= 11.502 Å, b= 4.153 Å, and c= 4.450 Å.28 Perspective view along b axis (Figure S2a) clearly reveals that Sn-Se bi-atomic slabs in the b-c plane are stacked along a axial mainly via van der Waals interaction.22,29,30 The Sn-Se bonding in one slab is strong ionic/covalent interaction. Recently, He et al. reported a Bicatalyzed growth of SnSe nanorods (NRs) arrays by a CVD route, in which dense 2D branches are found on the [100]-oriented 1D core of the NR, suggesting the lateral growth simultaneously accompanying with the 1D growth.31 Then, F.K. Butt et al. reported the synthesis of straight and smooth SnSe NWs via an Au-catalyzed VLS growth process.32 Up to now, catalyst-free vapor phase growth of 1D SnSe nanostructures has not yet been achieved. Herein, we report large scale catalyst-free synthesis of ultralong SnSe NWs via the vapor-phase growth process. The detailed structural characterizations show that as-synthesized SnSe NWs are high-quality single crystals with lengths up to millimeter scale and the aspect ratio over 3000, exhibiting a typical 1D growth features.

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Figure S2b shows the XRD pattern of the as-synthesized samples, in which all Bragg reflections can be well indexed to orthorhombic SnSe (International Centre for Diffraction Data(ICDD), Powder Diffraction File (PDF) No.48-1224), indicating that the as-collected products are singlephase SnSe. Obviously, the intensity of (400) diffraction peak is much stronger than those of other reflections, revealing a strong (100) preferred orientation facing the X-ray incident direction. The relative intensities of the XRD reflections are different with the previous data on SnSe nanorods31 and SnSe NWs32 that have the strongest reflection of (111), implying that the products synthesized here have different growth directions. The low-magnification SEM image in Figure 1a gives an overview of the products transferred to SEM conductive adhesives from the Si substrate, clearly showing that millimeter-length NWs have been synthesized. Further, Figure S3 gives an optical image of the as-grown products and one can see clearly that the large scale synthesis of SnSe NWs is achieved. SEM image (Figure 1b) recorded on the edge region of the sample further verify their typical ultra-length characteristics of the SnSe NWs. In addition, some NWs present the wavy shape along their longitudinal direction, indicating the good flexibility of the ultralong NWs. Figure 1c is a low-magnification SEM image of SnSe NWs asgrown on the Si substrate, showing that a large number of NWs randomly distribute on the substrate with high density.

High-magnification SEM image in Figure 1d reveals that the

diameters of the SnSe NWs range from tens to hundreds of nanometers. Note that the top of an individual SnSe NW (inset in Figure 1d) is flat. More detailed SEM observations indicate that most of the NWs have smooth surfaces, flat tops and uniform thickness along their axial directions. No particles can be observed on the tops of the NWs. Figure S4 gives a diameter distribution histogram of as-synthesized SnSe NWs, which reveals that their mean diameter is about 270 nm. So, it means that the aspect ratio of SnSe NWs can be over 3000, exhibiting a

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typical uniaxial growth features in such vapor-phase growth system. It is worth noting that the growth of ultralong SnSe NWs in our study has good reproducibility.

Figure 1. (a) The low-magnification SEM image of ultralong SnSe NWs transferred on SEM conductive adhesives. (b) The SEM image recorded on the edge region of some of ultralong SnSe NWs. (c) SEM image of high-density SnSe NWs as-grown on the Si substrate. (d) Further magnified SEM image of the NWs on the Si substrate (the inset shows the top of a NW). Figure 2a and 2b show a TEM morphological image and the corresponding HRTEM image of a SnSe NW, respectively. Based on the standard data of ICDD PDF NO.48-1224, the (011) and (011) planes of orthorhombic SnSe can be well distinguished along the longitudinal and lateral direction of the NW, respectively, suggesting that the growth direction of the SnSe NW is normal to (011) plane.

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The SAED pattern (inset of Figure 2b) of the NW can be indexed to orthorhombic SnSe with zone axis along [100], disclosing its single-crystal nature. It can be seen that there is a thin amorphous layer coated on the surface of the NW. Figure S5 schematically illustrates crystallographic atom arrangement of the SnSe NW deduced from HRTEM analysis. More HRTEM examinations on another SnSe NW (Figure S6) further indicate that the NW has the same growth direction normal to (011) plane. Such crystallographic feature of the SnSe NW will result in (100) plane preferred parallel to the substrate, which is consistent with the preferred (400) reflection observed in the XRD pattern (Figure S2b). The fast growth direction of the SnSe NWs synthesized here is normal to {011} planes, which is unusual considering the previous report30 on the surface energy of planes in SnSe under the equilibrium condition. It is known that the [100] direction should have the slowest growth rate due to the weak van der Waals interaction between (100) planes of SnSe.30 He et al reported that Bi-assisted VLS growth of cone-like SnSe NRs have [100] growth direction,31 but it can be well understood because that those SnSe NRs were constructed by the stack of lateral expanded {100} planes. Ma et al.30 calculated that {011} planes have a lower surface energy than that of {010} and {001} planes, suggesting that the expanding rate of {011} planes should be lower than that of another two planes. So it is speculated that the growth direction of the SnSe NWs should be mainly affected by the growth dynamic parameters rather than crystal structure. HRTEM analysis also verifies the SnSe NW is a high-quality single crystal. Figure 2c is a TEM bright field image depicting the straight and smooth morphology of a SnSe NW. The elemental distributions in the SnSe NW are investigated by EDS elemental mapping technique. As revealed by Figure 2d, Sn and Se uniformly distribute in the NW, indicating its homogeneous compositions. It is shown that there is trace O impurity in the sample, which may originate from

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the residual oxygen in the growth system. The quantitative elemental analysis by EDS (Figure S7) indicates the NWs are mainly composed of Sn and Se with an atomic ratio of about 48.13% to 41.15%. Taking into account the O impurity in the NW, it can be deduced that Sn atoms are less than the sum of Se and O atoms, meaning the existence of Sn vacancy in the SnSe NW. It has been reported that Sn vacancy is a kind of common intrinsic defect that results in the p-type conductivity of SnSe.27,33 Our electronic transport examination on individual SnSe NWs also revealed their p-type conductivity. The more investigations on the electronic property of the SnSe NWs are being carried out now.

Figure 2. (a) TEM image of a SnSe NW and (b) the corresponding HRTEM image taken from the circled region in (a) (the inset shows the SAED pattern). (c) The TEM image and (d) corresponding EDS elemental mapping images of the SnSe NW.

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Figure 3. (a) Raman spectrum, XPS spectra of Sn 3d (b) and Se 3d (c) of the SnSe NWs Raman scattering spectroscopy was used to examine the photon modes of the SnSe NWs. Figure 3a shows a Raman spectrum of the SnSe NWs, in which four peaks can be clearly observed at 71, 107, 126 and 150 cm-1, respectively. According previous work,22,34 the strongest peak at 107 cm-1 corresponds to B3g phonon mode and the other three peaks are due to Ag phonon modes. The Raman data also confirms orthorhombic crystalline structure of the SnSe NWs. Figure 3b and c display the XPS spectra of SnSe NWs. Sn 3d3/2, Sn 3d5/2 and Se 3d orbital peaks can be observed at 493.9 eV, 485.5 eV and 53.9 eV, respectively, which matches well with the previous reports on SnSe materials.35,36

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In order to understand the growth process of the SnSe NWs, the control experiments were carried out by varying the growth time from 15 min to 180 min with other growth parameters fixed. Figure 4 shows the morphologies of the products grown by 15, 30 and 180 min, respectively. It can be seen that there are a few of SnSe nanoparticles deposited on the substrate after 15–min growth. When the growth time increases to 30 min, the SnSe nanoparticles become larger and some 1D SnSe nanostructures are formed. The growth of the 1D SnSe structures is controlled by so-called VS mechanism that was found in many other semiconducting 1D nanostructures such as ZnO and SnS.37-40 As the deposition time further extended to 180 min, the high-density ultralong NWs are formed on the substrate. The as-synthesized NWs are smooth and straight. Based on the results of the time-dependent growth experiments, we can conclude that the formation of the SnSe NWs is main a spontaneous 1D oriented growth process controlled by the VS mechanism. The schematic growth process of the SnSe NWs is illustrated in Figure 4 and depicted below.

Figure 4. SEM images of the products with different growth durations: (a) 15 min, (b) 30 min, (c) 180 min. Briefly, the vaporized SnSe species are firstly carried to growth region by Ar flow and become supersaturated with the increase of concentration. Then the solid SnSe clusters will precipitate from the saturated vapor-phase system and nucleate to form nanoparticles deposited on the

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substrate (Figure 5a). After that, as illustrated in Figure 5b, the growth of SnSe 1D structure is initiated via the vapor-solid (VS) mechanism and grow along the normal direction of {011} plane. Generally speaking, the outer shape of a crystal grown under the equilibrium condition is controlled by the growth rates of deferent planes, which is determined by the surface energy γ of the planes. Obviously, [100] should be the slowest growth direction due to the weak van der waals bonds between {100} planes. Ma and co-wookers30 reported the 2D vapor-phase growth of SnSe nanosheets with top/bottom surfaces of {100} planes and side surfaces of {011} planes. The authors’ calculation indicates that γ(001)> γ(010)> γ(011), meaning that the growth rate of {011} planes should be slower than those of other two planes.30 In our case, the strong 1D growth direction of the SnSe NW is normal to {011} planes, which suggests that the growth of the NWs is under non-equilibrium condition and mainly controlled by growth dynamic parameters including gas flow, temperature, supersaturation, and so on. Finally, the SnSe species are absorbed by the foregoing deposited nanoparticles and gradually form SnSe ultralong NWs along specific direction (Figure 5c). Such VS growth of NWs41 seriously depends on the growth parameters, which has been reported by other literatures.42,43

Figure 5. (a)-(c) Schematic illustration of the growth process of SnSe NWs.

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Figure 6. (a)UV-vis-NIR absorbance spectrum of SnSe nanowires. (b) and (c) are the corresponding Tauc plots drawn by the direct and indirect band gap modes. It is known that there are both indirect and direct optical absorption in SnSe30,32. The optical band gap of the SnSe nanowires was examined by the UV-Vis-NIR absorbance spectrum (Figure 6a). The optical band gap the SnSe nanowires can be determined using Tauc plot: 44

(αhv) n = B (hv − Eg ) Where α, hv, Eg and B are absorption coefficient, photon energy, optical band gap and a constant, respectively, and n is 2 or 1/2 for the direct or indirect band gap mode. As revealed by Figure 6b and c, the direct and indirect band gaps of for the as-synthesized SnSe nanowires were determined to be 1.07eV and 0.93eV, respectively, which is in good agreement with the previous reports.30,32 The calculated optical band gap of the SnSe NWs implies their potential applications in NIR waveband optoelectronic devices. In summary, single-crystal ultralong SnSe NWs have been synthesized by a catalyst-free vapor phase growth. The length of the NWs can be up to millimeter size with the aspect ratio over 3000, showing strong 1D growth behavior. The formation of the ultralong SnSe NWs indicates that the lateral growth of SnSe has been effectively suppressed through finely tuning the vapor

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phase growth parameters. The growth direction of the NWs is normal to {011} planes of orthorhombic SnSe, which is mainly controlled by the vapor phase growth dynamic process. The optical band gap of the SnSe NWs is determined to be in the NIR waveband, suggesting their promising applications in NIR photodetectors and photovoltaic cells. ASSOCIATED CONTENT Supporting information The experimental section, the temperature dependence on the distance away from the furnace center; the perspective view along b axis of SnSe and XRD pattern of the synthesized SnSe NWs; the optical picture of as-synthesized SnSe NWs; the diameter distribution histogram of SnSe NWs; perspective view (011) plane of SnSe crystal structure; TEM and HRTEM images of SnSe NWs; the energy dispersive X-ray spectroscopy (EDS) of SnSe NWs. AUTHOR INFORMATION Corresponding Author * JiKang Jian: E-mail: [email protected] ACKNOWLEDGMENT This work was financially supported by National Scientific Foundation of China (Grant Nos. 51672051 and 51472052) and Science and Technology Program of Guangzhou, China (201707010251). REFERENCES 1. Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004,306,666-669.

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43. Chueh, Y. L.; Lai, M. W.; Liang, J. Q.; Chou, L. J.; Wang, Z. L. Adv. Funct. Mater. 2006, 16,2243-2251. 44. Butt, F. K.; Cao, C. B.; Khan, W. S.; Ali, Z.; Ahmed, R.; Idrees, F.; Aslam, I.; Tanveer, M.; Li, J. L.; Zaman, S.; Mahmood, T. Mater. Chem. Phys. 2012,137,565-570.

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Crystal Growth & Design

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Catalyst-free vapor phase growth of ultralong SnSe single-crystalline nanowires

Jiao Liu1, Jikang Jian1*, Zhiqiang Yu2, Zhihua Zhang3, Binglei Cao1, Bingsheng Du1

Synopsis Ultralong SnSe NWs were successfully synthesized from vapor phase without the assistant of catalyst. The as-synthesized SnSe NWs are high-quality single crystals with orthorhombic structure. The length of the NWs can be up to millimeter size with high aspect ratio above 3000, showing strong one-dimensional growth trend.

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