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Long-range orientation and atomic attachment of nanocrystals in 2D honeycomb superlattices

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Science  20 Jun 2014:
Vol. 344, Issue 6190, pp. 1377-1380
DOI: 10.1126/science.1252642

Nanoparticle lattices and surfaces

The challenge of resolving the details of the surfaces or assemblies of colloidal semiconductor nanoparticles can be overcome if several characterization methods are used (see the Perspective by Boles and Talapin). Boneschanscher et al. examined honeycomb superlattices of lead selenide nanocrystals formed by the bonding of crystal faces using several methods, including high-resolution electron microscopy and tomography. The structure had octahedral symmetry with the nanocrystals distorted through “necking”: the expansion of the contact points between the nanocrystals. Zherebetskyy et al. used a combination of theoretical calculations and spectroscopic methods to study the surface layer of lead sulfide nanocrystals synthesized in water. In addition to the oleic acid groups that capped the nanocrystals, hydroxyl groups were present as well.

Science, this issue p. 1377, p. 1380; see also p. 1340

Abstract

Oriented attachment of synthetic semiconductor nanocrystals is emerging as a route for obtaining new semiconductors that can have Dirac-type electronic bands such as graphene, but also strong spin-orbit coupling. The two-dimensional (2D) assembly geometry will require both atomic coherence and long-range periodicity of the superlattices. We show how the interfacial self-assembly and oriented attachment of nanocrystals results in 2D metal chalcogenide semiconductors with a honeycomb superlattice. We present an extensive atomic and nanoscale characterization of these systems using direct imaging and wave scattering methods. The honeycomb superlattices are atomically coherent and have an octahedral symmetry that is buckled; the nanocrystals occupy two parallel planes. Considerable necking and large-scale atomic motion occurred during the attachment process.

In oriented attachment, a single nanocrystal (NC) is formed from two adjacent NCs through atomically matched bond formation between two specific facets. Controlled oriented attachment is currently emerging as a route to form extended one- and two-dimensional single-crystalline semiconductors of II-VI and IV-VI compounds (15). These superlattices are of interest in optoelectronics. Truncated nanocubes of the Pb-chalcogenide family (fig. S1) have been recently used to create two-dimensional (2D) atomically coherent ultrathin quantum wells (4) as well as superlattices with square or honeycomb geometries (fig. S2) (5). The formation of such systems is remarkable, given that several demanding conditions must be fulfilled. The NC building blocks must be nearly monodisperse in size and shape, and attachment should only occur with a geometrically defined subset of NC facets. The long-range atomic and nanoscale order in such systems is far from understood. For extended, atomically coherent PbSe superlattices with honeycomb geometry, immediate questions emerge regarding the large-scale crystallographic orientation of the NCs, the role of surface passivation of specific facets, and the atomic mechanism of attachment.

Here, we report on the atomic and nanoscale analysis of atomically coherent PbSe, PbS, and CdSe honeycomb superlattices. Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) tomography, we show that the honeycomb structures are buckled—the NCs occupy two parallel planes—and hence show nanoscale analogy with the proposed atomic silicene honeycomb structure. The specific orientation of the NCs in the 2D superlattice extends over hundreds of unit cells, suggesting that such types of single crystals must be formed from a preordered state, for example at the suspension/air interface. Compared with the native building blocks, the NCs in the honeycomb structure are considerably elongated in the direction of the NC-NC bond. This finding points to bond formation via necking accompanied with a gradual release of the capping molecules and considerable atomic motion within the NCs. Moreover, the 2D honeycomb structures of PbSe with a rocksalt atomic lattice were robust enough to be transformed into 2D CdSe lattices with a zinc blende atomic lattice through cation-exchange and keep the nanoscale honeycomb geometry intact. This method opens a route to a new class of 2D semiconductors with tunable composition. In these structures, the nanoscale honeycomb geometry has been predicted to result in both valence and conduction bands that can be filled with Dirac-type charge carriers as in graphene but, unlike graphene, with strong spin-orbit coupling (6).

The PbSe NCs have the shape of a cantellated cube, approaching that of a rhombicuboctahedron (fig. S1), implying that the NCs are terminated with {100}, {110}, and {111} facets. We estimated the NC size from the radially averaged diameter of the TEM projections for >1000 particles (5.3 ± 0.4 nm) (fig. S3). The oriented attachment of these NCs resulted in structures with long-range periodicity, as visualized by means of an equilateral triangle spanning the same number of unit cells along each vertex in the HAADF-STEM image (Fig. 1A). Zooming in on the honeycomb structure (Fig. 1B) reveals that the Embedded Image axes of the NCs are perpendicular to the substrate and that the NC-NC bonds are perpendicular to three of the Embedded Image axes (a discussion on atomic contrast in HAADF-STEM images is available in fig. S4). This result is also corroborated by electron diffraction (ED) patterns (Fig. 1C). The structures have a high degree of crystallinity, as observed with the occurrence of sharp spots in ED patterns recorded on selected areas with a diameter of 200 nm (Fig. 1C; a full analysis of all peaks in the diffraction pattern is available in fig. S5 and tables S1 and S2) (7).

Fig. 1 Single-crystalline PbSe honeycomb structures created by means of oriented attachment.

(A) HAADF-STEM image of the honeycomb structure (bright on a dark background). The equilateral triangle shows the long-range ordering of the structure. Scale bar, 50 nm. (B) High-resolution HAADF-STEM image showing that the Embedded Image NC axes are perpendicular to the honeycomb plane, and three of the Embedded Image axes are perpendicular to the NC bonds. (Inset) Zoom-in on the atomic columns indicated by the blue box. Scale bar, 5 nm. (C) Electron diffraction pattern showing the high degree of crystallinity. TEM image in the background shows the area on which the ED pattern was recorded (the honeycomb appears dark on a bright background). Red line and inset show the orientation of the diffraction spots with respect to the honeycomb structure, confirming that the Embedded Image axes are perpendicular to the NC bonds. Scale bars, 50 nm (TEM); 5 nm−1 (ED). (D to I) Models of the honeycomb structure, with cantellated cubes as nanocrystals. The two inequivalent sites in the honeycomb lattice are indicated by yellow/red and blue/green NCs. Rectangles (orange and light green) represent {110} facets, triangles (red and dark green) represent {111} facets, and squares (yellow, blue) represent {100} facets. (D) and (E) are the top and side view of the trigonal structure, respectively. (F) and (G) are top and side view of the tetrahedral structure, respectively. (H) and (I) are the top and side view of the octahedral structure, respectively.

There are three different models for attachment of the NCs that result in a honeycomb structure with the nanocrystal Embedded Image axes perpendicular to the substrate: attachments via the {110}, {111}, or {100} facets, respectively (Fig. 1, D to I). The three structures appear similar from the top but have a very different 3D geometry. Attachment via {110} facets results in a planar, trigonal structure with bond angles between the NCs of 120° (Fig. 1, D and E). If the NCs attach via the {111} facets, the overall structure of the superlattice is slightly buckled, with a tetrahedral symmetry and NC bond angles of 109.5° (Fig. 1, F and G). Attachment via {100} facets results in a highly buckled geometry, with an octahedral symmetry and NC bond angles of 90° (Fig. 1, H and I). We used various imaging techniques to show that the experimental structure is of the octahedral type, but that the bond lengths are longer than expected on the basis of a simple block model.

In the trigonal model, the attachment takes place via the {110} facets, meaning that the Embedded Image axes are parallel to the NC bonds. However, both the high-resolution HAADF-STEM images and the ED patterns show that in our experiment, the Embedded Image axes are perpendicular to the NC bonds (Fig. 1, B and C), so we could discard the trigonal model. In the buckling of the honeycomb, the HAADF-STEM images showed additional scattering strength on the NC bonds, indicating a larger-than-average thickness of the sample at that position. In this respect, both the tetrahedral and octahedral model are buckled and have three of the Embedded Image NC axes perpendicular to the NC bonds, so it was impossible to discriminate between these remaining models by ED or 2D projections from high-resolution HAADF-STEM. Thus, we needed to use a combination of scanning tunneling microscopy (STM), grazing-incidence small-angle x-ray scattering (GISAXS), and electron tomography to fully resolve the honeycomb structure.

The STM measurements showed that the honeycomb structure is indeed buckled (Fig. 2, A and B). However, topography in STM is a complicated convolution of NC height and tip radius (fig. S6), so no quantitative height data could be extracted. We cannot measure bond lengths and angles directly in STM, but we could reliably determine the distance between next-nearest neighbors (NNNs) at the same height (8.5 ± 0.8 nm, for >500 measurements). From GISAXS (Fig. 2, C and D, and fig. S7), we observed scattering peaks arising from the hexagonal order of the holes in the honeycomb structure with relative positions of 1:Embedded Image:2:Embedded Image, as expected from a honeycomb structure (Fig. 2, C and D). Looking at the absolute peak positions in Fig. 2D, we found a hole-hole distance of 8.5 nm, which is in accordance with the NNN distances observed in STM. Assuming perfect honeycomb models, this distance implies a bond length of 5.2 nm for the tetrahedral and 6.0 nm for the octahedral honeycomb structure.

Fig. 2 STM and GISAXS measurements of the PbSe honeycomb structures.

(A) Constant-current STM measurement showing high (red star) and low (green triangle) nanocrystals forming a honeycomb structure. Feedback settings are 3.5 V and 20 pA. Scale bar, 20 nm. (B) Height profile along the blue line indicated in (A). The position of a high NC (star), low NC (triangle), and hollow site (diamond) are indicated. (C) GISAXS pattern of a honeycomb structure on top of a Si(100) substrate, under an angle of incidence of 0.7°. Scale bar, 1 nm−1. (D) Line trace in the horizontal direction indicated in (C), revealing the in-plane order. Red lines mark diffraction peaks with relative positions of 1:Embedded Image:2:Embedded Image, arising from the hexagonal order of the holes in the structure.

The tomographic reconstruction of a PbSe honeycomb structure obtained with electron tomography is shown in Fig. 3 (movie S1) (710). Slices through the reconstruction in the direction perpendicular to the substrate indicate that the two inequivalent NCs in the honeycomb unit cell are located on different heights (Fig. 3, B and C) (11). We performed automated particle detection (7) on the region of interest indicated in Fig. 3D. We remark that the effect of the missing wedge in our tomography data results in a blurring of the contrast along the z direction. In (7), we used simulated tomography data to show that the missing wedge in this case does not impede objective particle detection (fig. S8). From the resulting coordinates, we constructed a model resembling the tomographic reconstruction (Fig. 3E). Using a Voronoi method, we could measure important parameters such as bond angle, bond length, and NNN bond length (Fig. 3, F to H).

Fig. 3 HAADF-STEM tomography on a PbSe honeycomb structure.

(A) Overview of the honeycomb structure obtained by averaging the tomographic reconstruction in the direction perpendicular to the substrate. Scale bar, 10 nm. (B and C) Slices through the tomogram perpendicular to (A), along the (B) red or (C) green line indicated in (A). (D) Iso-volume rendering of the tomographic reconstruction in the region indicated with the blue box in (A). (E) Equally sized spheres plotted on the coordinates obtained by means of automated particle detection in the region indicated in blue in (A). (F) Bond angles measured from the coordinates in (E), showing a bond angle of 95 ± 5°. (G) Bond lengths to nearest neighbors, showing a bond length of 6.0 ± 0.5 nm. (H) Bond lengths to NNNs, showing a NNN bond length of 8.9 ± 0.6 nm.

The obtained bond angle of 95° ± 5° (Fig. 3F) shows that the honeycomb structure is of the octahedral type, with bonds between the {100} facets. We can compare the ratio between bond length (6.0 ± 0.5 nm) and NNN bond length (8.9 ± 0.6 nm) that we find with the theoretical values for the different structures—for the octahedral type, one would expect 1:Embedded Image; for the tetrahedral type, 1:Embedded Image; and for the trigonal type, 1:Embedded Image. The ratio we find here (1:1.48) is a bit higher than the expected ratio for an octahedral structure, which is in agreement with the bond angles being slightly higher than 90°. The same measurement on a tomogram of a CdSe honeycomb structure shows bond angles of 85° ± 6°, which is slightly lower than expected (figs. S9 and S10 and movie S2).

We established that the NCs attach via the {100} facets into a honeycomb structure with octahedral symmetry (Fig. 1, H and I). Next, we returned to the bond lengths of the honeycomb structure, as extracted from TEM (fig. S11), STM, GISAXS, and tomography. All four techniques give an independent measure of 6.0 nm for the NC bond lengths, which is an increase of 13% of the original NC diameter. The increased bond lengths shed light on the microscopic mechanism of facet-to-facet atomic attachment. Li et al. (12) showed that in the process of oriented attachment, the NCs are continuously rotating and moving in close proximity by means of Brownian motion to find an optimum configuration before enduring attachment takes place. We propose that during the rotation and Brownian motion as described by Li et al. (12), atomic-scale necking took place as a first step in the attachment. After necking had started, the capping molecules that remain bound to the facet were gradually removed, and the neck extended perpendicular to the bond axis through large-scale atomic motion. For PbSe NCs, considerable atomic reconfigurations have been reported (13, 14). The necking explains the elongation of the bond lengths, which also results in a considerably more open honeycomb structure than can be obtained from geometric attachment of rigid block models (Fig. 1H, as compared with the real structures in Figs. 1, A to C, 3, and 4). In the mechanism that we propose, not all of the capping molecules have to be released at once from a facet, resulting in pathways with lower activation energy. Oriented attachment accompanied with neck formation has been reported before as a pathway for the growth of various metal nanoparticles (15, 16).

Fig. 4 Single crystalline CdSe honeycomb structures created by cation exchange.

(A) HAADF-STEM image of the CdSe honeycomb structure. The honeycomb appears bright on a dark background. The equilateral triangle shows that the long-range ordering of the structure is retained. Scale bar, 50 nm. (B) High-resolution HAADF-STEM image showing that the orientation of the Se anion lattice with respect to the superlattice geometry is preserved. (Inset) Zoom-in on the area indicated by the blue box. Scale bar, 5 nm. (C) An ED pattern showing that the high degree of crystallinity is preserved. TEM image in the background shows the area on which the ED pattern was recorded (the honeycomb appears dark on a bright background). Red line and inset show the orientation of the diffraction spots with respect to honeycomb structure, confirming that the Embedded Image axes are perpendicular to the NC bonds. Scale bars, 50 nm (TEM); 5 nm−1 (ED).

Since the emergence of graphene (17), there has been a general interest in the properties of electrons confined in a honeycomb lattice (1823). We show that oriented attachment can form the basis of a generally applicable two-step method for the preparation of 2D semiconductors with nanoscale honeycomb geometry: assembly and oriented attachment of Pb-chalcogenide NCs with truncated cubic shape, followed by cation exchange. It has been predicted that honeycomb semiconductors of zinc blende compounds (such as CdSe) show a truly new electronic band structure, with a valence hole Dirac band and one or two conduction electron Dirac bands combined with strong spin-orbit coupling (6). We transformed PbSe honeycomb structures into CdSe superlattices via a cation exchange reaction (24, 25) and showed that honeycomb structures can also be prepared from PbS NCs (figs. S12 and S13).

The successful exchange of a PbSe honeycomb lattice into a CdSe honeycomb is presented in Fig. 4. The complete transformation of the PbSe lattice into CdSe is confirmed by means of energy-dispersive x-ray spectroscopy (fig. S14). HAADF-STEM and ED measurements show that the orientation of the Se anion lattice with respect to the honeycomb periodicity is preserved (Fig. 4, A to C). This result is in agreement with earlier described mechanisms in which the anion lattice is preserved during cation exchange (26, 27) and is corroborated with high-resolution HAADF-STEM measurements of honeycomb structures at intermediate stages of cation exchange (7).

Supplementary Materials

www.sciencemag.org/content/344/6190/1377/suppl/DC1

Materials and Methods

Figs. S1 to S14

Tables S1 to S2

References (28, 29)

Movies S1 to S2

References and Notes

  1. Materials and methods are available as supplementary materials on Science Online.
  2. The PbSe honeycombs suffer from beam damage, making it hard to obtain high-quality tomographic reconstructions. CdSe, on the other hand, is more stable under the electron beam, resulting in higher-quality tomographic reconstructions (Fig. 3 as compared with fig. S9, for example. Both images are obtained under the same conditions).
  3. Acknowledgments: We thank ESRF (Grenoble, France) for providing the synchrotron beamtime and the staff of ESRF beamline ID-01 for their support. We thank C. van Overbeek for his help on the PbS sample. This research is part of the programs “Control over Functional Nanoparticle Solids (FNS)” and “Designing Dirac Carriers in semiconductor honeycomb superlattices (DDC),” which are supported by the Foundation of Fundamental Research on Matter (FOM), which is part of the Dutch Research Council (NWO). The authors also acknowledge financial support from the European Research Council (ERC Advanced Grant 24691-COUNTATOMS and ERC Starting Grant #335078-COLOURATOM). The authors also appreciate financial support from the European Union under the Seventh Framework Program [Integrated Infrastructure Initiative 262348 European Soft Matter Infrastructure (ESMI)]. This work was supported by the Flemish Fund for Scientific Research (FWO Vlaanderen) through a Ph.D. research grant to B.G.
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