Bangle Rachel E, Li Hengming, Mikkelsen Maiken H
Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina 27708, United States.
ACS Nano. 2023 Dec 12;17(23):24022-24032. doi: 10.1021/acsnano.3c08915. Epub 2023 Nov 28.
The nonlinear conversion of photons from lower to higher energy is important for a wide range of applications, from quantum communications and optoelectronics to solar energy conversion and medicine. Triplet-triplet annihilation upconversion (TTA UC), which utilizes an absorber/emitter molecular pair, is a promising tool for upconversion applications requiring low intensity light such as photovoltaics, photocatalysis, and bioimaging. Despite demonstrations of efficient TTA UC in solution, practical applications have proven difficult, as thin films retard the necessary energy transfer steps and result in low emission yields. In this work, TTA UC emission from a thin film is greatly enhanced through integration into plasmonic nanogap cavities consisting of a silver mirror, a nanometer-scale polymer spacer containing a TTA molecular pair, and colloidally synthesized silver nanocubes. Mechanistic studies performed by varying the nanocube side length (45-150 nm) to tune the nanogap cavity resonance paired with simulations reveal absorption rate enhancement to be the primary operative mechanism in overall TTA UC emission enhancement. This absorption enhancement decreases the TTA UC threshold intensity by an order of magnitude and allows TTA UC emission to be excited with light up to 120 nm redder than the usable wavelength range for the control samples. Further, combined nanogap cavities composed of two distinct nanocube sizes result in surfaces which simultaneously enhance the absorption rate and emission rate. These dual-size nanogap cavities result in 45-fold TTA UC emission enhancement. In total, these studies present TTA UC emission enhancement, illustrate how the usable portion of the spectrum can be expanded for a given sensitizer-emitter pair, and develop both mechanistic understanding and design rules for TTA UC emission enhancement by plasmonic nanostructures.
光子从低能量到高能量的非线性转换对于从量子通信、光电子学到太阳能转换和医学等广泛的应用都很重要。三线态-三线态湮灭上转换（TTA UC）利用吸收体/发射体分子对，是一种有前途的工具，可用于需要低强度光的上转换应用，如光伏、光催化和生物成像。尽管在溶液中已证明TTA UC有效，但实际应用却很困难，因为薄膜阻碍了必要的能量转移步骤并导致低发射产率。在这项工作中，通过将薄膜中的TTA UC发射集成到由银镜、包含TTA分子对的纳米级聚合物间隔层和胶体合成的银纳米立方体组成的等离子体纳米间隙腔中，大大增强了发射。通过改变纳米立方体边长（45-150 nm）来调整纳米间隙腔共振并结合模拟进行的机理研究表明，吸收速率增强是整体TTA UC发射增强的主要作用机制。这种吸收增强将TTA UC阈值强度降低了一个数量级，并允许用比对照样品可用波长范围红120 nm的光激发TTA UC发射。此外，由两种不同尺寸的纳米立方体组成的组合纳米间隙腔产生的表面同时增强了吸收速率和发射速率。这些双尺寸纳米间隙腔使TTA UC发射增强了45倍。总的来说，这些研究展示了TTA UC发射增强，说明了如何为给定的敏化剂-发射体对扩展光谱的可用部分，并发展了对等离子体纳米结构增强TTA UC发射的机理理解和设计规则。
