Nanopatterned 2D Materials using Block Copolymer Nano-Lithography for Superior Thermoelectrics

Abstract

This study presents fabrication and characterization of nano patterned low dimensional materials fabricated from block copolymer (BCP) self-assembly and nano lithography and application to the thermoelectrics. Also the thermoelectric generator from conductive polymer and rubbery BCP is prepared and applied to the self-powered devices. Firstly, universal method to prepare perpendicular BCP nanopattern is introduced. Universal solution for 3 different BCP including polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA), polystyrene-b-polydimethylsiloxane (PS-b-PDMS) and polystyrene-b-poly(2-vinylprydine) (PS-b-P2VP) is adopted to produce perpendicular micro domain. Short time plasma treatment was applied to the top surface of BCP which initiate the crosslinking reaction to produce crosslinking layer. The crosslinking layer exhibits physical immobilization effect to both BCP blocks. The crosslinking layer also applied to the bottom layer replacing random copolymer brush, and the bottom layer can act as a neutral layer to the BCP film. The perpendicular nanopattern from 3 different BCP was observed with scanning electron microscope (SEM) and grazing intensity small angle x-ray scattering (GI-SAXS). BCP with plasma treatment of both top and bottom has fully-perpendicular orientation from the bottom to the top. The crosslinking layer on the top surface is more stiff than neat BCP, the wrinkle is induced after annealing process. In this reason, the thickness of crosslinking layer is controlled by varying the intensity and time of plasma treatment. <br/> Secondary, graphene nano structure was fabricated from BCP self-assembly. The large graphene up to 2 centimeter scale was fabricated from chemical vapor deposition (CVD) and nano patterned using PS-b-P2VP sphere nanopattern. Thermoelectric properties of the graphene nanomesh structure with controlled neck width is enhanced up to 40 times higher than pristine graphene. The Seebeck coefficient of the bilayer graphene nanomesh with 8 nm neck width shows the highest value of ~ 520 μV/K among the graphene-related materials. The thermal conductivity of suspended graphene nanomesh is reduced to ~78 W/m·K which is the lowest thermal conductivity for graphene nanostructure. Classical and quantum mechanical calculations supported my nanomesh approach, which can achieve high thermal properties based on reduced thermal conductivity and higher thermopower due to the confined geometry. Also the heterostructure of graphene nanostructure and transition metal dicalcogenides was introduced. Molybdenum disulfide (MoS2) has extremely high Seebeck coefficient and low thermal conductivity, however, very low electrical conductivity. To enhance the electrical conductivity selectively, graphene nanoribbon is adopted as an electron highways to the MoS2 thin film. The large area graphene nanoribbon was fabrication from block copolymer lithography, and was transferred to the MoS2 atomic layer. The graphene nanostructure significantly enhances the electrical conductivity of the MoS2 atomic layer more than 1000 time higher. We show that with heterostructure, the thermoelectric properties can be enhanced.<br/> Finally, PEDOT:PSS with coaxial strut was introduced for thermoelectric generator applications. The increment of the data communication between machine to machine, machine to human and human to human leads the requirement of collecting data from any kinds of sensors. One of the most important sensor is pressure sensor which can give information of blood pressure, heat beating rate or something. And the electric energy source is required to these kinds of electric devices, and the energy source is also needed to be flexible for the wearable electric devices. Recently, wearable thermoelectric generator based on organic materials are reported and the wearable TEG can generate several microwatt energies. However, to apply these wearable TEG, any kind of electric device should be assembled together with TEG. We introduced coaxial structure for thermoelectric generator. It is found that shell structure of SEBS give the mechanical strength and recovery properties to the foam and PEDOT:PSS is well-known thermoelectric polymer which have moderate thermopower and high electrical conductivity and used to the thermoelectric foam structure for thermoelectric generator. This SPM was assembled to the wearable thermoelectric generator and could generate 338 nW from the forearm.