Facile assembly of size- and shape-tunable IV-VI nanocrystals into superlattices

In comparison to the previous lengthy approaches, we described a general and simple strategy for engineering the superlattice assembly of IV-VI semiconductor nanocrystals (NCs) with tunable sizes and morphologies. Not only the well-studied spherical NCs but also some special-shaped NCs, such as the quasi-cubic, cubic, truncated octahedral, and octahedral, could self-assemble into well-ordered patterns, as demonstrated in PbS, PbSe, and PbTe. These results extended our proposed model about the configuration of ligand chains in the superlattice assembly. This powerful capability of assembling superlattices was dominated by a heat-treatment process, providing a significant and extensive direction in the engineering of morphology-tunable NC superlattices.     

Simple cubic self-assembly of PbS quantum dots by finely controlled ligand removal through gel permeation chromatography

The geometry in self-assembled superlattices of colloidal quantum dots (QDs) strongly affects their optoelectronic properties and is thus of critical importance for applications in optoelectronic devices. Here, we achieve the selective control of the geometry of colloidal quasi-spherical PbS QDs in highly-ordered two and three dimensional superlattices: Disordered, simple cubic (sc), and face-centered cubic (fcc). Gel permeation chromatography (GPC), not based on size-exclusion effects, is developed to quantitatively and continuously control the ligand coverage of PbS QDs. The obtained QDs can retain their high stability and photoluminescence on account of the chemically soft removal of the ligands by GPC. With increasing ligand coverage, the geometry of the self-assembled superlattices by solution-casting of the GPC-processed PbS QDs changed from disordered, sc to fcc because of the finely controlled ligand coverage and anisotropy on QD surfaces. Importantly, the highly-ordered sc supercrystal usually displays unique superfluorescence and is expected to show high charge transporting properties, but it has not yet been achieved for colloidal quasi-spherical QDs. It is firstly accessible by fine-tuning the QD ligand density using the GPC method here. This selective formation of different geometric superlattices based on GPC promises applications of such colloidal quasi-spherical QDs in high-performance optoelectronic devices.

Gel permeation chromatography can finely control ligand coverage of PbS quantum dots. Self-assembly of these QDs with different ligand density leads to the formation of 2D square, hexagonal and 3D simple cubic and face-centered cubic superlattices.     

Explicit all-atom modeling of realistically sized ligand-capped nanocrystals

We present a study of an explicit all-atom representation of nanocrystals of experimentally relevant sizes (up to 6 nm), "capped" with alkyl chain ligands, in vacuum. We employ all-atom molecular dynamics simulation methods in concert with a well-tested intermolecular potential model, MM3 (molecular mechanics 3), for the studies presented here. These studies include determining the preferred conformation of an isolated single nanocrystal (NC), pairs of isolated NCs, and (presaging studies of superlattice arrays) unit cells of NC superlattices. We observe that very small NCs (3 nm) behave differently in a superlattice as compared to larger NCs (6 nm and above) due to the conformations adopted by the capping ligands on the NC surface. Short ligands adopt a uniform distribution of orientational preferences, including some that lie against the face of the nanocrystal. In contrast, longer ligands prefer to interdigitate. We also study the effect of changing ligand length and ligand coverage on the NCs on the preferred ligand configurations. Since explicit all-atom modeling constrains the maximum system size that can be studied, we discuss issues related to coarse-graining the representation of the ligands, including a comparison of two commonly used coarse-grained models. We find that care has to be exercised in the choice of coarse-grained model. The data provided by these realistically sized ligand-capped NCs, determined using explicit all-atom models, should serve as a reference standard for future models of coarse-graining ligands using united atom models, especially for self-assembly processes.     

Three-dimensional nanocrystal superlattices grown in nanoliter microfluidic plugs

We studied the self-assembly of inorganic nanocrystals (NCs) confined inside nanoliter droplets (plugs) into long-range ordered superlattices. We showed that a capillary microfluidic platform can be used for the optimization of growth conditions for NC superlattices and can provide insights into the kinetics of the NC assembly process. The utility of our approach was demonstrated by growing large (up to 200 μm) three-dimensional (3D) superlattices of various NCs, including Au, PbS, CdSe, and CoFe(2)O(4). We also showed that it is possible to grow 3D binary nanoparticle superlattices in the microfluidic plugs.     

Comparing the structural stability of PbS nanocrystals assembled in fcc and bcc superlattice allotropes

We investigated the structural stability of colloidal PbS nanocrystals (NCs) self-assembled into superlattice (SL) allotropes of either face-centered cubic (fcc) or body-centered cubic (bcc) symmetry. Small-angle X-ray scattering analysis showed that the NC packing density is higher in the bcc than in the fcc SL; this is a manifestation of the cuboctahedral shape of the NC building block. Using the high-pressure rock-salt/orthorhombic phase transition as a stability indicator, we discovered that the transition pressure for NCs in a bcc SL occurs at 8.5 GPa, which is 1.5 GPa higher than the transition pressure (7.0 GPa) observed for a fcc SL. The higher structural stability in the bcc SL is attributed primarily to the effective absorption of loading force in specific SL symmetry and to a lesser extent to the surface energy of the NCs. The experimental results provide new insights into the fundamental relationship between the symmetry of the self-assembled SL and the structural stability of the constituent NCs.     

Thiocyanate-capped nanocrystal colloids: vibrational reporter of surface chemistry and solution-based route to enhanced coupling in nanocrystal solids

Ammonium thiocyanate (NH(4)SCN) is introduced to exchange the long, insulating ligands used in colloidal nanocrystal (NC) synthesis. The short, air-stable, environmentally benign thiocyanate ligand electrostatically stabilizes a variety of semiconductor and metallic NCs in polar solvents, allowing solution-based deposition of NCs into thin-film NC solids. NH(4)SCN is also effective in replacing ligands on NCs after their assembly into the solid state. The spectroscopic properties of this ligand provide unprecedented insight into the chemical and electronic nature of the surface of the NCs. Spectra indicate that the thiocyanate binds to metal sites on the NC surface and is sensitive to atom type and NC surface charge. The short, thiocyanate ligand gives rise to significantly enhanced electronic coupling between NCs as evidenced by large bathochromic shifts in the absorption spectra of CdSe and CdTe NC thin films and by conductivities as high as (2 ± 0.7) × 10(3) Ω(-1) cm(-1) for Au NC thin films deposited from solution. NH(4)SCN treatment of PbTe NC films increases the conductivity by 10(13), allowing the first Hall measurements of nonsintered NC solids, with Hall effect mobilities of 2.8 ± 0.7 cm(2)/(V·s). Thiocyanate-capped CdSe NC thin films form photodetectors exhibiting sensitive photoconductivity of 10(-5) Ω(-1) cm(-1) under 30 mW/cm(2) of 488 nm illumination with I(photo)/I(dark) > 10(3) and form n-channel thin-film transistors with electron mobilities of 1.5 ± 0.7 cm(2)/(V·s), a current modulation of >10(6), and a subthreshold swing of 0.73 V/decade.     

Linking Semiconductor Nanocrystals into Gel Networks through All‐Inorganic Bridges

For colloidal semiconductor nanocrystals (NCs), replacement of insulating organic capping ligands with chemically diverse inorganic clusters enables the development of functional solids in which adjacent NCs are strongly coupled. Yet controlled assembly methods are lacking to direct the arrangement of charged, inorganic cluster‐capped NCs into open networks. Herein, we introduce coordination bonds between the clusters capping the NCs thus linking the NCs into highly open gel networks. As linking cations (Pt²⁺) are added to dilute (under 1 vol %) chalcogenidometallate‐capped CdSe NC dispersions, the NCs first form clusters, then gels with viscoelastic properties. The phase behavior of the gels for variable [Pt²⁺] suggests they may represent nanoscale analogues of bridged particle gels, which have been observed to form in certain polymer colloidal suspensions.     

Temperature-Dependent Photoluminescence of Silver-Indium-Sulfide Nanocrystals in Aqueous Colloidal Solutions

The temperature dependence of the photoluminescence (PL) intensity of colloidal semiconductor nanocrystals (NCs) makes them an appealing option in bio-sensing applications. Here, we probed the temperature-dependent PL behavior of aqueous glutathione (GSH)-capped Ag−In−S (AIS) NCs and their core/shell AIS/ZnS heterostructures. We show that both core and core-shell materials reveal strong PL quenching upon heating from 10 to 80 °C, which is completely reversible upon cooling. The PL quenching is assigned to the thermally activated dissociation of complexes formed by ligands with the metal cations on the NC surface and the introduction of water into the NC coordination sphere. This unique mechanism of the thermal PL quenching results in a much higher temperature sensitivity of the aqueous colloidal AIS (AIS/ZnS) NCs as compared with previously reported analogs capped by covalently bound ligands. Our results are expected to stimulate further studies on aqueous ternary NCs as colloidal luminescent nano-thermometers applicable for ratiometric temperature sensing.     

Enhanced thermal stability and magnetic properties in NaCl-type FePt-MnO binary nanocrystal superlattices

We report the growth of NaCl-type binary nanocrystal (NC) superlattice membranes by coassembly of FePt and MnO NCs at the liquid-air interface. The constituent FePt NCs were converted into the hard magnetic L1(0) phase by thermal annealing at 650 °C without degradation of the long-range NC ordering. In contrast, both FePt-only NC superlattices and FePt-MnO disordered NC mixtures showed substantial FePt sintering under the same annealing conditions. Our results demonstrate that the incorporation of FePt NCs into binary superlattices can solve the problems of FePt sintering during conversion to the L1(0) phase, opening a new route to the fabrication of ordered ferromagnetic NC arrays on a desired substrate for high-density data storage applications.     

Excavated Cubic Platinum–Tin Alloy Nanocrystals Constructed from Ultrathin Nanosheets with Enhanced Electrocatalytic Activity

Excavated polyhedral noble‐metal materials that were built by the orderly assembly of ultrathin nanosheets have both large surface areas and well‐defined facets, and therefore could be promising candidates for diverse important applications. In this work, excavated cubic Pt–Sn alloy nanocrystals (NCs) with {110} facets were constructed from twelve nanosheets by a simple co‐reduction method with the assistance of the surface regulator polyvinylpyrrolidone. The specific surface area of the excavated cubic Pt–Sn NCs is comparable to that of commercial Pt black despite their larger particle size. The excavated cubic Pt–Sn NCs exhibited superior electrocatalytic activity in terms of both the specific area current density and the mass current density towards methanol oxidation.     

Soluble precursors for CuInSe2, CuIn(1-x)Ga(x)Se2, and Cu2ZnSn(S,Se)4 based on colloidal nanocrystals and molecular metal chalcogenide surface ligands

We report a new platform for design of soluble precursors for CuInSe(2) (CIS), Cu(In(1-x)Ga(x))Se(2) (CIGS), and Cu(2)ZnSn(S,Se)(4) (CZTS) phases for thin-film potovoltaics. To form these complex phases, we used colloidal nanocrystals (NCs) with metal chalcogenide complexes (MCCs) as surface ligands. The MCC ligands both provided colloidal stability and represented essential components of target phase. To obtain soluble precursors for CuInSe(2), we used Cu(2-x)Se NCs capped with In(2)Se(4)(2-) MCC surface ligands or CuInSe(2) NCs capped with {In(2)Cu(2)Se(4)S(3)}(3-) MCCs. A mixture of Cu(2-x)Se and ZnS NCs, both capped with Sn(2)S(6)(4-) or Sn(2)Se(6)(4-) ligands was used for solution deposition of CZTS films. Upon thermal annealing, the inorganic ligands reacted with NC cores forming well-crystallized pure ternary and quaternary phases. Solution-processed CIS and CZTS films featured large grain size and high phase purity, confirming the prospects of this approach for practical applications.     

Nonaqueous synthesis of TiO2 nanocrystals using TiF4 to engineer morphology, oxygen vacancy concentration, and photocatalytic activity

Control over faceting in nanocrystals (NCs) is pivotal for many applications, but most notably when investigating catalytic reactions which occur on the surfaces of nanostructures. Anatase titanium dioxide (TiO(2)) is one of the most studied photocatalysts, but the shape dependence of its activity has not yet been satisfactorily investigated and many questions still remain unanswered. We report the nonaqueous surfactant-assisted synthesis of highly uniform anatase TiO(2) NCs with tailorable morphology in the 10-100 nm size regime, prepared through a seeded growth technique. Introduction of titanium(IV) fluoride (TiF(4)) preferentially exposes the {001} facet of anatase through in situ release of hydrofluoric acid (HF), allowing for the formation of uniform anatase NCs based on the truncated tetragonal bipyramidal geometry. A method is described to engineer the percentage of {001} and {101} facets through the choice of cosurfactant and titanium precursor. X-ray diffraction studies are performed in conjunction with simulation to determine an average NC dimension which correlates with results obtained using electron microscopy. In addition to altering the particle shape, the introduction of TiF(4) into the synthesis results in TiO(2) NCs that are blue in color and display a broad visible/NIR absorbance which peaks in the infrared (λ(max) ≈ 3400 nm). The blue color results from oxygen vacancies formed in the presence of fluorine, as indicated by electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) studies. The surfactants on the surface of the NCs are removed through a simple ligand exchange procedure, allowing the shape dependence of photocatalytic hydrogen evolution to be studied using monodisperse TiO(2) NCs. Preliminary experiments on the photoreforming of methanol, employed as a model sacrificial agent, on platinized samples resulted in high volumes of evolved hydrogen (up to 2.1 mmol h(-1) g(-1)) under simulated solar illumination. Remarkably, the data suggest that, under our experimental conditions, the {101} facets of anatase are more active than the {001}.     

Role of surface ligands in the nanoparticle assemblies: a case study of regularly shaped colloidal crystals composed of sodium rare earth fluoride

Assembly of nanoparticles is a promising route to fabricate devices from nanomaterials. Colloidal crystals are well-defined three-dimensional assemblies of nanoparticles with long-range ordered structures and crystalline symmetries. Here, we use a solvent evaporation induced assembly method to obtain colloidal crystals composed of polyhedral sodium rare earth fluoride nanoparticles. The building blocks exhibit the same crystalline orientation in each colloidal crystal as indicated in electron diffraction patterns. The driving force of the oriented assembly is ascribed to the facet-selected capping of oleic acid molecules on {110} facets of the nanoparticles, and the favorable coordination behavior of OA molecules is explained by the steric hindrance determined adsorption based on the studies of the surface atomic structure of nanocrystals and molecular mechanics simulation of OA molecules. The capping ligands also provide hydrophobic interactions between nanoparticles and further direct the oriented assembly process to construct a face-centered cubic structure. These results not only provide a new type of building block for colloidal crystals, but also clarify the important role of surface ligands, which determine the packed structure and orientations of nanoparticles in the assemblies.     

Synthesis of nanocrystals with variable high-index Pd facets through the controlled heteroepitaxial growth of trisoctahedral Au templates

The shape-controlled synthesis of noble metal nanocrystals (NCs) bounded by high-index facets is a current research interest because the products have the potential of significantly improving the catalytic performance of NCs in industrially important reactions. This study reports a versatile method for synthesizing polyhedral NCs enclosed by a variety of high-index Pd facets. The method is based on the heteroepitaxial growth of Pd layers on concave trisoctahedral (TOH) gold NC seeds under careful control of the growth kinetics. Polyhedral Au@Pd NCs with three different classes of high-index facets, including concave TOH NCs with {hhl} facets, concave hexoctahedral (HOH) NCs with {hkl} facets, and tetrahexahedral (THH) NCs with {hk0} facets, can be formed in high yield. The Miller indices of NCs are also modifiable, and we have used the THH NCs as a demonstrative example. The catalytic activities of these NCs were evaluated by the structure-sensitive reaction of formic acid electro-oxidation. The results showed that the high-index facets are generally more active than the low-index facets. In summary, a seeded growth process based on concave high-index faceted monometallic TOH NC templates and careful control of the growth kinetics is a simple and effective strategy for the synthesis of noble metal NCs with high-index facets. It also offers tailorability of the surface structure in shape-controlled synthesis.     

Solvent‐driven symmetry of self‐assembled nanocrystal superlattices—A computational study

The preference of experimentally realistic sized 4‐nm facetted nanocrystals (NCs), emulating Pb chalcogenide quantum dots, to spontaneously choose a crystal habit for NC superlattices (Face Centered Cubic (FCC) vs. Body Centered Cubic (BCC)) is investigated using molecular simulation approaches. Molecular dynamics simulations, using united atom force fields, are conducted to simulate systems comprised of cube‐octahedral‐shaped NCs covered by alkyl ligands, in the absence and presence of experimentally used solvents, toluene and hexane. System sizes in the 400,000–500,000‐atom scale followed for nanoseconds are required for this computationally intensive study. The key questions addressed here concern the thermodynamic stability of the superlattice and its preference of symmetry, as we vary the ligand length of the chains, from 9 to 24 ? CH₂ groups, and the choice of solvent. We find that hexane and toluene are “good” solvents for the NCs, which penetrate the ligand corona all the way to the NC surfaces. We determine the free energy difference between FCC and BCC NC superlattice symmetries to determine the system's preference for either geometry, as the ratio of the length of the ligand to the diameter of the NC is varied. We explain these preferences in terms of different mechanisms in play, whose relative strength determines the overall choice of geometry. © 2012 Wiley Periodicals, Inc.     

Effect of metal ions on photoluminescence, charge transport, magnetic and catalytic properties of all-inorganic colloidal nanocrystals and nanocrystal solids

Colloidal semiconductor nanocrystals (NCs) provide convenient "building blocks" for solution-processed solar cells, light-emitting devices, photocatalytic systems, etc. The use of inorganic ligands for colloidal NCs dramatically improved inter-NC charge transport, enabling fast progress in NC-based devices. Typical inorganic ligands (e.g., Sn(2)S(6)(4-), S(2-)) are represented by negatively charged ions that bind covalently to electrophilic metal surface sites. The binding of inorganic charged species to the NC surface provides electrostatic stabilization of NC colloids in polar solvents without introducing insulating barriers between NCs. In this work we show that cationic species needed for electrostatic balance of NC surface charges can also be employed for engineering almost every property of all-inorganic NCs and NC solids, including photoluminescence efficiency, electron mobility, doping, magnetic susceptibility, and electrocatalytic performance. We used a suite of experimental techniques to elucidate the impact of various metal ions on the characteristics of all-inorganic NCs and developed strategies for engineering and optimizing NC-based materials.     

Nanocrystal superlattices with thermally degradable hybrid inorganic-organic capping ligands

Colloidal metallic and semiconductor nanocrystals (NCs) functionalized with metal chalcogenide complexes (MCCs) have shown a promise for designing materials that combine high carrier mobility with the electronic structure of strongly quantum-confined solids. Here we report a simple and general methodology for switching the repulsive forces responsible for colloidal stabilization of MCC-capped NCs from long-range electrostatic to short-range steric through the formation of tight ionic pairs with cationic surfactants. This noncovalent surface modification remarkably improved the ability of MCC-capped NCs to self-assemble into long-range ordered superlattices. These NCs are highly soluble in nonpolar solvents and compatible with various technologically relevant organic molecules and polymers. The hybrid inorganic-organic coating can be thermally decomposed at significantly lower temperatures compared to those required for removal of conventional organic ligands.     

Colloidal crystal microarrays and two-dimensional superstructures: a versatile approach for patterned surface assembly

A simple, fast, and robust approach to colloidal assembly on patterned surfaces was developed. The approach involves the rapid settling and dewetting of suspensions of spherical colloids on lithographically templated surfaces. Using this method, we can quickly and easily fabricate close-packed colloidal crystal microarrays of both silica and polystyrene spheres that range in size from 500 nm to 4.5 microm. The microarrays tend to induce the formation of monolayer colloidal crystals, which can be interconnected and removed from the templates as free-standing colloidal crystal slabs. The same approach can also be used to assemble two-dimensional colloidal crystal superlattices that can adopt a variety of structures. Graphite, kagome, body-centered cubic, open hexagonal, tetragonal, and linear chain structures can all be quickly accessed by adjusting the ratio of the sphere diameter to the template diameter.     

Self-Organization of PbS into Quantum Dots Superlattices

Orderedcolloidsor"colloidalcrystals",whichhavelong-rangespatialordef,havebeeninvestigatedextensively,andstillremainanactiveareaofresearchI.Selforganizationisageneralphenomenonofcolloidaldispersionswherecontroloverparticlesizeandstabilizationhasbeenachieved.Selforganizationrequiresonlyahard-sphererepulsion,acontro1ledsizedistribution,theinherentVanderWaaIsattractionbetWeenparticles,andameansofgentlydestabilizingthedispersion.Theinherenttendencyformonodisperselyophobiccolloidstoselforganizepro…