Ag substrate

Attention in this chapter will be mainly focused on che reaction of tecrahydrofuran (THF) with thin lithium films vapor-deposited in UHV on clean polycrystalline Ag substrates. The techniques employed in this study included TPD, XPS and AES. A parallel study of THF adsorption on Li layers deposited on Au substrates is currently being pursued by Ms. Guorong Zhuang using FTIRRAS (Fourier Transform Reflection Absorption Infrared Spectroscopy) in this laboratory. 

THF was damaged by che electron beam used in AES and the x-rays in XFS and this behavior was carefully examined. In addition, Li diffusion into che Ag substrate will be discussed. 

All of che THF/Ag and THF/Li/Ag experiments started with a clean Ag substrate, which was a polycrystalline Ag disk 1.0 cm in diameter and 0.1 cm Chick (see Section 2.4.3 for details about 

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Wang CF, Wang Q, Chen LD, Xu XC, Yao Q (2006) Electrodeposition of Sb2Tc3 films on Si(lOO) and Ag substrates. Electrochem Solid State Lett 9 C147-C149... 

UPD process and surface Cd-Ag alloying were also studied on polycrystalline Ag electrode [285, 286]. The surface alloy formation rate was similar to that for Ag(lOO) [286]. The dynamics of surface alloying promoted by Cd UPD was studied on irregular Ag substrates, dendritic, and columnar Ag deposits [291]. 

Lee etal. [129] have studied adsorption configuration and local ordering of sdicotungstate anions (STA) on Ag(lOO) electrode surfaces. Voltammetric studies have shown that STA passivates the Ag surface and thus slows down the electron transfer the dissolved redox species participate in. STA species is oriented with its fourfold axis perpendicular to the Ag(lOO) surface and the center of the STA molecule is located 4.90 A above the top layer of the Ag substrate. From the analysis of bond lengths, it has been found that four terminal O atoms are located near the hollow sites and that an Ag—O bond length is... 

Figure 13. (a) Wavelength vs. coupling angle for S-120 dye on a Ag substrate (b) the minimum reflectivity vs. wavelength for the same system. The measured points are shown as open circles and the solid curves calculated from bulk values of the dielectric function for S-120 dye. The measurements were made using the Kretschmann—Raether arrangement (19). 

As another example, oxide films on a vapor-deposited Ag substrate are presented [116]. Detailed XPS investigations show the development of Ag20 already 0.15 V below its equilibrium potential of E = 0.35 V [ 115]. Fig.50a presents the k-weighted Fourier Transform of the reflectivity-EXAFS, FT(ARk). of a 2.5 nm thick oxide film formed in 1 M NaOH at E = 0.40 V at an angle 0 = 0.09° relative to the surface. 

Electrodeposition of Bi onto Ag(lll) from aqueous acetate electroyte yielded an hexagonally close-packed superlattice, Ag(lll)(2N/3 x 2 /3) R30° Bi for which Bi = 7/12. The (2 /3 x 2v/3)/ 30° cells were present in parallel, strip-shaped domains averaging 8.5 silver unit meshes in width and unlimited length. In contrast, electrodeposition of Cu at Ag(lll) yielded no UPD peaks. While the Cu electrodeposit at Ag was stable at open circuit and was present as monoatomic layers as judged by attenuation of the Ag substrate Auger signal, it was somewhat disordered as judged by LEED. 

Further enhancement of the SERS can be achieved through precise control over the parameters at the metal particle size scale [10] Most SERS-active substrates were made from pure metallic nanostructures such as metal nanoparticles [33-35], metal particle arrays [5], roughened metal surfaces [36], or a combination with metal nanostructures and other nanomaterials [17, 18, 29, 37-39]. Recently, many strategies have shown the adsorbation of molecules on the surface of Ag and Au substrates for SERS applications [40]. SERS-active Ag nanostructures substrates are required to satisfy certain conditions with good reproducibility and stability [39]. For this reason, it is indispensable to develop and optimize the methods to prepare the SERS-active Ag substrates [41]. 

In recent years, there are many effective methods for the preparation of SERS-active Ag substrates. These include Langmuir-Blodgett (LB), self-assembly, lithography, and electrodeposition. 

The self-assembly technique has attracted much attention since they were observed by Decher in 1991 [49]. Self-assembly is the fundamental principle that provides the precise control of the resulting assemblies and the thickness of an individual layer on the nanometer scale by variation in the bulk concentration of the metal colloids suspension, deposition time, pH, and transport conditions [50]. Recently, the functionalization of metal nanoparticles has opened up new opportunities for the construction of nanostructured self-assembly films to fabricate novel SERS-active Ag substrates. 

The electrodeposition technology has proven to be the least expensive, effective, and readily adoptable method to deposit Ag substrates for reliable SERS substrates with good reproducibility. It allows the preparation of nanostructure patterns by controlling the amount of composition, deposition time, temperature, and applied potential. The SERS substrates prepared by electrodeposition were a good candidate for the fabrication of a reproducible substrate. In principle, most of the metals including Au [55], Cu [56], and Ag [57-59] can be electrodeposited from aqueous solutions. 

Recently, Sun et al. [60] proposed to fabricate Ag nanostructures by electrochemical deposition of Ag in anodic aluminum oxide templates with each pore diameter of 100 nm. The morphology of Ag substrates was characterized by EESEM. The length of the Ag nanowires is estimated to be about 2 pm from the EESEM images. In addition, the SERS enhancement factor calculated for Ag nano wires SERS substrates is approximately 10 in magnitude. The Ag nano wire arrays are expected to have important applications in the development of high sensitivity SERS-based substrates. 

Fig. 20.16 Simplified depiction of ike SERS-active Ag substrate. The dimension of...

Fig. 20.17 SERS spectra of 1,10-phenanthroline adsorbed on a rough Ag plate and on the sandwich-like Ag substrate, with the corresponding AFM micrographs. Exciting line 514.5 nm...

Gellini C, Muniz-Miranda M, Innocenti M, Carla F, Loglio F, Forest ML, Salvi PR (2008) Nanopattemed Ag substrates for SERS spectroscopy. Phys Chem Chem Phys 10 4555 558... 

The further growth following eq. (3.85) was assumed to take place by movement of Ag atoms through the vacancy-rich surface alloy and a simultaneous reversible Cd UPD on modified 2D Cd-Ag substrate surface. 

Preparing a clean surface is often a prerequisite for surface-science studies. UHV-based methods of sample preparation and characterization are established, and these may be exploited for studies of surfaces immersed in solution by interfacing an electrochemical cell with an UHV chamber. Samples can then be transferred from UHV and immersed into electrolyte solution under a purified-Ar atmosphere. However, even under these clean conditions, some metals oxidize or get contaminated prior to immersion. Other techniques for the preparation of clean surfaces that do not require UHV techniques are available for some metals. For example, flame annealing and quenching have been successfully used, but this procedure is probably limited to Au, Pt, Rh, Pd, Ir, and Ag substrates. In this technique, substrates are annealed in an oxygen flame and quenched in pure water. 

The experimental approaches used in this project have been the formation of fresh Li films on Ag substrate by vapor deposition in UHV, the exposure of the films to various amount of THF at different temperatures, and finally the characterizations of the formed layers using spectroscopic techniques. 

It was observed that THF did not adsorb on Ag at room temperature. However, THF-like TPD features were observed above room temperature if THF was condensed on a bare Ag substrate and then the TPD was initiated at 113 K. The adsorption of the THF was probably induced by the electrons from the filament used for heating che sample. In the last chapter, a suggestion will be made of replacing electron bombardment heating by resistive heating for future work. 

It was also observed in this work that Li and/or Li oxides appeared red under Ar ion bombardment. In the energy range of 500 to 2000 eV, the brightness of the red color increased with the ion energy. This provided another means to detect the presence of Li (or Li oxide). In one experiment, a clean Ag substrate was covered by a multilayer Li film (95% by AES) and then exposed to THF at 110 K. After about two hours (one hour for sample warm-up and the other for XPS characterization of the surface), the sample was placed under the Ar ion beam for cleaning. It took about 2 minutes to eliminate the Li layer (or Li oxide layer) on the surface by Ar ion bombardment... 

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