Int. J. Electrochem. Sci., 13 (2018) 1803 – 1812, doi: 10.20964/2018.02.11
International Journal of

ELECTROCHEMICALSCIENCEwww.electrochemsci.org

Structure and Electrochemical Properties of Nitrogen Doped
Diamond-like Carbon Film Synthesized by Low Temperature
Neutral Beam Enhanced Chemical Vapor Deposition
Qiuhe Wang1, Xijiang Chang1, 2*, Yoshiyuki Kikuchi2, 3, Kumi. Y. Inoue4, 5, Tomohiro Kubota2,
Tomokazu Matsue4, 5, 6, Toshihisa Nozawa3 and Seiji Samukawa2, 6
1

College of Electronic & Electric Engineering, Shanghai University of Engineering Science, Shanghai,
201620, China
2
Institute of Fluid Science, Tohoku University, Sendai, 980-8577, Japan3
TEL Technology Center Sendai, Tokyo electron limited, Sendai, 981-3137, Japan
4
Micro System Integration Center (μ-SIC), Tohoku University, Sendai, 980-0845, Japan
5
Graduate School of Environmental Studies, Tohoku University, Sendai, 980-8579, Japan
6
World Premier International Center Initiative, Advanced Institute for Materials Research, Tohoku
University, Sendai, 980-8577, Japan*
E-mail: 091031001@fudan.edu.cn
Received: 27 September 2017 / Accepted: 19 November 2017 / Published: 28 December 2017

We report nitrogen doping diamond-like carbon film synthesis by a new technique called neutral beamenhanced chemical vapor deposition. The deposition is processed in UV/electron damage-freeenvironment, and the substrate temperature is controlled at room temperature. The film structure wasinvestigated by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The
N/C ratio is 0.26, and doped nitrogen atoms exist in forms of amine and inside the aromatic ring,which helps increase film electrical conductivity. Cyclic voltammograms were carried out and theresults indicate the DLC: N film qualified as electroanalysis sensor material.

Keywords: DLC film, NBECVD, cyclic voltammograms

1. INTRODUCTION
Diamond-like carbon (DLC) has recently been focused on due to its unique characteristics suchas high hardness, wear resistance, chemical inertness, high electrical resistivity, and optical properties[1-4]. Because of these characteristics, DLC film has been widely applied in fields such as magnetic
storage disks, biomedical coatings, solar cells, automotive engineering and tribological applications [5-`Int. J. Electrochem. Sci., Vol. 13, 2018

180411]. As reported, nitrogen (N) doped DLC film (DLC: N) can obtain significantly lower electricalresistivity [12-16], which has led to a new direction of electrode materials for electrochemical
application. Carbon materials have been widely used in electroanalysis, especially for biologicalsensing, due to a number of advantageous properties such as a wide potential window and superior
biocompatibility [17-19]. DLC based film can be synthesized using many methods, including plasma
chemical vapor deposition (PECVD), sputtering, electron beam evaporation, pyrolysis of polymericthin films and ion beam deposition [20-27]. Results from these studies show that the carbon films moreor less require the electrochemical properties for sensor electrode material, but during the synthesis
processing, sometimes high temperatures over 600°C are applied, which is destructive to many
substrates of new advanced biosensors such as Bio-LSI [28]. New technique is required to deposit highquality DLC film with low temperature and furthermore, the film structure is expected to be controlled
via different experimental conditions. In this work, we present a new technique which can satisfy theserequirements and N doped conductive DLC film is deposited on a room temperature, which issignificant for applying such material on bio-sensor related applications.
2. EXPERIMENTAL
In our laboratory, we have employed a natural beam technique for series work on highly
selective and energy-controllable etching, oxidization, and low-k material film deposition [29-36]. The
advantages of the neutral beam technique for plasma processing are the UV/electron damage-free andenergy controllability of the neutral beam, which is found in CVD processing. Thanks to theseadvantages, the precursor dissociation can be tightly controlled, so then the film structure can be
designed [32-34]. Hence, the new CVD technique based on neutral beam, which we call neutral beamenhanced chemical vapor deposition (NBECVD), is a promising technique for the film deposition on
substrates that are thermal sensitive or easily damaged by exposure to plasma.
The DLC: N film deposition has been done with the same equipment used by Kikuchi et al [33].
Figure 1 shows the experimental setup and schematic illustration of deposition process.

(a)`Int. J. Electrochem. Sci., Vol. 13, 2018

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(b)
Figure 1. Schematic of NBECVD system and illustration of deposition process: (a) experimental setup;(b) illustration of DLC deposition processing with toluene as precursor.
The substrate is an 8-inch silicon wafer with a 100 nm platinum interlayer (sputter-deposited,
L-332S-FH; Avelva Corp., Japan), which is treated as a real sensor substrate to investigate theadhesion between DLC: N film and metal surface. The platinum surface is cleaned by the hydrogenneutral beam bombardment to remove attached gas molecules before the deposition. Toluene isselected as a precursor for the formation of a polycyclic aromatic hydrocarbon structure, which canprovide a conjugated system with delocalized π electrons for high electrical conductivity [33-34].
Argon, hydrogen, and nitrogen mixture gas with a ratio of 100:10:30 (sccm) is used for plasma
generation with a 3.25 kW microwave, and then a corresponding mixture neutral beam is obtained in
the deposition chamber by neutralization by a carbon aperture. Next, 150 W RF bias is added on theaperture to increase the neutral beam energy to break the benzene ring of toluene. Nitrogen doping isdone during film growth. After deposition of thirty minutes, the deposited DLC: N film thickness isfound to be in the range of 80-120 nm by measuring with scanning electron microscope (SEM, Hitachi
High-Technologies Corporation, S-5500) and ellipsometer (Gaertner, LSE-2A2W). During theprocessing, the cooling system under the substrate is set at -50°C. As a result, the temperature on thewafer upper surface is below 50°C, which can be realized as room temperature. A four-probe
resistivity system (Kokusai Semiconductor Equipment Corporation, VR-120s) is used to check the film,
and the average volume resistivity is 3.4×10-2 Ω•cm. To investigate the film structure properties,
photoelectron spectroscopy (XPS, KRATOS, AXIS Ultra DLD) and Fourier transform infrared (FTIR,
Thermo Scientific, Nicolet 6700) are employed.
To investigate the electrochemical properties of the DLC: N film, cyclic voltammetry
measurements were done by potentiostat (CompactStat, Ivium Technologies, Netherlands). The DLC:
N sample was prepared in the same conditions mentioned above and cut into 1 cm×2 cm pieces. We
used Elegrip Tape (GD-60-23A, Denka, Japan) pasting on the samples to form a circular electrode area`Int. J. Electrochem. Sci., Vol. 13, 2018

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with a diameter of 2.0 mm. The voltammograms of DLC: N electrode in H2SO4 (H2SO4, Wako Pure
Chemical Industries, Ltd., Japan) was done to check the potential window with a scan rate of 100 mV/s.
Also the cyclic voltammograms obtained at working electrodes of the DLC: N and glassy carbon (GC)electrode with 2 mM ferrocenemethanol (FMA, 97%, Sigma-Aldrich Co., USA) in 0.1 M KCl (Wako
Pure Chemical Industries, Ltd., Japan) solution, separately. The scan rate dependence of the cyclic
voltammograms with DLC: N electrode in FMA/KCl solution was studied with different scan rates
from 0.1 to 5.0 V/m.
3. RESULTS AND DISCUSSION
Figure 2 shows the FTIR spectrum of the N-DLC film deposited on a Si wafer with Pt
interlayer. The absorption bands in the range of 1000-1700 cm-1 are widely observed in CN related
papers [37-40], with several components at ~1570 cm-1 and 1370 cm-1, which correspond to the G and
D graphite peaks observed in Raman spectra. The major bonds are assigned as follows: ~1650 cm-1 as
C=N stretching vibration mode, ~1550 cm-1 as NH2 symmetric stretching mode, ~1440 cm-1 as CH2
bending mode, ~1400 cm-1 and ~1210 cm-1 as C–N starching mode, ~700 cm-1 as CH out of planebending at the edge of PAH, and C=C and C=N vibration modes. The absorption band around 2200
cm-1 is attributed to C≡N stretching mode [41-42]. Furthermore, imine (R–N=C), isonitrile (R–N+≡C-),and carboimide (N=C=N) groups can also give the absorption band here [43-45]. The N–H stretchingvibration mode and C–H vibration modes appear at 2350 cm-1 and 2356 cm-1, respectively.

Figure 2. FTIR spectrum of DLC: N film

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(a)

(b)
Figure 3. XPS spectra from DLC: N film: (a) C 1s; (b) N 1s.
The C 1s and N 1s photoelectron spectra of the N-DLC film are displayed in Fig. 3 (a) and (b),respectively. From the XPS data, the nitrogen concentration in the film was calculated, and the N/Cratio present in the film was 0.26. Because of the existence of nitrogen and aromatic content, the C 1s
center peak shifted to a higher bonding energy than that of DLC. The C 1s region was deconvoluted`Int. J. Electrochem. Sci., Vol. 13, 20181808

into four peaks at 285.6, 286.7, 288.1 and 289.3eV [46-48]. Similarly, the N 1s region was divided into
peaks at 399.7, 400.4 and 401.5 eV [48-49]. The peak at 400.4 eV can be assigned to sp2 NC3 in thearomatic-ring-like pyrrolic or pyridinic structures. The peak at 399.7 eV corresponds to an amine-likestructure, and that at 401.5 eV indicates the protonated nitrogen. For C 1s spectra, the binding energy
at 285.6 eV is assigned to sp2 C bound to a nitrogen atom inside an aromatic structure. The binding
energies at 286.7 and 288.1 eV are assigned to sp3 CN and sp2 CN binding, respectively. The peak at
289.3 eV can be assigned to the sp2 hybridized carbon in the aromatic ring attached to the NH2 group.
The XPS analysis is in good agreement with FTIR analysis and indicates that the nitrogen is present in
the DLC: N film in both forms of amine and ring structures. The nitrogen acted as a weak donor incommon DLC doping processing, but in our case, during the deposition processing, the participation of
nitrogen enhanced the formation of a PAH structure, in which the conjugated system with delocalized
π electrons can provide electrical conductivity for the film. As a result, the DLC: N film can express
higher electrical conductivity than that prepared by other methods [50-51].

Figure 4. Cyclic voltammograms depicting the working potentials windows obtained with DLC: N,
Au and Pt electrodes in 0.5 M H2SO4. Scan rates are 100 mV/s.
Figure 4 is the voltammograms of DLC: N in 0.5 M H2SO4 with a scan rate is 100 mV/s.
Results with Pt and Au electrodes at same solution are also shown as references. No electrochemical
processes occur on the DLC: N surface in the range between oxygen and hydrogen evolution, whichshows a flat potential window with a width about 1.8 V. This value is close to the results of DLC: N
electrode synthesized by the PECVD method [52-53]. Fig. 5 (a) shows the cyclic voltammograms
obtained at working electrodes of the DLC: N and GC electrode with 2 mM ferrocenemethanol in 0.1
M KCl solution. The DLC: N and GC electrodes show almost the same CV curves with ∆Ep of 85 mV,
which indicates the DLC: N film has good electrocatalytic activity for the redox reaction of ferrocene`Int. J. Electrochem. Sci., Vol. 13, 2018

1809[54]. The scan rate dependence of the cyclic voltammograms was studied, as shown in Fig. 5 (b). The
peak current increases in a linear relationship with the square root of scan rate in the range of 10-5000
mV/s, which indicates the process is diffusion-controlled [55]. With these preliminary results, such
DLC: N film can be competent as electrochemical electrode material, which is especially important forthe chips that cannot be processed under plasma environment directly or at high temperature.

(a)

(b)
Figure 5. Cyclic voltammograms in 2 mM ferrocenemethanol (FMA) in 0.1 M KCl solution: (a) CVs
of DLC: N and glassy carbon electrode with scan rate of 100 mV/s. (b) CVs of DLC: N
electrode at different scan rates.

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4. CONCLUSION
As a conclusion, neutral beam enhanced chemical vapor deposition is used for DLC film
synthesis. Toluene is selected as a precursor for the formation of a large polycyclic aromatichydrocarbon structure, which can provide a conjugated system with delocalized π electrons for highelectrical conductivity. Argon/nitrogen/hydrogen mixed plasma is employed for the neural beam
generation, and the neutral beam energy can be adapted in advance to control the deposited film
structure. XPS and FTIR analyses of the film were done, and the aromatic hydrocarbon structure wasconfirmed. The doped nitrogen atoms exist in the film in forms inside the aromatic-ring-like andamine-like structures. Cyclic voltammetry measurements were done with the DLC: N film to examine
its electrochemical properties. It has a wide potential window in 0.5 M sulfuric acid, which is similar
to a graphite electrode, and the cyclic voltammograms in 2 mM FMA solution is the same as that froma standard glassy carbon electrode. Thus, the DLC: N film is a superior electrochemical electrodematerial and can be a candidate for many sensor electrodes, especially bio-sensors, due to the good
biocompatibility of carbon. The remarkable advantages of this technique are that the deposition
processing is totally electron/UV damage resistant to sensor substrates and that the temperature is kept
at room temperature, so it can be used for deposition on heat-sensitive materials. Furthermore, thistechnique is expecting to be used in the improved fabrication of advanced electrochemical sensors
such as bio-LSIs.

ACKNOWLEDGEMENTS
This work was supported by National Natural Science Foundation of China (No. 11705115), Programfor Professor of Special Appointment (Young Eastern Scholar) at Shanghai Institutions of Higher
Learning (No. QD2016036) and Shanghai Local University Young Teacher Training Project (No.
ZZGCD16006).

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