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Jun 7, 2018 - inside the TEM, Sn particles smaller than 5 nm can be melted even at .... of Zn vapor is pretty low in the TEM chamber, only a small qua...
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Atomistic Interface Dynamics in Sn-catalyzed growth of Wurtzite and Zinc-blende ZnO nanowires Shuangfeng Jia, Shuaishuai Hu, He Zheng, Yanjie Wei, Shuang Meng, Huaping Sheng, Huihui Liu, Siyuan Zhou, Dongshan Zhao, and Jianbo Wang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00420 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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Atomistic Interface Dynamics in Sn-catalyzed growth of Wurtzite and Zinc-blende ZnO nanowires Shuangfeng Jia,†,* Shuaishuai Hu,† He Zheng,† Yanjie Wei,† Shuang Meng,† Huaping Sheng,† Huihui Liu,† Siyuan Zhou,† Dongshan Zhao,† and Jianbo Wang†,‡



School of Physics and Technology, Center for Electron Microscopy, MOE Key

Laboratory of Artificial Micro- and Nano-structures, and Institute for Advanced Studies, Wuhan University, Wuhan 430072, China



Science and Technology on High Strength Structural Materials Laboratory, Central

South University, Changsha 410083, China

KEYWORDS: wurtzite, zinc blende, ZnO nanowires, phase selection, in situ TEM

ABSTRACT: Unraveling the phase selection mechanisms of semiconductor nanowires (NWs) is critical for the applications in future advanced nano-devices. In this study, the atomistic vapor-solid-liquid growth processes of Sn-catalyzed wurtzite (WZ) and zinc blende (ZB) ZnO are directly revealed based on the in-situ transmission electron microscopy. The growth kinetics of WZ and ZB crystal phases in ZnO appear markedly different in terms of the NW-droplet interface, whereas the nucleation site as determined by the contact angle φ between the seed particle and the NW is found to be crucial for tuning the NW structure through combined

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experimental and theoretical investigations. These results offer an atomic-scale view into the dynamic growth process of ZnO NW, which has implications for the phase-controllable synthesis of II-VI compounds and heterostructures with tunable band structures.

The last decade has seen an exponential rise in the research into the phase-controlled synthesis of one-dimensional II-VI semiconductors,1-8 as driven by the increasing technological demands for versatile and durable high-performance electronic and photonic materials.4,9-14 To achieve this, a full picture of the growth kinetics and phase selection mechanism is desperately needed.15 The seed-catalyzed growth of nanowire (NW) is an ideal system for investigation, whereas multiple (meta)stable phases, such as the wurtzite (WZ) and zinc blende (ZB) structures with a wealth of physical properties can nucleate by simply tuning the growth conditions (source-material flux ratio, catalyst geometry, etc).15-17

Nonetheless, unlike other II-VI group compounds, only the WZ phase has been well-documented in ZnO nanostructures during the catalyst-assisted growth processes, although a metastable ZB structure was reported in ZnO thin films fabricated by the oxidation of zinc18,19 and ZnS,20 and heteroepitaxial growth on cubic substrates.21 Because the different stacking sequences in WZ and ZB phases with energy difference of ~50 meV/ZnO formula unit18,22 can lead to quite distinct band structures and thus optical and electronic properties,23-25 the lack of the ZB-phase availability in one-dimensional ZnO may post a limit for a wider engineering applications, which

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again requires a better understanding of the physics behind the catalyst-assist growth kinetics.

In general, Au and Sn metals have been demonstrated to catalyze the ZnO NWs growth. On the basis of post-growth observations, it was found that all the NWs adopt the WZ structure. Compared with the dominant growth direction in the NWs catalyzed by Au particle,26,27 the NWs growth assisted by the Sn particle show various growth directions, e.g. and < 1010 > ,28 implying that Sn catalysts are more effective in tuning the structures and thus properties of the ZnO NWs. Nonetheless, to our best, the role of Sn catalyst during the ZnO NW growth is still mysterious. In this context, the direct real-time observation of the atomic-scale growth process in Sn-catalyzed ZnO NWs is desperately needed.

Here, applying the in situ high resolution transmission electron microscopy (HRTEM),29-41 we directly observe the atomistic growth processes of Sn-catalyzed ZnO NWs in real-time during the electron beam (e-beam) irradiation of Zn2SnO4 (ZTO) which acts as the substrate. Surprisingly, both the WZ- and ZB- structure ZnO NWs nucleate based on the vapor-liquid-solid (VLS) growth mode. The distinct growth kinetics of the two phases are shown and well-interpreted according to a recent growth model proposed by F. M. Ross,15 whereas the droplet geometry plays a vital role in determining the structure. The ZTO nanowires were grown by chemical vapor deposition (CVD) methods in a horizontal quartz tube furnace using Zn, SnO2, and carbon powders as source materials. The experiments were performed inside the

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JEOL ARM-200F operated at 200 kV.

First of all, the 200 keV high energy e-beam illumination results in the formation of droplets on the ZTO surface. The nucleation of droplets is due to the low melting temperature of bulk Sn ~505 K. In addition, considering the high vacuum inside the TEM, Sn particles smaller than 5 nanometer can be melted even at room temperature.42 Similarly, it was reported that liquid Bi nanoparticles with high bulk melting temperature of 544 K formed on the SrBi2Ta2O9 surface under e-beam irradiation at room temperature.43,44

Afterwards, it is surprising to note that NW with diameter close to that of droplets would nucleate and grow at the droplet/ZTO liquid-solid interface (Supporting Information Figure S1). Consistently, the NW can be well-characterized based on the WZ or ZB structure of ZnO (Figure 1). In addition, we observed the crystalline fluctuation of catalyst particles (Supporting Information Figure S1b-c), i.e. the catalyst particles become liquid (melting) or crystalline (solidification) phases alternatively. It should be mentioned that since the NW mostly grows when the particle is liquid, we focus on discussing the VLS growth mechanism in this paper. As shown in Figure S2, it is believed that the catalyst is mainly consisted of Sn (Figure S2b). However, we cannot preclude the existence of Zn in liquid Sn since the dissolution of Zn is required for the following ZnO NW precipitation. Unfortunately, the tiny amount of Zn content cannot be well detected.

Figure 1a is a typical HRTEM image that shows the nucleation of WZ and ZB ZnO

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nanostructures. The crystallized catalyst can be well-indexed based on the tetrahedral Sn crystal structure (space group: I41/amd). It is noted that the tiny amount of Zn will show little influence on the lattice parameters of the solidified Sn particle.45 The catalyst is enclosed by (101), (121) , (020) and (121) crystal planes as marked in Figure 1b (see also Figure S3 about the structural model of Sn), which is different with the reported circular morphology of Sn catalyst at the tip of the WZ ZnO NWs.28,46,47 Enlarged views from Figure 1a marked by ‘c’ and ‘d’ are presented in Figure 1c-d, which can be indexed by the WZ-structure ZnO (space group: P63 mc ) with [1210]WZ ZnO axis and ZB-structure ZnO (space group: F 43m ) with [101]ZB ZnO axis, respectively. There is a slight lattice mismatch at the interface between the (0002)WZ and (111)ZB structure, but no dislocation or transition phase is found (Figure 1a and S4). The orientation relationships (ORs) of Sn, WZ and ZB ZnO are [101]Sn //

[1210]WZ ZnO // [101]ZB ZnO , (020)Sn // (0002) WZ ZnO // (111)ZB ZnO . The high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) image and the corresponding elemental maps confirm the existence of the separated Sn and ZnO phases (Figue S2). It is worthwhile to mention that both the OR between crystalline Sn and WZ ZnO and that between ZB and WZ ZnO are consistent with the previous results.18,28

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Figure 1. (a-b) A typical HRTEM image (a) and schematic illustration of the Sn-catalyzed WZ and ZB ZnO nanostructures (b). (c-d) Enlarged HRTEM images from the left and right parts of the NW marked by ‘c’ and ‘d’ in (a), showing the different WZ ‘ABABAB’ and ZB ‘ABCABC’ stacking sequences of the close-packed crystal planes.

More importantly, to understand the underlying mechanisms of the phase selection, the growth dynamics of ZnO NWs are further investigated. It is interesting to note that different interface dynamics occurred during the precipitation of WZ and ZB structures as illustrated in Figure 2. Figure 2a-c and 2d-f are time-sequenced images of the growth processes of WZ and ZB ZnO with growth directions of [0001]WZ and [111]ZB, respectively (see also Movie S1-S2, and Figure S5 about the schematic diagram of the growth processes of WZ and ZB ZnO). It is evident that the contact angle φ between liquid seed and the precipitated WZ NW is larger than 90° (Figure 2g), e.g. ~128° in Figure 2a. In contrast, the value of φ is around 90° during the ZB

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phase precipitation (Figure 2d and h). Such phenomenon was consistently found throughout the experiments, as illustrated in Figure 3a, whereas the averaged value of

φ is about 122° (black dotted line) and 90° (red dotted line) during the precipitation of WZ and ZB ZnO, correspondingly. Furthermore, the switch from ZB to WZ phase is observed (insets in Figure 3b, see also Movie S3), accompanied by an abrupt increase of an increase φ from nearly 90° to larger angle, signifying an undeniable effects of the seed particle shape on the growth kinetics.

In parallel with the observed changes in the contact angle, the nucleation site also changes during the switch from the ZB to WZ phase. To be specific, WZ ZnO always nucleated at the location away from the trijunction (at which solid, liquid and vapor meet) as commonly observed in other NWs (nucleation site ‘N’ in Figure 2g), forming an edge facet at the interior point of the catalyst (indicated by the white dotted line in Figure 2b-c and 3b); while the nucleation site for ZB ZnO is located at the trijunction (designated by ‘T’ in Figure 2g).

Figure 2.

Real-time atomistic growth process of WZ (a-c) and ZB ZnO (d-f). The

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arrows indicate the nucleation sites of the NW. (g-h) Schematic diagrams illustrating the different interface growth kinetics and NW-droplet morphologies during the nucleation of WZ and ZB ZnO, respectively.

Figure 3. (a) The statistics of the contact angle φ during the nucleation of WZ (black squares) and ZB ZnO (red squares). The averaged value of φ are indicated by black and red dotted line, respectively. (b) The evolution of φ during the phase switch from ZB to WZ ZnO. Insets are corresponding HRTEM images extracted from the video at 30 s and 62 s. The white dotted lines indicate the edge facet during growth.

To the best of our knowledge, for the first time, the metastable ZB ZnO is prepared via the VLS growth mechanism, compared to those formed due to the occurrence of stacking faults in WZ ZnO films.18,21 The in-situ observations indicate that both the NW geometry (contact angle φ) and interface dynamics (nucleation site) are quite different during the WZ and ZB NW growth. Our current observations can be perfectly explained by the recently proposed model for catalyst-assisted growth of semiconductor NWs,15,48-50 according to which, the difference in free energy ∆E between the nanostructures with an interior edge facet (Figure 2g) and with a sharp

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edge (Figure 2h), is related with the capillary forces c1 acting on the edge facet. It was reported that15

c1 = γ e

1 cos θ − γ vs − γ ls + γ vl sin φ . (1) sin θ sin θ

in which θ is the facet angle, γe is the liquid-solid interfacial energy at the edge facet,

γvs is the vapor-solid interfacial energy, γls is the liquid-solid interfacial energy at the main growth facet, and γvl is the vapor-liquid interfacial energy.

According to equation (1), the maximum capillary force acting on the edge facet is obtained when φ = 90° ( ∆E >0), which intends to eliminate the interior edge facet (Figure 2h) and favors the NW precipitation at the vicinity of the trijunction (Figure 2d-f).15,51 For φ > 90° , c1 decreases and the edge facet starts to emerge inside the seed particle (Figure 2g), leading to the NW precipitation away from the trijunction (Figure 2a-c).

Several models have been proposed to explore the effects of nucleation sites on the nucleated crystal phases.15,48-50 When nucleation occurs at the trijunction (with a sharp edge), the solid-vapor interface will play a key role15, which may reduce the nucleation barrier for the metastable phase, i.e. the ZB ZnO in our case.52 Under the circumstance that the nucleation at the interior point of the catalyst, the liquid-solid interface plays a critical role and the liquid-vapor interface plays no part, which may lead to the nucleation of WZ ZnO.15

Based on the discussion above, different contact angles φ at left (φ > 90ο) and right

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sides (φ ∼ 90ο) of the NW lead to the nucleation of WZ and ZB hybrid crystal structure (Figure S7), respectively, which again illustrates that the NW structure can be effectively tuned by changing the contact angle φ and thus the nucleation site. Similar phenomena are also observed in other hybrid ZB/WZ ZnO structures (Figure S8). It was reported that the droplet geometry described by the droplet aspect ratio h/d and the contact angle φ, is the key parameter in determining structure, but in an indirect way, via its effect on the nanowire edge morphology.15 In our experiment, the h/d value is almost consistent during the individual WZ or ZB/WZ hybrid structure

growth (nearly from 0.75 to 0.85, Figure S9), indicating that the droplet in our experiment keeps almost the same volume during the growth process; (2) However, regarding the precipitation of ZB phase, followed by the WZ structure growth, the h/d value does increase accompanied with the larger value of φ (from ZB (φ ∼ 90ο) to WZ (φ > 90ο)) (Figure S9 and S10). It should be noted that, ZB and WZ structures nucleate simultaneously during the growth of hybrid ZB/WZ structures with the same value of droplet aspect ratio h/d but different contact angles. Thus, it is speculated that the nucleated crystal phase is directly determined by the contact angle φ rather than the h/d value.

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Figure 4. The statistics of the edge facet angle θ of the WZ ZnO illustrated solid and dotted lines indicates the average value of θ measured in our experiments and the previous reports,28 correspondingly.

It is worthwhile to mention that, in the post-grown Sn-catalyzed WZ ZnO NWs ([0001]WZ ZnO growth direction), a neck region consistently occurs at the catalyst-NW interface, representing the final stage of the NW growth with the limited supply of the source materials.28 The averaged angle between the edge facet and the solid Sn is about 58° (dotted line in Figure 4) as reported in literature,28 which is closed to the averaged θ value of 61° during the growth of WZ ZnO in current experiments (solid line in Figure 4), suggesting the similar growth mechanisms. Simultaneously, the value of θ is closed to the angle between (1011) WZ ZnO and (0002)WZ ZnO crystal planes (~62°), which means that (1011) WZ ZnO could be the growth facets in the growth of WZ ZnO.

In our experimental configuration, it is believed that the Zn source for the growth of ZnO NW comes from the knock-on damage as well as the local sublimation of the

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Zn atoms induced by the e-beam irradiation, similar to the Al2O3 NW growth inside the high-vacuum TEM.53 Although the content of Zn vapor is pretty low in the TEM chamber, only a small quantity of Zn is required before Zn precipitates out, because less than 10% Zn can be adsorbed in liquid Sn based on the Zn-Sn phase diagram.45 Interestingly, ordered interfacial structures emerge at the liquid/solid interface (Figure 2 and S11), illustrating the local ordering of Zn atoms, similar to the ordered interfacial Bi atoms in the Bi liquid as well as the Al layers of Al droplets53. Afterwards, the nucleation of ZnO is accomplished by interfacial diffusion of oxygen through the ordered liquid Zn atoms.53

In conclusion, our in situ HRTEM observations have revealed the VLS growth processes of Sn-catalyzed WZ and ZB ZnO NWs. We find clear differences between the growth dynamics of the WZ and ZB ZnO phases, including differences in the contact angle φ and nucleation site. It is anticipated that the underlying growth mechanisms in Sn-catalyzed ZnO NWs can be extended to other II-VI or III-V semiconductor compounds that exhibit structural polytypism, which are relevant to the phase control of individual crystals and fabrication of high-performance electronic devices with tunable band structures. Meanwhile, the site-specific decoration of ZnO quantum dots on ZTO semiconductors has important application in the ultraviolet image sensors.54

ASSOCIATED CONTENT

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Supporting Information. The orientation relationship and elemental map of ZTO, Sn, WZ, and ZB ZnO; the microstructural analysis of droplet; the characterization of hybrid crystal structure with both WZ and ZB ZnO; the schematic diagram of the growth process of WZ and ZB ZnO; the local ordering of catalyst/NW liquid/solid interface; real-time videos for the atomistic growth of WZ and ZB ZnO.

AUTHOR INFORMATION

Corresponding Author Email: [email protected]

Notes The authors declare no competing financial internets.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51671148, 51271134, J1210061, 11674251, 51501132, and 51601132), the Hubei Provincial Natural Science Foundation of China (2016CFB446 and 2016CFB155), the Fundamental Research Funds for the Central Universities, the CERS-1-26 (CERS-China Equipment and Education Resources System), the China Postdoctoral Science Foundation (2014T70734), the Open Research Fund of Science and Technology on High Strength Structural Materials Laboratory (Central South University), and the Suzhou Science and Technology project (No. SYG201619). ABBREVIATIONS

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Nanowire, NW; WZ, wurtzite; ZB, zinc blende; HRTEM, high resolution transmission electron microscopy; e-beam, electron beam; ZTO, Zn2SnO4; VLS, vapor-liquid-solid; CVD, chemical vapor deposition; ORs, orientation relationships; HAADF, high-angle annular dark field; STEM, scanning transmission electron microscopy. REFERENCES (1) Zheng, H.; Wang, J.; Huang, J. Y.; Wang, J.; Zhang, Z.; Mao, S. X. Nano Lett.

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Figure 1 81x81mm (300 x 300 DPI)

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Figure 2 69x34mm (300 x 300 DPI)

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Figure 3 65x25mm (300 x 300 DPI)

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Figure 4 63x49mm (300 x 300 DPI)

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