Synthesis Of Heterostructured Core/shell Quantum Dots CuInSeTe/CdS For Ultrafast Carrier Dynamic Properties

Semiconductor nanocrystals,so-called quantum dots(QDs)have attracted intense interest due to their outstanding optical properties including size-tunable emission,broad absorption spectrum,and high photoluminescence quantum yield(PLQY),facilitating their widespread usages in various biomedical and optoelectronic technologies,such as biomedical imaging,luminescent solar concentrators(LSCs),light-emitting diodes(LEDs),photoelectrochemical(PEC)cells for hydrogen production and QDs-sensitized solar cells(QDSCs).However,one of the crucial challenges for colloidal QDs is the intrinsic instability induced by the high surface sensitivity,leading to low physical and chemical stability that have a deteriorative effect on the performance of a variety of QDs-based biomedical and optoelectronic devices.Core/shell heterostructure engineering is a promising technique to optimize and enhance the photophysical properties of QDs.The shell can efficiently passivate the surface traps/defects to alleviate the non-radiative recombination of photoexcited carriers,thus resulting in improved PLQY,reduced photobleaching and enhanced photophysical-and chemical stability.An attractive feature of core/shell QDs is that their band structure can be engineered via tuning the composition of the core/shell materials and size.Nevertheless,there are still some potential properties of core/shell QDs that are highly desirable to realize:i)The tunable near-infrared(NIR)emission,especially in the NIR-II window(i.e.emission at～1000-1700 nm),which is significant for deep tissue penetration of NIR photons and favorable for NIR biomedical imaging and optical communication etc.ii)The ultralong carrier lifetimes,which is beneficial to efficient charge separation/transfer,can further boost the efficiency of g-QDs-based photovoltaic devices,photocatalysts etc.iii)The fundamental understanding of charge carrier dynamic processes including ultrafast photoinduced charge generation/transport in g-QDs and corresponding applications.To address these problems,this thesis focused on developing the NIR core/shell structured CQDs and systematically studied the growth,optical properties,electron-hole wavefunction and photoelectrochemical(PEC)application of these QDs,as summarized below:(1)Considering the band offset in bulk materials,the Cd S was fabricated as the shell materials coated on the CITS core,to form the quasi-type II band structure in such core/shell g-QDs.Since most"giant"core-shell quantum dots are spherical,non-spherical cores have been reported in recent years.Shell-structured quantum dots are very favorable for electron-hole spatial separation.In addition,most of the emission spectra of"giant"core-shell quantum dots are generally in the ultraviolet-visible region,and"giant"core-shell quantum dots with emission in the near-infrared region are also very promising research directions to expand this field.The as prepared heterostructured CITS/Cd S g-QDs showing tunable NIR emitting with various shell thicknesses.Then the morphology and crystal structure of g-QDs were investigated at diverse growth stages and proposed the corresponding growth mechanism.(2)Combining time-resolved THz and transient absorption spectroscopy,the carrier dynamics in core QDs and core/shell QDs were studied.In order to investigate the effective of charge separation by the non-spherical structure of QDs,the exciton lifetime was measured via time-resolved emission spectroscopy.The theoretical simulation was used to calculate the electron-hole wave function distribution of the pyramid-shaped"giant"core-shell quantum dots in different spatial directions.The experiment and theory revealed the optical properties and quasi type II band structure.The generation,separation and transfer process of photoinduced carriers in core-shell quantum dots were explored by transient spectroscopy and time-resolved terahertz spectroscopy.The ultrafast carrier dynamics mechanism in QDs can guide optoelectronic application.(3)The prepared quantum dot-based films showed excellent optical limiting properties.The nonlinear optical properties of films was measured by Z-scan technique.The results have proved that the films have obvious nonlinear optical responses under1064 nm wavelength light pulses.The nonlinear optical property mechanism of quantum dot films was studied by time-resolved terahertz spectroscopy.The photoinduced carriers have longer scattering time because of the quasi type-II band structure.The film conductivity of quantum dots was simulated via the Drude-Smith model,and the scattering time of photoinduced carriers from core-shell quantum dots was calculated to be～108 fs,while the scattering time of photoinduced carriers from core quantum dots was only 70 fs,Therefore,the core-shell structure quantum dot films exhibited enhanced optical limiting properties.(4)The QDs-based photoelectrochemical cell were fabricated by EPD technique and the thick shell core-shell QDs photoelectrochemical cell has a saturation photocurrent of～4.5 m A/cm~2,which is 9 times that of the core quantum dot device.The stability test verified the as-prepared core/shell QDs-photoanodes secure better device stability than that of the bare QDs under photo-irradiation.To confirm the origination of photocurrent,the incident-photon-to-electron conversion efficiency(IPCE)spectra of core and core/shell QDs-based PEC cells were evaluated from visible to the NIR region,showing largely enhanced conversion efficiency of core/shell QD-based devices in the visible region and decreased efficiency in the NIR region with respect to core QDs-based devices,which is corresponding to the absorption spectra of QDs.The saturation photocurrent density of the quantum dot device increases with thickness of the shell layer increase,and the device performance is enhanced.