Characterization of CBD-CdS Doped with Some Rare Earths III (Eu3+,Ce3+) as Function of Synthesis Time

Rare earths (Europium, cerium)-doped CdS nanofilms are prepared using the growth technique chemical bath deposition (CBD) at the reservoir temperature of 70±2°C varying the synthesis time in a wide range. For the rare earths doped CdS nanofilms the synthesis time was ranged from 80 to 135 min. The rare earths molar concentration was in the range 0.0≤x≤3.47, which was determined by energy dispersive X-ray spectroscopy (EDS). The X-ray diffraction (XRD) analysis reveals that CdS nanofilms showed the zinc blende (ZB) crystalline phase. The nanocrystal size was ranged from 2.67 to 2.35 nm for the CdS and 1.84-2.33 nm for rare earth-doped CdS that were determined by the Debye-Scherrer equation from ZB (111) direction and it was confirmed by transmission electron microscopy (TEM). The doped CdS exhibits a direct band gap that diminishes with the increase of the synthesis time, from 2.50 to 2.42 eV, which was obtained by transmittance. The room-temperature photoluminescence of CdS presents the band-to-band transition at 431 nm, which is associated with quantum confinement because the grain size is less than its Bohr exciton radius and a dominant band at 523 nm, which is called the optical signature of interstitial oxygen. Eu3+-doped CdS photoluminescence shows the dominant radiative peak at 576 nm that is associated to the intra-4f radiative transition of Eu3+ ions, which corresponds to the magnetic dipole transition, (5D0→7F1). For the Ce3+-doped CdS the dominant radiative transitions are clearly redshifted. Additionally, other radiative peaks associated at structural defects are observed. The passivation of the CdS by rare earths was approximately of two orders of magnitude obtaining better results with cerium.

采用化学浴沉积（chemical bath deposition, CBD）技术，在70±2℃的浴槽温度下制备了掺铕（Europium）、铈（cerium）的硫化镉（CdS）纳米薄膜，并在宽区间内调控合成时长；其中掺稀土CdS纳米薄膜的合成时长范围为80~135 min。稀土元素的摩尔浓度区间为0.0≤x≤3.47，该浓度通过能量色散X射线能谱（energy dispersive X-ray spectroscopy, EDS）表征确定。X射线衍射（X-ray diffraction, XRD）分析显示，纯相CdS纳米薄膜呈现闪锌矿（zinc blende, ZB）晶型。通过德拜-谢乐公式（Debye-Scherrer equation）结合闪锌矿(111)晶面衍射峰，并辅以透射电子显微镜（transmission electron microscopy, TEM）验证，计算得到纯CdS纳米晶尺寸介于2.67~2.35 nm之间，掺稀土CdS的纳米晶尺寸则为1.84~2.33 nm。掺杂后的CdS仍为直接带隙半导体，其带隙值随合成时长增加逐渐降低，从2.50 eV降至2.42 eV，该结果通过透光率测试获得。室温下纯CdS的光致发光光谱呈现两处特征峰：一处为431 nm处的带间跃迁峰，对应量子限域效应——因晶粒尺寸小于其玻尔激子半径；另一处为主峰位于523 nm，该峰被称为间隙氧的光学特征峰。掺铕离子（Eu³+）的CdS光致发光光谱中，主峰位于576 nm，对应Eu³+离子的4f内层辐射跃迁，具体为磁偶极跃迁(⁵D₀→⁷F₁)。而掺铈离子（Ce³+）的CdS光致发光主峰则发生明显红移，同时还观测到与结构缺陷相关的其他辐射峰。此外，稀土元素对CdS的钝化效果提升约两个数量级，其中铈的钝化效果更为优异。