本工作通过反相溶剂扩散法成功生长出厘米级尺寸的Cs3Cu2I5单晶, 全面表征了其光学和闪烁性能, 并与利用布里奇曼法生长的Cs3Cu2I5单晶进行性能对比研究。利用溶液法生长的Cs3Cu2I5单晶不仅不潮解、无自吸收, 而且在X射线和γ射线辐照下具有高光产额以及低余辉特性。结果表明：利用溶液法和熔体法生长的Cs3Cu2I5单晶光学和闪烁性能相当。
1 实验方法
1.1 原料
CsI(99.999%, 购自Alfa)、CuI(99.999%, 购自Acros)、N,N-二甲基甲酰胺(DMF, ≥99.5%, 购自Greagent)、二甲基亚砜(DMSO, ≥99.0%, 购自Greagent)、甲醇(MeOH, 水含量≤5×10-5, 99.8%, 购自Adamas)。除非另有说明, 所有的试剂和溶剂都没有进行纯化。
1.2 晶体生长
在N2氛围下, 称取CsI (0.7794 g, 3 mmol)和CuI (0.3809 g, 2 mmol)溶解于12 mL的DMF/DMSO (体积比1 : 1)混合溶液中。然后加热至65 ℃并搅拌12 h至完全溶解, 形成前驱体溶液。用0.25 μm的聚四氟乙烯膜过滤至新的40 mL的透明玻璃容器中, 形成澄清的前驱体溶液。将该透明玻璃容器裹覆保鲜膜并扎孔, 置于盛有12 mL MeOH的100 mL密闭大瓶中。最后在65 ℃下恒温生长7 d, 得到透明棒状单晶(图1)。将良性溶剂与反相溶剂MeOH的体积比由1 : 1优化为5 : 3, 其它生长参数保持不变, 生长9 d得到厘米级块状Cs3Cu2I5单晶。
图1
图1
用反相溶剂扩散法生长Cs3Cu2I5晶体示意图
Fig. 1
Schematic diagram of Cs3Cu2I5 crystal grown by anti, solvent diffusion method
1.3 性能测试
使用Bruker D8 Advance X射线衍射仪, 在Mo Kα射线辐射下, 对研磨后的Cs3Cu2I5粉末进行粉末X射线衍射(XRD)分析。
使用Horiba FluoroMax-plus型荧光光谱仪测试了Cs3Cu2I5的室温激发和发射光谱。激发光源为氙灯。Cs3Cu2I5晶体的吸收和透过光谱由PE Lambda 950型分光光度计测得, 测试范围为200~800 nm。光致发光衰减动力学由牛津仪器公司FLS980型光谱仪的OptistatDN系统测得。
稳态X射线发射光谱是采用JF-10型携带式诊断X射线机作为激发源, 使用的管电压为50 keV, 管电流为0.5 mA, 利用积分球收集发射光, 通过光纤将收集到的光导入荧光光谱仪并采集数据。利用光谱仪的动力学模式测试其稳态X射线余辉性能, 以BGO和CsI:Tl晶体作为参比样品, 测试时间范围为100 s。使用ROSBTL 3DS热释光光谱仪测试晶体的热释光曲线, 温度范围为300~580 K, 加热速率为0.5 K/s。γ射线辐照下的脉冲高度谱由在137Cs辐照下, 滨松R2059光电倍增管收集发光信号, 高压为-1800 V, 成形时间为10 μs, 增益为4, 用单光子法标定绝对光输出。闪烁衰减曲线采用137Cs作为激发光源, 滨松R2059光电倍增管收集发光信号, 最终通过Tektronix DPO 5104数字荧光示波器采集。
2 结果与讨论
2.1 晶体生长与物相分析
利用反相溶剂MeOH缓慢扩散至前驱体溶液中, 降低Cs3Cu2I5在良性溶剂DMF/DMSO中的溶解度, 从而形成过饱和溶液, 促进晶核形成和晶体生长, 最终生长出毫米级尺寸的棒状Cs3Cu2I5单晶, 如图2(a)所示。通过减小反溶剂与良溶剂体积比, 减小反溶剂的扩散速率, 减缓晶体生长速率, 最终生长出的厘米级块状Cs3Cu2I5单晶, 如图2(b)所示。Cs3Cu2I5单晶在254 nm紫外灯辐照下, 晶体发出明亮的蓝光。图2(c, d)分别为利用溶液法和熔体法生长的Cs3Cu2I5单晶测试用样品。Cs3Cu2I5晶体中阴离子多面体[Cu2I5]3-的结构如图2(e)所示。Cs3Cu2I5是空间群为Pnma的正交晶系。Cu+离子占据四面体体心位点和三角形面心位点。两个Cu+离子和周围的五个I-组成一个[Cu2I5]3-多面体, 每个多面体都被Cs+隔开, 形成0D结构。溶液法生长的晶体粉末XRD图谱和Cs3Cu2I5单晶的XRD标准卡片对比, 如图2(f)所示。粉末的XRD衍射峰位置与标准卡片基本吻合[11], 且未出现多余的杂峰, 表明合成了纯Cs3Cu2I5物相。
图2
图2
Cs3Cu2I5晶体照片和结构
Fig. 2
Photographs and structures of Cs3Cu2I5 crystals
(a-b) Photographs of millimeter-sized rod-like Cs3Cu2I5 crystals and centimeter-sized bulk Cs3Cu2I5 crystals grown by solution method under sunlight and 254 nm ultraviolet light; (c) Photographs of Cs3Cu2I5 crystals grown by solution method for comparative study; (d) Photographs of Cs3Cu2I5 crystals grown by melt method for comparative study; (e) Structure of [Cu2I5]3- where blue and purple spheres represent Cu and I atoms, respectively; (f) X-ray powder diffraction (XRD) pattern of crystals grown by solution method
2.2 光学性能
为了研究溶液法生长的Cs3Cu2I5单晶的光学性能, 测试了其吸收光谱、荧光激发/发射光谱和荧光衰减动力学。从图中可以观察到一个位于309 nm的吸收峰(图3(a)), 通过对吸收截止边拟合得到其光学带隙为3.68 eV, 与利用熔体法生长的Cs3Cu2I5单晶的3.77 eV的光学带隙接近[11]。晶体在380~780 nm范围内的透过率均大于68%, 展现出优异的透光性(图3(b))。激发光谱中有一个308 nm的主峰和280 nm的肩峰, 发射峰位置在448 nm(图3(c)), 与利用熔体法生长的Cs3Cu2I5单晶峰位450 nm相近[11]。此外, 由荧光光谱可知, 溶液法制备的Cs3Cu2I5晶体几乎没有自吸收, 具有1.37 eV的大斯托克斯位移。溶液法生长的Cs3Cu2I5单晶只有一个发光中心(图3(d)), 该发光中心来源于单个无机团簇[Cu2I5]3-的自陷激子发射[14]。图3(e)为监测308 nm激发波长和448 nm发射波长下测试得到的时间分辨荧光光谱。采用单指数衰减函数拟合, 得到荧光衰减时间为943 ns, 略短于通过熔体法生长的Cs3Cu2I5单晶968 ns的荧光衰减时间[11]。
图3
图3
Cs3Cu2I5晶体的光学性能
Fig. 3
Optical properties of Cs3Cu2I5 crystals
(a) Absorption spectra of Cs3Cu2I5 crystals grown by solution method; (b) Transmittance spectra of Cs3Cu2I5 crystals grown by solution method; (c) Photoluminescence excitation (PLE) and emission (PL) spectra of Cs3Cu2I5 crystals grown by solution method; (d) PL and PLE contour mappings of Cs3Cu2I5 crystals grown by solution method; (e) Photoluminescence decay profiles of Cs3Cu2I5 crystals grown by solution method when λex=308 nm and λem=448 nm excited by nanoLED
2.3 闪烁性能
溶液法和熔体法生长的Cs3Cu2I5单晶的X射线激发发射光谱如图4(a)所示。在稳态X射线激发下, 二者的发射峰均位于448 nm。在相同测试条件下, 以相同尺寸的BGO晶体作为参比样品, 通过对比发射光谱的积分面积, 计算得到溶液法和熔体法生长的晶体的光产额均为32000 photons/MeV。图4(b)为晶体在X射线诱导下的余辉曲线。在稳态X射线辐照下, 与商用BGO和CsI : Tl单晶相比, 通过溶液法和熔体法生长的Cs3Cu2I5晶体都具有更低的余辉。为了研究溶液法和熔体法生长的Cs3Cu2I5晶体的缺陷特征, 以4 mm×4 mm×1 mm的CsI : Tl晶体作为参比样品, 测试它们的热释光曲线。如图4(c) 所示, 相同测试条件下, 在300~580 K的温度范围内, 溶液法与熔体法生长的Cs3Cu2I5单晶的热释光发光强度比CsI:Tl单晶低两个数量级。这说明与CsI:Tl单晶相比, 溶液法与熔体法制备的Cs3Cu2I5单晶具有更低的缺陷浓度。
图4
图4
Cs3Cu2I5晶体的闪烁性能
Fig. 4
Scintillation properties of Cs3Cu2I5 crystals
(a) X-ray excited RL spectra of Cs3Cu2I5 crystals grown by solution method and melt method with BGO as reference sample; (b) X-ray afterglow curves of Cs3Cu2I5 grown by solution method and melt method compared with that of BGO and CsI:Tl; (c) Thermoluminescence glow curves of Cs3Cu2I5 grown by solution method and melt method compared with that of CsI:Tl; (d) Pulse height spectra of Cs3Cu2I5 grown by solution method and melt method under 137Cs gamma-ray radiation as well as that of BGO; (e) Scintillation decay profiles of Cs3Cu2I5 grown by solution method and melt method under 137Cs gamma-ray radiation===Colorful figures are available on website
为了进一步考察溶液法制备Cs3Cu2I5晶体作为γ射线闪烁体的辐射探测性能, 利用γ射线脉冲高度谱结合单光电子峰法估算了两种方法制备的Cs3Cu2I5晶体的绝对光产额。图4(d)为溶液法和熔体法制备的Cs3Cu2I5晶体以及商用BGO(光产额为8000 photons/ MeV)作为参比样品在137Cs激发下的脉冲高度谱, 计算得到溶液法与熔体法生长的Cs3Cu2I5晶体的绝对光输出分别为(29000±3000) photons/MeV和(30000± 3000) photons/MeV。图4(e)为溶液法与熔体法生长的Cs3Cu2I5单晶在137Cs激发下的闪烁衰减时间曲线。利用双指数衰减函数拟合溶液法与熔体法生长的Cs3Cu2I5单晶衰减时间, 得到的主分量分别为947 ns (96%)和932 ns (97%)。主分量与荧光衰减时间相近, 该闪烁发光来源于自陷激子发光, 快分量来源有待进一步研究[11]。
3 结论
本工作采用反相溶剂扩散法生长出厘米级块状高质量Cs3Cu2I5单晶。该法制备的单晶具有优异的光学以及闪烁性能, 可用于X射线和γ射线探测。该晶体具有蓝光宽带发射, 来源于自陷激子发光, 因此具有1.37 eV的大斯托克斯位移。稳态X射线辐照下, 该单晶光产额为32000 photons/MeV。在137Cs源激发下, 溶液法生长的Cs3Cu2I5单晶的光产额为29000 photons/MeV, 主要闪烁衰减时间为947 ns。X射线下的余辉和热释光结果表明, 溶液法生长Cs3Cu2I5单晶具有极低的缺陷水平。这一研究结果证实了低成本溶液法可制备大尺寸高性能Cs3Cu2I5单晶。
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