P ervasive networks of wireless sensor and communication nodes have the potential to significantly impact society and create large market opportunities. For such networks to achieve their full potential, however, we must develop practical solutions for self-powering these autonomous electronic devices.Fixed-energy alternatives, such as batteries and fuel cells, are impractical for wireless devices with an expected lifetime of more than 10 years because the applications and environments in which these devices are deployed usually preclude changing or re-charging of batteries. There are several power-generating options for scavenging ambient environment energy, including solar energy, thermal gradients, and vibration-based devices. However, it's unlikely that any single solution will satisfy all application spaces, as each method has its own constraints: solar methods require sufficient light energy, thermal gradients need sufficient temperature variation, and vibration-based systems need sufficient vibration sources. Vibration sources are generally more ubiquitous, however, and can be readily found in inaccessible locations such as air ducts and building structures.We've modeled, designed, and built small cantilever-based devices using piezoelectric materials that can scavenge power from low-level ambient vibration sources. Given appropriate power conditioning and capacitive storage, the resulting power source is sufficient to support networks of ultra-low-power, peer-to-peer wireless nodes. These devices have a fixed geometry and-to maximize power output-we've individually designed them to operate as close as possible to the frequency of the driving surface on which they're mounted. Here, we describe these devices and present some new designs that can be tuned to the frequency of the host surface, thereby expanding the method's flexibility. We also discuss piezoelectric designs that use new geometries, some of which are microscale (approximately hundreds of microns). Problem overviewWe first analyze the wireless sensor nodes' power requirements, and then investigate the various sources that can fill those demands. Power demandAssuming an average distance between wireless sensor nodes of approximately 10 meters-which means that the radio transmitter should operate at approximately 0 dBm (decibels above or below 1 milliwatt)-the radio transmitter's peak power consumption will be around 2 to 3 mW, depending on its efficiency. Using ultra-low-power techniques, 1 the receiver should consume less than 1 mW. Including the dissipation of the sensors and Given appropriate power conditioning and capacitive storage, devices made from piezoelectric materials can scavenge power from low-level ambient sources to effectively support networks of ultra-low-power, peerto-peer wireless nodes.

The self-assembly of nanocrystals enables new classes of materials whose properties are controlled by the periodicities of the assembly, as well as by the size, shape and composition of the nanocrystals. While selfassembly of spherical nanoparticles has advanced significantly in the last decade, assembly of rod-shaped nanocrystals has seen limited progress due to the requirement of orientational order. Here, the parameters critically relevant to self-assembly are systematically quantified using a combination of diffraction and theoretical modeling; these highlight the importance of kinetics on orientational order. Through dryingmediated self-assembly we achieve unprecedented control over orientational order (up to 96% vertically oriented rods on 1cm 2 areas) on a wide range of substrates (ITO, PEDOT:PSS, Si 3 N 4 ). This opens new avenues for nanocrystal-based devices competitive with thin film devices, as problems of granularity can be tackled through crystallographic orientational control over macroscopic areas.Colloidal nanocrystals offer a potential route to realizing low-cost solution-processed electronic devices. In particular, with their size-tunable properties, single-crystallinity, and inexpensive synthesis, semiconductor nanoparticles could enable improved optoelectronic devices. To date, however, the performance of nanoparticle devices has been limited, in large part, by the number of interfaces that charge carriers encounter before they can be collected; each interface presents an opportunity for recombination and subsequent charge loss or

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Inorganic nanocomposites have been prepared by assembling colloidal nanocrystals and then replacing the organic ligands with precursors to an inorganic matrix phase. Separate synthesis and processing of the nanocrystal and matrix phases allows complete compositional modularity and retention of the superlattice morphologies for sphere (see scheme; top) or rod (bottom) assemblies.

Inorganic nanocomposites have recently emerged as a means of controlling material functionality by morphology and composition to give combinations of properties not generally found in homogeneous single-phase materials. For example, battery electrodes must efficiently conduct both electrons and ions to achieve high power, [1] whereas thermoelectric energy conversion is most efficient when electrical conductivity is high yet thermal conductivity is low. [2] However, the development of nanocomposites for such applications is hindered by the lack of a general fabrication method capable of controlling morphology over a wide range of compositions. Recently, exquisite control over colloidal nanocrystal assembly has been developed, including highly ordered superlattices, [3] binary nanocrystal assemblies, [4] and oriented nanorod assemblies. [5] Herein we show that such nanocrystal assemblies can be converted into inorganic nanocomposites by the postassembly replacement of organic ligands with inorganic chalcogenidometallate clusters (ChaMs). The nanocrystals and ChaMs [6] are synthesized and processed independently, so this approach affords complete compositional modularity. Critically, the morphology of the original nanocrystal assemblies, including oriented nanorod assemblies, is maintained in the resulting nanocomposites.Compelled by the unique properties achievable in inorganic nanocomposites, several approaches to their fabrication have been shown for specific applications. For example, spinodal decomposition and precipitation of a secondary phase of PbS within a PbTe matrix was used to generate a nanostructured thermoelectric composite. [2b] Whilst remarkable improvements in thermoelectric efficiency resulted, the ability to tune morphological characteristics such as the size of the nanoinclusions is limited, and the achievable compositions are severely restricted. A more general approach, which has been applied to battery electrodes, is to mechanically mill the component materials until they intermix on the nanoscale. [1b] The cost of generality, however, is a failure to reliably create intimate contact between the components, and morphology is again poorly controlled. Another elegant example is the co-assembly of solution-processed building blocks into ordered arrays of gold nanoparticles within a silica matrix, which provides thermal stability. [7] However, this method cannot be applied to arbitrary compositions as it relies on the carefully balanced interaction of the two components, together with a structure-directing surfactant, under dynamic solvent evaporation conditions. In a recent report, solution-phase ligand exchange in a strongly reducing environment was used to adsorb ChaMs to the surfaces of solvent-dispersed nanocrystals. [8] These ChaM-coated nanocrystals could then be deposited to form composite films, although the harsh conditions limit the compositional applicability, and general approaches to assemble the resulting charged nanocrystals are lacking. Underscoring these limitations, in onl...