Am in Nano Tracts Final

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Amin Meyghani

final DraftNANOTRACTS PAPER

Non Invasive, Multiscale 3D X-Ray Characterization of

Porous Functional Composites and Membranes, with

Resolution from MM to Sub 50 NM

_____________Condensation of the research

Purpose of the study:

Background:

S H Lau1, Wilson K S Chiu2, Fernando Garzon3, HauyeeChang1, Andrei

Tkachuk1, Michael Feser1, Wenbing Yun1

1Xradia Inc, Concord, CA, United States

2Dept. of Mechanical Engineering, University of

Connecticut, Storrs, CT, USA.

3 Los Alamos National Lab., Los Alamos, NM, USA

Investigation of a non-invasive novel X-ray computer tomographymethod for 3D reconstruction of the materials with resolution near

50nm.

Tomography refers to the reconstruction of 3 dimensional images

from the inner structure of materials by taking multiple 2-dimensional

cross-sections of the material and applying a mathematical algorithm

to the individual slices to generate the 3 dimensional images.

The main method discussed is the x-ray tomography method. In the x-

ray tomography method the sample is placed in front of the x-ray

source and is rotated with particular angles. The images are taken with

these time angles are either averaged or stacked up in order to give the

three-dimensional image of the sample.

In addition to x-ray tomography method, there are different methods

used by researchers in order to reconstruct the 3-D structure of the

materials. The common methods include:

1. Scanning Electron Microscope(SEM) tomography

2. Photoacoustic tomography

3. Atomic Force Microscope tomography(AFM)

4. Laser Scanning Confocal Microscope tomography



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In AFM tomography a cantilever type probe is used to raster scan the sample. The raw data from the

AFM microscope is the matrix of height values sampled across a surface with the size of the matrix

equal to the number of data points measured. The AFM is able to reconstruct the depths of the

calibration gratings within an accuracy of 1 nm.

Laser Scanning Confocal Microscope can be used for confocal data collection. LSCM has the ability

to determine the z-position details in the structure of the sample. To achieve that researchers use very

thin slices of the specimen(the optical slices) in order to reconstruct a 3-D model of the inner

structure of the sample. Based on the maximum pixel intensity of all the slices, the data from the 2D

images of the sample can be stored in matrices and by comparison of the z-values to the XY values,

the 3-D images can be produced.

On the contrary, the Scanning Electron Microscope can give images of the sample of high contrast

and a good depth of field. In SEM tomography the sample is placed on the stage of tomography and

it is rotated relative to the electron beam arriving to the surface of the sample. By tilting the sample

and collecting several 2-D images of the sample, the inner 3-D structure of the sample can be

obtained by finding the common points on the 2-D images. This process is called matching and uses

mathematical algorithms to find the 3-D coordinates of the point which finally leads to the

reconstruction of the 3-D inner structure of the sample.

Moreover, in the other method called Photoacoustic tomography, high optical contrast and high

ultrasonic resolution are combined in order to give the 3-D images that are not only good at having

high contrast but also good for giving high spatial resolution. In this tomography method the 3-D

images are reconstructed by the interaction of the laser and the sample and by the measurement of

the photoacoustic waves that are produced as the result of this interaction.

All of these methods mentioned above, have advantages and disadvantages that will be discussed in

the conclusions section. But, one of the problems of 3-D reconstruction of materials is characterizing

the porous structure. In the next section we will see how researchers have come up with a novel x-

ray tomography method to reconstruct the 3-D inner porous structure of materials. Furthermore, we

can see how they implemented this new technique to reconstruct the 3-D inner structure of soft

membranes as well.

Researcher’s Approach:

One of the keys of the researchers’ success is the implementation of Frenzel Zones. In their technique Frensel

Zone plates are used to focus light. We know that x-rays have low refraction indices (in different materials)

due to their high frequency. Therefore, using Frenzel zones, which work based on the diffraction of light, is a

better choice. The higher the diffraction, the better the resolution. In addition to that, the researchers have

applied two computer tomography (CT) systems(the novel MicroXCTTM and nanoXCTTM from Xradia Inc

(Concord, CA)), which enabled them to configure a multiscale CT solution to the 3-D reconstruction of the

materials. Below, the configuration of the micro CT system is demonstrated in figure 1:



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Figure 1. Schematic of the MicroXCT

“The MicroXCTTM uses a commercial microfocus x-ray source. The sample can be translated in x-y-z

directions in a high resolution precision rotation stage. High resolution and high contrast capability is

achieved with proprietary PhaseEnhanced TM x-ray imaging and detector optics- with effective detector

pixel size < 1 micron.”

Moreover, “Unlike other commercial 2D or 3D x-ray systems in the market, which uses geometric

projection thereby limiting resolution to the spot size of its source, the novel microCT configuration does

not require the use of small spot size x-ray source nor the need for the sample to be placed very close to

the source to achieve high resolution imaging.



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High resolution and high contrast images of a number of polymeric and composite materials with

relatively large sample dimensions and with thickness of several mm can be imaged to 1 to 3 micron

resolution, at large working distance, several mm from the sample.”

Figure 2. Fresnel Zone plates asfocusing optics for x-rays. The

Rayleigh resolution δ of a x-ray

microscope is proportional to theoutermost zone width Δrn of the zone

plate:δ = 1.22Δr n

“The Rayleigh resolution δ of a x-ray microscope is a function of the outermost zone width Δrn of

the objective zone plate:

δ = 1.22Δr n

The resolution of the zone-plate based x-ray microscope is independent of x-ray source spot size

and is ultimately limited by the outermost zone width of the zone plate (finer zones give higher

resolution).”

Figure 3. Schematic of sub-50 nmnanoXCT 3D X-ray

microscope with specialized highly

efficient reflectivecondenser, Fresnel Zone plate optics

and high resolution

imaging detector.



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In the figure the rotation axis can be seen in the middle that has the sample placed on it. Also, the Frensel

Zone plate evident as “objective” in the figure. In the figure above it can be seen that the Frensel Zone

plate is functioning similar to a lens and passing the x-ray through gold phase ring which is ultimatelydetected by the detector.

What Researchers Accomplished:

In this tomography method, researchers managed to successfully reconstruct the 3-D porous structure of

hard functional ceramics, solid oxide fuel cells and proton exchange membranes. In addition to that,

researchers managed to use Frensel zone plates as the objective lens and high resolution and high contrast

detector to achieve resolution of couple of millimeters to sub 50 nm. The significance of their

achievement lies within sample preparation. Modeling the 3-D inner structure of the porous materials has

become much easier with this method compared to SEM tomography where sample preparation is tedious

and the high vacuum might damage the sample. Therefore, this novel x-ray tomography is superior to

other tomography methods in terms of sample preparation, special resolution and image contrast.

Commentary on the research

In this section, we shall discuss the disadvantages and advantages of the different tomography methods

compared to the X-ray Tomography in terms of resolution and 3D reconstruction of materials.

Furthermore, we will suggest different sources for further expansion of the research.

As mentioned in the first section, there are different types of tomography which have their own strengths

and weaknesses. One of the other methods of tomography is neutron tomography. In neutron tomography

neutrons, that are neutral particles, interact with the nuclei of the sample while in x-ray tomography the x-



Amin Meyghani


rays interact with the electrons in the outer shells of the sample. In neutron tomography, neutrons can be

eliminated on relatively thick samples and the light emitted from the sample and absorbed by liquid

nitrogen for example. And by having the sample on a tunable disc that can be rotated from 0 to 180°,

projections are imaged with the parallel beam geometry. By using different cameras, different depth of

field can be produced. Both methods are nondestructive ways of reconstructing the 3-D inner structure of

materials. However, the contrast of the images obtained are different which is the result of different

interactions. In addition to that, these two methods of tomography can be used together with the SEM

tomography to produce better 3-D images. Considering the fact that the SEM images have high contrast,

SEM tomography can be used with the x-ray tomography. For example, the SEM can be used to fine tune

reconstructed images. In addition to that, they can be used for tuning the threshold in the x-ray

tomography. Furthermore, since SEM images can give detailed information from the surface of the

sample, they can be compared to the 3-D data of the volume and the surface obtained by x-ray

tomography. And by this comparison, the threshold estimation for reconstruction of the whole structure

can be done more accurately.

In addition to that, the x-ray tomography can be used with the transmission electron microscope (TEM)

tomography for 3-D reconstruction of materials. TEM tomography is well suited for studying small

biomolecules with nanometer resolution, but difficult for imaging large biological specimens due to

limited penetration. On the other hand, x-ray nanoxct can be used to image large range of biological

samples with sub cellulous cells. Therefore, improving the quality and the accuracy of the 3-D

reconstructed images from x-ray tomography.

Future works:

As suggested by the researchers, this study may be future extended to in situ characterization of intact

PEM fuel cells for observing changes in the internal morphology of the cell under operating condition or

under controlled environmental conditions such as freezing heat and different humidity conditions which

are encountered under normal vehicle operating environment. One suggestion that can help to extend the

future work is integration of x-ray tomography with Electron Cryotomography. This tomography method

is a novel technique based on electron microscopy, flash freezing and maintaining samples at for exampleliquid- nitrogen temperatures. One of the advantages of this method is that it can freeze the cells in such a

manner that no ice crystals are formed. Moreover, no chemical fixing or additional staining (as in SEM) is

required. Below, is the name of some articles which show the integration of x-ray tomography with

Electron Cryotomography, their advantages and disadvantages.



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Moreover, the uncertainty involved when tilting and rotating the samples for reconstruction of the

materials can be considered as another field that can be further studied.

1 - Integration of Crytomography and x-ray tomography:

S Heim, P Guttmann, S Rehbein, S Werner and G Schneider, “Energy-tunable full-_eld x-ray microscopy:Cryo-tomography and nano-spectroscopy with the

new BESSY TXM”, Journal of Physics: Conference Series 186 (2009).

2 – Uncertainty determination of rotation angles:

M. Ritter a, M. Hemmleb b, P. Faber c, B. Lich c, H. Hohenberg, “SEM/FIB STAGE CALIBRATIONWITH PHOTOGRAMMETRIC METHODS”.

References:

[1] J.D. Woodward a,b,n, B.T.Sewell, “Tomography of asymmetric bulk specimens imaged by scanning

electron microscopy.

[2] Lihong V. Wang*, Xueding Wang, Geng Ku, George Stoica, “HIGH-RESOLUTION FUNCTIONAL

PHOTOACOUSTIC TOMOGRAPHY”

[3] Peter Vontobel, Eberhard Lehmann, and William D. Carlson, “Comparison of X-ray and NeutronTomography

Investigations of Geological Materials”, IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 52,

NO. 1, FEBRUARY 2005

[4] Chris Kammerud, Besma Abidi, Shafik Huq, and Mongi Abidi, “ 3D NANOVISION FOR THE

INSPECTION OFMICRO-ELECTRO-MECHANICAL SYSTEMS”

[5] S Heim, P Guttmann, S Rehbein, S Werner and G Schneider, “Energy-tunable full-_eld x-raymicroscopy:

Cryo-tomography and nano-spectroscopy with the

new BESSY TXM”, Journal of Physics: Conference Series 186 (2009).

[6] Y. WANG1, C. JACOBSEN, J. MASER1 & A. OSANNA, “Soft X-ray microscopy with a cryo

scanning transmission

X-ray microscope: II. Tomography”, Journal of Microscopy, Vol. 197, Pt 1, January 2000.

[7] H. Ostadi K. Jiang P.D. Prewett, “Micro/nano X-ray tomography reconstruction

fine-tuning using scanning electronmicroscope images”, Published in Micro & Nano Letters

Received on 7th August 2008, Revised on 3rd September 2008

[8] M. Ritter a, M. Hemmleb b, P. Faber c, B. Lich c, H. Hohenberg, “SEM/FIB STAGE

CALIBRATION WITH PHOTOGRAMMETRIC METHODS”.

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