22B02
Study on changes of chemical structures at initial growth surface ofcarbon nanowalls
カーボンナノウォールの初期成長表面における化学構造の変化に関する研究
K. Yasuda1, H. Kondo1, M. Hiramatsu2, M. Sekine1, and M. Hori1,安田幸司, 近藤博基，平松美根男，関根 誠, 堀 勝
1. Department of Electrical Engineering and Computer Science, Nagoya University, Furo-cho, Chikusa-ku,
Nagoya 464-8603, Japan名大院工〒464-8603

名古屋市千種区不老町

2. Department of Electrical and Electronic Engineering, Meijo University, 1-501 Shiogamaguch,Tempaku-ku,
Nagoya 486-8502, Japan
名城大理工 〒486-8502 名古屋市天白区塩釜口 1-501
Elemental composition, chemical bonding state and crystalline structure of initial growth surface of carbon nanowalls(CNWs) were investigated by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). According to
Raman spectra, it is found that nanodiamond structure exists in initially deposited thin carbon film. XPS analyses reveal
that, when nanoislands grow on thin carbon film, chemical bonding state of C 1s changed from sp3 bonding to sp2
bonding, and composition rate of fluorine drastically increased. These results indicate that nucleation of nanographeneoccurs in previously-deposited thin carbon film before its morphological change.

1. Introduction
Vertically-grown graphene sheets, so-calledcarbon nanowalls (CNWs), have attracted muchattention as novel nanomaterials realizing new
functional devices in recent years.[2] It is due to thefact that the CNWs could possess several uniquecharacteristics, such as ultrahigh aspect ratio, large
surface-to-volume ratio, high carrier mobility and soforth. To control morphology of CNWs, it isimportant to clarify initial growth process of CNW,
especially nucleation of nanographene. We havefound that initial growth process of CNWs has 4steps. According to scanning microscopy (SEM), thesteps are as follows: (1) Deposition of thin carbon
film, (2) Formation of nanoislands on the thincarbon film, (3) Nucleation of nanographene on thenanoislands, and (4) Vertical growth of CNWs.
However, details of nucleation processes ofnanographene have not clarified yet.
In this study, elemental compositions, chemicalbonding states and crystal structures of initial growthsurfaces of CNWs were investigated by Raman
spectroscopy and X-ray photoelectron spectroscopy(XPS).
2. Experimental
CNWs were formed on Si substrates at a substratetemperature of 680ºC using a multi-beam chemical
vapor deposition system, which has two radicalsources and an ion source consisting of radio
frequency (13.56 MHz) inductively-coupled plasma
sources.[3] Flow rates of Ar, C2F6 and H2 gases were5, 10, and 6 sccm, respectively. The total gas
pressure was set at 2.5 Pa. RF powers applied to Ar+ion, CFx and H radical sources were 200 W. Anacceleration voltage of Ar+ ion was 100 V. Growthtimes were varied ranging from 2 to 50 min. Then,these samples were investigated by XPS and Raman
spectroscopy.
3. Results and discussion
Fig.1 shows SEM images of initial growth surfacesof CNWs. With increasing growth times from 2 to50 min, deposition of thin carbon film (2min),formation of nanoislands (5 min), nucleation (15min) and growth of nanographenes (22 min and 50min) were found in the SEM images. At that time,thicknesses of those samples increase from 0.8 to35 nm.
(a)

(b)

(c)

(d)

25 nm

Fig.1 SEM images of initial growth surfaces of
CNW: birds-eye view images at (a) 2min (b) 5 minand (c) 22 min, and (d) top-view image at 22 min.

Figure 2 shows Raman spectra of initial layer of3.0D-band

C-SiG-band

2.550 min2.022 min

1.5
1.0

O-O

0.5
0.05 min

2 min1100 1200 1300 1400 1500 1600 1700 1800

Wavenumber (cm-1)

Fig.2 Raman spectra of initial layer of CNWs

Figure 3 shows C 1s spectra of initial layer ofCNWs at growth time from 2 to 22 min. It is foundthat, between 2 min and 5 min, shapes of C 1sspectra drastically change and sp2-bond peaks(284.4 eV) become dominant. On the other hands,
after 5 min, the peak position didn’t change. This
result indicates that the nucleation of nanographeneoccurs at the growth times from 2 min (thin carbonfilm) to 5 min (nanoislands). It is consistent with theresult of Raman spectroscopy.
Figure 4 shows peak intensity ratio of F 1s/C 1s asa function of growth time. Between 3 min and 5 min,composition rate of F drastically increased. On theother hands, it didn’t change after 5 min. Thisbehavior corresponds with the change of C 1s
spectrum. Therefore, it is deduced that change influorine content has some relation with nucleation ofnanographene.

Normalized intensity (arb.unit)

1.02 min
5 min
9 min
22 min

0.8

sp2

sp3

C-Si

0.6
0.4
0.2
0.0
294

292

290

288

286284282

Binding energy (eV)Fig.3 C 1s spectra of initial layer of CNWs
at growth time of 2 to 22 min.
Peak intensity ratio of F 1s/C 1s

Normalized intensity (arb. unit)

CNWs at growth time of 2 to 50 min. G-band (1600cm-1) and D-band (1350 cm-1) were observed at allsamples. The peak at 1450 cm-1 and 1550 cm-1 arederived from C-Si and O-O bonding states,respectively. At only 2 min, there is peak at 1320
cm-1, which indicates existence of nanodiamond.
Between 5 and 22 min, shapes of spectra hardly
changed, however, the position of G-band peakslightly shifted to lower wavelength. In general, suchthe slight shift of G-band peak indicates increase ofsize of nanographene. From these results, at theinitial growth stage of CNWs, thin carbon film
which contains nanodiamond was firstly deposited,
then nanoislands which consist of nanographene
were formed.
Size of nanographene hardly
changes in the nanoislands. It starts to increase
with its vertical growth.

0.040
0.035
0.030
0.025
0.020
0.015
05

10 15 20 25 30 35 40 45 50 55

Growth time (min)Fig.4 Peak intensity ratio of F 1s/C 1s as afunction of growth time.
4. Conclusions
Elemental compositions, chemical bonding stateand crystal structure of initial growth surface ofCNWs were investigated. In this study, it was foundthat thin carbon film contains nanodiamondcomponents, and size of nanographene increased
with its vertical growth. On the other hand, XPS
analysis showed that, when nanoislands wereformed on the carbon film, chemical bonding stateof carbon was changed from sp3 bonding to sp2
bonding and composition rate of fluorine drasticallyincreased. These results indicate that, at thenucleation of nanographene, change of bonding
states occurs previous to morphological change.
References[1] S. Kondo et. al.: J. Appl. Phys. 106, 094302 (2009).
[2] M. Hiramatsu and M. Hori, “Carbon
Nanowalls:Synthesis and Emerging
SpringerWienNewYork,Springer-Verlag, 2010”[3] S. Kondo et. al.: Appl. Phys. Express 3 045102 (2010)