Experimental Study Of Dielectric Properties In Two-dimensional Transition Metal Dichalcogenides And CsPbBr₃ Perovskite At Terahertz Frequency

The Recent development of low-dimensional material systems presents their outstanding material characteristics and unique physics mechanism among mechanics,thermal and optoelectronic aspects.With the deepening of cognition and research,bright application prospect of low-dimensional material systems in nano photonics,bio-sensor,solar cell and so many other fields has been increasingly emerged.To date,much progress is mostly achieved at visible and near-infrared frequency while the understanding of the interaction between low-dimensional material and terahertz wave is relatively lacking.Lack of suitable material is one of the major problems faced by terahertz research over the last decades.As low-dimensional materials open up new approaches to tune carrier behavior and band structure and then to material characteristics,it is thought as a feasible method to solve the problem.To achieve this goal,two typical low-dimensional materials,including 2D transition metal dichalcogenides?TMDCs?and lead halide perovskite quantum dots?CsPbBr₃ Qds?,are experimentally and theoretically studied in this dissertation.The specific results are listed as follow:1.We built terahertz time-domain spectroscopy system based on photoconductive antenna.This system has parameters including spectrum range from 100 GHz to 2.5THz;¹00 fs time resolution,⁰.05 THz spectrum resolution and peak dynamic range over 65 dB.Using this instrument,we completed dielectric parameters measurement on WS₂ nanosheets and CsPbBr₃ Qds and obtained their frequency-dependent refractive index,extinction coefficient,dielectric constant and conductivity.Coupled with 800 nm continuous wave laser,it was also applied in the modulation measurement of WS₂-Si heterostructure THz modulator.2.A model for generation and propagation of photogenerated carriers in Vander Waltz heterostructure composed of two-dimensional materials and body semiconductors is derived in this part.After that,by combining the liquid-phase exfoliation material-preparation technology and terahertz modulation combination,we experimentally fabricated optically controlled terahertz modulator on the base of WS₂-Si heterostructure for the first time.Under low irradiated fluence of 50 mW,the modulation depth of this sample is twice as much as that of the graphene-based modulator and over 10 times than that of Si substrate.This effective modulation for terahertz is mainly due to high electron mobility in prepared WS₂ and the spatial separation of photogenerated electrons and holes.Besides this,as the coverage limit of traditional chemical vapor deposition?CVD?method is overcome in this work,our modulator presents better performance in modulation depth?94.8%,470mW?and modulation speed?>3 kHz?than other reported TMDC-based modulator.Moreover,the further experimental results show that this liquid-phase exfoliated THz modulator has many advantages such as high modulation depth and speed,not sensitive to THz polarization,easy-made and fine reproducibility and therefore present significant superiority in practical application.3.The dielectric properties of CsPbBr₃ Qds solution among 0.1².0 THz have been experimentally studied in this part of work.In combination with effective media theory,the complex dielectric constant of Qds solute was obtained?the real and imaginary part are 11.7 and 5.1,respectively?.This value is higher than those in common semiconductors such as silicon and glass,which reflects the strong Column interaction among carriers in CsPbBr₃ Qds.By fitting solution dielectric constant spectrum with Drude-Lorentz model,the plasma frequency is quite close to zero,indicating that the free carrier absorption is negligible in this case.The major absorption mechanism in Qds solution is phonon resonance absorption of CsPbBr₃ molecule.After that,the THz absorption spectrums between dark and photoexcitation conditions are comparatively studied.An abnormal phenomenon that no transmission changes among this two condtion were observed.Further analysis shows that the bounded exciton in CsPbBr₃ Qds has atom-liked discrete energy band.Those excited excitons cannot make an effective response to THz wave in measured frequency as the low photon energy of THz wave cannot excite them.This work reveals the potential and value of using terahertz wave to study the carrier properties in quantum dot materials.