One-dimensional inorganic nanostructures: synthesis, field-emission and photodetection

One-dimensional inorganic nanostructures have drawn prime attention due to their potential for understanding fundamental physical concepts and constructing nanoscale electronic and optoelectronic devices. This critical review mainly focuses on our recent research progresses in 1D inorganic nanostructures, including their rational synthesis and potential applications, with an emphasis on field-emitter and photodetector applications. Firstly, we will discuss the rational design of synthetic strategies and the synthesis of 1D nanostructures via a vapour phase approach. Secondly, we will present our recent progresses with respect to several kinds of important inorganic nanostructures and their field-emission and photoconductivity characteristics. Finally, we conclude this review with some perspectives/outlook and future research in these fields (212 references).     

Facile synthesis of optically functional,highly organized nanostructures: dye-surfactant complexes

Multiply charged dye molecules can be precipitated from water by complexation with oppositely charged surfactants. It is shown that this complex formation occurs with 1:1 stoichiometry in a highly cooperative fashion. The resulting solids show either a gel-like or a supramolecular fibrillar morphology with a very high degree of order on the nanoscale, as evidenced by small-angle X-ray scattering and pleochroic behavior under plane-polarized light.     

Advances in polymer and polymeric nanostructures for protein conjugation

Linear polymers have been considered the best molecular structures for the formation of efficient protein conjugates due to their biological advantages, synthetic convenience and ease of functionalization. In recent years, much attention has been dedicated to develop synthetic strategies that produce the most control over protein conjugation utilizing linear polymers as scaffolds. As a result, different conjugate models, such as semitelechelic, homotelechelic, heterotelechelic and branched or star polymer conjugates, have been obtained that take advantage of these well-controlled synthetic strategies. Development of protein conjugates using nanostructures and the formation of said nanostructures from protein–polymer bioconjugates are other areas in the protein bioconjugation field. Although several polymer–protein technologies have been developed from these discoveries, few review articles have focused on the design and function of these polymers and nanostructures. This review will highlight some recent advances in protein-linear polymer technologies that employ protein covalent conjugation and successful protein-nanostructure bioconjugates (covalent conjugation as well) that have shown great potential for biological applications.     

Interfacially formed poly(N,N-dimethylaminoethyl methacrylate)/polysulfone composite membranes for CO2/N2 separation

Interfacially formed poly(N,N-dimethylaminoethyl methacrylate)/polysulfone (PDMAEMA/PSF) composite membranes were developed for CO₂/N₂ separation. A layer of PDMAEMA was deposited on a microporous PSF substrate by the solution coating technique, followed by crosslinking with p-xylylene dichloride (XDC) at the interface between the PDMAEMA solid layer and the crosslinking solution. The hydrophilicity and surface free energy of the membranes were analyzed by contact angle measurements with different probe liquids. The permselectivity of the membrane was shown to be affected by the PDMAEMA deposition time, interfacial crosslinking reaction time, and the PDMAEMA and XDC concentrations in the polymer coating solution and the crosslinking solution, respectively. The composite membrane showed a CO₂ permeance of 85 GPU and a CO₂/N₂ ideal separation factor of 50 at 23 °C and 0.41 MPa of CO₂ feed pressure.     

H2 separation using defect-free, inorganic composite membranes

Defect-free, microporous Al(2)O(3)/SAPO-34 zeolite composite membranes were prepared by coating hydrothermally grown zeolite membranes with microporous alumina using molecular layer deposition. These inorganic composite membranes are highly efficient for H(2) separation: their highest H(2)/N(2) mixture selectivity was 1040, in contrast with selectivities of 8 for SAPO-34 membranes. The composite membranes were selective for H(2) for temperatures up to at least 473 K and feed pressures up to at least 1.5 MPa; at 473 K and 1.5 MPa, the H(2)/N(2) separation selectivity was 750. The H(2)/CO(2) separation selectivity was lower than the H(2)/N(2) selectivity and decreased slightly with increasing pressure; the selectivity was 20 at 473 K and 1.5 MPa. The high H(2) selectivity resulted either because most of the pores in the Al(2)O(3) layer were slightly smaller than 0.36 nm (the kinetic diameter of N(2)) or because the Al(2)O(3) layer slightly narrowed the SAPO-34 pore entrance. These composite membranes may represent a new class of inorganic membranes for gas separation.     

Effects of organic ligands, electrostatic and magnetic interactions in formation of colloidal and interfacial inorganic nanostructures

This paper discusses effects of organic ligands, electrostatic and magnetic interactions involved in morphological control of chemically synthesized inorganic nanostructures including colloid and planar systems. The special attention was concentrated on noble metal (gold and palladium) nanoparticles and nanostructures formed at the gas-liquid interface. The analysis of experimental data showed that electrostatic and ligand-related interactions influence very strongly on the metal nanostructure morphology. The hydrophobicity of ligand, charge and binding affinity to inorganic phase are important factors influencing the morphology of inorganic nanostructures formed in a layer at the gas/liquid interface by the interfacial synthesis method. The important point of this method is the quasi two-dimensional character of reaction area and possibilities to realize ultimately thin and anisotropic dynamic monomolecular reaction system with two-dimensional diffusion and interactions of precursors, intermediates and ligands resulting in planar growth and organization of inorganic nanoparticles and nanostructures in the plain of Langmuir monolayer. The morphology of resulting inorganic nanostructures can be controlled efficiently by variations of growth conditions via changes in state and composition of interfacial planar reaction media with the same precursor, and by variations of composition of adjacent bulk phases. The extreme anisotropy and heterogeneity of two-dimensional interfacial reaction system allows creating conditions when growing inorganic particles floating on the aqueous phase surface interact selectively with hydrophobic water-insoluble ligands in interfacial monolayer or with hydrophilic bulk-phase ligands, or at the same time with ligands of different nature present in monolayer and in aqueous phase. The spatial anisotropy of interfacial reaction system and non-homogeneity of ligand binding to inorganic phase gives possibilities for growth of integrated anisotropic nanostructures with unique morphologies, in particularly those characterized by very high surface/volume ratio, high effective perimeter, and labyrinth-like structure. In a case of magnetic nanoparticles dispersed in colloids specific magnetic dipolar interactions can result in formation of chains, rings and more complex nanoparticulate structures or separated highly anisotropic nanoparticles. Theoretical considerations indicate to the importance of system dimensionality in relation to the energy balance which determines specific features of structure organization in planar charged metallic and magnetic nanostructures. For example, a requirement of Coulomb energy minimum, the possibility of free electron redistribution and strengthened attractive interactions between particles in metallic nanostructures can explain formation of very branchy systems with extremely high "effective perimeter". The obtained experimental and literature data show that system dimensionality, organic ligand nature along with electrostatic and magnetic interactions are most important factors of morphological control of chemically synthesized inorganic nanomaterials. The understanding and appropriate exploitation of these factors can be useful for further developments of efficient nanofabrication techniques based on colloidal and interfacial synthetic methods.     

Colloidal chemical approaches to inorganic micro- and nanostructures with controlled morphologies and patterns

The controlled synthesis of inorganic micro- and nanostructures with tailored morphologies and patterns has attracted intensive interest because the properties and performances of micro- and nanostructured materials are largely dependent on the shape and structure of the primary building blocks and the way in which the building blocks are assembled or integrated. This review summarizes the recent advances on the solution-phase synthesis of inorganic micro- and nanostructures with controlled morphologies and patterns via three typical colloidal chemical routes, i.e., synthesis based on catanionic micelles, reactive templates, and colloidal crystal templates, with focus on the approaches developed in our lab. Firstly, catanionic micelles formed by mixed cationic/anionic surfactants are used as effective reaction media for the shape-controlled synthesis of inorganic nanocrystals and the solution growth of hierarchical superstructures assembled by one-dimensional (1D) nanostructures. Secondly, reactive template-directed chemical transformation strategy provides a simple and versatile route to fabricate both hollow structures and 1D nanostructures. Thirdly, colloidal crystals are employed as very effective templates for the facile solution-phase synthesis of novel inorganic structures with controlled patterns, such as three-dimensionally (3D) ordered macroporous materials and two-dimensionally (2D) patterned nanoarrays and nanonets. Finally, a brief outlook on the future development in this area is presented.     

Synergic nitrogen source route to inorganic fullerene-like boron nitride with vessel, hollow sphere, onion, and peanut nanostructures

In this paper we describe the large-scale synthesis of inorganic fullerene-like (IF-like) hexagonal boron nitride with vessel, hollow sphere, peanut, and onion structures by reacting BBr(3) with the synergic nitrogen sources NaNH(2) and NH(4)Cl at 400-450 degrees C for 6-12 h. The composition of products could be confirmed to be pure boron nitride with hexagonal structures by the XRD patterns and FT-IR, XPS, and EDXA spectra. The representative HRTEM images clearly reveal the layerlike features of the products. Here, the peanut-like structure of the IF-like BN is reported for the first time, and added to the list as one kind of new morphology of BN nanomaterials. The similarity in the structure between h-BN and graphite is responsible for the formation of IF-like BN with nanostructures of vessels, hollow spheres, peanuts, and onions.     

Self-assembled complexes of non-cross-linked amphiphilic polymeric ligands with inorganic species: highly active and reusable solid-phase polymeric catalysts

I present herein the development of highly active and reusable polymeric catalysts produced by self-assembly process of non-cross-linked amphiphilic polymeric ligands with inorganic species. Thus, PWAA 1 prepared from H3PW12O40 and poly[(N-isopropylacrylamide)-co-(acrylamide with ammonium salt)] is suitable for oxidation of alcohols, amines, and sulfides in aqueous hydrogen peroxide. PdAS 2 produced by self-organization of (NH4)2PdCl4 and poly[(N-isopropylacrylamide)10-co-diphenylphosphinostyrene] is an excellent recyclable catalyst for Suzuki-Miyaura reaction in water, water-organic solvent, and organic solvent. It is commercially available from Tokyo Kasei Kogyo (TCI). PdAS-V 3 assembled from (NH4)2PdCl4 and poly[(N-isopropylacrylamide)5-co-diphenylphosphinostyrene] provides recycling system of itself for Mizorogi-Heck reaction. TiSS 4 made from Ti(O-i-Pr)4 and poly(styryl-linked binaphtholate-co-styrene) promotes an enantioselective carbonyl-ene reaction as a recyclable catalyst.     

Rare-earth/inorganic/organic polymeric hybrid materials: molecular assembly, regular microstructure and photoluminescence

2-Hydroxynicotinic acid (HNA) was grafted by 3-(triethoxysilyl)propyl isocyanate (TEPIC) to achieve the molecular precursor HNA-Si through the hydrogen-transfer nucleophilic addition reaction between the hydroxyl group of HNA and the isocyanate group of TEPIC. Then, a chemically bonded rare-earth/inorganic polymeric hybrid material (A) was constructed using HNA-Si as a bridge molecule that can both coordinate to rare-earth ions (HNA-Si-RE) and form an inorganic Si-O network with tetraethoxysilane (TEOS) after cohydrolysis and copolycondensation processes. Further, three types of novel rare-earth/inorganic/organic polymeric hybrids (B-D) were assembled by the introduction of three different organic polymeric chains into the above system. First, methacrylic acid (MAA) [or methacrylic acid and acrylamide (ALM) in the molar ratio of 1:1] was mixed to polymerize (or copolymerize) with benzoyl peroxide (BPO) as the initiator to form poly(methacrylic acid) (PMAA) [or poly(methacrylic and acrylamide) (PMAALM)], and then PMAA or PMAALM was added to the precursor HNA-Si before the assembly of HNA-Si-RE, resulting in the hybrid materials HNA-Si-RE-PMAA (B) and HNA-Si-RE-PMAALM (C). Second, poly(vinylpyrrolidone) (PVP) was added to coordinate to the rare-earth ions by the carbonyl group in the complex HNA-Si-RE, to achieve the hybrid HNA-Si-RE-PVP (D). All of these hybrid materials exhibit homogeneous, regular, and ordered microstructures and morphologies, suggesting the occurrence of self-assembly of the inorganic network and organic chain. Measurements of the photoluminescent properties of these materials show that the ternary rare-earth/inorganic/organic polymeric hybrids present stronger luminescent intensities, longer lifetimes, and higher luminescent quantum efficiencies than the binary rare-earth/inorganic polymeric hybrids, indicating that the introduction of the organic polymer chain is a benefit for the luminescence of the overall hybrid system.     

Structural and optical properties of ZnMgO nanostructures formed by Mg in-diffused ZnO nanowires

ZnMgO nanostructures with wurtzite phase were prepared by thermal diffusion of Mg into the ZnO nanowires. As ZnO light-emitting devices have been operated by using ZnMgO layers as energy barrier layers to confine the carriers, it is essential to realize the characterization of ZnMgO particularly. In this work, the Mg content in Zn_1−xMg_xO alloy determined by X-ray diffraction (XRD) and photoluminescence (PL) shows a good coincidence. The variation of lattice constant and the blueshift of near-band-edge emission indicate that Zn²⁺ ions are successfully substituted by Mg²⁺ ions in the ZnO lattice. In Raman-scattering studies, the change of E₂(high) phonon line shape in ZnO:Mg nanostructures reveals the microscopic substitutional disorder. In addition to the host phonons of ZnO, two additional bands around 383 and 510 cm⁻¹ are presumably attributed to the Mg-related vibrational modes.     

Ionic liquids for the convenient synthesis of functional nanoparticles and other inorganic nanostructures

Ionic liquids are a new class of organic solvents with high polarity and a preorganized solvent structure. Very polar reactions can be carried out in these liquid in the absence of or with a controlled amount of water, and crystalline nanoparticles can be synthesized conveniently at ambient temperatures. The pronounced self-organization of the solvent is used in the synthesis of self-assembled, highly organized hybrid nanostructures with unparalleled quality. The extraordinary potential of ionic liquids in materials synthesis is described in this minireview and a physicochemical explanation is given.     

Multifunctional nanostructures based on inorganic nanoparticles and oligothiophenes and their exploitation for cellular studies

The combination of materials that possess different properties (such as, for instance, fluorescence and magnetism) into one single object of nanoscale size represents an attractive challenge for biotechnology, especially for their potential relevance in biomedical applications. We report here the preparation of novel bifunctional conjugates based on the linkage of inorganic nanoparticles to organic oligothiophene fluorophores (OTFs). In comparison to the organic dyes commonly used in bioimaging and more similarly to colloidal quantum dots, OTFs have broad optical absorption spectra, and therefore OTF fluorophores emitting at different colors can be excited with a single excitation source, allowing for easier multiplexing analysis. In this work we show the preparation of OTF-nanoparticle conjugates based on gold and iron oxide nanoparticles and their characterization using different techniques such as gel electrophoresis, photoluminescence spectroscopy, dynamic light scattering, and so on. In addition, by performing an in vitro study on human tumor cells we show that OTF-nanoparticle conjugates emitting at different colors can be used for multiplexing detection. Also, in the case of iron oxide-OTF conjugates, once uptaken by the cells, we show that they preserve both their fluorescent and their magnetic properties.     

Synthesis and application of inorganic/organic composite materials

A new type of inorganic-organic composite has been developed using organic monomeric or polymerizable silanes (with appropriate organic groupings like double bonds or epoxides) as monomers or in situ prepared or separately added nano-scale ceramic or metal particles. Due to the small size of the particulate phase, these composites are still highly transparent but show properties at least partially to be attributed to the inorganic phase. The introduction of special functions into these materials has been used for the fabrication of interesting functions like non-linear optical properties, low surface free energy coatings, controlled release properties or special mechanical properties (scratch resistance).     

Polymer science with transition metals and main group elements: Towards functional,supramolecular inorganic polymeric materials

Inorganic polymers are relatively unexplored because the efficient formation of macromolecular chains from atoms of transition metals and main group elements has presented a synthetic challenge. Nevertheless, these materials offer exciting opportunities for accessing properties that are significantly different from and which therefore complement those available with the well‐established organic systems. Inorganic block copolymers are of particular interest for the generation of functional, nanoscale supramolecular architectures and hierarchical assemblies using self‐assembly processes. This article focuses on research in my group over the past decade, which has targeted the development of new and controlled routes to inorganic polymers and their subsequent use in forming supramolecular materials as well as studies of their properties and applications. The use of ring‐opening polymerization (ROP) and transition‐metal‐catalyzed polycondensation approaches are illustrated. Controlled ROP procedures have been developed that allow access to polyferrocene block copolymers that self‐assemble into interesting nanoscopic architectures such as cylinders and superstructures such as flowers. The future prospects for inorganic polymer science are discussed, and a growing emphasis on the study of supramolecular inorganic polymeric materials is predicted. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 179–191, 2002     

Composite block copolymer stabilized nanoparticles: simultaneous encapsulation of organic actives and inorganic nanostructures

We describe the preparation and characterization of hybrid block copolymer nanoparticles (NPs) for use as multimodal carriers for drugs and imaging agents. Stable, water-soluble, biocompatible poly(ethylene glycol)-block-poly(epsilon-caprolactone) NPs simultaneously co-encapsulating hydrophobic organic actives (beta-carotene) and inorganic imaging nanostructures (Au) are prepared using the flash nanoprecipitation process in a multi-inlet vortex mixer. These composite nanoparticles (CNPs) are produced with tunable sizes between 75 nm and 275 nm, narrow particle size distributions, high encapsulation efficiencies, specified component compositions, and long-term stability. The process is tunable and flexible because it relies on the control of mixing and aggregation timescales. It is anticipated that the technique can be applied to a variety of hydrophobic active compounds, fluorescent dyes, and inorganic nanostructures, yielding CNPs for combined therapy and multimodal imaging applications.     

Binding ability and self-assembly behavior of linear polymeric supramolecules formed by modified beta-cyclodextrin

[structure: see text] The binding ability and self-assembly behavior of molecular interpenetration by newly synthesized mono[6-O-(4-formyl-phenyl)-beta-cyclodextrin has been investigated, revealing the formation mechanism of modified cyclodextrin from solution aggregation to solid linear polymeric supramolecules.     

Assembly of one dimensional inorganic nanostructures into functional 2D and 3D architectures. Synthesis, arrangement and functionality

This review will focus on the synthesis, arrangement, structural assembly, for current and future applications, of 1D nanomaterials (tubes, wires, rods) in 2D and 3D ordered arrangements. The ability to synthesize and arrange one dimensional nanomaterials into ordered 2D or 3D micro or macro sized structures is of utmost importance in developing new devices and applications of these materials. Micro and macro sized architectures based on such 1D nanomaterials (e.g. tubes, wires, rods) provide a platform to integrate nanostructures at a larger and thus manageable scale into high performance electronic devices like field effect transistors, as chemo- and biosensors, catalysts, or in energy material applications. Carbon based, metal oxide and metal based 1D arranged materials as well as hybrid or composite 1D materials of the latter provide a broad materials platform, offering a perspective for new entries into fascinating structures and future applications of such assembled architectures. These architectures allow bridging the gap between 1D nanostructures and the micro and macro world and are the basis for an assembly of 1D materials into higher hierarchy domains. This critical review is intended to provide an interesting starting point to view the current state of the art and show perspectives for future developments in this field. The emphasis is on selected nanomaterials and the possibilities for building three dimensional arrays starting from one dimensional building blocks. Carbon nanotubes, metal oxide nanotubes and nanowires (e.g. ZnO, TiO(2), V(2)O(5), Cu(2)O, NiO, Fe(2)O(3)), silicon and germanium nanowires, and group III-V or II-VI based 1D semiconductor nanostructures like GaS and GaN, pure metals as well as 1D hybrid materials and their higher organized architectures (foremost in 3D) will be focussed. These materials have been the most intensively studied within the last 5-10 years with respect to nano-micro integration aspects and their functional and application oriented properties. The critical review should be interesting for a broader scientific community (chemists, physicists, material scientists) interested in synthetic and functional material aspects of 1D materials as well as their integration into next higher organized architectures.     

Preparation and characterization of magnetic polymeric composite particles by miniemulsion polymerization

A water‐based magnetite ferrofluid, with an average size of about 10 nm, was prepared in a first step by the chemical coprecipitation of ferrous and ferric salts. Oil‐based styrene (St) magnetite ferrofluid was obtained by the acidification of the water‐based magnetite ferrofluid and the dispersion of the acidified magnetite in St. Magnetic polymeric composite particles (MPCPs) were prepared by miniemulsion polymerization in the presence of the oil‐based St magnetite ferrofluid with hexadecane as a hydrophobe, 2,2′‐azobisisobutyronitrile as an initiator, and sodium dodecyl sulfate as an emulsifier. Methacrylic acid was used as a comonomer, and hydroxyethyl cellulose and polyvinylpyrrolidone were used as aid stabilizers subsequently. With the aim of improving the encapsulation degree of magnetite, avoiding pure polymer particles and exposed magnetite particles, and obtaining the narrowest particle size distributions, the encapsulation conditions of magnetite were investigated in detail. The results show that miniemulsion polymerization is an effective method for encapsulating magnetite into a hydrophobic polymer successfully. Exposed magnetite particles and pure polymer particles can be avoided completely by the selection of the appropriate preparation conditions. All the resulting MPCPs exhibited superparamagnetism and possessed some magnetic response. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4187–4203, 2006