Exploring Nanostructures: Nanocharacterization Techniques Unveiled

1. Nanocharacterization (II)

2. “Seeing” at the nano-scale By “seeing” at the nano-scale, we mean (1) being able to literally see size, shape, and structure, (2) being able to determine composition, and (3) being able to determine physical and chemical properties of the tiny, tiny structures made by nanofabrication and used in nanotechnology

3. Seeing at the Nano-scale One way to see things at the nano-scale is to use beams of electrons. These beams can be used to let us see size, shape, structure and even composition.

4. A Beam of Electron Interacts many Ways with a Material when it Impinges • When a beam of electrons hits a material (specimen), a number of responses occur. Some of the electrons may go through the specimen (transmitted electrons). Some may bounce back (backscattered electrons) and new electrons may be knocked off the atoms of the specimen and come back (secondary electrons and Auger electrons). In addition, photons, including high energy photons (x-rays), generated by the relaxing of excited atoms may come back. • Each one of these responses can be exploited to “see” the specimen http://www.microscopy.ethz.ch/interactions.htm

5. Using the Transmitted Electrons to “See” Seeing by using the transmitted electrons is called transmission electron microscopy (TEM). It is called field emissiontransmission electron microscopy (FE-TEM) when the impinging beam of electrons is produced by quantum mechanical tunneling

6. Schematic of a TEM JEOL 2010F Semiconductor Material and Device Characterization, 2nd ed. Dieter K. Schroder, John Wiley & Sons, Inc.

7. Electron Gun is formed from filament, biasing circuit, a cap, and an extraction anode. • The gun creates a beam of electrons exiting from the assembly at a gun divergence semi-angle a. The thermionic current density, J, is related to the workfunction, F, via the Richardson relation • Theoretically the maximum resolution, d, is • For an electron rest mass mo and energy E in the beam, and taking into account relativistic correction

8. TEM Operation • When electron beam is transmitted to a specimen (thinner than 100nm), the incident electrons interact with the atoms in the specimen. • Using different apertures, bright field, dark field, and diffraction images can be obtained. • Atomic scale imaging and structural information (for crystalline material) can be obtained

9. Size, Shape, and Structure Observations using a TEM atomic resolution of silicon (white small dots represent Si atoms) silver nanowire http://www.engr.utexas.edu/che/nano/nanoMaterials/metalNanowires.cfm

10. Using the Backscattered and Secondary Electrons to “See” Seeing by using the backscattered and secondary electrons is called scanning electron microscopy (SEM). It is called field emissionscanning electron microscopy (FE-SEM) when the impinging beam of electrons is produced by quantum mechanical tunneling.

11. Schematic of a SEM Leica Leo 1530 Semiconductor Material and Device Characterization, 2nd ed. Dieter K. Schroder, John Wiley & Sons, Inc.

12. A Size and Shape Observation using an SEM Carbon nanotube http://www.jeol.com

13. Using the X-rays to “See” Seeing by using the X-rays produced by electron impingement is called X-ray Spectroscopy. There are two types: Energy Dispersive X-ray Spectroscopy (EDS) and Wavelength Dispersive X-ray Spectroscopy (WDS) . EDS analyzes the X-rays by their photon energy while WDA analyzes the X-rays by their wavelength.

14. An EDS Instrument • This is the X-ray detector needed. It is installed in electron microscope (TEM or SEM) • Using detection of the X-rays produced from the specimen, this instrument provides spatial maps of the elements present in the regions impacted by the electron beam. • Elemental mapping can be superimposed onto the electron microscopic images and quantitative elemental information (how much of a specific element is present) can be obtained from selected spots in the images. http://www.x-raymicroanalysis.com/pages/tutorial3/detectorprotection.htm

15. SEM Image of Costa Rican Sand quantitative analysis at interesting spots Colors assigned to different elements Ca Na Ni C element mapping based on the SEM image above http://virtual.itg.uiuc.edu/help/ A Size, Shape, and Composition Observation using EDS

16. Seeing at the Nano-scale Another way to see things at the nano-scale is to use beams of ions (charged atoms). These beams can be used to see composition.

17. One technique that uses ion beams is Secondary Ion Mass Spectroscopy (SIMS). This techniques uses the beam of ions to sputter off atoms of the material (sample/specimen) in the form of ions. These ions are analyzed to determine their mass thereby giving a “picture” of the elemental composition of the specimen

18. Schematic for SIMS showing Physics and Actual Tool Layout http://pprco.tripod.com/SIMS/general.htm http://www.gideonlabs.com/mass.htm

19. Seeing at the Nano-scale Another way to see things at the nano-scale is to use beams of light (beams of photons). These beams can be used to let us see size, shape, structure and composition.

20. An Example: Using Light Interaction with Vibrations to “See” One way of seeing by using light is to have the light (photons) excite mechanical vibrations (phonons) in molecules or solids. Molecules and solids can be finger printed by noting the energy taken out of a beam of light by exciting mechanical vibrations as the light scatters. This type of “seeing” can give structure and composition information. It is called Raman Spectroscopy.

21. Raman Spectroscopy Operation • The energy difference between the impinging light (blue lines) and the scattered light (red lines) is proportional to the energy used to excite mechanical vibrations in the sample (specimen). • Raman spectroscopy can provide information on the specimen`s chemical composition and structure.

22. An Example of Raman Spectroscopy Use Raman spectra obtained for a single-crystal silicon wafer, various size SiNW/Rs (widths from 30nm to 200nm), and the control region (a region with no silicon nanowires present) Y. Shan, A. K. Kalkan, C.Y. Peng, and S. J. Fonash, “From Si Source Gas Directly to Positioned, Electrically Contacted Si Nanowires: The Self-Assembling Grow-in-Place Approach”, Nano Letters, Vol. 4, p. 2085-2089, 2004