Exciton-Phonon Interactions Govern Charge-Transfer-State Dynamics in CdSe/CdTe Two-Dimensional Colloidal Heterostructures.
Exciton-Phonon Interactions Govern Charge-Transfer-State Dynamics in CdSe/CdTe Two-Dimensional Colloidal Heterostructures.
复制标题
DOI：
10.1021/jacs.8b05842
复制
发表时间：
2018-10
影响因子：
15
通讯作者：
Rajal Pandya;Richard Y. S. Chen;Alexandre Cheminal;Marion Dufour;Johannes M. Richter;Tudor H. Thomas;Shuroug Ahmed;A. Sadhanala;Edward P. Booker;G. Divitini;F. Deschler;N. Greenham;S. Ithurria;A. Rao
中科院分区：
文献类型：
--
作者：
Rajal Pandya;Richard Y. S. Chen;Alexandre Cheminal;Marion Dufour;Johannes M. Richter;Tudor H. Thomas;Shuroug Ahmed;A. Sadhanala;Edward P. Booker;G. Divitini;F. Deschler;N. Greenham;S. Ithurria;A. Rao
CdSe/CdTe核冠ii型纳米血小板异质结构是一种二维半导体，由于其易于制造、优异的发射产率和可调谐的特性，在发光技术中引起了人们的兴趣。尽管如此，这些复杂材料的激子动力学，特别是它们如何受到声子的影响，还没有得到很好的理解。在这里，我们使用飞秒振动光谱、温度分辨光致发光（PL）和温度相关结构测量相结合的方法来研究厚度为四层单层的CdSe/CdTe纳米薄片。我们发现，CdSe/CdTe界面上的电荷转移（CT）激子在两个不同的时间尺度上形成：最初来自超快（~ 70 fs）电子转移，然后在更长的时间尺度上（~ 5 ps）从域激子扩散到界面。我们发现CT激子在~ 120 cm-1处受到界面声子模式的影响，使它们定位在界面上。使用低温PL光谱，我们发现相同的声子模式是扩大CT PL的主要机制。当冷却到4 K时，总PL量子产率接近统一，其中约85%来自CT发射，其余来自发射亚带隙状态。在室温下，域激子向界面的不完全扩散以及CT激子和声子之间的散射将PL的量子产率限制在~ 50%。我们的研究结果提供了二维异质结构界面上激子-声子相互作用性质的详细图像，并解释了CT PL光谱的广泛形状和PL量子量产损失的来源。此外，他们提出，为了最大限度地提高PL量子产率，需要改进界面晶体结构的工程设计和向界面扩散域激子，例如，通过改变相对核/冕尺寸。
CdSe/CdTe core-crown type-II nanoplatelet heterostructures are two-dimensional semiconductors that have attracted interest for use in light-emitting technologies due to their ease of fabrication, outstanding emission yields, and tunable properties. Despite this, the exciton dynamics of these complex materials, and in particular how they are influenced by phonons, is not yet well understood. Here, we use a combination of femtosecond vibrational spectroscopy, temperature-resolved photoluminescence (PL), and temperature-dependent structural measurements to investigate CdSe/CdTe nanoplatelets with a thickness of four monolayers. We show that charge-transfer (CT) excitons across the CdSe/CdTe interface are formed on two distinct time scales: initially from an ultrafast (∼70 fs) electron transfer and then on longer time scales (∼5 ps) from the diffusion of domain excitons to the interface. We find that the CT excitons are influenced by an interfacial phonon mode at ∼120 cm-1, which localizes them to the interface. Using low-temperature PL spectroscopy we reveal that this same phonon mode is the dominant mechanism in broadening the CT PL. On cooling to 4 K, the total PL quantum yield reaches close to unity, with an ∼85% contribution from CT emission and the remainder from an emissive sub-band-gap state. At room temperature, incomplete diffusion of domain excitons to the interface and scattering between CT excitons and phonons limit the PL quantum yield to ∼50%. Our results provide a detailed picture of the nature of exciton-phonon interactions at the interfaces of 2D heterostructures and explain both the broad shape of the CT PL spectrum and the origin of PL quantum yield losses. Furthermore, they suggest that to maximize the PL quantum yield both improved engineering of the interfacial crystal structure and diffusion of domain excitons to the interface, e.g., by altering the relative core/crown size, are required.