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APPLICATIONS OF SCANNING ELECTRON MICROSCOPY (SEM) IN NANOTECHNOLOGY AND NANOSCIENCE

I. VIDA-SIMITI, N. JUMATE, I. CHICINAS, G. BATIN,

Technical University of Cluj-Napoca, Romania

Received May 28, 2004

Among the techniques of electron microscopy, scanning electron microscopy (SEM) represents a high-performance method of investigating structures and devices in the domain of nanometer dimensions.

This paper is a synthesis of the possible applications of SEM in the investigation of the nanometer domain, nanomaterials and nanotechnologies. It includes the results of the authors’ research work as well as a review of the literature regarding the study and analysis of nanostructured materials, nanotechnologies based on SEM, and the current trends in this cutting-edge field.

Our analysis and investigations presented in the paper represent contributions on the studies and researches regarding to some nanostructured materials: Zn layers electrochemically deposited, Au films deposited in vacuum, carbon nanotubes, organic nanomaterials obtained by sol-gel method.

Key words: scanning electron microscopy, nanomaterials, nanoscience, nano-technology.

1. INTRODUCTION

Starting with the last decade of the past century, terms like nanotechnology, nanoscience, nanoparticles, nanostructures, etc. have been more and more frequently encountered in the special literature of fundamental sciences, engineering, biology, medicine, etc.

The objective of nanoscience and nanotechnology is to study, create or apply materials, devices and systems that could control matter at nanometric or even atomic dimension.

The dimensional range of development of this science and technology is between 1–100 nm, with implications in all the fields of human activity in the near future.

This dimensional field, as a field of development of nanoscience and nanotechnologies, has upper and lower limits. These limits may be considered differently in nanoscience and nanotechnology, though the causes may be the same. In nanoscience, and especially in the case of nanomaterials, the lower limit is conventionally set at about 1 nm (3–4 atoms exist within this distance). This is

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accepted due to the fact that the limit of measurable length is about 0.3 nm (approximately the distance between the atoms of a crystal network). Under this measurable limit, quantic-type phenomena of position indetermination may occur.

The upper limit, conventionally set at about 100 nm, is given by the fact that above this limit the special properties of these types of materials do not manifest.

The modification of the properties of the nanostructured materials is principally the consequence of the following factors:

– The number of atoms within the limits between the grains is much higher (40–50%) than in the classical materials with polycrystalline structure;

– The appearance of quantic effects that begin to manifest within this dimension range.

The concept of nanotechnology refers to ultraprecision processing, construction of miniature machines in the nanometric range, molecular devices, nanorobots, supercomputers, applications in medicine, biology, etc.

Quantic effects, non-significant at a macroscopic level, strongly affect the material properties, the behaviour of mechanical or electronic devices in the nanometer domain.

2. TYPES OF MICROSCOPIC INVESTIGATIONS IN THE NANOMETRIC DOMAIN

The development of materials and technologies in the nanometer domain cannot be conceived without investigations based on microscopy.

The first transmission electron microscope (TEM) was devised in 1931, then the first SEM was devised in 1965, followed by dozens of types of microscopes devised for investigation of nanometric dimensions.

The most used types of microscopes in nanoscience and nanotechnology are electronic, based on the interaction of the electrons with the substance (Fig. 1), and scanning of the specimen surface (Fig. 2). The principle of functioning, their advantages and disadvantages are presented in the following:

a) The Transmission Electron Microscope – TEM – is based on the effect of the structural characteristics of the material to be investigated on the passage of an accelerated electron fascicle through a very thin specimen (electrons transmitted through the specimen, position 7, Fig. 1). The contrast of the image obtained is given by the difference between the absorption coefficients of the different specimen points.

– Maximal resolution is under 0.1 nm at × 106 magnitude. Thus atoms or molecules can be visualized.

3 Applications of SEM in nanotechnology and nanoscience 957

Fig. 1. – Interaction of electrons with the substance: 1 – incident electrons; 2 – Auger electrons; 3 – secondary electrons; 4 – backscattered electrons; 5 – characteristic X radiation; 6 – instrument measuring the absorbed electrons current; 7 – transmitted electrons; 8 – catodo-luminescence [1].

Fig. 2. – Diagram of a scanning probe microscope.

– The specimen shape: under 100 nm thin film. – It may also function with electron diffraction for phase identification,

precipitates, dislocations, etc. – Studies performed on: metallic, ceramic, plastic, biological and other

surfaces. – It may perform local chemical analyses with X-ray spectrophotometry. – Major disadvantage is that the specimen must be very thin (<100 nm),

which sometimes is difficult to achieve.

b) Scanning Electron Microscope – SEM – is based on the scanning of the specimen surface by an electron fascicle and the analysis of the signal (electromagnetic particles and waves) resulting from the interaction between the primary fascicle and the specimen (positions 3, 4, 5 and 6, Fig. 1).


The depth at which information on the specimen is obtained ranges between 1 nm (Auger electrons) and 5 µm (characteristic X radiation). With SEM the contrast may be of the following types: topographic contrast, atomic number contrast, magnetic contrast, etc.

– Maximal resolution is about 25 Å or 5 Å (field emission beam); – Magnifying by 20–500000 times – Atoms or molecules cannot be visualized – Specimens must have good electrical conductivity. Those without

electrical conductivity must be covered by a metallic layer prior to examination or examined in reduced vacuum in order to prevent electrostatic charge.

– Preparation of specimens is relatively easy as compared to TEM. – It may be provided with an X-ray spectrophotometer for local quantitative

(composition) chemical analysis.

c) Scanning Probe Microscope – SPB – (Fig. 2). This technique was developed starting from the conception of the Scanning Tunnelling Microscope (in 1981) and has a number of variants at present. The system consists of a sharp tipped probe electrode controlled by a three-dimensional (3D) positioning unit that includes a piezoelectric element of sub-nanometric resolution and accuracy and a laser optical device. The specimen to be examined is at a distance of about 0.1 nm. The physical magnitudes: tunnelling current, magnetic force, ionic capacity, atomic force, friction force etc. are measures by the interaction between atoms of the specimen and those of the probe.

SPM measures associated superficial properties by a simple but very precise mechanical process based on the scanning of the specimen surface.

The advantages of SPM:

– images obtained are tridimensional at atomic resolution (individual atoms may be distinguished;

– working environment: liquid, gas or vacuum; – temperature: (1–1000 K); – no special preparation of the specimen is required; – contrast does not depend on the atomic number; – no electromagnetic radiations are produced; – surface distributions of the mechanical, electronic, optic properties may

be obtained; – atoms may be manipulated individually in order to modify the surface

structure; – relatively low cost as compared to the other types of microscope.

The disadvantages of SPM:

– low speed of image obtainment (about 30 seconds);


– danger of surface deterioration because of the strong forces at the contact between the specimen and the probe tip;

– local chemical analyses cannot be performed directly.

If the interatomic forces between the specimen and the probe is measured, the SPM is called an Atomic Force Microscope. AFM types provide information regarding the surface topography, elasticity mode, behaviour at friction, magnetic structure, electrostatic, magnetic and Van de Waals forces, etc.

3. EXAMPLES OF APPLICATIONS OF SEM IN NANOTECHNOLOGY AND NANOSCIENCE

Some of the examples of application presented in the form of microphotographs are taken from literature, while others represent research work undertaken at the Department of the Materials Science and Technology, Laboratory of Electron Microscopy of the Cluj-Napoca Technical University.

The second group of examinations were performed using the SEM type Jeol 5600 LV, which has the following characteristic features:

– 3.5 nm resolution with secondary electrons; – × 300,000 magnitude; – examination of non-conductive specimens (ceramic, biological, medical

etc.) in reduced vacuum (up to 130 Pa) with backscattered electrons (maximal magnitude × 5000);

– local quantitative chemical analysis based on the characteristic X-ray spectrum for component elements ranging between Bor and Uranium, with 0.01% detection limits.

3.1. EXAMPLES OF INVESTIGATION OF NANOMATERIALS

Subsequently we made report on the images that represent our investigations on the nanostructured materials. For the other applications examples there are data below the figures.

Fig. 4 presents the image of Au film obtained by vacuum deposition. One can observe discreet structural formations having dimensions below 40 nm.

Figs. 5 and 6 present the microscopically and compositional analysis of a Zn layer deposited by electrolytic process. There were obtained nanostructured systems with dimensions ranged between 100 and 300 nm, for anticorrosive protection.

In Figs. 7 and 8 one can observe the nanoastructures of about 100 nm of some ompounds based on gold deposited on ceramic substrate. c


Fig. 9. – Carbon nanotubes (examination in the Electron Microscopy Laboratory, Cluj-Napoca Technical University).

Fig. 10. – Carbon nanotubes, high resolutionSEM micrograph (Jeol 7400 F microscope) [5].

Fig. 11. – Organic nanomaterial processed bythe sol-gel method (examination in the ElectronMicroscopy Laboratory, Cluj-Napoca Technical University).

Fig. 12. – Nanomaterial processed by the sol-gel method (examination in the Electron Microscopy Laboratory, Cluj-Napoca Technical University).

Fig. 13. – Au/Pd bi-layered metallic nanotubes: 1 – general view; 2a, 2b – details [8].


The shape and dimensions from nanometrically field of some carbon nanotubes one can observe in the images presented in Figs. 9 and 10.

Organic nanomaterials obtained by sol-gel method were investigated in the nanometrically range for different magnitudes and resolutions (Figs. 11 and 12).

Fig. 14. – Polymer nanotubes, longer than 100 µm and pores diameter

between 300 and 900 nm [11].

3.2. EXAMPLES OF INVESTIGATION IN NANOTECHNOLOGY

Fig. 15. – Electrostatic nano-device [3]. Fig. 16. – Nano-manipulator with thermal activation [4].


Fig. 17. – Gripper (115 nm size) for a nanorobotobtained by the CVD procedure and processed with ion fascicle [3].

Fig. 18. – Porous silica film with applications in separations, catalysis, sensors etc. [10].

Fig. 19. – Culture of rat cortical neurons on a silicon plate for

biocompatibility studies [9].


Fig. 20. – Optical network for X rays manufactured by nanolithography [12].

Fig. 21. – CoNi cylinders obtained by nano-lithography and electro-depositing for magnetic registration media [6].

4. CONCLUSIONS

– Future progress in the field of nanoscience and nanotechnologies is not possible without including the investigation methods provided by electron microscopy.

– The scanning electron microscopic methods make possible the overall examination of nanomaterials and nanodevices.

– The scanning electron microscopy (SEM), using EDS or WDS types X-ray analysers, allow local composition analyses and the distribution of elements on the surface examined.

– Preparation of specimens is relatively easy as compared to other microscopic methods.


– Examination of specimens is much faster as compared to SPM, STM, AFM or other methods.

– The specimen surface does not deteriorate during the microscopic examination.

– Scanning electron microscopy (SEM) is complementary to other types of microscopic investigation.

REFERENCES

1. ***, Invitation to the SEM World, JEOL Serving Advanced Technology, 50 p., (1996). 2. N. Taniguchi, Nanotehnologie, Ed. Tehnică, Bucureşti, p. 588 (2000). 3. Brian D. Jensen, Shaped Comb Fingers for Tailored Electromechanical Restoring Force,

Journal Of Microelectromechanical Systems, Vol. 12, No. 3, p. 373, (2003). 4. Xuefeng Wang, Loren Vincent, Minfeng Yu, A Thermally Actuated Three-Probe

Nanomanipulator for Efficient Handling of Individual Nanostructures, Micro Electro Mechanical Systems, 17 th IEEE Int Conf., (2004), p. 442–445.

5. ***, Materialstoday, January, p. 12 (2003). 6. A. Ross, M. Hwang, M. Shima, Micromagnetic Behavior of Electrodeposited Cylinder Arrays,

Physical Review B, Vol. 65, p. 14–17 (2002). 7. Y. Sun, Y. Xia, Growing Shapely Nanocrystals, Materialstoday, March, p. 12, (2003). 8. Lahav et. al., Nanoparticle Nanotubes, Angew. Chem. Int. Ed., Vol. 42, p. 5576–5579 (2003). 9. P. Gould, EU Firms up Funding Scheme, Materialstoday, p. 48–52, April (2004). 10. R. A. Pai et al.,Templates and Mesoporous Films, Science, p. 303–307 (2004). 11. ***, Polymer Nanotubes from Porous Templates, Materialstoday, p. 4, September (2002). 12. M. L. Schattenburg, Nanofabricated Metal Transmission Gratings, Space Nano-technology

Workshop, Tsukuba, Japan, November 27 (2001).

Fig. 3. Silver nanotubes [7]. Fig. 4. – Gold layer in vacuum, particles between 40–400 nm (examination in the Electron Micro-scopy Laboratory, Cluj-Napoca Technical University).

Fig. 5. – Zn layer deposited by electrolysis, 100–300 nm nanostructures (examination in theElectron Microscopy Laboratory, Cluj-Napoca Technical University).

Fig. 6. – X-ray radiation spectrum of the Zn layer (examination in the Electron Microscopy Laboratory, Cluj-Napoca Technical University).

Fig. 7. – Gold layer deposited on faience plate, nanostructures of approximately 100 nm (exami-nation in the Electron Microscopy Laboratory, Cluj-Napoca Technical University).

Fig. 8. – X-ray spectrum of the gold layer (exami-nation in the Electron Microscopy Laboratory, Cluj-Napoca Technical University).

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