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2004年,Geim和Novoselov首次从石墨中(微)机械剥离出石墨烯(Graphene)[1],从而开辟了二维材料研究和应用的崭新时代。随后相继发现了六方氮化硼(h-BN)、过渡金属二硫族化合物(TMD),过渡金属碳/氮化物(MXene)等二维材料[2-4]。通过数据库搜索,人们能够找到800多种本体层状材料,理论上可以从中剥离出稳定的单层结构[5]。二维材料特殊的结构赋予了它卓越的力学、热学、电学、磁学和光学性能,可作为优异的导热材料、耐高温材料、润滑材料、催化材料、储能材料、绝缘材料、防护材料、光电转换材料、可饱和吸收体等[3, 6-13]。机械剥离、化学气相沉积(CVD)、外延生长、球磨、超声/液相剥离、插层、模板辅助合成、溶剂热/水热等 [14-20]已广泛用于二维材料的制备。尽管制备方法种类繁多,不过始终面临着无法同时兼顾本征、普适和规模等方面的巨大难题。
在二维材料研究和应用过程中,一方面,人们致力于获得横向尺寸更大的高质量二维材料[16, 21],以充分发挥二维材料的结构和性能优势。如Liu等人[22]在铜箔上通过外延生长技术制备了100 cm2的单晶六方氮化硼;另一方面,人们不断尝试减小二维材料的横向尺寸,以逐渐显露其量子限域和边缘效应。本课题组[23]近期开辟了一条普适性的路径,利用硅球辅助球磨结合超声辅助溶剂剥离获得了横向尺寸小于5 nm,厚度小于1 nm的二维量子片(2D QSs)。创新性地将纳米硅球引入到球磨体系,由于硅球和玛瑙球的化学组成、密度、硬度等十分相似,由此构建了一套理想的二元协同球磨体系,进而通过全物理的方式首次实现了二维量子片的本征、普适和规模制备。
二维量子片因其独特的物理和化学性质已在诸多领域崭露头角,如催化、能源、生物成像、癌症治疗、非线性光学等[24-28]。由于具有强烈的量子限域和突出的边缘效应,二维量子片的研究和应用逐渐受到重视。本文介绍了二维量子片的基本概念、制备现状以及在光学领域的研究进展,特别回顾了近年来二维量子片在光致发光、非线性光学和固态发光器件等方面的重要进展,并且分析了二维量子片在光学领域的发展前景以及面临的主要挑战。
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量子片(quantum sheet)一词最早出现在一篇德国科学家报道的文献中[29],而与量子片一词最为接近的是二维量子点(2D quantum dot),最早出现在一篇英国科学家报道的文献中[30]。由于材料发展和制备技术的时代限制,量子片当时并没有引起广泛的关注。二维材料的出现为量子片的发展提供了不可多得的材料基础,不过本征、普适和规模制备方法的缺失,使得二维材料量子片的发展仍然极其缓慢。通过文献检索,不难发现现有研究普遍热衷于采用化学方法或化学/电化学前期处理获得追求的(二维材料)量子点而非真正的量子片,这显然不利于本征量子片的研发及其极限性能的探索。
这里需要重申一下二维(材料)量子片的概念。二维量子片作为二维材料和量子体系不断发展和交叉的产物(图1(a))[31],其横向尺寸一般小于20 nm,厚度通常在10层以下。本文对量子片概念进行了全新解析[31],首先,量子片概念不限于层状材料,非层状材料同样适用。其次,量子片概念同时包含几何结构和量子特性,例如,从几何结构来讲,量子片为二维量子尺度材料;从量子特性来讲,量子片同时具有面外和面内量子限域效应,而且为面外/面内各向异性。显然,二维量子片不仅保留了二维材料的本征特性,而且具有强烈的面外和面内量子限域效应以及突出的边缘效应。
二维量子片的研究和应用尚处于起步阶段,大多数集中于石墨烯量子片,而对其他量子片的关注相对较少(图1(b))[4]。不过,二维量子片的独特结构以及全新性质,使得它在诸多领域有着潜在应用价值(如图1(c))[24-28]。
图 1 (a)量子片所处体系的示意图[31];(b)二维量子片的文章发表数目(2007—2016)[4];(c)二维量子片的应用领域:医药[24],生物成像[25],催化[26],太阳能电池[27],非线性光学[28]等。(b)转载自文献[4],版权所有(2018)皇家化学学会。(c) 转载自文献[24],版权所有(2018)施普林格;转载自文献[25],版权所有(2018)施普林格;转载自文献[26],版权所有(2016)自然出版集团;转载自文献[27],版权所有(2018)施普林格;转载自文献[28],版权所有(2020)美国化学学会
Figure 1. (a) Schematic diagram of the system in which the quantum sheet is located[31];(b) number of journal publications on 2D QSs from 2007 to 2016[4];(c) application fields of 2D QSs:medicine[24], biological imaging[25],catalysis[26],solar cell[27],nonlinear optics[28],etc. (b) Adapted with permission ref. [4]. Copyright 2018, Royal Society of Chemistry; (c) Reproduced with permission ref. [24]. Copyright 2018, Springer. Reproduced with permission ref. [25]. Copyright 2018, Springer; Reproduced with permission ref. [26]. Copyright 2016, Nature Publishing Group; Reproduced with permission ref. [27]. Copyright 2018, Springer; Reproduced with permission ref. [28]. Copyright 2020, American Chemical Society.
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石墨烯是一种单原子层厚度的蜂窝状全碳二维材料(图2(a))[32],具有独特的光学和电学性质,其电子迁移率最高,然而零带隙的石墨烯限制了其在光电子领域的应用。当石墨烯的横向尺寸减小至20 nm以下时(GQSs),带隙逐渐打开,从而获得可调的光致发光性质 [33]。
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氮化硼与石墨烯晶体结构极为类似(图2(b)),由等量氮原子和硼原子通过共价键组成六角网络结构。与BN纳米片相比,BNQSs的报道要远远小于预期[34]。BNQSs除了具有氮化硼本征性质,如良好的电绝缘性、高温稳定性、机械强度高、导热性好、低毒性和化学稳定性,还具有量子尺度下的各种新奇特性。
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过渡金属二硫族化合物是一类发展迅速的二维材料,在单层TMD晶体中,一层过渡金属原子夹在两层硫族原子之间。TMD的带隙随着层数的减少而变得连续可调,如二硫化钼(MoS2)(图2(c)),可以通过控制其层数调节禁带宽度,当为单层时,MoS2将从间接带隙材料转变为直接带隙材料[35],这对设计新型发光二极管、光电探测器、太阳能电池具有重要指导意义[36]。单层二硫化钼具有强烈的光致发光特性,其量子产率(QY)是本体材料的104倍[37]。图2(d)、2(e)分别为硒化铟、氮化碳的原子结构。在量子尺度,MoS2 QSs又会产生新的光学性质,其中在非线性光学领域有着诱人的应用前景[38]。
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由于与碳同族,硅烯和锗烯的量子片也引起了人们的注意,然而相关的研究仅限于理论阶段[39]。黑磷(BP)同样是单元素二维材料(图2(f)),其带隙具有典型的层数依赖性,而且BP具有较高的载流子迁移率和灵敏的光电子响应,有望用于晶体管和光电探测器等领域[6, 40, 41]。其他二维量子片,如过渡金属氧化物量子片(TMO QSs)[42, 43], MXene QSs[2, 44],硫化铟量子片(In2S3 QSs)[45],硒化铟量子片(InSe QSs)[46],砷化铟量子片(InAs QSs)[47]等亟待人们深入研究。
图 2 二维材料的原子结构。(a)石墨烯[3];(b)氮化硼[3];(c)二硫化钼[48];(d)硒化铟[49];(e)氮化碳[50];(f)黑磷[41]。(a) 和(b)转载自文献[3],版权所有(2017)美国化学学会;(c) 转载自文献[48],版权所有(2011)自然出版集团;(d) 转载自文献[49],版权所有(2017)自然出版集团;(e) 转载自文献[50],版权所有(2017)皇家化学学会;(f) 转载自文献[41],版权所有(2014)自然出版集团
Figure 2. Atomic structures of 2D materials. (a) Graphene[3]; (b) BN[3]; (c) MoS2[48]; (d) InSe[49]; (e) C3N4[50]; (f) BP[41]. (a) and (b) Adapted with permission ref. [3]. Copyright 2017, American Chemical Society. (c) Adapted with permission ref. [48]. Copyright 2011, Nature Publishing Group. (d) Adapted with permission ref. [49]. Copyright 2017, Nature Publishing Group. (e) Adapted with permission ref. [50]. Copyright 2017, Royal Society of Chemistry. (f) Adapted with permission ref. [41]. Copyright 2014, Nature Publishing Group.
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二维材料的结构特殊,面内为强的化学键连接,层间靠弱的范德瓦耳斯力维系。近年来,诸多方法已经应用到制备二维量子片中。制备方法大体分为 “自下而上(Bottom-up)”和“自上而下(Top-down)”两类(图3,彩图见期刊电子版)。
“自下而上”法通常是利用各种前驱体在化学或物理作用下生成量子片,如CVD[16],水热/溶剂热[17],湿化学法[51],溶胶凝胶法[52]等。这些方法的优点在于可以精确地控制量子片的尺寸、形貌和进行表面功能化。Ma等人[33]通过氧化石墨烯(GO)在紫外线照射下和次氯酸钠反应生成GQSs,吸收率达到78%。该方法得到的GQSs表现出了强的荧光发射行为。Leong等人[35]利用过渡金属氧/氯化物和硫的前体在温和的水溶液条件下合成TMD QSs(图3(b)),包括MoS2、WS2、RuS2、MoTe2、MoSe2、WSe2和RuSe2,其反应几乎在瞬间(大约10~20 s)达到平衡。当然,“自下而上”的方法存在诸多局限性,如反应条件严苛,程序复杂,过程中容易被化学修饰,无法大规模生产等。
“自上而下”法是从其本体材料出发,破坏其面内的化学键和层间的范德瓦耳斯力获得量子片。如液相剥离[18],电化学剥离[53, 54],球磨[19],化学刻蚀[55],离子插层[20]等。He等人[56]通过化学剥离的方法制备高产量、单层、直径为4~8 nm的FePS3 QSs。首先通过将本体FePS3和N2H4在水热条件下进行插层反应,然后通过超声剥离得到FePS3 QSs。然而该方法仅适用于面内断裂强度较小的二维材料,且须考虑寻找合适的溶剂。常规的液相剥离能够破坏二维材料的层间作用力但很难破坏面内的化学键。即便使用高能球磨,其极限尺寸也只是百纳米级。这严重限制了二维量子片的制备,因而需要发展一种简单有效的方法实现二维量子片的普适和规模制备。
本文重点强调通过物理手段来获得本征状态的二维量子片,包括机械剥离、手工研磨、球磨、液相剥离、液氮/回流预处理、超薄切片等技术或手段[23, 57-63]。本征状态为原始/初始状态,其内部为单晶品质,无缺陷,无掺杂;边缘完全裸露,无覆盖,无配体,无官能化。这使得本征量子片能够同时表达其量子限域效应和突出的边缘效应,进而展现出非凡的光学、催化等性质。南洋理工大学张华课题组采用(湿法)手工研磨与超声剥离成功制备了一系列过渡金属二硫族化合物量子片[62](图3(d)),不过产率极低(低于1 wt%)。莱斯大学Ajayan和合肥工业大学吴玉程等人利用液氮/回流预处理和超声剥离(图3(e)~3(f))制备了各种二维量子片 [57, 63],液氮/回流预处理后能够减弱层间范德瓦耳斯力,产生细小的裂纹,在后续的超声中剥离出量子片,但产率(约为1 wt%)和分散浓度(约为0.0287 mg/mL)都极低。中国科学院化学研究所郑健等人利用超薄切片和超声剥离(图3(g))制备了高质量单层单晶二维量子片[59],不过所用本体层状材料须为大块单晶。显然,这些方法虽适于制备本征状态的二维量子片,却未能同时兼顾普适和规模。
图 3 二维量子片的制备方法。自下而上:(a)化学气相沉积[66];(b)湿化学法[35]。自上而下:(c)电化学剥离[54];(d)研磨结合超声剥离[62];(e)液氮预处理和超声剥离[57];(f)回流预处理和超声剥离[63];(g)超薄切片结合液相剥离[59]。(a) 转载自文献[66],版权所有(2016)美国化学学会;(b)转载自文献[35],版权所有(2019)自然出版集团;(c) 转载自文献[54],版权所有(2015)皇家化学学会。(d) 转载自文献[62],版权所有(2015)威立出版集团;(e)转载自文献[57],版权所有(2017)美国科学促进会;(f)转载自文献[63],版权所有(2019)爱思唯尔;(g)转载自文献[59],版权所有(2020)施普林格
Figure 3. The preparation methods of 2D QSs. Bottom-up: (a) CVD [66]. (b) Wet chemical method[35]. Top-down: (c) Electrochemical exfoliation[54]; (d) grinding combined with sonication exfoliation[62]; (e) liquid nitrogen pretreatment combined with sonication exfoliation[57]; (f) reflux pretreatment combined with sonication exfoliation[63]; (g) ultrathin section combined with liquid phase dissection[59]. (a) Reproduced with permission ref. [66]. Copyright 2016, American Chemical Society; (b) adapted with permission ref. [35]. Copyright 2019, Nature Publishing Group; (c) reproduced with permission ref. [54]. Copyright 2015, Royal Society of Chemistry; (d) reproduced with permission ref. [62]. Copyright 2015, Wiley-VCH; (e) reproduced with permission ref. [57]. Copyright 2017, AAAS; (f) adapted with permission ref. [63]. Copyright 2019, Elsevier; (g) reproduced with permission ref. [59]. Copyright 2020, Springer.
本课题组近期先后开发出狭义[60]和广义[23]的二维材料量子片制备方法,成为国际上第一个也是唯一一个成功实现二维量子片的本征、普适和规模制备的研究小组。狭义和广义方法均采用球磨和超声相结合的全物理方式,均由未经任何处理的本体层状材料出发,所得成品均为本征、无缺陷二维量子片(本征特性(无掺杂、无配体等)、单晶品质)。我们深刻意识到球磨过程中力矩和压强的决定作用,对球磨方法进行了全新变革,将球磨极限推进到前所未有的水平(见图4,彩图见期刊电子版)。狭义和广义方法分别采用了盐辅助球磨(图4(a))和硅球辅助球磨(图4(c)),简单球磨是无效的。我们提出了制备机理(图4(b)),认为球磨贡献是决定性的。在狭义和广义方法中,盐微晶粒和纳米硅球分别充当了力的(传递)载体角色,而纳米硅球相对于盐微晶粒具有更大优势,如结构/几何稳定、空间分辨率高、受力均匀、压强极大等。在广义方法中,我们首次提出了(硅球和玛瑙球)二元协同球磨效应。这种全新球磨方法可以在低能状态下产生极大压强(最大超过1000 GPa),从而高效打断任何化学键[31],规模制备出量子尺度材料[23, 64, 65]。例如,我们以极高(单次循环)产率(高于30 wt%)制备出GQSs、BNQSs、MoS2 QSs等。所得量子片平均(横向)尺寸为2~3 nm,厚度1−2层(图4(d))。通过简单的沉淀/离心操作即可实现量子片粉末的高效收集(图4(d)),而且(粉末状态)量子片可在多种溶剂中良好分散,分散浓度极高(最高可达5 mg/mL),为其溶液加工带来了极大便利。在广义方法的基础上,我们成功实现了(本征)曲率量子片的规模制备(图4(e))[64]以及二维纳米片的全尺寸(1~100 nm)可控制备[65]。这种本征、普适和规模制备方法的建立,使得在相同协议/标准上建设二维量子片完整数据库/图书馆成为可能,势必会加速二维量子片的全面和细致探索,为系统深入研究二维量子片的全新物理提供了可靠平台,同时为其工业化生产及应用铺平了道路[31]。
图 4 二维量子片的本征、普适和规模制备。(a)盐辅助球磨和超声辅助溶剂剥离[60];(b)量子片的制备机理示意图[60];(c)硅球辅助球磨和超声辅助溶剂剥离[23];(d)量子片分散液和粉体照片及对应的高分辨透射电镜照片[23];(e)从多壁碳纳米管制备石墨烯量子片[64]。(a-b)转载自文献[60],版权所有(2017)美国化学学会;(c-d)转载自文献[23],版权所有(2019)皇家化学学会;(e)转载自文献[64],版权所有(2020)美国化学学会
Figure 4. Universal and scalable production of intrinsic 2D QSs. (a) Salt-assisted ball-milling and sonication-assisted solvent exfoliation[60]; (b) schematic diagram of the fabrication mechanism of 2D QSs[60]; (c) silica-assisted ball-milling and sonication-assisted solvent exfoliation[23]; (d) photographs of the QS dispersions and powders and their HRTEM images; (e) robust strategy for tailoring multi-walled carbon nanotubes into GQSs[64]. (a-b) Reproduced with permission ref. [60]. Copyright 2017, American Chemical Society; (c-d) Reproduced with permission ref. [23]. Copyright 2019, Royal Society of Chemistry; (e) Reproduced with permission ref. [64]. Copyright 2020, American Chemical Society.
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二维量子片具有良好的化学稳定性、分散性、光学性能、生物相容性,易功能化等优点[4, 66],因而引起了人们极大的研究兴趣。二维量子片还具有从深紫外到近红外的发射波长多样性,可调谐性和宽带发射的性质[67]。而这些性质是由量子片的量子限域效应、表面效应和边缘效应所决定的。此外,二维量子片的晶格破缺为其光学、化学、电子活性增加了一个额外的维度[35]。
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光致发光(PL)是二维量子片的一个重要的性质,许多模型被相继提出来解释PL机制,包括尺寸主导的量子限域效应[68]、晶格缺陷[69]和边缘效应[70]等。二维量子片的荧光性质是由内部本征荧光和表面缺陷荧光的相互作用决定的,前者来自于量子限域、边缘效应和局域电子空穴对的辐射耦合,而后者则是由化学基团、元素掺杂和表面缺陷的状态引发的[71-73]。通过密度泛函理论(DFT)计算,证实了GQSs荧光特性对尺寸依赖的现象。二维量子片的光致发光性能测试结果见图5(彩图见期刊电子版)。可见随着直径从0.46 nm到2.31 nm逐渐增大,GQSs发射出从紫外(带隙为6.81 eV)到红外(带隙为1.64 eV)的连续光谱(图5(a))[74]。Rhee和Lee等人通过控制反应物的浓度实现不同尺寸GQSs的可控制备,带隙随着GQSs尺寸的增大逐渐减小,呈现出从蓝色到棕色的光致发光行为(图5(b))[75]。
二维量子片的PL性能受多种因素调控。官能团修饰是改变材料荧光性能的重要手段,通过不同官能团的修饰调控能带结构,为实现可见光谱的全面覆盖提供了依据[76]。Seo等人[20]通过锂离子插层制备出MoS2 QSs,展示了其作为荧光标记的功能,并且在特定体系中能发挥供体和受体双重作用,仅单调地改变修饰的官能团就可以调节其光致发光行为[77]。掺杂、内部缺陷导致的结构扭曲都可能破坏本征二维量子片的电子空穴对称性,进而影响其PL性能[78, 79]。如图5(c),氮元素掺杂的氧化石墨烯量子片表现出了P型和N型导电性[80]。Saitow等人[81]在室温下使用纳秒脉冲激光烧蚀合成了含有1%的硫空位的MoS2 QSs,显示出了高量子产率和优异的蓝色光致发光特性。二维量子片的PL性能还受所处环境的影响(如溶剂、pH、金属离子等)[82, 83]。Wang等人[83]用碱金属插层和液相剥离的方法制备的MoS2 QSs具有pH响应的光学性质,GQSs的发射光谱随着pH的增大发生红移,这种现象是不同的pH环境下量子片与周围官能团共同作用的结果。
二维量子片的PL行为通常表现出激发波长依赖性(图5(d)~5(f)),浓度依赖性(图5(g))和溶剂依赖性(图5(h))。我们对基于广义方法制备的三种本征状态二维量子片的PL行为进行了详细表征[23]。激发波长依赖性可能与量子片的尺寸分布有关。而浓度依赖性十分有趣。对于GQSs和MoS2 QSs,随着量子片分散浓度的增大,荧光强度出现先增大后减小的现象,这种浓度引起的PL淬灭一般认为是聚集诱导淬灭(ACQ)引起的。同时,这种浓度淬灭效应是带隙荧光机制的一个有力证据,这两种机制都促进了激发态的非辐射跃迁,导致了荧光淬灭。值得注意的是,BNQSs的荧光-浓度依赖性有所不同[23],它的荧光强度随着量子片分散浓度的增大而显著增强,与聚集诱导发光(AIE)现象相似,但是考虑到BNQSs的结构特点,将这种现象归因于边缘诱导荧光。随着氮化硼量子片分散浓度的增大,量子片之间的距离逐渐减小,相互作用增强,导致量子片与溶剂之间的相互作用变弱,向溶剂中转移的电荷减少,从而导致了荧光增强。上述荧光机理同样在二维量子片的复合薄膜体系得以体现,如图5(i)展示的一系列量子片-PMMA复合薄膜具有优越的固态荧光现象,复合薄膜的荧光-质量分数依赖性与分散液的荧光-浓度依赖性相似[23]。另外,溶剂依赖性(图5(h))非常独特:在质子性溶剂(H2O、EtOH等)中,荧光强度极弱;而在非质子性溶剂(THF、NMP等)中,荧光强度极强。考虑到三种本征量子片均带有负电荷(水体系zeta电势均为负值)[23],因此其与质子性和非质子性溶剂的直接相互作用强弱不同,从而导致在质子性溶剂中,(激发态到基态)非辐射衰减占据优势,而在非质子性溶剂中,(激发态到基态)荧光发射占据优势。这里需要说明的是,采用化学方法或化学/电化学前期处理等获得的非本征量子片,可能与传统的二维材料以及经典的胶体量子点一样,均无法充分实现本征边缘/表面与溶剂的直接相互作用,从而导致溶剂依赖性明显减弱。而且,非本征量子片、传统二维材料与经典量子点,可能均缺乏足够的边缘-边缘(或表面-表面)相互作用,从而导致浓度依赖性与本征量子片不同,类似AIE现象更罕见。
图 5 二维量子片的光致发光性能。(a)发射波长(nm)对GQSs尺寸的依赖关系[74];(b)不同尺寸石墨烯量子片的颜色变化[75];(c)元素掺杂的影响[80];(d-f)激发波长依赖性[23];(g)浓度依赖性[23];(h)溶剂依赖性[23];(i)固态荧光性能[23]。 (a) 转载自文献[74],版权所有(2015)皇家化学学会;(b) 转载自文献[75],版权所有(2014)美国化学学会;(c) 转载自文献[80],版权所有(2014)威立出版集团;(d-i)转载自文献[23],版权所有(2019)皇家化学学会
Figure 5. Photoluminescence of 2D QSs. (a) Dependence of emission wavelength (nm) on the size of GQSs[74]; (b) color changes of GQSs with different sizes[75]; (c) effects of elemental doping[80]; (d-f) excitation wavelength dependence[23]; (g) concentration dependence[23]; (h) solvent dependence[23]; (i) solid-state fluorescence[23]. (a) Reproduced with permission ref. [74]. Copyright 2015, Royal Society of Chemistry. (b) Adapted with permission ref. [75]. Copyright 2014, American Chemical Society. (c) Reproduced with permission ref. [80]. Copyright 2014, Wiley-VCH. (d-i) Reproduced with permission ref. [23]. Copyright 2019, Royal Society of Chemistry.
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激光的出现使得非线性光学(NLO)得到了巨大的发展,其内容涵盖了光与物质相互作用的基础研究,以及激光频率转换、光学切换等应用研究。近些年,越来越多的研究致力于揭示二维材料及二维量子片的非线性光学特性[84]。二维量子片在非线性光学中的应用见图6(彩图见期刊电子版)。此外,二维量子片的非线性光学成像以其良好的生物相容性、成像深度大、空间分辨率高、组织荧光背景小、减少生物组织的光损伤等优点在基础生物医学中备受关注[85, 86]。例如,与基于线性光学的单光子成像相比,N-GQSs的非线性成像穿透深度大(高达1800 µm)(如图6(d)),有望应用于监测深部生物组织的活性[87]。
图 6 二维量子片在非线性光学中的应用。(a)等离激元增强石墨烯量子片二阶非线性效应[89];(b)黑磷量子片的三阶非线性效应[94];(c)锑烯量子片的光学克尔效应[96];(d)N掺杂的石墨烯量子片的非线性生物成像[87];(e)量子片-PMMA复合薄膜的非线性饱和吸收性能[23]。(a) 转载自文献[89],版权所有(2015)美国化学学会;(b)转载自文献[94],版权所有(2016)威立出版集团;(c) 转载自文献[96],版权所有(2017)威立出版集团;(d)转载自文献[87],版权所有(2013)美国化学学会;(e)转载自文献[23],版权所有(2019)皇家化学学会
Figure 6. Application of 2D QSs in nonlinear optics.(a)Plasmon-enhanced GQSs second-order nonlinearity[89];(b)third-order nonlinearity of BPQSs[94];(c)Kerr effect of AQSs[96];(d)nonlinear biological imaging of N-GQSs[87];(e)nonlinear saturation absorption of QSs-PMMA hybrid films[23]. (a) Reproduced with permission ref. [89]. Copyright 2015, American Chemical Society. (b) Reproduced with permission ref. [94]. Copyright 2016, Wiley-VCH. (c) Reproduced with permission ref. [96]. Copyright 2017, Wiley-VCH.(d)Reproduced with permission ref. [87]. Copyright 2013, American Chemical Society.(e)Reproduced with permission ref. [23]. Copyright 2019, Royal Society of Chemistry.
经典理论适用于描述二维量子片的线性响应,而普遍低估了它的NLO响应[88, 89]。强偶极型的极化子激发在理解各种二维材料的光与物质相互作用中起着关键作用,如石墨烯中的等离极化激元[90]、氮化硼中的声子极化激元[91]。基于等离激元共振的石墨烯可用于增强其NLO响应,如图6(a)[89]是边长为8.4 nm的三角形GQSs,由两个中心能量为hω1 = 0.41 eV和hω2 = 0.66 eV的共线光脉冲照射,将从混合脉冲获得的光谱与每个单独脉冲激发的光谱叠加,这些脉冲表现出了以基频谐波(nω1,nω2)振荡的偏振特征(包括和频、差频、简并四波混频以及各种其他的谐波产生和混频组合)。近年来,黑磷的非线性光学性质也引起了较高关注[92],黑磷由折叠的蜂窝状晶格形成(图2(f)),在结构上就与其他二维材料不尽相同。研究发现,与石墨烯相比,黑磷具有更好的非线性饱和吸收特性[93]。Yu和Zhang[94]等人采用溶剂热法成功制备了平均尺寸为(2.1 ± 0.9) nm的BPQSs,如图6(b)所示,BPQSs的非线性饱和吸收性能优于黑磷纳米片(BPNSs),BPQSs具有较高的调制深度(BPQSs为36%,BPNSs为13.3%)和较低的饱和强度(BPQSs为3.3 GW cm−2,BPNSs为647.7 ± 60 GW cm−2)[94, 95]。然而,BP的稳定性欠佳,与磷同族(砷、锑、铋)的新型二维量子片展现了较高的稳定性。通过电化学剥离和声化学的方法制备的锑烯量子片(AQSs)是一种具有增强稳定性的新型光学克尔材料。研究表明,在可见光下(图6(c)),AQSs具有较大的非线性折射率(≈ 10−5 cm2 W−1 )[96]。
现阶段非线性光学材料的研究热点和难点包括(但不限于):(1)可实现超高非线性光学响应;(2)可在超低功率下激发非线性光学效应。我们课题组[23]利用自主开发的广义方法,制备出各种本征二维量子片,再通过溶液加工得到一系列量子片-PMMA复合薄膜(如图6(e))。GQSs、BNQSs、MoS2 QSs在同等条件下表现出截然不同的非线性饱和吸收性能,绝对调制深度分别为26%,4%,59%;相对调制深度分别为37.9%,31.7%,89.2%;饱和强度分别为23.8 kW/cm2,63.0 kW/cm2,6.7 kW/cm2。其中,MoS2 QSs的绝对调制深度在所有二维材料已知报道(相同激光波长下)中几乎并列最高(60%左右);其饱和强度首次低至sub-10 kW/cm2 (或sub-1 nJ/cm2),在所有二维材料已知报道(可见-近红外范围,不限波长)中最低,而且至少降低了1~2个数量级[97-100]。也就是说MoS2 QSs同时实现了非线性饱和吸收的超高响应和超低功率激发,为新型二维半导体量子片研发提供了强大动力。另外,我们[65]研究发现,MoS2的非线性饱和吸收性能表现出显著的尺寸依赖性。当其纳米片尺寸由300 nm逐渐降至2 nm,绝对调制深度由5.2%提高至59%,而饱和强度由298.5 MW/cm2降低至6.7 kW/cm2,分别达到惊人的1个数量级和4−5个数量级,这对于新型非线性光学材料和器件的研发具有重要指导意义。分析原因认为,MoS2 QSs的二维半导体本征特性、强烈的量子限域效应以及突出的边缘效应,可能都对其优异的非线性饱和吸收性能做出重要贡献。特别注意的是,所用量子片横向尺寸为2 nm,厚度为1~2层,因此只有表面晶胞和边缘晶胞,分别占比2/3和1/3左右,相对于表面晶胞,边缘晶胞处于非对称环境,而且为非平衡状态(晶格破缺、悬挂键、不饱和键、动态变化等),正是这种大量本征、裸露、不规则边缘,使得其非线性饱和吸收性能得到惊人提升。
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无机发光半导体与大多数有机发光材料相比,具有较高的发光效率和使用寿命以及良好的稳定性,这引起了人们对固态发光器件的研究兴趣。二维材料已在固态发光领域显示出了优异的性质:可调的能带结构,极高的载流子迁移率和极宽的光谱响应。因此,二维材料在高性能发光器件(激光器、发光二极管等)中得到了广泛应用[101]。
研究表明,二维量子片具有宽带吸收、强的非线性、超快响应和可饱和吸收特性,其作为宽带、高效、方便和多功能的饱和吸收体在超快激光的应用中有着广阔的前景(见图7,彩图见期刊电子版)[96, 102, 103]。以GQSs作为光活性介质,使用Ta2O5/SiO2设计的垂直腔面发射激光器(VCSEL)首次实现了室温下稳定的绿光发射。图7(a)描绘了GQSs-VCSEL的示意图,布拉格反射镜形成的光腔在空间上限制了GQSs的发射光子,从而形成了一个强大的光场[104]。TMDs具有很强的光和物质相互作用和较大的激子结合能,是激光器增益介质的理想选择[105]。已有研究设计了基于二硫化钼/石墨烯纳米复合材料的可拉伸结构的激光装置(图7(c)),在优化的MoS2 QSs掺杂浓度下,可以有效地增强器件的激光[106]。此外,激光可以通过外部应变调节,这为宽带和可拉伸激光器件的发展提供了一条有用的路径,图7(c)展示了PDMS/G/MoS2激光器件及褶皱结构之间多重散射的示意图。由于波长跨越整个可见光谱的多色激光器超出了单一材料的容量,所以白色激光的生产仍然具有一定的挑战性。近年来,过渡金属碳/氮化物(MXenes)展现了优异的特性[107-109],有研究构建了基于V2C MXene量子片(MQSs)的白色激光器,在最优的条件下,蓝光、绿光、黄光和红光被同时放大[110],图7(b)展示了在不同抽运强度下MQSs激光器发射出的不同光谱和在355 nm激光泵(4.8 mJ cm−2)下产生的白色激光。
图 7 二维量子片在固态发光器件中的应用情况。(a)基于GQSs的垂直腔面发射激光器[104];(b)基于V2C MXene量子片的白色激光器[110];(c)基于MoS2 QSs的可拉伸和宽带无腔激光器[106];(d)基于MoS2 QSs(组氨酸掺杂)的白色发光二极管[112]。(a) 转载自文献[104],版权所有(2019)美国化学学会;(b) 转载自文献[110],版权所有(2019)威立出版集团;(c) 转载自文献[106],版权所有(2020)威立出版集团; (d) 转载自文献[112],版权所有(2019)威立出版集团
Figure 7. Applications of 2D QSs in solid-state light emitting device. (a) Vertical cavity surface-emitting lasers based on GQSs[104];(b)white lasers with V2C MXene quantum sheets (MQSs)[110]; (c) stretchable and broadband cavity-free laser devices based on MoS2 QSs[106]; (d) white-light-emitting diodes based on histidine-doped MoS2 QSs[112]. (a) Reproduced with permission ref. [104]. Copyright 2019, American Chemical Society. (b) Reproduced with permission ref. [110]. Copyright 2019, Wiley-VCH. (c) Reproduced with permission ref. [106]. Copyright 2020, Wiley-VCH. (d) Reproduced with permission ref. [112]. Copyright 2019, Wiley-VCH.
发光二极管(LED)是固态照明行业中的关键元件,近年来得到了快速的发展。目前对于柔性发光器件的需求日益增长,而二维量子片被认为是最有前途的柔性发光材料之一[111]。二维量子片最重要的特性是具有宽光谱可调性(从紫外到近红外)和窄的发射峰(半高宽在30 nm左右),这是有机发光二极管(OLEDs)难以实现的。Chen等人[106, 112]研究了组氨酸掺杂MoS2 QSs在发光器件中的重要作用,他们设计了基于组氨酸掺杂MoS2 QSs的白色发光二极管,证实了单一发光材料可用于制造LED。图7(d)展示了该二极管的器件结构和SEM截面图以及在正向电压下的伏安(I-V)特性曲线,其表现出了良好的整流特性[112]。
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本文介绍了二维量子片的制备现状及其在光学领域的相关应用。自下向上和自上向下方法均可用于制备二维量子片,各有其优点和局限性。不同方法制备的二维量子片的结晶度、尺寸、晶相、厚度、表面性质、缺陷等并不完全一致。二维量子片作为二维材料和量子体系交叉的产物,吸引着人们不断发掘其奥秘。而先进的制造技术和精细的表征手段为我们研究二维量子片奠定了坚实的基础。二维量子片可调的光学性质可以通过制备不同尺寸、不同层数的二维量子片,构建量子片异质结构,对二维量子片进行表面修饰,或外部刺激(如温度、应变、电场、磁场等)等获得。
二维量子片在诸多领域已经展现出了美好的应用前景,但仍然面临巨大挑战。首先,尽管已经开发出二维量子片的本征、普适和规模制备方法[23],使得在相同协议/标准上建设二维量子片的完整数据库/图书馆成为可能[31],然而建设过程任重道远,需要大量的研究人员共同参与。其次,二维量子片的横向尺寸和厚度尚需独立开来,这样才能分别探讨面内和面外量子限域效应以及二者所带来的各向异性。另外,二维量子片的几何/边缘结构尚未做到原子级别精确可控,这就为相应的机理解析带来了困惑。此外,由于内部晶格、表面晶格与边缘晶格数目相当,因此二维量子片在相工程和电子态调控方面值得关注。随着纳米科技和材料研究的不断发展和进步,我们相信,二维量子片将在非线性光学、激光、光电器件、催化、能源、生物成像、癌症治疗、固体润滑等领域大放异彩。
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摘要: 以石墨烯为代表的二维材料因其独特的结构和优异性能而受到广泛关注。随着二维材料在无限小的方向不断发展,二维(材料)量子片逐渐引起人们极大的兴趣。二维量子片不仅保留了二维材料的本征特性,而且表现出量子限域和突出的边缘效应,为二维材料的潜在应用带来全新机遇。本文详细介绍了二维量子片的基本概念,制备现状与光学性能的研究进展,特别强调了二维量子片本征、普适和规模制备的实现及其重大意义。此外,重点关注了二维量子片的光致发光特性以及在非线性光学、固态发光器件等领域的应用。最后,分析了二维量子片的发展趋势以及面临的主要挑战。#共同第一作者 # These authors contributed equally to this workAbstract: Two-dimensional (2D) materials like graphene have attracted much attention due to their unique structures and exotic properties. With significantly reduced lateral sizes, 2D quantum sheets (2D QSs) are attracting an increasing level of interest. 2D QSs have opportunities for new applications because of their intrinsic characteristics of being 2D materials and having emerging quantum confinement and prominent edge effects. This review focuses on the conceptual interpretation of 2D QSs and the recent progress on their preparation and optical properties. Particular focus is given to the realization and significance of the universal and scalable production of intrinsic 2D QSs. In addition, the photoluminescence of 2D QSs and their applications in nonlinear optics and solid-state light-emitting devices are reviewed. At the end, the perspectives and challenges towards the future development of 2D QSs are discussed.
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Key words:
- two-dimensional materials /
- quantum sheets /
- photoluminescence /
- nonlinear optics
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图 1 (a)量子片所处体系的示意图[31];(b)二维量子片的文章发表数目(2007—2016)[4];(c)二维量子片的应用领域:医药[24],生物成像[25],催化[26],太阳能电池[27],非线性光学[28]等。(b)转载自文献[4],版权所有(2018)皇家化学学会。(c) 转载自文献[24],版权所有(2018)施普林格;转载自文献[25],版权所有(2018)施普林格;转载自文献[26],版权所有(2016)自然出版集团;转载自文献[27],版权所有(2018)施普林格;转载自文献[28],版权所有(2020)美国化学学会
Figure 1. (a) Schematic diagram of the system in which the quantum sheet is located[31];(b) number of journal publications on 2D QSs from 2007 to 2016[4];(c) application fields of 2D QSs:medicine[24], biological imaging[25],catalysis[26],solar cell[27],nonlinear optics[28],etc. (b) Adapted with permission ref. [4]. Copyright 2018, Royal Society of Chemistry; (c) Reproduced with permission ref. [24]. Copyright 2018, Springer. Reproduced with permission ref. [25]. Copyright 2018, Springer; Reproduced with permission ref. [26]. Copyright 2016, Nature Publishing Group; Reproduced with permission ref. [27]. Copyright 2018, Springer; Reproduced with permission ref. [28]. Copyright 2020, American Chemical Society.
图 2 二维材料的原子结构。(a)石墨烯[3];(b)氮化硼[3];(c)二硫化钼[48];(d)硒化铟[49];(e)氮化碳[50];(f)黑磷[41]。(a) 和(b)转载自文献[3],版权所有(2017)美国化学学会;(c) 转载自文献[48],版权所有(2011)自然出版集团;(d) 转载自文献[49],版权所有(2017)自然出版集团;(e) 转载自文献[50],版权所有(2017)皇家化学学会;(f) 转载自文献[41],版权所有(2014)自然出版集团
Figure 2. Atomic structures of 2D materials. (a) Graphene[3]; (b) BN[3]; (c) MoS2[48]; (d) InSe[49]; (e) C3N4[50]; (f) BP[41]. (a) and (b) Adapted with permission ref. [3]. Copyright 2017, American Chemical Society. (c) Adapted with permission ref. [48]. Copyright 2011, Nature Publishing Group. (d) Adapted with permission ref. [49]. Copyright 2017, Nature Publishing Group. (e) Adapted with permission ref. [50]. Copyright 2017, Royal Society of Chemistry. (f) Adapted with permission ref. [41]. Copyright 2014, Nature Publishing Group.
图 3 二维量子片的制备方法。自下而上:(a)化学气相沉积[66];(b)湿化学法[35]。自上而下:(c)电化学剥离[54];(d)研磨结合超声剥离[62];(e)液氮预处理和超声剥离[57];(f)回流预处理和超声剥离[63];(g)超薄切片结合液相剥离[59]。(a) 转载自文献[66],版权所有(2016)美国化学学会;(b)转载自文献[35],版权所有(2019)自然出版集团;(c) 转载自文献[54],版权所有(2015)皇家化学学会。(d) 转载自文献[62],版权所有(2015)威立出版集团;(e)转载自文献[57],版权所有(2017)美国科学促进会;(f)转载自文献[63],版权所有(2019)爱思唯尔;(g)转载自文献[59],版权所有(2020)施普林格
Figure 3. The preparation methods of 2D QSs. Bottom-up: (a) CVD [66]. (b) Wet chemical method[35]. Top-down: (c) Electrochemical exfoliation[54]; (d) grinding combined with sonication exfoliation[62]; (e) liquid nitrogen pretreatment combined with sonication exfoliation[57]; (f) reflux pretreatment combined with sonication exfoliation[63]; (g) ultrathin section combined with liquid phase dissection[59]. (a) Reproduced with permission ref. [66]. Copyright 2016, American Chemical Society; (b) adapted with permission ref. [35]. Copyright 2019, Nature Publishing Group; (c) reproduced with permission ref. [54]. Copyright 2015, Royal Society of Chemistry; (d) reproduced with permission ref. [62]. Copyright 2015, Wiley-VCH; (e) reproduced with permission ref. [57]. Copyright 2017, AAAS; (f) adapted with permission ref. [63]. Copyright 2019, Elsevier; (g) reproduced with permission ref. [59]. Copyright 2020, Springer.
图 4 二维量子片的本征、普适和规模制备。(a)盐辅助球磨和超声辅助溶剂剥离[60];(b)量子片的制备机理示意图[60];(c)硅球辅助球磨和超声辅助溶剂剥离[23];(d)量子片分散液和粉体照片及对应的高分辨透射电镜照片[23];(e)从多壁碳纳米管制备石墨烯量子片[64]。(a-b)转载自文献[60],版权所有(2017)美国化学学会;(c-d)转载自文献[23],版权所有(2019)皇家化学学会;(e)转载自文献[64],版权所有(2020)美国化学学会
Figure 4. Universal and scalable production of intrinsic 2D QSs. (a) Salt-assisted ball-milling and sonication-assisted solvent exfoliation[60]; (b) schematic diagram of the fabrication mechanism of 2D QSs[60]; (c) silica-assisted ball-milling and sonication-assisted solvent exfoliation[23]; (d) photographs of the QS dispersions and powders and their HRTEM images; (e) robust strategy for tailoring multi-walled carbon nanotubes into GQSs[64]. (a-b) Reproduced with permission ref. [60]. Copyright 2017, American Chemical Society; (c-d) Reproduced with permission ref. [23]. Copyright 2019, Royal Society of Chemistry; (e) Reproduced with permission ref. [64]. Copyright 2020, American Chemical Society.
图 5 二维量子片的光致发光性能。(a)发射波长(nm)对GQSs尺寸的依赖关系[74];(b)不同尺寸石墨烯量子片的颜色变化[75];(c)元素掺杂的影响[80];(d-f)激发波长依赖性[23];(g)浓度依赖性[23];(h)溶剂依赖性[23];(i)固态荧光性能[23]。 (a) 转载自文献[74],版权所有(2015)皇家化学学会;(b) 转载自文献[75],版权所有(2014)美国化学学会;(c) 转载自文献[80],版权所有(2014)威立出版集团;(d-i)转载自文献[23],版权所有(2019)皇家化学学会
Figure 5. Photoluminescence of 2D QSs. (a) Dependence of emission wavelength (nm) on the size of GQSs[74]; (b) color changes of GQSs with different sizes[75]; (c) effects of elemental doping[80]; (d-f) excitation wavelength dependence[23]; (g) concentration dependence[23]; (h) solvent dependence[23]; (i) solid-state fluorescence[23]. (a) Reproduced with permission ref. [74]. Copyright 2015, Royal Society of Chemistry. (b) Adapted with permission ref. [75]. Copyright 2014, American Chemical Society. (c) Reproduced with permission ref. [80]. Copyright 2014, Wiley-VCH. (d-i) Reproduced with permission ref. [23]. Copyright 2019, Royal Society of Chemistry.
图 6 二维量子片在非线性光学中的应用。(a)等离激元增强石墨烯量子片二阶非线性效应[89];(b)黑磷量子片的三阶非线性效应[94];(c)锑烯量子片的光学克尔效应[96];(d)N掺杂的石墨烯量子片的非线性生物成像[87];(e)量子片-PMMA复合薄膜的非线性饱和吸收性能[23]。(a) 转载自文献[89],版权所有(2015)美国化学学会;(b)转载自文献[94],版权所有(2016)威立出版集团;(c) 转载自文献[96],版权所有(2017)威立出版集团;(d)转载自文献[87],版权所有(2013)美国化学学会;(e)转载自文献[23],版权所有(2019)皇家化学学会
Figure 6. Application of 2D QSs in nonlinear optics.(a)Plasmon-enhanced GQSs second-order nonlinearity[89];(b)third-order nonlinearity of BPQSs[94];(c)Kerr effect of AQSs[96];(d)nonlinear biological imaging of N-GQSs[87];(e)nonlinear saturation absorption of QSs-PMMA hybrid films[23]. (a) Reproduced with permission ref. [89]. Copyright 2015, American Chemical Society. (b) Reproduced with permission ref. [94]. Copyright 2016, Wiley-VCH. (c) Reproduced with permission ref. [96]. Copyright 2017, Wiley-VCH.(d)Reproduced with permission ref. [87]. Copyright 2013, American Chemical Society.(e)Reproduced with permission ref. [23]. Copyright 2019, Royal Society of Chemistry.
图 7 二维量子片在固态发光器件中的应用情况。(a)基于GQSs的垂直腔面发射激光器[104];(b)基于V2C MXene量子片的白色激光器[110];(c)基于MoS2 QSs的可拉伸和宽带无腔激光器[106];(d)基于MoS2 QSs(组氨酸掺杂)的白色发光二极管[112]。(a) 转载自文献[104],版权所有(2019)美国化学学会;(b) 转载自文献[110],版权所有(2019)威立出版集团;(c) 转载自文献[106],版权所有(2020)威立出版集团; (d) 转载自文献[112],版权所有(2019)威立出版集团
Figure 7. Applications of 2D QSs in solid-state light emitting device. (a) Vertical cavity surface-emitting lasers based on GQSs[104];(b)white lasers with V2C MXene quantum sheets (MQSs)[110]; (c) stretchable and broadband cavity-free laser devices based on MoS2 QSs[106]; (d) white-light-emitting diodes based on histidine-doped MoS2 QSs[112]. (a) Reproduced with permission ref. [104]. Copyright 2019, American Chemical Society. (b) Reproduced with permission ref. [110]. Copyright 2019, Wiley-VCH. (c) Reproduced with permission ref. [106]. Copyright 2020, Wiley-VCH. (d) Reproduced with permission ref. [112]. Copyright 2019, Wiley-VCH.
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