InP/ZnS核壳量子点光学性质研究
Study on the Luminescence Properties of InP/ZnS Quantum Dot

作者: 张博文 , 王登魁 , 方 铉 , 房 丹 , 王新伟 , 唐吉龙 , 王晓华 , 魏志鹏 :长春理工大学高功率半导体激光国家重点实验室,吉林 长春;

关键词: 光致发光InP/ZnSTEM界面应力Photoluminescence InP/ZnS TEM Interface Stress

摘要:
InP量子点被认为是最有希望替代Cd基量子点的材料。然而对于InP量子点的制备和性质的研究远远小于Cd基量子点的研究,特别是其光学性方面的研究。本文采用一锅法制备了晶格质量良好,光学性能优异的InP/ZnS量子点,通过TEM图像确认制备出了的InP核的尺寸约为3 nm,晶格间距与闪锌矿结构的InP(111)晶面间距一致;通过变功率的光谱测试以及拉曼测试,认定InP/ZnS中存在两个发光来源,高能端的来源于自由激子的跃迁,低能端的来源于界面应力产生的缺陷。

Abstract: InP-based quantum dots are considered as the most promising candidate for Cd-based QDs. How-ever, the research on the fabricated and properties of InP quantum dots is far less than that of Cd-based QDs, especially their optical properties. In this paper, InP/ZnS quantum dots with good lattice quality and optical properties were prepared by one-pot method. The size of InP core was confirmed to be about 3 nm by TEM image. The lattice spacing was in agreement with that of InP(111). The results of power dependence photoluminescence and Raman spectra show that there are two luminescent sources in the InP/ZnS, the transition from the free energy exciton in the high energy end and the defects in the low energy end originating from the interface stress.

1. 引言

胶体量子点被广泛应用在显示 [1] 、探测 [2] 、生物荧光标记 [3] 、LED [4] 、激光 [5] 等领域。在众多的半导体材料中,Cd基量子点的研究最为深入,然而重金属的使用,给Cd基量子点的商业化生产带来很大的问题。在所有Cd基量子点的替代材料中,InP量子点被认为是最有希望的替代材料,因为它具有与CdSe相似的带隙,且带隙可调性覆盖整个可见光范围 [6] 。单纯的InP量子点由于表面的缺陷严重影响了其光学特性,一般采用包覆禁带宽度大的半导体壳层增强其发光,ZnS是InP量子点常用的包覆材料 [7] [8] [9] 。

本文采用一锅法制备了InP/ZnS核壳量子点,不经中间体的分离,直接获得结构复杂的量子点。通过TEM进行了表征;通过室温光致发光光谱认为其存在两个发光来源;通过变激发的光致发光光谱对量子点的发光来源进行了分析;并结合拉曼光谱分析了其缺陷的来源。

2. 实验

采用一锅法制备InP/Zns量子点,In源为InCl3,P源为三(二甲胺基)磷,S源为TOP-S,Zn源为ZnCl2与Zn。具体合成过程如下:将0.45 mmol (200 mg)的InCl3,2.2 mmol (122 mg)的ZnCl2,5 ml的油酰胺混合。反应混合物在100℃下搅拌0.5 h,之后加热到220℃ (在惰性气体的氛围中进行操作);在达到220℃时,快速注入0.25 ml的三(二甲胺基)磷;(注入P源后,开始形成InP,在3min时出现成核反应);在反应3 min时开始缓慢加入0.6 ml饱和的TOP-S(S源) (2.2 mmol S),17 min,注入1 ml的化学计量的TOP-S (2.2 mmol S);30 min,注入4 ml含有1 g Zn (硬脂酸盐)的十八烯;60 min,温度升高到240℃,65 min,注入0.7ml的化学计量的TOP-S(2.2 mmol S);90 min,注入4 ml含有1g Zn (硬脂酸盐)的十八烯;95 min,升温到260℃;150 min,停止反应,缓慢降温。制备出核约3 nm,壳约2 nm的InP/ZnS量子点,发光峰位在648 nm,半高宽为70 nm,量子产率约为45%。

我们对合成的量子点进行了室温下光致发光测试以及TEM表征。通过变激发的光致发光光谱和拉曼光谱,我们确认了InP/ZnS量子点中存在的发光来源。光致发光光谱测试采用448 nm的激光作为激发光源,iHR550光谱仪采集信号。拉曼测试采用的是LAB-RAM Infinity系统。

3. 结果与讨论

对合成的InP/ZnS量子点进行了TEM测试,如图1所示。TEM图中可以看到明显的具有晶格结构圆形图像,尺寸为3 nm左右,与InP核尺寸一致。插图为其中一个量子点图案的放大,通过对多组晶格

条纹的累计除以间隔数,我们得到晶格条纹的间隙为0.34 nm,这个数值与闪锌矿结构的InP的(111)晶面一致,说明我们合成了InP,且核壳量子点中InP核的晶格结构为闪锌矿结构的(111)晶面。右侧图片为合成的InP/ZnS量子点的结构示意图,通过合成时间的控制以及TEM图像可以得出InP核的尺寸约为3 nm,ZnS壳层的尺寸约为2 nm。

合成的InP/ZnS量子点室温下的光致发光光谱如图2所示,图中可以看出峰型不符合高斯分布,在低能端应该存在一个新的峰,采用分峰的方法对光致发光谱进行处理,发现其能很好的拟合成两条高斯线性。高能端的峰我们命名为A,其中心位于1.907 eV,半高宽为0.193 eV,低能端的峰我们命名为B,其中心位于1.762 eV,半高宽为0.189 eV。为了分析A峰与B峰的来源,我们对InP/ZnS量子点进行了变激发的光致发光测试。

图3为室温下InP/ZnS量子点变激发的光致发光测试,对图中的光谱进行分峰,并对发光峰的积分强度进行分析(如图3所示)。光致发光光谱的积分强度随功率的变化遵从如下公式 [10] [11] :

I = η I 0 α (1)

Figure 1. TEM image and structure of InP/ZnS QDs

图1. InP/ZnS量子点的TEM图片及其结构示意图

Figure 2. Room temperature photoluminescence of InP/ZnS QDs and two fitting Gaussian curves

图2. InP/ZnS量子点室温下的光致发光光谱测试及通过高斯拟合得到的曲线

式中I0是激光辐射功率,η为辐射效率,α的数值可判断发光的辐射机制。当1 < α < 2,发光来自于自由激子复合;当α < 1时,为杂质或者缺陷产生的发光 [11] [12] [13] 。从图中的拟合结果,我们得出低能端的B峰的α值为0.795(小于1),说明B峰为杂质或者缺陷产生的发光。A峰的数值为1.159,说明A峰来自于自由激子的复合。A峰随激发功率增大峰位基本没有变化,而B峰峰位随激发功率增大出现蓝移的现象也说明了其发光来源分别为自由激子的复合与局域态或缺陷发光。蓝移是能带填充效应导致的。弱激发时光生载流子数量少且获得能量少,只够填充到较低能级的局域带尾态;随着激发增强,载流子数量增多,大量填充到低能级局域态的电子和空穴来不及复合,就堆积起来逐渐填满基态,出现一定程度的饱和,多余的载流子填充到更高的能级,所以出现蓝移 [14] 。

为了进一步验证B峰缺陷的来源,我们对InP/ZnS量子点进行了拉曼谱的测试,如图4所示。InP

Figure 3. Power-dependent PL spectra of InP/ZnS QDs at 300 K. Insert picture shows the changes of integrated area of PL spectra with the change of excitation power

图3. 室温下,InP/ZnS量子点的变激发的光致发光谱。插图为积分面积随激发功率变化以及采用公式拟合的曲线

Figure 4. Raman spectra of InP/ZnS QDs

图4. InP/ZnS量子点的拉曼谱

体材料的TO与LO峰分别位于304 cm−1与345 cm−1,ZnS体材料的TO与LO峰分别位于280 cm−1与345 cm−1 [15] [16] 。图中的拉曼峰位于316 cm−1与342 cm−1,与体材料相比LO峰无明显变化,而TO峰与体材料出现的差异我们认为是由核壳界面处的应力导致的缺陷产生的 [17] 。InP晶格常数0.58687,ZnS晶格常数0.5410,晶格失配率为7.8% [18] ,晶格失配导致核径向和切线方向均收到压应力;壳径向受到压应力,切线方向受到张应力。所以,B峰为界面处的应力带来的晶格失配导致的缺陷发光。

4. 结论

我们采用一锅法制备了InP/ZnS量子点,通过TEM图像可以看出合成的InP核的晶格质量良好,且排列结构为闪锌矿的(111)晶面。通过变功率的光致发光光谱的测试,结合积分面积随激发功率变化的公式的拟合确认了InP/ZnS量子点中存在两个发光来源,一个为高能端的自由激子发光,另一个来源于缺陷发光。为了进一步验证缺陷的来源,对InP/ZnS量子点进行了拉曼测试,通过拉曼谱中TO峰的移动,可以认定缺陷是由于界面处应力导致的晶格失配产生的。本文详细研究了InP/ZnS量子点的发光来源,对其光学性质的研究具有一定的指导作用。

基金项目

国家自然科学基金(61474010, 61574022, 61504012, 61674021, 11674038, 61704011)、吉林省科技发展计划(20160519007JH, 20160520117JH, 20160101255JC, 20160204074GX, 20170520118JH)、长春理工大学科技创新基金(XJJLG-2016-11, XJJLG-2016-14)。

参考文献

[1] Bourzac, K. (2013) Quantum dots go on Display. Nature, 493, 283. https://doi.org/10.1038/493283a

[2] Bao, J. and Bawendi, M.G. (2015) A Colloidal Quantum Dot Spectrometer. Nature, 523, 67. https://doi.org/10.1038/nature14576

[3] Bruchez Jr., M. and Alivisatos, A.P. (1998) Semiconductor Nanocrystals as Fluorescent Biological Labels. Science, 281, 2013. https://doi.org/10.1126/science.281.5385.2013

[4] Sun, Q., Wang, Y.A., Li, L.S., et al. (2007) Bright, Multicoloured Light-Emitting Diodes Based on Quantum Dots. Nature Photonics, 1, 717-722. https://doi.org/10.1038/nphoton.2007.226

[5] Klimov, V.I., Mikhailovsky, A.A., Xu, S., et al. (2000) Optical Gain and Stimulated Emission in nanocrystal Quantum Dots. Science, 290, 314-7. https://doi.org/10.1126/science.290.5490.314

[6] Brunetti, V., Chibli, H., Fiammengo, R., et al. (2012) InP/ZnS as a Safer Alternative to CdSe/ZnS Core/Shell Quantum Dots: in Vitro and in Vivo Toxicity Assessment. Nanoscale, 5, 307. https://doi.org/10.1039/C2NR33024E

[7] Micic, O.I., Smith, B.B. and Nozik, A.J. (2000) Core-shell Quantum Dots of Lattice-Matched ZnCdSe2 Shells on InP Cores:  Experiment and Theory. Cheminform, 32.

[8] Bharali, D.J., Lucey, D.W., Jayakumar, H., et al. (2005) Folate-Receptor-Mediated Delivery of InP Quantum Dots for Bioimaging Using Confocal and Two-Photon Microscopy. Journal of the American Chemical Society, 127, 11364-11371. https://doi.org/10.1021/ja051455x

[9] Haubold, S., Haase, M., Kornowski, A., et al. (2001) Strongly Luminescent InP/ZnS Core-Shell Nanoparticles. Chemphyschem—A European Journal of Chemical Physics & Physical Chemistry, 2, 331. https://doi.org/10.1002/1439-7641(20010518)2:5<331::AID-CPHC331>3.0.CO;2-0

[10] Bergman, L., Chen, X.B., Morrison, J.L., et al. (2004) Photoluminescence Dynamics in Ensembles of Wide-Band-Gap Nanocrystallites and Powders. Journal of Applied Physics, 96, 675-682. https://doi.org/10.1063/1.1759076

[11] He, H., Yu, Q., Hui, L., et al. (2016) Exciton Localization in Solution-Processed Organolead Trihalide Perovskites. Nature Communications, 7, Article No. 10896. https://doi.org/10.1038/ncomms10896

[12] Cooper, D.E., Bajaj, J. and Newman, P.R. (1988) Photoluminescence Spectroscopy of Excitons for Evaluation of High-Quality CdTe Crystals. Journal of Crystal Growth, 86, 544-551. https://doi.org/10.1016/0022-0248(90)90773-E

[13] Schmidt, T., Lischka, K. and Zulehner, W. (1992) Excitation-Power Dependence of the Near-Band-Edge Photoluminescence of Semiconductors. Physical Review B Condensed Matter, 45, 8989. https://doi.org/10.1103/PhysRevB.45.8989

[14] Bao, J., Bell, D.C., Capasso, F., et al. (2008) Optical Properties of Rotationally Twinned InP Nanowire Heterostructures. Nano Letters, 8, 836-841. https://doi.org/10.1021/nl072921e

[15] Pietra, F., De, T.L., Hoekstra, A.W., et al. (2016) Tuning the Lattice Parameter of InxZnyP for Highly Luminescent Lattice-Matched Core/Shell Quantum Dots. Acs Nano, 10, 4754-4762. https://doi.org/10.1021/acsnano.6b01266

[16] Ramasamy, P., Kim, B., Lee, M.S., et al. (2016) Beneficial Effects of Water in the Colloidal Synthesis of InP/ZnS Core-Shell Quantum Dots for Optoelectronic Applications. Nanoscale, 8, 17159-17168. https://doi.org/10.1039/C6NR04713K

[17] Gheshlaghi, N., Pisheh, H.S., Karim, M.R., et al. (2016) Interfacial Strain Effect on Type-I and Type-II Core/Shell Quantum Dots. Superlattices & Microstructures, 97, 489-494.

[18] Kim, K., Jeong, S., Woo, J.Y., et al. (2012) Successive and Large-Scale Synthesis of InP/ZnS Quantum Dots in a Hybrid Reactor and Their Application to White LEDs. Nanotechnology, 23, Article ID: 065602. https://doi.org/10.1088/0957-4484/23/6/065602

文章引用: 张博文 , 王登魁 , 方 铉 , 房 丹 , 王新伟 , 唐吉龙 , 王晓华 , 魏志鹏 (2018) InP/ZnS核壳量子点光学性质研究。 材料科学, 8, 131-136. doi: 10.12677/MS.2018.83016

参考文献

[1] Bourzac, K. (2013) Quantum dots go on Display. Nature, 493, 283.
https://doi.org/10.1038/493283a

[2] Bao, J. and Bawendi, M.G. (2015) A Colloidal Quantum Dot Spectrometer. Nature, 523, 67.
https://doi.org/10.1038/nature14576

[3] Bruchez Jr., M. and Alivisatos, A.P. (1998) Semiconductor Nanocrystals as Fluorescent Biological Labels. Science, 281, 2013.
https://doi.org/10.1126/science.281.5385.2013

[4] Sun, Q., Wang, Y.A., Li, L.S., et al. (2007) Bright, Multicoloured Light-Emitting Diodes Based on Quantum Dots. Nature Photonics, 1, 717-722.
https://doi.org/10.1038/nphoton.2007.226

[5] Klimov, V.I., Mikhailovsky, A.A., Xu, S., et al. (2000) Optical Gain and Stimulated Emission in nanocrystal Quantum Dots. Science, 290, 314-7.
https://doi.org/10.1126/science.290.5490.314

[6] Brunetti, V., Chibli, H., Fiammengo, R., et al. (2012) InP/ZnS as a Safer Alternative to CdSe/ZnS Core/Shell Quantum Dots: in Vitro and in Vivo Toxicity Assessment. Nanoscale, 5, 307.
https://doi.org/10.1039/C2NR33024E

[7] Micic, O.I., Smith, B.B. and Nozik, A.J. (2000) Core-shell Quantum Dots of Lattice-Matched ZnCdSe2 Shells on InP Cores:  Experiment and Theory. Cheminform, 32.

[8] Bharali, D.J., Lucey, D.W., Jayakumar, H., et al. (2005) Folate-Receptor-Mediated Delivery of InP Quantum Dots for Bioimaging Using Confocal and Two-Photon Microscopy. Journal of the American Chemical Society, 127, 11364-11371.
https://doi.org/10.1021/ja051455x

[9] Haubold, S., Haase, M., Kornowski, A., et al. (2001) Strongly Luminescent InP/ZnS Core-Shell Nanoparticles. Chemphyschem—A European Journal of Chemical Physics & Physical Chemistry, 2, 331.
https://doi.org/10.1002/1439-7641(20010518)2:5<331::AID-CPHC331>3.0.CO;2-0

[10] Bergman, L., Chen, X.B., Morrison, J.L., et al. (2004) Photoluminescence Dynamics in Ensembles of Wide-Band-Gap Nanocrystallites and Powders. Journal of Applied Physics, 96, 675-682.
https://doi.org/10.1063/1.1759076

[11] He, H., Yu, Q., Hui, L., et al. (2016) Exciton Localization in Solution-Processed Organolead Trihalide Perovskites. Nature Communications, 7, Article No. 10896.
https://doi.org/10.1038/ncomms10896

[12] Cooper, D.E., Bajaj, J. and Newman, P.R. (1988) Photoluminescence Spectroscopy of Excitons for Evaluation of High-Quality CdTe Crystals. Journal of Crystal Growth, 86, 544-551.
https://doi.org/10.1016/0022-0248(90)90773-E

[13] Schmidt, T., Lischka, K. and Zulehner, W. (1992) Excitation-Power Dependence of the Near-Band-Edge Photoluminescence of Semiconductors. Physical Review B Condensed Matter, 45, 8989.
https://doi.org/10.1103/PhysRevB.45.8989

[14] Bao, J., Bell, D.C., Capasso, F., et al. (2008) Optical Properties of Rotationally Twinned InP Nanowire Heterostructures. Nano Letters, 8, 836-841.
https://doi.org/10.1021/nl072921e

[15] Pietra, F., De, T.L., Hoekstra, A.W., et al. (2016) Tuning the Lattice Pa-rameter of InxZnyP for Highly Luminescent Lattice-Matched Core/Shell Quantum Dots. Acs Nano, 10, 4754-4762.
https://doi.org/10.1021/acsnano.6b01266

[16] Ramasamy, P., Kim, B., Lee, M.S., et al. (2016) Beneficial Effects of Water in the Colloidal Synthesis of InP/ZnS Core-Shell Quantum Dots for Optoelectronic Applications. Nanoscale, 8, 17159-17168.
https://doi.org/10.1039/C6NR04713K

[17] Gheshlaghi, N., Pisheh, H.S., Karim, M.R., et al. (2016) Interfacial Strain Effect on Type-I and Type-II Core/Shell Quantum Dots. Superlattices & Microstructures, 97, 489-494.

[18] Kim, K., Jeong, S., Woo, J.Y., et al. (2012) Successive and Large-Scale Synthesis of InP/ZnS Quantum Dots in a Hybrid Reactor and Their Application to White LEDs. Nanotechnology, 23, Article ID: 065602.
https://doi.org/10.1088/0957-4484/23/6/065602

分享
Top