Synthesis of Graphene Nanoparticles and Their Application in Surface Raman Enhancement

Multilayer graphene-enclosed nickel nanoparticles have been prepared on the SiO₂/Si substrate using chemical vapor deposition. Rhodamine 6 G was used as a molecule probe to detect if the multilayer graphene-enclosed nickel nanoparticles can be applied in surface-enhanced Raman spectroscopy. The experimental results indicated that the Raman signals of C-C stretching within the benzene ring of rhodamine 6 G can be observed only on the multilayer graphene-enclosed nickel nanoparticle substrate rather than on reference substrates such as SiO₂/Si and Ni/SiO₂/Si. This suggests that the mechanism of Raman enhancement for rhodamine 6 G molecules on graphene is through the π–π bond coupling between them.     

Reliable determination of the few‐layer graphene oxide thickness using Raman spectroscopy

We report on a reference‐free Raman spectroscopy method for a precise thickness determination of the multilayered graphene oxide flakes. The method is based on the normalization of the total integral intensity of D and G Raman bands to the integral intensity of the second‐order optical phonon peak of the silicon substrate in the Raman spectrum. The normalization provides discrete ratio values corresponding to the number of graphene oxide layers in the respective flakes with the intensity linearly increasing with the number of layers. This provides a fast and robust determination of the thickness of graphene oxide flakes in terms of the layer number up to high values. A comparison with conventional spectrally resolved reflectivity mapping shows similar sensitivity, while selectivity to particular functional chemical groups is a bonus of the Raman‐based method. Copyright © 2015 John Wiley & Sons, Ltd.     

Graphenic Aerogels Decorated with Ag Nanoparticles as 3D SERS Substrates for Biosensing

A versatile and efficient surface-enhanced Raman scattering (SERS) substrate based on a hybrid aerogel composed of reduced graphene oxide (rGO) decorated with silver nanoparticles (AgNPs), suitable for highly sensitive label-free detection of chemical and biological species, is presented. The simple and low-cost one-pot hydrothermal synthesis allows obtaining of a 3D nanostructured spongy-like matrix that shows good spatial distribution of Ag nanoparticles in intimate contact with rGO flakes, characterized by means of several morphological, structural, and compositional techniques. The nanostructured material, tested by SERS analysis with both rhodamine 6G (R6G) and 4-mercaptobenzoic acid (MBA), shows a satisfying SERS efficiency, quantified in terms of minimum detectable concentration of 10⁻¹⁰ and 10⁻⁷ m , corresponding to on- and off-resonant excitation, respectively. The versatility of chemical/biochemical functionalization is successfully demonstrated by exploiting different routes, by immobilizing both protoporphyrin IX (PRPIX) and hemin (H) that take advantage of π−π non-covalent bonding with the graphene layers, as well as thiol-ended oligonucleotides (DNA probes/aptamers) directly grafted on the AgNPs. Finally, after the successful integration of the hybrid aerogel into a microfluidic chip, the biorecognition of miR222 is obtained demonstrating the reliability of the aerogel substrate as SERS platform for biosensing.     

退火法制备石墨烯-银纳米粒子及其增强拉曼实验

针对目前SERS基底上金属颗粒制备过程中存在的分布不均匀、易氧化和稳定性差等缺点，通过热蒸镀和高温退火获得分布均匀的SERS基底；同时结合石墨烯优良的光学性能、化学惰性、荧光猝灭以及本身的SERS增强等优点，制备了稳定的石墨烯-银纳米颗粒（GE/AgNPs）复合结构SERS基底。通过GE/AgNPs复合结构的拉曼光谱稳定性试验证明了石墨烯在GE/AgNPs结构中起到隔绝银纳米颗粒与空气直接接触及催化氧化银脱氧的作用，有利于SERS基底的时间稳定性。(1) 石墨烯、Ag纳米颗粒及其复合结构的制备。首先采用热蒸镀和高温退火的方法使Ag纳米颗粒均匀地沉积在SiO₂/Si基底上，再采用化学气相沉积法在Cu箔上制备少层石墨烯，并用湿法转移法将石墨烯转移到目标基底上，并实验研究了以不同的退火顺序对GE/AgNPs基底造成的影响。(2) 石墨烯、Ag纳米颗粒及其复合基底的表征。分别采用光学显微镜、扫描电子显微镜和拉曼光谱进行表征，得到转移后的纯石墨烯较完整地覆盖在SiO₂/Si基底上面，表面比较平整，但在少数地方仍然存在褶皱和杂质；SEM表征结果表明对于不同制备流程的GE/AgNPs复合结构上的Ag纳米颗粒基本呈球形。基本符合Ostwald熟化理论，通过对退火温度和时间的控制能获得平均粒径在40~60 nm的银颗粒，且分布较均匀。此外，在不同退火顺序中，石墨烯的加入对银纳米颗粒的扩散形成扩散势垒，从而出现较大的不规则的颗粒。(3) 基底稳定性试验和仿真分析。通过基底本身的Raman mapping测试，分析了石墨烯拉曼特征峰峰值和半高宽的变化，得知基底对石墨烯本身的拉曼增强效果主要来源于银纳米颗粒间的电磁场增强。同时采用浓度为10-6 mol·L-1的罗丹明6G (R6G)水溶液作为探针分子，对转移了石墨烯的GE/AgNPs复合基底和未转移石墨烯的Ag纳米颗粒基底进行了SERS稳定性实验。结果表明GE/AgNPs复合基底在1~33 d内衰减较缓慢，30 d后仍能探测到拉曼信号约为原来信号的35.1%~40.6%；而纯Ag基底上随着Ag纳米颗粒在空气中迅速氧化，基底的SERS性能显著下降，在30 d后只有原来信号的5.9%~11.3%。此外，通过实验得到覆盖了石墨烯之后的增强因子约为6.05×105。最后采用时域有限差分算法（FDTD）计算了复合结构的电磁场分布和理论增强因子，其理论增强因子可以达到5.7×105。实验和仿真结果的差异，主要是源于石墨烯的化学增强作用。     

Raman Spectroscopic Characterization of Graphene

Abstract

The recent progress using Raman spectroscopy and imaging of graphene is reviewed. The intensity of the G band increases with increased graphene layers, and the shape of 2D band evolves into four peaks of bilayer graphene from a single peak of monolayer graphene. The G band will blue shift and become narrow with both electron and hole doping, whereas the 2D band will blue shift with hole doping and red shift with electron doping. Frequencies of the G and 2D band will downshift with increasing temperature. Under compressed strain, the upshift of the G and 2D bands can be found. Moreover, the strong Raman signal of monolayer graphene is explained by interference enhancement effect. As for epitaxial graphene, Raman spectroscopy can be used to identify the superior and inferior carrier mobility. The edge chirality of graphene can be determined by using polarized Raman spectroscopy. All results mentioned here are closely relevant to the basic theory of graphene and application in nanodevices.     

Raman Spectrum of Epitaxial Graphene on SiC （0001） by Pulsed Electron Irradiation

We report new Raman features of epitaxial graphene （EG） on Si-face 4H-SiC prepared by pulsed electron irradiation （PEI）. With increasing graphene layers, frequencies of G and 2D peaks show blue-shifts and approach those of bulk highly-oriented pyrolytic graphite. It is indicated that the EG is slightly tension strained and tends to be strain-free. Meanwhile, single Lorentzian line shapes are well fitted to the 2D peaks of EG on SiC（O001） and their full widths at half maximum decrease with the increasing graphene layers, which indicates that the multilayer EG on Si-face can also contain turbostratic stacking by our PEI route instead of only AB Bernal stacking by a traditional thermal annealing method. It is worth noting that the stacking style plays an important role on the charge carrier mobility. Therefore our findings will be a candidate for growing quality graphene with high carrier mobility both on the Si- and C-terminated SiC substrate. Mechanisms behind the features are studied and discussed.     

石墨烯-银纳米颗粒复合结构制备工艺优化及表面增强拉曼光谱特性

在石墨烯-Ag纳米颗粒复合结构表面增强拉曼散射（SERS）基底常规制备工艺的基础上，提出了采用偶联剂吸附的方法来改善Ag纳米颗粒在目标基底上分布的均匀性；采用双层聚甲基丙烯酸甲酯（PMMA）来转移石墨烯，以减少石墨烯表面的缺陷；采用退火处理的方法来降低SERS基底的拉曼背景噪声，从而提高拉曼光谱的对比度。实验结果表明，采用优化制备工艺得到的复合结构SERS基底均匀性有较大提高，石墨烯G峰和2D峰的增强拉曼光谱对比度分别提高了54.9%和64.3%，罗丹明6G（R6G）分子在774和1 363 cm-1处的拉曼光谱强度随浓度变化关系的拟合优度（R2）分别达到了0.997 5和0.986 7。     

银/硅纳米孔柱阵列活性基底的SERS效应研究

以硅纳米孔柱阵列(Si-NPA)为衬底,采用浸渍法制备出一种具有规则表面形貌特征的银/硅纳米孔柱阵列(Ag/Si-NPA),并以R6G为探测目标材料,对其表面增强拉曼(SERS)效应进行了研究。结果表明,对于R6G浓度低至10-15M,Ag/Si-NPA均能表现出清晰的特征SERS峰。随着浓度的降低,R6G的荧光淬灭,所测拉曼光谱的基线降低,但特征峰峰位基本保持不变。在低浓度10-15M时得到的SERS光谱,理论上证明为单分子光谱。此外,Ag/Si-NPA活性基底具有较好的稳定性,在长达28天的自然老化过程中,Ag/Si-NPA能够保持对R6G较高的探测水平,光谱具有较好的信噪比和分辨率。Ag/Si-NPA是一种理想的SERS活性基底。     

An effective surface-enhanced Raman scattering template based on gold nanoparticle/silicon nanowire arrays

A large-scale Si nanowire array （SiNWA） is fabricated with gold （Au） nanoparticles by simple metal-assisted chemical etching and metal reduction processes. The three-dimensional nanostructured Au/SiNWA is evaluated as an active substrate for surface-enhanced Raman scattering （SERS）. The results show that the detection limit for rhodamine 6G is as low as 10-7 M, and the Raman enhancement factor is as large as 105 with a relative standard deviation of less than 25%. After the calibration of the Raman peak intensifies of rhodamine 6G and thiram, organic molecules could be quantitatively detected. These results indicate that Au/SiNWA is a promising SERS-active substrate for the detection of biomolecules present in low concentrations. Our findings are an important advance in SERS substrates to allow fast and quantitative detection of trace organic contaminants.     

SERS activity of carbon nanotubes modified by silver nanoparticles with different particle sizes

A two-dimensional (2D) surface-enhanced Raman scattering (SERS) substrate is fabricated by decorating carbon nanotube (CNT) films with Ag nanoparticles (AgNPs) in different sizes, via simple and low-cost chemical reduction method and self-assembling method. The change of Raman and SERS activity of carbon nanotubes/Ag nanoparticles (CNTs/AgNPs) composites with varying size of AgNPs are investigated by using rhodamine 6G (R6G) as a probe molecule. Meanwhile, the scattering cross section of AgNPs and the distribution of electric field of CNTs/AgNPs composite are simulated through finite difference time domain (FDTD) method. Surface plasmon resonance (SPR) wavelength is redshifted as the size of AgNPs increases, and the intensity of SERS and electric field increase with AgNPs size increasing. The experiment and simulation results show a Raman scattering enhancement factor (EF) of 10⁸ for the hybrid substrate.