Anisotropic Zinc Blende ZnSe Nanostructures: The Interface Chemistry

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Anisotropic Zinc Blende ZnSe Nanostructures: The Interface Chemistry and the Retention of Zinc Blende Phase during Growth Riya Bose, Goutam Manna, and Narayan Pradhan* Department of Materials Science and Centre for Advanced Materials, Indian Association for the Cultivation of Science, Kolkata 700032, West Bengal, India S Supporting Information *

ABSTRACT: Anisotropic growth in the zinc blende phase of nanomaterials in solution is normally less favorable in comparison to the wurtzite phase. Considering the case of ZnSe and using appropriate surface ligands for selective facet binding, herein we report the anisotropic growth leading to 1D rods, bullet-shaped and finally hemisphere-shaped nanostructures. A detailed study from the nucleation with magic-sized dots to all these structures has been performed, and their formation mechanism has been discussed. Interestingly, while such structures have existed to date mostly in wurtzite-type crystal phase with cell parameters a = b ≠ c, here for ZnSe they are formed uniquely in zinc blende phase with a = b = c.

1. INTRODUCTION Anisotropic crystal growth of different nanomaterials in solution remains one of the leading fields of research since the last two decades.1−7 Enormous investigations have been carried out to understand the basic fundamental aspects of this crystal growth. It has been established that the subtle balance between thermodynamically and kinetically controlled growths at corresponding reaction conditions governs the final architecture of the nanocrystals.8,9 Under thermodynamic growth regime, which is driven by low flux of monomer approach and sufficient supply of thermal energy, the most stable form of nanocrystals, i.e., zero-dimensional structures, is obtained. In contrast, under non-equilibrium kinetic growth conditions with a high monomer flux, preferential anisotropic growth is facilitated along kinetically most favorable direction/s having low activation energy.8,9 Again, as the kinetic energy barrier is inversely proportional to the surface energy of the nanocrystals, choice of the surface ligands also plays a crucial role in tailoring the geometry of the nanocrystals.7−12 With the control of these reaction parameters, wide varieties of shapes for different nanomaterials have already been designed.2,4,13−18 Among these, one- and two-dimensional growths are the most common which lead to rod/wire13,19,20 and disk/sheet21−23 shapes of anisotropic nanostructures, respectively. However, a close analysis of the literature reports reveals that, in addition to all these nanostructures, there are some peculiar shaped anisotropic structures which evolve because of different rates of crystal growth in different directions. Droplet-, pencil-/bullet-/tree-, and pyramidal-shaped structures belong to this category. A large number of such structures are also reported.2,7,14−18,24−33 Interestingly, it has been observed that these structures mostly possess wurtzite-type crystal phase or have the unit cell parameters a = b ≠ c. A list of such structures with references is provided in the Supporting Information (Table S1). This suggests that anisotropic growth for obtaining © 2013 American Chemical Society

such shapes is normally favored in the crystal structures with asymmetric unit cell parameters. These structures possess a polar axis and under certain reaction conditions the surface energy of the facet in the polar direction increases to an optimum extent. Hence to minimize the overall surface energy of the nanocrystals, new low energy facets are created which eliminate the existing high energy facets. As a consequence, the nanostructures achieve pencil−/bullet−/tree− or pyramidal− shapes.2,3 But the crystal structure with zinc blende (ZB) unit cell having the lattice parameters a = b = c possesses less polarity along any specific direction, and hence the anisotropic growth is less pronounced. Even though the growth of ZB 1D rods, wires, and some zigzag structures are reported in colloidal solution, they are rare in comparison to wurtzite 1D structures.19,34,35 In most cases, the ZB rods/wires are formed by oriented attachments.19,36 Herein, we extend this study for ZnSe and investigate the crystal growth for similar anisotropic nanostructures. ZnSe is one of the most important semiconducting materials with high bandgap (2.8 eV). Unlike other group II−VI semiconductor materials, the crystal growth and consequent shape/size/phase variation of ZnSe nanostructures are less explored. Among the existing nanostructures, the most widely studied shapes are 1D rods/wires with wurtzite phase and 0D spherical dots with ZB phase. 37−41 In addition, a few faceted and branched nanostructures are also reported which attain mixed ZB and wurtzite phases.40,42 We report here the fabrication of anisotropic hemisphereshaped ZnSe nanostructures and investigate their crystal growth in solution. It is observed that these structures are formed via 1D nanorods and bullet-shaped nanostructures. Surprisingly, Received: June 27, 2013 Revised: August 12, 2013 Published: August 13, 2013 18762

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three-necked flask and degassed for 15 min by purging Ar at 50−70 °C. Then the temperature was increased to 250 °C, and it was kept at that temperature for 10 min. After that, the temperature was cooled down and nanocrystals were collected. Purification. The as-synthesized nanocrystals were precipitated by centrifugation using excess acetone from the crude product and further purified using chloroform as solvent and acetone as non-solvent.The purified nanocrystals were dispersed in chloroform for further measurements.

contrary to all the reports in Table S1 (Supporting Information), in this case we have observed the retention of ZB phase in all these anisotropic nanostructures, and they are grown along the [111] direction. A thorough investigation suggests that the presence of strong surface binding ligands (thiol here) plays a key role here in directing the phase and shape of these anisotropic structures, and in their absence, ZnSe grows to form 1D rods/wires with wurtzite crystal phase (Figure S1, Supporting Information). These nanostructures are synthesized in a facile colloidal synthetic approach using Zncarboxylate as zinc and selenourea as Se precursor. Dispersing them in polar solvent alkylamine and introducing appropriate amount of alkylthiol as a strong surface binding agent, the shape evolution and phase control in ZnSe nanostructures are carried out with the function of reaction time and temperature. Samples obtained in successive stages from the reaction system show different shapes of ZnSe nanostructures. Further, a detailed investigation of the growth of these anisotropic structures of ZnSe in solution having ZB phase has been carried out and discussed in this article.

3. RESULTS AND DISCUSSION Figure 1a−1d shows the transmission electron microscopy (TEM) images of ZnSe nanostructures obtained from the

2. EXPERIMENTAL SECTION a. Materials. Zinc stearate (Zn(St)2, tech.), selenourea ((NH2)2CSe), octadecylamine (ODA, 97%), and 1-dodecanethiol (DDT, >98%) were purchased from Aldrich. All these chemicals were used without further purification. b. Methods. Synthesis of Zinc Blende ZnSe Dot-, Rod-, Bullet-, and Hemisphere-Shaped Nanostructures. To obtain different zinc blende ZnSe nanostructures, Zn and Se precursors are dissolved in alkylamine solvent along with required amount of thiol. On heating, different sizes and shapes of ZnSe nanostructures evolve with the course of the reaction time and change in reaction temperature. In a typical reaction, 0.15 mmol of Zn(St)2 (0.094 g), 0.3 mmol of selenourea (0.036 g), 3 g of ODA, and 0.4 mL of DDT were loaded in a three-necked flask and degassed for 5 min by purging Ar at room temperature. Then the reaction temperature was increased to ∼50−70 °C, and the purging of Ar gas was continued for another 10 min. Next, the temperature was increased to 170 °C, then 210 °C, 240 °C and finally fixed at 250 °C. Once the temperature reached our desired state, samples were collected for analysis, and the next state temperature was fixed for further heating. At 250 °C, samples were collected at different time intervals, and they were analyzed microscopically. After 10 min, we obtained the hemisphere-shaped ZnSe nanostructures with the best size distribution. Before 240 °C, mostly rods and bullet-shaped particles were formed. Continuous heating at 250 °C or increase of the reaction temperature to 280 °C−300 °C mostly defocused the size distribution, and the hemispheres turned to hexagonal pyramidal structures with generations of clear facets. It is worth mentioning here that we also tried the reaction with variation in the ratio of the Zn and Se precursor. An excess of Se:Zn ratio (>2:1) is found to broaden the size distribution of the hemispheres (Figure S2, Supporting Information), whereas use of less Se is unable to grow the nanostructures and particles remain much smaller and spherical. Synthesis of Wurtzite ZnSe 1D Nanostructures. To obtain 1D wurtzite nanorods/nanowires, the above reaction protocol was followed, but no thiol was introduced into the reaction system. In a typical reaction, 0.15 mmol of Zn(St)2 (0.094 g), 0.3 mmol of selenourea (0.036 g), and 3g of ODA were loaded in a

Figure 1. (a−d) TEM images of the samples collected at different temperature and time intervals from the reaction system. While the first two cases (a and b) show the rod- and mixture of rod- and bulletshaped structures, respectively, the second two cases (c and d) present the hemispheres with different diameters. More TEM images of intermediate samples have been provided in Figure S4 (Supporting Information). (e,f) TEM image of the hemisphere-shaped structures at different magnification. (g,h) HRTEM image of a single hemisphere in 0 and +20 degree X-tilted view. Insets show the atomic model. (i−k) HRTEM images of the intermediate rod- and bullet-shaped structures.

reaction system at different time and temperature intervals. Sample collected immediately when the temperature reached 170 °C showed rods with aspect ratio ∼2.5−3 (Figure 1a). At 210 °C, mixture of rods and bullets was observed (Figure 1b). When the temperature reached 240 °C, hemisphere shaped particles were formed (Figure 1c), and they further grew in size with increase in temperature to 250 °C and further annealing. Figure 1d presents the finally obtained hemisphere-shaped structures after annealing for 10 min at 250 °C. However, samples collected at the very beginning before rod formation showed mostly small size spherical particles (data not shown). Enlarged view of the TEM images of the final hemispheres is shown in Figure 1e−1f (more HRTEM images provided in Figure S3, Supporting Information). Figure 1g−1h shows the HRTEM images of a single hemisphere with zero (Figure 1g) and +20 degree (Figure 1h) X-tilting of the grid. The atomic models in the zero and tilted view of the hemispheres have also 18763

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been shown in the inset of the corresponding figures. From the tiled images it is clear that these structures have a flat bottom surface, and they are grown on top of it and appear like half of a sphere. Figure 1i−1k shows the HRTEM images corresponding to the rod- and bullet-shaped nanostructures. Successive absorbance spectra of the samples collected at different temperatures show the peak corresponding to magicsized ZnSe initially, and then the band edge moves to longer wavelength suggesting growth of the nanostructures (Figure 2).

Hence, these hemisphere-shaped nanostructures are formed from spherical magic-sized seeds which anisotropically grow to 1D rods and then via bullet-shaped nanostructures finally lead to hemispheres. Once they attain the hemisphere shape, further growth only increases the dimensions of these nanostructures. However, with prolonged annealing these hemispheres show broad size distribution, and their shapes are turned to hexagonal pyramidal structures (Figure S5, Supporting Information). To get a better insight into the formation of the hemispheres, the HRTEM images of the intermediate rod- and bullet-shaped structures and that of the hemisphere-shaped structures are analyzed. Figure 3a shows the HRTEM image of a single rod, and the inset shows the selected area FFT (fast Fourier transform) pattern. Figure 3b shows the HRTEM image of a bullet-shaped structure. The corresponding FFT pattern has been shown in Figure 3c. The calculated d-spacings of 0.32, 0.20, and 0.17 nm from their FFTs (Figure 3c and inset FFT of Figure 3a) signify the {111}, {022}, and {113} planes of bulk ZB ZnSe. This suggests that these intermediate rods and bulletshaped structures are also in ZB phase. Figure 3d shows the atomic model of the bullet-shaped structure which also shows {111}, {022}, and {113} planes. The atomic pattern reveals that these nanocrystals are grown along its polar [111] direction. Similarly, the HRTEM images of the hemisphere,

Figure 2. Stepwise absorption spectra obtained during the formation of hemisphere-shaped ZnSe nanostructures.

Figure 3. (a) HRTEM image of a single rod, and the inset shows the FFT pattern of the same which shows the ZB phase. (b) HRTEM image of a bullet-shaped structure and (c) its selected area FFT pattern. (d) Atomic pattern of the bullet-shaped structure. (e) HRTEM image of a hemisphere viewed along the [022] direction. (f) Selected area FFT pattern. (g) Atomic model of the hemisphere viewed along the [022] direction. (h) Hemisphere of ZnSe viewed along the [111] direction. (i,j) Enlarged area (white rectangle) of the HRTEM and selected area FFT pattern, respectively. (k) Presents the atomic model of the hemisphere viewed along the [111] direction. The FFT of all these images shows the zinc blende phase. 18764

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FFTs, and atomic models in different views are shown in Figure 3e−3k. The selected area FFT pattern (Figure 3f) of the image in Figure 3e suggests that the viewing axis is along the [022] direction, and the FFT (Figure 3j) of the image in Figure 3h represents the view along the [111] direction of ZB ZnSe. Figure 3g and 3k represent the atomic patterns of the hemispheres in the two viewing directions. While the {1̅11̅}, {002}, and {111̅} planes are viewed for the first case, only the {022} group of planes having the d-spacing of 0.20 nm is viewed in the latter case. These planes crossing each other at a 60° angle further confirms that the basal plane is (111) of ZB ZnSe. In addition to the HRTEM analysis, the XRD of the samples collected at different stages of the reaction also supports their ZB phase (Figure 4) and the peak positions correspond to

Figure 5. (a,b) TEM images of spherical ZnSe particle synthesized with excess thiol (five times the thiol concentration used to obtain the hemisphere-shaped nanostructures presented Figure 1).

final product ceasing the shape evolutions. This further confirms that the presence of a certain amount of thiol and their specific binding on the surface of the nanostructures here strongly control the entire anisotropic crystal growth. To reaffirm our proposed mechanism of prevention of selective facet growth by the strong surface binding ligand thiol, we further tried to synthesize ZnSe following a similar reaction methodology but in the presence of other strong binding ligands, like carboxylic acids or bidentate thiols. In all the cases we obtained zinc blende hemisphere shaped ZnSe (Figure S6, Supporting Information), but the best size distribution was obtained with dodecanethiol only. As both the final hemisphere structures and the intermediate bullet-shaped nanocrystals have (111) basal plane (Figure 3b and Figure 3h), it can be assumed here that the growth along [111] is restricted during the entire reaction. This suggests that thiols are mostly bonded to this facet which is populated with only Zn atoms, and they block the growth. This leads to the approach of Zn and then Se atoms alternatively along the polar (1̅1̅1̅) facet and allows the nanocrystals to grow along [1̅1̅1̅] direction. As a consequence, the 0D seed dots are elongated and changed to 1D nanorods. During this process, the {220} facets of the rods are created. However, at a certain stage, the growth along [1̅1̅1̅] direction proceeds at such a faster rate that adsorption of the atoms (either Zn or Se) of the next layer starts even before the previous layer covers the entire (1̅1̅1̅) facet. This creates new low energy facets, {020̅ } and {11̅ 1̅ }, and the crystal starts to grow on them to minimize the total surface energy of the system. With this process, the (1̅1̅1̅) facet slowly diminishes. This is the typical mechanism for the growth of the bullet or the tip of the pencil-shaped nanostructure.2,3 If the reaction is kept at this temperature (∼210 °C), the shape of the nanostructures does not change further to hemisphere. However, the important observation here is that the tip of the bullet-shaped nanocrystals does not remain sharp during growth as for the wurtzite structure.2 For the ZB bullets, the {02̅0} planes of the tip remain perpendicular to each other, as shown in Figure 6(a,b). Hence, with further growth also, mostly hemisphere-shaped structures evolve without formation of a sharp pyramid-like structure. This is expected here as the polarity along the [1̅1̅1̅] direction cannot increase in ZB phase like that in the [001] of wurtzite, and hence the rate of growth along the polar direction cannot be faster. This has been schematically shown in Figure 6c. With the increase of the reaction temperature, these bulletshaped nanostructures change to hemispheres. From the change in dimensions, it is clear that these nanostructures have further grown. It can be assumed that the increase of reaction temperature activates more precursors present in the reaction medium to generate more monomers which resume

Figure 4. Powder XRD pattern of the samples collected at different stages during the synthesis of the hemisphere-shaped nanostructures. The patterns for rods and different diameters of hemispheres are labeled, and these are self-explanatory.

(111), (220), and (113) planes of bulk zinc blende ZnSe. The decrease of the full width at half maxima of all the peaks suggests the growth of the nanocrystals during the shape conversions which occurred with the progress of the reaction. While the size of the rods was 5−7 nm, initially ∼10−15 nm sized hemispheres were formed which further increased their size up to ∼20−25 nm. Further, we investigate the reason behind this striking zinc blende anisotropic growth of the nanostructures. To understand this, details of the chemical components present in the reaction medium and their role at the interface of the nanocrystals are analyzed. When a control reaction is performed without thiols, it is observed that the reaction proceeds in a different way leading to 1D nanorods/nanowires with wurtzite phase (Figure S1, Supporting Information). In this case, initially similar magic-sized nanocrystals are formed, but during the reaction they convert to 1D wurtzite rods/wires (detailed discussion in later section). This suggests that thiol plays a crucial role in directing the anisotropic growth of ZB ZnSe. As thiols are relatively stronger surface binding ligands than alkylamine, it is expected that their selective binding on the nanocrystals’ surfaces leads to specific directional growth to obtain such structures. However, excess thiol slows down the reaction and restricts the crystal growth, and even under proper conditions only the spherical particles (Figure 5) remain the 18765

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seeds undergo oriented attachment and change their phase to wurtzite during the course of the reaction. However, thiols being strong capping agents, their presence on the nanocrystal’s surface restricts the atomic deformation and retains ZB phase of the seed crystals without allowing their phase change to wurtzite. So also the growth proceeds retaining the ZB phase throughout. The entire process starts with the mechanism of polar [111] direction growth similar to the [001] directional growth in wurtzite phase for obtaining similar shapes of nanostructures starting from the wurtzite seed growth.2 So the exception here is the ZB phase, and in spite of having less polarity of the structure, the growth patterns can compete with that in the wurtzite phase leading to several anisotropic structures.

4. CONCLUSIONS In summary, we report here that with proper manipulation of the chemical and physical parameters controlling the crystal growth, some of the anisotropic nanostructures of ZnSe, like rods, bullets, hemispheres, and pyramidal shapes in the ZB phase can be designed which are preferably obtained in the wurtzite phase otherwise due to preferential directional growth along the polar axis in the wurtzite phase. Use of thiol as a capping agent plays here the most crucial role in determining phase as well as governing the shape evolution of the nanocrystals. Obtaining the entire anisotropic shapes in ZB phase, which was less probable for most of the materials, is a step ahead and would undoubtedly provide a new dimension in the phase-dependent directional crystal growth in colloidal synthesis.

Figure 6. (a) and (b) HRTEM and enlarged view of the HRTEM image of a bullet-shaped ZnSe nanostructure, respectively. These are the prestage of the hemisphere-shaped structures. The planes of the tip of the bullet intersect each other at 90°, and the planes are labeled within the images. (c) Unit cells and the schematic models for the pencil-/bullet-shaped structures with wurtzite and zinc blende phase, respectively.

further growth process. The temperature of 240−250 °C is enough for the thiol (DDT) desorption, and the alkylamines are also more labile at this temperature. However, as the (111) facet with only Zn atoms is more populated with thiol ligands, apart from this direction the nanocrystals continue to grow along other directions as stated above. Hence, the size of the hemisphere continues to increase. A typical schematic model representing the shape evolution process via ZB seed dot, then rod, bullet to hemisphere structure has been shown in Figure 7. Details of the growth direction and facet evolutions are marked in each image of the figure. Further annealing develops the facets in these hemispheres because of material diffusion and turns them to hexagonal pyramidal structures (Figure S5, Supporting Information). The final size can increase up to ∼25 nm, and they can also form stable colloidal dispersion. However, the interesting part here is the retention of ZB phase throughout these shape evolutions. It has been reported in literature that the crystallographic phase of nucleating seed particles is one of the most crucial factors in determining the crystal phase of the final nanostructures. As we already found thiol to be the key parameter to determine the shape of the nanocrystals, we investigated the seed particles in both the reactions with thiol and without thiol to understand the change of phase. We found that in absence of thiol, initial ZB spherical

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ASSOCIATED CONTENT

S Supporting Information *

Details of the instrumentation and supporting figures have been provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 7. Schematic presentation of zinc blende hemisphere-shaped nanostructures via seed dot, rod, and bullet. The planes are labeled in each case and are self-explanatory. 18766

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dx.doi.org/10.1021/jp406358e | J. Phys. Chem. C 2013, 117, 18762−18767