Ludescher Lukas, Dirin Dmity N, Kovalenko Maksym V, Sztucki Michael, Boesecke Peter, Lechner Rainer T
Institute of Physics, Montanuniversitaet Leoben, Leoben, Austria.
Department of Chemistry and Applied Biosciences, ETH Zurich, Zurich, Switzerland.
Front Chem. 2019 Jan 22;6:672. doi: 10.3389/fchem.2018.00672. eCollection 2018.
To study the influence of the chemical and crystalline composition of core/shell NCs on their photoluminescence (PL) the mean structural profile of a large ensemble of NCs has to be retrieved in atomic resolution. This can be achieved by retrieving the chemical profile of core/shell NCs using anomalous small angle x-ray scattering (ASAXS) in combination with the analysis of powder diffraction data recorded by wide angle x-ray scattering (WAXS). In the current synchrotron based study, we investigate CdSe/CdS core/shell NCs with different core dimensions by recording simultaneously ASAXS and WAXS spectra. The CdS shells are grown epitaxial on nominal spherical CdSe cores with core diameters from around 3.5-5.5 nm. Three different CdSe shell thicknesses are realized by depositing around 4, 6, and 8 monolayers (MLs) of CdSe. We reveal that the epitaxial core/shell structure depicts a chemical sharp interface, even after a post growth annealing step. With increasing NC diameter, however, the CdSe/CdS NCs deviate significantly from a spherical shape. Instead an elliptical particle shape with pronounced surface facets for the larger core/shell NCs is found. In combination with the powder diffraction data we could relate this anisotropic shape to a mixture of crystal phases within the CdSe core. The smallest CdSe cores exhibit a pure hexagonal wurtzite crystal structure, whereas the larger ones also possess a cubic zincblende phase fraction. This mixed crystal phase fractions lead to a non-spherical shell growth with different thicknesses along specific crystallographic directions: The long axes are terminated by basal crystal faces parallel either to the - or -axis, the short axes by "tilted" pyramidal planes. By combining these structural data with the measured PL quantum yield values, we can clearly connect the optical output of the NCs to their shape and to their shell thickness. Above 6 ML CdS shell-thickness no further increase of the PL can be observed, but for large aspect ratio values the PL is significantly decreased. The gained understanding of the internal crystal structure on CdSe/CdS NCs is general applicable for a precise tuning of the optical properties of crystalline core/shell NCs.
为了研究核壳纳米晶体的化学和晶体组成对其光致发光（PL）的影响，必须以原子分辨率获取大量纳米晶体的平均结构轮廓。这可以通过使用反常小角X射线散射（ASAXS）结合广角X射线散射（WAXS）记录的粉末衍射数据分析来获取核壳纳米晶体的化学轮廓来实现。在当前基于同步加速器的研究中，我们通过同时记录ASAXS和WAXS光谱来研究具有不同核尺寸的CdSe/CdS核壳纳米晶体。CdS壳层外延生长在标称直径约为3.5-5.5nm的球形CdSe核上。通过沉积约4、6和8个单层（MLs）的CdSe实现了三种不同的CdSe壳层厚度。我们发现，即使经过生长后退火步骤，外延核壳结构仍呈现出化学清晰的界面。然而，随着纳米晶体直径的增加，CdSe/CdS纳米晶体明显偏离球形。相反，对于较大的核壳纳米晶体，发现了具有明显表面小面的椭圆形颗粒形状。结合粉末衍射数据，我们可以将这种各向异性形状与CdSe核内的晶相混合物联系起来。最小的CdSe核呈现出纯六方纤锌矿晶体结构，而较大的核也具有立方闪锌矿相分数。这种混合晶相分数导致沿特定晶体学方向具有不同厚度的非球形壳层生长：长轴由平行于-轴或-轴的基面终止，短轴由“倾斜”的锥面终止。通过将这些结构数据与测量的PL量子产率值相结合，我们可以清楚地将纳米晶体的光学输出与其形状和壳层厚度联系起来。当CdS壳层厚度超过6ML时，未观察到PL的进一步增加，但对于大纵横比的值，PL显著降低。对CdSe/CdS纳米晶体内部晶体结构的深入理解通常适用于精确调节晶体核壳纳米晶体的光学性质。
