【作者】 孙琦；

【导师】 张庆瑜；

【作者基本信息】 大连理工大学 ， 凝聚态物理， 2021， 博士

【摘要】 表面增强Raman光谱（SERS）是近年来受到广泛关注的痕量分子检测手段,在生物医学、环境监测、化学分析等领域展现出了巨大的应用潜力。SERS信号强度强烈地依赖于金属纳米结构产生的局域表面等离子共振（LSPR）,而LSPR的性质直接与金属纳米颗粒的尺寸、形状以及结构等因素有关。高导电率金属纳米材料在可见光范围内能够产生极强的LSPR现象,基于此类材料制备出的三维（3D）SERS基片由于结构种类丰富,制备成本低以及可重复使用等特点,极大地促进了SERS的应用与发展。本论文利用氧化锌纳米棒（ZnO-NR）阵列,通过沉积薄Ag层制备了一种高灵敏度、长保质期的Ag-ZnO-NR 3D SERS基片,分别通过实验分析和时域有限差分（FDTD）软件模拟两种方式研究了Ag-ZnO-NR 3D SERS基片的长效机制,并优化了Ag-ZnO-NR结构;通过Cl^-溶液浸泡,探究了Cl^-对Ag-ZnO-NR 3D SERS基片的改性以及Cl^-在Raman增强方面的作用。此外,以Ag-ZnO-NR 3D SERS基片为基础,设计了专门用于气相分子检测的SERS装置,探讨了Ag-ZnO-NR 3D SERS基片用于气相SERS检测的可行性及存在的相关问题。论文的主要研究工作及结果如下:（1）Ag-ZnO-NR 3D SERS基片的长效机制研究:在绒面硅、图形化蓝宝石及毛玻璃等多种基底上,采用磁控溅射和溶液法生长制备了Ag-ZnO-NR 3D SERS基片,并通过控制Ag的沉积量,优化了SERS基片的性能。利用罗丹明6G（R6G）作为探针分子,证明优化后的Ag-ZnO-NR 3D SERS基片的检测极限可达10^-14 M。同时,长效实验研究表明,在室温环境下,Ag-ZnO-NR 3D SERS基片可在空气中保存6年以上,SERS信号无明显变化。针对这种Ag-ZnO-NR 3D SERS基片具有的长效特性,通过制备Ag层部分覆盖的不同电阻率的ZnO薄膜,利用开尔文扫描探针显微镜证明,Ag层向高阻ZnO的电子转移可以使Ag层表面带正电,从而有效避免了Ag层的氧化,这种电子转移可能是决定Ag-ZnO-NR 3D SERS基片能够长期保持SERS活性的主要原因。在此基础上,通过制备Ag和ZnO-NR之间存在不同过渡层的Ag-ZnO-NR阵列证明,当Ag层和ZnO-NR之间的电子转移被SiO_x过渡层阻断后,Ag-ZnO-NR的化学稳定性急剧下降;而采用ZnO过渡层制备的Ag-ZnO-NR SERS基片,化学稳定性变化不大,进一步证明了Ag层向ZnO-NR的电子转移是保持Ag-ZnO-NR 3D SERS基片保持长期稳定的原因。此外,本文还探讨了Ag层结晶质量对Ag-ZnO-NR 3D SERS基片化学稳定性的影响。（2）Ag-ZnO-NR 3D SERS基片的FDTD模拟及结构优化:构建了Ag-ZnO-NR结构模型,利用FDTD方法计算了Ag-ZnO-NR的消光光谱和局域电磁场,结果表明,与纯Ag-NR不同,Ag-ZnO-NR的LSPR的偶极振荡模劈裂为高频（H-mode）和低频（L-mode）两个分支,其中L-mode对400-600 nm的Raman增强起主要作用。同时,ZnO-NR的半径、长度和Ag层的厚度都对LSPR频率和强度起着重要的调节作用。此外,将ZnO-NR替换成SiO₂-NR或TiO₂-NR时,Ag-SiO₂-NR和Ag-TiO₂-NR的消光光谱均发生不同程度的改变,说明介电材料的折射率也是调控这种3D SERS基片性能的重要参数。通过分析周期性Ag-ZnO-NR表面电磁场强度,证明这种3D SERS基片存在优化的ZnO-NR数密度约为83/μm²,与实验结果基本一致。此外,计算结果表明高密度的Ag-ZnO-NR之间的电磁场相互辐射在提高这种3D SERS基片灵敏度方面起着十分重要的作用。（3）Cl^-对Ag-ZnO-NR 3D SERS基片的改性及其在Raman增强中的作用:采用Cl^-溶液浸泡方法,实现了Ag-ZnO-NR 3D SERS基片的改性,探究了Cl^-浓度和基片浸泡时间对3D SERS基片形貌的影响及相关的改性机制。扫描电子显微分析和FDTD模拟结果表明,Cl^-溶液浸泡可以使Ag-ZnO-NR 3D SERS基片转变为Ag颗粒弥散分布在ZnO-NR顶部的3D SERS基片,从而有效避免Ag-ZnO-NR 3D SERS基片因Raman分析时激光焦斑过热而出现的探针分子的“碳化”现象,并显著降低了Raman光谱的背景噪音。此外,采用R6G分子和Cl^-的分离浸泡和共同浸泡两种方式分别加载探针分子,通过Raman分析和光致荧光光谱证明,Cl^-的存在可以增强R6G分子在Ag表面的吸附,是Cl^-或其它卤素离子可以产生化学增强的主要原因。因此,R6G分子和Cl^-的共同浸泡显著提高了SERS基片上Raman信号的信噪比。（4）Ag-ZnO-NR 3D SERS基片在气相分子检测中的应用:基于Ag-ZnO-NR 3D SERS基片,特别设计并制作了用于气相分子检测的SERS分析装置。利用甲基蓝为探针分子,以乙醇蒸气为载气,探究了Raman信号随甲基蓝分子沉积量的演化。研究结果表明,该装置可以实时在线检测气相分子,SERS检测极限约为6×10^-8 M。此外,分析了SERS基片在气相分子检测过程中存在的相关问题。更多还原

【Abstract】 Surface-enhanced Raman spectroscopy（SERS）has become a powerful tool of detecting trace molecules and exhibits many distinguishing characteristics,which can be exploited in the applications of biomedicine,environmental monitoring,and chemical analysis,thus receiving much attention in the past decades.The SERS of molecules confined in the vicinity of metal nanoparticles（NPs）or on a rough metal surface is due to the enhancement of electromagnetic fields generated by excitation of a localized surface plasmon resonance（LSPR）.Three-dimensional（3D）SERS substrates prepared with high-conductive metal nanomaterials,which can produce extremely strong LSPR in the visible light range,have greatly promoted the development of SERS application due to the rich structures,low cost and good repeatability.In this thesis,high-sensitive and ultra-stable 3D SERS substrates were prepared by depositing a thin layer of Ag film on ZnO nanorod（ZnO-NR）arrays.By a series of experiments and finite difference-time domain（FDTD）simulations,the mechanism of ultra-stability for Ag-ZnO-NR 3D SERS substrates was revealed and the SERS performance was optimized.By soaking the Ag-ZnO-NR arrays in chloride solutions,the 3D SERS substrates were found to be modified and thus the modification mechanisms were explored and the role of chloride ions in enhancing Raman signals was studied.In addition,the 3D SERS substrates were used to detect gas-phase molecules using a specially designed experimental device.The main results in this thesis are summarized as follows:（1）The stabilization mechanism of Ag-ZnO-NR 3D SERS substratesOn the texturized Si wafers,the patterned sapphire single-crystal chips,and the frosted glass slides,ZnO-NR arrays were grown by a solution method and the 3D SERS substrates were prepared by optimizing the thickness of deposited Ag layer.Using Rhodamine-6G（R6G）as the probing molecules,the Ag-ZnO-NR 3D SERS substrates exhibited good performance capable of detecting R6G as low as 10^-14 M.Moreover,the Ag-ZnO-NR 3D SERS substrates are ultrastable,with a shelf-time in air as long as more than 6 years.Using specially prepred samples of ZnO films with controlled conductivity,the Klevin scanning probe microscopy revealed that the electrons in the Ag layer are transferred to the high-resistance ZnO film,which probably plays an important role in stabilizing the 3D SERS substrates.The stabilitzation mechanism was further substantiated by studing the stability of Ag-ZnO-NR arrays with different buffered layers between the Ag films and ZnO-NRs,showing that the Ag-ZnO-NR arrays become unstable as the electron transfer is obstructed by SiO_x layer,whereas the stability of Ag-ZnO-NR substrates with ZnO buffer layer maintains nearly unchanged.As such,the stabilization mechanism of Ag-ZnO-NR 3D SERS substrates can be attributed to the electron transfer from Ag layer to the ZnO-NRs,thus avoiding Ag films from oxidation.In addition,the stability of Ag-ZnO-NR 3D SERS substrates was studied by exploring the crystallinity of Ag layer.（2）FDTD simulation and structure optimization of Ag-ZnO-NR 3D SERS substrates.By constructing an Ag-ZnO-NR model,the extinction spectra and local electromagnetic fields in the vicinity of Ag-ZnO-NR were calculated by an FDTD method.Differing from Ag-NRs,the Ag-ZnO-NRs exhibited a dipole LSPR split into high-and low-frequency modes（H-and L-modes）,in which the L-mode plays a dominant role in enhancing the Raman signals in the range of 400 to 600 nm.At the same time,the radius and length of ZnO-NRs as well as the the thickness of Ag layer were substantiated to play a role in tuning the LSPR frequency and the intensity.Using SiO₂-NR or TiO₂-NR to replace ZnO-NR,the extinction spectra of Ag-SiO₂-NR or Ag-TiO₂-NR were changed as well,indicating that the refractive index of dielectric materials is another parameter of tuning the LSPR frequency.The local electromagnetic fields of Ag-ZnO-NRs with periodic boundary conditions showed that the performance of Ag-ZnO-NR 3D SERS substrates can be optimized at a ZnO-NR denisity of～83/μm²,which is close to the experimental rsults in this work.In addition,the FDTD simulation revealed that the scattering of electromagnetic fields between different Ag-ZnO-NRs plays an crucial role in enhancing the Raman signals.（3）Effects of chloride ions on Ag-ZnO-NR modification and the Raman enhancement.Using chloride solutions,the Ag-ZnO-NR 3D SERS substrates were modified and the morphological evolution and modification mechanism were explored by changing the chloride concentration and the soaking time.Scanning electron microscopy and FDTD simulation showed that the chloride solutions are able to change the Ag-ZnO-NR arrays into Ag nanoparticles decorated ZnO-NR（Ag NP-ZnO-NR）arrays,which become nearly steady as the chloride solution is higher than a critical concentration and the soaking time is longer than10 min.The Ag NP-ZnO-NR arrays are able to effectively depress the thermalization of probe molecules during the signal collection and reduce the background noise of Raman spectra.By loading the R6G molecules onto SERS substrates in the presence and absence of chloride ions addition,the chloride ions were substantiated to be capable of enhancing adsorption of R6G molecules on the surface of Ag nanoparticles,which is the major reason for the chemical enhancement.The Ag NP-ZnO-NR 3D SERS substrates prepared by soaking Ag-ZnO-NR arrays in R6G and chloride mixed solutions are able to obviously enhancing the ratio of signal to noise.（4）Application of Ag-ZnO-NR 3D SERS substrates in detecting gas-phase molecules.On the basis of Ag-ZnO-NR 3D SERS substrates,a device for detecting gas-phase molecules was specially designed and used to detect the methyl blue molecules in ethonal vapor,showing that the Ag-ZnO-NR substrates are capable of on-line detecting the gas-phase molecules as low as 6×10^-8 M.更多还原