TiN纳米粒子增强CdSe/Al2O3异质结荧光的研究 
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1 引言
与传统的荧光方法相比,表面增强荧光(SEF)技术[1-2]具有较高的灵敏度和较低的检测限[3],是一种前沿荧光检测技术[4],在高精度生物传感器[5-6]、太阳能电池[7-8]和探测生物分子信息[9]等领域应用非常广泛。构建量子点异质结是增强异质结界面荧光的重要方法之一[10-13],是进一步扩大量子点异质结在光伏[14-16]、光催化[17-19]等领域应用的有效手段。该方法通过控制量子点的光生载流子在异质结中的输运特性,增加光生载流子的数量,减少非辐射重构,实现辐射体的表面荧光增强效应。
多孔Al2O3薄膜是一种重要的金属氧化物半导体材料[20-21],具有带隙可调、光吸收率高等优点,被广泛应用于催化[22]、光伏[23]、光传感器[24]等领域。尤其是它表面的氧空位和铝填位等缺陷[25-26]使其具有能够吸引带负电离子的特性,为制备量子点/氧化铝异质结提供了一种新的方法。但是原材料的表面缺陷[27]不仅抑制了异质结载流子输运,还提高了非辐射复合率,从而限制了它的应用。然而,在原材料表面增加一层电子导体是有效输运载流子和减少非辐射复合的重要方法[28]。
TiN作为过渡金属氮化物,是一种重要的表面等离子体材料[29],能够与现行的纳米加工技术兼容[30]。由于TiN的电子结构是由离子键、共价键和金属键结合而成的,氮的p轨道能级低于费米能级,导致了类似于金、银和其他贵金属的自由电子的运动[30-31],使得TiN纳米粒子具有类似于金纳米粒子的性质,如可以作为转移电子的导体[32]。Naldoni等[33]发现,与Au相比,TiO2纳米线上的TiN纳米粒子在太阳能水分解中产生的光电流提高了25%,TiN与TiO2之间形成欧姆接触,增强了热电子在界面上的传输。相比于金和银不能通过选择性干法刻蚀技术进行纳米结构化,TiN能与纳米加工技术相兼容。此外,TiN具有高硬度、高化学稳定性和高耐腐蚀性等优点[34],而且高温性能更好,其熔点高达2950 ℃,远高于Au、Ag等贵金属。这些良好的性能使其广泛应用于燃料电池[35]、微电子[36]和电容器[37]等领域。
本文采用电化学沉积方法和胶体自组装方法分别将TiN纳米粒子和表面带有羧基的CdSe量子点组装到多孔Al2O3薄膜表面,制备CdSe/TiN/Al2O3异质结,利用TiN纳米粒子良好的电子传输特性,将CdSe量子点的能量转移到Al2O3界面,实现TiN/Al2O3异质结界面荧光增强。
2 实验
2.1 材料和仪器
乙醇和正己烷为分析纯级别的试剂,分别购自天津富宇精细化工有限公司和阿拉丁试剂有限公司。铝基多孔Al2O3薄膜(上海纳腾公司)为蜂窝状孔分布结构,其孔尺寸约为70 nm,孔深约为5 μm。 实验中样品采用超声波清洗仪(深圳市语盟超声波清洗机设备厂,YM-031S)清洗。电化学工作站(瑞士万通PGSTAT302N)用于在多孔氧化铝表面沉积TiN纳米粒子。扫描近场光学显微镜(以色列,NanonicsMV4000)用于测量异质结表面荧光,其激发光采用中心波长为360 nm的连续激光(长春新产业,UV-360)。异质结的吸收光谱采用紫外-可见吸收光谱仪(日立,UV-4000)的反射模式测量;本文中的溶剂无特殊说明均为去离子水。CdSe量子点由本课题组前期所提方法[38-39]制备,单个量子点的尺寸约为3.5 nm,为闪锌矿结构,其数密度约为3×1018 mL-1。
2.2 TiN/Al2O3薄膜的制备
2.3 CdSe/TiN/Al2O3异质结的制备
图 2. CdSe/TiN/Al2O3异质结的流程示意图
Fig. 2. Schematic of CdSe/TiN/Al2O3 heterojunctions preparation
2.4 表征
以KCl溶液作为电解质,利用电化学工作站的循环伏安法测量TiN/Al2O3薄膜电学特性。以多孔Al2O3为背景,利用吸收光谱仪分别测量TiN/Al2O3和CdSe/TiN/Al2O3异质结的紫外-可见吸收光谱;采用扫描近场光学显微镜原位测量多孔Al2O3薄膜、TiN/Al2O3薄膜,以及CdSe/TiN/Al2O3异质结的荧光光谱和表面形貌。将CdSe量子点溶液放入比色皿中,利用中心波长365 nm的
LED光源激发,以光纤光谱仪(海洋光学,QE6500)收集荧光,其积分时间设置为100 ms。
3 结果与讨论
0.1 mol/L的KCl溶液作为电解液,三个TiN/Al2O3薄膜样品的伏安特性曲线如图3(b)所示,所有样品都有两组氧化还原峰,它们均在-1 V处存在一个显著的还原峰,-0.85 V处存在一个显著的氧化峰,且随着纳米TiN含量的增加,峰电流增大,说明TiN/Al2O3薄膜的导电性变强。它们还有一对较弱的氧化还原峰,分别在-0.22、-0.18、-0.16 V处出现还原峰,-0.26、-0.22、-0.21 V处出现另一个氧化峰,说明随着纳米TiN含量的增加,其峰电位绝对值越小。
3.1 TiN纳米粒子电化学沉积及TiN/Al2O3薄膜表面特性
图 3. TiN/Al2O3薄膜的电化学表征。(a)电化学沉积过程中,TiN/Al2O3薄膜的表面电势随时间变化曲线;(b) TiN/Al2O3薄膜的伏安特性曲线
Fig. 3. Electrochemical characterization of TiN/Al2O3 film electrodes. (a) Curves of the surface potential with deposition time on TiN/Al2O3 film electrodes; (b) volt-ampere curves of TiN/Al2O3 film electrodes
3.2 TiN/Al2O3薄膜和CdSe/TiN/Al2O3异质结的紫外—可见吸收特性
图 4. 50 μA沉积电流的TiN薄膜和CdSe/TiN薄膜的吸收光谱
Fig. 4. Absorption spectra of TiN film and CdSe/TiN film at 50 μA depositing time
量子点向红外方向有较大的延长(CdSe量子点吸收截止,而TiN/Al2O3还具有很强的吸收),由于CdSe/TiN/Al2O3的异质结效应,TiN/Al2O3的光生载流子被它们之间的表面势垒所阻挡,在界面处快速回落,故在580 nm附近的吸收峰明显变窄,且有明显蓝移,而400 nm附近的吸收峰相同,吸收增强明显,是CdSe量子点的吸收导致的。
3.3 CdSe/TiN/Al2O3异质结的表面增强荧光效应
为对比CdSe/TiN/Al2O3异质结的荧光光谱(FL),
图 5. 样品的荧光光谱表征。(a)多孔Al2O3和CdSe量子点溶液的荧光光谱;(b)不同沉积电流时,CdSe/TiN/Al2O3异质结的荧光光谱
Fig. 5. Fluorescence spectra characterization of the samples. (a) Fluorescence spectra of the porous Al2O3 film and CdSe quantum dot solution; (b) fluorescence spectra of CdSe/TiN/Al2O3 heterojunctions at different depositing currents
图 6. CdSe/TiN/Al2O3异质结光生载流子输运示意图
Fig. 6. Diagram of optical generating carriers transfer CdSe/TiN/Al2O3 heterojunction
3.4 CdSe/TiN/Al2O3异质结表面光谱地形图分析
为证实以上界面能量转移过程,本文使用扫描近场光学显微平台原位地测量了CdSe/TiN/Al2O3异质结的原子力显微镜(AFM)表面形貌和光谱形貌图,其异质结表面形貌如
图 7. TiN/Al2O3薄膜和CdSe/TiN/Al2O3异质结的AFM表面形貌。(a) TiN/Al2O3薄膜的AFM形貌;(b) CdSe/TiN/Al2O3异质结的AFM表面形貌
Fig. 7. AFM morphology of TiN/Al2O3 film and CdSe/TiN/Al2O3 heterojunction. (a) AFM morphology of TiN/Al2O3 film; (b) AFM morphology of CdSe/TiN/Al2O3 heterojunction
为测量自组装纳米线状结构区域与非线状区域之间的光谱变化,在
图 8. TiN/CdSe/Al2O3异质结的光谱形貌图。(a)(b) TiN/Al2O3界面的荧光峰分布及其强度;(c)(d) CdSe/TiN界面的荧光峰分布及其强度
Fig. 8. Optical spectral morphology of CdSe/TiN/Al2O3 heterojunction. (a)(b) Distribution of fluorescence peaks and its intensity from TiN/Al2O3 interface; (c)(d) distribution of fluorescence peaks and its intensity from CdSe/TiN interface
4 结论
本文采用电化学沉积法和胶体化学法制备了CdSe/TiN/Al2O3异质结,分别利用紫外-可见吸收光谱仪和扫描近场光学显微镜测量了CdSe/TiN/Al2O3异质结的吸收光谱和表面增强荧光效应。结果表明,CdSe量子点可以明显提高异质结的吸收效率,且CdSe量子点自组装结构越多的区域,能量转移效率越高,增强荧光越强;TiN纳米粒子的表面等离子体态作为CdSe量子点和多孔Al2O3的中间能级,将CdSe量子点的光生载流子转移到TiN/Al2O3薄膜界面与空穴复合向外辐射荧光,提高了异质结界面电子转移率,减少非辐射重构,增强了TiN/Al2O3薄膜界面的荧光,最大增强了2.5倍,且该荧光发生了明显的红移;随着TiN纳米粒子沉积厚度的增加,CdSe量子点和多孔Al2O3之间的距离增加,CdSe量子点的光生载流子在转移过程中更容易被损耗,导致异质结中多孔Al2O3界面的辐射荧光减弱,但该荧光峰值波长红移会更明显。这个荧光增强和能量转移的方法可广泛应用于控制光电控测、光显示和光传感等领域。
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Article Outline
刘鹏程, 昌梦雨, 白忠臣, 秦水介. TiN纳米粒子增强CdSe/Al2O3异质结荧光的研究[J]. 中国激光, 2020, 47(9): 0913001. Liu Pengcheng, Chang Mengyu, Bai Zhongchen, Qin Shuijie. Enhanced Fluorescence of CdSe/Al2O3 Heterojunctions Enabled by TiN Nanoparticles[J]. Chinese Journal of Lasers, 2020, 47(9): 0913001.
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