Department of Chemistry and Biochemistry and Renewable and Sustainable Energy Institute, University of Colorado , Boulder, Colorado 80309, United States.
Nano Lett. 2017 Feb 8;17(2):762-771. doi: 10.1021/acs.nanolett.6b03874. Epub 2017 Jan 3.
Femtosecond two-dimensional Fourier transform spectroscopy is used to determine the static bandgap inhomogeneity of a colloidal quantum dot ensemble. The excited states of quantum dots absorb light, so their absorptive two-dimensional (2D) spectra will typically have positive and negative peaks. It is shown that the absorption bandgap inhomogeneity is robustly determined by the slope of the nodal line separating positive and negative peaks in the 2D spectrum around the bandgap transition; this nodal line slope is independent of excited state parameters not known from the absorption and emission spectra. The absorption bandgap inhomogeneity is compared to a size and shape distribution determined by electron microscopy. The electron microscopy images are analyzed using new 2D histograms that correlate major and minor image projections to reveal elongated nanocrystals, a conclusion supported by grazing incidence small-angle X-ray scattering and high-resolution transmission electron microscopy. The absorption bandgap inhomogeneity quantitatively agrees with the bandgap variations calculated from the size and shape distribution, placing upper bounds on any surface contributions.
飞秒二维傅里叶变换光谱用于确定胶体量子点的静态带隙不均匀性。量子点的激发态吸收光，因此它们的吸收二维（2D）光谱通常具有正峰和负峰。结果表明，通过在带隙跃迁周围的 2D 光谱中分离正峰和负峰的节点线的斜率，可以稳健地确定吸收带隙不均匀性；该节点线斜率与吸收和发射光谱中未知的激发态参数无关。将吸收带隙不均匀性与电子显微镜确定的尺寸和形状分布进行比较。使用新的二维直方图分析电子显微镜图像，该直方图将主要和次要图像投影相关联，以揭示出长形纳米晶体，这一结论得到掠入射小角 X 射线散射和高分辨率透射电子显微镜的支持。吸收带隙不均匀性与从尺寸和形状分布计算的带隙变化定量一致，对任何表面贡献都施加了上限。
