The traditional semiconductor device manufacturing way is top-down process by using etching techniques for miniaturization of material. With the improvement of technology, component size is possible to reach sub-micron, but if it is decreased further to the nanometer range, manufacturing process will face very harsh conditions and cannot be achieved easily by the present technology.
And recently, bottom-up process based on atoms or molecules have been gradually raised.
In recent years, semiconductor nanomaterials researches are already mature in basic principles, preparation or applications and in particular CdSe semiconductor nanoparticles are well exploited [6,7,8] .
According to the section described above, nanoparticles have a high surface area/volume ratio so that surface defect strongly influences the optical properties of nanoparticles. By using a larger gap of the organic ligand coating on the surface of the nanoparticles[8,9,10], or inorganic material attached [11,12,13], we can form a core-shell structure. It can be used to enhance stability against chemical degradation, and thus reduce the electron hole pair non-radiative recombination with the surface defects as well as reduce the nanoparticles aggregation. Most importantly, it can increase the quantum yield. In the following, we will introduce the evolution of CdSe nanoparticles and the core-shell structure synthesis technology.
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1.2.1 Chemical Colloidal Method
In 1993, from the Massachusetts Institute of Technology Murray's team first use of “Chemical Colloidal Method” in the organic phase synthesis of II-VI semiconductor quantum dots.[6] The dimethylcadmiun (CdMe2) as cadmium precursor, produces a single size distribution and high crystallinity of CdSe quantum dots. Compared to earlier “Lithography and Etching Processes”[18], Murray proposed a method which has a good reproducibility, and the ability to prepare quantum dots out of uniform size distribution. Since the particle size can be regulated by the reaction time in this method, spectra show a narrow absorption peak as well as band-edge emission peak. Following this success, most of synthesis of nanoparticles is practiced by using chemical colloidal method. However, CdMe2 is toxic and explosive. Despite the use of a strong coordination ability of TOP (trioctylphosphine), TOPO (trioctylphosphine
Murray
Fig. 1-5 Important discoveries over the years.[6,14,15,16,17]
Peng improve chemical colloidal method
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oxide), protection agents and solvent also have the cost and toxic problems. In addition, the reaction temperature up to 250 ~ 300 degrees also limits the further application.
In 2001, an improved method was proposed by Peng et al. [17] of which a relatively low toxicity and stable CdO, Cd(AcO)2 and CdCO3 successfully used to synthesize CdSe, CdS, CdTe quantum dots with high crystalline quality quantum dots and low cost.
1.2.2 Core/Shell Structure
In 1994, Mews et al. proposed a method to produce "core-shell structure"
of the quantum dots by chemical colloidal method, in which a layer of organic (or inorganic) compounds (such as zinc sulphide, zinc selenide, etc.) is added to the surface of quantum dots. This method limits the energy of excitation and then reduces non-radiative energy loss and increase photochemical stability. On the other hand, this method can reduce the lattice mismatch, and increase the efficiency of quantum dot emission.
In 1996, Hines used CdSe quantum dots as the core with the outer nuclear layer of ZnS to form a core/shell structure of the quantum dots. Compared to original CdSe quantum dots, the quantum efficiency of CdSe/ZnS quantum dots was increased by 6 times. The red-shift of photoluminescence spectroscopy peak of CdSe/ZnS quantum dots confirms the formation of core-shell structure.
Core-shell structure can be divided into Type-I[15,19,20] and Type-II[21] quantum dots according to the energy levels. (see Fig. 1-6) CdSe/CdS, CdSe/ZnS, CdSe/ZnSe, CdS/ZnS, ZnSe/ZnS belong to Type I. For Type I quantum dots, the shell energy gap is larger than the core energy gap. The shell conduction band is
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higher than that of the core, but the shell valence band is lower than that of nuclear. On the other hand, for type-II including CdTe/CdSe, CdTe/CdSe, CdSe/ZnTe, ZnTe/CdSe, the conduction band and valence band of the shell is higher or lower than those of core.
Therefore, for Type-I quantum dots, the electron and hole are confined to the core and they have higher probability of recombination and reduced fluorescence lifetime, and consequently higher luminous efficiency; but the Type-Ⅱ quantum dots have the completely opposite characteristics.
(A)
(B)
Fig. 1-6 (A) Type-I core/shell structure (B) Type-II core/shell structure
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1.2.3 Hydrophilic Ligand
Even though the synthesis method of quantum dots in organic solvents has been developed quite completely, the reaction in toxic solvents and risk of pollution remain the most serious problems. So far, we have described that nanoparticlecs or core/shell structure by the traditional method are not soluble in water, indicating hydrophobic. Therefore, they are incompatible with the organism and their applications are quite limited. The search for the method to synthesize low toxicity, safe and water-soluble CdSe nanoparticle were gradually merged to change the quantum dot surface for hydrophilic modification or replacement. Goal is to synthesize low toxicity, safe and water-soluble CdSe nanoparticles.
In 1998, Nie and Chan proposed a method to modify CdSe / ZnS quantum dot surface by using mercaptoacetic acid and solved the problem of the water solubility and protein binding.[16] Zinc atoms of CdSe / ZnS outer layer bonding with the mercapto group let a polar-COOH coat the most outer layer and make CdSe / ZnS quantum dots soluble in water.
While the above method can convert quantum dots from organic phase to the aqueous phase, the process of replacement of functional groups will produce a lot of defect to cause fluorescence quench and cause nanoparticles to fuse together, which makes the quantum dot fluorescence decay extremely fast and initiates precipitate formation and lowering of the quantum yield.
Therefore, it is crucial to directly synthesize high-quantum-yield water-soluble quantum dots in biomedical or optoelectronic applications.
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1.2.4 Applications of CdSe nanoparticle
Compared to the general common fluorescent dyes, quantum dots have strong light stability and different particle size QD can emit different wavelengths of light with narrow spectral width and high output power. These unique optical properties cause a lot of attention in the quantum dots solar cells[22,23,24], quantum dots light emitting diodes[25,26], quantum dots biomedical sensors[27,28] , etc.