Inorganic, low bandgap semiconductors such as Bi2Te3 have adequate efficiency for some thermoelectric energy conversion applications, but have not been more widely adopted because they are difficult to deposit over complex and/or high surface area structures, are not eco-friendly, and are too expensive. As an alternative, conducting polymers have recently attracted much attention for thermoelectric applications motivated by their low material cost, ease of processability, non-toxicity, and low thermal conductivity. Metal-organic frameworks (MOFs), which are extended, crystalline compounds consisting of metal ions interconnected by organic ligands, share many of the advantages of all-organic polymers including solution processability and low thermal conductivity. Additionally, MOFs and Guest@MOF materials offer higher thermal stability (up to ~300 °C in some cases) and have long-range crystalline order which should improve charge mobility. A potential advantage of MOFs and Guest@MOF materials over all-organic polymers is the opportunity for tuning the electronic structure through appropriate choice of metal and ligand, which could solve the long-standing challenge of finding stable, high ZT n-type organic semiconductors. In our presentation, we report on thermoelectric measurements of electrically conducting TCNQ@Cu3(BTC)2 thin films deposited using a room-temperature, solution-based method, which reveal a large, positive Seebeck coefficient. Furthermore, we use time-dependent thermoreflectance (TDTR) to measure the thermal conductivity of the films, which is found to have a low value due to the presence of disorder, as suggested by molecular dynamics simulations. In addition to establishing the thermoelectric figure of merit, the thermoelectric measurements reveal for the first time that holes are the majority carriers in TCNQ@Cu3(BTC)2.
In this paper we demonstrate the potential for novel nanoporous framework materials (NFM) such as metal-organic
frameworks (MOFs) to provide selectivity and sensitivity to a broad range of analytes including explosives, nerve
agents, and volatile organic compounds (VOCs). NFM are highly ordered, crystalline materials with considerable
synthetic flexibility resulting from the presence of both organic and inorganic components within their structure.
Detection of chemical weapons of mass destruction (CWMD), explosives, toxic industrial chemicals (TICs), and volatile
organic compounds (VOCs) using micro-electro-mechanical-systems (MEMS) devices, such as microcantilevers and
surface acoustic wave sensors, requires the use of recognition layers to impart selectivity. Traditional organic polymers
are dense, impeding analyte uptake and slowing sensor response. The nanoporosity and ultrahigh surface areas of NFM
enhance transport into and out of the NFM layer, improving response times, and their ordered structure enables structural
tuning to impart selectivity. Here we describe experiments and modeling aimed at creating NFM layers tailored to the
detection of water vapor, explosives, CWMD, and VOCs, and their integration with the surfaces of MEMS devices.
Force field models show that a high degree of chemical selectivity is feasible. For example, using a suite of MOFs it
should be possible to select for explosives vs. CWMD, VM vs. GA (nerve agents), and anthracene vs. naphthalene
(VOCs). We will also demonstrate the integration of various NFM with the surfaces of MEMS devices and describe
new synthetic methods developed to improve the quality of VFM coatings. Finally, MOF-coated MEMS devices show
how temperature changes can be tuned to improve response times, selectivity, and sensitivity.
We have synthesized and tested new highly fluorescent metal organic framework (MOF) materials
based on stilbene dicarboxylic acid as a linker. The crystal structure and porosity of the product are
dependent on synthetic conditions and choice of solvent and a low-density cubic form has been
identified by x-ray diffraction. In this work we report experiments demonstrating scintillation
properties of these crystals. Bright proton-induced luminescence with large shifts relative to the
fluorescence excitation spectra were recorded, peaking near 475 nm. Tolerance to fast proton
radiation was evaluated by monitoring this radio-luminescence to absorbed doses of several hundred MRAD.