To date investigations of the dynamics of driven colloidal systems have focused on hydrodynamic interactions and often employ optical (laser) tweezers for manipulation. However, the optical fields that provide confinement and drive also result in electrodynamic interactions that are generally neglected. We address this issue with a detailed study of interparticle dynamics in an optical ring vortex trap using 150 nm diameter Ag nanoparticles. We term the resultant electrodynamically interacting nanoparticles a driven optical matter system. We also show that a superior trap is created by using a Au nanoplate mirror in a retro-reflection geometry, which increases the electric field intensity, the optical drive force, and spatial confinement. Using nanoparticles versus micron sized colloids significantly reduces the surface hydrodynamic friction allowing us to access small values of optical topological charge and drive force. We quantify a further 50% reduction of hydrodynamic friction when the nanoparticles are driven over the Au nanoplate mirrors versus over a mildly electrostatically repulsive glass surface. Further, we demonstrate through experiments and electrodynamics-Langevin dynamics (ED-LD) simulations that the optical drive force and the interparticle interactions are not constant around the ring for linearly polarized light, resulting in a strong position-dependent variation in the nanoparticle velocity. The nonuniformity in the optical drive force is also manifest as an increase in fluctuations of interparticle separation, or effective temperature, as the optical driving force is increased. Finally, we resolve an open issue in the literature on periodic modulation of interparticle separation with comparative measurements of driven 300 nm diameter polystyrene beads that also clearly reveal the significance of electrodynamic forces and interactions in optically driven colloidal systems. Therefore, the modulations in the optical forces and electrodynamic interactions that we demonstrate should not be neglected for dielectric particles and might give rise to some structural and dynamic features that have previously been attributed exclusively to hydrodynamic interactions.

Room temperature ferromagnetism has been observed in ZnO nanorods prepared by hydrothermal method. Saturation magnetization of ϳ0.004 emu/ g was measured in the nanorods with diameters of ϳ10 nm and lengths of below 100 nm, and the magnetization reduced with the increase of the size. Annealing of the samples at 900°C in air completely transformed the nanorods into twinning structures and weakened the magnetizations. The mechanism of morphology transformation was discussed. Analysis indicates that the interstitial zinc at the surface may contribute to the ferromagnetism in ZnO nanorods.As ideal candidate materials for spintronic devices, ZnObased diluted magnetic semiconductors ͑DMSs͒ have attracted much attention in recent years. In particular, onedimensional ͑1D͒ ZnO nanostructures which can serve as building blocks for nanoscale devices have been intensively studied. 1-9 Room temperature ͑RT͒ ferromagnetism ͑FM͒ has been achieved in several transition metal ͑TM͒ doped ZnO nanorods, 2-4 nanowires, 6-8 and nanoneedles. 9 On the other hand, RT FM has also been observed in pure ZnO nanoparticles and thin films, 10-12 which shows that FM may be an intrinsic characteristic of ZnO with low dimensionality. Recently, Banerjee et al. 11 reported the enhancement of RT FM in ZnO particles by 900°C annealing in air and related the phenomenon to oxygen vacancy clusters. However, the study on the RT FM in ZnO thin films confirmed that the source of the FM cannot be oxygen vacancies but defects on Zn sites. 12 So far, the understanding of FM origination in ZnO is still insufficient, partly because the reports of RT FM in pure ZnO are rare. Moreover, while annealing treatments were processed in several researches, 6,11 the impact of annealing on the morphology change was seldom considered. In this letter, we report on the RT FM in ZnO nanorods. Annealing of the samples at 900°C in air for 2 h can completely transform the nanorods into twinning structures, while adjusting the intrinsic defects and reducing the saturation magnetization ͑M s ͒.ZnO nanorods were synthesized by hydrothermal method. Zn͑Ac͒ 2 ·2H 2 O and NaOH with a mole ratio of 1:10 were added to absolute ethanol and pretreated in an ultrasonic water bath for 30 min, then hydrothermally treated at 120°C for 5 h. The products were collected by centrifugation and thoroughly washed with distilled water and ethanol, and finally dried at 70°C for 10 h. Two kinds of samples were prepared with different Zn͑Ac͒ 2 ·2H 2 O concentrations of 0.0125M and 0.05M, and denoted as S1 and S2, respectively. The as-prepared samples were annealed at 900°C in air for 2 h. The structure and morphology of the samples were characterized by x-ray diffraction ͑XRD͒, scanning electron microscopy ͑SEM͒, and transmission EM ͑TEM͒.Magnetic properties were measured using an alternative gradient magnetometer. Photoluminescence ͑PL͒ studies were carried out at RT with the excited wavelength of 325 nm. Figure 1 shows the XRD patterns of the as-prepared and annealed samples. All diffraction peaks corr...

ZnO spheres and pseudospherical structures have been synthesized by a surfactant-free wet chemical method. The products were formed by a dissolution/reprecipitation mechanism. Room temperature ferromagnetism has been observed in the products, which could be affected not only by annealing but also by ultrasonic treatment. Photoluminescence measurement reveals that the ferromagnetism may relate to the defects that cause the 469 nm emission, and the products mainly show blue emission in the visible band after annealing. The low-cost and high-yield synthesis method makes the products promising materials for optoelectroic and spintronic devices.