U. Philipose

High-temperature ferromagnetism in Mn-doped ZnO nanowires

We have observed ferromagnetism in dilute (∼1–4at.%) Mn-doped crystalline ZnO nanowires at temperatures up to 400K. Arrays of freestanding single crystal ZnO:Mn nanowires were fabricated by Au-catalyzed vapor-liquid-solid growth. Structure and compositional analyses revealed that Mn was incorporated into the ZnO lattice. From the observed saturation magnetization, the magnetic moment per Mn atom is estimated to be between 0.3μB and 1.2μB. Photoluminescence measurements show a strong suppression of defect related midgap emission, indicative of an interplay between Mn doping and native point defects.

Enhancement of band edge luminescence in ZnSe nanowires

In order to realize the full potential of nanowires for optical applications, it is essential to synthesize nanowires that can emit predominantly via band to band or band edge (BE) transitions. However, many compound semiconductor nanowires, irrespective of the method of their growth, contain a high density of native defects; these result in competing deep defect (DD) related emission, limiting their utility for optoelectronic device applications. The concentration of these native defect states depends on the gas phase stoichiometry. In this work, we report on the influence of gas phase stoichiometry on the structural and optical properties of single crystal zinc selenide (ZnSe) nanowires. We find that nanowires grown under stoichiometric conditions contain such defect states with associated weak BE emission and strong DD emission. However, nanowires grown under Zn-rich conditions were characterized by photoluminescence spectra dominated by strong BE emission while those grown under Se-rich conditions sho...

Electrical properties of Ohmic contacts to ZnSe nanowires and their application to nanowire-based photodetection

Multilayer Ti∕Au contacts were fabricated on individual, unintentionally doped zinc selenide nanowires with 80nm nominal diameter. Four-terminal contact structures were used to independently measure current-voltage characteristics of contacts and nanowires. Specific contact resistivity of Ti∕Au contacts is 0.024Ωcm2 and intrinsic resistivity of the nanowires is approximately 1Ωcm. The authors have also measured the spectral photocurrent responsivity of a ZnSe nanowire with 2.0V bias across Ti∕Au electrodes, which exhibits a turnon for wavelengths shorter than 470nm and reaches 22A∕W for optical excitation at 400nm.

Developing 1D nanostructure arrays for future nanophotonics

There is intense and growing interest in one-dimensional (1-D) nanostructures from the perspective of their synthesis and unique properties, especially with respect to their excellent optical response and an ability to form heterostructures. This review discusses alternative approaches to preparation and organization of such structures, and their potential properties. In particular, molecular-scale printing is highlighted as a method for creating organized pre-cursor structure for locating nanowires, as well as vapor–liquid–solid (VLS) templated growth using nano-channel alumina (NCA), and deposition of 1-D structures with glancing angle deposition (GLAD). As regards novel optical properties, we discuss as an example, finite size photonic crystal cavity structures formed from such nanostructure arrays possessing highQ and small mode volume, and being ideal for developing future nanolasers.

Origin of the red luminescence band in photoluminescence spectra of ZnSe nanowires

In this work, the origin of the deep level, defect related photoluminescence emission band in ZnSe is investigated. Using the dependence of the peak energy on excitation intensity, it was shown to originate from donor-acceptor pair recombination. The binding energy of the donor-acceptor pair was estimated to be 18±0.5meV and the shallow impurity Bohr radius was estimated to be 9.1±0.2nm. Using a postgrowth annealing treatment in a Zn atmosphere, the two species involved in the donor-acceptor pair recombination process were attributed to Zn vacancies and Zn interstitials.

Structure and photoluminescence of ZnSe nanostructures fabricated by vapor phase growth

The synthesis of ZnSe nanowires is a rapidly expanding research field with numerous applications. Key to realizing the potential of such nanowires is an ability to prepare structurally uniform defect-free material. Microstructure studies on as-grown nanostructures reveal that the structure of these nanowires are highly sensitive to the growth temperature, with higher temperature growth (750°C) resulting mostly in nanoribbons having different morphologies. Planar defects (mainly stacking faults and twins) are found to extend throughout the nanoribbons. At lower growth temperatures (650°C) structurally uniform nanowires are typically found. Photoluminescence (PL) measurements on crystalline nanowires show that the spectrum comprises two characteristic peaks corresponding to strong near band edge emission, and a weaker emission associated with defect states. However, the PL spectra of ZnSe nanoribbons which contain a high density of structural defects are dominated by defect related emission.

Giant anisotropy of second harmonic generation for a single ZnSe nanowire

The effect of second harmonic generation was experimentally investigated in ZnSe nanowires grown by the vapor-liquid-solid method. The effect dramatically depended on the angle between the nanowire axis and the linear polarization of the excitation light. The magnitude of the effect was 20 times stronger for the parallel polarization in comparison with the perpendicular one. The results were theoretically explained in terms of a large difference in dielectric constants between the nanowire and the environment, resulting in a strong orientation-dependent optical electric field in the nanowire.

Twinning modulation in ZnSe nanowires

ZnSe nanowires were grown on Si substrates by Au-catalyzed vapour phase growth at 725 °C. A detailed structural and microstructural investigation has been carried out using electron diffraction and high-resolution transmission electron microscopy (HRTEM). Modulated twins have been observed along the nanowire axial direction along the entire length of the nanowires. Faceting has been observed on the side surfaces of the wires with a larger twinning periodicity. The formation mechanism of these twinning-modulated nanowires is discussed. The optical properties are correlated with the microstructure of the nanowires. These twinning-modulated ZnSe nanowires might have great potential as building blocks for optoelectronic nanodevices.

Defect studies of ZnSe nanowires

During the synthesis of ZnSe nanowires various point and extended defects can form, leading to observed stacking faults and twinning defects, and strong defect related emission in photoluminescence spectra. In this paper, we report on the development of a simple thermodynamic model for estimating the defect concentration in ZnSe nanowires grown under varying Se vapour pressure and for explaining the results of our experimental findings. Positron annihilation spectroscopy was used successfully for the first time for nanowires and the results support predictions from the defect model as well as agreeing well with our structural and optical characterization results. Under very high Se vapour pressure, Se nodules were observed to form on the sidewalls of the nanowire, indicating that beyond a limit, excess Se will begin to precipitate out of the liquid alloy droplet in the vapour-liquid-solid growth of nanowires.

Ultrafast carrier dynamics in band edge and broad deep defect emission ZnSe nanowires

Ultrafast carrier dynamics of ZnSe nanowires grown under different growth conditions have been studied. Transient absorption measurements reveal the dependence of the competing effects of state filling and photoinduced absorption on the probed energy states. The relaxation of the photogenerated carriers occupying defect states in the stoichiometric and Se-rich samples are single exponentials with time constants of 3–4ps. State filling is the main contribution for probe energies below 1.85eV in the Zn-rich grown sample. This ultrafast carrier dynamics study provides an important insight into the role that intrinsic point defects play in the observed photoluminescence from ZnSe nanowires.