Inorganic–organic superlattice thin films for thermoelectrics

Abstract

Nanoscale layer-engineering is an attractive tool to tailor the performance of thermoelectric materials as it potentially allows us to suppress thermal conductivity without significantly hindering the electrical transport properties. By combining the state-of-the-art thin-film fabrication technique for inorganics, i.e. atomic layer deposition (ALD), with its emerging counterpart for the organics, i.e. molecular layer deposition (MLD), it is possible to fabricate in a single reactor oxideorganic thin-film superlattices in which periodically introduced single/few-molecule organic layers alternate with thicker thermoelectric oxide layers. In such fundamentally new types of superlattice materials the oxide-organic interfaces with notable property mismatch are anticipated to hinder the phonon transport and/or bring about charge confinement effects thereby enhancing the material´s thermoelectric figure-of-merit. The experimental data so far gathered for the (Zn,Al)O:HQ and (Ti,Nb)O2:HQ systems (HQ stands for hydroquinone) show significantly suppressed thermal conductivities. Here in this topical review we summarize the experimental and computational works carried out on these superlattice materials and discuss the future potential of the ALD/MLD-fabricated inorganic-organic superlattice and nanolaminate thin-film structures in thermoelectrics.

ALD/MLD processes for Mn and Co based hybrid thin films

“…5,6 While there are not much more than a handful of publications on purely organic MLD films, the interest towards developing new ALD/MLD processes for the inorganic-organic hybrid materials is rapidly growing. 4 Most of the early ALD/MLD works concern very few metal constituents (mainly Al, Zn, Ti and Zr) 1,4,[7][8][9][10][11][12][13] but in the most recent works the technique has been successfully extended to some 3d, 4f and alkali metal based hybrid thin films with interesting luminescence and electrochemical properties. [14][15][16][17] Hybrid thinfilm materials based on 3d transition elements with partiallyfilled d orbitals would be attractive due to their potential to show e.g.…”

“…Only very recently when carboxylic acids 10,23 were employed as organic precursors the first β-diketonate based ALD/MLD processes were developed for Cu(thd)2, 14 Li(thd), 17 Zn(CH3CO2)2 24,25 and Eu(thd)3. 15 In these depositions the organic precursor was aromatic with a rigid benzene ring structure which is beneficial for the film growth; 4 the aromaticity is also believed to help to preserve the electrical properties of the inorganic constituent, 11,12 giving the resilient hybrid materials exciting features in electronics. In literary, linear carboxylic acids have been employed as well to fabricate e.g.…”

Nanoscale Organic Thermoelectric Materials: Measurement, Theoretical Models, and Optimization Strategies

“…Generally, inorganic and organic materials show obvious mismatches in the phonon density of states; therefore, their thermal conductivity can be further controlled by carefully constructing internal interfaces, which may lead to dramatically decreasing of thermal conductivity at the thermal boundary of the inorganic–organic interface . As long as the structure length of different layers is properly set in the alternating layer structure of organic–inorganic materials, the phonon transmission can be effectively suppressed on the premise of not affecting or enhancing the electronic properties . Therefore, an organic–inorganic hybrid structure is expected to be a high performance thermoelectric material, and experiments have shown that such a structure can result in a greatly improved ZT value.…”

Flexible Thermoelectric ZnO–Organic Superlattices on Cotton Textile Substrates by ALD/MLD

“…[24,25] In combination with the ALD fabrication of inorganic materials, a very convenient and highly controllable route to nanostructuring is molecular layer deposition (MLD) to produce hybrid inorganic-organic materials (Figure 1d). [26][27][28][29][30][31][32] The combined ALD/MLD technique has been used to fabricate various nanoscale oxide-organic superlattices in a highly controllable fashion [33] and crystalline ZnO-organic superlattices fabricated using hydroquinone (HQ, benzene-1,4-diol, HOC 6 H 4 OH) as the organic precursor indeed show orderof-magnitude reduction of thermal conductivity. [34] The orderof-magnitude reduction of thermal conductivity has also been proven for analogous hybrid TiO 2 -organic superlattices fabricated by ALD/MLD, highlighting the wider applicability of the oxide-organic superlattice approach for the thermal engineering of metal oxides.…”