Photon upconversion via triplet–triplet annihilation creating light in the high-energy regime of the visible spectrum could prove particularly useful in applications such as photocatalysis. Quantum-confined semiconductor nanocrystals have previously been shown to function as efficient triplet sensitizers in green-to-blue upconversion schemes, but an improvement in the apparent anti-Stokes shift of the upconversion scheme will be beneficial for future commercial applications. Additionally, both zero- and two-dimensional quantum-confined sensitizers have been investigated but not one-dimensional nanomaterials. In this work, we fill a hole in the present field of photon upconversion by accomplishing both of these feats. Specifically, we introduce CdTe nanorods as a new class of triplet sensitizers for red-to-blue photon upconversion. When the triplet transmitter ligand (9-anthracenecarboxylic acid) and triplet annihilator (9,10-diphenylanthracene) are added to the nanorods, we observe efficient photon upconversion at a normalized upconversion efficiency of ηuc = 4.3% and a low threshold power of I th = 93 mW/cm2. The introduction of one-dimensional triplet sensitizers could yield future research that effectively harnesses the unique properties of these materials, allowing for new approaches for efficient photon upconversion, especially at large apparent anti-Stokes shifts.
Plasmonic semiconductor nanocrystals (NCs) are a new and exciting class of materials that enable higher control of their localized surface plasmon resonance (LSPR) than metallic counterparts. Additionally, earth-abundant and non-toxic materials such as copper iron sulfides are gaining interest as alternatives to heavy metal-based semiconductor materials. Colloidal bornite (Cu5FeS4) is an interesting but underexplored example of a heavy metal-free plasmonic semiconductor. This report details the hot-injection synthesis of bornite yielding NCs ranging from 2.7 to 6.1 nm in diameter with stoichiometric control of the copper and iron content. The absorbance spectra of bornite NCs with different Cu:Fe ratios change at different rates as the particles oxidize and develop LSPR in the near-infrared region. X-ray photoelectron spectroscopy results indicate that oxidation produces sulfates rather than metal oxides as well as a decrease in the iron content within the NCs. Additionally, increasing iron content leads to decreases in carrier density and effective mass of the carrier, as determined by the Drude model. This controlled synthesis, combined with a further understanding of the relationship between the particle structure and optical properties, will enable the continued development and application of these fascinating heavy metal-free plasmonic semiconductor nanoparticles.
Correlating changes in carrier density arising from incorporating dopant ions into the lattice of a plasmonic semiconductor nanocrystals (NCs) with the observed physical properties at the elemental level can improve our understanding of these materials. Here, we investigate Sn:In 2 O 3 (ITO) NCs, a well-known near-infrared plasmonic system, by analyzing the induced carrier density changes that occur in the optical response and the 119 Sn nuclear relaxation rates. The carrier density, as evaluated by a chemical titration method, is correlated to the Burstein−Moss shift and the plasmon frequency to evaluate the effectiveness of the Drude-Lorentz model. Comparison of the values for carrier density extracted from these methods suggests the Drude and Burstein−Moss models underestimate the actual carrier density, particularly at higher concentrations, and that the parabolic approximation to the band structure is not appropriate for the ITO samples. The error in the fits can be accommodated using a modification in the Drude-Lorentz model to incorporate an additional correction value to account for the change in the local band shape as the Fermi level is moved with increasing carrier incorporation. The chemical shift and broadening of the 119 Sn solid-state NMR features provide a direct measure of the effects of carrier density on nuclear spin relaxation pathways. The 119 Sn signal of ITO NCs exhibits an increase in the full width at half-maximum with increasing carrier density, which can be related to the carrier-dependent T 2 * effects. The experimental results indicate the simple models are empirically predictive but require further evaluation to be quantitative.
Understanding the role of dopant deactivation on plasmon frequency and extinction is important for the rational design of plasmonic semiconductor nanocrystals (PSNCs). Aliovalent dopants do not always contribute a free carrier to a localized surface plasmon resonance (LSPR) for many reasons, including the existence of a depletion region, the pinning of carriers at neutral defect sites, or even the formation of a secondary insulating microphase (inclusions) not observable in the powder X-ray diffraction (pXRD). Here, we investigate such possibilities and their role in determining the LSPR frequency of Al-, Ga-, and In-doped ZnO NCs. Elemental analysis, pXRD, and absorption measurements are utilized to examine the impact of dopant incorporation on the resulting properties. Both simple and advanced effective mass Drude models are used to fit the mid-infrared plasmons, while one-electron oxidant chemical titrations are used as an independent measure of the free electron concentrations. The results of these analyses indicate that dopant/host lattice mismatch leads to inefficient carrier generation for aliovalent substitution, potentially due to local spinel-like inclusions. Smaller dopant ions are more likely to incorporate interstitially and form spinel phases, which results in an increased number of pinned carriers. Improved size matching from Al3+ to In3+ results in increased substitution efficiency and subsequently higher free carrier concentrations and LSPR frequencies. Drude model correction factors are calculated for each sample and compared to the literature value for n-ZnO determined via full band structure calculations. Each dopant is shown to have a unique correction factor, further illustrating the effect of differing ionic radii on the resulting LSPR.
Triplet sensitization of rubrene by bulk lead halide perovskites has recently resulted in efficient infrared-to-visible photon upconversion via triplet-triplet annihilation. Notably, this process occurrs under solar relavant fluxes, potentially paving the way toward integration with photovoltaic devices. In order to further improve the upconversion efficiency, the fundamental photophysical pathways at the perovskite/rubrene interface must be clearly understood to maximize charge extraction. Here, we utilize ultrafast transient absorption spectroscopy to elucidate the processes underlying the triplet generation at the perovskite/rubrene interface. Based on the bleach and photoinduced absorption features of the perovskite and perovskite/rubrene devices obtained at multiple pump wavelengths and fluences, along with their resultant kinetics, our results do not support charge transfer states or long-lived trap states as the underlying mechanism. Instead, the data points towards a triplet sensitization mechanism based on rapid extraction of thermally excited carriers on the picosecond timescale.
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