Structural evolutions of CdSe nanocrystals in ripening process

Materials Chemistry and Physics 111 (2008) 513–516

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Structural evolutions of CdSe nanocrystals in ripening process Xinfeng Zhang, Zhihu Sun, Wensheng Yan, Feng Wei, Shiqiang Q. Wei ∗ National Synchrotron Radiation Laboratory, University of Science and Technology of China, 42 HeZuoHua Road, Hefei, Anhui 230029, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 12 November 2007 Received in revised form 17 March 2008 Accepted 2 May 2008 Keywords: Nanostructures XAFS Chemical synthesis

a b s t r a c t A size-selective separation method is used to synthesize CdSe nanocrystals (NCs) with different grain sizes. The structural evolutions of CdSe NCs in the ripening process are investigated by high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), and X-ray absorption fine structure (XAFS) spectroscopy. The results indicate that the structural disorder S2 of the Se–Cd bond in CdSe NCs rises from 0.0005, 0.0012 and 0.0034 A˚ 2 as the size of NCs increases from 1.9 to 2.7 and 3.1 nm. The small S2 of the 1.9 nm NCs unambiguously reveals that at this stage the CdSe NCs are well crystallized and almost free of interior defects. The unusual increase of S2 with size can only be interpreted by the interior defects rather than surface defects of NCs. The interior defects produced in the core of the CdSe NCs through the ripening process are accumulated. This leads to a rapid increase in their structural disorders for the large CdSe NCs. © 2008 Elsevier B.V. All rights reserved.

1. Introduction CdSe nanocrystals (NCs) with quantum confinement effects have been comprehensively studied from theoretical and experimental viewpoints recently [1–5], because it is easy to prepare the CdSe NCs with various grain sizes in a very narrow size distribution (<5%) by colloidal chemistry methods [6–10]. Furthermore, CdSe NCs in the grain size range of 1 and 5 nm show a tunable optical property with the band emission covering the whole visible light spectra (420–660 nm) [6]. Hence, the CdSe nanocrystal-based emitters can be widely used for optoelectronic devices and biomedical tags [11–13]. Optical properties of CdSe NCs are greatly affected by their structures as well as preparations [14]. A better understanding of growth mechanism will benefit the accurate control of structures and properties of the CdSe NCs. The NCs synthesized by the colloidal method involves simultaneously crystallization and growth process [15], therefore, it is difficult to separately measure one of these two processes. Recently, through a carefully designed experiment, Chen et al. investigated the isolated crystallization process of 2.0 nm CdSe NCs and found that the initially formed amorphous nanoparticles are crystallized well after a long reaction time [15]. A size-dependent structure study on CdS NCs with a diameter of 1.3–12 nm prepared by wet chemical reaction in the presence of organic stabilizer, has revealed that there is a cluster size of about

∗ Corresponding author. Tel.: +86 551 3601997; fax: +86 551 5141078. E-mail addresses: [email protected], [email protected] (S.Q. Wei). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.05.001

3.0 nm separating thermodynamically and kinetically controlled growth regimes [16]. In this paper, in order to separately investigate structural evolutions of CdSe NCs during the growth process, we designed such a method to obtain CdSe NCs with different sizes. CdSe NCs with a rather broad size distribution were synthesized in the same reaction batch firstly, and then a method of size-selective separation was used to isolate the different size NCs with a narrow distribution regime [17]. In this way the observed structure changes of CdSe NCs with various sizes can be mainly attributed to the grain growth process. The sizes of the selected NCs are determined by high-resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD), and the structural evolutions of the CdSe NCs were monitored by X-ray absorption fine structure (XAFS) spectroscopy. 2. Experimental CdSe NCs were prepared by aqueous synthesis method reported by Rogach et al. [17]. These thiol-capped CdSe NCs from aqueous synthesis could be separated into several monodispersive fractions of different sizes through post size-selective process. Briefly, 2 mmol CdCl2 was dissolved in 200 ml water with 1 ml thiolglycolic; the pH of the solution was adjusted to 11.2 with 1 M NaOH. Then the fresh 1 mmol NaHSe solution was quickly injected into the N2 saturated Cd2+ solution under fast stirring. The solution was refluxed at 100 ◦ C for 4 h. The CdSe NCs with three different sizes were obtained by a size-selective separation. Briefly, ethanol was added dropwise to the solution until it became turbid, then the precipitate was collected by centrifugation and was considered as sample 1, the supernatant was further disposed by the same procedure to obtain the samples 2 and 3. This post-preparative size-selective precipitation procedure allowed the separation of the initial colloidal solution into several fractions of NCs with narrowed size distributions. The sizes of the CdSe NCs in these three samples are 3.1, 2.7,

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X. Zhang et al. / Materials Chemistry and Physics 111 (2008) 513–516 Table 1 The structural parameters of CdSe NCs from XRD Sample size (nm)

2 1 1 1 (◦ )

˚ d1 1 1 (A)

˚ a (A)

1.9 2.7 3.1

25.7 25.6 25.5

3.47 3.48 3.49

6.00 6.03 6.05

Fig. 1. XRD patterns of CdSe NCs with different sizes.

and 1.9 nm, respectively, as demonstrated by the XRD and HR-TEM results shown below. The HR-TEM images were recorded by a JEOL-2010 electron microscope operated at 200 kV. XRD measurements were carried out on a Rigaku D/Max-RA diffractometer using the Cu K␣ radiation. The Se K-edge XAFS spectras were measured at the beamline of U7C of National Synchrotron Radiation Laboratory (NSRL). The storage ring of NSRL was operated at 0.8 GeV with a maximum current of 300 mA. The hard X-ray beam was from a three-pole superconducting Wiggler with a magnetic field intensity of 6 T. The fixed-exit Si(1 1 1) flat double crystals were used as monochromator. The XAFS data were recorded in a transmission mode with ionization chambers filled with Ar/N2 at room temperature. Data analysis was performed by using NSRL-XAFS 3.0 software package compiled by Zhong and Wei according to the standard procedures [18].

3. Results and discussion Fig. 1 displays the XRD patterns of the CdSe NCs obtained by a size-selective separation. It can be observed that the three samples have the same diffraction pattern, where the peaks at about 25.4, 42.0, and 49.7◦ correspond to the (1 1 1), (2 2 0), and (3 1 1) faces of the zinc-blende phase of CdSe (JCPDS file No. 19-0191). The average particle sizes of the three samples, estimated from the broadening of XRD patterns with Scherrer formula are 1.9, 2.7, and 3.1 nm, respectively. Moreover, the colors of the CdSe NCs are yellow, orange and red, which are consistent with the results reported by Rogach et al. due to the well-known quantum size effect [17]. A close examination reveals that the 2 shifts slightly to a larger value as the NCs size decreases, as seen in Table 1. This indicates a lattice contraction in the thiol-capped CdSe NCs. The analysis for 1.9, 2.7,

Fig. 3. (a) The EXAFS (k) functions and (b) the RSFs by Fourier transforming the k3 -weighted (k) functions of CdSe NCs with different sizes.

and 3.1 nm NCs shows a lattice contraction of 1.2%, 0.8% and 0.4%, respectively, relative to the bulk value. The TEM images of all three fractions of CdSe NCs are shown in Fig. 2. Nearly spherical CdSe NCs with sizes close to 2–4 nm can be resolved, which is consistent with the estimation from the XRD patterns. The existence of lattice planes on the HR-TEM image confirms the crystallinity of CdSe NCs. The size distributions determined by statistical evaluation of tens of particles from TEM are around 10%. This indicates that the size distributions of these NCs are rather narrow, which are similar to other reports [17,19].

Fig. 2. TEM overview image of three fractions: (a) 1.9 nm, (b) 2.7 nm and (c) 3.1 nm CdSe NCs (bar 20 nm) with a HR-TEM image (inset, bar 5 nm).

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Table 2 The structural parameters around Se atoms for CdSe NCs from EXAFS fitting Sample

Bond type

˚ R (A)

1.9 nm 2.7 nm 3.1 nm CdSe crystal

Se–Cd Se–Cd Se–Cd Se–Cd

2.582 ± 0.005 2.586 ± 0.005 2.606 ± 0.003 2.631

Fig. 3(a) demonstrates the Se K-edge normalized EXAFS spectrum for the CdSe NCs. The radial structural functions (RSFs) are derived from the experimental spectra following the standard procedure. The EXAFS function, , was obtained by subtracting the postedge background from the overall absorption and then normalized with respect to the edge jump step. The normalized (E) was transformed from energy space to k-space, The (k) data were multiplied by k3 to compensate for the damping of EXAFS oscillations in the high k-region. Fig. 3(b) shows the RSFs around Se atoms by Fourier transforming the k3 -weighted (k) functions. The RSFs curves have only one peak at 2.45 A˚ corresponding to the first shell Cd atoms in CdSe with zinc-blende structure. This is in agreement with that of the bulk CdSe reported by Carter et al. [20]. In order to obtain quantitative structural parameters, the first RSF peak was inversely Fourier transformed to k-space and then fitted by one Se–Cd coordination pair. A non-linear least-squares algorithm was applied for curve fitting of EXAFS in k-space between 2.5 and 12 A˚ −1 , following the standard EXAFS formula: (k) =

(E) − 0 (E) = 0 (E)

 Nj S2 fj (k)  0

j

k

0

∞

g(Rj ) Rj2

e−2rj /(k)

×sin[2kRj + ı(k) + 2C (k)] dR.

(1)

Here k is the photoelectron wave vector defined as k = [2m(E − E0 )/2 ]1/2 , E0 is the threshold energy, S02 is the amplitude reduction factor, R is the distance between the absorbing and scattering atoms, g(Rj ) is the pair distribution function of the scattering atom, (k) is the mean free path of the excited photoelectron, fj (k) and ı(k) are the backscattering amplitude and scattering phase shift of the scattering atom, respectively, and C (k) is phase shift of the absorbing atom. The theoretical amplitude and phase shift functions were generated by FEFF8 code [21]. In the fitting procedure, N, R and  2 were treated as variable parameters, whose best-fit results are summarized in Table 2. Seen from Table 2, the disorder factor  2 of Se–Cd bond is 0.0042 (0.0049 and 0.0071) A˚ 2 for the CdSe NCs of 1.9 (2.7 and 3.1) nm, respectively. Here the disorder  2 is a summation of thermal and static disorders. At room temperature the thermal disorder of Se–Cd bond in the CdSe NCs can be assumed to be that (0.0037 A˚ 2 ) of the bulk CdSe with a zinc-blende structure [22]. Therefore, the structural disorder S2 of the Se–Cd pair is 0.0005, 0.0012 and 0.0034 A˚ 2 for the CdSe NCs with the sizes of 1.9, 2.7 and 3.1 nm, respectively. Although the structure disorder factor S2 of the first Se–Cd coordination shell is 0.0005 A˚ 2 for the 1.9 nm NCs, it increases rapidly to 0.0012 and 0.0034 A˚ 2 for the 2.7 and 3.1 nm NCs. Generally, the structural disorder in NCs may come from surface or/and interior. The surface structural disorder should be larger than that of the interior, due to the existence of surface dangling bonds or different types of ligands around the surface atoms [15,23]. A number of studies on the size-dependent disorder of NCs have reported that the disorder increases as the size decreases, which is mainly ascribed to the surface defects [16,24]. For example, Hamad et al. reported the increased structural disorder as the size of CdSe nanocrystals decreased from 8 to 1.7 nm, and attributed it to a reconstruction of the surface in the spherical nanocrystals [24]. On

N 3.6 ± 0.3 3.7 ± 0.3 3.8 ± 0.2 4

 2 (×10−3 A˚ 2 ) 4.2 ± 0.1 4.9 ± 0.1 7.1 ± 0.1

the contrary, for the CdSe NCs in this study, the variation of structural disorder with size exhibits an opposite trend and can only be explained by the interior defects rather than surface defects. It is of interest to understand why the interior disorder can dominate over the surface disorder as the size of CdSe NCs increases in our samples. In order to minimize the surface effect on the disorder degree around the Se atoms in the CdSe NCs, the thiolglycolic stabilizing ligands and the excessive Cd2+ were used in the solution during the preparation reaction process. This would result in a nonstoichiometric ratio of Cd/Se (larger than 1) for the nanoparticles. This non-stoichiometry was also shown to occur in the growing of uncapped ZnS nanoparticles recently, which is generally induced by the excess atoms of surface shell [25,26]. In our study, the surface of CdSe NCs is terminated by Cd2+ ions which are further coordinated with thiol groups, and the majority of Se atoms are located at the core of NCs. Thus a stoichiometric Cd/Se ratio can be postulated for the core region of the particles, though the surface region of Cd–thiol shell is locally non-stoichiometric [25,26]. This indicates that the Se–Cd first coordination shell mainly shows the interior structure information of CdSe NCs from the Se K-edge XAFS. Table 2 further indicates that for all the CdSe NCs, the obtained Se–Cd coordination numbers are larger than 3.6, close to the value (4) of the perfect CdSe crystal. This is in good agreement with a lot of literature results indicating the interior nature of Se atoms ¨ for thiol-capped group II–VI nanoparticle [22,23,27]. Eychmuller and Rogach investigated the structure of the core and surface separately of thiol-capped group II–VI nanoparticle, which were found to be a spherical core–shell like system made of a CdTe (or CdSe) core with zinc-blende lattice and a heteroepitaxially overgrown CdS monolayer [27]. By Se K-edge EXAFS, Marcus et al. also studied the structures of CdSe and CdS clusters capped with –SeC6 HS or –SC6 HS terminations [22]. They found that the cluster interiors have bulklike Cd coordinations around the Se atoms, with distances reduced from the bulk values by a marginally detectable amount. All these results lead us to believe that the majority of Se atoms in CdSe NCs are located in the interior. Since all the CdSe NCs were synthesized from the same batch colloidal solution under the same reaction conditions by a sizeselective method, the effects of reaction conditions and stabilizer on the structures of NCs with different sizes are expected to be identical. Therefore, the size-dependent structural evolution of NCs mainly reflects their grain growth process. Concerning the growth process of colloidal NCs, there are typically two types of mechanisms, namely, the orient attachment process [28] and the Ostwald ripening process [6,29]. The orient attachment mechanism was proposed by Penn and Banfield recently for the growth of TiO2 [30], ZnO [31], CuO [32], ZnS [33] and some other nanoparticles [28] under hydrothermal conditions. The characteristic of this growth mechanism is that the primal units aggregate oriented along certain direction to form secondary crystalline structures which are normally irregular in their shape [28,33]. The ripening process, different from orient attachment, is a widely accepted growth mechanism for a large variety of colloidal NCs, which involves the diffusion of individual molecules from smaller aggregates to larger ones driven by the difference in the relative magnitude of the sur-

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face free energy [6,29]. Specially, the Ostwald ripening process has been believed to be the main path of group II–VI semiconductor nanocrystals by thiol stabilizers [17,19]. For our thiol-capped CdSe NCs, the HR-TEM images also provide evidence supporting the grain growth mechanism of the Ostwald ripening process. The growth of CdSe NCs from aqueous synthesis involves a fast nucleation process after quick injection of NaHSe solution and later a slow growth stages, which follows the two-stage growth mechanism proposed for precipitation method [34], microemulsion method [35], and hot-matrix method [36]. There are time constants for each stage, and the typical time of growth stages is 103 to 104 s [34–36]. The time scale of aqueous synthesis after initial fast injection is in the order of 104 s, which also evidences Ostwald ripening in the growth stages as Ostwald ripening should be very slow [36]. From the XAFS analysis, the ripening process increases the structural defects and disorder while the NCs grow in size, likely in such a way: the grain growth of CdSe NCs is through the deposition of Cd and Se ions on the surface of CdSe nucleates. The defects on the nucleates surface remain as the growth of NCs proceeds [17], and consequently, the disorder increases with the grain size. Another notable result in Table 2 is that the bond lengths RSe–Cd ’s in the NCs are obviously contracted relative to the value 2.631 A˚ in crystalline CdSe: the RSe–Cd is 2.582, 2.586 and 2.606 A˚ for the CdSe NCs of 1.9, 2.7 and 3.1 nm, respectively. This result shows a lattice contraction of 1.9%, 1.7% and 1.0% for the 1.9, 2.7 and 3.1 nm NCs, in good agreement with the XRD analysis. The similar trend of lattice parameters changing with the size of NCs was also reported by several authors. The contracted bond lengths indicate that the CdSe NCs are compressively strained. This phenomenon has been observed and predicted by many authors in similar systems [14,17,20,22,23], and has been interpreted as the result of lattice mismatch between the epitaxially grown CdSe and CdS in the thiolate layer. Elastic distortion of the interior lattice of the CdSe NCs provides a possibility to decrease the surface energy and to minimize the total free energy [37,38], which in turn strongly affects the crystallization process during the NCs growth. Among the three CdSe NCs samples, the 1.9 nm NCs is under the largest strain, and the crystallization in the core of a particle is the most complete, resulting in the smallest structural disorder of 0.0005 A˚ 2 as we have obtained. It is well known that for NCs grown by colloidal chemical method using stabilizing ligands, there is a uniquely stable size separating the thermodynamically and kinetically controlled grown regime [14–16,39]. Our results have unambiguously revealed that the CdSe NCs of 1.9 nm are well crystallized and almost free of interior defects, which may be governed by the thermodynamical growth process. As the NCs further grow beyond the stable size, the growth process becomes a kinetic one, through ripening process. More interior structural defects are produced and the crystallinity of the NCs is deteriorated, which leads to the rapidly increased structural disorder as shown in Fig. 3 and Table 2. 4. Conclusions In summary, we have investigated the ripening effect on the structures of colloidal grown CdSe NCs by XRD, HR-TEM and XAFS. The XAFS results indicate that the structural disorder S2 of CdSe NCs increases from 0.0005 to 0.0012 (0.0034) A˚ 2 with its size from 1.9 to 2.7 (3.1) nm during the ripening process of colloidal chemical preparation. The results provide direct evidence that CdSe NCs at the size of 1.9 nm are grown in a thermodynamic manner. For NCs

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