ABSTRACT
The ecofriendly tin selenide (SnSe) is expected to find multiple applications in optoelectronic, photovoltaic, and thermoelectric systems. This work is focused on the thermoelectric properties of thin films. SnSe single crystals exhibit excellent thermoelectric properties, but it is not so in the case of polycrystalline bulk materials. The investigations were motivated by the fact that nanostructuring may lead to an improvement in thermoelectric efficiency, which is evaluated through a dimensionless figure of merit, ZT = S2 σ T/λ, where S is the Seebeck coefficient (V/K), σ is the electrical conductivity (S/m), λ is the thermal conductivity (W/mK), and T is the absolute temperature (K). The main objective of this work was to obtain SnSe films via magnetron sputtering of a single target. Instead of common radiofrequency (RF) magnetron sputtering with a high voltage alternating current (AC) power source, a modified direct current (DC) power supply was employed. This technique in the classical version is not suitable for sputtering targets with relatively low thermal and electrical conductivity, such as SnSe. The proposed solution enabled stable sputtering of this target without detrimental cracking and arcing and resulted in high-quality polycrystalline SnSe films with unprecedented high values of ZT equal to 0.5 at a relatively low temperature of 530 K. All parameters included in ZT were measured in one setup, i.e., Linseis Thin Film Analyzer (TFA). The SnSe films were deposited at sputtering powers of 120, 140, and 170 W. They had the same orthorhombic structure, as determined by X-ray diffraction (XRD), but the thickness and microstructure examined by scanning electron microscopy (SEM) were dependent on the sputtering power. It was demonstrated that thermoelectric efficiency improved with increasing sputtering power and stable values were attained after two heating-cooling cycles. This research additionally provides further insights into the DC sputtering process and opens up new possibilities for magnetron sputtering technology.
ABSTRACT
For the first time, transition metal-based chalcogenides conforming to the definition of high entropy materials, are synthesized, with the multicomponent occupation being utilized on both cationic and anionic sublattices. The pentlandite-structured (Co,Fe,Ni)9S8 and (Co,Fe,Ni)9(S,Se)8 compositions are obtained using a two-stage, solid-state reaction method. Room temperature structural analysis (XRD, SEM, Raman) in both cases indicates the presence of a homogeneous, single-phase, Fm3[combining macron]m structure, with a profound effect of Se addition on the lattice parameters. The obtained materials possess an excellent electrical conductivity of 105 S m-1, and slightly negative Seebeck coefficient values, resulting from their metallic character, combined with a low thermal conductivity of 2.5 W m-1 K-1, especially when compared with conventional analogues. The optical measurements reveal very promising behavior in the UV/vis range. The electrochemical sensitivity towards hydrazine and acetaminophen is also presented, making them potentially interesting for sensor devices. Based on the DFT analysis of various sub-systems, the origins of the observed transport and optical behavior are explained. Furthermore, it is shown that the application of the high-entropy principle to both sublattices simultaneously allows for extensive tailoring of the band structure, allowing these materials to be optimized with respect to the given application, including thermoelectric and photoelectrochemical devices and catalysis, e.g., the hydrogen evolution reaction.
ABSTRACT
For the first time, an alternative way of improving the stability of Cu-based thermoelectric materials is proposed, with the investigation of two different copper chalcogenide-copper tetrahedrite composites, rich in sulfur and selenium anions, respectively. Based on the preliminary DFT results, which indicate the instability of Sb-doped copper chalcogenide, the Cu1.97S-Cu12Sb4S13 and Cu2-xSe-Cu3SbSe3 composites are obtained using melt-solidification techniques, with the tetrahedrite phase concentration varying from 1 to 10 wt.%. Room temperature structural analysis (XRD, SEM) indicates the two-phase structure of the materials, with ternary phase precipitates embed within the copper chalcogenide matrix. The proposed solution allows for successful blocking of excessive Cu migration, with stable electrical conductivity and Seebeck coefficient values over subsequent thermal cycles. The materials exhibit a p-type, semimetallic character with high stability, represented by a near-constant power factor (PF)-temperature dependences between individual cycles. Finally, the thermoelectric figure-of-merit ZT parameter reaches about 0.26 (623 K) for the Cu1.97S-Cu12Sb4S13 system, in which case increasing content of tetrahedrite is a beneficial effect, and about 0.44 (623 K) for the Cu2-xSe-Cu3SbSe3 system, where increasing the content of Cu3SbSe3 negatively influences the thermoelectric performance.
ABSTRACT
We report a study on a hybrid mode-locked fiber laser with two saturable absorbers: slow and fast, integrated in a single device. Amorphous antimony telluride (Sb(2)Te(3)) layer was deposited on side-polished fiber to form the slow saturable absorber due to the third order nonlinear susceptibility of Sb(2)Te(3). Additionally, an unsymmetrical design of the device causes polarization-dependent losses and together with polarization controller allows to use a nonlinear polarization evolution to form the artificial fast saturable absorber. Sub-200 fs soliton pulses with 0.27 nJ of pulse energy were generated in the hybrid mode-locked Er-doped fiber laser. Differences in the dynamics of mode-locked laser are further investigated with the use of slow and fast saturable absorbers solely, and compared with the hybrid device. Joint operation of two saturable absorbers enhances the laser performance and stability. The conducted experiments allowed to define roles of each mechanism on the pulse shaping in the laser cavity.
