Ion-track lithography for thermoelectric applications

This work presents a novel process to fabricate nanostructures for thermoelectric applications by a combination of a traditional silicon microfabrication techniques, electroplating, and submicron ion-track nanolithography. Polyimide (Kapton HN, PI2731), and PMMA (polymethyl methacrylate) were ion-track irradiated and wet etched. Bi2Te3 nanowires (80 and 120 nm diameter) were electroplated with preferential orientation in the (110) crystallographic planes and fine-grained microstructure. An increased need of powering low-power wireless systems and portable electronics requires an alternative source of electrical energy to replace batteries, which have limited shelf life and are bulky. One of the existing alternatives is thermoelectric power, which offers several advantages over other energy harvesting techniques: the devices are wholly solid-state, have no moving parts, long life and high reliability [1]. Drawbacks of existing thermoelectric generators are low efficiency and large size. Recent theoretical studies have predicted that efficiency is increased if the thermoelectric element diameter can be decreased to a size at which quantum confinement and interface scattering effects occur [2]. There are different approaches to miniaturize thermoelectric materials, e.g. by growth in porous alumina templates, and e-beam or micro-photolithography. These methods have limitations in size, density of thermoelectric elements, complexity, and high cost and they are not compatible with silicon microfabrication processes. Ion-track technology can overcome mentioned issues and produce low-cost templates for nanowires with high-aspect-ratio [3,4]. This work is aimed to develop a new generation of thermoelectric generators with high-aspect-ratio and high density nanoelements to target power harvesting from small temperature gradients. As templates we used polyimide and PMMA polymer samples and irradiated them with Pb, U, or Au projectiles of MeV to GeV kinetic energy and fluences of 5×10 or 5×10 ions/cm at the UNILAC. Track etching was studied in polyimide PI2731 photoresists (HD Microsystems), dry films (12.7 and 20 μm thick) of Kapton HN (Dupont), PMMA foils and PMMA 950A11 photoresist (Goodfellow Microchem). Polyimide samples were pre-etched in H2O2 solution at 60°C and subsequently etched in sodium hypochlorite (NaClO, 13%, pH~12.6) solution at 60°C, resulting in pore diameters between 30 and 120 nm for different etching times. Two etch solutions were used to fabricate PMMA nanotemplates: methyl-iso-butyl-ketone (MIBK/IPA) and GG developer (15% water, 60% butoxyethoxyethanol, 20% morpholin, 5 % aminoethanol). For the PMMA photoresist, these etching conditions resulted in pore diameters of <30 nm for the MIBK:IPA solution and ranging from 30 to 50 nm for the GG solution. Electroplating of Bi2Te3 and Bi0.5Sb1.5Te3 nanowires into polymide templates was carried out by cathodic electrochemical co-deposition of Bi, Sb, and Te powder dissolved in the aqueous nitric acid using a three-electrode cell with an Autolab potentiostat/galvanostat. For a range of applied potentials from -0.22 to 0.03 V vs a Saturated Calomel Electrode (SCE), the resulting Bi2Te3 compounds showed a stoichiometric composition with a grain size of the order of tens nanometers (Figure 1). X-ray Diffraction Spectroscopy (XRD) spectra revealed that the deposited nanowires are monophased polycrystalline Bi2Te3 of rhombohedral structure, R ̅3m.