Xiao Liu

Thiophene-based polymers, notably poly(3,4-ethylenedioxythiophene) (PEDOT) and its composite with poly(styrenesulfonate) (PEDOT:PSS), are attractive targets for thermoelectric research because of their high ZT values and ease of thin-film synthesis, but traditional solution-phase routes limit both chemical control and applicable form factors. Oxidative chemical vapor deposition (oCVD) is an alternative technique capable of producing highly conductive PEDOT films without the use of PSS, yet the thermoelectric properties of PEDOT produced through oCVD have not been as thoroughly investigated. Here, we report an enhancement in thermoelectric properties of oCVD-grown PEDOT films with V2O5 inclusions through treatment in polar aprotic, high-dielectric organic solvents (DMSO, EG, and DMF). In solution-phase deposited films, the performance enhancements from post-deposition treatments in such solvents are often attributed to depletion or segregation of the insulating PSS phase, but the similar enhancements in our PSS-free films show that other effects on the physicochemical nature of the PEDOT must also be present. Our observations provide key insights into routes to optimize vapor-phase deposited PEDOT as well as the potentially misunderstood effects of organic solvents on PEDOT in general.

The non-line-of-sight nature of initiated chemical vapor deposition (iCVD) makes it an amenable technique for depositing thin, conformal polymer films on three-dimensional (3D) high surface-area substrates. Adding to the versatility of iCVD, we process the deposited film in situ to impart additional functionality. Herein we describe fabricating submicron-thick positively charged polymer films with labile anions, a class of materials that brings ion-transport regulation to water desalination, gas separations, and energy-storage systems. Coatings of iCVD-deposited poly(4-dimethylaminomethylstyrene) (pDMAMS) on planar and 3D substrates are reacted with an alkylating/cross-linking reagent to yield quaternary ammonium-containing pDMAMS⁺. By optimizing deposition conditions, we demonstrate conformal coatings of pDMAMS and pDMAMS⁺ spanning a thickness range of 30–1000+ nm. Chemical and mechanical characterization confirms the successful quaternization of the iCVD-generated films, subsequent ion exchange of various counterions (Br⁻, HCO₃ ⁻, and OH⁻), and enhanced modulus of the crosslinked pDMAMS⁺ polymer. The use of grazing-incidence X-ray scattering reveals the films are noncrystalline and contain ion clusters with anion-dependent geometry. By understanding the impact of deposition conditions and functionalization on subsequent physicochemical-mechanical properties, we lay the groundwork for using iCVD-derived pDMAMS⁺ films in targeted applications.

The sensitivity of gravitational-wave detectors is limited by the mechanical loss associated with the amorphous coatings of the detectors’ mirrors. Amorphous silicon has higher refraction index and lower mechanical loss than current high-index coatings, but its optical absorption at the wavelength used for the detectors is at present large. The addition of hydrogen to the amorphous silicon network reduces both optical absorption and mechanical loss for films prepared under a range of conditions at all measured wavelengths and temperatures, with a particularly large effect on films grown at room temperature. The uptake of hydrogen is greatest in the films grown at room temperature, but still below 1.5 at.% H, which show an ultralow optical absorption (below 10 ppm) measured at 2000 nm for 500-nm-thick films. These results show that hydrogenation is a promising strategy to reduce both optical absorption and mechanical loss in amorphous silicon, and may enable fabrication of mirror coatings for gravitational-wave detectors with improved sensitivity.

Silicene, a 2D hexagonal structure of silicon, has shown great promise for next-generation advanced electronic devices due to recent progress. However, one major obstacle to its application and integration into devices is its strong tendency of rapid oxidation in an ambient environment. Enhancing the chemical inertness by means of chemical modification without comprising its unique structure and properties is vital. Here, we report the fabrication of hydrogenated silicene grown on polycrystalline Ag films by means of plasma-enhanced chemical-vapor deposition (PECVD) with proper hydrogen dilution. Our Raman spectroscopy and x-ray diffraction results show the existence of typical monolayer and few-layer silicene. The x-ray photoelectron spectroscopy demonstrates its oxidation resistance after a few minutes of air exposure in a class 100 clean room. We propose that PECVD silicene can be an alternative to molecular-beam-epitaxy silicene for potential low-cost and large-area silicene applications.

Nanocrystallization has been an important approach for reducing thermal conductivity in thermoelectric materials due to limits on phonon mean-free path imposed by the characteristic structural size. We report on thermal conductivity as low as 0.3 Wm⁻¹K⁻¹ of nanocrystalline silicon thin films prepared by plasma-enhanced chemical-vapor deposition as grain size is reduced to 2.8 nm by controlling hydrogen dilution of silane gas during growth. A multilayered film composed by alternating growth conditions, with layer thicknesses of 3.6 nm, is measured to have a thermal conductivity 30% and 15% lower than its two constituents. Our quantitative analysis attributes the strong reduction of thermal conductivity with decreasing grain size to the magnifying effect of porosity which occurs concomitantly due to increased mass density fluctuations. Our results demonstrate that ultrasmall grain sizes, multilayering, and porosity, all at a similar nanometer-size scale, may be a promising way to engineer thermoelectric materials.