Study On The Terahertz Spectroscopy Of Cellulose And Sulfonic Type Organics

Terahertz(THz)spectroscopy contains rich physical and chemical information and has been a promising candidate for use in materials identification,qualitative analysis,molecular conformation dynamics and many other fundamental and applied researches.Compared with inorganic materials,organics have attracted more interest in THz research.The numerous varieties of organics and their great flexibility in design make them potential materials for THz generation and detection.The collective vibrations of organics lie in THz regime,but they have long remained elusive and difficult to understand thoroughly,owing to themselves complexity in nature.This causes great limitations not only in the applications of THz spectroscopy in organics characterizations and analyses but also in the exploration of new materials favorable for the development of THz devices.In this thesis,cellulose and its allotrope,solid taurine and organic sulfonate DAST have been characterized experimentally by THz spectroscopy and theoretically by density functional theory(DFT).The THz finger peaks of these organics,as well as the origins of the vibrational modes,were identified in detail.The DFT-based methods toward to precise prediction of THz spectra in partially-crystalline polymer and hydrogen-bonded molecular crystal were also investigated and established.The main results presented in this work are outlined as following.(1)Regenerated cellulose films were prepared by an iced dissolution method and characterized by SEM,FTIR,and XRD.Terahertz time domain system(THz-TDS)and FTIR were employed to measure the THz spectra of the regenerated cellulose(cellulose II)films and degreased cotton(cellulose I?).Based on these,the THz characteristic peaks of regenerated cellulose films in 0-21 THz are experimentally identified for the first time.Moreover,a new approach for distinguishing the cellulose II and cellulose I? by THz spectra was developed.In particular,it was found that the increase of the THz transmittance with the decrease of the wavenumber is attributed to the existence of amorphous components in the regenerated cellulose films,while the THz absorption peaks come from the molecular vibrations of long-range ordered crystalline portion.As a result,a simplified modelling method for partially-crystalline cellulose II in the framework of quantum-mechanical calculation was established,by which the origins of vibrational modes of regenerated cellulose were systematically assigned.(2)Through choosing simple but representative sulfonic acid,taurine as model compound,revealing the difference between the calculated THz spectra using popular PBE-Fix and PBE-D2 method is mainly originated from the different treatment of intermolecular interaction.It was also disclosed that the dispersion correction play a crucial role in the accurate THz calculation of taurine.An unambiguous identification of the characteristic peaks of taurine in 3-12 THz was attempted for the first time.In addition,an unusual blue-shift with increasing temperature was observed at the 302 cm-1(9.1 THz)mode.We propose a mechanism based on a coupled-oscillators theory to account for it.The novel three coupled-oscillators model provides extra insights into the temperature dynamics of non-bonded(i.e.intermolecular)interactions.While reaffirming one's expectation of a flattening of the intermolecular potential with increasing temperature due to expansion of the crystalline lattice,it also offers the opportunity to better understand the importance of the relative changes of the intermolecular couplings in taurine that may also occur in other molecular systems.(3)Using a step-by-step optimization method with gradually increasing convergence accuracy,we successfully obtained the steady ground-state structure of DAST,providing support for the pursuit of further modes assignment.Results indicated that the vibration at 1.12 THz is originated from the optical phonon mode of cation and anion;the modes at 1.46 THz and 1.54 THz are related with the vibration of sulfonate(anion);while the vibration at 2.63 THz and 3.16 THz are assigned to the torsion of cation and rotation of cation.These results fill the gap of mode analysis of low-frequency vibrations of DAST,and provide some worthwhile information for the design and synthesis of DAST derivatives of large second-order optical nonlinear coefficient.