Optical Gain Based Sensing Using Colloidal Quantum Dots

Colloidal quantum dots(CQDs)are solution-processed semiconductor nanocrystals.They exhibit 3-dimensional quantum confinement due to the small size in all dimensions(<20 nm),which leads to many unique optical and transport properties compared with the corresponding bulk semiconductor materials.For example,size-dependent emission spectra,that is,the fluorescence emission from ultraviolet to near-infrared can be achieved by adjusting the size of CQDs.Besides,because of their extremely large surface-to-volume ratio,the fluorescence in CQDs is sensitive to their surrounding environments,the CQDs therefore can be useful optical probes of local environments.Although there are some reports using CQDs fluorescence to detect gas molecules,they generally suffer from low sensitivity and signal-to-noise due to the lack of signal amplification.In this thesis,we discuss how to improve the gas sensing performance by virtue of amplified spontaneous emission(ASE)of CQDs.Due to the optical gain property of ASE,we can simultaneously achieve high sensitivity and signal-to-noise in CQD-based gas sensing.The main research results of this thesis are listed as follows:1.Carrier dynamics model and rate equation are established according to the band structure of CQDs.The model can be utilized to analyze the carrier dynamics of excited state colloidal quantum dots including radiative and non-radiative processes.We also analyze the influence of nonradiative processes,such as Auger recombination and trap trapping,on the optical gain property.In addition,the relationship between the spontaneous emission intensity and stimulated emission intensity of CQDs and the average number of excitons is analyzed,respectively.It is theoretically concluded that colloidal quantum dot stimulated emission is more sensitive to the change of the average number of excitons than spontaneous emission.2.Experiments are conducted to verify the theoretical calculation results.Firstly,CdSe/CdS core-shell CQDs capped with organic oleic acid ligands were synthesized.By integrating the ASE excitation optical path with the gas-mixing path,we successfully realized a high-performance humidity sensor based on the intensity change of ASE of CQDs.Compared with the fluorescence intensity as the detection signal,the ASE intensity as the detection signal possess a 8 times higher sensitivity and has a higher signal-to-noise ratio.Whereas,the CQD films show poor reversibility due to high steric hindrance and small modal gain.3.Improving the sensing sensitivity of our humidity sensor by ligand exchange strategy.We can further improve the detection sensitivity and reversibility by substituting the long-chain organic ligands with the short chloride ligands.Compared with the sensitivity achieved by spontaneous emission from oleic-acid-passivated CQDs,we can achieve a 31 times enhancement in sensitivity at the ASE peak of chloride passivated CQD films.Moreover,the reversibility is also improved due to the reduced steric hindrance of short chloride ligands.Meanwhile,we found that the threshold power of ASE was reduced by 34%in a high humidity environment,providing an interesting avenue to the engineering of the lasing threshold.4.The mechanism behind the sensing and threshold reduction is analyzed.According to our transient absorption measurements,we reveal that there is some nanoradiative ultrafast trapping that drains the exciton in CdSe/CdS core-shell CQDs.The trapping can be mitigated by the surface absorbed water molecules and therefore the intensity of ASE or PL can be improved.