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Transmission and fluorescence modes

X-ray Absorption measurements can be performed in several modes: transmission, fluorescence and electron yield; where the two first are the most common. The choice of the most appropriate mode to use in one experiment is a crucial decision.

The transmission mode is the most used because it only implies the measure of the X-ray flux before and after the beam passes the sample. Therefore, the adsorption coefficient is defined as [link] . Transmission experiments are standard for hard X-rays, because the use of soft X-rays implies the use the samples thinner than 1 μm . Also, this mode should be used for concentrated samples. The sample should have the right thickness and be uniform and free of pinholes.

μ E = ln I 0 I size 12{μ rSub { size 8{E} } ="ln" left ( { {I rSub { size 8{0} } } over {I} } right )} {}

The fluorescence mode measures the incident flux I 0 and the fluorescence X-rays I f that are emitted following the X-ray absorption event. Usually the fluorescent detector is placed at 90° to the incident beam in the horizontal plane, with the sample at an angles, commonly 45°, with respect to the beam, because in that position there is not interference generated because of the initial X-ray flux ( I 0 ). The use of fluorescence mode is preferred for thicker samples or lower concentrations, even ppm concentrations or lower. For a highly concentrated sample, the fluorescence X-rays are reabsorbed by the absorber atoms in the sample, causing an attenuation of the fluorescence signal, it effect is named as self-absorption and is one of the most important concerns in the use of this mode.

Sample preparation for xas

Sample requirements

Uniformity

The samples should have a uniform distribution of the absorber atom, and have the correct absorption for the measurement. The X-ray beam typically probes a millimeter-size portion of the sample. This volume should be representative of the entire sample.

Thickness.

For transmission mode samples, the thickness of the sample is really important. It supposes to be a sample with a given thickness, t , where the total adsorption of the atoms is less than 2.5 adsorption lengths, µ E t ≈ 2.5; and the partial absorption due to the absorber atoms is around one absorption length ∆ µ E t ≈ 1, which corresponds to the step edge.

The thickness to give ∆ µ E t = 1 is as [link] . where ρ is the compound density, n is the elemental stoichiometry, M is the atomic mass, σ E is the adsorption cross-section in barns/atom (1 barn = 10 -24 cm 2 ) tabulated in McMaster tables, and E + and E - are the just above and below the energy edge. This calculation can be accomplished using the free download software HEPHAESTUS.

t = 1 Δμ = 1 . 66 i n i M i ρ i n i σ i E + σ i E size 12{t= { {1} over {Δμ} } = { {1 "." "66" Sum cSub { size 8{i} } {n rSub { size 8{i} } M rSub { size 8{i} } } } over {ρ Sum cSub { size 8{i} } {n rSub { size 8{i} } left [σ rSub { size 8{i} } left (E rSub { size 8{+{}} } right ) - σ rSub { size 8{i} } left (E rSub { size 8{ - {}} } right ) right ]} } } } {}

Total x-ray adsorption.

For non-concentrate samples, the total X-ray adsorption of the sample is the most important. It should be related to the area concentration of the sample (ρ t , in g/cm 2 ). The area concentration of the sample multiplied by the difference of the mass adsorption coefficient ( ∆µ E ) give the edge step, where a desired value to obtain a good measure is a edge step equal to one, (∆µ E /ρ)ρ t ≈ 1.

The difference of the mass adsorption coefficient is given by [link] , where ( µ E /ρ) i is the mass adsorption coefficient just above (E+) and below (E-) of the edge energy and f i is the mass fraction of the element i . Multiplying the area concentration, ρ t, for the cross-sectional area of the sample holder, amount of sample needed is known.

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
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Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
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Damian Reply
research.net
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sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
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nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
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characteristics of micro business
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Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
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for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
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in general
s.
Graphene has a hexagonal structure
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Source:  OpenStax, Physical methods in chemistry and nano science. OpenStax CNX. May 05, 2015 Download for free at http://legacy.cnx.org/content/col10699/1.21
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