Tunability of silicon clathrate film properties by controlled guest-occupation of their cages

Abstract

Type I and type II silicon clathrates are guest–host structures made of silicon polyhedral cages large enough to contain atoms that can be either inserted or evacuated with only a slight volume change of the structure. This feature is of interest not only for batteries or storage applications but also for tuning the properties of the silicon clathrate films. The thermal decomposition process can be tuned to obtain Na8Si46 and Na2<x<10Si136 silicon clathrate films on intrinsic and p-type c-Si (001) wafer. Here, from a unique synthesized NaxSi136 film, a range of resistivity of minimum four order of magnitude is possible by using post-synthesis treatments, switching from metallic to semiconductor behavior as the Na content is lowered. Extended exposition to sodium vapor allows us to obtain fully occupied Na24Si136 metallic films, and annealing under iodine vapor is a way to reach the guest-free Si136, a semiconducting metastable form of silicon with a 1.9 eV direct bandgap. Electrical measurements and resistance vs temperature measurements of the silicon clathrate films further discriminate the behavior of the various materials as the Na concentration is changing, additionally shouldered by density functional theory calculations for various guest occupations, further motivating the urge of an innovative pathway toward true guest-free type I and type II silicon clathrates.

Synthesis and Characterization of Type II Ge-Si Clathrate Films for Optoelectronic Applications

“…A systematic blue shift of the major peaks was observed for the samples with increasing Si ratios. This blue shift phenomenon is often correlated with a decrease in crystallinity or material subjected to compressive stress [20]. Hence, the presence of an amorphous phase, supported by the XRD measurement results, and lattice strain, due to substitutional effects of the Ge lattice with smaller Si atoms, were attributed to the observed blue shift of the Raman scattering spectra.…”

“…A type II clathrate structure allows for the removal of guest species without affecting its stability, leading to unique features such as a semiconductor with a direct band gap. Type II Si and Ge clathrates with low guest occupancies exhibit direct band gaps of 1.9 eV [20] and 0.7 eV [21], respectively, in contrast to the indirect band gap of diamond-structured Si and Ge. Hence, Si-and Ge-based type II clathrates are investigated for applications in photovoltaics [9,22,23], thermoelectric devices [21], and Li-ion batteries [24][25][26][27].…”

“…For powder samples, type II clathrates have been synthesized mainly using a two-step thermal decomposition method [34][35][36], an ionic liquid method [30], and an electrochemical method [26,[37][38][39]. On the other hand, for the film form, a two-step thermal decomposition method, primarily derived from the powder synthesis method, was used predominantly [3,20,23,32,33]. Recently, our team has reported a single-step thermal decomposition method using a specially designed setup [31,40,41].…”

“…Other techniques include electric field application coupled with annealing in the inert atmosphere for the removal of Na in a type II Ge clathrate film [32]. Recently, Vollondat et al demonstrated the removal of a Na guest from a type II Si clathrate film by annealing in an iodine vapor, whereas a fully occupied type II Si clathrate film was achieved through an extended exposition to sodium vapor [20]. For optical property analysis or to realize its potential in practical devices, the film form is preferred over powder.…”