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## **Meet the editor**

Dr. Sudheer Neralla is a Materials Scientist at the Jet-Hot High Performance Coatings, a nanotechnology-based coating industry in Burlington, NC, USA. He is also involved as an Adjunct Research Scientist with the NSF Engineering Research Center at North Carolina Agricultural and Technical State University, Greensboro, NC. Dr. Neralla received his doctoral degree in mechanical

engineering from the North Carolina Agricultural and Technical State University, Greensboro, NC. His research interests include synthesis of nanomaterials, thin films, nanoindentation, friction and wear, and corrosion study of coatings and biodegradable Mg-based alloys.

### Contents



Vahid Mohammadi and Stoyan Nihtianov


**Solid‐State Devices 233** A.M. Torres‐Huerta, M.A. Domínguez‐Crespo and A.B. López‐ Oyama

Chapter 10 **Plasma-Enhanced Chemical Vapor Deposition: Where we are and the Outlook for the Future 247** Yasaman Hamedani, Prathyushakrishna Macha, Timothy J. Bunning, Rajesh R. Naik and Milana C. Vasudev

### Preface

Chapter 7 **Silicon-Rich Oxide Obtained by Low-Pressure Chemical Vapor Deposition to Develop Silicon Light Sources 159**

Chapter 8 **High‐Density Plasma‐Enhanced Chemical Vapor Deposition of Si‐Based Materials for Solar Cell Applications 183**

Chapter 10 **Plasma-Enhanced Chemical Vapor Deposition: Where we are**

Aceves-Mijares

**VI** Contents

Oyama

H. P. Zhou, S. Xu and S. Q. Xiao

**Solid‐State Devices 233**

Chapter 9 **Applications of CVD to Produce Thin Films for**

**and the Outlook for the Future 247**

Rajesh R. Naik and Milana C. Vasudev

J. Alarcón-Salazar, R. López-Estopier, E. Quiroga-González, A. Morales-Sánchez, J. Pedraza-Chávez, I. E. Zaldívar-Huerta and M.

A.M. Torres‐Huerta, M.A. Domínguez‐Crespo and A.B. López‐

Yasaman Hamedani, Prathyushakrishna Macha, Timothy J. Bunning,

Over the past few years, chemical vapor deposition (CVD) methods have undergone signifi‐ cant changes and have embraced the technological updates to enable growth of novel mate‐ rials, including nanostructures, thin films, and multiphase materials with focus on electronic, physical, and optical properties. These updates have helped overcome some of the limitations, such as synthesis temperatures, materials, and field of applications, which would have limited CVD methods to high-temperature applications otherwise. One of the major advantages of CVD technique is its ability to coat any shape, and with the advent of metal-organic chemical vapor deposition (MOCVD) and plasma-enhanced chemical vapor deposition (PECVD) techniques, deposition temperatures have drastically reduced.

CVD has now evolved into the most widely used technique for growth of thin films in elec‐ tronics industry. Several books on CVD methods have emerged in the past, and thus the scope of this book goes beyond providing fundamentals of the CVD process.

The book is divided into two sections. Section 1 covers authors' works on the synthesis of various nanomaterials and thin films using CVD methods. This section includes Chapters 1 through 5.

Chapter 1 presents preparation and characterization of carbon nanofibers and composites by chemical vapor deposition method. Effect of synthesis temperature and of metal catalyst concentration on the electrochemical characteristics is studied.

Chapter 2 summarizes the nonclassical crystallization in the growth of thin films and nano‐ structures by CVD. Several variables such as surface conductivity, flow rate, substrate posi‐ tion, and nanoparticle size are studied in deposition of diamond, ZnO, and silicon.

Chapter 3 discusses the growth of HgCdTe heterostructures using MOCVD technique. Elec‐ trical and chemical characterization of HgCdTe structures is described and infrared photodi‐ odes were constructed using these heterostructures.

In Chapter 4, a parametric study of synthesis of bilayer graphene on copper using hot fila‐ ment chemical vapor deposition method is presented. Synthesis process and the parameters' effect are discussed.

In Chapter 5, a method for in situ observation of CVD is introduced. A langasite crystal mi‐ crobalance is used to evaluate the surface chemical reactions in a CVD reactor.

Section 2 is devoted to recent advances in materials synthesis using CVD and their applica‐ tions such as photodetectors, optical sources, solar cell, and solid-state devices.

#### XII Preface

In Chapter 6, a new method, PureB, is introduced for deposition of boron at low tempera‐ tures using CVD method for application as photodetectors. Different models behind PureB growth are discussed.

Chapter 7 presents work on the synthesis of silicon-rich oxide using low-pressure CVD method. Parameters affecting the stoichiometry of the silicon oxide are analyzed.

Chapter 8 reports high-density plasma CVD of Si-based materials for solar cell applications. High-frequency plasma-enhanced CVDmethod has been employed to overcome the limita‐ tions of traditional PECVD methods.

In Chapter 9, CVD methods for synthesis of thin films with application in solid-state devices are discussed. Growth of Pt-YSZ and Pt-ZrO2 ceramic-metallic composites were developed and evaluated in solid-state devices.

Chapter 10 provides an overview of plasma-enhanced chemical vapor deposition and recent advanced applications.

Through this book, an effort is made to bring together most recent works in the area of CVD. I would like to express my sincere thanks to all the participating authors of this book for their valuable contributions.

> **Dr. Sudheer Neralla** Jet-Hot High Performance Coatings NSF-Engineering Research Center North Carolina A&T State University Greensboro, NC-USA

**CVD: Overview and Synthesis of Micro/Nano Structures**

In Chapter 6, a new method, PureB, is introduced for deposition of boron at low tempera‐ tures using CVD method for application as photodetectors. Different models behind PureB

Chapter 7 presents work on the synthesis of silicon-rich oxide using low-pressure CVD

Chapter 8 reports high-density plasma CVD of Si-based materials for solar cell applications. High-frequency plasma-enhanced CVDmethod has been employed to overcome the limita‐

In Chapter 9, CVD methods for synthesis of thin films with application in solid-state devices are discussed. Growth of Pt-YSZ and Pt-ZrO2 ceramic-metallic composites were developed

Chapter 10 provides an overview of plasma-enhanced chemical vapor deposition and recent

Through this book, an effort is made to bring together most recent works in the area of CVD. I would like to express my sincere thanks to all the participating authors of this book for

**Dr. Sudheer Neralla**

Greensboro, NC-USA

Jet-Hot High Performance Coatings NSF-Engineering Research Center North Carolina A&T State University

method. Parameters affecting the stoichiometry of the silicon oxide are analyzed.

growth are discussed.

VIII Preface

advanced applications.

their valuable contributions.

tions of traditional PECVD methods.

and evaluated in solid-state devices.

### **Preparation and Characterization of Carbon Nanofibers and its Composites by Chemical Vapor Deposition**

Chang-Seop Lee and Yura Hyun

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/63755

### **Abstract**

Hydrocarbon gas or carbon monoxide was pyrolyzed by chemical vapor deposition (CVD), and carbon nanofiber (CNF) synthesis was performed using transition metals such as Ni, Fe, and Co as catalysts. When synthesizing carbon nanofibers using the CVD method, experimental variables are temperature, catalysts, source gas, etc. Especially, the particle size of the catalyst is the most important factor in determining the diameter of carbon nanofibers. Hydrocarbon gases, such as CH4, C2H4, benzene, and toluene are used as the carbon source, and in addition to these reaction gases, nonreactive gases such as H2, Ar, and N2 gases are used for transportation. Synthesis occurs at a synthesis temperature of 600–900°C, and catalyst metals such as Ni, Co, and Fe are definitely required when synthesizing CNFs. Therefore, it is possible to synthesize CNFs in selective areas through selective deposition of such catalyst metals. In this study, CNFs were synthesized by CVD. Ethylene gas was employed as the carbon source for synthesis of CNFs with H2 as the promoting gas and N2 as the balancing gas. Synthe‐ sized CNFs can be used in various applications, such as composite materials, electro‐ magnetic wave shielding materials, ultrathin display devices, carbon semiconductors, and anode materials of Li secondary batteries. In particular, there is an increasing demand for light-weight, small-scale, and high-capacity batteries for portable electron‐ ic devices, such as notebook computers or smartphones along with the recent issue of fossil energy depletion. Accordingly, CNFs and their silicon-series composites are receiving attention for use as anode materials for lithium secondary batteries that are eco-friendly, light weight, and high capacity.

**Keywords:** carbon nanofibers, transition metal catalyst, chemical vapor deposition, composite, Li ion batteries

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **1. Introduction**

Chemical vapor deposition (CVD) is widely used as a surface treatment technology for materials. CVD forms a solid-state thin film mostly on the surface and is used not only to produce high-purity bulk materials and powder but also to manufacture composite materi‐ als through infiltration techniques.

CVD is used to deposit a wide variety of materials. Most of the elements in the periodic table deposited in the pure element are formed by CVD technology. However, they are deposited mostly in the compound form rather than the pure element form. CVD can make precursor gases flow to one or more heated objects in a chamber to coat the desired compound. A chemical reaction occurs on the hot surface, and this leads to the deposition of a thin film on the surface. This reaction also produces the unreacted precursor gas and the chemical byprod‐ uct discharged from the chamber at the same time.

CVD can deposit many kinds of materials and can be applied to broad areas, so the syn‐ thesis condition is also diverse. CVD synthesis can occur in a high- or low-temperature reactor, the pressure ranges from sub-Torr pressures to above-atmospheric pressures, re‐ gardless of the kind of catalyst, and the reaction temperature can range from 200 to 1600°C to diversify the synthesis condition.

**Figure 1.** The schematic diagram of a tube-furnace CVD system.

Microfabrication processes widely use CVD to deposit materials in various forms, including monocrystalline, polycrystalline, amorphous, and epitaxial. These materials include silicon (SiO2, germanium, carbide, nitride, and oxynitride), carbon (fiber, nanofibers, nanotubes, diamond, and graphene), fluorocarbons, filaments, tungsten, and titanium nitride [1, 2].

CVD is a technology used to deposit a solid-state thin film on the substrates from vapor species through chemical reaction. One of the most unique characteristics of CVD synthesis is that chemical reaction plays an important role, so it is comparable to other thin film deposition technologies. **Figure 1** shows the schematic drawing of a typical tube-furnace CVD system. Gas flows are regulated by mass flow controllers (MFCs) and fed into the reactor through a gas-distribution unit. Chemical deposition takes place in the reactor, which is heated by outside heaters.

During the CVD process, the reaction gas is supplied to the reactor through mass flow controllers (MFCs) controlling the flow rate of the gas being passed. In addition, the mixture gas device mixes the gases evenly before they flow into the reactor. The chemical reaction occurs in the reactor and solid-state materials are deposited on the substrates. A heater is placed around the reactor to provide reaction at high temperatures. CVD is used not only to create a solid-state thin film on the surface and produce high-purity bulk materials and powder but also to manufacture composite materials through infiltration techniques. CVD is used to deposit various materials on solid surfaces.

A characteristic feature of CVD technique is its excellent throwing power, enabling the production of coatings of uniform thickness and properties with low porosity even on substrates with complicated shapes. Another characteristic feature is the possibility of localized or selective deposition on patterned substrates [1–4].

In this chapter, we describe the preparation process for carbon nanofibers (CNFs) and their silicon/silicon oxide composites using the chemical vapor deposition method and investigate the physicochemical and electrochemical characteristics of the prepared materials for the application of anode materials in Li secondary batteries.

### **2. Synthesis and characterization of CNFs on transition metal catalysts by CVD**

### **2.1. Preparation of transition metal catalysts**

**1. Introduction**

als through infiltration techniques.

to diversify the synthesis condition.

uct discharged from the chamber at the same time.

**Figure 1.** The schematic diagram of a tube-furnace CVD system.

Chemical vapor deposition (CVD) is widely used as a surface treatment technology for materials. CVD forms a solid-state thin film mostly on the surface and is used not only to produce high-purity bulk materials and powder but also to manufacture composite materi‐

4 Chemical Vapor Deposition - Recent Advances and Applications in Optical, Solar Cells and Solid State Devices

CVD is used to deposit a wide variety of materials. Most of the elements in the periodic table deposited in the pure element are formed by CVD technology. However, they are deposited mostly in the compound form rather than the pure element form. CVD can make precursor gases flow to one or more heated objects in a chamber to coat the desired compound. A chemical reaction occurs on the hot surface, and this leads to the deposition of a thin film on the surface. This reaction also produces the unreacted precursor gas and the chemical byprod‐

CVD can deposit many kinds of materials and can be applied to broad areas, so the syn‐ thesis condition is also diverse. CVD synthesis can occur in a high- or low-temperature reactor, the pressure ranges from sub-Torr pressures to above-atmospheric pressures, re‐ gardless of the kind of catalyst, and the reaction temperature can range from 200 to 1600°C

Microfabrication processes widely use CVD to deposit materials in various forms, including monocrystalline, polycrystalline, amorphous, and epitaxial. These materials include silicon (SiO2, germanium, carbide, nitride, and oxynitride), carbon (fiber, nanofibers, nanotubes, diamond, and graphene), fluorocarbons, filaments, tungsten, and titanium nitride [1, 2].

In this study, transition metal catalysts were prepared through the coprecipitation method and then used in the synthesis of CNFs. In order to prepare the metal catalysts with different compositions, the mass of the precursor was first calculated according to the ratio of the metal content required.

Solution A was composed of aluminum nitrate, which helps to generate alumina (Al2O3) to serve as a supporter for the transition metal catalysts in the transition metal nitrate, dissolved in distilled water. With the foregoing supporter working to capture the nanometal catalyst, the coagulation phenomenon occurs when the temperature is increased up to the temperature for the synthesis of carbon nanofibers without a supporter because of the unstable nanometal particles. The usage of a supporter helps carbon nanofibers grow without a clustered catalyst and thereby serves as a matrix that prevents catalyst coagulation.

Meanwhile, it is preferred to mix passive metals to control the interparticle coagulation of transition metals, such as Fe, Co, and Ni, which all have catalytic activity against the reaction gas during high-temperature reaction. This study employed a mixture of the foregoing Solution A and another Solution B, which was composed of ammonium molybdate and distilled water.

Solution C was made of ammonium carbonate, which served as a precipitator to precipitate the transition metals and the aluminum included in the foregoing Solution A. Precipitation was induced by gradual blending of the mixture composed of Solutions A and B and the mixture of Solution C. This step was followed by agitation for stability of the precipitation.

**Figure 2.** Preparation process transition metal catalysts.

These solutions were sufficiently stirred to stabilize the precipitates; moisture was removed by filtering; and they were dried for more than 12 h in a 110°C oven. Fully dried precipitates were made into powder, and this powder of a metal catalyst was used as the catalyst in the synthesis of carbon nanofibers. The preparation process for the catalysts is shown in **Figure 2** [5–13].

### **2.2. Synthesis of CNFs**

Meanwhile, it is preferred to mix passive metals to control the interparticle coagulation of transition metals, such as Fe, Co, and Ni, which all have catalytic activity against the reaction gas during high-temperature reaction. This study employed a mixture of the foregoing Solution A and another Solution B, which was composed of ammonium molybdate and

6 Chemical Vapor Deposition - Recent Advances and Applications in Optical, Solar Cells and Solid State Devices

Solution C was made of ammonium carbonate, which served as a precipitator to precipitate the transition metals and the aluminum included in the foregoing Solution A. Precipitation was induced by gradual blending of the mixture composed of Solutions A and B and the mixture of Solution C. This step was followed by agitation for stability of the precipitation.

These solutions were sufficiently stirred to stabilize the precipitates; moisture was removed by filtering; and they were dried for more than 12 h in a 110°C oven. Fully dried precipitates were made into powder, and this powder of a metal catalyst was used as the catalyst in the synthesis of carbon nanofibers. The preparation process for the catalysts is shown in

distilled water.

**Figure 2.** Preparation process transition metal catalysts.

**Figure 2** [5–13].

Chemical vapor deposition (CVD) method was employed to synthesize carbon nanofibers in horizontal tube furnace. The schematic diagram of the reaction apparatus, manufactured as metal heating element and horizontal quartz reaction tube in 80 mm (diameter) × 1400 mm (length), was demonstrated in **Figure 3**.

**Figure 3.** Schematic diagram of CVD apparatus for preparation of CNFs.

Flux of reaction gas was regulated by electronic mass flow controller (MFC); ethylene gas (C2H4) was used to grow carbon nanofibers; and 2H2 gas was used as promoting gas for gas phase reaction, whereas N2 gas was used for stabilization of reaction. Following are conditions of synthesis reaction.

A prepared metal catalyst was evenly spread on a quartz boat, placed into reactor under N2 atmosphere, and temperature was increased to 10°C/min. When the temperature was reached 700°C, it was maintained for 30 min; 20% H2 gas balanced with N2 gas were flown into all together; and then H2 gas balanced with N2 gas and 20% ethylene balanced with N2 gas were flown into the reactor for 1 h. Ethylene and H2 gas were shut off after the reaction was completed; N2 gas was passed with the reactor atmosphere inactive until room temperature was reached. Then, carbon nanofibers were synthesized [11, 14, 15].

### **2.3. Synthesis of CNFs on iron and copper catalysts**

### *2.3.1. Scanning Electron Microscope (SEM)*

Carbon nanofibers are synthesized when pyrolyzed hydrocarbons contact metal particles at high temperature. The microstructure of the synthesized carbon nanofibers was observed by SEM and is shown in **Figure 4**. As shown in **Figure 4(a)**–**(d)**, carbon nanofibers grew both in the case of synthesis by Fe catalyst only as well as with an Fe:Cu weight ratio of 7:3, 5:5, and 3:7. In addition, it is known that the fiber diameters average 25–35 nm. Since physical properties may vary depending on diameter size, the diameters of carbon nanofibers can be adjusted according to the weight ratio of catalysts to meet specific purposes.

It was found that the carbon nanofibers grew slightly in the SEM image in (e) but not in (f). Here, it is believed that Fe played the role of a positive catalyst, whereas Cu played the role of a negative catalyst [9, 11].

**Figure 4.** SEM images of CNFs synthesized from ethylene at 700°C under different concentrations of Fe and Cu cata‐ lysts. (a) Fe:Cu = 10:0, (b) Fe:Cu = 7:3, (c) Fe:Cu = 5:5, (d) Fe:Cu = 3:7, (e) Fe:Cu = 1:9, and (f) Fe:Cu = 0:10.

### *2.3.2. Brunauer–Emmett–Teller (BET)*

A comparison was performed by measuring the surface area (m2 /g) of respective carbon nanofibers using a measuring instrument for specific surface area. When the weight ratio of Fe and Cu was 3:7, the highest BET surface area was found to be 305 m2 /g. With weight ratios of 289, 264, 250, and 77 m2 /g, the respective BET surface areas were 10:0, 5:5, 7:3, and 1:9. Synthesized carbon nanofibers usually have wide specific surface area, which makes them good at storing energy, and thus, they can be used as electrodes materials for capacitor or lead storage batteries, or lithium ion secondary batteries [8, 16].

### *2.3.3. X-ray Diffraction (XRD)*

the case of synthesis by Fe catalyst only as well as with an Fe:Cu weight ratio of 7:3, 5:5, and 3:7. In addition, it is known that the fiber diameters average 25–35 nm. Since physical properties may vary depending on diameter size, the diameters of carbon nanofibers can be adjusted

8 Chemical Vapor Deposition - Recent Advances and Applications in Optical, Solar Cells and Solid State Devices

It was found that the carbon nanofibers grew slightly in the SEM image in (e) but not in (f). Here, it is believed that Fe played the role of a positive catalyst, whereas Cu played the role of

**Figure 4.** SEM images of CNFs synthesized from ethylene at 700°C under different concentrations of Fe and Cu cata‐

nanofibers using a measuring instrument for specific surface area. When the weight ratio of

/g, the respective BET surface areas were 10:0, 5:5, 7:3, and 1:9.

/g) of respective carbon

/g. With weight ratios

lysts. (a) Fe:Cu = 10:0, (b) Fe:Cu = 7:3, (c) Fe:Cu = 5:5, (d) Fe:Cu = 3:7, (e) Fe:Cu = 1:9, and (f) Fe:Cu = 0:10.

A comparison was performed by measuring the surface area (m2

Fe and Cu was 3:7, the highest BET surface area was found to be 305 m2

*2.3.2. Brunauer–Emmett–Teller (BET)*

of 289, 264, 250, and 77 m2

according to the weight ratio of catalysts to meet specific purposes.

a negative catalyst [9, 11].

**Figure 5** shows the XRD results for the change of crystal quality according to the Fe and Cu weight ratio. It was confirmed that both carbon nanofibers synthesized with Fe catalyst only and those synthesized with Fe:Cu catalysts at the weight ratios of 7:3 and 5:5 showed carbon peaks with the highest strength.

Most carbon peaks had high strength except for the nanofibers synthesized with Fe and Cu at the weight ratio of 1:9. Therefore, the ratio of pure carbon nanofibers with excellent crystal quality was confirmed to be high [9, 11].

**Figure 5.** Change in carbon nanofiber crystal quality according to weight ratio of Fe and Cu.

### **3. Synthesis and characterization of SiO2/CNF composites by CVD**

### **3.1. Synthesis and electrochemical performance of mesoporous SiO2–CNF composite on Ni foam**

### *3.1.1. Synthesis of catalysts and mesoporous SiO2*

Binders, electronic conducting additives, and current collectors constituting the electrode are very important factors in the manufacturing process for batteries because the overall perform‐ ance of the battery depends on the performance of these materials.

When the volume changes repeatedly during the adsorption and desorption of lithium, bonds of active materials become weaker or the conductive additives and contact resistance increase in the electrode. In particular, because active materials such as silicon or tin are used in highcapacity electrodes, the volume changes are even bigger and thus the bond strength between the collector and the anode active materials become weaker.

**Figure 6.** Deposition of catalysts and mesoporous SiO2 on Ni foam.

Therefore, in this study, we tried to increase the bond strength between the collector and the anode active materials as well as to improve the problem regarding the volume expansion of the electrode by synthesizing CNFs and mesoporous SiO2–CNF composites directly on the collector, Ni foam, using the CVD method without a binder [17, 18] (**Figure 6**).

### *3.1.2. Synthesis of CNFs and mesoporous SiO2–CNF composites*

CNFs and mesoporous SiO2–CNF composites were synthesized in the quartz reactor using chemical vapor deposition. The CVD apparatus used in this experiment is shown in **Figure 7**. C2H4/N2(20/80 vol%) gas was used as the carbon source for the synthesis of carbon nanofibers. H2/N2(20/80 vol%) and N2(99%) were used as the promotion gas for the gas phase reaction and the carrier gas, respectively.

**Figure 7.** Schematic diagram of CVD apparatus for the preparation of CNFs and mesoporous SiO2–CNF composites.

After the Fe–Cu catalyst or Fe–Cu/mesoporous SiO2 deposited on Ni foam was placed in the reaction furnace, the temperature was raised by 10°C/min while the nitrogen atmosphere was maintained. At 600°C, nitrogen and hydrogen gases were flowed while the temperature was maintained for 30 min. Hydrogen and ethylene gases were flowed for 10 min. After the reaction was completed, CNFs and mesoporous SiO2–CNF composites were synthesized by cooling to room temperature with nitrogen gas [17, 18].

### *3.1.3. Fabrication process of anode materials for lithium secondary batteries*

in the electrode. In particular, because active materials such as silicon or tin are used in highcapacity electrodes, the volume changes are even bigger and thus the bond strength between

10 Chemical Vapor Deposition - Recent Advances and Applications in Optical, Solar Cells and Solid State Devices

Therefore, in this study, we tried to increase the bond strength between the collector and the anode active materials as well as to improve the problem regarding the volume expansion of the electrode by synthesizing CNFs and mesoporous SiO2–CNF composites directly on the

CNFs and mesoporous SiO2–CNF composites were synthesized in the quartz reactor using chemical vapor deposition. The CVD apparatus used in this experiment is shown in **Figure 7**. C2H4/N2(20/80 vol%) gas was used as the carbon source for the synthesis of carbon nanofibers. H2/N2(20/80 vol%) and N2(99%) were used as the promotion gas for the gas phase reaction and

**Figure 7.** Schematic diagram of CVD apparatus for the preparation of CNFs and mesoporous SiO2–CNF composites.

collector, Ni foam, using the CVD method without a binder [17, 18] (**Figure 6**).

the collector and the anode active materials become weaker.

**Figure 6.** Deposition of catalysts and mesoporous SiO2 on Ni foam.

*3.1.2. Synthesis of CNFs and mesoporous SiO2–CNF composites*

the carrier gas, respectively.

A three-electrode cell was prepared by applying CNFs and mesoporous SiO2–CNF composites as anode active materials of lithium secondary batteries. Three-electrode cell was assembled in the glove box filled with Ar gas and was assembled as a half cell. The scheme for cell assembly is shown in **Figure 8**. Prepared active materials were used for the working electrode while lithium was used for the counter and reference electrode. A glass fiber separator was used as the separator membrane. 1 M LiClO4 was employed as the electrolyte and dissolved in a mixture of EC (ethylene carbonate):PC (propylene carbonate) in a 1:1 volume ratio [17, 19– 21].

**Figure 8.** Fabrication scheme of lithium secondary batteries.

### *3.1.4. Scanning Electron Microscope (SEM)*

SEM images of the CNFs and mesoporous SiO2–CNF composites synthesized on Fe–Cu, Fe– Cu/mesoporous SiO2, and mesoporous SiO2-deposited Ni foam using the CVD method were obtained. Analysis of the SEM images showed that CNFs and mesoporous SiO2–CNF composites grew on Fe–Cu, Fe–Cu/mesoporous SiO2, and mesoporous SiO2-deposited Ni foam. The average diameter of the grown CNFs was 25–100 nm [18] (**Figure 9**).

**Figure 9.** SEM images of CNFs and mesoporous SiO2–CNF composites. (a) CNF–BF/Fe–Cu/Ni foam, (b) CNF–BF/Fe– Cu/mesoporous SiO2/Ni foam, and (c) CNF–BF/mesoporous SiO2/Ni foam.

### *3.1.5. Transmission electron microscopy (TEM)*

TEM was measured to determine the development of porosity in the synthesized mesoporous SiO2 materials as well as the structure of the synthesized CNFs and mesoporous SiO2–CNF composites. Panel **Figure 10(a)** shows that mesoporous SiO2 with uniform porosity was synthesized. As Shown in **Figure 10(b)**, CNFs were synthesized with a hollow tube-like structure in various diameters. As shown in **Figure 10(c)**, the CNFs were surrounded by mesoporous SiO2 in the mesoporous SiO2–CNF composites. Panel **Figure 10(d)** shows the elemental mapping from analyzing Si and O atoms. The overall distributions of mesoporous silica were examined [18] (**Figure 10**).

**Figure 10.** TEM images of CNFs and mesoporous SiO2–CNF composites. (a) mesoporous SiO2, (b) CNF–BF/Fe–Cu/Ni foam, (c) CNF–BF/mesoporous SiO2/Ni foam, and (d) elemental mapping of CNF–BF/mesoporous SiO2/Ni foam.

### *3.1.6. Cycle performances*

**Figure 9.** SEM images of CNFs and mesoporous SiO2–CNF composites. (a) CNF–BF/Fe–Cu/Ni foam, (b) CNF–BF/Fe–

12 Chemical Vapor Deposition - Recent Advances and Applications in Optical, Solar Cells and Solid State Devices

TEM was measured to determine the development of porosity in the synthesized mesoporous SiO2 materials as well as the structure of the synthesized CNFs and mesoporous SiO2–CNF composites. Panel **Figure 10(a)** shows that mesoporous SiO2 with uniform porosity was synthesized. As Shown in **Figure 10(b)**, CNFs were synthesized with a hollow tube-like structure in various diameters. As shown in **Figure 10(c)**, the CNFs were surrounded by mesoporous SiO2 in the mesoporous SiO2–CNF composites. Panel **Figure 10(d)** shows the elemental mapping from analyzing Si and O atoms. The overall distributions of mesoporous

**Figure 10.** TEM images of CNFs and mesoporous SiO2–CNF composites. (a) mesoporous SiO2, (b) CNF–BF/Fe–Cu/Ni foam, (c) CNF–BF/mesoporous SiO2/Ni foam, and (d) elemental mapping of CNF–BF/mesoporous SiO2/Ni foam.

Cu/mesoporous SiO2/Ni foam, and (c) CNF–BF/mesoporous SiO2/Ni foam.

*3.1.5. Transmission electron microscopy (TEM)*

silica were examined [18] (**Figure 10**).

Charging–discharging characteristics were examined by employing a current of 100 mA/g in order to investigate electrochemical characteristics such as the capacity and cycle ability of the three-electrode cell synthesized by applying CNFs and the mesoporous SiO2–CNF composites synthesized in this study as anode active materials. The electrochemical characteristics of the three-electrode cell were examined with and without the binder.

As shown in **Figure 11(a)**, when CNFs synthesized following the deposition of Fe–Cu catalyst on Ni foam were used as anode active materials, the initial capacity (256 mAh/g) was reduced to 231 mAh/g after 30 cycles, resulting in a retention rate of 90.2%. As shown in **Figure 11(b)**, when mesoporous SiO2–CNF composites synthesized after the deposition of Fe–Cu catalyst and mesoporous SiO2 on Ni foam were used as anode active materials, the initial capacity (289 mAh/g) was reduced to 169 mAh/g after 30 cycles, for a retention rate of 58.5%. As shown in **Figure 11(c)**, when mesoporous SiO2–CNF composites synthesized after the deposition of mesoporous SiO2 on Ni foam were used as anode active materials, the initial capacity (2420 mAh/g) was reduced to 2092 mAh/g after 30 cycles. The retention rate was 86.4%.

**Figure 11.** Cycle performance of CNFs and mesoporous SiO2–CNF composites without binder up to 30 cycles. (a) CNF–BF/Fe–Cu/Ni foam, (b) CNF–BF/Fe–Cu/mesoporous SiO2/Ni foam, and (c) CNF–BF/mesoporous SiO2/Ni foam.

The discharging capacity of mesoporous SiO2–CNF composites was higher than that of CNFs due to the high theoretical capacity of Si. As shown in (b) and (c), the capacity varied depending on the preparation methods used for mesoporous SiO2–CNF composites. The mesoporous SiO2–CNF composites synthesized after Fe–Cu catalyst and mesoporous SiO2 deposited on Ni foam did not show relatively good performance compared to those synthesized without a binder. The reason could be that SiO2 could not play a role because more CNFs grew in the presence of the Fe–Cu catalyst.

On the other hand, as shown in **Figure 11(c)**, mesoporous SiO2–CNF composites were synthe‐ sized after mesoporous SiO2was deposited on Ni foam without a catalyst. The CNFs grew because the Ni foam served as the catalyst. Thus, panel (c) had a higher capacity due to the mesoporous SiO2 than panel **Figure 11(b)**, in which many CNFs were grown. The CNFs also grew appropriately. Thus, the retention rate was relatively high [18] (**Figure 11**).

### **3.2. Synthesis and electrochemical performance of SiO2/CNF composite on Ni-Cu/C-fiber textiles**

### *3.2.1. Deposition of catalysts*

The electrophoretic deposition method was used to deposit Ni and Cu catalysts onto C-fiber textiles, and a schematic diagram of the experimental apparatus used in electrophoretic deposition is displayed in **Figure 12**. The C-fiber textiles were used as the cathode and a carbon electrode was employed as an anode, with a distance of 85 mm between each electrode. Three experimental conditions were employed in depositing the catalyst onto the C-fiber textiles. Ni was deposited onto the C-fiber textiles with a nickel(II) acetate tetrahydrate aqueous solution (Ni) while Ni and Cu were deposited onto the C-fiber textiles with a mixed solution of nickel(II) acetate tetrahydrate and copper(II) acetate monohydrate (Ni–Cu). For the third condition, Cu

**Figure 12.** Electrophoretic deposition apparatus used in the deposition of catalysts.

was predeposited onto the C-fiber textiles and Ni was subsequently deposited onto the same C-fiber textile in a nickel(II) acetate tetrahydrate aqueous solution (Ni/Cu) [11, 14, 16, 22, 23].

### *3.2.2. Reduction*

The discharging capacity of mesoporous SiO2–CNF composites was higher than that of CNFs due to the high theoretical capacity of Si. As shown in (b) and (c), the capacity varied depending on the preparation methods used for mesoporous SiO2–CNF composites. The mesoporous SiO2–CNF composites synthesized after Fe–Cu catalyst and mesoporous SiO2 deposited on Ni foam did not show relatively good performance compared to those synthesized without a binder. The reason could be that SiO2 could not play a role because more CNFs grew in the

14 Chemical Vapor Deposition - Recent Advances and Applications in Optical, Solar Cells and Solid State Devices

On the other hand, as shown in **Figure 11(c)**, mesoporous SiO2–CNF composites were synthe‐ sized after mesoporous SiO2was deposited on Ni foam without a catalyst. The CNFs grew because the Ni foam served as the catalyst. Thus, panel (c) had a higher capacity due to the mesoporous SiO2 than panel **Figure 11(b)**, in which many CNFs were grown. The CNFs also

**3.2. Synthesis and electrochemical performance of SiO2/CNF composite on Ni-Cu/C-fiber**

The electrophoretic deposition method was used to deposit Ni and Cu catalysts onto C-fiber textiles, and a schematic diagram of the experimental apparatus used in electrophoretic deposition is displayed in **Figure 12**. The C-fiber textiles were used as the cathode and a carbon electrode was employed as an anode, with a distance of 85 mm between each electrode. Three experimental conditions were employed in depositing the catalyst onto the C-fiber textiles. Ni was deposited onto the C-fiber textiles with a nickel(II) acetate tetrahydrate aqueous solution (Ni) while Ni and Cu were deposited onto the C-fiber textiles with a mixed solution of nickel(II) acetate tetrahydrate and copper(II) acetate monohydrate (Ni–Cu). For the third condition, Cu

grew appropriately. Thus, the retention rate was relatively high [18] (**Figure 11**).

**Figure 12.** Electrophoretic deposition apparatus used in the deposition of catalysts.

presence of the Fe–Cu catalyst.

*3.2.1. Deposition of catalysts*

**textiles**

A reduction step was applied. This was done to convert the metal oxides on the surface of the C-fiber textiles into elemental nickel and copper using a tube furnace. H2 gas mixed with N2 gas was used for the reduction process, and the flux of the reaction gas was regulated by MFC. The reactor temperature was increased at the rate of 12°C/min, until it reached 700°C. Once the temperature reached 700°C, N2 gas mixed with 20% H2 gas was flowed into the reactor. This reduction process was performed for 2 h [23, 24].

### *3.2.3. Growth of CNFs*

CNFs were synthesized onto C-fiber textiles using the CVD method in a horizontal tube furnace after the reduction process was completed. The prepared metal catalyst was evenly spread on a quartz boat, which was then placed into the reactor under an N2 atmosphere, and the reactor temperature was increased to 12°C/min. Once the temperature reached 700°C, this temperature was maintained for 30 min; 20% H2 gas balanced with N2 gas was flowed into the reactor. Then, for 3 h, the H2 gas balanced with N2 gas and 20% ethylene balanced with N2 gas were flowed together into the reactor. The flow of ethylene and H2 gases was cut off after the reaction was completed. N2 was then passed through the reactor under an inactive reactor atmosphere to cool it down to room temperature [23, 24].

### *3.2.4. Oxidation and SiO2 coating on CNFs*

For SiO2 coating on the surface of CNFs, the hydroxyl group was introduced as an anchor group. This was performed by oxidizing the hydroxyl group for half an hour in 80°C nitric acid and rinsing with distilled water. Then, for the synthesis of a composite of SiO2-coated CNFs, TEOS was dissolved in ethyl alcohol followed by the dispersion of the CNFs grown on C-fiber textiles in the solution and addition of ammonia water for a 24 h reaction at 50°C [23, 24].

### *3.2.5. Fabrication of anode materials for lithium secondary batteries*

The as-prepared CNFs were grown on C-fiber textiles without any binders and the conducting compounds were used as working electrodes for the fabrication of a conventional threeelectrode cell. Lithium was used as the counter and reference electrode. A glass fiber separator was used as the separator membrane. 1 M LiClO4 was employed as the electrolyte and dissolved in a mixture of EC (ethylene carbonate):PC (propylene carbonate) in a 1:1 volume ratio [23].

### *3.2.6 Scanning Electron Microscope (SEM)*

SEM images of CNFs grown with the Ni (a), Ni–Cu (b), and Ni/Cu (c) catalysts deposited onto C-fiber textiles are shown in **Figure 13**. As shown in **Figure 13(a)**, Y-shaped CNFs were grown with an average diameter of 40 nm using the Ni catalyst only, representing the growth of CNF branches that stem from a single origin. Meanwhile, in **Figure 13(b)**, another type of Y-shaped CNFs stemming from a single catalyst in various directions is shown. This figure is relevant to the size of the catalysts created because of the differences in the average diameters. Fur‐ thermore, in **Figure 13(c)**, helically grown CNFs with a uniform diameter of 33 nm are shown. With Ni deposited onto the predeposited C-fiber textiles, no Y-shaped carbon nanofiber can be observed in **Figure 13(c)** due to the tendency of the catalyst deposit and the introduction of Cu to affect the growth mechanism of CNFs [23].

**Figure 13.** SEM images of CNFs grown on the catalysts Ni (a), Ni–Cu (b), and Ni/Cu (c) on C-fiber textiles.

### *3.2.7. Transmission electron microscopy (TEM)*

TEM images were analyzed in order to investigate the structure of the SiO2-coated layer in the SiO2/CNF composite after the growth of CNFs onto C-fiber textiles. These images are shown

**Figure 14.** TEM images of CNFs (a) and SiO2/CNF composite (b)–(d).

in **Figure 14**. As shown in **Figure 14(a)**, the TEM image of CNFs illustrates the multilayer graphite forming wires with a central microhollow. As for the TEM images from the SiO2/CNF composite **Figure 14(b)**–**(d)**, they represent the SiO2 from the output of TEOS hydrolysis, which was uniformly coated onto the CNFs to obtain a layered structure [23, 24].

### *3.2.8. Cycle performances*

branches that stem from a single origin. Meanwhile, in **Figure 13(b)**, another type of Y-shaped CNFs stemming from a single catalyst in various directions is shown. This figure is relevant to the size of the catalysts created because of the differences in the average diameters. Fur‐ thermore, in **Figure 13(c)**, helically grown CNFs with a uniform diameter of 33 nm are shown. With Ni deposited onto the predeposited C-fiber textiles, no Y-shaped carbon nanofiber can be observed in **Figure 13(c)** due to the tendency of the catalyst deposit and the introduction of

16 Chemical Vapor Deposition - Recent Advances and Applications in Optical, Solar Cells and Solid State Devices

**Figure 13.** SEM images of CNFs grown on the catalysts Ni (a), Ni–Cu (b), and Ni/Cu (c) on C-fiber textiles.

TEM images were analyzed in order to investigate the structure of the SiO2-coated layer in the SiO2/CNF composite after the growth of CNFs onto C-fiber textiles. These images are shown

Cu to affect the growth mechanism of CNFs [23].

*3.2.7. Transmission electron microscopy (TEM)*

**Figure 14.** TEM images of CNFs (a) and SiO2/CNF composite (b)–(d).

The SiO2/CNF composite was subjected to a repeated cycling test at a current density of 100 mA g−1 within a voltage window of 0.1–2.6 V. For comparison, the CNF electrode was tested at the same condition. The cycling performances of the CNFs and the SiO2/CNF composite electrodes for Li secondary batteries are shown in **Figure 15**. The early-stage discharge capacity of the CNF electrode was 300 mAh/g and a near-stable discharge capacity was maintained for 30 cycles. In the case of the SiO2/CNF composite, a comparatively high discharge capacity of 2053 mAh/g was observed in the second cycle, and the discharge capacity of the 29th cycle was significantly reduced to 1295 mAh/g, with 63% capacity retention as compared to that of the second cycle. This indicates that the discharge capacity of the CNF electrode nearly reached its theoretical capacity (372 mAh/g) and showed no decline. The SiO2/CNF composite had a high discharge capacity of 2053 mAh/g, but the cycle performance was not as good as that of the CNFs [23].

**Figure 15.** Discharge capacity of CNFs and SiO2/CNF composite.

### **4. Conclusions**

CNFs were synthesized by using CVD and the effects of synthesis conditions on the growth of CNFs were investigated by controlling the synthesis temperature and the concentration ratio of transition metal catalysts. Physiochemical and electrochemical characteristics of the grown CNFs were investigated using various spectroscopic and electrochemical techniques. Based on these CNFs, SiO2–CNF composites were synthesized, and the physiochemical characteris‐ tics of the SiO2/CNF composites as well as their electrochemical characteristics as anode materials of lithium secondary batteries were investigated.


### **Acknowledgements**

This research was financially supported by the Ministry of Education, Science Technology (MEST) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (No. 2015035858).

### **Author details**

of transition metal catalysts. Physiochemical and electrochemical characteristics of the grown CNFs were investigated using various spectroscopic and electrochemical techniques. Based on these CNFs, SiO2–CNF composites were synthesized, and the physiochemical characteris‐ tics of the SiO2/CNF composites as well as their electrochemical characteristics as anode

18 Chemical Vapor Deposition - Recent Advances and Applications in Optical, Solar Cells and Solid State Devices

**(1)** CNFs were synthesized by ethylene decomposition using CVD based on Fe and Cu catalysts. According to the SEM measurements, the CNFs had 15–35 nm diameter. In

using BET, the synthesized CNFs had the largest specific surface area of 77–305 m2

**(2)** CNFs were synthesized by ethylene decomposition using CVD with Co and Cu catalysts. According to the SEM measurements, the CNFs had 20–35 nm diameter. In addition,

**(3)** CNFs and mesoporous SiO2–CNF composites were synthesized using Fe–Cu binary catalysts with the CVD method. According to the results of SEM measurements, the average diameter of grown CNFs along with mesoporous SiO2 was 25–100 nm. According to the results of galvanostatic charging and discharging, the discharging capacity of mesoporous SiO2–CNF composites was higher than that of CNFs due to the high theo‐ retical capacity of Si. In particular, mesoporous SiO2–CNF composites synthesized without binders after mesoporous SiO2 was deposited on Ni foam showed the highest charging and discharging capacity and retention rate. The initial capacity (2420 mAh/g)

BET, the synthesized CNFs had the largest specific surface area of 178–306 m2

was reduced to 2092 mAh/g after 30 cycles for a retention rate of 86.4% [18].

**(4)** CNFs were grown with the CVD method onto C-fiber textiles, based upon Ni, Ni–Cu, and Ni/Cu catalysts, followed by TEOS hydrolysis to coat SiO2 onto the CNFs. The conclusion of the results is as follows. CNFs grown on Ni/C-fiber textiles were synthesized with a diameter of 40 nm and showed a consistent Y-shaped branch morphology. CNFs grown on Ni–Cu/C-fiber textiles were synthesized with a diameter of 300 nm and had a multi‐ directional Y-shaped branch morphology. CNFs grown on Ni/Cu/C-fiber textiles ap‐ peared to be the most uniform CNFs and had a diameter of 33 nm. Based on galvanostatic charge–discharge, the SiO2/CNF composites featured a much more excellent discharge capacity of 1295 mAh/g compared to the CNF, which remained at 304 mAh/g, after 29 cycles. Further, a fairly decent capacity retention, 63% compared to the first two cycles,

This research was financially supported by the Ministry of Education, Science Technology (MEST) and National Research Foundation of Korea (NRF) through the Human Resource

/g) of carbon nanofibers

/g) of the carbon nanofibers using

/g

/g [11, 14].

materials of lithium secondary batteries were investigated.

according to the measured specific surface areas (m2

was observed after 20 cycles [23].

Training Project for Regional Innovation (No. 2015035858).

**Acknowledgements**

[9, 11].

addition, according to the measured specific surface areas (m2

Chang-Seop Lee\* and Yura Hyun

\*Address all correspondence to: surfkm@kmu.ac.kr

Department of Chemistry, Keimyung University, Daegu, South Korea

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[21] Yura Hyun, Jin-Young Choi, Heai-Ku Park, Chang-Seop Lee. Synthesis and Electro‐ chemical Performance of Ruthenium Oxide-coated Carbon Nanofibers as Anode Materials for Lithium Secondary Batteries. Applied Surface Science. Forthcoming. DOI: http://dx.doi.org/10.1016/j.apsusc.2016.01.095

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[11] Yura Hyun, Eun-Sil Park, Karina Mees, Ho-Seon Park, Monika Willert-Porada, Chang-Seop Lee. Synthesis and Characterization of Carbon Nanofibers on Transition Metal Catalysts by Chemical Vapour Deposition. Journal of Nanoscience and Nanotechnol‐

[12] Eun-Sil Park, Jong-Ha Choi, Chang-Seop Lee. Synthesis and Characterization of Vaporgrown Si/CNF and Si/PC/CNF Composites Based on Co–Cu Catalysts. Bulletin of the

[13] Eunyi-Jang, Heai-Ku Park, Jong-Ha Choi, Chang-Seop Lee. Synthesis and Characteri‐ zation of Carbon Nanofibers Grown on Ni and Mo Catalysts by Chemical Vapor Deposition. Bulletin of the Korean Chemical Society. 2015;36(5):1452–1459. DOI:

[14] Eun-Sil Park, Jong-Won Kim, Chang-Seop Lee, Synthesis and Characterization of Carbon Nanofibers on Co and Cu Catalysts by Chemical Vapor Deposition. Bulletin of the Korean Chemical Society. 2014;35(6):1687–1695. DOI : 10.5012/bkcs.2014.35.6.1687

[15] Sang-Won Lee, Chang-Seop Lee. Growth and Characterization of Carbon Nanofibers on Fe/C-fiber Textiles Coated by Deposition-Precipitation and Dip-Coating. Journal of Nanoscience and Nanotechnology. 2015;15(9):7317–7326. DOI: http://dx.doi.org/

[16] Sang-Won Lee, Chang-Seop Lee. Electrophoretic Deposition of Iron Catalyst on C-Fiber Textiles for the Growth of Carbon Nanofibers. Journal of Nanoscience and Nanotech‐

[17] Yura Hyun, Heai-Ku Park, Ho-Seon Park, Chang-Seop Lee. Characteristics and Electrochemical Performance of Si-Carbon Nanofibers Composite as Anode Material for Binder-Free Lithium Secondary Batteries. Journal of Nanoscience and Nanotech‐

[18] Yura Hyun, Jin-Yeong Choi, Heai-Ku Park, Jae Young Bae, Chang-Seop Lee. Synthesis and Electrochemical Performance of Mesoporous SiO2-Carbon Nanofiber Composite as Anode Materials for Lithium Secondary Batteries. Materials Research Bulletin.

[19] Eun-Sil Park, Heai-Ku Park, Ho-Seon Park, Chang-Seop Lee. Synthesis and Electro‐ chemical Properties of CNF–Si Composites as an Anode Material for Li Secondary Batteries. Journal of Nanoscience and Nanotechnology. 2015;15(11):8961–8970. DOI:

[20] Eunyi-Jang, Heai-Ku Park, Chang-Seop Lee. Synthesis and Application of Si/Carbon Nanofiber Composites based on Ni and Mo Catalysts for Anode Material of Lithium

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20 Chemical Vapor Deposition - Recent Advances and Applications in Optical, Solar Cells and Solid State Devices

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### **Non-Classical Crystallization of Thin Films and Nanostructures in CVD Process**

Jae-soo Jung and Nong-moon Hwang

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/63926

### **Abstract**

Non-classical crystallization, where crystals grow by the building blocks of nanopar‐ ticles, has become a significant issue not only in solution but also in the gas phase synthesis such as chemical vapor deposition (CVD). Recently, non-classical crystalli‐ zation was observed in solution in-situ by transmission electron microscope (TEM) using a liquid cell technique. In various CVD processes, the generation of charged nanoparticles (CNPs) in the gas phase has been persistently reported. Many evidences supporting these CNPs to be the building blocks of thin films and nanostructures were reported. According to non-classical crystallization, many thin films and nanostructures which had been believed to grow by individual atoms or molecules turned out to grow by the building blocks of CNPs. The purpose of this paper is to review the development and the main results of non-classical crystallization in the CVD process. The concept of non-classical crystallization is briefly described. Further, it will be shown that the puzzling phenomenon of simultaneous diamond deposi‐ tion and graphite etching, which violates the second law of thermodynamics when approached by classical crystallization, can be approached successfully by nonclassical crystallization. Then, various aspects of non-classical crystallization in the growth of thin films and nanostructures by CVD will be described.

**Keywords:** chemical vapor deposition, non-classical crystallization, thin films, charged nanoparticles, gas phase nucleation

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **1. Introduction**

The theory of classical crystal growth was established based on the concept that the building block of crystals should be individual ions, atoms, or molecules. However, there have been some experimental results, which cannot be properly explained by this classical mechanism. Rather such experimental results strongly imply that crystals should grow by the building blocks of nanoparticles, whose way of crystal growth is called 'non-classical crystallization' [1–5]. Recently, non-classical crystallization was confirmed by in-situ transmission electron micro‐ scope (TEM) observations. Although non-classical crystallization is a relatively new and revolutionary concept in crystal growth, it has now become so established that a few related books have been published and its tutorial and technical sessions had been included respec‐ tively in the spring meetings of Materials Research Society (MRS) and European Materials Research Society (EMRS) in 2014. With the establishment of non-classical crystallization, many crystals that were believed to grow by atomic, molecular, or ionic entities turn out to grow actually by nanoparticles.

Non-classical crystallization can be applied to crystal growth not only in solution but also in the gas phase synthesis of thin films and nanostructures by chemical vapor deposition (CVD) and physical vapor deposition (PVD). Hwang et al. [6–10] extensively studied non-classical crystallization in the CVD process, publishing more than 80 SCI papers. They suggested that the electric charge carried by the nanoparticles played a critical role, by which the growth of thin films and nanostructures by the building blocks of nanoparticles is made possible. This is why they called this new growth mechanism in the gas phase synthesis 'theory of charged nanoparticles (TCN)'. According to this theory, charged nanoparticles (CNPs), which are spontaneously generated in the gas phase in most CVD processes, contribute to the growth of thin films and nanostructures. If nanoparticles are neutral, they undergo random Brownian coagulation, producing a very porous structure. If nanoparticles are charged, however, they deposit as dense films without voids. This is because CNPs undergo self-assembly and are liquid-like, resulting in epitaxial recrystallization.

There seem to be two reasons why this new growth mechanism has been unknown. The first reason would be that CNPs are invisible because their size is much smaller than the wavelength of visible light. The second reason would be that it is difficult to believe that CNPs can be the building blocks for the evolution of dense films and nanostructures. The generation of CNPs in the gas phase was experimentally confirmed in many CVD processes synthesizing such as diamond [11, 12], ZrO2 [13], Si [14], carbon nanotubes [15, 16], ZnO nanowires [17], and silicon nanowires [18]. The critical reason why these CNPs can be the building blocks of thin films and nanostructures is that the charge weakens the bond strength and makes nanoparticles liquid-like.

TCN was first suggested to explain the paradoxical experimental observation of simultaneous deposition of less stable diamond and etching of stable graphite. This phenomenon violates the second law of thermodynamics if approached by the classical concept of crystal growth by an atomic unit.

### **2. Non-classical crystallization**

**1. Introduction**

actually by nanoparticles.

liquid-like.

an atomic unit.

liquid-like, resulting in epitaxial recrystallization.

The theory of classical crystal growth was established based on the concept that the building block of crystals should be individual ions, atoms, or molecules. However, there have been some experimental results, which cannot be properly explained by this classical mechanism. Rather such experimental results strongly imply that crystals should grow by the building blocks of nanoparticles, whose way of crystal growth is called 'non-classical crystallization' [1–5]. Recently, non-classical crystallization was confirmed by in-situ transmission electron micro‐ scope (TEM) observations. Although non-classical crystallization is a relatively new and revolutionary concept in crystal growth, it has now become so established that a few related books have been published and its tutorial and technical sessions had been included respec‐ tively in the spring meetings of Materials Research Society (MRS) and European Materials Research Society (EMRS) in 2014. With the establishment of non-classical crystallization, many crystals that were believed to grow by atomic, molecular, or ionic entities turn out to grow

24 Chemical Vapor Deposition - Recent Advances and Applications in Optical, Solar Cells and Solid State Devices

Non-classical crystallization can be applied to crystal growth not only in solution but also in the gas phase synthesis of thin films and nanostructures by chemical vapor deposition (CVD) and physical vapor deposition (PVD). Hwang et al. [6–10] extensively studied non-classical crystallization in the CVD process, publishing more than 80 SCI papers. They suggested that the electric charge carried by the nanoparticles played a critical role, by which the growth of thin films and nanostructures by the building blocks of nanoparticles is made possible. This is why they called this new growth mechanism in the gas phase synthesis 'theory of charged nanoparticles (TCN)'. According to this theory, charged nanoparticles (CNPs), which are spontaneously generated in the gas phase in most CVD processes, contribute to the growth of thin films and nanostructures. If nanoparticles are neutral, they undergo random Brownian coagulation, producing a very porous structure. If nanoparticles are charged, however, they deposit as dense films without voids. This is because CNPs undergo self-assembly and are

There seem to be two reasons why this new growth mechanism has been unknown. The first reason would be that CNPs are invisible because their size is much smaller than the wavelength of visible light. The second reason would be that it is difficult to believe that CNPs can be the building blocks for the evolution of dense films and nanostructures. The generation of CNPs in the gas phase was experimentally confirmed in many CVD processes synthesizing such as diamond [11, 12], ZrO2 [13], Si [14], carbon nanotubes [15, 16], ZnO nanowires [17], and silicon nanowires [18]. The critical reason why these CNPs can be the building blocks of thin films and nanostructures is that the charge weakens the bond strength and makes nanoparticles

TCN was first suggested to explain the paradoxical experimental observation of simultaneous deposition of less stable diamond and etching of stable graphite. This phenomenon violates the second law of thermodynamics if approached by the classical concept of crystal growth by

### **2.1. Theory of charged nanoparticles in CVD**

Thin film growth by CVD is explained in text books as follows. Atoms or molecules are formed on the growing surface or in the gas phase as a result of chemical reactions of reactant gases. Those atoms or molecules are then adsorbed on a terrace, diffused to a ledge and become incorporated in the crystal lattice at the kink, which is called the terrace, ledge, and kink (TLK) model [19, 20]. This mechanism is called 'classical crystal growth mechanism'. Normally, a ledge of monoatomic height is regarded as a kink because the ledge is disordered or rough, consisting of lots of kinks. An atom on the terrace, which is called an adatom, has excess broken bonds but an atom at the kink has no excess broken bond. For this reason, a reversible transfer of atoms occurs only at kinks during condensation or evaporation. In other words, the interaction of atoms or molecules with the terrace is repulsive; however, their interaction with the kink is attractive. Because of this difference between the terrace and kink, atoms are only accommodated at the kink, which results in self-assembly of atoms or molecules. If the atomic interaction with the terrace should be attractive as well, there would be no atomic selfassembly, resulting in random packing of atoms and the growing film would become amor‐ phous.

In this paradigm of thin film growth, the maximum supersaturation, which would define the maximum growth rate, would be the one which triggers the onset of gas-phase nucleation. However, according to Hwang et al. [9, 10], the supersaturation that triggers the gas-phase nucleation turns out to be so low that the film growth rate without gas-phase nucleation is negligibly low and such processing conditions would be hardly adopted in the thin film industry. In other words, under the process conditions of commercially available thin films, the gas-phase nucleation occurs in general. This means that thin films are growing inevitably under the condition of gas-phase nucleation in most CVD and PVD processes.

It was believed that the gas-phase nucleation would be harmful to the thin film growth. Gas phase-generated nanoparticles may cause killer defects, resulting in device failure, due to small feature sizes, which decrease to <100 nm [21]. However, Hwang and Lee [10] suggested that the deposition behavior of gas phase-generated nanoparticles differs drastically depending on whether they are electrically charged or not. Neutral nanoparticles produce a porous skeletal structure, usually degrading the property of films. However, CNPs tend to be liquid-like and tend to deposit epitaxially, leaving no voids behind, producing dense films. The film micro‐ structures evolved by the deposition of liquid-like CNPs would be difficult to distinguish from those by the deposition of individual atoms or molecules.

Therefore, in order to grow a high quality film at a high deposition rate, it would be necessary to utilize the generation of CNPs in the gas phase. In accordance with this new understanding, Yoshida et al. [22] could grow high Tc superconducting (YBa2Cu3O7-x) films epitaxially at a rate as high as 16 nm/s by supplying YBa2Cu3O7-x particles using the plasma flash evaporation method. Cabarrocas [23, 24], Vladimirov and Ostrikov [25], and Nunomura et al. [26] also utilize the incorporation of gas phase-nucleated nanoparticles in the plasma-enhanced CVD (PECVD) process. During the deposition of silicon by PECVD, Cabarrocas [23, 24] deposited polymorphous films, where gas phase-generated crystalline silicon nanoparticles are incor‐ porated into the films. Polymorphous films have better stability and electrical properties than amorphous films.

Crystal growth mechanism by the building blocks of nanoparticles has a long history. For example, more than 40 years ago, Glasner et al. [27–30] suggested that nanometer-sized nuclei were generated in the solution with Pb2+ during the growth of KBr and KCl. They confirmed that the crystal was grown by self-assembly of the block nuclei in the solution. The crystallinity increased with decreasing size of nuclei. Sunagawa [31, 32], made a similar suggestion that the growth unit of synthetic diamond is not an atom but a much larger unit. These suggestions were not accepted in the crystal growth community largely because the experimental tools were not available at that time to confirm the generation of nanoparticles in solution or in the gas phase. Besides, it was believed that crystal growth by the building blocks of nanoparticles would produce aggregates of nanoparticles instead of dense structures. For example, Glasner et al. [27–30]'s suggestion was doubted and criticized [33] and has been neglected in the crystal growth community.

Such a way of crystal growth by the building blocks of nanoparticles is now well-established and called 'non-classical crystallization' [34, 35]. **Figure 1** compares crystalline pathways between classical and non-classical crystallization. The building blocks of classical crystalli‐ zation are atoms, ions, or molecules, which form nanoparticles (**Figure 1(a)**) [35]. As described in the classical nucleation theory, these nanoparticles may grow or shrink by the relative magnitude of surface and bulk energies. If nanoparticles reach the size of the critical nucleus, they can continue to grow into macro crystals by the attachment of an individual atom or molecule.

**Figure 1.** Schematic representation of classical and non-classical crystallization. (a) Classical crystallization. (b) orient‐ ed attachment of primary nanoparticles. (c) mesocrystal formation via self-assembly of primary nanoparticles covered with organics. Reprinted with permission from [35]. Copyright 2006 Elsevier.

**Figure 1(b)** shows the main course of non-classical crystallization, where an iso-oriented crystal grows by oriented attachment of primary nanoparticles, which can form a single crystal upon fusion of the nanoparticles. If the nanoparticles are covered by some organic components, they can form a mesocrystal by mesoscale assembly (path (c)). Cölfen and Antonietti [36] studied a mesocrystal as a superstructure of crystalline nanoparticles with external crystal faces on the scale of hundreds nanometers to micrometers. They also studied that the meso‐ crystal intermediates can lead to the synthesis of single crystals with included organic additives. During the synthesis process of single crystals, highly oriented nanoparticle-based intermediates could be observed as shown in **Figure 1(c)**. If these mesocrystal intermediates are heated at sufficiently high temperature, they can fuse into a single crystal.

(PECVD) process. During the deposition of silicon by PECVD, Cabarrocas [23, 24] deposited polymorphous films, where gas phase-generated crystalline silicon nanoparticles are incor‐ porated into the films. Polymorphous films have better stability and electrical properties than

26 Chemical Vapor Deposition - Recent Advances and Applications in Optical, Solar Cells and Solid State Devices

Crystal growth mechanism by the building blocks of nanoparticles has a long history. For example, more than 40 years ago, Glasner et al. [27–30] suggested that nanometer-sized nuclei were generated in the solution with Pb2+ during the growth of KBr and KCl. They confirmed that the crystal was grown by self-assembly of the block nuclei in the solution. The crystallinity increased with decreasing size of nuclei. Sunagawa [31, 32], made a similar suggestion that the growth unit of synthetic diamond is not an atom but a much larger unit. These suggestions were not accepted in the crystal growth community largely because the experimental tools were not available at that time to confirm the generation of nanoparticles in solution or in the gas phase. Besides, it was believed that crystal growth by the building blocks of nanoparticles would produce aggregates of nanoparticles instead of dense structures. For example, Glasner et al. [27–30]'s suggestion was doubted and criticized [33] and has been neglected in the crystal

Such a way of crystal growth by the building blocks of nanoparticles is now well-established and called 'non-classical crystallization' [34, 35]. **Figure 1** compares crystalline pathways between classical and non-classical crystallization. The building blocks of classical crystalli‐ zation are atoms, ions, or molecules, which form nanoparticles (**Figure 1(a)**) [35]. As described in the classical nucleation theory, these nanoparticles may grow or shrink by the relative magnitude of surface and bulk energies. If nanoparticles reach the size of the critical nucleus, they can continue to grow into macro crystals by the attachment of an individual atom or

**Figure 1.** Schematic representation of classical and non-classical crystallization. (a) Classical crystallization. (b) orient‐ ed attachment of primary nanoparticles. (c) mesocrystal formation via self-assembly of primary nanoparticles covered

with organics. Reprinted with permission from [35]. Copyright 2006 Elsevier.

amorphous films.

growth community.

molecule.

**Figure 2.** TEM images of the initial nucleation and growth of Pt3Fe nanowires in the molecular precursor solution. Re‐ printed with permission from [37]. Copyright 2012 Elsevier.

**Figure 1(c)** shows that the mesocrystal intermediates clearly reveal the kinetic path of crystal‐ lization. Therefore, the mesocrystal intermediates have an important role in revealing the mechanism of non-classical crystallization. However, when the kinetics follows the path of **Figure 1(b)**, it would be difficult to distinguish from a final morphology of the crystal whether it had grown by an individual atom or nanoparticle. This is why crystal growth by nanopar‐ ticles had a great resistance in the crystal growth community in the early years.

Recently, crystal growth by nanoparticles in solution could be directly observed by TEM using a liquid cell [37, 38], which provided direction evidences for non-classical crystallization. Liao et al. [37] show detailed real-time imaging to show how Pt3Fe nano-rods grow by nanoparticles in solution using a silicon nitride liquid cell for in situ TEM observation.

**Figure 2** shows images listing the growth process of a twisted Pt3Fe nanowire. In the initial stage of growth, many small nanoparticles are formed when the Pt and Fe precursors are reduced by electron beam illumination. Some of them grow by monomer attachment and others undergo coalescence. The nanoparticles were combined by coalescence and then relaxed into nanoparticles. Finally, the average size of these nanoparticles reached 5.3 ± 0.9 nm.

In the second stage, nanoparticles interact with each other to form nanoparticle chains. The nanoparticle chain is formed by shape-directed nanoparticle attachment with successive structural relaxation into straight Pt3Fe nano-rods and reorientation, revealing critical mech‐ anisms of the growth into nano-rods from nanoparticle building blocks. Therefore, even when nanoparticles attach without orientation, single-crystalline nano-rods are formed eventually. In the first stage of growth, nanoparticles meet each other, and form dimer. But if the dimer meets another nanoparticle, unlike the first stage of growth, the dimer does not coalesce into a sphere but forms a trimer by connecting the particle to the dimer end. The additional endto-end attachments generate a nanoparticle chain.

By in situ TEM observation using graphene liquid cells, Yuk et al. [38] carried out direct atomicresolution imaging to show how Pt crystals grow in solution. The microscope is operated at 80 kV with a beam intensity of 103 to 104 A/m2 maintained during nanocrystal growth. Upon locating a liquid pocket on the TEM grid, the beam intensity is optimized, which reduces the Pt precursor and initiates nanocrystal growth [39]. The use of graphene liquid cells made it possible to discern colloidal Pt nanoparticles with radii as small as 0.1 nm and to track their motion, which was not possible by previous cells with silicon nitride windows [39].

**Figure 3** shows the TEM images of the nanocrystals which are connected by a neck at the initial stage of coalescence. Neck growth occurs simultaneously with decreasing length (l) and thickness (t), which means that the atoms migrate to the neck region by surface diffusion [40]. After coalescence, the nanocrystal structure gradually reorganizes, evolving truncated surfaces.

**Figure 3** shows the detailed process how crystalline growth occurs by the building blocks of nanocrystals. Yuk et al. [38] mentioned in the supporting information that all images were collected under ambient conditions at 23°C. It should be noted that such enhanced kinetics of liquid-like coalescence at such a low temperature can never be expected from neutral

nanocrystals. Although the authors did not mention the role of charge, it should be noted that charging is unavoidable during TEM observation.

According to Hwang and Lee [10], the role of charge is critical in non-classical crystallization since it makes CNPs liquid-like. The liquid-like property of CNPs was deduced by Hwang et al. from the experimental observation that dense films are evolved by the deposition of CNPs. Considering that CNPs are liquid-like, it is expected that the bond strength should be weak‐ ened by the presence of charge. To check this possibility, we made an extensive literature survey and found the paper by Clare et al. [41], who studied the effect of charge on the bond strength in hydrogenated amorphous silicon. The main result of this paper and its implications are summarized in the following section.

**Figure 3.** Pt nanocrystal dynamics of coalescence. l, t, and n in the figure represent respectively the length along the center-to-center direction, the thickness in vertical direction to the length and the neck diameter. Reprinted with per‐ mission from [38]. Copyright 2012 Elsevier.

### **2.2. Effect of charge on the bond strength**

**Figure 1(c)** shows that the mesocrystal intermediates clearly reveal the kinetic path of crystal‐ lization. Therefore, the mesocrystal intermediates have an important role in revealing the mechanism of non-classical crystallization. However, when the kinetics follows the path of **Figure 1(b)**, it would be difficult to distinguish from a final morphology of the crystal whether it had grown by an individual atom or nanoparticle. This is why crystal growth by nanopar‐

Recently, crystal growth by nanoparticles in solution could be directly observed by TEM using a liquid cell [37, 38], which provided direction evidences for non-classical crystallization. Liao et al. [37] show detailed real-time imaging to show how Pt3Fe nano-rods grow by nanoparticles

**Figure 2** shows images listing the growth process of a twisted Pt3Fe nanowire. In the initial stage of growth, many small nanoparticles are formed when the Pt and Fe precursors are reduced by electron beam illumination. Some of them grow by monomer attachment and others undergo coalescence. The nanoparticles were combined by coalescence and then relaxed into nanoparticles. Finally, the average size of these nanoparticles reached 5.3 ± 0.9 nm.

In the second stage, nanoparticles interact with each other to form nanoparticle chains. The nanoparticle chain is formed by shape-directed nanoparticle attachment with successive structural relaxation into straight Pt3Fe nano-rods and reorientation, revealing critical mech‐ anisms of the growth into nano-rods from nanoparticle building blocks. Therefore, even when nanoparticles attach without orientation, single-crystalline nano-rods are formed eventually. In the first stage of growth, nanoparticles meet each other, and form dimer. But if the dimer meets another nanoparticle, unlike the first stage of growth, the dimer does not coalesce into a sphere but forms a trimer by connecting the particle to the dimer end. The additional end-

By in situ TEM observation using graphene liquid cells, Yuk et al. [38] carried out direct atomicresolution imaging to show how Pt crystals grow in solution. The microscope is operated at

locating a liquid pocket on the TEM grid, the beam intensity is optimized, which reduces the Pt precursor and initiates nanocrystal growth [39]. The use of graphene liquid cells made it possible to discern colloidal Pt nanoparticles with radii as small as 0.1 nm and to track their

**Figure 3** shows the TEM images of the nanocrystals which are connected by a neck at the initial stage of coalescence. Neck growth occurs simultaneously with decreasing length (l) and thickness (t), which means that the atoms migrate to the neck region by surface diffusion [40]. After coalescence, the nanocrystal structure gradually reorganizes, evolving truncated

**Figure 3** shows the detailed process how crystalline growth occurs by the building blocks of nanocrystals. Yuk et al. [38] mentioned in the supporting information that all images were collected under ambient conditions at 23°C. It should be noted that such enhanced kinetics of liquid-like coalescence at such a low temperature can never be expected from neutral

motion, which was not possible by previous cells with silicon nitride windows [39].

A/m2 maintained during nanocrystal growth. Upon

to 104

ticles had a great resistance in the crystal growth community in the early years.

28 Chemical Vapor Deposition - Recent Advances and Applications in Optical, Solar Cells and Solid State Devices

in solution using a silicon nitride liquid cell for in situ TEM observation.

to-end attachments generate a nanoparticle chain.

80 kV with a beam intensity of 103

surfaces.

The effect of a single negative or positive charge on the strength of silicon-silicon and siliconhydrogen bonds in the molecules SiH4 and Si2H2 was calculated by ab initio calculations. To determine the difference in the energy to break a single Si-H bond in SiH4, SiH4 + and SiH4 − calculations were done on six species: SiH3, SiH4, SiH3 − , SiH4 − , SiH3 + , and SiH4 <sup>+</sup> and the required energies were determined by comparing the bond strength of each species. Similar calculations were done with the species Si2H6, Si2H5, Si2H6 − , Si2H5 − , Si2H6 + , and Si2H5 <sup>+</sup> to observe the effect of a lower charge/size ratio and to examine the effect of charge on the Si-Si bond energy. The results of ab initio calculations are shown in **Table 1**.

When the atoms are embedded in a lattice, they will not be free to attain geometries resembling the optimized ion geometry, although they will be able to relax to some degree. Thus, the actual effects of charge on bond strength in hydrogenated amorphous silicon will be between those indicated by the unoptimized (adiabatic) and optimized (vertical) rows of **Table 1**. They are likely to be closer to those for the unoptimized rows.

Both positive and negative charges drastically weaken the bond strength of Si-Si and Si-H. The bond strength of Si-Si is weakened from 3.2 eV to 1.11 eV when Si2H6 is negatively charged. It is weakened to 1.6 eV when Si2H6 is positively charged. The bond strength of Si-H is weakened drastically from 3.9 eV to 0.98 eV when SiH4 is negatively charged. It is weakened to 0.3 eV when SiH4 is positively charged.


**Table 1.** Calculated bond strengths of Si-H and Si-Si. Reprinted with permission from [41]. Copyright 1993 Elsevier.

The effect of charge on the bond strength can be explained by a bond order in the molecular orbital theory. A bond order, which represents the strength or stability of bond, is the number of bonding electron pairs shared by two atoms in a molecule. A bond order is defined as half the difference between the number of bonding electrons and the number of antibonding electrons as expressed by the following equation,

$$\text{Bond order} = \frac{\#of \text{bonding electrons} - \#of \text{ antibonding electrons}}{2} \tag{1}$$

If a nanoparticle is charged negatively, electrons are added to the antibonding orbital. If a nanoparticle is charged positively, electrons are removed from the bonding orbital. Therefore, both positive and negative charges would decrease the bond order and thereby weaken the bond strength.

Weakening of bond strength by charge has very important implications because it means that diffusion or kinetics is enhanced. The new concept of charge-induced weakening of bond strength can explain the liquid-like property of CNPs, which was suggested by Hwang et al. [10]. This concept can also explain the rapid kinetics of coalescence in **Figures 2** and **3**. The concept of charge-enhanced kinetics can explain the enhanced chemical reactions of reactant gases even at low temperature in the PECVD process. It also explains the deposition of crystalline films at low temperature.

When the atoms are embedded in a lattice, they will not be free to attain geometries resembling the optimized ion geometry, although they will be able to relax to some degree. Thus, the actual effects of charge on bond strength in hydrogenated amorphous silicon will be between those indicated by the unoptimized (adiabatic) and optimized (vertical) rows of **Table 1**. They are

30 Chemical Vapor Deposition - Recent Advances and Applications in Optical, Solar Cells and Solid State Devices

Both positive and negative charges drastically weaken the bond strength of Si-Si and Si-H. The bond strength of Si-Si is weakened from 3.2 eV to 1.11 eV when Si2H6 is negatively charged. It is weakened to 1.6 eV when Si2H6 is positively charged. The bond strength of Si-H is weakened drastically from 3.9 eV to 0.98 eV when SiH4 is negatively charged. It is weakened to 0.3 eV

**Compound Si–H (eV) Si–Si (eV)** SiH4 (optimized) 3.9 —

(optimized) 0.98 —

 (optimized) 0.30 — Si2H6 (optimized) 3.5 3.2

(optimized) 1.02 1.11

(optimized) 1.59 1.6

(unoptimized) 1.35 —

(unoptimized) 0.09 —

(unoptimized) 1.34 1.3

(unoptimized) 1.49 1.6

**Table 1.** Calculated bond strengths of Si-H and Si-Si. Reprinted with permission from [41]. Copyright 1993 Elsevier.

The effect of charge on the bond strength can be explained by a bond order in the molecular orbital theory. A bond order, which represents the strength or stability of bond, is the number of bonding electron pairs shared by two atoms in a molecule. A bond order is defined as half the difference between the number of bonding electrons and the number of antibonding

2

If a nanoparticle is charged negatively, electrons are added to the antibonding orbital. If a nanoparticle is charged positively, electrons are removed from the bonding orbital. Therefore, both positive and negative charges would decrease the bond order and thereby weaken the

Weakening of bond strength by charge has very important implications because it means that diffusion or kinetics is enhanced. The new concept of charge-induced weakening of bond

*of bonding electrons of antibonding electrons* - <sup>=</sup> (1)

likely to be closer to those for the unoptimized rows.

electrons as expressed by the following equation,
