Andrea R. Tao

Langmuir-Blodgettry of nanocrystals and nanowires.

Although nanocrystals and nanowires have proliferated new scientific avenues in the study of their physics and chemistries, the bottom-up assembly of these small-scale building blocks remains a formidable challenge for device fabrication and processing. An attractive nanoscale assembly strategy should be cheap, fast, defect tolerant, compatible with a variety of materials, and parallel in nature, ideally utilizing the self-assembly to generate the core of a device, such as a memory chip or optical display. Langmuir-Blodgett (LB) assembly is a good candidate for arranging vast numbers of nanostructures on solid surfaces. In the LB technique, uniaxial compression of a nanocrystal or nanowire monolayer floating on an aqueous subphase causes the nanostructures to assemble and pack over a large area. The ordered monolayer can then be transferred to a solid surface en masse and with fidelity. In this Account, we present the Langmuir-Blodgett technique as a low-cost method for the massively parallel, controlled organization of nanostructures. The isothermal compression of fluid-supported nanoparticles or nanowires is unique in its ability to achieve control over nanoscale assembly by tuning a macroscopic property such as surface pressure. Under optimized conditions (e.g., surface pressure, substrate hydrophobicity, and pulling speed), it allows continuous variation of particle density, spacing, and even arrangement. For practical application and device fabrication, LB compression is ideal for forming highly dense assemblies of nanowires and nanocrystals over unprecedented surface areas. In addition, the dewetting properties of LB monolayers can be used to further achieve patterning within the range of micrometers to tens of nanometers without a predefined template. The LB method should allow for easy integration of nanomaterials into current manufacturing schemes, in addition to fast device prototyping and multiplexing capability.

Surface-Enhanced Raman Spectroscopy for Trace Arsenic Detection in Contaminated Water†

Low-level arsenic contamination of drinking water in Bangladesh, India, and parts of China presents an international public health crisis, with over 300000 deaths attributed to chronic poisoning in Bangladesh alone. In 1993, the World Health Organization set a provisional guideline of 10 ppb (0.01 mgL ) for maximum arsenic content in groundwater. However, exposure to arsenic at these concentrations still results in increased rates of skin, lung, urinary bladder, and kidney cancer. New technologies allowing reliable detection of arsenic below 10 ppb should instigate a stricter standard. Current technologies for laboratory analysis (e.g. inductively coupled plasma (ICP) MS, atomic fluorescence spectroscopy (AFS), HPLC-MS) allow detection at these levels, but they are neither readily available in developing countries nor capable of on-site field detection. The current state of field-compatible technologies has been reviewed, and there remains significant room for improvement. Even if current chemical field tests are improved to meet these standards, there are no examples of chemical indicators that can distinguish the oxidation state of the arsenic species. For exposure studies, this knowledge is necessary for toxicology, remediation, and monitoring of the effects within the local populations. By developing a highly active substrate for surface-enhanced Raman spectroscopy (SERS) that can be used in conjunction with portable Raman technology, many of these challenges can be surmounted. Since the discovery of SERS in the late 1970s there has been a continual push to maximize the Raman signal for molecules near nanostructured surfaces. SERS enhancement results from an intense local amplification of the electric field near a metal surface when collective oscillations of conduction electrons resonate in phase with the incident light. The size, shape, and proximity of nanostructures all affect the frequency and magnitude of the localized surface plasmons (LSPs), thus directly influencing the degree of Raman enhancement exhibited. LSPs have been directly observed using experimental techniques such as scanning near field and TEM-correlated dark field microscopy. These experiments, along with more conventional light-scattering techniques, demonstrate the dramatic effects that size and shape have on the LSPs. Recent studies on electromagnetic coupling between nanostructures that are nearly touching indicate that such collective effects can excite LSPs that lead to even higher electromagnetic enhancement. Although it is widely known that silver shows the strongest plasmonic response, gold is often used for sensing applications because of its chemical stability and compatibility with many laser excitation wavelengths. For our SERS sensor, we have introduced two key features that lead to better analytical capability under typical sensing conditions. First, dense arrays of silver nanocrystals are fabricated using Langmuir–Blodgett (LB) assembly. These close-packed monolayers exhibit broadband scattering profiles, making them compatible with many different excitation wavelengths. The second key feature is the surface passivation of the silver particles with adsorbed polymer. Surface-adsorbed poly(vinyl pyrrolidone) (PVP) serves dual purposes: it functions as the passivating ligand during nanocrystal synthesis, and it stabilizes the silver particles to oxidation while still facilitating interaction between silver and arsenate during sensing experiments. The PVP coating makes these silver nanostructures airand water-stable over much longer periods then other passivating ligands. The synthesis of the polyhedral silver nanoparticles proceeds by the polyol process, in which the metal-salt precursors and a polymer capping agent (PVP) are alternately added to a solution of pentanediol heated near reflux. In this way the pentanediol acts as both the solvent and the reductant for the reaction, while PVP imparts shape control as the particles grow. The final shape of the particles is dictated by the length of the reaction; the particles are progressively capped by more [111] faces (see Figure 1). This growth results in an increase of the particle size as the reaction progresses, starting with cubes which are 80–100 nm on an edge, then cuboctahedra with diameters of 150–200 nm, and finally octahedra with edge lengths of 250–300 nm. Typically these nanocrystals can be isolated as nearly monodisperse suspensions, and final purification is achieved by filtration through 0.45-mm Durapore filters. Homogeneity of [*] M. Mulvihill, K. Benjauthrit, Prof. J. Arnold, Prof. P. Yang Department of Chemistry University of California, Berkeley Berkeley, CA 94720 (USA) Fax: (+1)510-642-7301 E-mail: p_yang@berkeley.edu

Self-orienting nanocubes for the assembly of plasmonic nanojunctions

Plasmonic hot spots are formed when metal surfaces with high curvature are separated by nanoscale gaps and an electromagnetic field is localized within the gaps. These hot spots are responsible for phenomena such as subwavelength focusing, surface-enhanced Raman spectroscopy and electromagnetic transparency, and depend on the geometry of the nanojunctions between the metal surfaces. Direct-write techniques such as electron-beam lithography can create complex nanostructures with impressive spatial control but struggle to fabricate gaps on the order of a few nanometres or manufacture arrays of nanojunctions in a scalable manner. Self-assembly methods, in contrast, can be carried out on a massively parallel scale using metal nanoparticle building blocks of specific shape. Here, we show that polymer-grafted metal nanocubes can be self-assembled into arrays of one-dimensional strings that have well-defined interparticle orientations and tunable electromagnetic properties. The nanocubes are assembled within a polymer thin film and we observe unique superstructures derived from edge-edge or face-face interactions between the nanocubes. The assembly process is strongly dependent on parameters such as polymer chain length, rigidity or grafting density, and can be predicted by free energy calculations.

Localized Surface Plasmon Resonances of Anisotropic Semiconductor Nanocrystals

We demonstrate that anisotropic semiconductor nanocrystals display localized surface plasmon resonances that are dependent on the nanocrystal shape and cover a broad spectral region in the near-IR wavelengths. In-plane and out-of-plane dipolar resonances were observed for colloidal dispersions of Cu(2-x)S nanodisks, and the wavelengths of these resonances are in good agreement with calculations carried out in the electrostatic limit. The wavelength, line shape, and relative intensities of these plasmon bands can be tuned during the synthetic process by controlling the geometric aspect ratio of the disk or using a postsynthetic thermal-processing step to increase the free carrier densities.

Self-Organized Silver Nanoparticles for Three-Dimensional Plasmonic Crystals

Metal nanostructures that support surface plasmons are compelling as plasmonic circuit elements and as the building blocks for metamaterials. We demonstrate here the spontaneous self-assembly of shaped silver nanoparticles into three-dimensional plasmonic crystals that display a frequency-selective response in the visible wavelengths. Extensive long-range order mediated by exceptional colloid monodispersity gives rise to optical passbands that can be tuned by particle volume fraction. These metallic supercrystals present a new paradigm for the fabrication of plasmonic materials, delivering a functional, tunable, completely bottom-up optical element that can be constructed on a massively parallel scale without lithography.

A Nanocube Plasmonic Sensor for Molecular Binding on Membrane Surfaces

Detection and characterization of molecular interactions on membrane surfaces is important to biological and pharmacological research. Here, silver nanocubes interfaced with glass-supported model membranes form a label-free sensor that measures protein binding to the membrane. The technique utilizes plasmon resonance scattering of nanocubes, which are chemically coupled to the membrane. In contrast to other plasmonic sensing techniques, this method features simple, solution-based device fabrication and readout. Static and dynamic protein/membrane binding are monitored and quantified.

Tunable and directional plasmonic coupling within semiconductor nanodisk assemblies.

Semiconductor nanocrystals are key materials for achieving localized surface plasmon resonance (LSPR) excitation in the extended spectral ranges beyond visible light, which are critical wavelengths for chemical sensing, infrared detection, and telecommunications. Unlike metal nanoparticles which are already widely exploited in plasmonics, little is known about the near-field behavior of semiconductor nanocrystals. Near-field interactions are expected to vary greatly with nanocrystal carrier density and mobility, in addition to properties such as nanocrystal size, shape, and composition. Here we demonstrate near-field coupling between anisotropic disk-shaped nanocrystals composed of Cu2-xS, a degenerately doped semiconductor whose electronic properties can be modulated by Cu content. Assembling colloidal nanocrystals into mono- and multilayer films generates dipole-dipole LSPR coupling between neighboring nanodisks. We investigate nanodisks of varying crystal phases (Cu1.96S, Cu7.2S4, and CuS) and find that nanodisk orientation produces a dramatic change in the magnitude and polarization direction of the localized field generated by LSPR excitation. This study demonstrates the potential of semiconductor nanocrystals for the realization of low-cost, active, and tunable building blocks for infrared plasmonics and for the investigation of light-matter interactions at the nanoscale.

Changes in reflectin protein phosphorylation are associated with dynamic iridescence in squid

Many cephalopods exhibit remarkable dermal iridescence, a component of their complex, dynamic camouflage and communication. In the species Euprymna scolopes, the light-organ iridescence is static and is due to reflectin protein-based platelets assembled into lamellar thin-film reflectors called iridosomes, contained within iridescent cells called iridocytes. Squid in the family Loliginidae appear to be unique in which the dermis possesses a dynamic iridescent component with reflective, coloured structures that are assembled and disassembled under the control of the muscarinic cholinergic system and the associated neurotransmitter acetylcholine (ACh). Here we present the sequences and characterization of three new members of the reflectin family associated with the dynamically changeable iridescence in Loligo and not found in static Euprymna iridophores. In addition, we show that application of genistein, a protein tyrosine kinase inhibitor, suppresses ACh- and calcium-induced iridescence in Loligo. We further demonstrate that two of these novel reflectins are extensively phosphorylated in concert with the activation of iridescence by exogenous ACh. This phosphorylation and the correlated iridescence can be blocked with genistein. Our results suggest that tyrosine phosphorylation of reflectin proteins is involved in the regulation of dynamic iridescence in Loligo.

Supramolecular Precursors for the Synthesis of Anisotropic Cu2S Nanocrystals

Copper alkanethiolates are organometallic precursors that have been used to form Cu2S nanodisks upon thermal decomposition. Here, we demonstrate that molecular assembly of Cu alkanethiolates into an ordered liquid crystalline mesophase plays an essential role in templating the disk morphology of the solid-state product. To examine this templating effect, we synthesize Cu alkanethiolate precursors with alkane tails of varying chain length and sterics. We demonstrate that short chain precursors produce two-dimensional (2D) nanosheets of Cu2S, while longer-chained variants produce Cu2S nanodisks exclusively. This work provides new insights into the use of liquid crystalline phases as templates for nanocrystal synthesis and as a potential route for achieving highly anisotropic inorganic nanostructures.

Colloidal metasurfaces displaying near-ideal and tunable light absorbance in the infrared

Metasurfaces are ultrathin, two-dimensional arrays of subwavelength resonators that have been demonstrated to control the flow of light in ways that are otherwise unattainable with natural materials. These arrays are typically composed of metallic Ag or Au nanostructures shaped like split rings, nanowire pairs or nanorods (commonly referred to as meta-atoms) that are arranged to produce a collective optical response spanning an impressive range of properties, from the perfect absorption of incident light to superresolution imaging. However, metasurfaces pose major challenges in their fabrication over large areas, which can be prohibitively expensive and time consuming using conventional nanolithography techniques. Here we show that differently shaped colloidal nanocrystals can be organized into metasurface architectures using robust, scalable assembly methods. These metasurfaces exhibit extreme in-plane electromagnetic coupling that is strongly dependent on nanocrystal size, shape and spacing. Colloidal metasurfaces that display near-ideal electromagnetic absorbance can be tuned from the visible into the mid-infrared wavelengths.