Modification of Single Photon Fluorescence Emission of Single Quantum Dots with Different Substrates
1 引言
随着纳米半导体材料制备技术的不断成熟,与传统荧光染料相比,具备更高荧光稳定性,更强、更窄荧光辐射等光学特性的量子点[1]在生物标记[2-3]、荧光传感[4-5]、超分辨率成像[6-7]、单光子源构建[8-9]及半导体器件[10-12]等领域显示了越来越明显的优势。掌握和调控量子点发射荧光的特性变得尤为重要,如量子点发射荧光的单光子性、衰减或辐射速率、闪烁等特性。
目前在量子点相关研究方面,上海交通大学团队在水溶性量子点中加入巯基丙酸配体,实现了对量子点闪烁的调控[13]。研究人员发现,改变量子点的溶液环境[14]或在量子点的内核外加上壳层调节其厚度[15-16],同样可以调控量子点的闪烁行为。王早等[17]利用N型半导体纳米材料作为量子点的基质来抑制单量子点的闪烁。随着微纳加工技术的不断发展,研究人员将设计、加工的微纳结构与量子点相结合[18-20],或使用原子力显微镜(AFM)操纵纳米颗粒靠近量子点[21]以实现荧光增强,进而开展高灵敏度光学传感[22-23]等相关工作。
与将量子点置于特定的环境中或通过各种复杂的微纳加工技术将量子点与微纳结构结合相比,采用特定种类的基片来调控量子点发射荧光的特性,实验过程将得到极大简化。近年来也有不少小组研究不同基片上单量子点发射荧光的相关特性,如Stefani等[24]研究了不同基片(ITO、玻璃)上,不同激发功率下单量子点的闪烁特性。Jin等[25]为了研究量子点闪烁的动力学机理,对比分析了ITO、石英玻片、In2O3基片上量子点的闪烁行为。吴建芳等[26]基于界面电荷转移机理解释了PMMA、SiO2、ITO基片上量子点闪烁行为的差异。然而,对于平整基片上具有单光子发射特性的单量子点,其荧光发射特性的研究鲜有报道。
本文针对具有单光子发射荧光特性的单量子点,较为全面地研究其在不同基片上荧光发射的寿命、闪烁等特性,并与理论计算作对比,为单光子源构建、荧光成像及传感等相关应用提供数据和指导。
2 实验原理与方法
2.1 实验原理
实验基于时间相关单光子计数(TCSPC)技术测量量子点发射荧光的寿命,通过电子倍增电耦合器件(EMCCD,Ixon Ultra X-10854,Andor)进行荧光成像,对单量子点发射的荧光进行连续的数据采集,以记录和分析其闪烁行为、荧光寿命等,光学实验装置如图1所示。
皮秒激光器(PDL 800-B,PicoQuant)发出的激光(中心波长640 nm)经过滤色镜组中的激发片(ET620/60x-49006,Chroma)可获取更为有效的窄带激发光,激发光经显微物镜聚焦到载物台上的量子点使其发射荧光。量子点发射的荧光与反射回来的激发光经过滤色镜组中的二向色镜(T660lpxr,Chroma)与发射片(ET700/75m-49006,Chroma)可获得更纯净的透射荧光。透射荧光可以通过EMCCD进行荧光成像,也可以通过针孔被单光子雪崩二极管(PDM Series,MPD)采集,经TCSPC采集卡(PicoHarp 300,PicoQuant)信号处理得到荧光寿命等参量。考虑到正入射激光的聚焦光斑较小,搭建了一路斜入射激光,以大面积激发量子点发射荧光实现宽场荧光成像,便于寻找待观测的单量子点。
2.2 样品制备
首先把基片(石英玻片、硅片、金膜)放入烧杯中,加入适量乙醇,用超声振荡器(QT3120,天津瑞普电子仪器公司)清洗10 min,再用去离子水超声振荡清洗5 min,接着用氮气吹干基片上的液滴,最后用等离子体清洗机(PDC-32G,Mycro)清洗2 min。
将量子点(Q21361MP,Life Technologies Corporation)用去离子水稀释至1 pmol/L,取10 μL稀释过的量子点溶液,用匀胶机(WS-650MZ-23NPP,Mycro)以3000 r/min的转速旋转30 s,分别旋转涂布于石英玻片、硅片、金膜上。
3 结果与讨论
3.1 不同基片上量子点荧光寿命测量结果及分析
将基片上量子点样品放到显微镜载物台上进行宽场荧光成像,选取其中单量子点发射的荧光进行数据采集、分析。图2中(a)、(b)分别为石英玻片上单量子点发射荧光的强度随时间变化曲线以及荧光强度衰减曲线[对应图2(c)中红色圆圈内单量子点]。图2(b)中红色实线为荧光强度衰减的指数函数拟合曲线,拟合得到该量子点发射荧光的寿命为42 ns,图2(b)内插图为单量子点发射荧光的二阶相关函数曲线,在时间延迟为0的位置能够明显看到强度低于最大强度一半的凹陷,该凹陷表明该单量子点发射的荧光具有单光子性[27]。图2(d)给出了量子点发射荧光的光谱,从中可以看出荧光中心波长约为705 nm。
图 2. 单量子点荧光发射特性的测量。(a)石英玻片上单量子点发射荧光的强度随时间变化的曲线(采用雪崩二极管或EMCCD采集);(b)石英玻片上单量子点发射荧光的强度衰减曲线,内插图为该量子点发射荧光的二阶相关函数曲线(显示单光子发射);(c)宽场荧光成像,红色圆圈内为测试的单量子点;(d)量子点荧光光谱(荧光中心波长约为705 nm)
Fig. 2. Measurement of the fluorescence characteristics of single quantum dots. (a) Time trace of fluorescence intensity of single quantum dots on quartz glass substrate (recorded by single photon avalanche diode or EMCCD); (b) fluorescence decay curves of single quantum dots on quartz glass substrate (The inset shows the second-order correlation function that exhibits the anti-bunching signature of single photon emission for single quantum dots); (c) wide-field fluorescence imaging of quantum dots, the
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针对不同基片上具有单光子发射特性的单量子点,测量得到的荧光寿命分布如图3(a)~(c)所示,依次对应石英玻片、硅片、金膜,横坐标为基片上单量子点发射荧光的寿命,纵坐标为对应的量子点个数。设荧光寿命τ=±σ(
为被统计的一组量子点发射荧光的平均寿命,σ为该组量子点寿命的标准偏差)。经统计,得到石英玻片、硅片、金膜上量子点发射荧光的寿命分别为(44.93±6.218),(36.29±7.183),(28.97±3.874) ns。数据表明:金膜上量子点发射荧光的寿命最短,硅片次之,而石英玻片上的量子点发射荧光的寿命最长。由于荧光总的衰减速率(即总的辐射速率,包括远场辐射速率与非辐射速率两部分[28])反比于荧光寿命[4,21,29],对于石英玻片、硅片、金膜上的量子点,荧光总的衰减速率依次增大。
为了对不同基片上量子点单光子荧光发射的衰减速率有更深入的理解,数值模拟计算了不同基片上量子点发射荧光的总衰减速率。对于金膜,该总衰减速率包含表面等离子体的贡献。计算中将量子点看成一个点电流源,表示为J=δ(x,y,z-d)n,式中:δ为狄拉克函数;n为沿点电流源偏振方向的单位长度矢量;d为点电流源到基片表面的距离。引入距离d是考虑到量子点本身的大小,此外基片表面的粗糙程度、清洁程度等因素会影响量子点到基片表面的有效距离。分别计算了3种基片上的水平偏振、竖直偏振点电流源荧光发射的总衰减速率
式中:Re(En)为点电流源位置沿源偏振方向的电场分量的实部[30]。为了便于比较不同基片上量子点发射荧光的总衰减速率Γtot与其在自由空间中衰减速率Γair的差别,定义归一化衰减速率Γtot/Γair,Γair表示为
式中:η为真空中的波阻抗;na=1为空气的折射率;k0=2π/λ,取λ=705 nm为量子点发射荧光的中心波长。
图 3. 单量子点发射单光子荧光的寿命分布。(a)~(c)依次为石英玻片、硅片、金膜上单量子点的测量结果
Fig. 3. Fluorescence lifetime distributions of the single photon emission of single quantum dots. (a)-(c) show the measurement results for single quantum dots on quartz glass substrate, silicon substrate and gold film, respectively
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图 4. 不同基片上量子点发射荧光的总衰减速率的计算结果和AFM成像图(成像范围5 μm×5 μm)。(a1)、(a2)水平偏振、竖直偏振点电流源到石英玻片、硅片、金膜表面不同有效距离d下荧光的总衰减速率计算结果;(b1)~(b3)石英玻片表面、硅片表面、金膜表面的AFM成像图(均无量子点);(b4)石英玻片上涂覆量子点后的AFM成像图
Fig. 4. Simulated fluorescence decay rates of quantum dots on different substrates and AFM images (with range 5 μm×5 μm). (a1)-(a2) Simulated fluorescence decay rates of an electric point source of horizontal polarization (a1) or vertical polarization (a2) at different effective distances d away from the surface of quartz glass substrate, silicon substrate and gold film; (b1)-(b3) AFM images of the surface of quartz glass substrate, silicon substrate and gold film, respectively; (b4) AFM image of quantu
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对于荧光发射波长705 nm,石英玻片、硅片、金膜的折射率依次为1.45、3.7780+0.012i、0.1610+4.0504i[31]。采用非周期傅里叶模式法(a-FMM)[32-33])得到图4(a)的计算结果。a-FMM是严格耦合波分析(RCWA)[34]向非周期结构的推广,a-FMM通过在非周期结构中加入完美匹配吸收边界层,将非周期结构转化为周期性结构,然后采用RCWA对结构进行电磁场分析。
图4(a1)、(a2)表明:对于硅片和金膜,随着量子点到基片的有效距离d不断增大,量子点发射荧光总的衰减速率迅速降低,最后趋于平缓下降,可见制备更小的量子点和加工更低粗糙度的基片对荧光总衰减速率的提升有明显作用;对于石英玻片,量子点发射荧光的总衰减速率受量子点到基片有效距离的影响不大。前述实验结果表明,石英玻片、硅片、金膜上量子点发射荧光的总衰减速率依次增大,该相对大小关系与图4(a1)、(a2)给出的计算结果完全一致。图4(b1)~(b4)中AFM(Innova,Bruker)测试得到的表面轮廓图表明,清洗干净的基片表面也存在几个纳米的起伏(石英玻片表面算术平均粗糙度Ra=0.373 nm,表面最大轮廓高度差dvp=4.03 nm;对于硅片,Ra=0.138 nm,dvp=1.74 nm;对于金膜,Ra=0.414 nm,dvp=3.96 nm)。量子点本身的大小、基片表面的粗糙程度、清洁程度等因素均会增大量子点到基片的有效距离,因此,实验并未在不同基片上测得相差较大的量子点荧光寿命。考虑总衰减速率的具体数值,对于水平偏振量子点辐射源,如果有效距离接近10 nm[图4(b1)~(b4)表明该假设合理],图4(a1)内插图表明:此时硅片与金膜上量子点发射荧光的总衰减速率非常接近,且约为石英玻片上量子点发射荧光的总衰减速率的2倍,在采集的实验数据中有与上述数值吻合的实验数据。对于竖直偏振量子点辐射源,如果量子点到基片表面有效距离大于10 nm[量子点溶液中含有分散剂,并且如果基片清洗没有达到图4(b1)~(b3)所示的洁净度,则有可能使有效距离大于10 nm], 图4(a2)内插图表明,当d取值在10~20 nm时,金膜上量子点发射荧光的总衰减速率大于或接近硅片,并且大于或接近石英玻片上量子点发射荧光的总衰减速率的2倍,在采集的实验数据中仍有与上述数值吻合的实验数据。
由于金膜表面的表面等离子体存在损耗,当点电流源到金膜表面的距离d趋于0时,表面等离子体的损耗会导致点电流源荧光发射的量子产率很低[35-37],使得荧光强度降低,即发生荧光淬灭。为了从理论上证实该结论,计算石英玻片、硅片、金膜上点电流源荧光发射的量子产率,量子产率定义为[35-37]
式中:Γrad=
S·nda为远场辐射速率,A为包含点电流源的一个封闭曲面,a为封闭曲面元,S为时间平均能流密度矢量,n为曲面A的外法向矢量。图5中量子产率q的计算结果表明:当点电流源到基片表面的距离d趋于0时,对于金膜,q趋于很小的数值(水平、竖直偏振计算结果分别为0.0002,0.0019),即发生荧光淬灭;对于硅片,q取较小的数值(水平、竖直偏振计算结果分别为0.0531,0.0443),原因是硅存在光吸收(折射率虚部不为0);对于石英玻片,由于不存在光吸收(折射率虚部为0),q保持为100%,不会发生荧光淬灭。上述理论计算表明,对于实验中金膜上的量子点,可能存在发生(或接近)荧光淬灭的量子点,而由于前文提到的量子点本身大小、基片表面粗糙程度等因素会增大量子点到金膜表面的有效距离,故在实验中仍然能观测到大量未发生荧光淬灭的量子点。
图 5. 计算得到的点电流源荧光发射的量子产率与其到基片距离d的相关性。石英玻片、硅片、金膜上(a)水平偏振与(b)竖直偏振点电流源的计算结果
Fig. 5. Simulated quantum yield q of an electric point source as a function of the distance d between the electric point source and the substrates. The results are obtained for an electric point source of (a) horizontal polarization or (b) vertical polarization on quartz glass substrate, silicon substrate and gold film
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3.2 不同基片上量子点闪烁行为测量结果及分析
将EMCCD采集的单量子点荧光成像区域所有像元的计数相加,得到量子点发射荧光的强度随时间变化曲线,如图6所示。图6首行至末行依次为石英玻片、硅片、金膜上的量子点发射荧光强度数据;3列分别为3处单量子点的数据,其中由于存在环境杂散光、EMCCD暗电流噪声等,产生的暗计数(计数最小值)不为零。图6表明,与石英玻片和硅片相比,金膜上单量子点的持续发光时间最长,即金膜上单量子点荧光发射的闪烁现象被抑制,持续发光性能较好。而硅片与石英玻片上单量子点荧光发射闪烁行为差别不大,此外,石英玻片上单量子点的发光强度高于硅片。
图 6. 荧光强度随时间变化的曲线。(a1)~(a3)石英玻片上3处单量子点的数据;(b1)~(b3)硅片上3处单量子点的数据;(c1)~(c3)金膜上3处单量子点的数据
Fig. 6. Time trace of fluorescence intensity of single quantum dots. (a1)-(a3) Results of three single quantum dots on the quartz glass substrate; (b1)-(b3) results of three single quantum dots on the silicon substrate; (c1)-(c3) results of three single quantum dots on the gold film
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图 7. 单量子点荧光强度的统计分布。(a1)~(a3)石英玻片上3处单量子点的数据;(b1)~(b3)硅片上3处单量子点的数据;(c1)~(c3)金膜上3处单量子点的数据
Fig. 7. Fluorescence intensity statistical distribution of single quantum dots. (a1)-(a3) Results of three quantum dots on the quartz glass substrate; (b1)-(b3) results of three quantum dots on the silicon substrate; (c1)-(c3) results of three quantum dots on the gold film
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利用图6数据,得到不同基片上单量子点发光强度的统计分布,如图7所示,横坐标代表荧光强度,纵坐标代表对应该荧光强度的时间占比。图7表明,与石英玻片和硅片相比,金膜上单量子点发射荧光的强度分布较为分散并有较明显的两极分化,且主要分布于较大的荧光强度,进一步表明金膜上单量子点持续发光时间较长且荧光较强。石英玻片和硅片上量子点的荧光强度集中分布在强度较低的部分,但石英玻片上强度较大的部分也有少许分布,这表明石英玻片上单量子点持续发光时间较短,但发光时荧光强度较大。而硅片上量子点的荧光强度主要集中在与背景光(周围环境杂散光、设备暗电流噪声等)强度接近的区域,表明硅片上的单量子点持续发光时间较短,且发光时荧光强度较低,接近背景光强度。关于不同基片上单量子点的闪烁特性,可以用基于不同基片上界面电荷转移动力学机理进行解释[25-26],不同基片的导带具有不同的势能,当其势能大于量子点内核的势能时,在外界激发光激发下,量子点导带的电子很难转移到基片的导带,从而导致量子点持续发光时间较长。但目前关于量子点闪烁机理还有待进一步明确和深入研究。
4 结论
针对具有单光子荧光发射特性的单量子点,研究其在石英玻片、硅片、金膜等不同基片上的发光特性。实验结果表明,无结构的平整基片对量子点发射荧光的衰减速率、闪烁特性等具有调控作用,石英玻片、硅片、金膜上量子点发射荧光的衰减速率依次增大。数值模拟计算结果表明,加工制备较小的量子点和光滑表面的基片能够明显提升荧光衰减速率,靠近金膜表面的点辐射源会发生荧光淬灭。与石英玻片和硅片相比,金膜上量子点持续发光时间较长,量子点闪烁现象被抑制。本工作为荧光传感、超分辨率成像、单光子源构建等提供了可借鉴的实验依据和指导。
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