Tuning Work Function of Multi-Walled Carbon Nanotubes By Defects Creation
Authors: Gong Bo , Chen Jie , Ikematsu Akitoshi , Waki Keiko 
Published: February 2020

Carbon nanotube(CNT) is a very attractive material to various electronic devices and optoelectronic applications owing to its promising structural and electronic properties comparing with other carbon materials.1CNT has been successfully applied on field emission displays, light emitting diodes, solar cells, etc. To these electronic or optoelectronic devices which require modification of the junction interface to achieve a well-defined electronic energy level leading to a desired performance, work function is a critical parameter. It is of importance to develop a systematic methodology to manipulate CNT’s work function to match diverse energy structures.
Impacts of multi-walled carbon nanotube(MWCNT) surface functional groups on its work function has been poposed,2 which provides a potential strategy of tailoring an appropriate work function for different applications. In our previous study,3we found that creating defects on MWCNT benefits planting abundant functional groups on it, this would contribute to control work function in a wide range.
In this work, a surface functional group decoration methods associated with coating, defects creation, surface functional groups decoration and doping on MWCNT for tuning work function has been designed and carried out. Inductively coupled plasma mass spectrometry(ICP-MS) are employed to distinguish the impact of impure metal elements, since the raw CNT always contains some metal impurities which may have negative effect on investigating the origin of work function changes. Annealing MWCNT samples in different temperatures to remove certain functional group is conducted to identify which certain functional group responsible for the work function tuning. Photo-electron spectroscopy in air (PESA)* and ultraviolet photoelectron spectroscopy (UPS) are conducted to measure modified CNT’s work function directly and also provide information of density of state (DOS) near Femi level. Temperature programed desorption(TPD) and X-ray photoelectron spectroscopy(XPS) analysis provides estimation of different functional group amount and compelling evidence of doping and coating. Field emission scanning electron microscopy(FESEM) and transmission electron microscopy(TEM) are adopted to observe the morphology of modified MWCNTs. Structure disorder information is supplied by Raman spectroscopy.
It has been revealed that a 0.55 eV work function difference between different modified WMCNTs is achieved at current stage (Figure 1). The further modification parameters will be presented on the conference.

PESA*:
It is an instrument for Photoelectron Spectroscopy at atmospheric pressure. UV photons emitted from a deuterium lamp are monochromated by a grating spectrometer and focused on a sample. Then, photoelectrons emitted from the sample are counted by an open counter. In terms of metal, when the photon energy is smaller than work function, there is no electron reach counter. If the photon energy is larger than work function, the electron from sample starts to be counted by open counter which indicates the exact value of work function. After plotting data in a graph, the crossing point of the back ground and the yield line is a photoemission threshold energy, also called the work function or the ionization potential.
Acknowledgement:
This work partially was supported by COI STREAM from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Shin-Etsu Chemical Co., Ltd. Japan. and Riken Keiki Co., Ltd. Their contribution is greatly appreciated.
Reference:
1 S. Iijima, Nature, 1991, 354, 561–58.
2 H. Ago, T. Kugler, F. Cacialli, W. R. Salaneck, M. Shaffer, A. H. Windle and R. H. Friend, J. Mater. Chem. B, 1999, 103, 8116–8121.
3 K, Waki, R. A. Wong, H. S. Oktaviano, T. Fujio, T. Nagai, K. Kimoto and K. Yamada, Energy Environ. Sci., 2014,7, 1950-1958

Figure 1