Nanocrystal Solids: A Modular Approach to Design of Functional Materials

Research Summary 2007-2010

My current research interests lie in the development of novel materials through the assembly of functional nanoscale building blocks. During past years my group explored different routes which included nanomaterial synthesis, self-assembly and electronic studies of the nanocrystal arrays. These works have been summarized in more than 20 peer-reviewed publications. My group has developed a generalized synthetic methodology for combining inorganic nanocrystals and nanowires with the molecular metal chalcogenide (MCC) ligands [Science, 2009, 324, 1417* and cv publications cv1, cv3, cv5, cv6*; *=hardcopy enclosed]. That work turned colloidal nanocrystals into a versatile class of electronic materials, offering new directions for solution processed light-emitting, photovoltaic and thermoelectric devices. We also reported self-assembly of nanocrystals into long-range ordered superlattices with aperiodic quasicrystalline packing [Nature, 2009, 461, 964*].

Single- and multicomponent nanocrystal assemblies provide a powerful general platform for designing programmable solids with tailored electronic, magnetic, and optical properties. Unlike atomic and molecular crystals where atoms, lattice geometry, and interatomic distances are fixed entities, the nanocrystal arrays represent ensembles of “designer atoms” with potential for tuning their electronic structure and transport properties. Generally speaking, nanocrystal assemblies can be considered as a novel type of condensed matter, whose behavior depends both on the properties of the individual building blocks and on the many-body exchange interactions.

My research program combines chemical synthesis [cv2, cv5, cv6*, cv7*, cv10*, cv23, cv27, cv31] and self-assembly [cv1, cv4*, cv8, cv11*, cv13, cv19] with structural and electronic characterization of nanocrystal assemblies [cv3, cv7*, cv10*, cv12, cv14-15, cv17-18, cv20-26, cv28] (Figure 1). As two major research thrusts, we explore the chemistry and physics governing self-assembly of nanocrystals into long-range ordered superlattices and develop novel chemical strategies for strengthening the electronic coupling between chemically synthesized nanocrystals. The latter is necessary for efficient charge transport and, ideally, can lead to the formation of collective electronic states (minibands) in the nanocrystal superlattices.

I.       Self-assembly of colloidal nanocrystals into ordered superstructures.

Recent studies have demonstrated enormous structural diversity in multicomponent nanoparticle assemblies, leading to a multitude of complex phases combining semiconducting, metallic, and magnetic nanocrystals into long-range ordered binary nanocrystal superlattices (BNSLs) [1-3]. During last years we extensively studied BNSL formation [cv1, cv4*, cv8, cv11*], focusing on several aspects described below.

1.1.What is the driving force of nanocrystal self-assembly?

At present, there is no clear understanding of the processes that govern the assembly of colloidal nanoparticles into complex multicomponent structures. On the other hand, the physics of sphere packing is rather well understood, with numerous theoretical and experimental studies outlining the effects of entropy [4-6], pair potentials [7-11] and polydispersity [12] on the nucleation and growth of ordered phases. To apply this framework to colloidal nanocrystals, we need quantitative estimates for the terms contributing to the free energy (, Eq.1) associated with the disorder-to-order transition in nanocrystal assemblies. We studied the hierarchy of the energy scales acting during self-assembly of nanocrystals and have shown that the structural diversity of BNSLs is a result of the cooperative effect of the entropy-driven crystallization and the interparticle interactions. Both ΔU and TΔS terms associated with the superlattice formation have the same order of magnitude [cv4*]. Moreover, temperature can be used as the weighting factor for the internal energy (U) and entropy (S) contributions and allows tailoring the relative weights of the interparticle interactions and free-volume entropy during the formation of nanocrystal superlattices. An amazing example of this effect is shown in Figure 2. Free-energy calculations predict no stable binary phases for a mixture of hard spheres with the size ratio γ=0.74 [13], whereas PbSe and Pd nanocrystals with same γ formed six (!) different BNSL phases at different temperatures [cv4*]. From the practical side, temperature provides a convenient tool for directing self-assembly of nanocrystals toward desired BNSL structures.

1.2. BNSL structural complexity and self-assembly of quasicrystalline superlattices.

We studied the formation of amazingly complex binary structures from spherical iron oxide and gold nanoparticles (Figure 3) and revealed their topological connections to the Archimedean tilings, defined as the regular patterns of polygonal tessellation of a plane by regular polygons, where only one type of vertex is permitted [cv11*]. The Archimedian tilings are known as the unique pseudomorphic phases that can combine both periodic and aperiodic structural elements [14,15].

Careful exploration of BNSL phase diagrams allowed us to extend the borders of self-assembly to quasicrystalline nanoparticle superlattices. Quasicrystals generate sharp diffraction peaks while lacking any translational symmetry and often showing symmetry operations forbidden in classical crystallography. We demonstrated that different mixtures of colloidal nanocrystals can self-assemble into quasicrystalline structures with “forbidden” 12-fold rotational symmetry (Figure 4). Our work introduced a new class of materials with quasicrystalline order. The compositional flexibility of qualicrystalline BNSLs indicated that the quasicrystalline ordering could be a common phenomenon in nanocrystal solids. Our work also showed how the cooperative effect of the free-volume entropy and configurational entropy can lead to the formation of quasicrystalline phases from spherical particles, even in the absence of any directional forces [cv11*]. Nanoparticle quasicrystals helped solving a long standing problem about the possibility of a smooth transition between topologically different quasicrystalline and crystalline phases. We observed that such transition requires a thin “wetting layer” of (3³,4²) Archimedean tiling which matched both phases with a low concentration of interfacial defects.

The goals for future studies: We will further continue the mechanistic studies of nanocrystal self-assembly. We will apply a suite of advanced experimental techniques for in situ monitoring of the assembly process. In collaboration with Ismagilov group, we recently developed a microfluidic platform for combinatorial studies of the formation of single- and binary nanocrystal superlattices. Using facilities at the Advanced Photon Source (Argonne National Lab), we study the nanocrystal assembly using in-situ time-resolved small- and wide-angle X-ray scattering. My group will also design and build flow-through liquid cells for TEM imaging through a thin layer of a colloidal solution. By tracking the movements of individual particles, we will have the opportunity to directly observe how size and shape of the building blocks affect their assembly into single- and multicomponent superlattices.

Nanocrystal superlattices provide a convenient test bed for fundamental studies of the crystallization phenomena. Many key features of ordinary crystals, such as faceting, twinning, polymorphism, etc. have been observed in nanoparticle superlattices [2], suggesting that their assembly follows the same fundamental principles as crystallization of conventional atomic and molecular solids. Nanoparticle superlattices provide a unique chance to study these phenomena in real space and real time. For example, twinning of the fcc lattice can be studied using superlattices self-assembled from colloidal PbS nanocrystals [cv8*]. We will use nanoparticle superlattices to gain insights to the early stages of the formation of an epitaxial interface between two periodic lattices with precisely engineered lattice parameters and tailorable interaction potentials.

II.    Transforming colloidal nanocrystals into a novel class of electronic materials.

To employ colloidal nanocrystals in electronic and optoelectronic devices (e.g., in solar cells, light-emitting diodes or radiation detectors), it is necessary to establish efficient electronic communication between individual nanocrystals. The conduction in nanocrystal solids involves charge transfer between individual nanocrystals [16-18]. Surface ligands with long hydrocarbon chains, used for nanocrystal synthesis, form highly insulating interparticle barriers and result in a very weak exchange coupling between the nanocrystals. Charge transport can be improved by replacing bulky ligands with smaller capping molecules such as pyridine, n-butylamine, etc., so that the electronic coupling between adjacent nanocrystals increases [19,20]. Also, chemical treatments of nanocrystal solids with dilute solutions of hydrazine, methylamine or 1,2-ethaneditiol can significantly improve carrier mobility, up to about 1 cm²V^-1s^-1 [17,21-24]. Unfortunately, such small linking molecules are volatile and susceptible to oxidation, imparting instabilities in the electronic properties. My group has proposed entirely new chemical approach for designing electronic materials from colloidal nanocrystals.

2.1.            Colloidal nanocrystals with molecular metal chalcogenide surface ligands.

We identified a broad class of inorganic molecular species, chalcogenidometallate Zintl ions or molecular metal chalcogenide complexes (MCCs), that can be used as robust and electronically transparent surface ligands for almost all known colloidal metal and semiconductor nanostructures (Figure 5) [cv3, cv5, cv6*, cv10*]. The unique advantage of MCCs as the capping ligands for colloidal nanocrystals is the possibility to convert them into amorphous and crystalline metal chalcogenides with semiconducting, ferroelectric or phase-change properties, thus linking individual nano-building blocks into a macroscopic assembly of electronically coupled functional modules. This chemistry set new benchmarks for electrical conductivity and electron mobility in self-assembled nanocrystal solids. For example, the conductivity of a superlattice of Au nanocrystals capped by Sn₂S₆^4- ligands approached 1000 S cm^-1 [cv5], which is about four orders of magnitude higher than the highest previously reported value for Au nanocrystal arrays with conventional (organic) surface ligands [25]. We applied this synthetic methodology to several materials systems, targeting different potential applications of nanocrystal solids.

Spin-coated arrays of CdSe nanocrystals capped with In₂Se₄^4- MCC ligands exhibited electron mobility higher than 15 cm²V^-1s^-1, which was at least one order of magnitude higher than the carrier mobility in best solution-processed organic [26] and nanocrystal-based [16] devices [cv3]. I am not aware of any other approach that could provide a dense film of a direct gap semiconductor with comparable carrier mobility via a simple low temperature solution-based route. Moreover, our high mobility nanocrystal solids preserved the properties of quantum-confined semiconductors (Figure 6). High carrier mobility, combined with size-tunable electronic structure, makes MCC-capped nanocrystals very attractive for printable electronics, photovoltaics, and photodetector applications [cv3, cv10*].

Lead- and bismuth chalcogenide nanocrystals capped with Sb₂Te₇^4- MCCs were used as the soluble building blocks for nanostructured thermoelectric materials [cv6*]. Properly designed MCC bridges between nanocrystals can provide high electron mobility and very low thermal conductivity supressed by the interfacial phonon scattering, which is the combination required for high thermoelectric figures of merit (ZT’s). Thermoelectric properties of our nano-composite materials (ZT=0.7 and 0.6 at 300K and 523K, respectively) compared favorably to all previously reported solution-processed thermoelectric materials [cv6*], with the potential for further improvements. This chemical methodology has been licensed by Evident Technologies, Inc. (NY). This company launched a development effort toward its practical implementation for designing advanced thermoelectric materials.

2.2.            Charge transport in the arrays of metal-semiconductor nano-heterostructures.