Facile Synthesis of Semiconducting Ultrathin Layer of Molybdenum Disulfide

In this paper, we have reported a simple and efficient method for the synthesis of uniform, highly conducting single or few layer molybdenum disulfide (MoS2) on large scale. Scanning Electron Microscopy (SEM) and High Resolution Transmission Electron Microscopy (HRTEM) have been used for the confirmation of mono or few layered nature of the as-synthesized MoS2 sheets. X-ray Photoelectron Spectroscopy (XPS), X-Ray Diffraction (XRD) and Raman Spectroscopy have also been used to study the elemental, phase, and molecular composition of the sample. Optical properties of as-synthesized sample have been probed by measuring absorption and photoluminescence spectra which also compliment the formation of mono and few layers MoS2 Current-voltage (I-V ) characteristics of as-synthesized sample in the pellet form reveal that MoS2 sheets have an ohmic character and found to be highly conducting. Besides characterizing the as-synthesized sample, we have also proposed the mechanism and factors which play a decisive role in formation of high quality MoS2 sheets.

Mercury based drug in ancient India: The red sulfide of mercury in nanoscale

Mercury is one of the elements which had attracted the attention of the chemists and physicians of ancient India and China. Among the various metal based drugs which utilize mercury, we became interested in the red sulfide of mercury which is known in ancient Indian literature as rasasindur (alias rasasindura, rasasindoor, rasasinduram, sindur, or sindoor) and is used extensively in various ailments and diseases. Following various physico-chemical characterizations it is concluded that rasasindur is chemically pure α-HgS with Hg:S ratio as 1:1. Analysis of rasasindur vide Transmission Electron Microscopy (TEM) showed that the particles are in nanoscale. Bio-chemical studies of rasasindur were also demonstrated. It interacts with Bovine Serum Albumin (BSA) with an association constant of (9.76 ± 0.56) × 10³ M⁻¹ and behaves as a protease inhibitor by inhibiting the proteolysis of BSA by trypsin. It also showed mild antioxidant properties.

Fe3C-filled carbon nanotubes: permanent cylindrical nanomagnets possessing exotic magnetic properties

The present study aims to deduce the confinement effect on the magnetic properties of iron carbide (Fe3C) nanorods filled inside carbon nanotubes (CNTs), and to document any structural phase transitions that can be induced by compressive/tensile stress generated within the nanorod. Enhancement in the magnetic properties of the nanorods is attributed to tensile stress as well as to compression, present in the radial direction and along the nanotube axis, respectively. Finally, the growth of permanent cylindrical nanomagnets has been optimized by applying a field gradient. Besides presenting the growth model of in situ filling, we have also proposed the mechanism of magnetization of the nanotubes. Magnetization along the tube axis has been probed by confirming the pole formation. Fe3C has been selected because of its ease of formation, low TC and incompressibility.

Coherently embedded Ag nanostructures in Si: 3D imaging and their application to SERS

Surface enhanced Raman spectroscopy (SERS) has been established as a powerful tool to detect very low-concentration bio-molecules. One of the challenging problems is to have reliable and robust SERS substrate. Here, we report on a simple method to grow coherently embedded (endotaxial) silver nanostructures in silicon substrates, analyze their three-dimensional shape by scanning transmission electron microscopy tomography and demonstrate their use as a highly reproducible and stable substrate for SERS measurements. Bi-layers consisting of Ag and GeO_x thin films were grown on native oxide covered silicon substrate using a physical vapor deposition method. Followed by annealing at 800°C under ambient conditions, this resulted in the formation of endotaxial Ag nanostructures of specific shape depending upon the substrate orientation. These structures are utilized for detection of Crystal Violet molecules of 5 × 10⁻¹⁰ M concentrations. These are expected to be one of the highly robust, reusable and novel substrates for single molecule detection.

Synthesis and Characterization of Mn-Doped ZnO Nanocrystals

We report the synthesis and characterization of several sizes of Mn-doped ZnO nanocrystals, both in the free-standing and the capped particle forms. The sizes of these nanocrystals could be controlled by capping them with polyvinylpyrollidone under different synthesis conditions and were estimated by X-ray diffraction and transmission electron microscopy. The absorption properties of PVP-capped Mn-doped ZnO exhibit an interesting variation of the band gap with the concentration of Mn. Fluorescence emission, electron paramagnetic resonance, and X-ray absorption spectroscopy provide evidence for the presence of Mn in the interior as well as on the surface of the nanocrystals.

DC heating induced shape transformation of Ge structures on ultraclean Si(5 5 12) surfaces

We report the growth of Ge nanostructures and microstructures on ultraclean, high vicinal angle silicon surfaces and show that self-assembled growth at optimum thickness of the overlayer leads to interesting shape transformations, namely from nanoparticle to trapezoidal structures, at higher thickness values. Thin films of Ge of varying thickness from 3 to 12 ML were grown under ultrahigh vacuum conditions on a Si(5 5 12) substrate while keeping the substrate at a temperature of 600 °C. The substrate heating was achieved by two methods: (i) by heating a filament under the substrate (radiative heating, RH) and (ii) by passing direct current through the samples in three directions (perpendicular, parallel and at 45° to the (110) direction of the substrate). We find irregular, more spherical-like island structures under RH conditions. The shape transformations have been found under DC heating conditions and for Ge deposition more than 8 ML thick. The longer sides of the trapezoid structures are found to be along (110) irrespective of the DC current direction. We also show the absence of such a shape transformation in the case of Ge deposition on Si(111) substrates. Scanning transmission electron microscopy measurements suggested the mixing of Ge and Si. This has been confirmed with a quantitative estimation of the intermixing using Rutherford backscattering spectrometry (RBS) measurements. The role of DC heating in the formation of aligned structures is discussed. Although the RBS simulations show the presence of a possible SiO(x) layer, under the experimental conditions of the present study, the oxide layer would not play a role in determining the formation of the various structures that were reported here.

Stability of embedded indium nanoclusters in silica under thermal treatment and ion irradiation

The stability of embedded Indium (In) nanoclusters (NCs) in silica under thermal annealing and ion irradiation was investigated. The In NCs were prepared by implantation of 890 keV indium ions in silica matrix at room temperature. Post implantation annealing resulted in the shifting of the size distribution to higher side. On the other hand 140 keV Nitrogen ion irradiation at elevated temperature resulted in the reduction of NCs size, with significant narrowing of the size distribution. The paper discusses the results of the study in the light of the models pertaining to the stability of NCs under ion irradiation conditions.

Oxide mediated liquid-solid growth of high aspect ratio aligned gold silicide nanowires on Si(110) substrates

Silicon nanowires grown using the vapor-liquid-solid method are promising candidates for nanoelectronics applications. The nanowires grow from an Au-Si catalyst during silicon chemical vapor deposition. In this paper, the effect of temperature, oxide at the interface and substrate orientation on the nucleation and growth kinetics during formation of nanogold silicide structures is explained using an oxide mediated liquid-solid growth mechanism. Using real time in situ high temperature transmission electron microscopy (with 40 ms time resolution), we show the formation of high aspect ratio ( approximately 15.0) aligned gold silicide nanorods in the presence of native oxide at the interface during in situ annealing of gold thin films on Si(110) substrates. Steps observed in the growth rate and real time electron diffraction show the existence of liquid Au-Si nano-alloy structures on the surface besides the un-reacted gold nanostructures. These results might enable us to engineer the growth of nanowires and similar structures with an Au-Si alloy as a catalyst.

Formation of aligned nanosilicide structures in a MBE-grown Au/Si(110) system: a real-time temperature-dependent TEM study

Thin Au films (∼2 nm) were deposited on an Si(110) substrate epitaxially under ultra-high vacuum (UHV) conditions in a molecular beam epitaxy (MBE) system. Real-time in situ transmission electron microscopy (TEM) measurements were carried out at various temperatures (from room temperature to 700 °C), which shows the formation and growth of aligned gold silicide nanorod-like structures. The real-time selected-area electron diffraction patterns show the presence of silicon and unreacted gold at lower temperatures (up to 363 °C), while at higher temperatures only the signature of silicon has been observed. The diffraction analysis from room temperature cooled systems show the presence of gold silicide structures. Around 700 °C, 97% of the nanostructures were found to be aligned nanosilicide-rod-like structures with a longer side of ≈37 nm and aspect ratio of 1.38. For a high temperature annealed system (at 600 °C), selected-area diffraction (SAD) and high resolution lattice (after cooling down to room temperature) confirmed the formation of nano- Au(5)Si(2) structures. The alignment of gold silicide structures has been explained on the basis of lattice matching between the substrate silicon and silicide structures.

Flux dependent MeV self-ion-induced effects on Au nanostructures: dramatic mass transport and nanosilicide formation

We report a direct observation of dramatic mass transport due to 1.5 MeV Au(2+) ion impact on isolated Au nanostructures of average size ≈7.6 nm and height ≈6.9 nm that are deposited on Si(111) substrate under high flux (3.2 × 10(10)-6.3 × 10(12) ions cm(-2) s(-1)) conditions. The mass transport from nanostructures was found to extend up to a distance of about 60 nm into the substrate, much beyond their size. This forward mass transport is compared with the recoil implantation profiles using SRIM simulation. The observed anomalies with theory and simulations are discussed. At a given energy, the incident flux plays a major role in mass transport and its redistribution. The mass transport is explained on the basis of thermal effects and the creation of rapid diffusion paths in the nanoscale regime during the course of ion irradiation. The unusual mass transport is found to be associated with the formation of gold silicide nano-alloys at subsurfaces. The complexity of the ion-nanostructure interaction process is discussed with a direct observation of melting (in the form of spherical fragments on the surface) phenomena. Transmission electron microscopy, scanning transmission electron microscopy, and Rutherford backscattering spectroscopy methods have been used.

MeV gold ion induced sputtered nanoparticles from gold nanostructures: dependence of incident flux and temperature

The high-energy and heavy-ion induced sputtered particles from nanostructures under various conditions can result in variety of size distributions. 1.5 MeV Au2+ ions induced sputtering from isolated gold nanostructures deposited on silicon substrate have been studied as a function of incident ion flux (dose rate) and the sputter particle catcher at low temperature. At higher fluxes, a bimodal distribution of the sputtered particles has been observed. Cross-sectional transmission electron microscopy and Rutherford backscattering spectrometry measurements showed that the sputter particle size distribution depends on morphology at surface and interfaces. The results for the size distribution from a catcher at low temperature showed the less agglomeration of ejected clusters on the catcher grids, resulting in the lower-disperse size distribution.

Synthesis of gold nanorods in organic media

A seed mediated approach for the synthesis of anisotropic rod shaped gold nanoparticles in organic media (toluene) is demonstrated. Pre-formed gold nanoparticles stabilized in toluene by 4-hexadecylaniline (HDA) are used as seeds. These when reacted with 1-octadecylamine (ODA) hydrophobised chloroaurate ions in toluene lead to the formation of gold nanorods. ODA or alkylamines of different chain lengths which are the chloroaurate ion phase transfer agent have been found to play a key role in the formation of the nanorods. The gold nanorods that have a five-fold symmetry evolve from multiply twinned particles and are bound at the tips by [1 11] faces and at the sides by [100] faces. The gold nanorods have been shown to grow under the shape directing effect of the alkylamines which stabilize the high energy [100] faces. The concentration of the alkylamines has been found to play a critical role in the formation of the gold nanorods. Higher concentrations of the alkylamines lead to formation of spherical particles, at times of narrow size distribution.

Interfacial deposition of Ag on Au seeds leading to AucoreAgshell in organic media

A seed mediated procedure for the synthesis of hydrophobic Au(core)Ag(shell) nanoparticles in toluene is demonstrated. The reaction proceeds by way of the interfacial reduction of silver ions by 3-pentadecylphenol followed by their deposition on hydrophobized Au nanoparticles. Such a hitherto unreported interfacial seeded growth reaction leads to the formation of phase pure Au(core)Ag(shell) nanoparticles that retain the hydrophobicity of the seed particles and remain stable in toluene. Such core-shell structures are however not formed in the aqueous phase. The core-shell architecture was verified using TEM analysis and the formation process was studied by recording the UV-vis spectra of the organic phase nanoparticles as a function of time. TEM kinetics also showed gradual increase in the silver layer thickness. Conclusive evidence was however obtained on examination of the HRTEM images of the products formed. Elemental analysis using X-ray photoelectron spectroscopy of the Au(core)Ag(shell) nanostructure revealed the presence of metallic silver. Moreover changing the surface capping of the Au seed does not affect the formation of the Au(core)Ag(shell) nanostructure.

Synthesis of nanocrystalline tin oxide thin film by swift heavy ion irradiation

Nanocrystals of tin oxide were formed in e-beam evaporated films by swift heavy ion (SHI) irradiation. The nucleation of nanocrystals occurred due to electronic excitation by swift heavy ion. Nanophase thin films are characterized systematically by HRTEM, GAXRD, EDX, and UV/NIS absorption techniques. Nanocrystals having size of 8 nm radius are synthesized in different substrates during swift heavy ion irradiation and without subsequent annealing. SHI induced nanocrystallization could be achieved in both crystalline and non-crystalline substrates.

Morphology and size dependent optical properties of CdS nanostructures

CdS nanoparticles with different sizes ranging from 2.5 nm to 300 nm and nanorods with aspect ratio -32 were synthesized by simple solvothermal process with a view to explore the effect of size and shape on the optical properties of these nanoforms. Solvent, temperature and the Cd source played important role in determining the morphologies and sizes of the nanocrystals. Comparative study of the optical properties of these nanoforms showed systematic changes in the optical absorption spectra with the reduction in particle size. Nanorods showed bulk like properties. Photoluminescence and Raman studies were carried out to explore the size and morphology dependent optical properties of the CdS nanoforms.

Synthesis and Optical Properties of CdS Nanoribbons

Rapid production of single crystalline CdS nanoribbons with hexagonal wurtzite phase has been achieved by thermal evaporation of CdS powder on Si wafers. The flow rate of the carrier (Ar) gas along with the synthesis temperature plays an important role in defining the size and shape of the CdS nanoribbons. Scanning electron and transmission electron microscopic observations revealed the nanoribbons to have a flat end as well as side surfaces which will make it ideal for optoelectronic devices such as nanolasers and light emitting diodes based on individual nanoribbons. The nanoribbons have widths within 200-400 nm and lengths approximately a few hundred micrometers. Room-temperature photoluminescence measurements show green emission centered at approximately 525 nm which may be ascribed to the near band edge emission. The Raman spectra of the CdS nanoribbons show peaks around 304, 609, 915, and 1220 cm(-1) corresponding to the first-, second-, third-, and fourth-order longitudinal optical phonon modes, respectively.

Electron and phonon confinement and surface phonon modes in CdSe-CdS core-shell nanocrystals

The optical and vibrational properties of bare and CdS shelled CdSe nanocrystalline particles are investigated. To confirm the formation of such nanocrystals in our samples we estimate their average particle sizes and size distributions using TEM measurements. From the line profile analysis of the HRTEM images the core-shell structure in the particles has been confirmed. The blue shift in the optical absorption spectra, analysed using theoretical estimates based on the effective bond order model, establishes the electron confinement in the nanoparticles. The main aim of this paper is to show the unique characteristics of the nanocrystals (which are absent in the corresponding bulk material), such as confinement of optical phonons and the appearance of surface phonons. Making use of the dielectric continuum model we are able to match the experimental and theoretical values of the frequencies of the surface phonons. We believe that our studies using optical probes provide further evidence for the existence of core-shell structures in CdSe-CdS type materials.

Single crystalline nickel nanorods inside carbon nanotubes: growth behavior, structure, and magnetic properties

Nickel nanorods with diameters ranging from 5 to 10 nm, encapsulated inside the carbon nanotubes, are prepared using microwave plasma chemical vapor deposition. High-resolution transmission electron microscopy (HRTEM) studies reveal the perfect crystalline nature of the rods with d-spacing closely matching the (111) interplanar spacing of Ni. The (111) planes of the Ni nanorods are always aligned at 39.6 degrees with respect to the graphite planes of the nanotubes. The cosine component of the d-spacing along the direction of the graphite planes is found to be 1.6 A; exactly half the d-spacing between the graphite planes. The electron diffraction pattern shows clear spots corresponding to Ni structure. The field cooled and zero field cooled magnetization data reveal the reversibility of the magnetization of the Ni nanorods and show a blocking temperature of 195 K, which correspond to energy barrier of 0.4 eV/(V).