www.afm-journal.de FULL PAPER © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 4678 www.MaterialsViews.com wileyonlinelibrary.com Gustavo E. Fernandes,* Jin Ho Kim, Ashok K. Sood, and Jimmy Xu 1. Introduction Carbon nanotube (CNT) nanocomposites have attracted con- siderable attention in recent years because of their potential applications in electronics and sensing. [1] Electromagnetic radiation sensing via bolometeric operation [2] is one emerging application for these materials. [3,4–6] Bolometers operate via thermotransduction–electromagnetic radiation is absorbed and converted into heat, which in turn changes the bolometer mate- rial's electrical resistivity according to the temperature coeffi- cient of resistance (TCR). It has been shown that the responses of thin CNT films to electromagnetic radiation is dominated by bolometric effects. [7] Thin membranes of CNTs and CNT nano- composites with polymers and other materials have much to offer in terms of bolometric sensing. [4] They display strong and broad-band absorbance, which enables detection over broad ranges of the electromagnetic spectrum. [8] Their electrical and thermal conductivities can be tailored by appropriate selection of CNT chirality (semiconducting, metallic, or ratios thereof), type (single walled or multi-walled), doping and structure (e.g., crystalline vs. polycrystalline). [9] In addition, CNT membranes are known to have large strength-to- weight ratios, [10] which may facilitate the fabrication of robust standalone sus- pended structures that are required for heightened heat sensitivity. CNTs are also chemically inert to a large variety of chem- ical species. [11] Currently, reported TCR values for CNT membranes and nano- composites are in the neighborhood of -0.5%/ °C near room temperature, [3,4,12] while infrared responsivities as large as 500 V/W have been reported. [7] These fig- ures still compare unfavorably with those for vanadium oxide, the leading platform for uncooled bolometric detection, in which TCR values in excess of 3%/ °C [13] and responsivities in the tens of MV/W have been observed. [14] σ = σ 0 e - (T 0 / T ) α (1) The electrical resistance of a membrane of randomly dis- persed CNTs is known to be dominated by the tunneling of elec- trons between nearby CNTs. [15] This type of electrical transport is commonly described in the framework of the variable range hopping VRH model, [16,17–20] according to which the conduc- tivity of a CNT membrane is given by Equation (1), where T 0 is an activation energy (in units of temperature), T is the tempera- ture, and α is given by 1/( d+1), where d is the dimensionality that characterizes transport (e.g., α = 1/4 in three dimensions, 1/3 in two dimensions and 1/2 in one dimension). [19] T 0 con- tains information about the tunneling process, [20] including the mean CNT-CNT tunneling barrier height and width, and the mean contact area. T 0 also contains, via the permittivity, infor- mation about the properties of the non-conductive medium that fills the space between CNTs, as will be discussed in more detail later. A general expression for the TCR can be easily derived from Equation 1, and is given by TCR = α T 2 T 0 T α -1 - T 0 TT 0 (2) where T′ 0 is the derivative of T 0 with respect to T. It is clear from Equation (2) that the TCR, having terms proportional to both T 0 and T′ 0 , could be enhanced via nanoengineering of these two parameters. One way to accomplish such a task is via incorporation of non-conductive polymers into the CNT film. Polymers having strongly temperature dependent prop- erties, such as large thermal expansion coefficient or large temperature dependence of the permittivity could be particu- larly good candidates. In addition, polymers that experience Giant Temperature Coefficient of Resistance in Carbon Nanotube/Phase-Change Polymer Nanocomposites The temperature coefficient of resistance of a carbon nanotube nanocom- posite with the non-conductive phase-change hydrogel Poly(N-isopropy- lacrylamide) is studied. This nanocomposite is found to achieve the largest reported temperature coefficient of resistance, ≈-10%/ °C, observed in carbon nanotube-polymer nanocomposites to date. The giant temperature coeffi- cients of resistance results from a volume-phase-transition that is induced by the humidity present in the surrounding atmosphere and that enhances the temperature dependence of the resistivity via direct changes in the tunneling resistance that electrons experience in moving between nearby carbon nano- tubes. The bolometric photoresponses of this new material are also studied. The nanocomposite’s enhanced responses to temperature and humidity give it great potential for sensor applications and uncooled infrared detection. DOI: 10.1002/adfm.201300208 Dr. G. E. Fernandes, Dr. J. H. Kim, Prof. J. Xu Brown University School of Engineering Providence, RI 02912, USA E-mail: gustavo_fernandes@brown.edu Dr. A. K. Sood Magnolia Optical Technologies Inc. 52-B Cummings Park, Woburn, MA 01801, USA Adv. Funct. Mater. 2013, 23, 4678–4683