CuFeS₂ Quantum Dots and Highly Luminescent CuFeS₂ Based Core/Shell Structures: Synthesis, Tunability, and Photophysics

We report the synthesis of copper iron sulfide (CuFeS₂) quantum dots (QDs). These materials exhibit a tunable band gap that spans the range 0.5–2 eV (600–2500 nm). Although the as-prepared material is nonemissive, CuFeS₂/CdS core/shell structures are shown to exhibit quantum yields that exceed 80%. Like other members of the I–III–VI₂ family QDs, CuFeS₂ based nanoparticles exhibit a long-lived emission that is significantly red-shifted compared to the band gap. CuFeS₂ QDs are unique in terms of their composition. In particular, these QDs are the only band-gap-tunable infrared chromophore composed entirely of elements with atomic numbers less than 30

Optical Signatures of Impurity–Impurity Interactions in Copper Containing II–VI Alloy Semiconductors

We study the optical properties of copper containing II–VI alloy quantum dots (Cu_<i>x</i>Zn_<i>y</i>Cd_{1–<i>x</i>–<i>y</i>}Se). Copper mole fractions within the host are varied from 0.001 to 0.35. No impurity phases are observed over this composition range, and the formation of secondary phases of copper selenide are observed only at <i>x</i>_Cu > 0.45. The optical absorption and emission spectra of these materials are observed to be a strong function of <i>x</i>_Cu, and provide information regarding composition induced impurity-impurity interactions. In particular, the integrated cross section of optical absorption per copper atom changes sharply (from 1 × 10 ^–2 nm³ to 4 × 10 ^–2 nm³) at <i>x</i>_Cu = 0.12, suggesting a composition induced change in local electronic structure. These materials may serve as model systems to understand the electronic structure of I–III–VI₂ semiconductor compounds

Optical Transparency Enabled by Anomalous Stokes Shift in Visible Light-Emitting CuAlS₂‑Based Quantum Dots

We observe and study the anomalous Stokes shift of CuAlS₂/CdS quantum dots. While all known I–III–VI₂ semiconductor core/shell quantum dots show Stokes shifts in excess of 100 meV, the shift associated with CuAlS₂/CdS quantum dots is uniquely large, even exceeding 1.4 eV in some cases. CuAlS₂/CdS quantum dots are thus associated with cross sections less than 10^–17 cm² under the emission maximum. We investigate this anomaly using spectroscopic techniques and ascribe it to the existence of a strong type-II offset between CuAlS₂ and CdS layers. Besides their strong Stokes shift, CuAlS₂/CdS quantum dots also exhibit high quantum yields (63%) as well as long emission lifetimes (∼1500 ns). Because of the combined existence of these properties, CuAlS₂/CdS quantum dots can act as tunable, transparent emitters over the entire visible spectrum. As a demonstration of their potential, we describe the construction of a wide area transparent lighting device with waveguided optical excitation and a clear aperture of 7.5 cm²

Efficient Photosynthesis of Organics from Aqueous Bicarbonate Ions by Quantum Dots Using Visible Light

We synthesized CuAlS₂/ZnS quantum dots (QDs) composed of biocompatible, earth-abundant elements that can reduce salts of carbon dioxide under visible light. The use of an asymmetric morphology at a type-II CuAlS₂/ZnS heterointerface balances multiple requirements of a photoredox agent by providing a low optical bandgap (∼1.5 eV), a large optical cross section (>10^–16 cm² above 1.8 eV), spatial proximity of both semiconductor components to the surface, as well as photochemical stability. CuAlS₂/ZnS QDs thus have an unprecedented photochemical activity in terms of reducing carbon dioxide in the form of aqueous sodium bicarbonate under visible light, without the need for a cocatalyst, promoter, or sacrificial reagent while maintaining large turnover numbers in excess of 7 × 10⁴ per QD. Devices based on these QDs exhibit energy conversion efficiencies as high as 20.2 ± 0.2%. These observations are rationalized through our spectroscopic studies that show short 550 fs electron dwell times in these structures. The high energy efficiency and the environmentally friendly composition of these materials suggest a future role in solar light harvesting

Why Does CuFeS₂ Resemble Gold?

While several potential applications of CuFeS₂ quantum dots have already been reported, doubts regarding their optical and physical properties persist. In particular, it is unclear if the quantum dot material is metallic, a degenerately doped semiconductor, or else an intrinsic semiconductor material. Here we examine the physical properties of CuFeS₂ quantum dots in order to address this issue. Specifically, we study the bump that is observed in the optical spectra of these quantum dots at ∼500 nm. Using a combination of structural and optical characterization methods, ultrafast spectroscopy, as well as electronic structure calculations, we ascertain that the unusual purple color of CuFeS₂ quantum dots as well the golden luster of CuFeS₂ films arise from the existence of a plasmon resonance in these materials. While the presence of free carriers causes this material to resemble gold, surface treatments are also described to suppress the plasmon resonance altogether

Behavior of Methylammonium Dipoles in MAPbX₃ (X = Br and I)

Dielectric constants of MAPbX₃ (X = Br, I) in the 1 kHz–1 MHz range show strong temperature dependence near room temperature, in contrast to the nearly temperature-independent dielectric constant of CsPbBr₃. This strong temperature dependence for MAPbX₃ in the tetragonal phase is attributed to the MA⁺ dipoles rotating freely within the probing time scale. This interpretation is supported by ab initio molecular dynamics simulations on MAPbI₃ that establish these dipoles as randomly oriented with a rotational relaxation time scale of ∼7 ps at 300 K. Further, we probe the intriguing possibility of transient polarization of these dipoles following a photoexcitation process with important consequences on the photovoltaic efficiency, using a photoexcitation pump and second harmonic generation efficiency as a probe with delay times spanning 100 fs–1.8 ns. The absence of a second harmonic signal at any delay time rules out the possibility of any transient ferroelectric state under photoexcitation