Xinchang Pang

A general and robust strategy for the synthesis of nearly monodisperse colloidal nanocrystals

Colloidal nanocrystals exhibit a wide range of size- and shape-dependent properties and have found application in myriad fields, incuding optics, electronics, mechanics, drug delivery and catalysis, to name but a few. Synthetic protocols that enable the simple and convenient production of colloidal nanocrystals with controlled size, shape and composition are therefore of key general importance. Current strategies include organic solution-phase synthesis, thermolysis of organometallic precursors, sol-gel processes, hydrothermal reactions and biomimetic and dendrimer templating. Often, however, these procedures require stringent experimental conditions, are difficult to generalize, or necessitate tedious multistep reactions and purification. Recently, linear amphiphilic block co-polymer micelles have been used as templates to synthesize functional nanocrystals, but the thermodynamic instability of these micelles limits the scope of this approach. Here, we report a general strategy for crafting a large variety of functional nanocrystals with precisely controlled dimensions, compositions and architectures by using star-like block co-polymers as nanoreactors. This new class of co-polymers forms unimolecular micelles that are structurally stable, therefore overcoming the intrinsic instability of linear block co-polymer micelles. Our approach enables the facile synthesis of organic solvent- and water-soluble nearly monodisperse nanocrystals with desired composition and architecture, including core-shell and hollow nanostructures. We demonstrate the generality of our approach by describing, as examples, the synthesis of various sizes and architectures of metallic, ferroelectric, magnetic, semiconductor and luminescent colloidal nanocrystals.

Monodisperse Dual-Functional Upconversion Nanoparticles Enabled Near-Infrared Organolead Halide Perovskite Solar Cells.

Extending the spectral absorption of organolead halide perovskite solar cells from visible into near-infrared (NIR) range renders the minimization of non-absorption loss of solar photons with improved energy alignment. Herein, we report on, for the first time, a viable strategy of capitalizing on judiciously synthesized monodisperse NaYF4 :Yb/Er upconversion nanoparticles (UCNPs) as the mesoporous electrode for CH3 NH3 PbI3 perovskite solar cells and more importantly confer perovskite solar cells to be operative under NIR light. Uniform NaYF4 :Yb/Er UCNPs are first crafted by employing rationally designed double hydrophilic star-like poly(acrylic acid)-block-poly(ethylene oxide) (PAA-b-PEO) diblock copolymer as nanoreactor, imparting the solubility of UCNPs and the tunability of film porosity during the manufacturing process. The subsequent incorporation of NaYF4 :Yb/Er UCNPs as the mesoporous electrode led to a high efficiency of 17.8 %, which was further increased to 18.1 % upon NIR irradiation. The in situ integration of upconversion materials as functional components of perovskite solar cells offers the expanded flexibility for engineering the device architecture and broadening the solar spectral use.

1D nanocrystals with precisely controlled dimensions, compositions, and architectures

The ability to synthesize a diverse spectrum of one-dimensional (1D) nanocrystals presents an enticing prospect for exploring nanoscale size- and shape-dependent properties. Here we report a general strategy to craft a variety of plain nanorods, core-shell nanorods, and nanotubes with precisely controlled dimensions and compositions by capitalizing on functional bottlebrush-like block copolymers with well-defined structures and narrow molecular weight distributions as nanoreactors. These cylindrical unimolecular nanoreactors enable a high degree of control over the size, shape, architecture, surface chemistry, and properties of 1D nanocrystals. We demonstrate the synthesis of metallic, ferroelectric, upconversion, semiconducting, and thermoelectric 1D nanocrystals, among others, as well as combinations thereof.

Strictly Biphasic Soft and Hard Janus Structures: Synthesis, Properties, and Applications

Janus structures, named after the ancient two-faced Roman god Janus, comprise two hemistructures (e.g. hemispheres) with different compositions and functionalities. Much research has been carried out over the past few years on Janus structures because of the intriguing properties and promising potential applications of these unusually shaped materials. This Review discusses recent progress made in the synthesis, properties, and applications of strictly biphasic Janus structures possessing symmetrical structures but made of disparate materials. Depending on the chemical compositions, such biphasic structures can be categorized into soft, hard, and hybrid soft/hard Janus structures of different architectures, including spheres, rodlike, disclike, or any other shape. The main synthetic routes to soft, hard, and hybrid soft/hard Janus structures are summarized and their unique properties and applications are introduced. The perspectives for future research and development are also described.

Organic−Inorganic Nanocomposites by Placing Conjugated Polymers in Intimate Contact with Quantum Rods

Recent advances in the synthesis [ 1 ] and assembly [ 2 ] of nanocrystals (NCs) provide unique opportunities to exploit NCs for the development of next generation organic/inorganic hybrid solar cells as one of the most promising alternatives to Si solar cells to deliver effi cient energy conversion with inexpensive fabrication. [ 1 , 2 ] These conjugated polymer-based photovoltaic devices capitalize on the advantages peculiar to conjugated polymers (CPs), such as light weight, fl exibility, processability, roll-to-roll production, low cost, and large area, in conjunction with the high electron mobility and tunable optical properties of inorganic NCs. Of the organic/inorganic hybrids, poly(3-hexylthiopene) (P3HT) is one of the most extensively utilized CPs due to its excellent solution processability, environmental stability, high charge carrier mobility, and tailorable electrochemical properties. [ 3 , 4 ] CdSe quantum dots (QDs) are the most commonly investigated NCs because of their quantum-confi ned nature and well-matched energy level with P3HT. [ 5–11 ]

Semiconductor Anisotropic Nanocomposites Obtained by Directly Coupling Conjugated Polymers with Quantum Rods

Conjugated polymers (CPs) have received considerable attention as promising materials for use in organic photovoltaics, light-emitting diodes (LEDs), thin film transistors, and biosensors. Among various types of CPs, poly(3hexylthiopene) (P3HT) is one of the most widely studied organic semiconductors. P3HT possesses excellent solution processability, environmental stability, high charge-carrier mobility, and tailorable electrochemical properties. Owing to their quantum-confined nature, for quantum dots (QDs) such as cadmium selenide (CdSe), variation of the nanocrystal size provides continuous and predictable changes in fluorescence emission, thus rendering them useful for a wide range of applications in photovoltaic cells, 7] LEDs, biosensors, and bio-imaging. CP-based organic/inorganic hybrid solar cells (e.g., CP/QD composites) are favorable alternatives to inorganic solar cells as they have many advantages peculiar to CPs, such as light weight, flexibility, processability, roll-to-roll production, low cost, and large area. However, the CP/QD composites are most often prepared by simply physically mixing the CPs and QDs. This procedure, however, suffers from several severe problems, including microscopic phase separation and the existence of insulating interfacial layers, thereby reducing the interfacial area between CPs and QDs and thus limiting the performance of the resulting devices. Recently, various methods have been utilized to overcome these problems, such as the use of cosolvent mixtures or binary solvent mixtures and surface modification of QDs. The most elegant approach is to chemically tether CPs on the QD surface (i.e., preparing CP–QD nanocomposites), hence enabling direct electronic coupling between CPs and QDs. Notably, this strategy has only recently been developed and has been primarily implemented by ligand exchange, which permits the derivatization of the composite with a broad range of functional groups. However, ligandexchange chemistry suffers from incomplete surface coverage. In this context, recently P3HT–CdSe-QD nanocomposites have been synthesized by directly grafting vinyl-terminated P3HT onto a [(4-bromophenyl)methyl]dioctylphosphine oxide(DOPO–Br)-functionalized CdSe QD surface by a mild palladium-catalyzed Heck coupling without the need for ligand exchange. The ability to manipulate the shape of nanocrystals has led to quantum rods (hereafter referred to as nanorods; NRs) with diameters that range from 2 to 10 nm and lengths ranging from 5 to 100 nm. Owing to their intrinsic structural anisotropy, NRs possess many unique properties that make them potentially better nanocrystals than QDs for photovoltaics and biomedical applications. Photovoltaic cells made of NRs and CPs show an improved optical absorption in the red and near-infrared ranges that originates from the NRs. Moreover, the long axis of the NRs provides continuous paths for the transport of electrons, an advantage over QDs, in which electron hopping between QDs is required. The performance of photovoltaic cells can be further improved if NRs are vertically aligned between two electrodes to minimize the carrier transport pathways. It is worth noting that although CP–NR nanocomposites were recently produced by ligand exchange of CPs with insulating ligands that were initially attached to the NR surface, direct grafting of CPs onto anisotropic nanocrystals has not yet been explored. Herein, we report one simple yet robust route to CP–NR nanocomposites that displaces the need for ligand-exchange chemistry. In this strategy, the catalyst-free alkyne–azide cycloaddition, which belongs to the emerging field of click chemistry, was utilized in the preparation of P3HT–CdSeNR nanocomposites. As shown in Scheme 1, CdSe NRs were passivated with bromobenzylphosphonic acid (BBPA), which not only induced elongated growth but also functionalized the CdSe NR surface and led to the formation of BBPA–CdSeNRs. Subsequently, the aryl bromide groups of BBPA were converted into azide groups, thus forming N3–BPA–CdSe NRs. Finally, catalyst-free Huisgen 1,3-dipolar cycloaddition between ethynyl-terminated P3HT and N3–BPA–CdSe NRs successfully gave P3HT–CdSe NR nanocomposites without the introduction of any deleterious metallic impurity. Click reactions possess several attractive features, including an extremely versatile bond-formation process, no need for protecting groups, good selectivity, nearly complete conversion, and generally no need for purification. As such, it stands out as a promising method to simplify the synthetic procedure and opens opportunities to increase the grafting density for large-scale synthesis. The charge transfer occurred at the P3HT/CdSe NR interface and was confirmed by [*] L. Zhao, Dr. X. Pang, Prof. Z. Lin Department of Materials Science and Engineering Iowa State University, Ames, IA 50011 (USA) and School of Materials Science and Engineering Georgia Institute of Technology, Atlanta GA 30332 (USA) Fax: (+1)515-294-7202 E-mail: zqlin@iastate.edu

Graphene-Enabled Superior and Tunable Photomechanical Actuation in Liquid Crystalline Elastomer Nanocomposites.

Programmable photoactuation enabled by graphene: Graphene sheets aligned in liquid crystalline elastomers are capable of absorbing near-infrared light. They thereafter act as nanoheaters and provide thermally conductive pathways to trigger the nematic-to-isotropic transition of elastomers, leading to macroscopic mechanical deformation of nanocomposites. Large strain, high actuation force, high initial sensitivity, fast reversible response, and long cyclability are concurrently achieved in nanocomposites.

Organic-Inorganic Nanocomposites via Placing Monodisperse Ferroelectric Nanocrystals in Direct and Permanent Contact with Ferroelectric Polymers.

Organic-inorganic nanocomposites composed of polymers and nanoparticles offer a vast design space of potential material properties, depending heavily on the properties of these two constituents and their spatial arrangement. The ability to place polymers in direct contact with functional nanoparticles via strong bonding, that is, stable chemical interaction without the dissociation of surface capping polymers, provides a means of preventing nanoparticles from aggregation and increasing their dispersibility in nanocomposites, and promises opportunities to explore new properties and construction of miniaturized devices. However, this is still a challenging issue and has not yet been largely explored. Here, we report an unconventional strategy to create in situ organic-inorganic nanocomposites comprising monodisperse ferroelectric nanoparticles directly and permanently tethered with ferroelectric polymers by capitalizing on rationally designed amphiphilic star-like diblock copolymer as nanoreactors. The diameter of ferroelectric nanoparticles and the chain length of ferroelectric polymers can be precisely tuned. The dielectric and ferroelectric properties of nanocomposites containing different sizes of ferroelectric nanoparticles were scrutinized. Such bottom-up crafting of intimate organic-inorganic nanocomposites offers new levels of tailorability to nanostructured materials and promises new opportunities for achieving exquisite control over the surface chemistry and properties of nanocomposites with engineered functionality for diverse applications in energy conversion and storage, catalysis, electronics, nanotechnology, and biotechnology.

An Unconventional Route to Monodisperse and Intimately Contacted Semiconducting Organic–Inorganic Nanocomposites

We developed an unconventional route to produce uniform and intimately contacted semiconducting organic-inorganic nanocomposites for potential applications in thermoelectrics. By utilizing amphiphilic star-like PAA-b-PEDOT diblock copolymer as template, monodisperse PEDOT-functionalized lead telluride (PbTe) nanoparticles were crafted via the strong coordination interaction between PAA blocks of star-like PAA-b-PEDOT and the metal moieties of precursors (i.e., forming PEDOT-PbTe nanocomposites). As the inner PAA blocks are covalently connected to the outer PEDOT blocks, the PEDOT chains are intimately and permanently tethered on the PbTe nanoparticle surface, thereby affording a well-defined PEDOT/PbTe interface, which prevents the PbTe nanoparticles from aggregation, and more importantly promotes the long-term stability of PEDOT-PbTe nanocomposites. We envision that the template strategy is general and robust, and offers easy access to other conjugated polymer-inorganic semiconductor nanocomposites for use in a variety of applications.

A Simple Route to Hierarchically Assembled Micelles and Inorganic Nanoparticles

Earning their stripes: A hierarchical assembly of micelles composed of an amphiphilic diblock copolymer, poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP), were made by combining controlled evaporative self-assembly of the confined PS-b-P4VP toluene solution in a cylinder-on-Si geometry with spontaneous self-assembly of micelles. This method gave microscopic stripes of nanometer-sized PS-b-P4VP micelles within the stripes (see pictures).