Low-energy electron irradiation induced top-surface nanocrystallization of amorphous carbon film

doi:10.1016/j.apsusc.2016.05.042

Applied Surface Science

Volume 384, 30 October 2016, Pages 341-347

https://doi.org/10.1016/j.apsusc.2016.05.042 Get rights and content

Abstract

We report a low-energy electron irradiation method to nanocrystallize the top-surface of amorphous carbon film in electron cyclotron resonance plasma system. The nanostructure evolution of the carbon film as a function of electron irradiation density and time was examined by transmission electron microscope (TEM) and Raman spectroscopy. The results showed that the electron irradiation gave rise to the formation of sp² nanocrystallites in the film top-surface within 4 nm thickness. The formation of sp² nanocrystallite was ascribed to the inelastic electron scattering in the top-surface of carbon film. The frictional property of low-energy electron irradiated film was measured by a pin-on-disk tribometer. The sp² nanocrystallized top-surface induced a lower friction coefficient than that of the original pure amorphous film. This method enables a convenient nanocrystallization of amorphous surface.

Graphical abstract

Low-energy electron irradiation was proposed to nanocrystallize the top-surface of the as-deposited amorphous carbon film, and sp² nanocrystallites formed in the film top-surface within 4 nm thickness.

Introduction

Amorphous carbon film has been extensively studied since the early 1970s, because of their excellent properties such as low friction, high wear resistance and hardness [1], [2], [3]. Recently, amorphous carbon film was often served as raw material [4] and many attempts have been made to modify the structure of amorphous carbon film in nanoscale for improving or extending the properties of the film [5], [6]. Amorphous carbon could transform to nanocrystallite structure through many kinds of treatments such as thermal annealing [7], [8], laser annealing [9], ion irradiation [10], [11] and electron irradiation [12], [13]. High-energy electron irradiation (80–300 keV) has been found to induce the nanocrystallite structural transformation of amorphous carbon in transmission electron microscope (TEM). The transformation was suggested to be caused by an electronic excitation effect (inelastic electron scattering interaction) [13], [14]. Catalytic graphitization of amorphous carbon film could also be obtained by high-energy (200 keV) electron irradiation without external heating under TEM observation [15], [16]. However, this method was inapplicable for large-scale modification of amorphous carbon film. Lately, we proposed a low-energy (0–100 eV) electron irradiation technique based on electron cyclotron resonance (ECR) plasma, which is convenient for large-scale nanostructured carbon film fabrication [17]. By adjusting the electron irradiation energy and density during film growth, the formation of graphene nanocrystallite could be precisely controlled [18]. And post low-energy electron irradiation also showed a good potential to transform amorphous carbon to sp² nanocrystallite in the top-surface of carbon film [19]. However, the relationship between the structural transformation of amorphous carbon film and the post low-energy electron irradiation condition was unclear, which is necessary to be studied for optimizing the properties of carbon film.

In general, the formation of sp² nanocrystallite in amorphous carbon film was beneficial for the improvement of electrical conductivity [20], while the effect of sp² nanocrystallite on the frictional property of carbon film was quite complicated. On one hand, the weak Van der Waals bond force between the graphene sheets was helpful to achieve low friction [21], [22]. On the other hand, it might also lead to reductions of film wear life and hardness [23], [24]. Our previous study showed that the graphene nanocrystallite embedded in the whole carbon film should be controlled at a certain size in order to maintain good frictional performance [18]. So the nanostructural modification of amorphous carbon film should be high precise and controllable in order to obtain good frictional property.

In this study, top-surface nanostructure of amorphous carbon film was modified systematically with different electron irradiation densities and times. The evolution of film nanostructure was identified by Raman spectroscopy, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The friction coefficient and wear life were obtained by a Pin-on-Disk tribometer. The relationship between carbon nanostructure and frictional property was revealed by presenting a nanostructure-friction diagram.

Section snippets

Film deposition and low-energy electron irradiation treatment

Film deposition and low-energy electron irradiation treatment were performed by using an ECR plasma sputtering system [25]. The experimental procedure is illustrated in Fig. 1. Carbon film was deposited on silicon substrate (p-type <100>) with divergent electron cyclotron resonance (DECR) plasma sputtering method, as shown in Fig. 1(a). Silicon substrate was fixed onto the substrate holder after cleaned in acetone and ethanol bath by ultrasonic wave. The background pressure of the vacuum

Nanostructural transformation of amorphous carbon film

Fig. 2 shows the Raman spectra of the as-deposited carbon film and the films irradiated with different electron irradiation densities and times. For the as-deposited film, the spectrum exhibited a composite band centered around 1500 cm⁻¹ consisting of D and G bands, indicating the amorphous structure of the film. After low-energy electron irradiation, the shape of D band became more distinct with the increasing of irradiation density and time, which revealed the nanostructural change of the

Conclusion

Low-energy electron irradiation method was proposed to modify the nanostructure of the amorphous carbon film. There were two different nanostructural changes of amorphous carbon film after low-energy electron irradiation. One was the formation of sp² nanocrystallites in the film top-surface through inelastic scattering. The other was the bond transformation from sp³ to sp² occurred inside the carbon film by irradiation-induced heating. The nanostructured-induced frictional properties were

Acknowledgement

The authors would like to thank the National Natural Science Foundation of China (No. 51305332 and No. 51575359).

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Low-energy electron irradiation induced top-surface nanocrystallization of amorphous carbon film

Abstract

Graphical abstract

Introduction

Section snippets

Film deposition and low-energy electron irradiation treatment

Nanostructural transformation of amorphous carbon film

Conclusion

Acknowledgement

Tribol. Int.

Surf. Coat. Technol.

Carbon

Appl. Surf. Sci.

Appl. Surf. Sci.

Acta. Mater.

Surf. Sci.

Carbon

Carbon

Tribol. Int.

Appl. Surf. Sci.

Surf. Coat. Technol.

Appl. Surf. Sci.

Chem. Phys. Lett.

Ion-beam deposition of thin films of diamond like carbon

J. Appl. Phys.

Large-scale and patternable graphene: direct transformation of amorphous carbon film into graphene/graphite on insulators via Cu mediation engineering and its application on all-carbon based device

Nanoscale

Effects of pre-heat treatment on tribological properties of DLC film

Tribol. Lett.