Colloidal quantum dots (CQDs) are semiconductor nanoparticles that have unique size- and shape-dependent optical properties. Early research projects focused only on Group II-VI CQDs, which contain lead and cadmium. Although group II-VI CQDs provide high quality optical properties and stability, due to the restriction of hazardous substances (RoHS) by the EU, it is difficult to commercialize group II-VI CQDs.
Recently, research has been conducted to try to find a new type of non-toxic CQDs such as GaP. The first topic is 'Synthesis of InP colloidal quantum dots with white phosphorus for a new phosphine precursor'. Most colloidal quantum dots of InP are synthesized with tris(trimethylsilyl)phosphine [(TMS)3P], yielding monodisperse and highly crystalline InP CQDs.
The second topic is “Synthesis of colloidal quantum dots from In1-xGaxP alloys and their optical properties.” Although In1-xGaxP CQDs have been reported, the effect of indium/gallium ratio on optical properties has not been well investigated. In this topic, In1-xGaxP/ZnS (0 ≤ x ≤ 1) core shell CQDs with different In/Ga ratios were synthesized.
The resulting In1-xGaxP/ZnS alloy CQDs were characterized by X-ray diffraction (XRD) pattern, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-mass spectrometry (ICP-MS), UV-Vis absorption spectroscopy, photoluminescence (PL) spectroscopy, temperature-dependent photoluminescence (TDPL) and so on.
Introduction of quantum dots
Introduction of quantum dots
Optical properties; UV-Vis absorption, PL, TDPL, TRPL, PLQY
Core-shell, doping, alloy
Introduction of quantum dots
Quantum dots application light emitting diode
Quantum dots application for solar cell
In the early 1880s, Charles Fritts invented the first solar cell with a layer of selenium on a metal plate with a thin layer of gold on the selenium. Among them, CQDs have great advantages; high carrier mobility, large UV to IR absorption, and available solution process. After the first CQD28 solar cell in 2005, much research has been done to achieve high power conversion efficiency, and the efficiency increases dramatically (Figure 1.8).
The CQD's solar cells have different structures; Schottky, depleted heterojunction, depleted bulk heterojunction, CQD-sensitized solar cell, tandem structure and quantum funnel structure. The Schottky structure was the first CQDs solar cell to bring a thin CQDs film into contact with both transparent conductive oxide and metal. This structure is very simple and easy to manufacture, but has critical drawbacks; low hole barrier between CQDs film and metal, Schottky barrier and poor absorption.
Due to the low hole barrier, the excited hole is transferred to the metal and recombines, causing carrier leakage, and the Schottky barrier also reduces the open circuit voltage. The CQD31-sensitized solar cell has the same structure as the dye-sensitized solar cell, which uses a wide band-gap semiconductor as the transfer layer, and the adsorbed CQD generates excitons with incident light. Some carriers are transferred through the wide-bandgap semiconductor, while other carriers are transferred to the metal by the electrochemical reaction of the electrolyte.
CQDs have greater stability and a wide absorption range than dyes, resulting in higher efficiency CQD-sensitized solar cells. A depleted heterojunction is developed to combine the advantages of a Schottky solar cell and a CQD-sensitized solar cell. The depleted bulk heterojunction is an advanced structure of the depleted heterojunction, whose wide-bandgap semiconductor nanopillar reduces the exciton diffusion length.
By reducing the exciton diffusion length, exciton recombination decreases and thicker absorber film is available to increase current density. The graded bandgap of CQDs gives diversion power for carrier transfer to improve fill factor32. First is surface defect of CQDs which have new energy level between conduction band and valence band.
Synthesis of InP colloidal quantum dots with white phosphorus for new
Experimental session
- Materials
- Preparation of InP CQDs with white phosphorus
- Characterizations of InP CQDs
The Nozik group39, but do not show well the effect of the In/Ga ratio on the optical properties. In this topic, In1-xGaxP/ZnS (0 ≤ x ≤ 1) core-shell CQDs are synthesized with different In/Ga ratios and analyzed by different optical property analysis methods; UV-Vis spectroscopy, PL spectroscopy, TDPL spectroscopy and TRPL spectroscopy. Also XRD, XPS, ICP-MS provide information on the structure, binding energy and In/Ga ratio of each CQD.
In this subject, the band gaps of In1- xGaxP CQDs are controlled from 1.93 eV to 2.60 eV as In/Ga ratio changes, and the PLQY decreases as the Ga ratio increases. The XRD measurements were taken to determine the crystal structure of CQDs having different In/Ga ratios. There are several reports of In1-xGaxP CQDs40, but they do not show the effect of In/Ga ratio on optical properties.
The XRD patterns of InxGa1-xP shift from InP to GaP as the Ga ratio increases, and this result indicates that InxGa1-xP is an alloy structure. In In1-xGaxP, x is not the actual In/Ga ratio in the CQD, but the prior In(OA)3/Ga(acac)3 ratio. The actual In/Ga ratio is measured by ICP-MS, which is an efficient elemental analysis.
As the size of the CQDs decreases, the band gap of the CQDs becomes small, and this phenomenon makes it difficult to see the effect of the In/Ga ratio which changes the optical properties of the In1-xGaxP/ZnS alloy CQDs. Both the first exciton peak of the absorption spectrum and the peak of the PL spectrum shift to the blue region with increasing Ga ratio. In the absorption spectra, the first exciton peak shifts from 580 nm to 440 nm as the Ga ratio increases, and GaP does not show the first exciton peak.
Since bulk GaP has an indirect band gap that does not emit light, the band gap of In1-xGaxP/ZnS changes from direct to indirect band gap as the Ga ratio increases. These results show that the defect density increases as the Ga ratio increases, and in addition, these defects decrease the PLQY, increase the shoulder peak, and affect the blue shift. The XRD patterns gradually move from InP to GaP as the In/Ga ratio changes, and so do the XPS spectra.
Result and discussion
- Optical properties of InP CQDs
- TEM analysis of InP CQDs
Conclusions
The properties of CQDs can be controlled by the quantum confinement effect, which is related to the band gap. Therefore, to control the properties of CQDs, changing the band gap is necessary, and the alloy structure of CQDs can be one of the main strategies. Theoretically, by changing the In/Ga ratio, In1-xGaxP can have eV in the bulk case, and in the case of CQDs, a larger band gap can be achieved due to the quantum confinement effect.
The In1-xGaxP/ZnS alloy CQDs contain a higher actual In/Ga ratio than the In(OA)3/Ga(acac)3 precursor ratio, due to the lower reactivity of the Ga precursor. As the Ga ratio increases, the peak positions of the In 3d peaks shift to lower energy states and the full width half maximum (FWHM) of the peaks increases slightly. The Ga 3d peaks are located at 17.3 eV which represent the energy state of GaP42 and shift to higher energy level and also broaden with increasing Ga ratio.
This peak shift provides direct evidence that the In1-xGaxP/ZnS CQDs have a metal-P bonding state with a homogeneous In and Ga alloy state. The first is to change the forbidden band by changing the particle size; however, the particle sizes are controlled to remove the factor. The blue shift of the peaks in both spectra is a foregone conclusion that the band gap of GaP is larger than that of InP.
Randomization of crystal lattice also makes non-radiative recombination process and increases exciton-phonon coupling which can reduce PLQY as increase of Ga ratio. The large Ga ratio increases lattice randomness, defect density which increases shoulder peak and decreases PLQY. This subject presents a comprehensive overview of the optical properties of the In1-xGaxP/ZnS CQDs.
In1-xGaxP/ZnS CQDs (0 ≤ x ≤ 1) are successfully synthesized by hot injection method and ZnS shell is synthesized by simple injection of TOP solution, sulfur as sulfur source. Also TEM images show In1-xGaxP/ZnS CQDs have similar sphere shape and negligible particle size. In the rich state, there is no defect-induced PL, but with increasing Ga ratio, defect-induced PL occurs and this can be determined by TDPL.
Experimental session
- Materials
- Preparation for indium oleate, zinc oleate stock solution
- Preparation of In 1-x Ga x P/ZnS CQDs
- Characterization of InP CQDs
In0.5Ga0.5P/ZnS spectra show increased shoulder peak intensity, as well as a decrease in shoulder peak as temperature increases. Also, the In0.75Ga0.25P/ZnS spectra show a small blue shift and a large shoulder peak, which decreases as the temperature increases. Portable red-green-blue quantum dot light-emitting diode array with high-resolution intaglio printing.
Park, J; Kim, SW CuInS2/ZnS Core/Shell Quantum Dots by Cation Exchange and Their Blue-Shifted Photoluminescence. Panda, SK; Hickey, SG; Demir, HV Bright White-Light Emitting Manganese and Copper Doped Quantum ZnSe. Bright and efficient light-emitting colloidal quantum dot diodes using an inverted device structure.
Portable Red-Green-Blue Quantum Dot Light-Emitting Diode Array Using High-Resolution Gravure Transfer Printing. McDonald, SA; Constantatos, G; Zhang, S; Cyr, PW Solution-Processed PbS Quantum Dot Infrared Photodetectors and Photovoltaics. Kagan, CR; Murray, CB; Bawendi, MG Long-Range Resonance Transfer of Electronic Excitations in Close-Packed CdSe Quantum-Dot Solids.
Result and discussion
- XRD, ICP-MS, and TEM analysis for structure and elemental information
- XPS analysis for identification of alloy condition
- Optical properties of In 1-x Ga x P/ZnS with various In/Ga ratio
Conclusions
Amorphous TiO2 Thin Shell on CdSe Nanocrystal Quantum Dots Enhances Photocatalysis of Hydrogen Evolution from Water. Synthesis of fluorescent, radio-opaque and paramagnetic water-dispersed CdS: Mn/ZnS quantum dots: A multifunctional probe for biological imaging. Timonen, J.; Seppälä, ET; Ikkala, O From hot injection synthesis to heating synthesis of cobalt nanoparticles: Observation of kinetically controllable nucleation.
Synthesis of Cu-doped InP nanocrystals (d-dots) with ZnSe diffusion barrier as efficient and color-tunable NIR emitters. Kuo, Effect of n-type AlGaN layer on carrier transport and efficiency decay of blue InGaN light-emitting diodes, IEEE Photon. Lee, YL; Lo, YS Highly efficient quantum-dot-sensitized solar cell based on co-sensitization of CdS/CdSe.
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