Rakesh S. Moirangthem

Bio: Rakesh S. Moirangthem is an academic researcher from Indian Institutes of Technology. The author has contributed to research in topics: Surface plasmon resonance & Plasmon. The author has an hindex of 11, co-authored 46 publications receiving 349 citations. Previous affiliations of Rakesh S. Moirangthem include Academia Sinica & Max Planck Society.

Enhanced localized plasmonic detections using partially-embedded gold nanoparticles and ellipsometric measurements

Abstract: A cost-effective, stable and ultrasensitive localized surface plasmon resonance (LSPR) sensor based on gold nanoparticles (AuNPs) partially embedded in transparent substrate is presented. Partially embedded AuNPs were prepared by thermal annealing of gold thin films deposited on glass at a temperature close to the glass transition temperature of the substrate. Annealed samples were optically characterized by using spectroscopic ellipsometry and compare with theoretical modeling to understand the optical responses from the samples. By combining the partially-embedded AuNPs substrate with a microfluidic flow cell and dove prism in an ellipsometry setup, an ultrasensitive change in the LSPR signal can be detected. The refractive index sensitivity obtained from the phase measurement is up to 1938 degrees/RIU which is several times higher than that of synthesized colloidal gold nanoparticles. The sample is further used to investigate the interactions between primary and secondary antibodies. The bio-molecular detection limit of the LSPR signal is down to 20 pM. Our proposed sensor is label free, non-destructive, with high sensitivity, low cost, and easy to fabricate. These features make it feasible for commercialization in biomedical applications.

Cesium doped and undoped ZnO nanocrystalline thin films: a comparative study of structural and micro‐Raman investigation of optical phonons

Abstract: Undoped and cesium-doped zinc oxide (ZnO) thin films have been deposited on sapphire substrate (0001) using the sol–gel method. Films were preheated at 300 °C for 10 min and annealed at 600 and 800 °C for 1 h. The grown thin films were confirmed to be of wurtzite structure using X-ray diffraction. Surface morphology of the films was analyzed using scanning electron microscopy. The photoluminescence (PL) spectra of ZnO showed a strong ultraviolet (UV) emission band located at 3.263 eV and a very weak visible emission associated with deep-level defects. Cesium incorporation induced a blue shift of the optical band gap and quenching of the near-band-edge PL for nanocrystalline thin film at room temperatures because of the band-filling effect of free carriers. A shift of about 10–15 cm−1 is observed for the first-order longitudinal-optical (LO) phonon Raman peak of the nanocrystals when compared to the LO phonon peak of bulk ZnO. The UV resonant Raman excitation at RT shows multiphonon LO modes up to fifth order. Copyright © 2010 John Wiley & Sons, Ltd.

Optical cavity modes of a single crystalline zinc oxide microsphere.

Abstract: A detailed study on the optical cavity modes of zinc oxide microspheres under the optical excitation is presented. The zinc oxide microspheres with diameters ranging from 1.5 to 3.0 µm are prepared using hydrothermal growth technique. The photoluminescence measurement of a single microsphere shows prominent resonances of whispering gallery modes at room temperature. The experimentally observed whispering gallery modes in the photoluminescence spectrum are compared with theoretical calculations using analytical and finite element methods in order to clarify resonance properties of these modes. The comparison between theoretical analysis and experiment suggests that the dielectric constant of the ZnO microsphere is somewhat different from that for bulk ZnO. The sharp resonances of whispering gallery modes in zinc oxide microspheres cover the entire visible window. They may be utilized in realizations of optical resonators, light emitting devices, and lasers for future chip integrations with micro/nano optoelectronic circuits, and developments of optical biosensors.

Construction of a solar spectrum active SnS/ZnO p–n heterojunction as a highly efficient photocatalyst: the effect of the sensitization process on its performance

Abstract: In this paper, we present a highly efficient SnS nanoparticle sensitized zinc oxide (ZnO) nanorod based photocatalyst, which can degrade strong organic dyes under direct exposure to sunlight. The thermal decomposition technique was adopted to prepare crystalline ZnO nanorods, which were subsequently sensitized with a narrow bandgap semiconductor, namely tin sulfide (SnS), via in situ and ex situ sensitization processes. Here, SnS acts as a photosensitizer to expand the light absorption range from the UV-VIS-NIR spectral region. In addition, the formation of the SnS/ZnO heterostructure suppresses the electron–hole pair recombination via the formation of a p–n heterojunction and enhances the separation of photoexcited charge carriers. Thereafter, the photocatalytic activity of the pristine ZnO nanorods and sensitized SnS/ZnO nanorods was investigated in the degradation of three different strong dyes under sunlight illumination. It was observed that the in situ sensitization process helps in achieving a uniform coating of SnS over the surface of the ZnO nanorods, which resulted in an enhanced photocatalytic performance in dye degradation in comparison to the ex situ sensitized SnS/ZnO nanorods and pristine ZnO nanorods. The rate constants for the in situ sensitized SnS/ZnO nanorods determined from experimental data were found to be 0.0245 min−1, 0.0212 min−1 and 0.0139 min−1 in the photodegradation of Rhodamine 6G, Rhodamine B, and Methyl Orange dyes, respectively. Thus, we can infer from our experimental results that the in situ sensitized SnS/ZnO nanorods could be an economical and promising inorganic photocatalytic material, in addition to the TiO2 photocatalyst for photodegradation of toxic organic dyes present in water.

Nanomaterials enhanced surface plasmon resonance for biological and chemical sensing applications

Abstract: The main challenge for all electrical, mechanical and optical sensors is to detect low molecular weight (less than 400 Da) chemical and biological analytes under extremely dilute conditions. Surface plasmon resonance sensors are the most commonly used optical sensors due to their unique ability for real-time monitoring the molecular binding events. However, their sensitivities are insufficient to detect trace amounts of small molecular weight molecules such as cancer biomarkers, hormones, antibiotics, insecticides, and explosive materials which are respectively important for early-stage disease diagnosis, food quality control, environmental monitoring, and homeland security protection. With the rapid development of nanotechnology in the past few years, nanomaterials-enhanced surface plasmon resonance sensors have been developed and used as effective tools to sense hard-to-detect molecules within the concentration range between pmol and amol. In this review article, we reviewed and discussed the latest trend and challenges in engineering and applications of nanomaterials-enhanced surface plasmon resonance sensors (e.g., metallic nanoparticles, magnetic nanoparticles, carbon-based nanomaterials, latex nanoparticles and liposome nanoparticles) for detecting “hard-to-identify” biological and chemical analytes. Such information will be viable in terms of providing a useful platform for designing future ultrasensitive plasmonic nanosensors.

Optical Biosensors Based on Plasmonic Nanostructures: A Review

Abstract: This paper reviews fundamentals of optical affinity biosensors based on plasmonic nanostructures and discusses recent advances in the development of this technology, including plasmonic nanostructures and surface plasmon phenomena, advances in sensor instrumentation, and functional coatings. Examples of applications for both the detection of chemical and biological substances as well as the investigation of biomolecular interactions are also given.

Fluorescent and lasing whispering gallery mode microresonators for sensing applications

Abstract: Whispering gallery modes (WGMs) have been exploited for a broad range of sensing applications. However, the vast majority of WGM sensors consist of passive resonators, requiring complex interrogation systems to be employed, ultimately limiting their practicality. Active resonators containing a gain medium, allowing remote excitation and collection of the WGM-modulated fluorescence spectra, have emerged as an alternative to passive resonators. Although research is still in its infancy, recent progress has reduced the performance gap between the two paradigms, fueled by the potential for new applications that could not previously be realized. Here, recent developments in sensors based on active WGM microresonators are reviewed, beginning with a discussion of the theory of fluorescence-based and lasing WGMs, followed by a discussion of the variety of gain media, resonator architectures, and emerging sensing applications. We conclude with a discussion of the prospects and future directions for improving active WGM sensors.

The one phonon raman spectrum in microcrystalline silicon

Abstract: The red shift and the broadening of the Raman signal from microcrystalline silicon films is described in terms of a relaxation in the q-vector selection rule for the excitation of the Raman active optical phonons. The relationship between width and shift calculated from the known dispersion relation in c-Si is in good agreement with available data. An increase in the decay rate of the optical phonons predicted on the basis of the same model is confirmed experimentally.