Synthesis An Characterization Of Cu-Zn-In-S Nanocrystals

Luminescent Colloidal semiconductor nanocrystals are of significant technology interest as they impact many applications including light emitting diodes (LEDs), biomedical labeling,lighting, etc. While existing examples of light emitting semiconductor nanocrysal s such as II-VI, II-V, III-V, IV-IV are numerous, most suffer from a number of disadvantages such as containing highly toxic elements, using extremely expensive and hazard raw materials, needing to surface modification to improve their properties. At an extend work, I-III-VI semiconductor nanocrysal s such as CuInS₂ have been successfully prepared recently due to their avoiding some of disadvantages as described above. The progress on the synthesis of such nanocrysal emitters brought the quality of I-III-VI nanocrysal s, especially the optical properties (quantum yield of about 5%) could not up to a level comparable to that of CdSe NCs,Therefore, it is a big challenge for scientists to develop greener, stable, high emissive semiconductor nanocrystals now.CuInS₂ is a direct band gap semiconductor material with a band gap of 1.45 eV, which means that it would be possible to develop color-tunable CuInS₂ nanocrystal emitters from the visible to near-infrared(NIR) regions ,the near-infrared fluorescence materials are a better biological fluorescent labeling materials due to its near-infrared fluorescence is suitble for in vivo deep tissue imaging. More importantly, the nanoctrystals do not contain the toxic elements. At present, the ternary quantum dots have poor fluorescence quantum yield (<5%) and size distribution, poor chemical stability. Although our previous work have synthesized nearly monodisperse CuInS₂ (from 2 to 20nm) using greener approach. Nevertherless the resulted nanocrystals had poor quntum yield and stability. Forther studies indicxates that the emissive efficiency of nanocrystals can reach up to 30% and the stability of nanocrysals is also greatly improved after the growth of ZnS shell on the surface of nanocrysals. Therefore, it is still chanlenge on synthesis of CuInS₂ quantum dots with high quanum yield. ,It is found that the introduction of zinc can increase the fluorescence efficiency and stability of CuInS₂ Nanocrystals. In this article we systematically studied the reaction activity of three metal precursors under different experimental conditions, synthesized nearly monodisperse quaternary CuZnInS₃ nanocrystals with adjustable sizes and compositions. To improve the stability and quantum yield, we prepared CuZnInS₃ /ZnS core/shell nanocrystals. Preliminary application on preparation of QDs-LEDs white light emitter by using CuZnInS₃ nanoctysals. The main content of the thesis as following:1. To avoid phase separation on synthesis of multinary nanocrystals, we systematically studied the reaction activity of three metal precursors (Cu, Zn, In) under different experimental conditions. Experimental results showed that the formation temperature of CuxS, ZnS, In2S3 respectively are 80℃, 120℃, 160℃in the absence of dodecanethiol. It is noted that the reaction temperature is adopted to quatitaify the reactivity of three metal precursors. Thus, the reactivity of three metal precursors is Cu > In> Zn. When dodecanethiol is used as ligand, the formation temperature of Cu2S, ZnS, In2S3 respectively are 60℃, 100℃, 140℃(Cu> In> Zn), indicating that dodecanethiol further balanced the reactivity of three metals. Therefore, the formation temperature of quaternary nanocrystals is above 140℃. This conclusion is necessary for choosing sutable reaction temperatures to prepare quaternary CuZnInS₃ nanocrystlas.2. We study the effects of ligand, reaction additives, reaction temperature on the size control of quaternary CuZnInS₃ nanocrystals. The experimental results showed that the size of particles is independent of the variety of ligand concentrations and reaction additives. Based on the understanding the growth mechanism of quaternary nanocrystals, we finally synthesized nearly monodisperse CuZnInS₃ nanocrystals (2 nm - 7.5 nm) by aid of reaction temperature. The resulted nanocrystals have identical element composition (Cu:Zn:In =1:1:1 in particle), the fluorescence spectra can range from 620 nm to 780 nm, and the emissive efficiency can reach up to 65%. The experimental results reveal that the growth mechanism of quatunary nanoctysals is simiar to that reported on preparation of InAs nanocrystals where reaction temperature is dominatant the size of nanocrystals.3. In order to prepared quaternary nanocrystals with wide emission range, we preared the quaternary nanocrystals with different compositions by simple changing the ratio of precursors and particle sizes. As as result the as prepared nanocrysgtals exhigled a wide the emission range which cover the whole visible region to NIR (500 nm to 900 nm). Importantly, the photoluminescence quantum yield of nanocrystals can reach up to 70% without any surface modification such as wide band gap ZnS shell used. It is noted that quntum yield of nanocrystals varied with different elemental composition where the almost constant quantum yields of 70% was observed with increase of the ratio of Zn vs Cu from1:1 to 20:1, and subsequently decreased significantly below the ratio of 1:1. It should be noted that these particles had similar sizes as determined by TEM, and were under the radius of their Bohr exciton, which excluded the sizes contributed the variety of quantym yields of nanocrystals. This fact indicates that the notable characterization of multinary NCs differencing binary NCs is that quantum yields of particles fluctuated depending on their composition defects and surface defects, and improve the emission efficiency of nanocrystals. PL decay of the sample with Zn:Cu ratio of 1:2 and 3:1 were characterized, indicating that the extensive elimination of the internal defects and thus suppressed non-radiative recombination process for high Zn vs Cu ratuio. As a result the plain NCs presented emission efficiency as high as 70% 4. Stabilites experimental showed that the CuZnInS₃ nanocrystals possese well chemical photochemical and thermal stabilities. However, the nanocrysals with high ratio Cu vs Zn showed poor stabilities. To resolve this problem, a wide band gap semiconductor was used to passivate the surface of nanoctrysals for improvment the stabilities and quantum yields of nanocrystals. For example, CuZnInS₃/ZnS core/shell nanocrystals was prepared by using thermal- cycling and successive ionic layer adsorption and reaction (TC-SILAR). As a result, the stability of paticle was successfully improved. Meanwhile, the photoluminescence quantum yield of CuZnInS₃/ZnS core/shell nanocrystals can reach up to 60%. Finally, a simple QDs-LED lighting device was fabricated by coating CuZnInS₃ nanocrystals on the surface of the blue LEDs to produce war white light, indicating the feasibilities in lighting field.