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锗是一种非常重要的半导体材料,在光电领域中应用广泛。不同形貌或结构的锗材料,如块体锗、锗微纳米颗粒、锗纳米线等,具有不同的光学、电学性能。目前关于液相激光在溶液中烧蚀锗靶的研究还较少[14]。文中利用图1的实验装置进行烧蚀实验,激光脉冲能量设置为100 mJ,聚焦后光斑大小1 mm,烧蚀时间为20 min。烧蚀靶材为锗靶,溶液为去离子水。液相激光烧蚀锗靶后的产物XRD谱如图2所示。锗晶体标准的XRD峰位在27.3°、45.3°、 53.7°、 66.0 °和72.8°,分别对应(111)、(220)、 (311)、 (400) 和(331)晶面。液相激光烧蚀锗靶后产物的XRD谱与标准图谱峰位对应较好,说明产物为锗颗粒,且结晶性较好。烧蚀后的产物溶液及其紫外可见吸收光谱如图3所示。发现烧蚀后溶液颜色明显改变,由烧蚀前的澄清透明变为棕褐色,说明溶液中已悬浮很多锗颗粒。从产物溶液的紫外可见吸收光谱图上发现,在287 nm附近有明显的吸收峰。参考文献[15]的锗块体在288 nm有尖锐的吸收峰,在550 nm附近有一个平缓的小吸收鼓包。从锗的能带结构图[16]中可知这两个吸收峰是由于特定位置的直接带隙吸收跃迁产生的。对于不同尺寸的锗纳米晶还可能出现其他的吸收峰位,比如300 、350 nm[15]。无论什么形貌的锗材料,在300 nm左右总会出现一个吸收峰。这里利用液相激光烧蚀法制备的锗颗粒也在300 nm左右出现一个吸收峰,但它的半高宽较宽,吸收波段可一直延续到可见光波段。
图 2 激光水下烧蚀锗靶后的产物XRD谱
Figure 2. XRD patterns of the particle products obtained by laser ablation of Ge target in water
图 3 激光水下烧蚀锗靶后的产物溶液及其紫外可见吸收光谱
Figure 3. Solution product after laser ablation of Ge target in water and its uv-visible absorption spectra
图4和图5为液相激光烧蚀锗靶后产物颗粒的扫描电子显微镜图以及根据该图统计的粒径分布。从图中可以看到锗颗粒的基本形貌为球状,将SEM图局部放大后可以发现,颗粒并不是规则的球形,颗粒直径在0.3 ~2.9 μm之间,平均直径为1.1 μm。图5的粒径分布在0.9 μm,1.3 μm处都呈现了峰值,图4中也可明显看出大颗粒和小颗粒并存的情况。因此推测烧蚀过程中一方面靶材被烧蚀形成产物颗粒,由于周围压强、空泡等的影响使形成的颗粒大小不一。这些已形成的颗粒会悬浮于溶液中,随着烧蚀的不断进行,溶液中的颗粒浓度不断增加。另一方面激光束聚焦过程中会经过溶液,会和悬浮于溶液中的颗粒相互作用,并存在两种过程:一种大颗粒烧蚀变成小颗粒;另一种小颗粒熔融合并成大颗粒。两种情况都将影响最终产物的粒径分布。目前,大多数已报道的液相激光烧蚀法制备的锗颗粒的粒径为纳米或亚微米量级[17-18]。Marina Rodio等人使用15 mJ不同波长(1 064 、532 、355 nm)的ps激光在水溶液中烧蚀60 min制备了40 ~2 nm粒径的锗纳米晶[18]。Dongshi Zhang等人利用能量为75 μJ波长为532 nm的ps激光在流动的水和异丙醇溶液中制备0.2~1.8 μm粒径的锗颗粒[17]。因此,产物的粒径和实验中使用的激光参数以及溶液有很大关系。
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除了溶液中的锗颗粒产物,还具体研究了锗靶在激光烧蚀后的凹坑形貌,以及随着激光烧蚀的进行,凹坑深度的变化过程,如图6所示。图6(a)给出了激光烧蚀脉冲次数N=1、10、100、1 000时的显微镜照片。可以看出,凹坑的直径变化不大,这与激光光斑的直径有关。凹坑的深度在逐渐加深,N=1时,从显微镜图上可以清楚的看到凹坑底部形貌;N=10时,底部形貌变得不清晰;N=100时,隐约可以看到凹坑底部;N=1 000时,已看不到凹坑底部。为了得到凹坑的具体深度,利用白光共聚焦显微镜对靶材表面的3D形态进行了测试,见图6(b)。图6(c)给出了N=100时烧蚀凹坑的剖面轮廓,图6(d)给出了凹坑深度随激光烧蚀脉冲数量的变化。从图中可以看出凹坑成锥状,凹坑深度随激光脉冲次数的增加而增加。利用凹坑的锥状形貌,可以近似算出烧蚀凹坑体积和烧蚀质量,如表1所示。烧蚀质量也随着激光脉冲次数的增加而增加,但增加的比例却在下降,即烧蚀的效率在降低。整个烧蚀过程和机理类似于激光打孔,包括表面蒸发、表面凹陷、孔的形成等阶段。对于凹坑的变化过程这里的结果也与其他激光打孔文献类似见参考文献[16-17]。
图 6 不同数量的激光脉冲烧蚀后的靶材形貌
Figure 6. Target morphology after ablation of different number of laser pulses
表 1 不同数量的激光脉冲烧蚀凹坑体积和烧蚀产物质量
Table 1. Ablated volume and mass after ablation by different number of laser pulses
Number of laser pulses1 10 100 10 000 Ablated volume/mm3 0.00104 0.00461 0.03738 0.15626 Ablated mass/mg 0.00557 0.02466 0.20000 0.83601 烧蚀凹坑的变化过程可以反映激光烧蚀效率和溶液中微纳米颗粒的产率。从上面的结果中,可以看出随着激光烧蚀的进行烧蚀效率将不断下降。一方面是大家公认的原因,随着烧蚀的进行,溶液中的产物颗粒浓度不断上升,使到达靶面的激光能量下降,从而导致烧蚀效率降低;另一方面,随着凹坑的加深,激光需要进入凹坑内部进行烧蚀,由于烧蚀位置的变化以及凹坑的束缚使部分喷射的物质沉积在凹坑内部,最终导致烧蚀效率降低。因此,仅仅通过延长烧蚀时间很难达到提高微纳米颗粒产率的目的,建议在烧蚀一段合适的时间后通过改变激光烧蚀的位置来提高烧蚀效率。
Study on Nd:YAG ns pulsed laser ablation of Ge target in water
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摘要: 激光在液体中烧蚀靶材不仅能改变靶材表面形貌,还能在溶液中制备出微纳材料。作为一种“绿色”、低成本以及方便操作的材料制备方法受到许多学者的关注。利用Nd:YAG纳秒脉冲激光在水下烧蚀锗靶制备微米级和亚微米级锗颗粒。通过紫外-可见吸收光谱、X射线衍射谱(XRD)和扫描电子显微镜(SEM),研究了锗颗粒产物的特性。接着,具体研究了锗靶在激光烧蚀后的凹坑形貌和烧蚀质量。发现烧蚀质量随着激光脉冲次数的增加而增加,但增加的比例却在下降,即烧蚀效率在降低。最后,分析了烧蚀效率降低的原因,为提高液相激光烧蚀法制备材料的效率提供理论参考和可行方案。Abstract: Laser ablation in liquid can modify the target morphology, as well as fabricate micro-nano materials in the solution. Many researchers were attracted by this method, because it was chemically clean, low cost and simple operation. Here, germanium particles with micron and submicron scale were prepared by Nd: YAG nanosecond pulsed laser ablation in water. The characteristics of germanium particles were studied by ultraviolet-visible absorption spectrum (uv-vis), X-ray diffraction (XRD) and scanning electron microscope (SEM). Then, studied the target morphology and ablation quality were studued after laser ablation of the germanium target. It was found that the ablation quality increases with the number of laser pulses, but the increase rate decreases, which means the ablation efficiency decreases. Finally, the reasons for the decrease of ablation efficiency were discussed to provide theoretical reference and feasible strategy for improving the preparation rate of materials fabricated by laser ablation in liquid.
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Key words:
- laser ablation in liquid /
- germanium particles /
- target morphology /
- ablation efficiency
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表 1 不同数量的激光脉冲烧蚀凹坑体积和烧蚀产物质量
Table 1. Ablated volume and mass after ablation by different number of laser pulses
Number of laser pulses1 10 100 10 000 Ablated volume/mm3 0.00104 0.00461 0.03738 0.15626 Ablated mass/mg 0.00557 0.02466 0.20000 0.83601 -
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