We use self-consistent field theory (SCFT) to study the directed self-assembly of laterally confined diblock copolymers. In this study, we focus on systems in which the self-assembled lamellae are oriented parallel to selective sidewalls in a channel. While well-ordered, perfect lamellae are observed in narrow channels both experimentally and numerically, undesirable defective structures also emerge. We therefore investigate the energetics of two categories of isolated defects (dislocations and disclinations) for various segregation strengths and channel dimensions, and establish conditions that favor the formation of defects. We also determine the energy barrier and the transition path between defective and perfect states using the string method. We find that only a few kT of energy are necessary to overcome the kinetic barrier and remove a defect, sharply contrasting with the large gain in free energy (many tens of kT) that is necessary for the formation of a defect from the pristine state.
Optical nuclear magnetic resonance ͑ONMR͒ is a powerful probe of electronic properties in III-V semiconductors. Larmor-beat detection ͑LBD͒ is a sensitivity optimized, time-domain NMR version of optical detection based on the Hanle effect. Combining LBD ONMR with the line-narrowing method of POWER ͑perturbations observed with enhanced resolution͒ NMR further enables atomically detailed views of local electronic features in III-Vs. POWER NMR spectra display the distribution of resonance shifts or line splittings introduced by a perturbation, such as optical excitation or application of an electric field, that is synchronized with a NMR multiple-pulse time-suspension sequence. Meanwhile, ONMR provides the requisite sensitivity and spatial selectivity to isolate local signals within macroscopic samples. Optical NMR, LBD, and the POWER method each introduce unique demands on instrumentation. Here, we detail the design and implementation of our system, including cryogenic, optical, and radio-frequency components. The result is a flexible, low-cost system with important applications in semiconductor electronics and spin physics. We also demonstrate the performance of our systems with high-resolution ONMR spectra of an epitaxial AlGaAs/ GaAs heterojunction. NMR linewidths down to 4.1 Hz full width at half maximum were obtained, a 10 3 -fold resolution enhancement relative any previous optically detected NMR experiment.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.