摘要
量子点薄膜的制备和光学常数的准确测定对推动量子点在光电领域的应用发展具有重要意义。目前,对于通过机械剥离法和化学气相沉积法制备的二碲化钼(MoTe₂)单晶薄膜,其光学常数表征技术已较为成熟;然而,关于2H⁃MoTe₂量子点薄膜的光学常数研究仍鲜见报道。采用超声辅助液相剥离的方法制备出2H-MoTe2量子点,并通过改变辅助溶剂的种类与超声顺序,成功制备了两种尺寸的MoTe2量子点;基于椭圆偏振光谱技术,采用B⁃spline模型和Tauc⁃Lorentz模型分别研究了两种尺寸量子点薄膜的折射率、消光系数与介电常数等光学常数。结果显示,两种尺寸的2H-MoTe2量子点在可见光到红外波段内具有相近的折射率、消光系数和较宽的光谱吸收区,并且与MoTe2体材料相比,具有较低的介电常数。
关键词
量子点薄膜的制备及其光学常数的准确测定是量子点在光电器件领域应用的重要前提。二碲化钼(MoTe2)属于层状过渡金属二硫化合物,具有半金属(1T')相和半导体(2H)相两种结构,其所具备的光电特性和厚度依赖带隙可调等特点使其成为二维材料器件研究的热点之
椭圆偏振光谱法(spectroscopic ellipsometry,SE)是一种重要的光学技术,用于获得材料在宽光子能量范围内的光学常数和介电常数。相较于基于反射对比度或光致发光的方
Gul
本文使用两步液相剥离法将2H⁃MoTe2通过两种溶剂超声制备成量子点,并在石英片上旋涂成薄膜,通过椭圆偏振光谱法测量出2H⁃MoTe2量子点薄膜的n、k、ε1和ε2。
本文采用超声辅助液相剥离制备2H⁃MoTe2量子点,流程如

图 1 2H⁃MoTe2量子点制备流程图
Fig. 1 Flow chart of the preparation of 2H-MoTe2 quantum dots
10.5 nm量子点制备步骤为:准确称取2.0 g 2H⁃MoTe2粉末,加入装有60 mL N⁃N二甲基甲酰胺(DMF)溶液的烧杯中;将混合溶液置于超声仪中,超声仪功率为180 W,时间为10 h;然后,将得到的混合溶液2 000 r/min离心10 min,取上清液,9 000 r/min离心10 min,留沉淀,即得到10.5 nm的2H⁃MoTe2量子点。
3.0 nm量子点制备步骤为:准确称取2.0 g 2H⁃MoTe2粉末,加入装有60 mL 1⁃甲基⁃2⁃吡咯烷酮(NMP)溶液的烧杯中,之后将混合溶液置于超声仪中,超声功率为300 W,时间为24 h;然后,将得到的混合溶液8 000 r/min离心10 min,留沉淀;用无水乙醇进行离心清洗2次,将溶液中的NMP去除,倒入60 mL DMF混合沉淀,通过300 W超声24 h,2000 r/min离心10 min;取上清液,用50 mL容量瓶收集,即为2H⁃MoTe2量子点溶液。
将1.1部分制备的量子点溶液进行9 000 r/min离心20 min,收集沉淀,加入1 mL异丙醇,配成浓度为40 mg/mL的2H⁃MoTe2量子点墨水。然后,使用匀胶机进行旋涂,旋涂第一转速为400 r/min,时间为5 s,第二转速选择900 r/min,时间为40 s,在2 cm的正方形石英片上旋涂一次。
使用EMPYREAN型X射线衍射仪(x-ray diffraction,XRD)、Nicolet iS10型傅里叶变换红外光谱仪(fourier transform infrared spectroscopy,FTIR)和Renishaw⁃In⁃Via型拉曼光谱仪对2H⁃MoTe2量子点的物相组成进行分析。利用Tecnai G2 TF30 S⁃Twin型透射电镜(transmission electron microscope,TEM)对2H⁃MoTe2量子点的尺寸和颗粒分散性进行表征,用扫描电镜(scanning electron microscope,SEM)对薄膜表面形貌和厚度进行表征,并使用Shimadzu UV⁃3600 Plus型紫外⁃可见⁃近红外(ultraviolet-visible-near infrared,UV⁃Vis⁃NIR)分光光度计测量量子点薄膜在300~2 000 nm波段的吸收光谱。最后,使用RC2型椭偏仪进行2H⁃MoTe2量子点薄膜椭偏参量的测量。

图 2 2H⁃MoTe2量子点的粒径、形貌、结构和组分分析:(a)10.5 nm量子点TEM图;(b)3.0 nm量子点TEM图;(c)10.5 nm和3.0 nm量子点的HRTEM图;(d)10.5 nm量子点的XRD图;(e)10.5 nm量子点的Raman光谱;(f)10.5 nm量子点的FTIR光谱
Fig. 2 Size, morphology, structure and composition analysis of 2H⁃MoTe2 quantum dots: (a) TEM image of 10.5 nm quantum dots; (b) TEM image of 3.0 nm quantum dots; (c) HRTEM images of 10.5 nm and 3.0 nm quantum dots; (d) XRD pattern of 10.5 nm quantum dots; (e) Raman spectrum of 10.5 nm quantum dots; (f) FTIR spectrum of 10.5 nm quantum dots

图 3 2H⁃MoTe2量子点薄膜的SEM和UV-Vis-NIR吸收光谱:量子点薄膜俯视SEM图:(a)粒径为10.5 nm,(d)粒径为3.0 nm;量子点薄膜断面SEM图:(b)粒径为10.5 nm,(e)粒径为3.0 nm;量子点薄膜UV-Vis-NIR吸收光谱:(c)粒径为10.5 nm,(f)粒径为3.0 nm,插图为薄膜照片
Fig. 3 SEM and UV-Vis-NIR absorption spectrum of 2H⁃MoTe2 quantum dot films: the top-view SEM images of the quantum dot films: (a) the particle size is 10.5 nm, (d) the particle size is 3.0 nm; the cross-sectional SEM images of the quantum dot films: (b) the particle size is 10.5 nm, (e) the particle size is 3.0 nm; the UV-Vis-NIR absorption spectrum of quantum dot films: (c) the particle size is 10.5 nm, (f) the particle size is 3.0 nm (insets are the pictures of the films)

图 4 (a) 椭偏光谱仪装置的示意图;(b) 2H-MoTe2量子点薄膜模型
Fig. 4 (a) The schematic diagram of the spectroscopic ellipsometry device; (b) the models of 2H-MoTe2 quantum dot film
通过椭偏仪测量入射光和反射光的偏振变化参数,即相对于样品表面在p和s方向上的光之间的振幅φ和相位差Δ,实验通过测量φ和Δ,再由公式(
, | (1) |
, | (2) |
, | (3) |
, | (4) |
, | (5) |
其中,φ是振幅,Δ是p和s方向的光之间的相位差,θ是检测光的入射角度,IS和IC是光强;ρ是偏振变化,N是材料的复折射率,是空气的负折射率(=1)
椭偏仪测试的参数通常使用洛伦兹(Tauc⁃Lorentz)振子模
本论文两种尺寸量子点椭偏仪测试的参数分别通过B-spline模型和Tauc⁃Lorentz模型拟合,由于旋涂制备的薄膜会有微小的差距,椭偏分析时两种尺寸量子点薄膜厚度值均设定为300 nm。公式(
, | (6) |
, | (7) |
, | (8) |
其中,函数、和的上标表示B样条的次数,下标i是节点位置;ti和ti+p表示节点间距。
, | (9) |
其中E是入射光能量,A是振子强度,C是振子宽度,Eg是带隙,E0是振子中心能,可以结合K⁃K关

图5 平均尺寸为10.5 nm 的2H⁃MoTe2量子点的B-spline椭偏模型拟合结果:(a,b)为椭偏参量;(c)为n,k值的测试值与拟合值;(d)为介电常数ε1和ε2的测试值与拟合值
Fig. 5 The spectroscopic ellipsometry fitted results using B-spline model for 2H⁃MoTe2 quantum dot film with an average size of 10.5 nm: (a, b) the ellipsometric parameters; (c) the tested and the fitted values of n, k; (d) the tested and the fitted values of the dielectric constants ε1 and ε2

图 6 尺寸为3.0 nm的2H⁃MoTe2量子点薄膜的Tauc-Lorentz椭偏模型拟合结果:(a)和(b)为椭偏参量;(c)是n,k值的测试值与拟合值;(d)是介电常数ε1和ε2的测试值与拟合值
Fig. 6 The spectroscopic parameters fitted results using Tauc-Lorentz model of 2H⁃MoTe2 quantum dot film with the size of 3.0 nm: (a) and (b) are the ellipsometric parameters; (c) the tested and fitted values of n, k; (d) the tested and fitted values of the dielectric constants ε1 and ε2
两种尺寸的2H⁃MoTe2量子点薄膜的ε1值均比体材料2H⁃MoTe2小,ε2随波长增大而减小,并且尺寸为10.5和3.0 nm的量子点薄膜的ε2峰位随尺寸减小而出现红移,可能是量子点尺寸变小导致量子限域效应更加明
采用两步液相剥离法,制备了10.5 nm和3.0 nm的2H⁃MoTe2量子点,并旋涂制备了2H⁃MoTe2量子点薄膜;通过椭圆偏振光谱法中的B⁃spline和Tauc⁃Lorentz模型,分别对粒径为10.5 nm和3.0 nm的量子点薄膜进行拟合,并得出了两种尺寸2H⁃MoTe2量子点薄膜的n、k、ε1和ε2;相较于文献报道的单晶MoTe2材料的椭偏分析,首次提供了2H⁃MoTe2量子点薄膜材料的椭圆偏振光谱分析数据。随着波长的增加,粒径为10.5 nm的2H⁃MoTe2量子点薄膜材料在200~1 700 nm波长内时n(1.2~1.8)和ε1(1.5~3.0)总体呈现上升趋势,而k(0.1~0.3)和ε2(0.0~1.0)总体呈现下降趋势,直至1 700 nm处接近零;粒径为3.0 nm的2H⁃MoTe2量子点薄膜的n(约为3.2)和ε1(约为11)在1 200 nm处取得最大值,高于粒径为10.5 nm的 2H⁃MoTe2量子点薄膜的n值和ε1值;并且k与ε2在1 000 nm之后快速减小直至1 500~2 000 nm波长范围接近零。结果显示,2H⁃MoTe2量子点薄膜的光学常数与量子点尺寸关系较大,并且在可见光到红外波长区内两种尺寸的2H⁃MoTe2量子点薄膜都具有较高的n值、ε1值和较宽的光谱吸收区,表明其在红外光电探测领域具有潜在应用价值。
References
Chhowalla M, Shin H S, Eda G, et al. The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets [J]. Nature Chemistry, 2013, 5(4): 263-275. [百度学术]
Keum D H, Cho S, Kim J H, et al. Bandgap opening in few-layered monoclinic MoTe2 [J]. Nature Physics, 2015, 11(6): 482-486. [百度学术]
Lezama I G, Ubaldini A, Longobardi M, et al. Surface transport and band gap structure of exfoliated 2H-MoTe2 crystals [J]. 2D Materials, 2014, 1(2): 021002. [百度学术]
Ruppert C, Aslan O B, Heinz T F, et al. Optical properties and band gap of single- and few-layer MoTe2 crystals [J]. Nano Letters, 2014, 14(11):6231-6236. [百度学术]
Aspnes D E. Spectroscopic ellipsometry—past, present, and future [J]. Thin Solid Films, 2014, 571: 334-344. [百度学术]
Fujiwara H. Spectroscopic ellipsometry: principles and applications [M]. John Wiley & Sons, 2007. [百度学术]
Gulo D P, Hung N T, Sankar R, et al. Exploring optical properties of 2H-and 1T′-MoTe2 single crystals by spectroscopic ellipsometry [J]. Physical Review Materials, 2023, 7(4): 044001. [百度学术]
Fang Ming-Sheng, Gu Hong-Gang, Guo Zheng-Feng, et al. Temperature and thickness dependent dielectric functions of MoTe2 thin films investigated by spectroscopic ellipsometry [J]. Applied Surface Science, 2022, 605: 154813. [百度学术]
Munkhbat B, Wróbel P, Antosiewicz T J, et al. Optical constants of several multilayer transition metal dichalcogenides measured by spectroscopic ellipsometry in the 300-1700 nm range: high index, anisotropy, and hyperbolicity [J]. ACS Photonics, 2022, 9(7): 2398-2407. [百度学术]
Wang Ying, Xiao Jun, Zhu Han-Yu, et al. Structural phase transition in monolayer MoTe2 driven by electrostatic doping [J]. Nature, 2017, 550(7677): 487-491. [百度学术]
Chen Xin-Peng, Chen Xiang-Dong, Ding Xing, et al. Gas sensitive characteristics of polyaniline decorated with molybdenum ditelluride nanosheets [J]. Chemosensors, 2022, 10(7): 264. [百度学术]
Hamizi N A, Johan M R. Optical and FTIR studies of CdSe quantum dots [C]. 2010 3rd IEEE International Nanoelectronics Conference, Hong Kong, IEEE, 2010: 887-887. [百度学术]
Diroll B T, Gaulding E A, Kagan C R, et al. Spectrally - resolved dielectric functions of solution-cast quantum dot thin films [J]. Chemistry of Materials, 2015, 27(18): 6463-6469. [百度学术]
Shim Y, Okada W, Mamedov N, et al. Incoherent ellipsometry below energy gap of TlInS2 [J]. Thin Solid Films, 2005, 509(1):137-140. [百度学术]
Ito K, Yamada Y, Miura A A, et al. High-aspect-ratio mushroom-like silica nanopillars immersed in air: epsilon-near-zero metamaterials mediated by a phonon-polaritonic anisotropy [J]. RSC Advances, 2019, 9(29): 16431-16438. [百度学术]
Likhachev D V. Selecting the right number of knots for B-spline parameterization of the dielectric functions in spectroscopic ellipsometry data analysis [J]. Thin Solid Films, 2017, 636: 519-526. [百度学术]
Likhachev D V. Optimization of the dielectric-function modeling by B-splines in spectroscopic ellipsometry analysis: A hybrid approach [J]. Thin Solid Films, 2022, 762: 139545. [百度学术]
Wang Xin, Fan Xian-Guang, Xu Ying-Jie, et al. A baseline correction algorithm for Raman spectroscopy by adaptive knots B-spline [J]. Measurement Science and Technology, 2015, 26(11): 115503. [百度学术]
Cox M G. The numerical evaluation of B-splines [J]. IMA Journal of Applied mathematics, 1972, 10(2): 134-149. [百度学术]
Song Li-Yuan, Tang Li-Bin, Wang Shan-Li, et al. Study on spectral ellipsometry of SnTe nanofilm [J]. Journal of Infrared and Millimeter Waves, 2023, 42(05), 581-587. [百度学术]
宋立媛, 唐利斌, 王善力, 等. SnTe纳米薄膜的椭圆偏振光谱研究 [J]. 红外与毫米波学报, 2023, 42(05), 581-587. 10.11972/j.issn.1001-9014.2023.05.003 [百度学术]
Magnozzi M, Ferrera M, Piccinini G, et al. Optical dielectric function of two-dimensional WS2 on epitaxial graphene [J]. 2D Materials, 2020, 7(2): 025024. [百度学术]
Xie Jun-Fang, Zhang Di, Yan Xiao-Qing, et al. Optical properties of chemical vapor deposition-grown PtSe2 characterized by spectroscopic ellipsometry [J]. 2D Materials, 2019, 6(3): 035011. [百度学术]
Zeng Wei, Feng Li-Ping, Yu Yao-Chen, et al. Investigation of the layer-dependent optical properties of ultrathin BiOI by spectroscopic ellipsometry [J]. Journal of Alloys and Compounds, 2021, 850: 156676. [百度学术]
Chittihan D, Burasiri P, Lertvanthphol T, et al. Observations of the initial stages on reactive gas-timing sputtered TaO thin films by dynamic in situ spectroscopic ellipsometery [J]. Optical Materials, 2019, 92: 223-232. [百度学术]
Magnozzi M, Ferrera M, Piccinini G, et al. Optical dielectric function of two-dimensional WS2 on epitaxial graphene [J]. 2D Materials, 2020, 7(2):025024-025024. [百度学术]
Pu Yong-Jie, Liu Wen-Qiang, Liu Yun-Yong, et al. Enhancing effects of reduced graphene oxide on photoluminescence of CsPbBr3 perovskite quantum dots [J]. Journal of Materials Chemistry C, 2020, 8(22):7447-7453. [百度学术]
Gu Hong-Gang, Song Bao-Kun, Fang Ming-Sheng, et al. Layer-dependent dielectric and optical properties of centimeter-scale 2D WSe2: evolution from a single layer to few layers [J]. Nanoscale, 2019, 11(47):22762-22771. [百度学术]
Peci E, Magnozzi M, Ramó L, et al. Dielectric function of 2D tungsten disulfide in homo and heterobilayer stacking [J]. Advanced Materials Interfaces, 2023, 10(3): 2201586. [百度学术]
Patel D D, Desai P F, Bhacser D N, et al. Optical, thermal and surface microtopography studies of MoTe2 single crystals [C]. AIP Conference Proceedings. American Institute of Physics, 2013, 1536(1): 323-324. [百度学术]
Zhao Ming-Lin, Shi Yu-Jun, Dai Jun, et al. Ellipsometric study of the complex optical constants of a CsPbBr3 perovskite thin film [J]. Journal of Materials Chemistry C, 2018, 6(39): 10450-10455 [百度学术]
Tiutiunnyk A, Morales A L, Bertel R, et al. Electronic, optical, and magnetic properties of doped triangular MoS2 quantum dots: a density functional theory approach [J]. Physica Status Solidi (b), 2022, 259(4): 2100509. [百度学术]