Temperature dependent photoluminescence of single CdS nanowires

Temperature dependent photoluminescence (PL) is used to study the electronic properties of single CdS nanowires. At low temperatures, both near-band edge (NBE) photoluminescence (PL) and spatially-localized defect-related PL are observed in many nanowires. The intensity of the defect states is a sensitive tool to judge the character and structural uniformity of nanowires. As the temperature is raised, the defect states rapidly quench at varying rates leaving the NBE PL which dominates up to room temperature. All PL lines from nanowires follow closely the temperature-dependent band edge, similar to that observed in bulk CdS.Comment: 11 pages, 4 figure

Low temperature photoluminescence imaging and time-resolved spectroscopy of single CdS nanowires

Time-resolved photoluminescence (PL) and micro-PL imaging were used to study single CdS nanowires at 10 K. The low-temperature PL of all CdS nanowires exhibit spectral features near energies associated with free and bound exciton transitions, with the transition energies and emission intensities varying along the length of the nanowire. In addition, several nanowires show spatially localized PL at lower energies which are associated with morphological irregularities in the nanowires. Time-resolved PL measurements indicate that exciton recombination in all CdS nanowires is dominated by non-radiative recombination at the surface of the nanowires.Comment: 9 pages, 3 figures, to be published in Applied Physics Letter

Low temperature properties of a quantum particle coupled to dissipative environments

We study the dynamics of a quantum particle coupled to dissipative (ohmic) environments, such as an electron liquid. For some choices of couplings, the properties of the particle can be described in terms of an effective mass. A particular case is the three dimensional dirty electron liquid. In other environments, like the one described by the Caldeira-Leggett model, the effective mass diverges at low temperatures, and quantum effects are strongly suppressed. For interactions within this class, arbitrarily weak potentials lead to localized solutions. Particles bound to external potentials, or moving in closed orbits, can show a first order transition, between strongly and weakly localized regimes.Comment: 10 page

Decoherence in elastic and polaronic transport via discrete quantum states

Here we study the effect of decoherence on elastic and polaronic transport via discrete quantum states. The calculations are performed with the help of nonperturbative computational scheme, based on the Green's function theory within the framework of polaron transformation (GFT-PT), where the many-body electron-phonon interaction problem is mapped exactly into a single-electron multi-channel scattering problem. In particular, the influence of dephasing and relaxation processes on the shape of the electrical current and shot noise curves is discussed in detail under the linear and nonlinear transport conditions.Comment: 11 pages, 3 figure

Quantum Diffusion of H/Ni(111) through the Monte Carlo Wave Function Formalism

We consider a quantum system coupled to a dissipative background with many degrees of freedom using the Monte Carlo Wave Function method. Instead of dealing with a density matrix which can be very high-dimensional, the method consists of integrating a stochastic Schrodinger equation with a non-hermitian damping term in the evolution operator, and with random quantum jumps. The method is applied to the diffusion of hydrogen on the Ni(111) surface below 100 K. We show that the recent experimental diffusion data for this system can be understood through an interband activation process, followed by quantum tunnelling.Comment: In press at Phys.Rev.Let

Morphology of epitaxial core-shell nanowires

We analyze the morphological stability against azimuthal, axial, and general helical perturbations for epitaxial core-shell nanowires in the growth regimes limited by either surface diffusion or evaporation-condensation surface kinetics. For both regimes, we find that geometric parameters (i.e., core radius and shell thickness) play a central role in determining whether the nanowire remains cylindrical or its shell breaks up into epitaxial islands similar to those observed during Stranski-Krastanow growth in thin epilayers. The combination of small cores and rapid growth of the shell emerge as key ingredients for stable shell growth. Our results provide an explanation for the different core-shell morphologies reported in the Si-Ge system experimentally, and also identify a growth-induced intrinsic mechanism for the formation of helical nanowires.Comment: In press, Nano Letters (7 pages, 4 figures

Probing quantum confinement within single core-multishell nanowires

Theoretically core-multishell nanowires under a cross-section of hexagonal geometry should exhibit peculiar confinement effects. Using a hard X-ray nanobeam, here we show experimental evidence for carrier localization phenomena at the hexagon corners by combining synchrotron excited optical luminescence with simultaneous X-ray fluorescence spectroscopy. Applied to single coaxial n-GaN/InGaN multiquantum-well/p-GaN nanowires, our experiment narrows the gap between optical microscopy and high-resolution X-ray imaging and calls for further studies on the underlying mechanisms of optoelectronic nanodevices. © 2012 American Chemical Society.The authors thank Irina Snigireva and Armando Vicente Sole for their assistance with the SEM measurements and data processing using PyMca, respectively. We thank Remi Tocoulou and Peter Cloetens for their help and the ESRF for the beam time allocated. We also thank Andrei Rogalev for the valuable discussions and Gary Admans for the critical reading of the manuscript. This work has been partially supported by the NANOWIRING Marie Curie ITN (EU project no. PITN-GA-2010-265073), as well as by the EPIC-NANOTICS (TEC2011-29120-C05-04) and Q&C-LIGHT (S2009ESP-1503) from Spanish MEC and CAM, respectively.Martínez Criado, G.; Homs Puron, AA.; Alen, B.; Sans Tresserras, JÁ.; Segura Ruiz, J.; Molina Sánchez, A.; Susini, J.... (2012). Probing quantum confinement within single core-multishell nanowires. Nano Letters. 12(11):5829-5834. https://doi.org/10.1021/nl303178uS58295834121

Ultralow Thermal Conductivity of Isotope-Doped Silicon Nanowires

The thermal conductivity of silicon nanowires (SiNWs) is investigated by molecular dynamics (MD) simulation. It is found that the thermal conductivity of SiNWs can be reduced exponentially by isotopic defects at room temperature. The thermal conductivity reaches the minimum, which is about 27% of that of pure 28Si NW, when doped with fifty percent isotope atoms. The thermal conductivity of isotopic-superlattice structured SiNWs depends clearly on the period of superlattice. At a critical period of 1.09 nm, the thermal conductivity is only 25% of the value of pure Si NW. An anomalous enhancement of thermal conductivity is observed when the superlattice period is smaller than this critical length. The ultra-low thermal conductivity of superlattice structured SiNWs is explained with phonon spectrum theory.Comment: Nano Lett., ASAP Article 10.1021/nl0725998 S1530-6984(07)02599-4 Web Release Date: December 21, 200

Ge/Si nanowire mesoscopic Josephson junctions

The controlled growth of nanowires (NWs) with dimensions comparable to the Fermi wavelengths of the charge carriers allows fundamental investigations of quantum confinement phenomena. Here, we present studies of proximity-induced superconductivity in undoped Ge/Si core/shell NW heterostructures contacted by superconducting leads. By using a top gate electrode to modulate the carrier density in the NW, the critical supercurrent can be tuned from zero to greater than 100 nA. Furthermore, discrete sub-bands form in the NW due to confinement in the radial direction, which results in stepwise increases in the critical current as a function of gate voltage. Transport measurements on these superconductor-NW-superconductor devices reveal high-order (n = 25) resonant multiple Andreev reflections, indicating that the NW channel is smooth and the charge transport is highly coherent. The ability to create and control coherent superconducting ordered states in semiconductor-superconductor hybrid nanostructures allows for new opportunities in the study of fundamental low-dimensional superconductivity