Prat I. Effect of E-Beam on Raman Spectra of Carbon Nanotube and Graphene
Chapter 3. Effect of Low-Energy Electron Beam on Nanotubes and Graphene
3.2 Effect of Electron Beam
3.2.1 Hydrocarbon Generated by Low-Energy Electron Beam
Usually nanomaterials are several to hundreds of nm and are not visible through optical microscopes, as is known in Abbe's Diffraction Limit. The electron microscope can accelerate electrons from an electron gun, thus shortening the wavelength of an electron beam. In other words, using short wavelengths of electron beam, nanoparticles are easily seen. The SEM use low-voltage electronic beam (~30keV) to see easy. The SEM is one of the equipment used by many researchers because it is easy to prepare samples. The SEM is essential equipment for researchers who study nanomaterials that are not easily seen with optical microscopes. The SEM is especially important to study carbon nanotubes with diameters of nm and lengths of hundreds of micro-meter (μm) [117]. Some studies reported the importance of SEM to determine the quality of graphene [10]. In graphene research, information on the graphene layer can be obtained through SEM, because the information on the layer is consistent with optical microscopy and Raman spectroscopy [107, 118]. The SEM has many advantages, but it also has disadvantages. As see the Figure 3.3, it is the SEM images of carbon nanotubes (CNTs) using synthesis method of chemical vapor deposition (CVD). When I look
carefully at this image, I can see that there is a rectangle where the electron beam was irradiated for a long time by SEM. The rectangle is hydrocarbons due to electron beam irradiation [119]. Although the SEM is kept in high-vacuum condition, it is known that the hydrocarbon is deposited on the sample surface due to organic gases and the residual gas in the SEM chamber by the electron beam irradiation.
This phenomenon is the e-beam-induced deposition (EBID) and well understood and utilized by many Figure 3.3 SEM image of CNTs with hydrocarbon. After zooming into SEM, zoomed-out image shows that a rectangle is deposited. Upper part is irradiated by e-beam during 1min and bottom part is during 10 min. The bottom part looks thicker than upper part.
Upper
Bottom
10 μm
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researchers. For example, a study of the low thickness of the pattern by depositing the hydrocarbon on the substrate was reported [120]. To research CNTs strength and breaking mechanism, the hydrocarbon is also used to increase adhesion with substrate at the end of CNTs [121]. There is also a lot of research on the deposition of hydrocarbon according to the time and intensity of exposure to electron beam. When the voltage of the electron beam is high, the width of hydrocarbon deposited decreases and its thickness increases [119]. This means that the hydrocarbon was deposited thin and thick at the high voltage of the electron beam. When the current of electron beam increases, in addition, the width of the hydrocarbon increases. Conversely, when the electron beam current is low, the height of hydrocarbon increases.
Hydrocarbon 1
Hydrocarbon 2 (a)
(b)
(d)
(c)
(e)
20 μm
Figure 3.4 Thickness of hydrocarbon following the time. a) SEM images of hydrocarbon on the Si wafer. The hydrocarbon 1 was deposited during 20 min at 10.0K and 1.0kV. Its electron flux was 6.194 x 1022 cm-2. The hydrocarbon 2 was deposited during 10 min. Other condition was the same to hydrocarbon 1. its electron flux was 3.097 x 1022 cm-2. (b) AFM image of hydrocarbon 1. (c) Cross section thickness of hydrocarbon 1. The apex is the highest, the edges the next highest, and the center thickness of the hydrocarbon was the lowest. (d) AFM image of hydrocarbon 2. (e) Cross section thickness of hydrocarbon 2. Compared to hydrocarbon 1, the overall height of hydrocarbon 2 was lower than hydrocarbon 1.
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To measure the thickness of the hydrocarbon, the hydrocarbon was deposited on the empty wafer using Hitachi S-4800 SEM. If SEM examines electron beam at high magnification, it is possible to deposit hydrocarbon with high thickness quickly, but it is difficult to find it in optical microscopes.
However, irradiating an electron beam at a low magnification that is too takes a long time and does not have a high thickness. With considering time and thickness, the hydrocarbon was deposited on wafer at 10,000 magnifications and two hydrocarbons were deposited with different time. As see the Figure 3.4 (a), the hydrocarbon1 in the red rectangle was deposited at 1 kV for 20 min (electron flux:
6.194 x 1022 cm-2). The hydrocarbon 2 in the green rectangle was deposited at 1kV for 10 min (electron flux: 3.097 x 1022 cm-2). When the hydrocarbon is deposited without marker during SEM measurement, I cannot find the hydrocarbon. If I look at hydrocarbon in SEM image, I don’t know
Before annealing After 1 hr annealing (800oc)
After 4 hr annealing (800oc)
After 2 hr annealing (1050oc)
Figure 3.5 SEM image of CNTs and hydrocarbon during annealing process. Before annealing process, the hydrocarbons were blackened by SEM, but they appeared bright when heat treated.
The hydrocarbon was removed at 800 oC (After 1 hr annealing), but continuing heat treatment at 800 oC did not remove the hydrocarbon (After 4 hrs annealing). After increasing the temperature up 1050 oC, the hydrocarbon was removed (After 2 hrs annealing). The scale is 10 μm.
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whether hydrocarbon is deposited, or wafer is etched. That is, information on height can’t be obtained from SEM. The atomic force microscope (AFM) was used to have accurate thickness measurements.
As see the AFM images (Figure 3.4 (b, d)), the hydrocarbon in the edge section is thick and the hydrocarbon in the center section is generally thin. When comparing AFM images of hydrocarbon1 and 2, I see that hydrocarbon is similar over time in the area of the center, and that the edge thickness increases over time. The AFM image shows that sliding occurred during the measurement of SEM.
The hydrocarbon can be removed through the annealing process with high temperature. As before, to deposit hydrocarbon on CNTs, an electron beam was exposed at 1kV for 10 min (electron flux: 3.097 x 1022 cm-2) on CNTs synthesized by CVD technique. As see the SEM images of before annealing, the color of hydrocarbon was dark black, similar to the substrate in SEM image (Figure 3.5). With flowing nitrogen gas at 1.2 L/min, the hydrocarbon was annealed at 800 oC temperautre during 1 hr. After 1 hour of annealing process, the hydrocarbon was checked with SEM. As you see in the SEM images, some of the hydroCarbon was removed, but many of it still remained. The hydrocarbon color changed from dark balck to light gray in SEM images after anenaling. The same sample was annealed during additional four hours at the same temperature. However, even though the annealing time was longer at 800 oC, the hydrocarbon was not removed. The sample was annealed
additionally at 1050 oC for 2 hr at the same condition and then, the hydrocarbon was checked in SEM images. After 1050 annealing process, Most of the central part of the hydrocarbon has been removed.
The edge part was also removed, but some edge part still remained. The hydrocarbon in edge area was thick and needed more time. If the annealing temperature increase to remove the hydrocarbon clearly,
CH-
C- H-
Intensity
Sputtering Time (s)
0 5 10 15
100 μm
(a) (b)
Figure 3.6 TOF-SIMS analysis of hydrocarbon. (a). Optical image of hydrocarbon. It seems clear that hydrocarbon is on top of clean SiO2 wafer. (b). Depth profiles for hydrocarbon TOF-SIMS. H- ,C- and CH- ions were detected in Hydrocarbon. H- ions was detected at first and decreased following the sputtering time. CH- ions was detected at first more than C- ions, but its amount is not more than C- ions. C- ions showed the largest amount in terms of area.
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the CNTs also would be burned out during annealing process.
In many papers, electron beam contamination is called hydrocarbon, but no clear analysis has been found. Therefore, I used TOF-SIMS to analyze the elements of hydrocarbon. The hydrocarbon was deposited on clean SiO2 wafer at 1kV, 10 min (Electron flux: 1.238 x 1020 cm-2). Its area was 400 μm x 300 μm. Using the TOF-SIMS equipment, the analysis of the hydrocarbon was done. (Sputtering analysis, Cs dual sources, Energy: 0.5 keV, Current 30.00nA, Area: 150 x 150 μm2) As see the Figure 3.6 (a), the optical image showed that the hydrocarbon was well attached on SiO2 wafer. As see the data of TOF-SIMS analysis (Figure 3.6 (b)), the H-, C- and CH- ions was detected from the hydrocarbon according to sputtering time. Although CH2- ion was also detected, the intensity is very low (No data). Other ions were not found from the hydrocarbon except Si- and O- that the ions come from the SiO2 wafer. From the TOF-SIMS analysis, I confirm that the hydrocarbon consists of C and H atoms (Figure 3.6). When measuring the Raman spectra of the hydrocarbon, the Raman peak tends to increase background signal by 100 cm-1 to 3000 cm-1 signal. The story of the hydrocarbon affecting signals of Raman spectroscopy has a more complex. I will talk about this later in Part 3.4.
In summary, low-voltage electron beam in SEM makes it impossible to avoid Hydrocarbon deposition. Although the SEM is a high-vacuum condition, the hydrocarbon is deposited on the substrate due to residue gases in Chamber by electron beam. To remove the hydrocarbon, annealing process at high temperature over than 1000 oC is required. From the TOF-SIMS analysis, the hydrocarbon consists of C and H atoms. The hydrocarbon affects background signal of Raman spectroscopy.
3.2.2 Defect Generated in Nanotubes and Graphene by High-Energy Electron Beam
A TEM is an equipment that requires energy of more than 100 keV. Although the TEM is difficult to prepare samples and e-beam alignment require much time, but it provides a single atom level of resolution. It is one of the equipment that is frequently used by researchers using nanomaterials. In particular, graphene and CNTs researchers use TEM frequently to obtain information about structural information, but it is known that atomic damage is caused by the electron beam of TEM using high voltage [122-124]. In other words, TEM's images are often taken with the modified atomic structure by high voltage electron beam. In case of graphite, it is known that there is no structural damage or atom emission up to electron beam at 130 kV [125]. Although nanotubes have sp2 carbon like graphite, it is known to generate knock-on collision more than 86 keV. Compared to graphite, the collision occurs at lower energy. The ballistic ejection occurs when electron beam energy is more than139 keV or higher energy [126]. It's less energy than that of threshold of graphite. In the
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case of graphene, the threshold for e-beam-irradiated damage is also known as 80 to 140 keV [127].
In conclusion, structural defects in nanotubes and graphene often occur in TEM measurement, so it is necessary to use them well for researchers' experiments.