Taron Makaryan

One-step Solution Processing of Ag, Au and Pd@MXene Hybrids for SERS.

We report on one-step hybridization of silver, gold and palladium nanoparticles from solution onto exfoliated two-dimensional (2D) Ti3C2 titanium carbide (MXene) nanosheets. The produced hybrid materials can be used as substrates for surface-enhanced Raman spectroscopy (SERS). An approximate analytical approach is also developed for the calculation of the surface plasmon resonance (SPR) frequency of nanoparticles immersed in a medium, near the interface of two dielectric media with different dielectric constants. We obtained a good match with the experimental data for SPR wavelengths, 440 nm and 558 nm, respectively for silver and gold nanoparticles. In the case of palladium, our calculated SPR wavelength for the planar geometry was 160 nm, demonstrating that non-spherical palladium nanoparticles coupled with 2D MXene yield a broad, significanlty red-shifted SPR band with a peak at 230 nm. We propose a possible mechanism of the plasmonic hybridization of nanoparticles with MXene. The as-prepared noble metal nanoparticles on MXene show a highly sensitive SERS detection of methylene blue (MB) with calculated enhancement factors on the order of 105. These findings open a pathway for extending visible-range SERS applications of novel 2D hybrid materials in sensors, catalysis, and biomedical applications.

MoS2‐on‐MXene Heterostructures as Highly Reversible Anode Materials for Lithium‐Ion Batteries

Two-dimensional (2D) heterostructured materials, combining the collective advantages of individual building blocks and synergistic properties, have spurred great interest as a new paradigm in materials science. The family of 2D transition-metal carbides and nitrides, MXenes, has emerged as an attractive platform to construct functional materials with enhanced performance for diverse applications. Here, we synthesized 2D MoS2 -on-MXene heterostructures through in situ sulfidation of Mo2 TiC2 Tx MXene. The computational results show that MoS2 -on-MXene heterostructures have metallic properties. Moreover, the presence of MXene leads to enhanced Li and Li2 S adsorption during the intercalation and conversion reactions. These characteristics render the as-prepared MoS2 -on-MXene heterostructures stable Li-ion storage performance. This work paves the way to use MXene to construct 2D heterostructures for energy storage applications.

Growth kinetics and growth mechanism of ultrahigh mass density carbon nanotube forests on conductive Ti/Cu supports.

We evaluate the growth kinetics and growth mechanism of ultrahigh mass density carbon nanotube forests. They are synthesized by chemical vapor deposition at 450 °C using a conductive Ti/Cu support and Co-Mo catalyst system. We find that Mo stabilizes Co particles preventing lift off during the initial growth stage, thus promoting the growth of ultrahigh mass density nanotube forests by the base growth mechanism. The morphology of the forest gradually changes with growth time, mostly because of a structural change of the catalyst particles. After 100 min growth, toward the bottom of the forest, the area density decreases from ∼ 3-6 × 10(11) cm(-2) to ∼ 5 × 10(10) cm(-2) and the mass density decreases from 1.6 to 0.38 g cm(-3). We also observe part of catalyst particles detached and embedded within nanotubes. The progressive detachment of catalyst particles results in the depletion of the catalyst metals on the substrate surfaces. This is one of the crucial reasons for growth termination and may apply to other catalyst systems where the same features are observed. Using the packed forest morphology, we demonstrate patterned forest growth with a pitch of ∼ 300 nm and a line width of ∼ 150 nm. This is one of the smallest patterning of the carbon nanotube forests to date.

Plasmonic nanostructures fabricated using nanosphere-lithography, soft-lithography and plasma etching

Summary We present two routes for the fabrication of plasmonic structures based on nanosphere lithography templates. One route makes use of soft-lithography to obtain arrays of epoxy resin hemispheres, which, in a second step, can be coated by metal films. The second uses the hexagonal array of triangular structures, obtained by evaporation of a metal film on top of colloidal crystals, as a mask for reactive ion etching (RIE) of the substrate. In this way, the triangular patterns of the mask are transferred to the substrate through etched triangular pillars. Making an epoxy resin cast of the pillars, coated with metal films, allows us to invert the structure and obtain arrays of triangular holes within the metal. Both fabrication methods illustrate the preparation of large arrays of nanocavities within metal films at low cost. Gold films of different thicknesses were evaporated on top of hemispherical structures of epoxy resin with different radii, and the reflectance and transmittance were measured for optical wavelengths. Experimental results show that the reflectivity of coated hemispheres is lower than that of coated polystyrene spheres of the same size, for certain wavelength bands. The spectral position of these bands correlates with the size of the hemispheres. In contrast, etched structures on quartz coated with gold films exhibit low reflectance and transmittance values for all wavelengths measured. Low transmittance and reflectance indicate high absorbance, which can be utilized in experiments requiring light confinement.

Hybrids of carbon Nanotube Forests and Gold Nanoparticles for Improved Surface Plasmon Manipulation

We report the fabrication and characterization of hybrids of vertically-aligned carbon nanotube forests and gold nanoparticles for improved manipulation of their plasmonic properties. Raman spectroscopy of nanotube forests performed at the separation area of nanotube-nanoparticles shows a scattering enhancement factor of the order of 1 × 10(6). The enhancement is related to the plasmonic coupling of the nanoparticles and is potentially applicable in high-resolution scanning near-field optical microscopy, plasmonics, and photovoltaics.

Strong dipole-quadrupole coupling and Fano resonance in H-like metallic nanostructures

Under certain conditions of the incident light polarization direction a Fano resonance arises in small gold nanorods arranged in a H-like configuration. This stems from the coupling between a bright dipole plasmon mode excited in the horizontal rod and dark quadrupole plasmon modes in both vertical rods. We investigate these surface plasmon modes, and analyze the dependence of the Fano resonance on the geometry parameters such as rod size and interparticle separation, and refractive index of embedding medium. To describe the degree of this energy transfer, we introduce a new parameter: the Fano resonance efficiency. We calculate absorption cross-sections for visible and NIR spectrum in each element of the structure, and near-field distributions at different wavelengths. We show that Fano resonance in small H-like structures exhibits high sensitivity with respect to the refractive index of the host medium, outperforming the values for larger plasmonic structures based on nanorods already investigated.

Carbon nanotube forests as top electrode in electroacoustic resonators

We grow carbon nanotube forests on piezoelectric AlN films and fabricate and characterize nanotube-based solidly mounted bulk acoustic wave resonators employing the forests as the top electrode material. The devices show values for quality factor at anti-resonance of ∼430, and at resonance of ∼100. The effective coupling coefficient is of ∼6%, and the resonant frequencies are up to ∼800 MHz above those observed with metallic top electrodes. AlN promotes a strong catalyst-support interaction, which reduces Fe catalyst mobility, and thus enforces the growth of forests by the base growth mechanism.

Optical resonators based on carbon nanotube for photonics applications

By performing experiments and numerical modeling, we investigate the use of carbon nanotubes for photonics applications.

Preliminary investigation in optical resonators based on carbon nano-tube and coupling for optoelectronics

We investigate the use of carbon nano-tubes performed by chemical vapor deposition for photonics applications producing samples of various geometries on the same wafer and performing experiments and numerical modeling. Publisher’s Note: This paper, originally published on 5/2/14, was replaced with a corrected/revised version on 5/19/14. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.

Theoretical Study of Surface Plasmon Frequencies in a System of Two Coupled Spheres and Comparison with Experimental Data

We consider the problem of surface plasmon (SP) oscillations in a pair of coupled spherical metallic nanoparticles (MNP) analytically and compare the results with those obtained experimentally as well as by numerical methods: discrete dipole approximation (DDA), boundary integral method and T-matrix. The calculation of SP frequencies of the pairs of spherical MNPs with size less than SP wavelength is reduced to the electrostatic boundary problem and is solved analytically. Such reduction becomes impossible when the system size is comparable with SP wavelength and the retardation effects in this case must be accounted for. Since this problem does not allow exact solution we develop an approximate analytical approach in which we account for retardation effects within each of the spheres, neglecting it in the electromagnetic interaction between the spheres. We prove that this approximation is accurate for interparticle gaps down to 0.1 of sphere diameter. To check the validity of the approximation we performed also numerical calculations based on DDA method for the system of two dielectrically coated small spheres, and the pair of larger spheres for which the retardation effects are essential. Good agreement demonstrated in both cases indicates the applicability of presented analytical approach allowing quick calculation of SP frequencies of coupled spheres. The theoretical results are compared with known experimental data for the pairs of 42 nm and 87 nm particles. In the valuable for biological applications gap range 5÷50 nm there is a good agreement between experimental data and the results of our calculations.