Growth and transmission electron microscopy studies of nanomaterials 1 2

Other areas which will be discussed in this thesis include the relationship between the size of nanomaterials and the catalysts, the influence of growth conditions on the morphology and

Chapter 1 Introduction

Nanotechnology is rapidly emerging as an important research field all over the world because of its potential for revolutionizing every aspects of modern day science Technological bottlenecks in the roadmap for semiconductor technology are creating increasingly more complex problems for scientists which call for a paradigm shift in the fundamental concepts and architecture of electronic devices and materials processing It is inevitable that nanotechnology will play a central role

in this due to the new possibilities presented by dimensional materials, as well as the increasingly demanding manipulation and processing techniques required for the assembly and manufacture of these devices

Dimensional nanomaterials present fundamentally different physical concepts to conventional bulk materials because of their unique density-of-states as well as vibrational and electronic confinements Confinement effects are attributed

to the electronic and vibrational excitations with characteristic lengths comparable to the diameter of the crystallites As a result, substantial modifications of the density

of states and electronic structure should be expected The challenge therefore is to create nanomaterials with a monodispersion in their sizes, and then to study the correlation in properties, size and structure However, the synthesis of nanomaterials with controlled dimensions, desired shapes, as well as oriented growth on a substrate,

is technically non-trivial The growth of uniformly-sized dimensional nanomaterials requires careful control of catalyst size and dispersion matrix to prevent aggregation Ordered orientation of the nanomaterials on a substrate, or selective growth of the nanomaterials for making wire circuitry and devices, in a manner that is compatible with large scale industrial synthesis and conventional microelectronic processing methods, are technologically challenging problems A number of novel strategies have been developed to address these targets

This thesis is motivated by the challenge of synthesizing semiconductor nanomaterials that may have potential applications in electronic devices Using chemical vapor deposition, a range of semiconductor nanomaterials has been successfully synthesized These include BN nanocapsules, ZnS nanowires, SiC nanocones and CuInS2 thin films The internal microstructure of these materials will

be mainly studied using TEM and other characterization tools Other areas which will be discussed in this thesis include the relationship between the size of nanomaterials and the catalysts, the influence of growth conditions on the morphology and structure of final products, the growth mechanism of these nanomaterials, and the mechanism of morphology and phase transfer of these nanomaterials

In the following section, a brief introduction to the science and technology of nanomaterials will be presented Following which, several characterization

techniques will be discussed with particular emphasis on the application of TEM for the characterization of nanomaterials Finally a brief introduction to the following chapters will be presented

“In the great future -we can arrange the atoms the way we want; the very atoms, all the way down!”

-Richard Feynman, 1959 1.1 Nanotechnology

Nanotechnology can be broadly defined as the application of science to develop new materials and processes by manipulating molecules and atoms in the length scales of 1-100 nanometers In general, any technology related to features of nanometer scale: thin films, fine particles, chemical synthesis, advanced microlithography, and so forth can be classified as nanotechnology It is an interdisciplinary field which can impact the traditional disciplines of physics, chemistry and biology at the fundamental molecular and atomic level In terms of engineering, devices are constructed at the molecular scale and function at this scale However, nanotechnology is not simply a miniaturization in size, but an entire paradigm shift in physical concepts, system design and materials manufacturing This technology is expected to allow the construction of very compact and high performance computing devices or molecular sensors High hopes have been pinned

on nanotechnology to provide the impetus for breakthroughs needed in many areas which has reached conventional technological limits

1.2 Nanomaterials

Table 1.1 Examples of dimensional nanomaterials

2-D

Quantum wells

super-thin films (a) InAsP/InGaP multi-quantum wells (Campi R etc, J

Crys Growth) [2]; (b) wurtzite ZnS single-crystal nanosheets (Yu SH etc, Adv Mate.) [3]

1-D

nanowires nanorods nanotubes nanocones

(a) Au nanotubes (Sun Y, Nano Lett.) [4]; (b) Well aligned ZnO nanowires (Huang, M etc Science) [5]

(b ) (a)

The enabling of nanotechnology requires the use of nanomaterials, which refer to materials wherein at least one dimensional size is on nanometer scale (1-100 nm) According to the geometric dimensionality, nanomaterials can be categorized into three groups: Two dimensional (2-D), one dimensional (1-D) and zero dimensional (0-D) nanostructured materials Table 1.1 lists current examples of dimensional nanomaterials and their usage in technologies

Nanomaterials have unique optical, electronic and catalytic properties, which often depend strongly on their size and are very different from the corresponding bulk materials For example, the bandgap of CdS quantum dot can be tuned between 2.5 to 4 eV, while the irradiative rate for the lowest allowed optical excitations ranges from several nanoseconds down to tens of picoseconds when its size decreases to nanometer scale [1]

If the size of the crystal is small enough, quantum confinement due to discrete electron charge or energy levels can be observed macroscopically Quantum confinement means that electrons are trapped in a small area, like particles in a box model The nanomaterials show continuous energies and momentum in free directions, and confinement restricted in 1 (2D nanomaterials), 2 (1D nanomaterials) and 3 (0D nanomaterials) directions The energy separations of the confined states and their numbers depend on the materials property, the length scale and potential offsets Figure 1.1 illustrates the simple relationship between density of states and

confinement dimensions The accurate treatments of confinement require high-order calculations to account for the band structures However, most investigations of quantum confinement now focus on its optical effects, which is similar to the tunable optical excitations of CdS nanodots mentioned above

Figure 1.1 Density of states characterized by the confinement dimension

The interests in nanomaterials are sometimes motivated and sustained by the availability of powerful electron microscope for studying these materials It is only when materials are observed under these microscopes that the possibilities of generating nanosized particles in chemical reactions can be verified The greatly improved resolution and sensitivity of modern microscopes provides the opportunity for the “discovery” of carbon nanotubes by Prof Ijima [8] When Richard Feynman

1D

gave his classic talk in 1959, the highest attainable resolution of transmission electron microscopy (TEM) was only 1 nm The resolution of current state-of-art TEM has improved to 0.19 nm (the size of an average atom) when carbon nanotubes were discovered in 1991 The invention of other new instruments also promotes research on nanomaterials In the early 1980’s, the scanning tunneling microscope (STM) was invented at IBM-Zurich in Switzerland This was the first instrument that was able to “see” atoms in conductive and semiconductor materials A few years later, the Atomic Force Microscope (AFM) was invented, expanding the types of materials that could be investigated to insulating materials Currently, a large number of techniques, such as Scanning Electron Microcopy (SEM), small-angle X-ray Diffraction (SAXRD), scanning auger spectroscopy and Photoluminescence (PL) have been applied to help scientists to obtain more detailed information about nanomaterials

1.3 The applications of TEM on the study of nanomaterials

Among all the characterization instruments, TEM is one of the most powerful tools used routinely and has played the most important role in characterizing nanostructures Its ability for providing information on the internal microstructure of nanomaterials at resolution down to atomic level surpasses that of most other instruments One example is the determination of composition of

quantum dots The spatial resolution of XPS, STM and PL can not determine the compositional variations within the dot but TEM allows the determination of several levels of information: from the elemental mapping of the dot, to the study of the epitaxial interface between the dot and substrate, to the determination of the crystalline quality of the dot [9, 10]

TEM can be operated in various modes for the characterization of the structural and electronic properties of materials (table 1.2) Due to a large number of literatures and publications in this area, focus will be placed on the TEM techniques that are used in this thesis and some recent novel applications used in nanotechnology

Table 1.2 TEM techniques used in nanotechnology

Imaging Morphology, shape and internal structure 1.3.1

Diffraction pattern Internal crystal structure 1.3.1

EELS Chemical and electronic structure; elemental

distribution and phase mapping 1.3.2 Holographic mapping electric and magnetic fields 1.3.3

Observing dynamic phase transformation process

Observing the growth process of nanomaterials 1.3.4.2 in-situ TEM

Nanomeasurement of physical properties 1.3.4.3

1.3.1 Crystal structure of nanomaterials

The most important application of TEM is used for the characterization of the internal nanostructure of materials, a good example is the study of the structure

of carbon nanotube In High Resolution TEM images, the parallel fringes (0002) shown in figure 1.2 are the profile view of tube walls that is tangent to the electron beam The uniform spacing between the parallel fringes that corresponds to the tube walls is 0.34 nm, which indicates the structures are seamless and have a tubular structure [8]

Figure 1.2 HRTEM images of multi-walled carbon nanotubes (Iijima S Nature 354, 56, 1991)

Combined with diffraction patterns, TEM can provide direct imaging of the surface structures, chirality of nanotubes, and the distribution of atoms in one sheet layer Figure 1.3 shows a HRTEM image of a WS2 coated multi-walled carbon

nanotubes [11] The WS2 coating, verified by EDX, shows a fringe which is darker than the layers of carbon surrounding the carbon nanotube

Figure 1.3 HRTEM image of a WS2 partly coated MWCN (arrows indicate amorphous

WO3) (Whitby RLD etc, Chem Phys Lett 2002, 359, 121)

Figure 1.4a shows the diffraction pattern derived from the image in figure 1.3 The (0002) carbon plane is observed as a streak extending across the center of

the image (arrow A, Figure 1.4b) and some faint (1010) carbon spots (black dashed line, Figure 1.4b) are visible outside the ring of WS2 spots The horizontal spots spacing (arrow B) is ca 0.21 nm, corresponding to an armchair edge of (1010) Additionally, two sets of diffraction spots arising from front and rear regions appear

in hexagonal arrays, which match the diffraction pattern of the hexagonal WS2structure The two hexagonal arrays are rotated away from each other by ca 17° and are inclined at ca 8.5° to the CNT axis In other words, the WS2 coating is an 8.5° helical tube The hexagonal pattern, clearly seen between two walls, corresponds to the extension of the WS2 single sheet coating on the back of the CNT, as shown in

Figure 1.4c Individual spots correspond to W atoms The W–W distance is 3.1 Å which is consistent with the WS2 structure HRTEM simulations of WS2 tube were performed to clarify the complex hexagonal structure seen in the WS2 fully-coated area The combination of hexagonal patterns produced from front and rear halves are

in good agreement with experimental results (Fig 1.4a right side)

Figure 1.4 left: (a) Diffractogram obtained from a Fourier transform of the WS2-coated MWCN; (b) An indexed diffractogram, it is not possible to distinguish the front and back layers without further tilting experiments Right: (a) Enhanced TEM image of WS2-coated area; (b) A simulated (90, −14) WS2 tube; (c) Simulated HRTEM image for the front half of

a (90, −14) WS2 tube (E=400 kV, Cs=0.90 mm, def=−384.7 Å, div=0.50 mrad, Drms=100.0 Å) (d) Simulated HRTEM image for the rear-half of a (90, −14) WS2 tube (e) Simulated HRTEM image for the whole (90, −14) WS2 tube (Whitby RLD etc, Chem Phys Lett

2002, 359, 121)

In addition to obtain the structure of nanotubes, the structure of single crystalline nanowires can also be explained by TEM The crystal plane on the surface and the growth direction of one-dimensional nanowires play important roles

in determining their structural stability and influence the electronic and optical properties By changing the experimental parameters and synthesis methods, it is reasonable to achieve precise orientation control during nanowire growth for specific applications of nanowires

Figure 1.5 (a) HRTEM image of a ZnO nanobelt and its Fourier transform; (b) Optical diagram for Shadow image technique for determining the growth direction of a nanowire/nanobelt; (c) Optical diagram for imaging a nanowire under parallel beam illumination and recording the diffraction information from the nanowire by converging the electron beam and under-focusing the objective lens (Wang, Z.L J Phys Chem B 2000,

104, 1153.)

Therefore, it is necessary to determine the growth orientation of nanowires

by TEM It is often important to determine the rotation between the recorded diffraction pattern and the image due to a change in optical mode The most direct method for determining the nanowire growth direction is from a HRTEM image of

the nanobelt and its Fourier transform [12] Figure 1.5a is a HRTEM image of the ZnO nanobelt The Fourier transform of the image gives the growth direction of the

nanobelt clearly as [0110] Shadow imaging is another technique for determining the nanowire growth direction An image and a convergent beam diffraction pattern can be captured simultaneously in diffraction mode (Figure 1.5b), simply by changing the beam focus so that the beam cross over is either under or over the object In this case, the index of the diffraction spot g that is parallel to the nanobelt

is the growth direction, provided the incident beam is perpendicular to the nanowire The third technique for directly determining the nanobelt growth direction is presented in Figure 1.5c The first step is to form a bright field TEM image using a parallel illumination beam and record the image at in-focus condition (left, Figure 1.5c) The second step is to converge the beam onto the specimen, then to underfocus the objective lens so much that diffraction spots appear in image mode (right, Figure 1.5c) The next step is to record this image The index of the diffraction spot parallel to the nanobelt is the growth direction

Hence, TEM can be used not just as a routine characterization technique, but

as a means to improve the quality of nanomaterials based on the observation results

in mass production

1.3.2 Chemical and electronic structure of nanomaterials

Besides information on the crystal structure of nanomaterials, the chemical and electronic structure information can also be obtained at high spatial resolution Under the impact of the incident electrons, the electrons at the ground state atoms may be excited to a free or unoccupied electron level Then the quantum transitions associated with these excitations can provide quantitative chemical and electronic structure information The commonly used techniques for microanalysis in TEM are Energy Dispersive X-ray Spectroscopy (EDX) and Electron Energy Loss Spectroscopy (EELS) EDX is mainly sensitive to heavy elements while EELS is widely used for light elements

Figure 1.6 EELS C-K edge spectra acquired from diamond, C60 and graphite respectively, showing the sensitivity of EELS to bonding states and local electronic states (http://eels.kuicr.kyoto-u.ac.jp/eels.en.html)

The energy-loss near edge structure is sensitive to the phase of the materials, and can be served as a fingerprint for compound identification A typical example is the intensity variation in the σ* and π* peaks observed in the carbon K-edge [13] Diamond is sp3-hybridized so the EELS spectrum is dominated by σ* bonding, while π* bond appears in graphite due to sp2 hybridization Analogous σ* and π* peaks can also be used to identify the presence of cubic and hexagonal Boron Nitride phases in the compound

Spatially-resolved elemental mapping of the elemental distribution is another important method of analysis in TEM Energy filtered images are formed by selecting electrons with certain energy loss corresponding to the atomic inner-shell ionization edges

Now energy filtered TEM has been developed into a routine tool for chemical mapping of interfaces at high resolution, as well as characterizing microstructures in the industry Figure 1.7a shows a bright field image of the diamond particles grown by CVD [14] The diameter of the diamond particle is a few hundred nm, while the particle consists of smaller sub-grains of approximately 20-50 nm in diameter π* and σ* energy filtered maps was reordered to investigate chemical boding of the specimen It is found that the sp2 bonding is mainly localized

in the grain boundaries and the surface of the sub-grains This provides us useful information for obtaining high quality diamond in controlling the grain size of

nanocrystalline diamond particles and in optimizing the growth conditions in the CVD process

Figure 1.7 (a) The TEM image of the diamond particles The carbon K-edge EEL spectrum

of the whole area (b) shows a dominant σ* and a weak π* peak, which is originated from sp2bonding (c ) π* image and (d) σ* image (Okada, K J Appl Phys 2003, 93, 3120)

1.3.3 Holograph imaging of charged and magnetic nanocrystals

Electron holography which is applied to study magnetic materials is another novel application in TEM The electrons in the HRTEM are sensitive to the electrostatic charge or magnetic fields in nanocrystals Holography is based on the

(d) (b)

interference of a reference wave with a wave passing through the area of interest, from which both the amplitude and phase can be determined and the distortions due

to the electron lenses removed The development of high-brightness, high-coherence electron sources (field emission guns) have made it possible to obtain holograms using electron waves in TEM The most frequently used techniques for imaging the magnetic domain structures are Fresnel contrast, and Lorentz microscopy and off-axis electron holography [15]

Figure 1.8 shows the electron phase obtained from 4-nm diameter single crystalline Co nanowires using off-axis electron holography [16] The contours visible along the length of the isolated wire confirm that it is magnetized along its axis The fraction of magnetically active moments in a single wire is measured to be 1.01±0.19, indicating that the wire is fully magnetized throughout its diameter In contrast, the magnetic signal from the multi-branch wire is dominated by the junctions which are thicker in the electron-beam direction than the wires, and have a strong return flux around them Further line profiles confirm that the wires that approach each junction are magnetized along their length Thus, electron holograph

is a powerful technique for determining magnetic fields quantitatively

Figure 1.8 (a), (c) Mean Inner Potential (MIP) contribution to the phase shift for a single Co nanowire and multibrunch nanowire (b) Contours (~0.005 radian spacing) generated from the magnetic contribution to the phase shift for the nanowire, superimposed onto the MIP contribution (d) As for (b), but for the multibranch nanowire with a 0.015 radian contour spacing (e), (f) Line profiles obtained along line 1 from the MIP and magnetic contributions

to the phase shift across the isolated nanowire, respectively (f) Obtained by projecting (b) over a distance along the wire of 200 nm (g, h) As for (e, f), but for line 2 and the multibranch nanowire (Snoeck, E etc Appl Phys Lett 2003, 82, 88.)

1.3.4 Applications of in situ TEM

Ex situ TEM has been applied successfully to phase and structure identification of nanomaterials However, this type of study only provides static information relevant to an unchanging phase, and cannot provide the real-time information on the changes happening during a reaction, where metastable phases, and other dynamic reactions like melting, alloying, phase segregation, crystallization, diffusion, are happening

In situ TEM is ideal for conducting these experiments due to the real time observation at nanometer scale The whole sequences of events can be captured by a video camera, except that in this case, we are observing microscopic or nanoscopic features In situ TEM analyses have been applied to study the following problems:

Temperature and electron beam induced phase transformation to understand the structural stability of nanomaterials

Growth and phase transitions of nanomaterials

In situ measurement of properties of individual nanostructure

1.3.4.1 Temperature and electron beam induced phase transformation

In situ studies of the temperature and electron beam induced phase transformations and chemical evolution of nanocrystals are important for understanding the structural stability of nanomaterials In in situ TEM, a specimen can be cooled to the liquid nitrogen or liquid helium temperatures or heated to 1000

°C while being imaged Surprising events that challenged the commonly hold notions happen with a high frequency when such in situ TEM observations are made, which cause scientists to refine their understanding of materials

Figure 1.9 (a) Spherical concentric-shell carbon onion, generated under electron irradiation (1.25 MeV, 100 A/cm2) of a polyhedral graphitic particle at 1000 K In the core a diamond crystal of 2 nm in size has formed (b) Growth of a diamond crystal inside a carbon onion Image was taken after about 2 h of electron irradiation (1.25 MeV, 20 A/cm2) After about one further hour of irradiation (c) almost the whole particle has transformed into diamond (d) Interfacial region where the stacking in the graphitic shells can be deduced from the

<1100> cross fringes (Banhart F J Appl Phys 1997, 81, 3440.)

For example, the low pressure transformation of graphitic onions into diamond under electron beam irradiation at 1000 K has been observed by Banhart using in situ TEM [17] Normally, thermally activated processes at the interface give rise to the growth of graphite rather than diamond due to the gain in Gibbs free energy However, in this case, carbon atoms are displaced by ballistic knock-on events in the electron beam A higher displacement rate for carbon atoms bound on

b

c

a

d

sp2 sites (graphite) than on sp3 sites (diamond) has been proposed as a reason for the irradiation-induced reversal in phase stability This is governed by the dynamics of interstitials at the interface between the two phases: atoms aggregating to the diamond phase should survive a longer time until they are displaced again by electrons than those aggregating to the graphite phase [18] Figure 1.9 shows [19] that the diamond nucleates at the center of the onions, the interface between graphitic onion and diamond moves toward the graphite region, while the whole onion transforms into diamond The growth of the diamond core, once it has reached

a certain size, proceeds almost isotropically, i.e., without depending much on the crystal orientation This is the first time researchers observed the in situ growth of diamond under low-pressure and medium temperature

Other thermodynamic properties of nanomaterials, such as shape transformation, melting phenomenon of metal particles and stability of nanostructure also can be studied using in situ TEM

1.3.4.2 In situ TEM for observing the growth process of nanomaterials

In situ studies of solid-solid, vapor-solid and vapor-liquid-solid growth reactions are other important aspects for understanding the growth mechanism of nanomaterials

The well-known vapor–liquid-solid (VLS) process has now become a widely used method for generating one-dimensional nanostructures from pure and doped inorganic materials This process was first proposed in 1960s by Wagner [20] to study the growth of large whiskers in solution under optical microscope A typical VLS process starts with the dissolution of gaseous reactants into nano-sized liquid droplets of a catalyst metal, followed by nucleation and growth of single-crystalline rods and then wires [21] The steps are illustrated in the schematic map in Figure 1.10 The one-dimensional nanowire growth is induced and dictated by the liquid droplets, whose sizes remain essentially unchanged during the entire process of wire growth

Figure 1.10 Schematic illustration of vapor-liquid-solid nanowire growth mechanism including three stages (I) alloying, (II) nucleation, and (III) axial growth (Wu, Y.Y et al J

Am Chem Soc.2001, 123, 3165)

Until recently, the only evidence that nanowires grew by this mechanism was the presence of alloy droplets at the tips of the nanowires Yang Peidong’s group has first reported the real-time observations of Ge nanowire growth in an in situ TEM

[22], which demonstrate the validity of the VLS growth mechanism at nanometer scale The sequence of TEM during the growth of a Ge nanowire is shown in figure 1.11 which shows all the steps in figure 1.10

Figure 1.11 In situ TEM images recorded during the process of nanowire growth (a) Au nanoclusters in solid state at 500 °C; (b) alloying initiates at 800 °C, at this stage Au exists

in mostly solid state; (c) liquid Au/Ge alloy; (d) the nucleation of Ge nanocrystal on the alloy surface; (e) Ge nanocrystal elongates with further Ge condensation and eventually a wire forms (f) (Wu, Y.Y et al J Am Chem Soc 2001, 123, 3165)

The three stages can be clearly identified by TEM images:

(I) Alloying process (Figure 1.11(a)–(c)): Under 900 ˚C the Au metal remains solid without Ge vapor With an increasing amount of Ge vapor condensation and dissolution, Ge and Au form an alloy and liquefy (II) Nucleation, (Figure 1.11(d)–(e)): As the concentration of Ge increases in the Au-Ge alloy droplet, the process of nucleation of the nanowire begins The nucleation generally occurs at a Ge weight percentage of 50–60 % from the volume change

(III) Axial growth (Figure 1.11(d)–(f)): Once the Ge nanocrystal nucleates at the liquid/solid interface, further condensation/dissolution of the Ge vapor into the alloy increases the amount of Ge precipitation from the alloy The incoming Ge vapors diffuse and condense at the solid/liquid interface so that it is suppressing secondary nucleation events and pushed forward (or backward) to form nanowires

Figure 1.12 A series of video frames grabbed from observations of GaN decomposition at

1050 °C, showing the real-time GaN nanowire growth process The number on the bottom left corner of each frame is the time (millisecond) (Stach, EA et al Nano Lett 2003, 3, 867)

In 2003, Yang’s group directly observed the growth of GaN nanowires via a self-catalytic VLS mechanism using in situ TEM again [23] The series of video frames grabbed from observations of the GaN decomposition processes at elevated

temperatures is presented in Figure 1.12, which show these GaN nanowires nucleate and grow from Ga droplets formed during thermal decomposition of GaN at elevated temperatures in a vacuum This experiment further demonstrates the powerful capabilities of in situ TEM for observing real time reaction process and confirming the validity of the proposed mechanism

1.3.4.3 In situ measurement of the properties of individual nanostructures

Characterizing the properties of individual nanostructure is crucial for obtaining information specific to the nanostructure, as opposed to information averaged out or at a large area Based on electric field induced resonance excitation, Wang Zhonglin’s group have developed a new experimental approach for characterizing the mechanical properties of nanofibers [24] and the work function at the tip of a single carbon nanotube [25] using in situ TEM

They first manufactured nanobelt with diameters of 50-100 nm and length of 5-10 µm A TEM specimen holder is specially designed for applying a voltage across the nanobelt and its counter electrode One end of the nanobelt is glued on the copper wire, and the other end stands freely near the counter electrode An oscillating voltage with a tunable frequency is applied to the nanobelt When the frequency of the applied voltage matches the natural vibration frequency of the nanobelt, mechanical resonance of the nanobelt can be induced Thus the bending

module along certain direction can be calculated from the resonance frequency of that direction A stationary selected ZnO nanobelt is shown in Figure 1.13a By changing the frequency of the applied voltage, two fundamental frequencies are found in two orthogonal transverse vibration directions Figure 1.13b shows a harmonic resonance with its vibration plane nearly parallel to the viewing direction, and Figure 1.13c shows the harmonic resonance with the vibration plane closely perpendicular to the viewing direction The experimental results from ZnO nanobelts with different dimension size are summarized in table 1.3

Figure 1.13 (top left) A typical TEM image of a ZnO nanobelt and its electron diffraction pattern (inset); (top right) Schematic geometrical shape of the nanobelt (bottom) A selected ZnO nanobelt at (a) stationary, (b) the first harmonic resonance in x direction, υx1=622 kHz, and (c) the first-harmonic resonance in y direction, υy1=691 kHz (Bai XD etc Appl Phys Lett 2003, 82, 4806.)

Table 1.3 Bending modulus of the ZnO nanobelts Ex and Ey represent the bending modulus corresponding to resonance along the thickness and width directions

Fundamental frequency (kHz) Bending Modulus

(Gpa) Nano

Thick ness

T (nm) (±1) W/T υx1 υy1 υy1/ υx1 Ex Ey

Table 1.4 Summary of methods used for the synthesis of nanomaterials

Vapor-Liquid-Soli

d method

Vapor-solid method

Vapor

phase

• Large scale production

• Suitable for fabrication of

• Use of toxic precursors

MBE

Sol-gel Process Hydrothermal Synthesis Solvothermal Synthesis Reduction in Solution Thermal decomposition

phase

• Very simple

• large quantities

• applicable to all materials

• contamination from the milling hardware and environment

Mechanical and mechanochemical milling