First-principles study of work functions of double-wall carbon nanotubes

Abstract

Using first-principles density functional calculations, we investigated work functions (WFs) of thin double-walled nanotubes (DWNTs) with outer tube diameters ranging from 1nm to 1.5nm.The results indicate that work function change within this diameter range can be up to 0.5 eV , even for DWNTs with same outer diameter. This is in contrast with single-walled nanotubes (SWNTs) which show negligible WF change for diameters larger than 1nm. We explain the WF change and related charge redistribution in DWNTs using charge equilibration model (CEM). The predicted work function variation of DWNTs indicates a potential difficulty in their nanoelectronic device applications. PACS numbers: 31.15.Ar, 72.80.Rj, 73.30.+y * Electronic address: kjcho@stanford.edu 1 Double-walled nanotubes (DWNTs) have attracted considerable attention due to their novel electronic and mechanical properties [1]. Compared to single-walled nanotubes (SWNTs), they have higher mechanical stiffness and greater thermal stability which may be of benefit in field emission devices [2]. Their double shell structure offers the possibility of shielding the inner tube from external perturbations [3]. Such advantages make them very interesting for certain device applications.Utilization of DWNTs in early days were largely hindered by the unwanted carbon nanomaterials generated at the same time with DWNTs during fabrication processes. Over past few years, substantial improvements have been achieved in fabrication and purification techniques of DWNTs. Synthesis of high-quality DWNTs in large scale has been realized using high temperature pulsed arc discharge technique [4] and catalytic chemical vapor deposition [5]. In a more recent experiment, production of DWNTs with experimental yields more than 95% has been reported [6]. Thin DWNTs has also been successfully fabricated by by filling C 60 molecules in SWNTs followed by electron beam irradiation, yielding DWNTs with diameter distribution around 13 ∼ 14Å [7]. In addition, it has now become possible to identify atomic correlations between adjacent graphene layers using high-resolution transmission electron microscopy [8], enabling an unambiguous classification of DWNTs. All these point toward a possible application of DWNTs in nanoelectronic devices.Recently, the potential use of DWNTs as channel materials for field-effect transistors (FETs) has been successfully demonstrated [9], with better subthreshold swing factor as compared to SWNTs. Also, due to different inner/outer tube combinations and inter-layer interactions, DWNTs-based FET yields richer transistor characteristics [10]. For further large-scale integration of DWNT-based FETs, an understanding of DWNT work function is essential, since even work function change on the order of 0.1eV may lead to substantial changes in device characteristics [11].The encapsulation of molecules such as C 60 inside SWNTs are known to modify the electronic properties of carbon nanotubes [12]. Due to their concentric double layer structures, ultra small diameter ...

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