Krishnamurthy Sachidananda, Singh Ajay, Hu Zhongjian, Blake Anastasia V, Kim Younghee, Singh Amita, Dolgopolova Ekaterina A, Williams Darrick J, Piryatinski Andrei, Malko Anton V, Htoon Han, Sykora Milan, Hollingsworth Jennifer A
Materials Physics and Applications Division: Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos 87545, New Mexico, United States.
Department of Physics, The University of Texas at Dallas, Richardson 75080, Texas, United States.
ACS Nano. 2021 Jan 26;15(1):575-587. doi: 10.1021/acsnano.0c05907. Epub 2020 Dec 31.
We synthesized PbS/CdS core/shell quantum dots (QDs) to have functional single-emitter properties for room-temperature, solid-state operation in the telecom O and S bands. Two shell-growth methods-cation exchange and successive ionic layer adsorption and reaction (SILAR)-were employed to prepare QD heterostructures with shells of 2-16 monolayers. PbS/CdS QDs were sufficiently bright and stable to resolve photoluminescence (PL) spectra representing both bands from single nanocrystals using standard detection methods, and for a QD emitting in the O-band a second-order correlation function showed strong photon antibunching, important steps toward demonstrating the utility of lead chalcogenide QDs as single-photon emitters (SPEs). Irrespective of type, few telecom-SPEs exist that are capable of such room-temperature operation. Access to single-QD spectra enabled a direct assessment of spectral line width, which was ∼70-90 meV compared to much broader ensemble spectra (∼300 meV). We show inhomogeneous broadening results from dispersity in PbS core sizes that increases dramatically with extended cation exchange. Quantum yields (QYs) are negatively impacted at thick shells (>6 monolayers) and, especially, by SILAR-growth conditions. Time-resolved PL measurements revealed that, with SILAR, initially single-exponential PL-decays transition to biexponential, with opening of nonradiative carrier-recombination channels. Radiative decay times are, overall, longer for core/shell QDs compared to PbS cores, which we demonstrate can be partially attributed to some core/shell sizes occupying a quasi-type II electron-hole localization regime. Finally, we demonstrate that shell engineering and the use of lower laser-excitation powers can afford significantly suppressed blinking and photobleaching. However, dependence on shell thickness comes at a cost of less-than-optimal brightness, with implications for both materials and experimental design.
我们合成了硫化铅/硫化镉核壳量子点（QDs），使其具有功能性单发射体特性，可在室温下于电信O波段和S波段进行固态操作。采用两种壳层生长方法——阳离子交换法和连续离子层吸附与反应法（SILAR）——制备了具有2至16个单层壳层的量子点异质结构。硫化铅/硫化镉量子点足够明亮且稳定，能够使用标准检测方法分辨来自单个纳米晶体的代表两个波段的光致发光（PL）光谱，并且对于在O波段发射的量子点，二阶相关函数显示出强烈的光子反聚束现象，这是朝着证明硫属铅化物量子点作为单光子发射体（SPEs）的实用性迈出的重要一步。无论类型如何，能够进行这种室温操作的电信单光子发射体都很少。获得单个量子点光谱能够直接评估光谱线宽，与宽得多的系综光谱（约300毫电子伏特）相比，其约为70 - 90毫电子伏特。我们表明，非均匀展宽源于硫化铅核尺寸的分散性，这种分散性随着阳离子交换的延长而急剧增加。量子产率（QYs）在厚壳层（>6个单层）时受到负面影响，特别是受到SILAR生长条件的影响。时间分辨光致发光测量表明，采用SILAR时，最初的单指数光致发光衰减转变为双指数衰减，同时打开了非辐射载流子复合通道。总体而言，核壳量子点的辐射衰减时间比硫化铅核更长，我们证明这部分可归因于一些核壳尺寸处于准II型电子 - 空穴局域 regime。最后，我们证明壳层工程和使用较低的激光激发功率能够显著抑制闪烁和光漂白。然而，对壳层厚度的依赖是以亮度不太理想为代价的，这对材料和实验设计都有影响。
