Abstract
Free electrons on the metal surface will be driven to collective oscillation by incident light, exciting a strong electric field along the surface equivalent to an effective near-field confinement of incident light in the adjacency, well known as surface plasmons. Surface plasmons (SPs) are widely used in nanoscale light manipulation of specific wavelength, the light-matter interaction and so on, showing a promising application prospect in solar cells1, sensors2,3, nanoantennas4, imaging5, lasers6, light absorbers7, surface-enhanced Raman spectroscopy8 and others.
The non-radiative decay of SPs can excite one plasmonic hot electron. Then, this hot electron will experience continuous scattering with other electrons, exciting more and more hot electrons with redistribution of energy. Ultimately, all energy of hot electrons will be transferred to the lattice via electron-phonon scattering, called “cooling down” of hot electrons.9 Actually, these hot electrons can be fast extracted to semiconductors before complete transformation to heat through the Schottky junction. The hot electron injection can contribute to the performance of photodetection, photovoltaics, photocatalysis and photoelectrochemistry, attracting intense research interests.
This thesis consists of all my results based on plasmonic resonators and plasmonic hot electrons during my past more than three years. The main content is completed in the nano-optics group belonging to physical department in University of Otago under the supervision by Prof. Richard J. Blaikie and Dr. Boyang Ding, mainly supported by the Smart Ideas Fund by Ministry of Business, Innovation, and Employment, New Zealand through Contract No. UOOX1802 and the Marsden Fast-start Fund by Royal Society of New Zealand through Contract No. MFP-UOO1827.
Among my thesis, most of numerical simulation and experimental works are performed in our group lab in University of Otago including self-assembly preparation, optical interference lithography and angle-resolved absorption spectroscopy. Angle-resolved femtosecond transient absorption spectroscopy presented in Chapter 4 and 6 is performed in the lab of Westlake University belonging to Prof. Min Qiu. CsPbBr3-based photodetectors and corresponding I-V and I-T characteristics in Chapter 5 are performed in Prof. Qingyu Xu’s lab in Southeast University. CdSe/CdS colloidal quantum dots used in Chapter 6 are offered by Prof. Zeger Hens and Prof. Pieter Geiregat in Ghent University.
This thesis presents my researches on optoelectronic devices such as light absorbers, photodetectors and lasers based on SPs. Briefly, plasmonic resonators are used to manipulate the incident light at the subwavelength scale and excite hot electrons in order to obtain some specific properties. These results shown in following chapters are expected to provide an insight into further development of optoelectronic devices utilizing SPs and hot-electron injection.
The main research content of this thesis is divided into 5 chapters as below:
1. Chapter 2: Detailed background knowledge of surface plasmons, Scottoky junction and Plasmonic hot electrons is introduced.
2. Chapter 3: Primary experimental methods used in works of this thesis are demonstrated. They are Self-assembly methods, Optical interference lithography, Angle-resolved absorption spectroscopy and Angle-resolved femtosecond transient absorption spectroscopy.
3. Chapter 4: A type of bifacial light absorber based on free-standing SiO2-Ag core-shell resonators with highly symmetric structural configuration is developed, showing a high bifacial light absorption under a wide incident angle range of -40° to 40°. By changing the geometry size of the resonator arrays, the absorption band caused by the hybridized excitation of localized surface plasmons and whispery gallery modes in symmetric structures can be flexibly tuned. Angle-resolved transient absorption spectroscopy reveals that the energy absorbed by our absorbers ultimately transfers to excitation of hot electrons.
4. Chapter 5: We report a CsPbBr3-based photodetector engineered from a multilayer Si/Ag islands/CsPbBr3/PMMA system, showing an evidently enhanced photosensitization. On one hand, the photocurrent contribution from plasmonic hot-electron injection effectively extends the detection wavelength limit of our photodetectors much below the band edge of CsPbBr3, only depending on Schottky barrier. On the other hand, the surface plasmons on nanoscale silver islands can considerably improve the light harvesting ability of the CsPbBr3 layer, ascribed to the confinement of light in the adjacency of silver islands. Numerical simulations show the localized enhancement of light near silver islands, corresponding to the excitation of localized surface plasmon resonances (LSPRs). It shows a higher light intensity distribution inside the CsPbBr3 layer of the photodetector consisting of Si/Ag islands/CsPbBr3/PMMA with the photodetector with only Ag islands in accordance with their I−V characteristics. Ultimately, our plasmonic CsPbBr3–based photodetector presents a >10-fold increase in the photocurrent and a doubling of the operating lifetime.
5. Chapter 6: A zero-threshold optical gain in CdSe/CdS core/shell QDs is observed via plasmonic doping. Basically, plasmonic hot electrons injecting to single QD as pre-existing carriers fill unoccupied states in QDs’ conduction band, fully blocking the optical absorption. For bare QDs, their sub-wavelength sizes cause a rather low absorption rate to one pump photon, meaning a requirement of very high pump fluence to reach the threshold. In contrast, a surface plasmon excited by one pump photon can produced plenty of hot electrons, significantly reducing the threshold to zero. As a result, the plasmonic doping blocks the QD’s optical absorption, establishing population inversion with a very low pump power. Transient absorption spectroscopy demonstrates that we can achieve a near-zero threshold optical gain (~10-4) at 660 nm.