Synthesis Of Nanoparticle Networks By Femtosecond Laser Ablation Of Microparticles

The process of laser ablation has been adapted to generate nonoparticles from microparticles of the material, referred to as laser ablation of microparticles (LAM). The LAM process has been shown to generate finer nanoparticles than were previously possible through laser ablation of solid targets. In this thesis, a method of generating a 3D nanoparticle network using the LAM process has been proposed using a femtosecond laser. 3D naoparticles were successfully generated through ablation of microparticle samples of lead oxide, nickel oxide and zinc oxide. The size of the nanoparticles in the generated network was significantly reduced in comparison with similar networks generated through laser ablation of solid targets. The method has been further extended to generate a unique alloy nanomaterial through the ablation of the microparticle containing powders of two metals (Aluminum and Nickel Oxide).


Nanoparticles and Their Applications
The term nanoparticle refers to particles which have a diameter of 100nm or less. They have also been termed as ultra-fine particles. Nanoparticles have attracted a great deal of attention from the scientific community because of their enhanced optical, chemical and electrical properties in comparison to their macro counterparts. These enhanced properties are mainly due to large surface-to-volume ratio and the effect of quantum confinement. The advanced properties of the nanoparticles make them highly desirable for use in the development of different technological applications through advances in nanoscience and nanotechnology.
The small size of the nanoparticles is responsible for the difference in properties in comparison to bulk materials. Some examples of the different properties of the nanoparticles are lower melting temperature [1], increased solid-solid phase transition pressure [2], decreases ferroelectric phase transition temperature [3], higher self-diffusion coefficient [4], modified thermophysical properties [5] and enhanced catalytic activity [6].
The superior properties of the nanoparticles compared to their bulk materials makes them highly desirable for use in enhancing the existing technologies and developing emerging technologies for the 21 st century. Some of the applications of nanoparticles have been described below.

Nanoparticles as quantum dots
Nanoparticle as a system can be referred to as a quantum dot or a zero dimension (0-D) structure.
For such a system, the confinement of the electrons in a small domain results in new energy levels that are determined by quantum confinement effects. The creation of these new energy levels gives nanoparticles modified optoelectronic properties in comparison to the bulk material.
These optoelectronic property modifications can be utilized for the development and enhancement of several electronic and optical devices. One of the fields where such properties have been used is the field of light emitting diodes (LEDs). Nanoparticles of CdSe have been used in voltage controlled LEDs, where red or green color is emitted based on the voltage applied [7]. Quantum dots can also be used in memory storage devices, where they can be arranged in a 3D array to which information can be written to and retrieved from. They can also be utilized to reduce the response time of a lot of microelectronic devices thus increasing their working efficiency [8].

Biomedical Applications
The small size of the nanoparticles which is similar to that of biomolecules such as proteins

Gas Sensors
A gas sensor consists of a material with measurable physical properties (electrical or chemical) which change in the presence of a gas. A number of nanoparticle baser gas sensor systems have been developed. Palladium nanoparticles, in the size range of 10-15nm, were used for the detection of hydrogen gas in a Pd-H system based gas sensor [10]. The use of nanoparticles in gas sensors has been shown to increase the sensitivity and improved the gas selectivity in comparison to non-nanoparticle based sensors [11].

Nanoparticle Synthesis: Traditional Methods
Over the years, many methods have been used for the synthesis of nanoparticles. New methods are being researched and developed for the efficient generation of nanoparticles from a vast variety of materials. The generation of nanoparticles can be divided into two distinct approaches; top-down approach and bottom-up approach. The schematic in Figure 1-1 shows an illustration for the generation of the nanoparticles from the two approaches. In this section we discuss some of the methods for nanoparticle generation.

Inert Gas Condensation
It is one of earliest used methods for the synthesis of nanoparticles. It is well suited for the generation of nanoparticles from metals. Under this method, a solid material is heated till its evaporation point. The vapors are then mixed with a cool inert gas which rapidly cools them to form nanoparticles. Nanocomposites or other compounds can also be achieved by inserting a reactive gas in the environment. Extensive research has been carried out to better control the size of the generated nanoparticles. It has been shown that the flow and mixing of the cool gas and the hot vapors as well as the pressure and the molecular weight of the inert gas affects the size of the nanoparticles generated. This method has been used for generating nanoparticles in the size range of 5nm to a few hundred nm. [8]

Sputtering
Sputtering is the method of vaporizing material from the surface of a solid through bombardment of high-velocity ions of an inert gas (or electrons) in vacuum, causing an ejection of atoms or clusters. As early as in 1982, nanoparticles of Ti and Al with size as small as 10nm were produced by this method [13]. One of the advantages of this method is that the incident ion or electron beam only heats the target material and thus the generated nanoparticles comprises mainly of the target material.

Laser Vaporization
This technique uses a laser which evaporates a sample target in an inert gas flow reactor. The source material is locally heated to a high temperature enabling vaporization. The vapor is cooled by collisions with the inert gas molecules and the resulting supersaturation induces nanoparticle formation. This method has been used for generating nanoparticles in the size range of 6 -100nm from powders, single crystals and sintered blocks [14]. A modified method which combines laser vaporization of metal targets with controlled condensation in a diffusion cloud chamber was used to synthesize nanoscale metal oxide and metal carbide particles with a size range of 10-20 nm [15].

Mechanical Attrition
Mechanical attrition is a "top-down" method of generating nanoparticles. In this process, nanoparticles are formed in a mechanical device in which energy is imparted to a course grained material to reduce particle size. The final size of the particles obtained depends on the milling time that the material has been exposed to. At long enough milling times, nanoparticles with sizes of 10 -15nm has been obtained [16].

Nanoparticle Synthesis: Laser Ablation Methods
The development of powerful lasers has opened up many new areas where laser processing can be used. One of the areas where laser processing has rapidly grown is the field of nanotechnology; specifically the development of laser ablation based methods for nanoparticle synthesis. The laser ablation methods for nanoparticle synthesis have been reviewed in this section.

Laser Ablation
Laser ablation is a technique where laser pulses are used to ablate a solid target placed in a gaseous or liquid environment leading to nanoparticle formation in form of nanopowder or colloidal solution. It is a straight forward method of generating nanoparticles in comparison to other traditional methods. Some of the advantages of the laser ablation method are;  Does not require long reaction times, high temperature environment.
 Is free of multi-step chemical synthesis.
 Can produce nanoparticles from vast range of materials ranging from metals to polymers to dielectric materials  The use of toxic and hazardous chemical precursors is not required  Nanoparticles produced in a vacuum or liquid are free of contaminants and thus can be used for biomedical applications The first ever generation of nanoparticles by the use of laser ablation was reported in 1981, where a Q-switched Nd:YAG laser was used for the generation of Cu clusters, then characterised as ultrafine particles [17]. In this method, laser ablation was combined with supersonic expansion into vacuum. The method of laser ablation has since been greatly researched and refined for the generation of nanoparticles from a vast range of materials.
M. Fumitaka et. al. (2000) produced nanoparticles of silver through laser ablation of a silver plate in aqueous solution of surfactants. The silver metal plate was placed in an aqueous solution of the surfactant C n H 2n+1 SO 4 Na; different surfactants were used with n=8, 10, 12 and 14. The ablation was carried out with a Nd:YAG laser having a pulse width of 10ns. Nanoparticles with an average size of ~10nm were reported to have been produced [18]. Figure 1-3a gives an example of the nanoparticles generated through this method.
In another work, nanoparticles of gold were produced by ablation of a gold plate in an SDS solution using a Nd:YAG laser with a pulse width of 10ns. The variation in the size of the nanoparticles with varying concentrations of the SDS was studied. It was observed that the size of the gold nanoparticles decreased with an increase in the concentration of SDS [19].  size. In their work, a silver metal target placed in a liquid environment of deionized water was ablated at wavelengths of 1064nm, 532nm and 355nm at a laser fluence of 36J/cm 2 . It was observed that the size of the nanoparticles went from 29nm to 12nm with the decrease in the wavelength of the laser beam. Thus the wavelength of the laser beam was shown to have an effect on the size of the nanoparticles generated [22].
The generation of nanoparticles through laser ablation has been restricted only to the ablation of target in a liquid environment. Research has also been done on the generation of nanoparticles by laser ablation in vacuum and in a background gas environment (air or other gases). In one such study, a Ti:sapphire laser with a pulse width of 120fs was used in the ablation of silicon target in vacuum. The nanoparticles had a size distribution with a radius between 5nm and 25nm [23].  The above section provided a review of the work that has been done in the field of nanoparticle generation through laser ablation. The mechanism for the generation of nanoparticles by laser ablation will be discussed in Chapter 2.

Laser Ablation of Microparticles (LAM)
A new method of laser ablation has been studied in which the target material is not solid but is comprised of microparticles of the material of which nanoparticles are to be produced. This method is known as laser ablation of microparticles (LAM). The method of LAM capitalizes on the lack of a strong bonding between the microparticles to generate much finer nanoparticles at much lower laser energy than required for a solid feedstock target.
The LAM process has been used for the generation of nanoparticles from glass microspheres [25], metal microparticles [26,27] and alloys [26]. In the reported literature the microparticles were either applied on a substrate or were in a flowing aerosol when exposed to the laser. The generated nanoparticles were collected on a collector plate placed a certain distance away from the target surface. In all the cases, the nanoparticles were collected in a non-agglomerated state.
A significant reduction in the size of the generated nanoparticles was observed when compared to the size of the nanoparticles generated through ablation of solid targets. . It was observed that the generated nanoparticles ranged in size from 20nm to 190nm for all the three cases. It was also noted that for the lowest fluence, a peak in particle size distribution was observed at 80nm which shifted to 60nm for the highest fluence [25]. Figure 1-5 gives an example of the nanoparticles generated through this method.  in size. [28] The breakdown of the microparticles into nanoparticles has been explained on the basis of plasma breakdown and shockwave propagation through the microparticles. As per this theory, as the breakdown threshold of the material is achieved, a shockwave is generated. The shockwave propagates in two different directions; the primary shockwave propagates away from the material while the secondary shockwave propagates towards the material. As the shockwave travels through the feedstock, it compresses and heats the material to above its critical point. When the shockwave passes, the region right behind it has a much lower pressure which causes rapid condensation of the material. This rapid condensation leads to the formation of nanoparticles.
The major advantage of the LAM process is the reduction in the amount of laser energy required to completely vaporize the metal particle; for the gold particles it was observed that total laser energy absorbed for complete vaporization was 25% of the net energy required. Due to the loosely packed nature of the microparticles, less laser energy is required for the initiation of the ablation process than compared to a solid target.

Research Objectives
The LAM process has been successfully used for the generation of nanoparticles from microparticles. This research will focus on microparticle ablation using a Mega-Hertz repetition rate femtosecond laser, with the aim of producing nanostructure of reduced particle size. In particular, the following studies will be conducted:  Investigate mega-hertz repetition rate femtosecond laser ablation of microparticles. The nanostructure generated will be compared to similar structures generated through laser ablation of solid targets (Metals: Lead, Gold; Semi-conductor: Silicon; Dielectric: Glass)  LAM method for the generation of nanostructure will be examined for its use in the generation of an alloy nanostructure from pre-mixed two independent microparticle powders.
 This study will also probe the feasibility of the LAM process in generating a nanostructure by the interaction of two materials, one in solid phase and the other in powder phase.

Thesis Outline
Chapter 2 of the thesis will be focused on the explanation of the process of laser ablation of solids. It will also give a detailed insight into the generation of nanoparticles through laser ablation. The last section of the chapter will be focused on the process of laser ablation of microparticles (LAM).  dephasing. This is the Joule heating process, also known as inverse Bremsstrahlung. The seed electron can be accelerated enough that its kinetic energy exceeds the ionization potential of the bound electron. Therefore the next collision with a bound electron will result in an ionization event if the free electron transfers nearly all its energy to the bound electron, resulting in two free electrons with low kinetic energies. This is called impact ionization. This process will repeat itself, leading to an avalanche where the free-electron density grows exponentially from the very low seed electron density. When enough bound electrons are ionized by this avalanche process, plasma with a critical density is created, and the dielectric material is broken down and becomes absorbing. hence the heating, during the laser pulse depends on the pulse duration and the energy coupling coefficient. When melting or vaporization temperature is reached, the material is considered broken down and damaged. The breakdown is also accompanied by acoustic waves and optical plasma radiation. The rate of heating is determined by the rate of laser energy absorption and the rate of energy loss from the focus, mainly through thermal conduction away from the focus. The rate of laser energy absorption is approximately constant before the breakdown.

Nanoparticles generated by laser ablation
A schematic representation of the plume generation and the subsequent generation of nanoparticles is shown in

Summary
The advent of the laser technology opened new avenues for material processing. The laser ablation process provided an efficient method for micromachining of materials. Apart from micromachining, the laser ablation process has been used for the generation of nanoparticles from a large range of materials. The mechanism for the removal of material through the ablation process has been mentioned as the heating of the material to above its critical point leading to the ejection of material from the top surface of the material. The ultrashort laser pulses have been found to be more effective in material removal through ablation in comparison with short/long laser pulses.

Experimental Set-up
Experiments were conducted using a direct-diode-pumped Yb-doped fiber oscillator/amplifier

Sample Preparation
Under  The glue was later shown in the analysis to not have any effect on the generated nanostructure.

Sample Analysis
The ablated samples were analyzed for the nanostructure generated. The morphology was analyzed using a SEM (Hitachi S-5200 SEM) and a Transmission Electron Microscope (TEM) (Hitachi HD-2000 STEM). X-Ray Diffraction (XRD) analysis was also carried out on the ablated and non-ablated samples of lead oxide (Pb 3 O 4 ) and nickel oxide (NiO), to confirm that the stoichiometry was maintained during the ablation process. Energy Dispersive X-Ray (EDX) analysis was carried out to determine the elements present in the generated nanostructures.

Chapter 4: Femtosecond laser ablation of metallic microparticles 4.1 Introduction
Earlier research conducted at the Laser Micro and Nano Fabrication Research Facility at Ryerson University showed that the use of megahertz laser ablation can generate fibrous nanoparticle networks from metals, dielectrics and a wide range of other material [31][32][33]. A lot of research has been focused on the generation of nano-networks through laser ablation of bulk substrates.
The mechanism behind the generation of nano-networks through laser ablation has been found to be the agglomeration of the nanoparticles that are generate by the ablation of the substrate. Thus the size of the generated nano-network is dependent on the size of the agglomerated nanoparticles.
In this chapter, the study on the generation of very fine nanoparticle networks by the process of laser ablation of microparticles will be discussed. The results of the laser ablation of microparticle sample will be presented. The observed results will be compared with similar results obtained through laser ablation of bulk material. Also discussed in this chapter is the proposed mechanism for the reduction in size of the nanoparticles leading to the formation of very fine nanoparticle networks.

Experimental Parameters
The sample preparation methods are mentioned in Section 3.2. The prepared samples were ablated in an ambient air using a Yb-doped fiber amplified femtosecond laser. The ablation of all the samples was carried out with the same laser parameters. The laser parameters were set as follows:  Average power per pulse: 16W  Repitition rate: 25MHz  Dwell time: 0.1ms  In the traditional LAM process, the ablation process leads to the formation of nanoparticles only.

Results and Observations
A possible explanation for this is that the photo-ionization of the formed nanoparticles (due to the longer pulse duration of the nanosecond laser) prevented the agglomeration of the nanoparticles [26]. In the current experiment, the ultrashort pulse duration of the femtosecond laser does not provide enough time for the particles to be photo-ionized. Hence the generated nanoparticles agglomerate to form nanofibers. Thus the LAM process has been successfully used for the generation of nanofibers from microparticles.
A comparison was carried out between the laser ablation of the microparticle containing sample and those generated through ablation of bulk lead sample. The first noticeable difference was the

Discussion
The formation of the nanoparticle networks has been attributed to the agglomeration of the nanoparticles generated by the laser ablation of the microparticles. Hence the shift in the diameter of the nanoparticle networks can be explained by the shift in the size of the generated nanoparticles.
The explanation for the reduction of the nanoparticle size is obtained by analyzing the shockwave generated by the laser ablation process. One of the characteristics of the LAM process is the concentration of the generated shockwave onto a smaller volume of the particles [28]. A schematic for the generation of shockwave and its propagation is shown in     semiconductors, dielectrics) have been shown to be useful for a lot of applications [25,[36][37][38].

Conclusion
However, one realm of nanoparticles that has not been studied is capitalization of the advanced properties of a group of nanoparticles in a combination to achieve a new material/alloy with properties superior to those of the nanoparticles of the individual materials.
Over the years, non-conventional methods have been used for the synthesis of nanocrystalline alloys from immiscible metals. Methods such as ion beam mixing, sputtering, vapor deposition, thermal evaporation and laser ionization have been used for the synthesis of such alloys [39][40][41][42][43].
All these non-conventional methods take advantage of the theoretical predictions of lowered or suppressed phase separation at the nano scale [44,45]. One theory has suggested the lack of nucleation barrier for the formation of segregated species at the nanoparticle size regime [46]. Some effort has been made to produce such nanoparticle alloys through laser ablation of colloidal solutions or powder suspensions of materials [47][48][49][50][51]. In all of these methods some kind of liquid solution of the materials is irradiated with the laser to generate the nanoparticles which are a combination of the materials either in the form of a core-shell structure or a metastable alloy. These methods are limited by the preparation of such a liquid solution of the mixing materials.
In the previous chapter, the generation of a 3D nanostructure upon ablation of a microparticle containing powder has been demonstrated [52]. This chapter studies the use of the ablation process presented in the previous chapter to generate a 3D nanostructure composed of a mixture of two materials that initially do not exist in a combined form. The generation of a 3D nanostructure by the laser ablation of a mixture of Nickel Oxide (NiO) and Aluminum (Al) microparticle powders is presented. In extension to the above results, also reported is the generation of 3D nanostructure through ablation of Nickel Oxide (NiO) powder layer on an Aluminum foil; the generated nanostructure shows a different type of material.

Experimental Parameters
The sample preparation methods are mentioned in Section 3.2. The prepared samples were ablated in an ambient air using a Yb-doped fiber amplified femtosecond laser. The ablation of all the samples was carried out with the same laser parameters. The laser parameters were set as follows:  Average power per pulse: 12W  Repitition rate: 8MHz  Dwell time: 0.25ms

Results and Discussion
For the current study, a mixture of two microparticle containing powders was ablated and the nanostructure generated was analyzed. Figure 5-1 shows an illustrative depiction of the ablation process and the corresponding nanostructure generated. The ablated samples were analyzed under the SEM to study the generated nanostructure. Figure   5-2 shows the SEM images of the nanostructure generated by the laser ablation of a mixture of aluminum and nickel oxide microparticles. In order to explain the post ablation mixing of the two powders, the fundamental behind the process of laser ablation for material removal has to be re-looked into. The method of material removal by laser ablation has been explained by the heating of the target material above its boiling temperature by the laser pulses, followed by rapid cooling once the laser pulses stop.
When ablation of the target material is carried out in a background gas environment or in ambient air, the presence of the air/gas causes the re-deposition of the ablated material onto the target surface which does not take place for laser ablation in vacuum [53].  Another aspect of laser ablation that has been recently highlighted is the presence of a temperature gradient that exists across the surface of the target material [54]. The temperature at the point where the laser directly hits is the highest and it decreases as we move away from the center. There is also the existence of isotherms across the target surface [54]. Taking into account the above temperature gradient, the layer of mixture of aluminum and nickel oxide powders will be subject to different temperatures depending on the position from the point of laser impact. Thus there exists a variation in the extent of mixing of the two powders post ablation. Figure 5-6 shows the TEM images of the nanostructure generated in the area away from direct impact of the laser.  In extension of the above study, the microparticles of aluminum were replaced by an aluminum foil. A layer of the microparticles of nickel oxide was applied onto the aluminum foil. The sample was then ablated and analyzed for nanostructure generation. The process is illustrated in  For the current scenario, as shown in Figure 5-8, the laser was focused so as to ablate the microparticle layer and the aluminum foil simultaneously. The particles from the foil and the microparticle layer were ejected into the plume and upon subsiding of the laser pulses formed into nanoparticle networks. The generated networks showed certain extent of mixing between the two materials. Figure 5-9 shows the SEM image of nanostructure obtained by the ablation of NiO microparticles coated on an Aluminum foil.

Conclusion
In the current study, the process of laser ablation of microparticle for generating a nanostructure  Generated an alloy nanostructure by femtosecond ablation of two metallic microparticle powders.
 Demonstrated the use of femtosecond laser ablation for generating a nanostructure by ablation of two metals in different phases; one in microparticle powder form and the other in solid form.

Conclusion
Femtosecond laser ablation of bulk materials generates 3D nanoparticle networks when the laser pulse repetition rate is in the Mega Hertz (MHz) regime. The generated nanoparticles normally have diameters in the range of a few hundreds of nanometers. To further reduce the particle size, the process of laser ablation of microparticles (LAM); which has long been used for the generation of individual nanoparticles from microparticles, has been used for generating a 3D nanoparticle network by using lead oxide (Pb 3 O 4 ) microparticles, nickel oxide (NiO) microparticles and zinc oxide (ZnO) microparticles as precursors.
The increased efficiency of the LAM process for generating nanoparticle networks in comparison to laser ablation of bulk material was demonstrated with lead samples; wherein the size of the generated nanoparticles was approximately 60nm smaller than those generated from ablation of bulk lead. Reduced laser fluence for the LAM process; attributed to the loosely packed nature of the microparticles, provided for nanostructure generation at lowered laser energy level.
Apart from generation of nanostructure from microparticles of a single material, nanostructures were also obtained for ablation of a mixture of microparticle powders. The generated nanostructures showed mixing between the two microparticle materials (Al and NiO) during the ablation process; a fact highlighted by the EDX analysis. The difference in the boiling point of the two materials was proposed to be the cause of the mixing of NiO and Al during the condensation phase.

Suggestions for Future Works
The novel method of formation of an alloy nanoparticle network can be further expanded in various directions. The composition of the alloy obtained can be varied by changing the mixing ratio of the two powders. Also the effect of change in laser parameters can be studied. Research can also be carried out on the influence of the presence of a background gas on the alloy nanoparticle network obtained.