Nature Materials News & Views published: 14 October 2019 have cited the work of Prof. Matt Law’s group (Ref 6)
QUANTUM DOT SOLIDS
The path from ordered assemblies of quantum dots to epitaxially connected quantum dot solids is revealed with X-ray scattering and electron microscopy investigations.
The directed assembly of nanoscale building blocks into superstructures in which constituent building blocks can purposefully interact is of widespread scientific and technological interest. Scientists and engineers have been intrigued by the prospects of tailoring self-assembly processes to create materials whose properties and function can be controlled through the structure of the quantum dot building block (that is, size, shape and composition), the strength of the interaction between constituent particles, and the geometry of the superstructure. In particular, computational predictions of epitaxially connected colloidal quantum dot superlattices with long-range atomic coherence have generated significant interest as a platform for novel designer materials with rich electronic structure and experimentally accessible properties1.
However, recent charge transport measurements have shown that carrier delocalization is currently still limited by disorder within the epitaxially connected superstructure2,3,4. Gaining deeper insights into the relationship between electronic properties and crystallographic structure in these materials is predicated on access to high-fidelity supercrystals, which in turn demands a better understanding of the mechanism by which they form. The transformation of a colloidal quantum dot superlattice into an epitaxially connected quantum dot solid involves a complex choreography of several sub-processes, including changes in the position and orientation of dots within the superlattice sites. This choreography has previously been described as analogous to the metamorphosis of geometric tessellations made popular by the artwork of M. C. Escher5 (Fig. 1). What is astonishing, and was until now unknown, is how the nearly irreversible attachment of more than 104 proximate quantum dots is coordinated to enable the formation of micrometre-sized grains. Now, writing in Nature Materials, Alex Abelson and colleagues6 report the results of a detailed structural analysis that combines X-ray scattering, electron microscopy and electron diffraction to establish the transformation mechanism of epitaxially connected PbSe quantum dot solids. They demonstrate a transformation pathway in which the rhombically distorted body-centred cubic superlattice of quantum dots undergoes a slight deformation concurrent to rotation (10°) of the constituent dots to form the epitaxially connected quantum dot solid.
Creating epitaxially connected quantum dot superlattices involves a two-step process; the colloidal quantum dot building blocks are first assembled at a liquid/air interface, and then a chemical initiator is added to controllably remove the surface-bound protecting ligands and thereby trigger the formation of epitaxial bonds between proximate quantum dots. The organic ligands (in this case lead oleate) bound to the surface of the polyhedral quantum dot crystal are known to play a critical role in mediating the inter-particle interactions responsible for the initial self-assembly and the subsequent structure transformation. Infrared spectroscopy combined with theoretical binding energies to specific crystal facets allowed the researchers to describe how the ligand coverage changes throughout the transformation. Since the optical and electronic properties of quantum dots is very sensitive to their surface chemistry, the researchers performed two additional treatments to create environmentally and electronically robust superlattices. Exposure to lead iodide removed organic ligands from remaining facets and subsequent atomic layer deposition of amorphous alumina coated the exposed surface and infilled the void space of the superlattice.
Abelson and co-workers carefully combined insights from synchrotron-based grazing incidence X-ray scattering with real-space images obtained from transmission electron microscopy and electron diffraction to precisely determine how the structure of the superlattice evolves during formation of epitaxial connections between the constituent quantum dots. The presence of many higher-order reflections in the X-ray scattering pattern provides a very rich dataset, which allowed the researchers to precisely determine the rhombohedral distortion of the superlattice. Electron diffraction provided complementary insights into the exact crystallographic orientation of quantum dots within the superlattice. The researchers identified two distinct orientations that, after chemical treatment to initiate epitaxial fusion, led to the formation of two distinct topologies. More importantly, the detailed analysis of the structures before and after epitaxial fusion enabled the identification of the specific lattice deformation and quantum dot orientation involved in the transformation.
This mechanistic insight is an important advance in our understanding of how collective, multiscale topotaxy and epitaxy leads to the formation of mesoscale ‘crystals of crystals’. Whereas the proposed mechanism involves lattice deformation and quantum dot rotation, the detailed mechanistic description of the sequence of transformations remains an outstanding question. Understanding, and ultimately controlling, the complex interplay of thermodynamic and kinetic aspects of this transformation should enable the formation of even higher fidelity epitaxially connected superstructures. Beyond the specific model of polyhedral PbSe quantum dots demonstrated in this work it will be interesting to see how this mechanistic understanding can guide future efforts to create epitaxially connected quantum dot solids with other structures or compositions. From a processing perspective, important challenges related to the formation of macroscopic cracks due to volume reduction accompanying the transformation remain to be resolved. Multicomponent (such as binary or ternary) superlattices present a very intriguing future direction in this sense since advanced processing knowhow could enable the formation of superstructures in which one or both sublattices are epitaxially connected. The extent of connectivity between constituent dots in the superlattice thus presents a valuable degree of freedom in the fabrication and study of artificial materials with programmable optical and electronic structure.
- 1. Kalesaki, E. et al. Phys. Rev. X 4, 011010 (2014).
- 2. Whitham, K. et al. Nat. Mater. 15, 557–563 (2016).
- 3. Alimoradi Jazi, M. et al. Nano Lett. 17, 5238–5243 (2017).
- 4. Balazs, D. M. et al. Adv. Mater. 30, e1802265 (2018).
- 5. Whitham, K. & Hanrath, T. J. Phys. Chem. Lett. 8, 2623–2628 (2017).
- 6. Abelson, A. et al. Nat. Mater. https://doi.org/10.1038/s41563-019-0485-2 (2019).