DeJun Fu

JiaRui Liu

Rui Zhang

ShiXu Wang

ZaoDi Zhang

ZeSong Wang

Springer - Publisher Connector

An ultralow-energy negative cluster ion beam system and its application in preparation of few-layer graphene

   
 
© The Author(s) 2012. This article is published with open access at Springerlink.com csb.scichina.com   www.springer.com/scp 
                      
*Corresponding author (email: djfu@whu.edu.cn) 
Letter 
High-Energy Physics September 2012  Vol.57  No.27: 35563559 
 doi: 10.1007/s11434-012-5397-3  
An ultralow-energy negative cluster ion beam system and  
its application in preparation of few-layer graphene 
WANG ZeSong1, ZHANG ZaoDi1, ZHANG Rui1, WANG ShiXu1, FU DeJun1* & LIU JiaRui1,2 
1 Key Laboratory of Artificial Micro- and Nano-Materials of Ministry of Education of China, School of Physics and Technology, Wuhan University, 
Wuhan 430072, China; 
2 Texas Center for Superconductivity, University of Houston, Houston, Texas 77004, USA 
Received April 14, 2012; accepted July 8, 2012 
 
We developed a cluster ion beam system that produces negative cluster beams of C1–C10 with ion current of 4.5 nA–50 A at 
extraction voltages ranging from 6 to 20 kV. The system uses the injector of a tandetron accelerator and was established by in-
serting an electrostatic scanner on its ion-optical line and modifying its Faraday cup into a substrate holder. Utilization of clusters 
enables ultrashallow ion implantation at energies as low as 600 eV/atom without deceleration. Small carbon clusters C2 and C4 
were implanted into Ni/SiO2/Si substrates and following post-thermal treatment graphene was obtained. Raman spectroscopy 
showed characteristic 2D peaks with G-to-2D peak ratios revealing formation of 2–3 layers of graphene. The Raman data reveals 
clear effect of nonlinear cluster-surface interaction in ion beam synthesis of two-dimensional nanomaterials. 
ultralow energy, cluster beam, negative ion, ultrashallow implantation, grapheme, Raman scattering 
 
Citation:  Wang Z S, Zhang Z D, Zhang R, et al. An ultralow-energy negative cluster ion beam system and its application in preparation of few-layer graphene. 
Chin Sci Bull, 2012, 57: 35563559, doi: 10.1007/s11434-012-5397-3  
 
 
 
Graphene as a promising material has attracted much atten-
tion since 2004 for academic interest and technological im-
portance [1]. It has become a potential candidate material 
for fast electronic device applications, owing to its unique 
two-dimensional honey comb lattice structure and extraor-
dinary physical properties. Prior to device applications, con-
trollable process for deposition of graphene has to be estab-
lished. Up to date, mechanical exfoliation of highly oriented 
pyrolytic graphite [1], epitaxial graphitization of silicon 
carbide [2,3], chemical vapor deposition on single crystal 
transition metal such as Ru, Ni, Cu [4–6], and chemical 
reduction graphene oxide methods [7], have been reported. 
Most recently, ion implantation has been attempted to pre-
pare graphene samples [8,9]; however, in the reported pro-
cess, carbon ions at rather high energy (30 keV and 80 keV) 
were used, which would yield a broad depth-profile of car-
bon in the matrix and inevitably generate irradiation defects. 
To prepare good quality two-dimensional nanomaterials like 
graphene or topological insulator films, it is necessary to 
develop low- and ultralow-energy ion beam technologies.  
In this work, we designed an ultralow-energy cluster ion 
beam system based on the cesium sputtering negative ion 
source of the injector of a tandetron accelerator, and ex-
tracted carbon cluster ion beams (C1–C10). As an example, 
the carbon cluster beams were implanted into Ni films to 
form graphene layers, which proves ultrashallow implanta-
tion to be a successful process.  
The basic configuration of the modified injector consists 
mainly of a negative ion source, an electrostatic scanner and 
a sample carrier, as shown in Figure 1. The negative carbon 
cluster ion beam extracted from the ion source is analyzed 
by a switching magnet and scanned by an electric scanner 
prior to impinging on the substrate. Ni films with a thick-
ness of 50 nm deposited on 300 nm-thick SiO2/Si were used 
as substrates for the cluster ion implantation.  
The ion source is a Cs sputtering type negative source  
 Wang Z S, et al.   Chin Sci Bull   September (2012) Vol.57 No.27 3557 
 
Figure 1  Schematic diagram of the negative cluster ion beam system. 
consisting of a Cs reservoir, a cathode, an ionizer and an 
extractor. Since extraction of cluster ion beams is highly 
dependent on the density and morphology of the target [10], 
a high purity (99.999%) graphite rod was chosen for the 
sputtering. Cs atoms evaporated from the heated reservoir 
reach the high temperature region of the armored tantalum 
ionizer and lose their outmost electrons. The resultant Cs+ 
ions are accelerated to penetrate the condensed Cs layer on 
the cathode and sputter the carbon atoms off the graphite 
target. The carbon atoms and clusters passing through the 
Cs layer capture electrons and become negative cluster ions. 
They are then accelerated to the extractor by a voltage set 
between 6–20 kV. 
In Figure 1 the electrostatic scanner mounted between 
two 45° switching magnets was designed to form a 10 mm  
10 mm uniform area on the surface of the substrate, which 
is located 20 cm from the second magnet. The sweep volt-
age on the scanner is only 0–1000 V, and this novel design 
makes the system compact and efficient, in comparison with 
a previous layout [11]. C1–C10 cluster beams were trans-
ported by an extraction voltage of 20 kV to the substrate 
where ion currents of 30–50 A were recorded (Figure 2(a)). 
This means that multiplayer graphene growth needs only a 
few seconds of implantation. The mass spectra in Figure 2(b) 
and (c) show 8–10 times reduction of the current at 10 kV 
compared with Figure 2(a), and a further decrease at 6 kV 
where a few nA C5 clusters are recorded. This is still usable 
since a single atomic layer needs only a few 1015 cm2. For 
a substrate tilted 30°–45° from the beam line, implantation 
with C5 at 6 kV means an energy as low as 600 eV/atom, 
corresponding to a depth shallower than 3 nm.  
As an example of the application of the cluster beams 
and for simplicity, we implanted Ni films by C2 and C4 
clusters extracted by a voltage 20 kV, providing energy per 
atom of 10 and 5 keV, respectively.  
Figure 3 shows depth profiles of C in Ni films obtained 
by SRIM simulation [12], assuming that every atom of the 
Cn clusters carries an equal energy of 20 keV and simulating 
single atom-solid interaction rather than cluster-solid inter-
action. It is clearly seen that the different Cn clusters result 
in different C projectiles, with projected ranges between 
5–25 nm.  
Taking into account of the projectiles, solubility, and re- 
sputtering effect, the implant dose was set 8×1015 cm2 to 
prepare bilayer grapheme [8,9]. After implantation, the sam-
ples were annealed at 900°C for 50 min in vacuum and 
cooled to 725°C (at a rate of 5–8°C/min), followed by 
quenching in the air. 
Confocal micro-Raman spectroscopy (Renishaw RM- 
1000) was employed to characterize the graphene films us-
ing an Ar+ laser with a wavelength of 514.5 nm and a power 
at the sample surface below 0.4 mW. Figure 4 shows the 
Raman spectra of samples prepared by C2 and C4 implanta-
tion. The G bands at 1581–1584 cm1 and 2D bands at 2705– 
2712 cm1 represent typical sp2 structure of crystalline gra-
phene films. The Raman shift of the G peak is lower than 
the single layer graphene. The D bands at 1353–1357 cm1 
are associated with a high density of defects or grain 
boundaries in the samples arising from symmetry breaking. 
The intensity ratios of IG/ID peak (proportional to the in- 
plane crystalline size of the graphene lattice) and IG/I2D are 
9 and 2, respectively, correlated with defects and the num-
ber of graphene layers [13–15]. The results mean that the 
samples are of 2–3 layers of graphene without too many 
defects [9]. The full width at half maximum of the 2D peak 
is 79–80 cm1. The D peak located at 1622 cm1 and D+G at 
2947 cm1 are disorder activated features.  
The number of graphene sheets evaluated from Raman  
3558 Wang Z S, et al.   Chin Sci Bull   September (2012) Vol.57 No.27 
 
Figure 2  Mass spectra of negative carbon clusters produced by a cesium 
sputtering ion source at different extraction voltages (20 kV, 10 kV and   
6 kV).  
scattering is larger than expected, because the implant dose 
of 8×1015 cm2 corresponds to a double graphene density 
that should produce bilayer graphene on the Ni surface [8,9]. 
This difference means that the implantation and damaging 
process is governed by cluster-surface interaction and can-
not be taken as simple linear superposition of individual 
ion-solid interaction process. In fact, the implantation dam-
age increases nonlinearly with the number of atoms of a 
cluster [16]. During annealing at 700°C the damages were  
 
Figure 3  Depth profiles of various Cn clusters implanted in Ni films 
simulated by using the SRIM program.  
 
Figure 4  Raman spectra of graphene prepared by implantation of C2 and 
C4 clusters into Ni/SiO2/Si and annealed at 900°C for 50 min followed by 
quenching from 725°C.  
recovered and the Ni film re-crystallized, creating nuclea-
tion points and C atoms diffused out to the Ni surface from 
the bulk within a few seconds [17]. Carbon atoms segregat-
ed from the Ni film due to supersaturation during annealing 
and subsequent quenching, and diffused to the nucleation 
points, forming nanocrystalline graphene (Figure 3) [15,18].  
Figure 5 shows the Raman spectra of graphene prepared 
by C2 and C4 implantation annealed following a different 
procedure, i.e. annealing at 900°C for 50 min, and cool nat-
urally to room temperature in the furnace ambient. The very 
broad D and G peaks and high intensity ratio ID/IG indicate 
that more defects were induced in Ni films during this heat 
treatment. The IG/I2D peak ratio is almost the same as that in 
Figure 4, which shows stable multilayer graphene. At the 
same time, the D and G+D bands have grown dramatically, 
reaffirming generation of more serious disorder. The com-
parison shows that the annealing followed by quenching 
process is superior to the latter. For clarity, the peak posi-
tions and intensity ratios are summarized in Table 1, which 
shows that the process with larger clusters involves more 
evident nonlinear cluster-surface interaction effect.  
 Wang Z S, et al.   Chin Sci Bull   September (2012) Vol.57 No.27 3559 
 
Figure 5  Raman spectra of graphene by cluster ion implantation 
followed by annealing at 900°C for 50 min and natural cooling to room 
temperature.  
Table 1  Raman peak positions and intensity ratios of graphene 
Cluster used D (cm1) G (cm1) 2D (cm1) IG/ID IG/I2D 
C2 1355 1581 2706 2.85 2.05 
C4 1357 1584 2712 3.0 2.03 
 
In conclusion, we have developed a negative cluster ion 
beam system within the injector of a 1.7 MV tandetron and 
delivered 4.5 nA–50 A analyzed Cn cluster beams to the 
substrate holder. The cluster beam energies were 6–20 keV 
making it possible to conduct ultrashallow implantation 
down to a few nanometers. Small C2 and C4 clusters were 
implanted into Ni films to demonstrate the feasibility of 
graphene synthesis by the cluster ion beam technology. The 
Raman spectroscopy showed characteristic 2D peaks with 
IG/I2D ratios verifying formation of 2–3 graphene. The effect 
of different post-treatment schemes shows subtle but clear 
deviation in the structure of the resultant materials, which 
provides a good example for investigation of the nonlinear 
cluster-solid interaction.  
This work was supported by the International Cooperation Program of the 
Ministry of Science and Technology of China (2010DFA02010). 
1 Novoselov K S, Geim A K, Morozov S V, et al. Science, 2004, 306: 
666–669 
2 Berger C, Song Z, Li X, et al. Science, 2006, 312: 1191–1196 
3 Reina A, Jia X T, Ho J, et al. Nano Lett, 2009, 9: 30–35 
4 Marchini S, Günther S, Wintterlin J. Phys Rev B, 2007, 76: 075429 
5 Kim K S, Zhao Y, Jang H, et al. Nature, 2009, 457: 706–710  
6 Li X, Cai W, An J, et al. Science, 2009, 324: 1312–1314 
7 Eda G, Fanchini G, Chhowalla M. Nat Nanotech, 2008, 3: 270–274 
8 Garaj S, Hubbard W, Golovchenko J A. Appl Phys Lett, 2010 97: 
183103 
9 Baraton L, He Z B, Lee C S, et al. Nanotechnology, 2011, 22: 
085601 
10 Wang X M, Lu X M, Shao L, et al. Nucl Instr Meth B, 2002, 196: 
198–204 
11 Zhang Z D, Wang Z S, Wang S X, et al. Chin Phys Lett, 2012, 29: 
078101 
12 Ziegler J F. SRIM—The Stopping and Range of Ions in Matter (2008 
edition), www.srim.org  
13 Ni Z H, Wang Y Y, Yu T, et al. Nano Res, 2008, 1: 273–291 
14 Ferrari A C, Meyer J C, Scardaci V, et al. Phys Rev Lett, 2006, 97: 
187401 
15 Baraton L, He Z B, Lee C S, et al. Europ Phys Lett, 2011, 96: 46003 
16 Xie Z X, Wang X M, Lu X M, et al. AIP Conf Proc, 2001, 576: 
987–990 
17 Lander J J, Kern H E, Beach A L. J Appl Phys, 1952, 23: 1305–1309 
18 Ni Z H, Wang H M, Kasim J, et al. Nano Lett, 2007, 7: 2758–2763 
 
Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction 
in any medium, provided the original author(s) and source are credited. 
 


file:///data/core-remote/dit/data/Springer-OA/pdf/12e/aHR0cDovL2xpbmsuc3ByaW5nZXIuY29tLzEwLjEwMDcvczExNDM0LTAxMi01Mzk3LTMucGRm.pdf

An ultralow-energy negative cluster ion beam system and its application in preparation of few-layer graphene

Abstract

Similar works

Full text

Available Versions

Springer - Publisher Connector

Springer - Publisher Connector