Preparation And Optical/Photoelectric Properties Of Cu-Fe-S Quantum Dots

In past decades,fluorescent quantum dots?QDs?have been widely studied due to their unique properties such as tunable bandgap,excellent photoelectric conversion efficiency,high luminescence efficiency and soluble preparation.Luminescent colloidal semiconductor nanocrystals are of significant technology interest as they impact many applications including light emitting diodes?LEDs?,solar cells,biomedical labeling and biosensors.While existing examples of light emitting semiconductor nanocrystals such as II-VI,II-V,III-V,IV-IV have size tunable band gap,which show various emission spectra depending on the sizes.At an extend work,I-III-VI semiconductor nanocrystals such as CuInS₂ have been successfully prepared recently due to not only their size tunable bands but composition dependent optical properties.Recently,Cu-Fe-S QDs with bandgap from 0.5 eV to 2.1 eV,a member of I-III-VI family QDs,has been identified as a candidate in the optoelectronic applications due to their unique properties.However,no photoluminescence is observable from existed Cu-Fe-S QDs,although their colloidal nanoparticles have been reported in literature.To further insight into the optical properties,the synthesis and characterization of Cu-Fe-S QDs were systematically investigated in current thesis.The main content of the thesis are as follows:1.Both CuFeS₂ and Cu₅FeS₄ QDs were synthesized by hot injection method.The size of as prepared samples was controllable by changing the reaction temperature.As a result,the CuFeS₂ QDs was obtainable in a size range of 3-6 nm.Accordingly,their absorption peak can be tuned from 475 to 493 nm.In contrast,the Cu₅FeS₄ QDs with a size range of 4-7 nm were obtained,showing tunable absorption peak from 535-610nm.Importantly,these particles showed high monodispersity and good size distribution.To study the optoelectronic properties of these QDs,thin-film optoelectronic devices were fabricated with the two kinds of samples separately including CuFeS₂ QDs and Cu₅FeS₄ QDs.Experimental result indicated that the device based on CuFeS₂ QDs show a significant increase in current by ca.10 times under illumination relative to the dark state,indicating its potential application in the field of photovoltaic.2.Improvement of the optoelectronic properties of semiconductor nanocrystals is still a prominent research topic.One of the most important approaches is fabricating composite type-I core-shell structures which exhibit improved properties.In this chapter,CdS shell was used to passivate the CuFeS₂ QDs by preparation of CuFeS₂@CdS core-shell structure.The as-synthesized sample showed photoluminescence emission,which could be tunable by changing the core size and shell thickness.As a result,the as-prepared sample showed tunable emission from 780to 822 nm.Furthermore,the core-shell samples showed high quantum efficiency?reach up to 15%?,and good chemical and photochemical stability.3.A simple synthetic route for preparation of colloidal Cd-Cu-Fe-S QDs for the first time was developed by hot-injection method.The effects of different reaction conditions including reaction time,additional amount of precursors and reaction temperature were detailed investigated.As a result,quaternary Cd-Cu-Fe-S QDs exhibiting a strong size tunable photoluminescence were synthesized for the first time by tuning the reaction temperature from 120°C to 210°C.The preparation procedure involved cadmium acetate,copper acetate,iron chloride,and sulfur powder dissolved in oleylamine as precursors.The wavelength of the emission could be tuned from 630nm to nearly 1000 nm by only changing the size of the Cd-Cu-Fe-S QDs from 3.0 nm to 8.0 nm.Interestingly,these QDs possess a relatively high quantum yield of over 57%and excellent chemical stability without coating any wide-band-gap shell materials.The study on the optoelectronic properties of Cd-Cu-Fe-S QDs,where an order of photocurrent was enhanced under AM1.5 illumination,demonstrated their suitability as optically active components to fabricate optoelectronic devices.In conclusion,the work in this dissertation serves to explore the synthesis and optical/optoelectronic properties of Cu-Fe-S quantum dots.Controllable fabrication of Cu-Fe-S QDs and CuFeS₂@CdS core-shell QDs has been realized.And a one-step method has been developed to obtain Cd-Cu-Fe-S quaternary quantum dots.By controlling the size of the quantum dots,the fluorescence spectrum can be tuned to near infrared region.Through the study of the stability,we finally obtain stable fluorescent quantum dots with a strong tunable photoluminescence and a relative high quantum yield,demonstrating their suitability and potential in practical applications.