S1-3
Super-low-damage Neutral Beam Etching for Formation of Defect-free Graphene Nanostructure
and Development of Nanodevices
超低損傷中性粒子ビームエッチングによる無損傷グラフェンナノ構造の形成とデバイスへの展開

Seiji Samukawa1,2 and Takeru Okada1
1

寒川誠二、岡田健
Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan
2
WPI-AIMR, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan

Abstract
Graphene quantum dots have attracted much attention due to their potential for many applications, forexample, fluorescence markers in biological and opticaldevices. However, current understanding of graphene
quantum dots and their industrial applications are notsufficient because of their inhomogeneous size and difficulty of orientation. We propose a unique graphene
quantum dot fabrication method to overcome these
problems by using bio-template and the neutral beametching techniques. With this method, we succeeded infabricating graphene quantum dots with a
two-dimensional array structure on a substrate. Photoluminescence and Raman spectra suggest that our fabricated graphene quantum dots are highly oriented andexhibit a narrow size distribution, which will enable
them to be applied in technologies such as optoelectronics.
1. Introduction
Graphene is one of the most interesting materials due toits unique properties; hence, it has attracted much attention
since the discovery of a simple method of obtaining a single layer of graphene [1]. Recently, significant progress has
been reported on the synthesis and characterization of zero-dimensional graphene quantum dots (GQDs), whichemit visible light with quantum confinement and edge effect. Photoluminescence (PL) from GQDs is an especiallyinteresting issue. In fact, strong emission from GQDs in thevisible light region may lead to new possibilities of GQD
applications, e.g., fluorescence bio-markers and light emitting material [2]. Chemical synthesis is well known forsynthesis of GQDs because of its simplicity. The chemical
methods reported thus far involve using acid or heat to oxidize and cut graphene. Thus, size controllability is not high,leading to wide-range emission, which makes industrial use
difficult in terms of light source and handling, especiallyfor optoelectronics applications. Additionally, the wide
distribution of GQD size inhibits basic property investigation; so a homogeneous GQD fabrication method is required. In this paper, we propose a unique top-down nanofabrication method for GQD. Our top-down method isbased on the combination of the bio-template and neutralbeam (NB) techniques [2]. Protein is used as the etchingmask and damage-less etching is possible with the NB
technique, which involves no UV photons or high-energy
charged particles that cause defect formation during etching.

We report on our fabrication method and the optical properties of GQDs on a substrate with high size uniformity.
2. Experiment
Graphene was grown on copper film by using thechemical vapor deposition method [4]. The graphene wascoated with poly-methyl-methacrylate (PMMA) as a supporting material, and the copper film was etched using
FeCl3 solution for more than 4 hours. The sample wascleaned using HNO3 to completely remove the FeCl3. Thesample was then transferred to a sapphire substrate. Afterremoval of PMMA with acetone and annealing in H2/Ar
mixture atmosphere at 350°C for 4 hours, we obtainedgraphene on the substrate. For GQD fabrication, we usedthe combined bio-template and NBE technique. Ferritin isaligned closest to the packing structure on the substrate,
resulting in a two-dimensional dot array. To use it for GQD
fabrication, a 3 nm Si layer was deposited by electron beam
evaporation and oxidized using the NB technique. After
protein removal by annealing, the oxide layer between theFe cores was etched by NF3 radical treatment. Finally, NBEwas carried out using Cl2 gas. The structure was observed
using a scanning electron microscope (SEM), and micro
Raman spectroscopy, and PL measurement was done using
a custom-made system using a 532 nm CW laser.
3. Results and Discussion
Figure 1 shows SEM images of ferritin and an Fe core.
Under optimum conditions, ferritin is aligned closest to thepacking structure, as shown in Fig. 1 (a), and its orientation
is maintained even after the protein removal process. A
two-dimensional array of Fe cores was used as the etchingmask. After the Si cap layer was removed from the graphene, a dot structure was observed, as shown in Fig. 2.
The dots were isolated and in high density. The average
diameter, center to center between dots, and density were
11±4 nm, 19.27±7.21, and 3.2 x 1011 cm-2, respectively.
This is close to the ideal closest packing value. After NBE,
GQDs were obtained, as shown in Fig. 2. Homogeneousness was slightly lost in this process.
Raman spectrum measurement was carried out after Si
cap removal (Fig. 3). The G- and 2D-bands were clearlyobserved even after etching, indicating that the carbon
network of the Graphene was maintained during the process. The decrease in intensity is believed due to the decrease in the amount of Graphene in the measured area byetching. By comparing before and after etching, the D-band

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Intensity (arb. u.)

intensity appeared to increase. One of the possible originsof the D-band is the edge of the Graphene; thus, the intensity change in the D-band can be explained by the increasein the edge ratio in the measured area. In addition, thewidth of the G-band did not increase during the etchingprocess. Although it is not part of quantitative analysis,

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before etching

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Fig. 3 Raman spectra of Graphene and GQDs
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Fig. 1 SEM images of (a) ferritin and (b) Fe core
PL Intensity (arb. u.)

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Fig. 2 SEM image of GQDs with Si cap

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10 K 1.1nm

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defect formation was considered suppressed.
Figure 4 shows the PL spectra of the GQD array withdifferent measurement temperatures. Photoluminescence
was observed at 693 nm, which corresponds to 1.7 eV.
Peak intensity decreased with an increase in temperature,indicating thermal quenching. The peak position slightly
shifted to a lower energy level; however, the shift was lessthan 2 meV. Therefore, our GQDs emitted light independent of temperature. The PL signal was quite sharp and exhibited HMFW of 1.1 nm at 10 K. This sharpness of thepeak is due to the narrow size distribution of the GQDs.
The SEM and PL spectrum imply that the size distribution of our fabricated GQDs is in the range of 7 to 15 nm,
which is a narrower distribution than what was previously
reported [2,5]. Our fabricated size-controlled GQDs onsubstrate will be promising for optoelectronics applications.
4. Conclusion
By using the combined bio-template and NB technique,we succeeded in fabricating GQDs of a two-dimensional
array on a substrate. The GQDs exhibited narrow width of
PL and temperature independence. This GQD array has thepossibility of not only application for optoelectronics, butalso for fundamentally understanding the physics of GQDs.
Acknowledgements
We would like to express our sincere thanks to C-H. Huangand C. Y. Su for Graphene synthesis.

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Wavelength (nm)Fig. 4 PL spectra of GQDs at different temperatures

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Nature Materials, 6 (2007) pp. 183-191.
[2] Y. Li et al., “Nitrogen-Doped Graphene Quantum Dots withOxygen-Rich Functional Groups”, Journal of American
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[3] M. Igarashi et al., “Generation of high photocurrent inthree-dimensional silicon quantum dot superlattice fabricatedby combining bio-template and neutral beam etching forquantum dot solar cells”, Nanoscale research letters, 8 (2013)
pp. 228 (7 pages).
[4] C-H. Huang et al., “Ultra-low-edge-defect graphene nanoribbons patterned by neutral beam”, Carbon, 61 (2013 pp.
229–235.
[5] J. Peng et al., “Graphene Quantum Dots Derived from CarbonFibers”, Nanoletters, 12 (2012) pp. 844-849.