与其它贵金属(Au、Pt、Pd)相比,Ag具备无毒且价格较低、易制备、化学性质稳定等优点[16,17]。同时,光沉积法成本较低和工艺简单,通过调整沉积时间、光强和前驱体溶液配比即可在室温下实现Ag纳米粒子的可控制备[18~20]。目前,关于用金属Ag修饰半导体以加快电子-空穴对分离已有大量的研究工作。Liu等[21]通过将化学气相沉积与热蒸发相结合制备了Ag修饰ZnO阵列,能有效分离电子-空穴对。且与未修饰ZnO相比,其紫外探测性能显著提高。Devi等[22]合成了一种Ag修饰CeO2纳米棒光电探测器,Ag纳米粒子修饰能显著抑制CeO2纳米棒电子-空穴对的复合并提升其捕获电子的能力。Joshna等[23]制备的Ag修饰TiO2纳米管(TiO2 NTs),金属Ag显著减少了电子-空穴对的复合,Ag纳米粒子修饰的TiO2 NTs的光电流是纯TiO2 NTs的120倍。同时,在各种SnSe纳米结构材料中一维SnSe纳米管具有高电子传输效率、几何异向性和量子限域效应,更有利于提高红外探测性能[24~26]。因此,使用Ag修饰的SnSe纳米管有望制备出高性能红外探测器。本文用光沉积法在SnSe纳米管表面修饰金属Ag纳米粒子,在室温下合成Ag修饰SnSe(Ag/SnSe)纳米管并以Pt为对电极组装红外探测器,研究其在模拟红外光(830 nm)照射下的红外探测性能、光响应速度和循环稳定性,并讨论其机理。
1 实验方法
1.1 Ag/SnSe纳米管的制备
以Se纳米线为模板,用溶液法制备SnSe纳米管[26],然后将0.02 g的SnSe纳米管加到30 mL的0.05 mol/L 硝酸银溶液中并磁力搅拌使其分散均匀；用波长为365 nm的紫外光照射溶液,光沉积15 min后自然沉降5 min。然后将得到的样品用去离子水清洗并离心分离出Ag/SnSe纳米管,重复2次后将其在烘箱中低温干燥。
1.2 Ag/SnSe纳米管红外探测器的制备
将制备出的Ag/SnSe纳米管分散在无水乙醇中,然后旋涂到FTO导电面后烘干,将这一过程重复三次,获得Ag/SnSe纳米管薄膜。以负载在FTO表面的Ag/SnSe纳米管薄膜为工作电极,以镀Pt的FTO为对电极,通过热封膜将工作电极和对电极相连接,中间注入聚硫电解质溶液后密封。
1.3 性能表征
使用扫描电子显微镜(SEM,Hitachi SU-70)及附带的X射线能谱仪(EDS)表征样品形貌和化学成分。用透射电子显微镜(TEM,FEI,Tecnai G2 F20)和高分辨透射电子显微镜(HRTEM)观测样品的形貌。用X射线衍射仪(XRD,Bruker D8 Advance)表征样品的晶体结构。将组装好的Ag/SnSe纳米管探测器与Keithley 2400数字源表连接,使用830 nm的光作为模拟红外光源,测试其红外探测性能。
2 结果与讨论
图1
图1
SnSe纳米管和Ag/SnSe纳米管的SEM照片
Fig.1
SEM images of SnSe nanotubes (a, b) and Ag/SnSe nanotubes (c, d)
用EDS分析Ag/SnSe纳米管的元素组成,结果如图2所示。图中有源于SnSe纳米管的Se和Sn元素的特征峰,最强峰来自测试支撑Si衬底(用于样品形貌观察)。在2.99 keV处出现了Ag元素的特征峰,表明在SnSe纳米管表面沉积了Ag纳米粒子。
图2
图3
图3
Ag/SnSe纳米管的TEM和高分辨TEM照片
Fig.3
TEM (a) and high resolution TEM (b) images of Ag/SnSe nanotube
图4
在无外加偏压条件下用830 nm的光作为红外光模拟光源,开启红外光照射10 s后关闭红外光10 s作为一个测试周期,研究了Ag/SnSe纳米管探测器对红外光的探测性能,其结果如图5所示。在无红外光照射时Ag/SnSe纳米管红外探测器为静默状态,光电流密度几乎为零；开启红外光后器件瞬间产生光电流并快速攀升至最大值120 nA/cm2,然后逐渐稳定。关闭红外光后,光电流迅速衰减并恢复到初始状态。经过6次开关循环,电流密度曲线没有明显的变化,这表明,所组装的探测器具备高电流密度和较好的耐用性。同时,器件能在无偏压条件下稳定工作,表明其具有自供能特性。在相同的实验条件下测试了SnSe纳米管红外探测器的性能,其光电流密度只有46 nA/cm2,比Ag/SnSe纳米管红外探测器的光电流密度降低了61%。这表明,Ag纳米粒子修饰明显提高了SnSe纳米管探测器对红外光的探测性能。
图5
图5
SnSe纳米管和Ag/SnSe纳米管红外探测器在开/关红外光照射下的电流密度曲线
Fig.5
Time dependent current response of the SnSe and Ag/SnSe nanotubes infrared photodetector (IRPD) measured under on/off of IR light illumination
图6
图6
SnSe纳米管和Ag/SnSe纳米管红外探测器单个周期的光电响应特征曲线
Fig.6
Single-cycle photocurrent response of the IRPD based on SnSe and Ag/SnSe nanotubes
Ag/SnSe纳米管红外探测器的机理如图7所示。Ag/SnSe纳米管独特的鱼鳞状中空结构和大比表面积,显著提高了对红外光的吸收能力,进而提高了对红外光的利用效率。用红外光照射时SnSe纳米管吸收的光子能量高于其带隙,因此光生电子激发后从价带跃迁至导带并在价带留下空穴。电子-空穴对的快速复合严重影响了SnSe纳米管探测器的效率。而在SnSe表面负载Ag纳米粒子后,金属Ag与SnSe纳米管表面发生肖特基接触产生肖特基势垒,进而出现内建电场。在光生电子从SnSe表面迁至单质Ag的过程中促进载流子的分离,抑制了光生电子-空穴对的复合。同时,随着SnSe纳米管和电解液间电子-空穴对的分离[30],电子通过外部电路迁移至Pt电极并与电解液中的S x2-反应生成S2-和S x-12-。生成的S2-在电解液中扩散至SnSe纳米管与其表面的空穴反应生成S单质,S与S x-12-进一步反应生成S x2-[31]。S x2-和S2-没有被消耗而持续循环,使光生电子通过外电路产生电流。因此,Ag/SnSe纳米管红外探测器能实现对红外光的自供能探测。关闭红外光后没有光生电子产生,Ag/SnSe纳米管红外探测器迅速恢复到初始状态。
图7
图7
Ag/SnSe纳米管红外探测器的原理
Fig.7
Schematic illustration of the possible IR detection mechanism of the Ag/SnSe nanotubes IRPD
3 结论
用光沉积法将Ag纳米颗粒沉积在鱼鳞状中空SnSe纳米管表面,在室温下制备Ag/SnSe纳米管。Ag/SnSe纳米管表面粗糙致密,在表面能观察到微小的Ag纳米粒子。与SnSe纳米管探测器相比,用Ag修饰的SnSe纳米管红外探测器的光电流密度提高了约160%(达到120 nA/cm2)、光响应速度也明显改善、上升时间缩短至0.109 s、下降时间缩短至0.086 s,且具有较高的循环稳定性。
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