Silicon nano-particles: On Route to a Sustainable Mobility

Corrigendum

Some of the Figures and the corresponding text that appear in this work have been taken from already published articles. Although they have already been included in the References Lists, they are specified here once more in order to assign more precisely the contents with the original work.

List of Figures

3.2 Equivalent circuit modeling the electrical response of the SDG [1] 3.3 Waveforms of intrinsic silicon [1,2]

3.4 Fit of capacitor voltage VC[1,2] 3.5 Fit of the discharge current IS G [1,2] 3.6 Photographs of silicon spark discharges [1,2]

3.7 Particle size distributions for different gap spacings and discharge frequencies for intrinsic silicon [1,2]

3.8 Discharge voltage and current for doped Si rods [1,2]

3.9 Particle size distribution for intrinsic and doped Si electrodes [1,2]

3.10 TEM micrographs of nano-particles produced from doped silicon electrodes [1,2,5] 3.11 XRD pattern of nano-particles generated using doped silicon electrodes [1,2,5] 3.12 Colour of the particles as deposited on membrane filters [1,2]

4.1 Size effect on Si phase diagram based on capillary effects [3]

4.2 Flame photograph taken from one of the lateral optical window of the reactor [3,4] 4.3 Reaction zone consisting of a circular nozzle and a round laser beam [3,4] 4.4 Overlapping between gas velocity profile in the laminar mode and laser intensity

profile in the Gaussian mode [3,4] 4.5 Sketch of the proposed reaction zone [4] 4.6 Focusing action of a cylindrical lens [3,4]

4.7 Laser intensity profiles of the laser beam burned in a Perspex block [3,4] 4.8 3D impression of new nozzle [3,4]

4.9 Overlapping between flat gas velocity profile and laser intensity profile in the D-mode [3,4]

4.10 Schematic representation of the set-up [3,4] 4.11 2-D top view and cross section of the reactor [3,4] 4.22 Thermogravimetric analysis of Si-510 sample [3] 4.23 Infrared spectra of Si-510 sample [3]

5.2 Schematic diagram of the electro-spray process [5,6] 5.4 Cross section SEM images of the impacted layer [5] 5.8 AFM images of the impacted samples [5]

5.9 Typical SEM cross section of the electro-sprayed thin layer electrodes [5] 5.10 AFM images of the electro-sprayed thin layer electrodes [5]

6.1 XRD spectrum of the Si powder used for the electrode fabrication [5] 6.2 TEM micrographs of the Si powder used for the electrode fabrication [7]

6.3 SEM image of Si-CMC (3:1) sample deposited by electro-spray on Al substrate [8] 6.4 PCGA profile of the Si-CMC (1:1) sample deposited by electro-spray on a stainless

steel substrate [9]

6.5 Cycle performance of Si-PVdF (7:3) sample deposited by electro-spray on a coin cell bottom plate [8]

6.6 Cycle performance of Si-CMC (1:1) sample deposited by electro-spray on a coin cell bottom plate [8]

6.7 Cycle performance of Si-CMC-C (1:1:1) sample deposited by electro-spray on a coin cell bottom plate [8]

6.8 Proposed capacity fading mechanism for Si composite electrodes [8] 6.8 Proposed capacity fading mechanism for Si composite electrodes [8] 6.9 SEM image of Si-PVdF (7:3) sample [8]

6.10 SEM image of Si-CMC (1:1) sample [8] 6.11 SEM image of Si-CMC-C (1:1:1) sample [8]

6.12 Proposed CMC bonding mechanism with the Si nano-particles surface [8] 6.13 FTIR spectra of the dry powders used during the electrode preparation [8] 6.14 Voltage profiles of the galvanostatic tests performed on pristine Si-CMC (1:1)

electrodes deposited on SS substrate at C/20 rate [9]

6.15 Voltage profiles of the galvanostatic tests performed on surface treated Si-CMC(1:1) electrodes deposited on SS substrate at C/20 rate [9]

6.16 Cycle performance of Si-CMC (1:1) electrode deposited by electro-spray on a stainless steel disk, preliminary Li-treated and galvanostatically cycled at C/20 rate in a lithium cell [8]

6.17 SEM images of the Si-CMC (1:1) surface treated electrode [9]

6.18 Cycle performance of Si-CMC (1:1) electrode deposited via electro-spray on stainless steel disk, preliminary Li-treated and galvanostatically cycled at various C-rates [9]

[1] V. Vons, “Spark discharge generated nanoparticles for hydrogen storage applications”, PhD Thesis, TUDelft,2010.

[2] V. Vons, L. De Smet, D. Munao, A. Evirgen, E. Kelder, and A. Schmidt-Ott, “Silicon nanoparticles produced by spark discharge”, Journal Of Nanoparticle Research, vol. 13, no. 10, pp. 4867–4879, 2011.

[3] J. van Erven, The Improvement and Upscaling of a Laser Assisted Chemical Vapor Pyrolysis Reactor . PhD thesis, TUDelft, 2011.

[4] J. van Erven, D. Munao, Z. Fu, T. Trzeciak, R. Janssen, E. Kelder, and J. C. M. Marijnissen, “The Improvement and Upscaling of a Laser Chemical Vapor Pyrolysis Reactor”, Kona-Powder And Particle, vol. 27, pp. 157–173, 2009.

[5] D. Munao, J. V. Erven, M. Valvo, V. Vons, A. Evirgen, and E. Kelder, “Synthesis of silicon nano-particles for thin film electrodes preparation”, Mater Res Soc Symp Proc, vol. 1245, pp. 207–212, 2010.

[6] M. Valvo, “Electrospray-assisted synthesis methods of nanostructured materials for li-ion batteries”, PhD Thesis, TUDelft, 2010.

[7] D. Munao, M. Valvo, J. Erven, E. Garcia-Tamayo and E. Kelder. Aerosol technology and Si nano-composite electrode assembly for Li-ion batteries. MRS Proceedings, (2011) 1313 , doi:10.1557/opl.2011.1391.

[8] D. Munao, M. Valvo, J. Erven, E. Garcia-Tamayo and E. Kelder. Role of the binder on the failure mechanism of Si nano-composite electrodes for Li-ion batteries. Journal of Power Sources (2011), 196 (16), pp. 6695-6702.

[9] D. Munao, M. Valvo, J. Erven, E. Kelder, J. Hassoun and S. Panero. Silicon-based nanocomposite for advanced thin film anodes in lithium-ion batteries. Journal of Material Chemistry (2012), 22 (4), pp. 1556-1561

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Dit proefschrift is goedgekeurd door: Promotor: Prof. dr. S.J. Picken Copromotor: Dr. E. M. Kelder Samenstelling promotiecomissie:

Voorzitter Technische Universiteit Delft

Prof. dr. S.J. Picken Technische Universiteit Delft, promotor Dr. E. M. Kelder Technische Universiteit Delft, copromotor Prof. dr. Andreas Schmidt-Ott Technische Universiteit Delft

Prof. dr. Ernst Sudholter Technische Universiteit Delft Prof. dr. José Tirado Universidad de Córdoba

Prof. dr. Stefania Panero Università degli studi di Roma, La Sapienza Dr. Herman Kuipers Shell

The research described in this thesis was financially supported by the Shell/TU Delft Sustainable Mobility Program.

© David Munaó, 2012 ISBN: 978-94-6191-262-6

All rights reserved. The author encourages the communication of scientific contents and explicitly allows reproduction for scientific purposes, provided the proper citation of the source. Parts of the thesis are published in scientific journals and copyright is subject to different terms and conditions.

Art direction: Esther Beekman (www.estherontwerpt.nl) Lay-out in LaTeX: Zink Typografie (www.zinktypografie.nl) Printed by: Ipskamp drukkers BV, Enschede

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft op gezag van de Rector Magnificus

Prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op Maandag 7 mei 2012 om 12:30 uur

door David MUNAó Laurea in Chimica Industriale Università degli studi di Roma - La Sapienza

geboren te Rome, Italië

This thesis is approved by: Promotor: Prof. dr. S.J. Picken Copromotor: Dr. E. M. Kelder

Composition of Examination Committee:

Chairman Technische Universiteit Delft

Prof. dr. S.J. Picken Technische Universiteit Delft, promotor Dr. E. M. Kelder Technische Universiteit Delft, copromotor Prof. dr. Andreas Schmidt-Ott Technische Universiteit Delft

Prof. dr. Ernst Sudholter Technische Universiteit Delft Prof. dr. José Tirado Universidad de Córdoba

Prof. dr. Stefania Panero Università degli studi di Roma, La Sapienza Dr. Herman Kuipers Shell

The research described in this thesis was financially supported by the Shell/TU Delft Sustainable Mobility Program.

© David Munaó, 2012 ISBN: 978-94-6191-262-6

All rights reserved. The author encourages the communication of scientific contents and explicitly allows reproduction for scientific purposes, provided the proper citation of the source. Parts of the thesis are published in scientific journals and copyright is subject to different terms and conditions.

Art direction: Esther Beekman (www.estherontwerpt.nl) Lay-out in LaTeX: Zink Typografie (www.zinktypografie.nl) Printed by: Ipskamp drukkers BV, Enschede

Thesis

presented for the degree of doctor at Delft University of Technology under the authority of the Vice-Chancellor

Prof. ir. K.C.A.M. Luyben, Chairman of the Board of Doctorates,

to be defended in public in the presence of a committee on Monday 7 may 2012 at 12:30 o’clock

by David MUNAó Degree in Industrial Chemistry Università degli studi di Roma - La Sapienza

Born in Rome, Italy

Contents

1.2 Silicon and Nanotechnology 1.3 Synthesis

1.4 Applications 1.5 Sustainable Mobility 1.6 Scope of the Thesis

References

2 Porous Silicon - Electrochemical Etching

2.1 Introduction

2.2 Reaction Mechanism 2.3 Making Porous Silicon 2.4 Experimental

2.5 Results and Discussion 2.6 Conclusions

References

3 Spark Discharge Generation

3.1 Introduction 3.2 Theoretical aspects 3.3 Experimental

3.4 Results and Discussion 3.5 Conclusions

References

4 Laser assisted Chemical Vapor Pyrolysis

4.1 Introduction

4.2 Laser assisted CVP Process 4.3 Experimental

4.4 Results and Discussion

10 13 14 19 20 20 21 21 22 25 27 31 32 32 34 38 39 48 49 53 54 54 58 59 68 70 75 76 77 83 97 proefschrift_david manao.def.indd 8 17-4-12 11:31

5.3 Experimental

5.4 Results and Discussion 5.5 Conclusions

References

6 Si-based nanocomposite for advanced thin film anodes in Li-ion batteries

6.1 Introduction 6.2 Experimental

6.3 Results and Discussion 6.4 Conclusions References Summary Samenvatting Appendices A - Li-ion batteries References B - CO2 Lasers References

C - Programmable Logic Controller (PLC)

Acknowledgments Curriculum Vitae Publications 121 125 130 132 137 138 139 141 154 156 161 169 177 178 186 187 198 199 205 212 215 proefschrift_david manao.def.indd 9 17-4-12 11:31

1.1 Power and Energy Density of different electrochemical devices 2.1 Electrochemical cell used for the etching experiments 2.2 Wafer surface after etching

2.3 Dissolved silicon as function of the charge carriers passed 2.4 Schematic representation of the porous layer formation during

the electrochemical etching process 2.5 AFM images of etched silicon 2.6 TEM micrographs of etched silicon

2.7 TEM pictures of aggregated silicon nano-particles 2.8 PL spectra of sample etched at 80 mA/cm2

2.9 PL spectra of samples etched at 80, 40 and 20 mA/cm2

2.10 Absorption spectra of samples etched at 10, 20, 40 and 80 mA/cm2

3.1 Schematic representation of the Spark Discharge Generator connected with Inertial Impactor setup

3.2 Equivalent circuit modeling the electrical response of the SDG 3.3 Waveforms of intrinsic silicon

3.4 Fit of capacitor voltage VC 3.5 Fit of the discharge current ISG 3.6 Photographs of silicon spark discharges

3.7 Particle size distributions for different gap spacings and discharge frequencies for intrinsic silicon

3.8 Discharge voltage and current for doped Si rods

3.9 Particle size distribution for intrinsic and doped Si electrodes

3.10 TEM micrographs of nano-particles produced from doped silicon electrodes 3.11 XRD pattern of nano-particles generated using doped silicon electrodes 3.12 Colour of the particles as deposited on membrane filters

4.1 Size effect on Si phase diagram based on capillary effects

4.2 Flame photograph taken from one of the lateral optical window of the reactor 4.3 Reaction zone consisting of a circular nozzle and a round laser beam

4.4 Overlapping between gas velocity profile in the laminar mode and laser intensity profile in the Gaussian mode

4.5 Sketch of the proposed reaction zone 4.6 Focusing action of a cylindrical lens

4.7 Laser intensity profiles of the laser beam burned in a Perspex block

List of Figures

4.13 XRD spectra of the produced samples

4.14 Effect of the laser power on the crystallite size according to Scherrer’s formula 4.15 Representative TEM micrographs of the analyzed samples111

4.16 TEM micrograph of Si-728 sample 4.17 TEM images process analysis

4.18 Particle size distribution from TEM analyses

4.19 BET surface area estimation and relative equivalent diameter of the produced samples

4.20 Size analysis as a function of the laser power used during the synthesis experiments

4.21 Effect of the laser power on the internal structure of the particles 4.22 Thermogravimetric analysis of Si-510 sample

4.23 Infrared spectra of Si-510 sample

5.1 Schematic diagram of the inertial impaction process 5.2 Schematic diagram of the electro-spray process 5.3 SEM images of the impacted layer

5.4 Cross section SEM images of the impacted layer 5.5 SEM images of the fibrous structure

5.6 SEM images of the impacted layer

5.7 Profilometer scans of the impacted layers indicated in Figure 5.6 5.8 AFM images of the impacted samples

5.9 Typical SEM cross section of the electro-sprayed thin layer electrodes 5.10 AFM images of the electro-sprayed thin layer electrodes

6.1 XRD spectrum of the Si powder used for the electrode fabrication 6.2 TEM micrographs of the Si powder used for the electrode fabrication

6.3 SEM image of Si-CMC (3:1) sample deposited by electro-spray on Al substrate 6.4 PCGA profile of the Si-CMC (1:1) sample deposited by electro-spray on a

stainless steel substrate

97 98 100 101 101 102 104 105 105 106 107 118 124 126 126 126 127 127 128 129 130 141 142 142 143 11 proefschrift_david manao.def.indd 11 17-4-12 11:31

6.5 Cycle performance of Si-PVdF (7:3) sample deposited by electro-spray on a coin cell bottom plate

6.6 Cycle performance of Si-CMC (1:1) sample deposited by electro-spray on a coin cell bottom plate

6.7 Cycle performance of Si-CMC-C (1:1:1) sample deposited by electro-spray on a coin cell bottom plate 6.8 Proposed capacity fading mechanism for Si composite electrodes

6.8 Proposed capacity fading mechanism for Si composite electrodes 6.9 SEM image of Si-PVdF (7:3) sample

6.10 SEM image of Si-CMC (1:1) sample 6.11 SEM image of Si-CMC-C (1:1:1) sample

6.12 Proposed CMC bonding mechanism with the Si nano-particles surface 6.13 FTIR spectra of the dry powders used during the electrode preparation 6.14 Voltage profiles of the galvanostatic tests performed on pristine Si-CMC (1:1)

electrodes deposited on SS substrate at C/20 rate

6.15 Voltage profiles of the galvanostatic tests performed on surface treated Si-CMC (1:1) electrodes deposited on SS substrate at C/20 rate

6.16 Cycle performance of Si-CMC (1:1) electrode deposited by electro-spray on a stainless steel disk, preliminary Li-treated and galvanostatically cycled at C/20 rate in a lithium cell

6.17 SEM images of the Si-CMC (1:1) surface treated electrode

6.18 Cycle performance of Si-CMC (1:1) electrode deposited via electro-spray on stainless steel disk, preliminary Li-treated and galvanostatically cycled at various C-rates

B-1 Spontaneus emission process B-2 Absorption process

B-3 Stimulated emission process B-4 Three level laser diagram B-5 Four level laser diagram

B-6 The energy level diagram of a CO2 laser C-1 PLC Program loop 144 144 145 146 147 147 148 149 150 151 151 152 153 154 188 189 190 193 193 195 201 12 proefschrift_david manao.def.indd 12 17-4-12 11:31

3.1 Results of ICP-OES analysis

4.1 Particle geometrical mean size and standard deviation of the particle size distribution calculated from TEM experiments

5.1 Electro-sprayed sample composition

5.2 Electro-spray parameters used during the electrode deposition

68 103 123 123 13 proefschrift_david manao.def.indd 13 17-4-12 11:31

List of Symbols

List of Symbols

xxviii

α Numeric coefficient in Eq. 4.2.14

αs Transfer coefficient in the Butler-Volmer equation (A-10)

β∗n Collision rate between monomers and critical nuclei

βij Collision probability between the −ithand the −jthparticle

∆G Gibbs free energy ∆HM Latent heat of fusion

 Permittivity η Carnot efficiency η Overpotential

γLV Liquid-Vacuum surface energy

γSL Solid-Liquid surface energy

γSV Solid-Vacuum surface energy

λ Wavelenght

µ Reduced mass

ν Wavenumber

ν0 Kinematic viscosity

ρ(ν) Radiation density of photons of frequency ν ρL Liquid phase density

ρS Solid phase density

σ Conductance

14

xxix σ_∞ Surface energy of a flat interface

σg Geometric standard deviation τ Characteristic time of the particle

τij Transition lifetime between ithand jthenergy levels A, B, γ Experimental coefficients in Eq. 3.2.3

A21 Einstein coefficient for spontaneous emission

B21 Einstein coefficient for the absorption process

c Speed of light 299792458 ms−1

Cn∗ Critical nuclei volumetric concentration c0 Speed of sound

dg Electrodes gap distance Dj Nozzle diameter

dg Geometric average particle size E∗ Size dependent Band Gap Energy Eg Band Gap Energy

En Energy level of the nthquantum state

F Fenn number fs Spark frequency h Plank’s constant 6.602·10−34Js i Current density 15 proefschrift_david manao.def.indd 15 17-4-12 11:31

List of Symbols

xxx

J Flux of critical nuclei

k Boltzmann’s constant 1.38·10−23JK−1

M Mach number

me Electron mass mh Electron hole mass

n∗ _{Number of monomers needed to form a critical nucleus} Nn Number of atoms populating the nthenergy level O∗ Critical nuclei surface area

R Ideal gas constant 8.314462 J/mol K

r Particle radius Re Reynolds number Stk Stokes number

TE Melting point of the bulk phase TM Melting point

XT

ij Number of particles generated by the collision between the −ithand

the −jth_particle C Capacity Cth Theoretical capacity C/n C-rate d Diameter 16 proefschrift_david manao.def.indd 16 17-4-12 11:31

Introduction

Chapter 1

20

1.1

Preface

Silicon is a fascinating material for modern electronic devices and an intense research on various aspects, both on fundamentals as well as on its applications, has been carried out in the last decades. By tailoring down the size of the silicon devices, which are fabricated in the form of clusters, particles or quasi-one-dimensional structures such as nano-wires, new and interesting properties arising from the quantum confinement effects have been discovered. These, add new desired features to silicon, making it applicable, for instance, as an optical material. Bright luminescence in silicon was first discovered in the form of porous silicon and since then numerous studies have been carried out on nano-particles as well as nano-wires to understand the origin of photoluminescence and a broad variety of applications have emerged. In recent years it has become possible to produce nano-sized silicon with a controlled diameter. Such nano-structures have a huge potential for the development of silicon based devices such as, for instance, sensors, logical gates, memories, and systems for optoelectronic and biological applications.

Among these applications, one of the most recent and attractive fields in which sil-icon nano-particles can be used into, is energy storage [1]. The energy economy is playing a major role in the development of our modern society and is, in general, one of the most crucial aspect of human species evolution. Modern economics is strictly dependent on the energy source and its supply, as well as its utilization in both urban and industrial contexts [2]. Renewable energy sources are gaining popularity not only because their price per unit of energy is decreasing rapidly, but especially because the idea that relies behind their utilization implies the realistic view of a world in which a finite number of resources cannot sustain, obviously, an infinite progress of growth and consumption [3]. The use of renewable energy is strictly interconnected with an effi-cient way to store and convert the energy. Renewable energy sources in fact, like solar, wind or biomass are normally available during a limited period of the day/year, while the human energy consumption do not follow the same trend [4]. Another point that cannot be underestimated is the fact that the energy consumption is often delocalized with respect to the location where the energy is produced. From these perspectives, efficient ways of storing the energy are strongly required and demanded by the emerging energy economy.

1.2

Silicon and Nanotechnology

The area of nanotechnology is one of the most active field of science today. It is often seen as the area that could, in principle, lead to substantial progress in terms of finding new materials with new properties [5]. In particular, silicon nano-particles are found to be greatly attractive because of their significant technological implications. Among them, several interesting applications derive from the versatile and wide-ranging optical, electronic and biocompatible features that are arising from the quantum confinement of this material [6]. These properties, together with the fact that Si is an abundant and environmentally benign

21

Table 1.1: Synthesis methods to produce Si nano-structures.

Synthetic Route Example Size [nm] Remarks

Physical Techniques Ion implantation ∼ 3 Particles embedded in a matrix glass [7] Ball milling ≤ 100 Contamination from milling agents [8] Laser ablation ∼ 9 Colloidal solution of particles [9] Spark discharge 2-7 High purity [10]

Chemical Techniques Reduction 2-10 Colloidal solution of particles [11] Electrochemical Techniques Etching ∼ 1 H-passivated particles, batch process [12] Physico-Chemical Techquiques Plasma CVD 10-100 Size is tunable with the synthesis parameters [13]

Laser assisted Pyrolisys 20-70 SiOxsurface passivation [14]

element, substantially contributes to raise the attention of engineering, physics, chemistry, material science, biology and medical research towards the development of different syn-thesis routes, which lead to new silicon nano-structures.

1.3

Synthesis

It is important to keep in mind that a conventional method to produce Si nano-particles is somehow difficult to define, being nanotechnology a relatively new branch of science. Many different techniques can be used in order to synthesize Si nano-particles. They can be conveniently grouped into four main classes, according to the approach or synthetic route used during their preparation. Each route presents specific advantages and draw-backs, which are mainly related with the purity of the product, its size, its size distribution and its production rates. They are briefly mentioned in Table 1.1 for general comparison purposes.

Since different production methods lead to different products, each synthesis tech-nique is commonly associated with one or more applications of the Si nano-particles. In this respect, it is interesting to give a quick insight of all the possible effects of this product in our every day life with respect to the new properties arising from the size effect.

1.4

Applications

One of the most interesting properties found with the small size of the nano-silicon based structures is the photoluminescence effect. Photoluminescence has been observed with the discovery of porous silicon in the late 80’s, when porous silicon was usually fabricated with electrochemical etching techniques [15]. Nano-silicon based structures have been reported to luminesce efficiently in the near infrared, in the whole visible range and in the near UV [16]. Quantum confinement of charge carriers in Si was the first model proposed to explain porous silicon luminescence. Afterwards, many other alternative explanations have been proposed [17].

Chapter 1

22

Indeed, the interest in nano-silicon has renewed after the observation of its emis-sion properties, and the potential application areas of this material has spanned a wide area of technology, ranging from optoelectronics to sensors, biotechnology and energy conversion. Molecular level detection of gases [18] [19] and molecular species [20], bio-imaging [21, 22], memory devices [23], photodetectors [24] and nano-solar cells [25] are among the applications that have been achieved. The advantage of using such materials in the above mentioned devices relies in the key properties that the material can offer, such as, for instance: efficient electroluminescence, non linear optical properties, low refractive index, ambient sensitive properties and tunable chemical reactivity.

Silicon is of great interest in the energy storage field. Silicon, in fact, is considered as one the most promising chemical element for energy storage in modern electrochemical devices [26] due to the fact that it can alloy with lithium, the lightest and most electroneg-ative metal in the periodic table [27]. In this respect, energy storage based on lithium chemistry is one of the most advanced electrochemical system in terms of specific power and energy density (see Figure1_{1.1) [28]. Accordingly, Li-ion batteries could be significantly}

improved in terms of volumetric and gravimetric energy density by using silicon as a negative electrode material. Theoretically in fact, Si is able to store more than ten times the energy stored in a conventional electrode based on graphitic carbon. Being one of the most abundant element on the earth surface, Si is definitely one of the best candidate for developing a new generation of Li-ion batteries.

1.5

Sustainable Mobility

The concept of sustainable mobility derives from a more broad concept, called

sustainabil-ity, which accounts for the intimate relationship between the development of our modern

society and the quantity of raw materials available on the planet. From this very simple concept, it is clear how the withdrawal of fossil fuels as energy source is becoming, day by day, more relevant. The motivations that are behind this decision have an economical and ecological nature [29]. Nowadays, the main energetic source used by human kind are fossil fuels. The reasons for this can be attributed to their remarkable features, such as their high energy to volume ratio, their easy storage/transportability and their employment in relatively simple thermal machines. In particular, this last feature is the major reason why the automotive industry is so reluctant towards the use of new propellants. On the other hand, the combustion of fossil fuels holds several drawbacks, among which:

•Local pollution determined by NOx, SO2and carbon particles.

•Green-house gas production (CO2).

•They are not sustainable, as the amount of time needed for the fossilization of the organic matter is extraordinarily large compared with the time involved in the combustion process.

23

Figure 1.1: Power and Energy density for different electrochemical energy storage devices. Data shown in the Technology Roadmaps: Electric and Plug-in Hybrid Vehicles from the International Energy Agency.

The last point underlines the fact that the oil deposits are going to end while the energy demand is constantly rising. Clearly, new alternatives have to be considered in order to maintain and sustain our current energy need. The future scenario in which oil will be less abundant and more expensive stimulates the scientific world to research good alternatives for its main applications (i.e: energy production, transportation and chemicals production). In particular, important consequences can be foreseen in the secondary industry, if new and more efficient alternatives will not be adopted, not excluding a profound and extended global crisis [30]. The climatic importance of the main product of combustion, CO2, derives

from its own capability to absorb the infra red part of the solar spectrum, whereas the other atmospheric gases are transparent to it. In order to have a stable temperature on Earth, it is necessary that the net flux of incoming energy equals the net flux of outgoing energy. The last term is constituted by the energy emission in the infra red region. Since the incoming radiation wavelength is different from the outgoing one, it is clear how gases like CO2 are not only absorbing the incoming solar radiation, but also the outgoing one.

The energy absorbed is then re-emitted in all directions, also towards the earth surface, contributing to increase the total incoming radiation energy flux. This fact induces a change in the radiative equilibrium of the planet, which tends to be set to a higher temperature.

Chapter 1

24

This phenomenon is commonly referred as the greenhouse effect [31]. According to the climate mathematical models developed by the International Panel on Climate Change (IPCC)2, the Earth temperature will rise between 1.4 ◦C and 5.8◦C in the time period that spans from 1990 to 2100. It is widely believed that this will provoke several climate changes, with one of the most significative being the rise of the sea levels. These changes can contribute to increase the frequency of, for instance, floods, dry seasons or reducing biodiversity.

The general tendency that follows the development of novel technologies for the stor-age and conversion of the energy focuses on the attention on two crucial points: energy savings and the use of clean or sustainable energy sources. In order to understand clearly what is the meaning of the energy savings, apart from avoiding useless wastes of energy, it is fundamental to shift the attention to the efficiency of any energy conversion process. In this respect, thermodynamic laws strictly fix the efficiency of an ideal thermal machine with the Carnot rule:

η = 1 −TC TH

(1.5.1)

with TH and TC being the upper and lower temperature between which the machine is

working. It’s value, for the common temperatures used in combustion engines, is around 0.3. It is important to remember that this is an ideal value, being the Carnot machine an ideal device that works reversibly with subsequent equilibrium states. In real cases, this value goes easily down to 0.25, which means that the remaining 0.75 fraction of the chemical energy contained in the fuel is lost in heat formation. In order to have comparison data, it is worth expressing the efficiency of a vehicle in well-to-wheel terms, or in other words, in km driven per megajoule (km/MJ) of fuel consumed. The most efficient ordinary gasoline car made was the 1993 Honda Civic VX, which showed a well-to-wheel efficiency of 0.52 km/MJ for combined city and highway driving. On the other hand, one of the most sportive model of electric car, the Tesla Roadster, shows a well-to-wheel efficiency of 2.53 km/MJ3.

The very low efficiency of combustion engines, associated with the inherent toxicity of the combustion side-products, strongly indicates that the road transportation should be based on other technologies, such as, for instance, electric or hybrid vehicles. Elec-tric vehicles constitute a good example of the sustainable mobility concept. Their high efficiency (0.8 to 0.9 with the modern cell stacks), combined with the total absence of side-products, allows them to be the perfect choice for urban transportation, such as car and buses. Other advantages coming from the use of an electric motor would be, for instance, the possibility of recovering the deceleration energy via electro-magnetic brakes.

2 http://ipcc.ch/

25

1.6

Scope of the Thesis

The present work is part of a bigger framework, called the Sustainable Mobility Project, in which Academia and Industry are combining their intellectual and economical potential in order to find novel alternative solutions to be applied in the current transportation system. Specifically, the aim of the present thesis is to find a complete pathway of a new material, silicon nano-particles, from synthesis to the final designed application: a negative electrode for Li-ion batteries.

In this respect, the work has been structured as follows.

Chapters 2, 3 and 4 describe three different synthetic routes to obtain silicon nano-particles: Electrochemical Etching, Spark Discharge Generation and Laser assisted Chemical Vapor Pyrolisys, respectively. These techniques have been investigated in order to select the most suitable one with respect to the final designated application of the nano-powders. The synthesis techniques are thus reviewed in great detail, from their fundamental working principles to an accurate chemico-physical characterization of the obtained product. The most promising technique, in terms of purity of the product, size and size distribution of the particles as well as its great capabilities in terms of production rate, it is found to be the Laser assisted Chemical Vapor Pyrolisys (LaCVP). The results presented in Chapter 4 are the follow up of a previous research project carried out within the TUDelft, in which a LaCVP reactor was successfully upscaled to pilot levels [32].

Nevertheless, the results presented in the other two chapters are very interesting from a technological point of view. Spark Discharge Generation of silicon nano-particles proved to be an efficient aerosol method to produce nano-structures that are similar to the ones obtained by the Electrochemical Etching. The high purity of the powders, the absence of a liquid phase, the small size of the particles, the excellent particle size distribution, the appreciable production rate and the possibility of mixing different element in the Spark to obtain completely new alloys are all features that allow this method to be applicable for the fabrication of novel sensing devices.

Chapter 5 is a bridge between the synthesis step and the final application of the prod-uct. For most electronic and electrochemical applications, in fact, silicon nano-particles have to be incorporated in a real electrode assembly. In this chapter, two novel methods for electrode coating are proposed: the Inertial Impaction and the Electro-spray processes. While the first one can be easily coupled to any aerosol production method in order to print defined structures and patterns, the second one can offer great flexibility in terms of composite materials formation. Since battery electrode materials are often combined with other additive components, like polymeric binders or electronic conductivity enhancer, the Electro-spray technique was investigated for fabricating these composites structures. Moreover, this method can be easily upscaled to produce large surfaces by using roll-to-roll technology. In principle, it is possible to process the whole battery assembly in one single step.

Chapter 6 describes the electrochemical behavior of the silicon nano-composite elec-trodes in a half cell configuration (i.e. Si vs. Li). The first part of the chapter is dedicated to the explanation of the failure mechanisms of the silicon-based electrodes, while the second part offers viable solution to the capacity fading issue and the short lifetime of

Chapter 1

26

such electrodes. In this respect, a major role is played by the choice of an appropriate polymeric binder, which can accommodate the large volume expansion occurring dur-ing the electrochemical alloydur-ing of lithium. Moreover, a new chemical surface activation method of the electrode is proposed in order to overcome the characteristic capacity loss of the first lithium uptake process, in which a substantial part of the lithium coming from the counter electrode is lost irreversibly.

27

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[17] G. Bomchil, A. Halimaoui, I. Sagnes, P. Badoz, I. Berbezier, P. Perret, B. Lambert, G. Vincent, L. Garchery, and J. Regolini, “Porous silicon: material properties, visi-ble photo- and electroluminescence”, Applied Surface Science, vol. 65–66, no. 0, pp. 394–407, 1993.

[18] C. Baratto, G. Faglia, E. Comini, G. Sberveglieri, A. Taroni, V. La Ferrara, L. Quercia, and G. Di Francia, “A novel porous silicon sensor for detection of sub-ppm NO2 concentrations”, Sensors and Actuators B: Chemical, vol. 77, no. 1–2, pp. 62–66, 2001.

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[32] J. van Erven, The Improvement and Upscaling of a Laser Assisted Chemical Vapor

Porous Silicon -

Electrochemical Etching

Chapter 2

32

2.1

Introduction

In order to uniformly or referentially remove material from bulk material immersed in a solution, an etching technique is often used. In that regard, etching of silicon has been extensively explored due to its useful applications in the fabrication of electronic de-vices. Since the 1950s, when etching started to be used in device fabrication processes, numerous investigations have been carried out to develop and characterize the etch-ing systems for micro machinetch-ing, delineation of defects, surface polishetch-ing, and so on. An interesting side effect of the electrochemically etched silicon was the observation of room temperature fluorescence, which initiated intense research activity over the next decade [1]. The prospect of incorporating photoactive silicon with the current technology in the every-day used devices encouraged both academic and industrial researchers with hopes for new display technology or perhaps with new optoelectronic computing elements (i.e.: photonics). One aspect that made this research activity so attractive was the ease with which it could be performed. With common reagents and simple equipment it is possible to fabricate a nano structured material with interesting photo luminescent properties. Beyond the ease of making such material it was immediately understood that the formation mechanism of the porous structure is rather complex and many parameters are involved.

2.2

Reaction Mechanism

The first attempts of creating porous silicon were made with the intention of polishing a silicon surface. This work was carried out by Robbins and Schwartz, who investigated the chemical etching mechanism of a silicon surface and its kinetic with a mixture of HF and HNO3 [2–5]. Although they did not consider the presence of porous silicon films,

they identified the two major steps of the reaction, i.e. first the oxidation of the silicon surface and then the oxide dissolution in HF. Turner also studied the chemical etching mechanism [6]. He emphasized the idea that the chemical process was still electrochem-ically driven. When an oxidant species would approach the surface it would extract an electron from (or it would inject a hole into it) making that point a local cathode. The electronic hole would then migrate across the surface until it found a silicon atom which was ready to be attacked by a HF in a local region where progressive anode dissolution of the material would generate an etch pit. It was concluded that the dissolution of silicon at low current (where porous silicon is formed) consumed two electron holes for each Si atom according to:

33

and at higher current densities, where polishing of silicon occurs and when there were more electronic holes present than HF, the reaction proceeded by the following tetravalent mechanism:

Si + 4HF + 4h+→ SiF4+ 4H+ (2.2.2)

it is expected that SiF2, being very unstable, would lead to a disproportionation reaction

to produce stable SiF4and Si according to:

2 : SiF2 fast GGGGGGA Si0+SiF4(g) (2.2.3) Si0 slow GGGGGGGA +2H2O SiO2(s) +2H2(g) (2.2.4) SiO2(s) fast GGGGGGA +6HF H2SiF6(aq) +2H2O (2.2.5) Si0 fast GGGGGGA +6HF H2SiF6(aq) +2H2(g) (2.2.6)

From gravimetric analyses it was found that only the 20% of the dissolved silicon is partic-ipating to porous layer formation, the rest is involved in the formation of either the oxide or the hexafluoride. Theunissen performed SEM and XRD experiments on electrochemically formed porous Si, finding out that the material was still crystalline [7]. The crystalline nature of the film, together with the presence of etch channels in specific crystalline direction, has demanded a re-formulation of the reaction mechanism, which was given by Unagami [8] with the following principles:

•The formation of a porous layer is arising from the reactions described in 2.2.3, 2.2.4 and 2.2.6, however this reaction is only occurring at the beginning. Then, once the film has grown enough, the porous film starts to be resistant to further HF attack.

•The pores continued to be etched by a nondisproportionation reaction. The SiF2reacted

directly with additional HF to form the soluble H2SiF6. he suggested that the this silicic

acid was adsorbed on the walls of the pores and prevented further etching. In this way, etching could only occur at the bottom of the pores, leading to pore growth in the normal plane of the Si surface.

Details regarding the microstructure of porous silicon were reported in a systematic TEM study by Beale et al. [9]. They formed porous silicon from both p− and n−doped

ma-Chapter 2

34

terial over a wide range of doping levels, electrolyte concentrations and etching current densities. Generally speaking, it was found that the porosity increased with:

•decreasing HF electrolyte concentration

•increasing current densities

•increasing substrate resistivity

X-ray diffraction work [10] confirmed the crystalline nature of porous Si. This was observed for even very thick layers. A Rutherford backscattering (RBS) and nuclear reaction study [11] showed that porous silicon was indeed c-Si but that the walls were mostly covered by Hydrogen (50%), with small amounts of Carbon (≈3%), Oxygen (≈5%), and Fluor (≈1 %). The carbon probably was a result of atmospheric contamination. This clearly showed again the importance of SiH moieties but not of oxide, amorphous or crystalline, in defining the structure of porous Si. A clear, detailed understanding of the many processes occurring during porous silicon formation was given by work from Unno et al. [12] who noted the presence of two distinct reactions that were active simultaneously during the anodic etch-ing process. The first was the electrochemical process that led to the formation of porous Si. However, it was clear that another non-electrochemical process was also involved in the further etching of the porous silicon frame-work. They identified the chemical oxidation of the porous framework by H2O which was subsequently followed by direct chemical

attack by HF. Though this reaction was much slower than the electrochemical pathway, it was nevertheless measurable and reminded us of the presence of the traditional oxide forming reactions. Because of this, porous silicon density was shown to decrease with anodic etching time, soaking time, and decreasing HF concentration. The latter was a result of increased pH which would promote the oxidation step.

2.3

Making Porous Silicon

As listed in the following sections the main methods to produce porous silicon structures are either electrochemical or chemical.

2.3.1 Electrochemical Methods

The experimental arrangement is rather simple and it employs a basic electrochemical cell. Because HF is being used, glass is not an appropriate container as it will be vigorously corroded by the hydrofluoric acid. An inert container such as polyethylene or Teflon should be used. The counter-electrode can be a platinum wire or a graphite rod. A Pt foil is often used so that it provides a more uniform electric field across the sample, which should contribute to a more uniform etching process. A galvanostatic (constant current) or sometimes a potentiostatic (constant voltage) DC power supply is connected to the two electrodes. As this is an anodization process, the positive terminal is attached to the silicon working electrode. In more accurate electrochemical cells, a third reference electrode is also present to monitor and control the solution potential. The silicon sample needs to be made into an electrode. This can be accomplished rather simply by depositing a thin

35

layer of Al on the back side. When the back face of the silicon is degeneratively p−doped, an ohmic contact with the HF solution is formed without the deposition of a thin metal film. HF can be obtained commercially as an aqueous solution in various concentrations, the most common being approximately 49% HF. A cautionary word of safety is also appropriate here. HF is a particularly dangerous chemical and must be handled carefully and intelligently. HF is a particularly dangerous acid because of its unique ability among acids to penetrate tissue. The reason for this is the high electronegativity of the fluoride anion which holds onto the hydrogen cation tightly. The result is a weak acid that exists predominantly in the undissociated state compared to other acids. In the undissociated state the HF molecule is able to penetrate skin and soft tissue by non-ionic diffusion. Once in the tissue the F−anion is able to dissociate and cause liquefactive necrosis of soft tissue, bony erosion, as well as extensive electrolyte abnormalities by binding the cations Ca2+and Mg2+. This is unusual among acids which typically cause damage via the free H+ cations resulting in coagulative necrosis and poor tissue penetration. The ability to penetrate tissue is the reason why HF can cause severe systemic toxicity from even relatively small dermal exposures and why exposure to this compound should be treated with extreme caution. Workers and scientists within TUDelft have been instructed with a specific safety course before handling HF solution.

The main parameters that are influencing the porous silicon formation and structure are:

•HF concentration. The etching process depends upon a balance between the supply

of holes to the surface from the power supply and the arrival of F−etching ions from solution. When the process is limited by charge supply through the Si electrode, porous silicon is formed. The film porosity increases as the HF concentration is decreased.

•Current density. The other way to control this reaction is through the current density.

At low current density, holes arrive slower to the surface and the reaction to produce porous silicon is enabled. As the current density is increased, the film porosity increases until the system moves into the transition region and finally electropolishing sets in. For a fixed electrolyte solution, there is a certain critical current density Ipsabove which porous

silicon is not formed. For more concentrated HF solutions, Ipsis correspondingly larger,

providing a larger current range over which porosity can be obtained.

•Surfactants. Hydrogen gas is on of the products of the reaction (see equation 2.2.4).

The molecules agglomerate at the surface to form bubbles which attach themselves to the surface and grow inside until they are large enough to break: free and float to the top. Bubbles tend to introduce non-uniformity to the surface and their regular removal is important for a uniform film formation [13]. The most common method involves the addition of a surfactant, usually ethanol, though other materials have been used to decrease the surface tension of the solution so that the bubbles detach easier. Ultrasonic agitation and mechanical stirring have also been employed, though these steps are not universally required for quality film production.

•Substrate doping polarity and level. Silicon anodization in general and porous silicon

formation in particular depends upon the presence of holes in the substrate for the reaction to proceed. A space charge region is formed at the surface of a sample when

Chapter 2

36

it is brought into contact with an electrolyte solution. This arises because the mobile charge carriers are depleted from this region as the Fermi levels adjust throughout the system. When an anodic bias is applied to the sample, this space charge layer is very thick (several µm) in the case of lightly doped n−-type silicon and is very thin for heavily doped p+_{- and n}+_{-doped silicon. Porous silicon formation is relatively easy with p}−

, p+_,

and n+_{material, but production in n}−

requires considerably larger potential fields.

•Light. Charge carriers, including these reactively important holes, can be generated

by the absorption of light in the semiconductor. At higher light intensities, it has been found that with the higher photo generated carrier rate, the reaction can be inhibited on p−doped materials and this has been used in lithographic patterning of porous Si. Summarizing, for the basic production of quality porous Si, these general guidelines should be followed [14]:

•Use an electrolyte solution concentrated in HF (10%).

•Use moderately doped (1015_-1017_cm−3

) p−silicon substrates.

•Use a constant current density (10-100 mA/cm2_).

•Anodize in the dark.

2.3.2 Chemical Etching Methods

Since electronic holes are found to be necessary for producing porous Si, another way to introduce them is to provide an appropriate oxidant in the system. A variety of chemical solutions have been used for decades to achieve particular etching objectives on Si. Usually a mixture of nitric acid and HF, including eventually a few other components is used in that respect. The main parameters that are influencing the porous silicon formation and structure are the same listed in the previous section, including the use of solvents other than water and the concentration of the chemical oxidant.

2.3.3 Non-Chemical Methods for Si etching

Not only chemical methods have been employed for producing porous silicon. Spark ero-sion produces material that has all the features of porous silicon [15, 16]. Such treatment of a Si wafer using a high voltage produces porous silicon which is reminiscent of anodically formed porous silicon in all ways, structurally and spectrally. This material, because of its unique production environment, has helped to elucidate the nature of porous silicon and the mechanisms of its production. The formation of silicon nano-particles via spark discharge will be described thoroughly in Chapter 3. Nano-crystallites of silicon have been formed by various gas phase methods [17–20], often involving chemical deposition from silane like molecules. These nano-crystallites are clearly very different from the porous materials produced anodically and their synthesis via a specific silane decomposition technique will be presented in Chapter 4.

37

45 mm

20 mm

10 mm

80 mm

Figure 2.1: Electrochemical cell used for the etching experiments with top view and side views

respectively. Electrodes surface exposed to the electrolyte solutions was equal to 2 cm2_{and the distance}

between the electrodes was set to 4.5 cm. Experiments were performed at room temperature under normal laboratory light conditions.

Chapter 2

38

2.4

Experimental

In the present study, electrochemical anodization of the silicon wafer was carried out in a HF : H2O2: C2H5OH water solution. Reagents (HF 50% Fluka, H2O230% Merk, C2H5OH

100% Baker) were used as received, without any further purification. Hydrogen peroxide was chosen as a strong oxidant in order to increase the charge carrier density at the sur-face of the silicon. Moreover, it has been shown that, the incorporation of the peroxide into the system leads to smaller nanoparticles with respect to the ones produced in water [21]. Hydrogen peroxide is also accounted for the formation of an ideal mono-hydride stretching phase and the elimination of defects or impurities at the surface of porous silicon [22]. As a result, the particles are dominated by H-termination [23]. The volume ratio between the reagents was set as 1:1:2 respectively. Samples were electrochemically etched at a constant current density. 10, 20, 40 and 80 mA/cm2 were chosen for the produced samples. While the current density is kept constant by a DC current source (Keithly 6220) the voltage can go up to a threshold of 105V. The voltage value is determined by the total resistivity of the electrochemical system, which may vary during the etching process. Silicon wafer, p−type Boron-doped, (100) oriented, 500-550 µm thick, 2-5 Ωcm resistivity (Memc Electronic Materials SDN - corresponding to a doping level of≈ 3·1015

cm−3) was cut by means of a diamond tip into 1x8 cm electrode and placed in an electrochemical cell with a parallel symmetrical geometry. This allows for a a more uniform current distribution, and therefore, a more homogeneous etching reaction on the Silicon surface. A platinum electrode with the same dimension as Si, was used as counter electrode. A schematic representation of the electrochemical cell used is given in Figure 2.1. The fabrication of Si nano-particles from the porous layer formed on top of the Si surface is a two-step batch process. In the first step the porous layer is formed via electrochemical etching. The porous layer, as mentioned before, consists in thin wall-structures normally oriented to the Si surface. As these structures are weakly bonded to the rest of the surface, it is fairly easy to remove them with mechanical force. Wafers were first etched at a constant current density for 60 minutes, then removed from the electrolyte bath, washed with ethanol, dried with N2and immersed in an ultrasound ethanol bath for 15 minutes. The etched surface

area is shown in Figure 2.2. The nitrogen flow was dried in a glass column filled with P2O5powder before use. The mechanical action of the ultrasonic waves (provided by a

stainless steel finger, Dr. Hielscher GMBH, 100W 42kHz) leads to the detachment and the fragmentation of the porous structure into small nanoparticles, which are then collected in an ethanol suspension. The silicon wafer substrate is weighed before and after etching and the vibration process to determine the etching and the production rate of the silicon nanoparticles. Samples were weighed on a Mettler Toledo AE 240 scale with a precision of 0.01 mg. The surface morphology of the Si wafer was studied by means of Atomic Force Microscopy (AFM) on a NT-MDT NTEGRA scanning probe microscope in semi-contact mode, using a Si cantilever and tip (NT-MDT, Silicon: NSG 03), before and after the sonication step. Transmission Electron Microscopy (TEM) was carried out to study the morphology and size ethanol-suspended nano-particles. TEM was performed using a Philips CM30T electron microscope with a LaB6 filament as electron source operated at

39

Figure 2.2: Wafer surface after 60 minutes of electrochemical etching at 10 mA/cm2_.

and porous silicon on a carbon polymer (Quantifoil) supported by a copper grid, followed by drying at ambient condition in order to evaporate the ethanol. Ethanol suspended nano-particles were also used for optical measurements. After acquiring the blank spectrum for the ethanol on the UV-Vis Spectrophotometer (Perkin Elmer Lambda 40) the sample was measured in absorption mode in a spectral range between 210 and 400 nm. After having observed a broad absorption band between 220 and 340 nm, several excitation wave-lengths were chosen to induce photoluminescence in the samples, 240, 260, 280, 300 and 320 nm respectively. Photoluminescence spectra (Photon Technology International) were recorded by choosing the wavelength ranges were just above the excitation one (i.e for 240 nm the measure starts at 260 nm) and by avoiding overtone (i.e.: 480 nm in the case of 240 nm). The step size was set to 1 nm and the integration time to 1 sec.

2.5

Results and Discussion

The surface of the wafer after etching is shown in Figure 2.2. The colored path on its surface is the result of the interaction between the visible light and the porous layer structure. Clearly, the refractive index difference between the bulk silicon and the surface porous layer is generating a thin-film diffraction pattern-like. In particular, the interference pattern shows some regularities, which would suggest that by knowing the thickness of the porous layer with a good accuracy one could make a good estimation of its refractive index, and therefore, more information on the optical properties of the Si nano-particles would be obtained. The mass analysis results after the etching and the sonication process are listed in table 2.1. The Si nano-particles production rate is not very reproducible, making it not really linear with the current density used in the etching process. Clearly this is due to the efficiency of the mechanical removal of the porous network by sonication, which is performed by means of ultrasonic waves on the wafer surface. In contrast, the mass loss of the wafers by the etching process is very reproducible. The results have been plotted in Figure 2.3, where the amount of dissolved Si moles is plotted versus the electric charge during the etching process, which is calculated based on the surface area

Chapter 2

40

Table 2.1: Mass analysis of etching and vibration processes.

Current density Etched Area m∗Si m ∗∗ Si mSi m∗∗∗Si mSi etched nanoparticles (mA/cm2_{) (cm}2_{) (mg) (mg) (mg) (mg) (mg)} 10 2 862.49 857.59 4.9 857.51 0.08 10 2 857.51 853.63 4.88 852.59 0.04 20 2 982.38 971.88 10.5 971.83 0.05 20 2 971.83 961.33 10.5 961.2 0.13 40 2 923.36 904.24 19.12 904.17 0.07 40 2 904.17 884.11 19.94 884.11 0.12 80 1 1013.48 993.14 20.34 992.97 0.17 80 1 992.97 972.88 20.09 972.71 0.17

* mass of the substrate before etching. ** mass of the substrate after etching. *** mass of the substrate after sonication.

0 20 40 60 80 100

Current Density [mA/cm ]

0 5 10 15 20 25

Etc

hing Densit

y [mg/cm ]

Figure 2.3: Dissolved silicon as function of the charge carriers passed. The linear relation between etching density and current density is illustrated. It is explained by the mechanism of the electrochemical reaction. The electrochemical reaction requires the participation of electronic carriers, holes transfer across the silicon/electrolyte interface when the reaction occurs. The higher current density, the more electrons participate in the reaction per unit time and the more Si atoms are oxidized and consumed, as a result.

41 Si p -type HF, H2O2, CH3CH2OH → i , t

AFM

AFM

TEM

Figure 2.4: Schematic representation of the porous layer formation during the electrochemical etching process and its subsequent fragmentation into nano-particles agglomerates. Material microscopy analyses are highlighted in the dashed boxes.

and current density and the time of the experiment. It is worth noting that these mass analysis confirms the reaction mechanism proposed earlier (see equation 2.2.1). This can be seen by the linear dependency between the etching density (calculated by normalizing the etched mass with the surface in contact with the electrolytic bath) and the etched surface, as for each mole of silicon that has been dissolved in the electrolytic bath, two moles of electrons have been consumed in the process (Figure 2.3).

The materials have been characterized with different microscopy techniques in order to investigate the resulting morphology of the etched surface and the produced nano-particles respectively. The optical properties of the nano-nano-particles, their photolumines-cence have been observed by UV-Vis spectroscopy. The results are discussed in the following paragraphs.

2.5.1 AFM

Figure 2.4 shows schematically the process and where and when the various AFM and later on the TEM pictures are taken. AFM micrographs are shown in Figure 2.5. Samples chosen were etched at a current density of 10 and 20 mA/cm2 respectively. A typical morphology of the etched surface is observed: mountain-like structures are surrounded

Chapter 2

42

Figure 2.5: AFM images of samples etched at 10 (A, B) and 20 (C, D) mA/cm2_{respectively. The effect of}

the mechanical removal of the porous layer is visible by confronting the surface morphology before (A, C)

and after (B, D) the sonication step. Please note that the z scale changes fromµm to nm unit.

by valleys, where the electrochemical etching process took place in a vertical direction respect to the Si surface. The porosity of the network seems to be higher for the sample that has been etched at higher current density. Its structure is finer, revealing smaller pores and smaller areas of the mountain-like features. The higher porosity of the network is also observed in the images recorded after the sonication step. The wall height is drastically reduced, giving a good visual confirmation of the porous network removal. The underlying structure still offers a higher porosity for the sample etched at higher current, confirming the current density effect on the formation of the porous silicon. From these observations, smaller nano-particles are expected to be formed for the samples etched at higher current rates.

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Figure 2.6: TEM micrographs of etched Si. Figures A and B refer to the sample etched at 10 mA/cm2_while

Figures C and D refer to sample etched at 80 mA/cm2_.

2.5.2 TEM

Figure 2.6 shows TEM micrographs of different Si particles as removed from the substrate. It is important to notice that the Si particles, when removed by sonication are very porous. Here, the results of samples etched at a current density of 10 mA/cm2(Figures 2.6 A and B) and 80 mA/cm2 (Figures 2.6 C and D) are shown, respectively. For the low current density, large pieces of a porous layer are observed in Figure 2.6-A. By zooming in, Figure 2.6-B shows a crystalline structure with lattice fringes separated by the typical distance of the (111) silicon crystalline planes (JCPDS #75-0590). This single crystal seems to be part of the original Si substrate, detached from the surface by the mechanical action of the ultrasonic waves, as was shown earlier in Figure 2.4 In order to study the effect of the current density on the porosity of the network and on the resulting size of the