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Li Bin, Chen Xingfan, Liang Jing, Li Xueming, Tang Libin, Yang Peizhi. CoTe2 QDs: preparation, structure and optical properties (Invited)[J]. Infrared and Laser Engineering, 2021, 50(1): 20211021. doi: 10.3788/IRLA20211021
Citation: Li Bin, Chen Xingfan, Liang Jing, Li Xueming, Tang Libin, Yang Peizhi. CoTe2 QDs: preparation, structure and optical properties (Invited)[J]. Infrared and Laser Engineering, 2021, 50(1): 20211021. doi: 10.3788/IRLA20211021

CoTe2 QDs: preparation, structure and optical properties (Invited

doi: 10.3788/IRLA20211021
  • Received Date: 2020-11-05
  • Rev Recd Date: 2020-12-05
  • Available Online: 2021-01-22
  • Publish Date: 2021-01-22
  • In recent years, transition metal telluride (TMTs) has attracted extensive attention and research in the scientific field due to its unique crystal structure and excellent physical and chemical properties. In this paper, CoTe2 quantum dots (QDs) was prepared by ultrasonic method, the morphology and structure of the prepared CoTe2 QDs were characterized by TEM, AFM, EDS, XPS, XRD and FTIR. The optical properties of the prepared CoTe2 QDs were investigated by Spectrophotometer (UV-Vis), Photoluminescence (PL) and Photoluminescence Excitation (PLE). CoTe2 QDs shows good dispersion, uniform particle size and spherical morphology. The average diameter and height of the grains are about 3.1 nm and 2.9 nm respectively. CoTe2 QDs shows the obvious absorption in the infrared band, and the absorption value decreases with the increase of dilution concentration. When the wavelength of excitation light and emission light increases in turn, the PL and PLE peaks have a red shift, and they have an obvious Stokes shift effect. It shows that the photoluminescence of CoTe2 QDs is wavelength dependent. CoTe2 QDs has the photoluminescence characteristic of multicolor, different excitation light wavelength can emit different colors of light. The fluorescence quantum yield of QDs is 62.6%. The excellent optical characteristics of CoTe2 QDs, especially its absorption and luminescence characteristics in the infrared band, shows that it has important potential application value in infrared detection, laser protective coating, fluorescence imaging, multicolor luminescence and nano-photonic devices, and is expected to become a new type of infrared detection material.
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CoTe2 QDs: preparation, structure and optical properties (Invited

doi: 10.3788/IRLA20211021
  • 1. School of Energy and Environmental Sciences, Key Laboratory of Advanced Technique & Preparation for Renewable Energy Materials, Ministry of Education, Yunnan Normal University, Kunming 650500, China
  • 2. Kunming Institute of Physics, Kunming 650223, China
  • 3. Yunnan Key Laboratory of Advanced Photoelectric Materials & Devices, Kunming 650223, China

Abstract: In recent years, transition metal telluride (TMTs) has attracted extensive attention and research in the scientific field due to its unique crystal structure and excellent physical and chemical properties. In this paper, CoTe2 quantum dots (QDs) was prepared by ultrasonic method, the morphology and structure of the prepared CoTe2 QDs were characterized by TEM, AFM, EDS, XPS, XRD and FTIR. The optical properties of the prepared CoTe2 QDs were investigated by Spectrophotometer (UV-Vis), Photoluminescence (PL) and Photoluminescence Excitation (PLE). CoTe2 QDs shows good dispersion, uniform particle size and spherical morphology. The average diameter and height of the grains are about 3.1 nm and 2.9 nm respectively. CoTe2 QDs shows the obvious absorption in the infrared band, and the absorption value decreases with the increase of dilution concentration. When the wavelength of excitation light and emission light increases in turn, the PL and PLE peaks have a red shift, and they have an obvious Stokes shift effect. It shows that the photoluminescence of CoTe2 QDs is wavelength dependent. CoTe2 QDs has the photoluminescence characteristic of multicolor, different excitation light wavelength can emit different colors of light. The fluorescence quantum yield of QDs is 62.6%. The excellent optical characteristics of CoTe2 QDs, especially its absorption and luminescence characteristics in the infrared band, shows that it has important potential application value in infrared detection, laser protective coating, fluorescence imaging, multicolor luminescence and nano-photonic devices, and is expected to become a new type of infrared detection material.

    • 过渡金属碲化物(Transition-metal Tellurides, TMTs)有着独特的晶体结构和优异的物理化学特性,研究者因此对它们产生了极大的研究兴趣。目前TMTs在光电子学、催化、电极及磁性材料等诸多领域[14]具有广泛的应用前景。有研究表明,Ⅳ-Ⅶ族过渡金属与碲元素生成的TMTs主要为层状材料,而Ⅷ-Ⅹ族过渡金属与碲元素则是生成三维的TMTs晶体[5];理论和实验均表明,若改变TMTs的元素组成和形貌,其相应的物理、化学性质都会发生变化,且在较低维数下,它的物化性能会得到一定的增强[6-7]。CoTe2是TMTs中的一种,具有稳定的白铁矿型结构,也称为斜方碲钴矿(Mattagamite)[8],CoTe2晶体属正交晶系,空间群为Pnnm(58),具备良好的对称性,其晶体结构具体参数见表1。CoTe2具有半金属性质,取向为自旋向上的电子呈现金属性,取向为自旋向下的电子呈现非金属性,此时带隙为1.063 eV[9]。近年来通过制备CoTe2纳米材料,发现它具有诸多独特的性质,例如,Lu等人使用水热法制备出尺寸为20~50 nm的CoTe2纳米粒(NPs),可作为析氢反应(HER)高效、稳定的催化剂,稳定性可达2天及以上[9];Shi等人利用简单水热共还原路线成功合成了直径约为50 nm、长度为1 μm的CoTe和CoTe2纳米管,所得产物具有优异的磁性能[10];Yin等人利用单步水热工艺成功开发了一种新型CoTe2纳米膜电极,该电极在析氧反应/析氢反应(OER/HER)中具有优异的活性、耐久性和稳定性[11];Xie等人采用一步溶剂热路线,以乙二胺为溶剂,在高压釜中制备了直径为30 nm、长度大于1.2 μm的CoTe2纳米棒[12],通过能谱测试证实了CoTe2纳米棒为斜方晶系;Jiang等人通过溶剂热法合成了五种不同物质的纳米片,其中CoTe2纳米片的厚度约为10~15 nm,研究结果表明:其PL光谱存在很宽的发射带[13];Ian G. McKendry等人采用化学合成法制备了CoTe2纳米晶,发现它是一种优良的电化学水氧化反应(WOR)的催化剂[14];Huang等人合成了具有均匀多面体形状的CoTe2纳米复合材料,并首次将其作为染料敏化太阳能电池的电催化剂,该复合材料具有优异的光伏性能,光电转换效率(PCE)为9.02%[15]

      CoTe2Crystal parameters
      Crystal SystemOrthorhombic
      Space groupPnnm (58)
      Lattice parametersa=5.312 Å(1 Å=10−10 m ),b=6.311 Å,c=3.889 Å
      Angleα=90°,β=90°,γ=90°
      Z2

      Table 1.  Crystal parameters of CoTe2(PDF#011-0553)

      基于当前报道的CoTe2纳米材料研究现状,发现对于CoTe2量子点(QDs)的制备和光学性质的研究相对较少,目前CoTe2纳米材料制备主要采用水热法、溶剂热法等,但是这些合成方法所需实验条件相对较为严苛,液相超声剥离法(以下简称超声法)在常温常压下可在溶液中形成局部的高温高压,大幅度降低了实验反应条件的要求,同时它还具有污染小、操作方便、成本较低等优点。实验采用自上而下[16]的超声法制备CoTe2 QDs,并对制备得到的CoTe2 QDs进行测试表征,分析其形貌、结构特征,研究其近红外光学性能。

    • 实验采用的超声法主要步骤包括研磨、超声和离心,制备步骤如下:称取0.4 g CoTe2粉末(纯度≥99.95%)放至研钵中充分研磨2 h。然后将研磨后的样品转移至烧杯中,加入50 mL的N-甲基吡咯烷酮(NMP,纯度≥99.9%)作为分散剂混合均匀。将上述混合液置于超声仪(180 W)中累计超声4 h。将超声后的溶液转移至离心管,置于离心机中离心,离心机参数设置(转速: 5000 r/min,时间: 15 min),离心结束后收集上层清液。

    • CoTe2 QDs的尺寸、形貌、结构及元素组分分别使用透射电镜(TEM,Tecnai G2 TF30 S-Twin)、原子力显微镜(AFM,日本精工SPA-400)及能谱仪(EDS,NOVA NANOSEM 450)进行表征;物相组成及成键特性使用X射线光电子能谱(XPS,PHI Versa探针II)、X射线衍射仪(XRD,EMPYREAN,X射线源:Cu Ka,l=0.154178 nm)、傅里叶变换红外光谱仪(FTIR,Nicolet iS10)和拉曼光谱仪(Raman Renishaw-InVia)进行分析;光学性能使用紫外-可见分光光度计(UV-Vis, Shimadzu UV-3600)和荧光光谱仪(PL&PLE,Hitachi, F-4500)进行测试。

    • CoTe2 QDs的制备机理如图1(a)所示,CoTe2体材料在经过研磨、超声和离心三个流程后可以得到CoTe2 QDs;图1(a)的红色实线方框代表单个CoTe2晶胞;通过PDF#011-0553卡片分析得出CoTe2晶胞中轴向的Co-Te键长为2.550 Å,轴间Co-Te键长为2.598 Å,这与Afshar[17]等人报道的基本一致。

      Figure 1.  (a) The schematic diagram of the preparation of CoTe2 QDs; (b) TEM image (inset: the particle size distribution fitted with Gaussian distribution); (c) High-Resolution Transmission Electron Microscope (HR-TEM) image (inset: Lattice fringe analysis); (d) HR-TEM image (inset: FFT image); (e) AFM image; (f) Height analysis for locations 1,2 and 3 indicated in (e); (g) EDS spectrum

      图1(b)是CoTe2 QDs的TEM图和粒径分布直方图,图中的黑色小点即为CoTe2 QDs,可看出量子点分散性良好,通过对量子点尺寸进行分析发现其符合正态分布,高斯拟合得到CoTe2 QDs的平均直径(Wc约为3.1 nm,半峰宽(FWHM)为0.95 nm;图1(c)是CoTe2 QDs的高分辨率TEM图,对其中单个量子点使用Line Profile分析(图中右上角方框图),得出晶格间距d=0.182 nm,此时对应晶面是(221)面;图1(d)是CoTe2 QDs的高分辨率TEM图,选取其中一个量子点进行快速傅里叶变换分析(FFT),观察到CoTe2 QDs呈现立方晶型结构,表明CoTe2 QDs与CoTe2体材料的晶体结构相同。

      图1(e)为AFM测试结果,从图中可以看出量子点尺寸均一、分布均匀,从中随机选取三个量子点,标记为1、2、3,经粒径高度分析,其值分别为2.8 nm、2.8 nm和3.1 nm,平均高度为2.9 nm,如图1(f)所示,该结果与上述TEM的粒径分析相吻合;

      图1(g)是CoTe2 QDs的EDS元素分析图,EDS能谱可定性分析CoTe2 QDs中元素的相对含量,在去除硅衬底峰的干扰后,从图中可看到Co元素(33.2%)和Te元素(66.8%)的原子比例约为1∶2,这符合1个Co原子与2个Te原子相结合的结构模型。

      图2(a)为CoTe2 QDs的XPS全谱图, 从中可以清晰地看到Te 3d和Co 2p的XPS峰,C 1s可能是分散剂NMP的残余,O 1s可能是样品CoTe2表面被氧化的缘故或是分散剂NMP的残余;为了深入分析CoTe2 QDs中Te 3d和Co 2p的存在形式,进行XPS分峰拟合,Te 3d的XPS谱如图2(b)所示,共有3种组成成分, 分别为Te 3d5/2 (574.6 eV)、Te 3d3/2 (585.2 eV)和Te的氧化态(578 eV),根据峰值强度进行分析,样品氧化程度很低,Te 3d5/2态占主导;Co 2p的XPS谱如图2(c) 所示。 表明Co 2p主要存在形式为Co 2p3/2(772.9 eV), Co 2p1/2 (794.9 eV),而784.1 eV和806.2 eV处的峰是它们的Satellite Peak (卫星峰),这与参考文献[10, 18-19]报道的峰位位置相吻合。

      Figure 2.  (a) XPS full spectrum; (b) Te 3d XPS spectrum; (c) Co 2p XPS spectrum; (d) FTIR spectrum; (e) XRD diffraction pattern; (f) Raman spectrum;(g) UV-Vis absorption spectra at different concentrations (200-500 nm) (inset: photos of CoTe2 QDs under natural light and UV light); (h) UV-Vis absorption spectra at different concentrations (500-1200 nm); (i) UV-Vis absorption spectrum of the sample on a quartz substrate

      FTIR结果如图2(d)所示, 在指纹区507 cm–1处观测到Co-Te的伸缩振动吸收峰,其他伸缩振动吸收峰为有机溶剂NMP中的C、H和O所结合的化学键。CoTe2 QDs的XRD测试结果如图2(e)所示,经与标准PDF卡片(PDF#11-0553)对比可知,图中2θ=31.7°(d=0.282 nm)、2θ=33.0°(d=0.271 nm)和2θ=43.6°(d=0.207 nm)处的衍射峰分别对应于CoTe2样品的(111)、(120)和(211)晶面衍射峰,除此之外不存在其他的衍射峰,表明样品纯度较高;图2(f)为CoTe2 QDs的拉曼光谱图,在波数122.8 cm−1和140.7 cm−1处观察到Te-Te键的振动模式与碲元素[20]基本相同,可能对应于A1弯曲振动模式和拉伸振动模式或三角碲化物的E模[21-23],表明碲化物切实存在。

      CoTe2 QDs的UV-Vis吸收光谱如图2(g)图2(h)所示,图中清楚表明CoTe2 QDs在红外到紫外波段都存在吸收,且吸收强度随稀释浓度的增加而降低。此外,研究表明CoTe2 QDs溶液存在荧光效应,如图2(g)插图所示的溶液颜色变化过程,自然光照射下的样品溶液呈现淡黄色, 在波长为254 nm和365 nm的紫外光照射下, 样品分别呈现为淡紫色和蓝色。为了检测量子点薄膜的吸收特性,将CoTe2 QDs溶液在石英衬底上制备成膜,如图2(i)的插图所示,图2(i)为薄膜的UV-Vis吸收光谱,从中可以看出量子点薄膜在紫外到红外波段同样存在吸收,且相对于量子点溶液吸收更为明显;插图为UV-Vis吸收光谱的吸收值取对数之后的图像,其更能反映出量子点材料受波长的改变而发生变化的情况。

      光致发光谱(Photoluminescence, PL)是固定激发光波长,检测材料的发光光谱,可测得最强发射峰的位置。光致发光激发光谱(Photoluminescence Excitation, PLE)是固定待测材料荧光光谱的某一波段,改变激发激光波长,检测此波段强度对激发波长的响应。图3(a)图3(e)分别为CoTe2 QDs的PL和PLE图,由图可知,当激发光波长和发射光波长依次增加时,PL和PLE峰的峰位均呈现红移;归一化处理PL和PLE图,如图3(b)图3(e)所示,处理后的图像更能反映这一红移过程,其峰值所对应波长的变化为PL:400 nm→405 nm→409 nm→421 nm→434 nm→448 nm;PLE: 326 nm→329 nm→332 nm→340 nm→348 nm→358 nm;图3(c)图3(f)分别为PL与PLE峰波长和能量之间的关系,其红移的峰值能量范围为PL:3.12 eV → 2.76 eV, PLE: 3.80 eV → 3.46 eV,表明不同的激发波长会直接影响CoTe2 QDs的光致发光;通过比较PL和PLE,发现CoTe2 QDs具有明显的Stokes位移效应。图3(g)是CoTe2 QDs在近红外波段的PL图,当激发光波长从600 nm到750 nm (步长50 nm)时,CoTe2 QDs在近红外波段仍存在发光现象。图3(h)是CoTe2 QDs发射波长在近红外波段600~750 nm (步长50 nm)的PLE图,表明近红外波段的CoTe2 QDs发射光谱在不同波长都存在一定程度的响应。

      Figure 3.  Photoluminescence properties of CoTe2 QDs. (a) , (d) PL and PLE spectra; (b) , (e) Normalized PL and PLE spectra; (c) , (f) the relationship between the peak value and energy of PL and PLE; (g) near-infrared PL spectra; (h) near-infrared PLE spectra;(i) color coordinate

      通过上述对样品的表征测试,发现CoTe2 QDs具有较宽的光致发光光谱,表明可能存在光致多色发光的特性。量子点材料中电子接受外来能量后达到激发态,在电子回复至基态的过程中,会释放能量,这种能量通常以光的形式发射出去。量子点材料在不同的激发光波长激发下,电子会到达不同能级的激发态,在电子回复至基态的过程中,会发射出不同波长的光,呈现出不同的颜色。为了更直观地表现样品在不同激发波长下的发光过程,使用三刺激值公式 (1)[24]进行色坐标的计算:

      式中:$ \varphi \left(\lambda \right) $为颜色刺激函数, $ \varphi \left(\lambda \right)=T\left(\lambda \right) \cdot S\left(\lambda \right),S\left(\lambda \right) $为照明光源的相对光谱功率分布,$ T\left(\lambda \right) $为光谱透射率;$ \bar {x}\left(\lambda \right) $$ \bar {y}\left(\lambda \right) $$ \bar {\textit{z}}\left(\lambda \right) $分别为标准观察者光谱三刺激值; k值由照明光源确定比例系数。样品的色坐标数值采用公式(2)进行计算[25]

      式中:X+Y+Z=1;xyz为色坐标值。通过计算得出当激发波长从300 nm→500 nm (步长20 nm)时的色坐标xyz,如图3(i)中的AJ所示,相应的颜色发生了如下变化:紫调蓝-蓝色-蓝绿-绿色-黄调绿,表明样品具有优异的多色发光特性。

      荧光性能是衡量量子点材料的重要指标,因此,实验使用硫酸奎宁(Qs=0.48)[26]作为参比,计算了CoTe2 QDs的荧光量子产率(Qs),计算公式如公式(3)所示[27]

      式中:Q为荧光量子产率;A为特定激发光波长处的吸光度;I为荧光的发光峰积分面积;n为折射率;下标s为样品;r为参比。通过计算CoTe2 QDs的荧光量子产率可达62.6%,表明CoTe2 QDs具有优良的荧光性能。

    • 文中使用“自上而下”的液相超声剥离法成功制备出分散性良好、尺寸均匀、平均粒径约为3 nm的球形形貌CoTe2 QDs;根据UV-Vis测试,CoTe2 QDs从紫外到红外波段都存在吸收,吸收强度随稀释浓度的增加而降低;通过对PL和PLE的分析,CoTe2 QDs在红外波段存在光致发光,PL与PLE峰位有明显红移,存在明显的Stokes位移效应,表明光致发光具有激发波长依赖性;CoTe2 QDs存在光致多色发光特性,在不同激发光波长下可发出不同颜色的光;荧光量子产率可达62.6%。CoTe2 QDs优异的光学特性尤其是其在红外波段的吸收和发光特性,表明其在红外探测、激光防护涂层、多色发光、荧光成像和纳米光子器件等研究领域中具有重要的潜在应用价值。此外,该方法亦可用于制备其他高质量的碲化物量子点。

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