Jagmeet Singh Sekhon

Refractive Index Sensitivity Analysis of Ag, Au, and Cu Nanoparticles

The localized surface plasmon resonance (LSPR) spectrum of noble metal nanoparticles is studied by quasi-static approximation. Taking the sensitivity of LSPR shape to the size and shape of nanoparticle along with surrounding refractive index, parameters like refractive index sensitivity and sensing figure of merit have been determined. In the present analysis from the sensing relevant parameters, it is concluded that Ag represents a better sensing behavior than Au and Cu over the entire visible to infrared regime of EM spectrum.

Scattering Efficiency and LSPR Tunability of Bimetallic Ag, Au, and Cu Nanoparticles

Scattering efficiencies of Ag–Cu, Ag–Au, and Au–Cu alloy nanoparticles are studied based on Mie theory for their possible applications in solar cells. The effect of size (radius), surrounding medium, and alloy composition on the scattering efficiency at the localized surface plasmon resonance (LSPR) wavelengths has been reported. In the alloy nanoparticles of Ag1−xCux, Au1−xCux and Ag1−xAux; the scattering efficiency gets red-shifted with increase in x. Moreover, the scattering efficiency enhancement can be tuned and controlled with both the alloy composition and the surrounding medium refractive index. A linear relationship which is in good agreement to the experimental observations between the scattering efficiency and metal composition in the alloys are found. The effect of nanoparticle size and LSPR wavelength (scattering peak position) on the full width half maxima and scattering efficiency has also been studied. Comparison of Au–Ag, Au–Cu, and Ag–Cu alloy nanoparticles with 50-nm radii shows the optical response of Ag–Cu alloy nanoparticle with wide bandwidth in the visible region of the electromagnetic spectrum making them suitable for plasmonic solar cells. Further, the comparison of Ag–Cu alloy and core@shell nanoparticles of similar size and surrounding medium shows that Cu@Ag nanoparticle exhibits high scattering efficiency with nearly the same bandwidth.

Voltage induced local hysteretic phase switching in silicon

We report the observation of dc-bias induced 180° phase switching in silicon wafers by local-probe microscopy and spectroscopy. The switching is hysteretic and shows remarkable similarities with polarization switching in ferroelectrics as seen in piezoresponse force microscopy (PFM). This is always accompanied by a hysteretic amplitude vs. voltage curve which resembles the “butterfly loops” for piezoelectric materials. From a detailed analysis of the data obtained under different environmental and experimental conditions, we show that the hysteresis effects in phase and amplitude do not originate from ferro-electricity or piezoelectricity. This further indicates that mere observation of hysteresis effects in PFM does not confirm the existence of ferroelectric and/or piezoelectric ordering in materials. We also show that when samples are mounted on silicon for PFM measurements, the switching properties of silicon may appear on the sample even if the sample thickness is large.

DDA simulations of noble metal and alloy nanocubes for tunable optical properties in biological imaging and sensing

The selection of nanoparticles for achieving efficient contrast in biological imaging and sensing is based on their optical properties. In the present study, discrete dipole approximation (DDA) simulated Ag, Au and their alloy nanocubes of various edge length spectra are used to investigate the surface plasmon resonance (SPR) wavelength, resonant line-width, relative contribution of scattering in extinction, and sensitivity of the SPR peak position to the refractive index of the embedding medium. The optical properties are significantly altered by the nanocube alloy composition. The scattering to absorption ratio is enhanced and tuned in the visible regime by alloying the metals, which may be useful for biological imaging. The sensitivity of the SPR peak position to the bulk refractive index is enhanced in alloys compared to pure metals. According to the sensing figure of merit (FOM), there exists an optimum alloy nanocube edge length and spectral region to obtain the best sensing performance. Such endeavours have the potential to improve the sensing performance of the nanoparticles with tunable SPR wavelength of the desired spectral regime.

Direct evidence of strong local ferroelectric ordering in a thermoelectric semiconductor

It is thought that the proposed new family of multi-functional materials, namely, the ferroelectric thermoelectrics may exhibit enhanced functionalities due to the coupling of the thermoelectric parameters with ferroelectric polarization in solids. Therefore, the ferroelectric thermoelectrics are expected to be of immense technological and fundamental significance. As a first step towards this direction, it is most important to identify the existing high performance thermoelectric materials exhibiting ferroelectricity. Herein, through the direct measurement of local polarization switching, we show that the recently discovered thermoelectric semiconductor AgSbSe2 has local ferroelectric ordering. Using piezo-response force microscopy, we demonstrate the existence of nanometer scale ferroelectric domains that can be switched by external electric field. These observations are intriguing as AgSbSe2 crystalizes in cubic rock-salt structure with centro-symmetric space group (Fm–3m), and therefore, no ferroelect...

Ferroelectric-like response from the surface of SrTiO3 crystals at high temperatures

Since SrTiO3 has a high dielectric constant, it is used as a substrate for a large number of complex physical systems for electrical characterization. Since SrTiO3 crystals are known to be non-ferroelectric/non-piezoelectric at room temperature and above, SrTiO3 has been believed to be a good choice as a substrate/base material for PFM (Piezoresponse Force Microscopy) on novel systems at room temperature. In this paper, from PFM-like measurement using an atomic force microscope on bare crystals of (110) SrTiO3 we show that ferroelectric and piezoelectric-like response may originate from bare SrTiO3 at remarkably high temperatures up to 420 K. Electrical domain writing and erasing are also possible using a scanning probe tip on the surface of SrTiO3 crystals. This observation indicates that the role of the electrical response of SrTiO3 needs to be revisited in the systems where signature of ferroelectricity/piezoelectricity has been previously observed with SrTiO3 as a substrate/base material.

Cu Nanoparticle Plasmons to Enhance GaAs Solar Cell Efficiency

The absorption enhancement of GaAs solar cells with Cu nanoparticles have been studied by finite difference time domain (FDTD) method. The simulation results showed the 37% light absorption enhancement in comparison to bare. Further, it is also observed that the conversion efficiency of about 1.086 can be achieved at optimized radius and the period of the nanoparticles. Hence, the present result optimized the size and the separation between nanoparticles to get maximum conversion efficiency of GaAs solar cells.

Controlling the LSPR properties of Au triangular nanoprisms and nanoboxes by geometrical parameter: a numerical investigation

We have simulated the extinction spectra of Au triangular nanoprisms and nanoboxes by finite difference time domain method. It is found that the refractive index sensitivity increases linearly and near exponentially as the aspect ratio of nanoprisms increases and wall thickness of nanoboxes decreases, respectively. A sensing figure of merit (FOM) calculation shows that there is an optimum wall thickness for each edge length and height of the box, which makes them to be promising candidates for effective sensing applications. We have also shown that the higher FOM in triangular nanoboxes compared to the cubic nanoboxes and other solid structure is inherent in the shape of nanoparticles.

Dimensional and compositional dependent analysis of plasmonic bimetallic nanorods

The individual noble metal nanoparticles (NPs) are combined to form alloys with improved optical response, cost effectiveness and better stability. The selection of noble metal alloy NPs for their better use in plasmonic applications is being made on the bases of surface plasmon resonance peak position, its intensity and full width at half maxima (FWHM). Presently, the effect of metal composition (x), aspect ratio (R), size and metal type on the longitudinal plasmon resonance (LPR) of noble metal Ag–Au alloy nanorods (NRs) has been studied by applying modified Gans theory including finite wavelength effects and found that the LPR shifts towards the longer wavelength region with increase in aspect ratio and size of the NR. Moreover, a linear relationship which is in good agreement to the experimental results between the plasmon resonance and aspect ratio has been obtained. The aspect ratio and NR width-dependent absorption efficiency and FWHM have also been calculated. Further, a negligible effect of metal composition and its type is found on the LPR.

Effect of surrounding medium on light absorption characteristics of CdSe, CdS & CdTe nanospheres and their comparison

The absorption efficiency of cadmium compounds, prospective nanosolar cell materials, as spherical nanoparticles of radius less than 100 nm and effect of surrounding medium refractive index on them have been studied by Mie Plot. It is found that the absorption efficiency of CdSe, CdS, and CdTe increases with radius of the nanoparticle and also the UV-Visible absorption spectra shifts towards red. The maximum of absorption efficiency is observed at 80 nm radius and is in visible region for all the selected materials. Also with increase in surrounding refractive index, the absorption efficiency is found to be decreasing and absorption spectra is blue-shifted. For all these presently selected cadmium compounds, CdTe possess highest efficiency as compared to both CdS and CdSe.