Prof. Tapas Kumar Chini

Recent Results   ♣ Substrate Induced Symmetry Breaking in Penta-twinned Gold Nanorod Probed by Free Electron Impact ( J. Phys. Chem. C 2014, 118, 26284 ) Our experimental results and simulations demonstrate that the substrate plays a very crucial role in the observed plasmonic property of gold nanorod. We have shown that, in the visible domain of the spectrum, the plasmon mode gets split into two distinct peaks due to substrate induced hybridization of in-plane and out-of-plane modes. With increasing the refractive index of the substrate, the intensity of these hybridized modes increases.                                                                       Left Panel :  (a) Experimental CL spectra taken from different symmetry points of penta-twinned gold nanorod on Si (100) substrate. Morphology of the rod is shown in the inset. (b) 3D-FDTD simulated CL spectra of the same morphology as shown in inset of (a). In the inset panchromatic CL image has been shown. (c), (d), (e) Experimental monochromatic CL images at experimentally observed resonant wavelengths of 530 nm, 550 nm and 644 nm respectively. (f), (g), (h) FDTD simulated 2D-CL image acquired at simulated resonant wavelengths of 540 nm, 568 nm and 670 nm respectively. Length scale bar value in all the images is 200 nm.  Right Panel :   (a) Simulated CL spectra in the wavelength range 500-3000 nm for varying refractive index of the substrate. (b) Zoomed view of the spectra in the wavelength range 500-1000 nm showing spectral splitting in presence of the substrate of different index.               ♣ Effect of Inter Tip Coupling on the Plasmonic Behavior of Individual Multi-tipped Gold Nanoflower ( ACS Photonics 2014, 1, 1290 also Nanotechnology 2013, 24, 405704 )                                                                    We report the first experimental realization on the selective excitation of two closely lying tips from the same spherical core of a multitipped gold nanoparticle with flower-like morphology. The presence of strong multipeaked resonance in the near-infrared region of the far-field emission spectra shows a clear signature of tip to tip coupling and the coupled plasmon modes are shown to be originated from the interaction between two closely spaced tips with a narrow angular separation. Our analysis further estimates a range of angular separation between the tips that triggers the onset of the intertip coupling.                     Left Panel : (a) Experimentally acquired CL spectra from different tip apex regions marked as A−E of the AuNF. The inset SEM image shows the e-beam impact points with different colored dots. (b) The SE image of the same AuNF and the corresponding (c) panchromatic CL image along with the monochromatic CL maps for the selected resonance wavelengths (d) 530, (e) 670, (f) 736, (g) 780, (h) 800, and (i) 868 nm. The scale bar is 200 nm. Right Panel : (a) Normalized numerically calculated (3D-FDTD) spectrum (black curve) of the TTM structure and experimentally acquired CL spectrum (red curve) in the wavelength range 500−700 nm. Inset SEM image shows the e-beam is focused at the red colored position. (b) Difference between the calculated spectra of the TTM structure, with substrate (black curve) and without substrate (red curve). The spectra are calculated in the 500−900 nm range. (c) Simulated emission spectrum of different structures. The black curve represents the TTM structure. The blue and red curves represent the spectrum of the single tip model/STM structure for upper and lower tip, respectively. Simulated near-field intensity (|E|2) maps and the corresponding field maps at (d) 733 nm wavelength in the XZ plane, (e) 786 nm wavelength in the XZ plane, and (f) 786 nm wavelength in the YZ plane for TTM structure.                                    ♣ Localized Surface Plasmons of Trisoctahedral Gold Nanocrystals For Surface Enhanced Raman Scattering ( J. Phys. Chem. C 2016, 120, 27003 ) Trisoctahedral (TOH) shaped gold (Au) nanocrystals (NCs) have emerged as a new class of metal nanoparticles (MNPs) due to their superior catalytic and surface enhanced Raman scattering (SERS) activities. We examine the radiative localized surface plasmon resonance (LSPR) modes of an isolated single TOH Au NC using cathodoluminescence (CL) across the visible spectral range. We show pronounced enhancement in the Raman scattering on Rhodamine 6G (R6G)-covered TOH Au NPs aggregates on a Si substrate. It is believed that the hot spots between two adjacent MNP surfaces (“nanogaps”) can be significantly stronger than single particle LSPRs. Such “nanogaps” hot spots may have crucial role on the substantial SERS enhancement observed in this report. Consequently, the present study indicates that MNPs aggregates are highly desirable than individual plasmonic nanoparticles for possible applications in SERS based biosensing.              Left Panel :  (a) CL spectra from selected points marked as A-H of the studied TOH Au particle. The inset SEM image shows the electron beam impact points with different colored dots. (b) 3D-FDTD simulated spectra of the modelled TOH structure for beam impact at the points (A, B..., F). The electron beam is kept in 1 nm away from all the selected points during all numerical calculations, unless mentioned specifically. The SEM image and the corresponding panchromatic CL map/image of the same morphology are shown in (c) and (d) respectively. (e)-(h) Monochromatic CL maps/images at wavelengths 548, 560, 650 and 670 nm, respectively. Scale bar is 100 nm in all images. Right Panel : SERS spectra of R6G molecules in aqueous solution of 10−6 M concentration dispersed on SERS substrate and pure Si substrates. Arrow mark denotes the second order Raman mode of Si substrate. Inset represents the optical image of the SERS substrate at 100X magnification. Scale bar is 10 μm. ♣ Excitation of higher order Plasmonic modes of Indivdual Concave Gold Nanocubes on Silicon Substrae ( J. Phys. Chem. C 2017, 121, 731 ) Recently, concave nanocube (CNC) shaped metal nanoparticles (MNPs) with high index facets have drawn special attention due to their high chemical activity and large electromagnetic (EM) field enhancements. We explore the localized surface plasmon modes of individual gold (Au) CNCs sitting on silicon (Si) substrate using cathodoluminescence (CL) spectroscopy and imaging in scanning electron microscope (SEM). We have addressed several new aspects in the plasmonic response of concave gold nanocube. As for example, the experimental CL spectral and spatial data analyzed with detail finite-difference time-domain (FDTD) based simulations show the existence of edge quadrupolar mode as well as substrate-mediated hybridized corner quadrupolar and octupolar modes which have not been reported so far to the best of our knowledge for Au CNC particles. Our analysis revealed that large Au CNC particles with high concaveness constitute strong candidates for multifunctional surface enhanced Raman scattering (SERS) catalysis platforms as they exhibit strong localized electric fields for SERS and high index facets suitable for catalysis.           Left Panel : Evolution of edge (E) and corner (C1 and C2) modes observed experimentally in monochromatic CL map as a function of the particle size. Scale bar  is 200 nm in all the panels. Right Panel : (a) Schematic diagram of particle to substrate gap (G) variation. FDTD calculated CL spectra of an individual Au CNC of edge length (b) 170 nm (c) 225 nm on silicon substrate for different particle-substrate gaps. (d) Scattered electric field intensity distribution of CNC in vacuum for CQ mode in YZ plane (X=0). (d-f) Scattered electric field  intensity distribution in YZ plane (X=0)