Dominika Ziolkowska

Chromatic Mechanical Response in 2-D Layered Transition Metal Dichalcogenide (TMDs) based Nanocomposites

The ability to convert photons of different wavelengths directly into mechanical motion is of significant interest in many energy conversion and reconfigurable technologies. Here, using few layer 2H-MoS2 nanosheets, layer by layer process of nanocomposite fabrication, and strain engineering, we demonstrate a reversible and chromatic mechanical response in MoS2-nanocomposites between 405 nm to 808 nm with large stress release. The chromatic mechanical response originates from the d orbitals and is related to the strength of the direct exciton resonance A and B of the few layer 2H-MoS2 affecting optical absorption and subsequent mechanical response of the nanocomposite. Applying uniaxial tensile strains to the semiconducting few-layer 2H-MoS2 crystals in the nanocomposite resulted in spatially varying energy levels inside the nanocomposite that enhanced the broadband optical absorption up to 2.3 eV and subsequent mechanical response. The unique photomechanical response in 2H-MoS2 based nanocomposites is a result of the rich d electron physics not available to nanocomposites based on sp bonded graphene and carbon nanotubes, as well as nanocomposite based on metallic nanoparticles. The reversible strain dependent optical absorption suggest applications in broad range of energy conversion technologies that is not achievable using conventional thin film semiconductors.

Novel graphene oxide/manganese oxide nanocomposites

A new synthesis method of obtaining nanocomposites, consisting of manganese oxides nanoparticles embedded in carbonaceous matrix, is reported. The method is based on thermal processing of precursor consisting of lithium and manganese salts mixed with citric and acetic acids. The nanocomposite morphology, composition and structure can be tuned by selecting specific thermal processing routes (e.g. pressure, temperature, time, etc.). For instance, foam or microspheres morphology can be obtained by heating the precursor in a vacuum at moderately low temperatures (ca. 300–400 °C). Similarly, depending on the ambient pressure during heating above the recrystallization temperature (ca. 400–450 °C), the forming nanocomposite will consist of either MnO or LiMn2O4 nanoparticles, i.e. materials which are of importance for lithium-ion batteries as anodes and cathodes, respectively. Finally, the structure of the carbonaceous matrix can be tuned primarily by controlling the annealing temperature. For instance, annealing in the temperature range of about 600–800 °C can lead to the formation of graphene-related structures, such as modified graphene oxide. We used this method to produce example nanocomposites, performed their detailed characterization and proposed a mechanism to explain their formation.

Raman Spectroscopy of LiFePO_4 and Li_3V_2(PO_4)_3 Prepared as Cathode Materials

Structure of samples of lithium iron vanadium phosphates of different compositions were investigated by X-rays, electron microscopy and Raman spectroscopy. The investigated salts were mainly of olivine-like and NASICON-like structures. The X-ray diffraction and the Raman scattering show different crystalline structures, which is probably caused by difference between cores of the crystallites (probed by X-rays) and their shells (probed by the Raman scattering). Most of the Raman spectra were identified with previously published data, however in the samples with high vanadium concentration we have observed new, not reported earlier modes at 835 cm−1 and 877 cm−1, that we identified as oscillations related to V2O4− 7 or VO 3− 4 anions.