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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Article
  • Peer Reviewed
UC Riverside Previously Published Works (2017)
Molecular control of energy transfer is an attractive proposition because it allows chemists to synthetically tweak various kinetic and thermodynamic factors. In this Perspective, we examine energy transfer between semiconductor nanocrystals (NCs) and π-conjugated molecules, focusing on the transmitter ligand at the organic-inorganic interface. Efficient transfer of triplet excitons across this interface allows photons to be directed for effective use of the entire solar spectrum. For example, a photon upconversion system composed of semiconductor NCs as sensitizers, bound organic ligands as transmitters, and molecular annihilators has the advantage of large, tunable absorption cross sections across the visible and near-infrared wavelengths. This may allow the near-infrared photons to be harnessed for photovoltaics and photocatalysis. Here we summarize the progress in this recently reported hybrid upconversion platform and point out the challenges. Since triplet energy transfer (TET) from NC donors to molecular transmitters is one of the bottlenecks, emphasis is on the design of transmitters in terms of molecular energetics, photophysics, binding affinity, stability, and energy offsets with respect to the NC donor. Increasing the efficiency of TET in this hybrid platform will increase both the up- and down-conversion quantum yields, potentially exceeding the Shockley-Queisser limit for photovoltaics and photocatalysis.
    Cover page: Designing Transmitter Ligands That Mediate Energy Transfer between Semiconductor Nanocrystals and Molecules
    • Thesis
    • Peer Reviewed
    UC Riverside Electronic Theses and Dissertations (2017)

    The sequential absorption of two low energy photons, followed by the emission of one higher energy photon, is known as photon upconversion. This two-photon process has potential applications in biological imaging, solar energy conversion and photocatalysis etc. Two existing strategies to realize upconversion are an inorganic system based on lanthanide doping, and an organic system based on triplet-triplet annihilation. This dissertation introduces a novel hybrid molecule-nanocrystal system that upconverts photons across visible to near-infrared. It combines the advantages of high photostability and low excitation intensity, and overcomes the limits in the previous systems. Here, the absorption of low energy photons by the semiconductor nanocrystals (NCs) is followed by the energy transfer to molecular triplet states. Triplet-triplet annihilation then occurs to create high energy singlet states that emit the upconverted light. I show the introduction of functionalized acene molecules on NC surfaces greatly enhances the upconversion quantum yields (QYs) by up to three orders of magnitidue in different systems. As one of the efficiency limiting steps in upconversion, TET is systematically studied by modifying both NC donor and molecular acceptor. TET is characterized by steady state upconversion, time-resolved transient absorption and photoluminescence spectroscopies. TET is dependent on the anchoring groups of the transmitter molecules due to different binding affinities, docking geometries, sterics, intra-molecular spin-orbit couplings and molecule-NC triplet-triplet couplings. Transmitters with phosphonic acid, carboxylic acid and imidazole as binding groups show the best upconversion performance. For the NC donors, small sizes lead to high upconversion QYs due to the large driving force. While the growth of inorganic shells passivates surface traps, TET from core-shell NCs to transmitters is dependent on the shell composition and thickness. TET is improved when submonolayer shells suppress charge transfer and decrease exciton-phonon coupling.

      Cover page: Hybrid Molecule-Nanocrystal Photon Upconversion Across the Visible and Near-Infrared
      • Article
      • Peer Reviewed
      UC Riverside Previously Published Works (2016)
      We designed and synthesized a tetracene derivative 4-(tetracen-5-yl)benzoic acid (CPT) as a transmitter ligand used in PbS/PbSe nanocrystal (NC) sensitized upconversion of near infrared (NIR) photons. Under optimal conditions, comparing CPT functionalized NCs with unfunctionalized NCs as sensitizers, the upconversion quantum yield (QY) was enhanced 81 times for 2.9 nm PbS NCs from 0.021% to 1.7%, and 11 times for 2.5 nm PbSe NCs from 0.20% to 2.1%. The surface density of CPT controls the solubility of functionalized NCs and the upconversion QY. By increasing the concentration of CPT in the ligand exchange solution, the number of CPT ligand per NC increases. The upconversion QY is maximized at a transmitter density of 1.2 nm-2 for 2.9 nm PbS, and 0.32 nm-2 for 2.5 nm PbSe. Additional transmitter ligands inhibit photon upconversion due to triplet-triplet annihilation (TTA) between two neighboring CPT molecules on the NC surface. 2.1% is the highest reported QY for TTA-based photon upconversion in the NIR with the use of earth-abundant materials.
        Cover page: Ligand enhanced upconversion of near-infrared photons with nanocrystal light absorbers
        • Article
        • Peer Reviewed
        UC Riverside Previously Published Works (2017)
        Owing to the difficulty in comprehensively characterizing nanocrystal (NC) surfaces, clear guidance for ligand design is lacking. In this work, a series of bidentate bis(pyridine) anthracene isomers (2,3-PyAn, 3,3-PyAn, 2,2-PyAn) that differ in their binding geometries were designed to find the best complementary fit to the NC surface. The efficiency of triplet energy transfer (TET) from the CdSe NC donor to a diphenylanthracene (DPA) acceptor mediated by these isomers was used as a proxy for the efficacy of orbital overlap and therefore ligand binding. 2,3-PyAn, with an intramolecular N-N distance of 8.2 Å, provided the best match to the surface of CdSe NCs. When serving as a transmitter for photon upconversion, 2,3-PyAn yielded the highest upconversion quantum yield (QY) of 12.1±1.3 %, followed by 3,3-PyAn and 2,2-PyAn. The TET quantum efficiencies determined by ultrafast transient absorption measurements showed the same trend.
          Cover page: Complementary Lock‐and‐Key Ligand Binding of a Triplet Transmitter to a Nanocrystal Photosensitizer
          • Article
          • Peer Reviewed
          UC Riverside Previously Published Works (2017)
          Herein we report the first example of nanocrystal (NC) sensitized triplet-triplet annihilation based photon upconversion from the visible to ultraviolet (vis-to-UV). Many photocatalyzed reactions, such as water splitting, require UV photons in order to function efficiently. Upconversion is one possible means of extending the usable range of photons into the visible. Vis-to-UV upconversion is achieved with CdS/ZnS core-shell NCs as the sensitizer and 2,5-diphenyloxazole (PPO) as annihilator and emitter. The ZnS shell was crucial in order to achieve any appreciable upconversion. From time resolved photoluminescence and transient absorption measurements we conclude that the ZnS shell affects the NC and triplet energy transfer (TET) from NC to PPO in two distinct ways. Upon ZnS growth the surface traps are passivated thus increasing the TET. The shell, however, also acts as a tunneling barrier for TET, reducing the efficiency. This leads to an optimal shell thickness where the upconversion quantum yield (Φ'UC) is maximized. Here the maximum Φ'UC was determined to be 5.2 ± 0.5% for 4 monolayers of ZnS shell on CdS NCs.
            Cover page: CdS/ZnS core–shell nanocrystal photosensitizers for visible to UV upconversion
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