SPONSOR - The Slow-Positron System of Rossendorf
SPONSOR is a device for depth-resolved materials investigation with atomic resolution using Slow-Positron Doppler Broadening Spectroscopy. Positrons are generated through ß+ decays of the radioisotope 22Na and they are slowed down using a tungsten moderator foil. The obtained mono-energetic positrons are then accelerated towards the sample with typical energies in the range of 30 eV to ~ 36 keV.
Positron implantation spectroscopy
Magnetically guided positrons from a 22Na source of predetermined energies E (30 eV - 36 keV) are implanted at depths of up to a few micrometer into the sample. Then, the positrons reduce their energy within a few pico-seconds via inelastic scattering (thermalization) and diffuse through the lattice (10 ... 100 nm depending on trapping defects) until they annihilate with an electron. The resulting implantation profile can be described using an energy- and depth-depending Makhovian distribution .
The motion of positron-electron pairs prior to annihilation causes a Doppler broadening of the photopeak in the measured energy spectrum of the annihilation photons characterized by the lineshape parameters S and W. The value of S depends on the size and concentration of vacancy-like defects and is higher for an increasing amount of positrons being trapped at and annihilating in open-volume defects. The W parameter (“wing” or core annihilation parameter) is taken at the high-momentum region away from the center. Since the positron annihilation takes place with core electrons, the W parameter probes the chemical surrounding of the annihilation site. The parameters S and W are calculated as the normalized area of the curve in a fixed energy interval. The correlation between both parameters varies for different defect types.
Left: Implantation profiles for positrons in silicon for various incident energies.
Right: Definition of line shape parameters S and W for the 511 keV two-photon annihilation quanta.
Fitting routines like VEPFIT  can be used to obtain the positron diffusion length from the measured S-parameter curve. This parameter allows for further characterization of defects and can be used to determine the quality of a material. A combination of the diffusion length with the energy-dependent positron implantation profiles helps to obtain and visualize a depth-resolved defect profile.
Left: Example of an experimentally determined S parameter for varying positron implantation energies.
Right: Taking into account the depth distribution one derives a depth-dependent defect type variation (calculated with VEPFIT ) which treats the positron diffusion and trapping numerically.
Characteristics of the mono-energetic positron beam SPONSOR
- magnetically guided positron beam from an intensive 22Na source
- beam diameter: d ~ 4 mm
- accelerating potential for positrons: 30 eV to 36 keV, selectable
- annihilation energy resolution: (1.09 + 0.01) keV at 511 keV
- two Germanium detectors for coincident Doppler Broadening Spectroscopy
Schematics of the monoenergetic positron beam system SPONSOR for depth-resolved defect characterization.
- W. Anwand, G. Brauer, M. Butterling, H. R. Kissener, A. Wagner, Design and Construction of a Slow Positron Beam for Solid and Surface Investigations. Defect and Diffusion Forum, 331 (2012) 25-40. https://www.scientific.net/DDF.331.25
- A.F. Makhov, The Penetration of Electrons into Solids. 1. The Intensitiy of an Electron Beam, Transverse Paths of Electrons. Fiz. Tverd. Tela+, 2(9):1934–1941,1961.
- A. van Veen A, H. Schut, J. de Vries, R.A. Haakvoort, M.R Ijpma, Analysis of positron profiling data by means of VEPFIT, In Positron Beams for Solids and Surfaces, vol. 218. Edited by: Schultz PJ, Massoumi GR, Simpson PJ. New York: AIP; 1991:171–218.
Vacancy cluster in ZnO films grown by pulsed laser deposition
Wang, Z.; Luo, C.; Anwand, W.; Wagner, A.; Butterling, M.; Azizar Rahman, M.; Phillips, M. R.; Ton-That, C.; Younas, M.; Su, S.; Ling, F. C. C.
Undoped and Ga-doped ZnO films were grown on c-sapphire using pulsed laser deposition (PLD) at the substrate temperature of 600 oC. Positron annihilation spectroscopy study (PAS) shows that the dominant VZn-related defect in the as-grown undoped ZnO grown with relative low oxygen pressure P(O2) is a vacancy cluster (most likely a VZn-nVO complex with n=2, 3) rather than the isolated VZn which has a lower formation energy. Annealing these samples at 900oC induces out-diffusion of Zn from the ZnO film into the sapphire creating the VZn, which favors the formation
of vacancy cluster containing relatively more VZn. Increasing the P(O2) during growth also lead to the formation of the vacancy cluster with relatively more VZn. For Ga-doped ZnO films, the oxygen pressure during growth has significant influence on the electron concentration and the microstructure of the VZn-related defect. Green luminescence (GL) and yellow luminescence (YL) were identified in the cathodoluminescence study (CL) study, and both emission bands were quenched after hydrogen plasma treatment.
Keywords: Vacancy cluster; ZnO; pulsed laser deposition; positron annihilation spectroscopy; photo luminescence
Scientific Reports 9(2019), 3534
3D Local Manipulation of the Metal-Insulator Transition Behavior in VO2 Thin Film by Defect-Induced Lattice Engineering
Jia, Q.; Grenzer, J.; He, H.; Anwand, W.; Ji, Y.; Yuan, Y.; Huang, K.; You, T.; Yu, W.; Ren, W.; Chen, X.; Liu, M.; Facsko, S.; Wang, X.; Ou, X.
The ability to manipulate the metal-insulator transition (MIT) of metal oxides is of critical importance for fundamental investigations of electron correlations and practical implementations of power efficient tunable electrical and optical devices. Most of the existing techniques including chemical doping and epitaxial strain modification can only modify the global transition temperature, while the capability to locally manipulate MIT is still lacking for developing highly integrated functional devices. Here, lattice engineering induced by the energetic noble gas ion allowing a 3D local manipulation of the MIT in VO2 films is demonstrated and a spatial resolution laterally within the micrometer scale is reached. Ion-induced open volume defects efficiently modify the lattice constants of VO2 and consequently reduce the MIT temperature continuously from 341 to 275 K. According to a density functional theory calculation, the effect of lattice constant variation reduces the phase change energy barrier and therefore triggers the MIT at a much lower temperature. VO2 films with multiple transitions in both in-plane and out-of-plane dimensions can be achieved by implantation through a shadow mask or multienergy implantation. Based on this method, temperature-controlled VO2 metasurface structure is demonstrated by tuning only locally the MIT behavior on the VO2 surfaces.
Keywords: Metal–insulator transition VO2
Advanced Materials Interfaces 5(2018)8, 1701268
Investigating the structure of crosslinked polymer brushes (brush-gels) by means of Positron Annihilation Spectroscopy
Dehghani, E. S.; Aghion, S.; Anwand, W.; Consolati, G.; Ferragut, R.; Panzarasa, G.
Polymer brushes can be useful as small-scale reactors for the controlled synthesis of nanoparticles, an approach which is gaining increasing interest. In this context, chemical crosslinking of polymer brushes could be considered as a viable approach to control the size and size distribution of the formed nanoparticles. Here we describe the application of Positron Annihilation Spectroscopy (PAS) for the characterization of crosslinked polymer brushes (brush-gels). Poly(hydroxyethyl methacrylate) (PHEMA) brushes were obtained on silicon substrates by means of a surface-initiated atom transfer radical polymerization (SI-ATRP). Crosslinking was achieved during the polymerization by adding different amounts of diethyleneglycol dimethacrylate (DEGDMA) as a difunctional monomer. The resulting brushes, both un- and crosslinked, were then post-modified with carboxylic acid groups, allowing the in situ synthesis of silver nanoparticles after ion exchange with silver nitrate and reduction with sodium borohydride. The detailed characterization of such systems is notoriously challenging and PAS proved to be an effective, non-invasive technique to acquire insight on the structure of the brushes and of their nanoparticle composites.
Keywords: Polymer brushes nanoparticles Positron Annihilation Spectroscopy
European Polymer Journal 99(2018), 415-421