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,
𝑁
𝜙𝑓 = 𝑁 𝑒𝑚
𝑎𝑏𝑠
𝜙𝑓 = 𝑘
𝜙𝑓 =
𝑘𝑓,𝑟
𝑓,𝑟 +𝑘𝑓,𝑛𝑟
τ𝑓,𝑟 +τ𝑓,𝑛𝑟
τ𝑓,𝑟
(1)
(2)
(3)
In these equations 𝜙𝑓 is the fluorescence quantum yield of the excited emitter, 𝑁𝑒𝑚 is the number of
emitted photons, 𝑁𝑎𝑏𝑠 is the number of absorbed photons, 𝑘𝑓,𝑟 and 𝑘𝑓,𝑛𝑟 are the radiative and
nonradiative decay rates of the excited electronic state in the units of 𝑠 −1 and τ𝑓,𝑟 and τ𝑓,𝑛𝑟 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.
𝜙´𝑓𝑚 =
𝑘𝑓,𝑟 +𝑘𝑓𝑚,𝑟
𝑘𝑓,𝑟 +𝑘𝑓𝑚,𝑟 +𝑘𝑓,𝑛𝑟 +𝑘𝑓𝑚,𝑛𝑟
(4)
In this equation 𝜙´𝑓𝑚 denotes the modified fluorescence quantum yield of the fluorophore-metal
system, 𝑘𝑓𝑚,𝑟 and 𝑘𝑓𝑚,𝑛𝑟 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.