Recent advances in molecular electronics based on carbon nanotubes

Carbon nanotubes (CNTs) have exceptional physical properties that make them one of the most promising building blocks for future nanotechnologies. They may in particular play an important role in the development of innovative electronic devices in the fields of flexible electronics, ultra-high sensitivity sensors, high frequency electronics, opto-electronics, energy sources and nano-electromechanical systems (NEMS). Proofs of concept of several high performance devices already exist, usually at the single device level, but there remain many serious scientific issues to be solved before the viability of such routes can be evaluated. In particular, the main concern regards the controlled synthesis and positioning of nanotubes. In our opinion, truly innovative use of these nano-objects will come from: (i) the combination of some of their complementary physical properties, such as combining their electrical and mechanical properties, (ii) the combination of their properties with additional benefits coming from other molecules grafted on the nanotubes, and (iii) the use of chemically- or bio-directed self-assembly processes to allow the efficient combination of several devices into functional arrays or circuits. In this article, we outline the main issues concerning the development of carbon nanotubes based electronics applications and review our recent results in the field.     

金属有机配位聚合物荧光及小分子识别

金属有机配位聚合物不仅结构多样,而且在光学、电学、磁学、气体储存和分离等领域有着巨大的应用前景。众所周知,配位聚合物的性能与结构有着紧密的联系。当前晶体工程学的重要任务是选择合适的有机配体和金属离子,根据晶体工程原理和自组装规律,通过调控影响反应过程的各种因素,从而获得结构新颖、性能独特金属有机配位聚合物。利用金属有机配位聚合物的荧光性质在小分子识别方面已进行了广泛的研究。本文综述了金属有机配位聚合物的荧光产生原理及小分子识别方面的应用。     

Bottom-Up Molecular Tunneling Junctions Formed by Self-Assembly

This Minireview focuses on bottom-up molecular tunneling junctions – a fundamental component of molecular electronics – that are formed by self-assembly. These junctions are part of devices that, in part, fabricate themselves, and therefore, are particularly dependent on the chemistry of the molecules selected. The discussion covers the history of these junctions as well as recent advances. It is broken into the broad categories of conformal and rigid contacts, which place different constraints on the molecules used to form the junctions. The intention of this Minireview is to give an overview of research efforts in molecular electronics that is targeted at chemists, whose efforts are playing an increasingly important role in molecular electronics.     

Nanographenes as active components of single-molecule electronics and how a scanning tunneling microscope puts them to work

Single-molecule electronics, that is, realizing novel electronic functionalities from single (or very few) molecules, holds promise for application in various technologies, including signal processing and sensing. Nanographenes, which are extended polycyclic aromatic hydrocarbons (PAHs), are highly attractive subjects for studies of single-molecule electronics because the electronic properties of their flat conjugated systems can be varied dramatically through synthetic modification of their sizes and topologies. Single nanographenes provide high tunneling currents when adsorbed flat onto conducting substrates, such as graphite. Because of their chemical inertness, nanographenes interact only weakly with these substrates, thereby preventing the need for special epitaxial structure matching. Instead, self-assembly at the interface between a conducting solid, such as the basal plane of graphite, and a nanographene solution generally leads to highly ordered monolayers. Scanning tunneling spectroscopy (STS) allows the current-voltage characteristics to be measured through a single molecule positioned between two electrodes; the key to the success of STS is the ability to position the scanning tunneling microscopy (STM) tip freely with respect to the molecule in all dimensions, that is, both parallel and perpendicular to the surface. In this Account, we report the properties of nanographenes having sizes ranging from 0.7 to 3.1 nm and exhibiting various symmetry, periphery, and substitution types. The size of the aromatic system and the nature of its perimeter are two essential features affecting its HOMO-LUMO gap and charge carrier mobility in the condensed phase. Moreover, the extended pi area of larger substituted PAHs improves the degree of self-ordering, another key requirement for high-performance electronic devices. Self-assembly at the interface between an organic solution and the basal plane of graphite allows deposition of single molecules within the well-defined environment of a molecular monolayer. We have used STM and STS to investigate both the structures and electronic properties of these single molecules in situ. Indeed, we have observed key electronic functions, rectification and current control through single molecules, within a prototypical chemical field-effect transistor at ambient temperature. The combination of nanographenes and STM/STS, with the PAHs self-assembled in oriented molecular mono- or bilayers at the interface between an organic solution and the basal plane of graphite and contacted by the STM tip, is a simple, reliable, and versatile system for developing the fundamental concepts of molecular electronics. Our future targets include fast reversible molecular switches and complex molecular electronic devices coupled together from several single-molecule systems.     

Supramolecular Capsules Derived from Resorcin[4]arenes by H-Bonding and Metal Coordination: Synthesis,Characterization, and Single-Molecule Force Spectroscopy

Self-assembly by H-bonding and by metal-coordination of functionalized calix[4]arenes and cavitands to large supramolecular capsules is described. In addition, a new method of analyzing supramolecular recognition processes at the single molecule level is discussed. By measuring interaction forces in a hydrogen-bonded assembly using single-molecule force spectroscopy (SMFS), the dynamics of the self-assembly process can be evaluated. In the future, consequent application of this new technique will influence supramolecular design principles and the use of non-covalent interactions as construction elements in the field of nanotechnology.     

Exploring the complexity of supramolecular interactions for patterning at the liquid-solid interface

The use of self-assembly to fabricate surface-confined adsorbed layers (adlayers) from molecular components provides a simple means of producing complex functional surfaces. The molecular self-assembly process relies on supramolecular interactions sustained by noncovalent forces such as van der Waals, electrostatic, dipole-dipole, and hydrogen bonding interactions. Researchers have exploited these noncovalent bonding motifs to construct well-defined two-dimensional (2D) architectures at the liquid-solid interface. Despite myriad examples of 2D molecular assembly, most of these early findings were serendipitous because the intermolecular interactions involved in the process are often numerous, subtle, cooperative, and multifaceted. As a consequence, the ability to tailor supramolecular patterns has evolved slowly. Insight gained from various studies over the years has contributed significantly to the knowledge of supramolecular interactions, and the stage is now set to systematically engineer the 2D supramolecular networks in a "preprogrammed" fashion. The control over 2D self-assembly of molecules has many important implications. Through appropriate manipulation of supramolecular interactions, one can "encode" the information at the molecular level via structural features such as functional groups, substitution patterns, and chiral centers which could then be retrieved, transferred, or amplified at the supramolecular level through well-defined molecular recognition processes. This ability allows for precise control over the nanoscale structure and function of patterned surfaces. A clearer understanding and effective use of these interactions could lead to the development of functional surfaces with potential applications in molecular electronics, chiral separations, sensors based on host-guest systems, and thin film materials for lubrication. In this Account, we portray our various attempts to achieve rational design of self-assembled adlayers by exploiting the aforementioned complex interactions at the liquid-solid interface. The liquid-solid interface presents a unique medium to construct flawless networks of surface confined molecules. The presence of substrate and solvent provides an additional handle for steering the self-assembly of molecules. Scanning tunneling microscopy (STM) was used for probing these molecular layers, a technique that serves not only as a visualization tool but could also be employed for active manipulation of molecules. The supramolecular systems described here are only weakly adsorbed on a substrate, which is typically highly oriented pyrolytic graphite (HOPG). Starting with fundamental studies of substrate and solvent influence on molecular self-assembly, this Account describes progressively complex aspects such as multicomponent self-assembly via 2D crystal engineering, emergence, and induction of chirality and stimulus responsive supramolecular systems.     

Amphiphilic building blocks for self-assembly: from amphiphiles to supra-amphiphiles

The process of self-assembly spontaneously creates well-defined structures from various chemical building blocks. Self-assembly can include different levels of complexity: it can be as simple as the dimerization of two small building blocks driven by hydrogen bonding or as complicated as a cell membrane, a remarkable supramolecular architecture created by a bilayer of phospholipids embedded with functional proteins. The study of self-assembly in simple systems provides a fundamental understanding of the driving forces and cooperativity behind these processes. Once the rules are understood, these guidelines can facilitate the research of highly complex self-assembly processes. Among the various components for self-assembly, an amphiphilic molecule, which contains both hydrophilic and hydrophobic parts, forms one of the most powerful building blocks. When amphiphiles are dispersed in water, the hydrophilic component of the amphiphile preferentially interacts with the aqueous phase while the hydrophobic portion tends to reside in the air or in the nonpolar solvent. Therefore, the amphiphiles aggregate to form different molecular assemblies based on the repelling and coordinating forces between the hydrophilic and hydrophobic parts of the component molecules and the surrounding medium. In contrast to conventional amphiphiles, supra-amphiphiles are constructed on the basis of noncovalent interactions or dynamic covalent bonds. In supra-amphiphiles, the functional groups can be attached to the amphiphiles by noncovalent synthesis, greatly speeding their construction. The building blocks for supra-amphiphiles can be either small organic molecules or polymers. Advances in the development of supra-amphiphiles will not only enrich the family of conventional amphiphiles that are based on covalent bonds but will also provide a new kind of building block for the preparation of complex self-assemblies. When polymers are used to construct supra-amphiphiles, the resulting molecules are known as superamphiphiles. This Account will focus on the use of amphiphiles and supra-amphiphiles for self-assembly at different levels of complexity. We introduce strategies for the fabrication of robust assemblies through self-assembly of amphiphiles. We describe the supramolecular approach for the molecular design of amphiphiles through the enhancement of intermolecular interaction among the amphiphiles. In addition, we describe polymerization under mild conditions to stabilize the assemblies formed by self-assembly of amphiphiles. Finally, we highlight self-assembly methods driven by noncovalent interactions or dynamic covalent bonds for the fabrication of supra-amphiphiles with various topologies. Further self-assembly of supra-amphiphiles provides new building blocks for complex structures, and the dynamic nature of the supra-amphiphiles endows the assemblies with stimuli-responsive functions.     

Molecular self-assembly into one-dimensional nanostructures

Self-assembly of small molecules into one-dimensional nanostructures offers many potential applications in electronically and biologically active materials. The recent advances discussed in this Account demonstrate how researchers can use the fundamental principles of supramolecular chemistry to craft the size, shape, and internal structure of nanoscale objects. In each system described here, we used atomic force microscopy (AFM) and transmission electron microscopy (TEM) to study the assembly morphology. Circular dichroism, nuclear magnetic resonance, infrared, and optical spectroscopy provided additional information about the self-assembly behavior in solution at the molecular level. Dendron rod-coil molecules self-assemble into flat or helical ribbons. They can incorporate electronically conductive groups and can be mineralized with inorganic semiconductors. To understand the relative importance of each segment in forming the supramolecular structure, we synthetically modified the dendron, rod, and coil portions. The self-assembly depended on the generation number of the dendron, the number of hydrogen-bonding functions, and the length of the rod and coil segments. We formed chiral helices using a dendron-rod-coil molecule prepared from an enantiomerically enriched coil. Because helical nanostructures are important targets for use in biomaterials, nonlinear optics, and stereoselective catalysis, researchers would like to precisely control their shape and size. Tripeptide-containing peptide lipid molecules assemble into straight or twisted nanofibers in organic solvents. As seen by AFM, the sterics of bulky end groups can tune the helical pitch of these peptide lipid nanofibers in organic solvents. Furthermore, we demonstrated the potential for pitch control using trans-to-cis photoisomerization of a terminal azobenzene group. Other molecules called peptide amphiphiles (PAs) are known to assemble in water into cylindrical nanostructures that appear as nanofiber bundles. Surprisingly, TEM of a PA substituted by a nitrobenzyl group revealed assembly into quadruple helical fibers with a braided morphology. Upon photocleavage of this the nitrobenzyl group, the helices transform into single cylindrical nanofibers. Finally, inspired by the tobacco mosaic virus, we used a dumbbell-shaped, oligo(phenylene ethynylene) template to control the length of a PA nanofiber self-assembly (<10 nm). AFM showed complete disappearance of long nanofibers in the presence of this rigid-rod template. Results from quick-freeze/deep-etch TEM and dynamic light scattering demonstrated the templating behavior in aqueous solution. This strategy could provide a general method to control size the length of nonspherical supramolecular nanostructures.     

Contacting organic molecules by soft methods: towards molecule-based electronic devices

Can we put organic molecules to use as electronic components? The answer to this question is to no small degree limited by the ability to contact them electrically without damaging the molecules. In this Account, we present some of the methods for contacting molecules that do not or minimally damage them and that allow formation of electronic junctions that can become compatible with electronics from the submicrometer to the macroscale. In "Linnaean" fashion, we have grouped contacting methods according to the following main criteria: (a) is a chemical bond is required between contact and molecule, and (b) is the contact "ready-made", that is, preformed, or prepared in situ? Contacting methods that, so far, seem to require a chemical bond include spin-coating a conductive polymer and transfer printing. In the latter, a metallic pattern on an elastomeric polymer is mechanically transferred to molecules with an exposed terminal group that can react chemically with the metal. These methods allow one to define structures from several tens of nanometers size upwards and to fabricate devices on flexible substrates, which is very difficult by conventional techniques. However, the requirement for bifunctionality severely restricts the type of molecules that can be used and can complicate their self-assembly into monolayers. Methods that rely on prior formation of the contact pad are represented by two approaches: (a) use of a liquid metal as electrode (e.g., Hg, Ga, various alloys), where molecules can be adsorbed on the liquid metal and the molecularly modified drop is brought into contact with the second electrode, the molecules can be adsorbed on the second electrode and then the liquid metal brought into contact with them, or bilayers are used, with a layer on both the metal and the second electrode and (b) use of preformed metal pads from a solid substrate and subsequent pad deposition on the molecules with the help of a liquid. These methods allow formation of contacts easily and rapidly and allow many types of monolayers and metals to be analyzed. However, in their present forms such approaches are not technologically practical. Direct in situ vacuum evaporation of metals has been used successfully only with bifunctional molecules because it is too invasive and damaging, in general. A more general approach is indirect vacuum evaporation, where the metal atoms and clusters, emitted from the source, reach the sample surface in an indirect line of sight, while cooled by multiple collisions with an inert gas. This method has clear technological possibilities, but more research is needed to increase deposition efficiency and find ways to characterize the molecules at the interface and to prevent metal penetration between molecules or through pinholes, also if molecules lack reactive termination groups. This Account stresses the advantages, weak points, and possible routes for the development of contacting methods. This way it shows that there is at present no one ideal soft contacting method, whether it is because of limitations and problems inherent in each of the methods or because of insufficient understanding of the interfacial chemistry and physics. Hopefully, this Account will present the latter issue as a research challenge to its readers.     

Molecular electronics. Synthesis and testing of components

Molecular electronics involves the use of single or small packets of molecules as the fundamental units for computing. While initial targets are the substitution of solid-state wires and devices with molecules, long-range goals involve the development of novel addressable electronic properties from molecules. A comparison of traditional solid-state devices to molecular systems is described. Issues of cost and ease of manufacture are outlined, along with the syntheses and testing of molecular wires and devices.     

溶胶-凝胶过程中有机硅氧烷的自组装行为研究进展

在有机硅氧烷的溶胶-凝胶过程中,常常形成超分子自组装结构,这种自组装行为对前体水解/缩合反应过程具有非常重要的影响。本文从有机硅氧烷分子自身结构影响和外源分子的诱导作用两个方面出发,综述了近年来有关溶胶-凝胶过程中有机硅氧烷自组装行为研究方面的特色工作。分析了该领域未来发展的主要方向,指出设计合成含有独特官能团的有机硅氧烷为前体,引入外源分子,基于外源分子与前体分子之间的相互作用构筑有机硅氧烷超分子体系,利用水解/缩合过程与自组装体系之间的协同作用制备具有长程有序的硅基复合材料,将是该领域未来研究的重点。     

Self-assembly of organic molecules on montmorillonite

Smectite clays have been modified via ion exchange reactions with organic onium ions for five decades and are of industrial importance in diverse industries including oil well drilling, paint, grease, ink, cosmetics, environmental clean-up, polymer nanocomposites and pharmaceuticals. Over the past decade, a substantial amount of research has been conducted on other methods of organic modification of smectite clays. One method that has seen increased attention is surface treatment via ion–dipole bonding of organic molecules, oligomers or polymers to the exchangeable cation on the clay surface. Utilizing this method, a unique self-assembly of certain organic molecules has been discovered in which the molecules form rigid posts around each cation on the surface. This paper reports a parametric study coupled with molecular modeling of a series of three different families of ion–dipole bonding molecules. The bonding trends and controlling factors in the self-assembly of these molecules are described in detail. In general, it appears that the head group of the molecule is one of the principal factors controlling self-assembly but the chain length of the alkyl group also plays a role.     

卟啉基金属有机框架在光疗领域的应用

卟啉基金属有机框架是金属或金属团簇与卟啉配体或其家族化合物配位自组装形成的晶体结构,结合了卟啉类分子良好的光物理特性和生物相容性,具有周期性和可调控的结构,在光化学领域和生物医药领域具有应用潜力;尤其是能够充分发挥卟啉类分子的优越性能,可通过光动、光热等光辅助治疗方法实现杀伤有害细胞的效果。本文综述了卟啉基金属有机框架近年的发展及其在生物医学领域包括抗肿瘤、抗菌方向上的研究现状,重点介绍了近几年卟啉基金属有机框架在光敏剂基础上的改进以及衍生出的多样化的功能,对其发展前景做出了展望。     

Dynamic Continuum of Molecular Assemblies for Controlling Cell Fates

Biological systems have evolved to create a structural and dynamic continuum of bio-macromolecular assemblies for the purpose of optimizing the system′s functions. The formation of these dynamic higher-order assemblies is precisely controlled by biological cues. However, controlling the self-assembly of synthetic molecules spatiotemporally in or on live cells is still a big challenge, especially for performing functions. This concept article introduces the use of in situ reactions as a spatiotemporal control to form assemblies of small molecules that induce cell morphogenesis or apoptosis. After briefly introducing a representative example of a natural dynamic continuum of the higher-order assemblies, we describe enzyme-instructed self-assembly (EISA) for constructing dynamic assemblies of small molecules, then discuss the use of EISA for controlling cell morphogenesis and apoptosis. Finally, we provide a brief outlook to discuss the future perspective of this exciting new research direction.     

Controlled chain polymerisation and chemical soldering for single-molecule electronics

Single functional molecules offer great potential for the development of novel nanoelectronic devices with capabilities beyond today's silicon-based devices. To realise single-molecule electronics, the development of a viable method for connecting functional molecules to each other using single conductive polymer chains is required. The method of initiating chain polymerisation using the tip of a scanning tunnelling microscope (STM) is very useful for fabricating single conductive polymer chains at designated positions and thereby wiring single molecules. In this feature article, developments in the controlled chain polymerisation of diacetylene compounds and the properties of polydiacetylene chains are summarised. Recent studies of "chemical soldering", a technique enabling the covalent connection of single polydiacetylene chains to single functional molecules, are also introduced. This represents a key step in advancing the development of single-molecule electronics.     

Solution processable semiconducting organic single crystals

In this review we describe recent progress in fabrication, characterisation and measurements of solution processed organic single crystals based on small molecule semiconductors. We focus on single crystal applications using Field-Effect Transistors as building blocks for organic electronics.     

Nanospiral Formation by Droplet Drying: One Molecule at a Time

We have created nanospirals by self-assembly during droplet evaporation. The nanospirals, 60–70 nm in diameter, formed when solvent mixtures of methanol and m-cresol were used. In contrast, spin coating using only methanol as the solvent produced epitaxial films of stripe nanopatterns and using only m-cresol disordered structure. Due to the disparity in vapor pressure between the two solvents, droplets of m-cresol solution remaining on the substrate serve as templates for the self-assembly of carboxylic acid molecules, which in turn allows the visualization of solution droplet evaporation one molecule at a time.     

Two-Dimensional Chiral Molecular Networks from Achiral Building Blocks: A Computational Study

This article describes the application of computer simulations to explore the self-assembly of model achiral molecules on a solid surface leading to the creation of chiral overlayers. To that purpose the lattice gas Monte Carlo method is used to trace the spontaneous self-organization of cross- and tripod-shaped molecules which are represented by rigid planar structures comprising interconnected segments. The study focuses mainly on the influence of size and composition of the molecules on the morphology of the resulting superstructures. It is clearly demonstrated that the molecules, although intrinsically achiral, can assembly into globally chiral two-dimensional networks with regular cavities. Our simulations show also how the chiral networks can be obtained via co-assembly with much smaller molecules and how the additive fills the cavities. In this case, the mixed superstructure is further used as a model enantioselective adsorbent whose adsorptive properties are examined by simulating adsorption isotherms of a racemic mixture of a prototype chiral molecule. The results of this part indicate that the achiral molecular building blocks can be used to construct enantioselective surfaces with tunable adsorption properties.     

Molecular engineering of semiconductor surfaces and devices

Grafting organic molecules onto solid surfaces can transfer molecular properties to the solid. We describe how modifications of semiconductor or metal surfaces by molecules with systematically varying properties can lead to corresponding trends in the (electronic) properties of the resulting hybrid (molecule + solid) materials and devices made with them. Examples include molecule-controlled diodes and sensors, where the electrons need not to go through the molecules (action at a distance), suggesting a new approach to molecule-based electronics.     

Nanopillar,Nanowall, and Nanowire Devices for Fast Separation of Biomolecules

Nanostructures based on nanotechnologies have opened up a novel research field for the fast analysis of biomolecules with ultrahigh resolution, including the analysis of single biomolecules. Nanostructures for electrophoretic separation, especially, are an exciting topic among researchers in many areas, and their designs are widely expected to contribute to the goal of developing a single separation tool for a wide range of biomolecules. In this review, nanopillar, nanowall, and nanowire devices are introduced for fast separation of DNA molecules and protein samples, and the numerous advantages of these devices are described. This review also outlines the fabrication processes for nanostructures, including “top-down” and “bottom-up” nanofabrication approaches. Besides describing the fast separation of biomolecules, the electroosmotic flow (EOF) suppression effect, and its related online concentration technique in nanopillar devices, is reviewed. The nanowall devices have the unique feature that longer DNA molecules migrate faster than shorter ones, and that is completely different from the separation behavior of DNA molecules based on nanopillar devices. The feasibility is shown for self-assembly of the nanowire structure embedded in a microchannel on a fused silica substrate, as a means to separate DNA molecules. Applications of a newly-fabricated 3D network structure with spatial density control for the fast separation of a wide range of DNA molecules are also given.