chp5 nano materials characterization

8/4/2019 Chp5 Nano Materials Characterization

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Chapter 5

Nanomaterials: Characterization

5.1 Goals

Overview of characterization techniques.

Read:

P. S. Hale et al., J. Chem. Educ. 82 (5), 775 (2005). Growth kinetics and modeling ofZnO nanoparticles.

5.2 X-Ray Diffraction

Waves of wavelength comparable to the crystal lattice spacing are strongly scattered (diffracted).Analysis of the diffraction pattern allows to obtain information such as lattice parameter,crystal structure, sample orientation, and particle size. We will only mention that latticeparameters are obtained from the Bragg formula:

2d sin = n, (5.1)

where d is the lattice spacing.In a typical set-up, a collimated beam of X-rays is incident on the sample. The intensity

of the diffracted X-rays is measured as a function of the diffraction angle 2 (Fig. 5.1). Theintensities of the spots provide information about the atomic basis. The sharpness and shapeof the spots are related to the perfection of the crystal. The two basic procedures involveeither a single crystal or a powder. With single crystals, a lot of info about the structure

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62 CHAPTER 5. NANOMATERIALS: CHARACTERIZATION

Figure 5.1: A table-top XRD set-up and results for a w-ZnS nanowire.

can be obtained. On the other hand, single crystals might not be readily available andorientation of the crystal is not straightforward.

One disadvantage of XRD is the low intensity of diffracted beam for low-Z materials.

5.3 Optical Spectroscopy

Optical spectroscopy uses the interaction of light with matter as a function of wavelengthor energy in order to obtain information about the material. For example, absorption oremission (photoluminescence or PL) experiments with visible and UV light tend to revealthe electronic structure. Vibrational properties of the lattice (i.e., phonons) are usually inthe IR and studied either using IR absorption or Raman spectroscopy. Raman is an example


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5.3. OPTICAL SPECTROSCOPY 63

of an inelastic process whereby the energy of the incoming light is changed. The others areelastic processes where the intensity is changed. Typical penetration depth is of the orderof 50 nm. Optical spectroscopy is attractive for materials characterization because it is fast,nondestructive and of high resolution.

5.3.1 UV-vis spectroscopy

This technique involves the absorption of near-UV or visible light. One measures bothintensity and wavelength. It is usually applied to molecules and inorganic ions in solution.Broad features makes it not ideal for sample identification. However, one can determine theanalyte concentration from absorbance at one wavelength and using the Beer-Lambert law:

A = log(I

I0) = a b c, (5.2)

where a = absorbance, b = path length, and c = concentration. A schematic of the techniqueis shown in Fig. 5.2, together with a sample data.

Figure 5.2: Schematic of UV-vis spectrophotometer, an apparatus and a sample data on ZnO

nanoparticles. Absorption spectra for a colloid aged at 650

C. The spectra were recorded at0, 1, 3, 5, 11, 16, 30, 60, 90, and 120 min after immersion in the aging water bath [9].


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5.3.2 Absorption and photoluminescence

An example of PL data on semiconducting nanowires was previously given in Fig. 2.7. Ab-sorption results are shown in Fig. 5.3.

Figure 5.3: Absorption data from bulk cubic ZnS and w-ZnS nanowire.

5.3.3 Raman spectroscopy

The Raman process describes the excitation of vibrational modes (phonons) in the sampleusing light, whose frequency is then reduced due to energy conservation. Such a scatteringprocess is weak; experimentally, a laser is needed for good signal.

Raman spectroscopy has been used to characterize the chirality of carbon nanotubes(Fig. 5.4).

5.4 Optical Microscope

The basic principles of image formation is illustrated in Fig. 5.5.

5.5 TEM and SEM

Electron beams can be used to produce images. The basic operation in a transmissionelectron microscope (TEM) is for electrons to be generated from an electron gun, which arethen scattered by the sample, focussed using electrostatic lenses, and finally form images(Fig. 5.6).


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5.5. TEM AND SEM 65

Figure 5.4: Raman spectrum on CNTs [10].

Figure 5.5: Principles of optical image formation.

A typical accelerating voltage is 100 kV for which the electrons have mean free paths

of the order of a few tens of nm for light elements and a few hundreds of nm for heavyelements. These would be the ideal film thicknesses since much thinner films would leadto little scattering and much thicker ones would lead to too many scattering of the sameelectron resulting in a blurred image of low resolution.

The imaging mode can be controlled by the use of an aperture. If most of the unscatteredelectron is allowed through, the resulting image is called a bright-field image. If specificscattered beams are selected, the image is known as a dark-field image. Examples are shownin Fig. 5.7. In addition to forming images, a TEM can be used for chemical analysis andmelting-point determination.

If the TEM is operated in scanning mode, it is known as a scanning electron microscopeor SEM. Electrons scattered from the sample are collected on a CRT to form the image. Theresolution is a few nm and magnification is from 10 to 500,000 times. One such SEM isshown in Fig. 5.8.


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Figure 5.6: TEM.

5.6 Scanning-Tunneling Microscope

The scanning-tunneling microscope (or STM) is one of the most powerful microscopes avail-able. It provides atomic-scale resolution of surfaces and is also being developed to moveatoms on surfaces. According to its inventors, G. Binnig and H. Rohrer of IBM Zurich, itwas first operational in 1981. They won the 1986 Nobel Prize for this work.

The STM relies on a purely quantum-mechanical phenomenon: tunneling. The maincomponents are drawn in Fig. 5.9(a). The STM relies on the fact that electrons near surfaceshave wave functions which decay into the vacuum outside the surface boundary [Fig. 5.9(b)].The microscope consists on a conducting tip connected to a current-measuring circuit. Whenthe tip is in close proximity to the surface ( 1 nm), the decaying wave function from thesurface could overlap with the tip, i.e., the surface electron has a finite probability of beingin the tip. Since the latter is conducting, the electron under a voltage field ( 110 V) can


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5.6. SCANNING-TUNNELING MICROSCOPE 67

Figure 5.7: TEM. ZnS nanowires.


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Figure 5.8: SEM.


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5.6. SCANNING-TUNNELING MICROSCOPE 69

Figure 5.9: Principles of a scanning-tunneling microscope. (a) Set-up. (b) Physics.

then move creating a current. This current is known as a tunneling current whose magnitude(as for all tunneling currents) is very sensitive to the surfacetip separation (the region inbetween effectively acting as a barrier):

I V e2d, (5.3)

where is the wave number of the electrons. The current should also depend on the densityof electron states. A constant current would correspond to a constant altitude of the tip withrespect to the surface (Fig. 5.10). Hence a constant-current scan of the two-dimensional planereveals the surface structure. The tip motion can then be converted into a grayscale image.The other mode of operation of an STM is in constant height mode (Fig. 5.10). Note that,classically, the system consists of an open circuit, hence there should be no current.

The extreme sensitivity to the sampleprobe separation translates into a very high pre-cision in the vertical height ( 0.01 nm), with the separation being typically around 1 nm.Indeed, the piezoelectric mechanism has extension coefficients of A/V, resulting in veryaccurate vertical movement of the tip for low voltages (typically, mV for metals and V forsemiconductors due to the latters band gap). The lateral resolution, on the other hand, isdetermined by the tip size which can be down to a single atom and the best resolution is 0.1 nm. Prior to the STM, traditional microscopes have been based on light and electrons.


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Figure 5.10: STM. Operating modes.

The resolution of the latter types of microscopes is limited by diffraction effects; basically,the resolution wavelength /2. An example of an STM image is shown in Fig 5.11.

Example

visible 500 nm.e(1 keV) =

hp

= h2mE

0.1 nm.While the electron microscope can display atomic resolution, the high energy required

means that the primary electron beam penetrates deep into the material and is not sensitive

to surface features. Both of the above types of microscopes require focussing elements. Amajor new feature of the STM are the moving parts (controlled by piezoelectric materials)and the shielding of the latter from external vibrations (using electromagnetic damping viaeddy currents).

A limitation of the STM is the requirement for a conducting sample. A variation onthe STM for insulators is the atomic-force microscope (AFM) which operates on the atomicforce between the sample atoms and the probe atoms (in contrast to the tunneling currentfor the STM).

STM chemistry

The STM has also been used to carry out chemical reactions on surfaces. This relies on atipsample interaction. The force can be either attractive or repulsive.

Thus Hla et al. [1] induced all the steps of the Ullmann reaction with the STM tip


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5.7. ATOMIC FORCE MICROSCOPY 71

Figure 5.11: InAs surface structure.

(Fig. 5.12); this has the potential to lead towards single-molecule engineering. The reactionis

2C6H5I + 2Cu C12H12 + 2CuI.

Steps Chemical STMreaction reaction

dissociation of thermal electrons

iodobenzene 180 K 1.5 eV2 phenyl diffusion tip dragginggroupsassociation molecular excitation

electrons

5.7 Atomic Force Microscopy

The AFM differs from the STM in that what is being measured is the force between thesample and the tip. The AFM operates like a record player except that it has flexiblecantilevels, sharp tips, and a force feedback system (Fig. 5.13). The spring constant of thecantilever is of the order of 0.1 N/m which is about ten times more flexible than a slinky.


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Figure 5.12: STM-catalyzed chemical reaction.

Since no electric current is involved, the tip/sample does not have to be metallic. Thereare two modes of operation: contact mode whereby the sample-tip distance is so small thatthe important force is the core-core repulsive one, and noncontact mode where the force isthe van der Waals one. AFMs can achieve a resolution of 10 pm.

Examples of AFM images are shown in Fig. 5.14.


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5.7. ATOMIC FORCE MICROSCOPY 73

Figure 5.13: AFM. Schematic.


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Figure 5.14: Patterns formed using AFM in polymers, 80 nm deep. From Nature Materials2, 468472 (2003).

chp5 nano materials characterization

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