Review Cite This: Chem. Mater. 2018, 30, 54−63
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Entropic, Enthalpic, and Kinetic Aspects of Interfacial Nanocrystal Superlattice Assembly and Attachment Kevin Whitham,† Detlef-M. Smilgies,‡ and Tobias Hanrath*,# †
Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14850, United States Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York 14853, United States # Robert Fredrick Smith School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14853, United States ‡
S Supporting Information *
ABSTRACT: The combination of self-assembly and directed attachment of colloidal nanocrystals at fluid interfaces presents a scientifically interesting and technologically important research challenge. Remarkable strides have been made in the synthesis of polyhedral nanocrystals with precisely defined shapes and their self-assembly into highly ordered superstructures. We discuss the interplay of entropic and enthalpic driving forces and the kinetic aspects of interfacial selfassembly and attachment. We present in situ parallel smallangle X-ray scattering measurements and emerging insights into the complex choreography of interfacial transport processes involved in the formation of highly ordered epitaxially connected nanocrystal solids. New understanding emerging from in situ measurements provides process control and design principles for the selective formation of specific superlattice polymorphs. We discuss outstanding challenges that must be resolved to translate know-how from controlled assembly and attachment in the laboratory to scalable integration for emerging technological applications.
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INTRODUCTION The formation of complex mesoscale superstructures by directed assembly of nanoscale building blocks is of broad scientific and technological interest. Scientists and engineers have been captivated by the prospects of tailoring the interaction between constituent particles to direct self-assembly processes to create materials with properties and function by design.1,2 Recent research interest in self-assembly has received a profound boost from the availability of colloidal nanocrystal (NC) building blocks with precisely engineered size, shape, and composition.3,4 At the same time, advances in self-assembly and the ongoing expansion of the library of available NCs have created an exciting opportunity space for bottom-up solution strategies with important implications for a broad spectrum of technologies. The far-reaching prospects of this emerging class of metamaterials5 in catalysis,6−8 electronic,9,10 thermoelectric,11,12 magnetic,13 and photovoltaic14 applications have been captured in recent reviews. Thin films of NCs have been deposited by drop-casting,15 spin-casting,16,17 doctor-blade coating,18,19 spray-coating,20 and inkjet printing.17,21 Maenosono et al.22 reviewed a variety of wet coating techniques to form NC superlattices. Inkjetprinting23,24 and spray coating20 have significant potential for the high-speed and large-area processing of NC thin films; however, the solvent evaporation rates encountered in these techniques are generally too fast to allow NCs to assemble into © 2017 American Chemical Society
ordered superstructures. On the other hand, Langmuir− Blodgett,25,26 Langmuir−Schaefer,27,28 and other modifications of the liquid/gas interfacial assembly approach29 are characterized by slower evaporation rates, and absence of pinning enables the formation of highly ordered monolayers, although technical solutions for transferring these methods into industrial production conditions are still in their infancy. The film morphology is affected by thermodynamic variables, including interfacial energies between solvent, substrate, and colloids, and the potential of mean force (PMF), also known as pair potential, between colloids. Kinetic factors may also influence film morphology, including solvent evaporation rate and colloid diffusivity. One method that affords flexibility over the thermodynamic and kinetic variables is the formation of a Langmuir (or Gibbs) layer at a liquid−vapor (at liquid−vapor interface, but in equilibrium with the bulk solution) interface. Adsorption at the liquid−liquid interface is referred to as the Pickering effect,30 and corresponding Pickering emulsions are stabilized by colloids at the liquid−liquid interface. This method allows a greater range and ease of tuning interfacial energies between substrate and colloids or substrate and solvent. Dong et al. demonstrated binary superlattices with Received: October 6, 2017 Revised: December 2, 2017 Published: December 4, 2017 54
DOI: 10.1021/acs.chemmater.7b04223 Chem. Mater. 2018, 30, 54−63
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Figure 1. Polymorphism in self-assembled NC superstructures. (a) Self-assembled NC monolayer. (b) False-color dark-field transmission electron micrograph image differentiated by atomic ordering of the constituent NCs shows polycrystalline superlattice grains of a self-assembled multilayer superlattice. (c) NC orientation (top) and multiparticle interactions at the fluid interface (middle) dictates the NC superstructure. (bottom) TEM images of square, line, and puckered honeycomb superlattice polymorphs.40
grain sizes of hundreds of nanometers over cm2 area.31 Because the film forms at an interface rather than in the colloidal solution, film thickness can be tuned from a monolayer to multiple layers by controlling the colloid concentration. Highly ordered monolayer superlattices have been demonstrated using this technique.32,33 Self-assembled superlattices of isolated NCs and interconnected quantum dot solids at the heart of this paper are summarized in Figure 1. The polymorphism of different superlattice structures (monolayers, large and small-grain assemblies, and interconnected solids) illustrated in Figure 1 is remarkable. Because all structures are formed from the same NC building blocks, this figure illustrates the critical role of understanding and controlling the basic physical and chemical processes that underpin the formation of a specific polymorph. An important advantage of the assembly at fluid interfaces (in particular at the interface of two immiscible liquids) is that the assembly can be directly integrated with interfacial chemical reactions to enable, for example, the change or displacement of ligands bound to the NC surface. This approach has enabled recent advances in the formation of epitaxially connected nanocrystal networks.34−39 Understanding the NC orientation at the fluid interface and the nature of multiparticle interactions holds the key to directing the formation of superlattices with programmable symmetries. Figure 1c shows connected structures of NC assemblies oriented face-up ⟨100⟩, edge-up ⟨110⟩, and corner up ⟨111⟩. All of these structures have been experimentally observed;33,40 however, the relative role of entropic and enthalpic driving forces in the assembly of specific polymorphs has not yet been established. Kinetic Monte Carlo simulations of hard polyhedral particles by Escobedo and coworkers showed that different 2D assembly superlattice polymorphs are related to the shape of the constituent particles.41 In addition to the shape of the polyhedral NC core, packing frustrations of the
surface bound ligands also play an important role in determining the most stable superlattice structure.42 Beyond entropic considerations of the particle and ligands, enthalpic considerations of the energetically preferred orientation of the polyhedral NC relative to the plane of the fluid interface also need to be considered.33 In the following section, we describe the entropic processes and the experimental variables that control superlattice order. Although assembly of colloidal NCs is important to create ordered superlattices, many proposed applications rely on efficient interdot electronic coupling, which requires that the superlattice is transformed to produce an electronically active material and to facilitate efficient charge transport. Later, we show that oriented attachment is an interesting method to produce a conductive NC solid from an insulating one. Oriented attachment is an enthalpic process involving creation of epitaxial connections between NCs. Entropic Assembly. The narrow size distribution of colloidal PbSe NCs enables the self-assembly of structures with long-range order. For example, Figure 1a shows individual PbSe NCs with an average diameter of 7.3 nm. These interchangeable nanometer building blocks can self-assemble into structures micrometers in size. The structure and degree of order depend on several factors, including NC shape,43 ligand length,44 ligand−solvent interaction, NC concentration, solvent−substrate interaction, and solvent evaporation rate.45−47 Given the wide parameter space, predicting selfassembly behavior in general is not straightforward. The PMF describes the potential energy of a pair of NCs as a function of distance. The PMF is influenced by several forces, including Coulomb, van der Waals, excluded volume, and depletion.48 In the case of self-assembly at fluid interfaces, interactions between NCs must also consider capillary effects as shown by the work of Soligno et al.49 Kotov and coworkers recently described the challenges of decomposing the PMF into separate additive 55
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solvent evaporation can be exploited to form NC assemblies aligned with the honeycomb structure of the Marangoni cells was presented in the work by Stowell et al.55 With a liquid substrate, however, uniform and ordered superlattices form when NC diffusion is slower than evaporation. Bigioni et al.46 and Narayanan et al.47 showed that alkanethiol coated gold NCs monolayers form at the surface of an evaporating colloidal droplet when evaporation is sufficiently fast. Under these conditions, NCs are kinetically trapped at the liquid−vapor interface. When evaporation is slowed, superlattice clusters form in the bulk of the solvent as NC density increases. The same authors also showed that a minimum concentration of free alkanethiol was necessary for uniform film formation. Ordered monolayers at the air−solvent interface can also occur in thermal equilibrium with the bulk solution, as shown by Soligno et al.49 and Campolongo et al.57 for DNA-coated gold NCs. Characterization of the structure of self-assembled superlattices relies heavily on electron microscopy and X-ray scattering methods. In situ methods in particular have emerged as a critical and powerful characterization tool in the study of NC self-assembly processes.34,58−61 The ability to monitor the dynamics of self-assembly with subsecond temporal and micrometer spatial resolution under controlled conditions (e.g., controlled temperature and solvent vapor environment) has provided critical fundamental insights into the complex interplay of physical and chemical processes that govern the self-assembly. Wide- and small-angle scattering X-ray provides real-time information on the orientational ordering of constituent NCs and their longer range spatial order within the superlattice, respectively. The ability to monitor selfassembly processes on the surface of solid substrates as well as on liquid−liquid and liquid−air interfaces has proven to be immensely valuable. For example, Jiang et al.62 studied the interfacial self-assembly of monolayer (i.e., 2D) assemblies and described the dynamics phase transition in context of KTHNY theory. Weidman et al.63 showed the detailed pathway of structure formation in PbSe NC on a solid surface under controlled drying using simultaneous small- and wide-angle Xray scattering. Geuchies et al.34 used in situ grazing incidence small-angle X-ray scattering (GISAXS) to untangle the sequence of subprocesses involved in the self-assembly and subsequent oriented attachment of PbSe NC assemblies. NC assembly at fluid interfaces can be monitored in situ using parallel small-angle X-ray scattering (par-SAXS) at the ethylene glycol surface.34,57 Figure 2 summarizes our results from an in situ par-SAXS experiment that reveals self-assembly at the three-phase contact line between the NC suspension, solvent vapor, and ethylene glycol subphase. Evaporation of the droplet drives advection of NCs to the contact line, increasing the local volume fraction thus initiating self-assembly. In the absence of pinning or other instabilities of the contact line, a uniform film of densely packed nanocrystals is deposited on the interface. The in situ X-ray scattering data (Figure 2) initially show only the form factor of NCs dispersed in the suspension. As the suspending solvent evaporated, the three-phase contact line moved across the beam path, and we observed scattering from both sides of the contact line, from NCs in the liquid and in the self-assembled film. The final image shows scattering from the self-assembled film only. Film thickness could be controlled by adjusting the initial concentration of nanocrystals in the droplet.38,64 Thickness could be controlled down to a single monolayer.
contributions from these interactions if many-body interactions are important.50 In certain cases, however, the PMF can be simplified significantly. The simplest PMF used to describe the interaction of colloids is the hard sphere model. This model is appropriate when colloids exert only repulsive force at very short distance.43 In a good solvent, in which the colloid−solvent interfacial energy is less than the colloid−colloid interfacial energy, the hard sphere model can accurately predict structures seen in experiment.51 Long-chain hydrocarbon ligands are used to provide colloidal stability in nonpolar solvents. The entropy of ligand solvation results in a repulsive PMF between NCs.51 Because there is no attractive component to the PMF, selfassembly of colloidal crystals occurs by maximizing entropy of the colloid−solvent system. If the volume fraction taken up by colloids exceeds 0.545 (also known as the Kirkwood−Alder transition), close packing of the colloids maximizes the volume available to the solvent and therefore maximizes the entropy of the solvent.52 Therefore, the total free energy of the system is minimized by close-packing of the colloids.43 This method of self-assembly is commonly called evaporative (also known as EISA = evaporation-induced self-assembly)53 or convective selfassembly (CSA), where the volume fraction of colloids increases as the solvent is allowed to evaporate such as in the case of an evaporating droplet of a NC suspension.54 In addition, Marangoni currents induced by evaporation can transport material to the drying line (CSA).55 Polyhedral NCs are particularly interesting for self-assembly because they exhibit phase behavior far richer than that of basic spherical particles. The diversity of available polyhedral NC shapes has profoundly expanded the spectrum of possible superlattice symmetries. Theoretical and experimental studies have illustrated the assembly of polyhedral particles into the densest packing superstructures.23−25 The formation of NC superlattices at the air−fluid interface occurs under the influence of nonequilibrium processes and is hence controlled by their associated rates. The process in question is essentially the one by which the three-dimensional NC suspension becomes a quasi-2D film at the fluid interface. To understand this process, we need to consider several macroscopic processes, including: (i) solvent evaporation rate, (ii) NC diffusion from bulk to the interface, (iii) twodimensional NC diffusion at the liquid−gas and liquid−liquid interface, (iv) recession of the vapor−liquid interface, and (v) nucleation rates of specific ordered motifs (Figure 1c). Moreover, as detailed below, the NC−NC interaction and NC shape56 are also influenced by ligand desorption and rearrangement on the NC surface. Because evaporation is a nonequilibrium process, evaporative self-assembly is kinetically controlled. The substrate has a strong effect on film uniformity with solid substrates and liquid substrates showing opposite behavior. Rabani et al. investigated the morphology of NC thin films deposited on solid substrates.45 When the solvent evaporation rate is spatially uniform (outside of the binodal region) and the NC diffusion is much faster than the solvent− vapor phase boundary velocity, then the NC film grows by coarsening of NC aggregates. Aggregates form in a thin liquid layer on the solid substrate. When solvent evaporation is spatially inhomogeneous but NC diffusion still faster than the solvent liquid−vapor phase boundary motion, the resulting film is inhomogeneous with NCs collecting in boundaries between vapor cells. A nice example of how spatial temperature inhomogeneities arising from the Marangoni effect during 56
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added. The absence of structure in the vertical direction (except for form factor oscillations) shows that in both cases a monolayer NC film formed at the interface. In the presence of saturated hexane vapor, which has a vapor pressure of 160 mTorr at 26 °C,66 the width of the in-plane reflection is limited only by instrumental broadening. Given our experimental setup, this indicates average superlattice grain sizes greater than 200 nm.67 Without solvent vapor, the line width broadens to a full width at half-maximum (fwhm) of 0.138 nm−1. Using the Scherrer equation with a constant K = 0.9 gives an average grain size of 41 nm. Control over evaporation rate, free amphiphile concentration, and solvent vapor concentration can greatly improve superlattice film uniformity. However, films prepared from colloidal suspensions are quasi-crystalline with correlation lengths of a few hundred nanometers at best.38,68 It follows from the discussion regarding solvent vapor annealing that solvated ligands effectively result in a purely repulsive PMF similar to hard spheres, and therefore, assembly is driven by entropy. As the solvent vapor is removed, attractive van der Waals force between ligands causes distortion.62 Distortion of the superlattice can be observed by in situ GISAXS measurement of the interparticle distance and Scherrer broadening. Scattering by a superlattice during drying is shown in Figure 4. To slow the drying rate, the superlattice was contained within a sealed chamber that contained a reservoir of hexane. The superlattice was deposited on a substrate of ethylene glycol supported in a 10 × 10 × 5 mm rectangular Teflon well mounted on a temperature controlled stage at 26 °C (Figure 3d). After addition of hexane to the reservoir around the sample holder, the chamber was sealed and purged with nitrogen gas. Then, 3 μL of a 5 μm concentration of 7.3 nm diameter PbSe NCs in hexane was deposited on the ethylene glycol surface by an airtight syringe through a rubber septum. GISAXS patterns were collected every 72 s while hexane evaporated from the NC suspension. After most of the solvent had evaporated, a superlattice assembled at the ethylene glycol liquid surface. The superlattice structure was face centered cubic (fcc) with the {111} planes parallel to the liquid−vapor interface. Initially the interparticle distance was 10.48 nm, and the average superlattice grain size was 378 nm. As the solvent continued to evaporate, the interparticle distance reduced to 9.73 nm, and the average grain size reduced to 130 nm. For grain size analysis, we have to consider the instrumental broadening, which increases with the in-plane scattering vector q∥. However, the change in peak width observed is at least four times greater than the change expected from instrumental broadening.67 As solvent leaves the superlattice, the attractive van der Waals force between NC cores increases, causing the interparticle distance to decrease further. Concomitantly, the superlattice disorder increases. The {100} superlattice peak does not broaden homogeneously; rather, a broad shoulder develops from grains with smaller interparticle spacing. This suggests that the sample does not dry homogeneously and that there is a distribution of interparticle spacings. We could assume, for simplicity, that only two types of superlattice grains dominate rather than a continuous distribution. We considered two alternate hypotheses to support this interpretation. The first hypothesis is that the coexistence of distinct lattice constants can be rationalized by considering that a thin layer of solvent condensate is metastable during drying. Some superlattice grains are wetted by the condensate, and others have lost all
Figure 2. Temporal evolution of interfacial assembly of NCs from a colloidal suspension of hexane deposited on an immiscible ethylene glycol subphase can be probed by in situ par-SAXS.
The extent of long-range ordering of the NC assembly can be improved by solvent vapor annealing.61,65 During evaporation of an NC suspension, control of solvent vapor concentration is critical to produce highly ordered structures. The effect of solvent vapor concentration on disorder of a NC monolayer formed at a fluid interface is shown in Figure 3. To investigate
Figure 3. Effect of solvent vapor concentration on superlattice order. (a) Par-SAXS pattern from a NC superlattice formed at 26 °C in saturated hexane vapor. (b) Par-SAXS pattern from a NC superlattice formed without solvent vapor annealing. (c) Vertically integrated intensities show disorder induced line broadening. (d) Schematic illustration of in situ GISAXS chamber. The inset shows details of the sample stage; the NC film is assembled on the surface of an ethylene glycol subphase contained within a Teflon trough.
the effect of solvent vapor concentration on long-range superlattice order during evaporative self-assembly, par-SAXS data was collected in situ. During the experiment, 6 μL of a 5 μM concentration of 7.3 nm diameter PbSe NCs in hexane was deposited onto the surface of ethylene glycol within a sealed chamber at 26 °C. In one case, the chamber contained a reservoir of hexane, while in the other case, no hexane was 57
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Figure 4. Effect of drying on superlattice structure. (a) GISAXS pattern from a colloidal suspension of 7.3 nm diameter NCs. (b and c) Scattering patterns recorded just after assembly and after several minutes of drying, respectively. (d) Traces showing the location and width of the {100} peak during drying. Dashed red lines represent a fit to the data by an exponential background and one or two Lorentz peaks. (e) Evolution of interparticle distance during controlled drying.
typically passivated with long-chain ligands (Figure 5a); although these ligands aid to solubilize the NCs, they also present a critical impediment to efficient electronic coupling between dots. Cross-linking ligands such as ethaneditihol (Figure 5b) are used to reduce interparticle distance and improve electronic coupling.71,72 Whether bifunctional ligands actually cross-link NCs remains unclear. Choi et al. showed that interparticle distance scales with ligand length for a series of dithiols.73 However, Weidman et al. measured interparticle distances that were not strongly correlated to dithiol ligand length and showed that monothiol and dithiol ligands of similar length gave similar interparticle distances.74 Commonly, the native oleic acid ligands are exchanged for shorter ligands after deposition and drying of a superlattice film on a solid substrate. Due to the volume change during solid-state ligand exchange, microscopic defects are introduced, and long-range order is compromised.74 However, by allowing sufficient time for the exchange, short-range order can be preserved in thick superlattices (15 unit cells).74,75 Similar preservation of order during exchange on monolayer films has been demonstrated using a liquid substrate instead of a solid substrate.76 Short organic linker molecules reduce interparticle distance (Figure 5b) but do not enforce translational order. The order of the initial superlattice may be preserved by slow ligand
solvent. An alternative hypothesis is that the emergence of the shoulder peak in the integrated X-ray scattering data arise from the coexistence of intermediate mesophases, as predicted by Monte Carlo simulations.41 The actual influence of drying on the superlattice structure may be significantly more complicated because not only can the interparticle distance change, but also the grain size. Distortion of the superlattice by solvent loss is a major obstacle to the fabrication of highly ordered superlattices. One possible solution is to functionalize the ligands, thereby tuning the PMF in the absence of solvent. However, this poses a challenge to the ultimate goal of strong electronic coupling, which requires the NC cores to be as close as possible. An alternative solution would be to arrest the crystalline order of a solvated colloidal crystal by forming epitaxial connections between NCs before complete drying. To this end, a liquid substrate provides the possibility for introduction of chemical reagents before or during self-assembly to initiate epitaxial interdot bonding. In the next section, we describe such a process, called oriented attachment. Enthalpic Assembly. Coupling of semiconductor NCs using short organic ligands is a common approach for fabricating NC solids for optoelectronic applications.9,69,70 Figure 5 summarizes key strategies to enhance electronic coupling between NCs. As-synthesized colloidal NCs are 58
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translational disorder on structural correlations at hundreds of nanometers. Oriented attachment is the epitaxial bonding of two NCs along a single crystallographic plane (Figure 5c). The mechanism of oriented attachment has been studied in diverse NC systems, including metals, metal oxides, and semiconductors.78−84 Anisotropic nanostructures such as rods, wires, rings, and sheets have been formed by oriented attachment.85,86 Oriented attachment involves displacement of ligands, self-alignment of crystal facets, and epitaxial bonding. Schapotschnikow et al. simulated the attachment of ligandfree PbSe NCs by molecular dynamics and found the process occurs within tens of nanoseconds.87 In their simulation, the solvent was treated with a mean field approach (i.e., not explicitly); in the experimental system, the fusion is likely much slower due to limitations imposed by transport of solvent and desorbed ligand complexes. The molecular dynamics simulations showed that oriented attachment was driven by epitaxial bond formation.87 Thermal motion of the NCs resulted in, at first, a single epitaxial bond between an atom on each NC. This was followed by alignment of the NCs to each other and finally bonding of all atoms at the facets. The crystallographic direction of attachment can be controlled by altering the surface energies of different facets. This was demonstrated using cosurfactants in addition to the native ligands. Cho et al. showed that attachment of the {100} or {111} facets could be selected by introducing alkylphosphonic acids or alkyl-amines, respectively.85 Schliehe et al. observed attachment of {110} facets of PbS NCs in the presence of chlorinated solvents.86 These examples of oriented attachment were conducted in solution at elevated temperature. Alternative to oriented attachment in solution, Evers et al. showed that thermally induced oriented attachment of PbSe NCs could be accomplished in a thin film at a liquid−vapor interface.33 Without cosurfactants to influence the surface energies of different facets, attachment was predominantly through {100} facets. Although there was initial evidence that suggested attachment through {110} facets, high resolution transmission electron microscopy (HRTEM) later showed attachment only of {100} facets.35 Understanding which “sticky facets” are involved in the formation of the interdot bond and the symmetry of that facet in relation to the polyhedral NC define important design principles for the superlattice polymorphs shown in Figure 1. With a single species of ligand bound to the NC surface, attachment occurs at the facet with the least ligand binding energy. Density functional theory calculations have shown that the binding energy of lead oleate at the {100} surface of PbSe is about half that of the {111} surface.88 Therefore, at finite temperature or upon heating, ligands are displaced from the {100} surface at a greater rate than the {111}. This is a thermodynamic explanation for preferential attachment of PbSe NCs at the {100} facet. The dynamic equilibrium between ligands bound to the NC surface and their surrounding fluid is a key enabling aspect in the oriented attachment of NCs.49,50 NMR experiments by Hens and Martin estimated the exchange rate to be on the order of 50 s−1.51 One consequence of this equilibrium is that the ligand coverage can change as the NC comes in contact with a different fluid in which the ligand may be more (or less) soluble than in the initial solvent. Due to the facet-specific ligand chemistry, the interfacial ligand displacement can yield NCs with anisotropic (i.e., patchy) ligand coverage and
Figure 5. Coupling of NCs. (a) Colloidal PbSe NCs with their “native” oleate ligand shell. (b) Exchanging the long-chain ligands with shorter bifunctional linkers (in this case, ethane dithiol) reduces interparticle separation and enhances electronic coupling between particles. (c) Directed attachment can connect the dots via an epitaxial interdot bridge.
exchange. Alternatively, one can use the atomic structure of the NC to impose translational order on the superlattice through the NC−NC interaction. As discussed below, oriented attachment is one method to enforce atomic coherence between neighboring NCs. Oriented Attachment. A possible route to increase order in superlattices is by epitaxial attachment of faceted NCs in specific crystallographic directions. Crystallographic facets on the NC cores can be used to control NC orientation to achieve a higher degree of order than by entropic close-packing. This concept has been validated in recent experimental reports that demonstrated the formation of high quality superlattices in which constituent quantum dots are registered to within a single atomic bond length.35,38 The comprehensive characterization of these structures requires detailed insights into the translational and orientational order of NCs within the lattice sizes and information about the connectivity (i.e., interdot bond number and bond width). We previously demonstrated how complementary information from GISAXS, GIWAXS, and high-resolution aberration-corrected transmission electron microscopy can be integrated to provide a detailed insight into the long-range ordering in these confined-but-connected epitaxially connected superstructures (ECS).38 Savitzky et al.77 recently reported analytical methods to quantitatively characterize the propagating disorder in terms of a real paracrystal model and directly observe the dramatic impact of angstrom-scale 59
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Figure 6. Transformation of a preassembled superlattice of isolated NCs to a confined-but-connected NC solid. (a) TEM image of the bcc superlattice aligned with {110} superlattice planes parallel to the plane of the interface. (b) Electron diffraction pattern indicating the [110] alignment of constituent dots within the superlattice site. (c) Model illustrating alignment of isolated NCs with their lattice sites. (d) TEM image of the sc superlattice aligned with {100} superlattice planes parallel to the plane of the interface. (e) Electron diffraction pattern indicating the [100] alignment of constituent dots within the superlattice site. (f) Model illustrating alignment of connected NCs with CBC structure.
between neighboring dots within the assembly. Figure 6 illustrates the conversion of a preassembled NC superlattice into an ECS. In the preassembled body-centered cubic (bcc) superlattice, the (110) atomic plane of the constituent dots is parallel to the (110) plane of the superlattice, which in turn is parallel to the substrate. The introduction of the chemical trigger induces a chain of events, including the displacement of the ligands from the NC surface and a coordinated change in translation and orientation of the NCs to form the ECS structure (Figure 6b). The final ECS structure is defined by a simple cubic (sc) lattice in which the [100] axis of the constituent dots is again aligned with the [100] axis of the sc superlattice normal to the plane of the interface. Advances in synthesis and processing have opened up exciting directions in the bottom-up fabrication of novel materials with additional degrees of freedom over their tunable optical and electronic properties. Theoretical predictions of such materials forecast a rich electronic structure with important implications on future advances toward their application in optoelectronic devices. To date, these properties have eluded experimental validation due to the persistent disorder in these structures (both positional disorder of NCs in their lattice sites and inhomogeneities in the interdot bridge). Treml et al. recently showed that successive ionic layer absorption and reaction (SILAR) can enhance the interdot bonding within the NC assembly.93 How to connect the dots without disrupting the long-range order of the preassembled superlattice remains a persistent challenge. Future advances are predicated upon a deeper understanding of the interfacial transport processes (i.e., diffusion of NCs within the assembly and transport of chemical triggers and ligand complexes) and insights into the basic mechanism by which the preassembled superlattice is transformed into an epitaxially connected solid. Whether the transformation is a diffusionless process34 or involves distinct nucleation and growth steps37 is subject to ongoing investigation.
deprotected (i.e., sticky) crystal facets that can attach to similar facets on proximate NCs (Figure 1c). Displacement of ligands can be induced chemically instead of thermally. Lead−chalcogenide NCs are synthesized using an Xtype (anionic) ligand, the conjugate base of oleic acid comprising a carboxylic head and an alkyl tail.89 One method for chemical displacement of the X-type ligands is to introduce L-type (neutral electron donor) ligands. The L-type ligand coordinates with a surface metal atom (M) bound to two carboxylates (X) to displace L-MX2 from the surface. Charge neutrality is maintained by an L-type ligand bound to the NC.90 Anderson et al. classified the equilibrium constants for the displacement reaction 2L + NC − MX2 ↔ L − MX2 + NC − L for various L-type ligands. Small alkyl amines and diamines most readily displaced ligands from lead and cadmium chalcogenide NCs, while fewer ligands were displaced by larger diamines, alcohols, trialkyl amines, and trialkyl phosphines.90 Chemically induced oriented attachment was shown by Baumgardner et al. using dimethyl-formamide to displace oleate ligands from PbSe NCs.91 Attachment occurred preferentially at the {100} facet. Sandeep et al. also observed oriented attachment of PbSe {100} facets using tetramethylethylene-1,2diamine, hexylamine, butylamine, and ethylenediamine.92 However, these examples of chemically induced oriented attachment created structures that were not colloidally stable; therefore, oriented attachment was conducted by deposition of NCs onto a solid substrate followed by exposure to an L-type ligand solution. The resulting structures exhibited only shortrange order, up to several NCs. Walravens et al.39 recently showed that the ligand displacement potency of the chemical trigger to initiate interdot bonding (parametrized by pKb of the amine) plays the critical role in the transformation of the structure. Concurrent advances in the directed assembly and controlled interdot attachment enabled the formation of ECS that combine long-range ordering and intimate epitaxial bridges 60
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CONCLUSION The combination of self-assembly and directed attachment of colloidal NCs at fluid interfaces presents a scientifically interesting and technologically important research challenge. Remarkable strides have been made in the synthesis of polyhedral NPs with precisely defined shapes and their selfassembly into highly ordered superstructures. Recent advances by the investigators and others have revealed intriguing synergies between interfacial self-assembly and directed epitaxial attachment into ordered and connected superstructures. Access to nanocrystal superstructures with programmable symmetry opens new opportunities to create materials with properties by design. The coupled thermodynamic and kinetic principles governing the interfacial NP self-assembly and directed attachment present a rich albeit complex scientific problem. We described how the underlying entropic and enthalpic interactions as well as the kinetics of assembly and attachment influence the formation of epitaxially connected superstructures. Recent advances to the ability to control the assembly of NCs to within a single atomic bond length, limited only by the polydispersity of the quantum dot building blocks, have created exciting opportunities to create novel materials with properties by design. Future progress toward acclaimed technology scale-up is predicated on progress in scalable processing and integration of the assembled materials into devices.57
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04223. Additional in situ par-SAXS data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Detlef-M. Smilgies: 0000-0001-9351-581X Tobias Hanrath: 0000-0001-5782-4666 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Cornell Center for Materials Research with funding from the NSF MRSEC program (Grant DMR-1719875). K.W. was supported by the Basic Energy Sciences Division of the Department of Energy through Grant DE-SC0006647. T.H. acknowledges support from Grant NSF-DMR 1056943. We thank Fernando Escobedo and Ben Treml for valuable discussions. CHESS is supported by the National Science Foundation under Award DMR-1332208.
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