Energy Transfer Dynamics and Dopant Luminescence in Mn-Doped CdS/ZnS Core/Shell Nanocrystals

Mn-doped II-VI semiconductor nanocrystals exhibit bright dopant photoluminescence that has potential usefulness for light emitting devices, temperature sensing, and biological imaging. The bright luminescence comes from the 4T1→6A1 transition of the Mn2+ d electrons after the exciton-dopant energy transfer, which reroutes the exciton relaxation through trapping processes. The driving force of the energy transfer is the strong exchange coupling between the exciton and Mn2+ due to the confinement of exciton in the nanocrystal. The exciton-Mn spatial overlap affecting the exchange coupling strength is an important parameter that varies the energy transfer rate and the quantum yield of Mn luminescence. In this dissertation, this correlation is studied in radial doping location-controlled Mn-doped CdS/ZnS nanocrystals. Energy transfer rate was found decreasing when increasing the doping radius in the nanocrystals at the same core size and shell thickness and when increasing the size of the nanocrystals at a fixed doping radius. In addition to the exciton-Mn energy transfer discussed above, two consecutive exciton-Mn energy transfers can also occur if multiple excitons are generated before the relaxation of Mn (lifetime ~10^-4 - 10^-2 s). The consecutive exciton-Mn energy transfer can further excite the Mn2+ d electrons high in conduction band and results in the quenching of Mn luminescence. The highly excited electrons show higher photocatalytic efficiency than the electrons in undoped nanocrystals. Finally, the effect of local lattice strain on the local vibrational frequency and local thermal expansion was observed via the temperature-dependent Mn luminescence spectral linewidth and peak position in Mn-doped CdS/ZnS nanocrystals. The local lattice strain on the Mn2+ ions is varied using the large core/shell lattice mismatch (~7%) that creates a gradient of lattice strain at various radial locations. When doping the Mn2+ closer to the core/shell interface, the stronger lattice strain softens the vibrational frequency coupled to the 4T1→6A1 transition of Mn2+ (Mn luminescence) by ~50%. In addition, the lattice strain also increases the anharmonicity, resulting in larger local thermal expansion observed from the nearly an order larger thermal shift of the Mn luminescence compared to the Mn-doped ZnS nanocrystals without the core/shell lattice mismatch

Energy Transfer Dynamics and Dopant Luminescence in Mn-Doped CdS/ZnS Core/Shell Nanocrystals

Mn-doped II-VI semiconductor nanocrystals exhibit bright dopant photoluminescence that has potential usefulness for light emitting devices, temperature sensing, and biological imaging. The bright luminescence comes from the 4T1→6A1 transition of the Mn2+ d electrons after the exciton-dopant energy transfer, which reroutes the exciton relaxation through trapping processes. The driving force of the energy transfer is the strong exchange coupling between the exciton and Mn2+ due to the confinement of exciton in the nanocrystal. The exciton-Mn spatial overlap affecting the exchange coupling strength is an important parameter that varies the energy transfer rate and the quantum yield of Mn luminescence. In this dissertation, this correlation is studied in radial doping location-controlled Mn-doped CdS/ZnS nanocrystals. Energy transfer rate was found decreasing when increasing the doping radius in the nanocrystals at the same core size and shell thickness and when increasing the size of the nanocrystals at a fixed doping radius. In addition to the exciton-Mn energy transfer discussed above, two consecutive exciton-Mn energy transfers can also occur if multiple excitons are generated before the relaxation of Mn (lifetime ~10^-4 - 10^-2 s). The consecutive exciton-Mn energy transfer can further excite the Mn2+ d electrons high in conduction band and results in the quenching of Mn luminescence. The highly excited electrons show higher photocatalytic efficiency than the electrons in undoped nanocrystals. Finally, the effect of local lattice strain on the local vibrational frequency and local thermal expansion was observed via the temperature-dependent Mn luminescence spectral linewidth and peak position in Mn-doped CdS/ZnS nanocrystals. The local lattice strain on the Mn2+ ions is varied using the large core/shell lattice mismatch (~7%) that creates a gradient of lattice strain at various radial locations. When doping the Mn2+ closer to the core/shell interface, the stronger lattice strain softens the vibrational frequency coupled to the 4T1→6A1 transition of Mn2+ (Mn luminescence) by ~50%. In addition, the lattice strain also increases the anharmonicity, resulting in larger local thermal expansion observed from the nearly an order larger thermal shift of the Mn luminescence compared to the Mn-doped ZnS nanocrystals without the core/shell lattice mismatch

The Real Problem of Bishop Sentences

Bishop sentences such as “If a bishop meets a bishop, he blesses him” have long been considered problematic for the descriptivist (or E-type) approach of donkey anaphora (e.g. Evans, 1977; Heim, 1990; and Neale, 1990). Elbourne (2005) offers a situational descriptivist analysis that allegedly solves the problem, and furthermore extends its explanatory coverage to bishop sentence with coordinate subjects. However, I throw serious doubts on Elbourne’s analysis. Specifically, I argue that the purported solution is committed to the use of unbound anaphora, and it cannot sustain the claimed empirical adequacy

Intentional Identity Revisited

The phenomenon of intentional identity has bemused philosophical communities since Geach (1967). I argue that the phenomenon is ubiquitous and much more significant than previously acknowledged. The foundations of the problem are implicated in many other well-knownpuzzles, such as Kripke’s (1979) puzzles about beliefs. Thus, the need for a proper analysis is eminently pressing. I specify a template for generalizing intentional identity, identify the challenges involved, and argue that positing a level of representational entity in both philosophy of mind and language(e.g., mental files) is a promising approach to tackling the problem across the board

Contextualism and the Semantics of "Woman"

Contextualist accounts of “woman,” including Saul (2012), Diaz-Leon (2016), and Ichikawa (2020), aim to capture the variability of the meaning of the term, and do justice to the rights of trans women. I argue that (i) there is an internal tension between a contextualist stance and the commitment to trans-inclusive language, and that (ii) we should recognize and tackle the broader and deeper theoretical and practical difficulties implicit in the semantic debates, rather than collapsing them all into semantics. Moving on, I sketch three strategies to help us advance feminist philosophical endeavors, including how attending to contextual matters can lead us to further reflect on the meta-contextual, such as our role in shaping contexts and whether the working of language is indicative of a larger oppressive social structure

De se marking, logophoricity, and ziji

This chapter addresses the assumed connection between de se attitude ascription and logophoricity in the case of Chinese ziji. It is widely believed that logophors are among the paradigm cases of de se marking, and that long-distance ziji is logophoric. Drawing on a critical examination of a variety of analyses, this chapter argues that long-distance anaphora, de se interpretation, and logophoric marking are overlapping but distinct phenomena. Even if ziji is logophoric, it does not automatically trigger de se requirement. A de se-neutral analysis of ziji is consistent with pragmatic derivations of interpretations that emphasize the self. The findings point to a new approach to long-distance binding, and identify the blocking effect as the key issue for further research

Solving the Proportion Problem: A Plea for Selectivity

I argues that quantificational adverbs are unselective binders over individuals. The Lewisian analysis, however, fails to recognize the ambiguity in some quantificationally modified conditionals. That the Lewisian approach cannot predict some attested reading is known as the “proportion problem.” I propose a solution based on the following ideas: (a) quantificational adverbs bind selectively; (b) a singular indefinite and its anaphoric pronoun may introduce a plural discourse referent, and (c) plural predication is elusive

CO_2 Reduction Selective for C_(≥2) Products on Polycrystalline Copper with N-Substituted Pyridinium Additives

Electrocatalytic CO_2 reduction to generate multicarbon products is of interest for applications in artificial photosynthetic schemes. This is a particularly attractive goal for CO_2 reduction by copper electrodes, where a broad range of hydrocarbon products can be generated but where selectivity for C–C coupled products relative to CH_4 and H_2 remains an impediment. Herein we report a simple yet highly selective catalytic system for CO_2 reduction to C_(≥2) hydrocarbons on a polycrystalline Cu electrode in bicarbonate aqueous solution that uses N-substituted pyridinium additives. Selectivities of 70–80% for C_2 and C_3 products with a hydrocarbon ratio of C_(≥2)/CH4significantly greater than 100 have been observed with several additives. ^(13)C-labeling studies verify CO_2 to be the sole carbon source in the C_(≥2) hydrocarbons produced. Upon electroreduction, the N-substituted pyridinium additives lead to film deposition on the Cu electrode, identified in one case as the reductive coupling product of N-arylpyridinium. Product selectivity can also be tuned from C_(≥2) species to H_2 (∼90%) while suppressing methane with certain N-heterocyclic additives