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Professor Po Wen Chiu's Research on the Growth Mechanisms of Graphene Published in Nature Communications
Prof. Po Wen Chiu of the Department of Electrical Engineering and Institute of Electronic Engineering leads a research team investigating double-layer graphene. Using a scanning transmission electron microscope, they have become the first researchers to observe the process by which carbon atoms form into graphene. Their report on this innovative breakthrough has been published in the June issue of the prestigious British journal Nature Communications (DOI: 10.1038/ncomms5055).
 
Prof. Chiu often points out that the discovery of graphene led to many advances in the field of nanoscience. Firstly, it enabled researchers to use the single-atom thickness of graphene to verify the peculiar zero-mass fermions exhibited by low-energy electrons. Also, graphene is an excellent conductor of electricity and heat, making it highly suitable for high-speed electrical components, touch panels, and transparent conductive film.
 
As for practical applications, large-area graphene films need to be prepared through a process of chemical vapor deposition. The key factors in determining the quality of the graphene film are the grain boundaries and derivatives. Chiu says that understanding the growth mechanism of graphene and how to control its growth characteristics is the focus of his research. However, the graphene structure grows relatively quickly (at a rate of about 100–200 atoms per second), and is therefore difficult to observe.
 
In cooperation with Japan's AIST, Prof. Chiu used bilayer graphene in a single-crystal, small-layer border to form an epipole. When the temperature dropped from 1050° C to 500° C, with the remaining ultra-low hydrocarbon gases as a carbon source, it was possible to control the growth rate of the graphene. Then they used adsorption of single silicon atoms to catalyze the side-edge growth of the graphene. At the same time, they used a scanning transmission electron microscope to simultaneously observe the carbon atoms slowly crystallize one by one, in the process discovering how to use five-ring and seven-ring deficiencies to rotate the direction of the crystalline structure.
 
As Prof. Chiu puts it, "Inventiveness and persistence are the keys to breakthroughs in science and technology!" The keys to success are teamwork and a detailed division of labor, so that every member of the team works in a highly efficient manner. Recalling the process of the experiment, he says that each member of his team is a highly talented, has a high degree of enthusiasm for science, and is willing to spend lots of time and effort to develop new technology.
 
Team members Chun-Chieh Lu and Chao-Hui Yeh, widely acknowledged for their skill in growing graphene, were able to use the chemical vapor deposition method to achieve perfect crystal stacking. Yung-Chang Lin, whose graphene transfer technique is unsurpassed, has become a celebrity, and his contribution has been a key factor in the team's success. Finally, Zheng Liu's superb skill in the use of the electron microscope has been honed over a long period of time. In fact, the contributions of each and every member of the team were indispensable, and the entire team is very pleased to see their efforts recognized.
Professor Po Wen Chiu's team members

Professor Po Wen Chiu's team members

 


NATURE COMMUNICATIONS

In situ observation of step-edge in-plane growth of graphene in a STEM

Contributions : Zheng Liu, Yung-Chang Lin, Chun-Chieh Lu, Chao-Hui Yeh, Po-Wen Chiu, Sumio Iijima & Kazu Suenaga
 
 
Nature Communications 5, Article number: 4055 doi:10.1038/ncomms5055
 
Received 13 January 2014
 
Accepted 07 May 2014
 
Published 02 June 2014
 
Affliations: 
Professor Po-Wen Chiu Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
 
 
Abstract:
It is extremely difficult to control the growth orientation of the graphene layer in comparison to Si or III–V semiconductors. A direct observation of graphene growth and domain boundary formation in a scanning transmission electron microscope, with residual hydrocarbon in the microscope chamber being used as the carbon source for in-plane graphene growth at the step-edge of bilayer graphene substrate provides an important insight into growth orientation of graphene. We show that the orientation of the growth is strongly influenced by the step-edge structure and areas grown from a reconstructed 5–7 edge are rotated by 30° with respect to the mother layer. Furthermore, single heteroatoms like Si may act as catalytic active sites for the step-edge growth. The findings provide an insight into the mechanism of graphene growth and defect reconstruction that can be used to tailor carbon nanostructures with desired properties.
 
Text:
   Direct visualization of how the graphene network evolves during growth is highly desired in an effort to precisely understand the atomic processes of the growth mechanism. To realize well-controlled graphene nanodevices, identifying the orientation relationship between the seed crystal and the growing layer or detecting the influence of the catalytic atoms during growth would be of immense benefit. However, the extremely high CVD growth rate (on the order of micrometres per minute) makes atomic resolution analysis during CVD unfeasible.
   As the vacuum system in an electron microscope is not perfect, the residual hydrocarbon gas in the sample chamber can be used to grow graphene at high temperatures, although the properties of the hydrocarbon gases cannot be characterized. Here we show that graphene grows extremely slowly (several angstroms per minute) in a transmission electron microscope, allowing simultaneous growth and atomic resolution imaging. We visualize the in situ in-plane growth of graphene on the terrace of the first layer using aberration-corrected scanning TEM (STEM), and observe the activity of a single Si atom catalyst during graphene growth on an atomic scale. STEM has the added advantage in that the focused high-density electron beam is able to modify the structure of the sample, and thus, this method can be used to manipulate the graphene growth. Although previous reports have shown reknitting holes in monolayer graphene with the reknitted areas that are composed of many 5–7 defects showing an ‘amorphous’ characteristic, the mechanism of in-plane growth from a step-edge in this study is different from that of reknitting holes. Holes have a limited circumference without substrate, however, edges have a half opened space to grow graphene. More importantly, the newly grown graphene is not amorphous but crystalline in the current study. The relationship between the growth speed of graphene and the residual hydrocarbon gas pressure is also investigated.
Figure 1. Four categories of the growth model. From the left to the right panels are:  from ZZ to ZZ; from reconstructed ZZ (57) to AC; from AC to AC, and from reconstructed AC (57) to ZZ

Figure 1. Four categories of the growth model. From the left to the right panels are:  from ZZ to ZZ; from reconstructed ZZ (57) to AC; from AC to AC, and from reconstructed AC (57) to ZZ

 

Figure 2. (a) Scanning electron microscopy image of a single-crystal bilayer graphene, (b) marked by e-beam lithography, (c) transferred to a Mo TEM grid and (d) schematic drawing of the 2nd-layer graphene step-edge. Hydrocarbon accumulates to the e-beam scanning area. (e) The new growth graphene layer (2+ layer, light blue) at the BLG step-edge. Single Si atoms (pink) located at the step-edge. (f) ADF image in a perspective view showing the e-beam-induced growth from the step-edge of the bilayer graphene where the single Si atoms (brighter contrast) existed.

Figure 2. (a) Scanning electron microscopy image of a single-crystal bilayer graphene, (b) marked by e-beam lithography, (c) transferred to a Mo TEM grid and (d) schematic drawing of the 2nd-layer graphene step-edge. Hydrocarbon accumulates to the e-beam scanning area. (e) The new growth graphene layer (2+ layer, light blue) at the BLG step-edge. Single Si atoms (pink) located at the step-edge. (f) ADF image in a perspective view showing the e-beam-induced growth from the step-edge of the bilayer graphene where the single Si atoms (brighter contrast) existed.

 

 

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