Photoluminescence spectroscopy and lifetime measurements from self-assembled semiconductor-metal nanoparticle hybrid arrays

We present results of photoluminescence spectroscopy and lifetime measurements on thin film hybrid arrays of semiconductor quantum dots and metal nanoparticles embedded in a block copolymer template. The intensity of emission as well as the measured lifetime would be controlled by varying the volume fraction and location of gold nanoparticles in the matrix. We demonstrate the ability to both enhance and quench the luminescence in the hybrids as compared to the quantum dot array films while simultaneously engineering large reduction in luminescence lifetime with incorporation of gold nanoparticles. (C) 2010 American Institute of Physics. [doi:10.1063/1.3483162].

Structure, stoichiometry and spectroscopy of oxide superconductors

In the new oxide superconductors, structure and oxygen stoichiometry play the most crucial role. Thus, all the high-temperature oxide superconductors are orthorhombic perovskites with low-dimensional features. Oxygen stoichiometry in YBa2Cu3O7-δ has an important bearing on the structure as well as superconductivity. This is equally true in the La3-xBa3+xCu 6O14+δ system of which only the 123 oxide (x = 1) with the orthorhombic structure shows high Tc. Orthorhombicity though not essential, is generally found ; it is necessary for the formation of twins. The nature of oxygen and copper in the cuprates has been examined by electron spectroscopy. Copper in these cuprates is only in 1 + and 2 + states. It seems likely that oxygen holes are responsible for superconductivity of the cuprates as well as Ba(Bi, Pb)O3. High Tc superconductivity is also found in oxides of the Bi-(Ca, Sr)-Cu-O and related oxides possessing Cu-O sheets.

System effects in double‐channel gated‐integrator‐based deep‐level transient spectroscopy

The effects of the two sampling gate positions, and their widths and the integrator response times on the position, height, and shape of the peaks obtained in a double‐channel gated‐integrator‐based deep‐level transient spectroscopy (DLTS) system are evaluated. The best compromise between the sensitivity and the resolution of the DLTS system is shown to be obtained when the ratio of the two sampling gate positions is about 20. An integrator response time of about 100 ms is shown to be suitable for practical values of emission time constants and heating rates generally used.

Two-dimensional NMR spectroscopy of connected transitions by linear combination of flip angle dependent cosy: An alternative scheme for E.COSY

An alternative pulse scheme which simplifies and improves the recently proposed P.E.COSY experiment is suggested for the retention of connected or unconnected transitions in a coupled spin system. An important feature of the proposed pulse scheme is the improved phase characteristics of the diagonal peaks. A comparison of various experiments designed for this purpose, namely COSY-45, E.COSY, P.E.COSY and the present scheme (A.E.COSY), is also presented. The suppression of unconnected transitions and the measurement of scalar coupling constants and their relative signs are illustrated from A.E.COSY spectra of 2,3-dibromopropionic acid and 2-(2-thienyl)pyridine.

Cystine peptides. Disulfide conformations in fourteen membered loops investigated by Raman spectroscopy and circular dichroism

NHCH3 (X = Gly 1, Ala 2, Aib 3, Leu 4 and D-Ala 5), have been investigated by Raman and circular dichroism (CD) spectroscopy. Solid state Raman spectra are consistent with β-turn conformations in all five peptides. These peptides exhibit similar conformations of the disulfide segment in the solid state with a characteristic disulfide stretching frequency at 519 ± 3 cm-1, indicative of a trans-gauche-gauche arrangement about the Cα—Cβ—S—S—Cβ—Cα bonds. The results correlate well with the solid state conformations determined by X-ray diffraction for peptides 3 and 4. CD studies in chloroform and dimethylsulfoxide establish solvent dependent conformational changes for peptides 1, 3 and 5. Disulfide chirality has been derived using the quadrant rule. CD results together with previously reported nuclear magnetic resonance (n.m.r.) data suggest a conformational coupling between the peptide backbone and the disulfide segment

Precise measurement of hyperfine structure in the 2P(1/2) state of Li-7 using saturated-absorption spectroscopy

We report a precise measurement of the hyperfine interval in the 2P(1/2) state of Li-7. The transition from the ground state (D-1 line) is accessed using a diode laser and the technique of saturated-absorption spectroscopy in hot Li vapor. The interval is measured by locking an acousto-optic modulator to the frequency difference between the two hyperfine peaks. The measured interval of 92.040(6) MHz is consistent with an earlier measurement reported by us using an atomic-beam spectrometer Das and Natarajan, J. Phys. B 41, 035001 (2008)]. The interval yields the magnetic dipole constant in the P-1/2 state as A = 46.047(3), which is discrepant from theoretical calculations by > 80 kHz.

Photobleaching reduced fluorescence correlation spectroscopy

A dual beam excitation-depletion pulse technique is proposed for photobleaching reduced fluorescence correlation spectroscopy (FCS). Excitation pulse promote the molecules to the excited singlet state (S-1), a fraction of that population goes to energetically favorable metastable triplet state (T-1) due to strong intersystem crossing. The depletion pulse followed by excitation pulse instantaneously depletes the triplet states thereby recycling the bleached molecules back to the ground state (S-0). FCS study on diffusing Fluorescein and Rh6G molecules show more than 95% reduction in triplet state population and the associated photobleaching. (c) 2010 American Institute of Physics.

Terahertz Time Domain Spectroscopy to Detect Low-Frequency Vibrations of Double-Walled Carbon Nanotubes

We have measured the frequency-dependent real index of refraction and extinction coefficient (and hence the complex dielectric function) of a free-standing double-walled carbon nanotube film of thickness 200 nm by using terahertz time domain spectroscopy in the frequency range 0.1 to 2.5 THz. The real index of refraction and extinction coefficient have very high values of approximately 52 and 35, respectively, at 0.1 THz, which decrease at higher frequencies. Two low-frequency phonon modes of the carbon nanotubes at 0.45 and 0.75 THz were clearly observed for the first time in the real and imaginary parts of the complex dielectric function along with a broad resonance centred at around 1.45 THz, the latter being similar to that in single-walled carbon nanotubes assigned to electronic excitations. Our experiments bring out a possible application of double-walled carbon nanotube films as a neutral density filter in the THz range.

Transition metal oxide perovskites by photoelectron and x-ray absorption spectroscopy

X-ray and ultraviolet photoelectron spectroscopy as well as x-ray absorption spectroscopy have been employed to investigate transition metal oxide perovskites of the general formula ABOs (A=La or rare-earth ion, B=trivalent transition metalion). Systematics in the core levels and in the valence bands in the series of LaBOa compounds have been discussed. Lanthanum chemical shifts in the x-ray absorption spectra in this series show interesting trends. Photoelectron spectra of the solid solutions, LaNil_x Coxes, LaNix_x FexO8 and LaFel_x Coxes show that the rigid band model is applicable to these systems. It is shown that x-ray photoelectron spectroscopy can be employed to identify multiple oxidation states of transition metal ions in oxide perovskites.