between 2010 and 2040.[ 1 ] Sustainability is critical because current affordable energy, mainly from fossil fuels, is being rapidly depleted, while world demand increases. Even if the supply were unlimited, the use of fossil fuels is accompanied by environmental problems, such as pollution and the greenhouse effect. [ 2 ] Efforts to harness sustainable energy from solar, wind, nuclear, and other sources have shown promise, but cost and effi ciency remain signifi cant challenges to widespread adoption. Approximately 60% of all energy produced is wasted as heat, [ 3 ] which has driven the development of thermoelectric (TE) materials over the past two decades. [ 4 ] Heat is an abundant energy supply that can be harvested from a multitude of sources (engines, human body, etc.) with no moving parts. The TE performance is directly related to a dimensionless fi gureof-merit ( ZT = S 2 σ T κ −1 ), where S is the Seebeck coeffi cient, σ is the electrical conductivity, and κ is the thermal conductivity at a given temperature T . It is clear that a large σ and S , with a small κ , are desired to achieve high effi ciency ( ZT ≈ 1 corresponds to 4%-5% conversion effi ciency), [ 5 ] but there is a well-known confl ict among the three parameters that imposes limitations on traditional thermoelectric semiconductor development. [ 6 ] The best inorganic TE materials have achieved a ZT > 2, [ 7 ] but it should be noted that this value is measured at temperatures >600 K. The ZT of these materials at room temperature is <0.5, with a power factor (PF) <400 µW m −1 K −2 . These best commercially available materials are bismuth telluride-based alloys that are expensive and plagued by scarcity and toxicity concerns. Additionally, TE materials would need a ZT ≥ 3 to be effi cient enough to enter the power generation fi eld, making them commercially viable for more than niche applications. [ 8 ] Alternatively, low cost and lightweight materials that are printable (or paintable) could be useful even with relatively low conversion effi ciency.More recently, size effects in nanostructured systems such as nanowires, [ 9 ] quantum dots, [ 10 ] superlattices, [ 11 ] and a wide variety of composites with irregular nanosized inclusions have led to signifi cant improvements in thermoelectric efficiency over traditional bulk semiconductors. [ 12,13 ] For instance,In an effort to create a paintable/printable thermoelectric material, comprised exclusively of organic components, polyaniline (PANi), graphene, and double-walled nanotube (DWNT) are alternately deposited from aqueous solutions using the layer-by-layer assembly technique. Graphene and DWNT are stabilized with an intrinsically conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). An 80 quadlayer thin fi lm (≈1 µm thick), comprised of a PANi/graphene-PEDOT:PSS/PANi/DWNT-PEDOT:PSS repeating sequence, exhibits unprecedented electrical conductivity ( σ ≈ 1.9 × 10 5 S m −1 ) and Seebeck coeffi cient ( S ≈ 120 µV K −1 ) for a completely organic material. These t...

Relatively fast exposure times (5 s) to aqueous solutions were found to improve the gas barrier of clay-polymer thin films prepared using layer-by-layer (LbL) assembly. Contrary to the common belief about deposition time (i.e., the longer the better), oxygen transmission rates (OTRs) of these nano-brick-wall assemblies are improved by reducing exposure time (from 1 min to 5 s). Regardless of composition, LbL films fabricated using shorter deposition time are always thicker in the first few layers, which correspond to greater clay spacing and lower OTR. A quadlayer (QL) assembly consisting of three repeat units of branched polyethylenimine (PEI), poly(acrylic acid) (PAA), PEI and montmorillonite (MMT) clay is only 24 nm thick when deposited with 1 min exposure to each ingredient. Reducing the exposure time of polyelectrolytes to 5 s not only increases this film thickness to 55 nm but also reduces the oxygen transmission rate (OTR) to 0.05 cm3/(m2 day atm), which is 2 orders of magnitude lower than the same film made using 1 min exposures. A conceptual model is proposed to explain the differences in growth and barrier, which are linked to polyelectrolyte relaxation, desorption, and interdiffusion. The universality of these findings is further exemplified by depositing clays with varying aspect ratios. This ability to quickly deposit high-barrier nanocomposite thin films opens up a tremendous opportunity in terms of commercial-scale processing of LbL assemblies.

Exceptionally high hydrogen permselectivity, exceeding that of any polymeric or porous inorganic systems, is achieved using an ionically crosslinked multilayer polymer thin film.

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In an effort to impart light gas (i.e., H2 and He) barrier to polymer substrates, thin films of polyethylenimine (PEI), poly(acrylic acid) (PAA), and montmorrilonite (MMT) clay are deposited via layer-by-layer (LbL) assembly. A five "quadlayer" (122 nm) coating deposited on 51 μm polystyrene is shown to lower both hydrogen and helium permeability three orders of magnitude against bare polystyrene, demonstrating better performance than thick-laminated ethylene vinyl-alcohol (EVOH) copolymer film and even metallized polyolefin/polyester film. These excellent barrier properties are attributed to a "nanobrick wall" structure. This highly flexible coating represents the first demonstration of an LbL deposited film with low hydrogen and helium permeability and is an ideal candidate for several packaging and protection applications.