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.

采用化学浴沉积法（chemical bath deposition, CBD）于70±2℃的槽液温度下制备了掺杂稀土（铕、铈）的硫化镉（CdS）纳米薄膜，并通过宽范围调控合成时长实现变量设置。其中掺杂稀土的CdS纳米薄膜的合成时长介于80至135 min之间。稀土掺杂摩尔浓度范围为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）测定。X射线衍射（X-ray diffraction, XRD）分析结果显示，纯相CdS纳米薄膜呈现闪锌矿（zinc blende, ZB）晶相。通过德拜-谢乐公式（Debye-Scherrer equation）对闪锌矿(111)晶面进行晶粒尺寸计算，并结合透射电子显微镜（transmission electron microscopy, TEM）表征验证：纯CdS的纳米晶粒尺寸范围为2.67~2.35 nm，掺杂稀土后的CdS纳米晶粒尺寸则处于1.84~2.33 nm区间。掺杂后的CdS具有直接带隙结构，其带隙值随合成时长增加从2.50 eV降至2.42 eV，该结果通过透光率测试获取。室温条件下，纯CdS的光致发光谱在431 nm处出现带间跃迁峰，该现象源于量子限域效应，因其晶粒尺寸小于其玻尔激子半径；同时在523 nm处存在主导发射峰，该峰被称为间隙氧的光学特征峰。掺铕（Eu³+）的CdS光致发光谱在576 nm处呈现主导辐射峰，该峰对应Eu³+离子的4f轨道内辐射跃迁，即磁偶极跃迁(⁵D₀→⁷F₁)。对于掺铈（Ce³+）的CdS，其主导辐射跃迁峰发生了显著红移。此外，实验中还观测到与结构缺陷相关的其他辐射峰。稀土元素对CdS的钝化效果提升了约两个数量级，且铈的钝化效果更为优异。