RESUMO
Exploitation of optical confinement in nanoscale unveils a wealth of information about the structure, optical, electronic, and chemical properties of the materials. However, realizing such confinement by optical microscopy and spectroscopic techniques have remained challenging due to fundamental formulation that is related to the diffraction theory of light. A state-of-art technique, known as near-field scanning optical microscopy (NSOM) has the ability to break such diffraction limitation, as the spatial resolution depends on the near-field probe diameter and the distance between the probe and the surface. A home-built apertured NSOM (a-NSOM) developed in the beginning of NSOM discovery facilitated to investigate N-particles nano-assemblies, where N is two or more. Through surface-sensitive spectroscopy such as surface-enhanced Raman scattering (SERS) and surface-enhanced two-photon-induced photoluminescence (TPI-PL), a correlated optometrology was revealed by taking snapshots of shear-force topography, SERS and TPI-PL simultaneously in single-channel and multi-channel detection system. Here in this "Personal Account" we have decorated near-field optical confinement observed by a-NSOM in three constructs; archetype dimer, nano-assembly of few nanoparticles and long-range two-dimensional (2D) nano-assembly. In the case of dimer, optical confinement was localized and interstitial-dependent whereas coalescence of nearby confinements was reported in few particles nanoaggregate. In the case of 2D nano-assembly, optical confinements were more complex because a nanoparticle was surrounded by six or more adjacent nanoparticles. FDTD simulation were carried out to support and validate the experimental observations. Such observations in nanoscale taking snapshots of nanometric topography and surface-sensitive spectroscopic signal not only inspire us to understand optical confinements in near-field, but also implement the concept in designing miniaturized and efficient system.
RESUMO
Hydrogen (H2 ) is known as the key player in the alternative and renewable energy revolution and henceforth H2 production, transportation, storage and usage have been a major interest of current research. However, due to severe safety concerns, strategies are indispensable to devise superior H2 sensors, particularly selective and sensitive H2 sensors. In this personal account, three specific gas sensing constructs; zinc oxide (ZnO) nanostructures-, noble metal nanoparticles-decorated ZnO- and noble metal nanoparticles-decorated ZnO nanostructures on reduced graphene oxide (rGO)-based H2 sensors have been demonstrated. The dynamic response and H2 sensing characteristics of ZnO nanostructures-based H2 sensors were found to be improved compared to those of pristine ZnO. High-resolution field emission scanning electron microscopy (FESEM) confirmed the flower-like nanostructures that had higher surface area around the nanoscale petals. The mechanism behind the superior sensing characteristics of ZnO nanostructures-based H2 sensor has been demonstrated. Decoration of ZnO nanostructures with noble metal nanoparticles, particularly platinum (Pt) and gold (Au) was observed to be useful in achieving better H2 sensing performance compared to that of ZnO nanostructures. The Pt- and Au-decorated ZnO nanostructures followed the well-known "Spill-over" mechanism in enhancing the H2 sensing characteristics. Abundant free electrons/holes generation and higher conductivity are two important parameters for designing selective and sensitive gas sensors. In this context, a hybrid nanocomposite, rGO-ZnO has been developed and decorated with noble metal nanoparticles, particularly Pt and Au. The ultimate sensing material has been characterized and compared to those of pristine ZnO, ZnO nanostructures and Pt- and Au-decorated ZnO for H2 gas sensing applications. Such systemic and focus strategies is critical not only for developing efficient H2 gas sensors but also for better understanding the mechanisms underlying such superior performance.