EXCHANGE GRANT REPORT

Transcription

1 Indo Swiss Joint Research Programme (ISJRP) Research fellowships EXCHANGE GRANT REPORT Grant No.: RF09 Part 1 - General Information Project Title: Keywords: Start date: August 1, 2009 Duration: 9 months Sub wavelength spatial resolution for fluorescent centres in solids: Use of fluorescent nano-diamonds as Scanning Nearfield Optical Microscopy probes and SNOM study of thin films of CsBr:Eu 2+ X-ray Imaging phosphor Color centers, nanodiamonds, X-ray image phosphor, thin films, scanning near-field optical Microscopy (SNOM), Part 2 - Exchange Participant(s) Details Visiting Scientist Dr. Nagamani Suriya Murthy Radiological Safety Division Indira Gandhi Centre for Atomic Research (IGCAR) Kalpakkam INDIA suriyamurthy.nagamani@epfl.ch, suriya@igcar.gov.in Hosting Scientists Prof. Giovanni Dietler, Dr. Serguei Sekatski Laboratory of Physics of Living Matter Swiss Federal Institute of Technology (EPFL) 1015 Lausanne, SWITZERLAND serguei.sekatski@epfl.ch 1

2 Part 3 - Scientific & Technical Information 3.1 Purpose of visit The visit was aimed, 1. To study the luminescence properties of colour centres generated in fluorescent nano-diamond crystals. 2. To apply FND colour centres probes for Fluorescence Resonance Energy Transfer Scanning Near field Optical Microscopy (FRET SNOM). 2. To investigate the near-field images of photo-stimulated luminescence from X-ray storage phosphor CsBr:Eu 2+ using SNOM. 3. To prepare probes for single molecule imaging. 3.2 Short description of work carried out during the visit Fluorescent Nanodiamonds: For the generation of color centres in nanodiamond (Type-1b, size:<50nm), they were irradiated with charged particles/ions from accelerator. This includes 400 KeV proton irradiation at Minsk, Russia, 1 MeV proton, 130 KeV, 500 KeV and 1 MeV Helium ions (He + ) using 1.7 KV Tandetron accelerator at IGCAR, Kalpakkam. Helium and proton irradiation experiments were conducted to produce different kinds defect centres in nanodiamonds. The samples were subsequently subjected to vacuum annealing (10-3 torr) at 800 C for 2 hrs. During this process, the migration of nitrogen vacancies had taken place that resulted in the formation of negative nitrogen vacancy (N-V - ) defect centres. Topography and near-field images of fluorescent nanodiamond [irradiated by He + ions having energy of 130 KeV] were recorded by exciting the samples at 514 nm by Ar + ion laser ( mw, Spectra Physics). A 514nm Holographic Notch filter (Kaiser Optical Systems, Inc., USA) was inserted at the entrance of PMT to cut off the excitation laser light and the fluorescence was detected by the PMT MP942 (Perkin Elmer) working in a single photon counting mode as shown in Fig.1. Fig. 1 Experimental setup of SNOM in transmission mode for Nanodiamonds. Insert: A double resonant montage of (a)-tuning fork metallic case, metal case of a tuning fork, (c) fibre probe (d) thin glass rod connecting probe and tuning fork 2

3 Synthesis and Characterisation of X-ray storage phosphor: CsBr:Eu 2+ powder samples prepared under argon, air, vacuum and hydrogen atmospheres were compared for their luminescence intensities. Optimisation experiments with respect to synthesis temperature as well as atmosphere had been carried out. Temperature dependence of photoluminescence and photstimulated luminescence was studied. Effects of coodping to explore the possibility of shifting the stimulation spectra were investigated. Role of monovalent/divalent ions in enhancing fluorescence and photostimulated luminescence of this phosphor was examined. Thermo stimulated luminescence (TSL) glow curves structure was analysed and trap parameters were estimated. Prior to PSL and TSL measurements samples were subjected to gamma irradiation (1Gy). SNOM microscope used for simultaneous recording of topography and near-field images of PSL (CsBr:Eu 2+ ) by exciting at 633 nm by He-Ne laser (3.6 mw, JDS Uniphase, USA) is shown in Fig.2. Fig.2 Schematic view of TIRFM for the study of PSL of CsBr: Eu 2+ pellet by NSOM. Insert: A double resonant montage of (a)-tuning fork metallic case, metal case of a tuning fork, (c) fibre probe (d) thin glass rod connecting probe and tuning fork 3.3 Outcomes Results of studies on Nanodiamonds: Fig.3 shows the topographical and near-field images of fluorescent nanodiamonds which has absorption at 514 nm and efficient emission at 700 nm. Though the wavelength of emission light was not recorded, the near-field image of fluorescence from nanodiamonds is patently visible. This two distinctively observed luminescence spots confirm the fluorescent emission from nanoparticles. Increasing the power of laser light did not reduce emission property of defect centres which confirms the photostability of nanodiamonds. 3

4 (a) Fig.3 (a) 2D view of Near field image of fluorescence b) 3D reconstruction of Near-field image of fluorescence from 50 nm Fluorescent Nano Diamonds. Scanning size 2000 nm x 3500 nm with high 200 nm. Results of experiments on X ray image phosphor: Phosphor samples characterised using spectro-fluorimeter revealed that, the annealing under air atmosphere at 350 C is desirable for observing efficient PL and PSL from CsBr:Eu 2+. However, the samples obtained from hydrogen and vacuum exhibited weak PL and PSL signatures. This indicates that, complete reduction of Eu 3+ to Eu 2+ is occured only under air atmosphere. PSL emission intensity (Fig.4) measured after gamma irradiation was doubled for the samples annealed in air due to enhanced population of F-centres. Our experimental observations point to fact that presence of Eu 2+ fluorescence is essential to observe PSL in this phosphor. The effect of codoping of monovalent ions had significantly altered the thermoluminescence glow curve structure (Fig.5). Presence of lithium and silver shifted the glow peak from 250 C to 200 C and to 150 C respectively. In case of divalent ions, glow peaks shifted to low temperature. Appearance of single glow peak indicates the trap profile did not change as a result of monovalent as well as divalent codoping. Dose response linearity observed upto 100 Gy shows that this phosphor has good radiation hardness. As the gamma dose increased the PSL intensity decreased probably due to conversion of Eu 2+ to Eu 3+. Interestingly, X-ray irradiation (35 KeV) produced green color centres in this phosphor. Fig.6 shows the near-field images of PSL was recorded using SNOM in reflection mode. The red colour spots are the PSL emission observed upon excitation at 633 nm. 4

5 5.5x x x10 5 (a) Air (c) (a) 4.0x10 5 PSL intensity (a.u.) 3.5x x x x x x x wavelength (nm) Argonx2 Fig.4.Stimulation spectra of CsBr:Eu 2+ (λ emi -440 nm) (a)air, Argon atmospheres. TSL intensity (a.u) (f) (g) (e) temp/ o C Fig.5. Glow curves of (a)csbr:eu 2+, doped with Mg, (c)ca,(d) Sr, (e)in, (f)li, (g)ag (d) (a) Fig. 6 (a) Near-field Optical image of CsBr: Eu 2+ and 3D representation Scanning size 8000 nm x 8000 nm During this visit, I acquired the experience in generating the defect centres in solid state materials using particle accelerators which will certainly be useful for my future dosimetric studies. Besides, this visit also provided me an opportunity to learn technical skills of Scanning near field optical microscopy. 5

6 3.4 Future collaboration with host institution This programme had been very successful due to dynamic mutual collaboration and the expertise provided by host scientists. The synergism and interesting results generated from this joint research will provide an impetus to continue this collaboration in near future. There are other areas within scope of this project where further research can be pursued. Our objectives of single molecule imaging using nano-diamonds and near-field imaging of fluorescence induced by other defect centres (H3 and N-V 0 ) are worth to mention. X ray energy response studies of CsBr:Eu 2+ using SNOM is also a potential topic to venture. In view of this, we believe that there are better prospects for continuing this on-going research and the proposed visit of Dr.Sergei Sekatski to India as a part of follow-up activities will strengthen the collaboration. 3.5 Various comments It was fascinating to do research in a motivating environment at lpmv,epfl and the visit was professionally rewarding. I enjoyed Swiss s natural beauty, pleasant weather and good hospitality. The entire arrangements for my work and stay had been excellent. Acknowledgements: I express my sincere thanks to ISJRP (Swiss) and DST (India) for awarding this fellowship. I profoundly thank Prof.Giovanni Dietler and Dr.Serguei Sekatski for guiding me during the research period. I am indebted to Director,IGCAR and Dr.P.Chellapandi, Director,Safety Group, IGCAR for their motivation and permission to carry out this advanced research in Switzerland. I am very much grateful to Dr.B.Venkataraman,Head,RSD and Dr.B.S.Panigrahi,Head,RCS for their continuous encouragement and support. Thanks are due many to Dr.K.G.M.Nair, Dr.B.Sundaravel, IGCAR,India, Kalpakkam, and Dr.A.S.Kamyshan, Institute of Applied Physics Problems, State Belorussian University, Minsk for their help in successful irradiation of samples and Dr.Kanat Dukenbayev for SNOM experiments. 3.6 Publication A manuscript titled Luminescence studies of Photo Stimulated Phosphor CsBr:Eu 2+, N.Suriyamurthy, B.S.Panigrahi, Kanat Dukenbayev, Serguei Sekatski, Giovanni Dietler, has been submitted to 16 th Symposium on Solid State Dosimetry. 6