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DISSERTATIONS | JUHA-MATTI HUUSKO | FABRICATION AND CHARACTERIZATION OF... | No 293
SEMYON CHERVINSKII
FABRICATION AND CHARACTERIZATION OF SELF- ASSEMBLED PUBLICATIONS OF
THE UNIVERSITY OF EASTERN FINLAND
This Thesis is dedicated to the investigation of the optical and morphological properties
of silver nanostructures, which are self- assembled on the surface or in the volume
of ion-exchanged glass. The technique for two-dimensional self-arrangement of silver nanoislands on the glass surface was developed. Two approaches to the modification
of the resonant properties of nanoparticles were studied. It was also demonstrated that silver nanoisland films can be used in surface-
enhanced Raman spectroscopy.
SEMYON CHERVINSKII
FABRICATION AND CHARACTERIZATION OF SELF-ASSEMBLED GLASSY PLASMONIC NANOSTRUCTURES
Semyon Chervinskii
FABRICATION AND CHARACTERIZATION OF SELF-ASSEMBLED GLASSY PLASMONIC NANOSTRUCTURES
Publications of the University of Eastern Finland Dissertations in Forestry and Natural Sciences
No 293
University of Eastern Finland Joensuu
2017
Academic Dissertation
To be presented by permission of the Faculty of Science and Forestry for public examination in the Auditorium AG100 in Agora Building at the University of Eastern Finland, Joensuu, on December 8, 2017, at 12
o’clock noon.
Grano Oy Jyväskylä, 2017
Editors: Pertti Pasanen, Matti Vornanen, Jukka Tuomela, Matti Tedre
Distribution: University of Eastern Finland Library / Sales of publications
P.O.Box 107, FI-‐‑80101 Joensuu, Finland tel. +358-‐‑50-‐‑3058396
www.uef.fi/kirjasto ISBN: 978-‐‑952-‐‑61-‐‑2673-‐‑9 (nid.) ISBN: 978-‐‑952-‐‑61-‐‑2674-‐‑6 (PDF)
ISSNL: 1798-‐‑5668 ISSN: 1798-‐‑5668 ISSN: 1798-‐‑5676 (PDF)
Author’s address: Semyon Chervinskii University of Eastern Finland
Department of Physics and Mathematics P.O.Box 111
80100 JOENSUU FINLAND email: semen.chervinskii@uef.fi
Supervisors: Professor Yuri Svirko University of Eastern Finland
Department of Physics and Mathematics P.O.Box 111
80100 JOENSUU FINLAND email: yuri.svirko@uef.fi
Professor Seppo Honkanen University of Eastern Finland
Department of Physics and Mathematics P.O.Box 111
80100 JOENSUU FINLAND email: seppo.honkanen@uef.fi
Professor Andrey Lipovskii St. Petersburg Academic University Department of Physics and Technology of Nanoheterostructures,
194021 ST. PETERSBURG RUSSIA email: lipovsky@spbau.ru
Reviewers: Professor Zhipei Sun Aalto University
Department of Electronics and Nanoengineering Tietotie 3
02150 ESPOO FINLAND email: zhipei.sun@aalto.fi
Senior scientist Vladimir G. Melekhin
Ioffe Institute of Russian Academy of Science Centre of Nanoheterostructure Physics 26 Politekhnicheskaya
194021 ST. PETERSBURG RUSSIA email: melvol@hv.ioffe.rssi.ru
Opponent: Professor Amin Abdolvand University of Dundee
School of Science and Engineering Harris Bldg. 1.3
DD1 4HN DUNDEE SCOTLAND, UK email: a.abdolvand@dundee.ac.uk
Chervinskii, Semyon
Fabrication and characterization of self-‐‑assembled glassy plasmonic nanostructures.
Joensuu: University of Eastern Finland, 2017 Publications of the University of Eastern Finland Dissertations in Forestry and Natural Sciences 2017; 293 ISBN: 978-‐‑952-‐‑61-‐‑2673-‐‑9 (print)
ISSNL: 1798-‐‑5668 ISSN: 1798-‐‑5668
ISBN: 978-‐‑952-‐‑61-‐‑2674-‐‑6 (PDF) ISSN: 1798-‐‑5676 (PDF)
ABSTRACT
This Thesis is dedicated to the investigation of the optical and morphological properties of silver nanostructures, self-‐‑assembled on the surface or in the volume of ion-‐‑exchanged glass.
The structures were fabricated by enriching the subsurface layer of the soda-‐‑lime glass with silver ions using ion-‐‑exchange in AgxNa1-‐‑xNO3 (x = 0.01– 0.15) melt. The followed annealing in hydrogen resulted in formation of silver nanoparticles on the surface and (after a longer annealing time) in the subsurface layer of the silver-‐‑
enriched glass.
The most important result of this Thesis is the development of a technique for two-‐‑dimensional self-‐‑arrangement of silver nanoislands. In order to achieve this, the silver ions were redistributed within the subsurface layer through thermal poling of the glass with a profiled anodic electrode. By varying the shape and periodicity of the electrode, and the mode of the processing, it was possible to control the nanoislands’ size and to combine them in groups. This opens a way to create plasmonic molecules on the glass surface composed of one, two, three, or even more nanoparticles that are self-‐‑arranged in the prescribed fashion.
Another remarkable result is the reshaping of the nanoparticles in the subsurface layer under irradiation with femtosecond optical pulses. The transformation of the nanoparticles’ shape from spherical to spheroidal drastically affected the optical properties of the glass-‐‑metal nanocomposite. The model of this composite was built;
this allowed to reveal the dependence of the modified nanoparticles’ aspect ratio on the laser processing conditions.
The optical properties of silver nanoisland films on the glass surface were modified by coating them with a highly refractive amorphous TiO2 film. The coating red-‐‑shifted the surface plasmon resonance wavelength providing also additional protection of the nanoislands from the environment. The observed enhancement of the Raman signal from the molecules deposited on the silver nanoisland films creates opportunities to use them in surface-‐‑enhanced Raman spectroscopy.
ii
543.422.3, 543.424.2
CAB Thesaurus: surface plasmons, nanotechnology, ion exchange, poling, optics, photonics, silver, nanoparticles
ACKNOWLEDGEMENTS
I would like to express my sincere gratitude to my supervisors Prof. Yuri Svirko, Prof. Seppo Honkanen, and Prof. Andrey Lipovskii for their guidance in my PhD studies as well as in non-‐‑work-‐‑related matters. I consider myself extremely lucky to have so many advisers with different expertise, because there was always somebody to ask.
I wish to acknowledge the everlasting support in the experiments from Dr. Pertti Pääkkönen, Dr. Janne Laukkonen, and Dr. Tommi Itkonen. Their ability to explain obvious things patiently and difficult ones simply, which allowed me to learn a lot about performing experiments inside and outside the cleanroom, while their constant efforts on better organization of these experiments and on the equipment maintenance made this experience just great. The equally continuous support in administrative and organizational matters from Mrs. Hannele Karppinen, Mrs. Katri Mustonen, Dr. Noora Heikkilä, and Mrs. Marita Ratilainen can also not be underestimated. Being myself not the most accurate and organized person, I greatly appreciate the help of this team of our Department. To not forget Mr. Timo Vahimaa who, through his ‘invisible’ help with IT, literally connected me with all the above mentioned. I am thankful to the Prof. Seppo Honkanen and Prof. Timo Jääskeläinen, who led our team as the Head of the Department of Physics and Mathematics during my time here and who granted me the opportunity to work in such a nice environment.
I would also like to thank all my colleagues, who helped me in my studies as well as in basic practical matters. First of all, I would like to mention the nanocarbon-‐‑team which welcomed me to join both working and non-‐‑working activities from my first day on, and from whom I learned a lot about everyday life at the University and outside – Dr. Mikhail Petrov, Dr. Tommi Kaplas, Dr. Petr Obraztsov, and Dr.
Vyatcheslav Vanyukov. I greatly appreciate the gang of former and current PhD students whose company I enjoyed during my years here, it is impossible to list all, but I would like to mention at least Dr. Feruza Tuyakova, Mr. Salman Daniel, Mr.
Dmitrii Klyukin, Mr. Lutful Ahad, and Mr. Sergey Malykhin. I am thankful to Dr.
Olga Svirko for the help with the thin film depositions, to Dr. Victor Prokofiev for the interesting stories not only about experiments, and to Dr. Niko Penttinen for the introduction in ellipsometry. I would like to separately greet Dr. Antti Matikainen, with whom we started very fruitful studies on Raman scattering and who taught me how to operate ALD. And most of all I would like to acknowledge this cheerful guy with whom I started my research as an undergraduate, who contributed to most of my results and made it funnier both inside and outside the University, Mr. Igor Reduto.
I treasure the opportunities of collaboration in many laboratories, it greatly enhanced the variety and quality of my results and improved my competence. I am thankful to the group at the University of Southampton, headed by Prof. Peter
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the opportunity to gain my knowledge about the laser modification. I am grateful to the group of Prof. Martti Kauranen at Tampere University of Technology, and most of all to Mr. Kalle Koskinen and Dr. Godofredo Bautista, for sharing with me their outstanding expertise in second-‐‑harmonic generation. I am thankful to the group of Prof. Pavel Belov at ITMO University, first of all, to Dr. Anton Samusev, for fruitful collaboration, discussions, and constant motivation for finding new ways of thinking about my samples. Last but not least I wish to thank the group from my alma mater headed by Prof. Andrey Lipovskii, where I started my research in the field of plasmonics, and particularly to Dr. Alexey Redkov, Mr. Alexander Kamenskii, Mr.
Sergey Scherbak, and Mrs. Ekaterina Babich, for continuous and prosperous collaborative studies.
Besides all the bright individuals forming the team of our Department, I would like to acknowledge the hospitable people and environment at the whole University of Eastern Finland, and more than that – the whole Finnish society. I learned a lot from this country and admire many things here – not only nature, but also organization of work, particularly almost full absence of bureaucracy.
Regarding the Thesis, I am very grateful to my honorable reviewers Prof. Zhipei Sun and Dr. Vladimir Melekhin for their valuable remarks to the manuscript and to my friend Mrs. Rahel Beil, who put hundreds of forgotten articles in their places in the text.
Last but not least I would like to thank my friends who always supported me in various ways during my career. I am happy to have a brother, whose progress always encouraged me to try harder. And of course, I wish to thank from the bottom of my heart my parents and grandparents who always pushed me to be a better person and supported me even if it was not always clear what I am doing.
Joensuu, November 5, 2017 Semyon Chervinskii
LIST OF ABBREVIATIONS
AFM atomic force microscopy ALD atomic layer deposition GMN glass-‐‑metal nanocomposite SEM scanning electron microscopy
SERS surface-‐‑enhanced Raman spectroscopy SPR surface plasmon resonance
TEM transmission electron microscopy
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LIST OF ORIGINAL PUBLICATIONS
This thesis is based on data presented in the following articles, referrred to by the Roman Numerals I-‐‑V.
I S. Chervinskii, I. Reduto, A. Kamenskii, I. Mukhin, A.A. Lipovskii “2D-‐‑
patterning of self-‐‑assembled silver nanoisland films” Faraday Discussions 186, 107-‐‑121 (2016).
II S. Chervinskii, R. Drevinskas, D. V. Karpov, M. Beresna, A. A. Lipovskii, Yu.
P. Svirko & P. G. Kazansky “Revealing the nanoparticles aspect ratio in the glass-‐‑metal nanocomposites irradiated with femtosecond laser” Scientific Reports 5, 13746 (2015); 6, 18522 (2016).
III A. Redkov, S. Chervinskii, A. Baklanov, I. Reduto, V. Zhurikhina and A.
Lipovskii “Plasmonic molecules via glass annealing in hydrogen” Nanoscale Research Letters 9, 606 (2014); 10, 201 (2015).
IV S. Chervinskii, V. Sevriuk, I. Reduto, and A. Lipovskii, “Formation and 2D-‐‑
patterning of silver nanoisland film using thermal poling and out-‐‑diffusion from glass”, Journal of Applied Physics 114, 224301 (2013).
V S. Chervinskii, A. Matikainen, A. Dergachev, A. Lipovskii, and S. Honkanen
“Out-‐‑diffused silver island films for surface-‐‑enhanced Raman scattering protected with TiO2 films using atomic layer deposition” Nanoscale Research Letters 9, 398 (2014).
The above mentioned publications have been included at the end of this Thesis with their copyright holders’ permission.
Besides these publications, the author has the following articles which are not included in this Thesis
• K. Koskinen, A. Slablab, S. Divya, R. Czaplicki, S. Chervinskii, M.
Kailasnath, P. Radhakrishnan, and M. Kauranen “Bulk second-‐‑harmonic generation from thermally evaporated indium selenide thin films” Optics Letters 42 (6), 1076 (2017).
• S. Scherbak, N. Kapralov, I. Reduto, S. Chervinskii, O. Svirko, A. Lipovskii
“Tuning Plasmonic Properties of Truncated Gold Nanospheres by Coating”
Plasmonics 6, 1903–1910 (2017).
• E.S. Babich, S.A. Scherbak, F. Heisler, S.D. Chervinskii, A.K. Samusev, A.A.
Lipovskii. “Dark-‐‑field spectroscopy of plasmon resonance in metal nanoislands: effect of shape and light polarization”. Journal of Physics:
Conference Series 769, 012040 (2016).
• M. Som, S. Majumdar, N. Bachhar, G. Kumaraswamy, G.V.P. Kumar, V.N.
Manoharan, S. Kumar, M.G. Basavaraj, S. Kulkarni, R. Bandyopadhyay, S.
Punnathanam, H. Medhi, A. Srivastav, D. Frenkel, M. Tripathy, E. Eiser, L.
Gonzalez-‐‑Garcia, P.R. Chowdhury, J. Singh, V. Sridurai, A. Edwards, B.L.V.
Prasad, A.K. Singh, M. Bockstaller, N.S. John, J. Seth, M. Misra, C.
Chakravarty, V. Shinde, R. Bandyopadhyaya, J. Jestin, R. Poojari, N. Kotov, O. Gang, A. Karim, Y. Ju-‐‑Nam, S. Granick, S. Chervinskii and A. Tao
“Synthesis of Nanoparticle Assemblies: general discussion” Faraday Discussions 186, 123-‐‑152 (2016).
• I.V. Reduto, S.D. Chervinskii, A.N. Kamenskii, D.V. Karpov, A.A. Lipovskii
“Self-‐‑organized growth of small arrays of metal nanoislands on the surface of poled ion-‐‑exchange glasses” Technical Physics Letters 42 (1), 93-‐‑95 (2016).
• F. Heisler, E. Babich, S. Scherbak, S. Chervinskii, M. Hasan, A. Samusev, A.A. Lipovskii “Resonant optical properties of single out-‐‑diffused silver nanoislands” Journal of Physical Chemistry C 119 (47), 26692–26697 (2015).
• E.S. Piliugina, F. Heisler, S.D. Chervinskii, A.K. Samusev, A.A. Lipovskii
“Control of surface plasmon resonance in out-‐‑diffused silver nanoislands for surface-‐‑enhanced Raman scattering” Journal of Physics: Conference Series 661, 012034 (2015).
• I. Reduto, S. Chervinskii, A. Matikainen, A. Baklanov, A. Kamenskii, and A.
Lipovskii, “SERS-‐‑applicable silver nanoisland films grown under protective coating”, Journal of Physics: Conference Series 541, 012073 (2014).
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AUTHOR’S CONTRIBUTION
The author carried out the nanostructures fabrication in all the papers, and developed the technique allowing predetermined growth of 2D silver structures on the glass surface described in Papers I, III and IV. Besides that, the author performed optical absorbance measurements and analysis for papers I, II, IV and V. The author was significantly involved in microscopic studies and analysis in papers I and III-‐‑V, and participated in the laser modification (paper II) and atomic layer deposition experiments (paper V). The manuscripts for papers I, II, IV and V were prepared with main input by the author.
CONTENTS
INTRODUCTION ... 1
1
PLASMONS IN METAL-DIELECTRIC STRUCTURES ... 5
1.1
Plasma frequency ... 5
1.2
Surface plasmon polaritons at metal-dielectric interface ... 7
1.3
Localized surface plasmons ... 8
1.4
Effective media approximation for nanocomposite ... 10
2
EXPERIMENTAL TECHNIQUES ... 11
2.1
Plasmonic nanostructures fabrication and modification ... 11
2.1.1 Glass-metal nanocomposite ... 11
2.1.2 2D-patterned metal nanostructure ... 13
2.1.3 Atomic layer deposition ... 15
2.1.4 Femtosecond laser modification ... 16
2.2
Characterization of plasmonic nanostructures ... 18
2.2.1 Microscopy ... 18
2.2.2 Absorption spectroscopy ... 19
2.2.3 Raman spectroscopy ... 19
3
FABRICATED PLASMONIC NANOSTRUCTURES ... 21
3.1
Metal nanoisland films ... 21
3.2
Glass-metal nanocomposites ... 23
3.3
2D-structured metal nanoisland films ... 24
4
OPTICAL CHARACTERISTICS OF THE GROWN NANOSTRUCTURES ... 27
4.1
Absorption spectra of metal nanoisland films and glass-metal nanocomposites ... 27
4.1.1 Pristine samples ... 27
4.1.2 Samples coated with a highly refractive dielectric ... 29
4.1.3 Laser-modified bulk silver nanocomposites ... 31
4.2
Enhancement of the Raman scattering by self-organized silver nanoisland films ... 34
CONCLUSIONS AND PERSPECTIVES ... 37
REFERENCES ... 39
INTRODUCTION
Nanoplasmonics is a rapidly growing field studying optical phenomena originating from collective oscillations of conduction electrons (plasmons) in metal nanostructures and nanoparticles. Plasmons, for example, manifest themselves as a pronounced resonance in the absorption spectrum of a glass containing metal nanoparticles resulting in the glass coloring. This property of metal nanoparticles has been used for centuries providing a variety of glass colors for ceramic glazing and stained-‐‑glass artworks. However, the microscopic mechanisms of the glass coloring (see Figure 1) and its dependence on the nanoparticles nature and size have been understood only in the 20th century.
Figure 1. In a series of studies starting from 1959, it was revealed that the Lycurgus cup (4th century AD) colors in reflected (left) and transmitted (right) light originate from plasmon resonance in gold-silver nanoparticles [1]. © Trustees of the British Museum.
Fundamentals of plasmonics have been established in the beginning of the 20th century when Wood discovered anomalies in reflection spectra of metal gratings [2]
and Garnett explained coloring in glasses with embedded metal nanoparticles [3]. In the following years, Mie [4] and Sommerfeld [5-‐‑7] calculated the scattering cross section of the spherical particle and radiation pattern of the dipole situated near a conductive plane, respectively. However, strong interest of the optical community to plasmonic effects has emerged only during the second half of the 20th century when rapid development of the micro-‐‑ and nanotechnologies made both fabrication and investigation of nanoscale objects possible. It is worth noting that the rapid growth in plasmonic research has also been supported by advances in computing that
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enabled numerical simulation of the optical processes in complex subwavelength structures.
Arrays of metal nanoparticles on dielectric substrates and glass-‐‑metal nanocomposites (GMN), which are composed of metal nanoparticles embedded in bulk dielectrics, are often referred to as plasmonic structures. Being collective oscillations of unbounded electrons, plasmons appear under irradiation of metal-‐‑
dielectric or semiconductor-‐‑dielectric interface with light of particular resonance wavelength. For noble metals—gold, silver and copper—resonance wavelengths lay within optical or near-‐‑ultraviolet range depending on the dielectric matrix. One can distinguish localized and propagating surface plasmons. Localized surface plasmon resonance (SPR) is associated with the oscillation of electron gas confined within metal nanoparticles. SPR manifests itself as a sharp peak in the absorption and scattering spectra and in an enhancement of the electromagnetic field magnitude at the nanoparticle surface. Propagating plasmons, which are often referred to as plasmon polaritons, travel along a metal-‐‑dielectric interface and can at certain conditions be re-‐‑emitted into free space. This makes plasmon polaritons attractive for further miniaturization of integrated and optoelectronic circuits and for improvement of their performance, allowing signals’ processing at optical frequencies with subwavelength-‐‑sized elements. Recently, both passive and active photonic components (waveguides [8], junctions [9, 10], resonators [11], modulators and switches [12]) relying on the plasmon polaritons have been demonstrated.
Properties of the localized and propagating plasmons depend on the medium surrounding the metal nanoparticles or nanostructures. This implies that plasmonic structures are very attractive for sensing applications. Currently, plasmonic sensors—including integrated ones—are widely used for the detection of tiny amounts of analytes. These sensors rely on the SPR change caused by variations of permittivity in the vicinity of a metal surface [13-‐‑15] or by nanoparticles aggregation [16, 17]. More opportunities in sensing are offered through the increase of the electric field at the metal dielectric interface due to plasmons excitation that gives rise to an enhancement of photoluminescence [18], specifically of Raman scattering [19] and fluorescence [20] of the molecules situated in the vicinity of plasmonic structure. This phenomenon is employed in the surface-‐‑enhanced Raman spectroscopy (SERS) [21], as well as in tip-‐‑enhanced spectroscopy and microscopy [22].
SERS involves the measurement of the Raman spectrum of an analyte deposited on the plasmonic substrate. In most cases, the substrate is made from gold or silver, but other materials are also studied [21, 23]. SERS technique enables an enhancement of Raman scattering by many orders of magnitude. Although the theory predicts an enhancement as large as 1012 – 1014, in practice it usually does not exceed 107 [19].
Nevertheless, such an enhancement allows SERS to detect even traces of analytes.
Similarly, by placing a resonant nanoparticle/nanostructure at the tip of a scanning optical microscope one can enhance the intensity of fluorescence of the target molecules, thus improving the sensitivity [24-‐‑30]. The fluorescence enhancement can be also employed for visualization of biological tissues [17, 31-‐‑33],
DNA sequencing [16], and even in atomic optics [34]. It is worth mentioning that plasmonic nanostructures can be employed for finding, visualizing, and destroying cancer cells [33]. The enhancement of the local field provided by plasmon excitation leads to stronger luminescence [35] and absorption [36], increases nonlinear optical response [37], improves the efficiency, and broadens the spectral range of photocatalysis [38]. Strong light absorption and scattering by plasmons make plasmonic nanostructures prospective for applications in photovoltaic devices and solar cells [39, 40].
During the last decade, most attention has been paid to the application of plasmonic nanostructures in metamaterials. In particular, plasmonic materials were used to achieve the negative refractive index in the vicinity of the SPR [41, 42], hyperlensing [41, 43-‐‑46], cloaking [47, 48], and to demonstrate other rather unusual optical phenomena.
It is worth mentioning that SPR frequency and therefore optical properties of the plasmonic nanostructures can be tailored by changing size and shape of metal constituents [49]. Generally speaking, the control of the shape of metal nanoparticles is a powerful tool to govern resonant properties of plasmonic nanostructures.
At certain conditions, roughness and/or sharp edges of a nanoparticle, as well as squeezing the gap between adjacent nanoparticles may lead to an extremely high local electric field forming so-‐‑called “hot-‐‑spots”. They are of special interest because one may expect that hot spots can dominate Raman scattering, luminescence, and optical nonlinearity of plasmonic nanostructures [50].
A variety of methods to fabricate different plasmonic nanostructures have been developed [51]; however, the ever-‐‑increasing number of applications implies the diversity of fabrication techniques which can be divided into so-‐‑called “top-‐‑down”
and “bottom-‐‑up” techniques [52, 53]. The latter are used to fabricate structures of somehow predetermined morphology and rely on self-‐‑organization. An example of such “bottom-‐‑up” techniques is the growth of metal nanoparticles in ion-‐‑exchanged glasses via thermal annealing (considered in this Thesis) [54] or under irradiation with photon and/or ion beams [55, 56]. In contrast, in “top-‐‑down” techniques, morphological properties of the plasmonic structure are pre-‐‑determined by the method of fabrication. Examples of “top-‐‑down” techniques are optical and/or e-‐‑beam lithography and femtosecond micromachining, which enable to obtain a metal nanostructure of pre-‐‑defined geometry on the substrate [57].
It is worth noting that “top-‐‑down” techniques are hardly applicable for fabrication of ensembles of metal particles with lateral dimensions and inter-‐‑particle distance of a few nanometers [58]. Nevertheless, these techniques are very well suited for fabricating nanoparticle arrays of predefined shape and mutual arrangement (e.g.
nanogratings) [59]. In contrast, existing “bottom-‐‑up” techniques are capable to form very small and closely packed nanoparticles. However, the size and arrangement of these nanoparticles may vary in a wide range, cannot be predefined, and the nanoparticles do not necessarily have a strong adhesion to the substrate.
4
Figure 2. Scanning electron microscopy (SEM) images of different self-assembled silver nanostructures grown in the course of annealing after poling of ion-exchanged glass with a profiled electrode.
Thus, it is very important for plasmonics to develop new approaches to form plasmonic nanostructures, combining advantages of “bottom-‐‑up” (closely packed small nanoparticles) and “top-‐‑down” (predetermined nanoparticles size and mutual arrangements) techniques. In this Thesis, it is demonstrated that by combining the ion exchange and annealing in a reducing atmosphere with poling of the ion-‐‑
exchanged glass in a static electric field one can fabricate silver-‐‑based plasmonic nanostructures. In particular, we show that the size and mutual arrangement of silver nanoparticles can vary in a controllable way from several to hundreds of nanometers (Figure 2).
1 PLASMONS IN METAL-DIELECTRIC STRUCTURES
Collective oscillations of conduction electrons in metals are conventionally referred to as plasmons. Since the behavior of the electronic ensemble is determined by the number of degrees of freedom of individual electrons, the properties of plasmons depend on the system dimensionality. This implies that the plasmons may behave differently in bulk (3D) metals, at the metal-‐‑dielectric interface (2D), in metal wires (1D), and metal nanoparticles (0D). At optical frequencies, plasmons in metals can couple with photons to create quasiparticles called plasmon polaritons [60].
Glassy nanomaterials comprising of metal nanoparticles in glass matrix are of special interest because they possess a pronounced surface plasmon resonance (SPR) associated with collective oscillations of conduction electrons at the nanoparticles surface [60]. Sensitivity of the SPR to external influences offers a wide range of opportunities for photonic and optoelectronic devices [61, 62].
1.1 PLASMA FREQUENCY
In the space-‐‑frequency domain, the evolution of the electromagnetic field in a homogeneous, non-‐‑magnetic, isotropic medium with dielectric constant ε(ω) can be described by Maxwell equations [63]:
𝛻 ∙ 𝑬(𝒓, 𝜔) =0 (1)
𝛻 ∙ 𝑩(𝒓, 𝜔) = 0 (2)
𝛻×𝑬(𝒓, 𝜔) = 𝑖𝜔𝑩(𝒓, 𝜔) (3)
𝛻×𝑩 𝒓, 𝜔 = −𝑖324𝜀 𝜔 𝑬(𝒓, 𝜔) (4)
where E and B are electric field and magnetic induction, and it is assumed that there are no external electric charges. The electromagnetic field is determined by ε(ω), which should be obtained from microscopic theory accounting for electronic properties of the medium [64].
In metals, oscillations of conduction electrons which can freely travel through the crystal skeleton can be conventionally described in terms of the Drude model [65]:
64
674𝒓 + 𝛾676𝑟 = −<;𝑬 𝑡 (5)
where r is the displacement of the electron from its equilibrium position, γ is the rate of the momentum relaxation due to the electron collisions with a crystal skeleton and
6
impurities, e and m are electron charge and mass, respectively. For the harmonic electric field, 𝑬 𝑡 = 𝑬𝑒𝑥𝑝 −𝑖𝜔𝑡 + 𝑐. 𝑐., the amplitude of the electron oscillations of the frequency ω, 𝒓 𝑡 = 𝒓𝑒𝑥𝑝 −𝑖𝜔𝑡 + 𝑐. 𝑐. is given by the following equation [60]:
𝒓 =<(24;DEF2)𝑬. (6)
Correspondingly, macroscopic polarization of the medium (a dipole momentum of a unit volume) is given by the linear constitutive equation 𝑷 = −𝑁𝑒𝑬 = 𝜀I𝜒 𝜔 𝑬, where N is the free electron density and
𝜒 𝜔 = −L K;
M<(24DEF2)𝑬 (7)
is the susceptibility of the medium at frequency ω [51]. Since 𝜀 𝜔 = 1 + 𝜒 𝜔 [60], the following equation for the dielectric constant of the metal in the Drude model framework can be derived:
𝜀(𝜔) = 1 −242DEF2O4 , (8) where 𝜔P= 𝑛𝑒R/(𝜀I𝑚;) is the plasma frequency.
One can observe from Eq. (8) that at γ = 0, the imaginary part of the dielectric constant is zero, and no optical losses occur. Such materials are often referred to as simple metals. Since in simple metals the dielectric constant becomes zero at ω = ωU, this frequency corresponds to the collective oscillations of all electrons in a bulk metal.
Plasma frequency is an important parameter describing interaction of electromagnetic waves with metals. At ω < ωU, the dielectric constant is negative so the refractive index is imaginary. Waves incident on the medium in this frequency region do not propagate but will be totally reflected. At ω > ωU, the dielectric function is real and positive, i.e., electron gas is transparent and transverse electromagnetic waves can propagate in plasma. For metals, the plasma frequency is usually in the UV spectral range. For example, plasma frequency of silver is about 9 eV which corresponds to resonant wavelength of about 140 nm [66-‐‑68].
In real metals, the contribution of the ion lattice to the dielectric constant can be taken into consideration by introducing a dielectric constant εX (usually, 1 < εX< 10) [60]:
𝜀(𝜔) = 𝜀X−242DEF2O4 . (9)
It is worth noting that at high frequencies, the interband transitions may contribute to the value of the metal dielectric constant. In particular, the transitions to the d-‐‑
band about 4 eV modifies the dielectric function of silver in blue and near-‐‑UV range [51, 64].
1.2 SURFACE PLASMON POLARITONS AT METAL-DIELECTRIC INTERFACE
When an electromagnetic wave is incident on the metal surface, the oscillations of the electron density at the metal/dielectric interface can be coupled to the incident wave allowing excitation of the surface plasmon polariton.
Figure 3. The system geometry for the surface plasmon polariton (SPP) problem.
Let us consider a plane interface between a metal with a relative permittivity 𝜀(𝜔) given by Eq. (9) and a dielectric with a refractive index n. We choose the interface to coincide with the plane z = 0 of a Cartesian coordinate system (see Figure 3). The surface plasmon polaritons are homogeneous solutions of Maxwell’s equations (i.e.
eigenmodes) confined at the metal-‐‑dielectric interface. By taking into account the continuity of the tangential components of the electric field and normal components of the displacement at the interface, vectors of the electric field associated with plasmon polariton in metal (𝐸<(𝑧 < 0)) and dielectric (𝐸[(𝑧 > 0)) can be presented in the following form [51]:
𝑬<∝ 1; 0; − L 2^ 𝑒𝑥𝑝 𝑖𝑘`P𝑥 + 𝑖𝑘a<𝑧 , (10) 𝑬[∝ 1; 0; − L 2^ 𝑒𝑥𝑝 𝑖𝑘`P𝑥 − 𝑖𝑘a[𝑧 . (11) Here
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𝑘`P=23 L 2 D^L 2 ^44 (12)
is the wavevector of the surface plasmon polariton, propagating along x-‐‑axis of the Cartesian frame in Figure 3, while 𝑘a<= 𝑘`P L 2
^ and 𝑘a[= 𝑘`P ^
L(2) determine the decay of the electric field amplitude inside the metal and dielectric, respectively.
Surface plasmon polariton propagating along x-‐‑axis implies a real 𝑘`P. One may observe from (12) that this condition can be fulfilled if 𝜀 𝜔 and 𝜀 𝜔 +𝑛R are either both positive or both negative. On the other hand, in order to obtain a wave localized at the interface, the 𝑘a< and 𝑘a[ should be purely imaginary in both media leading to exponentially decaying solutions. This can only be achieved if 𝜀 𝜔 +𝑛R is negative. Therefore, the surface plasmon polariton at the metal-‐‑dielectric interface exists at 𝜀 𝜔 +𝑛R< 0. This condition can be readily fulfilled for an interface between noble metals (e.g. gold and silver), which have a large negative real part and a small imaginary part of the dielectric constant, and a dielectric (e.g. glass or air). It is worth noting that in the framework of the Drude model [65] the condition 𝜀 𝜔 +𝑛R= 0 is fulfilled at
𝜔`P= 2O
bD^4. (13)
This frequency corresponds to the collective oscillation of conduction electrons at an infinite plane interface.
1.3 LOCALIZED SURFACE PLASMONS
When metal nanoparticles are embedded in a dielectric, the free electrons are localized within the nanoparticles. The collective oscillations of the conduction electrons at the metal-‐‑dielectric interface are often referred to as localized surface plasmons. In order to describe these oscillations for a metal spherical nanoparticle with a radius of a ≪ λ one can employ the quasi-‐‑static approximation [63, 69]. In this approximation, Maxwell equations (1) and (3) reduce down to
𝛻 ∙ 𝑬 = 0, (14)
𝛻×𝑬 = 0. (15)
By substitution 𝑬 = −𝛻 ∙ 𝛷, the electric potential Φ can be found from the solution of the Laplace equation
𝛻R∙ 𝛷 = 0. (16)
The solution of this equation for a spherical particle with radius a and dielectric constant 𝜀 embedded into a dielectric (𝜀[) in presence of the external field along z-‐‑
axis of the laboratory Cartesian frame 𝐸 = {0,0, 𝐸I} can be presented in the following form [63]:
𝛷E^(𝒓, 𝜃) = −LDRLkLl
l𝐸I𝒓𝑐𝑜𝑠𝜃, (17) 𝛷op7(𝒓, 𝜃) = −𝐸I𝒓𝑐𝑜𝑠𝜃 +LDRLLqLl
l𝐸I𝑎k 3o`s𝒓4 , (18)
where r and θ are cylindrical coordinates, while subscripts “in” and “out” label potential inside and outside the sphere, and 𝜀[= 𝑛R. The second term on the right-‐‑
hand side of Eq. (18), which describes the electric potential produced by the particle, can be presented in terms of the dipole moment 𝑝 induced in the particle by the external electric field [60]:
𝛷op7(𝒓, 𝜃) = −𝐸I𝒓𝑐𝑜𝑠𝜃 +tuLP∙𝒓
MLl𝒓v. (19)
The dipole moment 𝑝 = 𝜀I𝜀[𝛼𝐸I, where α is the sphere polarizability [60]:
𝛼 = 4𝜋𝑎k LqLLDRLl
l. (20)
One can observe from Eq. (20) that at 𝜀 = −2𝜀[ the polarizability of the particle tends to infinity giving rise to the SPR. In the framework of the Drude model, SPR frequency can be presented in the following form [60]:
𝜔`P|= bDRL2O
l. (21)
The SPR frequency depends on the shape of the nanoparticle. In particular, for a complicated shape the polarizability is described by a tensor, i.e., the SPR resonance may depend on the direction of the electric field. In particular, the diagonal components of the polarizability tensor of an ellipsoid with radii ai, i=1,2,3 are [60]:
𝛼E= 4𝜋𝑎b𝑎R𝑎k LqLl
kLlDk}~(LqLl), (22)
where Li is the geometrical factor:
𝐿E=€•€R4€v [‚
(€~4D‚) (€•4D‚)(€44D‚)(€v4D‚) X
I . (23)
In paper II an example of spheroidal (𝑎b= 𝑎R≠ 𝑎k) nanoparticles was discussed in detail.
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1.4 EFFECTIVE MEDIA APPROXIMATION FOR NANOCOMPOSITE
Effective media approximations are widely used to calculate the optical properties of a composite consisting of metal nanoparticles embedded in a dielectric matrix. In the framework of this approach, the composite is considered as a ‘uniform’ (effective) medium of which the macroscopic properties reflect the ones of the composite. At the relatively low volume fraction of metal in the composite, its optical and electronic properties can be described in terms of the Maxwell Garnett approach [70]. Effective dielectric permittivity of the composite including metal nanospheres will be
𝜀„…= 𝜀[(LDRLl)DR†(LqLl)
(LDRLl)q†(LqLl). (24)
If inclusions in the composite have a shape differing from spherical, this results in more complicated expressions for the dielectric function [71]. In the framework of this study, the case of spheroidal inclusions was studied in experiments and numerically in paper II.
2 EXPERIMENTAL TECHNIQUES
2.1 PLASMONIC NANOSTRUCTURES FABRICATION AND MODIFICATION
2.1.1 Glass-metal nanocomposite
All plasmonic nanostructures studied in this Thesis were fabricated using ion exchange followed by thermal annealing in a reducing atmosphere. This “bottom-‐‑up”
technique allows one to fabricate metal nanoisland films on the glass surface and metal nanoparticles in the subsurface layer of the glass slab (Figure 4).
Ion exchange in glasses is known since the 9th century when it was used for manufacturing colorful glazings on ceramics in Mesopotamia and spread in the Mediterranean region by the 12th century [72, 73]. Modern history of ion exchange began about a century ago, when potassium exchange has started to be applied for chemical strengthening of glasses’ surface [74-‐‑77]. In the 1970s, the ion exchange was proposed to fabricate planar and stripe waveguides [78] and more complicated components of planar optics [79]. Another broad application area of ion exchange is gradient lens fabrication [80] suggested by Mikaelyan in 1951 [81]. Nowadays the gradient-‐‑index lenses, obtained by ion exchange in glass rods, are widely applied in connectors fabrication for fiber optics.
The ion exchange method relies on the replacement of ions, which were initially contained in the glass, with other ions supplied from external sources; this exchange is driven by the chemical potential [82]. In our studies, the silver ion exchange in soda-‐‑lime glass was used, i.e., silver ions replaced alkali (sodium) ions in the subsurface region of the glass slab. In the experiments, the glass was immersed in AgxNa1-‐‑xNO3 (x=0.01..0.15, 0.05 by default) melt at 325 °C for 5-‐‑60 minutes (Figure 4a). At this temperature, the glass remained solid, while the ions were already able to migrate. As a result, sodium ions in the subsurface layer of the glass were replaced by silver ones with the thickness of this layer and the silver concentration profile being dependent on the melt composition along with the duration of the process and its temperature [83].
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Figure 4. Sketch of the silver nanoisland films growth process. a) In ion exchange, soda-lime glass slide is immersed in AgxNa1-xNO3 (x=0.01-0.15) melt (325 °C, 5 minutes – hour), during this time silver ions from the melt replace sodium ones in the glass subsurface;; b) Hydrogen penetrating the glass reduces silver ions to atoms;; in the course of diffusion and self-
arrangement silver forms nanoparticles either on the glass surface or both on the surface and in the bulk of the glass. c) After annealing of the ion-exchanged glass slide in a hydrogen atmosphere (75-350 °C, 30 seconds - 3 hours) the silver nanoisland film grows. Inset: atomic force microscopy (AFM) image in the grown film example.
Thermal annealing in a reducing atmosphere, which was used in our experiments, is a well-‐‑known method for the formation of metal particles and clusters in ion-‐‑
exchange glasses [84-‐‑86]. Specifically, we used the annealing in hydrogen (in some cases air or water vapor [87]) atmosphere at temperatures of 75-‐‑350 °C during 30 seconds to 3 hours (Figure 4bc).
In the course of the annealing, hydrogen diffused into subsurface regions of the glass and reduced silver ions to neutral atoms by replacing silver at bonds with non-‐‑
bridging oxygen atoms [88]:
≡ Si − O − AgD+1
2HR→≡ Si − O − HD+ AgI
The metal reduction led to the formation of an oversaturated solid solution of neutral silver in the glass matrix, which tends towards a phase decomposition due to low solubility of atomic silver in glasses. The decomposition resulted in growth of the silver clusters and nanoparticles, thus, they were able to form self-‐‑organized silver nanoisland films on the glass surface and/or silver nanoparticles in the subsurface layer of the glass. The method can be employed for obtaining nanoparticles from other noble metals, in particular, from copper [89].
2.1.2 2D-patterned metal nanostructure
To obtain ensembles of nanoislands of a prescribed geometry the thermal poling of the glass slab at elevated temperature was performed after completing the ion exchange process. In thermal poling a heated glass plate is placed between two electrodes, and positive ions drift from the subsurface layer beneath an anodic electrode to the glass interior. Applying a profiled anodic electrode to the glass surface resulted in redistribution of metal ions in the subsurface layer and promoted nanoislands’ formation at locations dictated by the electrode profile (Figure 5) through the local poling of the glass. The technique described in this section was proposed in Paper IV and developed in Papers III and I.
In the last two decades, glass poling was studied extensively. It has been demonstrated that the migration of ions occurring within glass in the course of poling results in structural and compositional changes in its subsurface regions [90-‐‑94]. The ion redistribution lifts the inversion symmetry of glass giving rise to the second order nonlinearity [95, 96]. This study focused particularly on the migration of silver ions in silver-‐‑enriched glass under the applied electric field.
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Figure 5. Sketch of the 2D-patterned structures’ growth process. Ion exchange (a) performs as in Figure 4. Thermal poling of ion-exchanged glass using profiled anode (b) (200-350 °C, DC 500-600 V, 30 seconds - 30 minutes) results in an adjustment of the silver ions distribution (c). After annealing of the poled ion-exchanged glass silver nanoislands grow only in areas where the anodic electrode did not contact the surface (d).
In order to grow the silver nanoparticles in the predetermined areas of the glass surface we employed patterned anodic electrodes with submicron hollows on its surface and a polished nickel plate as a cathode. The anode was fabricated from a polished glassy carbon slab (3x100x100 mm3 with Ra < 50 nm, Svensk Specialgrafit AB), which was cut into pieces of a lateral size of 20x25 mm2 by electron beam lithography followed by ion plasma etching. Before the lithography, 50 nm of chrome as a mask were deposited by electron-‐‑beam evaporation in Kurt J. Lesker Company Lab 18 setup. After that, the chrome film was spin-‐‑coated with positive e-‐‑beam resist Allresist AR-‐‑P 6200 which was then baked. Electron beam exposure was done in the Vistec Lithography Ebeam EBPG5000 setup. Subsequently, the resist was developed using the SSE OPTIspin SST 120 setup; chrome was etched through the developed resist in the reactive ion etching setup Plasmalab 100, and then glassy carbon was etched through this chrome mask to form the final structure in the reactive ion etching station Plasmalab 80plus. Finally, the remains of chrome were chemically removed, and profiled glassy carbon was cleaned via ultrasonic washing, first in acetone and then in isopropanol. The depth of the formed hollows in the glassy carbon was 50-‐‑400 nm depending on the ion etching parameters.
Poling was performed by applying DC voltage of 500-‐‑600 V to the ion-‐‑exchanged glass heated at 200-‐‑350 °C; the duration of poling varied between 30 seconds and 3 hours, in some cases sample preheating was used. During the poling process, positively charged silver ions drifted into the glass volume, however the drift length under the hollows in the electrode was smaller than that under the contact areas (Figure 5b). It is worth noting that smaller hollows provided less ion distribution contrast.
Hydrogen annealing described in section 2.1.1 converted silver ions which were inhomogeneously distributed beneath the glass surface into the silver nanoislands on the glass surface. Redistribution of silver ions in the subsurface layer by poling allows a formation of silver nanoisland only in areas corresponding to hollows in the electrode (Figure 5c). Unfortunately, a lack of knowledge on the diffusion coefficients of silver atoms and hydrogen molecules in poled and unpoled glass did not allow us to develop a quantitative description of this process to compare with empirical parameters obtained in our experiments and used in the fabrication of the nanostructures.
2.1.3 Atomic layer deposition
Atomic layer deposition (ALD) is widely used for deposition of thin films, of which the thickness can be controlled at an atomic level. Besides thickness control, ALD is known for precise reproduction of the coated surface morphology. Currently, the ALD method is employed for deposition of thin layers and their combinations for photovoltaic, catalysis, microelectronics and other applications [97].
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The ALD method is based on alternate reagent feeding (compared to simultaneous for chemical vapor deposition) with following chamber purging with neutral gas [98, 99]. Due to this, the reaction between the delivered material components takes place only on the substrate surface where the atomic layer of this material is formed [100]. Reaction products remaining in the gas phase are removed from the chamber at the next purging step.
For additional protection of manufactured metal nanostructures and modification of their properties, an atomic layer deposition of thin layers of titanium dioxide was employed. This material was chosen for its high refraction index (n=2.3-‐‑3 in 300-‐‑1300 nm wavelength range) which strongly affects the plasmon resonance properties including wavelength and because of its applicability in photovoltaics [101]. The films were deposited at 0.07 nm/cycle rate at 120 °C in the Beneq TFS-‐‑200 (Beneq, Espoo, Finland) reactor using TiCl4 and water as precursors; between cycles the reactor chamber was purged with nitrogen. The deposited layers thickness range was from 3 nm (minimal thickness providing uniform coating) to 200 nm in accordance with the study tasks.
2.1.4 Femtosecond laser modification
Since the invention of the first lasers, they have been extensively used for materials processing. This is because the laser radiation allows one to concentrate extremely high densities of optical energy in the focal spot with a lateral size of as low as one micron. In transparent materials, the control of the spatial position of the focal spot makes it possible to modify the material with micron spatial resolution in three dimensions [102]. Lately, the possibility of modification within the volume attracted attention as a method of information recording with an extremely high density [103].
In the case of modification of glass with nanoparticles, laser irradiation can result in change of their shape and size which in turn affects the local resonant properties of the nanocomposite [104, 105]. Such nanocomposites comprising of elongated metal nanoparticles in a dielectric matrix possess intrinsic anisotropy [69], which provides linear and nonlinear dichroism and birefringence [106, 107]. It is worth noting that anisotropic composites can be conventionally fabricated by stretching the glass slabs with the embedded spherical nanoparticles [108]. However, such stretching does not allow one to control birefringence and dichroism on a submicron scale comparable to optical wavelength, which is needed for modern photonic devices. The required spatial-‐‑selective modification with a submicron resolution can be obtained by irradiation of the composites with a focused laser [109-‐‑112] or ion beam [113, 114].
Glass with embedded metal nanoparticles can be modified with femtosecond [115], picosecond [116], or nanosecond [111] laser pulses, or even with CW laser [117].
This modification results in changes of the nanoparticles’ shape from spherical to spheroidal (one of the main mechanisms is shown in Figure 6) with morphological properties depending on the modification parameters. The morphology of the GMN
