Synthesis of a size series of colloidal ZnTe/ZnSe (core/shell) quantum dots (QDs) is reported. Because of the unique Type-II characters, their emission can range over an extended wavelength regime, showing photoluminescence (PL) from blue to amber. The PL lifetime measures as long as 77 ns, which clearly indicates the Type-II characteristics. ZnTe/ZnSe (Core/Shell) QDs can be further passivated by ZnS layers, rendered in water, while preserving the optical and chemical stabilities and thus proved their potentials toward “nontoxic” biological or medical applications that are free from concerns regarding heavy-metal leakage. ZnTe/ZnSe Type-II QD/polymer hybrid organic solar cells are also showcased, promising environmentally friendly photovoltaic devices. ZnTe/ZnSe Type-II QD incorporated photovoltaic devices show 11 times higher power conversion efficiency, when compared to that of the control ZnSe QD devices. This results from the Type-II characteristic broad QD absorption up to extended wavelengths and the spatially separated Type-II excitons, which can enhance the carrier extractions. We believe that ZnTe/ZnSe-based Type-II band engineering can open many new possibilities as exploiting the safe material choice.
A model semiconductor-sensitizer layer of CdSe with under- or overlayers of CdS or ZnS by pre- or postadsorption was prepared on the surface of mesoporous TiO2 films by a series of successive ionic layer adsorption and reaction (SILAR) processes in solutions containing corresponding cations and anions. The growth of each semiconductor layer was monitored by taking UV−visible absorption spectra and high-resolution transmission electron microscopy (TEM) images. The all SILAR-prepared multicomponent sensitizer consisting of CdS/CdSe/ZnS layers was evaluated in a polysulfide electrolyte solution as a redox mediator in regenerative photoelectrochemical cells. The CdS and ZnS layers with the CdSe layer sandwiched in between were found to significantly enhance photocurrents. The best photovoltaic performance was obtained from the CdS/CdSe/ZnS-sensitizer with the ZnS layer on the top, yielding an overall power conversion efficiency of 3.44% with a mask around the active film and 3.90% with no mask. The effect of the mask on short-circuit current (J sc) and overall efficiency (η) measurements was shown to be increasingly critical in semiconductor-sensitized solar cells as they exhibit high photocurrents. The polysulfide electrolyte, which acted as an effective electron transfer mediator for CdS and/or CdSe sensitizers, was not as effective for PbS-based sensitizers prepared by the same SILAR process.
Temperature dependent photoluminescence (PL) spectroscopy in a range of 5 K to room temperature (RT, 290 K) and single dot blinking behavior were investigated for CdTe/CdSe (core/shell, C/S) quantum dots (QDs). The QDs show type-II characteristics as both of the valence and conduction band levels of the CdTe core are placed higher in energy than those of the CdSe shell. The thickness of the CdSe shell was varied to control the degree of type-II character, and bare CdTe QDs were used as controls. The CdTe/CdSe (C/S) QDs have unique PL properties including (i) high susceptibility to PL thermal quenching with an exciton dissociation energy as small as 18 meV, compared with 46 meV for the CdTe QD, (ii) smaller band gap change showing only half the reduction of the control within the temperature change, and (iii) up to 27% larger PL bandwidth broadening than the control. The unique temperature-dependent properties were enhanced as the type-II character was increased by the thicker CdSe shell. Single dot level PL intermittency characteristics were studied for quasi type-II CdTe/CdSe (C/S) QDs that have alloyed layers at the core-shell interface. The quasi type-II QDs exhibited more frequent PL intensity intermittence blinking on and off at 290 K when compared with the CdTe QDs. However, the blinking kinetics follows similar universal power law on/off probability distributions with the R on and R off exponents evaluated as 1.57 and 1.38, respectively.
Nearly monodisperse colloidal superstructures of cadmium chalcogenide quantum dots (QDs) are reported. The superstructures, which we named as supra quantum dot (SQD), are typically composed of hundreds of a-few-nanometer-sized QDs three-dimensionally (3D) assembled by oriented attachment. The synthesis route for SQD is quite universal and can be extended to CdS, CdSe, CdTe, and CdSeTe alloy. The size of SQD can be tuned from tens of nanometers to over a hundred nanometers. In the case of CdSe SQD, zinc-blende seeds (primary QDs) act as the building block for the formation of the 3D assembled structures, SQDs, with discrete intermediates nanostructures. Primary seeds, 4 nm tetrahedral shaped QDs, assembled into a large tetrahedron of 20 nm. The 20 nm tetrahedrons, in turn, self-assembled into a larger tetrahedron of 40 nm. The discrete-in-size and sequential assemblies were followed by conventional growth from the remaining precursors and ripening within the particles to result in spheroidal SQDs. SQDs allow surface ligand exchange without losing the structural integrity. Size-selective precipitation of SQDs can provide monodisperse SQDs that can assemble into ordered superlattices. The size and composition tunability of SQDs and their capability to form superlattices can provide a new solution-processable building block for superstructure with programmable physical and chemical properties.
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