Elsevier

Carbon

Volume 94, November 2015, Pages 1044-1051
Carbon

High definition conductive carbon films from solution processing of nitrogen-containing oligomers

https://doi.org/10.1016/j.carbon.2015.07.080Get rights and content

Abstract

Carbon films and coatings are industrially very versatile and have a multiplicity of applications, such as conductive coatings or chemical and thermal protection layers. However, in most cases they are fabricated by the expensive vapor process, which has prevented a wider commercialization of these products. Herein, carbon films are fabricated using an inexpensive polymer-solution-based or sol–gel coating process, exemplified by spin coating of thin layers. As carbon precursor, the oligomeric form of acrodam, a high nitrogen-containing and non-volatile compound which is known to have excellent carbon yields, was employed. The oligomer is synthesized from commercially available starting materials (diaminomaleonitrile (DAMN) and acrolein) and dissolves in various organic solvents. The carbonized films were found to be homogeneous, optically flat, void- and crack-free, and were fabricated with up to 800 nm thickness after a carbonization step. High conductivities (up to 334 S cm−1) were achieved at the carbonization temperature of 1000 °C.

Introduction

Carbon films are industrially versatile. Benefiting from its hardness, chemical inertness, low friction coefficient, high refractive index, high thermal and electrical conductivity [1], [2], [3], they have widespread applications as protective coatings for optical windows, automobile parts, biomedical coatings, computer hard disks and micro-electrochemical devices (MEMs), but also for the coating of filler particles and antistatic treatments [1], [4], [5]. The attraction of a conjugated conductive, rather homogeneous carbon films lies in the essential absence of grain boundary effects and minimization of contact resistance. In contrast, for conductive particles as the filler a contact resistance exists between adjacent particles, and the matrix separating the adjacent particles gives rise to additional resistance [7]. The potential application of such films are manifold and cover fields like transparent electrodes and TCO replacement materials (for thin films), surface protection, electric finishing, but also electrocatalysis.

For fabricating such carbon films, vapor processes such as physical vapor deposition (PVD) and chemical vapor deposition (CVD) are mostly employed to date [8]; with sputtering [9], pulsed laser deposition [10], electron cyclotron resonance [11], filtered cathodic vacuum arc process [12], [13] and plasma enhanced chemical vapor deposition [14], [15] as some more detailed examples. PVD and CVD techniques are used to produce thin and ultra-thin coatings of inorganic elements (metals, semiconductors, ceramics) as well as carbon and some organic compounds [16]. They are reliable methods benefiting from the vast number of examples, but their high costs mainly originating from the requirement of high-vacuum thus limiting their industrial applications to higher value products. In addition, the fabrication system becomes complicated and/or more hazardous when other elements such as nitrogen, boron, phosphor and metals [17], [18], [19], [20], [21], [22], [23] are to be introduced, which however can afford profitable properties to the film.

In solution processes on the contrary, incorporation of other elements can be relatively easy performed, e.g., by chemical composition of the soluble precursors. Moreover, solution based coatings can be applied in principle to any surface or object, which hardly can be inserted in high-vacuum chambers used for VD methods. Besides, large-area fabrication at one time is possible in less controlled environments because the coating can be done under ambient atmosphere. In spite of the inherent advantages of solution process, examples for fabricating carbon films are still limited [24], [25], [26], [27], [28], [29], [30]. Examples using graphene oxide (GO) [24], Carbon nanotubes (CNT) [25], formaldehyde resins [26], [27], [28], sucrose [29], polyacrylonitrile (PAN) [27] and nitrogen-containing photoresists [30] are disclosed so far. However, only some of them describe satisfactory homogeneity and conductivity. A conductivity of about 3000 S cm−1 is reported with a CNT-based film [25], but CNTs do not comply with the previously posed industrial constraints, mainly but not only due to the high price. A GO-based film was reported to exhibit 1000 Ω sq−1 at 80% transmittance, but hazardous agents had to be used for reduction, as it is mostly the case with GO materials [24]. As an inexpensive and safe example, a porous carbon film with the high conductivity of 23 S cm−1 could be obtained using a phenolic resol precursor [28]. We especially also want to point to the work of Müllen and coworkers who gave the first case of a transparent conductive graphene electrode [31] for dye sensitized solar cells, but also presented a stretchable, transparent polydopamine film [32] for a variety of applications. Seeking suitable carbon precursors for solution process suitable for industrial applications is nevertheless an attractive operation.

In this contribution, the oligomeric form of a nitrogen-containing precursor, acrodam (Fig. 1) [33], [34], was examined as precursor for carbon three-dimensional films using a solution process. Acrodam was recently employed to prepare high surface-area carbon particles synthesized in a salt flux in one step, which was disclosed to work as supercapacitor electrode with superior energy density [35]. There are some reasons backing the choice of this monomer:

  • The oligomer that can be prepared via consecutive addition and condensation of acrodam is known to be soluble in solvents such as tetrahydrofuran (THF) and acetone [33]; this could facilitate the finding of a suitable solvent system.

  • The carbonization yield for acrodam is relatively high; about 60% was reported in previous work [33]. The high carbonization yield facilitates the fabrication of a crack-free film with sufficient thickness because of the lower shrinkage and inner stresses.

  • The 800 °C carbonized particle as a pressed pellet is reported to have already a comparably high conductivity of 0.06 S cm−1 [34]; therefore, a further increase of conductivity in continuous films and the minimization of grain boundary effects was expected.

  • Acrodam oligomers are close relatives of polyacrylonitrile (PAN), the industrial carbonization of which is well established and for instance base of the mass production of carbon fibers. Acrodam, however, contains a higher content of nitrogen and much less sp3-carbons, which makes the carbonization products at medium temperatures more conductive. One might call polyacrodam the “PAN of conductive carbon structures”.

In this paper, carbon film fabrication by a sol–gel like spin coating process along with film characterization was carried out as an illustration of the possibilities of carbon coating with acrodam. We used silica waver as a flat model substrate, which are known to be terminated with a thin layer of silicium dioxide. Si-wavers can therefore be considered as a model of all flat, slightly polar oxidic substrates.

Section snippets

Materials

All the chemicals in this study including diaminomaleonitrile (DAMN) (Alfa Aesar, 98%), acrolein (Aldrich, 95%), trifluoroacetic acid (TFA) (Aldrich, 98%), triethylamine (Sigma–Aldrich, 99%), n-butyllithium (nBuLi) (Aldrich, 2.5 M in hexanes), Pluronic® P123 (EO20PO70EO20) (BASF), tetrahydrofuran (THF) (VMR, 99.8%), diethyl ether (Merck, 99.7%), hexane (VMR, 95%) and DMF (Sigma–Aldrich, 99.8%) were used as received without further purification. p-Type silicon (1 0 0) substrates were purchased from

Preparation of the carbon precursor for spin coating

Several kinds of oligomers were prepared using acrodam as the starting material by varying the oligomerization catalyst or employing thermal treatment (Table 1). When a small amount of trifluoroacetic acid (TFA) was introduced to the reaction mixture as a catalyst for the 24 h refluxing, the relative number and weight average molecular weights (Mn and Mw) were found to be 1186 and 2595 by gel permeation chromatography (GPC) using dimethylsulfoxide (DMSO) as eluent along with

Conclusion

In this contribution, conductive and conformal carbon films were fabricated by the spin coating method by using solutions of acrodam oligomers as a precursor. The oligomer could be easily synthesized by a one-pot procedure from DAMN and acrolein. The films were found to be homogeneous, flat, void- and crack-free, and could be fabricated in varying thicknesses up to 800 nm. They were found to include considerable amounts of sp3 carbon by EELS and SE analyses. High conductivities (up to 334 S cm−1)

Acknowledgments

Tomikazu Ueno and Tatsuya Sakai (JSR Corporation, Japan) are acknowledged for measurements with XPS, sheet resistance, and thickness by surface profiler. Dr. Manfred Erwin Schuster (Fritz-Haber-Institut) is appreciated for TEM and EELS measurements and for useful discussions.

References (41)

  • S. Liu et al.

    Study of the conductivity of nitrogen doped tetrahedral amorphous carbon films

    J. Non-Cryst. Solids

    (2007)
  • F.M. Wang et al.

    Metallic contacts to nitrogen and boron doped diamond-like carbon films

    Thin Solid Films

    (2010)
  • G.S. Cordeiro et al.

    Degradation of profenofos in an electrochemical flow reactor using boron-doped diamond anodes

    Diam. Relat. Mater.

    (2013)
  • M. Daranyi et al.

    Characterization of carbon thin films prepared by the thermal decomposition of spin coated polyacrylonitrile layers containing metal acetates

    Thin Solid Films

    (2011)
  • S. Waidmann et al.

    High-resolution electron energy-loss spectroscopy of undoped and nitrogen-doped tetrahedral amorphous carbon films

    Diam. Relat. Mater.

    (2000)
  • S.V. Singh et al.

    Hard graphite like hydrogenated amorphous carbon grown at high rates by a remote plasma

    J. Appl. Phys.

    (2010)
  • D. Feng et al.

    Free-standing mesoporous carbon thin films with highly ordered pore architectures for nanodevices

    J. Am. Chem. Soc.

    (2011)
  • D. Chung

    Materials for Electronic Packaging

    (1995)
  • J.G. Buijnsters et al.

    Hydrogen quantification in hydrogenated amorphous carbon films by infrared, Raman, and X-ray absorption near edge spectroscopies

    J. Appl. Phys.

    (2009)
  • C. Casiraghi et al.

    Raman spectroscopy of hydrogenated amorphous carbons

    Phys. Rev. B

    (2005)
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