No abstract

The wealth of technological patterning technologies of deca-nanometer resolution brings opportunities to artificially modulate thermal transport properties. A promising example is given by the recent concepts of "thermocrystals" or "nanophononic crystals" that introduce regular nano-scale inclusions using a pitch scale in between the thermal phonons mean free path and the electron mean free path. In such structures, the lattice thermal conductivity is reduced down to two orders of magnitude with respect to its bulk value. Beyond the promise held by these materials to overcome the well-known “electron crystal-phonon glass” dilemma faced in thermoelectrics, the quantitative prediction of their thermal conductivity poses a challenge. This work paves the way toward understanding and designing silicon nanophononic membranes by means of molecular dynamics simulation. Several systems are studied in order to distinguish the shape contribution from bulk, ultra-thin membranes (8 to 15 nm), 2D phononic crystals, and finally 2D phononic membranes. After having discussed the equilibrium properties of these structures from 300 K to 400 K, the Green-Kubo methodology is used to quantify the thermal conductivity. The results account for several experimental trends and models. It is confirmed that the thin-film geometry as well as the phononic structure act towards a reduction of the thermal conductivity. The further decrease in the phononic engineered membrane clearly demonstrates that both phenomena are cumulative. Finally, limitations of the model and further perspectives are discussed.

Thermoelectricity struggles with the lack of cheap, abundant, and environmentally friendly materials. Silicon could overcome this deficiency by proposing high harvested power density, simplicity, availability, harmlessness, CMOS compatibility, and cost reduction. However, despite its high Seebeck coefficient and electrical conductivity, silicon is an inefficient thermoelectric material due to a high thermal conductivity (κ). Modern nanofabrication techniques enable reduction of κ in silicon through attenuation of thermal phonons. In this letter, the design and the fabrication of nanostructured material onto κ measurement platforms are presented. The proposed fabrication process is versatile and ensures compatibility with CMOS technologies. The proposed devices enable precise κ measurement owing to a careful management of thermal losses. Characterization resulted in a two-fold (κ = 59 ± 10 W/m/K) reduction below bulk value for a 54-nm-thick plain silicon membranes. Further reduction is measured at κ = 34.5±7.5 W/m/K for membranes with phononic crystals.

The gas sensing properties of graphene back-gated field-effect transistor (GFET) sensors toward acetonitrile, tetrahydrofuran, and chloroform vapors were investigated with the focus on unfolding possible gas detection mechanisms. The FET configuration of the sensor device enabled gate voltage tuning for enhanced measurements of changes in DC electrical characteristics. Electrical measurements were combined with a fluctuation-enhanced sensing methodology and intermittent UV irradiation. Distinctly different features in 1/f noise spectra for the organic gases measured under UV irradiation and in the dark were observed. The most intense response observed for tetrahydrofuran prompted the decomposition of the DC characteristic, revealing the photoconductive and photogating effect occurring in the graphene channel with the dominance of the latter. Our observations shed light on understanding surface processes at the interface between graphene and volatile organic compounds for graphene-based sensors in ambient conditions that yield enhanced sensitivity and selectivity.