Monodisperse sub-10 nm gold nanoparticles by reversing the order of addition in Turkevich method--the role of chloroauric acid

The Turkevich method for synthesizing gold nanoparticles, using sodium citrate as the reducing agent, is renowned for its ability to produce biocompatible colloids with mean size >10 nm. Here we show that monodisperse gold nanoparticles in the 5-10 nm size range can be synthesized by simply reversing the order of addition of reactants, i.e. adding chloroauric acid to citrate solution. Kinetic studies and electron microscopic characterization revealed that the reactivity of chloroauric acid, initial molar ratio of citrate to chloroauric acid (MR), and reaction mixture pH play an important role in producing monodisperse gold nanoparticles. Reversing the order of addition also enhanced the stabilization of nanoparticles at high MR values. Remarkably, the system exhibits a 'memory' of the order of addition, even when the timescale of mixing is much shorter than the timescale of synthesis.     

A multistate switchable [3]rotacatenane

Rotacatenanes are exotic molecular compounds that can be visualized as a unique combination of a [2]catenane and a [2]rotaxane, thereby combining both the circumrotation of the ring component (rotary motion) and the shuttling of the dumbbell component (translational motion) in one structure. Herein, we describe a strategy for the synthesis of a new switchable [3]rotacatenane and the investigation of its switching properties, which rely on the formation of tetrathiafulvalene (TTF) radical π-dimer interactions-namely, the mixed-valence state (TTF(2) )(+.) and the radical-cation dimer state (TTF(+.) )(2) -under ambient conditions. A template-directed approach, based on donor-acceptor interactions, has been developed, resulting in an improved yield of the key precursor [2]catenane, prior to rotacatenation. The nature of the binding between the [2]catenane and selected π-electron-rich templates has been elucidated by using X-ray crystallography and UV/Vis spectroscopy as well as isothermal titration microcalorimetry. The multistate switching mechanism of the [3]rotacatenane has been demonstrated by cyclic voltammetry and EPR spectroscopy. Most notably, the radical-cation dimer state (TTF(+.) )(2) has been shown to enter into an equilibrium by forming the co-conformation in which the two 1,5-dioxynaphthalene (DNP) units co-occupy the cavity of tetracationic cyclophane, thus enforcing the separation of TTF radical-cation dimer (TTF(+.) )(2) . The population ratio of this equilibrium state was found to be 1:1. We believe that this research demonstrates the power of constructing complex molecular machines using template-directed protocols, enabling us to make the transition from simple molecular switches to their multistate variants for enhancing information storage in molecular electronic devices.     

Systematic measurements of M X-ray components X-ray fluorescence cross-sections for the elements with 77 ≤ Z ≤ 92 by employing laboratory source based energy dispersive X-ray fluorescence setup

The differential X-ray fluorescence (XRF) cross-sections for (Mξ₂, Mξ₁, Mδ₁), (Mδ₂, Mα_1,2 M₅-O₃), (Mβ, M₄-O_2,3), (Mγ, Mm₂, M₃-N₄, M₅-O_2,3), (Mm₁, M₃-N_6,7, M₃-O_4,5) and (Mm_2, M₂-N₆) group of M X-rays components have been measured for the elements with 77 ≤ Z ≤ 92 following photoionization by Mn K X-rays (E_Kαβ = 5.96 keV) obtained from ⁵⁵Fe radioisotope. The measurements were performed in annular source geometry at 126° emission angle using a low-energy Ge (LEGe) detector. The measured cross-section values are compared with theoretical values calculated using available sets of M_i (i = 1–5) photoionization cross-sections, radiative emission rates (F_ij), Coster-Kronig (f_ij), and fluorescence (ω_i) yields. The measured XRF cross-sections for the (Mξ₂, Mξ₁, Mδ₁), (Mm₁, M₃-N_6,7) and (Mm₂, M₂-N₆) groups of X-rays agree with the theoretical values within the experimental errors. The (Mβ, M₄-O_2,3) group of X-rays exhibit agreement with theoretical values within experimental uncertainty for all the elements under investigation except ₇₉Au and ₈₀Hg. The XRF cross-section for the (Mδ₂, Mα_1,2) group of X-rays are in general higher by ~20% for the elements with Z = 77–83 and exhibit agreement for the ₉₀Th and ₉₂U elements. For the (Mγ, Mm₂, M₃-N₄) X-ray group, the measured values are generally higher than the theoretical values, but the deviations are within experimental uncertainties. The large deviation in measured XRF cross-section for different M X-ray components from the theoretical ones are attributed to (i) poor separation of M X-ray components (ii) contribution of self-resonant Raman scattering (RRS) process and (iii) self-fluorescence of M₅ subshell by M_i subshell X-rays (i = 1–3).     

TSL, OSL and ESR studies in ZnAl2O4:Tb phosphor

ZnAl₂O₄:Tb phosphor was prepared by combustion synthesis. ZnAl₂O₄:Tb exhibits three thermally stimulated luminescence (TSL) peaks around 150, 275 and 350 °C. ZnAl₂O₄:Tb exhibits optically stimulated luminescence (OSL) when stimulated with 470 nm light.Electron spin resonance (ESR) studies were carried out to identify defect centres responsible for TSL peaks observed in ZnAl₂O₄:Tb. Two defect centres are identified in irradiated ZnAl₂O₄:Tb phosphor and these centres are assigned to V and F⁺ centres. V centre appears to correlate with the 150 °C TSL peak, while F⁺ centre could not be associated with the observed TSL peaks.     

Combined effect of bond and potential disorder in half-doped manganites

We analyze the effects of both bond and potential disorder in the vicinity of a first-order metal insulator transition in a two-band model for manganites using a real-space Monte Carlo method. Our results reveal a novel charge-ordered state coexisting with spin-glass behavior. We provide the basis for understanding the phase diagrams of half-doped manganites, and contrast the effects of bond and potential disorder and the combination of both.     

Systematic Study on System Size Dependence of Global Stopping: Role of Momentum-Dependent Interactions and Symmetry Energy

Using the isospin-dependent quantum molecular dynamical model, we systematically study the role of symme- try energy with and without momentum-dependent interactions on the global nuclear stopping. We simulate the reactions by varying the total mass of the system from 80 to 394 at different beam energies from 30 to 1000 Me V/nucleon over central and semi-central geometries. The nuclear stopping is found to be sensitive towards the momentum-dependent interactions and symmetry energy at low incident energies. The momentum-dependent interactions are found to weaken the finite size effects in nuclear stopping.     

Titanium dioxide synthesized using titanium chloride: size effect study using Raman spectroscopy and photoluminescence

Titanium dioxide nanocrystals were prepared by the wet chemical method and characterized by X‐ray diffraction (XRD), transmission electron microscopy (TEM), Raman scattering (RS) and photoluminescence techniques. The XRD pattern shows the formation of single phase anatase structure of average sizes ∼7 nm (sample A) and ∼15 nm (sample B) for two samples. Additionally, TEM and RS were used to confirm the anatase crystal structure for both samples. The PL spectra show that the intensity of the sample A is more than that of sample B, which has been attributed to defect(s) and particle size variation. A modified phonon confinement model incorporating particle size distribution function and averaged dispersion curves for two most dispersive phonon branch (Γ‐X direction) have been used to interpret the size effect in Raman spectra. The obtained Raman peak shift and full width at half‐maximum agree well with the experimental data. Our observations suggest that the phonon confinement effects are responsible for a significant shift and broadening for the Raman peaks. Copyright © 2009 John Wiley & Sons, Ltd.     

First principles study of electronic structure and carrier mobility in β-armchair antimony nanotubes

In recent work, we have investigated the structure and stability of β-armchair antimony nanotubes (SbNT) using density functional theory (DFT). We studied electronic properties like electronic band structure, density of states (DOS) and mechanical properties such as stiffness constant, Poisson's ratio, and mechanical strength for these nanotubes. We found that these nanotubes are energetically stable and semiconducting in nature with band-gap varying between 1.32 eV to 1.47 eV. We have also calculated effective mass and carrier mobility for these nanotubes. Furthermore, stiffness constant and mechanical strength of these nanotubes increases with increase in diameter. While, $(4,4)$ nanotube shows anomalously higher strength than other nanotubes. The results of effective mass and carrier mobility for these nanotubes shows that electrons have higher effective mass and therefore lesser mobility than holes for most of the nanotubes. Our calculations show that β-armchair antimony nanotubes (SbNT) could be use in nano-electronics.     

1,3-Diynes: A Versatile Precursor in Transition-Metal Catalyzed (Mediated) C−H Functionalizations

Transition metal-catalyzed C−H functionalization of diverse arenes with alkyne units has attracted enormous attention for decades since they provide straightforward access to various functionalization/annulations, which are commonly present in bioactive compounds and natural products. Recently, conjugated alkynes (1,3-diynes) have been utilized as key coupling partner in many C−H activation reactions due to their versatile characteristic properties. The presence of two C≡C bonds in conjugated 1,3-diyne brings the new diversity in synthetic transformations, such as chemo-, regioselective pathways, mono-bis functionalizations, cascade annulations, etc. Herein, we summarized the latest developments in the realm of transition-metal-catalyzed C−H functionalizations of diverse arenes with 1,3-diynes. Moreover, we highlighted the diverse transformations, conditions, mechanisms and applications of the corresponding reaction in detail.