Optical study of two-dimensional ternary and quaternary metal phosphorus chalcogenide crystals
Date of Issue2018
School of Physical and Mathematical Sciences
The exploration of functional two-dimensional (2D) materials beyond graphene has gone into eruption in recent years aimed at new materials and functionalities. The interest of individual layers of 2D materials mostly focuses on the unique electronic and optical properties to the bulk counterpart. In this thesis. a systematic study has been conducted on a new family of 2D semiconducting crystals- metal phosphorus chalcogenides (MPCs). mainly focusing on a variety of physical property of ultrathin MPCs. The investigation has started from the synthesis of high-quality single crystal by chemical vapor transport method followed by the preparation of atomically thin sample on Si02/Si substrate by mechanical exfoliation. Several compounds in this group are well crystallized with structural characterized techniques . A first principle calculation on cleavage energies of MPCs is calculated to confirm the feasibility and stability of monolayer structure. The transmission spectra of this series of crystal show a wide-range band gaps from 1.3 to 3.5 eV with element substitution, suggesting their potential on optoelectronic applications. Therefore, the MPC monolayers can be used to build the heterostructures by van der Waals stacking, which is significant in the fundamental research in optoelectronics. as well as unusual ferroelectric and magnetic properties. A comprehensive study on the lattice dynamics has been performed on MPC crystals by Raman spectroscopy. The similar Raman spectra in high-frequency region(> 200 em-') of MPCs suggest that those modes are originated from the internal vibration of [P2S6]4- anion , while the large differences in low-frequency region (< 200 cm-1) among MPCs are directly related to the metal ion vibrations. Similar Raman spectra in atomically thin FePS3 show that the Raman modes are mainly from intra-layer vibrations. The phonon dispersion relationship of FePS3 has been calculated by the first principle density function theory. A qualitative explanation of the dynamical behaviors of those materials was obtained to motivate further fundamental physical study, as well as the potential optical and optoelectronic 2D device based on this family of materials. We further demonstrate a systematic study on high-quality atomically thin iron phosphorus trisulfide (FePS3) crystal prepared by mechanical exfoliation. Raman spectroscopy is used to probe the spin-phonon coupling, spin interactions and magnetic phase transition on microscale mono- and few-layer FePS3 samples. The magnetic persistence shown in the thin sample suggests a promising future of ultrathin magnetic device, while the spin-phonon coupling in 2D materials promotes a better understanding on many critical problems in varies research fields , such as magnetic Raman scattering and possibly on iron based superconductivity. A calculation on total energy of magnetic phases is performed to support our experimental results. Our studies advocate the intriguing magnetic properties in 2D crystals and suggest FePS3 is a promising candidate material for future magnetic applications. The first observation of second harmonic generation (SHG) in 2D atomically thin silver indium phosphorous sulfide (AglnP2S6) crystals has been reported. The nonlinear signal facilitates the u e of thickness-dependent SHG intensity to investigate the lattice symmetry and the stacking type of this material. Moreover, the crystal-otientation dependent SHG of monolayer sample can be applied to probe the rotational symmetry and grain boundary of the AglnP2S6 lattice in plane. Besides, the ferroelectric property in CulnP2S6 flakes has been investigated by SHG based on the noncentrosymmetric lattice structure. The first observation ferroelectricity in 20 materials with the thickness below 50 nm has been demonstrated. Our temperature dependent Raman spectroscopy measurement of 20-nm CulnP2S6 flakes also supports the demonstration . An anisotropy in CulnP2S6, along layer has been detected by SHG signal. Our studies expand the 2D crystals family in nonlinear effect field, which opens considerable promise to in the functionalities and potential applications of 2D materials.