‡Department of Physics, University of Antwerp, Groenenborgerlaan 171, Antwerpen B-2020, Belgium.
ACS Nano. 2015 May 26;9(5):5326-32. doi: 10.1021/acsnano.5b00985. Epub 2015 Apr 15.
Monolayers of group VI transition metal dichalcogenides possess direct gaps in the visible spectrum with the exception of MoTe2, where its gap is suitably located in the infrared region but its stability is of particular interest, as tellurium compounds are acutely sensitive to oxygen exposure. Here, our environmental (time-dependent) measurements reveal two distinct effects on MoTe2 monolayers: For weakly luminescent monolayers, photoluminescence signal and optical contrast disappear, as if they are decomposed, but yet remain intact as evidenced by AFM and Raman measurements. In contrast, strongly luminescent monolayers retain their optical contrast for a prolonged amount of time, while their PL peak blue-shifts and PL intensity saturates to slightly lower values. Our X-ray photoelectron spectroscopy measurements and DFT calculations suggest that the presence of defects and functionalization of these defect sites with O2 molecules strongly dictate their material properties and aging response by changing the excitonic dynamics due to deep or shallow states that are created within the optical band gap. Presented results not only shed light on environmental effects on fundamental material properties and excitonic dynamics of MoTe2 monolayers but also highlight striking material transformation for metastable 2D systems such as WTe2, silicone, and phosphorene.
VI 族过渡金属二卤化物的单层具有可见光范围内的直接带隙，除了 MoTe2 之外，其带隙适当地位于红外区域，但它的稳定性特别令人感兴趣，因为碲化合物对氧气暴露非常敏感。在这里，我们的环境（时变）测量揭示了 MoTe2 单层的两个不同影响：对于光致发光较弱的单层，光致发光信号和光学对比度消失，就好像它们被分解了，但仍如 AFM 和拉曼测量所证明的那样保持完整。相比之下，强发光的单层在更长的时间内保持其光学对比度，而其 PL 峰蓝移，PL 强度饱和到略低的值。我们的 X 射线光电子能谱测量和 DFT 计算表明，缺陷的存在以及这些缺陷位点与 O2 分子的功能化强烈决定了它们的材料特性和老化响应，因为在光学带隙内创建的深或浅态改变了激子动力学。所呈现的结果不仅阐明了环境对 MoTe2 单层基本材料特性和激子动力学的影响，而且还突出了如 WTe2、硅酮和磷烯等亚稳态 2D 系统的显著材料转变。
