Fernández-Garrido S, Pisador C, Lähnemann J, Lazić S, Ruiz A, Redondo-Cubero A
Grupo de Electrónica y Semiconductores, Dpto. Física Aplicada, Universidad Autónoma de Madrid, C/ Francisco Tomás y Valiente 7, 28049 Madrid, Spain.
Nanotechnology. 2020 Nov 20;31(47):475603. doi: 10.1088/1361-6528/abadc8.
We analyse the morphological, structural and luminescence properties of self-assembled ZnO nanowires grown by chemical vapour transport on Si(001). The examination of nanowire ensembles by scanning electron microscopy reveals that a non-negligible fraction of nanowires merge together forming coalesced aggregates during growth. We show that the coalescence degree can be unambiguously quantified by a statistical analysis of the cross-sectional shape of the nanowires. The examination of the structural properties by x-ray diffraction evidences that the nanowires crystallize in the wurtzite phase, elongate along the c-axis, and are randomly oriented in plane. The luminescence of the ZnO nanowires, investigated by photoluminescence and cathodoluminescence spectroscopy, is characterized by two bands, the near-band-edge emission and the characteristic defect-related green luminescence of ZnO. The cross-correlation of scanning electron micrographs and monochromatic cathodoluminescence intensity maps reveals that: (i) coalescence joints act as a source of non-radiative recombination, and (ii) the luminescence of ZnO nanowires is inhomogeneously distributed at the single nanowire level. Specifically, the near-band-edge emission arises from the nanowire cores, while the defect-related green luminescence originates from the volume close to the nanowire sidewalls. Two-dimensional simulations of the optical guided modes supported by ZnO nanowires allow us to exclude waveguiding effects as the underlying reason for the luminescence inhomogeneities. We thus attribute this observation to the formation of a core-shell structure in which the shell is characterized by a high concentration of green-emitting radiative point defects as compared to the core.
我们分析了通过化学气相传输在Si(001)上生长的自组装ZnO纳米线的形态、结构和发光特性。通过扫描电子显微镜对纳米线集合体的检查表明，在生长过程中，不可忽略的一部分纳米线合并在一起形成了聚结聚集体。我们表明，可以通过对纳米线横截面形状的统计分析明确量化聚结程度。通过X射线衍射对结构特性的检查证明，纳米线以纤锌矿相结晶，沿c轴伸长，并且在平面内随机取向。通过光致发光和阴极发光光谱研究的ZnO纳米线的发光由两个带表征，即近带边发射和ZnO特有的与缺陷相关的绿色发光。扫描电子显微镜图像与单色阴极发光强度图的互相关显示：(i) 聚结节点作为非辐射复合的源，并且 (ii) ZnO纳米线的发光在单个纳米线水平上分布不均匀。具体而言，近带边发射来自纳米线核心，而与缺陷相关的绿色发光源自靠近纳米线侧壁的区域。对ZnO纳米线支持的光学导模的二维模拟使我们能够排除波导效应作为发光不均匀性的潜在原因。因此，我们将此观察结果归因于形成了一种核壳结构，其中与核心相比，壳的特征在于高浓度的发射绿色光的辐射点缺陷。
