X
‐ray emission generation constant from mono‐layer graphene measured using
STEM‐EDS
map detected with highly sensitive
EDS
system
Authors: Jimbo Yu , Sasaki Takeo , Sawada Hidetaka , Okunishi Eiji , Kondo Yukihito 
Published: December 2016

X‐ray generation constant of an atom by an electron irradiated is essential information for quantification of sample in X‐ray fluorescence spectroscopy. The generated X‐ray intensity (
I
) from a sample is described as

I = 1/A ∫
A
∫
t
P(
x
, z)I
k
(
x
) dzd
x

, where A is electron beam irradiated area,
t
is sample thickness,
P
is electron density and

I
k

is X‐ray intensity excited by an electron. To evaluate
I,
it is requested the accurate value of

I
k

. Then, to have accurate

I
k

,
the comparison between

I
k

values from theoretical calculation (

I
k theory

) and experimental result (

I
k exp

) is important. However, it is exacting to measure the

I
k exp

accurately using a multi‐layer specimen because of two difficulties, especially for low energy X‐rays. One is self absorption of emitted X‐ray, which can be judged from the 3D sample shape. And the other is counting a number of atoms included in an analysis area. Namely, it is difficult to grasp the 3D sample shape. The shapes of single atomic chain or single layer specimen are simple and ideal for the aimed experiment in this paper. In this paper, we report the way and results of directly measured X‐ray generation constant of carbon atoms, using a mono‐layered graphene sample, which is easy to estimate 3D shape from electron microscope images.

For experiment, highly sensitive detector system is requested, since the X‐ray signal from this sample is so thin and emit a little of X‐ray. We used a multiple silicon drift detector (SDD) system for the X‐ray measurement, since the SDD has become highly sensitive recently due to design flexibility of its shape and size to fit busy space around the sample of TEM. The microscope we used was an aberration corrected microscope (JEM‐ARM300F). For the new detection system with two SDD , a pair of objective lens pole pieces and a sample holder are re‐designed to make a distance between sample and detectors as short as possible, and each detector size is enlarged to be 100 mm
2
, resulting in the total detection solid angle of 1.6 sr.

Figure 1(c) shows the highly magnified ADF image of mono‐layer graphene from the square indicated by dashed line shown in Fig. 1(a). From the image we can recognize the area of mono‐layer graphens by clear lattice image and hole of the sample. An X‐ray elemental map shown in Fig. 1(b) was obtained simultaneously with ADF image shown in Fig. 1(a). The difference of X‐ray intensity between the hole and the mono‐layer graphene is apparent in Fig. 1(b). This leads that the X‐ray signal from mono layer with dual SDD detection system is well detectable under the dose, at which mono‐layer graphene maintains its structure. The experimental conditions for this experiment were followings: accelerating voltage = 80 kV, probe current = 169 pA, number of pixels = 128 × 128 and total acquisition time = 300 sec.

The

I
k exp

of carbon is estimated to be 3.29×10
‐23
photons*cm
2
/electrons, using the experimental conditions and X‐ray intensity from the mono‐layer graphene area on Fig. 1(b). This is described as

I
k exp

≈ 4π

I
total

/(

D
carbon
*S

*

D
electron

*
W
), where

I
total

is total X‐ray counts (photons) from scan area of mono‐layer graphene,

D
carbon

is density of carbon atoms (atoms/cm
2
) in a analyzed area,
S
is scan area of graphene (cm
2
), W is a solid angle of X‐ray detection and

D
electron

is density of electrons (electrons/cm
2
).

D
carbon

was estimated to be 3.82×10
15
atoms/cm
2
from typical atomic density for a unit area.
S
is 1.38×10
‐13
cm
2
. The amount of X‐ray counts (

I
total

) was measured to be 155 photons accumulated over the area of mono‐layer graphene. The density of electrons (

D
electron

) is simply calculated to be 7.22×10
22
electrons/cm
2
from experimental conditions. The theoretical generation constant (

I
k theory

) was calculated to be 1.85×10
‐23
photons*cm
2
, which was calculated as a product of ionization cross section (ionization atoms*cm
2
/electrons) by Bethe [2] and X‐ray fluorescence yield (photons/ionization atoms) by Burhop [3], which is shown as

I
k theory

≈
σ

k
W
k

, where

σ
k

is ionization cross section and calculated to be 2.70×10
‐20
ionization atoms*cm
2
for carbon,

W
k

is x‐ray fluorescence yield and calculated to be 7.11×10
‐4
photons/ionization atoms for C Ka. The experimental value (

I
k exp

) do not differ from one by pure calculation (

I
k theory

) so much. The difference may be due to error in the pure calculation, since the calculation contains considerable approximate expressions. In conclusion, with the highly sensitive detection system and 2D sheet sample, we could measure a physical constant with considerable accuracy. The similar 2D samples such as BN and MoS
2
sheets should be useful for measuring X‐ray generation constants.