Optical Properties Of Colloid PbSe Semiconductor Nanocrystals

PbSe nanocrystals (NCs) have the best PL quantum yield at the infrared region compared with other infrared-emitting NCs. In the past five years, researchers had paid more and more attention to the investigation of the PbSe NCs properties. However, many of its physical properties are still unclear and inaccurate. In addition, PbSe NCs are instability and toxicity so that many of its application are limited. In this thesis, electro-optical properties of PbSe NCs are investigated. Through the growth of the shell structure upon the PbSe NCs, the stability of PbSe NCs is improved dramatically and its toxicity is controlled.1. A series of colloid PbSe NCs with different particle sizes were prepared in the chemistry lab. The band gap structure was analyzed and size-dependent properties were recalculated.We have synthesized high-efficiency PbSe NCs with Yu's method. The PL quantum yield is up to 80%. Figure 1 shows the TEM image of 6.8 nm PbSe NCs and the absorption spectrum of PbSe NCs with different sizes. It is obvious that band gap of PbSe NCs is size-dependent. Fig.1 TEM image and absorption spectrum of PbSe NCsThe PL quantum yield of PbSe NCs could be improved by injecting an appropriate amount of TBP solution according to the experimental results. Figure 2 shows the absorption and PL spectrum of PbSe semiconductor NCs after the injection of TBP solution. It is obvious that PL quantum yield gets an improvement by injecting an appropriate amount of TBP solution. If the excess TBP solution is injected, PbSe NCs will be ruined and PL quantum yield will decrease dramatically. Fig. 2 Absorption and PL spectrum of PbSe NCs by injecting TBP solutionPbSe NCs have the following properties: a large Bohr radii (46nm), a small band gap (0.28eV), a small effective mass (0.07 m0). The current calculated band gap has large errors according to Brus's theory. The size quantum confinement energy and Coulomb interaction energy of PbSe NCs are calculated by EMA-FDW model as followed where the parameters are defined as m0ε(1re h )=ε1∞????ε1∞?ε10 ???×???1 ?exp(?reh/ρe)+2 exp(?reh/ρh)??? (6) Theoretical results show a good fit to the experimental data.2. We tested absorption spectra of PbSe NCs with different sizes under different temperatures. We found that the band gap of PbSe NCs is the size- and temperature -dependent. This effect has been analysized and explained theoretically.We measured absorption spectra of PbSe nanocrystalls with different sizes in the region from 23℃to 100℃. We found that the first absorption peak is the size- and temperature-dependent. The temperature-depentdent shift of the first absorption peak depends on the size of nanocrystals. When the particle size is less than a certain size, the absorption peak shifts to red with the increase of temperature and the temperature coefficient of band gap is above zero. When the particle size is more than a certain size, the absorption peak shifts to blue with the increase of temperature and the temperature coefficient is below zero. Figure 3 shows the absorption spectrum of PbSe NCs with different sizes in the region from 23℃to 100℃. Fig.3 Absorption spectrum of PbSe NCs with different from 23℃to 100℃We have set up a computational model to calculate exciton energy of PbSe NCs. The model includes several factors: nanocrystals thermal expansion-induced changes of the quantum-confined energy; temperature-dependent exciton effective mass; temperature–dependent exciton-phonon coupling energy. Here, the exciton-phonon coupling is quantitatively described. The temperature- and size-dependent band gap and its coefficient are described as follow Figure 4 shows the curve of temperature- and size-dependent band gap and its coefficient. There is a critical value of the size-dependent temperature coefficient. According to Eq. (7) and (8), the critical value is 4.88 nm. This value shows a good fit to the experimental results. Fig.4 The curve of temperature- and size-dependent band gap and its coefficient3. A new model is employed to calculate the molar extinction coefficient of PbSe NCs. The extinction coefficient of PbSe NCs can be determined exactly by this model. The experimental and theoretical results indicate that the extinction coefficient is the size-dependent.The colloid PbSe semiconductor NCs with different size were prepared in the chemistry lab. The relation of first exciton absorption peak position and size of nanocrystals is fixed. We demonstrate PbSe NCs surface layer is Pb atomic shell. By calculating the ratio of the number of Pb/Se atoms in PbSe NCs, the formula of molar concentration for PbSe NCs is givenTherefore, the extinction coefficient can be given as A =εexCl (11)We have established a theoretical function between the extinction coefficient and extinction cross section Extinction cross-section is a sum of absorption cross-sections and scattering cross-section The results indicate that the extinction coefficient is size-dependent.4. PbSe/CdSe and PbSe/CdSe/ZnSe NCs were synthesized by"the successive ion layer adsorption and reaction (SILAR)". The light-emitting stability of the nanocrystals is improved and toxicity was reduced.In this work, we employed the SILAR technology to prepare air-stable PbSe/CdSe and PbSe/CdSe/ZnSe NCs with strong quantum yield of 70% to improve the stability of PbSe NCs and reduce the its toxicity. Furthermore, the surface of PbSe/CdSe NCs was treated with a small amount of sodium borohydride (NaBH4) in hexanes at room temperature. The QY of PbSe/CdSe NCs was enhanced about 1.3 times by this procedure. The TEM images of PbSe, PbSe/CdSe, PbSe/CdSe/ZnSe NCs are shown in Figure 5.