Letter pubs.acs.org/NanoLett
Structure Identification of Two-Dimensional Colloidal Semiconductor Nanocrystals with Atomic Flat Basal Planes Dongdong Chen,† Yuan Gao,† Yiya Chen,† Yang Ren,‡ and Xiaogang Peng*,† †
Center for Chemistry of Novel & High-Performance Materials and Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China ‡ X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *
ABSTRACT: Discrete nature of thickness and flat basal planes of two-dimensional (2D) nanostructures display unique diffraction features. Their origin was uncovered by a new analysis method of powder X-ray diffraction, which reveals thickness and lattice orientation of the 2D nanostructures. Results indicate necessity of adoption of a different unit cell from the corresponding bulk crystal with the same internal atomic packing. For CdSe 2D nanostructures with zinc blende atomic packing, pseudotetragonal lattices are adequate, instead of face-centered cubic. KEYWORDS: two-dimensional, nanocrystals, simulation, thickness, unit-cell, XRD
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identification challenging. For instance, for the best-studied zinc blende CdSe 2D nanocrystals, one type of samples with the lowest energy absorption peak at ∼512 nm in their UV−vis absorption spectra have been reported by various groups. Though their extremely sharp absorption features made scientists believe their basal planes should be atomic flat, its thickness was reported in a large discrepancy, ranging from 6 monolayers,7 6.5 monolayers,15,16 4 monolayers (5 Cd planes, but expanded by 0.2 nm),17 and 5 monolayers,18 by different groups using different techniques. Furthermore, because carboxylates were the sole ligands for these CdSe 2D nanocrystals, we suggested that their basal planes should be either (001) or (111) facets.15 High-resolution transmission electron microscope (TEM) seemed to support (001) as the basal planes15,17 but TEM measurements could not be exhaustive. In fact, most of the 2D nanocrystals would not provide atomic-resolved TEM images because of curling,15 distortion,18 and other types of problems associated with ultrathin 2D nanocrystals. With CdSe 2D nanostructures as our model systems, this work shall explore possibilities to identify the structures of 2D nanostructures by simulation of their X-ray powder diffraction patterns that are often substantially different from those of bulk crystals with the same internal atomic packing. Figure 1a illustrates a series of UV−vis absorption spectra for these 2D nanostructures and the corresponding TEM pictures are in Figure 1c, which confirmed that they should be 2D
wo-dimensional (2D) single-crystalline materials, that is, with one or few atomic layers on their thickness direction and bulk-like extension on the lateral dimensions, have attracted increasing attention in recent years.1−4 Their diverse and interesting properties, such as electronic properties of graphene5 and topological insulators6 as well as quantum confinement effects in semiconductor 2D nanocrystals,7 often require the basal planes to be atomic flat. Such extremely thin yet atomically flat morphology dictates their properties to be highly thickness sensitive. Though several techniques, such as Raman spectrum for graphene,8 absorption and photoluminescence spectra for MoS2,9 and angle-resolved photoemission spectroscopy spectra for Bi2Se3,6 are reported for various systems, there remains a challenge in the field to develop a general method to identify their structure, including thickness, crystal lattice structure, crystal orientation, and so forth. For the most studied semiconductor nanocrystals, high quality colloidal cadmium chalcogenides 2D nanocrystals with their thickness direction in quantum confinement regime10,11 came to the field much later than the corresponding zerodimensional (so-called quantum dots)12 and one-dimensional (so-called quantum rods and wires).13,14 This is likely due to the difficulty to controllably break symmetry of highly symmetric structures of the corresponding close packed lattices, namely, either face-centered cubic (zinc blende) or hexagonal (wurtzite). Simultaneously, unlike those layered structures in bulk, such as graphene and MoS2-type transition metal dichalcogenides, there is no existing knowledge about the layered structures for the 2D cadmium chalcogenide nanocrystals from the bulk crystals. This makes their structure © XXXX American Chemical Society
Received: March 9, 2015 Revised: June 2, 2015
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Figure 1. (a) UV−vis spectra of a series of CdSe 2D nanostructures in zinc blende atomic packing. (b) Corresponding X-ray powder diffraction patterns of the samples by synchrotron. (c) Corresponding TEM images of the samples. The y axes in (a) and (b) are linear scale.
nanostructures with flat basal planes. Because of quantum confinement effects,7,11 sharp and discrete absorption spectra in Figure 1a indicate that each sample should be pure in thickness, which is further supported by the uniform contrast in each nanocrystal in Figure 1c. Furthermore, the thickness should increase as the sharp lowest-energy absorption peak in Figure 1a shifts to red, which allows us to label each sample with its lowest absorption peak. Usually, dependent on growth conditions, this representative peak might vary by ∼3 nm. Nevertheless, this labeling system is relatively simple and shall be used throughout this report. For four samples in Figure 1, they would be labeled as UV(394), UV(463), UV(512), and UV(552), respectively. For the XRD peaks, they shall be labeled by the supposed Miller indices of the corresponding planes in bulk crystals. In our early report, we noticed an unusual peak appeared at the (110) position for UV(463),15 which is forbidden for bulk zinc blende CdSe. It was suspected to be a result of distorted structure of the lattice. To confirm and further clarify this unusual feature, the entire series of CdSe 2D nanostructures were studied with high-resolution X-ray powder diffraction (XRD) using synchrotron technique (Figure 1b). The x axes in all diffraction patterns in this report shall be q (= 4πsinθ/λ) unless stated differently, instead of the commonly used 2θ, in order to eliminate the wavelength (λ) effects. The y axes in all diffraction patterns represent the diffraction intensity on linear scale. The high-resolution diffraction patterns by synchrotron in Figure 1b demonstrate that the signal at the forbidden (110) position of the bulk zinc blende crystals was real for UV(463). These high-resolution patterns further revealed that the (110) peak seemed to be very sensitive to the variation of the absorption spectrum, that is, the thickness of the 2D nanostructures. Careful inspection warned us that the entire low angle area (shaded area in Figure 1b) was significantly
different from a standard diffraction pattern for the bulk represented by the black lines on the x axis. Dilation along thickness and lateral directions and broadening caused by the limited crystal dimension could not explain such changes. The elementary laws of X-ray diffraction of crystals tell us that certain planes would not be represented in diffraction patterns because of systematic extinction. Given a crystal structure, its extinction rules are fixed. Appearance of the originally forbidden diffraction peaks should be a strong signature of departure of lattice structure from the normal one in bulk. For 2D nanostructures, variation of lattice structure should mostly come from the thickness direction, which is likely the main concern for 2D nanostructures. These considerations invited us to investigate the extinction effects of the CdSe 2D nanostructures by simulation of the XRD patterns in a layer-by-layer fashion along the thickness direction. The basis of simulation is the discrete form of Debye equation19 I (S ) = I 0
P(r ) f 2 (S ) ∑ k sin(2πrkS) 2πS k rk
where I(S) is the diffracted intensity, I0 is the incident intensity, f(S) is the scattering factor, S is the scattering parameter (S = 2sin(θ)/λ for X-rays of wavelength λ diffracted through angle θ), rk is the interatomic distance, and p(rk) is the number of times for a given interatomic distance rk occurring. The bond length was allowed to be adjustable and differentiated from one direction to another. The bulk bond length (2.63 Å) for the Cd−Se bond was used as the standard and the atoms were arranged along the different thickness directions and lateral directions. The comparisons between the experimental patterns and simulations were based on the peak positions, peak contours and approximate relative peak intensities. In this work, we focused mainly on the crystal B
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when the total thickness changes from 2 to 4 monolayers (Figure 2a). Specifically, for 2 monolayers of each type of atoms, the (111) peak is tilted substantially to the left and the peak maximum shifted to the low-angle direction from the standard position. This peak becomes symmetrical for 3 and 4 monolayers and the position shifts gradually back to the expected position for the bulk crystal. Interestingly, among three cases, the forbidden (110) peak only appears for the 2D lattices with 3 and 4 monolayers of either Cd atoms or Se atoms. This means that at least one type of atoms should possess 3 (or more) monolayers of atoms for UV(394). Therefore, the thinnest choice for UV(394) should be a system with 3 monolayers of Cd and 2 monolayers of Se, given the basal planes should be terminated with Cd atoms. The (200) peak is in-phase for pure (both Cd−Cd and Se− Se) interactions but out-phase for the mixed interaction. For a nanocrystal with 3 monolayers of Cd and 2 monolayers of Se atoms, that is, 2.5 monolayers of Cd−Se unit, cancellation caused by Cd−Se mixed interference is very significant for the (200) peak, which results in a low intensity for this peak. However, no cancellation due to the mixed interaction was observed for the usually forbidden (110) peak (Figure 2b and c). Figure 2d illustrates that the experimental diffraction pattern can be well simulated with a model of 2.5 monolayers of Cd− Se along the [001] direction. Slight deviation from 2.5 monolayers, either 2 monolayers, 3.5 monolayers, or 4 monolayers significantly worsens the simulation. This means that the assumption of flat basal planes terminated by pure Cd ions with carboxylate as the surface ligands15 is consistent with this experimental observation. The atomic packing parameters used here are similar to the bulk zinc blende CdSe with 2.1% dilation along the lateral dimensions and 6% contraction along the thickness direction. Figure 2 implies that the apparent changes of (111) peak, that is, gradual shift to the high angle, being increasingly symmetric, and narrowing of the peak for the samples from bottom to top in Figure 1b should be accountable by consideration of thickness increase. However, there is a surprising peak between (110) and (111) for UV(463), UV(512), and UV(552). This peak shifts toward the (111) peak as the thickness increases. This new peak cannot be simply caused by the lattice dilation because similar degree of lattice dilation is observed for all four samples (Figure S1, Supporting Information). This problem is resolved by considering the pair interactions between different monolayers. As mentioned above, the diffraction features around (110) peak should be resulted from the Cd−Cd interaction and the Se−Se interaction (Figure 2a) without much contribution from the mixed interactions (Figure 2b). Furthermore, the traces caused by the Cd−Cd and Se−Se interferences with the same monolayers were nearly the same if we neglected their intensity (Figure 2a). By choosing 5 monolayers of Se as the model, we divided the diffraction into two types, that is, from Se atoms in the same monolayer and by Se atoms in different monolayers. The latter type is the pair interaction between monolayers. On the basis of the distance between two monolayers, for a 5-monolayer slab (Figure 3a, inset), the pair interactions could be further divided into four groups. All diffraction patterns caused by four pairs of interactions (labeled as 1−2, 1−3, 1−4, and 1−5 in Figure 3a) and within the monolayer (1−1 in Figure 3a) are shown in Figure 3a. Each group may represent a family of pair
structures along the thickness direction. For the lateral directions, because the 2D nanocrystals could be easily rolled, the grain size employed in the simulations would be smaller than that observed by TEM. According to quantum confinement theory,11 UV(394) should be the thinnest one in the series in Figure 1. In terms of lattice variation, the thinnest 2D nanostructure should be most obvious and simplest to analyze. According to literature, the thickness direction of the CdSe 2D nanostructures should be most likely ⟨001⟩ axes, with a small chance being ⟨111⟩ axes.15 For the sample with thinnest thickness, namely UV(394), literature suggested its thickness in the range between 2 and 4.5 monolayers of Cd−Se atomic bilayer along the [001] direction,7,11,15−17 which was applied as our initial testing sets. We suggest that, either along the [001] or [111] direction, it is convenient to define “one monolayer of CdSe in 2D nanostructure with zinc blende atomic packing” as a single Cd−Se diatomic layer. Diffraction intensity of a CdSe zinc blende lattice could be decomposed into three terms: the interference of the X-rays scattered by the Cd atomic layers only (Cd−Cd interaction, Figure 2a), by the Se atomic layers only (Se−Se interaction,
Figure 2. Calculation results of diffraction patterns for CdSe 2D nanocrystals with different thicknesses (MLs: monolayers). (a) Contribution from Cd−Cd interaction and Se−Se interaction. The dotted line indicates the (111) peak position for each case. (b) Contribution from Cd−Se mixed interaction. (c) Calculated results for all three interactions for 2.5-monolayer (3Cd-2Se) slab. (d) Experimental results of the 2D CdSe nanocrystals (UV(394)) and fitting results with different thicknesses.
Figure 2a), and by the Cd and Se atomic layers in a mixed fashion (Cd−Se mixed interaction, Figure 2b). The calculation results reveal that, the pure interactions, namely either Cd−Cd or Se−Se interferences, are identical despite the intensity difference but the Cd−Se mixed interferences (Figure 2b) is significantly different. Although the mixed interaction is mostly insensitive to the thickness of the 2D nanocrystals (Figure 2b), both Cd−Cd and Se−Se pure interactions demonstrate substantial differences C
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reaches at least 4 monolayers along [001] direction, the monolayer pair interactions generate an additional peak at the high scattering angle next to (110) peak due to the appearance of 1−4 and 1−5 (Figure 3a). Additionally, results in Figure 3a further illustrate that, upon an increase of distance between the pair of monolayers, the second maximum shifts toward (111) from being close to (110). With Cd ions as the basal planes, we only need to consider 2D structures with half monolayer more than an integer number of Cd−Se diatomic layer (Figure 3b). By considering all diffraction effects discussed above, this half-integer model displays a nearly identical thickness-dependence of the diffraction patterns shown in Figure 1b. As shown in Figure 3b, there is no additional peak between (110) and (111) for 2.5-monolayer sample (3Cd-2Se or UV(394)). The calculated trace of 3.5-monolayer sample (4Cd-3Se, UV(463)) starts to display the additional feature, and the new feature shifts toward (111) for 4.5- and 5.5-monolayer samples (5Cd-4Se or UV(512), and 6Cd-5Se or UV(552)). These features fit the experimental results well (see comparison between the experimental diffraction pattern for UV(552) and the calculated one for 6Cd-5Se in Figure S4, Supporting Information). Figure 4 (top left) shows that the unit cell of the zinc blende CdSe bulk (face-centered cubic F4̅3m structure) contains four Cd atoms and four Se atoms. In comparison, Figure 4 (bottom left) illustrates that the number of Cd atoms in the unit cell for the 2.5-monolayer sample (UV(394)) increases to six while the number of Se atoms is the same as the conventional unit cell for zinc blende. Evidently, the variation of the unit cell for 3Cd-2Se and other types of CdSe 2D nanocrystals in Figure 1 is due to the discrete nature and limited monolayers of atoms along the thickness direction. This means that, for the CdSe 2D nanocrystals with their internal atomic packing similar to zinc blende, the unit cell should be considered as tetragonal ones (Figure 4). Considering the single-unit cell thickness of the 2D nanostructures, we shall describe these tetragonal unit cells as “pseudo unit cells”. With a tetragonal pseudo unit cell, one
Figure 3. (a) Calculated scattering patterns for interaction within the monolayer and intermonolayer pair interactions for a 5-monolayer Se slab (see inset), (b) Calculated scattering patterns of zinc blende 2D CdSe nanocrystals with different thicknesses.
interactions. For example, 1−2 pair means the monolayer pair interactions between adjacent neighbors, 1−3 represents the pair interactions between one monolayer and its next neighboring monolayer, and so on (see further details in Figures S2 and S3, Supporting Information). When the thickness is 3 monolayers or less, all three groups of monolayer pair interactions (1−1, 1−2, and 1−3 in Figure 3a) possess the same peak position at the (110) peak no matter the interference is constructive or destructive. This means that any 2D nanocrystals with 3 monolayers or less of the same type (Cd or Se) of atomic monolayers, there should be only one peak for the (110) plane. Interestingly, if one type of atoms
Figure 4. (a) Difference of unit cells between zinc blende CdSe bulk and 2.5-monolayer CdSe nanocrystal. (b) Special unit cells for zinc blende 2D CdSe nanocrystals with different lattice structures along the thickness direction. The dark red area in each case illustrates a unit cell perpendicular to [010] direction. D
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Nano Letters could understand the appearance of (110) diffraction peak for the 2D nanocrystals readily (see detailed analysis is Supporting Information and Figure S5). The lattice orientation of 2D nanocrystals with flat basal planes can also be readily identified by analyzing the XRD patterns. Our previous report15 suggested that the basal planes for the series of CdSe 2D nanocrystals in Figure 1 should be either (001) or (111) facets because of the ligands being carboxylates. Simulation of the XRD patterns using the above techniques can resolve this uncertainty. Results (Figure S6, Supporting Information) indicate that 2D nanocrystals with [111] axis as the thickness direction is either no existed or with a tiny fraction in the sample. The method discussed above can be applied not only for the cases with cubic bulk lattice but also for 2D nanocrystals with other types of bulk crystal lattices. Wurtzite CdSe is a hexagonal lattice, and its corresponding 2D nanocrystals were reported by several groups.10,20,21 The lowest-energy absorption peak for most studied 2D nanocrystals with their internal atomic packing similar to that of bulk wurtzite structure is ∼449 nm. To be distinguishable, this wurtzite-type of CdSe 2D nanocrystals shall be written as UVwz(449). Numerous reports demonstrated that, for the same chemical composition and diameter, the lowest-energy absorption peaks for zero-dimensional II−VI semiconductor nanocrystals (quantum dots) in zinc blende and wurtzite structures are similar to each other.22−24 Therefore, one would guess the thickness of UV wz (449) should be approximately between that of UV(394) and UV(463), that is, between ∼0.6 nm and ∼0.9 nm. However, this estimated value seems to be too small in comparison to the values suggested in literature (either 1.4 nm25 or 1.5−2.0 nm21) based on limited information from HRTEM and atomic force microscope. Conversely, an early report26 suggested the thickness of a similar CdSe 2D nanostructure being 4-monolayer Cd−Se without much detail. The experimental XRD pattern of UVwz(449) (data from ref 27, 2θ as the x axis in the diffraction pattern) is substantially different from the standard wurtzite pattern represented by the blue lines on the x-axis (Figure 5a). However, it could be simulated well using a ∼0.6 nm 2D nanocrystals with their atomic packing configuration similar to the bulk wurtzite structure (4-monolayer Cd−Se with 4% of lattice contraction along the thickness direction). Conversely, the simulated pattern with the 2D nanocrystals with either 1.4 nm thickness (Figure 5a) or other thickness values (Figure S7, Supporting Information) is apparently different from the experimental pattern.. The [0001] direction of wurtzite structure is well known as the easy axis for growth.13 It should thus be natural to assume the longest dimensionalso one of the lateral directionsis [0001]. This would end up two different types of CdSe 2D nanocrystals, which switches the thickness direction and the other lateral direction (Figure 5b, left). Simulation reveals two easily distinguishable XRD patterns (Figure 5b, right), and the experimental results are consistent with Type I. The Scherrer equation is commonly used for estimating the grain size for zero-dimensional and one-dimensional nanostructures. However, for 2D nanocrystals with only one or a few monolayers along their thickness direction, Scherrer equation is of limited significance. Along the thickness direction, the discrete nature of thickness becomes dominating and the thickness might exceed the accuracy of Scherrer equation. As for the lateral directions, the commonly occurred twisting15 and
Figure 5. (a) Experimental XRD pattern of the wurtzite-type 2D CdSe nanocrystals (UVWZ(449)) and fitting results with different thicknesses. (b) Calculated results of 4-monolayer CdSe nanocrystals with different orientations. The wavelength of the X-ray associated with the experimental diffraction pattern was 0.154 nm.
rolling25 in 2D semiconductor nanostructures make it difficult to correlate the crystal domain sizes and TEM images. Fortunately, the coherence length along the lateral directions is not of critical importance for uncovering structural information along the thickness direction of 2D nanostructures with flat basal planes using the method described in this work. In conclusion, quantitative simulation of XRD patterns of 2D nanostructures with flat basal planes can readily reveal their most important structural information, such as thickness, crystal orientation, and crystal structure. This quantitative simulation method, different from traditional simulation of spherical- and rod-shaped nanocrystals, focuses on the layer structure along the thickness direction. The discrete nature along the thickness direction of 2D nanostructures brings in unique and unambiguous features into their XRD patterns, which are highly sensitive to the exact thickness and orientaiton. Simultaneously, the discrete nature also means that the unit cell of the corresponding bulk crystals with the same internal atomic packing is quantitatively incorrect for the 2D nanostructures. For instance, for CdSe 2D nanostructures with their internal atomic packing similar to that of bulk zinc blende structure, their unit cell is better described as a tetragonal pseudocell with the c-axis dimension equal to the length of the thickness, instead of cubic lattice of zinc blende. This choice of pseudo unit cell is due to the limited periodicity along the c axis though it is compatible with possible lattice dilation/contraction along either thickness or lateral directions. Instead of being based on specific physical properties, such as Raman,8 UV−vis absorption,7,9,11 and photoluminescence,6,9 the method described here depends on the X-ray diffraction that is universally available for crystalline materials and, thus, should be generally applicable to 2D nanostructures. E
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(23) Capek, R. K.; Moreels, I.; Lambert, K.; De Muynck, D.; Zhao, Q.; Vantomme, A.; Vanhaecke, F.; Hens, Z. J. Phys. Chem. C 2010, 114, 6371. (24) Yu, W. W.; Wang, Y. A.; Peng, X. G. Chem. Mater. 2003, 15, 4300. (25) Son, J. S.; Wen, X. D.; Joo, J.; Chae, J.; Baek, S. I.; Park, K.; Kim, J. H.; An, K.; Yu, J. H.; Kwon, S. G.; Choi, S. H.; Wang, Z. W.; Kim, Y. W.; Kuk, Y.; Hoffmann, R.; Hyeon, T. Angew. Chem., Int. Ed. 2009, 48, 6861. (26) Huang, X.; Li, J. J. Am. Chem. Soc. 2007, 129, 3157. (27) Lim, S. J.; Kim, W.; Shin, S. K. J. Am. Chem. Soc. 2012, 134, 7576.
ASSOCIATED CONTENT
S Supporting Information *
Additional synthesis procedure, calculation method and simulation results. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b00940.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21233005) and Fundamental Research Fund for the Central Universities (Grant 2014FZA3006). We thank Dr. Zheng Li (Argonne National Laboratory) for inspiring discussions. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
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