HIGHLIGHTS OF RESEARCH

Improving blue colour emission in thin film electroluminescent devices by ion implantation

In the case of full-colour thin film electroluminescent (TFEL) devices, improving the blue colour phosphor is a key issue. At the moment, one way of achieving the blue colour is, e.g. filtering bluish-green emission of SrS:Ce phosphor by a cut-off blue filter resulting in low luminescence brightness in the blue region. By using ion implantation with, e.g. K and F ions and post heat treatment, we have been able to modify photoluminescence of the SrS:Ce films so that the spectrum shifts towards blue (about 10 nm), thus enhancing the blue portion emission. If this shift can be produced in a real electroluminescent device, this improvement would make the blue emission and white colour more satisfactory. Measurements with the electroluminescent devices will be carried out in the near future.

R. Lappalainen, J. Jokinen, W.-M. Li, M. Ritala, M. Leskelä and E. Soininen: Complementary analysis of ALE-grown SrS based thin film electroluminescent structures with ion beam methods, Nucl. Instrum. and Meth. in Phys. Res. B 132 (1997) 685-696

FIGURE 1. Luminescence excitation and emission (excited at 430 nm) spectra of a K (0.3 at.%) implanted SrS:Ce film compared to the SrS:Ce film made by the standard ALE process. Note the shift of emission spectrum towards blue (shorter wavelengths) due to potassium implantation.

FIGURE 2. Normalized emission spectra (lex = 420 nm) of fluorine implanted SrS:Ce/ATO glass sample annealed at 800 °C for 2 h and the similar reference sample without implantation. The implanted fluorine concentration is about 0.5 at.% and the Ce-doping with ALE is about 0.2 at.%.

Clouds without supersaturation

Traditional Köhler theory describes the equilibrium vapour-pressure relationship between liquid solution particles and humid air. We have recently presented the concept of multiphase-multicomponent Köhler theory, which reveals that stable cloud droplets of size 1-10 µm could exist in air with a relative humidity of less than 100%. This may explain the occurrence of persistent large-droplet fogs or smogs such as previously existed in London and which are now found in various heavily polluted locations, near the exits of chimneys and in the plumes of volcanoes.

a: Conventional Köhler curve for a 30 nm dry particle consisting of ammonium sulphate (NH4)2SO4) at 298.15 K. b: A similar particle containing additional 400 nm insoluble core (the amount of (NH4)2SO4) is the same as with a). c: Particle containing a slightly soluble calcium sulphate (CaSO4) core (radius 400 nm). d: Effect of an added, highly soluble nitric acid (HNO3) gas. Initial gas-phase concentration of nitric acid was 2 ppbv and the number concentration of aerosol particles was 500 cm­3.

M. Kulmala, A. Laaksonen, R.J. Charlson and P. Korhonen: Clouds without supersaturation, Nature 388 (1997) 336-337

Observations of solar wind plasma entry into the magnetosphere

The observations of one of the two Russian Interball satellites have revealed new information on solar wind plasma entry into the magnetosphere. The observations took place in January 1997 when a huge solar plasma cloud originating from a coronal mass ejection passed the Earth's magnetosphere. After the passage the collaborative Swedish-Finnish-Russian plasma instrument observed, for an exceptionally long time, impulsive plasma entry into the magnetosphere in a local time sector where such entry has not been reported previously. These observations are expected to have a significant impact on understanding solar plasma entry into the magnetosphere.

I. Sandahl, H. E. J. Koskinen, A. M. Mälkki, T. I. Pulkkinen, E. Yu. Budnik, A. Fedorov, A. L. Frank, J. B. Sigwarth, Dispersive magnetosheath-like ion injections in the evening sector on January 11, 1997, Geophys. Res. Lett., in press 1997

True value of the Debye mass in finite temperature QCD matter

Screening is a very important phenomenon in electrodynamics and plasma physics: if one puts a small test charge in a plasma, it attracts opposite charges around it so that seen from a long distance the whole system looks neutral, the original charge has been screened. At extremely high temperatures relevant for the very early universe, conventional electrodynamics must be extended to a more complicated non-Abelian gauge theory, containing quantum chromodynamics and electroweak theory. In these also the photon becomes effectively charged and the whole dynamics becomes much more complicated. The concept of screening has been discussed for years and an approximate result for the screening length is well known. However, a fully general formulation, permitting also a numerical computation of the result, has been lacking. This has now been accomplished, a general expression has been given and evaluated with numerical Monte Carlo simulations. The result is quite striking: in the region accessible for experiments it is actually larger by a factor of three that the known approximate result. This will have quantitative implications for both phenomenological analyses of heavy ion collisions and for cosmological phenomena.

K. Kajantie, M. Laine, J. Peisa, A. Rajantie, K. Rummukainen and M. Shaposhnikov, "Non-perturbative Debye mass in finite T QCD", Phys. Rev. Lett. 79 (1997) 3130