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ISBN 978-952-61-2133-8ISSN 1798-5668


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o 223

ANNI ERONEN

STUDIES ON GOLD NANORODS AND LUMINESCENCE ENHANCEMENT


This thesis provides information about gold nanorods; fabrication, controlled assembly, measurements and effects on luminescence enhancement. In addition, different gratings

and materials, for example epoxy, are studied for luminescence enhancement.

ANNI ERONEN

ANNI ERONEN

Studies on Gold Nanorods

and Luminescence

Enhancement

Publications of the University of Eastern Finland


No 223

Academic Dissertation

To be presented by permission of the Faculty of Science and Forestry for public

examination in the Auditorium E100 in Educa Building at the University of

Eastern Finland, Joensuu, on June, 1, 2016,

at 12 o’clock noon.

Department of Physics and Mathematics

Grano

Jyvaskyla, 2016

Editors: Prof. Jukka Tuomela, Dos. Pertti Pasanen, Prof. Pekka Toivanen

and Prof. Matti Vornanen

Distribution:

University of Eastern Finland Library / Sales of publications

[email protected]

http://www.uef.fi/kirjasto

ISBN: 978-952-61-2133-8 (printed)

ISSNL: 1798-5668

ISSN: 1798-5668

ISBN: 978-952-61-2134-5 (pdf)

ISSNL: 1798-5668

ISSN: 1798-5676

Author’s address: University of Eastern Finland


P.O.Box 111

80100 JOENSUU

FINLAND

email: [email protected]

Supervisors: Professor Markku Kuittinen, Ph.D.

University of Eastern Finland


P.O.Box 111

80100 JOENSUU

FINLAND


Dr Hanna Lajunen

University of Eastern Finland


P.O.Box 111

80100 JOENSUU

FINLAND


Reviewers: Professor Jens Gobrecht, Ph.D.

Paul Scherrer Institut

Laboratory for Micro- and Nanotechnology

ODRA/101

5232 Villigen

SWITZERLAND


Guoguo Gang, Ph.D.

Beijing Institute of Technology

School of Optoelectronics

100081

P.R. CHINA


Opponent: Professor Toralf Scharf, Ph.D.

Ecole Polytechnique Federale De Lausanne

EPFL STI IMT OPT , MC B3 301 (Microcity)

Rue de la Maladiere 71b, CP 526

CH-2002 Neuchatel 2

SWITZERLAND


ABSTRACT

This thesis consists of studies on gold nanorods and luminescence

enhancement. The orientation of gold nanorods into lithographi-

cally fabricated nanopatterns is optimized by changing the reaction

conditions: pH, surface materials, and morphology. The controlled

assembly of gold nanorods into nanopattern was significantly im-

proved. Furthermore, the effects of gold nanorods on photolumi-

nescence in aqueous solution are studied. The gold nanorods en-

hanced the luminescence, and we noticed that when the rod size

increased the luminescence enhancement increased as well. The

results allow predicting luminescence enhancement with different

gold nanorod sizes in aqueous solutions. The studies include nu-

merical calculations using the Fourier Modal Method, nanofabrica-

tion, and measurements with bispectrometer and spectrometer.

Luminescent materials were mixed with Bisphenol A epoxy.

Samples were micropatterned with two different gratings. The ef-

fects of the micropatterns on luminescence were studied and it was

observed that the gratings enhance the luminescence in most cases.

Also, a resonance waveguide grating was optimized for custom

made luminophore, then fabricated and the luminescence intensity

and its angle dependence were measured. The resonance waveg-

uide enhanced the luminescence 1.6-fold.

Universal Decimal Classification: 535.37, 620.3, 681.7.02

PACS Classification: 42.82.Cr,81.16.Nd, 78.55.Qr, 78.67.Bf, 78.67.Qa,

81.16.Dn

Keywords: optics; photonics; nanofabrication; microfabrication; photolu-

minescence; gold nanorod

INSPEC Thesaurus: optics; nanophotonics; nanostructured materials; nanorods;

gold; microfabrication; nanofabrication; nanolithography; nanopatterning;

pH; surface morphology; optical polymers; luminescence; photolumines-

cence

Yleinen suomalainen asiasanasto: optiikka; nanomateriaalit; kulta; litografia;

pH; morfologia; polymeerit; luminesenssi; fotoluminesenssi

Preface

Making of this thesis has been an interesting journey. Without go-

ing into too much details, a lot of things has happened during these

years. Many of these have changed me as person, and it is safe to

say that this project has also effected me.

I want to thank my supervisors Markku Kuittinen and Hanna

Lajunen for giving me support and advices during these years. My

thanks go to all co-workers in Physics and Chemistry departments,

especially to Markus; we did spend some serious time working on

our articles. I also want to thank the Ministry of Education for

funding my work through the Graduate School of Modern Optics

and Photonics and Emil Aaltonen foundation for financial support.

I thank my family and friends, especially Eemeli for supporting

me.

I’m kind of relieved that this project is (almost) over..

Joensuu April, 28, 2016

LIST OF PUBLICATIONS

This thesis consists of the present review of the author’s work in

the field of photonics and the following selection of the author’s

publications:

I M. Erola, A. Partanen, S. Okoro, S. Md. Rahman, H. La-

junen, S. Suvanto, M. Suvanto, T. T. Pakkanen and M. Kuit-

tinen, “Controlled assembly of gold nanorods on nanopat-

terned surfaces: Effects of surface materials, pH and surfac-

tant”, Microelectronic engineering 121, 76–79, (2014).

II A. Partanen, M. O. A. Erola, J. Mutanen, H. Lajunen, S. Su-

vanto, M. Kuittinen and T. T. Pakkanen, “Enhancing Effects

of Gold Nanorods on Luminescence of Dyes”, Journal of Lumi-

nescence 157, 126-130, (2015).

III A. Eronen, A. Harju, J. Mutanen, H. Lajunen, M. Suvanto, T.

Pakkanen, M. Kuittinen, “Micropatterned luminescent optical

epoxies”, Optics Express 23, 33419–33425, (2015)

Throughout the overview, these papers will be referred to by Ro-

man numerals. The author of the thesis was formerly A. Partanen

as listed in Papers I and II.

In addition the author has the following peer-reviewed journal

articles

• F. Joki-Korpela, J. Karvinen, B. Paivanranta, A. Partanen, M.

Suvanto, M. Kuittinen and T.T. Pakkanen, “Hydrophobic and

oleophobic anti-reflective polyacrylate coatings”, Microelectronic

Engineering 114, 38–46, (2014).

• A. Partanen, J. Vayrynen, S. Hassinen, H. Tuovinen, J. Mu-

tanen, T. Itkonen, P. Silfsten, P. Paakkonen, M. Kuittinen, K.

Monkkonen and T. Venalainen, “Fabrication of Terahertz Wire-

Grid Polarizers”, Applied Optics 51, 8360–8365, (2012).

• T. Katayama, S. Owada , T. Togashi , K. Ogawa , P. Karvinen ,

I. Vartiainen , A. Eronen , C. David , T. Sato , K. Nakajima , Y.

Joti , H. Yumoto , H. Ohashi , M. Yabashi, “A Beam Branching

Method for Timing and Spectral Characterization of Hard X-

ray Free-electron Lasers”, manuscript accepted to Structural

Dynamics

and international conference proceedings papers partly related to

the research work

• A. Partanen, M. O. A. Erola, H. Lajunen, S. Suvanto, T. Pakka-

nen and M. Kuittinen, “Use of gold nanorings for field en-

hancement”, Proc. of EOS Annual Meeting EOSAM 2014, 15.-

19.9.2014, Berlin, Germany.

• T. Katayama, P. Karvinen, I. Vartiainen, A. Partanen, K. Ogawa,

T. Togashi, Y. Inubushi, K. Tono, C. David, M. Yabashi, “Beam

branching system for single-shot spectral and timing analysis

of XFEL pulses”, Proc. of SPIE Optics + Photonics 17.-21.8.2014

San Diego, USA.

• A. Partanen, A. Harju, J. Mutanen, H. Lajunen, T. Pakkanen

and M. Kuittinen, “Luminescent optical epoxies for solar con-

centrators”, Proc. of SPIE Optics + Photonics 17.-21.8.2014 San

Diego, USA.

• A. Partanen, M. Erola, S. Okoro, S. Rahman, H. Lajunen, S.

Suvanto, M. Suvanto, T. T. Pakkanen and M. Kuittinen, “Di-

rected Assembly of Gold Nanorods”, Proc. of MNE 2013 16.-

19.9.2013 London, UK.

• A. Partanen, I. Koshevoy, T. Saastamoinen, J. Mutanen, H. La-

junen and M. Kuittinen, “Enhancement of Luminescence with

Resonance Waveguide Grating”, Proc. of 18th Micro-optics Con-

ference MOC’13, 27.-30.10.2013, Tokyo, Japan.

• T. Saastamoinen, J. Vayrynen, A. Partanen, H. Tuovinen, J.

Mutanen, K. Monkkonen and M. Kuittinen, “Design, fabrica-

tion, and characterization of hybrid structure”, Proc. of 18th

Micro-optics Conference MOC’13, 27.-30.10.2013, Tokyo, Japan.

• A. Partanen, I. Koshevoy, T. Saastamoinen, J. Mutanen, H.

Lajunen, M. Kuittinen, “Enhancement of Luminescence with

Resonance Waveguide Grating”, Proc. of EOS Annual Meeting

EOSAM 2012, 25.-28.7.2012, Aberdeen, Scotland.

• J. Mutanen, J. Vayrynen, S. Siitonen, A. Kauppila, A. Par-

tanen, M. Hayrinen, P. Paakkonen, H. Tuovinen, T. Itkonen,

M. Kuittinen, J. Niemi, and K. Monkkonen, “Combining UV-

replication techniques with injection moulded polymer op-

tics”, Proc. of Euspen 4.-8.6.2012, Stockholm, Sweden.



Monkkonen, T. Venalainen, “Fabrication of Terahertz Wire-

Grid Polarizers”, Proc. 8th EOS Topical Meeting on Diffractive

Optics DO 2012, 27.2.-01.03.2012, Delft, Netherlands.



Monkkonen, “Fabrication of Terahertz Wire-Grid Polarizer by

Direct Machining”, Proc. 17th Micro-optics Conference MOC’11,

30.10. - 2.11.2011, Sendai, Japan.

AUTHOR’S CONTRIBUTION

The publications selected in this dissertation are original research

papers on gold nanorods and luminescent materials. The ideas

of the papers originated during discussions of the author and co-

authors.

In papers II and III, the author has carried out numerical com-

putations and data-analysis. The author has done almost all of the

fabrication processes to fabricate the nanopatterns and gratings on

papers I and III. The author has participated in the measurements

of the samples in all the papers.

The author has written the manuscript to the papers I and III,

and the author has had a major role in the writing of paper II. In all

papers, the co-operation with the co-authors has been significant.

Contents

1 INTRODUCTION 1

2 ELECTROMAGNETIC THEORY OF LIGHT 3

2.1 Maxwell’s equations . . . . . . . . . . . . . . . . . . . 3

2.2 Wave and boundary value equations . . . . . . . . . . 4

2.3 Angular spectrum representation . . . . . . . . . . . . 6

2.4 Electromagnetic field in periodic structure . . . . . . 6

2.5 Fourier modal method . . . . . . . . . . . . . . . . . . 10

3 PROPERTIES OF USED MATERIALS 13

3.1 Refractive index . . . . . . . . . . . . . . . . . . . . . . 13

3.2 Bulk plasmons . . . . . . . . . . . . . . . . . . . . . . . 14

3.3 Surface plasmons . . . . . . . . . . . . . . . . . . . . . 14

3.4 Localized surface plasmons . . . . . . . . . . . . . . . 14

3.5 Luminescence definitions . . . . . . . . . . . . . . . . 15

3.6 Luminescent materials . . . . . . . . . . . . . . . . . . 16

3.7 Luminescence enhancement . . . . . . . . . . . . . . . 17

4 FABRICATION METHODS 19

4.1 Electron beam lithography . . . . . . . . . . . . . . . . 19

4.2 Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.3 Nanoimprint lithography . . . . . . . . . . . . . . . . 21

4.4 Preparation of gold nanorods . . . . . . . . . . . . . . 21

4.5 Preparation of luminescent liquids containing gold

nanorods . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.6 Fabrication of resonance grating . . . . . . . . . . . . 22

4.7 Fabrication of luminescent epoxies . . . . . . . . . . . 23

5 MEASUREMENTS 25

5.1 Transmittance measurements . . . . . . . . . . . . . . 25

5.2 Luminescence intensity measurements . . . . . . . . . 26

5.3 Ellipsometry . . . . . . . . . . . . . . . . . . . . . . . . 27

6 EXPERIMENTS AND RESULTS 29

6.1 Gold nanorod assembly . . . . . . . . . . . . . . . . . 29

6.2 Enhancing effects of gold nanorods on luminescence 32

6.3 Resonance waveguide gratings . . . . . . . . . . . . . 36

6.4 Luminescent epoxies . . . . . . . . . . . . . . . . . . . 37

7 CONCLUSIONS 43

1 Introduction

Gold has been used since the ancient Egypt in various applica-

tions for example in dentistry [1] and jewelry [2, 3]. Michael Fara-

day was the first to prepare a solution of colloidal gold in 1856

[4, 5]. Since that the range of applications for gold nanoparticles

has widened [6,7]. These applications include use in biological sen-

sors and catalysts [8,9], as a nonvolatile plastic digital memory [10],

in the surface enhanced Raman spectroscopy [11–14], in second har-

monic generation [15–18], in fluorescence enhancement [19,20] and

in sensing applications [21], just to name a few.

Some of the applications, such as optical antennas [22, 23], sur-

face enhanced Raman spectroscopy, and the fabrication of metasur-

faces [24–26], require a controlled assembly of the gold nanoparti-

cles. Orienting gold nanoparticles into different arrays, for example

lithographically patterned surfaces [27–33], is essential for such ap-

plications. Also, the possibility to tune the local electromagnetic

field is important [34]. The distance between plasmonic particles

along with the geometrical shape of assembly has been shown to

affect the local electromagnetic field [34,35]. The enhancement of lu-

minescence by plasmon resonances of gold nanoparticles has been

widely studied recently [36–40].

In 1888, Wiedemann classified luminescence into six different

classes: photoluminescence, thermoluminescence, electrolumines-

cence, crystalloluminescence, triboluminescence, and chemilumi-

nescence [41]. These definitions, which classify the types of lu-

minescence by the method of excitation, are still used. Lumines-

cence is by definition electromagnetic radiation generated by ma-

terial that is not a result of thermal radiation and photolumines-

cence is luminescence caused by ultraviolet (UV), visible or near

infrared (NIR) radiation [42,43]. Luminescence is widely utilized in

biological [44–47] and solar applications [48–51].

This thesis is divided into seven chapters. Chapters 2 and 3

Dissertations in Forestry and Natural Sciences No 223 1

Anni Eronen: Studies on Gold Nanorods and LuminescenceEnhancement

introduce the theory used in the research; first we dive into the

electromagnetic theory of light and then some properties of metals

are discussed. The fabrication processes and measurement setups

are presented in Chapters 4 and 5, respectively. Chapter 6 describes

the experiments and results of the research and finally in Chapter 7

some conclusions are drawn.

2 Dissertations in Forestry and Natural Sciences No 223

2 Electromagnetic theory of

light

In the electromagnetic theory, light is considered as electromag-

netic waves ignoring the particle-like properties, i.e., excluding the

atomic level processes. This chapter gives an overview of the prop-

agation of electromagnetic waves in a medium. Furthermore, light

in periodic structures is discussed.

2.1 MAXWELL’S EQUATIONS

In a homogeneous medium, a stationary and time-harmonic electric

field may be represented as a sum of monochromatic fields:

E(r, t) = R

{

∫ +∞

−∞E(r, ω) exp(−iωt)dω

}

, (2.1)

where r is the position, R is the real part and E(r, ω) is the angular

frequency (ω) dependent electric field. Analogous expressions can

be written for magnetic field H(r, t), magnetic induction B(r, t),

electric displacement D(r, t) and current density J(r, t). [52]

The time-harmonic field, E(r, ω), in homogeneous medium sat-

isfies the Maxwell’s equations [53, 54]:

∇× E(r, ω) = iωB(r, ω), (2.2)

∇× H(r, ω) = J f (r, ω)− iωD(r, ω), (2.3)

∇ · D(r, ω) = ρ f (r), (2.4)

∇ · B(r, ω) = 0 , (2.5)

where J f and ρ f are free current density and free charge density.

Material equations connect the macroscopic electric displacement

with the electric field and the magnetic induction with the mag-



netic field [55]. For the isotropic and linear material the constitutive

equations are:

D(r, ω) = ǫ(r, ω)E(r, ω), (2.6)

B(r, ω) = µ(r, ω)H(r, ω), (2.7)

J(r, ω) = σ(r, ω)E(r, ω), (2.8)

where ǫ(r, ω) is electric permittivity, µ(r, ω) magnetic permeability

and σ(r, ω) electric conductivity. [53, 56].

Equations for TE (transverse electric field) and TM (transverse

magnetic field) polarized light can be derived from Maxwell equa-

tions, if the permittivity is assumed to be independent in the direc-

tion of y-axis. For example, assuming that the EM field propagates

in xz plane in homogeneous media, Maxwell equations can be writ-

ten as follows:

Hx(x, z) = ik0

�

ǫ0µ0

∂∂z Ey(x, z)

Hz(x, z) = − ik0

�

ǫ0µ0

∂∂x Ey(x, z) ,

∂∂z Hx(x, z)− ∂

∂x Hz(x, z) = ik0ǫ(x, z)�

ǫ0µ0

Ey(x, z)

(2.9)

Ex(x, z) = − ik0ǫ(x,z)

�

µ0

ǫ0

∂∂z Hy(x, z)

Ez(x, z) = ik0ǫ(x,z)

�

µ0

ǫ0

∂∂x Hy(x, z) .

∂∂z Ex(x, z)− ∂

∂x Ez(x, z) = ik0ǫ(x, z)�

µ0

ǫ0Hy(x, z)

(2.10)

Upper equation group is for TE polarized light and the lower one

for TM polarized light. ǫ0, µ0 and k0 are electric permittivity in vac-

uum, magnetic permeability in vacuum and vacuum wavenumber,

respectively.

2.2 WAVE AND BOUNDARY VALUE EQUATIONS

The Maxwell equations are valid only in continuous homogeneous

medium, but the optical structures have boundaries between differ-


Electromagnetic theory of light

ent media. Therefore a set of boundary equations that connect two

media are defined as:

n12 · (B1 − B2) = 0 (2.11)

n12 · (D1 − D2) = 0 (2.12)

n12 × (E1 − E2) = 0 (2.13)

n12 × (H1 − H2) = 0 (2.14)

where vector n12 is defined as the unit normal of the boundary

between medium 1 and medium 2. The equations (2.11)–(2.14) are

called the electromagnetic boundary conditions.

The wave equations can be derived from Maxwell equations

(2.2)–(2.5) with constitutive equations (2.6)–(2.8). Assuming lin-

ear, isotropic but inhomogeneous media the wave equations can

be written as [57]:

∇× µ−1∇× E(r, ω)− k20ǫrE(r, ω) = iωµ0js and (2.15)

∇× ǫ−1r ∇× H(r, ω)− k2

0µr H(r, ω) = ∇× ǫ−1r js, (2.16)

where js is source current density, ǫr relative permittivity, and µr

relative permeability. In (piecewise) homogeneous media the Eqs.

(2.15) and (2.16) can be written as follows [57]:

(∇2 + k2)E(r, ω) = −iωµµ0j +∇ρ

ǫǫ0, (2.17)

(∇2 + k2)H(r, ω) = −∇× j, (2.18)

where k = (ω/c)√

µǫ and j and ρ are sources in the media. Assum-

ing no sources; j = 0 and ∇ρ = 0, and nonmagnetic media (µ = 1),

the Helmholtz wave equations can be written as:

∇2E(r, ω) + k20ǫrE(r, ω) = 0, (2.19)

∇2H(r, ω) + k20ǫr H(r, ω) = 0. (2.20)

The general solution for the wave equations can be constructed with

monochromatic plane waves using Fourier transformation. As the

name plane wave suggest, the propagating wavefront of a single

plane wave forms a plane.



2.3 ANGULAR SPECTRUM REPRESENTATION

Since the plane wave satisfies the Maxwell equations, the superpo-

sition of plane waves must also satisfy the Maxwell equations [58].

The aim of this representation is to describe the propagation of gen-

eral electromagnetic field in the homogeneous medium. Let this

general field be in the following form:

U(x, y, z; ω) =

=∫ ∫ +∞

−∞A(kx, ky; ω) exp [i(kxx + kyy + kz(z − z0))]dkxdky, (2.21)

where kx, ky and kz are the components of the wave vector k and

A(kx, ky; ω) =1

(2π)2

∫ ∫ +∞

−∞U(x, y, z0; ω) exp [−i(kx x + kyy)]dxdy

(2.22)

is the angular spectrum of the field in the plane z = z0. The prop-

agation direction of the field can be solved with the wave vector k.

The z component of the wave vector can now be presented as:

kz =

{

[k2 − (k2x + k2

y)]1/2, when k2

x + k2y ≤ k2,

i[(k2x + k2

y)− k2]1/2, otherwise.(2.23)

In Eq. (2.23) upper, real solution describes propagating wave, and

the lower one, imaginary solution evanescent wave. The length of

wave vector, k, is k = ω/c.

2.4 ELECTROMAGNETIC FIELD IN PERIODIC STRUCTURE

Let us consider a two-dimensionally periodic diffraction grating

with periods dx and dy and height h (Fig 2.1). Incident monochro-

matic plane wave with wavelength λ coming from z < 0 gener-

ates transmitted and reflected diffraction orders. The incident an-

gles of the plane wave are θi and φi (defined similarly as angles

θ and φ in Fig. 2.2) and the propagation angle of the transmitted

(m, n)th diffraction order is θtmn. The diffraction grating is placed



z

y

x

dx

dy

z =�0

z h=

n

n

i

t

θnm

Figure 2.1: Three dimensional grating with periods dx and dy.

between two homogeneous media with refractive indices ni and nt

for medium before and after the grating. The incident monochro-

matic electric field is of the form:

Ein(x, y, z < 0) = u exp [i(kix x + kiyy + kizz)], (2.24)

where u is the unit polarization vector

u =

cos Ψ cos θ cos ϕ − sin Ψ sin ϕ

cos Ψ cos θ sin ϕ + sin Ψ cos ϕ

− cos Ψ sin θ

(2.25)

and ki is the incident wave vector

ki =

kix

kiy

kiz

= k0ni

sin θi cos ϕi

sin θi sin ϕi

cos θi

. (2.26)

The x, y and z components of the incident wave vector are kix, kiy,

and kiz, respectively. Fig. 2.2 defines the angles Ψ, θ and ϕ. Accord-

ing to the pseudo periodicity of the field:

kxm = kix + 2πm/dx , (2.27)

kyn = kiy + 2πn/dy (2.28)

and



û

z

y

x

φ

θ

ψ

k

Figure 2.2: The angles ϕ, θ and Ψ, the wave vector k and unit polarization vector u.

kzmn =

[k2 − (k2xm + k2

yn)]1/2, when k2

xm + k2yn ≤ k2

i[(k2xm + k2

yn)− k2]1/2, otherwise.(2.29)

The equations (2.27) and (2.28) are the grating equations that define

the propagation directions of diffracted plane waves. The angles

ϕmn can be solved:

tan ϕmn =kyn

kxm=

ni sin θi sin ϕi + nλ/dy

ni sin θi cos ϕi + mλ/dx. (2.30)

For reflected diffraction orders

kzmn = k0ni cos θrmn (2.31)

and for transmitted diffraction orders

kzmn = k0nt cos θtmn. (2.32)

Grating equation

Inserting Eq. (2.31) and Eq. (2.32) into Eq. (2.29) we can derive the

grating equations:

n2r sin2 θr

mn = (ni sin θi cos ϕi + mλ/dx)2 + (ni sin θi sin ϕi + nλ/dy)

2

(2.33)



for reflected diffraction orders and

n2t sin2 θt

mn = (ni sin θi cos ϕi + mλ/dx)2 + (ni sin θi sin ϕi + nλ/dy)

2

(2.34)

for transmitted diffraction orders. If we assume one-dimensional

grating (dx = d, dy = ∞ and ϕi = 0) the grating equation becomes:

nt sin θm = ni sin θi + mλ/d. (2.35)

Diffraction efficiencies

The transmitted electric field is of the form:

Et(x, y, z > h) =∞

∑m=−∞

∞

∑n=−∞

tmn exp [i(kxm + kyn + kzmn(z − h))],

(2.36)

where

tmn =1

dxdy

∫ dx

0

∫ dy

0Et(x, y, h) exp[−i(kxmx + kyny)]dxdy, (2.37)

and the reflected electric field is of the form:

Er(x, y, z < 0) =∞

∑m=−∞

∞

∑n=−∞

rmn exp [i(kxm + kyn − kzmnz)], (2.38)

where

rmn =1

dxdy

∫ dx

0

∫ dy

0Er(x, y, 0) exp[−i(kxmx + kyny)]dxdy. (2.39)

The diffraction efficiencies ηmn for the reflected and transmitted

diffraction orders are defined as

ηrmn =

cos θrmn

cos θi�rmn�2 (2.40)

and

ηtmn =

nt cos θtmn

ni cos θi�tmn�2. (2.41)



2.5 FOURIER MODAL METHOD

We are interested in solving the complex amplitudes of the reflected

and transmitted diffraction orders Eqs. (2.36) and (2.38), and the

field distribution. For general gratings, accurate analytical solution

for efficiencies of diffraction orders is nonexistent. However, the

efficiencies of the diffraction orders can be calculated with several

computational methods such as differential [59,60], finite-difference

[61–63], finite-element [64,65] and integral methods [66,67]. The one

used in this work is Fourier Modal Method (FMM) [68–71].

The principle of FMM for one-dimensional binary grating with

z-invariant permittivity is simple: the grating profile is sliced in x

and z directions so that in every slice the refractive index is con-

stant as shown in Fig. 2.3. The sliced grating profile and the field

inside the grating (when 0 < z < h) is presented as Fourier series.

The refractive indices are assumed to be constant in every region,

assuming that when z < 0 the refractive index is real. Inside the

grating region and in the region z > h the refractive index can also

be complex. The field is then expressed as a superposition of the

waveguide modes and it is assumed to be pseudo periodic. Ac-

cording to the Floquet-Bloch theorem each mode can be expressed

as pseudoperiodic function:

Us(r) = ∑n

∑m

Us,nm exp[i(αmx + βny + γz)], (2.42)

where Us may be any scalar component of the magnetic or electric

field.

The eigenvalue equations are formed in the region 0 < z < h,

and Fourier coefficients and z-components of the wave vector are

solved. Since the x and y-components of the electric and magnetic

field are continuous at z = 0 and z = h the boundary conditions

can be formed. The mode presentation of the field inside the grat-

ing is matched at the boundaries with field presentations of Eqs.

(2.36) and (2.38) by applying boundary conditions. In addition, the

incident field has to be summed up with the reflected field when

solving the boundary value problem. The complex amplitudes of



the reflected and transmitted fields are solved with S-matrix algo-

rithm [72, 73] by using the field before and inside the grating and

the boundary conditions. It is also possible to use different algo-

rithms, such as so called R-matrix algorithm [72] and T-matrix algo-

rithm [74]. To ensure converged result, all the propagating modes

and sufficient amount of evanescent modes must be included into

calculations. FMM is also applicable to two-dimensional and arbi-

trary gratings.

When we are considering a grating with a period less than the

wavelength, the only propagating order is the zeroth diffraction or-

der. However, when calculating with FMM sufficient amount of

evanescent orders must be also included in the calculations to en-

sure the converged result. When implementing FMM on metallic

grating, the number of orders needed is higher than in the case of

dielectric gratings. The convergence of the solutions can be tested

by increasing the number of diffraction orders and verifying that

the sum of the diffraction efficiencies approaches 1 (the efficiencies

of the highest diffraction orders should be 0).

z

x

d

x

x

21

11

2

1

n

n

n

n

n

i

t

13

21 22

x12

n11 n12

Figure 2.3: An example of y-invariant grating geometry with period d. x11 and x12 are the

transition points in layer 1. Accordingly x21 is the transition point in the layer 2. Incident

light propagates along z-axis. Note that the number of transition points and layers is not

limited.


3 Properties of used materials

The optical properties of metals are characterized by the index of re-

fraction n and extinction coefficient k. Index of refraction is defined

by the ratio of the speed of light in vacuum c0 and speed of light in

medium c, whereas the extinction coefficient denotes the exponen-

tial decay of the wave propagating in the medium. For dielectric

materials the refractive index is real, and for absorbing materials,

such as metals, complex.

3.1 REFRACTIVE INDEX

The complex refractive index is defined as:

n = n + ik. (3.1)

Although not shown in the equation (3.1) the index of refraction

and extinction coefficient are both wavelength dependent. Hence

the value of complex refractive index varies depending on the wave-

length of the incident electromagnetic field. Complex dielectric con-

stant is defined as [75]

ǫ = ǫ1 + iǫ2, (3.2)

where

ǫ1 = n2 − k2 (3.3)

and

ǫ2 = 2nk. (3.4)

The index of refraction and extinction coefficient are usually

presented as wavelength dependent, however, the complex dielec-

tric constant is presented as angular frequency, ω, dependent. The

angular frequency and wavelength are connected as ω = 2πc/λ.

Since the electrons in metals’ conduction band can be assumed

to be free, the conduction band electrons can undergo coherent os-

cillation (plasma oscillation). The mode of the oscillation is depen-

dent on the dimensionality of the metal body. Bulk plasmons (or



volume plasmons) occur in three-dimensional metallic bodies, the

surface plasmon resonance occurs in the metal-dielectric interface

and localized surface plasmon resonance (LSPR, or nanoparticle

plasmon resonance) occurs in metallic nanoparticles in the presence

of optical spectra.

3.2 BULK PLASMONS

When considering metals usually measured values of dielectric con-

stant is used. The plasmon frequency is an important constant,

which describes the behavior of the metal. Plasmon frequency is

defined as

ω2p =

nee2

meffǫ0, (3.5)

where ne is the electron density, e is charge of the electron, meff ef-

fective mass of electron and ǫ0 vacuum dielectric constant. When

the frequency of light is below plasmon frequency, metal is opaque.

On the other hand, when the frequency is higher than the plasmon

frequency, metal is transparent. For gold, the real part of the di-

electric constant is negative in visible and near-infrared parts of the

spectrum.

3.3 SURFACE PLASMONS

A surface plasmon mode is a special surface-bound mode at the

interface between metallic and dielectric medium. This mode is

allowed by Maxwell equations. Surface plasmons travel along the

surface and decay exponentially into both media (Fig. 3.1). Sur-

face plasmon resonance is charge density oscillation in the interface.

Surface plasmon resonance can be excited by light. [76, 77]

3.4 LOCALIZED SURFACE PLASMONS

For metallic nanoparticles (size ≈ penetration depth) a clear distinc-

tion between surface and bulk plasmon vanishes. When applying


Properties of used materials

+++ +++

+++

--- ---

---

dielectric

metal

E

Emetal

SPP LSP

Figure 3.1: Scematic of Surface plasmon polariton (SPP), and localized surface plasmon

(LSP).

an external electric field to a metal nanoparticle in dielectric mate-

rial the field is homogeneous inside the particle and the electrons

in the nanoparticle are in collective oscillative motion. (Fig. 3.1)

The quantum of the electron oscillation in the case of highly local-

ized electron field is called localized surface plasmon or particle

plasmon. [76]

In the case of an infinitely long rod, only a field with a com-

ponent normal to the surface is capable of exciting plasmons [57].

When considering rods with a finite length, two sets of localized

surface plasmons can be excited depending on the orientation be-

tween field components and the rod surface. Transverse localized

surface plasmon resonance is caused by the electron oscillation in

the direction of rod’s shorter dimension. Similarly, the longitudi-

nal localized surface plasmon resonance is caused by the electron

oscillation in the direction of rod’s length. [78]

3.5 LUMINESCENCE DEFINITIONS

The general definitions of the colorimetry of luminescent materials

and luminescence are published by CIE [42, 43].

• Luminescence by definition is electromagnetic radiation gen-

erated by material that is not a result of thermal radiation.

• Photoluminescence is luminescence caused by UV, visible or

IR radiation. All luminescence measurements in this thesis

are based on photoluminescence measurements.



• Fluorescence is photoluminescence in which the decay is de-

termined by intrinsic time constant.

• Phosphorescence is luminescence delayed by storage of en-

ergy in intermediate level with time constant exceeding the

intrinsic one.

• Electroluminescence is luminescence caused by external elec-

tric field.

• Chemiluminescence is due to energy liberated in a chemical

reaction.

• Thermoluminescence is re-emission of previously absorbed

EM radiation upon heating.

• Luminescence emission spectrum is a spectral-power distri-

bution of the radiation emitted by luminescent material due

to (monochromatic or heterochromatic) excitation.

3.6 LUMINESCENT MATERIALS

Luminescent materials can be divided into groups based on the

type of the luminescent material [79]:

• Aromatic molecules

• Inorganic crystals

• Noble gases

• Simple inorganic molecules

• Inorganic ions

• Biological molecules: aromatic amino acids, vitamins, hor-

mones

• Aliphatic molecules


Properties of used materials

Aromatic molecules are used in dyes and paints, also in fabrics and

paper. Noble gasses and inorganic molecules are used in discharge

lamps.

Commercial luminescent dyes were used in papers II and III:

Erothrysin B (ErB, Sigma-Aldrich), Fluorescein (FL, Sigma-Aldrich),

2,5-diphenyloxazole (PPO, Alfa-aesar), Rhodamine B (RhB, Alfa-

aesar) and trans-Stilbene (trS, Sigma-Aldrich) in paper II and Py-

ronin B (PyrB, Sigma-Aldrich) and Direct Yellow 96 (DY96, Aldrich)

in paper III.

In addition to previously mentioned commercial dyes, some

synthesized organometallic luminophores made in chemistry de-

partment were used (see Section 6.3). The luminophore used with

resonance grating was synthesized analogously to the procedure

reported in Ref. [80].

3.7 LUMINESCENCE ENHANCEMENT

There are at least three different mechanisms for the luminescence

enhancement to happen near metal surfaces [81]:

1. Energy transfer quenching

2. Local incident field enhancement on the fluorophore

3. Increase in the intrinsic radiative decay rate of the fluorophore

Firstly, the energy transfer quenching is strongly dependent on

the distance between metal and fluorescent material and is valid

only at distances less than 50 A. Secondly, the enhancement of

the local field on the fluorophore increases the rate of excitation,

but since the radiative rate is not changed, the fluorescent lifetime

remains the same. The size of the local field and hence the en-

hancement factor are highly dependent on the size and geometry of

nanoparticles. Additionally, when the distance between a nanopar-

ticle and luminescent material increases, the field enhancement is

decaying rapidly [82]. Thirdly, as the intrinsic radiative decay rate



of the fluorophore increases, the fluorescence intensity rises (fluo-

rescent lifetime is decreased) which can lead to enhanced fluores-

cence.

In luminescence enhancement by metal nanoparticles, the en-

hancement is assumed to decrease as a function of distance in the

following order: quenching, increased local field excitation, and in-

creased intrinsic radiative decay rate [81, 83].


4 Fabrication methods

There are several different micro- and nanofabrication techniques,

like electron beam lithography, laser lithography, mechanical ruling

and nanoimprintlithography to list a few. In this chapter electron

beam lithography and nanoimprint lithography are discussed. In

addition, other processes used in this work are discussed, such as

etching and chemical processes.

4.1 ELECTRON BEAM LITHOGRAPHY

Electron beam lithography is a technique used to fabricate nano- or

microscale patterns on resist material. In the process high energy

electrons are focused on the resist surface. The higher the energy

of the electron beam is the straighter is the path of the electron

beam into the material. The resolution is affected by beam size,

scattering from the resist and back scattering from the substrate

(see Fig. 4.1). [84]

12

e-

resist

substrate

Figure 4.1: Scattering from the resist (1) and backscattering from the substrate (2).

The lithography process begins by drawing the pattern with a

software (for example AutoCAD) and transferring that pattern to

the electron beam writer. Also, the sample has to be prepared: The

sample is coated with a layer of resist by spin coating. Resist is

an electron beam reacting resist, traditionally a polymer. There are

two types of resists, positive and negative tone. With a positive tone

resist the exposed areas are removed in the resist development and



with a negative tone resist all but the exposed areas are removed in

the resist development.

The used electron beam writer was Vistec EBPG5000+ES HR,

with possible voltages being 50 kV and 100 kV and smallest beam

size 2.5 nm. Related to the research in Paper I, different nanopat-

terns, rings and lines were fabricated on negative tone HSQ-resist

(hydrogen silsesquioxane, XR-1541, Dow Corning). One of the

masks for Paper III was fabricated with electron beam lithography.

The exposed pattern area was 1 × 1 cm2 and the lines were 200 nm,

and the period of the grating structure 10 µm.

4.2 ETCHING

Etching is a process of transferring a pattern from a mask into a

substrate with chemical and/or physical material removal. Physi-

cally the material is removed by ion bombardment and chemically

via chemical reactions.

Etching processes can also be divided into wet and dry etch-

ing. Dry etching is done in gas phase and wet etching in liquid.

Dry etching tends to be anisotropic and wet etching (chemical etch-

ing) more isotropic (Fig. 4.2). Reactive ion etching (RIE) is partly

chemical and partly physical etching process where ions, photons

or electrons bombard the sample and cause chemical reactions on

the surface. [84]

Plasmalab 80 and Plasmalab 100 (Oxford Plasma Technology)

were used to etch the samples. Plasmalab 80 is used to etch oxides,

mask

substrate

Figure 4.2: Isotropic (left) and anisotropic etching (right).


Fabrication methods

nitrides, and silicon, and Plasmalab 100 for metal etching addition-

ally to the previously mentioned. Plasmalab 100 was used to etch

chromium in the fabrication process of a mask in Paper I. Addi-

tionally, silicon wet etching with KOH was used to form the blazed

grating profile in Paper III.

4.3 NANOIMPRINT LITHOGRAPHY

Nanoimprint lithography (NIL) is a process where pattern from a

mask (hard substrate) is copied into a liquid resist that is hard-

ened with UV-light or heat, or into a soft film (for example polymer

film). The pattern is typically copied to the resist or film with good

resolution and the mask can be used multiple times. [85] When im-

printing into soft film, the process temperature is raised above the

glass transition temperature of the film. Therefore this process is

often called hot embossing.

The used nanoimprinter was Eitre 3 (Abducat AB). In the paper

III the masks were copied into COP (cyclo-olefin polymer) film, and

the COP films were used in the fabrication of the epoxy samples.

4.4 PREPARATION OF GOLD NANORODS

In the papers I and II gold nanorods (GNRs) with different dimen-

sions made at the chemistry department were used. All of the gold

nanorods were made following the same principles: the silver(I)-

assisted seed-mediated synthesis protocol was employed for the

synthesis of GNRs [86, 87]. In similar way the synthesis of the

spherical gold nanoparticles was carried out by a seeding growth

approach [86]. HAuCl4 x 3H2O ( 99.5 %, Oy FF-Chemicals Ab) was

used as the gold precursor. AgNO3 (> 99 %, Sigma-Aldrich) was

used as the assistance of GNRs synthesis. Hexadecyltrimethylam-

monium bromide (CTAB, 98 %, Sigma) was used as the capping

agent in a seed solution and in growth solutions of the GNR syn-

thesis. The physical volume of GNRs was varied by controlling the

volume of seed solution in use. The GNRs were washed using cen-



a b c d

e f g h

Figure 4.3: SEM images of various GNRs. Each scale bars represent 25 nm. [Paper II]

trifugation and re-dispersed in deionized water. The dimensions

of GNRs were measured with a STEM mode of a Hitachi SEM S-

4800 scanning electron microscope. NH3 was used to adjust the

pH of the GNRs solution. The pH was measured with a Consort

P901 pH meter. The GNRs concentrations were determined using a

PerkinElmer Lambda 900 UV/Vis/NIR spectrometer [88].

4.5 PREPARATION OF LUMINESCENT LIQUIDS CONTAIN-

ING GOLD NANORODS

Three different series of aqueous solutions containing gold nanorods

were prepared. Each series had nine samples, eight with different

sized gold nanorods and one without nanorods. Two of the series

were prepared with stock solutions of luminescent materials: 294

g/l for Direct Yellow 96 and 44 g/l for Pyronin B. The third series

was used as a reference and it did not contain any added lumines-

cent material. All solutions were diluted to the volume where the

concentrations of Direct Yellow 96 and Pyronin B were 29.4 g/l and

4.4 g/l, respectively.

4.6 FABRICATION OF RESONANCE GRATING

The fabrication steps for the resonance waveguide grating were

straightforward. First a 50 nm layer of chromium was evaporated

on top of 57 nm thick SixNy layer on top of SiO2 wafer. The grating


Fabrication methods

lines with period of 294 nm and line width of 131 nm were then ex-

posed into XR-1541 electron beam resist (Dow Corning) by electron

beam lithography using Gaussian shaped electron beam patterning

tool. The pattern was then transferred first to chromium and then

to SiNx by dry etching. The luminophore was diluted in acetone

and applied on top of the grating by spin coating (Headway spin-

ner PWM101D, Headway Research Ltd.). The thickness of the film

was controlled by concentration and spin speed.

4.7 FABRICATION OF LUMINESCENT EPOXIES

Bisphenol A (Bisphenol A diglycidyl ether, tech. 80 %, Alfa-aesar)

was used as the epoxy resin. The epoxy resin was stirred at 80◦C

for 60 min prior curing in order to reduce the number of air bub-

bles. When fabricating the luminescent samples 0.016-0.178 mass-%

of different luminescent dyes were added to the epoxy resin (Table

4.1) and the mixture was stirred. 22.7 mass-% of Isophorone di-

amine (IPDA, Aldrich) as a curing agent was added to the mixture

of epoxy resin. Finally the luminescent dye was added and the

mixture was further stirred for 5 min. The epoxy was placed on a

silicone mold and cured at 80◦C for 120 min. The diameter of epoxy

discs was 2,5 cm and thickness 3 mm. The luminescent epoxy sam-

ples illuminated under D65 lamp (Gretag MacBeth Spectralight III,

simulating CIE standard illuminant D65, Ref. [89]) and UV lamp

are presented in Fig. 4.4.

Table 4.1: Used mass-% (compared to epoxy resin) of the luminescent material, the exci-

tation wavelengths, and the strongest emission wavelengths of the epoxy samples. [Paper

III]

Luminescent dye

Mass of

epoxy

resin [g]

Mass-% of

dye

Amount of

hardener [ml]

Excitation

wavelength

[nm]

Emission

wavelength

[nm]

Epoxy (DGEBA) 27.9 – 6.9 250 310

Erothrysin B 32.3 0.016 7.9 530 575

Fluorescein 26.6 0.063 6.5 510 535

2,5-diphenyl-oxazole 27.3 0.178 6.7 330 365

Rhodamine B 35.4 0.023 8.7 560 585

trans-Stilbene 37.2 0.111 9.2 320 355



Figure 4.4: The luminescence effect of the fabricated luminescent epoxy samples was

demonstrated by illuminating the samples with a D65 lamp (upper images) and a UV

lamp (lower images). In the figure: pure epoxy (a), Erythrosin B (b), Fluorescein (c),

2,5-diphenyloxazole (d), Rhodamine B (e) and trans-Stilbene (f) mixed with epoxy. [Paper

III]


5 Measurements

In this chapter different optical measurement and characterization

methods used in the work are presented. We have used transmit-

tance and absorbance measurements, luminescence intensity mea-

surements, and ellipsometric measurements.

5.1 TRANSMITTANCE MEASUREMENTS

The transmittances of aqueous gold nanorod solutions (rods ran-

domly orientated) were measured with unpolarized light using a

PerkinElmer Lambda 900 UV/Vis/NIR spectrometer. The trans-

mittance spectra (Vis/NIR) of GNRs presented in Fig. 5.1 show

two plasmon bands for each GNR solution: one at around 510 nm

arising from the light absorption in the transverse direction (diam-

eter) of the GNRs and the other at 600–850 nm from the absorption

in the longitudinal direction (length). The rod diameters vary from

7 nm to 18 nm and the lengths from 27 nm to 45 nm.

Figure 5.1: Measured transmittances of GNRs solutions with different dimensions. [Paper

II]



Figure 5.2: Absorbance spectra of organometallic luminophore with different concentra-

tions.

The absorbance spectra of the organometallic luminophore (sec-

tion 6.3) measured with UV/Vis/NIR spectrometer are presented

in Fig. 5.2. The absorbance is calculated as follows:

A = − log T, (5.1)

where T is the measured transmittance. Similarly the absorbances

and transmittances of luminescent epoxy pieces and pure epoksy

were measured (Fig. 5.3).

5.2 LUMINESCENCE INTENSITY MEASUREMENTS

The luminescence intensities of the samples containing luminescent

dyes were measured as a function of the excitation wavelength with

a custom-made bispectrometer [90–92]. The light from a 450 W

xenon lamp (Oriel M-66923 housing, Newport corporation, Irvine,

California and Osram XBO 450W bulb, Osram AG, Switzerland)

was directed to the sample through a Czerny-Turner monochroma-

tor (DTMc300, Bentham Instruments Ltd, United Kingdom). The


Measurements

Figure 5.3: Measured absorbances (left) and transmittances (right) of pure epoxy and

different dye-epoxy mixtures.

used peak width and the sampling interval of the excitation light

were both 5 nm. The emitted light was collected with a lens to an

optical fiber attached to a spectrograph detector (PMA-12, Hama-

matsu Photonics K.K., Japan). The spectrograph operates at the

wavelength range 200 – 950 nm with 0.72 – 0.76 nm spectral sam-

pling. In the used measurement geometry the angle of incidence

was 0◦ and the detection angle was 45◦. The beam spot size was 5

mm.

5.3 ELLIPSOMETRY

The refractive index of used epoxy was measured by a J. A. Wool-

lam variable angle spectroscopic ellipsometer (VASE) and the out-

put data was treated with the WVASE32 fitting program. Fig. 5.4

show real and imaginary parts of the refractive index for the epoxy.



[nm]

nk

Figure 5.4: Measured refractive index for epoxy.


6 Experiments and results

This chapter introduces the experiments done and the main results

obtained during research. Part of these results are not included in

the Papers I – III. First, gold nanorod assembly into nanopatterns

is discussed. Then luminescence enhancement with gold nanorods

and resonance waveguide gratings is presented. The last topic is

micropatterned luminescent epoxies.

6.1 GOLD NANOROD ASSEMBLY

In this work, controlling the assembly of GNRs was studied (Pa-

per I). The effects of surface materials, pH and reaction time on

the assembly were investigated with the use of simple methods,

such as scanning electron microscopy and contact angle measure-

ments. The gold nanorods were assembled on a nanopatterned

surface by applying a solution containing gold nanorods on top of

the nanopatterned surface.

Materials and reaction conditions

In order to reach optimal conditions for gold nanorod assembly,

various studies were made with different solution pH, different sur-

face materials, and reaction times.

Originally four different substrate materials were studied, ITO

coating on a silicon wafer, SiO2 wafer, silicon wafer and TiO2 coat-

ing on a silicon wafer. However at the very early stages of the stud-

ies, it became clear that SiO2 and TiO2 surfaces would not work

as well as ITO and silicon. We noticed also that the usage of SiO2

wafer would cause some problems in the fabrication process (in the

electron beam writing the conduction layer was an issue). There-

fore, Paper I discusses only ITO and silicon.

To improve the assembly on to the test surfaces, the surfaces



Figure 6.1: Gold nanorod surface coverage on different surface material as a function of

pH. The used reaction time was 1 minute. [Paper I]

were silanized with mercaptosilane. In the case of silicon, the

silanization improved the assembly. However, with ITO surface,

the opposite effect was seen (Fig. 6.1).

Results

In addition, the significance of the reaction time and pH of the

gold nanorod solution were studied. The most suitable conditions

for gold nanorod assembly were selected: ITO layer on top of the

silicon wafer, gold nanorod solution with pH 9, and reaction time

of 1 min.

The degree of orientation of gold nanorods in the nanopatterns

was studied with SEM (Figs. 6.2, 6.3). We obtained fine selectivity

between HSQ -resist layer and underlying ITO layer. The arrange-

ment of gold nanorods into different patterns was significantly im-

proved. Suitable reaction conditions, material choice, pH, and mor-

phology allowed controlled assembly into the nanostructures.


Experiments and results

Figure 6.2: Collection of SEM images showing GNR orientation into differently shaped

nanopatterns. [Paper I]

N��blow2

ITO

SiO2

Substrate

GNRs

Figure 6.3: Schematic illustration of the assembly of the GNRs into the nanostructure on

top of ITO layer. The SEM figure shows the assembly of GNRs into ring structure when

pH was 9. Note that the resist in the center part of the ring is shallower than the resist

surrounding the ring. [Paper I]



6.2 ENHANCING EFFECTS OF GOLD NANORODS ON LU-

MINESCENCE

In this study, a series of positively charged gold nanorods with

different sizes was synthesized, and mixed separately with two dif-

ferent luminescent materials, positively charged Pyronin B (PyrB,

Sigma-Aldrich) and negatively charged Direct Yellow 96 (DY96,

Aldrich) dyes. The electric charges of the luminescent dyes affect

the distance between the gold nanoparticles and thus the lumines-

cence enhancement [40, 93]. The effect of gold nanorods on the

luminescence properties of the dyes was measured with a bispec-

trometer, and the effects of GNR’s dimensions on the luminescence

intensity enhancement were studied.

Modeling

The electric and magnetic energy densities around gold nanorods

with different diameters were calculated with FMM (calculations

related to Paper II). The energy densities were normalized so that

the energy density of incident field is 1. Fig. 6.4 shows the energy

densities of gold nanorods with respect to their length (excitation

wavelength 400 nm). We can observe that the longer the rod, the

higher the energy density around the rod. Fig. 6.5 presents the

energy densities with respect to wavelength, with four different rod

diameters and seven lengths.

Data analysis

The measured luminescence intensity data was after careful consid-

eration corrected with gold nanorod and luminescent dye concen-

trations:

Ic =Im

(cdye × cGNR), (6.1)

where Im is the measured luminescent intensity, cdye is the concen-

tration of the dye and cGNR is the concentration of gold nanorods.



20 25 30 35 40 45 501

1.01

1.02

1.03

1.04

1.05

1.06

1.07

1.08

Length [nm]

Ene

rgy

dens

ity

wl = 400 nm

5 nm10 nm15 nm20 nm25 nm

Figure 6.4: The energy densities around gold nanorods with incident wavelength 400 nm

and with different rod diameters. Color shows rod diameters.

Figure 6.5: The energy densities around gold nanorods with respect to wavelength. Rod

diameter is given on top of figures and rod lengths in labels.



The measured luminescence intensities and corrected luminescence

intensities are presented in Fig. 6.6.

a) b)

c) d)

Figure 6.6: Measured luminescence intensities for Direct Yellow 96 (a) and Pyronin B (c),

and concentration corrected luminescent intensities for Direct Yellow 96 (b) and Pyronin

B (d).

Results

The concentration normalized luminescence enhancements for dif-

ferent gold nanorods are presented in Table 6.1. The luminescence

enhancements were plotted as a function of rod diameter, length,

surface area and volume (Fig. 6.7). The luminescence enhancement

for each luminescent dye increases as the diameter, length, surface

area and volume increases. It seems that the luminescence enhance-

ment increases with the size of the rod. These results allow us to

predict luminescence enhancement with different gold nanorods.



Table 6.1: Concentration normalized enhancement of luminescence for Direct Yellow and

for Pyronin B at emission wavelengths 485 nm and 570 nm using excitation wavelengths

400 nm and 555 nm, respectively. [Paper II]

Normalized Enhancement

DY96 PyrB

Rod1 42.6 31.5

Rod2 19.8 12.9

Rod3 14.4 9.97

Rod4 10.0 7.12

Rod5 8.86 6.02

Rod6 6.96 4.98

Rod7 5.27 3.74

Rod8 5.24 2.80

a) b)

c) d)

Figure 6.7: Concentration normalized luminescence intensity enhancement as a function

of diameter (a), length (b), surface area (c) and volume (d) of the GNRs. The error bars

were calculated using 95% confidence interval.



6.3 RESONANCE WAVEGUIDE GRATINGS

A Resonance waveguide grating (RWG) [94] was designed with

FMM, fabricated by electron beam lithography and measured with

bispectrometer. Earlier Karvinen et al. observed strong lumines-

cence enhancement with resonance grating [95, 96].

The RWG was designed to enhance the luminescence of organo-

metallic luminophore using FMM. The selected resonance wave-

length was 410 nm which is also primary excitation wavelength of

the used luminophore. The structure of the RWG is binary, the

substrate is silicon dioxide (SiO2), and the grating lines are sili-

con nitride (SiNx). The calculated energy densitiy and one period

cross-cut of the RWG described above with incident angles 0 and

45 degrees are presented in Fig 6.8. Optimization of the RWG gives

the period 294 nm, the width of the grating lines 131 nm, and the

height 57 nm.

The luminescence intensity was measured with different inci-

dent and measurement angles and with excitation wavelength

406 nm (Fig. 6.9). The luminescence enhancement was at maxi-

mum with 45 degree angle, being 1.59. The enhancement was much

lower than expected from the calculations. The calculated field in-

Figure 6.8: Calculated energy densities inside the RWG compared with the incident field

with incident angles 0 degrees (left) and 45 degrees (right). Figures show the cross-section

of a single period where the white lines denote the boundaries between the different mate-

rials (1 is air, 2 is SiNx and 3 is SiO2). The light propagates from left to right.



500 550 600 650 700 7500

500

1000

1500

2000

2500

3000

3500

Wavelength [nm]

Lum

ines

cenc

e in

tens

ity a

.u.

Excitation wavelength 406 nm

25o

30o

35o

40o

45o

Figure 6.9: Measured luminescence intensities for luminophore applied on top of resonance

grating. Legend shows the measurement angles. Excitation wavelength was 406 nm.

tensity with 0 degree incident angle was around 250-fold, and for

45 degree incident angle only 2-fold. However, the luminescence

enhancement is not directly linked to the field intensity. The lumi-

nescent intensity emitted by the luminophore might be saturated

in high field intensity, or the amount of luminophore on top of the

RWG was insufficient. Also the luminophore was custom made and

all the properties of the material are not known.

6.4 LUMINESCENT EPOXIES

In this work luminescent epoxies were fabricated and studied. The

refractive index of the used epoxy was measured with an ellipsome-

ter. Five different luminescent dyes were mixed into the epoxy prior

to the curing. Some of the epoxy samples were micropatterned. The

absorbance of the unpatterned epoxy samples was measured. Fur-

thermore, the luminescence intensities of the different epoxy sam-

ples were measured with bispectrometer.

The reflected diffraction orders propagating in 45-degree an-

gle were determined for triangular crest grating and blazed grat-

ing used in Paper III. The diffraction efficiencies of those orders

were then calculated using FMM. The results are presented in Ta-



Table 6.2: Diffraction efficiencies of the diffractive orders propagating closest to 45 degree

angle (measurement angle).

Triangular grating Blazed grating

Material Diff. order Efficiency Diff. order Efficiency

Epoxy +5 2.69 × 10−4 +24 9.50 × 10−7

PPO +4 1.78 × 10−4 +19 6.82 × 10−6

ErB +2 9.21 × 10−4 +12 3.21 × 10−5

FL +3 3.74 × 10−4 +13 9.15 × 10−6

RhB +2 1.10 × 10−4 +12 1.88 × 10−5

trS +4 9.54 × 10−4 +20 1.77 × 10−6

Blazed�grating

Figure 6.10: Calculated transmittances for triangular crest grating (left) and blazed grat-

ing (right).

ble 6.2. The data shows that the reflections from the gratings are not

affecting the luminescence measurements (note that the excitation

wavelengths are different for each sample). The calculated trans-

mittances for the triangular crest grating and blazed grating are

presented in Fig. 6.10. The triangular grating shows anti-reflective

properties, in between wavelengths 300 nm and 800 nm the trans-

mittance is over 99,5%. The profile of triangular grating allows

multiple reflections in the interface, thus increasing transmittance

and decreasing reflectance.

The micropatterning of the epoxy samples was successful. Some

of the samples were cut in half in order to get SEM pictures of the



Figure 6.11: SEM images of micro patterns imprinted on epoxy. On the left is triangular

crest grating and on the right a blazed grating. The white arrows show incoming light and

measurement angle of 45 degrees [Paper III].

cross-section of the gratings. Fig. 6.11 shows cross-sections of the

triangular crest grating and the blazed grating imprinted on epoxy.

The measured luminescence intensities of the unpatterned and

patterned luminescent epoxy samples at best excitation wavelengths

are presented in Fig. 6.12. The used epoxy resin has also lumi-

nescent properties [97] (see Fig. 6.12(a)). The luminescence emis-

sion spectra for pure epoxy, as well as for trans-Stilbene and 2,5-

diphenyloxazole (Figs. 6.12(b) and 6.12(c)) have lots of disturbances

due to the short exposure time (20 ms) that was used in order to

avoid saturation of the signal for more strongly luminescent sam-

ples. However, the exposure time and the other measurement pa-

rameters were kept constant throughout all measurements so the

luminescence intensity values are comparable among all results.

The luminescence was enhanced by the gratings in most of the

samples. The parameters of our test grating structures were not

specifically optimized to enhance the extraction of emitted light,

since each of the luminescent dyes would require optimization and

ultimately a grating with different parameters.

The luminescence intensities of the unpatterned epoxy sample

containing two different dyes, 2,5-diphenyloxazole and Rhodamine

B, are shown in Fig. 6.13 at two excitation wavelengths. Two sepa-

rate emission peaks are detected, one for 2,5-diphenyloxazole and

one for Rhodamine B, with same excitation and emission wave-

lengths as in samples containing only one dye. The use of two or



more dyes improves the UV to visible conversion and, in addition,

allows great variety of possible colors through color mixing.

The micropatterns were copied to the epoxy surface well, and

the method is simple and reproducible. Since the pattern transfer

was successful, it suggests that the same technique could be used

also for nanopatterning. Therefore, fabrication of anti-reflection,

hydrophobic and self-cleaning surface structures directly on the

epoxy surface (e.g. directly on top of the solar cell) is possible.



Figure 6.12: Measured luminescence intensities of samples with unpatterned and

patterned (triangular or blazed grating) for pure epoxy (a), trans-Stilbene (b), 2,5-

diphenyloxazole (c), Fluorescein (d), Erythrosin B (e) and Rhodamine B (f) mixed with

epoxy. For each dye the excitation wavelength is given. [Paper III]



Figure 6.13: Measured luminescence for epoxy containing both 2,5-diphenyloxazole and

Rhodamine B excited at wavelengths 335 nm and 560 nm (left) and the sample illuminated

with D65 lamp and UV lamp (right, top and bottom, respectively). [Paper III]


7 Conclusions

The orientation of gold nanorods into nanopatterns was studied.

The effect of pH on the GNR adhesion and assembly of GNRs

on different surfaces was studied. The suitable reaction conditions

were carefully selected in order to achieve selective arrangement of

gold nanorods. The arrangement of GNRs into different assembly

patterns was significantly improved by controlling pH and selecting

suitable surface materials. In the future, by further optimizing the

reaction conditions, controlled assembly can be achieved on large

scale (periodic) nanostructures. Luminescence enhancement with

arranged gold nanorods is a potential research subject in the fu-

ture.

The effects of gold nanorods on luminescence in aqueous solu-

tions were studied. Two different luminescent materials (Pyronin B

and Direct Yellow 96) and eight different sized gold nanorods were

used. The measured luminescence intensity data was concentration

corrected and the luminescence enhancements were studied. In-

creased luminescence enhancement was observed when the rod size

increases. Based on the observed dependence of the concentration-

normalized luminescence intensities on the length, the surface area,

and the volume of the gold nanorods it is possible to predict the lu-

minescence enhancement of certain sized gold nanorods for Direct

Yellow 96 and Pyronin B. Also other luminescent materials and rods

with a larger size scale could be studied.

Optical, luminescent epoxy samples with micropatterning were

fabricated. The fabrication process was simple and the success to

copy microstructures on the epoxy surface suggest the possibility of

nanofabrication and simple fabrication of anti-reflective, hydropho-

bic and self-cleaning epoxy structures, for example.

The luminescence of the epoxy pieces was measured with bis-

pectrometer. The micropatterns enhanced the luminescence in most

of the samples. However, to obtain better enhancement of the lu-



minescence and light extraction, the surface patterns should be op-

timized individually for each sample taking into account the emis-

sion wavelength and the refractive index of the dye-epoxy mix.

Based on the measurements, the epoxy showed autoluminescence

excited by UV light. This could enable the use of UV radiation

from the sun in solar concentrators since the epoxy converts UV

radiation into visible radiation. The existing solar concentrators

mainly work on the visible range of the spectrum. Furthermore, the

luminescence from the dyes converts UV radiation to visible

(trans-Stilbene, 2,5-diphenyloxazole), as well as gives the samples

a bright color. Through the color mixing, there is a considerable

amount of possible colors available to be used, for example, on top

of solar cells. In the future, nanopatterned epoxies could be fabri-

cated. Also, applications in the solar concentrators could be tested.


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ISBN 978-952-61-2133-8ISSN 1798-5668


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ANNI ERONEN

STUDIES ON GOLD NANORODS AND LUMINESCENCE ENHANCEMENT


This thesis provides information about gold nanorods; fabrication, controlled assembly, measurements and effects on luminescence enhancement. In addition, different gratings

and materials, for example epoxy, are studied for luminescence enhancement.

ANNI ERONEN

dissertations in forestry and natural sciences · 2016-05-20 · uef.fi publications of the...

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