金属纳米线阵列的制备及其表面增强光谱研究

【作者】 张利胜；

【导师】 张鹏翔； 方炎；

【作者基本信息】 昆明理工大学 ， 材料学， 2008， 博士

【摘要】 纳米线及其阵列材料是纳米材料应用研究中的热点。基于纳米线阵列为基底的表面增强拉曼散射(SERS)和针尖增强拉曼散射(TERS)研究对于形成新的一维增强基底,丰富SERS增强机制理论,拓展SERS应用领域,以及对纳米线自身电磁和界面特性的研究具有重大意义。本文首次在AAO模板上采用磁控溅射技术制备出了高质量的钛、铝和镍纳米线阵列,并首次将其应用于增强基底的SERS研究。通过实验与理论计算相结合的方式,系统地研究了在不同纳米线阵列的增强基底上存在着与传统基底上所不同的SERS增强机制,它同时存在着接触式增强；针尖增强和热点增强。本文主要的研究工作包括以下几点：采用二步阳极氧化法制备出孔洞排列规则,孔径均一的高质量AAO模板。分别用原子力显微镜、扫描电镜、X射线衍射、紫外-可见光吸收、光荧光和拉曼散射等手段对所制备AAO模板的各种性能进行了分析表征。随后还进一步研究了AAO模板的具体形成机理,根据阳极氧化过程中电流的变化把阳极氧化过程具体划分成了三个阶段。讨论了不同氧化电压对AAO模板孔洞直径大小的影响,对不同氧化电压所制备的模板孔径进行比较研究并拟合出了氧化电压与在该电压下制备模板孔径大小的经验关系式。采用激光分子束外延(LMBE)技术在AAO上限域生长成功的制备出银、镍纳米线和纳米线阵列。通过用扫描电镜、透射电镜和原子力显微镜等手段对所制备的纳米线及其纳米线阵列进行表征,可以看出采用该方法制备的纳米线粗细均匀、直径均一,大约100nm左右与所使用的AAO模板的孔径大小一致,纳米线长度可达2200nm,长径比为22。还可以看出,纳米线阵列表面总体比较平整,排列非常规则与AAO模板的孔洞排列一样呈正六边形排列。此外,每根纳米线端部不是平的而是针尖状的,因此用该方法所制备的银、镍纳米线阵列也是一个排列非常规则银、镍的针尖阵列。采用磁控溅射(MS)技术在AAO模板上限域生长成功制备出金、钛和铝纳米线阵列。通过用扫描电镜、原子力显微镜等手段对所制备的纳米线阵列进行表征,可以看出采用该方法制备的纳米线粗细均匀、直径均一大约100nm左右与所使用的AAO模板的孔径大小一致。纳米线阵列表面总体比较平整,排列非常规则与AAO模板的孔洞排列一样呈正六边形排列,同样是针尖阵列。采用真空蒸发镀膜技术在硅基片上制备出了表面平整的银颗粒状薄膜。用原子力显微镜对其进行表征,颗粒直径大小分布相对比较均匀,单个银颗粒直径大都分布在70-100nnm的范围内,这些小的银颗粒相互团聚成直径大约为500nm的大颗粒,而且这些大颗粒直径相对分布比较均匀。用激光烧蚀法制备出了近似球形的银、镍纳米颗粒,分别用透射电镜和紫外-可见吸收光谱对其表征可以看出粒径分布比较均匀,分散性好,直径大约在30nnm。在银、镍纳米线的增强基底上以苏丹红Ⅱ作为探针分子获得了很好的SERS谱图,通过与银、镍纳米颗粒上的SERS的比较,讨论了在银、镍纳米线上同时存在着接触式SERS增强和非接触式针尖TERS增强两种增强机制。以银纳米线阵列为基底,Rh 6G为探针分子得到了很好的SERS谱图,将其分别与在激光烧蚀法制备的银纳米颗粒和用真空蒸发镀膜法制备的银膜表面上的SERS谱进行分析比较,得出在银纳米线阵列上存在着与银纳米颗粒和银薄膜上不同的SERS增强机制,它同时存在着接触式SERS增强；TERS增强和热点增强机制。通过研究金纳米线阵列上NA分子在不同基底温度下SERS谱图相对强度的周期性变化,探讨了以金纳米线阵列为增强基底的SERS增强机制随基底温度的变化。首先,在110-150K的温度范围内SERS强度随着温度的升高而增大。因为探针分的振动强度在低温下很弱,随着温度的升高其振动强度会逐渐增加,所对应的拉曼散射频率强度也会随之增大。在150-270K的温度区域内SERS的相对强度呈现出先减小后增大的趋势。在这个范围内随着温度的升高探针分子的动能增大,吸附在金纳米线上的分子脱离金表面的几率越来越大,其SERS强度自然会随之减小。但是随着温度的进一步上升,分子将逐渐远离金表面而进入到金纳米线之间的热点中,在热点中探针分子的拉曼散射信号又将会被极大的增强,其SERS强度呈现出增大的趋势。在270-350K的范围内时,随着温度的升高SERS相对强度却又呈现出减小的趋势。随着基底温度的进一步上升,金纳米线内自由电子的无规则运动加剧。致使激发产生的表面等离子波的强度大大减弱,热点内局域电磁波的强度会随之大大减小,而处于热点内探针分子的拉曼散射强度随温度的上升而减小。分别以银、镍、钛、铝纳米线阵列作为SERS增强基底,用苏丹红Ⅱ作为探针分子,通过对上述增强基底上SERS谱图的比较,讨论以上各种纳米线阵列为基底的SERS增强效果。在以上几种纳米线阵列上都获得了很好的SERS谱图,说明以上几种纳米线阵列都是很好的SERS增强基底。特别是长期以来,国内外学术界普遍认为铝对于SERS而言没有增强效果,在铝的纳米线阵列上却可以得到很好的SERS谱。我们认为,当激发光照射到铝纳米线阵列上时,在每两根纳米线之间的区域会产生大量热点,处在热点中探针分子的拉曼散射信号将会被极大的增强。他是一种非接触式的热点增强机制,与传统的接触式SERS增强机制不同,所以当铝制备成纳米线阵列时就会有了很好的SERS增强效果。选用NA分子作为研究对象,有针对性的选用了六种不同的基组进行GaussianO3模拟计算,然后分别与实验值对比,确定出了实验所需的最佳计算基组。采用金纳米线阵列作为增强基底,NA作为探针分子获得了其SERS谱图。根据实验具体情况建立了两种不同的模型,分别代表金纳米线阵列上接触式增强和非接触式增强两种不同的SERS增强机制。使用DFT-B3PW91/LanL2DZ基组对上述两种模型进行计算,通过对实验值和计算值的分析比较,得出在金纳米线阵列上同时存这接触式和非接触式两种SERS增强机制。以银纳米线阵列增强基底作为电极,以对硝基苯甲酸(PNA)为探针分子,通过比较不同电位下SERS谱图的差异并结合Gaussian 03的模拟计算,探针分子在银纳米线阵列上的吸附方式随基底电位不同的变化方式。即：在0.1-0V时,探针分子随机的吸附在银纳米线阵列表面,主要以热点增强为主；0-(-0.3)V阶段,分子从随机吸附向以羧基吸附方式转变；(-0.3)-(-0.5)V阶段,探针分子的吸附方式从以羧基吸附为主导向以硝基吸附为主导转变。更多还原

【Abstract】 An investigation of nanowires and nanowires array-materials are more and more much attach importance in the area of applied material science and technology. The study of surface enhanced Raman scattering (SERS) effect on the substrate of metal nanowires and nanowires arrays exhibit more interesting variations. Because its. SERS enhanced mechanisms are very abundant, which include normal SERS, tip enhanced Raman scattering (TERS) and hot spots enhanced. It is very significative for enriching SERS mechanisms, out-spreading the application range of SERS and affording the method of study of material. In this paper, the titanium, aluminum and nickel nanowires arrays were prepared on the AAO templates by magnetic sputtering (MS); they were applied in the area of SERS as enhanced substrates, for the first time. The SERS enhancement-mechanisms were studied roundly on the metal nanowires arrays by integrate of experimentation with theoretics.In the dissertation, there are several segments which list as follows:The highly ordered anodic aluminum oxide (AAO) template was fabricated using aluminum anodizing in electrolytes with two-step method. Its performances were researched by some methods, which include AFM, SEM, XRD, visible-UV absorption spectrum, fluorescence spectrum and Raman spectrum. And its principle was discussed, which include three processes. We investigated the relation of AAO template apertures with anodic voltage, and then the function of it had been deduced.The nanowires and nanowires array-materials of silver and nickel were fabricated on AAO template by LMBE. Their forms were investigated by the means of SEM, AFM and TEM. The diameter which about 100nm was very homogeneous, and the length which about 2200nm was very uniformity. So the ratio of length with diameter of the nanowires was about 22. The exterior of the nanowires arrays was very smooth. Moreover, the end of the nanowires which fabricated on AAO template by LMBE was acuminate. So the nanowires arrays of silver and nickel were the tip arrays. The highly ordered nanowires arrays of gold, titanium and aluminum were fabricated on AAO templates by MS. The diameter was about 100nm which consistent with the templates’. And the ends of the nanowires which collocate in hexagon format were tips.The silver nano-film was fabricated on the silicon substrate by the method of vacuum vapor plating. The film was made up with the silver agglomerated particles which the diameter was about 500nm. And the silver agglomerated particles were conglobated with the nanoparticles which the diameter from 70 to 100nm. In this paper, we ablated silver plates in redistilled deionized water using pulsed laser and obtained ’chemically pure’ silver nanoparticles. Its diameter, which was good separate and uniform, was about 30nm.On the glasses covered with silver and nickel nanowires respectively, high quality surface enhanced Raman scattering (SERS) spectra of sudanⅡ(C18H16N20) were obtained. And the comparison of SERS on the silver and nickel nanoparticle indicated that there two enhanced mechanisms which include SERS and TERS on the substrates of the glasses covered with silver and nickel nanowires respectively.On the silver nanowires arrays, high quality SERS spectrum of probe molecules rhodamine 6G (Rh6G) was obtained. Comparison of SERS spectra on the substrates of silver nanowires array with silver film and colloids indicates that enhanced mechanism of SERS on the silver nanowires arrays different from the SERS on silver film or silver colloids. There are contiguously and non-contact enhanced mechanisms on the silver nanowires array.The change of SERS enhanced mechanisms on gold nanowires array at different temperature was studied by investigating the SERS spectra of probe molecules nicotinic acid (NA) relate with the different temperature on the substrate. In the first temperature range from 110 to 150K, the intensity of the molecule vibration was boosting up with temperature going up, so, the enhancement was ascending. In the second segment of 150-250K, the charge transfer enhancement was primary role. More and more molecules was disengaging from gold nanowires with temperature rising. Fewer and fewer molecules contributed to the effect of SERS, so, the intensity of SERS was diminishing in the range. However, in the range of 250-270K, electromagnetic enhancement became to leading function. The molecules movement was becoming strong with the raise of temperature. A lot of molecules moved to the hot spot where the extremely high enhancement of the electric field observed at the middle area of between each gold nanowies. And the molecules in the hot spot were enhanced largely, the intensity would be enhanced. In the fourth temperature segment from 270 to 350K, the electronic ruleless movement of gold nanowires was intensifying with the raise of temperature. The excitation of LSP produces a high local electromagnetic field in the middle area of between each gold nanowies that relate to the electronic ruleless movement of gold nanowires. The intensity of local electromagnetic field in the hot spot was receding with the enhancement of extent of the electronic ruleless movement. So, the intensity of SERS of the NA molecules which in hot spot would be lowering in the process of temperature from 207 to 350K.On the several kinds of nanowire arrays which silver, titanium, nickel, and aluminum, high-quality SERS spectra of probe molecules of the red dye SudanⅡwere obtained. In particular, the SERS spectrum of SudanⅡon an aluminum nanowire array is reported for the first time. Surface-enhanced Raman spectroscopy on the substrate of nanowire arrays is based on the extremely high enhancement of the electric field. observed in the area between the nanowires when excited with a laser beam which was called hot spots. The probe molecules should be enhanced when they are taken into the enhanced electric field between the nanowires. Under these conditions, the most enhanced molecules would not contact the nanowires.The optimal simulation method of Gaussian 03 was. confirmed by studying the computational spectra, which were simulated with six different methods, with the experimental spectrum of NA molecule. The investigation, which the experimenta SERS spectrum of probe molecule NA on the gold nanowires array and stimulant spectra which base on tow different models by the method of DFT-B3PW91/LanL2DZ, indicates that the presence, of contiguously and non-contact enhanced mechanisms on the gold nanowires array.We studied the SERS spectra of PNA on the silver nanowires array at different potential of the substrate. And the stimulant spectra which base on tow different models by the method of DFT-B3PW91/LanL2DZ were obtained. The investigation of the computational spectra and experimental SERS spectra indicates that the change of SERS enhanced mechanisms. It indicated that the hot-spot enhancement playing primary role and the probe molecule adsorbing on the silver nanowires at random in range of the potential from 0.1 to 0V. And in range of the 0-(-0.3) V, the adsorption of molecule transited from random to carboxylic adsorption. And the transformation, which carboxylic adsorption to nitro-adsorption, would occur in the range of the potential from (-0.3) to (-0.5) V.更多还原