Probing dipole and quadrupole resonance mode in non-plasmonic nanowire using Raman spectroscopy

Electric field enhancement in semiconductor nanostructures offers a possibility to find an alternative to the metallic particles which is well known for tuning the light-matter interaction due to its strong polarizability and size-dependent surface plasmon resonance energy. Raman spectroscopy is a powerful technique to monitor the electric field as its scattering depends on the electromagnetic eigenmode of the particle. Here, we observe enhanced polarized Raman scattering from germanium nanowires of different diameters. The incident electromagnetic radiation creates a distribution of the internal electric field inside the naowires which can be enhanced by manipulating the nanowire diameter, the incident electric field and its polarization. Our estimation of the enhancement factor, including its dependence on nanowire diameter, agrees well with the Mie theory for an infinite cylinder. Furthermore, depending on diameter of nanowire and wavelength of incident radiation, polarized Raman study shows dipolar (antenna effect) and quadrupolar resonances, which has never been observed in germanium nanowire. We attempt to understand this polarized Raman behavior using COMSOL Multiphysics simulation, which suggests that the pattern observed is due to photon confinement within the nanowires. Thus, the light scattering direction can be toggled by tuning the polarization of incident excitation and diameter of non plasmonic nanowire

One-step fabrication of GeSn branched nanowires

We report for the first time the self-catalyzed, single-step growth of branched GeSn nanostructures by a vapor–liquid–solid mechanism. These typical GeSn nanostructures consist of ⟨111⟩-oriented, Sn-rich (∼8 atom %) GeSn “branches” grown epitaxially on GeSn “trunks”, with a Sn content of ∼4 atom %. The trunks were seeded from Au0.80Ag0.20 nanoparticles followed by the catalytic growth of secondary branches (diameter ∼ 50 nm) from the excess of Sn on the sidewalls of the trunks, as determined by high-resolution electron microscopy and energy-dispersive X-ray analysis. The nanowires, with ⟨111⟩-directed GeSn branches oriented at ∼70° to the trunks, have no apparent defects or change in crystal structure at the trunk–branch interface; structural quality is retained at the interface with epitaxial crystallographic relation. The electrochemical performance of these highly ordered GeSn nanostructures was explored as a potential anode material for Li-ion batteries, due to their high surface-to-volume ratio and increased charge carrier pathways. The unique structure of the branched nanowires led to high specific capacities comparable to, or greater than, those of conventional Ge nanowire anode materials and Ge1–xSnx nanocrystals

Solution phase growth and analysis of super-thin zigzag tin selenide nanoribbons

Tin selenide (SnSe), a highly promising layered material, has been garnering particular interest in recent times due to its significant promise for future energy devices. Herein we report a simple solution-phase approach for growing highly crystalline layered SnSe nanoribbons. Polyvinylpyrrolidone (PVP) was used as a templating agent to selectively passivates the (100) and (001) facets of the SnSe nanoribbons resulting in the unique growth of nanoribbons along their b-axis with a defined zigzag edge state along the sidewalls. The SnSe nanoribbons are few layers thick (similar to 20 layers), with mean widths of similar to 40 nm, and achievable length of >1 mu m. Nanoribbons could be produced in relatively high quantities (>150 mg) in a single batch experiment. The PVP coating also offers some resistance to oxidation, with the removal of the PVP seen to lead to the formation of a SnSe/SnO (x) core-shell structure. The use of non-toxic PVP to replace toxic amines that are typically employed for other 1D forms of SnSe is a significant advantage for sustainable and environmentally friendly applications. Heat transport properties of the SnSe nanoribbons, derived from power-dependent Raman spectroscopy, demonstrate the potential of SnSe nanoribbons as thermoelectric material

Lattice dynamics of Ge1-xSnx alloy nanowires

Alloying group IV semiconductors offers an effective way to engineer their electronic properties and lattice dynamics. The incorporation of Sn in Ge permits a transition from an indirect to a direct bandgap semiconductor. Here, by combining polarization, laser power-dependent and temperature-dependent micro-Raman spectroscopy we explore the full lattice dynamics of Ge1−xSnx (x = 0.01, 0.06 and 0.08) alloy nanowires. In the high Sn content samples (x ≥ 0.06), a low-frequency tail and a high-frequency shoulder are observed which are associated with the F2g optical phonon mode of Ge (Ge–Ge mode). The new modes are assigned to the stretching of Ge–Ge bonds due to Sn-induced lattice relaxation and compression, respectively. The symmetry of the observed Raman modes has been studied by polarization-dependent Raman scattering. Nonlinear fitting of the laser power-dependent intensity of the high-frequency Ge–Ge mode in the Ge1−xSnx alloy nanowires with x = 0.06 and 0.08 suggests the activation of a third-order stimulated Raman scattering process, due to the high intensity localized electric field surrounding the Sn clusters. Finally, from the temperature-dependent Raman study, we have estimated the isobaric Grüneisen parameters for all the observed modes

Disorder-induced crossover of Mott insulator to weak Anderson localized regime in an argon-irradiated NdNiO3 film

We show that an introduction of disorder in a controlled way using 1 MeV argon (Ar) ion irradiation, suppresses the correlation driven metal-insulator transition (MIT) in NdNiO3 films. The films make a crossover to a heavily disordered conductor governed by weak localization (WL) and at even higher disorder, an Anderson localized state. The disorder (atomic displacement up to 2% of the total atoms) in the NdNiO3 films was created using 1 MeV Ar4+ ion irradiation. We show that the pristine films of NdNiO3 exhibit an MIT with the conduction process being governed by variable range hopping (VRH). For disorder up to 1% of the displaced atoms or lower, the insulating state arising from a gap in the density of states (DOS) at the Fermi level (E-F) as in a Mott insulator is suppressed and the conduction in the film shows a WL behavior with finite conductivity at temperature T -> 0. This behavior is expected ina disordered conductor that does not have a gap in DOS at E-F. At higher fluences the conductivity reduces substantially but the electrical conduction shows a power-law temperature dependence with a small but finite zero temperature conductivity Sigma (T = 0) which is expected in a solid with electrons that are Anderson localized. A similar experiment was performed on the La substituted NdNiO3 films (Nd1-xLaxNiO3) with x = 0.3 that are grown in the same way. La substitution in NdNiO3 suppresses the temperature driven transition and leads to a metallic state with critical composition at x approximate to 0.3. The pristine as well as films irradiated with lowest fluence shows metallic or marginally metallic behavior grown on LaAlO3 and SrTiO3 substrates, respectively. However, at higher fluences they too exhibit a convergence in electronic transport and Sigma shows a power-law temperature dependence at low T with Sigma (T = 0) ???0. Evidence of suppression of correlated behavior can also be seen in the irradiated films where the non-Gaussian nature of resistance fluctuation at T approximate to T-MI, a signature of correlated electron systems, is suppressed on irradiation that leads to collapse of the MIT. Evidence for progressing disordering of the films on irradiation were observed in Raman spectroscopy as well as x-ray studies that show the basic integrity of the NiO6 octahedra is preserved and the structure retains its crystallinity

Probing lattice dynamics in ST 12 phase germanium nanowires by Raman spectroscopy

The authors acknowledge financial support from Science and Engineering Research Board (SERB), India (File No. EMR/2017/ 002107). S.B., A.G., and J.D.H. acknowledge Science Foundation Ireland (Grant No. 14/IA/2513). Divya Srivastava would like to acknowledge CSC—the Finnish IT Center for Science for computational resources.Germanium (Ge) plays a crucial role in setting up important functionalities for silicon-compatible photonics. Diamond cubic germanium is an extensively studied semiconductor, although its other exotic forms, like BC8, ST8, ST12 phases, may possess distinct electronic properties. We have fabricated stable ST12-Ge nanowires via a self-seeded bottom-up three phase growth in a confined supercritical toluene environment. Here, we report on the direct evidence of the presence of the ST12 phase by a combination of Raman spectroscopy and first-principles calculations using density functional theory (DFT). It is important to remark that the DFT calculation predicts all the Raman active optical phonon modes of the P 4321 structure, and it is in very good agreement with the experimental results. The phonon dynamics as a function of temperature is investigated through Raman measurements at temperatures varying from 80 to 300 K. First-order temperature coefficients for all the observed Raman modes are estimated from the linear temperature dependence of the phonon shifts. A complete set of isobaric Grüneisen parameters is reported for all Raman modes of ST12-Ge nanowire, and the values are lower compared to the same for Si, dc-Ge bulk, and Ge nanowire. These results have important implications for understanding thermal properties of ST12-Ge nanowire.Peer reviewe

Engineering Multifunctionality in MoSe2 Nanostructures Via Strategic Mn Doping for Electrochemical Energy Storage and Photosensing

To achieve advanced functionalities in nanostructured MoSe2 for enhanced electrochemical charge storage and improved photosensing, here we propose an effective strategy, i.e., the substitutional doping of the heteroatom Mn. We achieve a 313% increase in specific capacitance for 6.2% of Mn doping compared to pristine MoSe2 at the scan rate of 5 mV/s in a three electrode configuration. For a two-electrode arrangement, also superior charge-storage performance is noted. The enhanced electrode performance can be attributed to the increase of electrical conductivity arising due to an increase of electron density for the n-type nature of Mn doping realized via an X-ray photoelectron spectroscopy study and density functional theory calculation. The latter one also unveils that Mn doping introduces catalytically active sites by disrupting homogeneous charge distribution over the topology of the MoSe2 basal plane contributing to better charge-storage performance. Mn doping-induced shift in the Fermi level of MoSe2 toward the conduction band also minimizes the contact barrier height signifying its improved capabilities for a photosensor device. Additionally, Mn doping causes alleviation of the charge-recombination process resulting in increase of photocarrier separation. As a result, we observe a 187% enhancement in the photocurrent and significantly higher responsivity and detectivity for 6.2% Mn-doped MoSe2 than its pristine counterpart. Our proposed doping strategy to modulate charge storage as well as photoresponse properties demonstrates high potential for MoSe2 along with other two-dimensional transition-metal dichalcogenides in developing next-generation energy-storage and optoelectronic devices

A rationally designed synthetic antimicrobial peptide against Pseudomonas-associated corneal keratitis: Structure-function correlation

Contact lens wearers are at an increased risk of developing Pseudomonas-associated corneal keratitis, which can lead to a host of serious ocular complications. Despite the use of topical antibiotics, ocular infections remain a major clinical problem, and a strategy to avoid Pseudomonas-associated microbial keratitis is urgently required. The hybrid peptide VR18 (VARGWGRKCPLFGKNKSR) was designed to have enhanced antimicrobial properties in the fight against Pseudomonas-induced microbial keratitis, including contact lens-related keratitis. In this paper, VR18\u27s modes of action against Pseudomonas membranes were shown by live cell Raman spectroscopy, live cell NMR, live-cell fluorescence microscopy and measures taken using sparsely tethered bilayer lipid membrane bacterial models to be via a bacterial-specific membrane disruption mechanism. The high affinity and selectivity of the peptide were then demonstrated using in vivo, in vitro and ex vivo models of Pseudomonas infection. The extensive data presented in this work suggests that topical employment of the VR18 peptide would be a potent therapeutic agent for the prevention or remedy of Pseudomonas-associated microbial keratitis

Growth and analysis of the tetragonal (ST12) germanium nanowires

New semiconducting materials, such as state-of-the-art alloys, engineered composites and allotropes of well-established materials can demonstrate unique physical properties and generate wide possibilities for a vast range of applications. Here we demonstrate, for the first time, the fabrication of a metastable allotrope of Ge, tetragonal germanium (ST12-Ge), in nanowire form. Nanowires were grown in a solvothermal-like single-pot method using supercritical toluene as a solvent, at moderate temperatures (290–330 °C) and a pressure of ∼48 bar. One-dimensional (1D) nanostructures of ST12-Ge were achieved via a self-seeded vapour–liquid–solid (VLS)-like paradigm, with the aid of an in situ formed amorphous carbonaceous layer. The ST12 phase of Ge nanowires is governed by the formation of this carbonaceous structure on the surface of the nanowires and the creation of Ge–C bonds. The crystalline phase and structure of the ST12-Ge nanowires were confirmed by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The nanowires produced displayed a high aspect ratio, with a very narrow mean diameter of 9.0 ± 1.4 nm, and lengths beyond 4 μm. The ST12-Ge nanowire allotrope was found to have a profound effect on the intensity of the light emission and the directness of the bandgap, as confirmed by a temperature-dependent photoluminescence study