While several potential applications of CuFeS 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 CuFeS 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 CuFeS quantum dots as well the golden luster of CuFeS 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.
We synthesized CuAlS 2 /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 CuAlS 2 /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 cm 2 above 1.8 eV), spatial proximity of both semiconductor components to the surface, as well as photochemical stability. CuAlS 2 /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 × 10 4 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.
We observe and study the anomalous Stokes shift of CuAlS/CdS quantum dots. While all known I-III-VI semiconductor core/shell quantum dots show Stokes shifts in excess of 100 meV, the shift associated with CuAlS/CdS quantum dots is uniquely large, even exceeding 1.4 eV in some cases. CuAlS/CdS quantum dots are thus associated with cross sections less than 10 cm under the emission maximum. We investigate this anomaly using spectroscopic techniques and ascribe it to the existence of a strong type-II offset between CuAlS and CdS layers. Besides their strong Stokes shift, CuAlS/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, CuAlS/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 cm.
This study describes the optoelectronic characteristics of CuFeS2/Si nanocrystal/bulk heterojunctions. These heterojunctions show a strong photocurrent response under ambient conditions upon excitation from a wide optical spectrum, from 460 to 2200 nm. The devices comprise of a heterojunction formed between heavily doped n‐type silicon (1–100 Ω cm) and copper iron sulfide (CuFeS2) nanocrystal films. Over the spectral range 460–2200 nm the device shows a fast response (20 µs at NIR wavelengths), along with responsivity and detectivity of 4.68 mA W−1 and 5.29 × 109 Jones at 1900 nm wavelength. The photocurrent is further observed to be a nonlinear function of power. These properties of the devices are discussed in terms of a defect filling mechanism. Besides their regular photoresponse described above, the devices also exhibit a slower photothermal response, allowing these to also sense hot objects (450 K; excess 6 mW incident onto the device) within the focal plane, thereby extending the useful sensing range of the devices deeper into the infrared.
CuAlS2/ZnS Quantum dots (QDs) are known to directly convert aqueous solutions of bicarbonate ions to oxygen and organic molecules such as formate with a remarkable efficiency even under sunlight. In cases, fairly complicated organic reaction products such as acetate and methanol have been observed when reactions are allowed to continue for longer periods of time. Here, we investigate the electron dynamics that occurs within CuAlS2/ZnS QDs and show that it is essentially dominated by ultrafast electron transfer (560 fs for 0.4 excitons per dot) to the surface. The electron dwell time in the conduction band increases exponentially (for example 872 fs for 1.4 excitons per dot) with the excitation fluence. This is reverse of the auger limited response of conventional QDs and is hypothesized to exhibit strong charge separation that lies at the root of the remarkable photocatalytic activity. We further investigate this system through multi‐pump experiments. We find that the system response to prior excitation changes over the period of nanoseconds, consistent with the charge reorganization in the system, well after the initial electron transfer. The results of these experiments are summarized in terms of a coulomb‐well interpretation.
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