Controllable Synthesis of Ag-In-Zn-S Quaternary Nanocrystals and Their Applications in Electroluminescent Light-emitting Diodes

Ag-In-Zn-S四元半导体纳米晶的可控制备及其在电致发光二极管中的应用 Controllable Synthesis of Ag-In-Zn-S Quaternary Nanocrystals and Their Applications in Electroluminescent Light-emitting Diodes 1 张京(1996-)，女，山东菏泽人，硕士研究生,2018年于西安理工大学获得学士学位，主要从事低毒无镉量子点材料合成及应用的研究。E-mail: 18121687@bjtu.edu.cn http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21349997&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21349998&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21349999&type= 张京 (1996-)，女，山东菏泽人，硕士研究生,2018年于西安理工大学获得学士学位，主要从事低毒无镉量子点材料合成及应用的研究。E-mail: 18121687@bjtu.edu.cn 18121687@bjtu.edu.cn 1 1 1 1 corresp E-mail: floodin@163.com E-mail: floodin@163.com 訚哲(1992-)，女，辽宁丹东人，博士，讲师,2020年于清华大学获得博士学位，主要从事碳基柔性电子材料及器件的制备和研究。E-mail: floodin@163.com http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21350001&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21350003&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21350004&type= 訚哲 (1992-)，女，辽宁丹东人，博士，讲师,2020年于清华大学获得博士学位，主要从事碳基柔性电子材料及器件的制备和研究。E-mail: floodin@163.com floodin@163.com 1 corresp awtang@bjtu.edu.cn awtang@bjtu.edu.cn 唐爱伟(1981-)，男，山东临沂人，博士，教授，博士研究生导师,2009年于北京交通大学获得博士学位，主要从事光电功能纳米材料的构筑及光电应用的研究。E-mail:awtang@bjtu.edu.cn http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21350008&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21350012&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21350017&type= 唐爱伟 (1981-)，男，山东临沂人，博士，教授，博士研究生导师,2009年于北京交通大学获得博士学位，主要从事光电功能纳米材料的构筑及光电应用的研究。E-mail: awtang@bjtu.edu.cn awtang@bjtu.edu.cn 北京交通大学　理学院，北京 　 100044 北京交通大学　理学院，北京 　 100044 School of Science, Beijing Jiaotong University, Beijing 100044, China School of Science, Beijing Jiaotong University, Beijing 100044, China Ag-In-Zn-S四元半导体纳米晶(以下简称AIZS NCs)不仅具有传统半导体纳米晶带隙可调、发光效率高等优异的发光特性，同时凭借其低毒和合成工艺简单等优点，在发光二极管、生物医学和光电转换等领域得到了广泛应用，成为传统镉基半导体纳米材料的有力竞争者之一。本文通过一步反应法制备出发光性能良好的AIZS NCs，并通过组分调控扩大其发光范围，使其发光颜色从绿光调至红光。在此基础上，采用注射法研究了AIZS NCs的形成过程，证明AIZS纳米晶的形成是阳离子交换反应发生的结果。为了进一步提高其发光性能，采用种子生长法继续在AIZS纳米晶中引入Zn源形成合金型AIZS-ZnS NCs，使其光致发光量子产率达到47%。最后，采用全溶液处理方法以AIZS-ZnS NCs作为发光层构筑了绿、黄和红三色电致发光二极管，其中黄光电致发光二极管的电流效率达到了1.07 cd·A -1 。 Ag-In-Zn-S quaternary semiconductor nanocrystals(AIZS NCs) not only have the excellent luminescent proeprties including the tunable optical band gap and high photoluminescence quantum yield and so on, but also possess low toxicity and simple synthesis technique. These NCs were widely used in the fields of light-emitting diodes, biomedicine, photoelectric conversion devices and so on, which was regarded as one of the substitutes for cadmium-based nanomaterials. In this paper, highly luminescent Ag-In-Zn-S NCs were synthesized by one-step reaction method, and the emission region was tuned effectively by varying the element ratios, which led to the emission color from green to red. On the basis of the reaction, the formation mechanism of AIZS NCs was studied through a hot-injection method, indicating that the partial cation exchange dominated the formation of AIZS NCs. In order to further optimize the luminescence performance, the alloyed AIZS-ZnS NCs were obtained by further incorporation of Zn source into AIZS NCs through a seeded-mediated growth method, which exhibited a high photoluminescent quantum yield of 47%. Finally, red, yellow and green electroluminescent light-emitting diodes were fabricated by using AIZS-ZnS NCs as the emissive layer, and the yellow device exhibited a current efficiency of 1.07 cd·A -1 . Ag-In-Zn-S 半导体纳米晶 控制制备 组分调控 电致发光二极管 Ag-In-Zn-S semiconductor nanocrystals controllable synthesis component tunable electroluminescent light-emitting didoes 1　引言 1 胶体半导体纳米晶(NCs)，又称为半导体量子点，特别是Ⅱ-Ⅳ NCs(CdSe,ZnS)、Ⅲ-Ⅴ NCs(InAs,InP)、Ⅳ-Ⅵ NCs(PbS,PbTe)，由于其独特的光电性质，在过去几十年中受到了学界的广泛研究和产业界的高度重视。凭借其成熟的合成技术，半导体纳米晶被广泛应用于光电子 ［ 1 1 - 3 3 1-3 ］ 和生物医学等领域 ［ 4 4 - 5 5 4-5 ］ 。然而，这几类半导体纳米晶的合成需要使用毒性较大的重金属元素，如Cd、Pb和Hg等，对环境产生不可逆的污染，所以这几类半导体纳米晶在实际应用中受到了很大限制。低毒Ⅰ-Ⅲ-Ⅵ NCs由于其具有丰富的材料储量和良好的发光性能等优点引起了人们极大的研究兴趣。其中，铜基硫族半导体纳米晶是研究最为广泛的材料之一，并已在光电器件和生物医学领域显示出较大的应用潜力 ［ 6 6 - 12 12 6-12 ］ 。但是，由于在高温时Cu + 离子具有较大的离子迁移率，在纳米结构调控方面有一定的难度。近年来，人们逐渐把研究目光转移到Ag基半导体纳米材料上。目前,Ag-In-S NCs(AIS NCs)和AIZS NCs由于其优异的光电特性而受到越来越多的关注。迄今为止，基于AIS NCs和AIZS NCs的高品质量子点合成及其在LEDs ［ 13 13 - 19 19 13-19 ］ 、太阳能电池 ［ 20 20 - 21 21 20-21 ］ 和生物成像 ［ 22 22 - 25 25 22-25 ］ 等领域的应用已经取得了一定进展。本文使用一步反应法合成了发光性能良好的AIZS NCs，通过组分调控的方式分别得到了绿色、黄色和红色AIZS NCs，并结合注射法的特点对其形成机制进行了探究。为进一步优化发光性能，在AIZS纳米晶中继续引入Zn源形成合金型AIZS-ZnS NCs。在此基础上，采用全溶液处理方法以AIZS-ZnS NCs作为发光层构筑了绿、黄和红光电致发光二极管，其中黄光器件的峰值电流效率达到了1.07 cd/A。 2　实验 1 2.1　AIZS NCs和AIZS-ZnS NCs合成 2 采用一步反应法合成AIZS NCs:将0.075 mmol硝酸银(AgNO 3 )、0.5 mmol 氯化铟(InCl 3 )、1 mmol乙酸锌(Zn(CH 3 COO) 2 )和2 mmol硫粉加入四口烧瓶中，再加入10 mL十八碳烯(ODE)、3 mL十二硫醇(DDT)、1 mL油酸(OA)和1 mL油胺(OM)。在N 2 保护下，将溶液升温至80 ℃保温30 min，然后升温至230 ℃，保温10 min后得到发光性能良好的绿光AIZS NCs。为了在AIZS NCs表面外延生长ZnS，待上述反应后，在230 ℃条件下注射Zn前驱体(1 mmol的Zn(CH 3 COO) 2 与1.5 mL ODE、0.1 mL DDT、0.2 mL OA以及0.4 mL OM加到小瓶中,170 ℃下加热30 min，使其充分溶解)，继续反应30 min便得到AIZS-ZnS NCs。为得到不同发光颜色的AIZS NCs，将Ag、In、Zn、S前驱体的比例从1.5∶10∶20∶40调至3∶10∶20∶40和6∶10∶20∶40。 2.2　电致发光二极管构筑 2 所有电致发光二极管器件都是在涂有氧化铟锡(ITO)的玻璃基板上制备的，先使用清洗剂清洗所有玻璃基板，再依次使用去离子水、丙酮、异丙醇将玻璃基板超声清洗30 min，最后对玻璃基板进行紫外线臭氧处理30 min。以5 000 r/min的速度将聚(3,4-乙二氧基-噻吩)/聚苯乙烯磺酸盐(PEDOT∶PSS)旋涂到玻璃基板上,PEDOT∶PSS的厚度为3040 nm，旋涂后在150 ℃下退火30 min。随后，将玻璃基板转移到充满氮气的手套箱中，依次将聚(9,9-二辛基芴-co-N-(4-丁基苯基-1)二苯基-三聚氰胺(TFB)溶液(8 mg/mL)、AIZS-ZnS NCs(12 mg/mL)、ZnO纳米颗粒(30 mg/mL)旋涂到PEDOT∶PSS上。旋涂各层后,TFB层在160 ℃下退火30 min,AIZS-ZnS NCs层静置10 min,ZnO纳米颗粒层在60 ℃下退火30 min。最后，在高真空沉积室中通过热蒸发的方式沉积Al阴极，有效面积为4 mm 2 。 2.3　性能表征 2 AIZS纳米晶的XRD谱图使用Bruker D8 Advance X射线粉末衍射仪进行表征，辐射源为Cu Kα( λ =0.154 056 nm)。材料的光致发光光谱采用FLUORAT-02-PANORAMA荧光光谱仪测定。材料的吸收光谱采用Ocean Optics USB 2000分光光度计进行测定。材料的发光寿命分析使用Omni- λ 300i光谱仪进行表征。材料组分使用电感耦合等离子体原子发射光谱进行分析。所有测试均在室温下进行。 3　结果与讨论 1 3.1　AIZS NCs的组分调控和发光性能 2 组分调控已被证实是调节银基硫族半导体纳米晶发光颜色的重要手段之一 ［ 26 26 - 28 28 26-28 ］ 。在本工作中，保持In、Zn前驱体和硫源的用量不变，通过调节Ag前驱体的用量可制备出绿光、黄光和红光AIZS NCs(如 图1(a) 图1(a) 插图所示)。为方便描述，将AgNO 3 的用量分别为0.075,0.15,0.3 mmol时所得产物标记为样品Ag-1.5、Ag-3、Ag-6。这3种颜色样品的吸收光谱和光致发光光谱分别如 图1(a)和1(b) 图1(a)和1(b) 所示。从 图1(a) 图1(a) 中可以看出3种不同Ag含量下所制备的AIZS NCs的吸收光谱都较宽且没有明显的激子吸收峰。随着Ag前驱体用量的增加，其吸收边缘有明显红移。光致发光光谱的结果如 图1(b) 图1(b) 所示，随着Ag前驱体用量的增加，其光致发光峰位从515 nm红移至685 nm，且光谱的半高全宽有所增加。为了进一步确认样品的晶型结构, 图1(c) 图1(c) 给出了3种样品的XRD谱图，从图中可以看出所有AIZS NCs都有3个明显的衍射峰，这是立方闪锌矿晶型的典型特征，且这3个衍射峰位于四方相AIS(JCPDS No.65-5163)和立方相ZnS(JCPDS No.77-2100)的相应衍射峰之间。随着Ag前驱体用量的增加，每个衍射峰都向低角度AIS方向移动，这说明得到的纳米晶不是AgInS 2 和ZnS纳米晶的混合物，而是合金结构的AIZS NCs。为了进一步研究3种样品的激发态寿命，采用时间分辨光致发光光谱研究了其发光寿命，结果如 图1(d) 图1(d) 所示。3种样品的平均寿命( τ av )可以通过三指数函数对光致发光衰减曲线进行拟合, τ av 可以根据下列公式进行计算: http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21350032&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21350036&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21350040&type= 其中, τ 1 、 τ 2 和 τ 3 表示不同类型发光的衰减时间, A 1 、 A 2 和 A 3 表示在 t =0时不同衰减时间的振幅大小。相应的拟合数据如 表1 表1 所示, τ 1 为快速的衰减指数, τ 2 和 τ 3 为较慢的衰减指数。快速的衰减组分来自表面缺陷态复合发光，而较慢的衰减组分来自纳米晶生长过程中产生的不同内部缺陷导致的施主-受主对复合发光 ［ 29 29 - 31 31 29-31 ］ 。根据公式(1)，可计算出绿色、黄色和红色AIZS NCs的平均寿命分别为168.9,274.4,682.7 ns。从上述结果可以看出，随着Ag前驱体用量的增加，其发光寿命有所增加，这与Cu-In-Zn-S纳米晶观察到的结果一致 ［ 10 10 ］ 。根据 表1 表1 中衰减时间的组分比例( α )可以看出,AIZS NCs的发光主要来自施主-受主对复合发光，表面缺陷态复合发光只占很少一部分。随着Ag前驱体用量的增加，施主-受主对复合发光所占比例增大，从而使其平均发光寿命明显增加。同时，随着Ag前驱体用量的增加, τ 1 组分比例从9.4%大幅下降至0.5%，说明表面缺陷态发光大幅减少，同时也说明Ag前驱体用量对AIZS NCs的发光性能有重要的影响。基于施主-受主对复合发光的特点,AIZS NCs的发光衰减速度很大程度上依赖于纳米晶发光能量的大小，在较高能量处发光纳米晶的发光寿命衰减较快，较低能量处发光纳米晶的发光寿命衰减较慢 ［ 29 29 , 32 32 - 36 36 32-36 ］ 。因此，随着Ag前驱体用量的增加,AIZS NCs逐渐向低能量方向移动，因而纳米晶的发光寿命衰减变慢，发光寿命有所增加。 1 1   1 1 τ 1 /ns 1 1 α 1 /% 1 1 τ 2 /ns 1 1 α 2 /% 1 1 τ 3 /ns 1 1 α 3 /% 1 1 τ av /ns 1 1 Ag-1.5 1 1 18.8 1 1 9.4 1 1 79 1 1 39.3 1 1 265.2 1 1 51.3 1 1 168.9 1 1 Ag-3 1 1 18.6 1 1 4.1 1 1 104.5 1 1 28.9 1 1 363.4 1 1 67.0 1 1 274.4 1 1 Ag-6 1 1 16.8 1 1 0.5 1 1 174.3 1 1 7.8 1 1 729.6 1 1 91.7 1 1 682.7 3.2　AIZS NCs的形成过程探究 2 为了探究AIZS NCs的形成过程，我们采用注射法合成AIZS NCs，即先在160 ℃下制备出AIS NCs，然后采用注射法将Zn前驱体溶液注入到AIS NCs中得到AIZS NCs，其他条件与一步反应法合成AIZS NCs相同。 图2(a) 图2(a) 给出了注射Zn前驱体溶液后随温度升高和时间延长所得样品的光致发光光谱，从图中可以看出注射Zn前驱体溶液之后，随着温度的升高，样品的光致发光峰持续蓝移。当温度升高至230 ℃后，反应时间延长20 min后样品并没有出现明显蓝移。 图2(b) 图2(b) 是通过外推吸收曲线的线性部分( αhν ) 2 得到的AIS NCs(160 ℃)和AIZS NCs(230 ℃)的光学带隙比较图 ［ 37 37 - 38 38 37-38 ］ ，可知随着Zn 2+ 的进入光学带隙有所增加。相应的XRD图谱表明，注射Zn前驱体溶液后，随着温度升高，样品的衍射峰朝较大2 θ 角度方向移动，即ZnS晶型方向移动，说明较高温度有利于Zn 2+ 向AIS晶格中扩散(如 图2(c) 图2(c) 所示)。结合ICP数据分析( 表2 表2 )可知，注入Zn前驱体溶液后纳米晶中的Ag + 和In 3+ 的含量持续减少，而Zn 2+ 的比例持续增加，证明是Zn 2+ 与Ag + 和In 3+ 之间发生了阳离子交换形成了AIZS NCs ［ 29 29 ］ 。 1 1 反应产物 1 1 Ag/% 1 1 In/% 1 1 Zn/% 1 1 AIS-160 ℃ 1 1 13 1 1 87 1 1 0 1 1 AIZS-160 ℃ 1 1 7 1 1 48 1 1 45 1 1 AIZS-200 ℃ 1 1 6 1 1 39 1 1 55 1 1 AIZS-230 ℃ 1 1 6 1 1 32 1 1 62 1 1 230 ℃-20 min 1 1 4 1 1 29 1 1 67 图2(d) 图2(d) 给出了AIS和AIZS纳米晶的时间分辨光致发光光谱。从图中可以看出注入Zn前驱体后纳米晶的发光寿命增加，结合 表3 表3 的拟合数据分析，引入Zn 2+ 后, τ 1 的组分有所减少，说明Zn 2+ 的引入钝化了纳米晶的部分表面缺陷，而 τ 2 和 τ 3 及其所占组分均有所增加，这是因为引入Zn 2+ 后由于部分阳离子交换反应使得纳米晶的内部缺陷增多，从而使得施主-受主对复合发光所占的比例增加，引起光致发光寿命增加 ［ 29 29 , 39 39 - 41 41 39-41 ］ 。 1 1   1 1 τ 1 /ns 1 1 α 1 /% 1 1 τ 2 /ns 1 1 α 2 /% 1 1 τ 3 /ns 1 1 α 3 /% 1 1 τ av /ns 1 1 AIS-160 ℃ 1 1 18.2 1 1 3.6 1 1 101.5 1 1 25.5 1 1 376.1 1 1 70.9 1 1 250.9 1 1 AIZS-230 ℃ 1 1 19.6 1 1 2.5 1 1 139.6 1 1 23.6 1 1 450.0 1 1 74.9 1 1 309.0 3.3　AIZS-ZnS NCs的发光性能研究 2 AIZS NCs表面存在大量的不饱和键及表面缺陷态，为光生载流子的非辐射复合提供了路径，不利于AIZS NCs的发光性能。为了解决这个问题，通常在纳米晶的表面外延生长宽带隙且与核芯纳米晶匹配的无机材料形成核壳结构或者合金型结构，用于钝化表面缺陷态从而提升其发光性能和稳定性 ［ 42 42 - 43 43 42-43 ］ 。但是，由于Ag离子在高温下也具有一定的离子流动性，因此在壳层的外延生长过程中也会发生部分阳离子交换反应。一般情况下,ZnS具有与AIZS 相似的晶体结构、晶格参数，并且具有相对较大的带隙(3.6 eV)，所以选择ZnS作为壳层材料。 图3(a) 图3(a) 给出了绿光、黄光和红光AIZS NCs在继续注射Zn源前后所得样品的光致发光光谱。图中虚线表示的是注射Zn前驱体后的样品。从图中发现所得样品的光致发光峰位置都有不同程度的蓝移，这与铜基硫族半导体纳米晶观察到的结果一致，说明在外延生长过程中发生了部分阳离子交换反应。以绿光AIZS NCs为例，在注射Zn前驱体后其相对光致发光量子产率达到47%。 图3(b) 图3(b) 给出了绿色AIZS纳米晶注射Zn源前后所得样品的时间分辨光致发光光谱，从图中可以看出注射Zn源后，其发光寿命有所增加，说明注射Zn源后样品的施主-受主复合所占比例增加，其内部发生了部分阳离子交换，这与光谱发生蓝移是一致的。相应的XRD图谱表明注射Zn源后，样品的XRD衍射峰位向高角度的ZnS晶型方向移动(如 图3(c) 图3(c) 所示)，这与文献报道的合金结构纳米晶的结果一致。 3.4　基于AIZS-ZnS NCs的电致发光二极管 2 采用全溶液处理方法构筑了基于不同颜色AIZS-ZnS NCs的电致发光二极管，器件结构如 图4(a) 图4(a) 所示，其结构为ITO/PEDOT∶PSS/TFB/纳米晶/ZnO/Al。该器件中AIZS-ZnS NCs作为发光层,PEDOT∶PSS 作为空穴注入层,TFB作为空穴传输层,ZnO纳米颗粒作为电子传输层。 图4(b) 图4(b) 为不同颜色电致发光二极管的电致发光光谱，右侧为不同颜色器件在驱动电压为5 V时的照片。 图4(c) 图4(c) 给出了不同器件的电流密度-亮度-电压曲线，从图中可以看出，当驱动电压超过2 V后器件的电流密度和亮度迅速增加，黄光器件的最大亮度接近500 cd·m -2 。器件的电流效率和外量子效率结果如 图4(d) 图4(d) 所示，黄光器件的峰值电流效率达到了1.07 cd·A -1 。虽然器件性能低于一些已报道的文献结果 ［ 13 13 , 17 17 ］ ，但值得一提的是，本工作中使用的AIZS-ZnS NCs没有经过任何配体交换和改进，并且是使用全溶液法构筑的电致发光二极管(不包括铝阴极)。我们有理由相信，这种全溶液法构筑的发光二极管的器件性能还有很大提升空间。 4　结论 1 本文采用一步反应法合成了发光性能优异的AIZS NCs，并通过调节Ag前驱体的用量对其发光颜色进行了调控，成功制备出了绿光、黄光和红光AIZS NCs。结合注射法合成AIZS NCs对其形成过程进行了探讨，证明AIZS NCs的形成是由于Zn 2+ 与Ag + 、In 3+ 之间发生了阳离子交换。进一步地，通过种子生长法在AIZS NCs中继续注射Zn源提升了发光性能，发光峰位置蓝移说明在ZnS壳层生长过程中伴随着部分阳离子交换反应发生。最后，采用全溶液法以不同颜色的AIZS-ZnS NCs作为发光层构筑了电致发光二极管，其中黄光器件的电流效率达到了1.07 cd·A -1 。 参考文献: SUN L F , CHOI J J , STACHNIK D , et al. . 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