Supramolecular Assembly of Single-Source MetalâChalcogenide

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Invited Feature Article

Supramolecular Assembly of Single-Source Metal-Chalcogenide Nanocrystal Precursors Stephanie C. Smith, Whitney Bryks, and Andrea R. Tao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01043 • Publication Date (Web): 28 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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Supramolecular Assembly of Single-Source Metal-Chalcogenide Nanocrystal Precursors

Stephanie C. Smith1, Whitney Bryks1,2, and Andrea R. Tao1,2,3*

1

Department of NanoEngineering, University of California, San Diego, 9500 Gilman Drive MC

0448, La Jolla, California 92093-0448

2

Department of Chemistry & Biochemistry, University of California, San Diego, 9500 Gilman

Drive, La Jolla, California 92093

3

Materials Science and Engineering, University of California, San Diego, 9500 Gilman Drive, La

Jolla, California 92093

*Email: [email protected]

Abstract In this Feature Article, we discuss our recent work in the synthesis of novel supramolecular precursors for semiconductor nanocrystals. Metal chalcogenolates that adopt liquid crystalline phases are employed as single-source precursors that template the growth of shaped solid-state nanocrystals. Supramolecular assembly is programmed by both precursor chemical composition and molecular parameters such alkyl chain length, steric bulk, and the intercalation of halide ions. Here, we explore the various design principles that enable the rational synthesis of these

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single-source precursors, their liquid crystalline phases, and the various semiconductor nanocrystal products that can be generated by thermolysis, ranging from highly anisotropic twodimensional nanosheets and nanodisks to spheres. I. Introduction Colloidal semiconductor nanocrystals (SNCs) are a technologically important class of materials because they possess electronic and optical characteristics  such as bandgap energy, carrier density, and dielectric response  that can be readily modulated through chemical synthesis. For example, it is well-known that the bandgap of colloidal SNCs synthesized by the hot-injection method (e.g. CdS, CdSe, CdTe) is highly size-dependent: by controlling the reaction time for nucleation and growth, one can achieve a range of SNC diameters giving rise to photoluminescence that is tuned to span the entire visible wavelength range.5 In a similar manner, manipulating the precursor injection rate can alter the shape of SNCs, from spherical nanocrystals to rod-like and mixed heterostructures or branched nanowires.1, 6-9 As a direct result of the ability to rationally and precisely control their chemical composition, size, and shape, these colloidal SNCs are currently exploited in wide applications ranging from display technology to biological imaging to photovoltaics.1, 3, 10-14 Colloidal SNCs have also been demonstrated as excellent nanomaterials for plasmonics, where light is propagated, manipulated, and confined by solid-state components that are smaller than the wavelength of light itself.15-17 Typically, plasmonic nanoparticles are composed of noble metals such as Ag and Au that support the excitation of localized surface plasmon resonances (LSPRs) in the visible wavelengths, where the conduction electrons of the metal oscillate in resonance with incident light to produce intense electromagnetic fields localized at the metal surface. Heavily doped SNCs that possess large free carrier densities support LSPR excitation in


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the infrared (IR),2, 18-25 and provide unprecedented opportunities for plasmonics in a new range of fields, including wireless telecommunication, remote sensing, bioimaging, and IR spectroscopy. (Figure 1) Because they possess free carrier densities that are tunable, SNCs could provide the foundation for active, “on-demand” plasmonic materials capable of signal modulation and switching. However, the use of heavily doped SNCs in plasmonics is severely limited by the ability to synthesize these nanomaterials with precise chemical and morphological control. The introduction of dopants into SNCs can result in surface segregation or can compromise the SNC crystal structure and stability.26 Moreover, few synthetic strategies for colloidal SNCs are capable of producing the highly anisotropic shapes that are desired for plasmonics.27 Coinage metal chalcogenides (Ag2E, Cu2–xE, where E = S, Se) have been synthesized using hot-injection chemistries typical for quantum dot formation with uniform sizes, but do not typically give rise to the formation of anisotropic shapes. For example, Brutchey et al. have demonstrated the use of solvent systems such as ionic liquids and thiol-amine mixtures to create metal chalcogenide nanocrystalline products;28-31 cation and anion exchange have also been used successfully to produce chalcogenide and pnictide nanocrystals;32-35 in the work of Vela et al., silylative deoxygenation of metal oxides with trimethylsilyl reagents has produced several different nanophases, including heterobimetallic structures.32 Choice of both metal salt anions36 and metal-ligand coordination chemistry37 have been demonstrated to have profound effect on crystal morphology and surface structures. Towards this end, precursor reactivity has been proposed as a knob for control the resulting nanocrystal shape and/or phase. Such a strategy seeks to promote reaction-limited nanocrystal growth along specific crystal direction, which has been successful in the formation of one- and two-dimensional nanocrystals of Cu sulfides such as nanorods and nanoplatelets.38, 39


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However, these strategies have yet to be extended to other metal chalcogenides. The ability to achieve shape control over metal chalcogenide nanocrystals utilizing rational precursor design is still highly desired.

Figure 1. Applications of shaped SNCs. (a) ZnO nanorods sensitized with CdS and CdSe quantum dots for photovoltaic applications.1 Adapted from Ref. 1. Copyright 2011 Springer. (b) Tuned localized plasmon resonances of copper sulfide nanodisks based on composition.2 Adapted from Ref. 2. Copyright 2014 American Chemical Society. (c) Nanoparticle temperature vs laser irradiation time of CuS nanodots for photothermal therapy.3 Adapted from Ref. 3. Copyright 2015 American Chemical Society. (d) Labeling of cell endosomes using ZnSe quantum dots.4 Adapted from Ref 4. Copyright 2014 American Chemical Society.

This Feature Article explores a novel method to synthesize colloidal SNCs via thermolysis of metal-organic coordination complexes that exhibit liquid crystalline behavior. In this strategy, the chemical and physical properties of the SNCs are programmed by a singlesource metal-organic liquid crystal (MOLC) precursor that contains stoichiometric amounts of


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metal and chalcogen atoms (to program SNC composition) and possesses supramolecular order (to template crystal phase and SNC morphology). Compound semiconductors such as Cu2-xS, Cu2-xSe and Cu2-xTe possess rich phase diagrams, supporting numerous stoichiometries and crystal structures, each with unique optical and electronic properties. In order to control the chemical composition of Cu chalcogenide SNCs, a single-source metal-organic precursor containing both the metal atom and the chalcogen can be decomposed by thermolysis to form the crystalline SNC. Because composition and morphology are dictated almost entirely by the single-source precursor, this synthetic strategy avoids the formation of equilibrium crystal phases and nanocrystal shapes frequently obtained by other colloidal synthesis methods that rely on selective adsorption of passivating agents or soft templating with surfactants.40, 41 In addition to metal alkanethiolates and alkaneselenolates, we and others have explored metal alkanoate systems.42-46 Specifically, we have synthesized and characterized Co, Cu, and Pb dodecanoates as precursors for forming their respective metal oxide nanoparticles. Though the precursors behave in an analogous fashion to the alkanethiolate system, metal alkanoate decomposition occurs at significantly higher temperatures than comparable alkanethiolates due to the lack of metal catalyzed C-S bond thermolysis. Out experiments indicate that this hinders our ability to carry out thermolysis within an ordered mesophase, since these precursors typically melt into an isotropic mesophase at elevated temperatures. First, we will focus on the design and synthesis of metal alkanethiolate and alkaneselenolate complexes, which function as precursors to metal sulfide or selenide SNCs upon pyrolytic decomposition. These precursor molecules are programmed to form supramolecular structures that behave as thermotropic liquid crystals, adopting MOLC mesophases upon heating. Next, we will discuss the requirements for metal chalcogenide


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crystallization and SNC formation, which is promoted via solventless thermolysis within an ordered MOLC mesophase. While many syntheses have been devised using similar metal alkanethiolates as single-source precursors for metal sulfide NCs in solvothermal reactions, such reactions dissolve the precursor, causing it to lose its supramolecular structure and consequently produce only spherical NCs.47-49 In contrast, solventless thermolysis of precursor compounds has been found to yield shaped particles,50, 51 though the effects of a mesophase on NC nucleation and growth were not well understood. Finally, we will discuss how this synthetic technique can be extended towards the development of an entire toolbox of SNCs with tailored optical, electronic, and chemical properties.

II. Metallomesogens A key characteristic of the MOLC precursors discussed here is the ability to support an ordered mesophase, the physical state of a liquid crystal (LC) material that is an intermediate between the crystalline (highly ordered solid) and isotropic (highly disordered liquid) states. LCs are generally composed of moderately-sized molecules which exhibit orientational (and sometimes also positional) order like a crystal, but are still free to move around like a fluid. LCs are typically classified as lyotropic or thermotropic, where thermotropic LCs show phase behavior that is temperature dependent and lyotropic LCs show phase behavior that is dependent on pressure or concentration. When a compound is capable of supporting a mesophase, it is classified as a mesogen, or a metallomesogen if the compound contains a metallic element. Metallomesogens have been studied for decades and encompass multiple types of chemical species which include lanthanides, metallocenes, macrocyclic metallomesogens, ionics, and polymers.52 Metallomesogens have been investigated extensively for applications which include


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electron paramagnetic resonance53 (for paramagnetic metallomesogens) and as ordered templates for mesoporous materials.54 In the present context, metallomesogens serve as templates for the growth of lowdimensional compound SNCs, such as Cu2S nanodisks, Ag2Se nanorods, and PbS nanooctahedra. We classify the metallomesogens used for SNC synthesis as MOLCs because there are two main requirements for these single-source precursors: i) the presence of a metalchalcogen bond that is retained after thermolysis, and ii) an organic component that enables the appropriate intermolecular forces to form and maintain supramolecular order at thermolysis temperatures. Long, linear alkane chains are often used as the organic component for the MOLCs described in our work because they facilitate strong attractive intermolecular interactions via van der Waals and hydrophobic forces, giving rise to supramolecular order. While various metal thiolates have been previously used as single-source precursors for the synthesis of metal sulfide nanoparticles such as Cu and Ag dodecanethiolates, these compounds are typically dissolved in solvent55 ,56, 57 or dispersed in polymer58 which prevents the formation of an ordered mesophase. In addition, several of these metal thiolates contain alkyl components such as bulky tert-butyl thiol59,60 or thiobenzoate61 which generally fail the steric requirements to produce an organized mesophase. Outside of very early work on silver alkanethiolates,62, 63 the mesogenic behavior of MOLC systems has been poorly explored, especially with regard to the formation of solid-state nanocrystal products obtained via thermolysis.


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Figure 2. Defining mesophase terminology. Lamellar solids such as the metal chalcogenolates discussed here take on a lamellar crystalline, rigid structure, seen in (a). This crystalline structure undergoes a phase change upon heating into a structured mesophase or an isotropic phase. Ordered mesophases include (b) lamellar smectic, which is still layered but more mobile, and (c) columnar, where the MOLC components form columns of discotic micelles. (d) The isotropic phase does not have any discernable structure. Adapted from Ref. 65. Copyright 2014 American Chemical Society.

To better understand the behavior of MOLC systems, we will first review the structure of the mesophases supported by these compounds, depicted in Figure 2, for a molecule that possesses a linear alkyl chain as the organic component. Typically, MOLCs self-assemble within solution to form a lamellar bilayer structure, which crystallizes as a solid precipitate. This lamellar crystalline phase (Figure 2a) adopts a high degree of order, observed in X-ray diffraction spectra by sharp periodic peaks that correspond to interlamellar spacings. As this crystalline MOLC melts,

molecules from the lamellar crystalline phase begin to lose

coordination within the metal-chalcogen network,63 though the layered ordering is loosely maintained. Molecules within this lamellar smectic phase (Figure 2b) are now able to rotate and successive layers are highly mobile with respect to each other. For a MOLC to exhibit a columnar mesophase (Figure 2c), the interactions which hold the lamellar structure together must be overcome by thermal kinetic energy.64 The rearrangement of molecules from a lamellar to a columnar structure is primarily caused by the generation of gauche conformers which cause


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alkyl chains to bend and kink, increasing the steric repulsion between neighboring chains. This rearrangement results in the formation of discotic micelles, where the metal-chalcogen headgroups adopt a ring-like coordination near the core and the alkyl tails project radially from this central ring.65 These discotic micelles can stack into columns, which pack hexagonally. A simplified depiction of a discotic micelle for Cu alkanethiolate is shown in Figure 2c. When stacked, the attractive interactions between alkyl chains still persist, albeit weaker than in the lamellar phases. When the MOLC is heated to temperatures at which the intermolecular interactions maintaining the supramolecular order of a given mesophase are overcome, the MOLC enters the isotropic phase (Figure 2d), which has no discernable order.

Figure 3. Analyzing birefringence using POM. POM images of three different compounds with lamellar smectic, columnar, and isotropic mesophases, respectively.

A key identifying property of MOLCs is their birefringence, which arises from supramolecular ordering that gives rise to a change in refractive index with respect to angle of orientation. This enables the characterization of MOLCs via polarized optical microscopy (POM). In POM, polarized light is passed through a thin layer of the MOLC and experiences constructive and destructive interference depending on the local MOLC orientation. Transmitted light is passed through a cross-polarized, producing visually vivid colors and characteristic patterns that can aid in the identification of some MOLC mesophases. Example of some typical POM images of MOLCs are shown in Figure 3. For the Ag thiolate in Figure 3b, the columnar


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mesophase can be readily identified by the characteristic fan-like structures observed via POM. However, direct imaging does not yield explicit phase information and typically is only indicative of whether an MOLC supports a mesophase or is isotropic. Because the isotropic phase has no supramolecular order, POM images appear dark since no light is transmitted through the cross-polarizer. A POM image of the isotropic phase shown in Figure 3c for Ag0.5Cu 0.5SeC12H25,

where the mixed Ag and Cu compounds form a frustrated system that lacks the high

degree of order shown by the individual Ag and Cu molecular components.

III. MOLC Design Requirements A new MOLC precursor is created and refined through a series of synthesis and characterization steps, as outlined in Figure 4. The first step in producing a MOLC precursor for shaped SNC synthesis is to select the appropriate metal salt and chalcogenol ligand for coordination. For ease of reaction, water- or alcohol-soluble salts including metal nitrates, sulfates, acetates, and halides are ideal, and can generally be purchased without issue. The chalcogenol can either be purchased (most alkanethiols discussed in this Article are commercially available) or synthesized (as is the case for alkylselenols, which are not typically commercially available). For our experiments, we adapted a previously reported method to obtain the relevant selenol.66 Typically, we have started with alkanethiols and alkaneselenols with linear, unbranched chains as a starting point since these tend to assemble into layered structures due to strong hydrophobic interactions. To facilitate a precipitation reaction between the metal ion and the chalcogenol, both need to be soluble in the reaction solvent and the chalcogenol must be deprotonated. A weak base such as triethylamine can be used for the deprotonation step if the pKa of the ligand is moderately high. If the metal salt and chalcogenol


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do not react to form a solid precipitate, then the properties of the reactants should be reassessed, including valency of the metal ion and the steric bulk of the chalcogenol ligand. For example, if the reaction product is a liquid, the chalcogenol ligand can be designed to have alkyl chains that facilitate more attractive intermolecular interactions in order to better stabilize the lamellar crystalline phase of the solid precursor. A relevant example of this scenario is Cu tertdodecanethiolate (CuSC(CH3)2C9H19), which has possesses bulky side-methyl groups that provide a strong steric hindrance to close-packing.65 Because of this steric hindrance and consequent lack of ample attractive intermolecular interactions, (CuSC(CH3)2C9H19) is isolated as an isotropic liquid. By replacing tert-dodecanethiol with its unbranched analog, 1dodecanethiol, the resulting Cu dodecanethiolate product can be isolated as a lamellar crystalline solid. If a solid precipitate forms upon reaction of the metal ion and chalcogenol, a room temperature powder XRD spectrum is obtained to determine its structure. For a compound to have promise as a MOLC precursor, it should adopt a lamellar crystalline structure at room temperature. The XRD spectrum characteristic of a lamellar solid will have periodically spaced low-angle peaks, which are attributed to multiple reflections correlated to the spacing between layers within the solid. If a potential precursor does not adopt this structure, another alkyl ligand should be chosen that better stabilizes the crystalline lamellar phase. Next, the thermal properties of the lamellar crystal are obtained using differential scanning calorimetry (DSC) to determine the temperatures of any existing MOLC phase transitions for the precursor, observed as an intense peak indicative of an endothermic event. Phase transitions from the lamellar crystalline to a lower-order phase usually occur at temperatures between 100°C and 200oC, depending upon the MOLC.67,


68

An additional

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endothermic peak is typically observed near 250oC, which corresponds to the vaporization of thermolysis byproducts. If strong endothermic peaks are observed between 100-250oC, this is usually indicative of a mesophase transition. If there is no evidence of a mesophase transition in the DSC thermogram, thermolysis can only be carried within the crystalline lamellar phase. If the MOLC melts into an isotropic phase prior to reaching the decomposition temperature, then no shape control is obtained and we have observed the formation of either spherical or irregularly shape nanocrystals. Ideally, the decomposition temperature of the MOLC needs to lie within a stable temperature range of the desired mesophase. If DSC characterization indicates the presence of a mesophase transition, further characterization can be carried out by temperature-dependent X-ray diffraction (XRD) to determine the various mesophases that are formed. If the spectra exhibit the same peaks indicative of a lamellar structure but after an endothermic event, the MOLC likely adopts a lamellar smectic phase. If the XRD of the mesophase indicates a hexagonally closepacked structure, the mesophase is likely columnar. However, since XRD is a bulk characterization technique with a spot size of ~5 mm, it may not be able to provide useful information if the crystallographic domains of the MOLC are too small. POM analysis may be used to at least confirm the adoption of a mesophase at a given temperature. To determine the decomposition temperature (Tdecomp), thermogravimetric analysis (TGA) is carried out to monitor mass loss as a function of temperature. The final mass loss also provides information about the final chemical composition of the sample as well as the amount of byproducts produced in thermolysis. For example, a mass loss of ~83% for a Cu dodecanethiolate sample indicates that Cu2S was formed, as this mass loss correlates to the vaporization of enough alkyl sulfides and disulfides such that there remained one mole of sulfur


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per two moles of copper.65 If Tdecomp does not occur within the mesophase of interest, the MOLC components must be modified to either stabilize or destabilize supramolecular ordering. If Tdecomp does lie in the correct range, the MOLC is a good candidate for a single-source precursor and solventless thermolysis can be carried out to produce shaped SNC products.

Figure 4. Decision tree for the rational design of a single-source MOLC precursor.

IV. Single-Source Precursor Precipitation There are few considerations in the precipitation of single-source metal chalcogenolate precursors beyond appropriate selection of the metal salt used as the starting material. For example, the precipitation of metal alkanethiolates is a facile reaction which has been reported extensively for various metals (e.g. AgSC8H17, CuSC18H37, AuSC18H37)62,


67, 69

and within

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various solvents (e.g. acetonitrile and THF).62,

69

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Generally, solutions of a chalcogenol and a

dissolved metal salt are combined under a high stir-rate via dropwise addition.65 Ethanol is a convenient solvent because it readily dissolves both nonpolar and polar chalcogenols, as well as water-soluble metal salts. The concentrations of chalcogenol to metal cation is either stoichiometric or involves a moderate excess of chalcogenol. In general, metal cations that are monovalent or possess similarly low oxidation states are preferred. For example, the precipitation of metal thiolates proceeds rapidly, with a typical reaction between a monovalent metal and an alkylthiol proceeding as: + ⇌ +

(1)

However, the acidification of the solution due to H+ generation can sometimes inhibit metal complexation. This can be overcome by selecting a metal salt where the counterion functions as a weak base such as acetate, or by adding a weak base such as triethylamine in a 1:1 molar ratio with the thiol.62 In addition, it should be noted that these precipitation reactions are limited to polarizable, soft metal ions, since alkyl sulfides (RS-) and alkyl selenides (RSe-) serve as soft bases. Metals cations (e.g. Au3+, Fe3+) with higher oxidation states are susceptible to reduction at the cost of oxidative disulfide formation. The addition of alkylthiols to Cu2+ has also been known to cause disulfide formation, as shown in Figure 5b. The initial redox reaction of the thiol with Cu2+ yields Cu+ ions and an insoluble disulfide species, which can be removed via reflux in toluene. Cu+ goes on to react with remaining alkylthiol molecules to form the corresponding Cu alkanethiolate, which can subsequently be used in thermolysis. Due to this oxidative disulfide formation, two equivalents of alkanethiol are used when working with Cu2+.


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Figure 5. Metal thiolate synthesis and thermolysis schemes. The reaction schemes for (a) Cu dodecanethiolate and (b) Ag dodecanethiolate.

A final consideration for precipitation of the single-source precursor is how the precursor will react under thermolysis conditions. The metal-chalcogen (M-Ch) bond in the precursor must be retained during the formation of the solid-state SNC product. Because the M-Ch bond is a strong covalent linkage and effectively withdraws electron density from the adjacent carbonchalcogen (C-Ch) bond, the C-Ch bond can be thermally cleaved at low temperatures. Thus, the formation of the SNC occurs through metal-catalyzed C-Ch bond thermolysis. However, for metals that possess high reduction potentials, C-Ch bond cleavage is outcompeted by M-Ch homolytic bond cleavage to form reduced M0. This results in the generation of metallic nanoparticles, rather than SNCs.64 For example, homolytic Ag-S bond cleavage occurs upon thermolysis of a AgSC12H25 MOLC, resulting in the formation of metallic Ag nanodisks (Figure 5b). This M-Ch bond cleavage can generally be avoided if the single-source precursor can be decomposed rapidly at a higher temperature to promote C-Ch bond cleavage.


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V. Supramolecular Assembly of MOLCs The supramolecular self-assembly of MOLCs is largely dictated by the intermolecular interactions between alkyl components that comprise the single-source precursors. In the crystalline lamellar phase, this can be manifested in both intra- and interlayer interactions. For example, decreased lamellar spacing typically coincides with lower melting transition temperatures whereas increased packing of alkyl chains within the lamellae tend to stabilize micellar phases. Here, we explore three main strategies for tailoring these types of interactions to control MOLC phase behavior: i) control of alkyl chain length, ii) chain conformation and sterics, and iii) intercalation of lamellar phases. These strategies are intended to provide increased control over the supramolecular behavior of single-source precursors, opening new MOLC phases at previously inaccessible temperatures.

Alkyl Chain Conformation Our work using Cu alkanethiolates as a model MOLC system has demonstrated that alkyl chain length plays an essential role dictating supramolecular order.65 MOLCs were prepared by precipitating Cu(NO3)2 with n-alkanethiols that possess chain lengths of n = 4, 8, 12 and 16 carbons. The room temperature XRD spectra of the crystalline lamellar MOLCs show periodically spaced low-angle peaks that are attributed to the multiple reflections characteristic of a lamellar solid (Figure 6c). Analysis of peak spacing indicates an increase in d-spacing from 1.59 to 4.55 nm as the alkane chain is increased from 4 to 16 methylene units, corresponding to a bilayer structure. For the MOLC with the longest alkane chain (CuSC12H25), thermal analysis by DSC shows a large endothermic peak at 132oC indicative of a melting to the columnar phase67, 68


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and a peak at 250oC corresponding to the vaporization of thermolysis byproducts (Figure 6d). Upon melting, the MOLC remains birefringent as observed by using POM, confirming that this melting does not result in an isotropic phase. For the MOLC with the shortest alkane chain (CuSC4H9), the thermal properties appear quite different. No melting transition was visually observed for CuSC4H9 prior to thermolysis at 200oC, though a distinct color change in the MOLC was observed. This color change is attributed to a transition from the crystalline lamellar phase to the lamellar smectic phase, which is known to occur for alkanethiolates with n= 4-10 carbons.63 This was confirmed by DSC analysis (Figure 6d) showing an endothermic peak near 175oC corresponding to this phase transition and a smaller peak above 200oC corresponding to isotropic melting and thermolysis.65 This phase transition is driven primarily by an elevated level of thermal disorder of the alkyl chains, where an increasing amount of entropy and steric forces overcome bonding in the lamellar phase. We observed that CuSC4H9 was also significantly less stable than CuSC12H25, showing susceptibility to oxidation and supramolecular disorder over time. This was attributed to weaker intralamellar interactions. Overall, these experiments showed that varying alkyl chain length can be used to control the mesophase a MOLC adopts. Specifically, that metal alkanechalcogenolates with longer alkane chains will favor the columnar phase and those with shorter alkane chains will favor the lamellar smectic phase.


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Figure 6. Analysis of short-and long-chain copper alkanethiolates. (a) A depiction of the lamellar crystalline to the lamellar smectic mesophase transition of copper dodecanethiolate (CuSC12H25) and (b) the lamellar crystalline to columnar mesophase transition of copper butanethiolate (CuSC4H9). (c) Room temperature XRD spectra of CuSC12H25 and CuSC4H9. (d) Thermal analysis of CuSC12H25 and CuSC4H9 using DSC. Panels b and c are adapted from Ref. 65. Copyright 2014 American Chemical Society.

Steric effects Strong intermolecular attraction is necessary for the formation of MOLCs, and steric hinderance is a contributing factor in limiting supramolecular assembly of single-source precursors. Both van der Waals and hydrophobic forces are strong drivers of MOLC assembly, as observed in the previous example of Cu alkanethiolates that exhibit bilayer-like stacking in the crystalline lamellar phase. However, the introduction of branched alkanethiols or bulky side groups disrupts this bilayer-like packing, as in the case of Cu tert-dodecanethiolate, which does not precipitate as a crystalline solid and instead separates in the reaction solution as an isotropic liquid at room temperature.65 In the absence of strong hydrophobic forces that drive alkyl chain packing, MOLCs also exhibit difficulty in supporting lamellar or columnar mesophases. For example, when Cu salt is precipitated with benzylmercaptan (HSCH2Ph), the aromatic phenyl


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group exhibits a packing structure much different than that of the linear alkanes due to the presence of strong π interactions.65 As a result, CuSCH2Ph is isolated as a solid precipitate, but possesses no lamellar order as determined by XRD analysis and exhibits no birefringence upon melting as determined by POM analysis. It can be concluded from these results that a disruption of efficient alkyl chain packing and favorable intermolecular forces via the steric effects precludes the formation of a structured mesophase and any templating capability of the MOLC.

Halide-mediated stability

Figure 7. Using Cl- ions to stabilize the lamellar mesophase. (a) Reaction schematic of CuSC10H20COOH with and without halide intercalation. (b) DSC thermograms of CuSC10H20COOH with and without halide intercalation. (c) Temperature-dependent XRD spectra showing that the lamellar structure is maintained even at elevated temperatures. Adapted from Ref. 70. Copyright 2016 American Chemical Society.


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For many classes of layered solids such as graphite and transition metal disulfides (e.g. MoS2), intercalation of atomic layers with guest molecules or ions can promote the cleavage or exfoliation of ultra-thin sheets due to decreased interlayer interactions. A similar intercalation strategy can be employed to tailor the interlayer interactions of lamellar MOLCs. For example, Cu alkanethiolates can be precipitated using Cu (II) acetate and functionalized alkanethiol ligands such as 11-mercaptoundecanoic acid (MUA) to form CuSC10H20COOH.70 XRD analysis for the precipitate showed that the interlayer d-spacing contracted by ~0.2 nm in comparison to CuSC12H25. We attributed this contraction to strong hydrogen bonding interactions between neighboring carboxylic acid groups. DSC analysis of CuSC10H20COOH (Figure 7b) showed a large endothermic peak just below 100oC, and temperature-dependent XRD analysis indicates that the crystalline lamellar solid transitions to a lamellar smectic phase. A smaller endothermic peak at 164oC is attributed to a phase transition from the lamellar smectic to columnar mesophase. The presence of the carboxylic acid groups in CuSC10H20COOH thus destabilizes the crystalline lamellar mesophase most likely due to frustrated alkane chain packing. We also found that the addition of Cl- ions to the precipitation reaction of CuSC10H20COOH resulted in incorporated Cl- ions, as observed by energy dispersive x-ray spectroscopy.70 The chemical formula of the resulting precipitate was [CuSC10H20COOH]·½Cl. We observed by DSC analysis that the endothermic transition at 164 oC was not present for the [CuSC10H20COOH]·½Cl., indicating the presence of Cl- ions stabilizes the lamellar smectic mesophase to the point of suppressing the columnar mesophase transition. For Cl– to stabilize the lamellar phase, it must either decrease the thermal disorder occurring among alkyl components or reinforce in-plane attractive forces. We find the latter explanation more compelling since Cl– coordination to Cu within the Cu–S planes represents another interaction which must be


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overcome prior to mesogenic rearrangement. During thermolysis, this lamellar template serves to encourage anisotropic nucleation and growth along the Cu–S planes, resulting in nanosheet formation. This mechanism complements those reported by Morrison et al., which provides evidence that the lamellar mesophase directs the formation of 2D PbS nanosheets via the oriented attachment of PbS nanoparticles.71

V. MOLC Thermolysis for Nanocrystal Formation

Figure 8. Solid-state nanocrystal products. Illustrations of the mesophases in which thermolysis was carried out and TEM images of the solid-state nanoparticle products from these respective thermolysis reactions. The TEM images are reproduced from the following: Cu9S5 nanosheets from Ref. 70. Copyright 2016 American Chemical Society; Cu2-xSe nanoribbons from Ref. 72. Copyright 2017 American Chemical Society; Cu2-xS nanodisks from Ref. 65. Copyright 2014 American Chemical Society; Ag2Se nanorods from Ref. 72. Copyright 2017 American Chemical Society; Ag2Se nanospheres from Ref. 72. Copyright 2017 American Chemical Society; AgCuSe with no shape from Ref. 72. Copyright 2017 American Chemical Society.


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The supramolecular order adopted by MOLC precursors is critical for garnering control over SNC morphology and nanocrystal shape. These SNC properties are templated during the nucleation and growth process by the formation of specific liquid crystalline phases (e.g. micellar, smectic, lamellar) that selectively limit reactant diffusion, thereby avoiding the formation of the equilibrium (i.e. pseudo-spherical) shapes frequently obtained by other colloidal synthesis methods. Uniquely, this approach has the potential to achieve extremely anisotropic SNCs that approach the two-dimensional atomic crystal limit. Below, we review the anisotropic SNC shapes that can be templated through MOLC thermolysis. Nanosheets and nanoribbons: If thermolysis is conducted with the MOLC stabilized in the lamellar smectic mesophase, the layered structure serves to templated anisotropic SNC growth along the metal-chalcogen planes. In our work, we have observed the formation of nanosheets70 and nanoribbons72 from the decomposition of lamellar smectic MOLCs. For example, thermolysis of [CuSC10H20COOH]·½Cl produced Cu9S5 nanosheets (Figure 7a) due to Cl- stabilization of the lamellar template, in comparison to CuSC10H20COOH thermolysis that results in nanodisks. Further characterization data via TEM, AFM, photoluminescence spectroscopy, UV-Vis-NIR, EDX, TGA and XRD can be found in our previous works.65, 70, 72 Others have also found that halide stabilization of the lamellar mesophase facilitates controlled growth of 2D nanosheets.73 Thermolysis of CuSeC12H25 takes place within the lamellar smectic mesophase of the precursor, where ultrathin 2D ribbons of Cu2-xSe are crystallized within the lamellar bilayer mesophase structure upon sustained thermolysis.72 This result is significant, as a direct synthesis of ultrathin copper selenide NCs has not been previously reported. Shape control is likely dictated by rapid in-plane diffusion within the lamellar MOLC. However, the uniformity of the resulting nanosheets and nanoribbons is limited by the size distribution of the MOLC


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domains, since reactant diffusion is likely hindered at grain boundaries between differently oriented lamellar domains. While thermal annealing can be employed to increase overall domain MOLC domain size, the production of these high aspect-ratio shapes with a tight size distribution is difficult to control. For this purpose, others have employed foreign metal ions to modulate morphology and crystal phase74 and exfoliation methods.75 Nanodisks: The discotic micelles that make up the columnar mesophase serve as nucleation points for the growth SNC disks or rods within the MOLC columns. Gradual thermal decomposition of the discotic micelles facilitates the transport of M-Ch units to the outer surfaces of the stacked particles, enhancing radial growth. This is consistent with other reports where columnar mesophases have been shown to template nanowires76 and nanoplates.77 In our work, we have found the columnar mesophase to yield Cu2-xS65 and Ag nanodisks from their respective precursors. Additional characterization data by AFM, XRD, TGA, and TEM can be found within our previous works.19, 65 We confirmed this templating mechanism for nanodisks by monitoring the thermolysis products of CuSC12H25 at various time intervals by transmission electron microscopy. Microscopy images show that SNC particles grow in an oriented columnar fashion within the dense MOLC matrix, consistent with enhanced radial growth. In general, the resulting nanodisks are observed to be monodisperse in size and shape, especially in comparison to nanosheets and ribbons. This likely results from fast reactant diffusion within the micellar columns of the MOLC matrix, where the radius of the column – not the overall MOLC domain size – is the critical determinant of nanodisk size. Nanorods: MOLCs in the columnar mesophase can also support the anisotropic growth of nanoparticles along the columnar axis. In the case of AgSeC12H25, thermolysis results in the formation of Ag2Se nanorods.72 XRD analysis of solid AgSeC12H25 indicates a lamellar bilayer


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spacing of ~3.44 nm,72 which is slightly larger than the analogous AgSC12H25 possesses a dspacing of ~3.35 nm and could lend insight into why this precursor templates nanorods instead of nanodisks. This increased interlayer spacing may hinder the transport of Ag-Se units between neighboring layers within a discotic micelle, allowing axial growth to out-compete the radial growth mechanism that produces nanodisks. It is also notable that at lower thermolysis termperatures the resulting nanorods were primarily composed of cubic-phase α-Ag2Se, which is not thermodynamically stable at room temperature and is rarely obtained.78 Higher thermolysis termperatures results in the formation of SNCs that are primarily orthorhombic β-Ag2Se. We suspect that the columnar MOLC mesophase is responsible for directing the crystallization of the thermodynamically unfavorable cubic phase, and that higher temperatures may weaken this effect by producing a mesophase which is less ordered or shorter lived. This result indicates that mesophase templating can be used to obtain both desirable morphologies and crystal phases. Further characterization via TEM, XRD, TGA, and EDX can be found in previous works.72 Like nanodisks, the resulting nanorods tend to possess uniform widths since the radial growth direction is templated by the micellar columns of the mesophase. However, growth along the axial nanorod direction component tend to be much more variable and results in a large distribution of nanorod aspect-ratios. Nanospheres: When thermolysis is carried out within an isotropic phase, the resulting nanoparticles fail to result in anisotropic SNCs as when thermolysis is carried out within a structured MOLC mesophase. Without the MOLC template to direct reactant diffusion, the SNCs grow into equilibrium morphologies such as spherical particles, as exemplified by the formation of Ag2Se nanospheres from AgSeC12H25 decomposition and AgCuSe quasi-spheres from Ag0.5Cu0.5SeC12H25 decomposition.72 However, the formation of spheres and quasi-spheres is


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still likely to result from diffusion-limited growth, which depends highly on the viscosity of the isotropic mesophase.

VI. Summary and Future Outlook

Figure 9. Future directions. (a) POM image of trivalent metal alkanethiolate In(SC12H25)3. (b) PbS octahedra nanoparticles from the thermolysis of Pb(SC18H37)2, a divalent metal alkanethiolate.

Thus far, we have carried out detailed structural and thermal characterization of Cu and Ag alkane chalcogenolates to better elucidate MOLC assembly and phase behavior, and to better understand the potential of these materials as templates for controlled SNC nucleation and growth. However, the ability to assemble these MOLC precursors into mixed mesogenic phases has the potential to enable the synthesis of SNCs with highly tailored plasmonic properties. We have shown that AgSeC12H25 and CuSeC12H25 co-assemble into a homogeneous supramolecular framework rather than form separate domains.72 The decomposition of this mixed-metal material, Ag0.5Cu0.5SeC12H25, yields AgCuSe nanoparticles rather than a mixture of Ag2Se and Cu2Se NCs or heterostructures. This result indicates the potential for precisely controlled chemical composition and SNC properties. Using a templated thermolysis strategy, it should also be possible to generate a library of single-source MOLC precursors that can be thermolyzed either neat or in a combinatorial fashion


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to generate SNCs with a wide range of compositions and carrier densities. This far, we have synthesized and characterized a large number of Cu thiolates and selenolates; however, thiolate complexes using such soft metals as Au, Hg, and Tl can be found going back through decades of inorganic synthesis.69, 79, 80 We and others have prepared metal thiolate and selenolate complexes with metals including Co2+, Ni2+, Zn, Pd2+, Ag+, Cd2+, In3+, Sn2+, Au+, Hg2+, Tl3+, Pb2+, and Bi3+. (Figure 10a) Our results indicate that only monovalent metals are capable of supporting a MOLC columnar mesophase. It has yet to be determined what types of MOLC mesophases divalent and trivalent metal ions support. Additionally, we have found that when a MOLC only supports an isotropic phase upon melting, thermolysis yields SNCs that adopt equilibrium morphologies as observed with Pb(SC18H37)2, whose thermolysis yields monodisperse PbS octahedra (Figure 10b). While a MOLC strategy may not achieve non-equilibrium PbS nanostructures obtained through solvent-based methods,81, 82 this strategy has the potential for the scaled-up production of equilibrium SNC products83 with tight size and shape distributions. Overall, the ability to generate MOLC precursors with precisely tuned chemical composition, stoichiometry, and long-range order has great potential for targeted solid-state nanomaterials synthesis. In addition to providing a novel synthetic pathway for achieving lowdimensional, anisotropic nanostructures, the thermolysis of MOLCs serves as a powerful new approach for the rational synthesis and design of SNCs.


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AUTHOR BIOGRAPHIES

Stephanie C. Smith is set to receive her M.S. in NanoEngineering at the University of California, San Diego (UCSD) in June 2018. She received her B.S in Chemistry and NanoEngineering and graduated magna cum laude from UCSD in 2017. During her time in the Tao research group, her primary research interests included the synthesis of inorganic nanoparticles via colloidal methods and self-assembled templates.


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Dr. Whitney Bryks currently works as an R&D engineer focused on microelectronics packaging at Intel Corporation. He received his Ph.D. in Chemistry from the University of California, San Diego in 2017. His doctoral research investigated the synthesis of inorganic nanostructures by self-assembled templates. He received his B.S. in Chemistry from the University of California, Santa Cruz in 2010 where his work centered on the novel synthesis of boronic esters and acids. His current work investigates surface chemistry interactions for new packaging materials.

Prof. Andrea R. Tao is currently an Associate Professor in the NanoEngineering Department at UC San Diego. She earned her A.B. in Chemistry and Physics from Harvard University in 2002 and her Ph.D. in Chemistry from UC Berkeley in 2007. Her research interests lie in the discovery and development of new nanomaterials and nanocomposites for plasmonics, where light is propagated, manipulated, and confined by nanocomponents that are smaller than the wavelength of light itself. She was the recipient of the 2008 International Union of Pure and Applied Chemistry Prize for Young Chemists, a 2013 RSC Emerging Investigator, a 2014 DARPA Young Faculty Award, and the 2015 Sloan Fellowship in Chemistry.


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Supramolecular Assembly of Single-Source MetalâChalcogenide

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