Kinetic Growth of Ultralong Metastable Zincblende MnSe Nanowires

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Kinetic Growth of Ultralong Metastable Zincblende MnSe Nanowires Catalyzed by a Fast Ionic Conductor via a Solution−Solid−Solid Mechanism Li Zhang†,‡,§,∥ and Qing Yang*,†,‡,§,∥ †

Hefei National Laboratory of Physical Sciences at the Microscale (HFNL), ‡Department of Chemistry, §Laboratory of Nanomaterials for Energy Conversion (LNEC), ∥Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei 230026, Anhui, P. R. China S Supporting Information *

ABSTRACT: The metastable semiconductor phase allows for the exploration of unusual properties and functionalities of abnormal structures, although it is often difficult to prevent thermodynamic transformations to lower energy structures from higher, unfavored energy states. Here, we show for the first time the preparation of high-quality ultralong metastable zincblende (ZB)−MnSe nanowires with a four-coordinate structure via solution−solid−solid (SSS) growth in a mild solution-phase synthetic environment (120−220 °C) in the presence of a trace amount of Ag(I). The metastable ZB-MnSe nanowires are stabilized kinetically due to the catalysis of early formed body-centered cubic (bcc) fast-ionic (superionic) Ag2Se nanocrystals from the Ag(I) source, and the ZB-MnSe nanowires grow epitaxially along the ⟨110⟩ axis rather than the ⟨111⟩ axis, as commonly observed for typical four-coordinate Grimm−Sommerfeld bonding solids. Our method provides a new route for the growth of metastable nanostructures. KEYWORDS: zincblende MnSe nanowire, metastable phase, solution−solid−solid growth mechanism, catalytic growth, superionic conductor, fast-ionic conductor, epitaxial growth, kinetic growth

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applications. Fortunately, recent progress has revealed that RSMnSe quantum wires can be fabricated using mesoporous silica as templates19 or via a chemical vapor deposition.20 It is noted that MnSe nanostructures with the four-coordinate nonequilibrium polymorph of wurtzite (Wur) structure have also been fabricated via solution phases, from which these MnSe nanostructures are commonly present in particles, including bullet-, shuttle-, and tetrapod-shaped nanocrystals.21−23 WurMnSe films were stabilized in early reports as a heterostructured components typically via epitaxial growth on GaAs substrate using group II−VI as a buffer24−26 or a solid-state solution.27,28 However, zincblende (ZB)-MnSe has rarely been reported. Several decades ago, Murray et al. reported the synthesis of ZB-MnSe powders via precipitation between manganese acetate and hydrogen selenide, which contained impurities of minor Wur-MnSe and 15 wt % elemental selenium.29 Notably, among various MnSe nanostructures, four-coordinate nonequilibrium MnSe nanowires other than those of stable rock salt19,20 have never been reported. Thus, it appears quite challenging to synthesize metastable MnSe nanowires at a bulk scale.

emiconductor nanowires, as typical one-dimensional (1D) nanostructures and important building blocks for nanodevices, have received increased attention in recent years due to their attractive physical and chemical properties, various functionalities and potential applications. Vapor−liquid−solid (VLS),1−3 solution−liquid−solid (SLS),4−6 and vapor−solid− solid (VSS)7−9 are the most successful models for the growth of diversified 1D semiconductor nanowires, typically using gold, non-noble metals, and alloys as catalysts. Recently, a novel strategy was reported for preparing 1D semiconducting nanostructures based on group-11 chalcogenides, which typically include Ag2S,10,11 Ag2Se,12 and Cu2−xS13,14 as effective catalysts. This strategy simplifies the procedures for the growth of 1D semiconductors, which is beneficial for exploring new nanostructures and even extending their functionalities and applications. However, the real catalytic growth mechanism was not well established until a recent report on a solution−solid− solid (SSS) mechanism15 proposed for the growth of 1D nanowires catalyzed by body-centered cubic (bcc)-Ag2Se nanocrystals, a typical fast-ionic (superionic) conductor with a high density of Ag+ vacancies.16−18 Among the known semiconductor nanowires, MnSe nanowires have rarely been reported to date because their thermodynamic stable structure is a six-coordinate cubic rocksalt (RS) form with a highly symmetric geometry that is prone to grow granular shapes. The difficulty of its preparation severely hampers the study of its functionalities and possible © XXXX American Chemical Society

Received: January 31, 2016 Revised: May 21, 2016

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Figure 1. (a) XRD pattern, (b) SEM image, and (c) low- and (d) high-magnification TEM images of ultralong ZB-MnSe nanowires prepared at 200 °C for 2 h (0.3 atom % Ag+/Mn2+ in stock solution), (e) and (f) HRTEM images of straight and zigzag nanowires, respectively. The red circle in (d) shows a typical ZB-MnSe nanowire ended by a Ag2Se terminus. Insets in (e) and (f) are the corresponding selected area electron diffraction (SAED) patterns for the HRTEM images, projected along the [1−1−1] direction (one of the ⟨111⟩ zone axes).

Here, we report for the first time a mild solution-phase synthetic route to synthesize ultralong metastable ZB-MnSe nanowires from the reaction of Mn(II) salt with dibenzyl diselenide in oleylamine in the presence of a trace amount of AgNO3 at 120−220 °C, as described in detail in the Supporting Information. To the best of our knowledge, this is the first report on the growth of MnSe nanowires in a metastable fourcoordinate structure. The metastable ZB-MnSe nanowires are stabilized kinetically via epitaxial growth along the ⟨110⟩ direction over the catalysis of the bcc-Ag2Se nanocrystals. Figure 1a shows the XRD pattern for the MnSe nanowires synthesized at 200 °C for 2 h (0.3 atom % of Ag+/Mn2+ in stock solution). The nanowires are of the four-coordinate metastable ZB phase (JCPDS Card, No. 27-0311). Moreover, there is a diffraction peak located at 24.63° (2θ), which can be indexed as stacking faults (marked with *). In general, the sample is of high quality, as there are no noticeable RS-MnSe, Wur-MnSe, or Ag2Se diffraction peaks (in either the stable orthorhombic30 (JCPDS Card, No. 24-1041) or the metastable

tetragonal phase15,31−33), even in the 20× (vertical) magnified profile (Figure S1). Notably, Ag2Se can be easily detected with the addition of slightly more silver source (∼3.0 atom %, Figure S2). Figure 1b shows a typical SEM image for the ultralong ZBMnSe nanowires, which are uniform in diameter with lengths of at least several tens of microns, as observed in the large area of the SEM image (Figure S3). This is the first report on the growth of ultralong ZB-MnSe nanowires. Figure 1c shows a typical TEM image of the ZB-MnSe nanowires, and most are straight except for a small zigzagged portion. The diameter of both the straight and zigzagged nanowires is approximately 35 nm (Figures 1d and S4). A Ag2Se nanoparticle tip is observed at the nanowire end (marked in red in Figure 1d), suggesting catalytic growth of the ZBMnSe nanowires. Few Ag2Se tips are observed unless the Ag(I) source loading is increased (Figure S5). Figure 1e shows a typical HRTEM image of an individual straight nanowire revealing its two-dimensional (2D) lattice structure with a B

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Figure 2. (a) A typical TEM image of an individual ZB-MnSe nanowire terminated by a Ag2Se nanocrystal tip with a clear interface between the tetragonal-Ag2Se {110} and ZB-MnSe {110} planes, determined via HRTEM (b); (c) HAADF−STEM image; and (d)−(f) STEM−EDX elemental mapping of Ag, Mn, and Se for a typical nanowire end. Scale bars: 25 nm.

2b). High-angle annular dark-field imaging in the scanning TEM (HAADF−STEM) image (Figure 2c) supports the TEM observations (Figure 2a,b). STEM energy dispersive X-ray spectroscopic (STEM−EDX) elemental mappings demonstrate that Ag is primarily present in the termini but Mn is primarily present in the stem compared to the even distribution of Se in all regions (Figure 2d−f). Meanwhile, traces of Mn and Ag are also detected in the termini tip and stem, respectively. Via the current method, high-quality ultralong metastable ZB-MnSe nanowires are fabricated only in the presence of a Ag source at temperatures of 120 to 220 °C (Figures 1 and S5− S7). Ag2Se is initially formed from the reaction between the Ag and Se sources before MnSe is formed because the solubility product constant of Ag2Se is much smaller than that of MnSe according to thermodynamic data in literature,36 and the early Ag2Se nanocrystals formed in situ serve as catalysts, which play an important role in the growth of the metastable ZB-MnSe nanowires instead of the thermodynamically stable RS-MnSe in the absence of the silver precursor, as shown in Figure S8. To confirm the catalytic effects of Ag2Se, we performed an

spacing of approximately 4.11 Å between the ZB-(110) planes, confirmed by the corresponding selected area electron diffraction (SAED) pattern (inset). Interestingly, the indexed SAED pattern reveals a typical hexagonal pattern from a set of {110} lattice planes, indicating that the MnSe nanowire is single-crystalline ZB phase grown along the ⟨110⟩ axis rather than the commonly observed ⟨111⟩ axis for typical fourcoordinate Grimm−Sommerfeld bonding nanostructures.1−9,34,35 Figure 1f shows an HRTEM image from the corner of a singular zigzag nanowire revealing that the zigzagged nanowire exhibited the same growth direction as the straight nanowire (see the inserted SAED pattern). Figure 2a shows a magnified TEM image for a typical MnSe nanowire terminated by a Ag2Se nanocrystal prepared at 200 °C for 2 h with 0.3 atom % Ag+/Mn2+. There is a clear heterogeneous conjunction-like interface between the MnSe nanowire stem and the Ag2Se nanocrystal termini in the nanowire end. The HRTEM image confirms the existence of the interface and reveals that both the termini and the stem are highly crystallized, as observed at room temperature (Figure C

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of the end part under election beam irradiation for different times, taking advantage of the high cation mobility of Ag(I) ions in bcc Ag2Se,16−18,30−33 and we observed that Ag2Se at the tip could be dispersed after a relatively long duration of election beam irradiation. Then, the alloyed MnSe in the tip dealloys from the tip and sustains the growth of the MnSe nanowire stem along the ⟨110⟩ direction. In detail, the Ag2Se nanocrystal termini tip is generally stable when it was irradiated for 420− 600 s (Figure 3a) compared to 0−360 s, which corresponds to the morphology shown in Figure 2a. After irradiation for more than 2100 s (Figure 3b), Ag2Se can no longer be observed in the tip due to the complete removal of mobile Ag(I) ions from the bcc-Ag2Se converted from the tetragonal phase after long durations of irradiation, as revealed by the STEM−EDX line scan of the nanowire end (Figure 3c). Interestingly, the interface between the catalyst tip and the nanowire stem shifts with the election beam irradiation until the Ag2Se tip is removed (Figure 3). Furthermore, variable-time HAADF−STEM images confirmed that the movement of the interface between the catalyst tip and the nanowire stem (Figure S12) sustains the forward growth of the MnSe nanowire catalyzed by bcc-Ag2Se (Figure S13). The HRTEM image in Figure S13a shows that the sustained MnSe nanowire at the end part takes the ZB form with a high quality of single crystals, which is supported by the corresponding ED detection (Figure S13b). The migration of Ag(I) ions in the bcc-Ag2Se catalyst tip catalyzes the growth of ultralong ZB-MnSe nanowires via a solid−solid regime between the interface of the fast ionic bcc-Ag2Se and ZB-MnSe because the aforementioned observed process is analogous to the SSS growth of the MnSe nanowires via the actual present solution synthetic route, except for the absence of solution feeding for a common TEM observation. However, the mass transfer of the Mn and Se sources occurs from the solution to the solid catalyst tip via the SSS regime for the continuous growth of the solid ZBMnSe nanowires (Figures 1 and S3−S7) due to the entropy contribution of Ag−Mn−Se solid-state solutions compared thermodynamically to the end-members of Ag2Se and MnSe. For the present SSS regime, the metastable ZB-MnSe nanowires were stabilized kinetically while simultaneously undergoing growth. Typically, early generated bcc-Ag2Se nanocrystals favor heterogeneous nucleation37,38 of ZB-MnSe via epitaxial growth on the bcc-Ag2Se catalyst due to very close lattice matching39,40 between Ag2Se (211)/MnSe(011) and Ag2Se (−222)/MnSe(100) along the [01−1] direction (Scheme 1b). In detail, 2dAg2Se (211) = dMnSe (011) with a lattice mismatch of 2.40% and 4dAg2Se (−222) = dMnSe (100) with a lattice mismatch of 2.37%, which suggests that they perfectly match to grow ZB-MnSe nanowires rather than RS- and/or Wur-MnSe nanowires via catalysis by bcc-Ag2Se nanocrystals. In addition to epitaxial growth, the incorporation of Ag (Figure 2d) promotes the kinetic stabilization of metastable ZB-MnSe nanowires, although a previous investigation revealed that treating MnSe with silver and selenium powders (less than 2 atom %) at 800 °C for 20 h leads to stable RS-MnSe.41 Ag doping, even for a very small proportion, promotes the stabilization of the metastable nanowires (Figure 1−3), consistent with the growth of other metastable superlattice nanostructures reported previously.3,34 This is in contrast to the high-pressure treatment that is commonly used for metastable crystal structures.42−44 Moreover, mild reaction conditions are kinetically favorable for metastable MnSe nanowires because increased reaction

alternative synthesis by loading pure Ag2Se nanocrystals instead of AgNO3 in the procedure (Figure S9a), and we also obtained ZB-MnSe nanowires (Figure S9b). However, we failed to grow ZB-MnSe nanowires using gold nanocrystals as a catalyst, as the obtained sample was just composed of granular RS-MnSe nanocrystals with cubic-like shapes (Figures S9c,d). These results confirmed that ZB-MnSe nanowires are only grown due to the catalytic activity of Ag2Se nanocrystals, even in trace amounts (Figure 1). Variable-temperature XRD and differential scanning calorimetry (DSC) investigations revealed that the metastable nanowires are grown by the catalysis of bcc-Ag2Se nanocrystals (Figures S10 and S11). During a catalysis growth process, source material is typically incorporated into growing nanowires via a catalyst, and the nanowire grows along the catalyst−nanowire interface due to transport of the source material. This phenomenon has been demonstrated for the growth of Si and/or Ge nanowires using gold as a catalyst via VLS,1,3 SLS,5 and VSS regimes7−9 and also for the fabrication of group III−V/II−VI nanowires via the SLS model.4,6 In the present study, the fast ionic conductor bccAg2Se nanocrystals promote the growth of 1D MnSe nanostructures via the SSS mechanism, which is attributed to the formation of a Ag−Mn−Se solid-state solution because the high density of Ag+ vacancies are favorable for Mn(II) cation incorporation at temperatures over approximately 104.5 °C (Figure S10, S11). The growth mechanism of ZB-MnSe nanowires catalyzed by bcc-Ag2Se via the SSS model (Scheme 1a) is directly supported by the following TEM investigations. We captured TEM images Scheme 1. (a) Schematic Growth of ZB-MnSe via the SSS Model Catalyzed by bcc-Ag2Se and (b) Schematic Image of Epitaxial Growth of ZB-MnSea with the {011} Facet on the bcc-Ag2Seb {211} Plane and the ZB-MnSe {100} Facet on the bcc-Ag2Se {222} of the Catalytic Interfacec

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JCPDS card No. 88-2344. bJCPDS card No. 76-0315. c2dAg2Se (211) = dMnSe (011); the lattice mismatch is 2.40%. 4dAg2Se (−222) = dMnSe (100); the lattice mismatch is 2.37%. D

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stability of the ZB-MnSe nanowires (Figures S5−S7, S10, and S14), consistent with a previous report.29 By understanding the growth mechanism, we fabricated highquality metastable ZB-MnSe nanowires via a synthetic route that can also be used for the growth of ultrathin MnSe nanowires (Figure S15), Ag2Se−MnSe nanorods/matchsticklike heteronanostructures (Figure S16) and even MnS nanowires (Figure S17) by varying the synthetic conditions. Conclusion. In conclusion, for the first time, we successfully fabricated ultralong metastable ZB-MnSe nanowires via a solution-phase reaction of Mn(II) salt with dibenzyl diselenide ((PhCH2)2Se2) in oleylamine in the presence of a trace amount of AgNO3 under mild conditions (120−220 °C). The metastable ZB-MnSe nanowires were grown based on an SSS mechanism catalyzed by bcc-Ag2Se nanocrystals via epitaxial growth along the ⟨110⟩ direction with a perfect lattice matching, kinetically. This route has been extended for the growth of ultrathin MnSe nanowires, Ag2Se−MnSe nanorods/ matchstick-like heteronanostructures and even MnS nanowires. In addition, the properties of the ZB-MnSe nanowires were investigated, revealing they are largely affected. Typically, the optical band gap (Eg) of the ZB-MnSe nanowires is estimated as approximately 3.6 eV, which is much larger than that of RSMnSe nanocrystals (2.5 eV, Figure S18). However, the Néel temperature of the ZB-nanowires is 41 K, which is much smaller than the reported data (Figure S19). The present study reports a new strategy for kinetically growing metastable semiconducting nanowires by taking advantage of the SSS growth regime.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b00419. Experimental procedures, a detailed analysis of the XRD patterns, SEM, TEM, HRTEM, and HAADF-STEM images, variable-temperature XRD, DSC scans, and the optical and magnetic properties of the products. (PDF)

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AUTHOR INFORMATION

Corresponding Author

* Tel.: +86-551-63600243. Fax: +86-551-63606266. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (51271173, 21571166) and the National Basic Research Program of China (2012CB922001).

Figure 3. Magnified TEM images of a typical MnSe stem terminated by a Ag2Se tip (at 200 °C for 2 h with 0.3 atom % Ag+/Mn2+) irradiated with an electron beam for various durations: (a) 420−600 s and (b) 2100 s. (c) The corresponding STEM−EDX line scan after 2100 s of irradiation.

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