Synthesis of advanced inorganic colloidal nanocrystals [Elektronische Ressource] / vorgelegt von Marco Zanella

-

English
305 Pages
Read an excerpt
Gain access to the library to view online
Learn more

Description

Synthesis of advanced inorganic colloidal nanocrystals Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften (Dr. rer. nat.) dem Fachbereich Physik der Philipps-Universiät Marburg vorgelegt von Marco Zanella aus Thiene (Italien) Marburg/Lahn, 2008 Vom Fachbereich Physik der Philipps-Universität Als Dissertation angenommen am: 21.05.2008 Erstgutachter: Prof. Dr. Wolfgang J. Parak Zweitgutachter: Prof. Dr. Wolfram Heimbrodt Tag der mündlichen Prüfung: 29.05.2008 Contents Abstract 1 I. ntroduction 3 I.1 Synthesis in solution 3 I.2 Sol-Gel 5 3Mic 6I.4 Hydrothermal process 7 I.5 Photo reduction and role of light in nanoparticles synthesis 7 I.6 Physical and Chemical Vapour Deposition 8 Refrnces 9 II. Nucleation, particles growth and ripening 11 I.1 Nucleation 1 2Growh 14I.3 Ripenig Refrcs 5 III. Magic Size Nanoparticles 16 Refrnces 24 IV. Hybrid naocrystal 25 IV.1 Synthesis of core-shell hybrid nanomaterials 25 IV.2 Synthesis of hetherodimers and oligomers 27 Refrnces 31 V. Conclusions and Perspectives 32 Refrences 4 VI. Publications 36 VI.

Subjects

Informations

Published by
Published 01 January 2008
Reads 14
Language English
Document size 21 MB
Report a problem








Synthesis of advanced inorganic colloidal nanocrystals




Dissertation

zur

Erlangung des Doktorgrades

der Naturwissenschaften

(Dr. rer. nat.)

dem

Fachbereich Physik

der Philipps-Universiät Marburg


vorgelegt von

Marco Zanella

aus

Thiene (Italien)


Marburg/Lahn, 2008






Vom Fachbereich Physik der Philipps-Universität

Als Dissertation angenommen am: 21.05.2008



Erstgutachter: Prof. Dr. Wolfgang J. Parak

Zweitgutachter: Prof. Dr. Wolfram Heimbrodt


Tag der mündlichen Prüfung: 29.05.2008


































Contents

Abstract 1

I. ntroduction 3
I.1 Synthesis in solution 3
I.2 Sol-Gel 5 3Mic 6
I.4 Hydrothermal process 7
I.5 Photo reduction and role of light in nanoparticles synthesis 7
I.6 Physical and Chemical Vapour Deposition 8
Refrnces 9

II. Nucleation, particles growth and ripening 11
I.1 Nucleation 1 2Growh 14
I.3 Ripenig
Refrcs 5

III. Magic Size Nanoparticles 16
Refrnces 24

IV. Hybrid naocrystal 25
IV.1 Synthesis of core-shell hybrid nanomaterials 25
IV.2 Synthesis of hetherodimers and oligomers 27
Refrnces 31

V. Conclusions and Perspectives 32
Refrences 4

VI. Publications 36

VI.1 Sequential Growth of Magic-Size CdSe Nanocrystals 37
VI.2 Blue light emitting diodes based on fluorescent CdSe/ZnS nanocrystals 55
VI.3 General Approach to II-VI Semiconductor Magic Size Nanocrystals 58

VI.4 Growth of colloidal hybrid nanoparticles of fluorescent group II/VI particles on top of
magnetic iron-platinum 86

VI.5 Synthesis and perspectives of complex crystalline nano-structures 114
VI.6 Design of an Amphiphilic Polymer for Nanoparticle Coating and Functionalization 122

VI.7 Size Determination of (Bio)conjugated Water-Soluble Colloidal Nanoparticles: A Comparison
of Different Techniques 170

VI.8 Biological Applications of Gold Nanoparticles 233

+++ +++VI.9 Chloroform- and water-soluble sol-gel derived Eu /Y O (red) and Tb /Y O (green) 2 3 2 3
nanophosphors: synthesis, characterization and surface modification 257

VI.10 Photoelectrochemical signal chain sensitive to superoxide radicals in solution 278

VI. Acknowledgements 301
VIII. Academic Curriculum Vitae 302
Abstract.

Colloidal nanocrystals are crystalline materials of nanometer size which are colloidally suspended
in a solution. Typical nanocrystals are made of few tens to some thousands atoms. Because of their
small size they exhibit properties different to the conventional bulk materials. In the nanosize
regime, in fact, it is not just the composition which determines the properties of a material but also
its size and shape. The possibility to control these parameters allows the fabrications of nanocrystals
whose properties can be exploited in several fields such as electronics, diagnostics, catalysis and
optoelectronics.

In this dissertation we will focus on semiconductive nanocrystals with particular attention to a new
synthesis process which allows us to have a better control on the size and thus the properties. In
particular we show that for small nanocrystals the growth is not continuous. Instead the
nanocrystals grow discretely, from one stable configuration to the next bigger stable configuration.
The possible stable configurations are termed "magic size clusters". For bigger particles growth is
continuous. We report the generalization of the process to grow magic size clusters for several
semiconductor materials. Also an application of magic size clusters of CdSe for the fabrication of
light emitters is reported.

The characterisation and application of particular semiconductive nanomaterials presented in this
work will led us to the synthesis of more complex nanostructures such as core@shell nanomaterials
and semiconductive-magnetic dimers. We demonstrate in particular the growth of II/VI
semiconductor materials on top of FePt nanocrystals. Thus dimeric nanocrystals with a magnetic
FePt domain and a II/VI domain are obtained. In these systems it is possible to combine together
properties of the different materials in order to fabricate nanoparticles presenting as well a magnetic
as a semiconductive domain.




Kolloidale Nanokristalle sind kristalline Materialien mit Nanometer-Größe die stabil in Lösung
suspendiert sind. Typische Nanokristalle enthalten einige 10 bis zu einigen 1000 Atomen. Aufgrund
ihrer kleinen Größe haben Nanokristalle unterschiedliche Eigenschaften als vergleichbare
Volumen-Materialien. Auf der Nanometer-Skala werden die Eigenschaften von Materialien nicht
nur durch deren Zusammensetzung, sondern auch durch ihre Größe und Form bestimmt. Die
Möglichkeit diese Parameter zu variieren ermöglicht die Herstellung von Nanokristallen deren
Eigenschaften sie für den Einsatz in verschiedenen Bereichen, wie Elektronik, Diagnose, Katalyse
und Optoelektronik, interessant machen.

In dieser Dissertation haben wir den Schwerpunkt auf Halbleiter-Nanokristalle gelegt. Dabei wurde
dem Syntheseprozess besondere Aufmerksamkeit gewidmet, so dass eine bessere Kontrolle der
Nanokristallgröße und damit der Eigenschaften ermöglicht wird. Besonders zeigen wir, dass für
sehr kleine Nanokristalle deren Wachstum nicht kontinuierlich verläuft. Hingegen wachsen kleine
Nanokristalle in diskreten Stufen, von einer stabilen Konfiguration zur nächst größeren stabilen
Konfiguration. Die stabilen Konfigurationen werden "magic size cluster" genannt. Für größere
Partikel ist der Wachstumsprozess wie gewohnt kontinuierlich. Wir beschreiben die
Verallgemeinerung des Wachstums von magic size clusters für verschiedene Halbleiter Materialien.
Als Anwendung von magic size clusters wird die Herstellung von Leuchtdioden beschrieben.
Die Charakterisierung und Anwendung von bestimmten Halbleiter Nanomaterialien die in dieser
Arbeit vorgestellt werden führt uns zur Synthese noch komplexerer Nanostrukturen wie
Kern@Hülle Konfigurationen and halbleitenden-magnetischen Dimer Strukturen. Besonders




beschreiben wir das Wachstum von II/VI Halbleitern auf der Oberfläche von FePt Nanokristallen.
Diese Dimere haben sowohl eine magnetische FePt Domäne als auch eine halbleitende II/VI
Domäne. So ist es möglich in diesem Systemen zwei verschiedene Eigenschaften in einem einzigen
Partikel zu kombinieren, da die Nanopartikel eine magnetische und eine halbleitende Domäne
besitzen.








































I. Introduction

In the last decade, new directions of modern research have emerged. One of these new fields
usually goes under the name “nanoscience and nanotechnology” and joins several areas of research
as engineering, physics, chemistry, material science and molecular biology. The research in this
direction has been triggered by the recent availability of new revolutionary instruments and
techniques which are able to improve our investigation abilities concerning the material properties
with a resolution close to atomic scale. Such technological advances have inspirited new pioneering
experiments which have revealed new physical properties and effects of matter at an intermediate
level between atomic and bulk.
The discovery of these properties, acquired by the materials at these size scale, have driven the
desire to fabricate materials with novel or improved characteristics suitable for future advancements
in electronics, optoelectronics, diagnostic and catalysis.
These new classes of materials are usually called “nanoscale materials”, or “nano-composites” and
their properties do not just depend on their composition and size but from their shape too.
Nanomaterials can appear in different forms, some of them are powders, some other can be
suspended into a solvent or be embedded in a solid material like glass or a polymer matrix. Their
form depends on the process used to synthesize them or their application. Anyway considering the
possible applications, in order to maximize the performances of a given nanomaterial some
techniques have been developed in order to pass from a form to an other one, for example
suspending some nanoparticles into a solvent starting from their powder. Some applications of
nanomaterials properties ranging from optic (i.e. optical filters [1,2], LASERs [3], LEDs [4]) to
molecular biology (i.e. labelling [5-7] , hyperthermia [8]) has already been realized.
For the fabrication of nanomaterials many techniques have been developed. Here we introduce
some of the most diffused.

I.1 Synthesis in solution

The synthesis of nanomaterials in solution is a wet-chemical approach which requires the reaction
of precursors injected into a hot reaction flask where some inorganic molecules can be present
dissolved into a coordinating solvent (Figure I.1). The temperature of this solution is sufficient to
decompose the reagents resulting into a super saturation of precursors in solution. This addition of
reagents, hence, raises the precursors concentration above a threshold called “nucleation threshold”
in which the precursors present in solution react forming nanocrystals nuclei. This process will be
discussed more extensively in the next chapter. For now we can approach the nanoparticle
nucleation and growth with a simple example. Let us consider the condensation of steam in water
drops at constant temperature. If we keep on increasing the humidity of a certain environment some
little drops of water start nucleating and the drops size can increase via water molecule addition or
coalescence since the single drops surfaces are not protected. The coalescence or agglomeration of
drops do not allow us to control their size.
Ar Ar
Inject
organometallic
Heating
temperature 350 mantle
controller
Mixture of surfactants

Figure I.1. Simple sketch of the apparatus for the synthesis of nanomaterials in solution.
In order to have a control in the nucleation and growth of nanoparticles some organic molecules are
used to stabilize the nuclei. These organic molecules, commonly known as surfactants (Figure I.1
and I.2), stick on their surface preventing agglomeration via steric repulsion.

b)
P=P=O
OC
C
P Se
P= P=
P=OO
O
P=O
P=O
Cd P=O P=O
P=O
P=
O
P= P=P=
O OO
P
Se



Figure I.2. a) Sketch of the growth of a nanoparticle of CdSe. The cadmium (red) and selenium
(violet) atoms are connected to the organic part forming the relative precursors which surround the
nanoparticle in solution. The nanocrystal is stabilized in solution by the surfactants, organic
molecule which sticks to its surface avoiding the agglomeration of different cores. b)High
resolution TEM micrograph of a spherical nanoparticle. Light grey dots are the nanoparticle atoms
(taken from [29]).

After the nucleation the nanoparticles start growing using the precursors left in solution. In general
the NCs size increases over time as more material is added to their surface. One more important
factor for the NCs growth, along with the precursor concentration, is the temperature since at higher
temperature the rate of atoms addition to the existing nuclei increases. The presence of the
surfactant ensure the stability of these particles and the solution itself. Surfactants can be even used
in order to drive the nanocrystal shape during the growth (Figure I.3). In fact surfactants can bind
more tightly to a NC surface than an other one changing the rate of atoms addition. Or an other
strategy is that one which uses two surfactant molecules one that binds tightly to a surface a one that
binds weakly to an other one, in order to permit a rapid growth in the second surface and a slow
growth in the first.
Reagents can be added to the solution (rapid or drop wise injection if liquid or poring them into the
reaction vessel if in powder form) or they can be added to the solution at a temperature in which no
reaction is going to occur. In the latter case the temperature will be then risen to the thermal
decomposition point to allow the nucleation. Adjusting the reaction parameters, such as reaction
time, temperature, precursors and surfactants concentration and type we can control composition,
size, shape and the quality of the product. As we will see in the next chapter a high ratio between
the concentration of surfactants and precursors in solution led to synthesis of very small
nanoparticles.
When the nanoparticles have the required size we can stop the synthesis just by quickly cooling
down the solution temperature. The particles are then isolated from the growth solution by adding a
solvent that is miscible with that one in which the nanoparticles are but it is incompatible with the
NCs surfactants. This incompatibility destabilize the solution and the NPs form big clusters in
solution which can be precipitated with a centrifuge or by decantation. The presence of the
surfactants on the NPs surface prevent their agglomeration during the precipitation process and the
addition of fresh solvent with which the surfactant are compatible led to a stable colloid. The synthesis in solution is so far one of the most used approach for the synthesis of nanoparticles
cause the high number of degree of freedom it allows. It is generally a low cost approach with
which is possible to synthesize nanomaterials of several composition ranging from pure metals,
semiconductors and insulators to their alloys or doped forms. It allows to finely tune the NCs size
(from about 1nm to several hundreds) and shape (i.e. dots, rods, tetrapods, disks Figure I.3).

a) b)
c) d)


Figure I.3. Some examples of nanocrystals shape control: a) CdSe nanospheres, b) CdSe
nanorods, c) CdTe tetrapods and d) Fe3O4 nanodisks, synthesized following the recipes reported in
[9].

An other strength point is the possibility to synthesize particles with a narrow size distribution,
fundamental requirement for several applications of these nanomaterials. The final product can be
delivered in solution or in a powder form. The possibility to have the sample in solution without
any further process make easy the step for mass production and application of nanocolloids in
biology and medicine.


For this thesis I have almost exclusively synthesized nanoparticles with the method just described,
but this is not the only available. Here are presented some among the most diffused techniques since
a complete dissertation is not the object of this thesis.




I.2 Sol-Gel
The sol-gel synthesis is a wet-chemical technique for the fabrication of nanocomposites (typically
metal oxides). Typical precursors for this kind of process are metal alkoxides and metal chlorides,
which are before hydrolized in solution (Sol) and then condensed by solvent evaporation at
relatively high temperature. Via these two steps the precursors form a colloid, a system composed
of solid particles (the size distribution is quite broad ranging from 1 nm to 1 μm) dispersed in a solvent which evolves into an inorganic network containing a liquid phase (Gel). The inorganoc
network is made of metal-oxo (M-O-M) or metal-hydroxo (M-OH-M) polymers. The last step of
the process is the drying which removes the liquid phase from the gel which led to a powder made
of porous material. Among all the ways we can harness this techniue ( i.e. metal-oxide thin films,
ceramic manifacture) there is the possibility to synthesize nano-micro particles and control their
chemical composition, as even a small quantity of dopant can be introduced in the solution and end
up in the product finely dispersed. An example of this approach is repoted in [10] where yittrium
oxide nanoparticles have been synthesized via sol-gel techique, they have been doped with
europium and gadolinium in order to make them red (europium) and green (gadolinium) fluorescent
under UV light. We developed even a preocess to suspend them in a solvent starting from the
nanoparticles powder. This further process made these materials suitable for future biological
application as labels for molecules.
An other very intetresting aspect of the sol-gel technique besides the synthesis of doped nano-micro
particles is the formation of porous solid matrices (aerogel and xerogel). To this class of materials
belong the world’s lightest materials and some of the toughest ceramics.

I.3 Micelles

A micelle is an aggregate of surfactant molecules dispersed in a liquid forming a colloid. The
surfactants constituting a micelle is basically made of two parts, one hydrophilic and the other one
hydrophobic. This particular composition allows to form two kind of micelles known as “normal”
and “inverse” depending on the solvent polarity (Figure I.4).


Figure I.4. Scheme of normal (left) and inverse (right) micelle. (Source Internet
http://en.wikipedia.org/wiki/Micelle)


If the main solvent is water the surfactants constituting the micelle will show the hydrophilic part
hiding the hydrophobic one on their nuclei. The opposite happens if the main solvent is
nonaqueous. The micelles formation from a solution in which some surfactant molecules are
suspended occur when the concentration of surfactant molecules exceeds the critical micelle
concentration (cmc). Further increases of the concentration can tune the dimension of the micelle
cores. Micelles are approximately spherical in shape but other shapes are also possible such as
ellipsoids and cylinders. Generally the micelle shape is tailored by using surfactant molecules with
different geometry but other factors can influence this feature such as surfactant concentration,
temperature, pH and ionic strength. The possibility to tune size and shape of the micelle cores let us
to tune the nanoparticles size and shape. Basically the reaction for the synthesis of nanoparticles by
using reverse micelles involves a precursor which is dissolved into a solution trapped in the micelle
cores since insoluble in the main solvent and an other one which is instead present in solution
outside the micelle. The reaction in this case take place by phase exchange. The different precursors
could be both trapped into the micelles, in that case the reaction occur via micelle coalescence. In the case of normal micelles there is no reactants confinement in fact it is the product that is trapped
into the micelle which act as a polymer that control the nanoparticle size and stabilize the solution.
With this technique several kinds of metal, metal oxide and semiconductive nanoparticles have been
successfully synthesized [11-15].


I.4 Hydrothermal process

Hydrothermal process or hydrothermal synthesis included the various techniques of crystallizing
substances from high temperature (150-700°C) and high pressure water solutions. This method
harness the solubility in water of almost all the inorganic substance under these condition and the
crystallisation of the dissolved material from the fluid. The crystal growth is performed in a
autoclave made of steel or titanium alloys with an hot and a cold end which maintain a temperature
gradient in the reaction chamber. The growth principle is simple: the reaction chamber is filled with
water and reagents, thanks to temperature gradient in the hotter part of the autoclave the reagents
are dissolved while in the cooler part there is the nucleation and consequent crystal growth. Usually
these kind of synthesis need few hours to several days to be completed.
With this technique it as been possible to produce nanoparticles which are not possible at room
temperature cause the peculiar structural properties that reagents and water can have at high
temperature and pressure condition. Tuning of parameters such as reaction time, temperature,
pressure, reagents type and concentration, allows the synthesis of different kinds of nanoparticles
with different size and shape and a good size distribution.
A very interesting example reported by Desvaux et al. [16] in the application of this technique is the
synthesis and the self-assembly of FeCo nanoparticles to form directly in the autoclave a mm scale
supercrystal (Figure I.5).


Figure I.5. SEM-FEG micrograph of a broken FeCo NPs supercrystal. (taken from [16])



I.5 Photo reduction and role of light in nanoparticles synthesis

Light can be used for the synthesis of nanoparticles too. Well known for the synthesis of carbon
nanotubes and fullerenes is the laser ablation technique which is a process of removing material
from a solid by irradiating it with a pulsing laser. The amount of material removed depends
basically on its optical properties and laser wavelength. Performing the laser ablation on a solid
dipped into a solvent in presence of some organic molecules is possible to produce colloidal
solution of nanoparticles which have the organic molecules as surfactants. Some examples of this
technique are the synthesis of metal nanoparticle colloids such as gold and silver [17-19].