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We show that photoluminescence properties of PbS nanocrystal thin films are directly related to film morphology and nanocrystal density. In densely packed PbS nanocrystal films, low-temperature donor-to-acceptor energy transfer is mainly responsible for the photoluminescence spectra narrowing and shift toward longer wavelengths. At elevated temperatures, back energy transfer is proposed to be responsible for an unusual photoluminescence intensity temperature dependence. In thin films with a low PbS nanocrystal density, the energy transfer is suppressed, and the effect is dramatically reduced. [ABSTRACT FROM AUTHOR]

We analyzed structural and electrical properties of axial Si-Ge nanowire heterojunctions produced by the vapor-liquid-solid growth method using Au nanoclusters as catalysts. The observed nonlinear current-voltage characteristics, strong flicker noise, and damped current oscillations with frequencies of 20–30 MHz are explained using a proposed Si-Ge nanowire heterojunction energy band diagram that includes energy states associated with structural imperfections, as revealed by transmission electron microscopy. [ABSTRACT FROM AUTHOR]

Describes the Science Fair Mentoring Program, one of the Science Connection programs of the Massachusetts General Hospital/James P. Timilty Middle School Partnership. This program is designed to enhance middle school science education, inform urban early adolescents about professions in the health field, and encourage them to pursue postsecondary study in the health sciences. Discusses planning, implementation, and evaluation of the program. (Author/AIM)

We present an experimental and theoretical study for the lattice vibrational (phonon) modes in the quasi-one-dimensional (or chain-like) antiferromagnet RbCoCl3 at low temperatures both above and below the two different magnetic phase transitions. Clear evidence is found for the role of spin-phonon interactions in providing a temperature-dependent contribution for the frequencies of the E1g and E2g symmetry phonons that occur with frequencies comparable to those of the spin wave excitations (magnons) in this compound. The behaviour in RbCoCl3, as studied here by Raman scattering experiments, is quite different from that typically observed in rutile-structure antiferromagnets where the spin-phonon coupling has been well characterized. The theory is modified to take account of the strong Ising-like component in the spin Hamiltonian. This enables the spin-phonon coupling parameters to be deduced, with the analysis also revealing the onset of an extra frequency shift for the phonons below the transition temperature TN1 = 28 K associated with magnetic ordering along the Co chains. [ABSTRACT FROM AUTHOR]

We present an experimental and theoretical study for the lattice vibrational (phonon) modes in the quasi-one-dimensional (or chain-like) antiferromagnet RbCoCl3 at low temperatures both above and below the two different magnetic phase transitions. Clear evidence is found for the role of spin-phonon interactions in providing a temperature-dependent contribution for the frequencies of the E1g and E2g symmetry phonons that occur with frequencies comparable to those of the spin wave excitations (magnons) in this compound. The behaviour in RbCoCl3, as studied here by Raman scattering experiments, is quite different from that typically observed in rutile-structure antiferromagnets where the spin-phonon coupling has been well characterized. The theory is modified to take account of the strong Ising-like component in the spin Hamiltonian. This enables the spin-phonon coupling parameters to be deduced, with the analysis also revealing the onset of an extra frequency shift for the phonons below the transition temperature TN1 = 28 K associated with magnetic ordering along the Co chains. [ABSTRACT FROM AUTHOR]

Author(s): G. F. Grom [1]; D. J. Lockwood [2]; J. P. McCaffrey [2]; H. J. Labbé [2]; P. M. Fauchet [1]; B. White, Jr [3]; J. Diener [4]; D. Kovalev [...]

The Science Fair Mentoring Program provides middle school pupils with an opportunity to learn about science under the guidance of a mentor. Mentors, research professionals working at the Massachusetts General Hospital, Massachusetts, encourage, inform and inspire teenage students from the James P. Timilty Middle School. Middle school science topics include single cell life, food chemistry and human body systems.

Detailed studies of the structural and optical properties of axial silicon-germanium nanowire heterojunctions show that despite the 4.2% lattice mismatch between Si and Ge they can be grown without a significant density of structural defects. The lattice mismatch induced strain is partially relieved due to spontaneous SiGe intermixing at the heterointerface during growth and lateral expansion of the Ge segment of the nanowire. The mismatch in Ge and Si coefficients of thermal expansion and low thermal conductivity of Si/Ge nanowire heterojunctions are proposed to be responsible for the thermally induced stress detected under intense laser radiation in photoluminescence and Raman scattering measurements. [ABSTRACT FROM AUTHOR]

The role of the interface potential on the effective mass of charge carriers is elucidated in this work. We develop a new theoretical formalism using a spatially dependent effective mass that is related to the magnitude of the interface potential. Using this formalism, we studied Ge quantum dots (QDs) formed by plasma enhanced chemical vapour deposition (PECVD) and co-sputtering (sputter). These samples allowed us to isolate important consequences arising from differences in the interface potential. We found that for a higher interface potential, as in the case of PECVD QDs, there is a larger reduction in the effective mass, which increases the confinement energy with respect to the sputter sample. We further understood the action of O interface states by comparing our results with Ge QDs grown by molecular beam epitaxy. It is found that the O states can suppress the influence of the interface potential. From our theoretical formalism, we determine the length scale over which the interface potential influences the effective mass. [ABSTRACT FROM AUTHOR]

We present a quantitative analysis of Raman scattering in various Si/Si1-xGex multilayered nanostructures with well-defined Ge composition (x) and layer thicknesses. Using Raman and transmission electron microscopy data, we discuss and model Si/SiGe intermixing and strain. By analyzing Stokes and anti-Stokes Raman signals, we calculate temperature and discuss heat dissipation in the samples under intense laser illumination. [ABSTRACT FROM AUTHOR]

Experimental results obtained previously for the photoluminescence efficiency (PLeff) of Ge quantum dots (QDs) are theoretically studied. A log-log plot of PLeff versus QD diameter (D) resulted in an identical slope for each Ge QD sample only when EG ~ (D² + D)-1 . We identified that above D ≈ 6.2nm: EG ~ D-1 due to a changing effective mass (EM), while below D ≈ 4.6 nm: EG ~ D-2 due to electron/hole confinement. We propose that as the QD size is initially reduced, the EM is reduced, which increases the Bohr radius and interface scattering until eventually pure quantum confinement effects dominate at small D. [ABSTRACT FROM AUTHOR]