Main-Group-Semiconductor Cluster Molecules as Synthetic

Mar 9, 2017 - This article is part of the Advances in Main-Group Inorganic Chemistry special issue. ... Main-group-semiconductor clusters are attracti...
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Main-Group-Semiconductor Cluster Molecules as Synthetic Intermediates to Nanostructures Max R. Friedfeld,† Jennifer L. Stein,† and Brandi M. Cossairt* Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States ABSTRACT: Main-group-semiconductor clusters are attractive atomically precise precursors for materials design. In particular, magic-sized clusters, those with elevated thermodynamic stability relative to other clusters of similar size, have been implicated as important intermediates in the synthesis of semiconductor nanostructures. A survey of the literature on the intermediacy of clusters in nanomaterial synthesis reveals two predominant mechanistic trends: monomer-driven growth and cluster assembly. In this Forum Article, we compare and contrast the systems in which these mechanisms are operative and attempt to extract the emerging design principles governing these transformations. Additionally, we highlight the gaps in our understanding of this emerging area of science and provide a roadmap for future reaction development.

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INTRODUCTION Main-group-semiconductor clusters are a class of well-defined molecular species that are intermediate in size between traditional small molecules and larger nanocrystals (Figure 1).1 The synthesis and spectroscopic characterization of such atomically well-defined clusters is of fundamental interest

because of their unique photophysical properties, which are unlike either discrete molecular units or the corresponding bulk materials. Many studies have suggested that such cluster species may exist on a continuum with quantum-confined semiconductor nanocrystals with respect to both their electronic and vibrational structure.2−5 This is exciting because such clusters lack the defect-ridden surfaces and inhomogeneous line broadening caused by sample polydispersity, allowing atomically precise clusters to act as an excellent model for exploring structure−function relationships and mechanistic questions within the field of colloidal semiconductor quantum dot (QD) chemistry. Main-group-semiconductor cluster molecules have been prepared selectively and have also been isolated from reactions designed to generate colloidal solutions of semiconductor nanomaterials. Many main-group-semiconductor cluster molecules have been structurally characterized in general.1,8−15 However, the synthesis and structural characterization of socalled magic-sized clusters (MSCs) (i.e., atomically precise clusters with elevated thermodynamic stability relative to other cluster sizes/structures that are often implicated as intermediates to larger nanostructures) is rare.7,16 Our laboratory became interested in such clusters because of their apparent impact on the synthesis of indium phosphide QDs. Specifically, we found that growth of InP QDs could be interrupted at temperatures lower than those used in QD growth, resulting in the isolation of stable magic-sized InP clusters and providing temporally distinct growth regimes for MSCs and QDs.17 It was

Figure 1. Representative examples of indium phosphide based species across the chemical length scales: (A) [In(CH2Ph)2P(SiMe3)2]2, a well-defined small molecule;6 (B) In37P20(O2CCH2Ph)51, an atomically precise main-group-semiconductor cluster;7 (C) an intentionally ill-defined nanostructure of InP composed of a crystalline inorganic core and a shell of organic passivating ligands (not shown); (D) the bulk zinc blende structure of InP. © 2017 American Chemical Society

Special Issue: Advances in Main-Group Inorganic Chemistry Received: February 3, 2017 Published: March 9, 2017 8689

DOI: 10.1021/acs.inorgchem.7b00291 Inorg. Chem. 2017, 56, 8689−8697

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Inorganic Chemistry shown that, upon heating to 400 °C, these MSCs convert to InP QDs, demonstrating that they act as competent singlesource precursors in the synthesis of larger nanomaterials (vide infra). Upon a change of the precursor from indium myristate to indium phenylacetate, the single-crystal X-ray structure of the MSC was determined, allowing for precise identification of the MSC stoichiometry, geometry, and electronic structure.7 Intrigued by the observation of molecularly well-defined clusters en route to larger nanocrystals, we sought to contextualize this finding by exploring the variety of mechanisms by which clusters have been seen to give rise to larger nanomaterials. In this Forum Article, we will explore the literature on the intermediacy of main-group-semiconductor cluster molecules as synthons for larger nanocrystal materials. Ultimately, we hope to draw relationships between the many disparate observations that have been made in this field to clarify emerging mechanistic trends. Two primary mechanisms will be highlighted: monomer-driven growth through continuous and quantized mechanisms and cluster assembly through templating or aggregative mechanisms (Scheme 1).

Figure 2. Absorbance spectra from the synthesis of CdSe nanorods in which CdSe MSCs form and appear to be consumed over the course of nanorod growth. It is proposed that MSCs dissociate to monomers that can nucleate at this temperature to larger nanostructures. Reprinted with permission from ref 18. Copyright 2010 American Chemical Society.

Scheme 1. Schematic Diagram Summarizing the Role of Cluster Molecules in Nanostructure Synthesis

kinetic profile supports a mechanism in which the MSC and monomers are in equilibrium and the forward and reverse rates that maintain this equilibrium are faster than the rates that form and deplete the monomers. The kinetic data suggest that the MSC reaction with monomers to give nanocrystals cannot be rate-determining.18 Similar conclusions have been made for the two-step nucleation of InP QDs. Kinetically persistent InP MSCs have been identified in the synthesis of InP at temperatures ranging from 100 to 400 °C.17,19 Recent structural characterization suggests that carboxylate-ligated MSCs display low pseudo-C2 symmetry, implicating the need for structural rearrangement and/or redissolution prior to conversion to zinc blende QDs (Figure 3).7 In addition to the in situ conversion of MSCs to QDs, purified samples of the MSCs have been used as robust single-source precursors.17 In the single-source-precursor reactions, the final QD size was correlated with the cluster concentration, while spectroscopic monitoring did not suggest an aggregative growth mechanism (e.g., a mechanism involving either quantized or continuous growth, which will both be discussed later). In combination, these data suggest that QD nucleation via cluster dissolution and monomer re-formation was likely in this case as well. Taken together, it is likely that these reports of kinetically persistent clusters en route to larger QDs are united in the fact that the cluster species must undergo ligand desorption, rearrangement, and/or complete or partial dissolution in a kinetically slow step of the reaction sequence. What remains largely unproven is the extent of cluster dissolution, whether into seeds or fully back to monomers, prior to QD formation. If dissolution to monomers is required, it is likely that in many cases the identity and reactivity of those monomers is different from those used to generate the clusters themselves. For example, in the synthesis of InP from In(O2CR)3 and P(SiMe3)3, an often implicated candidate for the reactive monomer species is [In(O2CR)2P(SiMe3)2]n;20−22 however, in the case of monomer generation from cluster dissolution, no source of +SiMe3 is present and thus the identity of the reactive monomer species must be different. Exogenous Cluster Seeds. Evidence for clusters (or cluster fragments) serving as seeds for QD growth comes most

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CLUSTERS “ON” THE REACTION PATH TO NANOCRYSTALS: MONOMER-DRIVEN GROWTH THROUGH CONTINUOUS AND QUANTIZED MECHANISMS Clusters have been observed by optical spectroscopy en route to larger nanocrystals in many syntheses of II−VI, IV−VI, and III−V QDs. In most reports, these clusters are designated either as direct intermediates or as off-path monomer reservoirs. Two notable examples in which the cluster-to-QD transformation has been directly discussed include CdSe nanorods and InP QDs.17,18 With respect to metal chalcogenide nanorod synthesis, MSCs have been suggested as local thermodynamic minima in the progression from precursors to nanorods and are typically observed only at the high monomer chemical potentials needed to access the rods themselves. For CdSe nanorods, the most persistent MSC is characterized by an intense absorption maximum at 350 nm and is believed to be a molecularly welldefined tetrahedral cluster of zinc blende CdSe (Figure 2).18 This zinc blende structure is notable because the final nanorods are exclusively wurtzite in structure, suggesting that MSCs are off-path in the reaction mechanism rather than intermediate between monomers and nanoparticles. Kinetic data demonstrate that, as the MSC concentration, [MSC], approaches zero, the rate of change of [MSC] does not decrease to zero. This 8690

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Inorganic Chemistry

that these clusters can serve as single-source precursors to wurtzite CdSe QDs that differ significantly in stoichiometry from the M:Se ratio in the starting clusters.24 This conversion was proposed to occur via an initial cluster nucleation step, followed by a structural rearrangement to a more thermodynamically favored structure. Several mechanisms for nucleus formation based on various degrees of cluster fragmentation could be envisioned. Similar to what was seen for in situ generated clusters in the synthesis of CdSe nanorods discussed previously, rearrangement of the cluster seed fragment was proposed as a necessary step to generate the final hexagonal structure of the QDs. Further support for cluster-seeded growth being a viable mechanism comes from Snee and co-workers, where additional molecular precursors were added to [M10Se4(SPh)16]4− clusters, generating a population of QDs wherein the QD number was equal to the number of clusters used (Figure 4B).25 Quantized Growth. In addition to the homogeneous growth mechanisms in which clusters evolve continuously in size en route to nanocrystals, MSCs have been observed to evolve in a series of specific cluster sizes toward larger QDs, most notably in II−VI semiconductor nanocrystals. This quantized, or step-growth, mechanism is most clearly characterized through optical spectroscopy in which the positions of discrete excitonic transitions remain constant over time while the peak intensities of smaller clusters decrease as the concentration of larger clusters increases (Figure 5).26 Although the conditions that differentiate between quantized growth and continuous growth have been studied widely across II−VI materials, it is difficult to draw general conclusions based on differences in synthetic methodologies and relative precursor conversion kinetics. Rosenthal and co-workers offer a thoughtful overview of synthetic factors that play a role in CdSe MSC quantized-growth reactions.27 There have been conflicting hypotheses regarding the mechanism of quantized growth. MSC families have been suggested to develop independently either from different nuclei,28,29 from cluster aggregation,30,31 or from monomer deposition on an existing cluster seed.16,26,32 Evidence supporting the monomer addition mechanism is most clearly demonstrated by Owen and co-workers.16 In their examination of CdSe MSCs, molecular precursors were combined at −78 °C and one CdSe cluster family with a lowest-energy electronic transition at 350 nm (CdSe350 nm) formed as the solution came to room temperature. Over time, this solution slowly formed

Figure 3. Molecular structure of In37P20(O2CCH2Ph)51 (hydrogen atoms omitted for clarity). (A) [In21P20]3+ core plus 16 surface indium atoms. (B) Complete single-crystal XRD structure including all ligands. Color legend: green, indium; orange, phosphorus; red, oxygen; gray, carbon. Adapted from ref 7. Copyright 2016 American Chemical Society.

clearly from the well-known molecular [M10Se4(SPh)16]4− clusters originally prepared by Dance and co-workers (M = Cd, Zn; Figure 4A).23 Strouse and co-workers demonstrated

Figure 4. (A) [Cd10Se4(SPh)16]4− clusters that serve as single-source precursors to QDs (some phenyl substituents are omitted for clarity). (B) Regression of the number of moles of clusters versus the number of moles of QDs synthesized. The inset shows the absorption spectrum of a sample prepared using diphenylphosphine selenide. Adapted from ref 25. Copyright 2013 American Chemical Society. 8691

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Inorganic Chemistry

inorganic core (Figure 6A). Hypothesizing that the larger clusters were simply tetrahedra expanded by additional layers of atoms, pair distribution function (PDF) analysis was performed to compare the overlay of experimental data with simulations of tetrahedra with one and two additional monolayers. Compared to previously predicted structures of CdSe MSCs,33−35 the simulations of expanded tetrahedra for all three sizes fit most accurately (Figure 6B). Additional experiments highlighting the necessary role of monomer in the evolution of CdSe MSC families were performed by Manna and co-workers.26 A solution containing a series of MSCs, similar to the mixture shown in Figure 5, was purified to remove any excess monomer and heated under the same conditions that were previously used to grow further sizes. With no free monomer present, optical peak positions and intensities remained unchanged, implying that smaller clusters were not aggregating to form larger clusters or dissolving to provide monomer reservoirs for growth. Aggregation of clusters is a potential pathway for clusters to form larger structures in which oriented attachment occurs through dipole interactions (vide infra).36 In these cases, the final products are often anisotropic (e.g., nanowires, nanoribbons, nanorods).29,37 The primary argument supporting cluster aggregation in the quantized growth of II−VI nanomaterials is the observation that absorbance peak maxima correspond to an integer increase in the particle volume.30,31,38 For example, Weiss and co-workers estimated the diameters of the CdSe clusters from absorbance data based on experimentally measured calibration curves and noted that the approximated volumes of the observed MSCs were multiples of the smallest cluster volume. This approximation holds in the case of the CdSe tetrahedra crystallographically characterized by Owen in which the volume also doubles as each base monomer layer is added.16 While the synthetic conditions used to access quantized growth have varied considerably, general trends can be developed. To reach the local thermodynamic minima of

Figure 5. Absorption spectra of CdSe MSCs demonstrating quantized growth over time. Reprinted with permission from ref 26. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA.

CdSe380 nm and CdSe408 nm. During the course of these studies, diffraction-quality single crystals of CdSe350 nm were obtained, demonstrating that this cluster has a tetrahedral Cd35Se20

Figure 6. (A) Two views of the tetrahedral Cd35Se20 core structure (Cd, green; Se, orange). (B) Experimental PDF data (blue) with simulated PDFs (red) overlaid for the three different clusters studied. Adapted with permission from ref 16. Copyright 2014 American Chemical Society. 8692

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Figure 7. TEM images of CdSe nanoribbons and schematic illustration of the formation of CdSe nanosheets. (A) Low-magnification TEM image of CdSe nanoribbons. (B) TEM image showing a parallel array of nanoribbons with uniform thickness. (C) Twisted image showing a ribbon-shaped structure. Inset showing a selected-area electron diffraction pattern of the circled area. (D) HRTEM image. (E) Schematic illustration of the synthesis of lamellar-structured CdSe nanosheets. (F) Schematic illustration of the synthesis of separated single-layered CdSe nanosheets. Adapted with permission from refs 42 and43. Copyright 2006 American Chemical Society. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA.

surface.41 In this fashion, amines may be able to displace surface cations and reduce the activation barrier to cluster growth. Last, the identity of molecular precursors likely impacts the rate at which the supersaturation of monomers occurs. For example, syntheses employing more reactive precursors [diphenylphosphine selenide or Se(SiMe3)2] report cluster growth at lower temperatures than those utilizing trioctylphosphine selenium or elemental selenium.16,26,28,39 The conditions outlined above are themes observed across the current literature but are not necessarily the precise, ideal conditions for stepwise growth in all semiconductor nanocrystal materials. Balancing these factors may exclude the pathway of continuous growth, but once this regime is entered, it is unclear whether nanoparticles develop from MSCs acting as seed nuclei or dissolve into monomer reservoirs.

each cluster size, monomers must be present at a critical concentration, which is, in turn, impacted by the nature of the surfactants, growth temperature, reaction concentration, and precursor reactivity. The simplest case is demonstrated by de Mello Donega and co-workers, in which they varied the precursor concentration over otherwise identical reaction conditions to produce ZnTe MSCs.32 They observed only a single absorbance maximum at 330 nm at the lowest concentration, while doubling the amount of precursor introduced a second maximum at 354 nm that increased in intensity with additional precursor. The apparent stepwise growth of a larger cluster can be attributed to the presence of more monomer that would be able to layer onto the ZnTe330 nm clusters. A consistent observation throughout the literature of quantized MSC growth has been the appearance of larger clusters in higher temperature syntheses. Presumably elevated temperatures improve the solubility of monomers and allow for overcoming increasing activation barriers.16,29,31,32,39 Additionally, the impact of ligand chain length was explored by Yu and co-workers, who demonstrated with UV−vis spectroscopy that evolution of a series of possible MSC families depended on the total number of carbons in the chain (C#).29 CdSe MSCs at 395 nm could only be isolated at low temperatures with C10 ligands; increasing the temperature or switching to C14 accessed a cluster at 463 nm via quantized growth, while C24−28 produced the largest cluster at 513 nm, again likely a result of monomer solubility. The presence of amines has generally been associated with the lowering of the temperature necessary to synthesize InP and CdSe nanostructures.17,40 This is consistent with studies of CdSe MSCs in which the addition of amine to a solution of preformed CdSe MSCs at room temperature induces growth to a new larger species31 or the exchange of trioctylphosphine oxide with amine produces a different series of sizes through quantized growth.28 Owen and co-workers have demonstrated the lability of CdSe nanocrystal surfaces in which cadmium carboxylates can be displaced by amines that coordinate to the

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CLUSTER ASSEMBLY: TEMPLATING AND AGGREGATIVE MECHANISMS Another reaction pathway by which clusters have been demonstrated to convert to semiconductor nanostructures is through a cluster assembly mechanism. In this growth mechanism, small clusters are formed in initial growth stages and can often be isolated and characterized. In later, temporally distinct phases of growth, these clusters aggregate in a precise fashion to make structured, dimensionally controlled nanomaterials. The factors invoked in controlling the morphology of the resulting nanomaterials include dipole−dipole orientation, controlled and selective ligand binding to different crystal faces of the cluster, and the destabilizing effect of different cosolvent mixtures. Characteristic aspects of this growth mechanism include the synthesis of discrete sizes of 1D or 2D nanomaterials and the manifestation of certain size dimensions from the parent clusters in the resulting nanomaterials. Multiple studies have demonstrated that CdSe nanomaterials can grow in an anisotropic fashion according to a cluster assembly mechanism. The formation of 2D structures (e.g., rods, platelets, ribbons, belts) has been demonstrated to be guided by the geometric dimensions within precursor lamellar 8693

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growth mechanism, small nanoclusters assemble to make 1D or 2D nanostructures. This growth mechanism is often accomplished by destabilizing the clusters using weak ligands and employing low-temperature annealing conditions to facilitate kinetic growth. In contrast to lamellar templated growth, dipole−dipole interactions in the clusters are considered to be the main driving factor in this mechanism. For example, highly uniform CdTe nanowires have been synthesized using low temperatures and weak ligands that destabilized the surface of the cluster precursors.36 Kotov and co-workers accomplished this by destabilizing water-soluble CdTe nanoclusters by removing strongly binding mercaptoacetic acid by washing with pH = 9 buffered water solutions followed by precipitation of the clusters with methanol. When initially synthesized clusters with diameters ranging from 2.5 to 5.4 nm are subjected to growth conditions, the diameters of the resultant nanowires are the same as those of the initial precursors. Highresolution transmission electron microscopy (HRTEM) images indicated that the ⟨001⟩ direction of the CdTe wurtzite crystal lattice lies along the long axis of the nanowires. The growth mechanism of this cluster assembly was demonstrated to occur in two steps. First individual clusters begin to aggregate in a linear fashion through dipole−dipole alignment, forming socalled “pearl necklace” aggregates, as evidenced by TEM images taken during the early stages of growth.36 These protorods then undergo an annealing, or smoothing, process that results in crystallization of the wires into the wurtzite material. The authors suggest that this annealing process does not occur through Ostwald ripening (i.e., the clusters do not dissociate during growth) because of the instability of the monomers in water. Other II−VI systems have been demonstrated to undergo cluster conversion to nanomaterials in nonaqueous systems. Peng and co-workers have synthesized CdSe nanowires from clusters using cadmium acetate, dodecylamine, and selenourea that, under similar conditions but at lower temperatures, can form lamellar sheetlike structures.42,44 At these temperatures (120−180 °C), the chain length of the amine was shown to influence the diameter of the initial clusters and thus the diameter of the resulting nanowires (ranging from 1.5 to 6 nm).57 Evidence for an oriented attachment mechanism has also been observed in the conversion of ZnS clusters to nanorods.58 Elemental sulfur dissolved in a long-chain primary amine reacts with a highly reactive zinc precursor, Et2Zn, forming 5 nm nanocrystals that bypassed the lamellar ZnX 2 (amine) n structures. These isolated ZnS nanocrystals stabilized with hexadecylamine, upon dispersal in hexane at 60 °C, spontaneously aggregate into nanorods with 5 nm diameter. According to length distribution histogram analysis, the nanorod length populations reached maxima in increments of 5 nm (the cluster diameter), providing evidence for an oriented attachment growth mechanism. Well-defined examples of oriented attachment have also been noted in IV−VI materials. Weller and co-workers identified 2.8 nm PbS QDs that, in the presence of chlorinated solvents such as dichloroethane, undergo oriented attachment along the less stable (110) facets of the clusters, forming intermediate “eggtray” structures that aggregate into uniform sheetlike structures (Figure 8).59 Using controlled-growth conditions, the nonselective growth of large particles (pathway A, Figure 8) is avoided in favor of the oriented attachment mechanism (pathway B then C, Figure 8). The selectivity for 2D growth

structures formed from the reaction of long-chain primary amines and cadmium halide or acetate salts. These sheet-like structures consist of alternating layers of cadmium coordination polymers passivated with long-chain alkylamines (Figure 7). van der Waals forces between the hydrocarbon amine chains stabilize the structures and ensure uniform spacing. The sheet-spacing dimensions of the [CdX2(L)n]m lamellae are maintained upon the introduction of reactive selenium precursors at low temperature. For example, the 1.4 nm thickness of a lamellar structure prepared from CdCl2 and oleylamine is maintained upon the addition of either a selenocarbamate42 or selenium powder43 at temperatures lower than 100 °C (Figure 7). Buhro and co-workers have demonstrated that lamellar structures formed from cadmium halides and primary amines can be used to make, in combination with reactive selenium precursors, ultrasmall CdSe clusters whose size dimensions are manifested in 2D nanoplatelets that grow from these clusters. Using this low-temperature templated strategy, CdSe clusters are isolated that are assigned as (CdSe)13 and, separately, (CdSe)34.44 While the cluster stoichiometry is inferred from mass spectrometric and elemental analysis experiments, it should be noted that, in other reports, families of nonstoichiometric CdSe clusters have been demonstrated to absorb and emit light at similar wavelengths.16,41 When (CdSe)13 clusters were heated to 64−85 °C, conversion to highly ordered nanostructures was observed. These so-called quantum belts displayed a d spacing of 2.64 nm in the low-angle X-ray diffraction (XRD) pattern, consistent with the formation from a lamellar structure. The kinetics of the conversion of (CdSe)34 clusters to quantum platelets was monitored by UV−vis absorption and found to occur via a first-order process without the formation of an intermediate. These data indicate that these clusters are direct intermediates, without dissolution to monomers, on the reaction pathway in the synthesis of quantum platelets from precursors.45 Hyeon and co-workers demonstrated that similarly prepared (CdSe)13 clusters could incorporate up to 10% Mn2+ into the crystal lattice. When these doped clusters were subjected to growth conditions, uniformly doped nanoribbons of 1.4 nm thickness were observed, indicating that the lamellar structure was maintained even after cation exchange.46 The use of long-chain amines in facilitating low-temperature cluster formation and controlled growth has been observed in the synthesis of other II−VI materials as well, including CdS,47,48 ZnS,48−50 ZnSe,48,51 and ZnTe.48,52 However, the presence of amines is not necessary to facilitate the formation of these lamellar structures. For example, Sarma and co-workers reported that cadmium acetate with excess decanoic acid reacts with selenium powder at elevated temperatures to make 1.4and 1.6-nm-diameter clusters, which, upon further exposure to annealing conditions, assemble to form nanowires with 1.6 nm diameter.53 The long-chain fatty acid in the reaction may serve to act as a template in making lamellar structures that stabilize cluster assembly. This strategy of using long-chain fatty acids instead of amines to facilitate unidimensional growth has been employed in the synthesis of CdS quantum disks48,54 and CdS and CdTe nanoplatelets,48,55,56 although, in some cases, the mechanism of growth could not be determined. In the absence of lamellar stabilizing structures, another growth mechanism that has been demonstrated with II−VI and IV−VI materials is the assembly of small clusters to form nanowires via an oriented attachment mechanism. In this 8694

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aggregative growth mechanisms can be distinguished from one another primarily by temperature: low temperatures are typically required for templation to maintain the structural integrity of the soft template, which is often held together by noncovalent interactions. Furthermore, in a given nanostructure reaction, evaluating the mechanism of cluster evolution is nontrivial, and as we have discussed above, several conflicting conclusions have been presented for the same reaction systems. To clearly distinguish between growth from monomers versus an aggregative mechanism, we suggest that cluster isolation, purification, and reexposure to the reaction growth conditions (in both the presence and absence of additional monomers) be a minimum requirement for an unambiguous conclusion. In addition, it is found that perturbing the reaction temperature (and/or concentration) is often sufficient to move between mechanisms, likely because of the influence of these parameters on the structure and reactivity of the cluster intermediates themselves, and thus should be explored in new chemistries implicating cluster intermediates. The use of clusters as intermediates to nanostructures and bulk phases is a field in its infancy with a bright future. The use of clusters as platforms for introducing dopants and defects in a controlled way, for example, has been little explored. The use of clusters of varying reactivity and compositions is also ripe for methodological development in the area of heterostructure growth. Finally, the use of ligand stabilization/destabilization strategies to predictively direct shape is a promising direction. Ultimately, the success of these emerging complex synthetic schemes will require a more detailed understanding of the structure−function relationships for the clusters themselves, which we believe is an area of emerging importance that merits the attention of synthetic chemists and spectroscopists alike.

Figure 8. Schematic illustration of large-particle (A) and sheet formation (B and C) from small PbS QDs. Reprinted with permission from ref 59. Copyright 2010 American Association for the Advancement of Science.

is rationalized by the dense and ordered packing of oleic acid ligands on the (100) surfaces of the clusters, which limit 3D growth. Similarly, small clusters of lead selenide have been identified by Murray and co-workers as intermediates in the formation of PbSe nanowires.60 The specific orientation of the dipole moment of the nanoclusters was determined to be crucial in observing oriented attachment growth. For example, when oleic acid and n-tetradecylphosphonic acid were used as the stabilizing ligands during growth experiments, cluster attachment was observed only with clusters whose dipole moments were oriented along the ⟨100⟩ axis.

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AUTHOR INFORMATION

Corresponding Author

CONCLUSIONS AND OUTLOOK Atomically precise main-group cluster molecules play an important role in the formation of semiconductor nanostructures. It seems reasonable to go so far as to implicate them as universal intermediates in nanomaterial assembly; however, their relative stabilities are highly dependent on their structure and surface chemistry and thus on the precise conditions of the reactions in which they are formed. Moreover, their role in understanding the nucleation of larger nanostructures is only relevant if the clusters have sufficient stability to appreciably build up in concentration during the growth process. As has been discussed, the mechanisms for cluster to nanostructure conversion can be broadly categorized into two types: growth from monomers (in either a continuous or a quantized fashion) and aggregative growth (in either templated or oriented attachment mechanisms). Understanding what conditions lead clusters down one type of path or another is an important step in the predictive design of nanostructure reactions. In general, on the basis of the above discussion, we can make several broad observations. First, clusters with physical structures that differ significantly from those of their final products tend to be long-lived and observable (and often isolable) during nanostructure growth. Next, quantized growth of clusters often requires high monomer availability, which is enabled by the use of highly reactive precursors, the addition of solubilizing agents (amines, long alkyl chains, etc.), elevated reaction concentrations, and increased temperatures. Finally,

*E-mail: [email protected]. ORCID

Brandi M. Cossairt: 0000-0002-9891-3259 Author Contributions †

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the David and Lucile Packard Foundation (M.R.F.), the National Science Foundation under Grant CHE-1552164 (J.L.S.), and the University of Washington (B.M.C.) for support during the time that this Forum Article was written.

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REFERENCES

(1) Corrigan, J. F.; DeGroot, M. W. Large Semiconductor Molecules. In The Chemistry of Nanomaterials: Synthesis, Properties and Applications; Rao, C. N. R., Muller, A., Cheetham, A. K., Eds.; Wiley-VCH: Weinheim, Germany, 2004; Vol. 2, pp 418−451. (2) Corrigan, J. F.; Fuhr, O.; Fenske, D. Metal Chalcogenide Clusters on the Border between Molecules and Materials. Adv. Mater. 2009, 21, 1867−1871. (3) Soloviev, V. N.; Eichhofer, A.; Fenske, D.; Banin, U. Molecular Limit of a Bulk Semiconductor: Size Dependence of the “Band Gap” in CdSe Cluster Molecules. J. Am. Chem. Soc. 2000, 122, 2673−2674.

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DOI: 10.1021/acs.inorgchem.7b00291 Inorg. Chem. 2017, 56, 8689−8697