Investigation of the Optical Properties and Ultrafast Plasmonic Dynamics of Digenite (C₉S₅) Thin Films

Digenite (Cu₉S₅, or Cu_1.8S) is a sub-stoichiometric phase of copper sulfide (Cu₂S) characterized by a high carrier concentration due to its defect-heavy structure. Similarly to other copper chalcogenides, Cu₉S₅ is a degenerate p-type semiconductor whose nanocrystals (NCs) can display localized surface plasmon resonances (LSPRs) located in the near infrared (NIR) spectral region. The reason for this is that sub-stoichiometric copper chalcogenides like Cu_2-xS, Cu_2-xSe and Cu_2-xTe (with 0&lt;x&lt;1) are characterized by carrier concentrations comprised between 10¹⁹ and 10²² cm^-3, so that their LSPRs can be easily tuned in the IR by varying the amount of copper vacancies in the material.¹ Other important characteristics displayed by Cu₉S₅ are its high conductivity, non-toxicity, the abundance of its elemental components, and the low costs of fabrication.² These features make Cu₉S₅ a promising candidate for integration into hot-electron-harvesting photovoltaic devices with absorption in the NIR region. LSPRs have been extensively studied for Cu_2-xS NCs,¹ but the possibility of realizing thin films of Cu₉S₅ aggregated nanoparticles displaying the same properties of colloidal NCs has not been investigated yet.<br/><br/>In this work, the static and transient optical properties of drop-cast thin films of Cu₉S₅ NCs were studied. The samples were prepared starting from layered bulk Cu₉S₅ obtained through reactive annealing of copper.² A small amount of material was powdered in finer grains through a mortar and dispersed at different concentrations in various solvents, including ethylene glycol, DMF and DMSO. The most stable dispersion for drop-casting, which resulted to be the one in DMF with a concentration of 7.5 mg/L, was deposited on pre-heated glass slides. The samples were then characterized through ultraviolet photoelectron spectroscopy (UPS), scanning electron microscopy (SEM), and both static and ultrafast transient absorption (TA) spectroscopy. The results of UPS confirmed the p-type character of Cu₉S₅ and allowed to retrieve the work function of the material (3.7 ± 0.1 eV), while the static absorption measurements showed a broad peak in the NIR region, starting at 1 μm and progressively increasing towards longer wavelengths, reaching a peak around 2.2 μm. SEM measurements allowed to associate this broad resonance to the presence of flakes with variable sizes of few hundreds of nm. Ultrafast differential transmission measurements were then performed on the samples to study carrier dynamics in Cu₉S₅ following photoexcitation. The samples were pumped at 50 kHz repetition rate with 50-fs-long pulses centered around 1.65 μm, in the region of the broad NIR plasmonic resonance of the material, and probed in the 1.1-1.6 μm spectral range. The measurement was performed with a newly developed high sensitivity interferometric spectrometer exploiting a birefringent delay line interferometer, an amplified InAsSb detector and lock-in detection. The high temporal resolution and sensitivity granted by the setup allowed us to fully resolve the relaxation dynamics of the photoexcited holes, which are dominated by a fast decay (τ₁ ~ 485 fs) associated with hole-phonon scattering, followed by a long-lived bleaching (τ₂ &gt;&gt; 50 ps) associated to the trapping of holes/electrons in defect states. The plasmonic properties of Cu₉S₅ are therefore confirmed also for a thin film of aggregated NCs, and this opens the way to the possibility to harvest energy from the NIR portion of the solar spectrum by coupling Cu₉S₅ NCs films to a suitable hole acceptor material, thus trying to achieve hot-hole transfer.<br/><br/><b>Funding</b><br/><br/>European Research Council (grant agreement No. [816313]).<br/><br/><b>References</b><br/><br/>1 - I. Kriegel et al., "Tuning the Excitonic and Plasmonic Properties of Copper Chalcogenide Nanocrystals," <i>J. Am. Chem. Soc.</i> <b>2012</b>, <i>134</i> (3), 1583–1590.<br/>2 - A. Itzhak et al., "Digenite (Cu9S5): Layered p-Type Semiconductor Grown by Reactive Annealing of Copper," <i>Chem. Mater.</i> <b>2018</b>, <i>30</i> (7), 2379–2388.