Aqueous Dye Diffusion in Thin Films of Water-Soluble Poly(Vinyl Pyrrolidone) Copolymers: A Dynamic Secondary Ion Mass Spectrometry Study

High Raman enhancing shape-tunable ag nanoplates in alumina: a reliable and efficient SERS technique

Shape-tunable Ag nanoplates in alumina enable very strong SERS performances showing Raman enhancement factor >1 × 10(9), and allows for easy detection of analyte methylene blue having a concentration in the picomolar range. Raman enhancements have been systematically studied during the Ag nanoparticle-shape evolution from spherical nanoparticles to hexagonal nanoplates with sharp corners to truncated triangular nanoplates in alumina sol. Large SERS enhancement has been observed because of the uniform dispersion and embedment of Ag nanoplates in a relatively high dielectric alumina network where analyte molecules are held. This new approach gives uniform and strong SERS signal with reproducibility.

Differential SERS activity of gold and silver nanostructures enabled by adsorbed poly(vinylpyrrolidone)

We report that poly(vinylpyrrolidone) (PVP), a common stabilizer of colloidal dispersions of noble metal nanostructures, has a dramatic effect on their surface-enhanced Raman scattering (SERS) activity and enables highly selective SERS detection of analytes of various type and charge. Nanostructures studied include PVP-stabilized Au-Ag nanoshells synthesized by galvanic exchange reaction of citrate-reduced Ag nanoparticles (NPs), as well as solid citrate-reduced Ag and Au NPs, both before and after stabilization with PVP. All nanostructures were characterized in terms of their size, surface plasmon resonance wavelength, surface charge, and chemical composition. While the SERS activities of the parent citrate-reduced Ag and Au NPs are similar for rhodamine 6G (R6G) and 1,2-bis(4-pyridyl)ethylene (BPE) at various pH values, PVP-stabilized nanostructures demonstrate large differences in SERS enhancement factors (EFs) between these analytes depending on their chemical nature and protonation state. At pH values higher than BPE's pK(a2) of 5.65, where the analyte is largely unprotonated, the PVP-coated Au-Ag nanoshells showed a high SERS EF of >10(8). In contrast, SERS EFs were 10(3)- to 10(5)-fold lower for the protonated form of BPE at lower pH values, or for the usually highly SERS-active cationic R6G. The differential SERS activity of PVP-stabilized nanostructures is a result of discriminatory binding of analytes within-adsorbed PVP monolayer and a subsequent increase of analyte concentration at the nanostructure surface. Our experimental and theoretical quantum chemical calculations show that BPE binding with PVP-stabilized Au-Ag nanoshells is stronger when the analyte is in its unprotonated form as compared to its cationic, protonated form at a lower pH.

Cluster secondary ion mass spectrometry of polymers and related materials

Cluster secondary ion mass spectrometry (cluster SIMS) has played a critical role in the characterization of polymeric materials over the last decade, allowing for the ability to obtain spatially resolved surface and in-depth molecular information from many polymer systems. With the advent of new molecular sources such as C(60)(+), Au(3)(+), SF(5)(+), and Bi(3)(+), there are considerable increases in secondary ion signal as compared to more conventional atomic beams (Ar(+), Cs(+), or Ga(+)). In addition, compositional depth profiling in organic and polymeric systems is now feasible, without the rapid signal decay that is typically observed under atomic bombardment. The premise behind the success of cluster SIMS is that compared to atomic beams, polyatomic beams tend to cause surface-localized damage with rapid sputter removal rates, resulting in a system at equilibrium, where the damage created is rapidly removed before it can accumulate. Though this may be partly true, there are actually much more complex chemistries occurring under polyatomic bombardment of organic and polymeric materials, which need to be considered and discussed to better understand and define the important parameters for successful depth profiling. The following presents a review of the current literature on polymer analysis using cluster beams. This review will focus on the surface and in-depth characterization of polymer samples with cluster sources, but will also discuss the characterization of other relevant organic materials, and basic polymer radiation chemistry.

Molecular Depth Profiling of Multilayer Polymer Films Using Time-of-Flight Secondary Ion Mass Spectrometry

The low penetration depth and high sputter rates obtained using polyatomic primary ions have facilitated their use for the molecular depth profiling of some spin-cast polymer films by secondary ion mass spectrometry (SIMS). In this study, dual-beam time-of-flight (TOF) SIMS (sputter ion, 5 keV SF(5)(+); analysis ion, 10 keV Ar(+)) was used to depth profile spin-cast multilayers of poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate) (PHEMA), and trifluoroacetic anhydride-derivatized poly(2-hydroxyethyl methacrylate) (TFAA-PHEMA) on silicon substrates. Characteristic positive and negative secondary ions were monitored as a function of depth using SF(5)(+) primary ion doses necessary to sputter through the polymer layer and uncover the silicon substrate (>5 x10(14) ions/cm(2)). The sputter rates of the polymers in the multilayers were typically less than for corresponding single-layer films, and the order of the polymers in the multilayer affected the sputter rates of the polymers. Multilayer samples with PHEMA as the outermost layer resulted in lowered sputter rates for the underlying polymer layer due to increased ion-induced damage accumulation rates in PHEMA. Additionally, the presence of a PMMA or PHEMA overlayer significantly decreased the sputter rate of TFAA-PHEMA underlayers due to ion-induced damage accumulation in the overlayer. Typical interface widths between adjacent polymer layers were 10-15 nm for bilayer films and increased with depth to approximately 35 nm for the trilayer films. The increase in interface width and observations using optical microscopy showed the formation of sputter-induced surface roughness during the depth profiles of the trilayer polymer films. This study shows that polyatomic primary ions can be used for the molecular depth profiling of some multilayer polymer films and presents new opportunities for the analysis of thin organic films using TOF-SIMS.

Controlling surface roughness in vapor-deposited poly(amic acid) films by solvent-vapor exposure

A series of vapor-deposited poly(amic acid) (PAA) films were exposed to dimethyl sulfoxide (DMSO) vapors to investigate sorption kinetics and surface smoothing phenomena. Gravimetric sorption and secondary-ion mass spectrometry (SIMS) results are both consistent with frontal (case II) diffusion. These experiments suggest that the solvent front is defined by a sharp interface that delineates the swollen material from the unswollen material. Solvent-vapor smoothing was studied by first depositing PAA onto rough aluminum surfaces, and then, during solvent-vapor exposure, the surface topology was continuously monitored using a light interference microscope. The resulting time-dependent power spectra indicate that high-frequency defects smooth faster than low-frequency defects. This frequency dependence was further investigated by depositing PAA onto a series of sinusoidal surfaces and exposing them to solvent vapor inside a flow channel. The sinusoidal amplitudes decay exponentially with time, with decay constants that are proportional to the surface frequency. To explain the physics of surface smoothing, a two-parameter model is presented and agrees qualitatively with experimental data.

Impact Energy Dependence of SF5+-Induced Damage in Poly(methyl methacrylate) Studied Using Time-of-Flight Secondary Ion Mass Spectrometry

Ion-induced damage of polymers is a critical factor in the depth profiling of polymer surfaces using polyatomic primary ions. In this study, time-of-flight secondary ion mass spectrometry was used to measure the damage of spin-cast poly(methyl methacrylate) (PMMA) films under 5-keV Cs(+) and 2.5-8.75-keV SF(5)(+) bombardment. Under 5-keV Cs(+) bombardment, the characteristic PMMA secondary ion intensities decreased rapidly for primary ion doses above 5 x 10(13) ions/cm(2). The damage profiles of PMMA under SF(5)(+) bombardment contained three distinct regions as a function of SF(5)(+) ion dose: a surface transient, an extended quasi-stabilization of the characteristic PMMA secondary ion intensities, and the decay of these intensities as the silicon substrate was reached. The PMMA film sputtered in a controlled manner for SF(5)(+) ion doses up to 4 x 10(14) ions/cm(2), with the maximum ion dose limited by the thickness of the PMMA film. Furthermore, the chemistry at the bottom of the sputter crater was significantly less modified by SF(5)(+) bombardment when compared with Cs(+) bombardment. The sputter rate was linearly correlated with the SF(5)(+) impact energy while the damage to the PMMA film varied minimally with the SF(5)(+) impact energy. These results were compared with Monte Carlo (SRIM) calculations of the penetration depth and vacancy production for SF(5)(+) at different impact energies. Since the SF(5)(+) impact energy only affected the sputter rate, selection of the appropriate SF(5)(+) impact energy for polymer depth profiling depends solely on the desired sputter rate.