Self-Organized Arrays of Gold Nanoparticles: Morphology and Plasmonic Properties

129 Pages·2012·4.32 MB·English

by  Luca Anghinolfi (auth.)

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Luca Anghinolfi
Self-Organized Arrays
of Gold Nanoparticles
Morphology and Plasmonic Properties
Doctoral Thesis accepted by
the University of Genoa, Italy
123
Author Supervisor
Dr. LucaAnghinolfi Dr. Francesco Bisio
Universityof Genoa Universityof Genoa
Italy Italy
ISSN 2190-5053 ISSN 2190-5061 (electronic)
ISBN 978-3-642-30495-8 ISBN 978-3-642-30496-5 (eBook)
DOI 10.1007/978-3-642-30496-5
SpringerHeidelbergNewYorkDordrechtLondon
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(cid:2)Springer-VerlagBerlinHeidelberg2012
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Supervisor’s Foreword
Metals largely owe their characteristic optical response, like the almost unitary
long wavelength reflectivity, to the interaction of electromagnetic radiation with
their free electrons. As soon as a metal is manufactured in the form of a nano-
metric-sized particle, however, the physical confinement that the particle’s
boundary exerts on the free electron gas leads to a dramatic change in its optical
response.Metallicnanoparticleswithsufficientlylowlosses,undertheinfluenceof
an electromagnetic field, can exhibit in fact a resonant enhancement of their
polarizability that is completely absent in bulk counterparts and leads to a highly
peculiar optical response. Such a behavior, that goes under the name of localized
surfaceplasmonresonance(LSPR),leadstoastrongabsorptionofradiationbythe
particles, and to extremely strong enhancements of the incident field on a typical
length scale of just a few nanometers away from the particle surface. These
exciting features have disclosed the possibility of designing materials with tailor-
made optical response and most importantly have provided a method for
harnessing the propagation of the electromagnetic radiation on subwavelength
scales, with heavy applicative implications.
In his thesis, Luca Anghinolfi has addressed the potential of applying a totally
self-organization-based approach to the fabrication of spatially extended
2-dimensional systems of closely spaced metallic nanoparticles exhibiting LSPR
intheiropticalresponse.Thethesisworkhasexperimentallyexploredtheextentto
which macroscopic, externally tunable fabrication parameters can influence the
morphology and the arrangement of the systems’ components at the nanometric
scale effectively enabling the morphology-mediated, a priori control of the
plasmonic response. The control of optical characteristics such as width and
energy of the LSPR and the degree of birefringence of the two-dimensional
plasmonic medium is experimentally demonstrated, and its underlying mecha-
nisms explained by means of ad hoc models.
The thesis work has addressed what likely represents the simplest and most
commonly available plasmonic materials, gold, proving the general feasibility of
the method and disclosing the potential of the self-organization approach. Based
on this work, the use of even more sophisticated fabrication schemes opens new
v
vi Supervisor’sForeword
exciting possibilities for these self-organized systems. Recent research addressed
the use of materials other than gold as the plasmonic counterpart, like the pure
metals Ag and especially Aluminium, that allowed taking the plasmonic
functionality into the deep ultraviolet region. Alloy nanoparticles with a freely
variable composition add a further degree of freedom to the tunability of the
systems’ response, allowing to flexibly manipulate the plasmonic response over a
very large frequency range. Substrates with broadband tunable LSPR can be
exploited as flexible supports for plasmon-mediated excitations of deposited
molecules or quantum dots, where the capability of addressing, within a single
system,in-andoff-resonancebehaviorshasthepotentialtohighlighttheplasmon
role in mediating the optical excitation.
Genoa, March 2012 Dr. Francesco Bisio
Acknowledgments
First of all, I would like to thank Dr. Francesco Bisio and Prof. Lorenzo Mattera
for making this work possible and for their precious guidance. A big thank you
also to Prof. Maurizio Canepa for his useful advice and discussions. Thanks to
Dr. Riccardo Moroni for his assistance in the experiments, and to Ennio Vigo for
the technical support during the realization of the preparation chamber. Grateful
thanks also to Dr. Mirko Prato for his precious help on ellipsometry.
I gratefully acknowledge the group of biophysics, in particular Prof. Ornella
Cavalleri, for allowing me to use the AFM and DLS instrumentations and access
thechemistrylaboratory.AparticularthankyoualsotoDr.AmandaPencoforher
kind assistance in the AFM measurements.
IdeeplythankthegroupofMariaDelPuertoMoralesatCSIC/ICCMofMadrid
for the collaboration in the synthesis of the magnetic nanoparticles.
Finally, I would like to thank Laura Caprile, Michael Caminale, Dr. Elena
Gatta, and Chiara Toccafondi for making my thesis a pleasant experience.
vii
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1 Light and Matter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1.1 Dipole Oscillator Model. . . . . . . . . . . . . . . . . . . . . . . . 17
2.1.2 Light Refraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.1.3 Reflection from Thin Films . . . . . . . . . . . . . . . . . . . . . 24
2.2 Heterogeneous Media. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.3 Optical Properties of Metallic Nanostructures. . . . . . . . . . . . . . 28
2.3.1 Quasi-Static Approximation . . . . . . . . . . . . . . . . . . . . . 29
2.3.2 Beyond the Quasi-Static Approximation . . . . . . . . . . . . 33
2.3.3 Far Field Extinction Spectra
of Nanoparticles Ensembles . . . . . . . . . . . . . . . . . . . . . 36
2.3.4 Effects of EM Interactions. . . . . . . . . . . . . . . . . . . . . . 40
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3 Experimental Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.1 Preparation Chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.2 Optical Characterization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.2.1 Spectroscopic Ellipsometry . . . . . . . . . . . . . . . . . . . . . 50
3.2.2 Spectroscopic Reflectometry and Transmittance . . . . . . . 53
3.3 Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4 Self-Organized Nanoparticle Arrays: Morphological Aspects. . . . . 59
4.1 Growth and Characterization of Lithium Fluoride Substrates . . . 59
4.2 2D Arrays of Gold Nanoparticles . . . . . . . . . . . . . . . . . . . . . . 65
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
ix
x Contents
5 Self-Organized Nanoparticle Arrays: Optical Properties . . . . . . . . 71
5.1 Optical Properties of Lithium Fluoride Substrates . . . . . . . . . . . 71
5.2 Optical Properties of Gold Nanoparticles Arrays. . . . . . . . . . . . 74
5.2.1 Gold Nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
5.2.2 From Au Nanowires to Au Nanoparticles. . . . . . . . . . . . 78
5.2.3 Gold Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
6 Modelling and Analysis of the Optical Properties . . . . . . . . . . . . . 87
6.1 Lithium Fluoride Nanostructures. . . . . . . . . . . . . . . . . . . . . . . 87
6.1.1 LiF Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6.1.2 Nanostructured LiF Templates . . . . . . . . . . . . . . . . . . . 89
6.2 2-Dimensional Arrays of Gold Nanoparticles . . . . . . . . . . . . . . 92
6.2.1 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
6.2.2 Optical Anisotropy of Self-Organized
Au Nanoparticles Arrays . . . . . . . . . . . . . . . . . . . . . . . 98
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
7 Composite Magnetic-Plasmonic Media Based
on Au/LiF Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
8 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
Chapter 1
Introduction
Inaworldinwhich theinformationand communication technology has gradually
assumed a pivotal role for scientific, technological and social purposes, there is
a constantly growing demand for smaller and faster devices able of manipulating
information, typically in the form of electronic, magnetic or optical signals. The
request of smaller and faster devices has inevitably led to a constant process of
miniaturizationoftheirelementarycomponents,therebybringingalongahugeload
of scientific and technological challenges that researchers and engineers have to
face.Nowadays,forexample,state-of-the-arttechnologyrequiresexcellentcontrol
of structures having typical lateral dimension of the order of approximately ten
nanometers,nottoofaroffmoleculardimensions.
While conventional lithographic methods [1–6] always represent the standard
choiceforthefabricationoftechnologicalnanostructures,thechallengesassociated
withtheconstantsizereductionhavepromotedintenseresearchaimedatexploiting
alternativestructuresforthefabricationofnanosizedsystems[7–20].
Among the various strategies, in the past years we have witnessed a growing
interest to the world of colloid and cluster science, leading to the consolidation of
nanoparticles(NPs)asconvenientstructuralelementsfortheconstructionoffunc-
tionalinterfaces[10, 14, 21–28].Infact,nanoparticlescanbefashionedfrommany
different materials, presenting a wide diversity of electronic, optical, catalytic and
magneticproperties,whichinmanycasesoriginatefromthereduceddimensionality
ofthesystemsandthusarenotfoundinthebulkcounterparts[29–34].
While the properties of individual NPs can be exploited in a variety of ways
[8, 21, 23, 26, 35–47], other interesting functionalities arise when the NPs are
assembledinsuchawaythateachone“feels”thepresenceofneighbouringparticles
viasomemutualinteraction.Typicalexamplesofsystemsexhibitingsuchcollective
functionalityareassembliesofmagneticparticles[14,27,28,34,48–50],interacting
withoneanotherthroughtheirmagneticdipolarfieldor,mostrelevantforthisthesis,
assembliesofmetallicparticlescharacterizedbypeculiaropticalfunctionalities[24,
25, 38, 51–60].
L.Anghinolfi,Self-OrganizedArraysofGoldNanoparticles,SpringerTheses, 1
DOI:10.1007/978-3-642-30496-5_1,©Springer-VerlagBerlinHeidelberg2012