Terahertz Photoelectric Properties Of Carbon Dots And Their Effects On Sonoluminescence

Carbon dots(CDs)are a class of zero-dimensional carbon-based materials with size less than 10 nanometers.Due to the quantum local effect and quantum confinement effect,they have excellent physical and chemical properties,such as tunable photoluminescence(PL)from deep ultraviolet(UV)to near-infrared(NIR),broad band absorption,ultrafast carrier transfer,versatile surface chemistry,to mention but a few.These outstanding properties make CDs become the promising materials for various applications,including optoelectronic materials and devices(e.g.,LED and solar energy conversion),chemistry,biomedicine and so on.It has developed into a multidisciplinary research hotspot joint physics,chemistry,materials,biology,medicine and so on.In this dissertation,aim to the practical application of CDs in optoelectronic devices,CDs in powder form and dispersed in solid matrix were prepared and their photoelectric properties were studied.The terahertz(THz)time domain spectroscopy(TDS)was first applied to study the dynamic characteristics of CDs carriers,and the key material physical parameters of CDs were measured by optical method.The interaction mechanism between CDs and light was also explored.At the same time,based on the superior physical and chemical properties of CDs,they were applied to the exploration of sonoluminescence mechanism.The main research contents and results of this dissertation are as follows:(1)The THz photoelectric properties of fluorescent carbon dots were studied.Via THz TDS measurement taking temperature as a variable,insulator-to-semiconductor transition with increasing THz frequency and temperature is observed in carbon dot powder.Relevant real part and imaginary part of photoconductance are obtained.And,key material physical parameters of CDs,such as carrier concentration,carrier relaxation time,carrier localization factor etc.are determined.(2)The THz magneto-optoelectronic physical properties of fluorescent carbon dots were studied.The real and imaginary parts of the left-handed circularly polarized and right-handed circularly polarized photoconductances of CDs in a solid matrix(e.g.,PVA)under magnetic field were experimentally measured.Through THz magnetooptical measurement,the carrier effective mass of CDs can be determined under the condition of non-cyclotron resonance,as well as other key material physical parameters such as carrier concentration and carrier relaxation time.The related characterizations and analyses shows that the carboxyl group with a large proportion on the surface contributes to P-type CDs,and the carrier effective mass of studied CDs in the PVA matrix can reach 2.25me(where me is electron rest mass).(3)The influence of carbon dots on the effect of multi-bubble sonoluminescence(MBSL)was studied.The MBSL effects in pure water as well as in CD aqueous solutions(CDAS)were comparatively studied.Through the measurement of SL spectra,it is observed that SL color changes from blue in water to orange light in CDAS,and its blackbody radiation temperature decreased dramatically.Based on the characterization results of CDs before and after SL,it is found that a larger content of-COOH groups are introduced to CDs through chain-like oxidation reactions assisted with hydroxyl radicals generated during SL.Thus,the suppression of the OH’ by the presence of the CDs is the main mechanism for the changes happened to SL spectra.(4)The influence of carbon dots on the effect of single bubble sonoluminescence(SBSL)is studied.Through comparative studies towards SBSL effect in pure water as well as in CDs aqueous solution,we observe emission from two mechanisms in SBSL spectrum,a continuous spectrum is from blackbody radiation of plasma,and characteristic spectra from excited molecule or atom,which can be well fitted by a combination of a blackbody radiation formula and multiply Gaussian functions.We find that the presence of CDs significantly reduce the blackbody radiation temperature of SL and introduce new characteristic emission peaks.The chemical changes of CDs after SL treatment indicate that the carboxyl groups on the surface of CDs are involved in the plasma formation process in SL.The modulation of plasma species by CDs provides new information for understanding the mechanism of SL.The innovations of this dissertation are:(1)The THz TDS is first applied to the study of the carrier dynamic of CDs,and the insulation-semiconductor transition of CDs is observed with the increase of THz frequency and temperature.The effective mass of the carrier of CDs is determined experimentally under the condition of non-cyclotron resonance.The key material physical parameters of CDs,such as carrier concentration,effective mass,relaxation time and localization factor,are obtained by means of optical and contactless measurement,which provides scientific research basis for the application of CDs in optoelectronic devices.(2)Applying CDs to the study of sonoluminescence effect is first tried,different modulation effects of CDs on MBSL and SBSL are observed,and different emission spectra are observed,which provides important information for understanding the microscopic mechanism of SL.