Micro-Raman for Local Strain Evaluation of GaN LEDs and Si Chips Assembled on Cu Substrates.

Integrated circuits are created by interfacing different materials, semiconductors, and metals, which are appropriately deposited or grown on substrates and layers soldered together. Therefore, the characteristics of starting materials and process temperatures are of great importance, as they can induce residual strains in the final assembly. Identifying and quantifying strain becomes strategically important in optimizing processes to enhance the performance, duration, and reliability of final devices. This work analyzes the thermomechanical local strain of semiconductor materials used to realize LED modules for lighting applications. Gallium Nitride active layers grown on sapphire substrates and Si chips are assembled by soldering with eutectic AuSn on copper substrates and investigated by Raman spectroscopy in a temperature range of -50 to 180 °C. From the Raman mapping of many different samples, it is concluded that one of the leading causes of strain in the GaN layer can be attributed to the differences in the thermal expansion coefficient among the various materials and, above all, among the chip, interconnection material, and substrate. These differences are responsible for forces that slightly bend the chip, causing strain in the GaN layer, which is most compressed in the central region of the chip and slightly stretched in the outer areas.

Raman study of the polybenzimidazole-phosphoric acid interactions in membranes for fuel cells.

Poly(2,5-benzimidazole) (AB-PBI) membranes are investigated by studying the FT-Raman signals due to the benzimidazole ring vibration together with the C-C and C-H out-of- and in-plane ring deformations. By immersion in aqueous ortho-phosphoric acid for different time periods, membranes with various doping degrees, i.e. different molar fractions of acid, are prepared. The chemical-physical interactions between polymer and acid are studied through band shifting and intensity change of diagnostic peaks in the 500-2000 cm(-1) spectral range. The formation of hydrogen bonding networks surrounding the polymer seems to be the main reason for the observed interactions. Only if the AB-PBI polymer is highly doped, the Raman spectra show an additional signal, which can be attributed to the presence of free phosphoric acid molecules in the polymer network. For low and intermediate doping degrees no evidence for free phosphoric acid molecules can be seen in the spectra. The extent of the polymer-phosphoric acid interactions in the doped membrane material is reinvestigated after a period of one month and the stability discussed. Our results provide insight into the role of phosphoric acid as a medium in the conductivity mechanism in polybenzimidazole.

Effect of high pressure CO2 on the structure of PMMA: a FT-IR study.

Conformational changes in polymer films exposed to high-pressure CO(2) have been investigated with Fourier transform infrared (FT-IR) spectroscopy. The experimental setup, based on a custom-made stainless steel optical cell with CaF(2) windows, allows measurements in a CO(2) environment for pressures up to 6 MPa, in a temperature range from 293 to 353 K and in the mid-infrared (1000-4000 cm(-1)). Poly(methyl methacrylate) (PMMA), a polymer with a side group (C-type), was studied to monitor the spectral changes as a function of CO(2) pressure and was compared to poly(D,L-lactic-co-glycolic acid) (PLGA), a polymer without a side group (B-type). By monitoring the characteristic carbonyl bands, conformational changes that occur due to molecular interactions between the high-pressure CO(2) and the polymers were explored at a constant pressurization rate (0.02 MPa/min) and temperature. Spectral changes are observed only for PMMA, where the vibrational band at 1680 cm(-1) disappears with increasing pressure. The spectra of PLGA do not show any significant change in the presence of high pressure CO(2) in the investigated range. The behavior of the absorbance peak as a function of pressure and temperature highlights the presence of dynamic cross-links (DCs) between the side groups of PMMA films obtained by solvent casting below the glass transition temperature of the polymer. The spectral features are correlated using a model that accounts for CO(2) diffusion and the relaxation kinetics of the polymer chains in the thin film. The disappearance of the vibrational band attributed to the DCs for PMMA is related to the glass transition temperature, and a retrograde vitrification phenomenon is observed. This approach can be considered a useful alternative to magnetic suspended balance for the study of polymer-gas systems.

Broadband electric spectroscopy at high CO2 pressure: dipole moment of CO2 and relaxation phenomena of the CO2-poly(vinyl chloride) system.

Broadband electric spectroscopy (BES) is a technique that shows promise in studying the interactions of dense or supercritical gases with polymers, particularly with respect to chain mobility. Polymers that are treated with dense gases show a reduction in the viscosity, glass transition, and melting temperature. A high pressure cell for BES has been constructed that can be used from ambient temperature and pressure to 353 K and 15 MPa and over a frequency range from 20 Hz to 1 MHz. In the past, the dielectric constant of CO(2) was determined by measurements at only one or two frequency values. New instrumentation and technology allow this experiment to be expanded to cover a wider frequency range. BES measurements of CO(2) do not show any relaxation peaks in the permittivity from 20 Hz to 1 MHz and 1 to 6 MPa. By these measurements, the CO(2) dielectric constant was evaluated between 0.1 and 6 MPa. Cell testing with poly(vinyl chloride) (PVC) at 323 K and CO(2) pressures from 0.1 to 13 MPa indicate an increase in the chain segmental motion at high pressures resulting from a reduction in the glass transition temperature of the PVC-CO(2) system due to plasticization by CO(2).

First time-resolved EPR observation of Nafion photochemistry.

Time-resolved EPR spectra of UV-irradiated Nafion reveal the formation of spin-polarized excited triplet states and allow the detection of photoinduced triplet-triplet energy transfer processes through hydrogen bonds between water and sulfonic acid groups.

Time-resolved EPR observation of synthetic eumelanin-superoxide radical pairs.

TR-EPR spectra of UV irradiated synthetic eumelanin in equilibrium with air oxygen, reveal the formation of melanin-superoxide radical pairs and allow to determine both the electron dipolar interaction D and the exchange interaction J between the two radicals.

Time-resolved EPR investigation of [70]fulleropyrrolidine nitroxide isomers.

A novel [70]fulleropyrrolidine functionalized with a nitroxide radical has been synthesized. After pulsed photoexcitation, time-resolved electron paramagnetic resonance (EPR) spectra have been recorded in liquid solution at standard X-band (9.5 GHz) and W-band (95 GHz) microwave frequencies. The spectra exhibit strongly electron spin polarized ground and excited states, the latter being arising from the spin coupling of the nitroxide's electron spin with the fullerene's excited triplet state. The EPR parameters such as g-values and hyperfine coupling constants have been discussed in terms of the spin Hamiltonian for excited doublet, triplet and quartet states. On the basis of the strength of the exchange interaction between the fullerene triplet and the radical group, two, out of a possible four [70]fulleropyrrolidine isomeric monoadducts, have been characterized.

Investigation of water structure in nafion membranes by infrared spectroscopy and molecular dynamics simulation.

The structure and interactions of water species in hydrated Nafion membranes as a function of water content were investigated on the basis of medium-infrared spectral analysis and molecular dynamics (MD) simulations. The spectral decomposition of the FT-IR data in the stretching OH region was performed on different levels of hydration of the sulfate functional groups (lambdaH2O/RSO3- = 2-22). Quantum mechanical calculations of two model systems [perfluoroethanesulfonic acid/(H2O)6 cluster] and a [perfluorobutanesulfonic acid/(H2O)6 crystal] were carried out in order to account for the band assignments of Nafion in the stretching OH region (2500-4000 cm-1). Our findings indicated that the secondary structure of water species in Nafion can be accurately explained in terms of our reactive force field for water. The distinction between "surface" and "bulk" water contributions in Nafion membrane pores is proposed along with a quantitative estimate of the different types of OH groups present in the system. The average pore size was calculated and supported by the spectral results.