Kevin Critchley

Light-Controlled Self-Assembly of Semiconductor Nanoparticles into Twisted Ribbons

Nanoparticles, Lightly Twisted The helical structures that are widespread in natural macromolecules result from well-coordinated bonding interactions and affect their physical properties in striking ways. To obtain helical nanoparticles, Srivastava et al. (p. 1355, published online 11 February) slowly oxidized cadmium-tellurium under visible light and assembled ribbons of nanostructure. The ribbons were persuaded to twist into helices because they were doped with cadmium sulfide nanoparticles, which underwent surface oxidation and caused localized stresses that could only be relieved by a conformational change. The pitch of the twisted ribbons that were produced could be controlled by the intensity of illumination applied. This behavior offers promise for application in the development of materials with interesting optical properties. The photooxidation of cadmium sulfide nanoparticles within cadmium telluride nanoparticle ribbons causes surface stresses that lead to twisting. The collective properties of nanoparticles manifest in their ability to self-organize into complex microscale structures. Slow oxidation of tellurium ions in cadmium telluride (CdTe) nanoparticles results in the assembly of 1- to 4-micrometer-long flat ribbons made of several layers of individual cadmium sulfide (CdS)/CdTe nanocrystals. Twisting of the ribbons with an equal distribution of left and right helices was induced by illumination with visible light. The pitch lengths (250 to 1500 nanometers) varied with illumination dose, and the twisting was associated with the relief of mechanical shear stress in assembled ribbons caused by photooxidation of CdS. Unusual shapes of multiparticle assemblies, such as ellipsoidal clouds, dog-bone agglomerates, and ribbon bunches, were observed as intermediate stages. Computer simulations revealed that the balance between attraction and electrostatic repulsion determines the resulting geometry and dimensionality of the nanoparticle assemblies.

Transparent Conductors from Layer-by-Layer Assembled SWNT Films: Importance of Mechanical Properties and a New Figure of Merit

New transparent conductors (TCs) capable of replacing traditional indium tin oxide (ITO) are much needed for displays, sensors, solar cells, smart energy-saving windows, and flexible electronics. Technical requirements of TCs include not only high electrical conductivity and transparency but also environmental stability and mechanical property which are often overlooked in the research environment. Single-walled carbon nanotube (SWNT) coatings have been suggested as alternative TC materials but typically lack sufficient wear resistance compared to ITO. Balancing conductance, transparency, durability, and flexibility is a formidable challenge, which leads us to the introduction of a new TC figure of merit, PiTC, incorporating all these qualities. Maximization of PiTC to that of ITO or better can be suggested as an initial research goal. Fine tuning of SWNT layer-by-layer (LBL) polymeric nanocomposite structures makes possible integration of all the necessary properties. The produced TC demonstrated resistivity of 86 Omega/sq with 80.2% optical transmittance combined with tensile modulus, strength, and toughness of the film of 12.3+/-3.4 GPa, 218+/-13 MPa, and 8+/-1.7 J/g, respectively. A new transparent capping layer to conserve these properties in the hostile environment with matching or better strength, toughness, and transparency parameters was also demonstrated. Due to application demands, bending performance of TC made by LBL was of special interest and exceeded that of ITO by at least 100 times. Cumulative figure of merit PiTC for the produced coatings was 0.15 Omega(-1), whereas the conventional ITO showed PiTC<0.07 Omega(-1). With overall electrical and optical performance comparable to ITO and exceptional mechanical properties, the described coatings can provide an excellent alternative to ITO or other nanowire- and nanotube-based TC specifically in flexible electronics, displays, and sensors.

Multiparameter Structural Optimization of Single-Walled Carbon Nanotube Composites: Toward Record Strength, Stiffness, and Toughness

Efficient coupling of mechanical properties of SWNTs with the matrix leading to the transfer of unique mechanical properties of SWNTs to the macroscopic composites is a tremendous challenge of todays materials science. The typical mechanical properties of known SWNT composites, such as strength, stiffness, and toughness, are assessed in an introductory survey where we focused on concrete numerical parameters characterizing mechanical properties. Obtaining ideal stress transfer will require fine optimization of nanotube-polymer interface. SWNT nanocomposites were made here by layer-by-layer (LBL) assembly with poly(vinyl alcohol) (PVA), and the first example of optimization in respect to key parameters determining the connectivity at the graphene-polymer interface, namely, degree of SWNT oxidation and cross-linking chemistry, was demonstrated. The resulting SWNT-PVA composites demonstrated tensile strength (σ(ult)) = 504.5 ± 67.3 MPa, stiffness (E) = 15.6 ± 3.8 GPa, and toughness (K) = 121.2 ± 19.2 J/g with maximum values recorded at σ(ult) = 600.1 MPa, E = 20.6 GPa, and K = 152.1 J/g. This represents the strongest and stiffest nonfibrous SWNT composites made to date outperforming other bulk composites by 2-10 times. Its high performance is attributed to both high nanotube content and efficient stress transfer. The resulting LBL composite is also one of the toughest in this category of materials and exceeding the toughness of Kevlar by 3-fold. Our observation suggests that the strengthening and toughening mechanism originates from the synergistic combination of high degree of SWNT exfoliation, efficient SWNT-PVA binding, crack surface roughening, and fairly efficient distribution of local stress over the SWNT network. The need for a multiscale approach in designing SWNT composites is advocated.

The Role of Order, Nanocrystal Size, and Capping Ligands in the Collective Mechanical Response of Three-Dimensional Nanocrystal Solids

Chemically synthesized PbS, CdSe, and CoPt(3) nanocrystals (NCs) were self-assembled into highly periodic supercrystals. Using the combination of small-angle X-ray scattering, X-ray photoelectron spectroscopy, infrared spectroscopy, thermogravimetric analysis, and nanoindentation, we correlated the mechanical properties of the supercrystals with the NC size, capping ligands, and degree of ordering. We found that such structures have elastic moduli and hardnesses in the range of approximately 0.2-6 GPa and 10-450 MPa, respectively, which are analogous to strong polymers. The high degree of ordering characteristic to supercrystals was found to lead to more than 2-fold increase in hardnesses and elastic moduli due to tighter packing of the NCs, and smaller interparticle distance. The nature of surface ligands also significantly affects the mechanical properties of NCs solids. The experiments with series of 4.7, 7.1, and 13 nm PbS NCs revealed a direct relationship between the core size and hardness/modulus, analogous to the nanoparticle-filled polymer composites. This observation suggests that the matrices of organic ligands have properties similar to polymers. The effective moduli of the ligand matrices were calculated to be in the range of approximately 0.1-0.7 GPa.

Near‐Bulk Conductivity of Gold Nanowires as Nanoscale Interconnects and the Role of Atomically Smooth Interface

The electronics industry has consistently decreased the dimensions of structural components and they are now well into the nanoscale range. Naturally, a significant portion of the chip is composed of interconnects. Besides, the engineering problems associated with short wavelength lithography to achieve smaller components, the performance of increasing number of interconnections has become one of the biggest limiting factors in device performance. [1,2] The power loss, signal degradation, interconnection delays, and other performance limitations related to interconnects should be minimized. The importance of such a task can be seen from the perspective of power dissipation by computation elements. The energy dissipation density in electronic chips approaches that in nuclear reactors. [3,4] Therefore, the decrease of the resistivity of metal interconnects is one of the key challenges in the design of nanoscale electronic circuits. Intense studies on methods of preparation of interconnects by advanced lithographic techniques including e-beam lithography lead to the preparation of Au, Pd, Pt, and Cu nanowires (NWs) with resistivities much greater than the bulk metal value (see Supporting Information). [5‐11] The reason for the drastic increase in resistivity for NWs is that charge carriers experience grain boundaries reflections and surface scattering. The smaller the diameter of the conductor, the greater this effect becomes. Even NWs with diameters much above the nanometer scale can exhibit resistivities as high as two orders of magnitude greater that the bulk. [12]

Four-probe electrical transport measurements on individual metallic nanowires

This work presents nanoscale four-probe measurements on metallic nanowires using independently controlled scanning tunnelling microscope tips. This technique has allowed us to follow the change in resistance with probe separation. Gold, zinc and nickel nanowires were grown by electrodeposition within porous polycarbonate membranes. Their structure and composition were studied by transmission electron microscopy. Four-probe electrical transport measurements were taken using four independently controlled scanning tunnelling microscope tips positioned using a high resolution scanning electron microscope. Multiple I–V measurements were taken at varying tip separations, on each nanowire, and the change in resistance with separation was observed to be in good agreement with predictions based on the nanowire geometry. The resistivity values of the nanowires were found to be close to bulk values.

LBL Assembled Laminates with Hierarchical Organization from Nano- to Microscale: High-Toughness Nanomaterials and Deformation Imaging

Layer-by-layer assembly (LBL) can generate unique materials with high degrees of nanoscale organization and excellent mechanical, electrical, and optical properties. The typical nanometer scale thicknesses restrict their utility to thin films and coatings. Preparation of macroscale nanocomposites will indicate a paradigm change in the practice of LBL, materials manufacturing, and multiscale organization of nanocomponents. Such materials were made in this study via consolidation of individual LBL sheets from polyurethane. Substantial enhancement of mechanical properties after consolidation was observed. The resulting laminates are homogeneous, transparent, and highly ductile and display nearly 3x higher strength and toughness than their components. Hierarchically organized composites combining structural features from 1 to 1 000 000 nm at six different levels of dimensionality with a high degree of structural control at every level can be obtained. The functionality of the resulting fluorescent sandwiches of different colors makes possible mechanical deformation imaging with submicrometer resolution in real time and 3D capabilities.

Diffusional Self-Organization in Exponential Layer-By-Layer Films with Micro- and Nanoscale Periodicity

The layer-by-layer (LBL) assembly technique is currently one of the most widely utilized methods for the preparation of nanostructured, multilayered thin films. The structure of LBL films is typically controlled by varying the deposition sequence of adsorbed layers, leading to stratified assemblies. For specific, non-spherical inorganic LBL components, such as sheets, or axial nanocolloids, such as nanotubes, nanowires, nanowiskers, or nanorods, the structure of the films can also be controlled by their orientation. As such, clay nanosheets spontaneously adsorb almost exclusively in the orientation parallel to the substrate, whilst assembly of axial nanocolloids under conditions of shear or dewetting results in partial alignment of the fibrous components. Morphological or structural control of the multilayers can also be imparted by the choice of the assembly method (e.g. spin coating versus dip coating), the assembly conditions, or post-assembly processing of the assembly. The shape and surface morphology of the assemblies can also be tailored by the structure or shape of the substrate, as has been shown in the preparation of hollow capsules or sculptured/perforated membranes. Both polymers and nanoparticles exhibit strong tendencies toward self-organization. This effect has not been utilized in the LBL assemblies, except for the recent observation by Yoo et al. of the organization of rod-shaped viruses on the surface of a film consisting of a few bilayers. Overall, the need for more sophisticated degrees of structural organization is quite extensive and commensurate with the increasingly complex applications for which they are being prepared. Importantly, this control must be possible on a nanometer and a micrometer scale. In principle, the LBL approach does allow such a broad-scale control, but microscale films require deposition of a great number of layers in traditional LBL. It would be exceptionally advantageous to design a method that can lead to well-organized materials combining fast deposition and hierarchical nano-, micro-, and macroscopic levels of organization. To achieve this aim, a degree of smartness and the presence of elements of selforganization in the film will most likely be required. Layered systems with alternating microand nanostrata of a stiff and an elastic nature might be particularly interesting because of mechanical properties associated with the distribution of stress in hierarchical structures and predicted theoretically unique mechanical properties. Exponentially grown LBL (e-LBL) films are multilayers in which polymer chains retain their mobility and diffuse through the deposited strata. The degree of mobility makes possible to observe self-organization phenomena in such structures. Herein, we show that a system with alternating nanometerand micrometer-scale layers of predominantly inorganic (stiff) and polymeric (elastic) layers forms upon LBL deposition of poly(diallyldimethylammonium chloride) (PDDA), poly(acrylic acid) (PAA), and sodium montmorillonite clay nanosheet (MTM) multilayers. Despite the expectations of fairly homogeneous coatings in the framework of both traditional and exponential LBL deposition, the deposition sequence (PDDA/PAA/PDDA/MTM)n (n is the number of deposition cycles), results in well-defined indexing of the films after the first few cycles, with a periodicity of (1.7 0.4) mm for 10 min deposition, and superimposed organization of MTM sheets at the interfaces with 0–10 nm spacing (Figure 1). The indexing can be further controlled by varying the deposition times for polyelectrolytes (Supporting Information, Figure S1). A typical assembly included alternate immersion of a glass slide into solutions of the polycation PDDA and an [*] Prof. N. A. Kotov Departments of Chemical Engineering, Materials Science and Engineering, and Biomedical Engineering University of Michigan, Ann Arbor, MI 48109 (USA) Fax: (+1)734-764-7453 E-mail: kotov@umich.edu

Surface functionalisation for the self-assembly of nanoparticle/polymer multilayer films

The use of organosilanes as surface functionalising materials has been investigated as a precursor to the adsorption of ligand stabilised gold nanoparticles and the build-up of nanoparticle/polymer multilayer films. The purpose of surface functionalisation here is to produce a uniform surface with the maximum positive charge possible to enable the efficient adsorption of negatively charged gold nanoparticles. It is generally acknowledged that the characteristics of the first layer are important in determining the quality of the subsequent multilayer film and hence careful attention has been paid to its optimisation. Three aminosilanes have been investigated together with various methods for their deposition. The degree of nanoparticle adsorption in the resulting films was characterised using atomic force microscopy and X-ray photoelectron spectroscopy. The surface potential of the aminosilane films was also measured to provide information regarding the surface charge density. Our results show a strong correlation between the nanoparticle density and the initial surface charge density. Films of 3-aminopropyltriethoxysilane adsorbed from toluene yielded the highest level of nanoparticle adsorption.

Expanding 3D geometry for enhanced on-chip microbubble production and single step formation of liposome modified microbubbles

Micron sized, lipid stabilized bubbles of gas are of interest as contrast agents for ultra-sound (US) imaging and increasingly as delivery vehicles for targeted, triggered, therapeutic delivery. Microfluidics provides a reproducible means for microbubble production and surface functionalisation. In this study, microbubbles are generated on chip using flow-focussing microfluidic devices that combine streams of gas and liquid through a nozzle a few microns wide and then subjecting the two phases to a downstream pressure drop. While microfluidics has successfully demonstrated the generation of monodisperse bubble populations, these approaches inherently produce low bubble counts. We introduce a new micro-spray flow regime that generates consistently high bubble concentrations that are more clinically relevant compared to traditional monodisperse bubble populations. Final bubble concentrations produced by the micro-spray regime were up to 10(10) bubbles mL(-1). The technique is shown to be highly reproducible and by using multiplexed chip arrays, the time taken to produce one millilitre of sample containing 10(10) bubbles mL(-1) was ∼10 min. Further, we also demonstrate that it is possible to attach liposomes, loaded with quantum dots (QDs) or fluorescein, in a single step during MBs formation.