Plasmon-coupled emission and spectral modification of photostable fluorophores Siim Pikker Table of contents LIST OF PUBLICATIONS INCLUDED IN THE THESIS LIST OF ABBREVIATIONS 1. INTRODUCTION 2. LITTERATURE OVERVIEW AND THEORETICAL BACKGROUND 2.1 Fundamental principles of fluorescence of photostable emitters 2.1.1 Fluorescence of rare earth impurity centres 2.1.2 Fluorescence of nitrogen vacancies in nanodiamonds 2.2 Fundamental principles plasmonic enhancement of emission 2.2.1 Metal enhanced fluorescence 2.2.2 Surface plasmon coupled emission 2.2.3 Fluorescence enhancing plasmonic substrates 3. MEASUREMENT OF PLASMON ENHANCED EMISSION 3.1 Sample preparation and characterization 3.1.1 Sol-gel derived rare earth doped oxides 3.1.2 Fluorescent nanodiamonds with nitrogen vacancies 3.1.3 Atomic layer deposition of doped oxide films 3.2 Experimental measurement setup 3.3 SPCE of photostable thin films 3.4 … 4. RESULTS AND DISCUSSION 4.1 Plasmonic effects on emitters 4.1.1 Plasmonic enhancement of absorption 4.1.2 Plasmonic enhancement of emission 4.1.3 Plasmon assisted exciton energy transfer SUMMARY IN ESTONIAN REFERENCES ACKNOWLEDGEMENTS PUBLICATIONS CURRICULUM VITAE LIST OF PUBLICATIONS INCLUDED IN THE THESIS 1. Dolgov, L.; Kiisk, V.; Matt, R.; Pikker, S.; Sildos, I. (2012). Tailoring of the spectral–directional characteristics of rare-earth fluorescence by metal–dielectric planar structures. Applied Physics B-Lasers and Optics, 107(3), 749 - 753. 2. Dolgov, L.; Kiisk, V.; Reedo, V.; Pikker, S.; Sildos, I.; Kikas, J. (2011). Sensitizing of Sm3+ fluorescence by silver dopant in the TiO2 films (article). Central European Journal of Physics, 542 - 546. 3. Kiisk, Valter; Kangur, Triin; Paalo, Madis; Tätte, Tanel; Lange, Sven; Pikker, Siim; Sildos, Ilmo (2011). Structural and luminescence characteristics of SnO2:Eu and SnO2:Eu,Sb nanophosphors upon annealing at high temperatures. Materials Chemistry and Physics, 130(1-2), 293 - 298. 4. Dolgov, L.; Reedo, V.; Kiisk, V.; Pikker, S.; Sildos, I.; Kikas, J. (2010). Structure and fluorescent properties of TiO2:Sm3+–Ag composite. Optical Materials, 32, 1540 - 1544. LIST OF ABBREVIATIONS Bg LSP CCD MEF SERS SPCE SPASER SP RT Band-gap Localized surface plasmon Charge-coupled device Metal enhanced fluorescence Surface-enhanced Raman spectroscopy Surface plasmon-coupled emission Surface plasmon amplification by stimulated emission Surface plasmon Room temperature 1. INTRODUCTION The optically generated collective electron density waves on metal-dielectric boundaries known as surface plasmon polaritons or simply surface plasmons have been of great scientific interest since their discovery.  Being electromagnetic waves themselves, surface plasmons and localized surface plasmons (SP-s localized on small metallic features) interact strongly with light and give rise to the vibrant colours in gold and silver colloids. This effect is caused by the strong absorption and scattering of light with plasmonic particles.  Resonant generation of SP waves can give rise to very high electric fields in the vicinity of the evanescent plasmon waves. These strong electric fields near the metal surfaces have been used in novel applications like surface enhanced Raman spectroscopy, metal enhanced fluorescence, plasmonic lithography, plasmonic trapping of particles etc. Resonant coupling of surface plasmons to fluorescent emitters can strongly modify the emitted intensity, the angular distribution and the polarization of the emitted radiation and even the spontaneous radiative decay times of the luminescent emitters. The phenomenon of radiative lifetime shortening is most intriguing as it causes enhanced emission and increased photostability of fluorophores and is therefore of high practical importance. More exotic applications of plasmon coupled emissions are the loss compensation of surface plasmon waveguides, SPASER-s, plasmon assisted lasing, single molecule fluorescence measurements, SPCE in biological sensing, optical qbit designs etc. Plasmonics is an exponentially growing research subject and so it is understandable that also the number of different applications is exponentially growing. The subject of this thesis is the spectroscopic study of the plasmon coupled photostable emitters. Fluorescent emitters have long been of enormous scientific and practical importance with applications in various scientific disciplines and in everyday life e.g.in lighting and display technologies. There exists also a growing need for new stable, brighter and more efficient fluorescent emitters and devices as traditionally used fluorescent organic molecules tend to suffer of strong photo-bleaching behaviour and sometimes unwanted photoblinking. In this thesis I study plasmonically coupled photostable rare earth dopant containing inorganic matrices and fluorescent nitrogen vacancies in nanodiamond hosts. … 2. LITTERATURE OVERVIEW AND THEORETICAL BACKGROUND 2.1 Fundamental principles of fluorescence of photostable emitters 2.1.1 Fluorescence of rare earth impurity centres Rare earth elements (Tablex) can form stable compounds with partially filled electronic shells, which give them unique optical properties. These partially filled d and f shells can undergo spectrally narrow electronic transitions and cover a spectral range from the vacuum-ultraviolet to the far-infrared. These narrow optically active transitions are basis for a whole range of practical applications and cover also the visible spectrum. In rare earth compounds the 4f electrons are localized near the ion and as they are also “shielded” by fully occupied shells by 5s and 5p electrons. In rare earths the lowest energy 4f electrons are spatially not the outermost ones. This unique feature gives the rare earths the atom like narrow spectral lines as they interact with the host matrix only weakly. The transparent host matrixes implanted with trivalent rare-earth impurities are of great potential in the fields of laser gain media, luminescent materials and in waveguide and sensory applications. In the present study we look predominantly at the Sm3+ doped TiO2 system and analyse the modification of the radiative properties by plasmonic interaction with silver nanoparticles and smooth gold film and core-shell nanoparticles. To understand the fluorescent system we draw the energy scheme of Sm3+ doped TiO2 system. [joonis viide Brik] In FIGX and FiGY you can see the high resolution low-temperature (10 K) photoluminescence spectra of Sm3+ doped TiO2 with anatase and rutile structure. 2.1.2 Fluorescence of nitrogen vacancies in nanodiamonds Although defect free pure diamond is a wide band-gap semiconductor (with Bg = 5.5 eV) and is colourless in the visible spectrum, defects and incorporated impurity ions can form more than 500 identified colour centres. Diamond is known to form optically active centres with many enclosed chemical elements, including Ag, As, B, Co, Cr, H, He, Li, N, Ne, Ni, O, P, Si, Zn, Zr, Ti, Tl, W and Xe. [Viide rm] In the entire list of colour centres nitrogen impurities have probably gained the most scientific interest. Although nitrogen can form different complexes in diamond, two of the most important and most studied fluorescent complexes are the charge neutral nitrogen vacancy complex NV0 and the negatively charged nitrogen vacancy complex NV-. The NV- state has been under intense scientific interest, because of its unique electronic structure, which enables room temperature control over its spin states, it has a long spin coherence time and it is suitable for a single photon emission. These properties make the NV- complex an extraordinary research subject for quantum computing experiments. (viide raamat 2001) Figure NRX shows the excitation spectra of the NV0 and NV- complexes. The NV- complex shows a strong purely electronic transition line at 637 nm (1.945 eV), which is the zero phonon line (ZPL). The ZPL is associated to be the dipole transition between the electronic ground state 3A2 and the excited state 3E. [Viide] The NV0 complex has a strong ZPL at 575 nm (2.156 eV). This transition has been associated to be the transition between the ground state 2E and the excited state 2A1. [Viide] The zero phonon lines of the NV0 and NV- complexes are both accompanied with noticeable phonon sidebands in the excitation and emission spectra. As shown in figure NRY the room temperature fluorescence spectra of nitrogen containing diamond film exhibits both ZPL-s. Normally both charge states exist in the same sample. Both states are present also in small nanoparticle samples and there are recent reports of photochromic switching between the NV0 and NV- charge states. [viide] 2.2 Fundamental principles plasmonic modification of fluorescence 2.2.1 Surface enhanced fluorescence Fluorescence is a luminescence process, where a light induced excitation within a substance will decay via emission of light. It is also called photoluminescence. Fluorescence is the light emission and electronic relaxation from an excited singlet electronic state and differs from phosphorescence, where the excited state is a triplet. [Viide] The process of fluorescence (the absorption and emission of light with some intermediate energy transfer and loss mechanisms) can be described by the Jabłoński diagram formalism. [Viide] Figure XXX shows a simplified Jabłoński diagram with the processes involved in the fluorescence phenomena. The different stages involved in the light energy absorption, transformation and emission are described by rate equations. The fluorescence and internal conversion processes are also described by the internal conversion lifetime τic and the fluorescence lifetime τf. The fluorescence lifetime is defined as the average relaxation time from the excited electronic site to the electronic ground state. The typical lifetimes for fluorescence in molecules are in the order of nanoseconds and the internal conversion times are in the picosecond scale. The overall fluorescence quantum yield can be used to describe an emitter’s efficiency for radiative decay. The fluorescence quantum yield is defined by the ration of the number of emitted photons to the number of absorbed photons. Three equivalent equations for the calculation of fluorescence quantum yield for an isolated emitter are given with eq 1, 2 and 3, = = = , , +, τ, +τ, τ, (1) (2) (3) In these equations is the fluorescence quantum yield of the excited emitter, is the number of emitted photons, is the number of absorbed photons, , and , are the radiative and nonradiative decay rates of the excited electronic state in the units of −1 and τ, and τ, are the radiative and nonradiative decay times of relaxation processes for the fluorophore . It is known that metal structures (nanoparticles, nanostructures and thin films) can modify the relaxation process by modifying the local density of states for the electromagnetic interaction. [viide] In a simplified model one can think of it so that for an excited fluorescent emitter extra decay channels are formed. The excited fluorescent emitter without the nearby metal surface has the possibility for radiative or nonradiative decay. The addition of a metal surface generates two extra decay paths. There exists now a channel for metal mediated radiative relaxation and also for metal mediated nonradiative decay. The metal modified system also has now an altered overall fluorescence quantum yield. ´ = , +, , +, +, +, (4) In this equation ´ denotes the modified fluorescence quantum yield of the fluorophore-metal system, , and , are the metal mediated radiative and nonradiative decay rates of the fluorophore. 2.2.2 Surface plasmon coupled emission 2.2.3 Fluorescence enhancing plasmonic substrates 3. MEASUREMENT OF SPCE RADIATION 3.1 Sample preparation and characterization For this thesis three general types of different fluorophores were studied. First types of investigated structures were organic dye doped polymer thin films and the measurements of those structures were done as a reference to previously published data. The second type of nanostructures that were investigated consisted of rare earth ion doped metal oxide thin films, which were supported on thin metallic films or contained various types of metal nanoparticles. Both the metal films and the nanoparticles incorporated into the oxide films were chosen to support surface or localized plasmon waves. The third type of fluorophores were NV complex containing fluorescent nanodiamonds, which were placed on a commercially available nanostructured gold substrate, which supports plasmon waves and are used in surface enhanced Raman spectroscopy. 3.1.1 Sol-gel derived rare earth doped oxides The exact procedures of the fabrication of rare earth ion doped metal oxide samples are described in detail in the referred articles included in the thesis. Here I describe only the fundamental fabrication technique of the sol-gel process used for the production fluorescent rare earth doped metal oxides. Host titanium dioxide was prepared by sol-gel method with hydrolysis and polycondensation of Ti(OBu)4 (Alfa Aesar, 98 wt %). The precursor was obtained after mixing of distilled water and n-butanol (YAKEMIA OY) with Ti(OBu)4 (mole ratio 1:24:1.6) and subsequent stirring for 1 h at 21°C. This was followed by incorporation of commercially available Ag nanoparticles (Nanoamor) or the silica-gold core-shell nanoparticles fabricated by us. [VIITED] The next step involved the doping of the nanoparticle containing precursor with samarium salt Sm(NO3)2·H2O (1 mol % or 2 wt % relatively to the weight of the TiO2 film). 3.1.2 Fluorescent nanodiamonds with nitrogen vacancies Investigations of the plasmon modified emission of fluorescent NV centre containing nanodiamonds were done using commercially available fluorescent nanodiamonds (FND) with the size of 140 nm. The nanodiamonds were purchased from (...) . The FND-s had SPECIFICATIONS. The FND-s were drop and spin coated onto commercially available micro- and nanostructured KLARITE 303 surface enhanced Raman spectroscopy (SERS) substrates. The substrates consist of lithographically structured Si wafer, which was covered with a thin roughened Au film. The structures consist of ordered inverted pyramidal features as depicted on FIG XXX FIG XXX. A scanning electron micrograph of the nano and microstructured KLARITE 303 SERS substrate used in the plasmon coupled emission experiments with NV centres in nanodiamonds. It was found that the traditional spin coating technique was more suitable for the placement of FND-s into the pyramidal microstructures. The slow drying process with the simple drop coating method was not suitable for the placement of the FND-s into the cavities of the structure, because of the surface tension induced self-organisation of the FND-s. As a result of the slow drop coating method most of the FND-s were placed in the areas between the resonant cavities. 3.2 Experimental measurement setup 3.2.1 3.3 SPCE of photostable thin films 3.4 Spectral reshaping of NV centre luminescence 3.4.1 Single nanodiamond spectral reshaping A) B) C) Fig XXX. A) Depicts a fluorescence microscopy image of a structured Klarite ssubstrate area containing only a two nanodiamonds. B) Depicts a fluorescence microscopy image of an unstructured region of the Klarite substrate covered with FND-s. C) depicts the recorded fluorescence spectra of the FND-s in the centre of images A) and B). Fig XXX. Fluorescence spectra of single nanoparticles deposited on the smooth Au regions of the Klarite SERS substrate. Although the intensities vary from particle to particle the spectral shape is unchanged. The only particle, whose spectrum is noticeably different from the others, is for the particle no 6. Their spectrum corresponds to a higher proportion of the NV0 centres inside the particle. All spectra contain the distinctive ZPL features for the NV0 and NV- charge states. The sharp line at ca 810 nm is a unfiltered laser line. The inset shows the fluorescent image obtained using λex=532 nm and shows the nanoparticles for the corresponding spectra. 3.4.2 Spectral reshaping of ensemble of nanodiamonds A) B) Fig XXX. A) A microscope image of a structured region of the Klarite substrate half of which is covered with NV centre containing FND-s and has a greyscale overlay map of the fluorescence intensity. B) A microscope image of a smooth region of the Klarite substrate half of which is covered with NV centre containing FND-s and has a greyscale overlay map of the fluorescence intensity. The scalebar in both images is 20 μm. FIG XXX Modification of NV centre luminescence spectra of fluorescent nanodiamonds. The black spectrum depicts the luminescence of nanodiamonds placed on the smooth Au film region on the Klarite substrate. The blue modified spectrum depicts shows a modified shape. The strong modifications overlap with the plasmon associated features in the reflectance measurements of the Klarite substrate. 4. RESULTS AND DISCUSSION SUMMARY IN ESTONIAN REFERENCES ACKNOWLEDGEMENTS PUBLICATIONS CURRICULUM VITAE List of publications 1. Oja Acik, I.; Dolgov, L.; Krunks, M.; Mere, A.; Mikli, V.; Pikker, S.; Loot, A.; Sildos, I. (2014). Surface plasmon resonance caused by gold nanoparticles formed on sprayed TiO2 films. Thin Solid Films, 553, 144 - 147. 2. Repän, T.; Pikker, S.; Dolgov, L.; Loot, A.; Hiie, J.; Krunks, M.; Sildos, I. (2014). Increased efficiency inside the CdTe solar cell absorber caused by plasmonic metal nanoparticles. Energy Procedia, 1 - 5. [ilmumas] 3. Heinsalu, S; Dolgov, L; Mamykin, S; Pikker, S; Utt, K; Sildos, I (2013). Core-shell silica-gold nanoparticles for control of fluorescence in the sol-gel films activated by rare earth. International Conference Functional Materials and Nanotechnologies Tartu, Estonia April, 21 – 24, 2013. , 2013, PO19 - PO19. 4. Dolgov, L; Heinsalu, S; Kiisk, V; Pikker, S; Repan, T; Sildos, I; Mamykin, S; Oja Acik, I; Mere, A; Krunks, M; Mikli, V (2013). Noble metal nanoparticles for improvement of fluorescent and photovoltaic materials. Summer school International research and practice conference: Nanotechnology and Nanomaterials (Nano-2013), 25.08-01.09.2013, Bukovel, Ukraine. (Toim.) L. Yatsenko. Eurosvit, 2013, 352 - 352. 5. Heinsalu, S; Dolgov, L; Mamykin, S; Pikker, S; Kiisk, V; Sildos, I (2013). Using of gold nanoparticles for plasmonically enhanced fluorescence of titania films activated by rare earth. Summer school International research and practice conference: Nanotechnology and Nanomaterials (Nano2013), 25.08-01.09.2013, Bukovel, Ukraine. (Toim.) L. Yatsenko. Eurosvit, 2013, 352 - 352. 6. Loot, A; Dolgov, L; Pikker, S; Lõhmus, R; Sildos, I (2013). Goniometric Setup for Plasmonic Measurements and Characterization of Optical Coatings. O. Fesenko, L. Yatsenko, M. Brodin (Toim.). Springer Series (119 - 134).Springer 7. Kelp, G.; Tätte, T.; Pikker, S.; Lõhmus, R.; Mändar, H.; Rozhin, A.; Hanschmidt, K.; Mäeorg, U.; Natali, M.; Persson, I. (2013). Structure and properties of nanocolloidal SnO2 watersols applied in preparation of optical quality micro- and nanosheres. FM&NT-2013, Tartu, Eesti, 21.-24. aprill 2013. , 2013. 8. Dolgov, L.; Kiisk, V.; Pikker, S.; Loot, A.; Sildos, I. (2012). Fluorescence of TiO2:Sm3+ composite Stimulated by Plasmon Waves (abstract). International conference "Functional materials and nanotechnologies 2012", April 17-20, 2012, Riga. , 2012, 61 - 61. 9. Repän, T.; Pikker, S.; Dolgov, L. (2012). Numerical simulations of planar metal-dielectric plasmonactive systems. Summer school "Nanotechnology: from fundamental research to innovations", 26 August - 2 September 2012, Bukovel, Ukraine. Eurosvit, 2012, 42 - 42. 10. Loot, A.; Pikker, S.; Dolgov, L. (2012). Automatized setup for plasmonic measurements based on prism coupling technique. Summer school "Nanotechnology: from fundamental research to innovations", 26 August - 2 September 2012, Bukovel, Ukraine. Eurosvit, 2012, 40 - 40. 11. Dolgov, L.; Kiisk, V.; Pikker, S.; Loot, A.; Sildos, I. (2012). Fluorescence of TiO2:Sm3+ composite stimulated by plasmon waves. IOP Conference Series: Materials Science and Engineering, 38, (012006-1) - (012006-6). 12. Dolgov, L.; Kiisk, V.; Matt, R.; Pikker, S.; Sildos, I. (2012). Tailoring of the spectral–directional characteristics of rare-earth fluorescence by metal–dielectric planar structures. Applied Physics B-Lasers and Optics, 107(3), 749 - 753. 13. Kiisk, Valter; Kangur, Triin; Paalo, Madis; Tätte, Tanel; Lange, Sven; Pikker, Siim; Sildos, Ilmo (2011). Structural and luminescence characteristics of SnO2:Eu and SnO2:Eu,Sb nanophosphors upon annealing at high temperatures. Materials Chemistry and Physics, 130(1-2), 293 - 298. 14. Kommel, L.; Mikli, V.; Traksmaa, R.; Saarna, M.; Pokartilov, A.; Pikker, S.; Kommel, I. (2011). Influence of the SPD Processing Feartures on the Nanostructure and Properties of a Pure Niobium. In: Nanomaterials by Severe Plastic Deformation: NanoSPD5: 5th International Conference on Nanomaterials by Severe Plastic Deformation held in Nanjing, China on March 21-25, 2011. (Toim.) Jing Tao Wang, Roberto B. Figueiredo, Terence G. Langdon. Trans TechScitec Publications Ltd, 2011, (Materials Science Forum ; 667-669), 785 - 790. 15. Kiisk, V.; Kangur, T.; Paalo, M.; Tätte, T.; Pikker, S.; Sildos, I. (2011). Optical characterization of sol-gel-derived SnO2:Eu nanopowders annealed at high temperatures. physica status solidi (c), 8(9), 2641 - 2644. 16. Dolgov, L.; Kiisk, V.; Reedo, V.; Pikker, S.; Sildos, I.; Kikas, J. (2011). Sensitizing of Sm3+ fluorescence by silver dopant in the TiO2 films (article). Central European Journal of Physics, 542 - 546. 17. Dolgov, L.; Reedo, V.; Kiisk, V.; Pikker, S.; Sildos, I.; Kikas, J. (2010). Structure and fluorescent properties of TiO2:Sm3+–Ag composite. Optical Materials, 32, 1540 - 1544. 18. Dolgov, L.; Kiisk, V.; Reedo, V.; Pikker, S.; Sildos, I.; Kikas, J. (2010). Sensitizing of Sm3+ fluorescence by silver dopant in the TiO2 films. Functional Materials and nanotechnologies 2010. (Toim.) Sternbergs, A.; Muzikante, I.; Grinberga, L.., 2010, 172.