Chemical Sensors Volume 2: Nanostructured Materials

v

CONTENTS

PREFACE TO CHEMICAL SENSORS: FUNDAMENTALS OF SENSING MATERIALS xi PREFACE TO VOLUME 2: NANOSTRUCTURED MATERIALS xiii ABOUT THE EDITOR xv CONTRIBUTORS xvii

1 INTRODUCTION TO NANOMATERIALS AND NANOTECHNOLOGY 1G. KorotcenkovB. K. Cho

1 What Are Nanomaterials? 1

2 A Brief History of Nanotechnology 7

3 What Distinguishes Nanomaterials from Bulk Materials? 10

4 Nanomaterials Manufacturing 14

5 Nanotechnology and Its Possibilities 17

6 Nanotechnology: Future Trends 19

7 Acknowledgments 23

References 24

2 QUASI-ONE-DIMENSIONAL METAL OXIDE STRUCTURES: SYNTHESIS, CHARACTERIZATION, AND APPLICATION AS CHEMICAL SENSORS 29

Pai-Chun Chang Dongdong LiJia G. Lu

1 Introduction 29

2 Synthesis of Q1D Nanomaterials 30

vi CONTENTS

2.1 Vapor-Phase Growth Methods 312.2 Solution-Phase Growth Methods 412.3 Template-Based Growth Methods 43

3 Electrical Transport Properties and Optical Characteristics 473.1 Nanowire Field-Eff ect Transistors and Electrical Properties 473.2 Photoluminescence Characteristics 52

4 Metal Oxide Nanowire Chemical Sensors 534.1 Sensor Device Fabrication 554.2 Mechanism of Nanowire Sensor Detection 564.3 Other Types of Q1D Structured Sensors 70

5 Summary and Future Outlook 72

References 73

3 CARBON NANOTUBES AND FULLERENES IN CHEMICAL SENSORS 87G. P. KotcheyA. Star

1 Introduction 871.1 History of Fullerenes and Carbon Nanotubes 871.2 Structure of Fullerenes 881.3 Structure of Carbon Nanotubes 88

2 Synthesis of Fullerenes and Carbon Nanotubes 892.1 Synthesis of Fullerenes 892.2 Synthesis of Carbon Nanotubes 90

3 Properties of Carbon Nanotubes 943.1 Physical/Mechanical Properties 943.2 Electronic Properties 943.3 Spectroscopic Properties 96

4 Chemical Modifi cation and Functionalization of Carbon Nanotubes 984.1 Introduction 984.2 Noncovalent Functionalization 984.3 Covalent Functionalization 100

5 Solid-State Electrical Conductivity CNT Sensors 1015.1 Nanotube FET for Gas-Sensing Applications 1015.2 NO2 Detection Using Resistivity Measurements 1025.3 Gas and Vapor Detection Using Functionalized CNTs 1035.4 Chemicapacitors 105

CONTENTS vii

5.5 Nanotube FETs for Detecting DNA Hybridization 1085.6 Employing NTFETs for Protein Detection 1105.7 Conductometric Glucose Biosensor 114

6 Raman Sensors 1156.1 A Surface-Enhanced Raman Scattering (SERS)–Based pH Sensor 1156.2 “Multicolored” Raman Probes for Biological Imaging and Detection 117

7 Optical Sensors 1207.1 Employing SWNTs as Fluorophores for Long-Term Optical Glucose Sensing 1207.2 Employing Spectroscopic Properties of SWNTs to Detect DNA Hybridization 122

8 Electrochemical Sensors 1238.1 Employing Electrochemistry to Monitor DNA Hybridization 1238.2 Electrochemical-Based Glucose Sensing 126

9 Field-Emission Sensors 1289.1 A CNT-Based Triode Sensor Th at Employs the Field-Emission Eff ect to Detect Gas Density 128

10 Electromechanical Resonators 13010.1 Nanomechanical Nanotube Resonators for the Detection of Evaporated Chromium Atoms 13010.2 Surface Acoustic Wave (SAW) Devices Th at Employ Buckminsterfullerene (C60) for the Detection of Toxic Organic Vapors 132

11 Outlook 133

References 135

4 SENSORS BASED ON MONOLAYER-CAPPED METALLIC NANOPARTICLES 141U. TischH. Haick

1 Introduction 141

2 Synthesis of MCNPs and Deposition of Solid MCNP Films 1422.1 Synthesis of MCNPs 1432.2 Surface Functionalization of Metal Nanoparticles 1442.3 Methods of MCNP Film Deposition 145

3 Four Good Reasons to Use Monolayer-Capped Metallic Nanoparticles for Chemical Sensing 147

3.1 Controllable Chemical Composition 147

viii CONTENTS

3.2 Controllable Size and Shape 1473.3 Controllable Nanoparticle Assembly 1493.4 Biocompatibility 151

4 Chemical Sensors Based on MCNPs 1514.1 Basic Principles 1514.2 “Lock-and-Key” Sensor Versus “Electronic Nose” 1524.3 Th e Role of the Number of Nanoparticles in Chemical Sensing 153

5 Categories of MCNP-Based Chemical Sensors 1535.1 Optical Sensors 1535.2 Chemiresistors 1705.3 Electrochemical Sensors 1805.4 Piezoelectric Sensors 185

6 Concluding Remarks 188


References 190

5 POROUS SEMICONDUCTORS: ADVANTAGES AND DISADVANTAGES FOR GAS SENSOR APPLICATIONS 203

G. Korotcenkov

1 Introduction 203

2 Porous Semiconductors: Principles of Fabrication and Properties 2052.1 Principles of Porous Silicon Fabrication 2052.2 Properties of Porous Silicon 2082.3 Techniques for Forming the Porous Silicon Layer 2102.4 Porosifi cation of Standard Semiconductors 220

3 Gas Sensors Based on Porous Semiconductors—Approaches and Characteristics 226

3.1 Capacitance-Type Gas Sensors 2263.2 Gas Sensors Employing Photoluminescence Quenching 2323.3 Sensors Based on Optical Measurements 2373.4 Conductometric-Type Gas Sensors 2433.5 Gas Sensors Based on Schottky Barriers and Heterostructures 2503.6 Gas Sensors Based on Measurement of Contact Potential Diff erence 2573.7 Gas Sensors Based on Simultaneous Control of Several Parameters

of the Porous Material 2583.8 Disadvantages of Porous Semiconductor Gas Sensors 259

CONTENTS ix

3.9 Surface Modifi cation of Porous Semiconductors to Improve Gas-Sensing Characteristics 265

4 Advantages of Porous Silicon for Applications in Micromachining Sensor Technology 269

5 Outlook 274


References 276

6 ORDERED MESOPOROUS FILMS AND MEMBRANES: SYNTHESIS, PROPERTIES, AND

APPLICATIONS IN GAS SENSORS 291M. Tiemann

1 Introduction 291

2 Porosity in Resistive Gas Sensors 2922.1 Categories of Porosity 2922.2 Gas Diff usion in Porous Materials 2932.3 Porous Films for Selective Gas Sensing 2932.4 Other Porosity-Related Nanostructural Aspects 296

3 Synthesis Methods 2973.1 Mesoporous Metal Oxides by Conventional Synthesis Methods 2973.2 Mesoporous Materials by Supramolecular Structure Directors 2993.3 Mesoporous Materials by Structure Replication 302

4 Summary 303

References 304

7 CHEMICAL SENSORS BASED ON ZEOLITES 311R. MoosK. Sahner

1 Introduction 311

2 Zeolites—Properties and Applications 312

3 Zeolites as an Auxiliary Phase in Chemical Sensors 3163.1 Zeolites as Host Materials 3163.2 Zeolites as Filters 3193.3 Zeolites as Preconcentrators 3213.4 Zeolites as Templates 321

x CONTENTS

4 Zeolites as the Functional (Sensitive) Phase 3224.1 Adsorptivity 3224.2 Ionic Conductivity 3234.3 Catalytic Activity 326

5 Conclusion 328

References 328

8 NANOCOMPOSITES: FROM FABRICATION TO CHEMICAL SENSOR APPLICATIONS 335RajeshT. AhujaD. Kumar

1 Introduction 335

2 Types of Nanocomposites 337

3 General Approaches to Nanocomposite Fabrication 338

4 Metal Oxide–Based Nanocomposites 3394.1 Synthesis 3394.2 Properties 3404.3 Application in Chemical Sensors 341

5 Polymer-Based Nanocomposites 3445.1 Synthesis 3445.2 Properties 3465.3 Application in Chemical Sensors 347

6 Carbon Nanotube–Based Nanocomposites 3496.1 Synthesis 3506.2 Properties 3526.3 Application in Chemical Sensors 353

7 Noble Metal–Based Nanocomposites 3557.1 Synthesis 3577.2 Properties 3587.3 Application in Chemical Sensors 359

8 Outlook 361

9 Acknowledgment 361

References 362

INDEX 369

xi

PREFACE TO CHEMICAL SENSORS:

FUNDAMENTALS OF SENSING MATERIALS

Sensing materials play a key role in the successful implementation of chemical and biological sen-sors. Th e multidimensional nature of the interactions between function and composition, preparation method, and end-use conditions of sensing materials often makes their rational design for real-world applications very challenging.

Th e world of sensing materials is very broad. Practically all well-known materials could be used for the elaboration of chemical sensors. Th erefore, in this series we have tried to include the widest pos-sible number of materials for these purposes and to evaluate their real advantages and shortcomings. Our main idea was to create a really useful “encyclopedia” or handbook of chemical sensing materials, which could combine in compact editions the basic principles of chemical sensing, the main properties of sensing materials, the particulars of their synthesis and deposition, and their present or potential ap-plications in chemical sensors. Th us, most of the materials used in chemical sensors are considered in the various chapters of these volumes.

It is necessary to note that, notwithstanding the wide interest and use of chemical sensors, at the time the idea to develop these volumes was conceived, there was no recent comprehensive review or any general summing up of the fundamentals of sensing materials Th e majority of books published in the fi eld of chemical sensors were dedicated mainly to analysis of particular types of devices. Th is three-volume review series is therefore timely.

Th is series, Chemical Sensors: Fundamentals of Sensing Materials, off ers the most recent advances in all key aspects of development and applications of various materials for design of chemical sensors. Regarding the division of this series into three parts, our choice was to devote the fi rst volume to the fundamentals of chemical sensing materials and processes and to devote the second and third volumes to properties and applications of individual types of sensing materials. Th is explains why, in Volume 1: General Approaches, we provide a brief description of chemical sensors, and then detailed discussion of desired properties for sensing materials, followed by chapters devoted to methods of synthesis, deposi-tion, and modifi cation of sensing materials. Th e fi rst volume also provides general background informa-tion about processes that participate in chemical sensing. Th us the aim of this volume, although not ex-haustive, is to provide basic knowledge about sensing materials, technologies used for their preparation, and then a general overview of their application in the development of chemical sensors.

xii PREFACE TO CHEMICAL SENSORS: FUNDAMENTALS OF SENSING MATERIALS

Considering the importance of nanostructured materials for further development of chemical sen-sors, we have selected and collected information about those materials in Volume 2: Nanostructured Materials. In this volume, materials such as one-dimension metal oxide nanostructures, carbon nano-tubes, fullerenes, metal nanoparticles, and nanoclusters are considered. Nanocomposites, porous semi-conductors, ordered mesoporous materials, and zeolites also are among materials of this type.

Volume 3: Polymers and Other Materials, is a compilation of review chapters detailing applications of chemical sensor materials such as polymers, calixarenes, biological and biomimetric systems, novel semiconductor materials, and ionic conductors. Chemical sensors based on these materials comprise a large part of the chemical sensors market.

Of course, not all materials are covered equally. In many cases, the level of detailed elaboration was determined by their signifi cance and interest shown in that class of materials for chemical sensor design.

While the title of this series suggests that the work is aimed mainly at materials scientists, this is not so. Many of those who should fi nd this book useful will be “chemists,” “physicists,” or “engineers” who are dealing with chemical sensors, analytical chemistry, metal oxides, polymers, and other materials and devices. In fact, some readers may have only a superfi cial background in chemistry and physics. Th ese volumes are addressed to the rapidly growing number of active practitioners and those who are interested in starting research in the fi eld of materials for chemical sensors and biosensors, directors of industrial and government research centers, laboratory supervisors and managers, students and lecturers.

We believe that this series will be of interest to readers because of its several innovative aspects. First, it provides a detailed description and analysis of strategies for setting up successful processes for screen-ing sensing materials for chemical sensors. Second, it summarizes the advances and the remaining chal-lenges, and then goes on to suggest opportunities for research on chemical sensors based on polymeric, inorganic, and biological sensing materials. Th ird, it provides insight into how to improve the effi ciency of chemical sensing through optimization of sensing material parameters, including composition, struc-ture, electrophysical, chemical, electronic, and catalytic properties.

We express our gratitude to the contributing authors for their eff orts in preparing their chapters. We also express our gratitude to Momentum Press for giving us the opportunity to publish this series. We especially thank Joel Stein at Momentum Press for his patience during the development of this project and for encouraging us during the various stages of preparation.

Ghenadii Korotcenkov

xiii

Nanomaterials and nanotechnology are new fi elds of science and technology. Fundamentally, nanotech-nology is about manipulating and making materials at the atomic and molecular levels. It is expected that nanotechnology will change solid-state gas sensing dramatically and will probably gain importance in all fi elds of sensor application over the next 10 to 20 years. Nanotechnology is still in its infancy, but the fi eld has been a hot area of research globally since a few years ago. It has been found that with reduc-tion in size, novel electrical, mechanical, chemical, catalytic, and optical properties can be introduced. As a result, it has been concluded that one-dimensional structures will be of benefi t for developing new-generation chemical sensors that can achieve high performance. Th erefore, in the last decade, the study of 1-D materials has become a primary focus in the fi eld of chemical sensor design. Synthesis of new nano objects and exploitation of their extraordinary properties is the goal and dream of many researchers engaged in the fi eld of sensor design. In addition, it has also been established that 1-D structures may be ideal systems in which to study the nature of chemical sensing eff ects.

Although many people consider this a brand-new technology yielding cutting-edge applications for consumer products, researchers have in fact been working in the fi eld of catalysis and gas-sensing eff ects for decades. What we call nanoparticle technology today actually began in the era of the 1950s–1980s, and chemical catalysis and gas sensor research and development have been conducted at the nanoscale ever since. Initially, research labs used the technology to increase the effi ciency of heterogeneous catalysis and to improve the sensitivity of solid-state gas sensors. Nanoclusters of metal catalysts and nanograins of metal oxides with dimensions less than 10 nm were the main objects of research.

Th e recent development of advanced tools for characterizing materials at the nano- or subnanoscale has provided scientists with new insights for understanding and improving existing devices and clues for ways to design new nanostructured materials to make better catalysts and sensors. Recent research has thus led to new types of prospective nanostructured materials.

It is obviously diffi cult to cover all aspects of a dynamic research area such as nanotechnology. Of course, it is not possible to analyze in one volume every nanoparticulate matter and its role in the revolu-tion of materials for chemical sensor applications. However, we have tried to cover this fi eld more or less completely. Th is book, together with Volume 1, includes as much as possible the recent advances and breakthroughs in the area of nanomaterials for chemical sensors as achieved by research groups all over the world. Th ese contributions have led to the emergence of some general guidelines.

PREFACE TO VOLUME 2: NANOSTRUCTURED MATERIALS

xiv PREFACE TO VOLUME 2: NANOSTRUCTURED MATERIALS

Th is book includes eight chapters written by researchers who are at the forefront of their fi eld, which address the role of nanomaterials in chemical sensors. One-dimensional metal oxide structures, carbon nanotubes, fullerenes, and metal nanoparticles are the objects of detailed analysis in the present volume. Processing, properties, and applications of porous semiconductors, zeolites, nanocomposites and ordered mesoporous materials are also discussed in this volume. A brief history of nanotechnology, particulars of nanomaterial properties, specifi city, and future trends in nanotechnology can be found in this volume as well.

Ghenadii Korotcenkov

xv

ABOUT THE EDITOR

Ghenadii Korotcenkov received his Ph.D. in Physics and Technology of Semiconductor Materials and Devices in 1976, and his Habilitate Degree (Dr.Sci.) in Physics and Mathematics of Semiconductors and Dielectrics in 1990. For a long time he was a leader of the scientifi c Gas Sensor Group and manager of various national and international scientifi c and engineering projects carried out in the Laboratory of Micro- and Optoelectronics, Technical University of Moldova. Currently, he is a research professor at Gwangju Institute of Science and Technology, Gwangju, Republic of Korea.

Specialists from the former Soviet Union know G. Korotcenkov’s research results in the study of Schottky barriers, MOS structures, native oxides, and photoreceivers based on Group III–V compounds very well. His current research interests include materials science and surface science, focused on metal oxides and solid-state gas sensor design. He is the author of fi ve books and special publications, nine invited review papers, several book chapters, and more than 180 peer-reviewed articles. He holds 16 patents. He has presented more than 200 reports at national and international conferences. His articles are cited more than 150 times per year. His research activities have been honored by the Award of the Supreme Council of Science and Advanced Technology of the Republic of Moldova (2004), Th e Prize of the Presidents of Academies of Sciences of Ukraine, Belarus and Moldova (2003), the Senior Research Excellence Award of Technical University of Moldova (2001, 2003, 2005), a Fellowship from the International Research Exchange Board (1998), and the National Youth Prize of the Republic of Moldova (1980), among others.

xvii

Tarushee Ahuja (Chapter 8)Department of Applied ChemistryDelhi College of EngineeringUniversity of DelhiBawana RoadDelhi-110042, India

Pai-Chun Chang (Chapter 2)Department of Physics and Department of Electrical EngineeringUniversity of Southern CaliforniaLos Angeles, California 90089-0484, USA

Beongki Cho (Chapter 1)Department of Material Science and Engineering and Department of Nanobio Materials and ElectronicsGwangju Institute of Science and TechnologyGwangju, 500-712, Republic of Korea

Hossam Haick (Chapter 4)Department of Chemical Engineering and Russell Berrie Nanotechnology Institute Technion—Israel Institute of TechnologyHaifa 32000, Israel

Ghenadii Korotcenkov (Chapters 1 and 5)Department of Material Science and EngineeringGwangju Institute of Science and TechnologyGwangju, 500-712, Republic of KoreaandTechnical University of MoldovaChisinau, 2004, Moldova

CONTRIBUTORS

xviii CONTRIBUTORS

Gregg P. Kotchey (Chapter 3)Department of ChemistryUniversity of Pittsburgh and the National Energy Technology LaboratoryPittsburgh, Pennsylvania 15260, USA

Devendra Kumar (Chapter 8)Department of Applied Chemistry Delhi College of EngineeringUniversity of DelhiBawana RoadDelhi 110042, India

Dongdong Li (Chapter 2)Department of Physics and Department of Electrical EngineeringUniversity of Southern CaliforniaLos Angeles, California 90089-0484, USA

Jia Grace Lu (Chapter 2)Department of Physics and Department of Electrical EngineeringUniversity of Southern CaliforniaLos Angeles, California 90089-0484, USA

Ralf Moos (Chapter 7)Department of Functional Materials University of Bayreuth95440 Bayreuth, Germany

Rajesh (Chapter 8)Liquid Crystal and Self Assembled Monolayer SectionNational Physical Laboratory (CSIR)Dr. K.S. Krishnan MargNew Delhi 110012, India

Kathy Sahner (Chapter 7)Functional Materials LaboratoryUniversity of Bayreuth95440 Bayreuth, Germany

Alexander Star (Chapter 3)Department of ChemistryUniversity of Pittsburgh and the National Energy Technology LaboratoryPittsburgh, Pennsylvania 15260, USA

CONTRIBUTORS xix

Michael Tiemann (Chapter 6)Department of ChemistyFaculty of ScienceUniversity of PaderbornWarburger Strasse 100D-33098 Paderborn, Germany

Ulrike Tisch (Chapter 4)Department of Chemical EngineeringTechnion—Israel Institute of TechnologyHaifa 32000, Israel

1

CHAPTER 1

INTRODUCTION TO NANOMATERIALS AND NANOTECHNOLOGY

G. KorotcenkovB. K. Cho

1. WHAT ARE NANOMATERIALS?

Recent scientifi c research has discovered an important role for nanotechnologies in fi elds such as informa-tion technology, materials science, biology, medicine, engineering, electronics, physics, fi ber-optic com-munication networks, chemistry, computer simulations, aerospace technology, advanced materials tech-nology, and chemical engineering (Feiner 2006; Wang et al. 2006; Mamalis 2007; Rickerby and Morrison 2007; Davenas et al. 2008; Meyer et al. 2009; Zhang and Webster 2009). As a result, a revolution of sorts has occurred in the search for approaches to attaining needed properties of materials: We have reached an era in which the control of physical, chemical, and biological properties of materials often takes place at the molecular and supramolecular levels. Just a few years ago, this ability seemed unrealistic.

According to the International Union of Pure and Applied Chemistry (IUPAC), nanomaterials are defi ned as materials having sizes smaller than 100 nanometers (1 nm = 10−9 m) along at least one dimension (length, width, or height) (Wikipedia 2009). For comparison, Table 1.1 lists the dimensions (sizes) of a number of well-known objects. In the range of crystallite sizes (<100 nm), and especially in the range smaller than 10 nm, large diff erences in the properties of nanomaterials and nanoparticles have been observed. For example, the exciton diameter in a semiconductor may be tens or hundreds of nanometers, the distance between domain walls in a magnet may be hundreds of nanometers, etc. So, nanostructures with characteristic size smaller than 10–100 nm can be considered as a special physical state, because the properties of materials formed with the participation of nano-sized structural elements

2 CHEMICAL SENSORS: FUNDAMENTALS. VOLUME 2: NANOSTRUCTURED MATERIALS

are not identical to the properties of bulk substance (Rosei 2004; Schwarz et al. 2004; Roduner 2006; Fahlman 2007). For instance, opaque substances may become transparent (copper); inert materials may exhibit catalytic properties (gold); stable materials may become combustible (aluminum); insulators may become conductors (silicon). Nanocrystallites of inorganic solids have been shown to exhibit size-dependent properties, such as higher energy gaps and nonthermodynamic behavior (Alivisatos 1996a; Murray et al. 2000; Schwarz et al. 2004). Th erefore, by considering structure at the nanoscale as a physi-cal variable, it is possible to greatly expand the range of performance of existing chemicals and materials. Once it is possible to control feature size and shape, it is also possible to enhance material properties and device functions beyond what are already established. For example, by creating nanometer-scale structures, it is possible to control fundamental properties of materials such as their melting temperature (Buff at and Borel 1976; Gulseren et al. 1995). It is known, for instance, that for grains smaller than

OBJECT SIZE

Resolution of electron microscope ~0.1 nmIndividual atom <0.2 nmSimple gas molecules (N2, CO, O2, etc.) ~0.3 nmWater molecule ~0.3 nmTh ickness of grapheme layer ~0.3–0.5 nmConstants of crystallographic unit cells Simple chemical elements 0.5–0.8 nm Inorganic compounds 0.5–1.5 nm Complex organic compounds 2–3 nmDiameter of DNA double-helix molecule ~2.5 nm

NANOMATERIALS 1–100 nm

Wall thickness of carbon nanotubes ~2–12 nmNanoclusters, nanoparticles 1–10 nmNanocrystals 4–100 nmNanowires, nanotubes Diameter 1–100 nm Length 102 up to 106 nmMagnetic domain ~hundreds nmSmallest feature in an integrated circuit ~150–200 nmResolution of light microscope >200 nmQuantum-dot transistor ~300 nmVisible spectra ~400–800 nmDiameter of biological cells ~(1–10) × 103 nmDiameter of human hair ~(5–10) × 104 nmTh ickness of a book page ~105 nmPet fl ea ~106 nmHead of a pin ~106 nm

Table 1.1. Characteristic dimensions of some commonplace objects

INTRODUCTION TO NANOMATERIALS AND NANOTECHNOLOGY 3

about 20–40 nm, the melting temperature of a crystal is inversely proportional to its eff ective radius (see Figure 1.1). Usually, this eff ect is considered to be a consequence of the large fraction of atoms with low coordination numbers that are present in solids with high surface-to-volume ratios.

Th ere is also experimental verifi cation of size-dependent thermal melting in thin one-dimensional nanostructures, principally owing to the diffi culty of fabricating free-standing rods or wires with dia-meters smaller than 5–10 nm (Law et al. 2004). Photo-induced melting and fragmentation of metal nanorods in solution have been studied in detail using femto- to nanosecond light pulses (Link et al. 1999, 2000). Also, a large melting-point depression was reported in the case of germanium nanowires (10–100 nm in diameter) encased in carbon sheaths (Wu and Yang 2001). Transmission electron mi-croscopy (TEM) has showed that the ends of sheathed Ge nanowires began to melt 280°C below the bulk melting temperature of Ge. Th e same eff ect was also observed for CdS nanoparticles (Alivisatos 1996b).

Th e magnetic properties of solids can exhibit size dependence as a consequence of several eff ects, including the infl uence of surfaces, the onset of carrier confi nement, and the reduction of structure size below that of a single magnetic domain (Law et al. 2004). It is possible to enhance or even induce magnetic behavior by changing the dimensionality of a system. For example, the broken symmetry of a surface can generate giant magnetic anisotropy energy in magnetic adatoms (Gambardella et al. 2003). Th e study of electron transport in suspended chains of atoms (Rodrigues et al. 2003) suggests that cer-tain nonmagnetic systems, such as Pd and Pt, become magnetic in such a geometry.

Other properties of nanomaterials, such as charge capacity and even their color, are also sensitive to size and therefore can be changed even without changing the materials’ chemical composition (Rosei 2004; Schwarz et al. 2004). For example, gold is known to be a shiny, yellow noble metal that does not tarnish, has a face-centered cubic structure, is nonmagnetic, and melts at 1336 K. However, a small sam-ple of the same gold is quite diff erent: 10-nm particles absorb green light and thus appear red (Eustis and El-Sayed 2006). Th e melting temperature decreases dramatically as the size goes down (<5 nm) (Schmid 2001). Moreover, bulk gold ceases to be noble, and 2- to 3-nm nanoparticles are excellent catalysts (see

Figure 1.1. Size dependence of the melting points of (a) In and (b) Sn particles. (Reprinted with permis-sion from Unruh et al. 1993 and Lai et al. 1996. Copyright 1993 and 1996 American Physical Society.)


Figure 1.2), which also exhibit considerable magnetism. At this size the particles are still metallic, but smaller ones turn into insulators (Roduner 2006). As we can see from results presented in Figure 1.3, in this range of cluster size we observe an increase of band gap. A metal-to-nonmetal transition is apparent as the cluster size approaches ~4.0 nm in diameter (~400 atoms/cluster), the approximate size at which onset of catalytic activity is observed for CO oxidation.

2.0 3.0 4.0 5.0 6.0Average cluster diameter, nm

0.3

0.1

0.2

1.0

2.0

3.0

4.0

1

2 3

Au/TiO2

Figure 1.2. Specifi c activity for CO conversion as a function of average Au cluster diameter on Au/TiO2 at (1, 2) 300 K and (3) 350 K. The number of surface sites was computed using cluster morphol-ogy determined by STM. Ptotal = 40 torr, CO/O2 = 1/5. [Data from (1) Valden et al. 1998a, (2) Valden et al. 1998b, and (3) Haruta, 1997.]

Average cluster diameter, nm0.0. 2.0 4.0 6.0 8.0

0.6

0.0

0.9

0.3

1.2

1.5 Au/TiO2(110)

Figure 1.3. Gold cluster band gap measured by scanning tunnel spectroscopy as a function of particle size on Au/TiO2(110). (Adapted with permission from Chen and Goodman 2006. Copyright 2006 Elsevier.)


As has been established for carbon nanotubes, band gap also can be controlled through a change of nanomaterial size (Robertson 2007). It was found that in the case of a semiconducting single-wale nanotube (SWNT), its electrical band gap Eg varies roughly inversely with the diameter d and is given in tight binding as

2

gaEd

æ ö÷ç= ÷ç ÷çè ø (1.1)

where is the π-matrix element between adjacent carbon atoms and a is the C–C bond length. Given that a sample of SWNTs contains a range of diameters, this variability of nanotube band gap adds com-plexity to electronic design.

For semiconductor nanomaterials the situation is diff erent. A detailed study of the eff ect of di-mensionality on confi nement in InP nanodots and nanowires (Yu et al. 2003) showed that the size dependence of the band gap in wires is weaker than in dots by the amount expected from simple theory. However, the absolute band-gap shifts in InP nanodots (ΔEg ≈ 1/d 1.35) and nanowires (ΔEg ≈ 1/d 1.45) did not follow the particle-in-a-box prediction (i.e., 1/d 2) (Efros and Rosen 2000), demonstrating that ac-curate treatment of confi nement requires higher-order calculations to account for band structure (Law et al. 2004).

As can be seen in Figures 1.4 and 1.5, the decrease of particle size is also accompanied by a strong change of optical properties. In fact, we observe strong transformations in both absorption and photolu-minescence spectra. Th e absorption energy of quantum dots is shifted to higher frequency with decreas-ing diameter of the dots, with a dependence of 1/r 2. Th is is readily observed from the refl ected colors of quantum dots with varying diameters, shifting from blue to red with increasing size. With decreasing

Figure 1.4. UV/vis absorbance spectra of ZnO sols. Dashed lines show the procedure to determine 1/2 for a particle size of 27 Å. (Reprinted with permission from Meulenkamp 1998. Copyright 1998 American Chemical Society.)


size, the position of both surface plasmon adsorption and the fl uorescence peak are shifted to shorter wavelengths as well (see Figure 1.5). For the smallest of metallic nanoclusters, with dimensions ca. <2 nm, the surface plasmon absorption disappears. Since so few atoms comprise discrete nanoclusters of this size, the spacing between adjacent energy levels becomes comparable to the thermal energy kT—especially at lower temperatures and smaller nanocluster diameters (Fahlam 2007).

Mechanical properties are size-dependent as well. Many of the mechanical properties of nano-materials are diff erent from those of the bulk materials, including hardness, elastic modulus, fracture toughness, scratch resistance, and fatigue strength, among others. One of the most familiar mechanical phenomena involving size dependency is the Hall-Petch eff ect, which is characteristic of polycrystalline solids (Law et al. 2004). Th e yield strength and hardness of a microstructured polycrystalline mate-rial typically increase with decreasing grain size, owing to the progressively more eff ective disruption of dislocation motion by grain boundaries. However, studies on solids composed of nanoscale grains suggest that the Hall-Petch relation breaks down at a critical grain size, below which a material softens. Atomistic modeling carried out by Schiotz and Jacobsen (2003) points to a transition from dislocation-mediated yielding to grain boundary sliding at very small crystallite sizes as the primary explanation for the anomalous maximum in the strength of metallic polycrystalline solids.

Phonon transport is expected to be greatly impeded in nanomaterials (i.e., d < Δ, where d is the dia meter and Δ is the phonon mean free path), due to increased boundary scattering and reduced phonon group velocities stemming from phonon confi nement (Law et al. 2004). Detailed models of phonon heat conduction in cylindrical (Zou and Balandin 2001) and rectangular (Lu et al. 2003)

Figure 1.5. Photoluminescence spectra of ZnS nanoparticles and ZnS clusters in zeolite Y (ZnS/Y). For photoluminescence spectral measurement of ZnS nanoparticles, the excitation at exc = 280 nm was used. (Reprinted with permission from Chen et al. 1997. Copyright 1997 American Institute of Physics.)


semiconducting nanowires predict a large decrease (>90%) in the lattice thermal conductivity of wires that are tens of nanometers in diameter. Size-dependent thermal conductivity in nanostructures presents a major hurdle in the drive toward miniaturization in the semiconductor industry. However, poor heat transport is actually advantageous for thermoelectric materials.

Th ese interesting phenomena occurring at this nanometer length scale are of interest to physicists, chemists, biologists, electrical and mechanical engineers, and computer scientists, making research in nanotechnology a frontier activity in materials science. Th e study of such nanomaterials creates great opportunities for fundamental science in condensed-matter physics, solid-state chemistry, materials sci-ence, electrical engineering, biology, and other disciplines. Nanoscience closes the gap between indi-vidual atoms and bulk materials.

It is a mistake to think that nanomaterials are just ordinary materials in the form of nanopar-ticles. As we will show in this volume, nanomaterials do not represent just one universal material; they encompass a very wide range of diff erent materials. Among nanomaterials are fullerenes (Chapter 3); one-dimensional metal oxide structures (Chapter 2); metal and metal oxide nanoclusters and nanopar-ticles (Chapter 4); and porous and mesoporous semiconductors and metal oxides (Chapters 5, 6, and 7). Among them are also included composites with components formed from nanomaterials (Chapter 8). Th e geometry of nanomaterials can be a variety of shapes (sphere, wire, rod, tube, ring, etc.) and is dependent on the synthesis method (see Figure 1.6).

Th e basic composition of nanocomponents can be either organic (dendrimers, polymers, etc.), inorganic (metals, metal oxides, metal hydroxides, etc.), carbon (carbon nanotubes, fullerenes, etc.), or a combinations of these materials. In addition, more complex systems are possible if surface function-alization is used to control surface charge and interactions (Rosei 2004; Suh et al. 2009). As follows from the above-mentioned listing, many nanomaterials are not just separate particles; in many cases they represent complicated microobjects, which are nanostructured on the surface or in the volume. So, among nanomaterials we can fi nd materials which have structured components with at least one dimension, two dimensions, and three dimensions less than 100 nm. Materials that have one dimension in the nanoscale (and are extended in the other two dimensions) are layers such as a thin fi lms or sur-face coatings. Materials that are nanoscale in two dimensions (and extended in one dimension) include nanowires and nanotubes. Materials that are nanoscale in three dimensions are particles, for example, precipitates, colloids, and quantum dots (tiny particles of semiconductor materials). Nanocrystalline materials made up of nanometer-sized grains also fall into this category (properties of these kinds of nanomaterials are discussed in Volume 1 of this series).

2. A BRIEF HISTORY OF NANOTECHNOLOGY

Th e term nanotechnology was fi rst suggested in 1974 by N. Taniguchi. Th e fi rst report about creation of these new materials was published in 1985 (Kroto et al. 1985). In this report was announced the discovery of a new type of carbon compound in nature—the fullerenes (C60). In 1996 the Nobel Prize was awarded for this discovery. Th e fi rst carbon nanotubes were created in 1991 (Iijima 1991). Th e fi rst National Nanotechnology Initiative was announced in the United States in 2000 (http://nano.gov). Th us, one can see that we are at the beginning of a new era in materials science. However, the


publication of this book, as well as the appearance of many others (e.g., Nalwa 2004; Koehler and Fritzsche 2004; Gogotsi 2006; Bhushan 2007; Hornyak et al. 2008) dedicated to various aspects of syn-thesis, study, and application of nanomaterials in diff erent fi elds means that even in such a short period of time it has been possible to take a considerable step forward and obtain impressive results. According to some forecasts, by the year 2015 the cost of nanotechnological production should exceed $1 billion dollars (Kohler et al. 2005; Bhushan 2007).

Among other important discoveries in nanotechnology development, one can also note the inven-tion of the scanning tunnel microscope (STM) in 1982 (Bennig and Rohrer 1982) and the scanning atomic power microscope in 1986 (Bennig et al. 1986; Noble Prize in 1992). Th ese new microscopes allowed observation of the atomic-molecular structure of monocrystal surfaces in the nanometer size range. It has also turned out that these new microscopes are useful not only in study of the atomic-mo-lecular structure of substance (Adam et al 2006; Sugimoto et al., 2008); they have also found application

Figure 1.6. ZnO nanostructures synthesized by thermal evaporation of solid powders: (a) nano-combs; (b) tetraleg; (c) hexagonal disks; (d) nanopropellers; (f) nanospirals; (g) nanosprings; (h) single-crystal nanoring; (i) combination of rods, bow, and ring. (Reprinted with permission from Wang 2005. Copyright 2005 Royal Society of Chemistry.)


in the construction of nanostructures. Th e atom (molecule) manipulation used in this process is il-lustrated schematically in Figure 1.7. In this process, the tip of the microscope is positioned above the adsorbate (atom or molecule) and then brought to approach the surface (to a distance of 0.2–0.4 nm), until a bond is formed between the tip and the adsorbate. Th e tip “feels” atoms as increases in current. Th e bond between the atoms and the tip must be strong enough that the adsorbate follows the move-ment of the tip parallel to the surface.

At present, three main parameters can be adjusted to manipule atoms/molecules on surfaces using the STM:

• Th e electric fi eld between the tip and the sample • Th e tunneling current of the electrons (increasing the current creates a bond between the tip and

the atom and therefore allows the atom to be moved) • van der Waals or chemical forces that act between the tip and the sample and that can be adjusted

by varying the tip–sample separation

In working with single atoms or molecules, one distinguishes between lateral and vertical manipula-tion. Lateral manipulation means that the particle is moved with the tip along the surface to the desired position, without losing contact with the surface. Vertical manipulation means that the particle is picked up by the tip and moved, on the tip, to the desired position. Th ere it is dropped back to the surface. While only the forces between tip and sample are important for reliable lateral manipulation, the electric fi eld and the tunneling current play important roles in vertical manipulation.

Th ese techniques have made it possible to build up artifi cial nanostructures at the atomic scale. Th e technical demands in using the STM in this way are high: It must be possible to control the tip position in the range of fractions of an atom’s size. Also, stability against drift must be so high that it is possible to work with a built-up artifi cial structure for days. Because of the thermal mobility of most adsorbates, the STM has to work at low temperatures (down to 4 K).

In this volume we will not be discussing the opportunities to use these microscopes for the fabrication of chemical sensors. However, since new technologies are being developed very quickly, we expect that in the near future, technologies of self-organization (Scanlon and Aggeli 2008) and manipulation of nano-structures will become an important part of the manufacture of nano- and micro-scale chemical sensors.

Tip Tip

Atom (adsorbate)

Substrate

Figure 1.7. Operation and manipulation of atoms by STM.


3. WHAT DISTINGUISHES NANOMATERIALS FROM BULK MATERIALS?

As we indicated earlier, while most microstructured materials have similar properties to the correspond-ing bulk materials, the properties of materials with nanometer dimensions are signifi cantly diff erent from those of atoms and bulk materials. Among the characteristics of nanomaterials that distinguish them from bulk materials, it is important to note the following: (1) large fraction of surface atoms; (2) high surface energy; (3) spatial confi nement; (4) reduced numbers of imperfections that do not exist in the corresponding bulk materials (Cao 2004).

Th e use of nanomaterials provides the following advantages. First, all nanomaterials consist of very small particles. Th is is the fi rst advantage of nanomaterials and nanotechnologies, promoting attainment of superminiaturization. Because they are small, nanostructures can be packed very closely together. As a result, on a given unit of area one can locate more functional nanodevices, which is very important for nanoelectronics. Th eir high packing density has the potential to bring higher area and volume capacity to information storage and higher speed to information processing (because electrons require much less time to move between components). Th us, new electronic device concepts, smaller and faster circuits, more sophisticated functions, and greatly reduced power consumption can all be achieved simultane-ously by controlling nanostructure interactions and complexity.

Second, because of their small dimensions, nanomaterials have large specifi c surface areas, accelerat-ing interactions between them and the environment in which they are located. For example, nanopar-ticles with a radius of 2.5 nm and a density of 5 g cm–3 have a surface of 240 m2 g–1 when assuming a ball-like shape. For comparison, a dense (compact) material with a weight of 1 g and the same density has a surface area of 2 × 10−6 m2. Th us, nanoparticles have a much larger surface area per unit of mass compared with larger particles. Because growth and catalytic chemical reactions occur at surfaces, this means that materials in nanoparticle form will be much more reactive than the same mass of material made up of larger particles.

A strong increase in the participation of surface atoms in the physical and chemical properties of nanomaterials is another consequence of a decrease in particle size. It is known that the volume of an ob-ject decreases as the third power of its linear dimensions, but the surface area decreases only as its second power. Th erefore, when materials are in the form of nanoparticles, their surface area-to-volume ratio, i.e., the ratio between surface and bulk atoms, increases. Th is eff ect is especially strong when the sizes of nanomaterials are comparable to the Debye length (Ogawa et al. 1982; Luth 1995). Simple calculations show that a particle of size 30 nm has 5% of its atoms on its surface; at 10 nm, 20% of its atoms; and at 3 nm, 50% of its atoms (Royal Society 2004) (see Figure 1.8).

It is known that atoms on the surface of nanoparticles have unusual properties, and (relatively speaking) there are a lot of them. Th ese surface atoms make nanoparticles very diff erent from just small particles, because not all bonds of surface atoms with neighboring atoms are enabled. For atoms on uneven surfaces, nonsaturation of the bonds is even higher. For this reason, corner atoms normally have the highest affi nity to form bonds to adsorbate molecules, followed by edge and in-plane surface atoms, a fact that is of great importance for catalytic activity. Alternatively, because of their low stabilization due to low coordination, edge and in particular corner atoms are often missing on single crystals, even in thermodynamic equilibrium (Roduner 2006). Recently, size-dependent variation in oxidation state


and lattice parameter has been reported for cerium oxide nanoparticles (Deshpande et al. 2005) (see Figure 1.9).

As a result of the changes that occur in particles with a decrease of particle size, nanomaterials can have extremely high biological and chemical reactivity (Eustis and El-Sayed 2006). For example, catalytically active nanomaterials allow accelerating either chemical or biochemical reactions by tens of

Figure 1.8. Surface/volume ratio as a function of particle size. (Reprinted with permission from Burda et al. 2005. Copyright 2005 American Chemical Society.)

Figure 1.9. Semilog plot of lattice parameter as a function of particle size. (Reprinted with permis-sion from Deshpande et al. 2005. Copyright 2005 American Institute of Physics.)


thousands, and even a million times. Th is attribute explains even 1 g of nanomaterial can be more eff ec-tive than 1 ton of a similar but macro substance. As will be shown later, the high chemical and catalytic activity of nanomaterials opens up fundamentally new opportunities for elaboration next-generation chemical sensors.

Another aspect we must consider is that the free surface is a place of accumulation (sink) of crystal-lographic defects. At small particles sizes, the surface concentration of such defects increases consider-ably. Classical calculations of van Hardeveld and Hartog (1969) showed that the largest changes of pro-portions between facets, edges, corners, and microdefects at the surface occur between 1 and 5 nm. As a result, strong lattice distortion and even a change of lattice type can take place on the surface layer (see Figure 1.10). In fact, due to accumulation of structural defects and chemical impurities on the surface, we can observe purifi cation of the bulk area of the nanoparticles.

An important specifi c characteristic of nanomaterial properties (we mean here polycrystalline ma-terials with grain size less than 40 nm) is an increase of the role of interfaces with decrease of the size of grains or crystallites in nanomaterials. Experimental research has shown that the state of grain boundaries has a nonequilibrium character, conditioned by the presence of the high concentration of grain boundary defects. Th is nonequilibrium is characterized by extra energy of the grain boundaries and by the presence of long-range elastic stress (see Figure 1.10). At the same time, the grains have ordered crystallographic structure, while the grain boundary defects act as a source of elastic strains. Nonequilibrium of the grain boundaries initiates the occurrence of the lattice distortion, the change of interatomic distances, and the appearance of suffi cient displacement of atoms, right up to loss of an ordered state.

Another important factor peculiar to nanoparticles is their tendency to aggregation. Th e possibility of migration (diff usion) of either atoms or groups of atoms along the surface and the boundaries, as well

Figure 1.10. Semilog plot of lattice strain as a function of particle size. (Reprinted with permission from Deshpande et al. 2005. Copyright 2005 American Institute of Physics.)


as the presence of attractive forces between them, often leads to processes of self-organization into vari-ous cluster structures. Th is eff ect has already been used for creation of ordered nanostructures in optics and electronics.

One more important aspect of nanomaterial properties is connected with the fact that, during transport processes (diff usion, electro- and thermal conductivity, etc.), there are certain eff ective lengths of free path of a carrier of this transport (Le), such as phonon and electron mean free paths, the Debye length, and the exciton diff usion length for certain polymers (see Figure 1.11). While proceeding to sizes smaller than Le, transport speed starts to depend on both the size and the shape of the nanomaterial; generally, the transport speed increases sharply. For example, in some cases the length of the electron free path can be considered as Le. As can be seen in Figure 1.11, chemically synthesized nanowires 5–100 nm in diameter allow experimental access to a rich spectrum of mesoscopic phenomena.

Nanomaterials are unique because these substance exist in a specifi c “nanosize” state. Th e prin-cipal characteristics of nanomaterials are conditioned by not only by their small the size, but also by the appearance of new quantum mechanical eff ects in a dominating role at the interface (Esaki 1991; Gimzewski and Welland 1995; Serena and Garcia 1997; Ciraci et al. 2001). Th ose quantum size eff ects occur at a critical size, which is commensurate with the so-called correlative radius of one or another physical phenomena, for example, with the length of the free path of electrons or photons, the length of coherence in a superconductor, sizes of magnetic domains, and so on. As a rule, quantum size eff ects appear in materials with crystallite sizes in the nanorange D < 10 nm. As a result, in nanomaterials with characteristic size, one can expect the appearance of eff ects which cannot be observed in bulk materials. In particular, quantum eff ects may appear in the form of an oscillating change of electric properties—for example, conductivity—or the appearance of new energetic states for electrons (see Figure 1.12). At

Critical Magnetic Single Domain Size

Phonon Mean Free Path

Exciton Diffusion Length in Polymers

Debye Screening Length

Exciton Bohr Radius

Fermi Wavelength in Metals

Characteristic Length, nm0.1 1.0 10 100 1000

Figure 1.11. A few characteristic length scales for condensed systems at 300 K. (Data from Law et al. 2004.)


nanoscale dimensions the normally collective electronic properties of the solid become severely distort-ed, and the electrons at this length scale tend to follow the ‘‘particle-in-a-box’’ model, so that they often require higher-order calculations to account for band structure (Law et al. 2004). Th e electronic states are more like those found in localized molecular bonds than in macroscopic solids. Th e main implica-tion of this confi nement is a change in the system total energy, and hence the overall thermodynamic stability. Th ese eff ects thus create conditions for the elaboration of chemical sensors based on entirely new principles.

Absence of point defects is another characteristic property of some nanomaterials, such as nano-tubes. For example, because of the absence of point defects, individual carbonic nanotubes have strength that exceeds the strength of the best steel. At the same time, they are much lighter in weight than steel, such that nanotubes may be 6 times lighter in weight and 50–100 times stronger than steel.

4. NANOMATERIALS MANUFACTURING

Th e synthesis, characterization, and processing of nanostructured materials are part of an emerging and rapidly growing fi eld (Hullmann 2006). As we will show, a variety of dry and wet technologies can be em-ployed in the manufacture of nanomaterials (Edelstein and Cammarata 1998; Knauth and Schoonman 2002; Cao 2004; Rao et al. 2004; Grimsdale and Müllen 2005; Bochenkov and Sergeev 2005; Lu et al. 2006; Bhushan 2007; Weber et al., 2008). As shown in Figures 1.13 and 1.14, there are two ap-proaches to nanomaterial manufacturing. Using top-down methods, manufacturers selectively modify the starting material, much as an artist creates a sculpture from a slab of marble. Top-down manufactur-ing, perfected by the semiconductor industry over the last 30 years, involves precision engineering and lithography to grind or cut bulk materials into tiny pieces and to etch or print nanoscale patterns onto them. Photolithography and electron-beam lithography are examples of top-down approaches that are used extensively in the semiconductor industry to fabricate electronic integrated circuits. Th e top-down

Size

Figure 1.12. Schematic of electron-level transforma-tion, starting from discrete levels of atoms forming a molecule having bonding and antibonding levels to “bulk crystal” with continuous bands. (Reprinted with permission from Alivisatos 1997. Copyright 1997 Elsevier.)


approach limits the dimensions of devices to what is technically achievable using lithography, however. At present, lithographic techniques can create device features as narrow as 130 nm, and the industry sees the road ahead pretty well drawn up for line widths down to ~50 nm. Th is continued progress does not come without a price, however; the cost of new fabrications is growing extremely quickly, at a pace that may limit continued progress simply because devices and circuits may become too expensive to be economically viable (Samuelson 2003).

Using the bottom-up method, fabricators seek to build larger and more complex systems based on small functional building blocks. Th is approach is based on using growth methods such as atom by atom, layer by layer, or molecule by molecule (Harry et al. 2006). Bottom-up manufacturing assembles

Nanomaterials manufacturing

Bottom-up approach Top-down approach

Chemical synthesis

Self- assembly

Positional assembly

Lithography (el.; photo)

Cutting, etching, grinding

Particle, clusters,

molecules

Crystals, films, tubes

Experimental atomic or

molecular devices

Electronic devices chip

masks

Precision engineered

surfaces

Figure 1.13. Technological approaches used for nanomaterial manufacturing.

•

• •

••••• •

••

• •••

••

•

• ••

••

•

“Top-down” “Bottom-up”

Ball-milling, Lithography/etching Chemical synthesis

Nanomaterials Molecular precursors (<<1 nm)

Figure 1.14. Comparison of “top-down” and “bottom-up” approaches to nanomaterials preparation (synthesis).


nanostructures from scratch through conventional chemical synthesis, self-assembly (harnessing natural physical or chemical bonding to assemble structures, like crystal formation), and positional assembly (in which atoms or groups are manipulated individually into a structure) (Dowling 2004). Presently, atoms, molecules, clusters, and nanoparticles can be used as functional building blocks for fabricating advanced and totally new phases of condensed matter on the nanometer length scale. Th ere is only one requirement to these blocks: Th ey must have the ability to self-assemble spontaneously when a chemical or physical trigger is applied. Th ere are many organic and inorganic materials which have self-assembly properties that can be exploited by properly devised design features to build structures with controlled confi gurations (Smith et al. 2006; Scanlon and Aggeli 2008). One examples of this approach is shown in Figure 1.15.

Th e assembly of components into functional devices is amenable to diverse approaches, from micro-fabrication techniques to self-assembled chemical methods. Th e optimal size of the unit components that participate in self-assembly depends on the particular property to be engineered: By altering the dimensions of the building blocks and controlling their surface geometry, chemistry, and assembly, it is possible to tailor functionalities in unprecedented ways (Rosei 2004). In the case of devices comprising a single molecule or processed from a single crystal (e.g., single polymers, certain microfabricated struc-tures, grafted polymeric structures, or precipitated particulates), assembly is often not an issue. However, integration of multiple, individually microfabricated components is periodically necessary and may at times drive the need for assembly, even for silicon- and other semiconductor-based devices. Indeed, for semiconductor and polymeric devices, standard top-down techniques can sometimes be employed in concert with bottom-up device assembly methods to achieve better and/or more effi cient device con-struction approaches, and in some instances this combination of approaches may be the only feasible

Figure 1.15. Example of self-assembly of colopymer blocks containing hydrophobic and hydrophilic parts. (Idea from Smith et al. 2006.)


way to create a device. Furthermore, many hybrid nanodevices contain multiple, chemically diverse components that must be precisely assembled to sustain their functional, cooperative contributions to device performance (Smith et al. 2006).

In addition to various chemical methods of synthesis, in the frame of the bottom-up approach a va-riety of vacuum deposition and nonequilibrium plasma chemistry techniques are used to produce nano-structured fi lms, layered nanocomposites, and nanotubes. Atomically controlled structures are produced using molecular-beam epitaxy and organometallic vapor-phase epitaxy. Various wet chemical routes are used for synthesis of one-dimensional structures and nanosize powders.

Although bottom-up processes are less developed and understood, they hold great promise for the future. Th ese new techniques allow us to mimic nature and self-assemble nanowire materials and devices at extremely downscaled dimensions, making it as easy to fabricate a 5-nm as a 200-nm nanoelectronic device (Samuelson 2003). Th ere is, however, a signifi cant obstacle along this path to device fabrica-tion—namely, that the requirement for self-assembly is very diff erent from that for device utilization. Self-assembly requires mobility, whereas device utilization requires stability. More detailed discussion of this problem has been given by Harry et al. (2006).

5. NANOTECHNOLOGY AND ITS POSSIBILITIES

Nanotechnology is a brainchild of modern fundamental science. It is a very complicated professional area, uniting the eff orts of professionally qualifi ed chemists, physicists, mathematicians, materials sci-entists, physicians, computer scientists, and so on. Nanotechnology in general comprehension is not “a technology” but “a set of technologies,” yielding a set of technical breakthroughs that will seep into many diff erent markets. For example, in the world of nanotechnology one can identify three broad branches: nanomaterials, nanodevices, and nanotools (Kohler et al. 2005). Nanotools include fabrica-tion techniques, analysis and metrology instruments, and software for nanotechnology research and development.

Th ere is no doubt that a nanotechnology will require extensive human investment for many long years ahead. Nanotechnology will be a strategic branch of science and engineering in the coming decades, which will fundamentally restructure the technologies used in manufacturing, medicine, defense, energy production, environmental management, transportation, communications, computing, and education. Already, recent achievements in nanotechnology testify to the opportunities to create new generations of functional materials (Rosei 2004). For example, metallic/magnetic nanoparticles and quantum dots may be used in quantum electronics for storing individual bits of information or as biological transport for targeted drug delivery, providing enhanced drug effi cacy and reduced toxicity. Iron, cobalt, or iron oxide nanoparticles and several other magnetic nanoparticles and alloys are also promising candidates for application in the medical treatment of cancer, magnetic storage, and magnetic resonance spectros-copy. Engineering better medicine represents a grand challenge that will allow us to diagnose diseases at an early stage and provide optimal treatment to individual patients. Many of the applications in this area may not be realized for 10 years or more (owing partly to the rigorous testing and validation regimes that will be required). In the much longer term, the development of nanoelectronic systems that can detect and process information may lead to the development of an artifi cial retina or cochlea (Dowling


2004). Of course, achievement of this objective will require additional advances in nanosensors and nanodevices, genomics, and bioinformatics (Bailey et al. 2004).

Nanoelectronic devices based on new nanomaterials systems and new device structures will con-tribute to the development of a next generation of nanophotonics as well as micro- and nanoelectronics (Samuelson 2003; Harry et al. 2006; Robertson 2007). For example, because of their high electronic mobility, structural fl exibility, and capability of being tuned from p-type to n-type conductivity by the application of a gate voltage, grapheme and carbon nanotubes are considered a potential breakthrough in terms of carbon-based nanoelectronics. We need to note that single-electron transistors (Tans et al. 1997; Postma et al. 2001) and fi eld-eff ect transistors (Tans et al. 1998; Martel et al. 1998; Keren et al. 2003) based on single-wall carbon nanotubes have already been designed. Prototype simple logic circuits of carbon nanotubes have already been demonstrated (Rueckes et al. 2000; Bachtold et al. 2001; Collins et al. 2001; Derycke et al. 2001). Further, nanotechnology was also expanded extensively to other fi elds of interest due to the novel properties of nanomaterials discovered and to be discovered. For example, nanowires have started to be used in nanophotonics, laser, solar cells, resonators, and high-sensitivity sensors. Quantum dots and the electrons that can be trapped in their discrete energy levels are of great interest for quantum information processing. Th e spin state of these trapped electrons could act as carri-ers of quantum information or “qubits” (Robledo et al. 2008). Nanoparticles are remarkable candidates for application in functional coatings, energy storage, and as catalysts (Eustis and El-Sayed 2006). For example, recently synthesized TiSi2 nanobelts show excellent results during photosplitting of H2O into H2 and O2 upon the absorption of visible light. Th erefore, nanomaterials such as TiSi2 nanonets might be attractive for energy-harvesting applications such as H2 production from solar energy. Nanostructured thin fi lms can be used in light-emitting devices, high-effi ciency displays, and photovoltaics (Law et al. 2004; Kim et al. 2008). Th e nanorods in solar cells function as additional light absorbers and charge-separation interfaces. Lasers and light-emitting diodes (LED) from both quantum dots and quantum wires are very promising for the future of optoelectronics as well.

Carbon nanohorns provide a unique combination of strength, electrical conductivity, high surface area, and open gas paths, making them an ideal next-generation electrode for various fuel-cell applica-tions. Carbon nanotubes are also being considered as a possible material for the storage of hydrogen (Atkinson et al. 2001; Zandonella 2001), and they make excellent tips for scanning probe microscopes. Silicon nanoparticles have been shown to dramatically expand the storage capacity of lithium-ion batter-ies without degrading the silicon during the expansion/contraction cycle that occurs as power is charged and discharged. Zinc, zinc oxide, and silver nanoparticles can be used as antimicrobial, antibacterial, antibiotic, and antifungal agents when incorporated in coatings, fi bers, polymers, fi rst-aid bandages, plastics, soap, and textiles. Tungsten oxide nanoparticles are being used in dental imaging because they are suffi ciently radioopaque (impervious to radiation) for high-quality x-ray resolution. Nanocrystalline zinc selenide, zinc sulfi de, cadmium sulfi de, and lead telluride are candidates for the next generation of light-emitting phosphors. Fluorescent nanoparticles are being used by biologists to stain and label cellular components (Bailey et al. 2004). By changing the size of the quantum dot, the color emitted can be controlled. With a single light source, one can see the entire range of visible colors, an advantage over traditional organic dyes. Nanosized titanium dioxide and zinc oxide are currently used in some sun-screens, because they absorb and refl ect UV rays and yet are transparent to visible light, and so they are more appealing to consumers (Dowling 2004). Nickel–metal hydride batteries made of nano crystalline


nickel and metal hydrides are envisioned to require less frequent recharging and to last longer because of their large grain boundary (surface) area (Royal Society 2004). Nanotechnology may off er non–energy-intensive solutions to water desalination and purifi cation. About one-sixth of the world’s population does not have suffi cient access to water, and over one-third of the people today suff er from poor sanita-tion due to unavailability of water. In fact, more deaths are caused by the lack of clean water than by wars. Nanoporous membranes or fi bers can be designed to fi lter out salt, pollutants, and bacteria from sea water, recycled water, or any available water source.

Th ese are only a few of the fastest-developing nanotechnologies; numerous other potential applica-tions of nanomaterials have already been or will be discovered. Th e new concepts of nanotechnology are so broad and pervasive that they may be expected to infl uence science and technology in ways that are currently unpredictable. For example, over the last decade, nanomaterials have been highlighted as promising candidates for application in regenerative medicine to replace traditional tissue engineering materials. Unique properties of nanomaterials have helped to improve the growth of various tissues over what is achievable today. Zhang and Webster (2009) have shown that nanomaterials can be used for bone, cartilage, vascular, neural, and bladder tissue engineering applications as well.

6. NANOTECHNOLOGY: FUTURE TRENDS

As we have shown, nanotechnology has already produced great achievements in design of new func-tional materials and devices. We anticipate even greater progress in the future. However, we have to admit that every next step will require greater combined eff orts by thousands of highly skilled scientists and engineers, and greater fi nancial investments in expensive fundamental research. Th is fundamental research must answer a huge number of questions, including the following:

• Is there a sharp boundary between the bulk state of a substance and the nanocrystalline state? How fast does the transformation occur, and at what stage of atomic association is the formation of one or another property of bulk crystals completed? Is there some critical grain or particle size below which the properties that are characteristic of nanocrystal become apparent and above which the properties of bulk material occur?

• What new and novel quantum properties will be enabled by nanostructures, especially at room temperature?

• How will the use of nanomaterials infl uence human health? Do nanomaterials bioaccumulate? What is the fate, transport, and transformation of nanosized materials after they enter the environment? What is the danger of exposure via skin absorption, ingestion, or inhalation (Fahlam 2007)?

Th e greater specifi c surface area of nanoparticles may lead to increased rates of absorption through the skin, lungs, or digestive tract and may cause unwanted eff ects to the lungs as well as other organs. However, the particles must be absorbed in suffi cient quantities in order to pose health risks (Lauterwasser 2007). As of now, in the realm of occupational health, much is unknown about the ways in which people may be exposed to nanomaterials during their manufacture and use in the workplace, and the potential health implications of such exposure.


Th erefore, estimation of the potential health risks associated with these new materials requires understanding of the mechanisms of ill health, identifi cation of some property or metric of the material which relates exposure to the material to health risk, and some method for measuring exposure in relation to that metric (Royal Society 2004; Borm et al. 2006; Weyer, et al. 2009; Zhang and Webster 2009). We suspect that the environmental and health hazards of nanomate-rials will prove no more serious or diffi cult to manage than those of existing particulate sources such as diesel exhaust or asbestos. However, ignoring or dismissing outright the concerns of the public in this or any other area of emerging technology is socially irresponsible, unbalanced sci-ence (Law et al. 2004).

• Why do nanocomponents show increased catalytic activity in specifi c reactions in a narrow range of sizes?

For example, the gold has maximum catalytic activity to CO low-temperature oxidation at a particle size of ~3 nm (Valden et al. 1998). Th e highest ethanol selectivity of Rh-based cata-lyst during ethanol synthesis by CO2 hydrogenation was obtained when the size of Rh particles was about 2.5 nm (Kusama et al. 1997). High selectivity of catalytic activity is also typical for nanoparticles of such well-known catalysts as palladium, platinum, nickel, and cobalt (see Figure 1.16) (Bezemer et al. 2006; Villani et al. 2006; Weber et al. 2006; Zhou and Lee 2008; Hoxha et al. 2009).

Such high sensitivity of catalytic activity to the sizes of small particles shows the importance of developing specifi c methods and techniques suitable for synthesis and deposition of nano-particles accurate to several atoms or several decades of atoms. A very narrow distribution of nanoparticle sizes is necessary not only for heterogeneous catalysis but also for chemical sensor

Figure 1.16. Infl uence of cobalt particle size on activity in the Fischer-Tropsch reaction normalized to the cobalt loading (220°C). (Reprinted with permission from Bezemer et al. 2006. Copyright 2006 Americal Chemical Society.)


elaboration. As is known, the incorporation of catalytically active additives to the gas-sensing matrix is one of the main approaches to improving their exploitation parameters (see Volume 1 of this series).

• How can we communicate signals from the nanoscale to the macroscopic world? (Fahlam 2007). • How can stabilization of the size and shape of nanoparticles, which have a tendency to agglomer-

ate and coalesce, be attained (Born et al. 2006)?

Considering these yet-unanswered questions, one can conclude that in spite of the progress that has been achieved, nanotechnologies and nanomaterials cannot resolve all our problems; and their de-velopment will take time even under the conditions of the “nanotechnology race” that is now occurring all over the world. For example, early publications included some sensational results and conclusions. Primarily, these concerned potential opportunities to change radically the properties of substances and materials by transition to the nanostate. Hopes were very high. Many published articles included such phrases as “the astonishing new science that will transform the world,” or “nanotechnology will change the future of your business.” It was expected that, by determining the novel properties of materials and systems at the nano scale, nanotechnology could aff ect the production of virtually every human-made object—everything from automobiles and tires to computer circuits, to advanced medicines and tissue replacement—and lead to the invention of objects yet to be imagined. However, accurate research has shown that many results testifying to unusual properties of nanomaterials were connected not with the decrease of the particle sizes, but with large amounts of impurities such as oxygen, nitrogen, hydrogen, as well as with large areas of interface, and the structure’s nonequilibrium. In addition, it turned out that, in producing devices from nanomaterials, it is very diffi cult to keep grain size small and attain the desired eff ect.

Diffi culties in manipulation of nanosized objects also appeared. For example, to fabricate the struc-tures shown in Figure 1.17, we need to cut, separate, select, put, and fi x nanowires on a certain place and then make electrical contact with an electrode. For these procedures it is necessary to elaborate

Figure 1.17. SEM micrographs of structure for transport measurements of ZnO nanowires. (Reprinted with permission from Heo et al. 2004. Copyright 2004 Elsevier.)


fundamentally new technological and measuring equipment (Mamalis 2007). Nor will nanotechnol-ogies be incorporated into products and devices without the development of scalable, cost-eff ective manufacturing techniques that retain and preserve the properties of the nanoscale material in the fi nal product. For example, researchers over the past few years have demonstrated electronic devices that are faster and smaller but that are quite often characterized by fundamentally diff erent (and possibly better) operational characteristics than existing technologies (Samuelson 2003). However, existing technologies follow well-established manufacturing paradigms known as a “top-down” approach, while nanoscience is typically based on a bottom-up approach. Herein lies the primary challenge in the development of nanotechnology: A chemically fabricated transistor, no matter how good it is, cannot just be slipped into the next generation of microprocessors. Such a transistor would not be compatible with top-down manufacturing (Heath 1999).

Another outstanding scientifi c challenge that needs to be addressed urgently is the integration and interfacing problem (Law et al. 2004). Th e ability to create high-density arrays is not enough: How to address individual elements in a high-density array and how to achieve precise layer-to-layer registration for vertical integration are just two of the many challenges still ahead. Equally important is precise control of the size uniformity, dimensionality, growth direction, and dopant distribution within semiconductor nanostructures, as these structural parameters will ultimately dictate the functionality of the nanostructures. In particular, the physical signifi cance of the dopant distribution and the inter-facial junction, and their implications in device operation and performance, will likely require careful reexamination and/or redefi nition at the nanometer-length scale. Lastly, accurate theoretical simulations appropriate to the above-mentioned mesoscopic regime are becoming feasible with the enhanced com-puting power available and should assist our understanding of many of these size- and dimensionality-controlled phenomena (Law et al. 2004).

Moreover, potential products and applications will not be realized unless there is a market for them (Royal Society 2004). Th erefore, nobody can predict the time when the potential of nanotechnology will be realized completely. It is really diffi cult to give a detailed time scale, because most of elaborations are at such an early stage of development. However, the whole promise of nanoscience is that it will eventually produce a bottom-up manufacturing paradigm for the inexpensive fabrication of electronic devices, sensors, motors, etc. Such a paradigm may allow for the fabrication of mesoscopically complex, atomically precise, true three-dimensional architectures (Heath 1999).

Th us, admitting that the technologies falling under the umbrella term “nanotechnology” have the chance to transform many aspects of our lives, we have to evaluate sensibly the opportunities in nano-materials and nanotechnologies. We also should not have big expectations for their use. Perhaps some applications may never be realized, whereas unanticipated scientifi c breakthroughs may lead rapidly to developments not foreseen at the time of our study. Perhaps opportunities for nanotechnology in the development of materials and technologies will have concrete limitations. For example, a nano level of structure exists in any material, but in most of them, nanosize does not play a role in the development of functional properties. Th erefore, it is useless to turn to nanosizes in such materials.

In many ways, the future of nanotechnology will depend on the ability to engineer two- and three-dimensional systems constructed from complex components such as macromolecules, biomolecules, nanostructured solids, etc. No doubt the principles on which these systems are built should be based on the principles of self-assembly or positional assemby (CRN 2005), and not on technologies based


on manipulation of individual atoms (Oyabu et al. 2003). Th ere are several possible approaches to such molecular manufacturing (Merrill and Sun 2009). A particularly promising nanofabrication strategy is the use of surface chemistry to direct the assembly of one-dimensional nanostructures onto litho-graphically patterned substrates (Law et al. 2004). By modifying the surfaces of the nanowires and the substrate with self-assembled monolayers (SAMs), it is possible to control the attractive and repulsive interactions that dictate where and how nanowires attach to the substrate and to each other. Th ese inter-actions can be of a van der Waals, hydrophobic/hydrophilic, electrostatic, or covalent nature. Mallouk and colleagues have demonstrated the selective adhesion of thick gold nanowires to appropriately func-tionalized gold surfaces using both electrostatic and covalent linkages in solution (Martin et al. 2002). However, it is necessary to note that most of the possibilities discussed in numerous papers, especially popular ones, have never been studied. Naturally, elaboration of such technologies is not a momentary process; it is a long and complex one, which would give results only after fundamental research.

People are often not able to be patient in awaiting realization of their hopes. However, all scientifi c revolutions, even with great names, are made step by step. Th erefore, in spite of the fact that nanoscale materials are already incorporated into hundreds of consumer products, including food, packaging, cos-metics, clothing, and paint, we have to admit that to come to a real industrial-scale realization of the so-called nano-revolution, it is necessary to wait a little. For example, Th ompson and Parthasarthy (2006) have recently considered the question of introducing nanomaterials into logic and memory devices and suggest that 20 years may be needed to develop the necessary processes to do this economically. Th is estimate was based on the time to introduce low-k dielectrics and Cu interconnects.

However, it is necessary to add that all we have said above does not apply to the fi eld of chemical sensors. Th e results presented in this series and in numerous reviews (Comini 2006; Agu et al. 2008; Comini et al. 2008; Kauff man and Star 2008; Korotcenkov 2008; Liu 2008; Weber et al. 2008; Xiao and Li 2008) clearly testify that the use of nanomaterials and nanotechnologies in chemical sensors design is progressing well and has already produced considerable improvement in desired parameters. Th erefore, the development of nanomaterials and nanostructures with novel functionalities and innova-tive properties for high-performance chemical sensing is the most promising current trend in solid-state sensing technology. As will be shown in following chapters, nanotechnology has already reached a fairly mature stage in nanomaterials processing, including synthesis, functionalization, and hybridization with sensing nanodevices, to fabricate chemical sensors and sensor arrays with high sensitivity and high speci-fi city. Nanomaterials in chemical sensors present new possibilities for controlling the properties of signal transducers, new signal transduction technologies, new matrices for immobilization of biomolecules, redox mediators, markers, indicators, etc., and lower detection limits.

7. ACKNOWLEDGMENTS

Th is work was supported by the World Class University (WCU) Program at the Gwangju Institute of Science and Technology (GIST) through a grant (Project No. R31-20008-000-10026-0), by the Ministry of Science and Technology (MEST) of the Korean Government, and by the Korean Science and Engineering Foundation (KOSEF) (Grant No. 2009-0078928). G. Korotcenkov and B. K. Cho are grateful also to the Korean BK21 Program for support of their scientifi c research.


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